Tectonomagmatism in continental arcs: evidence from the Sark arc complex

Tectonophysics 352 (2002) 185 – 201 www.elsevier.com/locate/tecto

Tectonomagmatism in continental arcs: evidence from the Sark arc complex Wes Gibbons*, Teresa Moreno1 Department of Earth Sciences, Cardiff University, Cardiff CF10 3YE, Wales, UK Received 27 January 2001; accepted 12 December 2001

Abstract The island of Sark (Channel Islands, UK) exposes syntectonic plutons and country rock gneisses within a Precambrian (Cadomian) continental arc. This Sark arc complex records sequential pulses of magmatism over a period of 7 Ma (ca. 616 – 609 Ma). The earliest intrusion (ca. 616 Ma) was a composite sill that shows an ultramafic base overlain by a magma-mingled net vein complex subsequently deformed at near-solidus temperatures into the amphibolitic and tonalitic Tintageu banded gneisses. The deformation was synchronous with D2 deformation of the paragneissic envelope, with both intrusion and country rock showing flat, top-to-the-south LS fabrics. Later plutonism injected three homogeneous quartz diorite – granodiorite sheets: the Creux – Moulin pluton (150 – 250 m; ca. 614 Ma), the Little Sark pluton ( > 700 m; 611 Ma), and the Northern pluton (>500 m; 609 Ma). Similar but thinner sheets in the south (Derrible – Hogsback – Dixcart) and west (Port es Saies – Brecqhou) are interpreted as offshoots from the Creux – Moulin pluton and Little Sark pluton, respectively. All these plutons show the same LS fabric seen in the older gneisses, with rare magmatic fabrics and common solid state fabrics recording syntectonic crystallisation and cooling. The cooling rate increased rapidly with decreasing crystallisation age: >9 Ma for the oldest intrusion to cool to lower amphibolite conditions, 7 – 8 Ma for the Creux Moulin pluton, 5 – 6 Ma for the Little Sark pluton, and < 3 Ma for the Northern pluton. This cooling pattern is interpreted as recording extensional exhumation during D2. The initiation of the D2 event is suggested to have been a response to the intrusion of the Tintageu magma which promoted a rapid increase in strain rate (>10 14 s 1) that focussed extensional deformation into the Sark area. The increased rates of extension allowed ingress of the subsequent quartz diorite – granodiorite sheets, although strain rate slowly declined as the whole complex cooled during exhumation. The regional architecture of syntectonic Cadomian arc complexes includes flat-lying ‘‘Sark-type’’ and steep ‘‘Guernsey-type’’ domains produced synchronously in shear zone networks induced by oblique subduction: a pattern seen in other continental arcs such as that running from Alaska to California. D 2002 Published by Elsevier Science B.V. Keywords: Continental arc; Syntectonic plutonism; Cadomian; Channel Islands

1. Introduction *

Corresponding author. Tel.: +44-29-2087-4330; fax: +44-292087-4326. E-mail address: [email protected] (W. Gibbons). 1 Present address: School of Biosciences, Cardiff University, Cardiff CF10 3US, Wales, UK.

This paper describes an example of Cadomian syntectonic magmatism from the island of Sark (Channel Islands, UK) and offers insight into the interactions between, and relative rates of, mid-crustal processes within Andean-style continental arcs. There is con-

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tinuing interest in the links between crustal deformation and syntectonic magmatism, in both collisional (e.g., Brown and Solar, 1999) and arc (e.g., McCaffrey et al., 1999) settings. A common connection between deformation, pluton ascent, and emplacement is well established, although the relative dominance of the mechanisms involved, the rates at which these processes occur, and the time taken to complete tectonic and magmatic events are still debated (e.g., Paterson and Tobisch, 1992; Tobisch et al., 1995; Bodorkos et al., 2000). The crystallisation of plutons intruded into actively deforming crust can be isotopically dated to within a million years or so. Despite the accuracy of such data, however, the actual time involved in the freezing of a magma through its rheologically critical melt percentage to full crystallisation is likely to be much less (104 – 105 years) than the error bars on isotopic dates (Paterson and Tobisch, 1992; Karlstrom et al., 1993). Uncertainty similarly exists over the length of time required for individual deformation events. Given ‘‘typical’’ crustal orogenic strain rates of 10 13 –10 15 s 1, estimates of the time taken to complete individual strain events range from 107 to 105 years (Piffner and Ramsay, 1982). Higher strain rates, more typical of mylonitic crustal shear zones (White and Mawer, 1992), will impose deformation fabrics over shorter timescales more comparable to those required for pluton crystallisation. Thus, a U –Pb date from one ‘‘syntectonic’’ pluton reveals little about the length of time over which deformation occurred in the intrusion and its envelope. A comparison between the rates of magmatic versus deformation processes becomes more feasible when several successive intrusion events are involved (e.g., Bodorkos et al., 2000). Polyphase syntectonic plutonism is commonplace within arc settings where magma supply is normally prolonged, episodic, and linked to tectonic controls such as strike slip faulting at oblique margins, or regional extension induced by subduction rollback. In the Sark arc complex, successive calcalkaline plutonic sheets intruded a deforming gneissic envelope during a single deformation event in which strain rate apparently progressively declined. Whereas the earliest intrusion was deformed into mylonitic orthogneisses at near-solidus temperatures, the latest intrusion shows plutonic textures modified by only a weak LS fabric. The evidence for this, and the timing of the processes involved, are described below.

2. Sark geology Sark lies in the English Channel (La Manche) within the archipelago of the Channel Islands (Fig. 1, inset). Although politically part of the UK, geologically the Channel Islands belong to the Cadomian rocks of NW France and lie within an elongate terrane running NE – SW from Cap de la Hague in Normandy to Tregor in Brittany (the Tregor – La Hague Terrane of Strachan et al., 1990: Fig. 1, inset). Throughout most of the Cadomian outcrop in NW France the oldest exposures are Late Precambrian calc-alkaline plutons and volcanics, arc basin sediments (Brioverian), and metamorphic rocks ranging up to migmatite grade and associated with trans-arc shear zones (see D’Lemos et al., 1990a and references therein). The Tregor – La Hague terrane is slightly different and especially significant for two main reasons. Firstly, it is the only area to preserve relics of Paleoproterozoic (>2 Ga) metamorphic basement (Adams, 1967, 1976; Calvez and Vidal, 1978; Auvray et al., 1980; Samson and D’Lemos, 1998). This pre-Cadomian basement is commonly referred to as Icartian, after the 2061 F 2 Ma metagranitoid Icart gneiss on Guernsey (Samson and D’Lemos, 1998). Secondly, the islands provide continuous seacliff exposures of syntectonic and posttectonic calc-alkaline plutons ranging from gabbro to granite, including spectacular examples of magmamingled composite intrusions (Elwell et al., 1962; Roach, 1971; Topley et al., 1982, 1990; Brown et al., 1990; Power et al., 1990; Power and Gibbons, 1994). Despite its small size, the island of Sark exposes one of the most varied metamorphic complexes to be found in the entire Cadomian Orogen (Fig. 1). Like the neighbouring islands of Guernsey and Alderney, Sark contains excellent examples of Late Precambrian (‘‘early Cadomian’’), foliated, grey plutons of quartz dioritic to granodioritic composition (Power et al., 1990). Unlike the other islands, however, Sark lacks post-tectonic Cadomian intrusives and instead preserves a metamorphic envelope of gently dipping paragneissic country rocks into which the foliated plutons were intruded. These semi-pelitic paragneisses include layers and lenses of basic amphibolitic orthogneisses sometimes interlayered with leucogneisses (Fig. 2A), and containing deformed ultramafic bodies. Both orthogneisses, and foliated plutons are essentially sheet-like in form and parallel

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Fig. 1. Geological map of Sark.

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Fig. 2. Tintageu banded orthogneiss. (A) Typical banded gneiss showing strong, flat banding of leucogneiss and amphibolite. Protolith igneous textures have been destroyed by tectonic transposition, Port a la Jument. (B) Isoclinal folding of boudinaged tonalitic component within banded gneiss, Pegane Bay. (C) Oblique foliation lenses indicating top-to-the-south shearing, Telegraph Bay. (D) Lens of mafic protolith preserving igneous texture within banded gneiss, Les Autelets.

to the main foliations within the paragneissic envelope. The Sark paragneisses show an early fabric (S1) folded and locally re-foliated by flat or gently dipping/ plunging D2 structures (Sutton and Watson, 1957; Gibbons and Power, 1975; Tribe et al., 1996). Although areas of relatively low strain and psammitic composition (such as at Banquette, Gibbons and Power, 1975) locally preserve bedding, most of the gneisses show migmatitic banding intensely folded by F2. Rootless isoclines, pervasive gently plunging L2 lineations, and top-to-the-south asymmetric kinematic indicators (Tribe et al., 1996) all testify to strong D2 shearing deformation. Similar structures, such as isoclinally folded leucocratic bands (Fig. 2B) and asymmetric kinematic indicators (Fig. 2C) are also seen in the Sark amphibolitic orthogneisses. Most exposures of these orthogneisses show a strong deformational

overprint, producing banded amphibolites containing no remnant of igneous textures (Fig. 2A), although this is sometimes preserved in less deformed competant mafic lenses (Fig. 2D). These orthogneisses are commonly mylonitic in texture, with strong LS fabrics and asymmetric top-to-the-south kinematic indicators (Fig. 2C). The later quartz diorite – granodiorite Sark plutonic sheets are mostly less deformed but they too usually display foliations and lineations, and (rarely) flatlying folds that are parallel to the D2 structures in their gneissic envelope (Fig. 3A). Locally, the plutons crosscut similar, but more intense, fabrics in adjacent orthogneisses (and orthogneiss xenoliths), an observation that led to differences of interpretation over the timing of intrusion relative to regional D2 (Power and Gibbons, 1980; Tribe et al., 1996). The general consensus now is that these plutons were intruded at

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Fig. 3. Textures within the syntectonic quartz dioritic Creux – Moulin pluton. (A) Strong, sub-solidus foliation at the base of the intrusive sheet, Les Burons. (B) Deformed enclave within foliated Creux – Moulin pluton, west side of Les Burons. (C) Deformed xenolith of Tintageu banded orthogneiss, Port du Moulin.

various stages during a protracted D2 Cadomian event (cf. Tribe and D’Lemos, 1996; Miller et al., 1999). The polydeformed nature of the Sark paragneisses, combined with observations such as xenoliths of folded, banded amphibolite immersed in some of the Sark quartz diorites (Fig. 3B), led to the complex being viewed as comprising Cadomian plutons intruding a much older basement. Early publications thus interpreted the undated Sark gneisses as part of an ancient ‘‘Pentevrian’’ basement more than 2 Ga old (Gibbons and Power, 1975). The argument for the antiquity of the Sark gneisses was weakened by a subsequent undoing of the case for a ‘‘Pentevrian’’ event in the type locality of Penthievre in Brittany (D’Lemos et al., 1990b). Increasing recognition that other supposedly ancient (>2 Ga) metamorphic rocks throughout the Cadomian area were in fact Late

Precambrian in age, continued to undermine the case for an ancient basement on Sark. An40Ar/39Ar hornblende plateau age of 607.2 F 1.2 Ma for a Sark amphibolitic banded orthogneiss was interpreted as dating post-metamorphic cooling after Cadomian deformation (Dallmeyer et al., 1991; Tribe et al., 1996). This view was subsequently supported by a 1.6 Ga Nd model age for the same Sark orthogneiss, and confirmed by the publication of high quality zircon and titanite dates (Samson and D’Lemos, 1998; Miller et al., 1999). These dates showed that not only are the Sark tonalitic and granodioritic rocks Neoproterozoic (ca. 609 –614 Ma), but the same is true of the leucogneissic component of the banded orthogneiss (ca. 616 Ma). On the other hand, confirmation of a >2-Ga Icartian age from the type locality on Guernsey (Samson

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and D’Lemos, 1998) has kept alive the possibility that Palaeoproterozoic basement exists on Sark. Metagranitoid augen gneisses, some of which are very similar to the Guernsey Icart gneiss, are known from two localities (both difficult to access) in eastern Sark, in the bay north of Maseline Jetty, and on the rocky islet of Les Burons (Gibbons, 1975). These Sark metagranitic gneisses, and the main paragneissic envelope that forms much of the island, remain undated. If ‘‘Sark Icartian’’ rocks do exist, however, they cannot now include the amphibolitic orthogneisses which must have been formed and deformed during the Cadomian orogeny (Dallmeyer et al., 1991; Samson and D’Lemos, 1998; Miller et al., 1999). The recently published U – Pb zircon data demand a re-examination of these amphibolitic gneisses, little detailed work on which has been published since their description by Sutton and Watson (1957).

3. Amphibolitic orthogneisses Concordant, metric to decimetric amphibolite bands within paragneisses occur at several levels within the Sark arc complex (Sutton and Watson, 1957; Gibbons, 1975; Fig. 2). The most prominent of these units is the Tintageu banded orthogneiss which occurs as a flat, up to 75-m thick sheet, underlain by foliated quartz– diorite (Creux –Moulin pluton) and overlain by migmatitic paragneisses. It is exposed in the coastal cliffline from Maseline Bay to Derrible Point on the southeast, and from Les Autelets to Port a la Jument in the northwest (Fig. 1). This unit is composite and comprises: (i) ultramafic serpentinite lenses; (ii) hornblendite and amphibolite, (iii) intermediate feldspathic leucogneiss, (iv) leucotonalite – leucogranodiorite. The ultramafic component of the Tintageu banded orthogneiss occurs only at the base of the unit, and is now preserved as boudin-like, deformed and metamorphosed lenses up to 10 m long and surrounded mostly by leucogneiss. These are the ‘‘ultramafic balls’’ of Sutton and Watson (1957) and typically contain a remnant nucleus of antigorite and chrysotile (interpreted as serpentinised dunite) surrounded by alteration zones rich in talc, actinolite, and chlorite. They are best exposed in Pegane Bay, south of Port du Moulin (Fig. 1), but occur at several localities along

the east and west coasts (Gibbons, 1975). The hornblendic component, which is normally the dominant lithology, is best viewed around the type locality of Tintageu on the northwest coast (Fig. 1). It varies from dark green hornblendite to hornblende – feldspar amphibolite that sometimes preserves deformed microgabbroic textures within low strain lenses (Fig. 2D). It is this component that yielded the 40Ar/39Ar hornblende plateau age of 607.2 F 1.2 Ma (Dallmeyer et al., 1991). The pale, fine to medium grained leucogneiss component is present throughout the unit, although is particularly abundant near the base and very well exposed on the sea stack of Tintageu. This is the ‘‘recrystallised mylonite. . .tonalitic. . .Tintageu leucogneiss’’ of Miller et al., 1999 (equivalent to the ‘‘Port du Moulin leucogneiss’’ of Samson and D’Lemos, 1998, collected from the north side of Pegane Bay), and has yielded a U – Pb zircon date of 616 + 42 Ma. The fourth component of the Tintageu banded orthogneiss is also leucocratic but more obviously igneous, commonly preserving undeformed plutonic textures. This granitoid component is usually aligned in bands parallel to the amphibolitic and leucogneissic components, but can be seen also to crosscut them (Fig. 4A). As with most exposures of Sark amphibolites, the Tintageu banded orthogneiss is pervasively deformed, with a strong, flat-lying D2 foliation (Fig. 2A). Locally, the fabric is intense enough to be fairly described as mylonitic (cf. Samson and D’Lemos, 1998; Miller et al., 1999), and includes a prominent N –S trending mineral lineation that is parallel to the L2 fabric in the paragneissic envelope (Tribe et al., 1996). Given the recent U – Pb data on the Tintageu banded orthogneisses it is now clear that both this mylonitic shearing deformation in the orthogneisses and the D2 deformation of the overlying paragneisses are Cadomian (i.e., Neoproterozoic) in age. The ultramafic lenses occurring along the base of the Tintageu banded orthogneiss are interpreted as boudins of deformed and metamorphosed olivinebearing early fractionates within a layered sill. The mixture of tonalitic leucogneiss and mafic amphibolite could also reflect original igneous layering, or the heterogeneity of two magmas existing prior to their crystallisation and deformation. Samson and D’Lemos (1998) suggested that the protolith was likely a netveined or igneous sheeted complex, but concluded

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Fig. 4. Remnants of igneous texture within the Tintageu banded orthogneiss, Les Autelets. (A) Granitoid vein both cross cuts and parallels preexisting gneissic banding. (B) Granitoid veining invading low strain area between deforming boudins. (C) Intimate mingling between mafic and intermediate magmas preserved in area of relatively low strain. (D) Mingled mafic-intermediate textures with more mobile granitoid component showing lobate margins with more viscous mafic component.

that the deformational overprint at their sample locality (near Tintageu on the north side of Pegane Bay) has eradicated any definitive evidence for the origin of these rocks. This observation holds true for most exposures of Sark amphibolitic orthogneisses, but in a few places a combination of low strain and the preservation of detailed textures in wave-washed rocks allows a view of the igneous protolith. These clearly indicate that the crystallisation of the Tintageu sheet involved magma mingling followed by deformation while temperatures were still high enough to allow some melt to exist. Microgabbroic and micromeladioritic textures are sometimes preserved, albeit deformed, in mafic boudins (Fig. 2D), the areas of low strain between which have attracted granitoid melt (Fig. 4B). These granitoid veins represent the youngest igneous event, and both cut and run parallel to the fabric in their orthogneissic host rock (Fig. 4A). In many places, however,

this same granitoid component is deformed along with the earlier gneissic components. A window into the nature of the early, untectonised protolith to the orthogneisses is revealed by Fig. 4C in which the mafic and intermediate components display the complex textures typical of mingled magmas in composite, net-veined intrusions. Fig. 4D shows how the later, granitoid component has intruded through the previously crystallised, but still very hot, mafic-intermediate host. Irregular, lobate contacts between the intrusive granitoid veins and the slightly older mafic material provide clear evidence for little rheological contrast between the two lithologies. These textures are strongly reminiscent of the mingled igneous relationships found in the post-tectonic Cadomian Bordeaux Diorite Complex in neighbouring Guernsey, 10 km to the west (Topley et al., 1990). In these Bordeaux rocks, the lack of a deformational overprint has preserved exquisite textures var-

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iously recording mutual mobility, mingling and mixing of meladioritic to leucodioritic, tonalitic and (less commonly) granodiorite magmas (Brown et al., 1980; Topley et al., 1982). Magmas partially mechanically mixed before freezing into the ‘‘Inhomogenous Suite’’ of the Bordeaux Diorite Complex (Brown et al., 1980) produced the same complex patchy textures seen in the protolith to the Tintageu banded orthogneiss (Fig. 4C). The Bordeaux rocks also show in abundance the same lobate, crenulate, cuspate and piped contacts between viscous melanocratic crystal mushes and more mobile leucocratic components preserved only rarely in the Tintageu banded orthogneisses (Fig. 4D). The Sark amphibolitic orthogneisses are therefore interpreted as early Cadomian mafic to intermediate composite intrusions that underwent a top-to-thesouth shearing deformation while they remained close to their solidus temperature. Textures such as those recording granitoid melt migrating into the area between mafic boudins indicates deformation took place in the presence of at least some melt. Whether the wall rock was being deformed during the initial intrusion of the Tintageu banded orthogneiss is unknown, although it is reasonable to invoke a tectonic mechanism operating to allow magma ingress. What is clear is that once intruded and more-or-less crystallised, the hot, layered and mingled sill became a focus for strong deformation during which the composite igneous textures were transposed into banded gneiss. The eradication of original igneous textures by tectonic transposition of composite plutons is a welldocumented phenomenon (compare, for example, Fig. 6 in Bodorkos et al., 2000). High temperature, pervasive transposition of composite, mingled igneous intrusions produces strongly deformed banded orthogneisses with similar (or even more intense) fabrics to those of their original envelope. Similarly, although in a different tectonic setting, the hot obduction of the Lizard ophiolite (Cornwall, UK) induced footwall melting, magmatic mingling, and deformation to produce the Kennack Gneiss (Sandeman et al., 2000). Such gneiss units can be difficult to interpret, as illustrated by past controversies over the origin of the Kennack Gneiss. In the same way, the Sark amphibolitic orthogneisses have been previously described incorrectly as pre-Cadomian ‘‘Pentevrian’’ (Gibbons and Power, 1975), or ‘‘Icartian’’ (Tribe et

al., 1996) basement. Instead, these rocks represent the first Cadomian magmatic event in the Sark arc complex and their emplacement was swiftly followed by strong shearing deformation. This deformation was the so-called ‘‘D2’’ event traditionally defined within the Sark and Guernsey gneisses: in fact, D2 in the paragneisses is D1 in the orthogneisses.

4. Foliated quartz diorite – granodiorite plutons The intermediate, early Cadomian Sark plutons form three main sheets (Creux – Moulin, Little Sark and Northern plutons) that intrude an orthogneissic and paragneissic envelope (Fig. 5). In addition to these three plutons there are further sheets on the south coast (Derrible Bay, Hogs Back and Dixcart Bay), on the west coast (Port es Saies, Telegraph Bay, Moie du Mouton), and on the neighbouring island of Brecqhou (Fig. 1). The Dixcart – Hogs Back –Derrible sheets are interpreted to be the same body, forming a transgressive sill that is connected to the underlying Creux– Moulin pluton (Fig. 5). The west coast and Brecqhou quartz diorites are interpreted as northward continuations of the Little Sark sheet (Fig. 1). The structurally lowest and oldest sheet is the 150- to 200-m thick Creux –Moulin pluton which lies immediately below the Tintageu banded orthogneisses. It is well exposed along the southeast coast around Creux Harbour, from Maseline Jetty to the east side of Derrible Point, and along the northwest coast from Les Autelets and beyond Port du Moulin to Port a la Jument (Fig. 1). The gently west-dipping base of this biotite – hornblende quartz diorite is only exposed on the outlying rocky archipelago of Les Burons to the east of Creux Harbour (Fig. 1). The contact on Les Burons shows the pluton to have intruded underlying paragneisses as a concordant sheet (Fig. 6A). The host rocks are amphibolitic orthogneiss and biotite granitoid gneiss underlain by recrystallised mylonitic biotite gneiss with feldspar augen (Fig. 6B), in turn, underlain by banded paragneiss with minor amphibolite. All these rocks, but particularly the annealed mylonitic zone close to the overlying quartz– diorite, display a strong LS fabric (Fig. 6B) with gently north plunging mineral lineations and rare top-to-the-south kinematic indicators in feldspar augen.

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Fig. 5. Composite section looking west and illustrating the architecture of the Sark arc complex. Several plutonic sheets intrude a gneissic envelope. The oldest sheet (616 Ma) was deformed into the Tintageu banded orthogneiss before the intrusion of the Creux – Moulin pluton (614 Ma), the Little Sark pluton (611 Ma), and the Northern pluton (609 Ma). The undated Hogsback sheet is tentatively correlated with the Creux – Moulin pluton. Beneath the Creux – Moulin pluton lies a thin ( < 10 m) layer of deformed granitoid gneiss interpreted as a partially melted, deformed, and recrystallised country rock contact. Granitoid, mylonitic augen gneisses above the Tintageu banded orthogneiss may have a similar origin or could be pre-Cadomian.

This country rock LS fabric is equivalent to the D2 event described on Sark by previous workers (e.g., Tribe et al., 1996), and is also seen in the overlying quartz diorite (Fig. 3A). Tribe and D’Lemos (1996) interpreted the microfabrics from this pluton (their ‘‘Port du Moulin sheet’’) as recording solid state deformation and recrystallisation. This fabric is found pervading all exposures of the Creux – Moulin pluton, but is most intense near its margins. Thin ( < 1 m), vein-like offshoots of the quartz – diorite sometimes intrude the overlying amphibolitic orthogneisses (Tintageu banded orthogneiss), as seen north of Port du Moulin, and these have proved especially susceptible to strong solid state deformation.

Mafic enclaves occur within the Creux – Moulin plutonic sheet, especially near the base (in Les Burons) and near the top (e.g., Les Laches, just south of Creux Harbour, and Port du Moulin). Whereas some of these are small, deformed, comagmatic enclaves that appear unrelated to the nearby wall rock (Fig. 3B), others show folded banded orthogneiss fabrics clearly derived from the adjacent Tintageu banded orthogneiss (Fig. 3C). On Les Burons, the quartz diorite has engulfed a sheet of the orthogneiss which thins northwards and disappears into a series of xenolithic lenses isolated within the pluton. In at least one place along the basal contact, thin, discontinuous remnants of amphibolite lie between the quartz diorite and the

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Fig. 6. (A) Geological map of Les Burons (see Fig. 1 for location). The west dipping quartz dioritic Creux – Moulin pluton shows a lower contact with country rock gneisses. Sheet-like and lensoid amphibolitic xenoliths of the Tintageu banded orthogneiss lie within the quartz diorite. (B) The contact between paragneiss and quartz diorite shows a zone of mylonitic granitoid gneiss interpreted as annealed mylonitic migmatites.

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underlying paragneisses (Fig. 6). The most spectacular example of the entrainment of disrupted orthogneissic xenoliths occurs within the Hogs Back sheet (Fig. 5) where the intrusive quartz diorite is crowded with blocks of the deformed amphibolite. Complex interaction between the two lithologies, such as veining, partial assimilation of the Tintageu banded orthogneiss, and the preferential growth of hornblende crystals at the contacts between amphibolite and quartz diorite, suggests high temperature interaction between the two lithologies (Gibbons, 1975; Power and Gibbons, 1980). The Little Sark pluton is also quartz dioritic in composition and forms the thickest of the intrusive sheets exposed on Sark (>700 m). It has intruded at a higher structural level within the Sark arc complex (Fig. 5) and its top is not exposed. An exception to its generally uniform quartz dioritic composition occurs along the base of the sheet where a more granitoid facies up to 7 m thick separates the main pluton from the underlying paragneisses. It also contains an abundant population of comagmatic enclaves including long slivers of microdiorite and smaller, more mafic pods aligned along the foliation. The west dipping, north plunging LS fabric is concordant with the D2 fabric in the underlying paragneisses, but is generally weaker than that within the Creux– Moulin pluton. Microstructures in this pluton show magmatic textures reworked by continued deformation at temperatures below the solidus (Tribe et al., 1996). The NNW dipping Northern Sark pluton (>500 m) is broadly similar to that of Little Sark, containing a similar enclave population, but is slightly more compositionally evolved (quartz diorite to granodiorite) and is the least foliated of all the Sark intrusive sheets, locally with unfoliated pegmatite veins cutting the main plutonic fabric (Power and Gibbons, 1980).

5. Tectonomagmatic evolution From the above description it is clear that four main Cadomian intrusive sheets invaded the Sark paragneisses. The most deformed of these intrusions is the Tintageu banded orthogneiss which is clearly older than the foliated Creux – Moulin quartz diorite – granodiorite sheet. The remaining two quartz diorite – granodiorite sheets have intruded at a higher level and

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are thicker and less deformed. The Northern Sark pluton is the most evolved and least foliated of the three quartz diorite – granodiorite sheets. The field evidence for the relative ages of intrusion is therefore entirely consistent with the U –Pb zircon dates published by Samson and D’Lemos (1998) and Miller et al. (1999). The crystallisation of the composite igneous protolith to the Tintageu banded gneiss took place at 615.6 + 4.2 2.3 Ma. This was followed in turn by the intrusion of the Creux – Moulin pluton (613.5 + 2.3 1.5 Ma), the Little Sark pluton (613 + 2.3 1.5 Ma), and the Northern pluton (608.7 + 1.1 0.7 Ma). Whereas it is theoretically possible that the deformation of each of these igneous intrusions represents a separate tectonic event, this is considered extremely unlikely. All fabrics in the igneous rocks are concordant with the D2 structures in the paragneissic envelope. Furthermore, the earliest pluton is the most deformed, and successively later plutons are progressively less deformed (Tribe, 1994). The most obvious interpretation is therefore one of a progressive ‘‘D2’’ event in which strain rate declined from the extreme, hot deformation of the Tintageu banded orthogneiss to the mild deformation of the rapidly cooling Northern pluton. The time period during which the four plutonic sheets intruded and crystallised was around 7 Ma. Solid state ductile and brittle deformation continued beyond this time, as recorded in the youngest plutons by boudinaged, microcracked hornblende and plagioclase, dynamically recrystallised quartz (and locally feldspar), and shear plane biotite aggregates (Tribe and D’Lemos, 1996). Given the fact that the construction of the Sark arc complex was syntectonic, questions clearly arise over how plutonism and deformation may have been linked. The evidence from Sark indicates that intrusion and crystallisation of the composite mafic-intermediate Tintageu magma at ca. 616 Ma initially produced delicate magma-mingled textures subsequently overprinted by strong deformation. This deformation started before the intrusion of the Creux – Moulin pluton at ca. 614 Ma, while the rocks were still hot enough to allow minor amounts of leucocratic magma to exist within the deforming pile: leucotonalitic veins are both deformed by, and crosscut, the main deformational fabric. Some of the most intense fabrics associated with this deformation are found within the Tintageu orthogneisses themselves,

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suggesting that top-to-the-south shearing was preferentially focused into the hot, recently crystallised sill. Estimates for the rate at which mafic to intermediate magmas move through the crust to their crystallisation site indicate that such events are geologically very fast ( < 1 Ma). Paterson and Tobisch (1992) suggest likely average rates of upward magma movement are around 2 m year 1 and that at such rates moderately sized plutons would intrude and freeze in a few tens of thousands of years at most. By comparison, much longer timescales are required for regional deformation at orogenic strain rates (ca. 10 14 s 1: Piffner and Ramsay, 1982). Shearing deformation producing 50 – 70% shortening at such strain rates would take a few (2 –4) million years (Paterson and Tobisch, 1992). However, much higher strain rates like those more typical of mylonitic zones (10 12 s 1 or greater) would accomplish the same deformation in 20,000 – 40,000 years or less. The intense, commonly mylonitic LS fabric imposed on most of the Tintageu banded orthogneiss is more in accord with such strain rates. The overview interpretation for the Tintageu igneous protolith that emerges from these considerations is one involving emplacement, crystallisation and deformation in rapid, overlapping succession over a period of much less than 1 Ma. The

specific local cause for initial intrusion of the Tintageu magma is unknown. Presumably, on a regional scale, subduction-generated, mantle-derived magma intraplated the crust in response to far field effects such as oblique plate convergence and/or slab rollback. The input of a hot magmatic sheet, such as that represented by the Tintageu rocks, into the crust is likely to lead to a dramatic, albeit transient, increase in strain rate (Sandiford et al., 1991). It is suggested that the intrusion of the Tintageu magma triggered, or at least accelerated, the D2 deformation event seen on Sark (Fig. 7A). Within this model, the intruding sheet surpasses its rheologically critical melt percentage and freezes in its final resting place. The crystallising pluton conducts heat into the envelope, in places heating the surrounding paragneisses beyond their solidus to produce a narrow contact zone of granitoid gneisses (such as seen on Les Burons) and migmatites (such as in Derrible Bay). This creates a hot, weak, flat layer within the crust into which the regional strain becomes focussed. Thus, the intrusion sows the seed of its own subsequent deformation and enhances local orogeny. The localised acceleration of strain rate within the arc by a vanguard pluton such as the Tintageu sheet creates the opportunity for continued magma uprise.

Fig. 7. (A) Time – deformation (t – d) path illustrating the tectonomagmatic evolution of the Sark arc complex. Initial injection of the Tintageu magma (T) heats the crust and induces a rapid increase in strain rate, triggering a D2 orogenic event in the country rock envelope. The newly crystallised Tintageu plutonic protolith becomes transposed into a banded orthogneiss (TBO). Subsequent syntectonic intrusions at ca. 613 Ma (Creux – Moulin pluton: C), ca. 611 (Little Sark pluton: L), and ca. 608 Ma (Northern pluton: N) transiently re-heat the complex and re-energise the D2 event during generally waning strain rate and exhumation. (B) Temperature – time (T – t) path illustrating the cooling paths of the four Cadomian intrusive events on Sark represented in A. Cooling rate increases rapidly with decreasing age of pluton: a phenomenon attributed to extensional exhumation during D2. (figure developed from ideas and data in White and Mawer, 1992; Dallmeyer et al., 1991; Samson and D’Lemos, 1998, and Miller et al., 1999).

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Large volumes of more homogeneous calc-alkaline magma opportunistically invade the deforming, dilating area, creating a thick stacked sill complex. Magmatic thickening of the crust would compete with deformation-driven extension. Structural analysis of Sark and Guernsey shows that D2 was a response to oblique dextral shear that produced both steep (Guernsey-type) and flat (Sark-type) belts (Tribe et al., 1996). A component of dilational transtension during this shearing would have provided the space necessary for the sill complex. On Sark, the first such sill was that of the Creux– Moulin pluton, which intruded beneath and mostly parallel to the older, by now deformed (but still deforming) Tintageu banded gneiss. In places the Creux – Moulin pluton broke through the overlying orthogneiss, entraining masses of hot xenoliths such as those frozen within the Hogs Back quartz diorite sheet. Having crystallised, the Creux –Moulin pluton continued to deform in the solid state at temperatures higher than those of the greenschist facies for over 7 Ma, and, in doing so, developed a strong post-magmatic foliation. This foliation, although pervasive and especially strong at the sill margins, is much less intense than that within the overlying Tintageu banded gneiss. This is interpreted as recording a slowing of strain rate from a pre-614 Ma maximum as the intrusions and their envelope slowly cooled. Immediately below the Creux – Moulin pluton high strain fabrics within the (once partially melted) granitoid and augen gneisses became progressively annealed. The arrival of the much thicker Little Sark pluton, intruding at a higher level and recording still less strain, took place around 611 Ma, as the Creux – Moulin and Tintageu rocks were still cooling through amphibolite facies temperatures. Another 2 – 3 Ma saw the intrusion of the Northern pluton, the youngest, least deformed, and most evolved of the plutonic sheets preserved on Sark. Titanite ages on the three quartz– diorite – granodiorite sheets all lie around 606 Ma (Miller et al., 1999), indicating that the time taken for each of the Sark intrusions to cool to around 550 jC decreases with decreasing age (Fig. 7B). The oldest and structurally deepest intrusions remained well above greenschist facies conditions for longer: >9 Ma for Tintageu banded orthogneiss, ca. 7 – 8 Ma for the Creux– Moulin pluton; ca. 5– 6 Ma for the Little Sark pluton; < 3 Ma for the Northern pluton. Titanite and 40Ar/39Ar data predictably record cooling

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rates to have declined with time for each intrusion (Dallmeyer et al., 1991; Miller et al., 1999). The Creux – Moulin pluton cooled to mid-amphibolite conditions at crudely averaged rates of around 36 jC/Ma, whereas equivalent rates for the Little Sark and Northern plutons are estimated as 48 jC/Ma and 95 jC/Ma. Much higher average cooling rates for the younger plutons in a series of magmatic pulses is a phenomenon noted in other arcs (e.g., Renne et al., 1993). Cretaceous plutons in central Sierra Nevada, for example, intruded over a period of around 12 Ma with the oldest intrusions taking 10 Ma to cool to around 500 jC, younger ones taking 5 – 6 Ma, and the youngest taking 1– 3 Ma (Tobisch et al., 1995). The underlying cause for the rapid cooling of such arc plutonic complexes is interpreted to be due to exhumation accompanying plutonism (Pickett and Saleeby, 1993; Renne et al., 1993; Tikoff and de Saint Blanquat, 1997). In the case of Sark, an exhumation rate of 1– 2 mm/year, like that calculated for the deeper levels of the Sierra Nevada Batholith (Pickett and Saleeby, 1993), would have brought the sill complex >7 Km closer to the surface between the initial intrusion of the Tintageu orthogneiss protolith and the final crystallisation of the Northern pluton. Other calculations for synplutonic exhumation in continental arcs suggest even faster rates may be common (e.g., Renne et al., 1993).

6. Arc complex architecture The Sark arc complex offers a window into the deforming mid-crust of a continental arc during a phase of calc-alkaline mafic to intermediate syntectonic plutonism. It reveals the Cadomian arc to have been constructed by the same processes active in more recent and better documented examples such as the huge Mesozoic continental arc running from SE Alaska to Baja California (e.g., Tobisch et al., 1995; Brown et al., 2000). Plutonic phases within such arcs commonly last from 5 to 10 Ma and are broadly syntectonic. In the southern Coast Belt of British Columbia, for example, plutonic phases at 104 –95 and 92 – 84 Ma produced sheet-like intrusions during oblique plate convergence (Brown et al., 2000). A particularly illuminating comparison may be drawn between Sark and the Khyex sill complex of

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Fig. 8. Block diagram placing Sark in the context of the younger, better preserved, and closely comparable arc setting of the Khyex Sill Complex (KSC) in British Columbia (Crawford et al., 1999). The 67 – 63 Ma KSC forms a gently dipping zone of sheeted syn-D2 mafic to intermediate sills up to 2 km thick and separated by orthogneiss and paragneiss screens. To the west the complex steepens into the gneisses and steep tabular plutons ( < 500 m) of the coeval Arden Lake Plutonic Complex (A) comparable to the early Cadomian geology of Guernsey (see text). To the east the KSC is sheared against gabbro – tonalite – granodiorite plutons of the Quottoon Plutonic Complex (QPC). The KSC is interpreted as emplaced within a network of ductile shear zones during arc transtension (Crawford et al., 1999).

the central Coastal Mountains in British Columbia (Crawford et al., 1999). The Khyex exposures comprise a series of gently dipping sills of coarse grained tonalite, leucotonalite and granodiorite (with minor gabbro) separated by screens of paragneiss and orthogneiss (Fig. 8). Individual sills measure up to 2 km thick and the complex was intruded over a period of at least 4 Ma (67 –63 Ma: Crawford et al., 1999). On a large scale, the sill complex runs parallel to the country rock structure, although locally individual plutons cut the surrounding gneissic fabric. The gneissic screens are overprinted by a D2 L –S fabric that is also present in the sills which show a range of magmatic to subsolidus microstructures. As with Sark, subsolidus structures are more prominent near the intrusion margins. Fig. 8 places an outline of Sark within the context of this better preserved, younger continental arc exposure. Note that the Khyex sill complex lies within a network of regional, sinistral, ductile shear zones that juxtapose it to the west against a steeply dipping terrane known as the Arden Lake

plutonic complex. The latter exposures comprise steep to vertical, north-striking orthogneisses and paragneiss that host tabular plutons up to 500 m thick. There is a striking similarity between this geology and that of southern Guernsey (Tribe et al., 1996). It is argued that whereas the Khyex sill complex belongs to a flat lying ‘‘Sark-type’’ arc terrane, the Arden Lake plutonic complex belongs to a steep ‘‘Guernsey-type’’ arc terrane. The emplacement of the Khyex sill complex has been interpreted as having been accompanied by arcparallel transtensional shearing (Crawford et al., 1999). The connection between strike slip faulting and plutonism, especially if aided by slab rollback, is now well established as a mechanism to allow magma to move upward from the crust – mantle interface. Various models invoked to offer space for upwardly invading magma include tensional cracks and extensional jogs along strike slip faults (Hutton, 1982, 1992; Glazner, 1991), tensional dilation between ‘‘P’’ shears (Tikoff and Teyssier, 1992), extensional

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duplexes insected by feeder dykes (Grocott et al., 1994), or some more subtle interplay of several such processes (Tobisch et al., 1995). The architecture of the Sark plutons belongs to a sub-type of arc magmatic complexes in which successive, shallow dipping sills intrude and thicken the crust above obliquely subducting oceanic plates (cf. southern Coast Belt of British Columbia: Brown et al., 2000). Such sill complexes are presumably fed by dykes in contiguous, deforming steep zones and are typically ‘‘syntectonic’’ in that they intrude an actively deforming crustal envelope.

7. Conclusions (1) The Sark arc complex comprises a series of Neoproterozoic (Cadomian) calc-alkaline mafic to intermediate plutonic sheets emplaced into a gneissic envelope of unknown age. The oldest (616 Ma) of these sheets has been deformed into the Tintageu banded orthogneiss, previously interpreted as ancient (Icartian) basement. The existence of Icartian (>2 Ga) ages within the metasedimentary and metagranitoid gneissic envelope on Sark remains possible but unproven. (2) There were four main Cadomian intrusive pulses on Sark, these taking place over a period of around 7 Ma. These intrusions are, from oldest to youngest, the Tintageu magmatic protolith (now banded gneiss ca. 75 m thick), the Creux –Moulin pluton (150 – 250 m thick; 614 Ma old), the Little Sark pluton (>700 m thick; 611 Ma old), and the Northern pluton (>500 m thick; 609 Ma). Thinner sheets that crop out in the south (Derrible –Hogs Back– Dixcart) and west (Port es Saies – Brecqhou) are interpreted as offshoots from the Creux– Moulin pluton and Little Sark pluton, respectively. (3) Whereas the youngest three plutons (Creux – Moulin, Little Sark, and Northern) comprise essentially homogeneous quartz diorite –granodiorite, the oldest intrusion (Tintageu) has a composite ultramafic – mafic – intermediate composition. A basal ultramafic unit (now boudinaged and serpentinised) is interpreted as a fractionally crystallised dunitic base to a layered sill. Above this, coeval mafic and intermediate magmas record mutual mobility, mingling to produce complex textures that were subsequently tectonically transposed into banded gneisses. This

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deformation occurred soon after bulk crystallisation, while the intrusive sill was still hot enough to allow minor melting of a leucocratic component that both crosscut and was deformed by the developing tectonic fabrics. (4) All Cadomian intrusions on Sark exhibit flat LS fabrics that are parallel to those produced by a coeval ‘‘D2’’ event in the paragneissic envelope. The most intense fabrics are exhibited by the Tintageu rocks, which commonly have the appearance of mylonitic gneisses. The subsequent three more homogeneous intrusions are progressively less deformed with decreasing age. Magmatic and high temperature solid state fabrics are only rarely seen, partly because of the overprint of continued ductile – brittle deformation under lower amphibolite and greenschist facies conditions. (5) Cooling of the Sark intrusions took place more rapidly with decreasing age. Whereas the oldest intrusion took >9 Ma to cool into lower amphibolite facies, successive pulses taking less time to reach a titanite blocking temperature estimated at around 550 jC: 7– 8 Ma for the Creux –Moulin pluton, 5 –6 Ma for the Little Sark pluton, and < 3 Ma for the Northern pluton. The cause for this is suggested to be extensional exhumation over a ‘‘D2’’ event that lasted around 10 Ma. (6) It is suggested that the intrusion of the maficintermediate Tintageu sheet produced a hot, weak layer in the crust the presence of which induced a rapid, transient increase in strain rate (>10 14 s 1). This model interprets the emplacement of the initial intrusion to have kick-started a local orogenic event that focussed crustal extension into the Sark arc complex. The increased rates of extension allowed subsequent injection of more evolved (and thicker) quartz dioritic – granodioritic sheets as strain rate slowly declined. (7) The ‘‘Sark-type’’ arc complex, comprising a series of deformed concordant plutonic sheets invading flat lying country rock gneisses, is seen in other examples of continental arcs, such as the Mesozoic Khyex sill complex of British Columbia. By comparison, a ‘‘Guernsey-type’’ arc complex represents steep zones within the continental arc (cf. the Arden Lake plutonic complex in British Columbia). Such flat and steep zones develop synchronously between and within anastamosing shear zone networks produced

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in response to far field effects such as oblique plate convergence.

Acknowledgements The authors are grateful for reviews by Damian Nance and Luis Eguiluz, and for accommodation and local advice given by Gee Guille.

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