THE RHYD Y BONT OPHIOLITE
OF ANGLESEY, WALES
Coloured inset map (2005) in the centre is taken from the web site owned by Paul Kabrna at http://www.kabrna.com/cpgs/anglesey/anglesey.htm .
Map to the left is from Church, 1980, and the map to the right from Collins, A.S., 2004 CLICK TO ENLARGE
A Google Earth .kmz file for Anglesey, including several map overlays, can be downloaded from: http://instruct.uwo.ca/earth-sci/fieldlog/Google_Earth/
A complete list of references (chronologic
order) for Anglesey can be downloaded from http://instruct.uwo.ca/earth-sci/fieldlog/cal_napp/napp/new_eng_maritimes/Anglesey/anglesey_refs_chrono.doc
This amended bibliography was derived from: Phillips, E.R. 2009. The Geology of Anglesey, North Wales: project scoping study. British Geological Survey, Internal Report IR/09/05. pp 47.
Extracted from: Church, W.R. 1980. Late Proterozoic Ophiolites. Association mafiques-ultramafiques dans les orogenes. Colloques Internationaux du CNRS, Grenoble 1977, 105-117.
late Proterozoic Monian
of Anglesey (Map), Wales, form part of an early Paleozoic
horst, the Irish Sea Landmass, separating the Cambro-Ordovician of
the Welsh basin from that of the southern margin of the
Proto-Atlantic ocean. In southern Ireland equivalents to the
Monian unconformably overlie basement gneisses of Proterozoic age.
The most noteable feature of the Monian from a plate tectonic point of view is the occurence of glaucophane-lawsonite bearing blueschists among the mafic rocks of the Gwna Group of southeastern Anglesey. Chemically the mafic rocks show affinity to oceanic basalt, but are relatively fractionated with mean TiO2, Zr, Y, Ni, Cr, and FeO/MgO values of 2.15 (wt. per-cent) 127, 37, 54, 146 (ppm)and 1.6 (wt. ratio) and very low values of La (Thorpe. 1972a). The mafic rocks, which occur as lenticular masses within mica schists of the Gwna Group, are not directly associated with either serpentinites or gabbros, and their ophiolitic affinity is therefore in doubt. Ophiolitic rocks do occur however within semi-pelitic rocks of the underlying New Harbour Group, north of Rhoscolyn on the island of Ynys Gybi. Ultramafic rocks of the complex, here referred to as the Rhyd-Y-Bont ophiolite (click to see location) are serpentinized cumulate rocks including dunite, poikilitic harzburgites, and websterites, and, according to Wood (1974), wehrlite. The peridotitic rocks are in contact with fine grained to coarse, even pegmatoid, leucogabbros, within which, in spite of their strongly deformed and altered state, it is still possible to recognise fine grained tabular mafic bodies which could represent diabase dikes intrusive into the gabbro. The contact zone between the ultramafic cumulates and the gabbro is occupied by a pegmatoid clinopyroxenite which can be seen in a number of localities to inject the ultramafic rocks. There is clear evidence therefore for a discordance separating the ultramafic cumulate and clinopyroxenite/gabbro units, a feature characteristic of the internal zone ophiolites of Newfoundland. Due to extensive alteration the Rhyd Y Bont rocks are difficult to define chemically. However, the clinopyroxenites, which appear to be the freshest rocks within the complex, are characterised by very low Ti contents. In this respect the Rhyd Y Bont complex closely resembles the Betts Cove and Gander Lake (Shoal Pond) ophiolites of Newfoundland, and the Asbestos-Thetford ophiolite of Quebec (Church and Riccio, 1974; Church, 1977).
The Rhyd Y Bont complex could represent a section of dismembered oceanic lithosphere which following obduction
was emplaced by gravity sliding into a marginal deep
water basin accumulating muds, cherts, and submarine volcanic
rocks. Such a conclusion does not contradict the contention
of Maltman (1975, p. 602) that "the emplacement of the ultramafic
material into the semi-pelitic rocks was an early event, preceding
the first period of deformation". A major objection to the
hypothesis of emplacement by obduction would however appear to lie
in the demonstration by Maltman that an aureole of epidote
hornfels surrounds the Pwllpillo serpentinite body. However, the
absence of high temperature minerals in the aureole contradicts
the possibility suggested by Maltman that the aureole was formed
by the intrusion of a basaltic liquid, whereas the presence of
epidote rich veins within the gabbro suggests that the
epidotisation of the surrounding rocks and the formation of such
assemblages as talc-chlorite-carbonate amphibole is the result of
hydrothermal reactions, perhaps related to the exothermal
serpentinisation of the ultramafic rocks. The presence of
ultramafic rocks with as much as 7 weight percent Al203 but only
1.0 weight percent CaO also suggests that there has been
considerable major element mobilisation during metamorphism of the
ultramafic complex (Table II).
Some support for the view that the ophiolites might have been emplaced as large blocks during deposition of the New Harbour Group lies in the presence of clasts of bright green quartz-serpentine material possibly representing altered ultramafic rock within the melange unit at Cemaes Bay. Shackleton (1969)
meta-gabbro with finer grained
patch (dike) of metadiabase
and Wood (1974) agree that the Gwna melange is probably a
submarine slide deposit (olistostromes) which, on the basis of the
sense of rotation of blocks within the melange, Wood. (1974)
suggested may have been derived from the east. Since Wood also
indicates that the volcanogenic greywackes of the South Stack
Series stratigraphically below the New Harbour Group were also
derived from the east (southeast) it is conceivable that the
oceanic source area of the Rhyd Y Bont ophiolite also lay in this
direction. Obduction of the oceanic lithosphere in
Anglesey may have taken place well in advance of the main orogenic
movements responsible for the development of the
glaucophane-bearing assemblages of southeast Anglesey. Baker
(1974, p. 451) states that the glaucophane formed later than the hornblende schists of this region, an
observation in accord with that of Shackelton (1969) who also
noted that the metamorphic minerals of the Anglesey schists did
not all develop simultaneously, and that successive parageneses
could be distinguished. As in the Western Alps (Dal Piaz et al.,
1972) the glaucophane may have formed at some time later than the
closure of the ocean basin and emplacement of the ophiolite.
Potentially, the lawsonite-glaucophane bearing rocks formed
following overthrusting of a slab of crystalline rocks which has
subsequently been removed by erosion, as in the case of the
Austro-alpide sheet which must have once covered the whole of the
Western Alps but which is now only seen in the form of remanants
such as the Dente Blanche nappe.
Chemical composition of serpentinite and pegmatoid clinopyroxenite from the
Rhyd Y Bont ophiolite, Anglesey. Analyst: Mme L. Savoyant. U.S.T.L., Montpellier
I. 2. 1. 2.
SiO2 41.41 34.48 45.25 39.8
TiO2 0.05 0.07 0.05 0.08
Al2O3 6.41 6.66 7.0 7.69
Fe2O3 3.89 5.11 4.25 5.90
FeO 2.87 1.83 3.13 2.12
MnO 0.13 0.08 0.14 0.09
MgO 27.25 37.60 29.75 43.4
CaO 9.49 0.80 10.35 0.92
Na20 0.11 0.01 0.12 0.01
K20 0.01 0.01 0.01 0.01
P2O5 0.03 0.03 0.03 0.03
L.O.I. 7.93 13.34
TOTAL 99.58 100.02 100.00 100.00
1. Coarse grained olivine-clinopyroxene rock, PB73.I, Rhyd-Y-Bont, Anglesey.
2. Bastite-serpentinite, PB72.2, Rhyd-Bont
If the Rhyd Y Bont complex can reasonably be interpreted as a fragment of oceanic crust obducted from the southeast, then it is possible that during Upper Proterozoic times Anglesey lay to the northwest of an oceanic basin. The interpretation of the latter as a marginal basin as in the model proposed by Baker (1973) runs counter to the evidence of lateral facies variations indicating the presence of a foreland towards the northwest (cf. Shackelton, 1969, p. 9). Another difficulty stems from the absence of calc-alkaline andesites in late Proterozoic sequences of western Britain, since such rocks should have been erupted in quantity prior to, during and after the opening of the marginal basins. While the Malvernian and Uriconian-volcanic rocks might be considered to fill this role, their Ti characteristics (Thorpe, 1972 b) would seem to be atypical of subduction related volcanic assemblages. The Rhyd Y Bont complex may therefore represent oceanic material formed within a primary intracontinental rift separating Anglesey from southern Britain during the late Proterozoic.
CURRENT PREFERRED INTERPRETATION:
The discovery that the South Stack - Rhoscolyn succession was a young as Late Cambrian (Collins, 2004) and not Late Proterozoic coeval with the arc volcanism of Southern Britain relieved the necessity of deriving the South Stack from some continental area to the northwest. The work of Phillips (1991) showed that the South Stack could be derived from the southeast following peneplanation of a Late Proterozoic continental volcanic arc, whereas the New Harbour represents an upward coarsening sequence that could have been derived from the northwest. The similar average composition of the finer grained New Harbour facies and the coarse grained Skerries unit indicates that the difference between the two units was result of mechanical differention of coarse and fines rather than differentiation by chemical weathering. The demonstration by Kawai et al. (2006) that the blueschist belt could be zoned and that the zonation could be traced into the low grade Gwna on either side of the blueschist belt also means that the northern and northwest Gwna that gradationally overlies the New Harbour Group (Kohnstamm, 1980) cannot, if both facts are true, be correlated with the Gwna of eastern Anglesey. The pillow lavas of the blueschist Gwna belt can be interpreted as the remains of a Late Proterozoic 'early' obducted ophiolite through which the blueschist unit was extruded as a spot singularity at 560 Ma. More recently, Kawai (2006) has stated that "U-Pb data from detrital zircons extracted from the New Harbour Group, Anglesey, UK, indicate that the maximum depositional age of the New Harbour Group is 472+/-30Ma." It would seem therefore that both the South Stack and New Harbour were deposited in the interval 501 - 472 Ma, and that the younger Rhyd Y Bont obduction event could be equivalent to the Ganderian obduction event of the Avalonian margin of the Newfoundland Appalachians.
Historiographic bibliography (arranged chronologically):
Sharpe 1846. Recorded the presence of jaspers and serpentinites on Anglesey.
Blake, J.F. 1888. The occurrence of glaucophane-bearing rocks in Anglesey. Geol. Mag. 3, 5, p.125-127.
Raisin 1893. Recognised gabbros and "spheroidal
basalts" (pillow lavas).
Greenly, E. 1919. The Geology of Anglesey. Memoir of the Geological Survey, UK. Greenly's map of NW Anglesey
p. 100 - 109, 320-322.
p. 100 THE SERPENTINE-SUITE
"composed chiefly of serpentine and gabbro...Coarse pyroxenites are present, and certain dolerites of the north appear to belong to the same suite of intrusions. Associated with them are .....andalusite hornfels..."
SERPENTINE - " The homogeneous granular material is a true dunite..; thence, by the coming in of enstatite, it passes into a saxonite...; agian, where diallage is also present, it becomes a lherzolite; while some varieties may be regarded as chromite-serpentine. .....it would not be easy to lay down lines for these types upon a map."
p. 101 Serpentinisation - "At Mynachdy is a serpentine rich in minute garnets, in clusters and veinlets together with stars of antigorite..."
p. 102 "..cannot be trace for more than a few yards in any direction. The diallage-rock ... The enstatite-rock.......Websterites are also found..." Enstatite-gabbro ... augite, kaolinised feldspar, serpentine after enstatite, and granules doubtfully ascribed to garnet and perovskite....links the pyroxenites to the true gabbros."
p. 103. DOLERITES .... deeply amphibolitized, but containes cores of brown augite and are ophitic. ...They have chilled selvages and are thus less peepseated than the gabbros, but are certinaly apophyses form an unseen and doubtless gabbroid intrusion.
p. 107 "DEFORMATION and METAMORPHISM ...For it is so riddled with planes of gliding such that the lenticular cores of massive rock are seldom as much as a few yards in length, and usually a foot or a few inches only.
p. 108 MARGINAL ROCKS Andalusite-mica hornfels. ---- A wide halo of induration that surrounds the dolerites of the north is desribed on pp 320-321. ...chiefly composed of 'criss-cross' white and brown mica in which are porphyroblastic pseudomorphs, now cheifly composed of white mica, but sometimes idiomorphic, zonal, and with outlines as of andalusite.
p. 109 Chronology when this thermal influence was exerted the foliation of the New Harbour Beds was inperfectly developed, though it had been initiated, and the same is the case with the epidote-hornfels, as well as with the andalusite-mica-hornfels of the north.
Fitch, F.J., Miller, J.A., and Meneisy, M.Y., 1963. Geochronological investigations on rocks from North Wales. Nature, 199, p. 449. None of the measured K-Ar ages exceeded 400 Ma.
Moorbath, S. and Shackelton, R.M. 1966. Isotopic ages from the Precambrian Mona Complex of anglesey, North Wales (Great Britain). EPSL, 1, p. 113-117. Muscovite - whole rock age of c. 580 Ma for the Coedana Granite and related semi-pelitic hornfels,
Wright, A.E. 1969. Precambrian rocks of England, Wales and southeast Ireland. American Association of Petroleum Geologists Memoirs, 12, p. 93–109. (The following ages are taken from Dr Alan E. Wright, 2003. Problems in dating the English and Welsh Late Precambrian. Charnia - the website of Section 'C' of the Leicester Literary and Philosophical Society, March 12 2004.
Zircons Inherited in the 566 Ma Charnian 590
Arvonian Fachwen 572
Arvonian Padarn Tuff 605
Coedana granite 630-570
Coedana Gneiss >650
Rushton Schist 667
Shackelton, R.M. 1969. The Pre-Cambrian of North Wales, p. 1-18, in Wood, A. (ed), The Precambrian and Lower Paleozoic rocks of North Wales. Univ. Wales Press.
Dewey, J.F. 1969. Evolution of the Appalachian/Caledonian orogeny. Nature, 222, p. 124-129. According to Dewey the Monian accumulated along the eastern flank of the Proto-Atlantic Ocean and represent an off shore facies of the west facing margin; the Monian (Gwna) volcanicity represents a continental margin arc formed above a subduction zone dipping SE beneath the Monian. The arc is erupted west of the older Mona sedimentary succession.
Bird, J.M., Dewey, J.F., and Kidd, W.S.F. 1971. Proto-Atlantic ocean crust and mantle: Appalachian/Caledonian ophiolites. Nature Phys. Sci., 231, 28-31. The authors suggest that the Welsh Lower Paleozoic sediments accumulated in a small ocean basin opened between the Irish Sea Horst (bearing the Monian) and the main foreland (carrying the Longmyndian)
Thorpe, R.S. 1972. Possible subduction zone origin for two Pre-Cambrian Calc-Alkaline plutonic complexes from southern Britain. BGSA, 83, p. 3663-3668. Thorpe interprets the Malverian and Jounston plutonic complexes as products of the Monian orogeny.
Thorpe, R.S. 1972. The geochemistry and correlations of the Warren House, the Uriconian and the Charnian Volcanic rocks from the English Pre-Cambrian. Proc. Geol. Assoc., 83, p. 269-285. The Uriconian is calk-alkaline and were therefore formed during subduction closure rather than during the opening phase of the Proto-Atlantic as favoured by Dewey (1969).
Thorpe, R.S. 1972. Ocean floor basalt affinity of Precambrian glaucophane schist from Anglesey. Nature Phys. Sci., 240, p. 164-166. Recognised that "An origin for the Mona Complex at ....a plate margin origin .....has been made by Dewey (1969), and that Dewey and Bird (1971) had proposed that ophiolites "may represent thrust slices of ocean crust and upper mantle emplaced at destructive plate margins." There is no intimation that Thorpe had learned of the obduction model of ophiolite emplacement and foreland basin formation as developed for the Oman (Reinhardt, 1969), Papua- New Guinea (Davies (1968), and Appalachian ophiolites (Stevens, 1969; 1970).
CLICK TO ENLARGE
Baker, J.W. 1973. A marginal Late Proterozoic ocean basin in the Welsh region. Geol. Mag., 110, p. 447-455. Baker was cognisant of the Southern Uplands and Western Newfoundland obduction.foreland basin model but (p. 451) concluded that the Dewey's subduction model was more applicable to the Mona rocks, even although "it is difficult to envisage a lithospheric slab dipping down southeastwards from the shallower and subaerial side of the Monian outcrop towards the deeper-water side." Baker favoured the idea that a subduction zone passed beneath the Mona Complex and its basement from the SE side" , and that consequently "the deeper water basin was underlain by a consumable oceanic lithospheric plate (Fig. 2). .... The later serpentinites of Holy Island could have been released from the deeper levels of the zone towards the N.W." This was the basis of Baker's proposal that Anglesey represented a continental micro-continent separated from southern Britain by an ocean that was consumed by way of NW directed subduction beneath the Monian microcontinent.
Wood, M. and Nicholls, G.D. 1973. Precambrian Stromatolitic Limestones from Northern Anglesey. Nature, 241, p. 65. The stromatolites suggest an age of 900+/- 300 Ma for the Cemais Bay unit of the Monian
Church, W.R. and Riccio, L. 1974.
The Sheeted Dike Layer of the Betts Cove Ophiolite Complex does
not represent spreading: Discussion. Can. Jour. Earth Sci., 11, p.
1499-1502. "The contact between ultramafic cumulates and gabbro is
not gradational but represents a major discontinuitywithin the
ophiolite: the basal clinopyroxenite of the gabbro unit injects
the ultramafics.... A similar discontinutiy is present
within the ophiolites of the Sherbrook-Thedfor region of Quebec,
the Ballantrae ophiolite of Scotland (Church and Gayer 1973), the
poorly preserved late Proterozoic ophiolite of Rhyd Y Bont, Anglesey (Unpublished
data), and the early Proterozoic Bou Azzer ophiolite of Morocco
(Leblanc, 1972; Church and Young, 1974)."
Wood, D.S. 1974. Ophiolites, melanges, blueschists, and ignimbrites: early Caledonian subduction in Wales?, p. 334-343, in Dott, R.H.Jr. and Shaver, R.H., (eds), Modern and Ancient Geosynclinal Sedimenation. Soc. Econ. Pal. Min. Spec., Pub 19. This paper represents Dennis Wood's incursion into the plate tectonic geology of Anglesey. He suggested that the Lower Monian sediments were deep-water and were supplied from the southeast and transported axially along a NE-SW trough. He followed Shackleton in interpreting the Gwna melange as being olistostromal, emplaced towards the west. He posited that the "serpentinites were not emplaced cold... because they preserve good igneous textures and have cause appreciable contact effects in the enclosing sediments."
Thorpe, R.S. 1974. Aspects of magmatism and plate tectonics in the Precambrian of England and Wales. Geol. Jour., 9, 2, p. 115-136. "Basic and ultrabasic igneous masses were emplaced into the Monian sedimentary rocks either prior to, or in an interval during their deformation...." Analyses of gabbro and serpentinite from the Mona complex.
Shackleton, R. M. 1975. Precambrian rocks of Wales, p. 76-82 in Special Report No. 6: Precambrian, Geol. Soc. London. "A sheet of gabbros and ultramafic rocks, now folded and disrupted, was introduced into the New Harbour Group ....before most of the deformation." Presented in Nov. 1973 at the Ordinary eneral Meeting of the Geol. Soc."
Maltman, A.J. 1975. Ultramafic rocks in Anglesey - their non-tectonic emplacement. Jour. Geol. Soc. London, 131, p. 593-605. Reference to Dewey (1969), Bird, Dewey and Kidd (1971), Thorpe (1972), Baker (1973) and Church and Riccio (1974). Although Dewey had changed his mind about the mechanism of ophiolite emplacement, Maltman maintains the view of Dewey, Baker and Thorpe that the ultramfic and mafic rocks "were magmatic at the time of their emplacement." "the contemporary view that (the emplacement) has resulted from major tectonic emplacement involving thrusting and shearing is not substantiated, "the model of plate subduction has to be modified to allow magamatic ascent of the appropriate materials." This view was largely dependent on Maltman's contention that the ultramafic - mafic bodies were margined by a zone of epidote hornfels.
Shackleton, R.M., 1975. Precambrian rocks of North Wales. In: Harris, A.L., Shackleton, R.M., Watson, J., Downie, C., Harland, W.B. & Moorbath, S. (eds) A correlation of Precambrian rocks in the British Isles. Geological Society, London Special Reports, 6, 76–82.
Church, W.R. 1976. The Rhyd Y Bont complex of Anglesey as late Proterozoic oceanic crust (Manuscript). This paper sought to establish that the Rhyd Y Bont ophiolite could be reconciled with an Appalachian type obduction model of the kind that had been earlier rejected by Baker, Maltman and Thorpe, and that there were interesting similarities between the Rhyd Y Bont ophiolite and other obducted ophiolites such as Betts Cove, Gander, Ballantrae, and Morocco. The paper was rejected largely on the basis of Malman's belief in the existance of "a progressive mineral replacement and hornfelsic and hornfelsic texture about the bodies which reasonably fit a metamorphic aureole." Also, "The paper relies to an inordinate extent on analogues that ... are extremely tenuous." The paper was eventually published by the French CNRS in 1980.
Church, W.R. 1976. Letter to Maltman dated April 16th 1976. "In your paper you mention that the pelitic qtz-biot-chlor country rock changes to an epidote-rich assemblage. How can you account for such a radical change in bulk chemistry in terms of contact metamorphism? Is it possible that the epidote-hornfels also represents an allochthonous unit welded onto the base of the peridotite as is found in other ophiolites?"
Shackleton 1976. Letter to Church - "I had always accepted the aureole - which was described by Greenly .... as genuine in spite of its mineralogical peculiarity. I think that I was influenced by the preservation within it of simpler structures. However, your suggestion that it is possibly hydrothermal is interesting, and the appearance of stratigraphic simplicity, with the mafic-ultramafic complex at a constant horizon, may be deceptive."
Maltman, A.J. 1977.
Serpentinites and related rocks of Anglesey. Geological Journal.,
12, 2, 113-128. (Summary of Maltman's Ph.D.
"The ultramafic rocks were emplaced as peridotite magmas immediately prior to D1...Various metagabbroic rocks ...may be comagmatic with the peridotite although intruded rather later." "..a sheeted dyke complex appears to be completely absent...It is easier to view the rocks as part of an intrusive igneous complex (perhaps associated with plate consumption) than as translated oceanic crust." (Click the following image to see Maltman's map of the Rhyd Y Bont complex.)
CLICK TO ENLARGE
Thorpe, R, S. 1978.
Tectonic emplacement of ophiolitic rocks in the Precambrian Mona
Complex of Anglesey. Nature 276, p. 57. The Mona Complex of
Anglesey is a thick succession (over 5,000 m) of metasedimentary
and locally metavolcanic rocks (the 'Bedded Succession'), gneisses
of uncertain age and granite intrusions. The Monian Bedded
Succession consists largely of flysch-type sediments deposited in
a progressively shallowing sedimentary trough. The succession
includes ferruginous chert and manganiferous shale, basaltic
pillow lava and a serpentinite−gabbro suite of intrusions. The
younger part of the succession has a mélange of regional extent,
now interpreted as an olistostrome4. The sedimentary
succession was complexly deformed and locally metamorphosed in
high temperature/pressure (sillimanite−almandine facies) and low
temperature/pressure (lawsonite−glaucophane facies) conditions,
and intruded by granite during the latest Precambrian5.
The structure and rock types present clearly indicate that the
Mona Complex formed near to a late Precambrian destructive plate
margin. In such a setting the association of deep ocean sediment,
pillow basalt and a serpentinite−gabbro suite might represent
progressively deeper oceanic crustal layers, and it has been
proposed that these igneous rocks are fragments of the oceanic
crust and upper mantle tectonically emplaced at the Monian
destructive margin. However, recent detailed studies of the
serpentinite−gabbro suite by Maltman suggest that these rocks were
emplaced magmatically rather than tectonically (for example, ref.
11). Here, I review the character of the Monian ophiolitic
association and argue that the
serpentinite−gabbro suite represents fragments of oceanic
crust tectonically emplaced during oceanic subduction
Thorpe references Dewey (1969) , Hugh Davies (1971) Woodcock and Robertson (1977) and Coleman (1977) as his authorities and as a counter to Maltman's hornfels defense, attributes the hornfelsing to frictional heating and residual heating from the parent oceanic crust, and idea he attributes to Woodcock and Robertson. Without elaborating on the already well established current concept of obduction foreland basins (Stevens, 1970), Thorpe allows that the ophiolites "might have slumped into position like the components of the Gwna melange." He takes no position the direction of subduction related to the emplacement
Maltman, A.J. 1979. Tectonic emplacement of ophiolitic rocks in the Precambrian Mona Complex of Anglesey. Nature, 277, p.327. Maltman's paper is a rebuttal of Thorpe (1977), in which he claims that "the serpentinite and gabbro show no semblance of a layered or sequential arrangement, thre is no sheeted dyke complex, and although cherty sediments and pillowed basalt do appear on Anglesey they are distant, both geographically and stratigraphically from the intrusive suite." He also disagrees that the suite'provides evidence of local rapid uplift, erosion and transport'. The country rocks are, on all sides, the same, distinctive deep-water lithology ... Debris from the suite is not found.....The melange ...occurring elsewhere ...also seems to contain no fragments of the serpentinite-gabbro suite, which may still been at depth." "A comparison with other, such as Tethyan ophiolites would imply emplacement by obduction, although Thorpe seems to envisage some (unspecified) mechanism associated with plate subduction."
Barber, A.J. & Max, M.D. 1979. A new look at the Mona Complex (Anglesey, North Wales). Journal of the Geological Society, London, 136, 407–432.
Muir, M.D. et al. 1979.
Paleontological evidence for the age of some supposedly
Precambrian rocks in Anglesey, North Wales, JGS, 136, 1, p. 61-64.
Present evidence for a Lower Cambrian age for the Llanddwyn
Spilitic Group pillow lavas.
Church, W.R., 1980. Late Proterozoic ophiolites, p.105-117. In Allegre, C.J. and Aubouin, J. eds., Orogenic Mafic and Ultramafic Association, Colloques Internationaux du C.R.N.S., Grenoble 1977, No. 272. (full text given above)
Kohnstamm, M.A. 1980. Discussion on "A new look at the Mona Complex (Anglesey, North Wales). Jour. Geol. Soc., 137, 513-514. (Full text and reply by Barber and Max given below)
Gibbons, W., 1981.
Glaucophanic amphibole in a Monian shear zone on the mainland of N
Wales. JGS, 138, 2, p. 139-143.
Glaucophanic amphibole occurs within recrystallized basic volcanic rocks derived from the Gwna Group at Penrhyn Nefyn in the Lleyn Peninsula. The glaucophanic rocks are restricted to the ‘Penmynydd Zone of Metamorphism’ which at Nefyn is interpreted as a narrow blastomylonite belt produced by the shearing of the Gwna Group against a body of tonalite. The blue amphibole coexists with blue-green amphibole and grades into actinolite greenschist away from the focus of shearing. Fragments of Gwna Group (dated as Lower Cambrian) and blastomylonitic ‘Penmynydd’ lithologies lie in the Cambro–Ordovician sediments of the Welsh Basin. Therefore, the metamorphism of parts of the Gwna Group during the ‘Penmynydd’ (subduction zone?) shearing is interpreted as having occurred in Cambrian times.
Kohnstamm, M.A. and Mann, A. 1981. Trancurrent faulting and Pre-Carboniferous Anglesey. Nature, 293, 762.
Gibbons, W., 1983. Stratigraphy, subduction and strike-slip faulting in the Mona Complex of North Wales – a review. Proceeding of the Geologists Association, 94, 147–163. Suggested that the evidence in Anglesey for palaeo-subduction including the blueschists was weak
Gibbons, W. & Mann, A., 1983.
Pre-Mesozoic lawsonite in Anglesey, northern Wales: preservation
of ancient blueschists. Geology, 11, 3–6.
First reported lawsonite in mafic blueschists,
Beckinsale, R.D., Evans, J.A., Thorpe, R.S.,
Gibbons, W. & Harmon, R.S. 1984.
Rb-Sr whole-rock ages (18O values and geochemical data for the
Igneous Complex and the Parwyd gneisses of the Mona Complex of Llyn, N. Wales. Journal of the Geological Society, London, 141, 701–709. Sarn complex igneous rocks - 549+/-19; Parwyd gneisses - 542+/-17; resetting event at 458+/-16.
Davies, G.R., Gledhill, A. & Hawkesworth, C. 1985. Upper crustal recycling in southern Britain: evidence from Nd and Sr isotopes. Earth and Planetary
Science Letters, 75, 1–12.
Hora´ k, J. M. & Gibbons, W., 1986. Reclassification of blueschist amphiboles from Anglesey, North Wales. Mineralogical Magazine, 50, 533–535. Reclassified the blueschist amphibole in particular as crossite and barroisite.
Gibbons, W. & Gyopari, M., 1986. A greenschist protolith for blueschists on Anglesey, U.K. In: Blueschists and Eclogites, (eds Evans, B. W. & Brown, E. H.), Geological Society of America. Memoir, 164, 217–228. Proposed an anticlockwise P–T trajectory for the blueschists formed as a result of the subduction of oceanic crust.
Dallmeyer, R.D. & Gibbons, W. 1987. The age of blueschist
metamorphism in Anglesey, North Wales: evidence from 40Ar/39Ar
mineral dates of the
Penmynydd schists. Journal of the Geological Society, London, 144, 843–850.
Gibbons, W. 1987. The Menai Strait Fault Stystem: an early Caledonian terrane boundary in North Wales. Geology, 15, 744–747.
Tietzsch-Tyler, D. & Phillips, E.R. 1989. Correlation of the Monian Supergroup in NW Anglesey with the Cahore Group in SE Ireland. Journal of the Geological Society, London, 146, 417–418
Gibbons, W. 1990a. Pre-Arenig Terranes of northwest Wales. In: Strachan, R. A. & Taylor, G.K. (eds) Avalonian and Cadomian Geology of the North
Atlantic. Blackie, Glasgow, 28–48.
Gibbons, W., 1989. Suspect terrane definition in Anglesey, North Wales. Geological Society of America, Special Papers, 23, 59–65.Gibbons (1989) interpreted the tectonic belts in Anglesey in terms of suspect terranes.
Gibbons, W. 1990b.
Transcurrent ductile shear zone and the dispersal of the Avalon
superterrane. In: D’Lemos, R.S., Strachan, R.A. & Topley, C.G.
Cadomian Orogeny. Geological Society, London, Special Publications, 51, 401–423.
Gibbons, W. & Hora´k, J.M. 1990. Contrasting metamorphic terranes in northwest Wales. In: D’Lemos, R.S., Strachan, R.A. & Topley, C.G. (eds) The
Cadomian Orogeny. Geological Society, London, Special Publications, 51, 401–423. Interpreted the contiguous, fault-bounded metamorphic belts as suspect terranes on the margin of Avalonia.
Gibbons, W. and Ball, M.J. 1991. A discussion of Monian Supergroup stratigraphy in northwest Wales. JGS, 148, 1, p. 5-8. New borehole data and trench exposures in northern Anglesey show the Gwna Group melange resting directly upon disrupted sedimentary rocks of the New Harbour Group. Field evidence from Anglesey and the Lleyn peninsula shows that units previously given formal lithostratigraphic status in the higher part of the Monian Supergroup occur as clasts within the Gwna Group melange. The subdivision of the Monian Supergroup is therefore simplified to comprise only three groups: the South Stack (lowest), New Harbour, and Gwna Groups (highest). The disruption of the upper part of the New Harbour Group is interpreted to have been effected during emplacement of the Gwna Group melange.
Tucker, R.D. & Pharaoh, T.C. 1991. U-Pb zircon ages for the Late Precambrian igneous rocks in southern Britain. Journal of the Geological Society, London, 148, 435–648.
Phillips, E. 1991.
The lithostratigraphy, sedimentology and tectonic setting of the
Monian Supergroup, western Anglesey, North Wales. Jour. Geol.
Soc., 148, p. 1079-1090. "Palaeocurrent data obtained from
cross lamination and sole structures within the South Stack Group
are illustrated in Fig. 9. Their bimodal distribution is
interpreted as recording both lateral and axial transport within a
northeast to southwest trending basin (cf. Wood 1974), with a primary
of detritus from the southeast. The variability
of the data may be due to a radial sediment dispersal pattern."
"The Bodefwyn Formation (Khonstamm 1980) comprises a sequence of
deformed and quartz-veined pelites and subordinate psammites
similar in character to the least deformed New Harbour
metasediments of southwestern
Anglesey. The overlying Lynas Formation is dominated by massive, coarse- to fine-grained volcaniclastic metasandstones (possible Facies B fluxo-turbidites) and interbedded pelites which are locally organized into coarsening and thickening upward cycles (scale 5 m). A poorly to moderately developed parallel-lamination is locally preserved within the metasandstones. The Lynas Formation grades upward into the Skerries Formation which can be subdivided into two distinct facies: the Church Bay Tuffs characterized by massive, fine grained tuffaceous pelites with subordinate, laterally impersistent metasandstones; and the Skerries Grits which are dominated by massive, poorly-bedded volcaniclastic metasandstones interbedded with subordinate tuffaceous pelites and conglomerates. The increase in the sandstone to mudstone ratio and the appearance of conglomerates within the Skerries Formation is consistent with a more proximal environment of deposition (cf. Greenly 1919). The outcrop pattern of the Skerries Formation in northern Anglesey (Fig. 1) may be interpreted as recording the interdigitation of a proximal
and more distal facies of the New Harbour Group (Fig. 3). Furthermore, the observed facies changes within the New Harbour Group are consistent with these sediments having been derived from the north to northwest (Figs 3 and 8) (cf. Greenly 1919; Shackleton 1969, 1975). It is concluded that the New Harbour Group was also deposited in a turbidite fan system, but in contrast to the South Stack Group, sandstone deposition was apparently dominated by structureless
fluxo-turbidites." (NOTE: they could also have been derived by resedimentation of passive margin sediments that were pushed from SE to NW during an oceanic obduction event.)
Church, W.R. 1992. Discussion on the trace of the Iapetus suture in Ireland and Britain CJES 149, p. 1048-1049. - "If the GRUB line ophiolites of Newfoundland to the south of the amalgamated oceanic arcs of the Exploits zone are not remnants of Iapetan oceanic crust, what do they represent? Petrographically and chemically they are more similar to primitive arc ophiolites than to oceanic crust, and could have formed above a northwesterly dipping subduction zone within the southernmost part of Iapetus. In this respect they are the mirror image of the Betts Cove - Ballantrae - Highland Border ophiolites of the northern Iapetan margin. In the British Caledonides, a possible analogue to the GRUB ophiolites is the Rhyd Bont ophiolitic fragment within the New Harbour fore-deep succession of Anglesey."
Horak, J.M. 1993.
The Late Precambrian Coedana & Sarn Complexes. PhD thesis,
University of Wales, Cardiff.
Noble, S.R., Tucker, R.D. & Pharaoh, T.C. 1993. Lower Palaeozoic and Precambrian igneous rocks from eastern England and their bearing on late
Ordovician closure of the Tornquist Sea: constraints from U-Pb and Nd isotopes. Geological Magazine, 130, p. 835–846.
Oldroyd, D.R. 1993.
The Archaean Controversy in Britain: Part III—The rocks of
Anglesey and Caernarvonshire. Annals of Science, 50, 6, 523-584.
A detailed account is given of the development of the Archaean Controversy in Caernarvonshire and Anglesey. Sedgwick had found no base for his Cambrian in North Wales, but had intimated that some of the unfossiliferous rocks of the Lleyn Peninsula and Anglesey might be older than his Cambrian. He also described two ‘ribs’ of igneous rock: one running from Caernarvon to Bangor; the other inland, parallel to the first and crossing the Llanberis Pass at Llyn Padarn. The early Surveyors (especially Ramsay) supposed that these ‘ribs’ had altered the surrounding rocks, and the resulting ‘Altered Cambrian’ could be traced across the Menai Strait to Anglesey, where it formed the various metamorphic rocks of that island. This view (which thus denied the occurrence of Precambrian on Anglesey) was challenged by the usual coalition of ‘amateurs’ (Hicks, Hughes, Bonney, Callaway, Blake, etc.) with attempts being made to recognize a sequence of Archaean rocks in North Wales similar to that in Pembrokeshire. However, vigorous debate occurred amongst the ‘Archaean’ geologists themselves, especially about a rock at Twt Hill, Caernarvon, and about a claimed unconformity at the base of the Cambrian in the Llanberis Pass, perhaps adjacent to the Llyn Padarn ‘rib’. (This was at first regarded as a Precambrian ‘island’ like those claimed at St David's, the Malverns, etc.) Vigorous debate took place about the location of the claimed Llanberis unconformity, but the ‘Archaeans’ were united in regarding the metamorphic rocks of Anglesey as Precambrian (or Archaean). Eventually, the greywackes of Anglesey, and around Bangor and Caernarvon, were identified as Ordovician, not Cambrian. Very detailed map-work in Anglesey was carried out privately by Edward Greenly, and his results were published by the Survey in the form of a map and a high-quality Memoir. Greenly utilized for Anglesey tectonic ideas derived from his earlier fieldwork in the Scottish Highlands, so that although his mapping and stratigraphical divisions have proved to be of permanent value it is believed that his structures for Anglesey were mistaken and his stratigraphic sequence inverted. The Survey eventually abandoned its earlier (Ramsay) model of Anglesey without too much difficulty; and the igneous ‘rib’ of Llyn Padarn is now construed as an ignimbrite. However, a definite base for the Cambrian has still not been found in Caernarvonshire, and the stratigraphical evidence for the Precambrian age of the Anglesey rocks (by perceived unconformity with known Cambrian strata) remains incomplete. The study reveals the great difficulty experienced by early geologists when working in unfossiliferous rocks, the evidence from included fragments proving particularly uncertain.
Gibbons, W., Tietzsch-Tyler, D., Horák, J.M. & Murphy, F.C. 1994. Precambrian rocks in Anglesey, southwest Llyn and southeast Ireland. In: Gibbons, W. & Harris, A.L. (eds) A revised correlation of Precambrian rocks in the British Isles. Geological Society, London, Special Reports, 22, p. 75–83.
Durham, J. and Rigby, I. 1995.
Anglesey Field Weekend with Walton Hall - "A Melange of Monian and
where we viewed the chaotic-looking and colourful assemblage of
blocks and fragments of sandstone, limestone, quartzite, dolerite,
serpentinite and jasper."
Gibbons, W. & Hora´k, J. M. 1996. The evolution of the Neoproterozoic Avalonian subduction system: Evidence from the British Isles. In: Nance, R.D. &
Thompson, M.D. (eds) Avalonian and related peri-Gondwanan terranes of the circum-Atlantic. Geological Society of America Special Papers, 304, p. 269–280.
Hora'k, J.M., Doig, R., Evans, J.A. & Gibbons, W. 1996. Avalonian magmatism and terrane linkage: new isotopic data from the Precambrian of North Wales.
Journal of the Geological Society, London, 270, p. 598–604.
New U-Pb isotope data from the Precambrian (Sarn Igneous Complex) of North Wales establishes that calc-alkaline plutonic rocks of the same age occur on either side of the Menai Strait fault system. Zircon fractions from the Sarn Igneous Complex gabbro give a near concordant U-Pb age of 615 ± 2 Ma which is interpreted as the first accurate age of igneous crystallization and supersedes previous dating attempts using the Rb-Sr method. The new U-Pb age is the same, within error, as recent zircon dates obtained from the Arfon Group ignimbrites along strike from the Sarn Igneous Complex, and from the Coedana Complex granite on Anglesey. These data emphasize the importance of what appears to have been the major phase of calc-alkaline magmatism at 630-600 Ma recognizable along the Avalonian arc in Britain. Newfoundland and elsewhere. Despite the similar ages and tectonic setting of the Coedana and Sarn plutonic rocks, however, they remain petrogenetically and geochemically distinct and are not directly linked as part of the same igneous complex. Instead the Coedana Complex granite is interpreted as having been generated in the Avalonian arc during a widespread magmatic event that is also recorded in Britain not only by the Sarn Igneous Complex and Arfon Group, but by volcanic and plutonic rocks in central England. Investigation of granite clasts in the Gwna Group melange (Monian Supergroup) on Anglesey failed to produce zircons adequate for dating purposes but revealed Rb-Sr. whole-rock geochemical and petrographic characteristics similar to the nearby Coedana Complex and suggest linkage between fault-bounded 'Monian' terranes on Anglesey. In the light of these new data rocks on either side of the Menai Strait terrane boundary are interpreted as belonging to the same arc system that was dismembered. dispersed and laterally duplicated by transcurrent faulting after late Precambrian magmatism.
van Staal, C.R., Sullivan, R.W. & Whalen, J.B. 1996. Provenance and tectonic history of the Gander Zone in the Caledonian/Appalachian orogen: implications for the origin and assembly of Avalon. In: Nance, R.D. & Thompson, M.D. (eds) Avalonian and Related Peri-Gondwanan Terranes of the Circum North Atlantic. Geological Society of America, Special Papers, 304, 347–367.
Ernst, W. G., Maruyama, S. & Wallis, S., 1997. Buoyancy-driven, rapid exhumation of ultra high pressure metamorphosed continental crust. Proceedings of the National Academy of Science, 94, 9532–9537.
Joanne K. Prigmore, Andrew J. Butler, and Nigel H.
Woodcock 1997. Rifting
during separation of eastern Avalonia from Gondwana; evidence from
subsidence analysis Geology; v. 25; no. 3 March, p. 203-206.
Subsidence curves for Cambrian-Ordovician sequences from the Anglo-Welsh segment of the paleocontinent of Avalonia reveal two periods of regionally enhanced basement subsidence: Early Cambrian (545-518 Ma) and Late Cambrian to early Tremadocian (505-490 Ma). The earlier event may record transtension following the Avalonian-Cadomian orogeny. The second event may be a transtensional precursor to the late Tremadocian volcanic arc on Eastern Avalonia. However, paleomagnetic, faunal, volcanic, and sedimentary evidence suggests that the main separation of Eastern Avalonia from Gondwana occurred after middle Arenigian time. Rifting during separation is probably recorded by localized middle Arenigian to Llanvirnian (480-462 Ma) subsidence along the Welsh basin margin, but rifting must have occurred mainly on the now-obscured southern margin of the Avalonian continent. Pronounced Caradocian (462-449 Ma) subsidence is associated with back-arc rifting after separation from Gondwana.
K. DAVIDEK , E. LANDING, S. A. BOWRING, S. R.
WESTROP, A. W. A. RUSHTON, R. A. FORTEY and J. M. ADRAIN. 1998. New uppermost Cambrian U–Pb
date from Avalonian Wales and age of the Cambrian–Ordovician
boundary. Geol. Mag., 135, 303-309.
A crystal-rich volcaniclastic sandstone in the lower Peltura scarabaeoides Zone at Ogof-ddû near Criccieth, North Wales, yields a U–Pb zircon age of 491+/- 1 Ma. This late Late Cambrian date indicates a remarkably young age for the Cambrian–Ordovician boundary whose age must be less than 491 Ma. Hence the revised duration of the post-Placentian (trilobite-bearing) Cambrian indicates that local trilobite zonations allow a biostratigraphic resolution comparable to that provided by Ordovician graptolites and Mesozoic ammonites.
Hudson, N.F.C. and Stowell, J.F.W. 1998. On the deformation sequence
in the New Harbour Group of Holy Island, Angelsey, North Wales.
Geo; Jour., 32, 2, p. 119-129.
Four phases of deformation are recorded by minor structures in the New Harbour Group (NHG) of southern Holy Island. The regional schistosity in these rocks is a differentiated crenulation cleavage of D2 age. An earlier preferred orientation (S1) is commonly preserved as crenulations within the Q-domain microlithons of the S2 schistosity and is demonstrably non-parallel to bedding. F3 folds are widely developed in S2 and, to a lesser extent, in bedding. S3 crenulation cleavage is sporadically developed but can be intense locally. A major antiformal fold exists in the NHG near Rhoscolyn. This fold is of D3 age since it clearly deforms S2 schistosity and is consistent with the vergence of F3 minor structures. All planar structures are deformed by folds of D4 age.
Carney, J.N., Hora´k, J.M., Pharaoh, T.C., Gibbons,
W., Wilson, D., Barclay, W.J., Bevins, R.E., Cope, J.C.W. &
Ford, T.D. 2000. Precambrian
England and Wales. Geological Conservation Review Series, Joint Nature Conservation Committee, Peterborough, p. 20.
Hora´k, J.M. & Gibbons, W. 2000. Anglesey and the Lleyn Peninsula. In: Carney, J.N. (ed.) Precambrian Rocks of England and Wales. Geological Conservation Review Series, Joint Nature Conservation Committee, Peterborough, 20, p. 145–149.
Constenius, K. N., Johnson, R. A., Dickinson, W. R., and T. A. Williams. 2000. Tectonic evolution of the Jurassic-Cretaceous Great Valley forearc, California: Implications for the Franciscan thrust-wedge hypothesis. GSA Bulletin, 112, 11, p. 1703-1723. http://instruct.uwo.ca/earth-sci/300b-001/cord1franmodels.jpg
Pharaoh, T.C. and Carney, J.N. 2000. Introduction to the Precambrian rocks of England and Wales. http://www.jncc.gov.uk/pdf/V20Chap1.pdf
ARMSTRONG, H.A. & OWEN, A.W. 2001. Terrane evolution of the
paratectonic Caledonides of northern Britain. Journal of the
Geological Society, London, 158, p. 475-486.
Compston, W., Wright, A.E. & Toghill, P. 2002. Dating the Late Precambrian volcanicity of England and Wales. Journal of the Geological Society, London,
159, p. 323–339.
Compston et al., JGS, Vol. 159, 2002, p. 323–339. Journal of the Geological Society, London, Vol. 160, 2003, pp. 329–330.
Hora'k, J., 2002.
Discussion on dating the Late Precambrian volcanicity of England
and Wales by Compston et al., JGS, Vol. 159, 2002, p. 323–339.
Journal of the Geological Society, London, Vol. 160, 2003, pp.
329–330. Jana Hora´k writes:
Whilst new data constraining the evolution of the Avalonian Superterrane in Southern Britain are most welcome, the conclusion that Compston et al. (2002) draw from this data, raise some points that merit comment.
Firstly, the conclusion that the new age data for the Arfon Group enables a redefining of the Sarn Complex terrane affinity is questioned. In particular the statement ‘it also now seems unlikely that the Sarn Complex should be compared with the Padarn area as the Parwyd Gneiss has yielded a Rb–Sr date of 542 +/- 17 Ma (Beckinsale et al. 1984). This is not now regarded as the age of the amphibolite facies metamorphism (Hora´k et al. 1996) but must indicate a considerable retrogression event in the gneissose basement in Late Precambrian–Cambrian time’ does not appear to have any relevance to any of the data presented.
The Parwyd Gneiss is represented by a single exposure of retrogressed garnet amphibolite and felsic gneiss, which lies entirely within the Llyn Shear Zone. Although previously included within the Sarn Complex, recent work has defined this as a separate unit of uncertain affinity (e.g. Gibbons & Hora´k 1990; Gibbons & McCarroll 1993). In contrast to the gneisses, the Sarn Complex is a poorly exposed heterogeneous plutonic complex, of which the most northwestern margin has been involved in ductile and brittle deformation. The Rb–Sr isotopic data age for the Parwyd gneisses has no bearing on the affinity or age of the Sarn Complex and furthermore the published U–Pb age of 615 +/- 2 Ma for a gabbro (not granite, as stated by Compston et al.2002) from the Complex (Hora´k et al., 1996) falls well within error for the age of the basal part of the Padarn Tuff Formation (614 +/- 2 Ma, Tucker & Pharaoh 1991). In addition to this, although the affinity of the Parwyd Gneiss is uncertain, a Sm–Nd model age for the felsic gneiss of 1.5 Ga (Davies et al. 1985) shows greater affinity to the model ages of plutonic rocks in Southern Britain (Davies et al. 1985; Noble et al. 1993), than data for the Monian Gneisses (Sm–Nd model ages, paragneiss 1.8 Ga, amphibolitic gneiss 1.1 Ga, Hora´k 1993).
A second point relates to the four undated outliers on Anglesey that Greenly (1919) correlated with the Arfon Group. If the Bwlch Gwyn tuff, correlated with the Padarn Tuff, dated at 614 +/-2 Ma (Tucker & Pharaoh 1991), lies unconformably on the blueschist-bearing shear zone (dated at 550 Ma, Dallmeyer & Gibbons 1987) then the age data clearly question this correlation. However, Hora´k & Gibbons (2000) clearly indicate that the Bwlch Gwyn Tuff (and Baron Hill Formation) forms ‘isolated exposures within the Berw Shear Zone, which represent fragments of the Cymru Terrane that were tectonically interleaved with the Monian Composite Terrane following cessation and dismemberment of the Avalonian arc’. As Compston et al. (2002) do not include any age constraint data for any of the four outliers, the data presented does not appear to have any bearing on, or progress the discussion related to, the affinity of these units.
The statement that ‘modern terrane analyses has made little advance on the correlation problems of the first attempt at plate tectonics interpretation of the area’ belies an appreciation of the value of terrane analysis, not as an end in itself, but as a means of identifying similarities or differences between adjacent blocks and establishing the common elements (if any) between them. In particular the identification of the Menai Strait Fault System as containing a major ductile, sinistral, shear zone has revealed the relative motion the Monian Composite Terrane and the Avalonian terranes of Southern Britain. (e.g. Gibbons 1987, 1990a, b; Gibbons & Hora´k 1996). Furthermore terrane analysis has provided a meaningful context in which to place recent radiometric ages (e.g. Tucker & Pharaoh 1991).
Finally, Compston et al. (2002) use Monian Composite Terrane, Monian Superterrane and Mona Complex interchangeably throughout the text, to refer to the Monian rocks of Anglesey and NW Llyˆn, with little regard to the difference in meaning of these three terms. This is both confusing and misleading. Although Monian Composite Terrane (Carney et al. 2000) is preferred to Monian Superterrane, as current thinking suggests that the component Monian terranes had amalgamated prior to juxtaposition with mainland Wales along the blueschist belt shear zone (Gibbons & Hora´k 1996), it is acknowledged that usage of composite terrane versus superterrane is a valid topic for discussion. The use of the term ‘Mona Complex’ however cannot be condoned. As explained by Gibbons & Hora´k (1990) the Monian rocks cannot be considered as having formed without any significant displacement between them as originally envisaged by Greenly (1919). This is clearly demonstrated by the contrasting metamorphic pathways recorded by the three terranes. ‘Mona Complex’ is therefore an anachronistic term and it is more helpful to refer to the three individual terranes (Monian Supergroup, Coedana Complex and Blueschist Terrane) which collectively comprise the Monian Composite Terrane.
W. Compston, A. E. Wright & P. Toghill reply:
We thank Hora´k for her discussion of the Arfon Group data presented in our paper. The relevance of our Arfon data to terrane correlation with the Sarn Complex and the Parwyd Gneiss depends entirely on the interpretation placed on the retrogressive metamorphism of the Parwyd Gneiss. We were assuming that this event was indicative of the age of some of the major tectonism suffered both by it and the Sarn Complex (as seems to have been the case for the Ercall Granophyre in the Welsh Borderland). As the Arfon Group seems to have been in a stable tectonic environment and unmetamorphosed between about 572 Ma and the top of Lower Cambrian times it still seems most likely that the Parwyd Gneiss, at least, was remote from Arfon until the Menai Strait Fault System had finally amalgamated these terranes. We recognize that the Sarn Complex is of identical age to the Padarn Tuff, but whether it belongs to the same terrane depends upon the date of any events subsequent to its emplacement, and these have not yet been determined.
A similar argument applies also to the four outliers, previously regarded as either Cambrian or correlatives of the post-Padarn Tuff Arfon Group, that occur on Anglesey. The reasonable assumption from Dallmeyer & Gibbon’s (1987) dating of the blueschists, is that the sedimentary, metamorphic and tectonic activity affecting the Monian Supergroup and the Blueschist Terrane spanned the period of deposition of the Arfon Group. We have no personal experience of the four inliers, but based purely on the earlier published evidence, they seem to be later than, or at least unaffected by, the tectonism and metamorphism affecting the Mona Complex and cannot therefore be correlatives of the Fachwen. We would agree, of course, that if any or all of them are simply fault bounded slivers within the Monian Composite Terrane, as suggested by Hora´k & Gibbons (2000), then it becomes possible to correlate them with almost any other lithostratigraphic group.
We use Mona Complex as rock stratigraphic term, admittedly old fashioned but much shorter than alternative phrases, for the rocks of the Monian Supergroup, Coedana Complex and the blueschists, as the terms Monian Composite Terrane and Monian Superterrane are not rock-stratigraphic terms. Hora´k uses ‘Monian rocks’ in the same way, but this is open to confusion as more logically referring only to rocks of the Monian Supergroup. Lastly, we defend our assertion that modern terrane analysis has made little advance on the correlation problems of the first attempt at a plate tectonic interpretation of the area. We do not question the value of terrane analysis, being some of its earliest disciples, nor the advances it has made in our understandingof the Avalonian or any other orogenic belt, but those of us working at the time of the plate tectonic revolution essentially used terrane analysis (‘identifying similarities or differences between adjacent blocks and establishing common elements (if any) between them’) to achieve correlations (Wright 1969), which are still being proved essentially correct by the much more accurate dating methods now being used.
Stone, P. and Evans, J.A. 2002.
Neodynium characteristics of Ordovician sediment provenance on the
Avalonian margin of the Iapetus ocean. Scot. Jour Geology, 38, 2,
Whole-rock epNd data from early Ordovician, Skiddaw Group (English Lake District) sandstones support correlation of the northern (and oldest) part of the group’s outcrop (epNdrange of 4.1 down to 8.5) with the Manx Group (Isle of Man) (epNd range of 4.7 down to 7.2). Both groups were deposited on the Avalonian margin of the Iapetus Ocean, with constituent sediment derived from an earlier, possibly Precambrian, continental margin and volcanic arc situated further south in the Avalonian–Gondwanan hinterland. The Nd isotope data from the Skiddaw Group sandstones show a trend of increasing provenance maturity with time, possibly the effect of arc unroofing. The trend is diachronous, with relatively mature sediment being deposited in the south of the Lake District during the Arenig (epNd range for sandstones is - 7 down to - 9.3), whilst generally more juvenile sands fed into the north (epNd ranges up to -4.1). The mature sands subsequently extended northward and Llanvirn sandstones from the Skiddaw Group show the most consistently mature provenance characteristics (epNd range of -7.6 to -8.7). Manx Group data are relatively homogeneous with no clear temporal trends. Mudstone data from the Skiddaw Group divide into two populations separated by the Causey Pike Fault; there is no overlap in mudstone Nd across the fault. The more mature mudstones (EpNd more negative than -8.1) lie to the south of the fault, an unexpected result in that the sequence there contains juvenile volcaniclastic interbeds. The EpNd data rule out the adjacent parts of Avalonia as a possible sediment provenance. Instead, a more distant, Gondwanan provenance seems likely, with implications for basin geometry and the timing of rifting along the southern margin of the Iapetus Ocean.
PHILLIPS, E.R., EVANS, J.A. & STONE, P. et al.,
2003. Detrital Avalonian
zircons in the Laurentian Southern Uplands terrane, Scotland, UK.
Geology, 31, p. 625-628.
Treagus, J. E., Treagus, S.H. Droop, G.T.R. 2003. -Superposed deformations and their hybrid effects on the Rhoscolyn Anticline unravelled. London, Journal of the Geological Society, London, 160, p. 117–136
Wood, Margaret 2003. Precambrian Rocks of the Rhoscolyn Anticline. A Field Guide dedicated to Dennis Wood, 24p. Written in both Welsh and English.
Contains photographs, descriptions (with Lat, Long GPS readings) of critical outcrops along the coast of Rhoscolyn. Also a useful set of maps
Murphy, J.B., Pisarevsky, S.A., Namce, D.A., and
Keppie, D. 2004.
Neoproterozoic—Early Paleozoic evolution of peri-Gondwanan
terranes: implications for Laurentia-Gondwana connections. Int.
Jour. Geology, 93, p. 5.
Abstract: Neoproterozoic tectonics is dominated by the amalgamation of the supercontinent Rodinia at ca. 1.0 Ga, its breakup at ca. 0.75 Ga, and the collision between East and West Gondwana between 0.6 and 0.5 Ga. The principal stages in this evolution are recorded by terranes along the northern margin of West Gondwana (Amazonia and West Africa), which continuously faced open oceans during the Neoproterozoic. Two types of these so-called peri-Gondwanan terranes were distributed along this margin in the late Neoproterozoic: (1) Avalonian-type terranes (e.g. West Avalonia, East Avalonia, Carolina, Moravia-Silesia, Oaxaquia, Chortis block that originated from ca. 1.3 to 1.0 Ga juvenile crust within the Panthalassa-type ocean surrounding Rodinia and were accreted to the northern Gondwanan margin by 650 Ma, and (2) Cadomian-type terranes (North Armorica, Saxo-Thuringia, Moldanubia, and fringing terranes South Armorica, Ossa Morena and Tepla-Barrandian) formed along the West African margin by recycling ancient (2–3 Ga) West African crust. Subsequently detached from Gondwana, these terranes are now located within the Appalachian, Caledonide and Variscan orogens of North America and western Europe. Inferred relationships between these peri-Gondwanan terranes and the northern Gondwanan margin can be compared with paleomagnetically constrained movements interpreted for the Amazonian and West African cratons for the interval ca. 800–500 Ma. Since Amazonia is paleomagnetically unconstrained during this interval, in most tectonic syntheses its location is inferred from an interpreted connection with Laurentia. Hence, such an analysis has implications for Laurentia-Gondwana connections and for high latitude versus low latitude models for Laurentia in the interval ca. 615–570 Ma. In the high latitude model, Laurentia-Amazonia would have drifted rapidly south during this interval, and subduction along its leading edge would provide a geodynamic explanation for the voluminous magmatism evident in Neoproterozoic terranes, in a manner analogous to the Mesozoic-Cenozoic westward drift of North America and South America and subduction-related magmatism along the eastern margin of the Pacific ocean. On the other hand, if Laurentia-Amazonia remained at low latitudes during this interval, the most likely explanation for late Neoproterozoic peri-Gondwanan magmatism is the re-establishment of subduction zones following terrane accretion at ca. 650 Ma. Available paleomagnetic data for both West and East Avalonia show systematically lower paleolatitudes than predicted by these analyses, implying that more paleomagnetic data are required to document the movement histories of Laurentia, West Gondwana and the peri-Gondwanan terranes, and test the connections between them.
HOSSEIN HASSANI, STEPHEN J. COVEY-CRUMP and ERNES T.H. RUTTER Department of Earth Sciences, University of Manchester, Manchester, UK. 2004. On the structural age of the Rhoscolyn antiform, Anglesey, North Wales. Geol. J. 39: 141–156 http://scholarsportal.info.proxy1.lib.uwo.ca:2048/pdflinks/07012118062926841.pdf
Collins, A.S. 2004. Provenance and age constraints of the South Stack Group, Anglesey, UK:U–Pb SIMS detrital zircon data. Journal of the Geological Society, London, Vol. 161, 2004, pp. 743–746. email@example.com
Abstract: U–Th–Pb Secondary Ion Mass Spectrometry (SIMS) data from detrital zircons extracted from the South Stack Group, Anglesey, UK, indicate that: (1) the maximum depositional age of the Holyhead Formation (South Stack Group, Monian Supergroup) is 501+/-10 Ma; (2) the Monian Supergroup was deposited between c. 500 and 475 Ma and is part of the Cambrian–Lower Ordovician succession found in southern Britain and Ireland; (3) Avalonia was a major sediment source (age maxima at 543–552 and 604–627 Ma); (4) Amazonia probably also provided zircons (common Neoarchaean– Mesoproterozoic grains) weakening suggestions that Avalonia had rifted off Gondwana by Cambrian times.
U–Th–Pb isotopic data indicate entrained detrital zircons from the South Stack Group range in age from 501 +/- 10 Ma to 3005 +/- 3 Ma (Figs 2 & 3). Both samples display two age maxima (Fig. 3) in the latest Neoproterozoic (543 and 604 Ma in the Holyhead Fm., 552 and 627 Ma in the South Stack Fm). Other age concentrations seen in both samples include c. 1250– 1300 Ma, c. 1500 Ma, 1950–2200 Ma, 2500 Ma and 2600– 2750 Ma (Fig. 3). The Holyhead Formation sample also contains a number of middle Neoproterozoic and pre-2750 Ma grains, which are absent from the South Stack Formation sample.
Depositional age constraints. The age of the Monian Supergroup was previously constrained as Neoproterozoic to Lower Ordovician by (1) the presence of upper Proterozoic to Lower Ordovician fauna within melange blocks (Muir et al. 1979; Gibbons et al. 1994) and (2) and an unconformably overlying Upper Arenig (Fennian) overstep sequence (Beckly 1987). The accepted consensus was that the supergroup was most likely to have been deposited in late Neoproterozoic–Early Cambrian times (Gibbons & Hora´k 1990; Strachan 2000) based, in part, on the degree of deformation that is not seen in the Lower Palaeozoic rocks of Wales. The Arenig is correlated with the unnamed Lower Ordovician International Commission on Stratigraphy stage above the Tremadocian that is dated by Gradstein et al. (2004) as between 478.6+/-1.7 Ma and 471.8+/-1.6 Ma. The youngest .90% concordant detrital South Stack Formation zircon has a 206Pb/238U age of 522+/- 6 Ma (Early Cambrian), whereas the youngest analysed zircon in the overlying Holyhead Formation has a 206Pb/238U age of 501+/-10 Ma (Mid Cambrian). Taken with the previously published data, these new dates indicate that the Holyhead and Rhoscolyn Formations of the South Stack Group and the overlying New Harbour and Gwna Groups were deposited between Mid Cambrian and late Arenig times, i.e. between c. 500 and 475 Ma (using the timescale of Gradstein et al. 2004). The South Stack Formation could have been deposited in the Early Cambrian, but the conformable nature and similar depositional environments shared between the formations of the South Stack Group (Phillips 1991) suggest that this is unlikely. Correlations, source provenance and palaeogeographic implications. The new age constraints demonstrate that the Monian Supergroup was deposited after the formation and cooling of the blueschists in the Berw shear zone that separates the Monian–Rosslare Terrane (Ganderia) from Avalonia (Fig. 1). The Monian Supergroup now appears to form part of the widespread Cambrian– Lower Ordovician siliciclastic succession found in SE Ireland (Bray and Ribband Groups), north and south Wales (Dolgelly and Festiniog Beds, Mawddach Group, Lingula Flags), the Welsh Borderlands (Shoot Rough Road Shales, Shineton Shales) and central England (Stockingford Shale) (Woodcock 2000), suggesting that the Avalon Superterrane was amalgamated before Mid Cambrian–Early Ordovician times. The more intense deformation in the Monian Supergroup is here interpreted as localized intra-Avalon shearing.
U–Pb isotopic detrital zircon data from Neoproterozoic Avalonian rocks have been used to help position Neoproterozoic Avalonia along the Gondwanan margin (Keppie et al. 1998), whereas data from Cambrian Avalonia has been used to suggest changes in drainage systems through time that may reflect rift initiation (Murphy et al. 2004).
The dual late Neoproterozoic age maxima of 543–552 and 604–627 Ma seen in both samples closely match the two phases of late Neoproterozoic volcanic activity in British Avalonia of 590–620 Ma and 550–575 Ma (Compston et al. 2002) and overlaps with c. 700–540 Ma volcanism in Canadian Avalonia (Nance et al. 1991; Bevier et al. 1993). The pre-Neoproterozoic grains seen in the Monian Supergroup samples have a similar spread to detrital zircon from Neoproterozoic rocks of the Avalon Superterrane of Nova Scotia, Canada (Keppie et al. 1998) suggesting that they may derive from a similar source region. Keppie et al. (1998) used the presence of c. 700–550 Ma and 1.3–1.0 Ma age populations in the Nova Scotia samples to suggest that this source was either Amazonia or the Mexican Oaxaquia terrane. In particular, the lack of c. 1.0–1.7 Ga sources in NW Africa argues against reconstructions that place Avalon adjacent to the West African craton. Detrital zircons from the Lower Cambrian Wrekin Quartzite in Shropshire, England, (c. 535 Ma, Compston et al. 2002) also show distinct age maxima at c. 540–560 Ma and 600–620 Ma (Murphy et al. 2004), similar to the data described here. Murphy et al. (2004) used the paucity of older zircons in Cambrian samples when compared to the Neoproterozoic samples of Keppie et al. (1998) to suggest that Avalon drainage basins may have been more restricted in Cambrian times, possibly due to rifting of the Avalon Superterrane from Amazonia. The South Stack Group data presented here are from younger rocks than those discussed by Murphy et al. (2004) and would therefore be expected to show a similar restricted age-profile if a Cambrian rift model was correct. However, many Mesoproterozoic, Palaeoproterozoic and Neoarchaean detrital grains occur in the Anglesey samples suggesting that the lack of older detritus in the samples analysed by Murphy et al. (2004) is likely to simply be due to sampling bias, and that the Avalon Superterrane rifted off Gondwana after deposition of the South Stack Group, probably in Early Ordovician times (Prigmore et al. 1997).
Samson , S.D., D'Lemos, R.S.,
Miller, B.V. & Hamilton, M.A. 2005.
Neoproterozoic palaeogeography of the Cadomia and Avalon terranes:
constraints from detrital zircon U–Pb ages, J. Geol. Soc., Lond.
162 , 65-71.
Abstract: Detrital zircons from three Neoproterozoic sandstone units from the Cadomia terrane of northern France and the Channel Islands yield ages in three broad groups: late Neoproterozoic (650–600 Ma), early Palaeoproterozoic (2.4–2.0 Ga) and Archaean (.2.5 Ga). The lack of Mesoproterozoic zircon crystals, combined with the high abundance of grains between 2.20 and 2.00 Ga, corresponds closely to the ages of exposed rocks in the West Africa Craton, and thus it is suggested that Cadomia was in close proximity to West Africa by c. 580 Ma. In contrast, the main age groups of detrital zircon from the Neoproterozoic Avalon terrane are Mesoproterozoic and there is a distinct gap of ages between 2.40 and 2.05 Ga. These significant differences suggest that the two terranes were in different locations relative to major Gondwanan cratons in latest Neoproterozoic time.
Late Proterozoic Age
Carolina (1) 633 -----------------------------------------------------547
Cadomia (1) 615-600 585-570 540
Avalonia (1) 630-------------600 570---------550
Anti-Atlas (5) 762 753 743 650 640 614 605 586 575 560
Age 633-547 615-600, 585-570, 540 630-600, 570-550 629-575
Nd Juvenile Evolved ->less so Intermediate Nd comp Underthrust 400-370 Ma epNd -4.8 - -9.3
Det zirc1 MesoProt in NeoProt >2.5;2.4; 2.2-2.0 (2) 1.65-1.5; 1.25-1.15 2.0 .88;1.05;1.5 620-520,1.2-.9; (1.4-1.0), 2.2-1.5; Few Archean
Det zirc2 2.3-2.0 in Cambrian 650-600 (2)
Source of data: (1) Samson et al., 2004; (2) Samson et al., 2005; (3) Murphy et al, 2004; (4) Greenough et al. 1999; see http://instruct.uwo.ca/earth-sci/fieldlog/cal_napp/napp/new_eng_maritimes/gander.htm for references; (5) see http://instruct.uwo.ca/earth-sci/fieldlog/pan_african/maroc/maroc.htm
Terrane Ganderia (Ellsworth)(6) Anglesey(7) Avalonia Newfoundland(9)
666 Coedana(8); <501South
Det zircon 2.09-1.97; 1.50, 1.21; 680-630; 545; 507 >2.5; 2.2-1.95; 1.5; 1.3-1.25; (800); 627-543 580 in L. Prot and Cambrian
(6) REUSCH, D.N., VAN STAAL, C.R., and MCNICOLL, V.J., 2004. ; (7) Collins, A.S., 2004; (8) Strachan et al., 2007; (9) Pollock, J., 2007
http://www.kabrna.com/cpgs/anglesey/anglesey.htm - 2005 P. Kabrna, coloured geological map of Anglesey
KAWAI, T., WINDLEY, B. F., TERABAYASHI,
YAMAMOTO, H., MARUYAMA, S. AND ISOZAKI,
Y., 2006. Mineral
isograds and metamorphic zones of the Anglesey blueschist belt, UK: implications for the
metamorphic development of a Neoproterozoic subduction–accretion
complex. Journal of Metamorphic Geology, 24, 7, p. 591.
The 560–550 Ma blueschists and associated rocks in Anglesey, UK were derived from a subduction–accretion complex. The blueschist unit is divided into three mineral zones by two newly mapped metamorphic isograds; zone I sub-greenschist facies, (crossite isograd), zone II blueschist facies, (barroisite isograd), zone III epidote-amphibolite facies. The zones and isograds dip gently to the east, and decrease in metamorphic grade from the central high-pressure zone III to lower grade zones II and I to the west and east. The P–T conditions estimated from zoned amphibole indicate an anticlockwise P–T path following adjustment to a cold geotherm. This path is well preserved in the compositional zoning of Na–Ca amphibole that have a core of barroisite surrounded by a rim of crossite, although this is only locally developed. The sense of subduction was to the east and exhumation to the west, as indicated by the metamorphic isograds. The symmetrical arrangement of the metamorphic zones with the deepest high-pressure rocks in the middle suggests an isoclinal antiformal structure that formed by wedge extrusion during exhumation in the subduction zone.
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Kawai, T (firstname.lastname@example.org) Maruyama, S., 2006. A new geotectonic division of
Anglesey islands, West UK. AGU Fall Meeting, Paper T41C-1586. http://www.agu.org/meetings/fm06/fm06-sessions/fm06_T41C.html
A new geologic division of the Anglesey island is proposed herein, based on the new observation of structural relationship and recognition of Pacific-type accretionary complex. In addition, new zircon dates are reported from New Harbour Group. Laser Ablation-ICP-Mass Spectrometry (LA-ICP-MS), U-Pb data from detrital zircons extracted from the New Harbour Group, Anglesey, UK, indicate that the maximum depositional age is 472+/-30Ma. From the top to the structural bottom, the oldest (615Ma; Tucker & Pharaoh, 1991) unit of Coedana granite-gneiss with supracrustals, three types of accretionary complexes with a blueschist unit (560- 550Ma; Dallmeyer & Gibbons, 1987) at structural middle, the youngest accretionary complex New Harbour Group, and the structural bottom of South Stack Group (501+10Ma; Collins & Buchan,2004) are the new framework of tectonic division of Anglesey island. All boundaries are low-angle thrusts. Three accretionary complexes are composed by the structural top of the Gwna Group, the blueschist belt at structural middle and the structural bottom of olistostrome-type accretionary complex. These subdivisions are newly proposed. A series of geologic mega-units were formed successively downward by (1) southeastward subduction of oceanic lithosphere, (2) ridge subduction, and (3) by collision of micro-continent. Subduction started in 677-614Ma to form both accretionary complex and Pacific-type volcano-plutonism to increase continental crust. Subduction tectonically eroded the hanging wall of the Avalonian continent. At around 560-550Ma, mid-ocean ridge subducted to exhume the blueschist belt to the surface. The successive subduction of oceanic lithosphere formed the New Harbour accretionary complex, and finally the South Stack micro-continent collided at sometime in Ordovician age. The large-scale olistostrome developed on the northern margin of Anglesey Island at this time.
granite-gneiss (650 Ma arc lid of the Avalonian margin overthrust
to the northwest)
Gwna (top)/560 Ma Blueschist (mid-ocean ridge subduction)/Olistostrome (bottom)
New Harbour Group (473+/-30Ma younger Cambro-Ordovician? subduction accretionary complex)
Structural bottom South Stack Group (microcontinent underthrust to the southeast in the Ordovician)
Strachan, R.A.1; Collins, A.S.2; Buchan, C.; Nance, R.D.3;
Murphy, J.B.4; 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, pp. 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.
Treagus, J.E. 2007. Metamorphic zones in the Anglesey blueschist belt and implications for the development of a Neoproterozoic subduction-accretion complex: discussion. Journal of Metamorphic Geology. 25(5), 507-508.
Kawai, T. 2007. Metamorphic zones in the Anglesey blueschist belt and implications for the development of a Neoproterozoic subduction-accretion complex: reply. Journal of Metamorphic Geology. 25(5), 509-510.
(NOTE: The two maps which are the objects of this discussion can be accessed as Google Earth overlays at http://instruct.uwo.ca/earth-sci/fieldlog/Google_Earth/Anglesey.kmz )
Schofield, D.I.; J A Evans; I L Millar; P R Wilby; J A Aspden, 2008. New U-Pb and Rb-Sr constraints on pre-Acadian tectonism in North Wales, Journal of the Geological Society, v.1 65, p. 891-894.
Twt Hill granite 615.2 +/-1.3 penecontemperaneous with the Padarn tuff at 614 +/-2 (Tucker and Pharaoh, 1991); Rb-Sr age of 491+/-12 Ma indates resetting of the isotopic system at this time;
Treagus, Jack, 2009. Anglesy Geology - A Field Guide., (ISBN 0-9546966-2-X) "Eleven sites concern the low-grade metamorphic rocks of the Mona Complex, most involving the spectacularly displayed polyphase deformation, others include stromatolitic limestone and the famous melange. Two involve Ordovician conglomerates, one the Ordovician and Silurian mineralised rocks of Parys Mt and others feature folded Devonian red-beds, pot-holes in the Carboniferous Limestone and a section through a Pleistocene drumlin.
Phillips, E.R. 2009. The Geology of Anglesey, North
Wales: project scoping study. British Geological Survey,
Internal Report IR/09/05. pp 47. An indispensable report
on all aspects of the geology of Anglesey; bibliography is the
best currently available.
Discussion on ‘A new look at the Mona Complex (Anglesey, North Wales): J. geol. Soc. London, Vol. 137, 1980, p. 513-514.
KOHNSTAMM writes: Barber & Max (1979), in their ‘New look at the
Mona Complex’, explain the geology of their ‘Northern Region’
(which includes the
Northern and Western Regions of Greenly 1919) by invoking 3 tectono-sedimentary units and a mylonite belt. The truth of this model depends not only on the
controversial lower contact of their middle, or New Harbour Unit, but also on its upper contact. The authors failed to reply to the point raised by Professor
Wood in the discussion of their paper, that field mapping and petrographic evidence show the upper contact to be of a ‘purely hallucinatory nature’. In fact,
the contact does exist, but only as a gradation between the upper, or Cemlyn Unit and its tectonized equivalent. The structure of the Northern Region (in the
original sense of Greenly) of the Mona Complex is explained by Barber & Max as 3 parallel belts - the Cemlyn Unit in the NW, the New Harbour Unit in the
centre, and the mylonite belt along the SE margin. It requires emphasising that, while the authors make important corrections to Greenly’s stratigraphy in demonstrating
that the New Harbour Unit is essentially a region of phyllonites rather than a stratigraphic unit, they persist in considering it a separate sedimentary
unit. This results in their need to thrust the unit to the middle of the succession.
Barber & Max claim that ‘all the contacts between the Cemlyn and New Harbour Units are faults’ and their map shows a single dashed line along the contact. Where this line reaches the coast at Bull Bay, although at least one fault is present, there is nevertheless a ‘perfectly gradational passage’ (Greenly 1919). Lithologies indistinguishable from grits of their Cemlyn Unit can be found throughout their New Harbour Unit (e.g. S of Amlwch Port, grid ref. 453924). Sheared areas, identical with their New Harbour Unit, are seen within their Cemlyn Unit. On Mynydd y Garn (grid ref. 322901) undeformed Gwna Group passes gradually into a New Harbour lithology by development of a mylonitic foliation. Under the microscope all stages in the formation of foliation and feldspar sericitization can be demonstrated in a gradation between Cemlyn grit and New Harbour phyllonite. The apparent change from psammite to pelite a result of the alteration of albite to phengite, with corresponding change in chemical composition.
petrographic work demonstrate that the
‘New Harbour Tectono-sedimentary Unit’ is a tectonized
equivalent of the Cemlyn Unit. Its unity is purely
tectonic, not sedimentary or stratigraphic. Where its
original sedimentary lithologies are preserved, they cannot be
distinguished on chemical, mineralogical or sedimentological
grounds from the Cemlyn Unit. Grit, green
and purple phyllite, ultrabasic rocks and
greenstones all occur in both units.
That the Cemlyn-New Harbour contact runs parallel to the
mylonite belt is not coincidence - it is a result of increasing
deformation towards the SE. Both the New Harbour Unit
and Mylonite Belt show the same foliation, N-S lineation and
intrafolial folds. The authors do not explain the difference
between mylonitic New Harbour Unit and the mylonite, derived
the rocks of the New Harbour Group’, that forms the Mylonite Belt. The name ‘New Harbour’ would be better abandoned completely, to avoid confusion between
a stratigraphic unit and a tectonic belt.
DRS A. J. BARBER & M. D. MAX
reply: We would go a long way towards accepting the
interpretations put forward by Mr Kohnstamm, as they provide
for some of the doubtful points in our own attempt to reinterpret the geology of Anglesey (Barber & Max, 1979). As we indicated in our Figs 1
and 2 and also in the text (p. 416), we were uncertain of the nature of the contact between the Cemlyn and New Harbour Tectonic Units. As Kohnstamm points
out, this contact is represented by a dashed line in Fig. 1, and is shown as a structural discordance, possibly a thrust or unconformity, in Fig. 2. It is unfortunate that
Fig. 2 also carried stratigraphic connotations. As we made clear in the text, the Cemlyn and New Harbour Units were distinguished only on structural grounds. In view of our own failure to resolve this relationship satisfactorily we welcome Kohnstamm’s interpretation of the New Harbour Unit as a zone of high strain
affecting rocks of the Cemlyn Unit. His interpretation of the lithological contrast between the Cemlyn and New Harbour Units as largely due to the effects of
deformation is also acceptable to us. We had appreciated that the lamination and ‘flaser-bedding’ seen in the New Harbour Unit were the effect of deformation
but we had not fully appreciated the importance of the breakdown of feldspar in the production of the pelitic component of the phyllitic schists.
studies in Anglesey have also caused us to modify our
interpretation of the lower contact of the New Harbour Unit,
where it rests on the South
Stack Unit in the hinge of the Rhoscolyn Fold (Barber & Max, 1979, p. 415). Since this fold is seen to fold bedding in the South Stack Unit, while it folds the
dominant schistosity in the New Harbour Unit, we presumed, following Shackleton (1969), that the Rhoscolyn Fold was a first phase fold in the South
Stack Unit, but occurred later in the structural sequence of the New Harbour Unit. To account for this discrepancy we postulated that the contact between
the South Stack and New Harbour Units was a structural discordance, along which the already deformed New Harbour Unit was thrust over the undeformed
South Stack Unit.
paper was prepared we have discovered that the dominant asymmetrical minor folds
in the South Stack Unit, overturned to the SE and congruent to
the Rhoscolyn Fold, are in fact second
phase folds in the structural sequence of this unit.
In many examples along the coast section to the W of Rhoscolyn
the asymmetrical folds deform an earlier cleavage in the pelitic
bands, which intersects the bedding, represented by the
quartzitic, bands, at a low angle.
Cleavage/bedding relationships show that this earlier cleavage is related to a large scale recumbent anticline, overturned towards the NW.
Small scale folds in pelitic and psammitic bands of the South Stack Unit provide analogues of the relationships seen between the quartzitic Rhoscolyn Formation and the ‘pelitic’ New Harbour Group in the hinge of the Rhoscolyn Fold, N of Borth Wen (Barber & Max, 1979, p. 415). The dominant foliation which is folded around the fold hinge is not related to the formation of the Rhoscolyn Fold as Maltman (1975), for example, has suggested, but is related to the earlier phase of deformation. The New Harbour Unit can be regarded as 3 very thick pelitic layer as Powell (in discussion of Barber & Max, 1979, p. 427) proposed. It is therefore no longer necessary to invoke a thrust plane at this contact.
Nevertheless, the contact between the South Stack Unit and the New Harbour Unit still represents an important structural and stratigraphic boundary. At the contact the lithology changes abruptly from alternating psammitic and pelitic units to an extensive unit composed of impure greywacke with tuffaceous beds, pillow lavas and cherts, and intruded (whether by igneous or tectonic processes) by basic and ultrabasic bodies (Maltman 1975). The boundary evidently marks an abrupt change in palaeo-environmental conditions and also represents the lower margin of an extensive zone of very high strain, in which a strong mylonitic schistosity and an intense stretching lineation are the dominant structural features.
One important implication
of these revised interpretations is that if the New Harbour
Unit is the deformed equivalent of the Cemlyn Unit and forms a
continuous stratigraphic sequence with the South Stack Unit, then the whole of the Bedded Succession in the Mona Complex may possibly be of Cambrian
age (cf. Muir et al. 1979), with only the quartzite and limestone blocks in the melange representing fragments of late Precambrian units.