Volcanostratigraphy of arc volcanic sequences in the Kohistan arc, North Pakistan: volcanism within island arc, back-arc-basin, and intra-continental tectonic settings

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Journal of Volcanology and Geothermal Research 130 (2004) 147^178 www.elsevier.com/locate/jvolgeores

Volcanostratigraphy of arc volcanic sequences in the Kohistan arc, North Pakistan: volcanism within island arc, back-arc-basin, and intra-continental tectonic settings Michael G. Petterson a , Peter J. Treloar b; b

a British Geological Survey, Kingsley Dunham Centre, Keyworth, Nottingham NG1 5GG, UK School of Earth Sciences and Geography, Kingston University, Penrhyn Road, Kingston-upon-Thames, Surrey KT1 2EE, UK

Received 6 January 2003; accepted 24 July 2003

Abstract The Kohistan arc was initiated, offshore of Asia, during the mid-Cretaceous above northward subducting, Tethyan oceanic crust. The arc sutured to Asia c. 90 Ma ago. Subduction of oceanic crust beneath the arc continued until Indian Plate continental rocks began to underthrust the arc c. 50 Ma ago. The arc shows an evolutionary history from the juvenile stages of an intra-oceanic island arc, through a thickened Andean-style volcanic arc accreted to a continental margin, to an arc underplated by older continental crust. Extrusive volcanic activity spanned the midCretaceous to Oligocene. This paper presents new and detailed lithostratigraphic data relating to two volcanic groups. The mid-Cretaceous Chalt Volcanic Group (CVG) documents volcanism in the last stages of the island arc phase. The Eocene-Oligocene Shamran Volcanic Group (SVG) documents Andean margin to post-Himalayan collision volcanism. The CVG comprises two formations, formally defined here. The back-arc Hunza Formation is dominated by subaqueous back-arc effusive basalt, andesite and boninite volcanism with a brief phase of subaerial silicic volcanism. The intra-arc Ghizar Formation comprises basalt and andesite-dominated crystalline and volcaniclastic rocks produced by subaerial and subaqueous calc-alkaline arc stratovolcano and shield eruptions. Two facies are present: a basalt and andesite lava flow-dominated sequence and a volcaniclastic-dominated sequence with characteristics that indicate effusive-explosive volcanism and subsequent volcanic sediment reworking and deposition within both subaqueous and subaerial settings. A stratovolcanic centre in the Ishkoman Valley contains abundant proximal volcanic lithofacies suggestive of Strombolian^Vulcanian explosive eruptive activity. The SVG, which unconformably overlies deformed rocks of the CVG, crops out in relatively small, high-altitude outliers. Previous suggestions that it has a large outcrop area in western Kohistan are unfounded. The SVG is an undeformed sequence of reddened, dominantly silicic volcanic rocks comprising mainly andesitic to dacitic and rhyolitic lavas, parataxitic and eutaxitic welded silicic ignimbrites, poorly sorted volcaniclastic sandstones, conglomerates and tuffs, and wellsorted, very fine-grained vitric tuffs. The SVG records highly evolved explosive and effusive volcanism within a mature Andean continental margin to post-Himalayan collisional environment. Primary magmas were probably generated at c. 40^30 Ma within relict metasomatised Tethyan mantle wedged between the Kohistan arc above and the underplating Indian Plate below. ; 2003 Elsevier B.V. All rights reserved.

* Corresponding author. E-mail addresses: [email protected] (M.G. Petterson), [email protected] (P.J. Treloar).

0377-0273 / 03 / $ ^ see front matter ; 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0377-0273(03)00287-7

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Keywords: back-arc basin; explosive volcanism; boninites; Kohistan; Pakistan

1. Introduction The Kohistan arc, located in the NW Himalaya, and exposed to the west of the Nanga Parbat syntaxis, was initiated as an intra-oceanic island arc between Eurasia and India during the Cretaceous. The arc was sutured to Asia at c. 100 Ma (Searle et al., 1999; Treloar et al., 1989) thus becoming an Andean margin type volcanic arc, before being underthrust by the leading edge of continental India at about 50 Ma (Rowley, 1997) after which volcanic activity began to decay. Since its recognition as an arc trapped between the Indian and Asian plates (Tahirkheli et al., 1979), the Kohistan arc sequence in northern Pakistan has been the subject of much research (e.g. Arbaret et al., 2000; Anckiewicz and Vance, 2000; Bard, 1983; Bignold and Treloar, 2003; Burg et al., 1998; Coward et al., 1982, 1987; Danishwar et al., 2001; George et al., 1993; Jan, 1988; Jan and Howie, 1981; Jan and Windley, 1990; Khan et al., 1989, 1993, 1997, 1999; Miller et al., 1991; Petterson and Windley, 1985, 1991, 1992; Petterson et al., 1990, 1993; Pudsey, 1986; Pudsey et al., 1985; Ringeutte et al., 1999; Robertson and Collins, 2002; Sullivan et al., 1993; Treloar et al., 1989, 1990, 1996; Yamamoto and Nakamura, 1996, 2000; Yamamoto and Yoshino, 1998; Yoshino et al., 1998). Volcanism can be divided into two main phases: a phase of intraoceanic basaltic and andesitic volcanism which predated to suturing to Asia and a phase of dominantly felsic volcanism, which mostly postdated suturing to Asia and continued until after the initiation of subduction of Indian Plate continental crust. Suturing to Asia postdated emplacement of the Matum Das pluton at c. 104 Ma (Petterson and Windley, 1985) and predated emplacement of the Chilas complex at 85 Ma (Treloar et al., 1996). Magmatism during the Andean stage was dominated by emplacement of dioritic to granodioritic plutons of the Kohistan batholith accompanied by subaerial silicic volcanism. The very

earliest stages of pluton emplacement shortly predated suturing to Asia, the latest stages postdate collision with India and persisted into the Oligocene. The Nanga Parbat syntaxis separates two halves of the arc complex. The Kohistan part to the west of the syntaxis is better known than the Ladakh part, sometimes known as the Dras arc, to the east. Correlations with Kohistan are apparent in the more westerly part of the Ladakh section (Rolland et al., 2000; Robertson and Collins, 2002) but become more tenuous further east (Clift et al., 2000, 2002). The suggestion by Rolland et al. (2000) that the arc may have been attached to the Eurasian margin at its eastern end, and thus may not be regarded as a true oceanic arc, is based on a limited isotopic dataset and has been disproved on chemical grounds by Clift et al. (2002), who found no continental in£uence in the Dras arc. Previous research, centred on ¢eld mapping, geochemical and geochronological programmes has de¢ned a geochronologically constrained history of the later stages of intra-oceanic arc development and subsequent events which is largely based on dating of plutonic rocks. A simple stratigraphy of the earliest stages of intra-oceanic arc has been established, although the details of lithologic distribution in the more inaccessible mountainous and tribal regions remain uncertain. Despite this, large gaps in knowledge remain which relate, in particular, to the early evolution of the intra-oceanic arc and the volcanism that postdates collision with continental India. This is partly a function of a lack of detailed descriptions of key stratigraphic sequences. This paper concentrates on two volcanic units within the Kohistan terrane. New detailed ¢eld descriptions and interpretations permit characterisation of their volcanological processes. The older of these two units, the Cretaceous Chalt Volcanic Group (CVG), is the stratigraphically highest, volcanic-dominant lithological unit within the intra-oceanic arc stratigraphy (Fig. 1) and

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Fig. 1. Simpli¢ed geological map of Kohistan (from Treloar et al., 1996) showing the location of Fig. 2.

represents volcanism during late stages of the island arc history. This paper represents the ¢rst attempt at a rigorous de¢nition of the internal stratigraphy of the CVG with the objective of enhancing correlations within the CVG and testing the hypothesis, ¢rst suggested by Petterson et al. (1990), that it can be divided into two lithologically distinct formations. The younger of the two units, the Tertiary Shamran Volcanic Group, represents volcanism within an Andean-type margin after ¢nal stages of ocean closure. Here, this previously poorly described unit is described and its magmatic evolution interpreted.

2. The Chalt and Shamran Volcanic Groups The CVG crops out in an E^W to NE^SW

trending arcuate belt, 330 km long and c. 30 km wide. It rests conformably upon the metasedimentary Jaglot Group (Treloar et al., 1996) and is conformably overlain by the carbonate, siliciclastic and volcaniclastic turbidites of the Albian-Aptian Yasin Group (Pudsey, 1986; Robertson and Collins, 2002). The CVG is tightly folded by highly asymmetric, N-vergent folds, with a strong, penetrative, bedding-parallel cleavage which strikes E^W and dips steeply north. Deformation is related to suturing of the arc to Asia (Coward et al., 1982). The accepted interpretation of the CVG, based on brief descriptions which provide little lithological detail and limited facies and genetic interpretation (Coward et al., 1982; Pudsey et al., 1985; Pudsey, 1986; Petterson et al., 1990) is that it is the product of lava-dominated subaqueous eruptions within an immature to early-

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mature island arc. The best-known section is in the Hunza Valley, where descriptions merely note a basalt-dominated lava sequence with local rhyolitic intercalations. To date, little attempt has been made to map volcanic facies within the CVG to determine whether it is possible to subdivide it and, if so, on what basis. Geochemical data show the CVG rocks in the Hunza region to have a more primitive, tholeiitic, geochemical signature than those in the Ghizar Valley to the west, which have an evolved, calc-alkaline geochemistry (Petterson et al., 1990; Bignold et al., 2001; Bignold and Treloar, 2003). On the basis of the geochemical di¡erences Petterson et al. (1990) tentatively subdivided the CVG into the Hunza Volcanics that crop out in the Hunza Valley north of Gilgit, and the Western Volcanics that crop out west of Gilgit. Here we test that hypothesis further and formally de¢ne the internal stratigraphy of the CVG. The Shamran Volcanic Group (SVG) is a remote outlier of £at-lying Tertiary volcanic rocks that includes highly evolved felsic units which rest unconformably upon the steeply dipping CVG and the older parts of the Kohistan batholith and is itself intruded by younger members of the Kohistan batholith (Danishwar et al., 2001). The outcrop area of the SVG is presently poorly constrained. Sullivan et al. (1993) suggested that a large area, previously mapped as part of the CVG around Shamran in the Upper Ghizar Valley, was of a younger, uncleaved sequence of evolved volcanic rocks which they termed the ‘Shamran Volcanic Group’ (SVG). On the basis of one K^Ar age (Treloar et al., 1989) they correlated this unit with the Early Eocene, 55 O 2 Ma, Utror Volcanic Formation of the Kalam region (Fig. 1). By contrast Danishwar et al. (2001) demonstrate that the outcrop area of the SVG is limited to a relatively small area around the village of Teru. Reconnaissance mapping data presented here broadly con¢rm this conclusion, but enable us to re¢ne the map of Danishwar et al. (2001). Danishwar et al. (in review) argue that the SVG, which they term the ‘Teru Volcanic Formation’, is an approx. 3-km-thick sequence of calc-alkaline to shoshonitic basalts, andesites and rhyolites, some pyroclastic, with a subduction-related geo-

chemical and isotopic signature. Late Eocene to Oligocene Hornblende Ar^Ar ages (Danishwar et al., in review) indicate that the SVG are the youngest volcanic rocks within the arc, and cannot be correlated with the Utror Formation. In spite of a limited outcrop area, the SVG is an important stratigraphic unit within the Kohistan terrane as it records the style and geochemistry of volcanism within an Andean to continental collision (or entirely continental collisional) tectonic setting. Its original outcrop area would have been signi¢cantly larger than that of today, with much of the SVG eroded during late Tertiary uplift.

3. Field data Field work was designed to produce a map of volcanic lithofacies within the CVG and SVG and to describe key sequences within both volcanic units with the aim of constraining their volcanogenic histories. Reconnaissance ¢eld work was performed in the Hunza, Bagrot, Ghizar, Ishkoman, Yasin, and Shandur areas of N. Pakistan (Fig. 2). Field data, and samples for petrologic and geochemical analysis, were collected along road and trek traverses and critical sections logged. The key results are presented below in sequence from north to south: (Hunza and Bagrot valleys; Figs. 2 and 3) and then from east to west: (Lower Ghizar, Ishkoman, Yasin, Upper Ghizar, and Shandur (Fig. 2). The locations of individual lithological logs (Figs. 5, 6, 7, 9 and 10) are shown in Fig. 2. Although the CVG has been deformed and metamorphosed to lower amphibolite facies conditions, the deformation is strongly partitioned in such a way that many units are only weakly deformed. This variable deformation state allows for precise description of volcanogenic features from throughout the sequence. 3.1. Hunza Valley section The lithological transition between the lower part of the turbiditic siliciclastic-carbonate Yasin Group and the upper part of the CVG is exposed

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Fig. 2. Sketch map showing the geology of the northern Kohistan. The map is based on our ¢eld data, satellite imagery that locates the outcrop of the SVG, and data from Petterson and Windley (1985), Pudsey et al. (1985) and Sullivan et al. (1993) that document the outcrop of the Kohistan batholith. The map also indicates the various ¢eld localities mentioned in the text and the location of Fig. 3.

from Chalt to Rahimabad, along the Karakoram Highway (KKH). That part of the Yasin Group exposed here (Fig. 3) comprises a unit, about 500 m thick, of steeply dipping, thickly bedded limestone overlain by a similar thickness of thinly bedded, turbiditic, micritic limestone containing slump folds. The preservation of sedimentary features, lack of internal deformation within units and absence of tectonic repetitions suggest that the limestones in this section have seen little tectonic thickening. These underlie a heterogeneous sequence of tightly folded chlorite-bearing, green slates that are the metamorphic products of ¢negrained siltstones and shales containing variable amounts of ma¢c volcanic ash. Where the ash content is low, garnet and biotite are present in the metamorphic assemblage. Rare silicic layers indicate a more felsic ash component, which is much less abundant than the ma¢c contribution. The precise location of the boundary between the steeply dipping CVG and the Yasin Group is open to interpretation. The boundary is taken here at a point where there is a change in sedimentation style from volcaniclastic- and siliclastic-dominant below to carbonate-dominant above.

In this analysis, basaltic and andesitic pillow lavas set in a carbonate matrix de¢ne the very lowest part of the Yasin Group in the KKH section, with the base of the Group marked by a basaltic peperite sill c. 3 m thick (locality 1, Fig. 3, left lower part). The peperite has a cuspate morphology with numerous, lenticular apophyses of basalt intrusive into laminated, calc-silicate sediments. The sill contains cm- and dm-sized xenoliths of the host rock. Within a relict £uidised margin a cm or so wide, all bedding structures in the host sediment are homogenised (cf. Kokelaar, 1982). Coincidence of the sill with the base of the calcareous sequence and the top of a zone of sediments with a higher volcaniclastic component than those above suggests that it marks a change in both sediment source and depositional environment. The volcaniclastic sediments beneath the peperite sill mark the ¢nal waning cycle of a protracted period of volcanism represented by a systematic volcanic stratigraphy described below. We note that Robertson and Collins (2002) place the contact 300 m further down section where there is a lithological change between mainly volcanic and volcaniclastic units below and volcaniclastic sand-

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Fig. 3. Map of part of the Hunza (left lower part) and Bagrot (right lower part) valleys showing the distribution of di¡erent rock units within the Yasin Group and upper CVG.

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Fig. 4. (a) Stretched, cleaved and epidotised CVG rocks, locality 2, Hunza Valley. (b) Wispy shard-¢amme structures within a 30-cm-thick welded tu¡ unit, Chalt Volcanic Formation, locality 5, Hunza Valley. (c) Laminated epiclastic turbidite tu¡s with hornblende garbenschiefer, locality 5, Hunza Valley. (d) Porphyritic and amygdaloidal basaltic andesite with epidotised phenocrysts and amygdales, locality 6, Bagrot Valley.

stones and siltstones above. The latter sequence (locality 2, Fig. 3, left lower part) comprises beds 10^50 cm thick with a modal thickness of 20^25 cm. Most beds display normal-size grading with a clast population that varies from 7 mm at the base of graded units to 6 3 mm at the top. These rocks have a strong bedding-parallel cleavage. They pass downward into a 20^40-m-thick sequence characterised by autobrecciated basalt and andesite lava £ows separated by siltstone screens. We prefer to take the boundary at the point where calcareous sediments become important as it is more likely that the change from volcanic rocks to volcaniclastic sediments is diachronous both spatially and temporally. Pillowed andesitic and basaltic lavas are the dominant lithologies at locality 3 (Fig. 3, left low-

er part). These lava £ows, between 1 and 20 m thick, are cleaved with epidotised pillows stretched into elongate bodies 30^60 cm long and 10^20 cm wide (Fig. 4a). Some pillows contain zeolite amygdales. In general, the basalts are melanocratic and recrystallised with strong shear fabrics. Quartz amygdales, measuring 8U4 mm, are stretched and aligned parallel to cleavage planes. Andesites are mesocratic and strongly foliated, with abundant quartz, feldspar and hornblende crystals. Coward et al. (1982) described felsic rocks from the Hunza Valley section through the CVG. These steeply dipping rocks are restricted to a narrow zone (Fig. 3, left lower part). The most northerly rhyolite sheet (locality 5, Fig. 3, left lower part) contains ma¢c enclaves, and is interleaved with

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Fig. 5. Lithological logs through a number of thin welded tu¡ units in the Hunza Valley, locality 5. Unit 1 displays a ‘model’ ignimbrite stratigraphy with a basal layer 1 ¢amme-rich zone, a layer 2, which exhibits inverse grading of ¢amme, and a layer 3 possible co-ignimbrite ash deposit. Units 2 and 3 are more massive and homogeneous eutaxitic welded lapilli tu¡s.

basalt £ows and cut by ultrama¢c sills. South of this, felsic volcanic rocks crop out within a 1-kmwide zone (Fig. 3). Pyroclastic rocks are exposed at locality 5 (Fig. 3, left lower part) where a rhyolite lava and ignimbrite sequence overlies an andesite tu¡. The pyroclastic units are 10 cm^1 m thick, welded, pumice lapilli tu¡s. The most complete ignimbrite is unit 1 (Fig. 5).

This 30-cm-thick unit, has a basal, ¢ne-grained, ¢amme-rich zone c. 4 cm thick (layer 1), which grades into the main body of the ignimbrite which is 20 cm thick (layer 2). Layer 2 shows inverse grading of ¢amme with the lower part containing very thin, wispy, chloritised ¢amme set in a crystal- and lithic-rich, poorly sorted ash matrix (Fig. 4b). The upper part contains larger ¢amme set in

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a poorly sorted crystal-lithic-rich ash matrix. Layer 3 is 3 cm thick and comprises ¢ne-grained, well-sorted tu¡. Unit 1 is a thin ignimbrite deposit that displays many features of internal pyroclastic £ow stratigraphy : a basal zone enriched in ¢amme; a main body displaying inverse grading of ¢amme; and an upper well-sorted, ¢ne-grained ash layer possibly representing a co-ignimbrite ash fall (e.g. Sparks, 1976; Wilson, 1986; Cas and Wright, 1987; Watanabe et al., 1999). Unit 2, which is 1 m thick, and unit 3, which is 1.5 m thick, (Fig. 5) show a less complete internal stratigraphy. They comprise poorly sorted, welded pumice-lapilli tu¡s, with aligned ¢amme and unvesiculated dacite lithic clasts set in an eutaxitic fabric. Fiamme, between 1 cm by 2 mm and 40 by 4 cm in size, form up to 30% of the rock, and are weakly reverse graded. Some ¢amme themselves

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comprise eutaxitic ignimbrite clasts with cuspate margins, and dark reaction rims. The matrix comprises medium- to coarse-grained, crystal-rich ash. Volumetrically the bulk of the section comprises steeply dipping felsic to intermediate lavas and volcaniclastic units with occasional metrescale ignimbrites. Some of the tu¡ units, could be interpreted as well-graded, primary or reworked epiclastic units (e.g. Kokelaar, 1990; Carey, 2000; Millward et al., 2000). Fig. 4c shows mudstone and siltstone units interbedded with laminated rhyolitic tu¡s that contain garbenschiefer crystals of metamorphic hornblende. The rhyolitic lavas and tu¡s pass stratigraphically downward into ¢ne-grained andesitic tu¡s. South of the volcaniclastic unit, the KKH traverses a 4-km-thick sequence of vertical to steeply dipping layers of pillowed and non-pillowed ba-

Fig. 6. Lithological logs through the CVG in the Bagrot Valley. Locality 7 illustrates a sequence of tu¡s which are normally graded and cross-bedded. The graded tu¡s are interleaved with more massive, homogeneous tu¡s. The locality 9 log illustrates a lava-dominated stratigraphy which contains a range of compositions from basalt to rhyolite. Thin tu¡ sequences are interbedded with the lavas.

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salt and andesite (locality 6, Fig. 3), some of which are boninitic in composition (Petterson et al., 1990; Bignold et al., 2001). The southern margin of these basalts is intruded by the 102 Ma Matum Das tonalite pluton that predates suturing of Kohistan to Asia (Treloar et al., 1989). 3.2. Bagrot Valley CVG rocks, stratigraphically lower than those exposed in the Hunza Valley, are exposed along strike to the east in the Bagrot Valley. The Upper Bagrot Valley, near the snout of the Hinarche glacier (locality 7, Fig. 3, right lower part), is dominated by steeply dipping basalt and andesite units 1^20 m thick, interleaved with screens of meta-tu¡ and meta-greywacke 10 cm to several metres wide. The basalts and andesites are highly cleaved and epidotised and contain hornblende phenocrysts often retrogressed to epidote and/or actinolite. Long axes of deformed amygdales parallel the stretching direction in the cleavage planes (Fig. 4d). Where in contact with volcaniclastic rocks, the margins of amygdaloidal basalt sheets are often autobrecciated. The autobrecciation is probably caused by variations in viscosity within the basaltic lava unit as it was emplaced within the lava pile. This particular unit exhibits aa-style behaviour with a partially solidi¢ed outer crust being continuously and actively brecciated by a central liquid magma core. Fig. 6 shows a lithological log through one of the tu¡ screens. These comprise green, ungraded, medium to coarse, crystal-rich ash and normal-graded, coarse to ¢ne ash with basal, gravel-grade, lag deposits. Low-angle cross-bedding is present in the upper parts of the graded units. The graded units indicate, as in the Hunza Valley, sequences young to the north. Opposite Hope Village (locality 8, Fig. 3, right lower part) the volcanic stratigraphy is dominated

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by north-younging, steeply dipping, cleaved and veined, epidote-rich units of pillowed basalt and laminated ¢ne-grained basaltic ash. These rocks are intruded by gabbros and granites. Further south, near Sinakhar (locality 9, Fig. 3, right lower part), the volcanic rocks are dominated by basalt sheets with subordinate andesite and rhyolite (Fig. 6). The units vary in thickness from 1 to 10 m. There are many contacts between volcanic layers. Where intervening tu¡ is present, it is predominantly ¢ne-grained and rhyolitic. One rare lithofacies has a medium-grained ash matrix with 20-cm-long clasts of welded lapilli tu¡ rimmed by hornblende. 3.3. Ghizar Valley west of Gilgit At Henzal (locality 10, Fig. 2) the CVG is dominated by tu¡aceous sequences. Three logs (Fig. 7) illustrate a range of tu¡ lithotypes from uniform, well-sorted, ¢ne-grained silicic tu¡s to coarse to ¢ne, normally graded intermediate to felsic tu¡s, to moderately sorted, gritty felsic tu¡s containing rip-up clasts (Fig. 7). Bed thicknesses are between 6 10 cm and 3 m. Some units have turbiditic characteristics and probably represent epiclastic units. The well-sorted felsic tu¡s probably represent distal, water-lain, rhyolite ash (e.g. Millward et al., 2000). A series of garnet-bearing psammites crop out stratigraphically beneath these steeply dipping tu¡s. These are interlayered with rare calc-silicate horizons and thin units of silt-grade felsic rocks which probably represent distal rhyolitic ash deposits. The garnetiferous psammites and calc-silicates are typical lithologies of the Gilgit Formation which stratigraphically underlies the CVG and it is likely that the base of the CVG is exposed here. Outcrops around Sher Qila (locality 11, Fig. 2) comprise steeply dipping, strongly cleaved, massive and pillowed basalts with thinly bedded to

Fig. 7. Lithological logs through the CVG along the Ghizar Valley, west of Gilgit, at Henzal and Sher Qila (localities 10 and 11). The Henzal lithological logs document a range in tu¡aceous litho-sequences varying from massive, ¢ne-grained acid tu¡ to weakly graded and moderately sorted intermediate to siliceous tu¡s. Bedding thicknesses vary between 3 m and 10 cm. The Sher Qila log shows a uniform sequence of thinly bedded ¢ne-grained, green tu¡s with weak grading and coarser, basal tu¡ lag deposits.

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Fig. 8. (a) Recumbently folded Yasin Group limestone and underlying CVG rocks (exposed on the left of the white-coloured folded limestone) near Hatoon village, upper Ishkoman. (b) Pyroclastic units at Ishkoman illustrating andesite juvenile fall clasts with impact-indentation structures, and bedded tu¡ and lapilli tu¡ sequences (locality 12). (c) Close up of an andesite clast within a fall-origin welded lapilli tu¡ forming indentation structures in a lower, bedded tu¡ unit. Note the ragged nature of some of the larger clasts (locality 12). (d) High density of large ragged andesite clasts (spatter bombs) forming a welded agglutinate lapilli tu¡ (locality 12). (e) Welded fall-lapilli tu¡ with highly ragged andesite clasts indicating an original hot and plastic form prior to welding. This is evidence for a proximal-fall nature of the deposit (locality 12). (f) Well-graded coarse tu¡ sequences with erosive bases and rip-up clasts (base to the right of the photograph), locality 12, Ishkoman.

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thickly laminated, ¢ne-grained green tu¡s (Fig. 7). The tu¡s are well-sorted and exhibit a penetrative, ¢ne-grained primary lamination or bedding. Some units are weakly graded with medium-grained, lag material at their base. These probably represent distal, water-lain, ash fall sequences (e.g. Cas and Wright, 1987; Kokelaar, 1990; McPhie et al., 1993; Carey, 2000; Millward et al., 2000). Between Sher Qila and the Ishkoman Valley the volcanic rocks are dominated by steeply dipping ma¢c tu¡s which are well sorted and parallel bedded, with bedding thicknesses between 8 mm and 4 cm. Sequences between Gilgit and Ishkoman are deformed by folds with amplitudes of up to 1 km. The sequences generally young north-

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ward, although with local younging reversals due to the folding. As the folds have a strong N-vergent asymmetry there are no major stratigraphic repetitions. Rocks of the CVG are well exposed around Hatoon in the Ishkoman Valley (locality 12, Fig. 2). Here the contact between the CVG and the overlying Yasin Group is tightly folded with synforms hosting Yasin Group carbonate rocks preferentially sheared out (Fig. 8a). The volcanic rocks are mainly comprised of pyroclastic and volcaniclastic units and basalt and andesite lavas. In the pyroclastic rocks, volcanic bombs, with associated impact structures, are common, indicative of a nearby active volcanic centre. The impact

Fig. 9. Lithological logs through the CVG in the Ishkoman Valley (locality 12). (a) A lower basalt limestone unit overlain by ¢ne-grained crystal-rich tu¡s and silicic welded tu¡s, an ignimbrite, and a nearby welded volcanic breccia. (b) a series of tu¡s and lapilli tu¡s, some of which contain impact ballistic bombs with associated sag structures. Parallel-bedded fall deposits are also recorded. (c) Lapilli tu¡s which contain abundant wispy-ragged ¢amme. (d) Volcaniclastic sedimentation within a water-saturated environment with soft-sediment deformation and high-energy current activity producing imbrication of c. 7-cm-long dense lithic clasts.

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structures are the result of the deposition of highmomentum ballistic bombs onto bedded tu¡. The tu¡ responded to impact by locally collapsing and forming concave upward dish structures which envelop the base of the bomb (e.g. Figs. 8b,c and 9). Fig. 8b^f documents some of the important lithological elements of the Ishkoman sequence. Fig. 8b,c shows subangular, andesitic porphyry volcanic bombs with associated impactindentation structures in the underlying laminated tu¡s and lapilli tu¡s. They also show alternating well-bedded, mesocratic and melanocratic tu¡ units with low-angle cross-bedding structures suggestive of a fallout or surge origin, and poorly sorted, welded lapilli tu¡ units containing lithic clasts of porphyritic andesite, diorite, crystal-rich and ¢ne-grained tu¡, and marble in a tu¡aceous matrix. Some of the lapilli have ragged or cuspate edges. There are also examples (not shown) of interlocking ragged lapilli welded against each other. This facies is interpreted as representing proximal pyroclastic fallout with juvenile lapilli and ash, some of which was hot and highly plastic, forming agglutinate-spatter deposits, along with lithic clasts picked up from country rocks (e.g. Cas and Wright, 1987; McPhie et al., 1993; Druitt et al., 1999). Fig. 8d,e shows further features of this proximal fall facies. Fig. 8d shows a welded lapilli tu¡ in which the bulk of the lapilli comprises ragged, lenticular, andesite lithic clasts in a poorly sorted, lapilli tu¡ matrix. The lapilli, which measure up to 30 by 8 cm, are concentrated within a 30-cm-thick unit and form more than half the rock. The lapilli-rich units grade into ¢ner-grained, welded lapilli tu¡ with carbonate ¢amme pseudomorphs. Fig. 8e, a close up of this welded volcanic agglutinate fall deposit shows the highly ragged morphology of the larger andesite clasts. These rocks are interpreted as proximal welded spatter-agglutinate deposits (e.g. McPhie et al., 1993). Fig. 8f shows a series of cleaved

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tu¡ units which show : well-developed graded bedding; an erosive base to the lowermost coarse unit which contains ripped-up lithic clasts derived from the underlying medium-grained tu¡; and a well-sorted, massive medium-grained tu¡aceous underlying unit. This facies indicates a subaqueous origin for the deposits, with fall into water proximal to a subaerial volcanic centre (Carey, 2000; Millward et al., 2000). Locally, where intrusive into limestone units, basaltic sheets have peperitic margins. The presence of thin limestone horizons within the sequence suggests shallow water marine conditions. Fig. 9 shows four representative stratigraphic logs from Ishkoman. Important features include the following. Heterolithic populations of juvenile and accidental clasts suggest vent-clearing processes that deposited a welded tu¡-breccia containing a mixture of juvenile and country rock blocks (e.g. Fig. 9a). Inverse grading of ¢amme is present in ignimbrite units (e.g. Fig. 9a). Sag structures occur beneath volcanic bomb impacts indicating a proximal origin (e.g. Fig. 9b). Parallel bedding in welded tu¡s suggests a fallout origin (e.g. Fig. 9c). Flame structures indicate loading onto a wet, poorly consolidated substrate (Fig. 9d). Ragged juvenile ¢amme with delicate wispy edges indicate hot, plastic emplacement (e.g. Fig. 9c). Finally, imbricated basaltic clasts within a tu¡ matrix are suggestive of water current activity (Fig. 9d). Fig. 9a documents a sequence typical of weak e¡usive basaltic volcanism with thin ( 6 1.5 m) basaltic lavas interbedded with limestone layers of similar thicknesses indicating periodic lava eruption into a shallow, subaqueous environment. This sequence is overlain by a, c. 10-m-thick, massive, crystal-rich, basaltic-andesite volcaniclastic sequence which is in turn overlain by a dacitic to rhyolitic welded ash horizon and a 1-m-thick ignimbrite. The upper package represents an up-

Fig. 10. Lithological logs through the CVG in the Upper Ghizar Valley (localities 13^20). At Gakuch (locality 13) and Phander (locality 20) the sequences are dominated by thin^medium-bedded and laminated ¢ne-grained tu¡s (some exhibiting normal grading) respectively, with one welded lapilli tu¡ present at Gakuch. The Raoshun (locality 14) locality contains a variety of deposits including welded volcanic breccias, tu¡s and lapilli tu¡s and a basalt. At Dehimal Bridge (locality 16) the stratigraphy is dominated by basalt sheets, most of which are pillowed. Note the absence of intervening tu¡s or sediment.

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ward transition from a shallow subaqueous to a subaerial setting and from ma¢c e¡usive to felsic and increasingly explosive volcanism. The uppermost part of Fig. 9a shows a proximal, heterolithic, welded volcanic breccia which probably resulted from a high-energy explosive eruption. Fig. 9b shows a sequence of medium-bedded tu¡s which range from well-sorted and ¢negrained, through poorly to moderately sorted, medium- to coarse-grained to coarse with lithic clasts up to 8 cm long and associated impact structures. These pyroclastic rocks are interpreted as proxi-

mal Strombolian^Vulcanian explosive deposits, with grain size variations possibly re£ecting varying degrees of fragmentation and sorting caused by varying explosivity of eruptive phases and variable clast transport distance within a subaerial volcanic plume (Nairn and Self, 1978; Kokelaar, 1983; Donoghue et al., 1997; Morrissey and Mastin, 2000; Vergniolle and Mangan, 2000; Adams et al., 2001; Arrighi et al., 2001). The upper part of Fig. 9b documents a series of ¢ne-grained, wellsorted tu¡s and coarse lapilli tu¡s with multiple lithi types. Parallel bedding indicates a fallout ori-

Fig. 11. Lithological photographs of the CVG in the Upper Ghizar Valley. (a) Raoshun (locality 14). Volcanic breccia with hornblende-andesite and limestone clasts set in a tu¡aceous matrix; (b) Shamran Village (locality 19). Subvertical homogeneous thinly bedded^laminated, green ¢ne^medium-grained tu¡s; (c) Phander (locality 20). Cleaved, subvertical, well-laminated and well-sorted tu¡s.

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gin while impact structures and ballistic bombs, including accidental and juvenile lithic clasts, suggest proximal, possibly vent-clearing, explosive eruptions typical of Strombolian^Vulcanian eruptions. Fig. 9c shows a series of four, 20^40-cm-thick, poorly sorted, welded lapilli tu¡s which contain non-vesiculate, porphyritic, andesite lithic clasts and elongate vesicular ¢amme with delicate, wispy-ragged edges indicating plasticity during deposition. These deposits document accumulation of explosively erupted spatter-agglutinate clasts which are typical of proximal Strombolian^Vulcanian eruptions (Cas and Wright, 1987; Morrissey and Mastin, 2000). The uppermost, very ¢negrained, laminated tu¡ may represent deposition of distal, ¢ne ash within a lacustrine environment. Fig. 9d documents an aqueous depositional environment. Fine-grained tu¡aceous units at the base contain £ame structures in their higher levels and are interbedded with coarse andesitic tu¡s suggesting rapid deposition of volcaniclastics onto water-saturated volcanic siltstones and tu¡s. The coarser upper unit contains imbricated basalt-andesite clasts within a tu¡aceous matrix indicating strong, subaqueous current activity. These features indicate that a signi¢cantly sized body of lacustrine or £uvial water, whose water currents were strong enough to imbricate lapilli up to 7 cm in size, was situated close to the Ishkoman volcanic centre (IVC). The lithological data from Hatoon suggest that the Ishkoman area was once an active, probably subaerial, stratovolcano with Strombolian^Vulcanian eruptive behaviour. 3.4. Upper Ghizar Valley and Yasin At Gakuch, and west of Gakuch (locality 13, Fig. 2), CVG rocks are dominated by a homogeneous sequence of steeply dipping, thinly bedded, well-sorted, ¢ne-grained, parallel-bedded tu¡s (Fig. 10). Bed are between 10 and 50 cm thick although most are 6 20 cm thick. One outcrop shows a welded lapilli tu¡ with thin ¢amme measuring 2U0.1 cm, and lithic clasts up to 1 cm long set in a very poorly sorted tu¡ matrix. Volcaniclastic rocks exposed near Raoshun (locality 14, Fig. 2) are £anked by Yasin Group

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limestones to north and south and probably represent part of the upper CVG sequence exposed within an anticlinal fold core. A log of part of this steeply dipping sequence (Fig. 10) shows the lowermost 50 cm to comprise a volcanic breccia with clasts of hornblende-andesite and limestone set in a crystal-rich tu¡aceous matrix (Fig. 11a). This unit is overlain by a 4-m-thick, crystal-rich, thinly bedded, ¢ne-grained ma¢c tu¡ which in turn is overlain by an aphanitic basalt with plagioclase phenocrysts. The uppermost unit is a 1.5-m-thick ignimbrite that displays typical internal ignimbrite stratigraphy (e.g. Sparks, 1976) including normal grading of andesite lithics, inverse grading of ¢amme, and an upper co-ignimbrite ash deposit. Not shown on the log is a nearby, peperitic, hyaloclastic andesite sill intrusive into a calcareous sandstone. In the Yasin Valley (locality 15, Fig. 2), outcrops are dominated by 5^30-m-thick lavas and sills of basalt, andesite and dacite. In contrast with much of the sequence west of Gilgit, these outcrops contain a low proportion of volcaniclastic material. There are many lava^lava contacts with no intervening sediment, indicating high effusion rates. The dominant composition is basaltic andesite and the rocks are only weakly cleaved. Pillowed, massive and aphanitic sheets are present. Some of the coarser sheets have feldspar or hornblende phenocrysts. Outcrops at Dehimal Bridge (locality 16, Fig. 2) are dominated by steeply dipping, moderately cleaved, pillowed basalt lavas, one almost 30 m thick, with subordinate andesite and basaltic andesite. The key features of this sequence are shown in Fig. 10. One of the basalt sheets has an autobrecciated upper margin. Pillow structures indicate a northward younging direction. Typical volcanic textures include amygdales, plagioclase and hornblende phenocrysts, melano- to mesocratic, ¢ne-grained to cryptocrystalline groundmasses, glassy pillow carapaces and autobreccias. The roadside outcrops between Dehimal Bridge and Sasat (locality 17, Fig. 2) are dominated by steeply dipping lavas and sills which are predominantly basalt, basaltic andesite and andesite in composition, with minor dacite. The basalt and andesite sheets show a variety of features includ-

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Fig. 12. Examples of the lithological variation of the SVG. (a) Blocks of bilithic volcanic breccia in scree near Pingal (locality 18); (b) blocks of heterolithic volcanic breccia in scree near Pingal (locality 18); (c) blocks of volcanic gravel and sandstone grade beds of in scree near Pingal (locality 18); (d) view, looking northward, from Phander showing the £at-lying nature of the unconformity that separates Shamran volcanic rocks from underlying, subvertical Chalt volcanic rocks (locality 20). (e) Block of coarse lapilli tu¡/volcaniclastic sandstone containing andesite lithic clasts up to 8 cm in diameter (locality 20); (f) block of poorly sorted coarse-grained lapilli tu¡/volcaniclastic sandstone (locality 20); (g) parataxitic acid lapilli tu¡ (locality 21); (h) ‘hornstone’-type lithofacies with alternating dark- and light-coloured, vitric, very ¢ne-grained, very well-sorted, parallel-laminated tu¡ (locality 21). Note the fresh, uncleaved nature of all the Shamran samples.

ing autobrecciation, hyaloclastites and pillows with glassy carapaces. Sheets are between 1^2 and c. 10 m thick and are interbedded with thin horizons of poorly sorted, basaltic lapilli tu¡s that are medium- to coarse-grained and contain abundant basalt to basaltic andesite lithic clasts. Bedding is vertical to subvertical, and youngs northward. The valley £oor east of Pingal (locality 18, Fig. 2) is covered by debris contained within an alluvial fan. Loose blocks within this fan are of uncleaved and unmetamorphosed volcanic rocks which are lithologically distinct from the green,

cleaved rocks of the CVG. Fig. 12 shows typical lithofacies that include red-, probably haematised, and green-coloured rocks. A number of lithofacies are present. These include: monolithic to bilithic volcanic breccias with subangular, 1^5-cm-sized clasts of plagioclase-phyric and aphanitic andesite, set in a pink tu¡aceous matrix (Fig. 12a); heterolithic volcanic conglomerates with rounded, 1^6-cm-sized clasts of andesite, dacite, and sandstone, set in a ¢ne- to medium-grained, red volcanicastic matrix (Fig. 12b); ¢ner-grained volcanic breccias of predominant gravel and pebble grade (Fig. 12c); ¢ne-grained green tu¡s with no clasts,

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Fig. 12 (Continued).

and poorly sorted volcaniclastic sandstones. These uncleaved rocks are interpreted as part of the SVG. There are no obvious indications of outcropping SVG rocks in this area and the loose blocks are interpreted as having been transported within the alluvial fan from the mountains to the south, probably from altitudes of s 4000 m, which is at least 1000 m above the local valley £oor level. CVG lithologies crop out around, and within, Shamran Village (locality 19, Fig. 2). These comprise homogeneous, well-sorted, ¢ne- to mediumgrained green tu¡s with a pronounced near-vertical bedding-parallel cleavage (Fig. 11b). The tu¡s are thinly bedded with bed thicknesses of 1^5 cm. Cross-bedding is locally present. This same rock type crops out westward, to the east of Phander. The tu¡s represent ash fallout into water reworked by local currents (Kokelaar, 1990, How-

ells et al., 1991; McPhie et al., 1993; Carey, 2000; Millward et al., 2000). The unconformity that separates the CVG from the overlying SVG can be mapped at Phander (locality 20, Fig. 2). The lithologies which crop out in the valley £oor at Phander are cleaved, well-sorted, ¢ne-grained, green tu¡s with steeply dipping bedding and cleavage (Fig. 11c). Beds are 4^8 mm thick with grading indicative of northward younging (Fig. 10). These cleaved rocks are part of the CVG. The unconformity is located at an altitude of 3500^4000 m on the south-facing slopes of the mountains situated north of Phander (Fig. 12d). The unconformably overlying uncleaved SVG rocks are predominantly red with horizontal to subhorizontal dips. As at Pingal, there are no exposures of the SVG at the valley £oor level, although SVG rocks have been transported in alluvial fans from the moun-

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tains to the north. Fig. 12e shows a selection of SVG lithologies present in the alluvial fan. The samples retrieved from the fan are fresh, nonmetamorphosed and undeformed. Many lithologies are haematised with red-bed characteristics. A wide spectrum of lithologies includes compact basalt, andesite and dacite lavas ; primary volcanic breccia and welded, acid lapilli tu¡s. The igneous rocks are dominated by andesites and dacites. The volcanic rocks include strongly reddened, porphyritic andesites with feldspar phenocrysts up to 5 mm long set in a ¢ne-grained microcrystalline groundmass; weakly welded ignimbrites containing large, ragged ¢amme measuring up to 20U0.5 cm; and £ow-banded, aphyric rhyolite-dacites. Clastic facies include volcaniclastic sandstones, conglomerates and breccias. Red volcaniclastic shales are interbedded with matrix-supported, pebbly sandstones. These latter are moderately to poorly sorted and contain a heterolithic assemblage of subrounded clasts which include diorite, andesite, tu¡, sandstone, and siltstone. The larger clasts are 6^10 cm in size. Fig. 12e shows a poorly sorted, reddened, volcaniclastic conglomerate with rounded, andesitic clasts measuring up to 5^8 cm set in a very poorly sorted, volcaniclastic sandstone matrix. Fig. 12f shows a eutaxitic lapilli tu¡ with darker lapilli up to 0.5 cm long set in a coarse tu¡ matrix. Fig. 12g shows a rhyolitic, parataxitic, highgrade ignimbrite with high-aspect-ratio ¢amme up to 17 cm long by 1 cm wide set in a ¢ne-grained, vitric tu¡ matrix. This sample also displays a degree of local £ow-folding and rotated lithic clasts indicating intra-ignimbrite £ow-shearing. These types of lithological characteristics are most typical of high-grade, highly silicic, rheomorphic and lava-like ignimbrites (e.g. Branney et al., 1992; McPhie et al., 1993; Millward et al., 2000). Fig. 12h shows a ‘hornstone’ volcanic facies typical of very well-sorted, distal vitric ash that settles into lacustrine bodies (e.g. Millward et al., 2000). Valley £oor exposures between Phander and Teru (locality 21, Fig. 2) are of the CVG. These are of highly cleaved basalt and andesite sheets with interbedded basaltic lapilli tu¡s. The unconformity between the CVG rocks in the valley £oor and the SVG rocks above can be seen high on the

mountain slopes to the north of the valley at an altitude of s 3500 m. A traverse up a steeply inclined, N-trending valley to the north of Teru to an altitude of 3000 m failed to reach outcrop of the SVG. Cleaved CVG basalts and andesites are exposed along the length of the traverse. Loose blocks sourced from the SVG include welded acid lapilli tu¡s with parataxitic fabrics (Fig. 12g) and a sequence of well-sorted, banded, ‘hornstone’-type vitric tu¡s (Fig. 12h). The unconformity between the SVG and the CVG can be observed from a distance on the north-facing slopes of mountains situated to the south and south^east of Teru at an altitude of c. 4000 m. Exposures within Chamorkhan Gul (locality 22, Fig. 2) are of cleaved, subvertical basalt and andesite £ows 5^10 m thick, with minor basaltic and andesitic breccias, lapilli tu¡, and welded lapilli tu¡. These CVG rocks pass northward into calcareous rocks of the Yasin Group, exposed within a synformal keel (Pudsey et al., 1985). The Shandur Pass area (locality 23, Fig. 2) is poorly exposed. On the eastern side of the pass, east of Shandur Lake, the unconformity between the SVG and CVG is present on the south-facing slopes on the northern side of the pass. Danishwar et al. (2001) also map it on the north-facing slopes on the southern side of the pass. West of Shandur Lake, the geology is dominated by schists and granite gneisses suggesting that this is a basement high and that the CVG is locally faulted out. However, CVG lithologies, dominated by highly cleaved and metamorphosed (amphibolite grade) basalt and basaltic andesite sheets (c. 5 m thick) with interbedded basaltic tu¡s, are exposed at Sor Laspur (locality 24, Fig. 2) at the bottom of the pass on its west side.

4. Interpretation of volcanic stratigraphy : discussion and synthesis The main ¢eld data are summarised in Fig. 2. The stratigraphy of the northern part of the arc is divisible into four distinct units. The CVG is divisible into the Hunza and Ghizar formations on the basis of variations in volcanic facies and geochemistry. This classi¢cation is justi¢ed in detail

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below but can be summarised as follows. The Hunza Formation is predominantly a basalt-andesite submarine, e¡usive volcanic series with a primitive arc-tholeiite geochemistry (Petterson et al., 1990; Bignold and Treloar, 2003). The Ghizar Formation is a heterogeneous, andesite-dominant package of volcanic rocks with an evolved calcalkaline arc geochemistry (Petterson et al., 1990), which was formed by explosive and e¡usive volcanism within subaerial and subaqueous environments. The Yasin Group overlies both formations of the CVG. The SVG unconformably overlies the Ghizar Formation and, possibly, parts of the Yasin Group. As it is exposed only at s 4000 m the extent of the SVG outcrop is constrained largely by long-distance observations in the ¢eld. 4.1. Chalt Volcanic Group: Hunza Formation Lithological data presented here, together with geochemical data (Petterson et al., 1990; Bignold et al., 2001; Bignold and Treloar, 2003), suggest the volcanic rocks exposed in the Hunza and Bagrot valleys are part of a stratigraphic unit distinct from rocks of the CVG that crop out west of Gilgit. It is proposed here that these two stratigraphic units be termed the Hunza Formation and the Ghizar Formation respectively and that these are stratigraphically coeval, geochemically distinct, divisions of the CVG. The Hunza Formation overlies the Jaglot Group (Treloar et al., 1996) and is overlain to the north by the Yasin Group (Pudsey, 1986). It is bounded to the east by the Nanga Parbat syntaxis although there may be correlative sequences of small areal extent in Baltistan to the east of the Nanga Parbat syntaxis. Rolland et al. (2000) describe a basaltic unit near Skardu (their northern unit) which has many geochemical similarities with the Hunza Formation, in particular £at REE patterns and low (Ce/Yb)N values. By contrast, although Robertson and Collins (2002) describe units to the east of Nanga Parbat lithologically similar to the Gilgit Formation and Yasin Group (the Burje-La and Pakora formations respectively), there are apparently no units in the area that they mapped which are lithologically similar to the Ghizar Formation. The Hunza Formation is bounded to the west by the

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Ghizar Formation with the contact probably located in the Naltar Valley. It is uncertain whether the contact is gradational or fault controlled. Fig. 3 summarises the key elements of the CVG stratigraphy within the Hunza Valley. The rocks have been deformed by tight to isoclinal, upright folds with a wavelength of 500^1000 m. As such original stratigraphic thicknesses cannot be estimated, the thicknesses noted are thus those of tectonically thickened sequences. The Hunza Valley section represents an apparent stratigraphic thickness of c. 9 km. Exposure is almost continuous throughout the section with an average dip of 70^80‡ towards the NNE. In general, rocks young to the north, although there are local younging variations in close proximity to upright isoclinal folds. The amount of thickening is di⁄cult to estimate. There are no major repetitions due to folding as is evidenced by the consistent northward younging direction and the geometry of the mappable folds. In this steeply dipping sequence the folds are highly asymmetric and verge northward with only short north-dipping limbs. Any signi¢cant thickening must therefore be due to internal thrusting of the unit during early stages of suturing prior to steepening of the rocks. However, there are no obvious thrust zones, although they might be di⁄cult to recognise in a dominantly ma¢c sequence with no clear marker horizons. Any mylonitic fabrics which dated from thrusting may have been annealed during later metamorphism. That any shortening deformation was strongly partitioned is indicated by the widespread preservation of essentially undeformed volcanic features. Given the lack of evidence for internal thrusting it is unlikely that the Hunza Valley sequence has been more than doubled in thickness during deformation. The lowermost 4 km of exposed rock comprise basaltic and andesitic £ows, whilst the upper 5 km comprise a variety of lithologies including basalt and andesite £ows, bedded ma¢c to felsic tu¡s, rhyolites, dacites and ignimbrites. The most felsic component of the stratigraphy is around 1 km thick and immediately overlies the lower 4 km of basaltic and andesitic £ows. A 2.5-km-thick basalt and basaltic tu¡-dominated sequence overlies the rhyolites and felsic tu¡s. In turn this is

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overlain by an uppermost tu¡-dominated sequence. Thus the ratio of volcaniclastic material to e¡usive sheets increases with stratigraphic height within the Hunza Valley sequence. Twelve km of consistently northward younging, thickened volcanic stratigraphy, which represents the stratigraphic downward continuation of the Hunza Valley section, is exposed in the Bagrot Valley (Fig. 2). The bulk of this section is similar in character to the base of the Hunza Valley section. It dominantly comprises basalt and andesite sheets with varying amounts of interbedded tu¡aceous material although the volcaniclastic component increases within the lowermost 2^3 km of the section where a mixture of basaltic and rhyolitic lavas and tu¡s is present. When the Bagrot and Hunza Valley sections are combined they comprise a 21-km-thick, consistently north-younging volcanic sequence, albeit tectonically thickened. Although it is unlikely that the Hunza Valley section has been more than doubled in thickness during deformation, the amount of thickening in the Bagrot Valley section could be greater as exposure is less continuous and high strain zones more widely developed. As in the Hunza Valley section, the general northward younging nature of the rocks suggests that any thickening is the result of thrusting rather than folding. This suggests that the present day thickness is the result of telescoping of back-arc basin crust early in the history of suturing, although it is not possible to quantify the thickness of that crust. Of the 21 km thickness, about 14 km (66%) is composed of basalt, basaltic andesite and andesite sheets, with varying amounts of interbedded volcaniclastic material. At least 1^2 km (9%) consists of dacite and rhyolite sheets and pyroclastic rocks with the remaining 5 km (25%) largely comprised of basaltic to intermediate volcaniclastic rocks. The bulk of the Hunza Formation thus represents e¡usive basaltic to basaltic andesite volcanism with associated pyroclastic activity. The stratigraphic history of the Hunza Valley section implies volcaniclastic activity with minor lava £ows, passing upward into a phase of lava-dominated activity before concluding with another phase of volcaniclastic-dominated activity, initially acidic

in nature but becoming more basic with time. Many of the lava £ows are pillowed and there are numerous examples of £ow^£ow contacts with little or no intervening ash. This facies is interpreted as the product of submarine volcanism with high e¡usion rates and limited seawater^ magma interaction. The c. 1-km-thick felsic volcanic unit may be the product of a magmatic fractionation cycle with evolved silicic magmas drained from a strati¢ed subarc magma chamber. Subsequently the magma chamber re¢lled with primitive basaltic magma and volcanism reverted to eruptions of basalt and basaltic andesite lavas. Thin, c. 1^2-m-thick welded ignimbrite £ows do not ¢t easily with a back-arc submarine setting. Submarine ignimbrites have been documented in arc settings (e.g. Howells et al., 1986, 1991) but these tend to be thicker, larger-volume deposits, which originate in subaerial settings and move down slope into aqueous depositional environments retaining a high enough temperature and cohesion for a su⁄ciently long period of time to weld. It is unlikely that the thin Hunza Formation ignimbrites could have welded in a subaqueous setting (e.g. Sparks et al., 1980; Cas and Wright, 1991; Mandeville et al., 1996; Freundt et al., 2000). These units thus indicate that part of the Hunza Formation volcanic ¢eld was subaerial, on an ephemeral basis at least. As the appearance of subaerial volcanoes coincides with the mostevolved period of volcanism within the Hunza Formation, it is likely that the back-arc basin supported a number of large, evolved stratovolcanic edi¢ces for a limited period. The ratio of tu¡ to lava reaches a maximum within the upper Hunza Formation suggesting a change in eruptive style as volcanism waned. This culminated in the deposition of the sedimentdominated Yasin Group (Pudsey et al., 1985; Pudsey, 1986; Robertson and Collins, 2002). During the waning stage, volcanism was dominated by small- to medium-volume pyroclastic eruptions, possibly into shallow water or in subaerial settings. Tu¡s represent both primary welded fallout material and distal clastic deposits reworked by turbidites within shallow, subaqueous basins (Cas and Wright, 1987; McPhie et al., 1993; Carey, 2000; Millward et al., 2000). The presence

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of peperites and volcaniclastic units within the lowermost Yasin Group suggests that episodic, but minor, volcanism continued into Yasin Group times. To the east, however, in Baltistan, the local equivalent of the Yasin Group, the Pakora Formation, contains much greater volumes of lava £ows, coarse lava breccias and tu¡s (Robertson and Collins, 2002). This suggests that the marginal basin setting remained volcanically active in some areas later than in others. This underlies the logic in placing the boundary between the Chalt and Yasin groups at the base of the calcareous sequence. 4.2. Ghizar Formation Like the Hunza Formation, the Ghizar Formation overlies the Jaglot Group (Treloar et al., 1996) and is overlain to the north by the Yasin Group (Pudsey, 1986), although all of these contacts are steeply dipping. Three distinct facies are recognised within the Ghizar Formation : the Ishkoman Volcanic Centre; a tu¡-dominated facies; and a basalt and andesite sheet-dominated facies. Ishkoman Volcanic Centre (IVC) is centred on the Ishkoman Valley (Fig. 2) as documented by lithofacies present at Hatoon which are dominated by proximal fallout deposits. These include: volcanic bombs up to 10 cm in diameter with associated impact structures ; abundant andesite lapilli within welded tu¡ sequences which have highly cuspate, whispy morphologies indicating hot and plastic emplacement within host fallout tu¡s; agglutinate-spatter deposits and very coarse volcanic breccias with lithic clasts up to 50 cm long, usually associated with proximal volcanic settings (Cas and Wright, 1987; Hackett and Houghton, 1989; McPhie et al., 1993). Other key eruptive environment indicators include: tu¡s with low-angle cross-bedding structures indicating surge activity (e.g. Wilson, 1986; Cas and Wright, 1987; McPhie et al., 1993; Valentine and Fisher, 2000); inverse grading of ¢amme indicating pyroclastic £ow activity (e.g. Sparks, 1976); and epiclastic volcaniclastic units with graded bedding, erosive bases and rip-up clasts of underlying units. The IVC is interpreted as a subaerial island arc stratovolcano centre similar to modern arc volca-

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noes such as Ruapehu or Ngauruhoe, New Zealand (e.g. Nairn and Self, 1978; Hackett and Houghton, 1989; Donoghue et al., 1997). The volcanic sequences at Ishkoman represent proximal andesitic to dacitic, pyroclastic eruptions dominated by fallout activity with accompanying minor surges and pyroclastic £ows. The presence of accidental, as well as juvenile, clasts suggests that there were numerous vent-clearing eruptions which mixed country rock lithologies and juvenile magma. Some of this material fell directly into lakes or local marine basins, or was transported there by £uvial systems. Turbidite currents reworked the ash and lapilli within subaqueous settings and deposited volcaniclastic turbidite deposits (Cas and Wright, 1987; Kokelaar et al., 1990; McPhie et al., 1993; Carey, 2000; Millward et al., 2000). Flame structures and rip-up clasts testify to the loading of successive units on wet sediment and the erosive nature of the turbidity currents. In summary, these features are interpreted as indicating an explosive volcanic centre which produced a range of pyroclastic activity and exhibited variable degrees of explosivity. Original dispersal directions and distances of erupted material cannot be estimated, although the presence of moderately sorted tu¡s east and west of the Ishkoman Valley, which may be broadly correlated with the Ishkoman centre, suggests dispersion over 10^50 km. The data suggest that the volcanic centre was a subaerial cone with Strombolian and Vulcanian eruptive styles (e.g. Cas and Wright, 1987; McPhie et al., 1993; Bertagnini et al., 1999; Morrissey and Mastin, 2000; Vergniolle and Mangan, 2000; Adams et al., 2001). The tu¡-dominated facies crop out mainly between Gilgit and Gupis and around Phander (Fig. 2). There is a gradation between this facies and the coherent lava £ow-dominated facies with, in places, an ash fall to lava £ow ratio close to 1:1. Typical lithologies within this facies include: ¢negrained, laminated intermediate to silicic tu¡, sometimes with basal lag units; normally graded felsic tu¡; massive vitric tu¡; welded lapilli tu¡ and cross-bedded tu¡s. The tu¡-dominated assemblages document medial to distal pyroclastic fall and reworking of fall deposits by subaqueous turbidity currents. That the outcrop of the tu¡-

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dominated assemblages is in close geographic proximity to the IVC suggests that the tu¡s may represent distal products of this volcano. The massive vitric tu¡s may represent highly distal fall deposits enriched in vitric material as a consequence of convective sorting. The occasional welded lapilli tu¡s and ignimbrites are probable pyroclastic £ow deposits indicative of a subaerial setting. The basalt and andesite sheet-dominated facies crops out between Yasin and Phander and also west of Phander (Fig. 2). This facies is dominated by large thicknesses of basalt and andesite lava £ows interleaved with tu¡. Lava compositions range from basalt to dacite and rhyolite, with basaltic andesite to andesite being the modal composition (Bignold et al., in prep.). This facies has similar physical characteristics to the main facies of the Hunza Formation in that it contains pillow lavas, many £ow^£ow contacts and minor, though variable, contents of intervening volcaniclastic material. However, this facies has a chemistry distinctive from that of the Hunza Formation with a higher proportion of dacitic and rhyolitic rocks and a distinctly di¡erent geochemical signature (Petterson and Windley, 1991 ; Bignold et al., in prep.). The pillow lavas provide evidence that at least part of the volcanism occurred within a subaqueous setting. The generally high ratio of lava £ows to ash fall sediments is an indicator of a proximal position relative to an eruptive centre as silicic lavas in particular rarely £ow far from the vent (e.g. Cas and Wright, 1987). It is likely that the basalt and andesite sheet-dominated facies represents lavas and sills coalescing from adjacent stratovolcanoes. Eruptive styles were predominantly e¡usive and e¡usion rates probably high. The stratovolcanoes may have erupted largely within a subaqueous setting but were probably periodically emergent. 4.3. Shamran Volcanic Group The SVG crops out as a series of outliers unconformably overlying the steeply dipping amphibolite facies rocks of the CVG, as well as parts of the Kohistan batholith and, possibly, the Yasin

Group (Fig. 2). It is undeformed, uncleaved and unmetamorphosed, and thus clearly postdates suturing of Kohistan to Asia along the Shyok Suture. Field data presented here suggest that the SVG crops out in a number of high-altitude outliers between Shamran Village and Shandur Pass (Fig. 2), although its original extent has been much reduced by erosion. The area over which the SVG was erupted is unknown. That it does not crop out in the hills that £ank the Ghizar Valley, to the east of Shamran and Phandur villages, or in the hills that £ank the Hunza Valley could imply that its areal extent was very restrictive and that it never extended that far. Alternatively, its areal extent could have been much greater than that indicated in Fig. 2, with its current exposure a function of present day erosion levels further east being below that of the basal unconformity. In any event, the ¢eld data presented here do not support the extensive distribution of the SVG postulated by Sullivan et al. (1993). Danishwar et al. (in review) present hornblende Ar^Ar ages of 43.8 O 0.5 and 32.5 O 0.4 Ma for the SVG. These ages are younger than that of postmetamorphic cooling in N. Kohistan (Treloar et al., 1989). The are also consistent with the ¢eld relationships in which the £at-lying, undeformed and unmetamorphosed rocks of the SVG unconformably overlie the steeply dipping strongly cleaved and metamorphosed rocks of the SVG. Thus they probably date extraction of the primary SVG magmas from the mantle, extraction that postdates initial stages of collision of India and Kohistan. Danishwar et al. (2001, their ¢gure 5) suggest that the SVG crops out in the valley £oor at Teru and SW of Teru in the uppermost parts of the Ghizar Valley just east of the Shandur Pass. Our ¢eld data show this to be incorrect. Instead, the £at-lying rocks of the SVG lie above a subhorizontal unconformity which crops out at between 500 and 1000 m above the Ghizar Valley £oor from Shamran westward (e.g. Fig. 12d). At Shamran, Pingal and Teru, valley £oor exposures are of steeply dipping, cleaved rocks of the CVG. SW of Teru, valley £oor exposures are of granitoid rocks of the Kohistan batholith that are in-

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trusive into the CVG. The sections drawn across this terrain by Danishwar et al. (2001), which suggest that the SVG is steeply dipping, are clearly incorrect given the subhorizontal layering of units within the SVG documented here, and the fact that the SVG never crops out at valley £oor level. The corollary of this is that the log through the SVG described by Danishwar et al. (2001, their ¢gure 7) is also erroneous. Essentially their log is a traverse across, what they infer to be, a steeply dipping unit. Not only is the unit subhorizontal, but the log takes no account of topographic height. The full thickness of the SVG is not the 3 km indicated by Danishwar et al., but is a function of the maximum altitude of the mountains in the region. From valley £oor to mountain top is c. 1.5 km and this is the minimum thickness

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of the SVG. A combination of our ¢eld observations and the data reported by Danishwar et al. (2001) suggests the base of the SVG to be of dominantly felsic pyroclastic rocks, up to 1 km thick, overlain by basaltic and then andesitic volcanic rocks. The ratio of pyroclastic, acid andesitic and rhyolitic rocks to basalts is about 5:1 suggesting that the SVG is a highly evolved silicic volcanic sequence. The range and association of lithofacies that we have collected from the SVG (ignimbrite, rhyolite, andesite, volcanic breccia, volcanic sandstone) suggest that the they were deposited within an intra-continental, largely subaerial setting with ephemeral lakes and active £uvial systems. Volcanic centres were predominantly andesitic and dacitic in composition and eruptive styles varied

Fig. 13. Palaeogeographical model for the eruptive setting of the CVG. The IVC is built on the £anks of the arc. The Ghizar Formation was deposited in a within-arc setting, probably synchronously with construction of the IVC, and with deposition of the Hunza Formation within a back-arc setting.

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between e¡usive and highly explosive and included eruption of lavas, pyroclastic £ows and ash. These rocks form part of a continuum of subaerially extruded felsic volcanics that characterise the Late-Cretaceous to Eocene^Oligocene evolution of the arc. They are clearly younger than the, dominantly felsic, Utror volcanics of the Kohistan arc (Sullivan et al., 1993) with which they cannot be correlated. It is, however, unclear as to whether they correlate temporally with the Dras 2 unit in Ladakh with which they show many lithological similarities. (Robertson and Degnan, 1986). The Dras 2 unit could be the temporal equivalent of the Utror volcanics, the SVG or neither, although all three units clearly document a late-stage, felsic volcanism which either shortly predates or postdates collision between continental India and the Kohistan^Ladakh arc.

5. Eruptive palaeogeography and tectonic environment 5.1. Chalt Volcanic Group Fig. 13 and Table 1 summarise a palaeogeographic model for the eruptive setting of the CVG. In this model the Ghizar Formation is interpreted as part of the main body of the Kohistan arc volcanic structure and the Hunza Formation as a back-arc basin developed along the northern margin of the arc. This is consistent with the interpretations of Rolland et al. (2000) and Robertson and Collins (2002), based on geochemical and lithological data respectively from analogous rocks to the east of the Nanga Parbat syntaxis. The greater amount of olistromal debris in the sections described from the sections further east suggests that the Hunza Formation back-arc

Table 1 Chalt & Shamran Volcanic Group Facies and Eruptive Environments Volcanic unit

Key lithofacies

Eruptive environment

Shamran Volcanic Group ‘Red-bed’ volcanic sandstones and conglomerates. Primary andesites, rhyolites, ignimbrites and breccias.

Intra-continental andesitic-dacitic stratovolcano and £anking plains with £uvial and lacustrine systems.

Ghizar Formation Proximal facies: andesitic bombs with impact (Chalt Volcanic Group): structures, spatter-agglutinate deposits, welded Ishkoman Volcanic fall, surge and?£ow deposits, reworked tu¡s Centre

Andesite stratovolcano with Strombolian^Vulcanian eruptive activity. Predominantly subaerial. Fluvial and lacustrine reworking of deposits in ring-plain surrounds.

Ghizar Formation (Chalt Volcanic Group): Tu¡-dominated assemblages

Zone of pyroclastic and epiclastic sediment accumulation adjacent to stratovolcano centres. Subaqueous and subaerial.

Medial to distal andesitic to rhyolitic tu¡s and lapilli tu¡s, both primary and reworked. Occasional thin ignimbrites. High tu¡: coherent sheet ratio.

Ghizar Formation Basalt, andesite and dacite sheets (lavas and (Chalt Volcanic Group): sills). Associated tu¡s. High coherent sheet: Basalt-andesite sheettu¡ ratio. dominated assemblages

Coalescence of andesitic volcanic centres. E¡usive and explosive volcanic eruptions. Moderate e¡usion rates. Predominant subaqueous setting.

Hunza Formation Primary and epiclastic basalt-andesite tu¡s, (Chalt Volcanic Group): associated lavas and peperite sills (continues Waning eruption cycle into Yasin Gp)

Low eruption rate. Predominant pyroclastic volcanism. Subaqueous reworking. Subaqueous and? subaerial settings.

Hunza Formation Dacite-rhyolite sheets and interbedded primary Stratovolcano-eruptions. Volcanoes become (Chalt Volcanic Group): tu¡s. Thin ignimbrites and welded tu¡s. ephemerally subaerial. Magma chambers produce large Dacite-rhyolite unit volumes of acid magma. Hunza Formation (Chalt Volcanic Group): Basalt, basaltic andesite and boninite sheets

Monotonous sequence of basalt, basaltic andesite, andesite and boninite sheets. Predominant basaltic composition. Limited amount of intersheet tu¡.

Submarine (pillow lavas). High e¡usion rate. Limited magma fractionation in subvolcanic magma chambers. Back-arc basin setting

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may be more proximal than either of them. Subaerial volcanoes produced a spectrum of geological deposits from proximal breccias, welded tu¡ and agglutinate deposits to medial coarse-grained volcaniclastic rocks and basalt-andesite lavas. Distal deposits were dominated by ¢ner-grained and better-sorted volcaniclastic sequences, particularly within intra- or fore-arc subaqueous basins bordering the subaerial arc volcanoes. On the basis of sedimentological data (Robertson and Collins, 2002) and the MORB-like geochemistry of the ma¢c rocks (Rolland et al., 2000, 2002; Bignold and Treloar, 2003) the Hunza Formation is interpreted as having evolved within a back-arc setting (Fig. 13). It is £anked to the south by within-arc volcanic sequences of the Ghizar Formation and Jaglot Group. Back-arc development was probably a result of lithospheric stretching that could have been caused by either subduction zone rollback, subduction of an active spreading ridge or plume activity behind the arc. Khan et al. (1997) suggested that the presence of boninitic lavas implied a fore-arc setting for the Hunza Valley Formation. However, boninites have been recorded in a number of back-arc settings (cf. Falloon et al., 1992; Me¡re et al., 1996) and it is likely that their sparse recognition in modern back-arc settings is largely a function of sediment cover. Boninitic lavas form a minor part

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of the volcanic sequence (Petterson et al., 1990; Bignold et al., 2001). These would be consistent with high heat £ows in the back-arc basin. The back-arc volcanoes were probably low-angle shield volcanoes and ¢ssure-centred volcanoes which erupted proli¢c volumes of extensive basaltic and andesitic, with minor boninitic lavas. Occasionally the volcanoes became subaerial and erupted relatively small volumes of highly evolved silicic lavas and pyroclastic rocks. Fig. 14 shows the plate tectonic setting of the CVG. The main Kohistan arc formed above a north-dipping subduction zone (after e.g. Petterson and Windley, 1985). It remains unclear how far from the southern margin of Asia the arc was originated. Rolland et al. (2000) suggested on the basis of a limited geochemical dataset that the Kohistan^Ladakh arc was attached to the Asian margin at its eastern end and that the arc was never far from the Asian continental margin. Two lines of evidence suggest this to be unlikely. Having recognised that the contact between the Ladakh section of the arc and Asia is marked by a zone of tectonic melange, Robertson and Degnan (1986) argued that the whole arc system was allochthonous with respect to Asia. In addition, Clift et al. (2002) showed that there was no continental in£uence on the major, trace and isotopic chemistry of the rocks of the Ladakh arc.

Fig. 14. Plate tectonic setting for eruption of the CVG. The main arc, of which the Ghizar Formation and the IVC are part, was erected above a north-dipping subduction zone. By contrast the Hunza Formation was deposited within an eruptive setting developed within a back-arc basin.

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Fig. 15. Schematic palaeogeographical model for the eruptive setting of the SVG. The Group was likely deposited within rapidly developing rift systems, possibly developed above deep-seated crustal structures.

Hence, there are currently no chemical or sedimentological constraints on the width of the ocean separating the arc from the southern margin of Asia. It could have been as much as thousands of kilometres as suggested by Khan et al. (1997). How wide or extensive was the back-arc basin is similarly unclear. The ¢eld data presented here together with geochemical data presented by Petterson et al. (1990), Bignold and Treloar (2003) and Rolland et al. (2000, 2002) show the present day remnant of the back-arc basin to be lenticular in shape, narrowing to both east and west, with a maximum thickness between Gilgit and Skardu. This could represent the original shape and extent of the back-arc basin: short-lived, narrow and of limited lateral extent. Alternatively the basin could have been much wider with much of it subducted beneath the Asian margin as suturing commenced. The thickness of the Hunza Formation sequence preserved in the Hunza and Bagrot valleys suggests that full back-arc spreading, with development of true oceanic crust, had commenced prior to suturing. This would imply that

the original basin was much larger than the remnant preserved today. 5.2. Shamran Volcanic Group Fig. 15 and Table 1 summarise a palaeogeographic eruptive model for the SVG. Intra-montane extensional rift, graben or caldera structures probably formed above deep-seated, basement crustal structures. These volcanic rocks were probably derived from silicic magma formed within strati¢ed crustal magma chambers and erupted as large-volume silicic ignimbrites and related rocks from steep-sided silicic stratovolcanoes onto deformed rocks of the early Kohistan arc. (e.g. Wilson, 1986; Cas and Wright, 1987; Howells et al., 1991; McPhie et al., 1993; Freundt et al., 2000; Millward et al., 2000). Caldera structures probably formed in response to magma chamber evacuations. A complex sequence of volcanic facies systems probably formed within caldera-graben structures including intra-caldera ignimbrites and breccias, dacite and rhyolite

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Fig. 16. Plate tectonic setting for eruption of the SVG. The SVG reprsents post-collisional silicic volcanism, probably within rift systems. The primary magma was sourced from relict, subduction metasomatised Tethyan mantle. The silicic volcanism represents mixing of this with melts generated within the pre-existing arc crust.

domes, coarse, £uvial point-bar and deltaic sands, and ¢ner-grained, well-sorted lacustrine sands and silts. Fig. 16 shows the tectonic setting of the SVG. The Eocene^Oligocene hornblende ages demonstrate that the SVG was erupted after initiation of India^Asia collision at 50^55 Ma. It is unlikely, given the convergence rates (Patriat and Achache, 1984) between India and Asia and the width of the currently exposed arc, that the leading edge of the continental margin of the Indian Plate would have underplated the northern margin of the Kohistan arc before eruption of the Oligocene SVG. This is consistent with mantle-type Sr and Nd isotopic signatures reported by Danishwar et al. (in review) which suggest little contamination from Indian Plate continental crust. The SVG Sr^Nd isotopic signature is close to that of the I-type Matum Das, Gilgit and Shirot granitoids of the Kohistan batholith (Petterson et al., 1993). The alkaline and shoshonitic nature of some of the SVG magmas may thus be largely a function of primary magma extraction from an extensively metasomatised, residual Tethyan mantle wedge sitting above subducted Tethyan oceanic crust. The isotopic signature of the volcanic rocks sug-

gests that metasomatism of the mantle wedge was a function solely of subduction processes prior to subduction of the leading edge of continental India. Extension within the arc, possibly as a result of subduction zone rollback as the more buoyant Indian Plate continental crust began to enter the subduction zone, could have caused intra-montane rifts to form, into which highly evolved and di¡erentiated magmas were extruded.

Acknowledgements Field work and analytical costs were supported by the Royal Society of London. Giray Ablay, Alastair Robertson and Yann Rolland provided detailed reviews of the manuscript.

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