Mikami, H. M., and Digman, R. E., 1957, The bedrock geology of the Guilford 15-minute quadrangle and a portion of the New Haven quadrangle: State Geological and Natural History Survey of Connecticut, Bulletin 86, 99 p.

Barr, S. M., Davis, D. W., Kamo, S., and White, C. E., 2003, Significance of U-Pb detrital ages in quartzite from peri-Gondwanan terranes, New Brunswick and Nova Scotia, Canada: Precambrian Research, v. 126,p. 123–145.

Fortey, R. A., Cocks, L., and Robin, M., 2003, Palaeontological evidence bearing on global Ordovician-Silurian continental reconstructions: Earth-Science Reviews, v. 61, p. 245–307.

Ludman, A., and Berry, H. N., IV, 2003, Bedrock geology of the Calais quadrangle, Maine: Maine Geological Survey, Open-file report 03-97, scale: 1:100,000.
McLaughlin, K. J., Barr, S. M., Hill, M. D., Thompson, M. D., Ramezani, J., and Reynolds, P. H., 2003, The Moosehorn Plutonic Suite, southeastern Maine and southwestern New Brunswick: age, petrochemistry, and tectonic setting: Atlantic Geology, v. 39, p. 123–146.

McNicoll, V. J., Whalen, J. B., and Stern, R. A., 2003, U-Pb geochronology of Ordovician plutonism, Bathurst mining camp, New Brunswick, in Goodfellow, W. D., McCutcheon, S. R., and Peter, J. M., editors, Massive sulfide deposits of the Bathurst mining camp, New Brunswick, and northern Maine: Economic Geology Monographs, v. 11, p. 203–218.

Moench, R. H., and Aleinikoff, J. N., 2003. Stratigraphy, geochronology. and accretionary terrane settings of two Bronson Hill arc sequences, northern New England: Physics and Chemistry of the Earth, v. 28,p. 113–160.

van Staal, C. R., Wilson, R. A., Rogers, N., Fyffe, L. R., Langton, J. P., McCutcheon, S. R., McNicoll, V., and Ravenhurst, C. E., 2003, Geology and tectonic history of the Bathurst Supergroup, Bathurst mining camp, and its relationships to coeval rocks in southwestern New Brunswick and adjacent mine; a synthesis: Economic Geology Monograph, v. 11, p. 37–60.

Van Wagoner, N. A., and Dadd, K. A., 2003. A Silurian age for the Passamaquoddy Bay volcanic sequence in southwestern New Brunswick; implications for regional correlations: Geological Society of America,Abstracts with Programs, v. 35, p. 79.

David P. West, Jr., Heather M. Beal, and Timothy W. Grove, 2003. Silurian deformation and metamorphism of Ordovician arc rocks of the Casco Bay Group, south-central Maine. CJES 40 887-905.

The Casco Bay Group in south-central Maine consists of a sequence of Late Cambrian to Early Ordovician interlayered quartzofeldspathic granofels and pelite (Cape Elizabeth Formation) overlain by Early to Late Ordovician back-arc volcanic (Spring Point Formation) and volcanogenic sedimentary rocks (Diamond Island and Scarboro formations).   These rocks were tightly folded and subjected to low-pressure amphibolite-facies metamorphism in the Late Silurian.This phase of deformation and metamorphism was followed by the development of a variety of structures consistent with a period of dextral transpression in Middle Devonian – Early Carboniferous time. Previously dated plutons within the sequence range in age from 422–389 Ma and record a period of prolonged intrusive activity in the region. Similarities in age, volcanic rock geochemistry, and lithologic characteristics argue strongly for a correlation between rocks of the Casco Bay Group and those in the Miramichi belt of eastern Maine and northern New Brunswick. The Cape Elizabeth Formation correlates with Late Cambrian to Early Ordovician sediments of the Miramichi Group (Gander Zone) and the Spring Point through Scarboro formations correlate with Early to Late Ordovician back-arc basin volcanics and volcanogenic sediments of the Bathurst Supergroup. The folding and low-pressure metamorphism of the Casco Bay Group is attributed to Late Silurian to Early Devonian terrane convergence and possible lithospheric delamination that would have resulted in a prolonged period of intrusive activity and elevated temperatures at low pressures. Continued convergence and likely plate reconfigurations in the Middle Devonian to Carboniferous led to widespread dextral transpression in the region.

Discussion: Upper Casco Bay Group correlatives

Past efforts to correlate rocks of the Casco Bay Group with other rocks in the northern Appalachians have been hindered by a lack of information on depositional ages and information on tectonic setting. However, recent isotopic age determinations (Aleinikoff et al. 1993; Tucker et al. 2001), geochemical analysis of metamorphosed volcanic rocks (Lawrence and Beane 2001; West et al. 2003), and detailed mapping (Pankiwskyj 1996; Hussey and Berry 2002; West 2002) have provided new insight into the tectonic setting of the Casco Bay Group. Previously suggested Casco Bay Group correlatives to the south (e.g., Massabesic Gneiss Complex in New Hampshire and Nashoba Terrane in Massachusetts) would seem to be untenable given large age differences (in the case of the Massabesic: Aleinikoff et al. 1995) and basic lithologic differences (in the case of the Nashoba: Hepburn et al. 1995). Correlations with Avalonian and Peri Gondwanan terranes to the east (e.g., St. Croix belt, Ellsworth Terrane, coastal New Brunswick terranes) are also unlikely for similar reasons (Berry and Osberg 1989; Rankin 1994; Stewart 1998; White and Barr 1996; Johnson and McLeod 1996).The most likely correlatives with the upper part of the Casco Bay Group are found along strike to the northeast in the Miramichi belt (see Fig. 1) of eastern Maine (Ludman 1991; Ludman et al. 1993) and northern New Brunswick (van Staal 1987, 1994; van Staal and Fyffe 1995). Although comparisons of these rocks are hindered by differences in regional metamorphic grade (amphibolite facies in the Casco Bay Group versus largely greenschist facies in the Miramichi) and the effects of severe tectonic thinning in the Casco Bay Group, close similarities are seen and will be discussed in detail later in the text.

Interlayered quartzofeldspathic granofels and pelites of the Cape Elizabeth Formation are lithologically very similar to a thick sequence of interbedded quartzofeldspathic wackes and pelites in the Miramichi belt (Baskahegan Lake Formation in Maine (Ludman 1991) and the Miramichi Group in New Brunswick (van Staal and Fyffe 1995)). These rocks are Late Cambrian to Early Ordovician in age based on the presence of trace fossils (Pickerill and Fyffe 1999) and radiometric ages from overlying units. As discussed earlier in the text, the only constraint on the age of the Cape Elizabeth Formation is that it is older than the overlying 469 ± 3 Ma Spring Point Formation. Thus a Late Cambrian to Early Ordovician age for the Cape Elizabeth Formation is plausible. Rocks of the Miramichi Group (including the Baskahegan Lake Formation) have been interpreted by van Staal et al. (1996) to represent a Late Cambrian to Early Ordovician passive margin sequence deposited on the eastern margin of the Iapetus Ocean (i.e., Gander Zone of Williams 1979). Metamorphosed late Arenig to Llanvirn (time scale of Tucker and McKerrow 1995) volcanic rocks of the Spring

Point Formation overlie the Cape Elizabeth Formation and these rocks are likely correlatives to volcanic rocks of similar age in the Bathurst Supergroup (van Staal et al. in press) that overlies the Miramichi Group in the Miramichi belt. Geochemistry from volcanic rocks of the Bathurst Supergroup (Rogers et al. in press; Rogers and van Staal in press) suggests a back arc basin tectonic setting (Tetagouche–Exploits back arc basin of van Staal et al. 1991), which is similar to the geochemical signature for rocks of the Spring Point Formation in Casco Bay (Lawrence and Beane 2001) and south central Maine (West et al. 2003). Recent work in the Miramichi belt, summarized by van Staal et al. (in press), has revealed a more complex tectonic history for the Bathurst Supergroup and necessitated a four fold subdivision of these rocks. Although the Spring Point Formation is most similar in age and lithologic characteristics to the California Lake Group (as described in Rogers et al. in press), van Staal et al. (in press) suggest that the Tetagouche–Exploits back arc basin contained a number of widely separated distinct blocks that were later juxtaposed during theLate Ordovician to Early Silurian closure of the basin. Given the complexity of this tectonic setting, the Spring Point volcanic rocks may not correlate directly with any of the newly defined members of recognized units in the Bathurst Supergroup of the Miramichi belt, but similarities in age, lithology, geochemical characteristics, and structural position within the orogen argue for a general correlation with these rocks.

It is important to note that the Miramichi Group (Cape Elizabeth equivalent) and the Bathurst Supergroup (Spring Point equivalent) are separated by a disconformity in New Brunswick (van Staal and Fyffe 1995). Similarly, the Cape Elizabeth equivalent (Baskahegan Lake Formation) in the Miramichi belt of eastern Maine is unconformably overlain by Ordovician volcanic rocks (Bowers Mountain Formation: Ludman et al. 1993), which have been correlated with theBathurst Supergroup. To date, no unconformity has been recognized between the Cape Elizabeth and Spring Point formations in Maine. However, this Ordovician unconformity is quite subtle and has only been recognized relatively recently in the Miramichi belt, thus its recognition, if it exists, would be very difficult in Maine where amphibolite facies metamorphism and ductile shearing are ubiquitous. Metavolcanic rocks of the Spring Point Formation are overlain by thin but distinctive black, graphitic quartzite and quartz–mica schist (Diamond Island Formation), followed by a thicker sequence of predominantly pelitic schist (Scarboro Formation). In the southern Miramichi belt in New Brunswick, Oak Mountain Formation volcanic rocks are overlain by Caradocian shale and feldspathic wacke of the Belle Lake Formation (Fyffe et al. 1983; Pickerill and Fyffe 1999). Similarly, in the Bathurst Supergroup of the northern Miramichi belt, volcanic rocks are overlain by Llanvirn–Caradoc sedimentary assemblages (van Staal et al. in press). Thus, Late Arenig to Llanvirn volcanic rocks overlain by dominantly sedimentary sequences are common to both the Casco Bay and Miramichi belts. In summary, Ordovician rocks of the upper Casco Bay Group correlate most favorably with rocks of similar age in the Miramichi belt of eastern Maine and adjacent New Brunswick. These rocks have been interpreted by van Staal et al. (1991, 1996) to represent a Late Cambrian to Early Ordovician passive margin sequence (Miramichi Group) unconformably overlain by Middle to Late Ordovician back arc volcanic rocks and associated volcanogenic sedimentary rocks (Bathurst Supergroup). A similar interpretation is favored for the upper part of the Casco Bay Group in Maine with the extensive Cape Elizabeth Formation representing the passive margin sequence, the Spring Point Formation representing the back arc volcanics, and the Diamond Island and Scarboro formations representing overlying back arc basin sediments.

Implications for middle Paleozoic orogenesis

Tectonic models of middle Paleozoic (Acadian) orogenesis in northern New England and adjacent Canada have evolved through the years as progressively more data has become available (e.g., Bradley 1983; Osberg et al. 1989; Ludman et al. 1993; Eusden et al. 1996; van Staal and de Roo 1995; Bradley et al. 2000; Tucker et al. 2001). While many of the earlier models attributed all middle Paleozoic tectonism to the Devonian Acadian orogeny, recent work (e.g., Bevier and Whalen 1990; West et al. 1992; 1995; van Staal and de Roo 1995: Bradley et al. 2000; Tucker et al. 2001) suggests a prolonged period of Early Silurian to Late Devonian deformation, metamorphism, and intrusive activity. Because the region lacks ophiolitic sutures that are important aspects of most tectonic reconstructions, deciphering the temporaland spatial distribution of deformation and metamorphism is critical to unraveling the tectonic history of the region. Upright isoclinal folding and an associated pervasive schistosity (D1 deformation) are the dominant deformational features preserved in rocks of the upper Casco Bay Group in south central Maine. Microstructural analysis indicates that an episode of low pressure amphibolite facies metamorphism was synchronous with this deformational event. Porphyritic shonkinite of the 418 Ma Lincoln Sill cuts the map scale D1 fold in the Casco Bay Group (Fig. 3) and thus the folding and associated metamorphism must be pre Devonian. However, both 40Ar/39Ar hornblende (West et al. 1995) and U–Pb sphene ages (Tucker et al. 2001) from the area range from 390 to 380 Ma and suggest this deformation and metamorphism is significantly younger. The following two models are proposed to explain this apparent contradiction:

(1) Rocks of the Casco Bay Group in south central Maine are polymetamorphic having been initially metamorphosed during folding prior to the intrusion of the 418 Ma Lincoln Sill and then re metamorphosed in the Middle Devonian.Although obvious textural evidence (e.g., pseudomorphs) for polymetamorphism in the field area is not widespread, polymetamorphism is prevalent in much of coastal, south central, and western Maine (Novak and Holdaway 1981; Berry 1987; Guidotti 1989; Guidotti and Holdaway 1993; Grover and Lang 1995), and it is somewhat unusual to not find it here. There are suggestions of multiple stages of garnet growth in some areas (e.g., Fig. 9e) and obvious pseudomorphs of earlier minerals might not be expected if the pressure–temperature conditions of a second metamorphic event were similar to an earlier metamorphic episode (see the “transition zone” of Guidotti and Johnson 2002).

(2) The rocks were folded and metamorphosed in the Late Silurian prior to intrusion of the Lincoln Sill and remained hot for a period of nearly 40 million years, when they finally cooled below hornblende and sphene blocking temperatures (500–550°C). This would seem unlikely given the low pressure nature of the metamorphism in this region, however, numerous plutons in the area span this age range (Haskell Hill = 408 Ma; Mixer Pond = 400 Ma; North Searsmont = 389 Ma) and an episode of lithospheric delamination proposed by Tucker et al. (2001) would likely result in a prolonged period of elevated heat flow in the region. At present, each of these models is plausible and further detailed geochronological investigations (e.g., in situ monazite dating) will be needed to distinguish between these and other possibilities.

North trending, asymmetric Z folds characterize D2 deformation in the upper Casco Bay Group and these features have been attributed to a significant period of regional dextral transpression in Middle Devonian to Early Carboniferous time (West and Hubbard 1997). Dextral transpression during this time interval is well documented throughout the northern Appalachians (e.g., Kirkwood 1995;

van Staal and de Roo 1995; Ludman and West 1999) and likely reflects changing plate configurations following Silurian– Devonian collisions along the eastern margin of Laurentia. 40Ar/39Ar muscovite cooling ages from the study area indicate the region cooled below 350°C in the Early Carboniferous (West et al. 1995) and regional penetrative deformation following this time period is unlikely. Localized D3 dextral shear bands and asymmetric boudinage are likely a later manifestation of the dextral transpression discussed earlier in the text and may be a precursor to the development of the Norumbega fault system in the Carboniferous (West and Hubbard 1997; Ludman and West 1999).

Conclusions : (1) Similarities in age, volcanic rock geochemistry, and lithologic characteristics argue strongly for a correlation between rocks of the upper Casco Bay Group in south central Maine and rocks exposed in the Miramichi belt of eastern Maine and northern New Brunswick. In this model, interlayered quartzofeldspathic granofels and pelites of the Cape Elizabeth Formation correlate with Late Cambrian – Early Ordovician passive margin sediments of the Miramichi Group (Gander zone). Overlying volcanic rocks of the Spring Point Formation and sedimentary rocks of the Diamond Island and Scarboro formations correlate with Arenig to Caradoc back arc basin lithologies of the Bathurst Supergroup. (2) Late Silurian deformation and low pressure amphibolite  facies metamorphism of the upper Casco Bay Group in south central Maine can be attributed to convergence and associated terrane accretion. A model of lithospheric delamination proposed by Tucker et al. (2001) could explain a prolonged period of intrusive activity coincident with a period of elevated temperatures at low pressures. (3) Structures consistent with dextral shear deformation are pervasive and are correlated with a period of regional dextral transpression in Middle Devonian to Early Carboniferous time in the northern Appalachians.

Wintsch, R. P., Kunk, M. J., Boyd, J. L., and Aleinikoff, J. N., 2003, P-T-t paths and differential Alleghanian loading and uplift of the Bronson Hill terrane, south-central New England: American Journal of Science, v. 303, p. 410–446.

39th Ann. GSA Northeastern Meeting, T25, 2004

Southern New Brunswick consists of fault-bounded belts of rocks which show both similarities and contrasts in Late Proterozoic - Early Paleozoic evolution. The number of distinct terranes represented by these belts, and their affinity to the classical Avalon and Gander zones of the northern Appalachian orogen, remain controversial, in spite of a large body of accumulated field, petrochemical, and geochronological data. Myriad Carboniferous and younger faults in the area obscure the original nature of boundaries between rock packages, and it is likely that many of the original components are missing. Remnants of high-pressure metamorphic belts provide evidence for the presence of former terrane boundaries. P-T estimates for the Hammondvale metamorphic suite (HMS) on the northwestern margin of Caledonia terrane indicate peak metamorphic conditions at 9.5-12 kbar and 580-420°C. The HMS is interpreted to represent a remnant of an accretionary complex formed in association with ca. 620 Ma subduction in Caledonia terrane, and its presence confirms that the now-adjacent Brookville terrane was not part of the Caledonia (Avalon) terrane at that time. The Pocologan metamorphic suite at the boundary between the Brookville and Kingston terranes had peak P-T conditions of 9.5 kbars and 550°C, probably at ca. 420 Ma. A complex later history is indicated by 40Ar/39Ar cooling ages as young as ca. 320 Ma. Modelling of magnetic and gravity data, constrained by magnetic susceptibility and density measurements from surface samples, indicates that present fault boundaries are sub-vertical or dip slightly to the southeast, although more shallow structures indicative of northwesterly thrusting are evident in some areas. The models require geophysically distinct bodies at depth (ca. 6 km or more) that are interpreted to correspond to Avalon, Brookville (Bras d'Or???), and Ganderia basements. Tracing terranes of southern New Brunswick into Maine is complicated by abundant Silurian and Devonian igneous units, and by apparent offset to the southeast to Grand Manan Island, where rock types and ages show elements of Avalon, Brookville, New River, and/or St. Croix terranes, in fault-bounded packages. Correlation with the mainland of New Brunswick or Nova Scotia is not yet resolved.

Initial Nd and Sr isotopic composition of Paleozoic granites from the Medial New England Terrane (MNET) and the Composite Avalon Terrane (CAT) in Maine indicate the basement beneath these terranes changed dramatically after the Silurian. Two major episodes of Paleozoic magmatism are documented in this area. "Silurian" magmatism is bimodal, and characterized by distinctive epizonal plutonic and volcanic igneous complexes (e.g., Vinalhaven, South Penobscot). Silurian magmatism is known southeast of the Norumbega Fault Zone (NFZ). Silurian granites exhibit a large variation in eNd values (2.5 to -8) and 87Sr/86Sr)i ratios (0.703 to 0.713).

"Devonian to Carboniferous" magmatism is dominantly granitic and includes stocks of (1) peraluminous granite (e.g., Waldoboro); 2) metaluminous granite (e.g., Deer Isle, Mt. Waldo); and (3) Large batholiths of weakly peraluminous to metaluminous granite with megacrystic alkali feldspar (e.g., Lucerne). Devonian magmatism is wide spread throughout the area and is found on both sides of the NFZ. Devonian granites exhibit a narrow range in eNd (0.8 to -2) and a large range in 87Sr/86Sr)i ratios (0.701-0.710). On a Nd-Sr isotopic correlation diagram the fields for Silurian and Devonian granites partially overlap, however, they exhibit distinctly different slopes. Silurian granites define the field with the steeper slope. The most radiogenic Silurian granite yields a TDM age of 1.8 Ma. Devonian granites define the field with a shallow slope. The most radiogenic Devonian granite yields a TDM age of 1.4 Ma. This is compatible with Silurian and Devonian granites being derived from partial melting of distinct basement source regions. Silurian granites and Devonian granites intrude both MNET and CAT. This suggests 1) "terrane-bounding" faults in coastal Maine juxtapose distinct upper crustal terranes but not distinct basement terranes and that the upper crustal terranes were assembled prior to the Silurian melting event. 2) The source region for Silurian granites was not "tapped" during the Devonian melting event, either because of its refractory nature, or because the Silurian granites and their (GANDER??) host terranes are both allochthonous and were displaced over the MNET (AVALON???) basement source region prior to the Devonian melting event.

The (Tally Pond) group comprises Cambrian island-arc felsic pyroclastic rocks with intercalated mafic volcanic rocks and epiclastic volcanic and sedimentary rocks. ...arc plutonic rocks of the Crippleback Lake Quartz Monzonite form the basement to the Tally Pond group. A sequence of dominantly pillowed mafic volcanic rocks, mafic to andesitic volcaniclastics, and intercalated felsic volcanic rocks are nonconformable on the Crippleback Lake Quartz Monzonite. U-Pb zircon geochronology indicates that the felsic volcanic rocks were erupted at 509 ± 1 Ma. Dykes and stocks of medium-grained gabbro-diorite intrude all of the rocks of the Tally Pond group. These mafic intrusions yielded a U-Pb zircon age of 465 ± 2 Ma. Major, trace and rare earth element geochemistry indicates that the volcanic rocks have geochemical affinities consistent with a volcanic arc paleotectonic environment. Mafic intrusive rocks show no evidence of arc-related volcanism. Pb isotope data from VMS occurrences in the Tally Pond group contrast with those from the Notre Dame Subzone and are comparable to data from the Exploits subzone. The youngest rocks in the study area are conglomerates and coarse-grained sandstones of the Silurian Rogerson Lake Conglomerate which unconformably overlies rocks of the Tally Pond group; the conglomerate is dominated by volcanic clasts derived from the underlying Exploits subzone sequences. The Early Cambrian rocks of the Tally Pond group are analogous to rocks of the Brookville and Bras d’Or terranes in New Brunswick and Nova Scotia. Previous workers have suggested that the Brookville and Bras d’Or terranes represent the eastern margin of the Central Mobile Belt of the Appalachians (Gander Zone). Correlation of the Tally Pond group with these terranes, suggests that the Tally Pond group formed along the peri-Gondwanan margin of Iapetus. The Silurian Rogerson Lake Conglomerate that unconformably overlies the Tally Pond group indirectly indicates that these terranes were accreted to Laurentia by Late Ordovician.

In central Newfoundland, a large, Upper Cambrian (~494 Ma) ophiolitic thrust sheet, the Gander River Ultrabasic Belt (GRUB), constitutes one of the most striking features of the northern Appalachian orogen. Gander Zone arenites and shales of Cambro-Tremadoc age occupy the footwall southeast of the GRUB line and the Cormacks window. Arenig strata that unconformably overlie both the ophiolite and Gander Zone sediments, combined with a 474 Ma stitching pluton, demand a Tremadoc/early Arenig age of emplacement (Penobscot orogeny). In the simplest scenario, the ophiolite formed in a back-arc setting southeast of the Cambrian Victoria arc; Gander Zone arenites accumulated along the southeasterly passive margin; and hot ophiolite was obducted shortly after its formation.

In New Brunswick and Maine, a thousand kilometers southeast (length of Java), discontinuous Cambrian marine igneous rocks (509 Ma Ellsworth, 502 Ma Castine, 497 Ma Lawson Brook, 493 Ma East Scotch) of the Ellsworth and Annidale terranes (EAT) comprise bimodal assemblages largely devoid of arc or continental signature. Rare arc-like rocks, however, suggest an ensimatic back-arc setting. The structurally lower St. Croix terrane comprises, in downward order, Caradocian shales and orthoquartzites (Kendall Mountain), quartzofeldspathic wackes (Woodland), and Tremadocian black shales and basalts (Calais-Penobscot). Both assemblages display top-to-NW sense-of-shear in early structures that must be post-Tremadoc and possibly post-Caradoc. A white-weathering, pin-striped arenite from Ellsworth Falls, close to highly sheared rocks at the northwestern limit of the Ellsworth terrane, yielded dominantly 545±4Ma (n=28; 73%) and lesser younger (507±6 Ma) and older Neo- (ca. 630 Ma, 680 Ma), Meso- (1.21 Ma, 1.50 Ma), and Paleoproterozoic (1.97-2.09 Ma) zircons, an age distribution typical of Gander Zone arenites.

New SHRIMP-analyses of zircon grains from Cambrian arenites in New Brunswick and Maine constrain the location of the Gander-Avalon terrane boundary, which is critical to understanding the Early Silurian Salinic and Early Devonian Acadian orogenies. Whereas peri-Gondwanan Ganderia and Avalonia have similar arc-dominated Neoproterozoic basements, their Lower Paleozoic rocks and tectonic evolutions differ significantly. In Ganderia, Middle Cambrian through Tremadoc arenites and shales overlie Lower Cambrian arc rocks, which were locally accompanied by coeval tectonism. In Avalonia, however, shale-dominated Lower Cambrian to Ordovician strata indicate a stable platform. Arenite samples collected from the Baskahegan Lake, Calais, Ellsworth, and Matthews Lake Formations contain abundant Early Cambrian and Late Neoproterozoic detrital zircon grains derived, probably, from local basement as well as important populations of Mesoproterozoic (1.2 -1.6 Ga) and Early Paleoproterozoic (2.0 - 2.3 Ga) zircon grains, which suggest an Amazonian provenance. We propose that the Gander-Avalon zone boundary coincides with the Caledonia Fault in New Brunswick, lies southeast of Grand Manan Island (Stanley Brook granite is c. 535 Ma), which is due to sinistral offset along the Oak Bay fault, and emerges southeastward from the Gulf of Maine as the Bloody Bluff Fault.

At ~430 Ma, the bulk of Ganderia accreted to Laurentia along its northern margin due to closure of the Middle Ordovician Tetagouche backarc basin, which caused the Salinic orogeny. Around this time (440-425 Ma), the coastal volcanic/Mascarene arc/backarc complex developed on its southern margin above a northwest-subducting Avalonian plate. Inversion of the Mascarene backarc basin at ~ 420 Ma signals the start of the Avalonia-Ganderia collision, which caused the Acadian orogeny.

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Black, L. P., Kamo, S. L., Allen, C. M., Davis, D. W., Aleinikoff, J. N., Valley, J. W., Mundil, R., Campbell, I. H.,Korsuch, R. J., Williams, I. S., and Foudoulis, C., 2004, Improved 206Pb/238U microprobe geochronologyby the monitoring of a trace-element-related matrix effect; SHRIMP, ID–TIMS, ELA–ICP–MS andoxygen isotope documentation for a series of zircon standards: Chemical Geology, v. 205, p. 115–140.

Bothner, W. A., Laird, J., Escamilla, C. J., Kerwin, C. W., Schulz, J., and Loveless, J. P., 2004, EDMAPS/newmaps in southeastern New Hampshire: Geological Society of America, Abstracts with Programs,v. 36, p. 57.

Bream, B. R., Hatcher, R. D., Jr., Miller, C. F., and Fullagar, P. D., 2004, Detrital zircon ages and Nd isotopic data from the Southern Appalachian crystalline core, Georgia, South Carolina, North Carolina, and Tennessee; new provenance constraints for part of the Laurentian margin: Geological Society of America, Memoir, v. 197, p. 459–475.

Burg, J. P., Kaus, B. J. P., and Podladchikov, Y. Y., 2004, Dome structures in collision orogens: Mechanical investigation of the gravity/compression interplay, in Whitney, D. L., Teyssier, C., and Siddoway, C. S.,editors, Gneiss domes in Orogeny: Geological Society of America Special Paper, v. 380, p. 47–66.

Churchill-Dickson, L., 2004, A late Silurian (Pridolian) age for the Eastport Formation, Maine; a review of the fossil, stratigraphic, and radiometric age data: Atlantic Geology, v. 40, p. 189–195.

Daniels, D. L., and Snyder, S. L., 2004, New England states aeromagnetic and gravity maps and data: A website for distribution of data: U.S. Geological Survey Open-File Report 2004–1258, http://pubs.usgs.gov/of/2004/1258/index.htm.

Ericksson, K. A., Campbell, I. H., Palin, J. M., Allen, C. M., and Bock, B., 2004, Evidence for multiple recycling in Neoproterozoic through Pennsylvanian sedimentary rocks of the central AppalachianBasin: Journal of Geology, v. 112, p. 261–276.

Fayon, A. K., Whitney, D. L., and Teyssier, C., 2004, Exhumation of orogenic crust: Diapiric ascent versus low-angle normal faulting, in Whitney, D. L., Teyssier, C., and Siddoway, C. S., editors, Gneiss domes in Orogeny: Geological Society of America Special Paper, v. 380, p. 129–139.

Karabinos, P., Morris, D., and Rayner, N., 2004, Silurian tectonism in the western New England Appalachians: Geological Society of America, Abstracts with Programs, v. 36, n. 2, p. 91.

Reusch, D. N., van Staal, C. R., and McNicoll, V. J., 2004, Detrital zircons and Ganderia’s southern margin, coastal Maine: Geological Society of America Abstracts with Programs, v. 36, n. 2, p. 129

Thomas, W. A., Becker, T. P., Samson, S. D., and Hamilton, M. A., 2004, Detrital zircon evidence of a recycled orogenic foreland provenance for Alleghanian clastic-wedge sandstones: Journal of Geology, v. 112,p. 23–37.

Tohver, E., Bettencourt, J. S., Tosdal, R., Mezger, K., Leite, W. B., and Payolla, B. L., 2004, Terrane transfer during the Grenville orogeny: tracing the Amazonian ancestry of southern Appalachian basementthrough Pb abd Nd isotopes: Earth and Planetary Science Letters, v. 228, p. 161–176.

Tollo, R. P., Aleinikoff, J. N., Borduas, E. A., Hackley, P., and Fanning, C. M., 2004, Petrologic andgeochronologic evolution of the Grenville Orogen, northern Blue Ridge Province, Virginia, in Tollo,R. P., Corriveau, L., McLelland, J., and Bartholomew, M. J., editors, Proterozoic tectonic evolution of theGrenville Orogen in North America: Geological Society of America Memoir, v. 197, p. 411–434.

van der Velden, A. J., van Staal, C. R., and Cook, F. A., 2004, Crustal structure, fossil subduction, and the tectonic evolution of the Newfoundland Appalachians; evidence from a reprocessed seismic reflection survey: Geological Society of America Bulletin, v. 116, p. 1485–1498.

van Staal, C. R., McNicoll, V. J., Valverde-Vaquero, P., Barr, S. M., Fyffe, L. R., and Reusch, D. N., 2004, Ganderia, Avalonia, and the Salinic and Acadian orogenies: Geological Society of America Abstractswith Programs, v. 36, n. 2, p. 128.

Vermeesch, P., 2004, How many grains are needed for a provenance study?: Earth and Planetary ScienceLetters, v. 224, p. 441–451.

Walsh, G. J., Aleinikoff, J. N., and Fanning, C. M, 2004, U-Pb geochronology and evolution of Mesoproterozoic basement rocks, western Connecticut, in Tollo, R. P., Corriveau, L., McLelland, J., and Bartholomew,M. J., editors, Proterozoic tectonic evolution of the Grenville Orogen in North America:Geological Society of America Memoir, v. 197, p. 729–753.

West, D.P., Jr, Coish, R.A. and Tomascak, P.B. 2004. Tectonic setting and regional correlation of Ordovician metavolcanic rocks of the Casco Bay Group, Maine: evidence from trace element and isotope geochemistry Geol. Mag. 141, 125–140.

p. 136 "This suggests that Early to Middle Ordovician Tetagouche-Exploits basin (van Staal,Winchester & Bedard, 1991;vanStaal et al.1998) can be traced well into southern Maine."

p. 138 the following tectonic model is proposed for the Casco Bay Group in Maine. (1)Arc volcanism along the Gander continental margin begins in Early Ordovician time and the Cape Elizabeth Formation represents volcanogenic sediment shed from this growing volcanic arc. (2) Crustal thinning and rifting of this continental arc begins about 470 Ma and bimodal volcanic rocks of the Spring Point Formation are erupted during the early stages of this rifting episode. Rocks of the Cushing Formation and Falmouth-Brunswick sequence may represent continued arc magmatism on the trench side of the back-arc basin. (3)Metasedimentary rocks of the Diamond Island and Scarboro formations (above the Spring Point Formation) reflect sedimentation within the back-arc basin. Late Ordovician (?) or Silurian compressional tectonic events subsequently closed the back-arc basin and juxtaposed the various elements of the arc–back-arc basin complex.

Whitney, D. L., Teyssier, C., and Vanderhaeghe, O., 2004, Gneiss domes and crustal flow, in Whitney, D. L.,Teyssier, C., and Siddoway, C. S., editors, Gneiss domes in Orogeny: Geological Society of America Special Paper, v. 380, p. 15–33.

Yin, A., 2004, Gneiss domes and gneiss dome systems, in Whitney, D. L., Teyssier, C., and Siddoway, C. S.,editors, Gneiss domes in Orogeny: Geological Society of America Special Paper, v. 380, p. 1–14.

Becker, T. P., Thomas, W. A., Samson, S. D., and Gehrels, G. E., 2005, Detrital zircon evidence of Laurentian crustal dominance in the Lower Pennsylvanian deposits of the Alleghanian clastic wedge in easternNorth America: Sedimentary Geology, v. 182, p. 59–86.

Dorais, M. J., Nelson, W. R., and Tubrett, M. N., 2005, What caused the Acadian Orogeny, New England Appalachians? A provenance study of the Carrabassett Formation, Central Maine Basin: GeologicalSociety of America, Abstracts with Programs, v. 37, n. 1, p. 31.

Karabinos, P., 2005, Age and style of emplacement of basement uplifts in the northern Appalachians: Geological Society of America, Abstracts with Programs, v. 37, p 20.

Tomascak, P. B., Brown, M., Solar, G. S., Becker, H. J., Centorbi, T. L., and Tian, J., 2005, Source contributions to Devonian granite magmatism near the Laurentian border, New Hampshire and Western Maine, USA: Lithos, v. 80, p. 75–99.

Tremblay, A., and Pinet, N., 2005, Diachronous supracrustal extension in an intraplate setting and the origin of the Connecticut Valley-Gaspe´ and Merrimack Troughs, Northern Appalachians: Geological Magazine,v. 142, p. 7–22.

van Staal, C. R., 2005, The Northern Appalachians, in Selley, R. C., Robin, L., Cocks, M., and Plimer, I. R., editors, Encyclopedia of Geology: Oxford, Elsevier, v. 4, p. 81–91.

Wintsch, R. P., Aleinikoff, J. N., Dorais, M. J., Unruh, D. M., and Walsh, G. J., 2005c, Melt weakening in crustal-scale tectonic wedging, southern New England: EOS Transactions, v. 86, n. 52, p. V21A-0591.

Wintsch, R. P., Aleinikoff, J. N., Unruh, D. M., and Walsh, G., 2005a, Evidence for tectonic wedging of Avalon terrane rocks into the Gander zone, southern New England: Geological Society of America Abstractswith Programs, v. 37, p. 31.

Wintsch, R. P., Aleinikoff, J. N., Webster, J. R., and Unruh, D. M., 2005b, The Killingworth complex: A Middle and Late Paleozoic magmatic and structural dome, in McHone, N. W., and Petersone, M. J.,editors, Guidebook for fieldtrips in Connecticut, New Haven, CT: New England Intercollegiate Geological Conference, 97th annual meeting, State Geological and Natural History Survey of Connecticut,Guidebook n. 8, p. 305–324.

Valverde-Vaquero, P., van Staal, C. R., McNicoll, V., and Dunning, G. R., 2006, Mid-Late Ordovician magmatism and metamorphism along the Gander margin in central Newfoundland: London, Journal of the Geological Society, v. 163, p. 347–362.

Walsh, G. J., Scott, R. B., Aleinikoff, J. N., and Armstrong, T. R., 2006, Preliminary Bedrock geologic map of the Old Lyme quadrangle, New London and Middlesex Counties, Connecticut: U.S. Geological SurveyOpen File Report 2006-1296, scale 1:24:000.

Zagorevski, A., Rogers, N., van Staal, C. R., McNicoll, V., Lissenberg, C. J., and Valverde-Vaquero, P., 2006, Lower to Middle Ordovician evolution of peri-Laurentian arc and backarc complexes in Iapetus; constraints from the Annieopsquotch accretionary tract, central Newfoundland: Geological Society of America Bulletin, v. 118, p. 324–342.

American Journal of Science 2007 v. 307, January

JOHN N. ALEINIKOFF, ROBERT P. WINTSCH, RICHARD P. TOLLO, DANIEL M. UNRUH, C. MARK FANNING, and MARK D. SCHMITZ. 2007. AGES AND ORIGINS OF ROCKS OF THE KILLINGWORTH DOME, SOUTH-CENTRAL CONNECTICUT: IMPLICATIONS FOR THE TECTONIC EVOLUTION OF SOUTHERN NEW ENGLAND. American Journal of Science, v. 307, 1, p. 63–118.

The Killingworth dome of south-central Connecticut occurs at the southern end of the Bronson Hill belt. It is composed of tonalitic and trondhjemitic orthogneisses (Killingworth complex) and bimodal metavolcanic rocks (Middletown complex) that display calc-alkaline affinities. Orthogneisses of the Killingworth complex (Boulder Lake gneiss, 456 6 Ma; Pond Meadow gneiss, 460 Ma) were emplaced at about the same time as eruption and deposition of volcanic-sedimentary rocks of the Middletown complex (Middletown Formation, 449 4 Ma; Higganum gneiss, 459 4Ma). Hidden Lake gneiss (339 3Ma) occurs as a pluton in the core of the Killingworth dome, and, on the basis of geochemical and isotopic data, is included in the Killingworth complex.
Pb and Nd isotopic data suggest that the Pond Meadow, Boulder Lake, and Hidden Lake gneisses (Killingworth complex) resulted from mixing of Neoproterozoic Gander terrane sources (high 207Pb/204Pb and intermediate Nd) and less radiogenic (low 207Pb/204Pb and low Nd) components, whereas Middletown Formation and Higganum gneiss (Middletown complex) were derived from mixtures of Gander basement and primitive (low 207Pb/204Pb and high Nd) sources. The less radiogenic component for the Killingworth complex is similar in isotopic composition to material from Laurentian (Grenville) crust. However, because published paleomagnetic and paleontologic data indicate that the Gander terrane is peri-Gondwanan in origin, the isotopic signature of Killingworth complex rocks probably was derived from Gander basement that contained detritus from non-Laurentian sources such as Amazonia, Baltica, or Oaxaquia. We suggest that the Killingworth complex formed above an east-dipping subduction zone on the west margin of the Gander terrane, whereas the Middletown complex formed to the east in a back-arc rift environment. Subsequent shortening, associated with the assembly of Pangea in the Carboniferous, resulted in Gander cover terranes over the Avalon terrane in the west; and in the Middletown complex over the Killingworth complex in the east. Despite similarities of emplacement age, structural setting, and geographic continuity of the Killingworth dome with Oliverian domes in central and northern New England, new and published isotopic data suggest that the Killingworth and Middletown complexes were derived from Gander crust, and are not part of the Bronson Hill arc that was derived from Laurentian crust. The trace of the Ordovician Iapetan suture (the Red Indian line) between rocks of Laurentian and Ganderian origin probably extends from Southwestern New Hampshire west of the Pelham dome of northcentral Massachusetts and is coverd by Mesozoic rocks of the Hartford basin.

R. P. WINTSCH, J. N. ALEINIKOFF, G. J. WALSH, W. A. BOTHNER, A. M. HUSSEY, II, and C. M. FANNING. 2007.  SHRIMP U-Pb EVIDENCE FOR A LATE SILURIAN AGE OF METASEDIMENTARY ROCKS IN THE MERRIMACK AND PUTNAM-NASHOBA TERRANES, EASTERN NEW ENGLAND. American Journal of Science, n. 307, 1, p. 119–167.

U-Pb ages of detrital, metamorphic, and magmatic zircon and metamorphic monazite and titanite ..... from the Merrimack and Putnam-Nashoba terranes of eastern New England. ....deposited in the middle Paleozoic above Neoproterozoic basement of the Gander terrane and juxtaposed by Late Paleozoic thrusting in thin, fault-bounded slices. The correlative Hebron and Berwick formations (Merrimack terrane) and Tatnic Hill Formation (Putnam-Nashoba terrane), contain detrital zircons with Mesoproterozoic, Ordovician, and Silurian age populations. On the basis of the age of the youngest detrital zircon population ( 425 Ma), the Hebron, Berwick and Tatnic Hill formations are no older than Late Silurian (Wenlockian). The minimum deposition ages of the Hebron and Berwick are constrained by ages of cross-cutting plutons (414 3 and 418 2 Ma, respectively). The Tatnic Hill Formation must be older than the oldest metamorphic monazite and zircon ( 407 Ma). Thus, all three of these units were deposited between 425 and 418 Ma, probably in the Ludlovian. Age populations of detrital zircons suggest Laurentian and Ordovician arc provenance to the west. High grade metamorphism of the Tatnic Hill Formation soon after deposition probably requires that sedimentation and burial occurred in a fore-arc environment, whereas time-equivalent calcareous sediments of the Hebron and Berwick formations probably
originated in a back-arc setting. In contrast to age data from the Berwick Formation, the Kittery Formation contains primarily Mesoproterozoic detrital zircons; only 2 younger grains were identified. The absence of a significant Ordovician population, in addition to paleocurrent directions from the east and structural data indicating thrusting, suggest that the Kittery was derived from peri-Gondwanan sources and deposited in the Fredericton Sea. Thus, the Kittery should not be considered part of the Laurentian-derived Merrimack terrane; it more likely correlates with the early Silurian Fredericton terrane of northeastern New England and Maritime Canada.

GREGORY J. WALSH, JOHN N. ALEINIKOFF, and ROBERT P. WINTSCH. 2007. ORIGIN OF THE LYME DOME AND IMPLICATIONS FOR THE TIMING OF MULTIPLE ALLEGHANIAN DEFORMATIONAL AND INTRUSIVE EVENTS IN SOUTHERN CONNECTICUT. American Journal of Science, v.  307, 1, p. 168–215.

.... the Lyme dome, southern Connecticut ...... Detrital zircon geochronology in combination with ages on intrusive rocks brackets the deposition of quartzite in the core of the dome sometime between ca. 925 and 620 Ma. Granite and granodiorite intruded the Neoproteorozic metasedimentary rocks in the core of the dome at ca. 620 to 610 Ma. Four major early Permian events associated with the Alleghanian orogeny affected the rocks in the Lyme dome area. Syn-tectonic migmatization and widespread penetrative deformation (D1, ca. 300 – 290 Ma) included emplacement of alaskite at 290 +/- 4 Ma during regional foliation development and aluminosilicate-orthoclase metamorphic conditions. Rocks of the Avalon terrane may have wedged between Gander cover rocks and Gander basement in the core of the Lyme during D1. Limited structural evidence for diapiric uplift of the Lyme dome indicates that diapirism started late in D1 and was completed by D2 (ca. 290 – 280 Ma) when horizontal WNW contractional stresses dominated over vertical stresses. Second sillimanite metamorphism continued and syn-tectonic D2 granite pegmatite (288 +/-4 Ma) and the Joshua Rock Granite Gniess (284 +/-3 Ma) intruded at this time. North-northwest extension during D3 (ca. 280 – 275 Ma) led to granitic pegmatite intrusion along S3 cleavage planes and in extensional zones in boudin necks during hydraulic failure and decompression melting. Intrusion of a Westerly Granite dike at 275 +/-4 Ma suggests that D3 extension was active, and perhaps concluding, by ca. 275 Ma. Late randomly oriented but gently dipping pegmatite dikes record a final stage of intrusion during D4 (ca. 275 – 260 Ma), and a switch from NNW extension to vertical unloading and exhumation. Monazite and metamorphic zircon rim ages record this event at ca. 259 Ma. The evolution of the Lyme dome involved D1 mylonitization, intrusion, and migmatization during north-directed contraction, limited late D1 diapirism, D2 migmatization during WNW contraction with associated flexural flow and fold interference, D3 NNW horizontal extension and decompression melting, and final D4 vertical extension and rapid exhumation. Late regional uplift, extension, and normal faulting at higher crustal levels may have been caused by diapiric rise of the lower crust, below the structural level of the Lyme dome. The rocks record no evidence of Acadian metamorphism or deformation, suggesting that the Gander zone here was not tectonically juxtaposed with Avalon until the Alleghanian orogeny.

Lin, SHOUFA; DONALD W. DAVIS, SANDRA M. BARR, CEES R. VAN STAAL YADONG CHEN, and MARC CONSTANTIN 2007. U-Pb GEOCHRONOLOGICAL CONSTRAINTS ON THE EVOLUTION OF THE ASPY TERRANE, CAPE BRETON ISLAND: IMPLICATIONS FOR RELATIONSHIPS BETWEEN ASPY AND BRAS D’OR TERRANES AND GANDERIA IN THE CANADIAN APPALACHIANS: American Journal of Science, v. 307, p. 371-398.

"New U-Pb zircon ages from nine samples of igneous and sedimentary rocks in the Aspy terrane, Cape Breton Island, show that Neoproterozoic rocks form major part of the terrane and confirm that the terrane was affected by a major Silurian-Devonian tectonothermal event. A rhyolitic crystal tuff, a leucotonalite pluton  and a felsic sheet yield ages of 618.8 +/- 0.6 Ma, 619.7 +/- 0.9 Ma and 573.5 +/- 2.7 Ma,  respectively. A metasedimentary unit contains zircon ranging in age from ca. 546 to ca. 1520 Ma. A diorite has an age of 428.6 +/- 1.9 Ma, and another diorite unit and a quartz porphyry also have probable Late Silurian ages. A third diorite yields an age of 373.0 +/- 0.5 Ma, part of a widespread bimodal igneous event represented by both volcanic and plutonic rocks throughout the Aspy region. Geochronological results and field relationships indicate that the Neoproterozoic rocks are similar to those in the Bras d’Or terrane and form the basement to the (Ordovician-) Silurian rocks in the Aspy terrane, and that the Aspy terrane is probably correlative with rocks in the Hermitage flexure of southern Newfoundland where Silurian metasedimentary and metavolcanic rocks lie unconformably on similar Neoproterozoic rocks. Likely correlative rocks also occur in the Kingston terrane of southern New Brunswick. The results of this study support the idea that the Aspy-Kingston terrane is part of Ganderia and that Neoproterozoic rocks in the Bras d’Or terrane and correlative rocks in southwestern Newfoundland and elsewhere represent exposed basement to Ganderia in the Canadian Appalachians."

Rankin, D. W., Coish, R. A., Tucker, R. D., Peng, Z. X., Wilson, S. A., and Rouff, A. A., 2007. Silurian extension in the upper Connecticut Valley, United States and the origin of middle Paleozoic basins in the Quebec Embayment: American Journal of Science, v. 307, p. 216–264.

Zagorevski, A., van Staal, C. R., McNicoll, V., and Rogers, N., 2007. Upper Cambrian to upper Ordovician island arc activity in the Victoria Lake supergroup, central Newfoundland: tectonic development of the northern Ganderian Margin: American Journal of Science, v. 307, p. 339–370.

"The Exploits Subzone of the Newfoundland Appalachians comprises remnants of Cambro-Ordovician peri-Gondwanan arc and back-arc complexes that formed within the Iapetus Ocean. The Exploits Subzone experienced at least two accretionary events as a result of the rapid closure of the main portion of the Iapetus tract: the Penobscot orogeny (c. 480 Ma), which juxtaposed the Penobscot Arc (c. 513 – 486 Ma) with the Gander margin, and c. 450 Ma collision of the Victoria Arc (c. 473 – 454 Ma) with the Annieopsquotch Accretionary Tract that juxtaposed the peri-Laurentian and peri-Gondwanan elements along the Red Indian Line. The newly recognized Pats Pond Group forms a temporal equivalent to other Lower Ordovician intra-oceanic complexes of the Penobscot Arc. The Pats Pond Group (c. 487 Ma) has a geochemical stratigraphy that is consistent with rifting of a volcanic arc. An ensialic setting is indicated by low epNd values (epNd 0.3 to -0.5) near the stratigraphic base and its abundant zircon inheritance (c. 560 Ma and 0.9 – 1.2 Ga).  The spatial distribution of Tremadocian arc – back-arc complexes indicates that the Penobscot arc is best explained in terms of a single east-dipping subduction zone. This model is favored over west dipping models, in that it explains the distribution of the Penobscot arc elements, continental arc magmatism, and the obduction of back-arc Penobscot ophiolites without requiring subduction of the Gander margin or subduction reversal.
       The newly recognized Wigwam Brook Group (c. 454 Ma) disconformably overlies the Pats Pond Group and records the youngest known phase of ensialic arc volcanism (
Nd – 4.1) in the Victoria Arc, which is also related to east-dipping subduction. Thus the Penobscot and the overlying Victoria Arc are reinterpreted in terms of a single, relatively long-lived east-dipping subduction zone beneath the peri-Gondwanan microcontinent of Ganderia. The cessation of arc volcanism towards the top of the Wigwam Brook Group and the subsequent syn-tectonic sedimentation in the Badger Group constrain the arrival of the leading edge of Ganderia with the ensialic arc complexes to the Laurentian margin to c. 454 Ma."

Pollack, J. C., Wilton, D. H. C., van Staal, C. R., and Morrissey, K. D., 2007. U-Pb zircon geochronological constraints on the Late Ordovician-Early Silurian collision of Ganderia and Laurentia along the Dog Bay Line: The terminal Iapetan suture in the Newfoundland Appalachians: American Journal of Science, v. 307, p. 399–433.

Strachan, R.A.; Collins, A.S.; Buchan, C.; Nance, R.D.; Murphy, J.B.; D'Lemos, R.S. 2007. Terrane analysis along a Neoproterozoic active margin of Gondwana: insights from U-Pb zircon geochronology. Journal of the Geological Society, Volume 164, Number 1, 2007, p. 57-60(4).
The tectonic affinities of terranes in accretionary orogens can be evaluated using geochronological techniques. U-Pb zircon data obtained from paragneisses of the Coedana Complex (Anglesey) and the Malverns Complex, southern Britain, indicate that they were deposited during the mid- to late Neoproterozoic and have a comparable Amazonian provenance. Metamorphism of the Coedana gneisses occurred at 666 ± 7 Ma, similar to the age of metamorphism in the Malverns Complex. Anglesey therefore probably evolved in proximity to the Avalonian basement of mainland southern Britain during the mid- to late Neoproterozoic and is not a `suspect terrane' relative to the remainder of Avalonia.

Van Staal. C. R., 2007, Pre-Carboniferous metallogeny of the Canadian Appalachians, in Goodfellow, W. D., editor, Mineral Resources of Canada: A Synthesis of Major Deposit-types, District Metallogeny, the Evolution of Geological Provinces, and Exploration Methods, Jointly by the Mineral Deposit Division of the Geological Association of Canada and the Geological Survey of Canada.

Geological Society of America, Northeast section meeting, Durham, NH, 2007 - http://gsa.confex.com/gsa/2007NE/finalprogram/ - link to program

Monday, 12 March 2007
Rev. James W. Skehan, S.J. — Geologist, Teacher, Mentor, Priest: A Jesuit Journey I

GEOLOGICAL CASE FOR AVALON OFF NORTH AFRICA AT 595 MA: THOMPSON, M.D.,  zircon arguments
Avalon terranes have commonly been pictured as originating along the Amazonian margin of Gondwana based on Nd isotopic signatures of crystalline rocks and 1.0-1.3 Ga detrital zircons that could have derived from South American source belts. We suggest that the geological evidence can also be reconciled with the peri-Gondwanan African position implied by our paleopole from ca. 595 Ma Lynn-Mattapan volcanic rocks in the Southeastern New England Avalon Zone.
Source rocks that could have generated positive initial ?Nd values observed in Avalonian igneous rocks of maritime Canada were first pointed out in the Tocantins Province of central Brazil. Comparable results are also published, however, for Pan African belts closer to our paleomagnetically preferred site. These include the Anti-Atlas orogen that was active along the northern margin of the west African craton during Gondwana assembly, as well as the Trans-Saharan suture between the West African craton and the Saharan paleocontinent on the east.
Mesoproterozoic-age detrital zircons for which there are no source belts in the West African craton might be longitudinally derived from other terranes along the Gondwanan margin, or even from a terrane separating Avalonia from West Gondwana. Possible candidates are Middle American terranes like Ouaxaquia (eastern Mexico) where igneous and metamorphic basement rocks have yielded U-Pb zircon dates between ca. 1.1 Ga and 0.9 Ga. A second aspect of detrital zircon suites deposited in proximity to West Africa is the expected presence of grains in the ca. 2.27-2.05 Ga interval marking events loosely known as the Eburnean orogeny. Zircons of this age are rare in Avalonian-cycle sandstones, but they have been identified in quartzite of the Blackstone Group which predates Avalonian arc magmatism in southeastern New England. High concentrations of detrital zircon ages around 2.0 Ga are also reported from quartzite clasts in conglomerates from the Caledonia and Mira terranes in New Brunswick and Nova Scotia, respectively. It appears from these examples that older Avalonian sedimentary sequences could have been supplied in part by source areas of Eburnean age, but that transport patterns changed with the onset of Avalonian tectonism so that ca. 2.0 Ga Paleoproterozoic zircons are present only locally as recycled grains.

METAMORPHISM AND FAULT ZONE ACTIVITY IN THE NASHOBA TERRANE, EASTERN MASSACHUSETTS:MARKWORT, Ross J.

2-6 10:15 AM MONAZITE AGES FOR METAMORPHISM AND FAULT ZONE ACTIVITY IN THE NASHOBA TERRANE, EASTERN MASSACHUSETTS: MARKWORT, Ross J., new evidence for the tectonic history of the Nashoba terrane.
The oldest monazite core age encountered, ~ 450 Ma (Late Caradoc), is from a grain within a garnet porphyroblast in a mylonitic schist that may represent a detrital grain. A second core age of 438 Ma (early Llandovery) in a now mylonitized granite likely represents an original igneous crystallization age. The first regional metamorphism (M1) affecting the terrane occurred in the interval ~ 430-420 Ma [Wenlock-Ludlow). A second metamorphic event (M2), associated with widespread migmatization, occurred between ~ 410 Ma and 390 Ma (L Dev.) and may be separable into two phases, the older coincident with the youngest phase of the Andover Granite. Sinistral ductile shearing accompanied both the M1 and M2 events. Following M1 and M2, peak metamorphic assemblages in the shear zones were overprinted during several intervals from ~ 375-340 Ma (Late Dev - Early Carb) and the shearing style shifted from ductile to more brittle-ductile conditions.

Rev. James W. Skehan SJ — Geologist, Teacher, Mentor, Priest: A Jesuit Journey II
1:05 PM CONSTRAINTS ON THE EVOLUTION OF THE AVALONIAN MANTLE AND TECTONIC IMPLICATIONS: MURPHY, J. Brendan,

12-2 1:25 PM A REVIEW OF COASTAL MAINE STRATIGRAPHY: BERRY, Henry N. IV, The following stratigraphic columns are used, from east to west. (1) coastal volcanics, (2) Ellsworth, (3) Islesboro, (4) Rockport, (5) St. Croix, (6) Benner Hill, (7) Fredericton, (8) Casco Bay, (9) East Harpswell, (10) Falmouth-Brunswick, (11) central Maine, (12) Merrimack, and (13) Shapleigh.
I. Progress since the 1985 State map: (1) may lack Devonian strata entirely; includes coeval volcanic-plutonic complexes; sub-Bar Harbor unconformity at West Gouldsboro. (2) Ellsworth is Cambrian; Lamoine Granite Gneiss carries Ellsworth foliation; Castine unconformity is within the Cambrian. (5) Battie may = Matthews Lake; Jam Brook is fault-related; Clarry Hill schist is west of Sennebec Pond fault. (6) Formations are defined; Prison Farm in thrust contact. (7) Appleton Ridge = Digdeguash. (8) is M. Ord. (9) is Late Ord, exposed in window through Boothbay thrust. (10) is separate from 8. (11) Hutchins Corner Fm defined, may rest on 10; Waterville/Vassalboro thrust on Sangerville. (12) is Sil., may rest on 8; may be partly equivalent to 11. (13) equivalent to Rangeley sequence; east-bounding thrust in question.
II. Remaining questions (for me): (1) Relationship to 2? (2) Deer Isle ultramafic? Calderwood Fm? (3) Islesboro Fm age? (4) Ogier Point = Islesboro? Rockport ls congl = Ashburn? (5) Rhyolites at Owls Head? Origin of Battie congl? (6) Pillow basalts at Friendship? (7) Cross River Fm? (8) Sheepscot Pond Gn? Passagassawakeag Gn? Kingdom Bog mem. of Scarboro? (9) Relationship to 7? (10) Mt. Ararat Fm? (11) Relationship to 10? Vassalboro vs. Hutchins Corner SW of Augusta? Rocks near Sebago pluton, in Standish? (12) Berwick Fm? (13) Relationship to 11?
Assignment of these sections to various terranes by workers from away remains unsatisfying because of so many loose ends and mis-matched stratigraphy. Diagnostic Avalon faunas (Acado-Baltic) lie to the east. Celtic faunas lie to the northwest. A peri-Gondwanan origin in eastern Iapetus seems likely, with accretion by late Silurian and significant modification by Devonian thrusting.

12-3 1:45 PM 40 YEARS OF MAPPING AND STILL WE DON'T KNOW: LUDMAN, Allan,

12-5 2:25 PM METAMORPHIC EVOLUTION OF HP-LT TECTONITES IN THE BRUNSWICK SUBDUCTION COMPLEX, NEW BRUNSWICK AND ITS IMPLICATIONS FOR THE SALINIC OROGENY IN NEW ENGLAND: VAN STAAL, Cees,
The Silurian Salinic orogeny, dynamically distinct from the Early Devonian Acadian orogeny, is still contentious in New England, although the name and hypothesis of such an event derives from an unconformity recognised by Boucot in the Central Maine belt of northern Maine during the 1960's. The Brunswick complex comprises a well preserved association of latest Ordovician-Early Silurian forearc terranes, an association that is only partially preserved elsewhere. Its metamorphic core is represented by exhumed slices of underplated backarc oceanic and continental rocks. Underplating was sequential and started at c. 447 Ma with underplating of a seamount at epidote-blueschist depth (400oC-7Kb). Mineral zonation (act-bar-gln-act) suggests an anticlockwise hairpin P-T-t path reflecting subduction initiation beneath the c. 460 Ma Fournier ophiolite and subsequent subduction-related refrigeration. Accretion of the Spruce Lake block at c. 442 Ma imbricated the overlying Fournier ophiolite and partially extruded the intervening blueschists. The ophiolite slices above the blueschists were metamorphosed to greenschist conditions (360oC-4.8 Kb). Underplating of the Spruce lake block beneath the blueschist, led to metamorphism characterised by winchite ± Na-augite (350oC-6.2 Kb) or actinolite ± pumpellyite (350oC-5.5 Kb), but zonation (Ep & Am) and microstructures suggest a clockwise P-T path, which is also shown by the structurally underlying greenschist nappes of the Tetagouche block (380oC-5.7 Kb), which were underplated between 435-430 Ma. Both blocks are characterised by late-D1 biotite or stilpnomelane porphyroblasts, which are ascribed to Salinic collision with the Gander margin at c. 430 Ma. Tectonism in the accretionary wedge is directly linked to tectonism in the Matapedia forearc and the Fredericton foredeep. Llandovery exhumation of the Fournier ophiolite and its late Llandovery reburial beneath the onlapping Matapedia forearc relates to underplating of the Tetagouche block and subduction hinge-retreat. The blueschists and Spruce Lake nappes were exhumed during the Salinic Wenlock collision, but reburied beneath a latest Silurian/Early Devonian clastic wedge shortly thereafter. The latter represents the northwards advancing Acadian orogenic wedge.

2:45 PM Break
12-6 3:00 PM FORMATION OF NEW ENGLAND GNEISS DOMES: KARABINOS, Paul,
Northward extrusion of quartz-feldspar-rich core gneisses beneath mica-rich Silurian units during Acadian shortening explains many of the critical observations. The domes are surrounded by high-strain zones (HSZ), which decoupled deformation in the core gneisses from that in the overlying nappes of Silurian metasediments. Units were dramatically thinned or omitted in the HSZ. Sense of shear indicators suggest that rocks above the HSZ were displaced southwest relative to rocks below it. P-T paths of rocks from below the HSZ in the Chester dome indicate decompression of several kbars during metamorphism, whereas rocks above the HSZ record nearly isobaric conditions. These observations are consistent with normal-sense displacement between the core of the domes and the mantling sequence during Acadian deformation. During extrusion, the core gneisses cut upsection into the nappes of Silurian rocks and the thickness of the intervening Lower Paleozoic section was dramatically reduced.

12-7 3:20 PM POST-ORDOVICIAN CONTRASTS IN THE GEOLOGICAL EVOLUTION OF THE NORTHERN AND SOUTHERN APPALACHIAN OROGEN: HIBBARD, James,
The New York promontory serves as the divide between the northern and southern segments of the Appalachians. The first order character of the pre-Silurian crustal building blocks of the orogen is essentially uniform along the length of the orogen. Following Late Ordovician-Silurian sinistral oblique accretion of Carolinia and Ganderia along the Appalachian margin, the northern and southern segments of the orogen appear to record distinctly different histories. The northern Appalachians contain a robust middle to late Paleozoic lithotectonic record of Silurian to Early Devonian tectonism with an extensional component followed by Acadian, Neoacadian, and Alleghanian events across most of the orogen. Although the tectonic details of this record are controversial, e.g. subduction polarity and bulk kinematics, it is generally agreed that this record relates to the accretion of the peri-Gondwanan tracts of Avalonia and Meguma and the culminating interaction of Gondwana with Laurentia.

 The southern Appalachian middle to late Paleozoic lithotectonic record is sparse, with relevant strata preserved as clastic basins and wedges along the western margin of the orogen and magmatic rocks of ambiguous origin in the hinterland; this meager record reflects Neoacadian and Alleghanian events. The Alleghanian event is explicitly tied to the collision of Gondwana with Laurentia, but the nature of the Neoacadian event is unknown, as no counterpart to either Avalonia or Meguma is recognized in the southern orogen. The contrast in lithotectonic evolution between the northern and southern segments of the orogen during the middle to late Paleozoic appears to be related mainly to the limited distribution of Avalonia and Meguma. This difference implies that the oceanic tract trailing Carolinia and Ganderia, i.e. the Rheic Ocean, was more complex than the older Iapetus Ocean in that it harbored first-order lateral variations.

12-9 4:00 PM SUMMARY OF ORIGIN AND ACCRETION OF THE ARGENTINE PRECORDILLERA TERRANE: THOMAS, William A.,

13-7 3:20 PM NORTHERN APPALACHIAN ACCRETION OF AVALONIA: EVIDENCE FROM CHEMOSTRATIGRAPHY AND K/AR DATING OF ILLITIZATION IN THE SILURIAN ARISAIG GROUP, NOVA SCOTIA: BURGREEN, Blair N.

Tuesday, 13 March 2007
The Neo-Acadian Orogeny and Implications for Tectonic and Depositional Setting of Devonian–Carboniferous Rocks in the Appalachian Orogen

26-3 9:00 AM THE BOUNDARY BETWEEN THE VERMONT AND NEW HAMPSHIRE TERRANES: AN ACADIAN CONUNDRUM: CHENEY, John T.
A long recognized conundrum of New England geology is the enigmatic relationship between the Acadian orogeny in Vermont and New Hampshire. The difference is manifest in seemingly synchronous yet different metamorphic styles of different Siluro-Devonian stratigraphies. The Buchan style metamorphism of New Hampshire, with counterclockwise P-T paths and regional high-T, low-P metamorphism contrasts markedly with the Barrovian metamorphism of Vermont, with its clockwise P-T paths culminating in maximum burial depths of ca 30-35 km.
The VT & NH terranes meet in the vicinity of the Chicken Yard line - the traditional stratigraphic boundary - forming the Connecticut Valley Metamorphic Low (CVML). In the CVS of Vermont and along strike in Massachusetts, the array of accumulated ion microprobe 208Pb/232Th spot ages from matrix and inclusion monazite grains is largely confined to 350-390 Ma. 40Ar/39Ar cooling ages from muscovite in these same rocks range from 330 Ma to 350 Ma. In Massachusetts, Cambro - Ordovician rocks have temperature-time histories that are identical to Siluro-Devonian units. The age signature of the VT metamorphism continues east across the CVML into the garnet zone in the NH stratigraphic sequence.
To the east, the Bronson Hill terrane consists of a sequence of nappes, each with a unique P-T path, that are stacked in an inverted metamorphic sequence and punctuated by late gneiss domes. Each of the domains of the Bronson Hill is separated by a post metamorphic fault that records a significant break in peak P and/or T and each unit has its own characteristic monazite age array. The lowest and western most unit, the big staurolite domain, contains monazite as old as ca 360 Ma, similar to the Vermont monazite ages, and, most significantly, 320 Ma monazite as inclusions in staurolite and 280 Ma matrix monazite grains. Muscovite 40Ar/39Ar ages along and east of the BHT range from 335 Ma at Fall Mtn. to around 250-290 Ma over much of central NH. Accordingly, the rocks of the big staurolite domain underwent metamorphism after the VT rocks cooled through the muscovite closure temperature and after the peak of metamorphism in the overlying NH nappes, presumably during juxtaposition of the two terranes during the Alleghanian and certainly not during the Acadian.

26-4 9:20 AM THE NEOACADIAN OROGENY IN THE SOUTHERN AND CENTRAL APPALACHIANS: A KINEMATIC MODEL LINKING MIDDLE DEVONIAN–EARLY MISSISSIPPIAN ACCRETION OF THE CAROLINA SUPERTERRANE, OROGENIC CHANNEL FLOW, AND FORELAND SEDIMENTATION: MERSCHAT, Arthur J.
The Neoacadian orogeny in the southern and central Appalachians (SCA) is characterized by 380-340 Ma metamorphism, deformation, and plutonism in the Blue Ridge and Inner Piedmont (IP). The IP is the core of the Neoacadian orogen in the SCA and records Late Dev.-Miss. closure of the Rheic remnant ocean basin, and high-grade metamorphism (sillimanite I and II) of Siluro-Dev. pelite and psammite. The IP is a large, composite, sillimanite-grade terrane that extends from near the VA-NC border, to central AL and consists of the eastern Tugaloo, Milton, and Cat Square (CSt) terranes. The IP is bounded to the west by the Brevard fault zone and the east by the central Piedmont suture. The CSt is bounded by the younger-over-older Brindle Creek fault to the west and the central Piedmont suture to the east. It consists of a unique sequence of Siluro-Dev. metapsammite and pelitic schist intruded by Dev. anatectic granitoids (Toluca Granite, ~378 Ma, and Walker Top Granite, ~366 and ~407 Ma). Rare mafic and ultramafic rocks occur in the eastern CSt and may represent relict oceanic lithosphere on which CSt sediments were deposited. Minimum sediment thickness is estimated at 4 km (13,000 ft). Detrital zircons indicate CSt rocks have a maximum age of ~430 Ma, with both Laurentian (1.1, 1.4, 1.8, 2.8 Ga) and peri-Gondwanan Carolina superterrane (Ct) (500, 600 Ma) provenance. Intrusion of the Concord and Salisbury plutonic suites into the Ct and position of the Smith River allochthon above the IP support Dev. subduction of the CSt and Tugaloo terranes beneath the Ct. Thus, the CSt was a Siluro-Devonian remnant ocean basin between Laurentia and the approaching Ct. Net estimates of SW-directed dextral strike-slip displacements of the Brevard fault zone range from 250 to 450 km. Palinspastic restoration of the IP delimits the location of the CSt basin to the Pennsylvania embayment, and links the mid-Dev. to Miss. deformation in the Neoacadian core with SW-migrating pulses of the diachronous Acadian-Neoacadian clastic wedge. Location and SW-migration of the clastic wedge in concert with structural patterns in the IP support transpressive NW-directed collision of the Ct with the New York promontory and zippered closing of the basin from NE to SW. Subduction of the CSt and parts of the Tugaloo terranes beneath the Ct resulted in a SW-directed orogenic channel.

26-5 9:40 AM EVALUATION OF THE GEOCHRONOLOGIC EVIDENCE FOR THE TIMING OF PALEOZOIC OROGENIC EVENTS ALONG THE WESTERN FLANK OF THE SOUTHERN APPALACHIANS: MILLER, Brent,

A comprehensive database was constructed from 528 journal articles, abstracts, and other sources (1959 to 2006) providing 454 Ar/Ar, EPMA, K-Ar, Rb-Sr, Sm-Nd, and U-Pb, ages. All ages taken together at face value show a distinct Taconian maximum at 460-480 Ma, an asymmetric Alleghanian peak at ~340 Ma tailing off to 250 Ma, and between the two lies a broad swath of Acadian or Neoacadian ages. This general pattern is retained through a stepwise quality control process that yields a surprisingly small subset of highly robust ages. The Ar/Ar dataset reveals virtually no Taconian cooling ages, little distinction between Acadian and Neoacadian cooling ages, and an Alleghanian 335 Ma spike. Only 18 U-Pb zircon ages have bearing on the distinction between Neoacadian vs. Acadian plutonism and metamorphism. Ion-microprobe ages from two Acadian granites (one of which, Rabun, is now known to be Alleghanian) are questionable. Two Spruce Pine intrusions have high-precision Acadian ages of ca. 377 Ma. Fourteen reported ages, evenly divided between Acadian and Neoacadian, await publication before their significance can be evaluated.

Although there are many potential biases in this type of evaluation, it does highlight historical trends in thinking about the timing of tectonothermal events and it potentially points the way to future progress in better deciphering the history of southern Appalachian orogenic effects. The database is available in spreadsheet format from the author's website.

26-6 10:15 AM PALEOMAGNETIC OVERPRINTING AS EVIDENCE OF NEO-ACADIAN TECTONISM IN NORTHERN APPALACHIAN PERI-GONDWANAN TERRANES: THOMPSON, M.D.,

26-7 10:35 AM NEOACADIAN DEFORMATION WITHIN THE MEGUMA TERRANE: HORNE, Richard J. The Early Devonian Torbrook Formation provides a minimum age on fold development and 40Ar/39Ar ages generally support a middle Devonian (ca 380-390 Ma) age. The Late Devonian (ca. 375 Ma) South Mountain Batholith (SMB) postdates the main folding event, it however records subtle internal strain consistent with folding accompanying transpression on the terrane-bounding fault. Flexural slip locally deforms porphyroblasts within the contact aureole of the SMB and 40Ar/39Ar ages for some auriferous saddle reef veins coincide with igneous ages. Shear zones parallel to the regional fold axial trend accompanied and may have played a role in granite emplacement. Rapid uplift following granite emplacement may explain the change to the late, brittle, flexural-slip folding.

26-8 10:55 AM THE NEOACADIAN OROGENY IN THE MEGUMA TERRANE, NOVA SCOTIA, CANADA: WHITE, Chris E.
The ca. 380 Ma and 376-372 Ma plutons and high-temperature/low-pressure metamorphism may have been related to delamination of a shallow-dipping Rheic Ocean lithosphere following accretion of Meguma to Avalonia.  Rapid uplift and erosion in the Meguma terrane between 370 and 360 Ma resulted in deposition of the Upper Devonian to Lower Carboniferous Horton Group in the northern parts of the terrane. Uplift coincided with intrusion of the younger (ca. 363 and 357 Ma) plutons in southern Meguma terrane and reflects continued northwestward subduction of Rheic Ocean lithosphere under the Meguma terrane as Gondwana approached from the southeast. Widespread ca. 320 Ma deformation in southern Nova Scotia maybe related to the final juxtaposition of Gondwana.


26-9 11:15 AM NEOACADIAN PLUTONISM IN THE NEWFOUNDLAND APPALACHIANS: A POORLY CONSTRAINED BUT ECONOMICALLY IMPORTANT MAGMATIC EVENT: KERR, Andrew
As currently understood, "NeoAcadian" plutonism in Newfoundland is largely confined to an arcuate belt within the eastern Gander Zone and adjacent Avalon Zone. It appears to be spatially associated with this boundary, although it postdates juxtaposition of the two zones. Compared to extensive Silurian and early Devonian magmatism in the Dunnage and Gander zones, this late plutonism is spatially displaced towards the southeast, i.e., outboard with respect to Laurentia. The compositional spectrum of late plutonism is bimodal but felsic-dominated. Granites are typically silica-rich, strongly potassic, and metaluminous to variably peralkaline in character, showing strong regional enrichments in U, Th and F. The Nd isotopic signatures of late granites provide a striking illustration of the fundamental (crustal-scale) importance of the Gander-Avalon boundary, which here demarcates discrete lower crustal blocks.

26-10 11:35 AM WHAT IS THE ACADIAN OROGENY?: MURPHY, J. Brendan
We suggest that collision of all peri-Gondwanan terranes be referred to as the Salinic orogeny, with Ganderian, Avalonian and Carolinian stages for those who interpret these terranes to have collided independently. We suggest that the term Acadian orogeny be reserved for events related to subduction of the Rheic Ocean, with Meguma and Carolinian stages for those who believe that these terranes collided independently from within that ocean. In this scheme a separate Neo-Acadian orogeny would be unnecessary.

Structural Geology and Tectonics
30-8 19 TIMING OF METAMORPHISM AND DEFORMATION IN SOUTHEASTERN PENNSYLVANIA AND NORTHERN DELAWARE: BLACKMER, Gale C.,

New 40Ar/39Ar age data from southeastern Pennsylvania and northern Delaware, together with published monazite data, suggest that development of the dominant regional fabrics and cooling from metamorphic conditions happened in the Devonian, with only localized, minor white mica growth during the Pennsylvanian. The study area is a portion of the central Appalachian Piedmont consisting of greenschist to granulite facies metamorphic rocks, including three provisional divisions of the Wissahickon Formation, separated into structural blocks by major NE-trending faults. Units were sampled as follows, from SE to NW: type Wissahickon Fm SE of the Rosemont Fault (Block 1); Baltimore Gneiss, Mt. Cuba Wissahickon fm, and Setters Fm between the Rosemont and Street Road faults (Block 2); Glenarm Wissahickon fm between the Street Road and Embreeville faults (Block 3); Peters Creek Schist and Octoraro Fm between the Embreeville and Martic faults (Block 4); and Antietam and Harpers formations, undivided, NW of the Martic Fault (Block 5). Amphiboles from amphibolite to granulite facies rocks in Block 2 yield 40Ar/39Ar ages indicating cooling through the ~500ºC isotherm at ~400-375 Ma, suggesting that the Baltimore Gneiss (basement) and the Mt. Cuba Wissahickon fm were at roughly the same structural level at about this time. In comparison, monazite that grew during formation of the regional fabric in garnet-grade rocks in Block 3 yields microprobe ages of ~400 Ma. White mica from lowermost greenschist facies rocks in Block 5 yields a growth age similar to the amphibole cooling and monazite growth ages. This may indicate a time of thrusting on the Martic Fault. White micas from Blocks 1-4 yield 40Ar/39Ar cooling ages of about 365 Ma in all units. We interpret these ages to represent cooling through ~350ºC. No regional patterns are apparent. Age spectra of white micas from the vicinity of the Pleasant Grove-Huntingdon Valley shear zone, within Block 4, are more complex and suggest minor recrystallization during Alleghanian shearing. Regionally, white mica ages from Block 4 are comparable with results from correlative units in the Potomac Terrane of Virginia and Maryland. The amphibole cooling ages from Block 2 are similar to amphibole ages from the Sykesville and Laurel formations of the Potomac Terrane.


30-11 22 TRACE ELEMENT EVIDENCE FOR LATE ORDOVICIAN TO EARLY SILURIAN CRUSTAL THICKENING, SOUTHWEST CONNECTICUT: PROCTOR, Brooks P.1, WINTSCH, Robert
The Beardsley orthogneiss is .....zircon yield 446 ± 2 Ma (Sevigny and Hanson, 1993)......the Pumpkin Ground (PG) orthogneiss is .... zircon yield 428 ± 2 Ma (Sevigny and Hanson, 1993). ......These results suggest that the Beardsley orthogneiss developed in a Late Ordovician volcanic arc setting, which from plate tectonic reconstructions was likely on the eastern margin of Laurentia. About 20 m.y. later, the S-type PG orthogniess developed over a transitional crust from VAG to WPG. This is consistent with merging of the Gander and Laurentian lower crusts, and may reflect tectonic activity during the Salinic orogeny.

30-12 23 ASTHENOSPHERIC UPWELLING AND SLAB ROLL-BACK OF THE SUBDUCTING AVALON PLATE: GEOCHEMICAL EVIDENCE FROM THE LEBANON GABBRO, EAST-CENTRAL CONNECTICUT: BOWMAN, Jeffrey D.

Wednesday, 14 March 2007
Isotopic and Other Indicators of Sediment Provenance and Basement Character

45-2 9:00 AM SM-ND ISOTOPIC AND WHOLE-ROCK CHEMICAL COMPOSITIONS OF LATE NEOPROTEROZOIC AND CAMBRIAN SEDIMENTARY AND METASEDIMENTARY ROCKS OF THE CALEDONIAN HIGHLANDS (AVALONIA), SOUTHERN NEW BRUNSWICK: SATKOSKI, Aaron Preliminary Nd isotopic results from this study combined with previously published data show that the Hammondvale Metamorphic Suite and metasedimentary rocks of the Broad River Group have negative eNd values, whereas the sedimentary rocks of the Coldbrook and Saint John groups show more positive eNd values. The Broad River Group and Hammondvale Metamorphic Suite samples fall well outside the Avalonian igneous isotopic window, suggesting a non-Avalonian source. In contrast, Coldbrook Group and Saint John Group samples fall within the main Avalonian isotopic window, suggesting a substantial Avalonian crustal component. Previously published Nd isotopic data from igneous units in the Broad River and Coldbrook groups have mostly positive eNd values, consistent with the Coldbrook and Saint John group sediments being derived from those rocks. The positive values from the igneous units, however, are not consistent with the more negative values for the Broad River and Hammondvale metasedimentary rocks. These sediments must have had a large, isotopically mature source, presumably outside Avalonia.

45-3 9:20 AM PROVENANCE STUDIES IN THE MEGUMA TERRANE, SOUTHERN NOVA SCOTIA, CANADA: BARR, Sandra M. Petrographic and chemical data suggest that the source area included calc-alkalic tonalitic and granodioritic rocks; the derived sediment experienced little transport and was poorly sorted during rapid deposition in an active continental margin environment. Previously reported detrital zircon ages cluster at 700-550 Ma and 2150-2000 Ma, compatible with African or Amazonian sources. Available epsilon Nd data are highly negative, and more compatible with an Amazonian source. The source of detrital muscovite is not yet resolved but preliminary data indicate ages of 600-560 Ma. Similarities in age and isotopic compositions suggest some links between sediment provenance in the Meguma terrane and now-adjacent Avalonia.

45-4 9:40 AM REINTERPRETING THE PROVENANCE OF EASTERN LAURENTIAN LATE PROTEROZOIC RIFT FILL: NEW WHOLE-ROCK PB ISOTOPIC CONTRAINTS: BREAM, Brendan R. Recent work demonstrates that some cratons have distinct whole-rock Pb isotopic signatures.     New whole-rock Pb isotopic data were obtained for 20 samples from a previous detrital zircon and Nd isotopic study. Of these samples, 17 have 207Pb/204Pb values > 15.65 and most 206Pb/204Pb values are between 18.2 and 20.2. The metasedimentary rocks have higher 207Pb/204Pb values for given 206Pb/204Pb values relative to rocks from the mid-continent ~1.4 Ga Granite-Rhyolite and 1.0-1.3 Ga Grenville terranes of New York and Texas. This distinct Pb isotopic signature of the metasedimentary rocks precludes incorporation of significant components of Laurentian material and necessitates an alternate source.
        Amazonia is the southeastern Laurentia conjugate rift margin in most Rodinia reconstruction and it contains rocks with similar U/Pb ages and Nd and Pb isotopic signatures to samples from the southern Appalachians. The U/Pb detrital ages and Nd and Pb isotopic signatures of the metasedimentary sequence are also strikingly similar to those of the underlying southern and central Appalachian basement (scAB). Distinct scAB signatures were used to argue that it was an exotic block that was transferred from Amazonia to Laurentia during the Grenville Orogeny. Thus, Blue Ridge and western Inner Piedmont metasedimentary sequences were mostly derived from: (1) a post-Rodinian Laurentian margin that included accreted Amazonian material (scAB); (2) the Amazonian conjugate margin; or (3) both margins. All three scenarios imply that during Rodinia breakup Laurentia included a rifted margin dominated by extended Amazonian blocks and possibly a continental interior that was submerged, topographically subdued, or isolated in terms of its drainage pattern.

45-5 10:15 AM GRAPTOLITE BIOSTRATIGRAPHY AND K-BENTONITE TEPHROCHRONOLOGY FROM A CORE SECTION THROUGH THE UTICA, TRENTON, AND BLACK RIVER GROUPS NEAR BALSTON SPA, NEW YORK: ROLOSON, Melissa. These relations indicate that the base of the Utica here is substantially older than it is farther west in the central Mohawk Valley..........

45-6 10:35 AM AVALONIA: A NEOPROTEROZOIC LOW-18O TERRANE: POTTER, Joanna. ...... We attribute this 18O-depletion to pervasive, post-magmatic hydrothermal alteration. This alteration likely occurred during the last stages of Neoproterozoic igneous activity, and was related to rifting as Avalonia separated from the Gondwanan supercontinent at ca. 560-550 Ma. The development of rift-wrench extensional basins allowed large-scale fluid infiltration and hydrothermal alteration of the Avalonian crust. As this systematic 18O-depletion is not observed in the Neoproterozoic igneous rocks of the other peri-Gondwanan terranes, its occurrence implies that Avalonia remained separate from Ganderia until at least ~550 Ma.

45-7 10:55 AM MAGMA SOURCE CHARACTERISTICS OF CAMBRIAN VOLCANIC ROCKS IN THE ELLSWORTH TERRANE, PENOBSCOT BAY AREA, MAINE: SCHULZ, Klaus J. The Ellsworth terrane in the Penobscot Bay area of Maine consists of Cambrian bimodal volcanic rocks of the Castine Volcanics (~502 Ma), Ellsworth Schist (~509 Ma), and North Haven Greenstone. The volcanic rocks are divided into four geochemical types based on data for immobile trace elements: two types of tholeiitic basalt (Tb-1a, Tb-1b) and two types of rhyolite (Rhy-1, Rhy-2). Here we report Nd isotope data that provide further constraints on the source characteristics of these bimodal volcanics.
Tb-1a basalts, present in all units of the Ellsworth terrane, have compositions transitional between N-MORB and E-MORB, including slightly depleted LREE and primitive mantle-normalized trace element patterns (PMNP) with no Th or Ta anomalies. They have large positive εNd500 values of +7.9 to +8.6 (mean +8.2; n = 6) indicating a long-term depleted mantle source and no crustal contamination. Tb-1b basalts, present only in the Castine Volcanics, are compositionally similar to E-MORB, including enriched LREE and PMNP with no Th or Ta anomalies. They have lower positive εNd500 values of +5.6 and +7.0 (mean +6.3; n = 2) and TDM model ages of 452 and 718 Ma. Rhy-1 rhyolites, present mainly in the Castine Volcanics, have tholeiitic compositions with immobile trace element ratios overlapping those of the Tb-1b basalts. Their positive epsilonNd500 values (mean +6.5; n = 3) also overlap those of the Tb-1b basalts, supporting a genetic relationship. Rhy-2 rhyolites, present mainly in the older Ellsworth Schist, are calc-alkaline with enriched LREE and PMNP with positive Th and negative Ta and Ti anomalies. Relatively low epsilonNd500 values near zero (-0.9 to +0.5) and TDM model ages ranging from 1067 to 1130 Ma indicate an evolved crustal component in the source area of the Rhy-2 rhyolites.
The geochemical and Nd isotope data support a non-arc setting for the Ellsworth terrane. Further, the compositional similarity of the Ellsworth basalts to those found in rift basins in the present Gulf of California suggests that the Ellsworth evolved in a similar tectonic regime. The older Rhy-2 rhyolites may reflect crustal contamination/ melting during initial crustal rifting. Such a rift setting is compatible with the Baja rift model proposed for the Neoproterozoic-Cambrian evolution of peri-Gondwanan terranes (Keppie et al., 2003, Tectonophysics 365: 195-219).

45-8 11:15 AM U-PB DETRITAL ZIRCON GEOCHRONOLOGY OF AVALONIA: CONSTRAINTS ON THE OPENING OF THE RHEIC OCEAN: POLLOCK, Jeff Avalonia comprises Neoproterozoic-early Palaeozoic magmatic arc terranes that extend from eastern Massachusetts to the type area in eastern Newfoundland. Most of Avalonia is at low metamorphic grade, generally mildly deformed, and consists of volcanic, plutonic and sedimentary rocks that record at least five distinct groupings of pre-Iapetan tectonomagmatic and depositional events at ca. 760 and 730 Ma, 685-670 Ma, 635-590 Ma, and 590-545 Ma. The Neoproterozoic rocks are overlain by a terminal Neoproterozoic-Early Ordovician cover of fine-grained siliciclastic rocks containing Acado-Baltic faunas.
U-Pb geochronology on detrital zircons from seven samples from the major lithotectonic elements in Avalonia was completed in order to address multiple first-order questions concerning the evolution of both Avalonia and the Rheic Ocean. The data from Neoproterozoic-Early Cambrian strata (Conception, Musgravetown, and Signal Hill groups and Random Formation) are dominated by Ediacaran (c. 580 Ma) ages and suggest derivation from the underlying arc-volcanic sequences. Although the Arenig Bell Island Group comprises mostly Ediacaran and Cambrian zircons, it also contains significant quantities of Mesoproterozoic and Palaeoproterozic zircons. This change in provenance may be related to the rifting and isolation of Avalonia from Gondwana in the Early Ordovician.

45-9 11:35 AM ANCIENT LAURENTIAN DETRITAL ZIRCON IN THE SOUTHERN UPLANDS TERRANE, BRITISH CALEDONIDES: WALDRON, John W.F. Zircons from a sample deposited early in the depositional history (Kirkcolm Fm.) display a range of U-Pb ages from Paleoarchean to late Ordovician, including the oldest such grain yet recorded from the British Isles. Neoarchean (2.8 - 2.5 Ga), Paleoproterozoic (2.0 - 1.7 Ga), and Mesoproterozoic (1.5 - 1.0 Ga) age populations suggest sources in Laurentia, such as the Grenville and Trans-Hudson orogens. The overall age distribution is comparable to metasedimentary rocks of the Laurentian margin of Iapetus in the Taconian orogen of W. Newfoundland. Samples with younger depositional ages show increasing amounts of Grenville-age zircon (ca. 1.0 Ga) and a general trend of reduced Archean input. Several analyses from the Glenlee Fm. plot on a discordia line suggesting overprinting of Archean zircon in the Early Paleozoic, consistent with tectonothermal reworking of Laurentian detritus in the Grampian orogen.

Field Guides - NEGSA 2007, Durham, NH.

Hon, R. , Hepburn, J.C. & Lair, Jo. 2007. Siluro-Devonian igneous rocks of the easternmost three terranes in southeastern New England: examples from NE Massachusetts and SE New Hampshire.Guidbook to field trips in New Hampshire, adjacent Maine and Massachusetts, 42nd Ann Meet. NEGSA, March 11 2007, p. 23-43 (20).   http://instruct.uwo.ca/earth-sci/fieldlog/cal_napp/napp/new_eng_maritimes/Nashoba_Avalon/NEGSAFT_F4.pdf

Swanson, M.T., 2007. Structure of Late Paleozoic brittle dextral strike-slip faults in Coastal Maine exposures. Guidbook to field trips in New Hampshire, adjacent Maine and Massachusetts, 42nd Ann Meet. NEGSA, March 11 2007, p. 3-18 (16).

Ulf Linnemann, R. Damian Nance, Petr Kraft, Gernold Zulauf. 2007. The Evolution of the Rheic Ocean: From Avalonian-Cadomian Active Margin to Alleghenian-Variscan Collision. GSA Spec paper, 592 p. ISBN: 9780813724232.

Ribeiro, António; Munhá, José; Dias, Rui; Mateus, António; Pereira, Eurico; Ribeiro, Luísa; Fonseca, Paulo; Araújo, Alexandre; Oliveira, Tomás; Romão, José; Chaminé, Hélder; Coke, Carlos; Pedro, Jorge, 2007. Geodynamic evolution of the SW Europe Variscides. Tectonics, Vol. 26, No. 6, 14 December 2007   The place of Avalonia in the Variscides of SW Europe.   http://instruct.uwo.ca/earth-sci/fieldlog/Variscan_Hercynian/Ribeiro07/Rib07_f10.jpg   http://instruct.uwo.ca/earth-sci/fieldlog/Google_Earth/  Variscides.kmz

10-1 1:05 PM GEOCHEMISTRY AND SM-ND ISOTOPIC SIGNATURE OF THE 0.76 GA BURIN GROUP: A COMPOSITIONAL EQUIVALENT OF THE BASEMENT FOR LATE NEOPROTEROZOIC AVALONIAN MAGMATISM?: MURPHY, J. Brendan, Department of Earth Sciences, St. Francis Xavier University, Antigonish, NS B2G 2W5, Canada, bmurphystfx.ca, MCCAUSLAND, Phil J.A., Earth Sciences, University of Western Ontario, Biology and Geology building, 1151 Richmond St, London, ON N6A 5B7, Canada, O'BRIEN, Sean J., Geological Survey of Newfoundland and Labrador, Newfoundland and Labrador Department of Natural Resources, St. John's, NF A1B, Canada, PISAREVSKY, Sergei, University of Edinburgh, School of Geosciences, Edinburgh, EH9 3JW, United Kingdom, and HAMILTON, Michael, Jack Satterly Geochronology Laboratory, Department of Geology, University of Toronto, Toronto, ON M5S 3B1, Canada

10-2 1:25 PM AVALONIA'S FOUNDATION? PRELIMINARY PALEOMAGNETISM AND U-PB ZIRCON GEOCHRONOLOGY OF THE MID-NEOPROTEROZOIC BURIN GROUP, NEWFOUNDLAND: MCCAUSLAND, Phil J.A., Earth Sciences, University of Western Ontario, Biology and Geology building, 1151 Richmond St, London, ON N6A 5B7, Canada, pmccausluwo.ca, MURPHY, J. Brendan, Department of Earth Sciences, St. Francis Xavier University, Antigonish, NS B2G 2W5, Canada, HAMILTON, Michael A., Geology, Univ of Toronto, Toronto, ON M5S 3B1, Canada, PISAREVSKY, Sergei, University of Edinburgh, School of Geosciences, Edinburgh, EH9 3JW, United Kingdom, and O'BRIEN, Sean J., ?Geological Survey of Newfoundland and Labrador, ? Newfoundland and Labrador Department of Natural Resources, St. John's, NF ?A1B, Canada

"single published U-Pb zircon age of 763 +/- 2 Ma on a gabbro sill (Wandsworth), "

"moderately SW-dipping mafic dykes intrude gabbro which has a preliminary U-Pb zircon age of 764.5 +/- 2.1 Ma, "

10-3 1:45 PM THE PENOBSCOTTIAN ARC SYSTEM OF COASTAL MAINE AND SOUTHERN NEW BRUNSWICK: FYFFE, Leslie R.1, JOHNSON, Susan C.2, and MCLEOD, Malcolm J.2, (1) New Brunswick Natural Resources, Geological Surveys Branch, PO Box 6000, Fredericton, NB E3B 5H1, Canada, les.fyffegnb.ca, (2) New Brunswick Natural Resources, Geological Surveys Branch, PO Box 5040, 207 Picadilly Road, Sussex, NB E4E 5L2, Canada

10-4 2:05 PM REFINED AGES OF PALEOZOIC PLUTONS AS CONSTRAINTS ON AVALONIAN ACCRETION IN SOUTHEASTERN NEW ENGLAND: THOMPSON, Margaret D., Geosciences Department, Wellesley College, 106 Central Street, Wellesley, MA 02481, mthompsonwellesley.edu and RAMEZANI, Jahandar, Dept. of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139

" CA-TIMS single-zircon U-Pb (weighted mean 206Pb/238U) dates of 425.97 ± 0.64 Ma for the Cape Ann Granite and 378.08 ± 0.62 Ma for the Peabody Granite represent significant refinements in both precision and accuracy over previous multi-grain results (450 ± 25 Ma and 380 ± 15 Ma, respectively). "

10-5 2:25 PM ZIRCON-BASED PROVENANCE STUDIES: GOING ALL THE WAY REQUIRES MORE THAN JUST A DATE: HIETPAS, Jack¹, SAMSON, Scott1, CHAKRABORTY, Suvankar2, and MOECHER, David2, (1) Department of Earth Sciences, Syracuse University, 204 Heroy Laboratory, Syracuse, NY 13244, jhietpassyr.edu, (2) Department of Earth and Environmental Sciences, University Of Kentucky, Lexington, KY 40506

10-6 3:05 PM PRESSURE-TEMPERATURE-TIME CONDITIONS, ND AND PB ISOTOPIC COMPOSITIONS AND DETRIAL ZIRCON GEOCHRONOLOGY OF THE MASSABESIC GNEISS COMPLEX, NEW HAMPSHIRE: DORAIS, Michael J.1, WINTSCH, Robert P.2, KUNK, Michael J.3, ALEINIKOFF, John4, UNDERDOWN, Christine1, and KERWIN, Charles M.5, (1) Geological Sciences, Brigham Young University, Provo, UT 84602, doraisbyu.edu, (2) Geological Sciences, Indiana University, Bloomington, IN 47405, (3) U.S. Geological Survey, 926A National Center, Reston, VA 20192, (4) U.S. Geol Survey, Denver, CO 80225, (5) Dept. of Earth Sciences, Univ of New Hampshire, 56 College Rd, Durham, NH 03824-3589 10-7 3:25 PM

10-7 3:25 PM SIGNIFICANCE OF DETRITAL ZIRCON AGES FROM THE WESTBORO QUARTZITE, AVALON TERRANE, EASTERN MASSACHUSETTS: HEPBURN, J. Christopher, Department of Geology and Geophysics, Boston College, Chestnut Hill, MA 02467-3809, hepburnbc.edu, FERNÁNDEZ-SUÁREZ, Javier, Departmento de Petrologia y Geoquímica, Universidad Complutense, Madrid, 28040, JENNER, George A., Dept. of Earth Sciences, Memorial University of Newfoundland, St. John’s, NF A1B3X5, Canada, and BELOUSOVA, Elena A., GEMOC, Dept. of Earth and Planetary Sciences, Macquarie University, Sydney, NSW, 2109, Australia

10-8 3:45 PM PROVENANCE OF THE MEGUMA TERRANE, NOVA SCOTIA: WHITE, Chris E.1, WALDRON, John W.F.2, BARR, Sandra M.3, SIMONETTI, Antonio4, and HEAMAN, Larry M.4, (1) Nova Scotia Department of Natural Resources, PO Box 698, Halifax, NS B3J 2T9, whitecegov.ns.ca, (2) Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, AB T6G2E3, Canada, (3) Geology, Acadia Univ, Wolfville, NS B4P 2R6, Canada, (4) Dept. of Earth and Atmospheric Sciences, University of Alberta, Edmonton, AB T6G 2E3, Canada

10-9 4:05 PM PROVENANCE AND TECTONIC SETTING OF LATE NEOPROTEROZOIC AND CAMBRIAN SEDIMENTARY AND METASEDIMENTARY ROCKS IN AVALONIA OF SOUTHERN NEW BRUNSWICK: SATKOSKI, Aaron M.1, BARR, Sandra M.1, and SAMSON, Scott D.2, (1) Earth and Environmental Science, Acadia Univ, Wolfville, NS B4P 2R6, Canada, aaron.satkoskiacadiau.ca, (2) Earth Sciences, Syracuse University, 204 Heroy Geology Laboratory, Syracuse Univeristy, Syracuse, NY 13244

" Neoproterozoic though Cambrian clastic sedimentary and/or metasedimentary rocks occur in the Hammondvale Metamorphic Suite (> ca. 620 <680 Ma) and Broad River (ca. 620 Ma), Coldbrook (ca. 560-542 Ma) and Saint John (ca. 540-490 Ma) groups in the Avalonian Caledonia terrane of southern New Brunswick. The lithological, major and trace element chemical, and Nd isotopic compositions of the sedimentary rocks provide new constraints on the provenance and environment of deposition of these units and hence on the tectonic evolution of this typical part of Avalonia. Nd isotopic and whole-rock chemical data show that the Hammondvale Metamorphic Suite and metasedimentary rocks of the Broad River Group have negative eNd values, were derived from recycled sedimentary and mafic igneous sources, and were deposited in intra-arc basins as part of the ca. 620 volcanic-arc complex. In contrast, sedimentary rocks of the Coldbrook Group show generally positive eNd values and likely were derived from Avalonian felsic to intermediate igneous sources and deposited in rift basins associated with 560-550 Ma arc extension. Samples from the overlying Saint John Group have felsic to mafic igneous sources, but are characterized by negative eNd values and likely were deposited as part of a newly forming passive margin sequence.

Many metasedimentary and sedimentary samples from the Hammondvale Metamorphic Suite, Broad River Group, and Saint John Group fall outside the normal range for Avalonian igneous rocks, whereas Coldbrook Group samples fall mainly in the typical Avalonian igneous range, suggesting a substantially larger Avalonian crustal component in their evolution. Based on their mostly positive eNd values, the sedimentary units in the Coldbrook Group were derived from associated volcanic units as well as the older Broad River Group igneous units. The more negative eNd values for samples from the Hammondvale Metamorphic Suite and Broad River Group indicate a large, isotopically mature source from more interior locations in Gondwana. Based on overlapping detrital muscovite ages and similar Nd isotopic values, the lower Saint John Group units were derived from the Hammondvale Metamorphic Suite, whereas the middle to upper units, which have more evolved Nd isotopic values, appear to require a source outside of the Caledonia terrane. "

10-10 4:25 PM PROVENANCE STUDIES OF CAMBRIAN SEDIMENTARY ROCKS IN AVALONIA, SOUTHERN NEW BRUNSWICK AND CAPE BRETON ISLAND, NOVA SCOTIA, CANADA: BARR, Sandra M.1, WHITE, Chris E.2, HAMILTON, Michael A.3, REYNOLDS, Peter H.4, and SATKOSKI, Aaron M.1, (1) Department of Earth and Environmental Science, Acadia University, Wolfville, NS B4P 2R6, Canada, sandra.barracadiau.ca, (2) Natural Resources, P.O. Box 698, Halifax, NS B3J 2T9, Canada, (3) Jack Satterly Geochronology Lab, Dept. of Geology, Univ of Toronto, Toronto, ON M5S 3B1, (4) Earth Sciences, Dalhousie Univ, Halifax, NS B3H 3J5, Canada

10-11 4:45 PM DETRITAL ZIRCON CONSTRAINTS ON THE PALEOZOIC PROVENANCE OF PERI-GONDWANAN TERRANES IN SOUTHERN MEXICO: NANCE, R. Damian, Department of Geological Sciences, Ohio University, Athens, OH 45701, nanceohiou.edu, MILLER, Brent V., Department of Geology and Geophysics, Texas A&M University, College Station, TX 77843, KEPPIE, J. Duncan, Departamento de Geología Regional, Instituto de Geología, Universidad Nacional Autónoma de México, México, DF 04510, Mexico, MURPHY, J. Brendan, Department of Earth Sciences, St. Francis Xavier University, Antigonish, NS B2G 2W5, Canada, and DOSTAL, Jaroslav, Department of Geology, St. Mary's University, Halifax, NS B3H 3C3, Canada

47-1 8:00 AM GEOCHRONOLOGY AND GEOCHEMISTRY OF THE ANNIDALE GROUP, SOUTHERN NEW BRUNSWICK, CANADA: CAMBRO-ORDOVICIAN ARC-RELATED MAGMATISM ALONG THE MARGIN OF GANDERIA: JOHNSON, Susan C.1, MCLEOD, Malcolm J.1, FYFFE, Les R.2, and DUNNING, Greg R.3, (1) Geological Surveys Branch, New Brunswick Department of Natural Resources, P.O. Box 5040, Sussex, NB E4E 5L2, Canada, susan.johnsongnb.ca, (2) Geological Surveys Branch, New Brunswick Department of Natural Resources, P.O. Box 6000, Fredericton, NB E3B 5H1, Canada, (3) Department of Earth Sciences, Memorial University of Newfoundland, St. John's, NF A1B 3X5, Canada

"Late Cambrian - Early Ordovician rocks of the Annidale Group are preserved within a northeast-trending, fold-thrust belt along the northwestern margin of the peri-Gondwanan New River terrane in southern New Brunswick. The group comprises a complex assemblage of marine mafic-intermediate volcanic and sedimentary rocks, rhyolite flows and domes, gabbro and plagiogranite. New U/Pb data indicates that tectonic interleaving of the c. 497 – 489 Ma Annidale Group and its juxtaposition with New River basement occurred prior to c. 476 Ma, the age of the Stewarton gabbro that stitches the faulted contact. Younger (c.478-472 Ma) felsic volcanic rocks and associated rhyolite domes are interpreted to lie unconformably on the Annidale Group.

Geochemical signatures of the Annidale Group are indicative of supra-subduction zone magmatism. Mafic-intermediate volcanic rocks are subalkaline, mostly tholeiitic to transitional basaltic andesites that exhibit both arc and non-arc geochemical signatures. Most samples show weak to strong negative Nb and Ta and positive Th anomalies on extended rare earth plots and fall clearly within volcanic-arc fields on trace-element discrimination diagrams. One exception is a group of extreme LREE-depleted basalts associated with plagiogranite intrusions, which display concave-upwards REE patterns typical of N-MORB affinity, but with uncharacteristically low TiO2 (0.48-0.54%) more typical of arc tholeiites. These basalts are also magnesian-rich (13.7 – 17.2% MgO) and strongly enriched in Cr (1180-4910 ppm) and Ni (390-880ppm).

These new data bearing on the age and tectonic setting of the Annidale Group are in accord with recent models for the tectonic development of the Exploits Subzone in Newfoundland and confirm that Latest Cambrian – Early Ordovician magmatism in the Annidale Group can be related to the development of the Penobscot arc along the Gander margin. Latest Arenig to Middle Ordovician (post-Penobscottian) magmatism, possibly related to the initiation of the Victoria arc, affected both the Annidale Group and adjacent Neoproterozoic – Early Cambrian New River terrane. A recent 40Ar/39Ar age of 444 +/- 5 Ma for metamorphic muscovite in schistose felsic tuff of the Annidale Group indicates that unroofing of the arc volcanics was coincident with the final closure of Iapetus."

47-3 8:40 AM MIDDLE CAMBRIAN TO EARLY ORDOVICIAN ARC – BACK-ARC DEVELOPMENT ON THE LEADING EDGE OF GANDERIA, NEWFOUNDLAND APPALACHIANS: ZAGOREVSKI, Alexandre^1, VAN STAAL, Cees R.2, ROGERS, Neil1, and MCNICOLL, Vicki1, (1) Geological Survey of Canada, 601 Booth St, Ottawa, ON K1A 0E8, Canada, azagorevnrcan.gc.ca, (2) Geological Survey of Canada, 625 Robson Street, Vancouver, BC V6B 5J3, Canada

"Ganderia, during its transfer from Gondwana to Laurentia, must have perturbed the global carbon cycle at least twice, first when it underthrust seafloor in the earliest Ordovician (Penobscot orogeny) and second as a side effect of late Ordovician arc-arc collision along the Hurricane Mountain suture (Red Indian Line). Ganderia's modern cousin, the Norfolk Ridge ribbon continent east of Australia, jammed the subduction zone of the Loyalty arc during the latest Eocene; the obducted New Caledonian seafloor is a viable candidate for precipitating growth, via pCO2 drawdown, of the Antarctic ice sheet. The "carbonator" refers to a scenario such as would result if a large mass of seafloor was translated through present-day Mt. Cook: surprisingly large factors, including volume of eroded material (for New Caledonia, <600,000 km3), exhumation rate, high acid-neutralizing capacity of peridotites (<9 mol CO2 kg-1), hydrothermally-enhanced carbonation rate (>1000-fold increase at 185 vs. 25ºC), and fraction dissolved, combine to produce surprisingly large flux (>0.1 Tmol yr-1). The last factor is most uncertain, and ferruginous, dolomitic carbonates in foreland basin sequences that occur beneath chromite-bearing sandstones should be examined as candidates for the actual carbon sequestered by the dissolution of the adjacent seafloor. The emplacement of single large igneous provinces has been linked to CO2-forced global warming. Conversely, seafloor exhumation, such as took place in New Caledonia and along the GRUB line, may create "point CO2 sinks" large enough to cool the globe. "

47-10 11:20 AM EVOLUTION OF THE PERI-GONDWANAN MARGIN OF SOUTHERN BRITAIN: SCHOFIELD, D.I.1, DAVIES, J. R.2, and WILSON, D.1, (1) British Geological Survey, Kingsley Dunham Centre, Keyworth, Nottingham, NG12 5GG, United Kingdom, disbgs.ac.uk, (2) British Geological Survey, Tongwnlais, Cardiff, CF15 7NE, United Kingdom