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The Lower Proterozoic
     Notwithstanding the limited occurrence of shelf sediments at North Spirit Lake and elsewhere in the  Superior Province, the Lower Proterozoic of Canada is distinguished from the Archean by the extensive  distribution of passive margin shelf marine sandstones (and even stromatolitic carbonates), deposited along the edges of relatively stable Archean age cratons.
    Structural Provinces of North America.
    Also see

     The Archean Superior Province is surrounded on all sides by the shallow water clastic sequences of (in anti-clockwise order):
     the Huronian (2.5-2.2 Ga) and Animikie (2.2-1.8 Ga, see Appendix A below) Supergroups of the Southern Province;
     the Kaniapiskau Supergroup (2.3 - 1.88 Ga, volcanism) of the Labrador trough;
     the Povungnituk Group of the Cape Smith Thrust belt (2.0-1.84 Ga) of Ungava;
     and the Richmond Gulf-Nastapoka-Belcher Groups (2.025 - 1.8 Ga) of the Belcher Islands region.

     To the west of the early Proterozoic Churchill ocean, passive margin and intracontinental arenaceous sedimentary sequences ssociated with the Wyoming, Hearne, Rae, and Slave Archean cratons are represented by the Hurwitz Group (2.45 - 2.11 Ga) of northern Saskatchewan and the Medicine Bow Group (2.3 Ga) of Wyoming; the Foxe Group located within the Archean Rae Province; and, to the northwest, northeast, and southeast, respectively, of the Slave Craton Province, the Coronation (1.98 - 1.84 Ga), Goulburn (2.02-1.91 Ga). and Great Slave (1.93 Ga) Supergroups. The stratigraphic sequences of the Hurwitz and Medicine Bow Groups bear considerable resemblance to that of the Huronian of the Southern Province. Corresponding sandstone dominated terranes are also found  in the Ketilidian  and Jatulian terranes of  the Greenland and Baltic shields, respectively (see Appendix B below), as well as in South America,  Africa, and Western Australia.

  The early Proterozoic extension of Archean crust, leading in some cases to the formation of passive margins,  is recorded by the intrusion of dike swarms at the following times:

        Mafic intrusives in the Huronian       2.491+5 Ga; Huronian volcanism 2450+25 Ma
                                                                      Volcanics EpNd = 0 to -6
        East Bull Lake                                   2.490-2450 Ga EpNd = chondritic
        Hearst/Matachewan                           2.454+2 Ga
        Murray-Creighton granites                 2.39-2.33 Ga
        Nipissing                                           2.219+4 Ga; EpNd = -2 to -6
        Fort Francis                                       2.120+67 Ga; Rb-Sr
        Kapuskasing                                      2.014 Ga
        Biscotasing                                    c. 2.1 Ga
        Marathon                                           2.1 Ga
        Preissac                                             2. 04 Ga
        Sudbury norite                                    1.85 Ga; EpNd = -8
        Wanapitei complex                            1.74 Ga; EpNd = 0 to -8

        The oldest early Proterozoic sedimentary basins are represented by the Huronian, Hurwitz and Snowy  Pass Supergroups, all of which are characterized by the presence of very similar lithologies, including lower clastic  units containing detrital pyrite and uraninite, diamictites of glacial origin (Gowganda), aluminous quartzites,  chromiferous green quartzites (Lorrain), and iron bearing sediment (see Appendix C below) at the very highest levels. Although very  similar successions, the Huronian and the Hurwitz-Medicine Bow Mountain rocks are located on opposite sides of  the Churchill ocean, and this fact has led to the suggestion that the Medicine Bow rocks represent the southern  margin of the Huronian basin rifted at c. 2.1 Ga and transposed across the Churchill ocean. Such an explanation  however does not explain the similarity of the Huronian and the Hurwitz. Furthermore, similar lower Proterozoic  sequences are also found as shelf sequences in Africa, Brazil, India, and Scandinavia.
        Comparative stratigraphy of the Huronian and Medicine Bow (Snowy Pass Supergroup) Early Protoerozoic successions - lprotmedbow1.jpg
        Major Precambrian geological elements of central North America - lprotmedbow2.jpg
        Organisation of the Archean Superior and Wyoming provinces during the Early Proterozoic - lprotmedbow3.jpg
        Stratigraphic succession of the Hurwitz Group - lprothurwitz.jpg

        The progressive appearance of a negative Eu anomaly in the the REE patterns of Huronian sedimentary rocks witnesses the progressive emergence and subjection to erosion of Eu-depleted K-granite terranes formed during the last stages in the collisional cratonization of the Archean Abitibi belt to the north.
        Variation in REE abundances in Archean and Huronian rocks - lprothurree.jpg
        Plots of Sm-Nd versus sedimentation and provenance age for sediments deposited since the Archean - lprotsmnd.jpg
         The principles involved in the generation of Eu-depleted granitoids by the partial melting of Archean tonalite are illustrated in the following data diagrams (Condie et al, 1985):
        Generalized geologic map of southern India - lprotmysore.jpg
        Chondrite-normalized REE patterns for Archean tonalites and granites - lprotmysoreree.jpg
        Similar changes occur in the Th/Sc and Sm/Nd chemical characteristics of early Proterozoic sediments.
       Plots of Th/Sc versus sedimentation age for sediments deposited since the Archean -  lprotthsc.jpg

        Oceanic terranes are represented by the Kisseynew-La Ronge arc volcanic (including boninite) complexes of the Churchill  Province, the Wisconsin-Yavapai-Mazatzal volcanic provinces, the obducted Purtiniq ophiolite of the Cape  Smith fold belt of Ungava, the Wopmay arcs west of the Coronation 'miogeosyncline', and the Taltson magmatic  plutonic arc of the North West Territories.
        Within the Cape Smith fold belt of Ungava, the oldest rocks (Povungnituk Group) include shales,  quartzite, carbonate and iron formations overlain by rift volcanic rocks clearly of WP character. This unit is  structurally overlain by a pyroxene- and plagioclase-phyric basalts and rocks with komatiitic affinity (Chukotat)  and MORB chemistry marking the development of an oceanic rift. This rift sequence is structurally overlain by a  deep water turbidite facies (Spartan Group) and at the top of the structural sequence by the primitive low-Ti  Purtiniq ophiolite klippe. Arc rocks are potentially represented by the Parent Group of basic to intermediate flows and pyroclastic rocks locally present below the ophiolite, but they may also represent fractionated rift volcanic rocks. Similar komatiitic rocks and associated ultramafic material also occurs along the Superior margin  (Thompson-Fox River belts) of the Churchill ocean.
    Geological map of the Cape Smith fold and thrust belt, Ungava Peninsula - lprotungava.jpg
    Discrimination diagrams for rocks of the Cape Smith fold and thrust belt - lprotungfinger.jpg

    The best documented collisional complex in the Lower Proterozoic is that of the Trans-Hudsonian system of Manitoba- Saskatchewen (NATMAP Shield Margin Project: Canadian Journal of Earth Science, 1999, volumes 36, numbers 2 and  11; Zwanzig, H.E., 1999). The Trans-Hudson, which extends from south Dakota, through Saskatchewan and Manitoba to northern Quebec, is a 400 km wide collage of juvenile oceanic terranes (Reindeer zone) and re-worked parts of the bounding Archean cratons. Collision of opposing Archean terranes at 1.84-1.8 Ga has caused the intervening oceanic terranes to be thrust stacked over the basal metaplutonic rocks and paragneisses of the 3.2-2.4 Ga Sask craton.  The oceanic terranes of the lower part of the thrust stack are composed of 1.92-1.87 juvenile arc rocks, including boninites, 1.88-1.84 Ga stitching plutons, 1.87-1.85 Ga volcano-sedimentary units, and 1.85-1.84 Ga alluvial-fluvial sandstones, whereas the upper part of the stack is composed of marine turbidites (1.85-1.84 Ga)  and coeval distal facies of alluvial-fluvial sandstones. Collision of the amalgamated arc terranes with the Superior craton took place at about 1.81 Ga and led to sinistral transpression of the eastern Trans-Hudson.

Diagram of interactive crustal blocks in Trans-Hudson Orogen (Zwanzig, 1999, Fig 11).

    According to Zwanzig (1999) the tectonic history of the south flank of the Flin Flon and Kisseynew Domains representing the southern margin of the Trans-Hudson Orogen involved the following sequence of events:
    1) 1.85-1.825 Reindeer Zone collision with the Sask craton in the southwest, provoking southerly directed overthrusting of the Flin Flon - Glennie Complex (1.92-1.87 Ga Amisk collage) over the Sask Craton; this potentially represents an early D0 phase of obduction accretion. Rocks to the north of the accretionary complex are dominantly sedimentary and represented by the Burntwood Group turbidites overlain by or as a facies of the continental arenites, conglomerates, quartzites, and continental arc-volcanic rocks, of the Missi Group. Burntwood sediments coeval with and to the north of the Missi arc volcanics would have been deposited in a fore-arc basin of the arc system.  They were ultimately involved in  F1 folding and thrusting towards the northeast (in present-day coordinates), commensurate with their sense of subduction beneath the Missi arc.

Tectonic assemblages of the Flin Flon Belt, southern exposed section of the Trans-Hudson orogen (Lucas, Syme and Ashton, 1999)

Gravity-magnetic composite image with geological contact overlay, Flin Flon Belt (Broome and Viljoen, 1999)

 N-S section through the Trans-Hudson orogen between the Lynn Lake and Flin Flon arcs (Zwanzig, 1999).

    2) 1.82 collision with the Superior Craton in the southeast, and overturning on the margin of the Flin Flon - Glennie Complex during overriding by the Burntwood Group and possibly by the rocks in the Snow Lake area; formation of large F2 folds, now recumbent but originally west verging, the File River nappe; doming; and formation of associated foliation.
    3) 1.81-1.79 overriding by the northern part of the Reindeer Zoneattached to the Hearne Craton, with south and southwesterly mid-crustal flow in the central Kisseynew Domain, driving the Flin Flon - Glennie Lake Complex farther over the SaskCraton: F3 folds with associated S3 foliation co-planar with S2; northeast stretching; folding is upright at the Flin Flon Beltmargin.
    4) 1.8-1.77 intracontinental convergence of the Kisseynew DomainandSuperior Craton; final overthrusting by the northeast part of the Reindeer Zone; formation of upright folds, steep shears, and retrograde folation S4.
    5) development of  north northeast-trending and east to northeast- trending faults.

 Geologic map of and N-S section  through part of the south flank of the Flin Flon Belt (Zwanzig, 1999, Fig 4)

 East-west sections drawn as downplunge projections, in segments with relatively uniform plunge (Zwanzig, 1999, Fig. 5).

        On the south side of the Superior Province, arc rocks are represented by the Wisconsin magmatic terranes 
south of the Niagara fault of Michigan/Wisconsin. North of the fault occurs a continental margin represented by 
the following assemblages:

                NORTHWEST                                                       SOUTHEAST
Locality:       Cuyuna                  Gogebic         Marquette               Crystal                   Rabbit Lake
                                                        Range                   Falls                      Allochthon?

 Northern       Virginia G              
ANIMIKIE GP     Biwabik IF 1873 +/-3 Ma 
    |           Pokegama              
    |    --------------------------------------------
    |           (Rabbit Lake S,IF,V)?
    =           (Trommald IF)?                                                  (U. Paint River)|
    |           (Mahnomen G)?                                                   (Riverton IF)   |
    |           <<<<<< Thrust                           U. Michigammee*         (L. Paint River)|        ?Allochthonous?
    |           Trout Lake C                            Bijiki IF               (Badwater V)    |
 Southern       Little Falls G,S,V                      L. Michigammee S        Michigammee*S
MILLE LAC GP    Glentown IF, V.         BARAGAR GP      Clarksburg V.           Hemlock V1874+/-9 Ma    Rabbit Lake S,IF,V)?
                                                        Greenwood IF   
                 Denham Q                               Goodrich Q     

                                        Tyler G
                                        Ironwood IF     Negaunee IF             Vulcan IF                 (Trommald IF)?
                MENOMINEE GP            Siamo S 
                                        Palms Q         Ajibik Q                Felch Q                   (Mahnomen G)?
                CHOCOLAY GP             Chocolay Gp     Chocolay Gp             Chocolay Gp
    Table 1. Tentative correlation of the Lower Proterozoic rocks of Michigan, Wisconsin, and Minnesota.  Q = quartzite; C = carbonate;  S = shale; G = Greywacke (foreland turbidite); IF = iron formation ('starved basin'); V = volcanic; * = 'Province 1' southerly derived Michigammee Formation;

       The locally developed Chocolay Group includes Gowganda-like tillites (Fern Creek Fm) overlain by aluminous quartzites (Sturgeon Fm) and carbonates, and is potentially equivalent to the Huronian Gowganda-Lorrain shelf sequence. If so, the succeeding rocks of the Menominee Group are likely unconformable on the Chocolay. The Menominee, Baragar, Mille Lac, and Animikie Groups are characterized by the presence of sedimentary iron-formations (IF).
      Rocks of the Lower Michigammee turbidite succession have Nd/Nd characteristics indicating that they were derived from the Superior crust, whereas the Upper Michigammee rocks have Nd/Nd (epsilonNd -0.8 to +1.5) characteristics indicating that they were being fed by debris from the colliding Wisconsin arc to the south at about 1860 Ma. Similarly in the Medicine Bow Mountain section of Wyoming, paragneisses of the Cheyenne Belt separating the Huronian-like Snow Pass Supergroup rocks from arc rocks of the early Proterozoic Colorado Province (Yavapai) also have Nd/Nd characteristics indicating derivation from a mixed provenance. As such the Cheyenne and Michigammee rocks represent foreland basin sediments laid down in advance of the overriding Wisconsin and Colorado arcs.

    Comparative sections of the Lower Proterozoic geology of Michigan and Wyoming
    Early Protoerozoic plate tectonic development of Wisconsin and northern Michigan  - lprotmichigan.jpg
    NNE-SSW (Lake Michagan) seismic profile across the Penokean orogen of northern Michigan - lprotseis.jpg
    Distribution of Early Proterozoic structural provinces of the southern and southwestern United States - lprotmazat1.jpg

        Other younger Lower Proterozic systems such as the Coronation, Thelon, and Labrador Trough, also contain sedimentary sequences exhibiting the change from rift to shelf to foreland basin to molasse reflecting changes in tectonics from rift to drift to arc accretion to continent collision. It would therefore seem that Lower Proterozoic tectonic settings resemble those of younger Paleozoic orogenic systems.


                The Early Proterozoic of the Fennoscandian Shield

        The Archean of the Fennoscandian Shield is composed of:

        tonalite terranes older than 2843 Ma;
        greenstone belts                 2790-2750 Ma old;
        granodiorites                      2740-2690 Ma;
        granodiorites                      2676
        pegmatites                         2642

        The earliest Proterozoic activity at 2450 Ma led to the formation of structurally controlled basins filled  with volcano-sedimentary sequences of granitoid breccias, arkoses , basalts, andesites, and conglomerates (Sariola).
       The activity was coeval with basic magmatism at 2440 Ma (20 bodies) and focussed along rift zones within areas of the Archean as layered mafic intrusions with pseudo boninitic affinities (E-W Tornio-Narankavaara and  Kuhmo-Suomussalmi belts of Finland). The mafic magmatism was accompanied by potassic rapakivi-type granite  intrusion dated at 2435+12. The mafic complexes were faulted, uplifted and eroded before the extrusion of the  Perapohja and Lapponian volcanic rocks at 2330 Ma. Further rifting involving the deposition of clastics, including quartzites, and volcanics, in a narrow inland  sea led to the formation of the Kainuu basin. Intrusion of low-Al tholeiitic sills and Fe-tholeiitic dikes along listric shear zones took place at 2200 Ma, with dikes serving as feeders to the Jatulian mafic volcanic suite. The  intracratonic Hoytianinen basin system devloped at about 2100 Ma as a result of extensional tectonics of a passive  margin in an asymmetrical rift or half graben above a major detachment fault extending deep into the lithosphere.  In the sedimentary sequence, there is ab abrupt change in facies from psammites and dolostones to carbonaceous  rocks, accompanied by a dramatic change in C isotope ratios in the carbonates, interpreted ato reflect the change in concentration of atmospheric oxygen at 2200-2100 Ma.
        At 2040 Ma the Otanmaki layered mafic complex with its Fe-T-V oxide deposits and associated rapakivi  anorogenic intrusions was formed at the same time as the sulphide-bearing Keivitsa layered complex and the  ultramafic Kummitsoiva - Sattasvaara - Karasjokk  komatiitic volcanic belt of Lapland.
        Further rifting was marked by the ultramafic Outokumpu assemblage and the 1970 Ma Jormu ophiolite of  the Kainuu basin.
        The crust at the Sveccofennian - Archean boundary is very thick (46-65 km). Most of the thickness  variation is in the high velocity lower crust. Thermal resetting of K-Ar ages to 1850-1800 Ma and fluid mediated  resetting of Rb-Sr ages to 1800-1760 Ma has taken place in Archean rocks marginal to the Sveccofennian system.
        The early Proterozoic tectonic activity of the Sveccofennian system shows clear parallels with the early  Proterozoic development of the Southern Province of the Canadian Shield.


Lower Proterozoic Iron Formations

        Banded Iron formations (BIFs) are thought to be chemical sediments containing relatively little detrital material.  Consequently it is believed that the positive Eu anomalies of Archean BIFs reflects the greater mobility of divalent  Eu under conditions of relatively low oxygen fugacity, and/or the preponderance of higher temperature oceanic hydrothermal  systems. Within a reducing environment Eu would participate as a divalent ion and would be uncoupled from the other  trivalent REE ions. High temperature hydrothermal fluids would preferentially remove Eu into oceanic solution relative  to the other REE, and chemical sediments of iron and silica would therefore show relative Eu-enrichment. Such a process would  be advantaged by oceanic water itself having a low oxygen (sulphate) content.
        In the case of Lower Proterozoic BIFs, sustained periods of volcanicity caused production of chert - carbonate  - silicate BIF rather than the normal and dominant hematite - magnetite - chert BIF. During volcanic periods, the  normally high capacity of sunlight to precipitate ferric iron directly by photolytic oxidation of ferrous iron,  and indirectly by reaction of ferrous iron with photosynthetic oxygen, was modified by turbidity in the atmosphere,  by dust, and, in the water, by the formation of colloids from the reactive ash. Surface-precipitated ferric hydroxyoxide redissolved in  the presence of decaying organic material in the subphotic zone, were precipitated as ferrous carbonates and  silicates when solubilities were exceeded. Despite decreased sunlight, an increase in volcanic activity would increase the availability of nutrients and therefore the production of biogenic matter, consequently cutting off the deposition of  ferric oxide deposits. Conditions favourable for the formation of early Proterozoic BIF deposits included a high  level of ocean floor formation and concommitant hydrothermal activity, relatively elevated deep sea water temperatures  (higher than 20 degrees C), the existence of photosynthetic plankton, the absence of silica secreting organisms, and  a low oxygen to anoxic atmosphere allowing the build up of high iron in oceanic waters.


The Early Proterozoic of the Hamersley Province of Western Australia

        Martin, D. McB., 1999. Depositional setting and implications of Paleoproterozoic glaciomarine sedimentation in the Hamersley Province, Western Australia. BGSA, 111, 2, 189-203.
        Powell, C. McA., et al., 1999. Synorogenic hydrothermal origin for giant Hamerslay iron oxide ore bodies. Geology, 27, 2, 175-178.
        Powell, C. McA. and Horwitz, R.C., 1994. Late Archean and Early Protoerozoic tectonics and basin formation of the Hamersley Ranges. Geol. Soc. Australia (WA Division) Excursion Guidebook No. 4, 53 p.
        Parker, A.K. et al. Mafic dyke swards of Austalia, in Mafic dyke swarms, eds., Halls, H.C. and Fahrig, W.F., Geol. Assoc. Canada Spec. paper 34, p. 401-417.

        Geological map and schematic evolution of the Early Proterozoic Hamersley Province - lprotaustham1.jpg
        Magnetic anomaly map of Australia - austmag.jpg
        Simplified map of mafic dike swarms in the Yilgarn and Hamersley blocks - austhamdikes.jpg
        Kimberley Block
        Map of the Archean Kimberley block, Western Australia - austkimb.jpg
        Lachlan - Tasman orogen
        Map of Tasmania and the southern Lachlan orogenic belt - austtasman.jpg


Structural Provinces of North America.


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