geotectonic framework of the blueschist unit on anglesey–lleyn, … · 2014. 2. 18. · the...

Precambrian Research 153 (2007) 11–28

Geotectonic framework of the Blueschist Unit on Anglesey–Lleyn,UK, and its role in the development of a Neoproterozoic

accretionary orogen

T. Kawai a,∗, B.F. Windley b, M. Terabayashi c, H. Yamamoto d, S. Maruyama a,S. Omori a, T. Shibuya a, Y. Sawaki a, Y. Isozaki e

a Department of Earth and Planetary Sciences, Tokyo Institute of Technology, O-okayama 2-12-1, Meguro, Tokyo 152-8551, Japanb Department of Geology, The University of Leicester, Leicester LE1 7RH, UK

c Department of Safety Systems Construction Engineering, Kagawa University, Kagawa 761-0396, Japand Department of Earth and Environmental Sciences, Kagoshima University, Kagoshima 890-0065, Japan

e Department of Earth Science & Astronomy Graduate School of Arts and Sciences,The University of Tokyo, 3-8-1 Komaba, Meguro, Tokyo 153-8902, Japan

Received 27 April 2006; received in revised form 10 November 2006; accepted 12 November 2006

Abstract

A 560–550 Ma, 5 km × 25 km Blueschist Unit extends NE–SW on the island of Anglesey, Wales, UK, and continues to thesouthwest for more than 70 km along the northern coast of the Lleyn Peninsula. It has the shape of a shallow-dipping slab or sheet upto a few kilometres thick that was exhumed from the subduction zone and emplaced into the accretionary complex to the northwest.Today the top boundary of the slab is commonly a thrust that was originally a low-angle normal fault at its top in the subduction zone;above it is an accretionary complex and a unit of high-grade gneisses. The bottom boundary is today a thrust (that was originallya thrust), below which is a unit of low-grade or unmetamorphosed rocks belonging to an olistostrome-type accretionary complex.This thrust was originally at the bottom of the slab when it was exhumed in the subduction zone. The sandwiched assemblagewas created by the tectonic insertion of the blueschist between the other units. Folding, particularly during late doming of thetectonic sandwich, rotated the top normal-fault boundary into a thrust. Later, high-angle, NE–SW-trending normal faults displacedthe tectonic stratigraphy in horst and graben.

A 200 km long and 200 km wide, 680–450 Ma calc-alkaline volcano-plutonic belt extends southeastwards from the blueschistbelt via northern Wales to central England, suggesting that the subduction of oceanic lithosphere was to the southeast, and this isconsistent with the extrusion of the thin high-pressure slab into the accretionary complex to the northwest. This took place on thewestern accretionary margin of Avalonia.

Finally, we outline the key steps involved in unravelling the structure of a Pacific-type accretionary complex.© 2006 Elsevier B.V. All rights reserved.

Keywords: Anglesey; Lleyn Peninsula; Blueschist; Top boundary; Bottom bo

∗ Corresponding author. Tel.: +81 3 5734 2618;fax: +81 3 5734 3538.

E-mail address: [email protected] (T. Kawai).

0301-9268/$ – see front matter © 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.precamres.2006.11.002

undary

1. Introduction

Blueschists (BS), locally with eclogites, form animportant component of many accretionary, Pacific-typeand collision-type orogens, and their study provides keyinformation not only on their metamorphic history, but

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12 T. Kawai et al. / Precamb
also on their structure, and accordingly on their subduc-tion tectonics and polarity (e.g. Miyashiro, 1973, 1994;Ernst, 1975, 1977). Blueschist–eclogite units were typ-ically exhumed by a process of wedge extrusion as thin(ca. 1 km width) slabs that were emplaced as shallow-dipping structures over or into adjacent accretionarywedges, the prime modern analogue of which is theSanbagawa high-pressure metamorphic unit in Japan(Maruyama, 1990; Maruyama et al., 1996; Masago etal., 2005). Well-investigated, post-Cretaceous examplesaround the circum-Pacific demonstrate that wedge extru-sion was the result of a shallowing subduction anglecaused by an approaching buoyant mid-oceanic ridgeat the leading edge of the circum-Pacific continentalmargin, and was coincident with widespread, landward,calc-alkaline arc magmatism (Maruyama et al., 1996;Maruyama, 1997).

Blake (1888) first discovered glaucophane in schists
on the island of Anglesey, Wales (Fig. 1). Greenly (1919)made the classic description of the BS unit (Fig. 2),Gibbons and Mann (1983) first reported the presenceof lawsonite, Horak and Gibbons (1986) classified the
Fig. 1. Simplified map of Wales and Central England showing location of mzircon ages, interpreted to date the crystallization of plutonic or volcanic rocHorak et al., 1996; 15–16, Noble et al., 1993). Position of Fig. 2 indicated.

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blueschist amphiboles, Gibbons and Gyopari (1986)described an anticlockwise PT trajectory, and Gibbons(1981) described glaucophane on the adjacent LleynPeninsula. Dallmeyer and Gibbons (1987) reported an40Ar/39Ar mineral age on phengite of 560–550 Ma forthe blueschists which they interpreted as the metamor-phic crystallization age. In spite of these metamorphicstudies of the blueschists, and of structural-tectonicinvestigations of these and associated rocks in Angle-sey (Wood, 1974; Barber and Max, 1979; Shackleton,1975), the structural framework of the Anglesey–Lleynblueschists is unknown, particularly with regard to thatof modern analogues.

Our recent investigations of the accretionary orogenof Anglesey–Lleyn have led to the definition of new min-eral isograds and metamorphic zones in the BlueschistUnit (Kawai et al., 2006), and in particular to a studyof the top and bottom boundaries of the high-pressure
unit, in order to work out its structural evolution fol-lowing the deep-seated blueschist facies metamorphism.The aim of this paper is to describe the structure ofthe Anglesey–Lleyn Blueschist Unit, and to produce
agmatic arc rocks in outcops and boreholes. Squares refer to U–Pbks (1–7, Tucker and Pharaoh, 1991; 8–13, Compston et al., 2002; 14,

T. Kawai et al. / Precambrian Research 153 (2007) 11–28 13

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ig. 2. Simplified geological map of Anglesey Island and Lleyn Penins980). Location of Figs. 3–5 indicated.

new tectonic model that explains its subduction andxhumation history. These results have major implica-ions for the tectonic environment of the Anglesey–Lleynccretionary orogen on the active continental mar-in of Avalonia in the Neoproterozoic and earlyalaeozoic.

. The Anglesey–Lleyn Blueschist Unit

The main geological units on Anglesey and Lleyn arehown in Figs. 1 and 2. The Blueschist Unit on Angleseys bordered on both sides by the accretionary complex ofhe Gwna Group. The olistostromal New Harbour Group
nd the passive margin sediments of the South Stackroup occur to the northwest, and the Coedana granite
s situated in the Central Anglesey gneisses. The Centralnglesey Schist Unit that occurs on the eastern side of

ing the main tectonic units (modified after British Geological Survey,

the Coedana granite is mainly composed of mica schistsand minor metabasite lenses.

The Blueschist Unit extends NNE to SSW from theeastern side of Anglesey Island to the southwestern endof the Lleyn Peninsula, a total distance of about 70 km(Fig. 2). On Anglesey the unit is bounded on its west-ern and eastern sides by late steep, NE–SW-strikingBerw and Menai Strait (Fig. 3A) faults, respectively.We will demonstrate that the original attitude of theBlueschist Unit in relation to its bordering rocks wassub-horizontal. At a small outcrop the top boundaryof the Blueschist Unit is overlain by unmetamorphosedGwna Group rocks and Holland Arms gneisses (Fig. 3B).
The bottom boundary crops out at the western endof the Lleyn Peninsula where schists are underlainby unmetamorphosed or low-grade rocks belongingto an accretionary complex. The three key areas,

14 T. Kawai et al. / Precambrian Research 153 (2007) 11–28

ey (Kawdary be

Fig. 3. (A) Metamorphic zonal map of the Blueschist Unit in Anglesgneisses, Gwna Group and Blueschist Unit. (C) Sketch of the bounBlueschist Unit from D to D′.

where the boundaries can be defined, are describedbelow.

3. The Blueschist Unit on Anglesey, and its topboundary

On Anglesey the metamorphic grade of the BlueschistUnit has been sub-divided into three mineral zones: I, IIand III that are separated by crossite and barroisite iso-grads (Fig. 3, after Kawai et al., 2006). The appearance

ai et al., 2006). (B) Sketch of the boundary between Holland Armstween the Arfon Group and Gwna Group. (D) Cross-section of the

of crossite defines the beginning of Zone II. Zone I hasa chlorite–epidote mineral assemblage. But, the basalticlavas of the Gwna Group to the east have only chlorite,which we ascribe to thermal hydration metamorphism onthe ocean floor; i.e. they were not metamorphosed by theregional metamorphism that affected Zones I–III. These
chloritic basaltic lavas correspond to similar unmetamor-phosed Gwna Group basaltic lavas in SW Lleyn.
In Fig. 3D, the spatial distribution of the mineralzones, passing from east to west, indicates a structurally

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ownwards, westward increase of P and T from Zones Io II towards a central structural horizon (Zone III), andhen a corresponding decrease farther westwards fromII to I (Kawai et al., 2006).

The Blueschist Unit (5 km × 25 km) in Zone IIIontains lenses of well-preserved glaucophane-bearingetabasic rocks up to 5 km long, as shown on the geolog-

cal map of Anglesey (British Geological Survey, 1980),he wall-rocks of which mostly consist of chloritic and

icaceous schists without glaucophane. Many metaba-ic lenses consist of hornblende schist, some of whichn less retrogressed areas contain glaucophane. Mineralineations defined by well-developed Na-amphibole onoliation surfaces and crenulation fold axes trend NNE-SW (Fig. 3A). Zone II (3 km × 11 km) on the easternide of Zone III also contain lenses up to 1.5 km long oflaucophane-bearing metabasic rocks.

The top boundary of the Blueschist Unit is exposed onts eastern and western margins (Fig. 3). On the eastern

argin tuffs of the 614–572 Ma Arfon Group (Tuckernd Pharaoh, 1991; Compston et al., 2002) rest abovethrust on unmetamorphosed accretionary rocks of the

astern Gwna Group (Greenly, 1919) (Fig. 3C), whichn turn rest on an extensional fault above the Blueschistnit.On the western margin in a 200 m × 1000 m area

oliated, micaceous Holland Arms gneisses, whichave a Rb–Sr whole-rock isochron age of 595 ± 12 MaBeckinsale and Thorpe, 1979), and NS-trending sub-orizontal isoclinal fold axes (Fig. 3B), are separatedrom the Blueschist Unit by a high-angle extensionalault that is marked as thrust on Fig. 3B in its rotatedosition. Although the original relationship betweenhe Holland Arms gneiss and the Blueschist Unit isnexposed, the attitude of shallow-dipping rocks inhe klippe suggests that surrounding, weakly metamor-hosed accretionary rocks of the Gwna Group rest onchists of the Blueschist Unit with a sub-horizontal tec-onic contact, which means in turn that the high-gradeneisses also overlie the Gwna Group with a tectonicontact. Further lithological characteristics and struc-ural relationships on this top boundary are well observedn the southwest Lleyn Peninsula to be describedelow.

. The Peninsulas of Porth Dinllaen and Penrhynefyn

Gwna Group accretionary rocks and blueschist–hengite schists (Schist Unit) crop out on the Porth Din-laen Peninsula and Penrhyn Nefyn Peninsula on theorthern coast of the Lleyn Peninsula (Fig. 4A and B).

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High-angle secondary faults trending NW and NE sep-arate the two groups of rocks.

The Gwna Group, best exposed on Porth DinllaenPeninsula, mostly consists of a 1 km-thick pile of pillowlavas, massive basaltic flows and minor pillow brec-cias, the pillows being largely undeformed. The lavascommonly contain inter-pillow lenses of red chert orlimestone. Hydrothermal mineralization is recorded insulphide-rich massive basalts and quartz veins. The fac-ing directions of the pillows are shown in Fig. 4B. Onthe western side of Penrhyn Nefyn (Fig. 4A) pillowlavas are associated with volcanic breccias. We correlatethese undeformed volcanic rocks with comparable unde-formed pillow lavas and associated rocks of the GwnaGroup in southwest Lleyn (Figs. 7A and 8A) and onLlanddwyn Island (Fig. 3).

The Schist Unit has two main outcrops, eastern andwestern, and contains several types of schistose rocks.In the eastern unit on Penrhyn Nefyn (Fig. 4A) themost common lithology is a meta-volcanic basic schistthat consists of epidote, chlorite, actinolite, and albite(Gibbons, 2000) and contains relict massive basalticflows, deformed flattened pillows up to 5 cm across, andblueschists in which crossite replaces actinolite as in themain blueschists of Anglesey (Gibbons, 1981; Kawai etal., 2006). Adjacent phengite schists (Fig. 4A) containquartz and albite (Gibbons, 2000); we confirm the pres-ence of phengite. These glaucophane–phengite schistsbelong to the northern end of the Blueschist Unit ofthe Lleyn Peninsula shown in Fig. 2. Adjacent tonaliticschists contain low strain zones with a well-preservedtonalitic texture; Gibbons (2000) correlated these rockswith tonalite belonging to the nearby Sarn Complex.

The western Schist Unit on the western side of PorthDinllaen peninsula (Fig. 4B) contains much chloriticschist, which we interpret as a mafic volcaniclastic sed-iment. The schist contains many lenses up to 2 m longthat demonstrate crude stratigraphic zones through theschists, each some tens of metres thick. Passing fromeast to west a zone with red chert lenses is followed suc-cessively by zones rich in sandstone lenses and then inlimestone lenses. We consider that the lenses are derivedfrom broken up beds of chert, sandstone and limestone.In the top of this chloritic melange there is a 50-m blockof homogeneous metabasalt, above which is a beddedsuccession of turbidites and sandstones. We interpretthe western Schist Unit as accretionary rocks overlainby clastic sediments. Although the present contact of
the pillow lavas with the western Schist Unit is a steepfault, we presume that the original relationship was a sub-horizontal fault, as seen on the SW Lleyn Peninsula, andtherefore that this is a top boundary of the Schist Unit.


Fig. 4. (A) Geological map and cross-section of Penrhyn Nefyn Peninsula to the east of the Porth Dinllaen Peninsula. (B) Geological map andcross-section of the Porth Dinllaen Peninsula.

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. SW Lleyn Peninsula; bottom and topoundaries of the Schist Unit

Our geological map of the southwestern end of theeninsula (Figs. 2 and 5) shows two blocks of the Schistnit (that do not contain glaucophane here), the struc-

ural relationships of which with surrounding rocks areescribed below.

.1. Bottom boundary

A klippe of Zone I schist crops out on a 550 m-highill that rises steeply from the sea at the western end of therea shown in Fig. 5. It consists of steeply dipping slaty

ig. 5. Geological map and cross-section of the southwest Lleyn Peninsula snit. See text for details. Positions of Figs. 6 and 7 indicated. On the cross-ith Ordovician sedimentary rocks to the east. The horst contains the bottom

omplex. The matrix of the latter consists of mudstone that contains olistoliraben is composed of two units, a Gwna Group accretionary complex and th

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mudstone that contains relicts of metabasic greenschist(predominately basaltic tuff), sandstone, mudstone, andrare chert and limestone.

The Schist Unit is separated from an underly-ing unmetamorphosed olistostrome-type accretionarycomplex by a sub-horizontal, phyllonite-bearing thrust(Fig. 6A). The steeply dipping schists are truncated bythe sub-horizontal thrust. This is the bottom boundaryof the Anglesey–Lleyn Blueschist Unit, as discussedin detail later. Fig. 6B and C shows thrusts near the
boundary between the Schist Unit and the underlyingrocks.
Below the thrust the steeply dipping, unmeta-morphosed olistostrome-type melange (accretionary

howing relationships at the bottom and top boundaries of the Schistsection, high-angle secondary faults have created a horst and graben

of the Schist Unit and an underlying olistostrome-type accretionaryths (exotic clasts) of basalt, limestone, quartzite and sandstone. Thee top of the Schist Unit.


Fig. 6. Four photos showing structural relationships in southwest Lleyn at localities marked in Fig. 5. (A) Bottom thrust boundary of the Schistbelt. This is an oblique view looking to the SE of a hillside showing a klippe of Zone I Schist Unit resting on an east-dipping thrust boundary on a

(B andatrix in

virtually unmetamorphosed, olistostrome-type accretionary complex.the north. View towards the east. (D) Quartzite clasts in a mudstone m

complex) has a matrix of mafic mudstone that containsmany, mostly 5–30 cm-size, lenses of quartzite (max-imum is 17 m × 25 m; Fig. 6D), red chert, dolomiticlimestone, and basaltic greenstone (Fig. 6D). The olis-tostromal mudstone melange passes stratigraphicallydownwards to ca. 35 m-thick sandstone that makes upthe controversial Gwyddel Beds. Many authors (e.g.Shackleton, 1975; Roberts, 1979) described these asbedded acidic tuffs. However, from our observationsin the field and in thin section we conclude they arebedded sandstones, in agreement with Gibbons andMcCarroll (1993). The bedded sandstones are under-lain conformably by a well-preserved, but small, sectiondisplaying ocean plate stratigraphy. A 1 m-thick bed ofhemi-pelagic mudstone passes downwards to 7 m of bed-
ded red chert (marked on Fig. 5) that is underlain by a1 m-thick basalt (not marked on Fig. 5).
All the main units described above are transected bysecondary high-angle extensional faults (Figs. 5 and 6A)

C) Thrusts at the bottom of the Schist Unit that indicates thrusting tothe olistostrome-type accretionary complex shown in Fig. 5.

that have the effect of uplifting and exposing the bottomboundary of the Schist Unit in a horst (Fig. 5).

5.2. Top boundary

The top boundary of the Schist Unit is exposed in thebay of Parwyd Llech-y-menyn (Fig. 5); Fig. 7 shows thegeological map and Fig. 8 some relevant photographs.Gwna Group rocks are here thrust over mafic chloriticschists without glaucophane belonging to Zone I of theSchist Unit (Figs. 5 and 7A).

The top boundary surface of the Schist Unit is aca. 50 m-thick thrust duplex that dips 45◦ northwest(Fig. 8B). Several parallel or divergent northerly dippingthrusts within the Gwna Group are in the hanging wall of
the thrust duplex (Fig. 7A). Fig. 8C and D shows minorsouth-vergent thrusts within the major thrust duplex.
The Gwna Group largely consists of pillow basalts,at least 20 m thick (Figs. 7B and 8A) that are overlain


Fig. 7. (A) Geological map of the central graben on the coast in southwest Lleyn extending eastwards from the bay of Porth Felen to the bay ofP . 8 indiG ared Lles

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ared Llech-y-menyn, as shown on Fig. 5. Position of photos of Figroup accretionary complex in the hanging wall of the main thrust at P

tratigraphy that has been imbricated in the Gwna Group.

y 2 m of brown pelagic limestone succeeded by 5 m ofed-bedded chert; this represents ocean plate stratigraphyFig. 7B). The Gwna Group is located in a graben thatas subsided differentially with respect to its neighbour-ng horst blocks, thereby preserving the top boundary.he western margin of the Gwna Group is occupied bysecondary, high-angle NS-striking normal fault that

ips 70◦ east, marking the western side of the graben,hereas the eastern margin is occupied by a major thrust

hat dips 45◦ west. On the eastern side of Parwyd bayFig. 5) a 30 m-wide strip of retrogressed garnetiferousneiss (the Parwyd gneiss) is overlain unconformablyy Ordovician sediments (Matley, 1928; Roberts, 1979;

cated. The map shows many thrusts that have imbricated the Gwnach-y-menyn. (B) Geological columnar section of primary ocean plate

British Geological Survey, 1994; Gibbons, 2000), Theeastern boundary of the Parwyd gneiss is marked bya sub-vertical fault that has a small amount of thrustmovement that has brought up the gneisses (section ofFig. 5). We suggest that an original thrust, probably sub-sidiary to the major thrust on the western side of the bay,was reactivated by the normal fault at the eastern endof the graben, but insufficiently to remove all the thrustoffset.

We agree with Gibbons (1983) who considered thatthe Schist Unit is comparable to the Blueschist Unit onAnglesey and showed that it extends along the LleynPeninsula to the schists on Penrhyn Nefyn. The upper


st Lleythwestwnd D) S

Fig. 8. Four photos of key features at localities on the coast in southwe(B) Top boundary of the Schist Unit which is a thrust that dips 45◦ norseveral parallel or divergent faults in the hanging wall (Fig. 7A). (C athrust in (B).

schists in Fig. 5 contain clasts of limestone, basalticlava, quartzite, red shale and chert. The schists increasein metamorphic grade eastwards from mafic chloriticschists at Pared Llech-y-menyn to phyllites, and ulti-mately to micaceous and basic, actinolitic schists atTrwyn Bychestyn headland—see Fig. 5 (Matley, 1928;Shackleton, 1956; Gibbons, 2000). This increase ingrade is downwards from the top towards the centre ofthe Schist Unit.

Combining the overall geometry of this top boundarywith that of the bottom boundary on Anglesey and Lleyn(Fig. 9A), we conclude that, when the Blueschist-bearingSchist Unit moved from depth towards the surface, it was
juxtaposed at a crustal level with the overlying GwnaGroup along an extensional fault and with the underlyingolistostrome-type accretionary complex along a thrust(Fig. 9B).
n marked in Fig. 7. (A) Undeformed pillow basalt in the Gwna Group.ard at the bay of Pared Llech-y-menyn. Looking to the NE. There are

outheast-verging minor thrust duplexes a few metres above the major

6. The magmatic arc

Fig. 1 shows key geological outcrops and boreholeswith their isotopic ages in Wales, the Welsh borderlandand English Midlands. We emphasize the many calc-alkaline igneous rocks that belong to this 200 km-widemagmatic arc that occurs as inliers within Phanerozoiccover sedimentary rocks. Reliably dated rocks in this arcformed in four main stages (Gibbons and Horak, 1996).

6.1. 680–670 Ma

Early arc magmatism. The Malvern Complex mainly
comprises calc-alkaline granodiorite, diorite, tonaliteand granite (Thorpe et al., 1984), and has a U/Pb zir-con age of 677 ± 2 Ma and a U–Pb monazite age of670 ± 10 Ma (Tucker and Pharaoh, 1991).


Fig. 9. Simplified cross-sections across Anglesey–Lleyn showing the present-day structure and tectonic evolution of the Blueschist–Schist Unit.(A) The present-day structure that has been divided by secondary high-angle faults into an anticline in Anglesey and a horst and graben in Lleyn.(B) Simplified cross-section of the original structure of the Blueschist–Schist Unit. Removal of the high-angle secondary faults has reconstructedthe original tectonic stratigraphy that has been folded after exhumation. The three oblong blocks refer to the areas where we observe the present-daystructures marked directly above in (A). (C) Cartoon showing the exhumation of the BS unit as it was transported from SE to NW in the uppersubduction zone (see also in Fig. 10C). The Blueschist Unit is being tectonically juxtaposed and sandwiched between unmetamorphosed or low-grade accretionary complexes; it has a thrust at its bottom and an extensional fault at its top that enable it to be exhumed during wedge extrusion.After exhumation, the folding into an anticline and syncline changed the orientation of the faults; the extensional fault at the bottom boundarybecame a thrust which we observe today in (A). (D) Geological columnar section showing the original tectonic stratigraphy between the four units(olistostrome-type accretionary complex, Schist Unit, Gwna Group, and Holland Arms gneiss).


Fig. 10. Simplified cross-sections along a NW–SE profile from Anglesey through Wales to Central England marked in Fig. 1, to illustrate thegeodynamic evolution of the active western margin of Avalonia from about 680–450 Ma. See text for more details. (A) First, minor magmatism inthe Malverns at 677 Ma. (B) Main arc activity at 620–600 Ma at Sarn, Arfon, Glinton, Orton and Chaldecote. The crustal melt Coedana leuco-graniteformed as a result of ridge subduction. (C) A ductile wedge of an accretionary complex, a few kilometres thick, was subducted, recrystallized underblueschist facies conditions, and then exhumed to a mid-upper crustal level, possibly assisted by a shallowing subduction angle. Metamorphic zonesand isograds indicate that the wedge has an antiformal isoclinal structure. (D) The South Stack and New Harbour Groups were underthrust intothe base of the accretionary wedge during final stages of subduction in the Cambrian. This underthrusting led to doming of the already accretedcomplex, as result of which the Coedana granite and gneisses became isolated from the western margin of Avalonia from which they were derived.Arc magmatism continued in the Tremadoc to Caradoc in NW Wales.

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.2. 620–600 Ma

Rocks that formed in this main period of arc mag-atism are: (a) the Sarn Igneous Complex in the Lleyneninsula that consists of calc-alkaline monzogranite,abbro, granodiorite, diorite, and tonalite and has a07Pb/206Pb zircon age of 615 ± 2 Ma (Gibbons andorak, 1996; Horak et al., 1996); (b) the 4 km-thickolcaniclastic Arfon Group in NW Wales that containslasts of Monian (Anglesey) metasedimentary rocks ands overlain by fossiliferous Cambrian slates (Shackleton,975; Horak et al., 1996). A rhyolitic ash-flow tuff hasU–Pb zircon age of 614 ± 2 Ma (Tucker and Pharaoh,991), and an ignimbrite has a SHRIMP zircon age of05 ± 1.6 Ma (Compston et al., 2002); (c) felsic tuffs inhe Glinton and Orton boreholes have U–Pb zircon agesf 616 ± 6 and 612 ± 21 Ma, respectively (Noble et al.,993); (d) the Coedana granite on Anglesey (Greenly,919) is a leuco-granite that contains muscovite, biotitend garnet, retains locally a pre-granite foliation, lackschilled margin, has granitic veins rich in garnet, and

lose to its contact with mica-tourmaline hornfels theranite in places contains tourmaline. Greenly (1919)oncluded that “the affinities of the Coedana graniteo some metamorphic granites are close”. In contrast,ibbons and Horak (1996) suggested that the Coedanaranite was a product of arc magmatism. We suggesthat it is a crustal melt granite, the high temperature forhich was provided by ridge subduction (Fig. 10B). Theranite has a U–Pb zircon age of 613 ± 4 Ma (Tuckernd Pharaoh, 1991); (e) the Chaldecote volcanic rocksith bedded tuffs, similar to those of the Charnianupergroup (Pharaoh et al., 1987), are intruded bydiorite at Nuneaton that has a U–Pb zircon age of

03 ± 2 Ma (Tucker and Pharaoh, 1991).

.3. 575–550 Ma

Late arc magmatic rocks include: (a) a tuff in theolcaniclastic Arfon Group in NW Wales (Shackleton,975) that has a SHRIMP U–Pb zircon age of72 ± 1.2 Ma (Compston et al., 2002); (b) in the Welshorderland a rhyolitic lava in the Uriconian Grouponsisting of basalts, basaltic andesites, dacites and rhy-lites (Thorpe et al., 1984) has a U–Pb zircon age of66 ± 2 Ma, and the Ercall granophyre that intrudeselsic lavas has a U–Pb zircon age of 560 ± 1 MaTucker and Pharaoh, 1991). Bentonite and tuff in the
ongmyndian Supergroup (Pauley, 1990) have SHRIMPircon ages of 567 ± 2.9 and 556 ± 3.5 Ma, respectivelyCompston et al., 2002); (c) in the Malvern Hills ahyolitic tuff occurring within a calc-alkaline tholei-
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itic basalt–andesite–rhyolite association in the WarrenHouse volcanic group (Thorpe et al., 1984) has a U–Pbzircon age of 566 ± 2 Ma (Tucker and Pharaoh, 1991);(d) in the English Midlands the Charnian Supergroupcontains felsic tuffs, andesites and dacites, agglomer-ates and granodioritic intrusions (Pharaoh et al., 1987).Compston et al. (2002) obtained SHRIMP zircon ageson tuffs of 566 ± 3 and 559 ± 2.0 Ma.

6.4. Tremadoc–Caradoc (ca. 488–448 Ma)

Arc volcanoes, associated with at least six major rhy-olitic calderas, resurgent and collapsed calderas, twoandesitic stratovolcanoes, Strombolean cones, and intra-arc graben, developed on already accreted crust at adestructive plate margin in Wales (Gibbons, 1998). Theygave rise to voluminous basic, intermediate and particu-larly felsic rocks with an overall calc-alkaline chemistryin the late Tremadoc in NW Wales and in the lateTremadoc or early Arenig in SW Wales (Kokelaar, 1988).In the Arenig to Caradoc bimodal basalt-rhyolite lavaserupted in a back-arc basin in central Wales, but calc-alkaline arc-type lavas erupted again in the Llanvirn toCaradoc on the southeastern side of the Welsh basin(Woodcock and Strachan, 2000). Although subsequentlate Caledonian deformation inverted fractures inheritedfrom the basement, created slate belts, and shortened thewidth of the early Palaeozoic arc belt, it did not destroythe primary volcanic structures in the arc rocks, and it didnot affect sufficiently the distribution of the magmaticrocks to prevent recognition of the arc (Kokelaar, 1988).

7. Discussion

In the first plate tectonic synthesis of the geotectonicdevelopment of the British Isles, Dewey (1971) con-sidered, reasonably, that the Caledonian and Hercyniancollisional orogenies were the most important eventsresponsible for formation of the continental crust. In thefollowing decades later plate tectonic models similarlyregarded collisional orogeny as predominant, becausethe role of accretionary orogens was little appreciatedinternationally at that time. This imbalance was accen-tuated by the subsequent emphasis placed on terraneanalysis (e.g. Gibbons, 1987, 1989), which overplaysthe role of map interpretation rather than cross-sectionalanalysis; in contrast, the unravelling of a Pacific-typeaccretionary margin requires a critical understanding
of cross-sectional structure based on detailed mapping.In the following sections, we outline the key tectonicprocesses involved in the formation of, and steps usedin the unravelling of, a Pacific-type accretionary com-

rian Re
plex, and the significance of Anglesey–Lleyn for thepalaeogeographic reconstruction of the Avalonian activecontinental margin.

The protoliths, not only of the Anglesey–LleynBlueschist Unit, but also the overlying and underlyingunits, are all accretionary in origin, having formed bysubduction of an oceanic plate.

7.1. Ocean plate stratigraphy (OPS) inaccretionary complexes

Ocean plate stratigraphy (MORB basalt–chert–hemi-pelagic mudstone–clastic turbidite–sandstone–conglo-merate) records the travel history of an oceanic platefrom a ridge to a trench (Isozaki et al., 1990; Matsudaand Isozaki, 1991), and in an accretionary orogen it maybe preserved in zeolite to granulite and blueschist faciesrocks (e.g. Kimura et al., 1996; Okamaoto et al., 2000).OPS has played a major role in working out the palaeo-history of many accretionary complexes in the westernand southwestern Pacific (e.g. Kimura et al., 1992;Wakita and Metcalfe, 2005), and of the early accretonarystage of collisional orogens of the Caledonides of Ireland(Ryan and Dewey, 2004), and the Himalaya of Tibet(Ziabrev et al., 2004). Although OPS may be presentat the top of an ophiolite, in most accretionary orogensonly the OPS is preserved, because the ultramafic rocks,gabbros and sheeted dykes are typically subducted inPacific-type plate margins, leaving only the peeled-offOPS to be accreted (Isozaki et al., 1990; Kimura andLudden, 1995). We wish to acknowledge the fact thatthe only author who previously identified and correctlyinterpreted ocean plate stratigraphy in Anglesey–Lleynwas Roberts (1979, p. 6), although he did not call it assuch. We have recognized numerous examples of OPSin Anglesey and Lleyn. In Japan and California the pre-dominant pelagic chert deposit of OPS in a trench isup 100–200 m thick, representing 100–200 m.y. dura-tion of travel time from a mid-oceanic ridge to a trench(Isozaki and Blake, 1994; Isozaki, 1996). In contrast, inAnglesey–Lleyn the best-preserved OPS has up to about5–10 m of bedded chert (intact and not reduced by defor-mation), suggesting a very short travel time in the oceanicdomain and buoyant subduction of a mid-oceanicridge immediately before the blueschist formation.Metabasalts in the Gwna Group of Anglesey–Lleyn haveMORB-type geochemistry (Thorpe, 1993).

The sense of younging of OPS is one of several critical
methods to estimate the polarity of subduction (Isozaki,1996; Maruyama, 1997), and on Anglesey–Lleyn suchyounging consistently suggests eastward subductionbefore and after exhumation of the Blueschist Unit.
search 153 (2007) 11–28

7.2. Top and bottom boundaries of a Blueschist Unit

In order to work out the tectonic framework and evo-lution of a Blueschist Unit, it is necessary to defineits top and bottom boundaries. Both boundaries of theBlueschist–Schist Unit crop out on Lleyn Peninsula, andthe top boundary on Anglesey. Although Gibbons (1987)defined the Menai Strait fault as a terrane boundary, itclearly does not separate rocks of different type and ori-gin, and has not displaced appreciably the upper andlower parts of the Schist Unit.

Fig. 9A shows a cross-section of the present-daystructure of the Blueschist–Schist Unit from Angle-sey to Lleyn. After removing the secondary high-anglefaults, Fig. 9B illustrates the overall original structureof the Schist Unit as a tectonic slab, after its exhuma-tion and after weak folding. A slab exhumed duringwedge extrusion in a subduction zone is floored by athrust and roofed by a normal fault (Maruyama, 1997).Fig. 9C shows the structural relations on the top andbottom boundaries of the high-pressure unit as it wasexhumed at the top of the subduction zone; it wasseparated from overlying and underlying rock unitsby an extensional fault and a thrust, respectively (seealso Fig. 10C). Note that the top extensional fault isstill preserved as such in eastern Anglesey (Fig. 9A),but that it has been rotated by the folding into apresent-day thrust in SW Lleyn (Fig. 9A). Domingand folding of a tectonic slab is to be expected in anaccretionary plate margin, because of buoyancy of thenewly accreted, juvenile, hydrated crust and of subse-quent compressional deformation (e.g. Maruyama et al.,2002).

7.3. Original nappe structure

Most blueschist and bordering units in accretionaryorogens are displaced by secondary normal faults(Maruyama et al., 1996). After removing the effects ofthe late high-angle faulting, the original nappe structureand tectonic stratigraphy within the subducted tectonicslab can be reconstructed. In Anglesey–Lleyn the nappepile has the following ascending units: an olistostrome-type accretionary complex, the BS-bearing unit, theGwna Group accretionary complex, and on top theHolland Arms gneisses (Fig. 9D), which we suggest con-tinue westwards as the Coedana gneisses of Anglesey(Fig. 10). We speculate that the Central Anglesey Schist
Unit is imbricated, repeated, but retrogressed examplesof the Blueschist Unit. We make this suggestion becauseof the remarkable similarity in lithology and internalstructure of these rock groups. We think that Greenly

rian Re

(mt‘mtlgt

vpgAd

imydinsaplaa

7

atufdpitsa(bas

7o

o


1919) realized this possibility because he noticeablyarked the Central Anglesey Schist Unit (Fig. 2) and

he Blueschist Unit (Fig. 3) with the same symbol, asPenmynydd Zone of Metamorphism’ on his geologicalap (this is British Geological Survey, 1980) and in his

ext. If this idea is correct, it would support our corre-ation of the Holland Arms gneisses with the Coedananeisses, which we have made independently on struc-ural grounds.

Note that all the units were metamorphosed underery low-grade conditions such as the zeolite orrehnite–pumpellyite facies, except for the high-gradeneisses at the structural top and the Blueschist Unit.ll four units are separated by low-angle faults that wereisplaced by high-angle secondary faults (Fig. 5).

The nappe structure can be explained by two stagesn the growth history of the Avalonian-early Caledonian

argin through plate subduction. Firstly, the units thatoung downwards were formed or inserted successivelyownwards by underplating in the accretionary wedgen the subduction zone on the hanging wall of Avalo-ia. Secondly, the Blueschist Unit was inserted into atructurally central position or intermediate level of thebove wedge. The BS unit was most likely exhumedreferentially from mantle depths to an upper crustalevel as a wedge during extrusion into the shallow-levelccretionary complex (Maruyama, 1990; Maruyama etl., 1996).

.4. Exhumation tectonics of the Blueschist Unit

The metamorphic assemblages, deformation fabricsnd senses of shear along the boundary planes betweenhe Blueschist Unit and the overlying and underlyingnits strongly indicate that the Blueschist Unit movedrom a mantle depth of ca. 35 km into an upper crustalepth of <5–6 km where the accretionary complex wasresent (Figs. 9C and 10C). Asymmetrical kinematicndicators on the top and bottom boundaries demonstratehat the Blueschist Unit was extruded as a solid tectoniclab to the northwest, where it had a thrust at its bottomnd an extensional fault at its top (Fig. 9C). Masago et al.2005) reported similar geometry in the Cretaceous San-agawa BS in SW Japan from contrasting senses of shearlong the top and bottom boundaries of the high-pressurelab.

.5. Role of Pacific-type orogeny in relation to the
rigin of continental crust in the UK
A Pacific-type orogeny creates firstly, huge volumesf new juvenile crust of MORB-, OIB-types that accrete

search 153 (2007) 11–28 25

to an active margin in a subduction–accretion complex,and secondly, a coeval calc-alkaline volcano-plutonicarc, such as the ca. 200 km-wide Cretaceous belt aroundthe Pacific. In principle therefore, the Anglesey–LleynBlueschist Unit should be accompanied by a coeval calc-alkaline magmatic arc to the east.

Since Dewey (1969), it has been widely acceptedthat southeastward subduction of oceanic lithospherefrom a NE–SW-trending plate margin sited somewherenorthwest of Anglesey gave rise to arc-type magma-tism in a 200 km-wide belt extending from westernWales to central England starting in the late Precam-brian (Thorpe, 1972, 1974; Thorpe et al., 1984; Pharaohet al., 1987; Gibbons and Horak, 1996; Horak et al.,1996) and continuing in the Ordovician in early Cale-donian times (Fitton and Hughes, 1970; Wood, 1974;Phillips et al., 1976; Fitton et al., 1982; Kokelaar, 1988;Woodcock and Strachan, 2000). We follow this longtradition, but add embellishments of a Japanese-typesubduction–accretion complex with exhumation of ahigh-pressure subducted slab, and of ridge subduction.

Oblique convergence led to the formation of strike-slip faults that imbricated and dismembered thesubduction system with the result that the Angleseyblueschists, belonging to the accretionary prism, werecaught between slivers of dismembered arc, as demon-strated by Gibbons and Horak (1996). Whilst we agreewith the above subduction–arc relationships, we con-sider that the structural relations between the blueschists,the accretionary units and the magmatic arc can be morerealistically interpreted in terms of a western Pacific ana-logue model, rather than by a displaced terrane model ata subduction front.

In Fig. 10, we illustrate four successive stages inthe accretion and tectonic evolution of the active con-tinental margin (Fig. 1). The earliest evidence for arcmagmatism is provided by the Malvern granodioriteintrusion at 677 ± 2 Ma (Fig. 10A), but the main periodwas at 620–600 Ma with formation of the Glinton,Orton and Arfon Group tuffs, the Sarn Complex dior-ite, and the Chaldecote diorite (Fig. 10B). The crustalmelt Coedana granite formed at 613 ± 2 Ma by subduc-tion ridge metamorphism. In the period 575–550 Ma(Fig. 10C) late magmatic arc rocks include the Ercallgranophyre, Uriconian rhyolite, Warren House rhyolitictuff, and Longmyndian bentonite and tuff (Fig. 1).In Tremadoc to Caradoc times arc volcanism tookplace across a wide extent of Northwestern to Central
Wales.
We suggest that westward wedge extrusion of theBlueschist Unit (Fig. 10C) was a result of a shallowingsubduction angle caused by the previously subducted

rian Re
buoyant mid-oceanic ridge (Fig. 10B) that facilitatedextrusion of the deep-seated, ductile, high-pressureaccretionary wedge to a shallow structural level at575–550 Ma.

Subsequent doming uplifted the Blueschist Unit andassociated rocks in central Anglesey, and gave rise tohigh-angle faults that cut and sliced the BS belt intolenses, and to high-angle secondary faults such as theMenai Strait fault and the Berw fault that have com-plicated the original structural relationships betweenthe Blueschist Unit, the subduction–accretion complex,and the coeval arc (Fig. 10D). The doming was likelycaused by underthrusting of crust to the northwest bythe youngest rocks to be accreted, the platform-type sed-iments of the South Stack Group that have youngestdetrital zircon ages of 501 ± 10 Ma (Collins and Buchan,2004). Subduction and underthrusting of the South StackGroup at the bottom of the accretionary complex jackedup the Blueschist Unit, and likely shifted the trench posi-tion farther northwestwards. The first, Caledonian-age,arc magmatism in NW and SW Wales (Fig. 1) was in theTremadoc, and arc activity continued through the Arenigand Llanvirn culminating in the Caradoc (Fig. 10D).

The apparent isolation of the high-grade gneisses andassociated 613 Ma Coedana granite of central Angleseyhas led to two main concepts concerning their tectonicorigin; as a basement to the accretionary “cover” rocks(Greenly, 1919), and as a suspect terrane (Gibbons andHorak, 1990). In contrast, we propose that these rocksbelong to a klippe that originally formed part of theAvalonian continent (Fig. 10D).

In summary, the tectonic events of the BS and asso-ciated rock units in Anglesey–Lleyn took place in thefollowing order: (1) evolution of ocean plate stratigraphyfrom ridge to trench, (2) formation of an accretionarycomplex in the footwall of the Avalonian continentalmargin, (3) development of the accretionary complex ofthe Gwna Group, (4) formation of an olistostrome-typeaccretionary complex, (5) subduction of accretionarymaterial to mantle depths to form blueschist–epidoteamphibolite facies metamorphism, and exhumation toan upper crustal level and (6) doming and related exten-sional brittle faulting.

Acknowledgements

We thank Margaret Wood for useful information anddiscussions on the geology of Anglesey. Field survey
and collection of rock samples used in this study werecarried out with Masanaka Watanabe, who is gratefullyacknowledged. We thank Reiko Hanada for encourage-ment.
search 153 (2007) 11–28

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