Geotectonic Framework of the Blueschist Unit on Anglesey–Lleyn, UK, and Its Role in the Development of a Neoproterozoic Accretionary Orogen T

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.Sawakia, Y. Isozaki e a Department of Earth and Planetary Sciences, Tokyo Institute of Technology, O-okayama 2-12-1, Meguro, Tokyo 152-8551, Japan b Department of Geology, The University of Leicester, Leicester LE1 7RH, UK c Department of Safety Systems Construction Engineering, Kagawa University, Kagawa 761-0396, Japan d 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 the southwest for more than 70 km along the northern coast of the Lleyn Peninsula. It has the shape of a shallow-dipping slab or sheet up to 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 originally a 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 assemblage was created by the tectonic insertion of the blueschist between the other units. Folding, particularly during late doming of the tectonic sandwich, rotated the top normal-fault boundary into a thrust. Later, high-angle, NE–SW-trending normal faults displaced the 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 blueschist belt via northern Wales to central England, suggesting that the subduction of oceanic lithosphere was to the southeast, and this is consistent with the extrusion of the thin high-pressure slab into the accretionary complex to the northwest. This took place on the western 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 boundary

1. Introduction

Blueschists (BS), locally with eclogites, form an ∗ Corresponding author. Tel.: +81 3 5734 2618; important component of many accretionary, Paciﬁc-type fax: +81 3 5734 3538. and collision-type orogens, and their study provides key E-mail address: [email protected] (T. Kawai). information not only on their metamorphic history, but

0301-9268/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.precamres.2006.11.002 12 T. Kawai et al. / Precambrian Research 153 (2007) 11–28 also on their structure, and accordingly on their subduc- blueschist amphiboles, Gibbons and Gyopari (1986) tion tectonics and polarity (e.g. Miyashiro, 1973, 1994; described an anticlockwise PT trajectory, and Gibbons Ernst, 1975, 1977). Blueschist–eclogite units were typ- (1981) described glaucophane on the adjacent Lleyn ically exhumed by a process of wedge extrusion as thin Peninsula. Dallmeyer and Gibbons (1987) reported an (ca. 1 km width) slabs that were emplaced as shallow- 40Ar/39Ar mineral age on phengite of 560–550 Ma for dipping structures over or into adjacent accretionary the blueschists which they interpreted as the metamor- wedges, the prime modern analogue of which is the phic crystallization age. In spite of these metamorphic Sanbagawa high-pressure metamorphic unit in Japan studies of the blueschists, and of structural-tectonic (Maruyama, 1990; Maruyama et al., 1996; Masago et investigations of these and associated rocks in Angle- al., 2005). Well-investigated, post-Cretaceous examples sey (Wood, 1974; Barber and Max, 1979; Shackleton, around the circum-Pacific demonstrate that wedge extru- 1975), the structural framework of the Anglesey–Lleyn sion was the result of a shallowing subduction angle blueschists is unknown, particularly with regard to that caused by an approaching buoyant mid-oceanic ridge of modern analogues. at the leading edge of the circum-Pacific continental Our recent investigations of the accretionary orogen margin, and was coincident with widespread, landward, of Anglesey–Lleyn have led to the definition of new min- calc-alkaline arc magmatism (Maruyama et al., 1996; eral isograds and metamorphic zones in the Blueschist Maruyama, 1997). Unit (Kawai et al., 2006), and in particular to a study Blake (1888) first discovered glaucophane in schists of the top and bottom boundaries of the high-pressure on the island of Anglesey, Wales (Fig. 1). Greenly (1919) unit, in order to work out its structural evolution fol- made the classic description of the BS unit (Fig. 2), lowing the deep-seated blueschist facies metamorphism. Gibbons and Mann (1983) first reported the presence The aim of this paper is to describe the structure of of lawsonite, Horak´ and Gibbons (1986) classified the the Anglesey–Lleyn Blueschist Unit, and to produce

Fig. 1. Simpliﬁed map of Wales and Central England showing location of magmatic arc rocks in outcops and boreholes. Squares refer to U–Pb zircon ages, interpreted to date the crystallization of plutonic or volcanic rocks (1–7, Tucker and Pharaoh, 1991; 8–13, Compston et al., 2002; 14, Horak´ et al., 1996; 15–16, Noble et al., 1993). Position of Fig. 2 indicated. T. Kawai et al. / Precambrian Research 153 (2007) 11–28 13

Fig. 2. Simpliﬁed geological map of Anglesey Island and Lleyn Peninsula showing the main tectonic units (modiﬁed after British Geological Survey, 1980). Location of Figs. 3–5 indicated. a new tectonic model that explains its subduction and the Coedana granite is mainly composed of mica schists exhumation history. These results have major implica- and minor metabasite lenses. tions for the tectonic environment of the Anglesey–Lleyn The Blueschist Unit extends NNE to SSW from the accretionary orogen on the active continental mar- eastern side of Anglesey Island to the southwestern end gin of Avalonia in the Neoproterozoic and early of the Lleyn Peninsula, a total distance of about 70 km Palaeozoic. (Fig. 2). On Anglesey the unit is bounded on its western and eastern sides by late steep, NE–SW-striking 2. The Anglesey–Lleyn Blueschist Unit Berw and Menai Strait (Fig. 3A) faults, respectively. We will demonstrate that the original attitude of the The main geological units on Anglesey and Lleyn are Blueschist Unit in relation to its bordering rocks was shown in Figs. 1 and 2. The Blueschist Unit on Anglesey sub-horizontal. At a small outcrop the top boundary is bordered on both sides by the accretionary complex of of the Blueschist Unit is overlain by unmetamorphosed the Gwna Group. The olistostromal New Harbour Group Gwna Group rocks and Holland Arms gneisses (Fig. 3B). and the passive margin sediments of the South Stack The bottom boundary crops out at the western end Group occur to the northwest, and the Coedana granite of the Lleyn Peninsula where schists are underlain is situated in the Central Anglesey gneisses. The Central by unmetamorphosed or low-grade rocks belonging Anglesey Schist Unit that occurs on the eastern side of to an accretionary complex. The three key areas, 14 T. Kawai et al. / Precambrian Research 153 (2007) 11–28

Fig. 3. (A) Metamorphic zonal map of the Blueschist Unit in Anglesey (Kawai et al., 2006). (B) Sketch of the boundary between Holland Arms gneisses, Gwna Group and Blueschist Unit. (C) Sketch of the boundary between the Arfon Group and Gwna Group. (D) Cross-section of the Blueschist Unit from D to D. where the boundaries can be defined, are described of crossite defines the beginning of Zone II. Zone I has below. a chlorite–epidote mineral assemblage. But, the basaltic lavas of the Gwna Group to the east have only chlorite, 3. The Blueschist Unit on Anglesey, and its top which we ascribe to thermal hydration metamorphism on boundary the ocean floor; i.e. they were not metamorphosed by the regional metamorphism that affected Zones I–III. These On Anglesey the metamorphic grade of the Blueschist chloritic basaltic lavas correspond to similar unmetamor- Unit has been sub-divided into three mineral zones: I, II phosed Gwna Group basaltic lavas in SW Lleyn. and III that are separated by crossite and barroisite iso- In Fig. 3D, the spatial distribution of the mineral grads (Fig. 3, after Kawai et al., 2006). The appearance zones, passing from east to west, indicates a structurally T. Kawai et al. / Precambrian Research 153 (2007) 11–28 15 downwards, westward increase of P and T from Zones I High-angle secondary faults trending NW and NE sep- to II towards a central structural horizon (Zone III), and arate the two groups of rocks. then a corresponding decrease farther westwards from The Gwna Group, best exposed on Porth Dinllaen III to I (Kawai et al., 2006). Peninsula, mostly consists of a 1 km-thick pile of pillow The Blueschist Unit (5 km × 25 km) in Zone III lavas, massive basaltic flows and minor pillow brec- contains lenses of well-preserved glaucophane-bearing cias, the pillows being largely undeformed. The lavas metabasic rocks up to 5 km long, as shown on the geolog- commonly contain inter-pillow lenses of red chert or ical map of Anglesey (British Geological Survey, 1980), limestone. Hydrothermal mineralization is recorded in the wall-rocks of which mostly consist of chloritic and sulphide-rich massive basalts and quartz veins. The fac- micaceous schists without glaucophane. Many metaba- ing directions of the pillows are shown in Fig. 4B. On sic lenses consist of hornblende schist, some of which the western side of Penrhyn Nefyn (Fig. 4A) pillow in less retrogressed areas contain glaucophane. Mineral lavas are associated with volcanic breccias. We correlate lineations defined by well-developed Na-amphibole on these undeformed volcanic rocks with comparable unde- foliation surfaces and crenulation fold axes trend NNE- formed pillow lavas and associated rocks of the Gwna SSW (Fig. 3A). Zone II (3 km × 11 km) on the eastern Group in southwest Lleyn (Figs. 7A and 8A) and on side of Zone III also contain lenses up to 1.5 km long of Llanddwyn Island (Fig. 3). glaucophane-bearing metabasic rocks. The Schist Unit has two main outcrops, eastern and The top boundary of the Blueschist Unit is exposed on western, and contains several types of schistose rocks. its eastern and western margins (Fig. 3). On the eastern In the eastern unit on Penrhyn Nefyn (Fig. 4A) the margin tuffs of the 614–572 Ma Arfon Group (Tucker most common lithology is a meta-volcanic basic schist and Pharaoh, 1991; Compston et al., 2002) rest above that consists of epidote, chlorite, actinolite, and albite a thrust on unmetamorphosed accretionary rocks of the (Gibbons, 2000) and contains relict massive basaltic eastern Gwna Group (Greenly, 1919)(Fig. 3C), which flows, deformed flattened pillows up to 5 cm across, and in turn rest on an extensional fault above the Blueschist blueschists in which crossite replaces actinolite as in the Unit. main blueschists of Anglesey (Gibbons, 1981; Kawai et On the western margin in a 200 m × 1000 m area al., 2006). Adjacent phengite schists (Fig. 4A) contain foliated, micaceous Holland Arms gneisses, which quartz and albite (Gibbons, 2000); we confirm the pres- have a Rb–Sr whole-rock isochron age of 595 ± 12 Ma ence of phengite. These glaucophane–phengite schists (Beckinsale and Thorpe, 1979), and NS-trending sub- belong to the northern end of the Blueschist Unit of horizontal isoclinal fold axes (Fig. 3B), are separated the Lleyn Peninsula shown in Fig. 2. Adjacent tonalitic from the Blueschist Unit by a high-angle extensional schists contain low strain zones with a well-preserved fault that is marked as thrust on Fig. 3B in its rotated tonalitic texture; Gibbons (2000) correlated these rocks position. Although the original relationship between with tonalite belonging to the nearby Sarn Complex. the Holland Arms gneiss and the Blueschist Unit is The western Schist Unit on the western side of Porth unexposed, the attitude of shallow-dipping rocks in Dinllaen peninsula (Fig. 4B) contains much chloritic the klippe suggests that surrounding, weakly metamor- schist, which we interpret as a mafic volcaniclastic sed- phosed accretionary rocks of the Gwna Group rest on iment. The schist contains many lenses up to 2 m long schists of the Blueschist Unit with a sub-horizontal tec- that demonstrate crude stratigraphic zones through the tonic contact, which means in turn that the high-grade schists, each some tens of metres thick. Passing from gneisses also overlie the Gwna Group with a tectonic east to west a zone with red chert lenses is followed suc- contact. Further lithological characteristics and struc- cessively by zones rich in sandstone lenses and then in tural relationships on this top boundary are well observed limestone lenses. We consider that the lenses are derived on the southwest Lleyn Peninsula to be described from broken up beds of chert, sandstone and limestone. below. In the top of this chloritic melange´ there is a 50-m block of homogeneous metabasalt, above which is a bedded 4. The Peninsulas of Porth Dinllaen and Penrhyn succession of turbidites and sandstones. We interpret Nefyn the western Schist Unit as accretionary rocks overlain by clastic sediments. Although the present contact of Gwna Group accretionary rocks and blueschist– the pillow lavas with the western Schist Unit is a steep phengite schists (Schist Unit) crop out on the Porth Din- fault, we presume that the original relationship was a sub- llaen Peninsula and Penrhyn Nefyn Peninsula on the horizontal fault, as seen on the SW Lleyn Peninsula, and northern coast of the Lleyn Peninsula (Fig. 4A and B). therefore that this is a top boundary of the Schist Unit. 16 T. Kawai et al. / Precambrian Research 153 (2007) 11–28

Fig. 4. (A) Geological map and cross-section of Penrhyn Nefyn Peninsula to the east of the Porth Dinllaen Peninsula. (B) Geological map and cross-section of the Porth Dinllaen Peninsula. T. Kawai et al. / Precambrian Research 153 (2007) 11–28 17

5. SW Lleyn Peninsula; bottom and top mudstone that contains relicts of metabasic greenschist boundaries of the Schist Unit (predominately basaltic tuff), sandstone, mudstone, and rare chert and limestone. Our geological map of the southwestern end of the The Schist Unit is separated from an underly- peninsula (Figs. 2 and 5) shows two blocks of the Schist ing unmetamorphosed olistostrome-type accretionary Unit (that do not contain glaucophane here), the struc- complex by a sub-horizontal, phyllonite-bearing thrust tural relationships of which with surrounding rocks are (Fig. 6A). The steeply dipping schists are truncated by described below. the sub-horizontal thrust. This is the bottom boundary of the Anglesey–Lleyn Blueschist Unit, as discussed 5.1. Bottom boundary in detail later. Fig. 6B and C shows thrusts near the boundary between the Schist Unit and the underlying A klippe of Zone I schist crops out on a 550 m-high rocks. hill that rises steeply from the sea at the western end of the Below the thrust the steeply dipping, unmeta- area shown in Fig. 5. It consists of steeply dipping slaty morphosed olistostrome-type melange´ (accretionary

Fig. 5. Geological map and cross-section of the southwest Lleyn Peninsula showing relationships at the bottom and top boundaries of the Schist Unit. See text for details. Positions of Figs. 6 and 7 indicated. On the cross-section, high-angle secondary faults have created a horst and graben with Ordovician sedimentary rocks to the east. The horst contains the bottom of the Schist Unit and an underlying olistostrome-type accretionary complex. The matrix of the latter consists of mudstone that contains olistoliths (exotic clasts) of basalt, limestone, quartzite and sandstone. The graben is composed of two units, a Gwna Group accretionary complex and the top of the Schist Unit. 18 T. Kawai et al. / Precambrian Research 153 (2007) 11–28

Fig. 6. Four photos showing structural relationships in southwest Lleyn at localities marked in Fig. 5. (A) Bottom thrust boundary of the Schist belt. 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 virtually unmetamorphosed, olistostrome-type accretionary complex. (B and C) Thrusts at the bottom of the Schist Unit that indicates thrusting to the north. View towards the east. (D) Quartzite clasts in a mudstone matrix in the olistostrome-type accretionary complex shown in Fig. 5.

complex) has a matrix of mafic mudstone that contains that have the effect of uplifting and exposing the bottom many, mostly 5–30 cm-size, lenses of quartzite (max- boundary of the Schist Unit in a horst (Fig. 5). imum is 17 m × 25 m; Fig. 6D), red chert, dolomitic limestone, and basaltic greenstone (Fig. 6D). The olis- 5.2. Top boundary tostromal mudstone melange´ passes stratigraphically downwards to ca. 35 m-thick sandstone that makes up The top boundary of the Schist Unit is exposed in the the controversial Gwyddel Beds. Many authors (e.g. bay of Parwyd Llech-y-menyn (Fig. 5); Fig. 7 shows the Shackleton, 1975; Roberts, 1979) described these as geological map and Fig. 8 some relevant photographs. bedded acidic tuffs. However, from our observations Gwna Group rocks are here thrust over mafic chloritic in the field and in thin section we conclude they are schists without glaucophane belonging to Zone I of the bedded sandstones, in agreement with Gibbons and Schist Unit (Figs. 5 and 7A). McCarroll (1993). The bedded sandstones are under- The top boundary surface of the Schist Unit is a lain conformably by a well-preserved, but small, section ca. 50 m-thick thrust duplex that dips 45◦ northwest displaying ocean plate stratigraphy. A 1 m-thick bed of (Fig. 8B). Several parallel or divergent northerly dipping hemi-pelagic mudstone passes downwards to 7 m of bed- thrusts within the Gwna Group are in the hanging wall of ded red chert (marked on Fig. 5) that is underlain by a the thrust duplex (Fig. 7A). Fig. 8C and D shows minor 1 m-thick basalt (not marked on Fig. 5). south-vergent thrusts within the major thrust duplex. All the main units described above are transected by The Gwna Group largely consists of pillow basalts, secondary high-angle extensional faults (Figs. 5 and 6A) at least 20 m thick (Figs. 7B and 8A) that are overlain T. Kawai et al. / Precambrian Research 153 (2007) 11–28 19

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 of Pared Llech-y-menyn, as shown on Fig. 5. Position of photos of Fig. 8 indicated. The map shows many thrusts that have imbricated the Gwna Group accretionary complex in the hanging wall of the main thrust at Pared Llech-y-menyn. (B) Geological columnar section of primary ocean plate stratigraphy that has been imbricated in the Gwna Group. by 2 m of brown pelagic limestone succeeded by 5 m of British Geological Survey, 1994; Gibbons, 2000), The red-bedded chert; this represents ocean plate stratigraphy eastern boundary of the Parwyd gneiss is marked by (Fig. 7B). The Gwna Group is located in a graben that a sub-vertical fault that has a small amount of thrust has subsided differentially with respect to its neighbour- movement that has brought up the gneisses (section of ing horst blocks, thereby preserving the top boundary. Fig. 5). We suggest that an original thrust, probably sub- The western margin of the Gwna Group is occupied by sidiary to the major thrust on the western side of the bay, a secondary, high-angle NS-striking normal fault that was reactivated by the normal fault at the eastern end dips 70◦ east, marking the western side of the graben, of the graben, but insufﬁciently to remove all the thrust whereas the eastern margin is occupied by a major thrust offset. that dips 45◦ west. On the eastern side of Parwyd bay We agree with Gibbons (1983) who considered that (Fig. 5) a 30 m-wide strip of retrogressed garnetiferous the Schist Unit is comparable to the Blueschist Unit on gneiss (the Parwyd gneiss) is overlain unconformably Anglesey and showed that it extends along the Lleyn by Ordovician sediments (Matley, 1928; Roberts, 1979; Peninsula to the schists on Penrhyn Nefyn. The upper 20 T. Kawai et al. / Precambrian Research 153 (2007) 11–28

Fig. 8. Four photos of key features at localities on the coast in southwest Lleyn marked in Fig. 7. (A) Undeformed pillow basalt in the Gwna Group. (B) Top boundary of the Schist Unit which is a thrust that dips 45◦ northwestward at the bay of Pared Llech-y-menyn. Looking to the NE. There are several parallel or divergent faults in the hanging wall (Fig. 7A). (C and D) Southeast-verging minor thrust duplexes a few metres above the major thrust in (B). schists in Fig. 5 contain clasts of limestone, basaltic 6. The magmatic arc lava, quartzite, red shale and chert. The schists increase in metamorphic grade eastwards from maﬁc chloritic Fig. 1 shows key geological outcrops and boreholes schists at Pared Llech-y-menyn to phyllites, and ulti- with their isotopic ages in Wales, the Welsh borderland mately to micaceous and basic, actinolitic schists at and English Midlands. We emphasize the many calc- Trwyn Bychestyn headland—see Fig. 5 (Matley, 1928; alkaline igneous rocks that belong to this 200 km-wide Shackleton, 1956; Gibbons, 2000). This increase in magmatic arc that occurs as inliers within Phanerozoic grade is downwards from the top towards the centre of cover sedimentary rocks. Reliably dated rocks in this arc the Schist Unit. formed in four main stages (Gibbons and Horak,´ 1996). Combining the overall geometry of this top boundary with that of the bottom boundary on Anglesey and Lleyn 6.1. 680–670 Ma (Fig. 9A), we conclude that, when the Blueschist-bearing Schist Unit moved from depth towards the surface, it was Early arc magmatism. The Malvern Complex mainly juxtaposed at a crustal level with the overlying Gwna comprises calc-alkaline granodiorite, diorite, tonalite Group along an extensional fault and with the underlying and granite (Thorpe et al., 1984), and has a U/Pb zir- olistostrome-type accretionary complex along a thrust con age of 677 ± 2 Ma and a U–Pb monazite age of (Fig. 9B). 670 ± 10 Ma (Tucker and Pharaoh, 1991). T. Kawai et al. / Precambrian Research 153 (2007) 11–28 21

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 reconstructed the original tectonic stratigraphy that has been folded after exhumation. The three oblong blocks refer to the areas where we observe the present-day structures marked directly above in (A). (C) Cartoon showing the exhumation of the BS unit as it was transported from SE to NW in the upper subduction 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 boundary became 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). 22 T. Kawai et al. / Precambrian Research 153 (2007) 11–28

Fig. 10. Simplified cross-sections along a NW–SE profile from Anglesey through Wales to Central England marked in Fig. 1, to illustrate the geodynamic evolution of the active western margin of Avalonia from about 680–450 Ma. See text for more details. (A) First, minor magmatism in the Malverns at 677 Ma. (B) Main arc activity at 620–600 Ma at Sarn, Arfon, Glinton, Orton and Chaldecote. The crustal melt Coedana leuco-granite formed as a result of ridge subduction. (C) A ductile wedge of an accretionary complex, a few kilometres thick, was subducted, recrystallized under blueschist facies conditions, and then exhumed to a mid-upper crustal level, possibly assisted by a shallowing subduction angle. Metamorphic zones and isograds indicate that the wedge has an antiformal isoclinal structure. (D) The South Stack and New Harbour Groups were underthrust into the base of the accretionary wedge during final stages of subduction in the Cambrian. This underthrusting led to doming of the already accreted complex, 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. T. Kawai et al. / Precambrian Research 153 (2007) 11–28 23

6.2. 620–600 Ma itic basalt–andesite–rhyolite association in the Warren House volcanic group (Thorpe et al., 1984) has a U–Pb Rocks that formed in this main period of arc mag- zircon age of 566 ± 2Ma(Tucker and Pharaoh, 1991); matism are: (a) the Sarn Igneous Complex in the Lleyn (d) in the English Midlands the Charnian Supergroup Peninsula that consists of calc-alkaline monzogranite, contains felsic tuffs, andesites and dacites, agglomer- gabbro, granodiorite, diorite, and tonalite and has a ates and granodioritic intrusions (Pharaoh et al., 1987). 207Pb/206Pb zircon age of 615 ± 2Ma (Gibbons and Compston et al. (2002) obtained SHRIMP zircon ages Horak,´ 1996; Horak´ et al., 1996); (b) the 4 km-thick on tuffs of 566 ± 3 and 559 ± 2.0 Ma. volcaniclastic Arfon Group in NW Wales that contains clasts of Monian (Anglesey) metasedimentary rocks and 6.4. Tremadoc–Caradoc (ca. 488–448 Ma) is overlain by fossiliferous Cambrian slates (Shackleton, 1975; Horak´ et al., 1996). A rhyolitic ash-flow tuff has Arc volcanoes, associated with at least six major rhy- a U–Pb zircon age of 614 ± 2Ma(Tucker and Pharaoh, olitic calderas, resurgent and collapsed calderas, two 1991), and an ignimbrite has a SHRIMP zircon age of andesitic stratovolcanoes, Strombolean cones, and intra- 605 ± 1.6 Ma (Compston et al., 2002); (c) felsic tuffs in arc graben, developed on already accreted crust at a the Glinton and Orton boreholes have U–Pb zircon ages destructive plate margin in Wales (Gibbons, 1998). They of 616 ± 6 and 612 ± 21 Ma, respectively (Noble et al., gave rise to voluminous basic, intermediate and particu- 1993); (d) the Coedana granite on Anglesey (Greenly, larly felsic rocks with an overall calc-alkaline chemistry 1919) is a leuco-granite that contains muscovite, biotite in the late Tremadoc in NW Wales and in the late and garnet, retains locally a pre-granite foliation, lacks Tremadoc or early Arenig in SW Wales(Kokelaar, 1988). a chilled margin, has granitic veins rich in garnet, and In the Arenig to Caradoc bimodal basalt-rhyolite lavas close to its contact with mica-tourmaline hornfels the erupted in a back-arc basin in central Wales, but calc- granite in places contains tourmaline. Greenly (1919) alkaline arc-type lavas erupted again in the Llanvirn to concluded that “the affinities of the Coedana granite Caradoc on the southeastern side of the Welsh basin to some metamorphic granites are close”. In contrast, (Woodcock and Strachan, 2000). Although subsequent Gibbons and Horak´ (1996) suggested that the Coedana late Caledonian deformation inverted fractures inherited granite was a product of arc magmatism. We suggest from the basement, created slate belts, and shortened the that it is a crustal melt granite, the high temperature for width of the early Palaeozoic arc belt, it did not destroy which was provided by ridge subduction (Fig. 10B). The the primary volcanic structures in the arc rocks, and it did granite has a U–Pb zircon age of 613 ± 4Ma(Tucker not affect sufficiently the distribution of the magmatic and Pharaoh, 1991); (e) the Chaldecote volcanic rocks rocks to prevent recognition of the arc (Kokelaar, 1988). with bedded tuffs, similar to those of the Charnian Supergroup (Pharaoh et al., 1987), are intruded by 7. Discussion a diorite at Nuneaton that has a U–Pb zircon age of 603 ± 2Ma(Tucker and Pharaoh, 1991). In the first plate tectonic synthesis of the geotectonic development of the British Isles, Dewey (1971) con- 6.3. 575–550 Ma sidered, reasonably, that the Caledonian and Hercynian collisional orogenies were the most important events Late arc magmatic rocks include: (a) a tuff in the responsible for formation of the continental crust. In the volcaniclastic Arfon Group in NW Wales (Shackleton, following decades later plate tectonic models similarly 1975) that has a SHRIMP U–Pb zircon age of regarded collisional orogeny as predominant, because 572 ± 1.2 Ma (Compston et al., 2002); (b) in the Welsh the role of accretionary orogens was little appreciated borderland a rhyolitic lava in the Uriconian Group internationally at that time. This imbalance was accen- consisting of basalts, basaltic andesites, dacites and rhy- tuated by the subsequent emphasis placed on terrane olites (Thorpe et al., 1984) has a U–Pb zircon age of analysis (e.g. Gibbons, 1987, 1989), which overplays 566 ± 2 Ma, and the Ercall granophyre that intrudes the role of map interpretation rather than cross-sectional felsic lavas has a U–Pb zircon age of 560 ± 1Ma analysis; in contrast, the unravelling of a Pacific-type (Tucker and Pharaoh, 1991). Bentonite and tuff in the accretionary margin requires a critical understanding Longmyndian Supergroup (Pauley, 1990) have SHRIMP of cross-sectional structure based on detailed mapping. zircon ages of 567 ± 2.9 and 556 ± 3.5 Ma, respectively In the following sections, we outline the key tectonic (Compston et al., 2002); (c) in the Malvern Hills a processes involved in the formation of, and steps used rhyolitic tuff occurring within a calc-alkaline tholei- in the unravelling of, a Pacific-type accretionary com- 24 T. Kawai et al. / Precambrian Research 153 (2007) 11–28 plex, and the significance of Anglesey–Lleyn for the 7.2. Top and bottom boundaries of a Blueschist Unit palaeogeographic reconstruction of the Avalonian active continental margin. In order to work out the tectonic framework and evo- The protoliths, not only of the Anglesey–Lleyn lution of a Blueschist Unit, it is necessary to define Blueschist Unit, but also the overlying and underlying its top and bottom boundaries. Both boundaries of the units, are all accretionary in origin, having formed by Blueschist–Schist Unit crop out on Lleyn Peninsula, and subduction of an oceanic plate. the top boundary on Anglesey. Although Gibbons (1987) defined the Menai Strait fault as a terrane boundary, it 7.1. Ocean plate stratigraphy (OPS) in clearly does not separate rocks of different type and ori- accretionary complexes gin, and has not displaced appreciably the upper and lower parts of the Schist Unit. Ocean plate stratigraphy (MORB basalt–chert–hemi- Fig. 9A shows a cross-section of the present-day pelagic mudstone–clastic turbidite–sandstone–conglo- structure of the Blueschist–Schist Unit from Angle- merate) records the travel history of an oceanic plate sey to Lleyn. After removing the secondary high-angle from a ridge to a trench (Isozaki et al., 1990; Matsuda faults, Fig. 9B illustrates the overall original structure and Isozaki, 1991), and in an accretionary orogen it may of the Schist Unit as a tectonic slab, after its exhuma- be preserved in zeolite to granulite and blueschist facies tion and after weak folding. A slab exhumed during rocks (e.g. Kimura et al., 1996; Okamaoto et al., 2000). wedge extrusion in a subduction zone is floored by a OPS has played a major role in working out the palaeo- thrust and roofed by a normal fault (Maruyama, 1997). history of many accretionary complexes in the western Fig. 9C shows the structural relations on the top and and southwestern Pacific (e.g. Kimura et al., 1992; bottom boundaries of the high-pressure unit as it was Wakita and Metcalfe, 2005), and of the early accretonary exhumed at the top of the subduction zone; it was stage of collisional orogens of the Caledonides of Ireland separated from overlying and underlying rock units (Ryan and Dewey, 2004), and the Himalaya of Tibet by an extensional fault and a thrust, respectively (see (Ziabrev et al., 2004). Although OPS may be present also Fig. 10C). Note that the top extensional fault is at the top of an ophiolite, in most accretionary orogens still preserved as such in eastern Anglesey (Fig. 9A), only the OPS is preserved, because the ultramafic rocks, but that it has been rotated by the folding into a gabbros and sheeted dykes are typically subducted in present-day thrust in SW Lleyn (Fig. 9A). Doming Pacific-type plate margins, leaving only the peeled-off and folding of a tectonic slab is to be expected in an OPS to be accreted (Isozaki et al., 1990; Kimura and accretionary plate margin, because of buoyancy of the Ludden, 1995). We wish to acknowledge the fact that newly accreted, juvenile, hydrated crust and of subse- the only author who previously identified and correctly quent compressional deformation (e.g. Maruyama et al., interpreted ocean plate stratigraphy in Anglesey–Lleyn 2002). was Roberts (1979, p. 6), although he did not call it as such. We have recognized numerous examples of OPS 7.3. Original nappe structure in Anglesey and Lleyn. In Japan and California the predominant pelagic chert deposit of OPS in a trench is Most blueschist and bordering units in accretionary up 100–200 m thick, representing 100–200 m.y. dura- orogens are displaced by secondary normal faults tion of travel time from a mid-oceanic ridge to a trench (Maruyama et al., 1996). After removing the effects of (Isozaki and Blake, 1994; Isozaki, 1996). In contrast, in the late high-angle faulting, the original nappe structure Anglesey–Lleyn the best-preserved OPS has up to about and tectonic stratigraphy within the subducted tectonic 5–10 m of bedded chert (intact and not reduced by defor- slab can be reconstructed. In Anglesey–Lleyn the nappe mation), suggesting a very short travel time in the oceanic pile has the following ascending units: an olistostrome- domain and buoyant subduction of a mid-oceanic type accretionary complex, the BS-bearing unit, the ridge immediately before the blueschist formation. Gwna Group accretionary complex, and on top the Metabasalts in the Gwna Group of Anglesey–Lleyn have Holland Arms gneisses (Fig. 9D), which we suggest con- MORB-type geochemistry (Thorpe, 1993). tinue westwards as the Coedana gneisses of Anglesey The sense of younging of OPS is one of several critical (Fig. 10). We speculate that the Central Anglesey Schist methods to estimate the polarity of subduction (Isozaki, Unit is imbricated, repeated, but retrogressed examples 1996; Maruyama, 1997), and on Anglesey–Lleyn such of the Blueschist Unit. We make this suggestion because younging consistently suggests eastward subduction of the remarkable similarity in lithology and internal before and after exhumation of the Blueschist Unit. structure of these rock groups. We think that Greenly T. Kawai et al. / Precambrian Research 153 (2007) 11–28 25

(1919) realized this possibility because he noticeably to an active margin in a subduction–accretion complex, marked the Central Anglesey Schist Unit (Fig. 2) and and secondly, a coeval calc-alkaline volcano-plutonic the Blueschist Unit (Fig. 3) with the same symbol, as arc, such as the ca. 200 km-wide Cretaceous belt around ‘Penmynydd Zone of Metamorphism’ on his geological the Pacific. In principle therefore, the Anglesey–Lleyn map (this is British Geological Survey, 1980) and in his Blueschist Unit should be accompanied by a coeval calc- text. If this idea is correct, it would support our corre- alkaline magmatic arc to the east. lation of the Holland Arms gneisses with the Coedana Since Dewey (1969), it has been widely accepted gneisses, which we have made independently on struc- that southeastward subduction of oceanic lithosphere tural grounds. from a NE–SW-trending plate margin sited somewhere Note that all the units were metamorphosed under northwest of Anglesey gave rise to arc-type magma- very low-grade conditions such as the zeolite or tism in a 200 km-wide belt extending from western prehnite–pumpellyite facies, except for the high-grade Wales to central England starting in the late Precam- gneisses at the structural top and the Blueschist Unit. brian (Thorpe, 1972, 1974; Thorpe et al., 1984; Pharaoh All four units are separated by low-angle faults that were et al., 1987; Gibbons and Horak,´ 1996; Horak´ et al., displaced by high-angle secondary faults (Fig. 5). 1996) and continuing in the Ordovician in early Cale- The nappe structure can be explained by two stages donian times (Fitton and Hughes, 1970; Wood, 1974; in the growth history of the Avalonian-early Caledonian Phillips et al., 1976; Fitton et al., 1982; Kokelaar, 1988; margin through plate subduction. Firstly, the units that Woodcock and Strachan, 2000). We follow this long young downwards were formed or inserted successively tradition, but add embellishments of a Japanese-type downwards by underplating in the accretionary wedge subduction–accretion complex with exhumation of a in the subduction zone on the hanging wall of Avalo- high-pressure subducted slab, and of ridge subduction. nia. Secondly, the Blueschist Unit was inserted into a Oblique convergence led to the formation of strike- structurally central position or intermediate level of the slip faults that imbricated and dismembered the above wedge. The BS unit was most likely exhumed subduction system with the result that the Anglesey preferentially from mantle depths to an upper crustal blueschists, belonging to the accretionary prism, were level as a wedge during extrusion into the shallow-level caught between slivers of dismembered arc, as demon- accretionary complex (Maruyama, 1990; Maruyama et strated by Gibbons and Horak´ (1996). Whilst we agree al., 1996). with the above subduction–arc relationships, we consider that the structural relations between the blueschists, 7.4. Exhumation tectonics of the Blueschist Unit the accretionary units and the magmatic arc can be more realistically interpreted in terms of a western Pacific ana- The metamorphic assemblages, deformation fabrics logue model, rather than by a displaced terrane model at and senses of shear along the boundary planes between a subduction front. the Blueschist Unit and the overlying and underlying In Fig. 10, we illustrate four successive stages in units strongly indicate that the Blueschist Unit moved the accretion and tectonic evolution of the active con- from a mantle depth of ca. 35 km into an upper crustal tinental margin (Fig. 1). The earliest evidence for arc depth of <5–6 km where the accretionary complex was magmatism is provided by the Malvern granodiorite present (Figs. 9C and 10C). Asymmetrical kinematic intrusion at 677 ± 2Ma(Fig. 10A), but the main period indicators on the top and bottom boundaries demonstrate was at 620–600 Ma with formation of the Glinton, that the Blueschist Unit was extruded as a solid tectonic Orton and Arfon Group tuffs, the Sarn Complex dior- slab to the northwest, where it had a thrust at its bottom ite, and the Chaldecote diorite (Fig. 10B). The crustal and an extensional fault at its top (Fig. 9C). Masago et al. melt Coedana granite formed at 613 ± 2 Ma by subduc- (2005) reported similar geometry in the Cretaceous San- tion ridge metamorphism. In the period 575–550 Ma bagawa BS in SW Japan from contrasting senses of shear (Fig. 10C) late magmatic arc rocks include the Ercall along the top and bottom boundaries of the high-pressure granophyre, Uriconian rhyolite, Warren House rhyolitic slab. tuff, and Longmyndian bentonite and tuff (Fig. 1). In Tremadoc to Caradoc times arc volcanism took 7.5. Role of Pacific-type orogeny in relation to the place across a wide extent of Northwestern to Central origin of continental crust in the UK Wales. We suggest that westward wedge extrusion of the A Pacific-type orogeny creates firstly, huge volumes Blueschist Unit (Fig. 10C) was a result of a shallowing of new juvenile crust of MORB-, OIB-types that accrete subduction angle caused by the previously subducted 26 T. Kawai et al. / Precambrian Research 153 (2007) 11–28 buoyant mid-oceanic ridge (Fig. 10B) that facilitated References extrusion of the deep-seated, ductile, high-pressure accretionary wedge to a shallow structural level at Barber, A.J., Max, M.D., 1979. A new look at the Mona Complex, 575–550 Ma. Anglesey, North Wales. J. Geol. Soc. Lond. 136, 407–432. Beckinsale, R.D., Thorpe, R.S., 1979. 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