the formation and bulk composition of modern juvenile continental crust: the kohistan...

Chemical Geology xxx (2012) xxx–xxx

CHEMGE-16340; No of Pages 18

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Chemical Geology

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The formation and bulk composition of modern juvenile continental crust:The Kohistan arc

O. Jagoutz a,⁎, M.W. Schmidt b

a Dept. of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, USAb Institut of Geochemistry and Petrology, ETH Zurich, 8092 Switzerland

⁎ Corresponding author.E-mail address: [email protected] (O. Jagoutz).

0009-2541/$ – see front matter. Published by Elsevier Bdoi:10.1016/j.chemgeo.2011.10.022

Please cite this article as: Jagoutz, O., Schmiarc, Chem. Geol. (2012), doi:10.1016/j.chem

a b s t r a c t
a r t i c l e i n f o
Article history:Received 21 June 2011Received in revised form 27 October 2011Accepted 28 October 2011Available online xxxx

Editor: K. Mezger

Keywords:Continental crustArc magmatismKohistan arc

The intraoceanic Kohistan arc, northern Pakistan, exposes a complete crustal section composed of infracrustalbasal cumulates formed at ≤55 km depth, a broadly basaltic/gabbroic lower crust, a 26 km thick calc-alkalinebatholith, and 4 km of a volcanoclastic/sedimentary sequence. The bulk composition of the Kohistan arc crustis approximated by estimating the relative volumes of exposed rocks through detailed field observations,in particular along a representative km-wide transect across the arc, through geobarometric constrains todetermine the unit thicknesses, and through satellite images to estimate their lateral extent. We separatedthe arc into three major units: lower, mid-, and mid- to upper crust, which contain a total of 17 subunitswhose average compositions were derived from employing a total of 594 whole rock analyses. Thevolume-integrated compositions of each unit yield the bulk composition of the arc crust. While the detailsof the resulting bulk composition depend slightly on the method of integration, all models yield an andesiticbulk supra Moho composition, with an average SiO2 of 56.6–59.3 wt.% and XMg of 0.51–0.55. The Kohistan arccomposition is similar to global continental crust estimates, suggesting that modern style arc activity is thedominant process that formed the (preserved) continental crust. A slight deficit in high field strength andincompatible elements in the Kohistan arc with respect to the global continental crust can be mitigated byadding 6–8 wt.% of (basaltic) intraplate type magmas. Our results document that infra arc processes, evenin a purely oceanic environment, result in an overall andesitic crust composition in mature arcs, contraryto the widely accepted view that oceanic arcs are basaltic. Bulk crust differentiation from a basaltic parentoccurs through foundering of ultramafic cumulates. Our results imply that secondary reworking processessuch as continental collision are of secondary importance to explain the major element chemistry of thebulk continental crust composition.

Published by Elsevier B.V.

1. Introduction

Based on trace element similarities, it is widely accepted thatmodern continental crust is dominantly created at convergentmargins (Rudnick, 1995; Barth et al., 2000). In subduction zones,fluid assisted melting of upwelling wedge mantle leads to primitive,dominantly basaltic melts, which ascend and fractionate (Kushiro,1990; Grove et al., 2003). The so-called “andesite model” of continen-tal crust growth (Taylor, 1967) holds that arcs produce andesitic bulkcrust compositions, in accord with the broadly andesitic bulk compo-sition of the continental crust (Rudnick and Gao, 2003). Estimates ofthe bulk composition of oceanic arcs however, indicate that the bulkcomposition of island arcs may be closer to basalt than to andesite(Smithson et al., 1981; Kay and Kay, 1986; Holbrook et al., 1999), im-plying that additional reworking processes that transform the basaltic

.V.

dt, M.W., The formation and bgeo.2011.10.022

island arc crust into andesitic continental crust are essential (Leeet al., 2007). Alternatively, Kelemen (1995) has proposed thatisland–arc crust may contain a substantial proportion of high-Mgandesites characterized by an XMg (XMg=Mg/(Fe+Mg)) that ishigher than for modern average andesites and is therefore appropri-ate to explain the high XMg of the bulk continental crust.

To differentiate primitive basaltic melts and a possibly initiallymore basaltic bulk arc crust toward an evolved continental crust, anultramafic fraction must be separated from the evolving melt. Mostprobably, this happens through accumulation and foundering ofgravitationally unstable, mafic to ultramafic, dense cumulates precip-itated at the base of the crust (Kay and Kay, 1993; Jull and Kelemen,2001; Jagoutz et al., 2011). Alternatively, partial melting of thelower crust could lead to an eclogitic s.l. residue that may gravitation-ally slump into the mantle (Tatsumi and Kogiso, 2003). Understand-ing the details of these processes and thus the origin of continentalcrust is hampered by our limited knowledge of the composition andstructure of volcanic arcs, in particular, their deeper sub-batholithicparts, which are rarely exposed and accessible.

ulk composition of modern juvenile continental crust: The Kohistan

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In order to test such hypotheses of continental crust formation,it is crucial to constrain the composition and deep structure ofarcs, ideally those where crust forms anew without interaction withpreexisting continental crust. The ideal site for such an undertakingis the Cretaceous to Tertiary intraoceanic Kohistan island arc, whereall levels of the arc are exposed, from ultramafic cumulates at thebasis of the crust to granulite facies lower crust to a mid- andupper-crustal batholith, and finally to surficial volcanoclastics andsediments (Tahirkheli, 1979).

In this paper we provide a quantitative estimate of the bulkcomposition of the Kohistan arc crust. This is accomplished throughreconstruction of the Kohistan arc section by approximating the pro-portions of each lithology based on geobarometric results combinedwith volumes derived from the present day exposure, integration ofover 500 bulk rock analyses and, finally, calculation of the bulk crustcomposition. Finally, we discuss this composition and its direct impli-cations for the formation of the continental crust.

2. Geological setting

The Kohistan arc (Fig. 1), exposed in NE Pakistan, is a fossilCretaceous to Tertiary arc complex (Tahirkheli, 1979; Bard, 1983)formed over a presumably north dipping subduction zone in theequatorial part of the Tethyan ocean. The arc was obducted and sand-wiched between the colliding Indian and Eurasian plates duringthe Himalayan orogeny. It is separated from the Eurasian marginby the Shyok or Karakoram–Kohistan sutures and from the Indiancontinent by the Indus suture zone (Fig. 1). Juvenile oceanic arcmagmatic activity, constrained by U–Pb zircon ages, and Hf, Pb, Ndand Sr isotopic data occurred dominantly between 110 and 50 Ma(Jagoutz et al., 2009).

The large-scale geodynamic evolution of the arc is only in partwell constrained. While it is well established that the Kohistan–Indiacollision occurred around ~50 Ma (Garzanti et al., 1987; Gaetani andGarzanti, 1991), debate circles around the exact timing of the collisionbetween the Kohistan arc and Eurasian margin, along the ShyokSuture. The closure of the Shyok Suture has been considered topredate the India–Eurasia collision (~104–85 Ma, Petterson and

Karakoram-K

Mingora

Chitral

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(Ortho-) Amphibolites

Mastuj

Dir

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(Jijal, Sapat)

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Yasin detrital series

Volcanosedimentary groups(Dir, Utror, Shamran and Chalt)Metasediments

Plutonic rocks (Kohistan batholith)

Gabbro-norite with ultramafite

Gabbro/diorite plutons with ultramafite

Mantle Ultramafite

Besham

JijA

(Meta-) Diorite

Felsic Intrusions

Jij

(Para- & Ortho-) Amphibolites

73°0'00''72°0'00''E

35°0'00''N

Fig. 1. Geological map of the Kohistan arc. The map illustrates well the dominance in outcropthe Southern Plutonic Complex. Geobarometry attributes similar thicknesses to the Gilgit C

Please cite this article as: Jagoutz, O., Schmidt, M.W., The formation and barc, Chem. Geol. (2012), doi:10.1016/j.chemgeo.2011.10.022

Windley, 1985; Treloar et al., 1996). However, the constraints on thetiming of collision have been questioned (Jagoutz et al., 2009) andpaleomagnetic data indicate that India collided first with Kohistan(b50–60Ma, Bard, 1983; Khan et al., 2008) before colliding withEurasia. The importance of this finding for this study is that, untilcollision with the Karakorum block, which then constituted thesouthern margin of Asia, the Kohistan arc must have developed in anintraoceanic setting. Irrespective of which collision scenario is correct,the absence of an old continental basement, the paleomagnetic data andthe lack of evidence for any contamination of the Kohistan intrusives by(ancient) continental crust until b50Ma (see discussion below) indicatesformation of the Kohistan arc far removed from continental crust. Thus,the pre-50 Ma Kohistan arc represents juvenile continental crust.

Intra-arc extension followed by sandwiching between the Karakorumand Indian plate and the underplating of the Indian subcontinent led tocomplete exposure of a continuous section of arc crust fromuppermantlerocks in the south to unmetamorphosed sediments in the north (Burget al., 2006). Recent advances in understanding the complex geologyof the Kohistan arc and increasingly available P–T determinations(Yoshino et al., 1998; Ringuette et al., 1999; Yoshino and Okudaira,2004; Enggist, 2007) indicate that, overall, the exposed thickness ofthe Kohistan agrees reasonably well with geobarometrically derivedpressures, suggesting that the crustal section is intact (Figs. 2–4).Major faults are absent in the east (Fig. 1), but some larger scale faultsare present in the west (e.g., Dir-Kalam fault, Jagoutz et al., 2009;Sullivan et al., 1993; Treloar et al., 1996). Estimated equilibration pres-sures range from ~1.5 GPa at the base of the garnet gabbro, which con-stitutes the Moho, to ~0.8 GPa at the top of the Southern PlutonicSequence (Fig. 2, see figure legend for references). This pressure differ-ence of 0.7 GPa correlateswell with the inferred lower crustal thickness(~26 km, Table 1). The batholith itself is continuously exposed from itsdeepest intrusion levels of ~0.8 GPa in the southeast to epithermal plu-tons in the northwest (Enggist, 2007) and continues into volcanoclastic,carbonate and terrigenous sedimentary rocks (Fig. 4). This picture iscomplicated by sedimentary sequences contained in and intruded bythe batholith at much deeper levels (Fig. 1). Between the lower andmiddle to upper crust of the Kohistan arc, the extension-related Chilasgabbronorite complex (Fig. 3, Jagoutz et al., 2006, 2007) is exposed.

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ohistan Suture

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area of the mid- to upper crust batholith with respect to the lower crust represented byomplex and the Southern Plutonic Complex.



Kiru

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hbl-diorite 1.4 km

lower Kiru sequence 3.2 km

Leucogranites 1.7 km

Chilas complex (Fig.3)

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0.85±1.1

E07

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Fig. 2. Profile across the Southern Plutonic Complex along the Indus transect (simplified after Burg et al., 2005). For each unit the preserved thickness is given in km.Geobarometric calculations in grey ellipsoids are in GPa and from Enggist (2007) (E07), Ringuette et al. (1999) (R99), Yamamoto (1993) (Y93), Yoshino et al. (1998) (Y98).

3O. Jagoutz, M.W. Schmidt / Chemical Geology xxx (2012) xxx–xxx

3. The building blocks of the Kohistan arc

The arc can be divided from south to north into three major com-plexes: The Southern Plutonic Complex (SPC), the Chilas Complex andthe Gilgit Complex (Fig. 1). The SPC (Fig. 2) represents the upper man-tle/lower arc crust, is beautifully exposed along the Indus River in theJijal area, and is dominated by ultramafic to mafic rocks (Jan and Howie,1981; Miller et al., 1991). The Gilgit Complex (Fig. 4) represents themid- and upper arc crust, and is dominated by the calc-alkaline Kohistanbatholith, whichmakes up the northern two thirds of the Kohistan block.This batholith is predominantly composed of quartz-, quartzmonzo- andgranodiorites, but ranges in composition from picro-basaltic dikes to leu-cogranite stocks and sheets (Petterson andWindley, 1985, 1986). Volca-noclastic sequences and terrigenous silicic and carbonate sediments are

Fig. 3. Profile across the Chilas ComPressure estimates same symbols as


associatedwith the plutonic units, which in part intrude these sediments.The SPC is separated from the batholith by the Chilas Complex (Fig. 3), anextension-relatedmafic–ultramafic intrusionmainly composed of gabbro(-norite) associated with km-scale ultramafic rocks, minor diorites, andsubordinate tonalites (Jagoutz et al., 2006, 2007).

3.1. The Southern Plutonic Complex: the lower arc crust

3.1.1. Ultramafic cumulatesThe southernmost part of the SPC (Fig. 2) is composed of minor

serpentinite associated with the Indus suture, followed up-sectionby a series of ultramafic cumulates that range from dunites to wherlitesto olivine–clinopyroxenites and (ol)–websterites (Table 2), locally withtiny amounts of garnet. Harzburgites which represent possible residual

plex (after Jagoutz et al., 2007).in Fig. 2 and Jagoutz et al. (2007) (J07).



Fig. 4. Geobarometrical constrained section through the Kohistan crust including the Southern Plutonic Complex (lower left, details in Fig. 2), the Chilas Complex (middle, details in Fig. 3) andthe Gilgit Complex (upper right). The vertical axis is drawn to scale and all units to the thickness derived from geobarometry. Note the difference in the proportions of the garnet-gabbro andamphibolite (Kamilla andKiru) units in the Southern Plutonic Complexwith respect to Fig. 2which contains the directly observable preserved thicknesses. Geobarometry on theChilas Complexintrusives did not reveal any noticeable pressure gradient and thus the Chilas Complex is drawn as a horizontal transect. TheGilgit Complex consists in its lower 80% of plutons encompassing allcompositions fromgabbros andqtz-diorites to granodiorites andgranites at all levels, althoughmoremafic compositions are relativelymore abundant in the lower intrusion levels. The batholithis overlain by a volcaniclastic sequences and thenby sedimentswhich in turn are also intruded. Histograms show the frequency distribution (SiO2wt% (grey bars) andMg# (brownbars) on thelower andupper horizontal axes, respectively) for the different subunitsmentioned in the text. Vertical axes are number of analyses. Note that the frequency distributionofMg# is shownupsidedown for graphical reasons.


Please cite this article as: Jagoutz, O., Schmidt, M.W., The formation and bulk composition of modern juvenile continental crust: The Kohistanarc, Chem. Geol. (2012), doi:10.1016/j.chemgeo.2011.10.022


Table 1Field proportions of the main building blocks of the Kohistan arc and their relative contribution to the different integration models.

Unit Subunit rel. surface proportionsof main units

relative weightingfactor of subunits

mappedthickness [km]

Barometricthickness [km]

Ultramafic cumulatesDunite 0.067Wherlite 0.167Websterite 0.267Hbldite/grtite 0.5Total 1.00 3.5

Southern Plutonic ComplexGarnet metagabbro (incl .10% hbldite) 4.5 12.7Sarangar gabbro 2.7 4.3Lower Kiru sequence 3.2 3.0Hbl-diorite 1.4 0.8Main Kiru Sequence 6.9 2.3Leucogranites 1.7 0.4Kamilla amphibolites 5.5 1.1Total 0.17 25.9 24.6

Chilas ComplexGabbronorite sequence 0.95Ultramafic sequence 0.05Total 0.20 1.00

Gilgit ComplexPlutonic rocks 26Mafic (SiO2 b57 wt.%) 0.07Intermediate (SiO2 57–70 wt.%) 0.67Leucocratic (SiO2 >70 wt.%) 0.09Volcanics 0.17 4Total 0.63 1.00 30

Kohistan arc total 1.00 54.6


mantle are described (Miller et al., 1991; Bouilhol et al., 2009) but arenot observed in the Jijal transect. Cr-diopside-rich dykes cross-cut spi-nel bands within dunite bodies. Dunites (Fo88-92) decrease in abun-dance up-section and variable modes of olivine and pyroxene(s)define compositional banding. Amphibole becomes increasingly abun-dant up-section as pyroxene-rich ultramafic rocks grade intohornblende-, clinopyroxene- and garnet-rich cumulates with importantmodal variations. The replacive relationship between pyroxene-rich ul-tramafic rocks and dunites and the irregular transition from schlierento dyke-shaped clinopyroxene-bearing garnetite and hornblenditehave been interpreted by Garrido et al. (2007) as indicating numerouspercolative intrusive events and associated fractionation and melt-rock reactions within a semi-consolidated crystal mush.

3.1.2. Garnet granulite gabbrosThefirst occurrence ofmodally important plagioclase defines a sharp,

intrusive contact with granulite-facies garnet-gabbros and is inferred toconstitute the sub-arc Moho (Burg et al., 2005). Outcrop-scale modalvariation of garnet ranges from garnetite to rare, garnet-free gabbro.On average, garnet constitutes ~20–30 vol.%, the remaining mineralogybeing composed of plagioclase, clinopyroxene, amphibole and minorclinozoisite and oxides. Up to hundred meter-sized hornblendite bodiesoccur throughout the garnet gabbro (Arbaret et al., 2000; Burg et al.,2005), become less abundant up-section and constitute ~10% of thevolume.

3.1.3. Sarangar gabbroUp-section, the garnet-bearing gabbro is intruded by the 98.9±

0.4 Ma Sarangar metagabbro (Schaltegger et al., 2002). This gabbrois mainly composed of clinopyroxene, plagioclase and ~10–30 vol.%hornblende with minor garnet and in turn is intruded by hornblenditeand tonalite dykes. It forms the lowest part of the Southern Amphibolites(Burg et al., 2005) and corresponds to the Patan Complex of Miller et al.(1991). At the contact between Sarangar and garnet-gabbro a complicat-ed network of amphibole- or garnet-rich zones is present and has been


attributed to fluid induced reactions (Yoshino et al., 1998) or partialmelting (Garrido et al., 2006).

3.1.4. Kiru sequenceThe Sarangar gabbro is followed by a complex magmatic sequence,

the so-called Kiru amphibolites. The strongly sheared amphibolite-facies sequence comprises numerous mutually intrusive, variously de-formed meter to tens-of-meter scale meta-gabbros, diorites, tonalitesand minor hornblendites and plagioclase–quartz–amphibole pegma-tites. Calc-silicate enclaves are locally present (Yoshino and Satish,2001). A sheet-like, 1.4 km thick homogeneous hornblende diorite(Fig. 2) intruded the otherwise strongly variable Kiru sequence at91.8±1.4 Ma (Schaltegger et al., 2002).

3.1.5. Granite sheetsThe Kiru and Kamilla sequences are separated and intruded by

several muscovite+zoisite bearing leucogranite sheets. One singlesheet is 700 m thick (Fig. 2) but smaller, tens of meter-scale dykesoccur throughout the entire Kiru and Kamilla sequences. The thickgranite sheet yielded a U–Pb TIMS zircon age of 97.1±0.2 Ma(Schaltegger et al., 2002) whereas LA-ICPMS U–Pb zircon dates indi-cate ages as young as 75.7±1.4 Ma (Yamamoto et al., 2005). The es-timated cumulative thickness of all granites in the SPC amounts to~1.7 km.

3.1.6. Kamila amphibolitesThe uppermost portion of the Southern Amphibolites, the Kamila

amphibolites of Treloar et al. (1996), is a sequence of various(meta-)plutonic bodies with gabbroic, dioritic, tonalitic and graniticcompositions intruded into a lower amphibolite-facies volcano-sedimentary sequence. A granite has been dated at 97.1±0.2 Ma(Schaltegger et al., 2002). To the north, the Chilas Complex intrudesthe Kamila sequence. The Kamilla sequence also contains some finegrained and sometimes pillow-textured metabasalts which haveMORB geochemistry and are interpreted as remnants of the former



Table 2Average compositions of the subunits of the Kohistan arc. A more detailed version of the table can be found in the electronic supplement.

Southern Plutonic Complex Chilas Complex Gilgit Complex

Dunite Wherlite Websterite Hbldite/grtite

Garnet granulitegabbros (incl10% hbldite)

SarangarGabbro

LowerKiruSequence

Hbl-diorite

Main KiruSequence

Leucogranites KamillaAmphibolites

GabbronoriteSequence

UltramaficSequence

Maficplutonic

Intermediateplutonic

Felsicplutonic

Volcanics

Mean Mean Mean Mean Mean Mean Mean Mean Mean Mean Mean Mean Mean Mean Mean Mean Mean

N 6 4 7 7 45 17 18 3 13 24 18 133 52 22 41 23 161Model #1 (wt.%) 3.0 1.8 2.1 0.9 4.5 1.1 3.6 19.0 1.0 4.7 41.8 5.8 10.7Model #2 (wt.%) 18.6 6.3 4.4 1.2 3.3 0.6 1.6 19.0 1.0 3.3 29.2 4.0 7.5Model #3 (wt.%) 23.2 7.8 5.5 1.5 4.1 0.7 2.0 4.1 36.5 5.0 9.4

Major elements [wt.%]SiO2 41.38 49.45 52.89 42.64 48.59 52.52 51.09 57.95 50.83 72.01 49.61 53.11 43.69 53.64 64.49 74.37 56.83TiO2 0.03 0.04 0.11 1.05 0.84 0.94 1.01 0.64 0.62 0.19 0.92 0.73 0.18 1.04 0.64 0.19 0.75Al2O3 0.59 1.22 2.32 16.56 19.01 18.30 19.34 19.40 18.74 15.60 18.94 19.09 5.33 17.71 16.29 14.18 15.82FeOT 11.34 7.61 6.20 13.84 11.33 9.45 9.75 5.92 6.82 2.15 9.31 7.56 12.59 8.08 4.69 1.58 7.81MnO 0.15 0.14 0.13 0.24 0.22 0.18 0.18 0.14 0.14 0.10 0.16 0.14 0.20 0.17 0.10 0.06 0.17MgO 45.74 31.73 21.14 11.74 6.66 5.24 4.94 2.84 7.28 0.72 6.63 5.88 31.31 5.38 2.31 0.43 6.10CaO 0.67 9.66 17.13 12.44 11.27 10.21 10.63 8.10 12.86 3.88 10.99 9.75 6.28 8.79 4.99 2.25 7.77Na2O 0.10 0.16 0.08 1.36 1.88 2.58 2.60 4.37 2.35 4.05 2.42 3.03 0.38 3.22 3.99 4.04 3.28K2O 0.13 0.12 0.40 0.25 0.40 0.16 1.22 0.80 0.57 0.03 1.67 2.28 2.98 1.29P2O5 0.02 0.08 0.18 0.22 0.25 0.22 0.07 0.21 0.13 0.01 0.32 0.22 0.06 0.18Total 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0

XMg 0.88 0.88 0.86 0.60 0.51 0.50 0.47 0.46 0.66 0.37 0.56 0.58 0.82 0.54 0.47 0.33 0.58

Trace elements [ppm]N 2–6 1–3 5–7 5–7 22–44 6–17 4–18 1–3 2–13 16–24 8–18 6–83 1–52 16–22 24–41 7–23 18–116V 38.8 58 173 494 329 256 276 89.5 179 31.7 239 170 106 209 105 23.2 185Cr 4366 3277 3266 131 52 79.3 50.6 247 193 22.6 108 198 1755 98 29.8 24.2 211Co 30.9 26.4 31.5 5.29 42.5 31.8 117 26.0 12.7 6.31Ni 1529 997 242 61.2 24.8 20.9 18.0 17.0 96.5 5.53 56.0 89.4 775 48.6 19.2 6.06 67.6Cu 42.4 101 7.00 48.2 25.7 67.6 63.3 91.2 58.8 35.6 7.54 60.1Zn 73.2 69.2 69.0 58.2 29.2 56.3 68.1 68.6 77.5 57.8 26.1 67.7Ga 17.9 22.9 14.0 20.0 16.8 17.5 18.0 6.32 15.4 14.4 13.8Rb 0.039 0.069 0.101 0.434 0.571 2.18 1.33 3.14 3.18 30.2 19.4 11.7 1.43 47.3 65.2 81.0 32.3Sr 0.591 6.04 8.86 104 235 282 387 427 207 323 416 388 96.1 555 484 202 304

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Y 0.043 0.494 2.54 19.6 15.4 20.0 20.1 17.8 16.1 10.6 19.5 15.6 5.01 21.1 17.1 13.3 23.6Zr 0.026 0.116 0.514 6.49 11.07 43.2 48.5 104 30.9 98.7 59.4 59.3 7.88 102 176 95.7 96.4Nb 0.028 0.030 0.024 0.183 0.424 1.34 1.46 2.89 1.30 3.42 2.78 3.42 0.89 8.08 7.32 7.95 5.28Cs 0.004 0.143 0.196 0.035 0.052 0.104 0.058 0.12 0.094 0.479 0.444 0.781 1.64 2.21 1.37 2.03Ba 0.602 1.56 3.09 16.5 49.3 136 72 119 41.1 503 375 132 17.2 316 429 407 164La 0.001 0.029 0.052 0.341 1.75 6.08 6.05 8.57 3.90 10.1 17.3 7.15 1.73 19.8 23.5 11.5 10.5Ce 0.008 0.069 0.156 1.51 4.32 14.2 15.1 21.4 9.76 19.3 33.7 15.5 2.62 42.8 44.8 20.4 23.2Pr 0.001 0.014 0.045 0.363 0.659 1.96 2.08 2.90 1.40 2.04 3.90 1.58 0.140 5.18 4.76 2.71 5.51Nd 0.008 0.085 0.328 2.69 3.86 9.55 10.3 13.4 7.07 8.09 16.7 8.80 1.91 22.8 18.6 8.26 12.4Sm 0.008 0.042 0.195 1.27 1.36 2.68 2.78 2.95 2.16 1.68 3.58 1.85 0.266 4.70 3.96 2.30 3.33Eu 0.003 0.020 0.095 0.672 0.718 0.953 1.04 0.990 0.742 0.491 1.24 0.778 0.107 1.29 1.03 0.495 1.05Gd 0.009 0.074 0.373 2.29 1.99 3.24 3.32 3.04 2.56 1.61 3.81 2.72 0.35 4.30 3.59 2.36 3.69Tb 0.001 0.014 0.072 0.446 0.377 0.545 0.549 0.480 0.411 0.248 0.584 0.358 0.132 0.654 0.539 0.475Dy 0.011 0.108 0.547 3.512 2.518 3.58 3.66 3.09 2.73 1.66 3.70 2.28 0.453 4.04 3.29 2.73 4.18Ho 0.003 0.024 0.118 0.795 0.581 0.763 0.757 0.660 0.596 0.331 0.741 0.476 0.129 0.771 0.647 0.588 1.20Er 0.009 0.068 0.333 2.26 1.61 2.27 2.20 1.93 1.63 1.04 2.05 1.29 0.28 2.24 1.91 1.83 2.40Tm 0.002 0.011 0.047 0.210 0.250 0.314 0.309 0.290 0.227 0.141 0.285 0.206 0.050 0.304 0.270 0.317 0.397Yb 0.012 0.068 0.287 2.10 1.62 2.09 2.01 1.92 1.46 1.03 1.91 1.29 0.29 2.11 1.87 2.06 2.43Lu 0.003 0.012 0.046 0.346 0.252 0.339 0.325 0.315 0.247 0.158 0.292 0.195 0.051 0.309 0.289 0.321 0.370Hf 0.004 0.008 0.035 0.308 0.303 0.94 0.78 0.682 2.65 0.910 1.41 0.89 2.43 3.73 3.23 2.03Ta 0.001 0.002 0.010 0.021 0.037 0.076 0.075 0.125 0.076 0.250 0.140 0.132 0.018 0.389 0.406 1.05 0.482Pb 0.071 0.245 0.125 0.168 0.814 4.08 1.88 3.70 1.05 5.56 2.26 4.84 3.32 7.27 9.44 13.2 7.10Th 0.002 0.005 0.004 0.004 0.022 0.254 0.197 0.145 0.255 2.40 2.76 1.48 1.89 4.05 6.18 7.73 3.07U 0.002 0.005 0.003 0.003 0.017 0.086 0.070 0.040 0.074 0.396 0.499 0.163 0.062 1.136 1.92 1.69 0.896

Isotopic compositionsa87/86SrInitial 0.70420 0.70369 0.70393 0.70392 0.70382 0.70375 0.70331 0.70366 0.70371 0.70404 0.70392 0.70445 0.70432 0.70398εSrI −3 −10 −6 −7 −8 −9 −15 −10 −10 −5 −7 0 −2 −6143/144NdInitial 0.512827 0.512798 0.512805 0.51276 0.512758 0.512816 0.51286 0.51283 0.512787 0.512781 0.5127663 0.51275 0.51277 0.51280εNdI 6.2 5.6 5.7 4.9 4.8 5.8 6.8 6.3 5.4 4.9 4.6 3.8 4.1 5.7206/204PbInitial 18.48 18.40 18.51 18.50 18.41 18.26 18.17 18.40 18.47 18.44 18.51 18.66 18.06207/204PbInitial 15.61 15.57 15.58 15.58 15.58 15.53 15.53 15.56 15.56 15.64 15.61 15.63 15.55208/204PbInitial 38.63 38.52 38.68 38.67 38.52 38.33 38.26 38.59 38.60 38.60 38.48 38.79 38.12176/177HfInitial 0.28311 0.28312 0.28301 0.283 0.28305 0.28305εHfI 14.2 14.40 10.40 11.630 11.10 10.79

a Samples for which no age information was available the initial isotopic composition were calculated at t=100 Ma.

7O.Jagoutz,M

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Table 3Bulk composition of the main building blocks of the Kohistan arc.

Ultramafic cumulates SPCa SPCb Chilas Complex Batholith

Average σmean Average σmean Average σmean Average σmean Average σmean

N 24 138 138 185 86Major elements [wt.%]SiO2 46.42 >0.42 52.16 0.82 50.52 0.71 52.64 0.65 64.60 0.98TiO2 0.56 0.24 0.78 0.07 0.84 0.05 0.70 0.06 0.62 0.07Al2O3 9.14 0.62 18.68 0.48 18.86 0.30 18.40 0.54 16.19 0.24FeOT 10.60 0.86 8.41 0.41 9.97 0.30 7.81 0.32 4.66 0.40MnO 0.19 0.07 0.17 0.01 0.19 0.01 0.14 0.01 0.10 0.01MgO 19.84 0.99 5.86 0.48 6.03 0.26 7.16 0.52 2.38 0.23CaO 12.44 0.81 10.79 0.44 10.92 0.25 9.58 0.40 5.03 0.31Na2O 0.73 0.42 2.56 0.15 2.28 0.11 2.90 0.16 3.92 0.15K2O 0.06 0.05 0.41 0.08 0.24 0.03 0.54 0.08 2.30 0.24P2O5 0.01 0.00 0.18 0.06 0.14 0.02 0.13 0.01 0.21 0.02Total 100.0 100.0 100.0 100.0 100.0

XMg 0.77 0.55 0.52 0.62 0.48

Trace elements [ppm]V 305 58 223 17 279 10 166 16 105 12Cr 1773 170 113 25 78.3 9.6 276 97 35.3 8.9Co 24.3 3.9 13.5 1.3 36.1 6.4 13.2 1.5Ni 363 30 47.6 10.4 30.7 3.6 124 49 20.4 3.2Cu 46.1 13.1 27.8 7.5 64.6 18.5 34.6 5.7Zn 49.3 4.7 31.9 1.8 68.2 4.8 56.1 3.7Ga 15.6 1.4 9.28 0.69 17.4 2.3 14.4 0.63Rb 0.258 0.144 7.61 2.32 2.59 0.53 11.2 4.5 65.3 8.4Sr 55.3 30.3 306 31.9 275 16 373 27 459 49Y 10.6 6.3 17.3 2.1 17.0 1.3 15.1 1.8 17.0 2.8Zr 3.40 2.35 45.4 6.3 29.8 2.9 56.7 11 161 23Nb 0.105 0.061 1.71 0.31 1.03 0.12 3.30 2.16 7.45 1.22Cs 0.094 0.053 0.184 0.051 0.092 0.017 0.742 0.556 2.06 0.48Ba 9.37 5.66 162 54 90.6 13.4 126 20 417 67La 0.189 0.075 7.52 2.38 4.30 0.60 6.88 0.97 21.9 2.8Ce 0.807 0.318 16.3 4.3 10.0 1.2 14.9 2.1 41.9 4.7Pr 0.196 0.077 2.07 0.46 1.37 0.14 1.51 0.36 4.57 0.49Nd 1.45 0.52 9.63 1.83 6.91 0.64 8.46 1.15 17.8 2.1Sm 0.694 0.151 2.47 0.34 2.00 0.15 1.77 0.400 3.84 0.45Eu 0.365 0.048 0.899 0.101 0.830 0.045 0.744 0.113 1.00 0.102Gd 1.26 0.30 2.85 0.34 2.53 0.16 2.60 1.26 3.52 0.48Tb 0.245 0.092 0.466 0.052 0.441 0.027 0.346 0.075 0.542 0.088Dy 1.92 0.90 3.05 0.33 2.92 0.19 2.18 0.46 3.30 0.57Ho 0.433 0.233 0.648 0.069 0.642 0.043 0.458 0.097 0.652 0.127Er 1.23 0.72 1.83 0.19 1.82 0.12 1.24 0.27 1.93 0.36Tm 0.120 0.102 0.260 0.026 0.268 0.020 0.198 0.042 0.278 0.057Yb 1.14 0.74 1.71 0.17 1.75 0.12 1.24 0.26 1.91 0.35Lu 0.187 0.127 0.275 0.027 0.278 0.021 0.188 0.039 0.294 0.054Hf 0.165 0.053 0.797 0.161 0.562 0.100 1.39 0.36 3.56 0.63Ta 0.013 0.003 0.096 0.016 0.063 0.008 0.126 0.042 0.475 0.157Pb 0.163 0.048 2.13 0.49 1.774 0.57 4.77 0.91 9.66 1.29Th 0.004 0.002 0.873 0.405 0.271 0.090 1.50 0.84 6.16 1.23U 0.003 0.001 0.174 0.066 0.070 0.015 0.158 0.104 1.82 1.02Isotopic compositionsc87/86SrInitial 0.70391 0.00013 0.70374 0.00007 0.70374 0.00007 0.70399 0.00004 0.70437 0.00017εSrI −7 2 −9 1 −9 1 −6 1 −1 2143/144NdInitial 0.51282 0.00001 0.512801 0.000017 0.512801 0.000017 0.512771 0.000018 0.512746 0.000039εNdI 6.0 0.3 5.6 0.3 5.6 0.3 4.7 0.3 3.7 0.8206/204PbInitial 18.46 0.03 18.41 0.05 18.41 0.05 18.44 0.01 18.58 0.11207/204PbInitial 15.59 0.01 15.57 0.01 15.57 0.01 15.64 0.01 15.62 0.01208/204PbInitial 38.60 0.05 38.56 0.06 38.56 0.06 38.60 0.01 38.63 0.27176/177HfInitial 0.283116 0.000009 0.283116 0.000009 0.283010 0.283048 0.000028εHfI 14.3 0.2 14.3 0.2 10.4 11.2 1.1

a SPC: Southern Plutonic Complex excluding ultramafic cumulates and using the mapped thickness of the subunits.b SPC: Southern Plutonic Complex excluding ultramafic cumulates and using the barometric thickness of the subunits.c Due to the limited number of published analyses available, the isotopic composition of the different units has only been averaged and not weighted for the volume of the units.


oceanic crust, on which the arc was build, or of a back arc basin (Khanet al., 1993a; Jagoutz et al., 2009).

The petrology of the SPC sequence and, in particular the garnet-gabbro is either interpreted to result from the high pressure crystalliza-tion of hydrous arc magmas (Miller et al., 1991; Ringuette et al., 1999;


Müntener et al., 2001; Alonso-Perez et al., 2009) or from metamorphicdehydration reactions, in combination with partial melting (Yamamotoand Yoshino, 1998; Garrido et al., 2006). Garnet is a major constituentof this section in the garnetites, often exceeds 30 vol.% in gabbros, andis present inmuch lower abundance in silica-rich granitoids. Importantly,garnetites and a few primitive garnet gabbros have enriched HREE, in



Gilgit Complex Bulk plutonic crust Kohistan arc modeled bulk composition#1 #2 #3

Average σmean Average σmean Average σmean Average σmean Average σmean

247 409 570 570 385Major elements [wt.%]63.28 0.84 59.53 0.61 59.26 0.54 56.56 0.47 57.54 0.560.64 0.06 0.67 0.04 0.68 0.04 0.73 0.03 0.73 0.0416.13 0.21 17.16 0.21 17.02 0.19 17.56 0.18 17.35 0.185.19 0.34 6.08 0.26 6.26 0.23 7.44 0.20 7.34 0.230.11 0.01 0.12 0.01 0.13 0.01 0.15 0.01 0.15 0.013.01 0.21 4.11 0.20 4.32 0.18 4.93 0.17 4.37 0.175.50 0.27 7.15 0.22 7.21 0.20 8.26 0.17 7.94 0.193.81 0.13 3.43 0.10 3.42 0.09 3.08 0.08 3.12 0.092.13 0.20 1.55 0.14 1.52 0.12 1.13 0.09 1.28 0.110.20 0.02 0.19 0.02 0.19 0.01 0.17 0.01 0.17 0.01100.0 100.0 100.0 100.0 100.0

0.51 0.55 0.55 0.54 0.51

Trace elements [ppm]119 10 141 8 146 7 186 7 191 765.2 12.6 104 23 115 21 112 20 71.1 8.211.0 1.3 20.0 2.0 18.3 1.6 16.9 1.5 12.1 0.928.4 3.6 48.6 11.2 50.8 10.1 48.3 9.9 29.5 2.638.9 6.0 43.5 5.9 45.3 5.6 40.1 5.3 33.9 4.758.1 3.3 57.5 2.6 58.6 2.3 50.7 1.9 46.3 2.012.0 0.5 15.3 0.7 13.7 0.6 12.1 0.6 10.8 0.459.7 7.1 42.2 5.0 41.2 4.3 29.5 3.2 34.0 3.9433 42 411 30 399 26 364 20 362 2418.2 2.4 16.7 1.7 17.4 1.5 17.1 1.2 17.7 1.4150 19 115 14 113 12 88.0 8.9 95.8 10.77.09 1.03 5.42 0.86 5.41 0.75 4.15 0.63 4.36 0.62.06 0.42 1.41 0.31 1.48 0.28 1.09 0.22 1.17 0.23374 56 302 41 288 35 222 25 246 3119.9 2.4 15.8 1.7 15.2 1.5 11.7 1.1 12.9 1.338.7 4.0 31.0 2.9 30.2 2.5 23.6 1.9 25.8 2.34.73 0.50 3.41 0.31 3.64 0.32 2.88 0.24 3.22 0.2816.9 1.80 14.2 1.3 14.0 1.1 11.6 0.9 12.4 1.03.76 0.39 3.12 0.29 3.14 0.26 2.73 0.20 2.97 0.231.00 0.09 0.920 0.070 0.93 0.062 0.89 0.049 0.926 0.0543.55 0.42 3.18 0.40 3.24 0.36 2.99 0.32 3.09 0.240.450 0.073 0.484 0.055 0.432 0.047 0.426 0.037 0.446 0.0423.45 0.49 3.00 0.35 3.13 0.31 3.01 0.24 3.21 0.280.745 0.126 0.607 0.078 0.671 0.079 0.651 0.061 0.699 0.0722.01 0.31 1.76 0.22 1.83 0.19 1.79 0.15 1.93 0.180.298 0.049 0.257 0.035 0.272 0.031 0.267 0.024 0.285 0.0282.00 0.30 1.72 0.21 1.80 0.19 1.76 0.15 1.89 0.170.307 0.046 0.267 0.033 0.278 0.029 0.273 0.023 0.294 0.0273.30 0.52 2.54 0.38 2.49 0.32 1.93 0.24 2.07 0.290.476 0.133 0.324 0.092 0.342 0.080 0.257 0.059 0.290 0.0739.23 1.27 7.12 0.79 7.13 0.79 5.65 0.62 5.87 0.755.64 1.03 4.10 0.75 4.00 0.65 2.88 0.49 3.22 0.571.67 0.85 1.13 0.60 1.11 0.51 0.79 0.375 0.947 0.468

Isotopic compositionsc

0.70425 0.00038 0.70394 0.00017 0.70411 0.00023 0.70402 0.00017 0.70402 0.00021−2 5 −7 2 −4 3 −6 2 −5 3

0.512773 0.000084 0.512791 0.000018 0.51278 0.00005 0.51278 0.00004 0.51279 0.000054.5 1.6 5.2 0.5 4.8 1.0 5.0 0.7 5.0 0.9

18.41 0.20 18.44 0.06 18.42 0.12 18.42 0.09 18.41 0.1115.60 0.04 15.58 0.02 15.60 0.02 15.60 0.02 15.58 0.0238.46 0.51 38.55 0.08 38.51 0.31 38.52 0.23 38.51 0.28

0.283048 0.000036 0.283079 0.000033 0.28305 0.00002 0.28306 0.00002 0.28308 0.0000210.9 1.5 12.4 1.4 11.4 0.9 12.0 0.7 12.5 0.8


accordance with textural evidence for magmatic garnet accumulation(Ringuette et al., 1999), but some garnet-bearing gabbros and dioriteshave metamorphic textures and unfractionated whole rock HREEpatterns, similar to those seen in the hornblende gabbros. This hasbeen interpreted as evidence that some garnet in those rocks is ofmeta-morphic origin (Yamamoto and Yoshino, 1998; Garrido et al., 2006).This interpretation is in line with hornblende-rich gabbros that havevery similar trace element compositions to the “garnet-gabbros” andwhich are found in mid-crustal levels of the Kohistan arc in the vicinityof the Chilas Complex (Jagoutz et al., 2006).


3.2. The Chilas Complex: extension related intrusions

The Chilas Complex (Fig. 3) is an ultramafic–mafic complex(Jagoutz et al., 2006). The mafic gabbronorite sequence, which in-cludes olivine-bearing gabbronorite, gabbronorite, (quartz-)dioriteand minor anorthosite is modally layered to homogeneous rocks.Within the mafic series, ortho- and clinopyroxene form the bulk of themafic minerals, withminor olivine. Hornblende is rare in the gabbronor-ite sequence, but hundreds ofmeterwide pegmatitic hornblende-gabbrostocks, with up to half meter long poikilitic hornblende crystals, occur




within the Chilas unit. Ultrabasics form km-scale circular to slightly elon-gated bodies within the mafic unit. These ultrabasics are composed ofcores of fine-grained, fairly homogeneous dunite, surrounded by minorlherzolite and pyroxenite. The ultramafic rocks have complex subverticalcontact relationships with the surrounding gabbronorite sequence andhave been interpreted as km-scale upper mantle melt conduits, feedingthe mafic part of the Chilas Complex, that were emplaced diapiricallyinto the lower crust (Jagoutz et al., 2006, 2007). The main gabbronoriteintrusion has been dated by U–Pb zircon to ~85 Ma (Zeitler, 1985;Schaltegger et al., 2002).

3.3. The Gilgit Complex: arc batholith and volcanosedimentary cover

This plutonic complex (Fig. 4) is composed of a compositionallyvariable intrusive suite of minor, mostly hornblenditic ultramaficrocks, a few fine-grained gabbros, dominantly quartz-, quartzmonzo-and granodiorite, granite and minor tonalite. Geochronological infor-mation is scarce in the Kohistan batholith, but the majority of reliableU–Pb zircon ages range from 112 to 30 Ma (Jagoutz et al., 2009). Onenotably older date at 154 Ma of the Matum Das gneissic tonalite(Schaltegger et al., 2004) could indicate significant oldermagmatic activ-ity. The greater part of the dated granitoids (~90%) predates the India–Kohistan–Eurasia collision and represents juvenile rocks derived fromsubduction related melts, and is the result of hydrous high to mediumpressure fractionation (Jagoutz, 2010). Few granites, distinctly differentin their isotopic composition, postdate collision(s) and do not belongto the intraoceanic subduction stage of the Kohistan arc s.s. (Heubergeret al., 2007). As discussed below, such postcollisional granites havebeen excluded from our bulk estimate.

Individual intrusions of the two dominant rock types, quartz- toquartzmonzodiorites and granodiorites are relatively small. The dee-per section is dominated by 5–100 m wide dikes, forming complexnetworks and mutual cross-cutting relations. These dikes includethe entire compositional range. Magma mingling, magmatic erosion,and co-magmatic flow deformation are abundant. In the northernshallower levels of the batholith, km sized stocks and bodies are com-mon, often displaying internal flow alignment of feldspars, xenoliths,and magmatic schlieren. The lithological variability is pronounced onan outcrop scale and only a few larger ‘homogeneous’ plutons aremappable, mostly composed of leucogranites or subordinate granodi-orites. In many outcrops, intermediate plutonic rocks with planar fab-ric form the structurally oldest rock type. Syn-magmatic mingling andmixing of magmas are common over length scales varying from cen-timeters to hundreds of meters, even within small plutons and dikes.These features demonstrate that, on a local scale, the entire spectrumfrom gabbroic to granitic magmas coexisted. Mineralogically, horn-blende is the dominant mafic mineral, but several samples alsocontain clinopyroxene, typically enclosed within hornblende. In thedeeper portions of the batholith (≥0.5 GPa), a crystallization se-quence (cpx)→hbl→plag→epidote→qz→kfsp is observed, while inthe northern, shallower part, at ≤0.4 GPa, the sequence is (opx,cpx)→plag→hbl→qz→kfsp (Enggist, 2007).

Frequent, late leucogranitic dikes crosscut the entire batholith, butare more abundant at shallower levels. Basaltic dykes are generallyscarce and based upon their generally sharp contact relationships andfine-grained texture, they intruded mostly when the host was coolenough to behave brittle.

The Kohistan plutons are overlain and intrude into a ~4 km thickvolcanoclastic sequence (Fig. 4), which remains poorly studied(Petterson and Treloar, 2004). Bulk compositions of the volcanoclasticsrange from basalts to dacites/rhyolites and are dominantly basaltic an-desites and andesites (Bignold and Treloar, 2003; Bignold et al., 2006).The volcanoclastic series ranges from ca. 110 to 55 Ma (for a summarysee Burg, 2011), based on fossiliferous horizons, direct Ar/Ar dating, orfrom the ages of intruding plutons.


4. Relative proportions and bulk compositions of the arc buildingunits

Thebulk compositions of each of the threemain units of theKohistanarc are estimated individually, using an approach best adopted to theirvariably constrained geology and geometries.

4.1. Southern Plutonic Complex

This complex is best studied along the Indus River and can be dividedinto the seven major geological units described above. The compositionof each unitwas calculated (Table 1) averaging 4–45whole rock analyses(e-Table 1). The relative thickness of each unit was constrained using thedetailed SW–NE section from Burg et al. (2005) and available geobaro-metric constraints (Yamamoto, 1993; Yoshino et al., 1998; Ringuetteet al., 1999; Yoshino and Okudaira, 2004; Enggist, 2007). Pressure esti-mates for the Kohistan arcMoho vary significantly, nevertheless, averagemineral rim compositions in the deepest-seated garnet–granulite gabbrounit yield ~1.5 GPa (Fig. 2). We used this value to derive a depth to theMoho of ~54–55 km using an average density of ρ=2.8 g/cm3 for thelower crust and 2.7 g/cm3 for the middle and upper crust. Even thoughthe pressure difference between the base of the arc and the top of theSouthern Plutonic Complex (~0.7 GPa) correlates well with the mappedthickness of the complex (~26 km), the relation between mapped thick-ness and geobarometric constraints varies significantly within the com-plex (Fig. 2). Within the garnet granulite gabbro, pressure decreasesfrom 1.5 GPa at the base to 1.2 GPa at the top, corresponding to a baro-metrically derived thickness of ~13 km, whereas the mapped thicknessis only ~4.5 km. As this section does not contain structural or lithologicaldiscontinuities, we interpret this mismatch to result from homogeneousthinning (e.g., during extension or delamination, see below). For the Sar-angar gabbro and lower Kiru sequence, the geobarometric pressure differ-ence correspondswell to themeasured thickness. In the upper part of theSPC, post-metamorphic thickening is indicated by equilibration pressuresthat decrease only from ~1 GPa to 0.8 GPa corresponding to ~5.8 kmcompared to a mapped thickness of ~14 km. Thus, for the integration ofthe Southern Plutonic Complex bulk chemistry (Table 2), we employthicknesses derived from the sections and map in model #1, whereasgeobarometrically inferred thicknesses (Table 1) of the units of theSouthern Plutonic Complex are used in model #2.

4.2. Chilas Complex

The bulk composition of the Chilas Complex (Table 2) was derivedby reintegrating the gabbronorite and the ultramafic sequence. Thebulk composition of the ultramafics was calculated by averagingwhole rock compositions, taking into account the abundance of du-nites (80% of the ultramafics) and the minor importance of lherzolitesand pyroxenites (20%) within the ultramafic conduits.

The average of the Chilas gabbronorite (Table 1 and e-Table 1) waspreviously estimated by Jagoutz et al. (2006) using two differentapproaches. For both methods, they assumed that the parentalmagma of the gabbronorite corresponds to the Chilas gabbronoritebulk composition. This choice is justified by the in-situ fractionationmechanism that this almost dry magma experienced (Jagoutz et al.,2006, 2007), and the conservation of both mafic cumulates andmore evolved diorites, which could be reintegrated into the parentalmelt composition (see Jagoutz et al., 2006, for more details). The dif-ferent methods take advantage of whole rock trace element data,which indicate that both adcumulates (with a positive Eu anomalydue to accumulation of plagioclase) and orthocumulates (withoutEu anomaly) are present. In one method, the parental magma of thegabbronorite sequence is estimated from LA-ICP-MS data on clinopyr-oxene and plagioclase present in the most primitive cumulates, whichallow calculation of the equilibrium melt phase. The second methodassumes that orthocumulates approximate the liquid composition of




the parental melt of the gabbronorite sequence (Jagoutz et al.,2006).

In this study we averaged the 133 Chilas gabbronorite analysesavailable in the literature (see e-Table 1) as this allows for a morerobust statistical error approximation which is more difficult usingthe approach of Jagoutz et al. (2006). All three methods yieldcomparable bulk compositions for the Chilas gabbronorite (Table 2and e-Table 1), which correspond to basaltic andesite (SiO2

53.1–55.6 wt.%; Na2O+K2O~3.6–4.0 wt.% and XMg~0.55–0.58).Finally, for the average bulk composition of the Chilas Complex we

estimate, based on field and satellite image observations, that the ul-trabasic rocks make up ~5% of the Chilas Complex and the gabbronor-ite sequence the remaining 95%. Our estimated bulk composition ofthe Chilas Complex (Table 3 and e-Table 1) is basaltic andesite (SiO2

52.6 wt.%; Na2O+K2O~3.4 wt.% and XMg~0.62).

4.3. Gilgit Complex

Based on Al-in-hornblende barometry, intrusion pressures of thebatholith decrease from S to N from 0.9 to 0.2 GPa. The thickness of thebatholith can thus be estimated to ~26 km (Enggist, 2007). From intru-sion pressures of 0.20–0.23 GPa at the base of the volcanoclastic pile(Enggist, 2007) and from mapped profiles in Ishukman and neighboringvalleys (Petterson and Treloar, 2004; Rosenberger and Spaar, 2006) weconstrain the thickness of the volcanoclastic unit to ~4 km; the remaininguppermost 2–3 km are comprised of carbonate and terrigenous sedi-ments (Fig. 4), which are not included in the bulk arc composition as nowhole rock composition of these units are available.

The average Kohistan batholith composition is the most difficult toapproximate, as the geology is the least constrained and the batholithis characterized by numerous mutual intrusions of highly variablerock types on an outcrop scale. Only a limited number of modernXRF and ICP-MS major and trace element studies exist (Jagoutz etal., 2009), most published analyses are XRF only and sample locationsare often unknown. Furthermore, most studies focused on themost ac-cessible part of the batholith near Gilgit in the NE and near Dir andKalam in the SW (Fig. 1) (e.g., Khan et al., 2007), and only isolatedwhole rock analyses exist in other areas of the batholith (Pettersonand Windley, 1985). Additionally, regional field-based geologicalmaps of the different dominant intrusive units exist only around Gilgit(Petterson and Windley, 1985), Gupis (Khan et al., 1993b) and theDir-Kalam area (Fig. 1) (Sullivan et al., 1993; Jagoutz et al., 2009) leav-ing the majority (>90%) of the batholith essentially unstudied.

To overcome this drawback, we sampled during two field campaigns(2005 and 2006) most accessible parts of the batholith, and analyzed 79new bulk rocks by XRF and ICP-MS (e-Table 1, electronic supplementfor analytical details). Based on field observations, we determined the rel-ative volumes of the different rock units in each outcrop andmapped theregional variability of the dominant rock types.We extended our local ob-servations into inaccessible areas using remote sensing techniques onLandsat 7 ETM+ satellite images. Using a combination of bands 754and principal component analyses of the different bands, we outlined re-gionsdominated by a certain reflectivity level. It is important to note thatthese regions do not reflect individual igneous bodies, as each of thesemapped areas is composed ofmultiple intrusions. Based on field and sat-ellite data, we were able to reliably map three different principal unitscharacterized by low,mediumandhigh reflectivity. The lowest reflectiv-ity correlates with areas where mafic compositions (gabbro/diorite)dominate and the highest reflectivity with bright, un-weathered leuco-granites. The highest reflectivity units composed of leucogranitesamount to 11% and the more mafic compositions to ~9%. The interme-diate reflectivity unit, making up 80% of the surface of the batholith isdominated by intermediate compositions. Using this approach, weused our new whole rock analyses with known field locations to calcu-late the bulk composition of the different reflectivity units by initiallyaveraging whole rock analyses available in the specific units according


to the volumes estimated in the field. To extend the dataset with an ad-ditionally 26 whole rock analyses that are available, but with unknownlocation, we averaged those composition with units with similar bulkcomposition. As a result, we grouped rocks with a SiO2 b57 wt.% in thelow reflectivity unit, rocks with SiO2 between 57 and 70 wt.% into the in-termediate reflectivity unit and rocks with SiO2 >70 wt.% into the highreflectivity unit (Table 2). This approach results in an average batholithcomposition with SiO2 64.6 wt.% (Table 3). Alternatively, we estimatedthe abundance of the different compositions simply based on local fieldobservations, which resulted in 5% mafic rocks (gabbro/diorite), 30%quartz- and quartzmonzodiorite, 13% tonalites, 25% granodiorites and12% granites. The latter exclude leucogranite stocks and dikes that are es-timated to comprise 15% of the outcrops. These estimates of relative pro-portions of different intrusives are comparable to those of Petterson andWindley (1985). We then calculated averages for each rock type and aweighted average for all intermediate compositions, which results in abulk composition of the batholith of 62.1 wt.% SiO2 (given as batholith av-erage #2 in the electronic appendix).

As a third alternative (batholith average #3), we have simply calcu-lated the average of all available bulk analyses that do not belong to thebasaltic dikes or the leucogranites; these range from 53 to 70 wt.% SiO2

and yield an average batholith of 62.2 wt.% SiO2 (e-Table 1). The logicof this approach is that leucogranites and basaltic dikes are texturallyand compositionally clearly distinct and each homogeneous, while forall other units internal heterogeneity is a major issue. The various ap-proaches to estimate the bulk batholith result in similar compositions(62.1–64.6 wt.% SiO2 and an XMg of ~0.49, e-Table 1). Trace elementvariations between the different estimates are generally within 20%.Our preferred approach (Table 3), relies on the reflectivity units andyields 64.6 wt.% SiO2 and an XMg of 0.48.

An additional complexity in calculating the bulk composition of thearc batholith arises from the presence of volumetrically minor post-collisional granites. These granites formed at b50–40Ma, subsequent tothe India–KLA collision (for a summary of ages see Burg, 2011) and arethought to stem from Proterozoic continental crust or sediments(Heuberger et al., 2007). Macroscopically, these granites aremostly char-acterized by abundant mega-crystic K-Feldspar and/or an aluminousphase (e.g., muscovite or garnet) thatmanifests their generally peralumi-nous character. Geochemical data indicate that most post-collisionalgranites have higher concentrations of incompatible trace elements andhigher Dy/Yb ratios compared to pre-collisional granitoids. Accordinglywe used geochemical characteristics (e.g., La >20 ppm) to separate thetwo groups. For rocks for which the intrusion age is known these criteriaresulted in a 98% separation of the two groups. Whatsoever, the effect ofthe post-collisional granites on the bulk composition is negligible for themajor elements. All average concentrations above (Tables 1–3) have thepost-collisional granites excluded.

Mostly on top of the batholith, a pile of volcanoclastic rocks also needsto be accounted for as part of the igneous arc crust (Fig. 4). Asmentionedabove, only a few partial profiles through these sequences exist and de-tailed chemical stratigraphy has not yet been obtained. We, thus, havesimply averaged the available 161 whole rock compositions, yielding anandesite with an SiO2 of 56.8 wt.% and an XMg of 0.58 (Table 2). Addingthe 4 km of volcanoclastics, the bulk Gilgit Complex (intrusives+extrusives) results to 63.3 wt.% SiO2 and an XMg of 0.51 (Table 3).

5. Bulk crust integration models

We have calculated three different Kohistan arc model bulkcompositions: The first approach relies on surface exposure of theunits and mapped thicknesses. The second and third approaches usethe geobarometrically derived thicknesses for the Southern Plutonicand the Gilgit Complexes with or without adding the Chilas Complex(Table 1). In all three approaches the compositions used for the Chilasand Gilgit Complexes are identical, and only supra-Moho units are in-cluded in the Southern Plutonic Complex.




Model bulk composition #1was calculated using themeasured thick-nesses of the various units for the Southern Plutonic Complex (Fig. 2),and surface exposure areas of the three main complexes. From satelliteimages, we estimated the distribution of the Southern Plutonic, Chilas,and Gilgit Complexes as 0.17, 0.20, and 0.63 respectively. This surfaceexposure approach is motivated by the lack of a pressure differenceacross the Chilas Complex. This model views the Chilas Complex,which intruded during initial back arc rifting, as an integral part of thearc crust (Jagoutz et al., 2011).

Model bulk composition #2 was calculated from geobarometricallyderived thicknesses of the Southern Plutonic and Gilgit Complexes,holding them representative of the composition of the pre-rifted arcand then adds a fraction of the Chilas Complex proportional to its sur-face proportion. As above, we consider the Chilas Complex to make up20% of the total crust and the remaining arc 80%. The difference betweenmodel compositions #2 and #1 is essentially equivalent to allowing inmodel #1 for some high-density garnet–hornblende–cpx cumulatesand garnet-gabbros to founder from the base of the arc.

Model bulk composition#3: This approach integrates the Southern Plu-tonic andGilgit Complexes as inmodel #2 but does not include the ChilasComplex. The logic of this model follows (Jagoutz, 2010) who has shownthat the Southern Plutonic Complex is geochemically complementary tothe plutonic/volcanic Gilgit Complex. This composition reflects a Kohistanarc excluding any additions during intra-arc rifting, i.e. the extension-related intrusion of the Chilas Complex. It also integrates proportionallythe largest amount of high density cumulates or gabbroic rock.

6. Results

The preserved and geobarometrically inferred thicknesses of all unitsof theKohistan arc are given in Table 1, their average geochemical compo-sitions in Table 2, and the differentmodel bulk compositions for the Kohi-stan arc in Table 3 (the entire dataset is given in e-Table 1). Thegeochemical characteristics of the different compositions are normalized

Fig. 5. Comparison of the model #1 and #2 bulk Kohistan arc crust compositions to the bulkMcLennan (1985, 1995). Our preferred estimate is model #1 which corresponds to the actred: model #2 calculated with geobarometric thicknesses. (For interpretation of the referenc


to the bulk continental crust estimate from Rudnick and Gao (2003)and further compared to those of Wedepohl (1995) and Taylor andMcLennan (1985, 1995) (Fig. 5).

All three models for the bulk composition of the Kohistan arc yieldan andesitic composition. SiO2 varies, depending on the model, be-tween 56.6 and 59.3 wt.%; K2O is 1.1–1.5 wt.%; Na2O 3.1–3.4 wt.% andXMg 0.51–0.55. These calculated compositions are significantly moresilica-rich than the first bulk estimate for the Kohistan arc by Millerand Christensen (1994). The singlemost important factor for this differ-ence is the employed thickness of the Kohistan batholith. WhereasMiller and Christensen (1994) inferred the vertical extent of the batho-lith to be ~10 km, the intrusion pressures of Enggist (2007), derivedfrom Al-in-hbl barometry, constrain the thickness of the batholith to26 km. Secondly, our bulk compositions are based on a significantly en-hanced understanding of the overall geology of the arc, and third, on amore extensive whole rock dataset which now encompasses the entirearc (594 compared to 23 analyses in Miller and Christensen, 1994).

Our new bulk Kohistan composition is similar to the bulk continentalcrust (BCC) of Rudnick andGao (2003). The difference betweenKohistanand the BCC is generally within 10–20 rel.% for the major elements(Fig. 5), comparable to the overall differences between various bulkcrust estimates (see Fig. 5 and Rudnick and Gao, 2003). Al2O3 is slightlyhigher in the bulk Kohistan arc at 17.0–17.6 wt.% compared to the BCC(15.9 wt.%). The high Al2O3 content may indicate a relative suppressionof plagioclase in the Kohistan primary magma and, thus, generallymore hydrous parental magmas or higher pressure fractionation. TheKohistan bulk composition has a significantly higher Na2O/K2O wt%-ratio (2.2–2.7) compared to the BCC (1.7) a result of both lower K2Oand higher Na2O contents in Kohistan compared to the BCC.

The transition metals (Mn, V, Cr, Co, Ni, Cu, Zn) in the Kohistanbulk crust are generally within b60% deviation with respect to theRudnick and Gao BCC concentrations (Fig. 5). In particular, our bulkNi content of the Kohistan arc is within 20% of the estimates for theBCC for the models that include the Chilas Complex.

continental crust (BCC) of Rudnick and Gao (2003), Wedepohl (1995) and Taylor andually preserved Kohistan crust. Blue: model #1 calculated with preserved thicknesses;es to color in this figure legend, the reader is referred to the web version of this article.)




The rare earth and large ion lithophile element (REE and LILE)compositions of the Kohistan arc are comparable to that of the BCC,but the more incompatible elements (LILE and light REE) are system-atically ~20–60% lower in the Kohistan bulk crust then in the Rudnickand Gao BCC (Fig. 5). The incompatible trace element concentrationof the Kohistan bulk depends strongly on the integration model ofthe Kohistan batholith. As mentioned above, our different integrationapproaches for the batholith result in a variability of ~20% for trace el-ement concentrations which translates to ~10–15% variability in theKohistan bulk. Accordingly the observed depletion of the differentKohistan bulk models compared to the BCC is significant with respectto the uncertainties in our models. The modeled bulk Kohistan arccompositions have slightly negative Eu/Eu* anomaly of 0.89–0.95(Eu/Eu*=Eu/(0.5*(Sm+Gd))), comparable to the estimate of thebulk continental crust (0.87).

High field strength elements (HFSE) are generally lower in theKohistan arc than the BCC estimate of Rudnick andGao (2003). However,for Zr and Hf the difference between the bulk Kohistan arc composition ismarginal compared to the bulk continental crust estimate of e.g. Taylorand McLennan (1985, 1995) (Fig. 5) making the significance of the dis-crepancy model dependent. For Nb and Ta the difference between thebulk Kohistan and various BCC estimates is always significant, illustratinga robust discrepancy between the bulk crust as produced in the Kohistanarc and the bulk continental crust.

Fig. 6. (a,b) Comparison between plutonic and volcanic arc rocks from Kohistan. The extrusiof the ultramafic cumulates being present only in the plutonic record. The increased abundaanalyses represent dm to m thick dikes of insignificant volume within the batholith, thus moSymbols labeled V and P indicate the average composition of the volcanic and plutonic cruKohistan units and the bulk continental crust of Rudnick and Gao (2003) normalized to the astan plutonic crust and Kohistan primitive arc magma (Jagoutz, 2010) normalized to the av


7. Discussion

7.1. Differences/similarities between plutonic and volcanic crust

The continuous exposure of the Kohistan arc allows us to comparethe overall plutonic record with the overall volcanic record. The vol-canic record spans a similar time range as the plutonic record, never-theless, the two are not necessarily contemporaneous as the temporalevolutions in each igneous suite are still poorly understood. The fol-lowing comparison thus concerns the time integrated compositionaldifferences and similarities between the intrusive and extrusivesuites of the Kohistan arc.

Overall, the volcanic rocks show a comparable spread in major el-ement composition as the plutonic rocks (e.g. Fig. 6a, b) except for theprimitive SiO2-poor ultramafic cumulates absent in the arc lavas.There is no indication of much higher abundances of more evolvedcompositions in the plutonic compared to the volcanic record as hasbeen postulated for e.g. the Aleutian arc (Kelemen et al., 2003). Theaverage composition of the volcanic record yields 56.8±1.2 wt.%SiO2 (Table 2), while the average composition of the Kohistan pluton-ic rocks results in 59.5±0.6 wt.% SiO2 (Fig. 6, Table 3). The average ofthe volcanics is calculated unweighted, as no detailed profiles are avail-able, and thus per se biased by the availability of analyses. Further field-work would be necessary to constrain a weighted average of the

ve and intrusive rocks show similar ranges in XMg and SiO2 content, with the exceptionnce of high silica plutonic rocks (~75 wt.% SiO2) contains a sampling bias, as most of thest of this high-SiO2 peak does not relate to increased volumes of these rocks in the field.st, respectively. (c) Trace element concentrations of the bulk composition of differentverage volcanics from Kohistan. (d) Trace element concentrations of the average Kohi-erage volcanics from Kohistan.




volcanics. A significant difference between the Kohistan and the Aleutianrecords also relates to the fact that in Kohistan the deeper portions of thearc are accessible, revealing a pressure dependence of the plutonic rocksuite: Field observations suggest, that the intrusives from the deeperpart of the eastern transect are generallymoremafic (andmore depletedin incompatible elements) than the average volcanic suite compositionand the intrusives from the higher levels of the batholith.

The Kohistan batholith and accordingly also the estimated bulkcomposition of the Kohistan arc as well as the bulk plutonic part ofthe arc, is on average strongly enriched in the more incompatible el-ements compared to the average of the volcanic suite (Fig. 6). This en-richment becomes marginal when comparing the average batholith tothe average of the evolved volcanics with SiO2 >60 wt.%. Compared toevolved volcanic rocks, the Kohistan batholith is only slightly enrichedin Ba, U, Nb Sr, Zr and Eu. This indicates that even evolved plutonicrocks in Kohistan generally have a cumulative enrichment in plagio-clase and accessory phases such as zircon and possibly oxides. It re-mains to be tested if the volumetric representation of the differentvolcanic compositions is equal to that of the plutonic rocks (with the ex-ception of the ultramafic cumulates).

7.2. The intraoceanic setting of the Kohistan arc

The andesitic composition of the Kohistan arc is strikingly similar tothe estimated bulk continental crust composition. Based on our currentknowledge, there is no field or geochemical evidence for involvementof an old continental basement in the formation of the Kohistan arc, ex-cept for the very minor post-collisional b50–40 Ma granites derivedfrom the underplating Indian continent (Heuberger et al., 2007). Thus,the generally accepted interpretation is that Kohistan represents apaleo-intra-oceanic arc (Tahirkheli et al., 1979). However, the crustalthickness and isotopic composition of this arcmight be taken as evidencefor Kohistan representing a continental margin.

7.2.1. Crustal thicknessThe inferred paleo-depth of the Moho in the Kohistan arc (~54–

55 km) is significantly more than any currently active intra-oceanicarc, which have crustal thicknesses of b35 km. It is however, essentialto elucidate the meaning of the “thickness” of modern intra-oceanicarcs: Inmany active arcs a sharply definedMoho based on a PmP reflec-tor is absent and the Moho seems to be rather transitional making theprecise location of the Moho in active arcs model dependent(Shillington et al., 2004). Additionally, an arc Moho does not necessarily

Fig. 7. (a) εNd and εSr for Pacific and Indian MORB, Aleutian Lavas (from Georoc database (arc. The bulk isotopic compositions for the Kohistan arc are given by stars (black: model #1(b) Relative enrichment of the Aleutian and Kohistan arc compared to their regional depletKohistan arc and the Aleutian differs significantly, the relative enrichment between the arc


delineate a mantle/crust boundary but rather the boundary betweenmafic and ultramafic cumulates or restites (e.g., Miller and Christensen,1994; Tatsumi et al., 2008). Even so the exact location of the paleo-Moho in Kohistan is disputed (Miller and Christensen, 1994; Burg et al.,1998; Kono et al., 2009), themeasured increase in seismic velocity acrossthe inferred Kohistan paleo-Moho (from 7.2–7.5 to >8.0 km/s above ref)is at significantly higher VP than the seismic velocity changes observed fortheMoho in active arcs. In theMariana arc, theMohowas interpreted byTakahashi et al. (2008) to correspond to the PmP reflection that correlateswith an increase in VP from 7.3 to 7.7 km/s in thewestMariana ridge andfrom6.9 to 7.6 km/s in theMariana arc. In both areas, this reflector locatesat depths of 17–20 km (Takahashi et al., 2008). Comparable changes inseismic velocities are present at various structural levels in Kohistan,one between the batholith and the Chilas Complex (Chroston andSimmons, 1989) at a paleo-depth of ~21 km, and one between the Saran-gar gabbro and the garnet gabbros (Miller and Christensen, 1994) at~40 km paleo-depth. It should further be noted, that e.g. the garnet-gabbros have an average VP of 7.9 (Chroston and Simmons, 1989;Miller and Christensen, 1994), a value that is typically attributed toman-tle lithologies, and in the interpretation of the Mariana arc structure byTakahashi et al. (2008) to an anomalously slow mantle. In the Marianaarc, further deeper reflectors are identified at 30–40 km depth and typ-ical mantle VP values of >8 km/s are only inferred for depths ≥50 km(Takahashi et al., 2008), similar to the depth of the Kohistan arc. Thesecomplexities make a direct comparison between crustal thicknesses in-ferred from an interpreted seismicMoho in active arcs and from the ex-posed Kohistan paleo-Moho difficult. The Kohistan profile implies thatsignificant amounts of gabbros and mafic to ultramafic cumulatesmight be present in active island arcs below an interpreted Moho.

The observed dimensions of the Kohistan arc allow calculation of anaverage crust production rate, as the arc section is complete. Assumingan initially 6 km thick oceanic crust onwhich the Kohistan arcwas built,a 120–150 km wide arc, which is the present distance from the Dir-Kalam volcanics in the south to the Kohistan–Karakoram Suture zoneand comparable to currently active intra-oceanic arcs (Dimalantaet al., 2002), and simplifying the shape of the arc to a simple rectangulargeometry (i.e., Aleutian, Holbrook et al., 1999), the Kohistan arc crustaverage production rate results to 48–72 km3/km along strike/Ma.This estimate is comparable to crust production rate estimated for theIzu–Bonin arc (80 km3/km/Ma), the Mariana arc (70 km3/km/Ma), theTonga and Vanuatu arc (60–80 km3/km/Ma, Dimalanta et al., 2002;Taira et al., 1998), the Aleutians (55–89 km3/km/Ma, Holbrook et al.,1999; Jicha et al., 2006) and the New Hebrides (87–95 km3/km/Ma,

http://georoc.mpch-mainz.gwdg.de/georoc)) and the igneous rocks from the Kohistan; red: model #2). Data and the source for Kohistan are given in the electronic appendixed MORB mantle (DMM) component. Even so the absolute isotopic composition of theand their inferred DMM component is comparable.


http://georoc.mpch-mainz.gwdg.de/georoc



Greene et al., 1994). All these crust production rates are higher than therecently questioned average arc magma addition rates of 23–33 km3/km/Ma (Reymer and Schubert, 1984).

7.2.2. Isotopic composition and DUPAL type mantle sourceThe isotopic composition of the different bulk Kohistan arc models

results to εSr (−4 to −6) εHf (11.4–12.5), εNd (4.8–5.0), 208/204Pb(~38.5), 207/204Pb (~15.6) and 206/204Pb (~18.4) which is moreevolved than modern pacific intraoceanic arcs (e.g., Aleutian, Fig. 7).This somewhat evolved isotopic composition could be taken as evi-dence for involvement of a yet undetected old continental basementduring arc formation. However, one has to consider the relative isoto-pic enrichment of the arc compared to its inferred (depleted) mantlecomponent. The depleted mantle component of the Mariana andAleutian arcs is similar to typical pacific MORB-type mantle (e.g.,Straub et al., 2004) whereas the depleted mantle component of theKohistan arc is considered to be paleo-Tethyan MORB type mantle(Khan et al., 1997) thought to have isotopic signatures similar to presentday IndianMORB typemantle (Mahoney et al., 1998). The paleo-Tethyanand Indian MORB type mantle are characterized by the so-called DUPALisotopic anomaly relatively enriched in isotopic composition comparedto Pacific MORB type mantle (Hart, 1984). This interpretation is basedon the Pb isotopic composition of Kohistan arc rocks and associatedophiolitic slivers, on the inferred paleomagnetic location of the arc duringits formation (Zaman and Torii, 1999) and on the fact that the DUPALexisted at least since Jurassic times and also characterized the TethyanMORB-source mantle. In support of this idea, we note that the relativeisotopic enrichment in Kohistan with respect to the DUPAL type mantleis similar to that seen in the Aleutian and Marianas arcs with respect tothe Pacific depleted mantle component (Fig. 7).

In summary, we consider neither the “unusual” thickness of theKohistan arc nor its isotopic composition as a contradiction to anevolvement as an intra-oceanic arc until collision with the Eurasianand Indian margins. There is no conclusive evidence for the involve-ment of old continental basement during arc formation, and thus,we agree with previous studies that the Kohistan arc is the best pre-served example of an obducted intra-oceanic island arc (Tahirkheli,1979).

7.3. Toward an andesitic bulk composition of intraoceanic island arcs

The composition of the Kohistan arc represents the best availableestimate for modern juvenile continental crust formed in oceanicarc settings. The fact that the Kohistan arc is andesitic in compositionchallenges the view that oceanic arcs are generally basaltic, a hypothesisthat was based on estimates from the Kohistan, Talkeetna, Mariana andAleutian arcs (Pearcy et al., 1990; Miller and Christensen, 1994;Holbrook et al., 1999; Tatsumi et al., 2008) and on the fact that primitivearc lavas are dominantly basaltic in composition (Kelemen et al., 2003).

Regarding paleo-arcs in which deeper parts are exposed, our im-proved estimate of the bulk Kohistan arc excludes a basaltic composi-tion as has been proposed by Miller and Christensen (1994) on thebase of a much smaller data set. For the Talkeetna arc, Hacker et al.(2008) showed that significant preservation gaps exist, thus rendering es-timates of the Talkeetna arc bulk composition stronglymodel dependent.

The bulk compositions of active arcs have mainly been estimatedon the basis of seismic profiles (Holbrook et al., 1999). The seismicprofiles of e.g. theMariana and Izu–Bonin arc clearly show the presenceof a mid-crust layer with an evolved tonalitic/andesitic composition(Kodaira et al., 2007). As the upper and lower crust layers are inter-preted as basaltic/gabbroic in composition (Takahashi et al., 2008;Tatsumi et al., 2008) the bulk of the Mariana–Izu–Bonin arc crust ismore evolved than basaltic. To re-compose a basaltic bulk arc, Tatsumiet al. (2008) proposed that the anomalously slow mantle below theMoho could be reinterpreted as mafic to ultramafic cumulates, possiblytransformed through remelting processes.


It has been shown that a range of cumulate and mantle rocks canhave comparable seismic velocities in the 6.8–7.8 m/s range (Behnand Kelemen, 2003, 2006) making estimates of bulk arc compositionbased on seismic profiles strongly dependent on the interpretation ofthese velocities. Secondly, such mafic to ultramafic cumulates or resti-tes are gravitationally unstable (Jagoutz et al., 2011). Their founderinginto the mantle is expected to commence when dense cumulate layersreach >2–4 km thickness (Behn et al., 2007), such a delaminationwouldmove the bulk composition of the arc towardmore evolved com-positions. Thus, while we endorse that primitive arc magmas are gener-ally basaltic (Kelemen et al., 2003), bulk arc compositions themselvesmay well be more evolved.

In summary, there are no conclusive data supporting the view thatoceanic arcs are generally basaltic and therefore could not be thelocus of (andesitic) continental crust formation without involvingmajor additional reworking processes. Rather, a range of bulk compo-sition of oceanic arcs from basaltic to andesitic seems to exist. An an-desitic Kohistan arc bulk composition has far-reaching implicationsfor crust formation in subduction zone settings.

7.4. A contribution of intra-plate type magmas to the continental crust?

As can be seen in Fig. 5, the Kohistan arc crust lacks ~5–60% of theHFSE elements Ti, Zr, Hf, Ta, and Nb with respect to average continen-tal crust. In accordance with previous models this mismatch can beexplained by a contribution of intra-plate magmas to the long timeaverage of the bulk continental crust (e.g., Barth et al., 2000), possiblyin the form of large igneous provinces such as the Deccan or Siberiantraps or accretion of oceanic plateaus formed from OIB.

To compensate for the HFSE element depletion in Kohistan, an addi-tion of 20–30% of continental flood basalts would be needed. Thisamount reduces to a more realistic ~6–8 wt.% if the HFSE depletion iscompensated by addition of ocean island type basalts (OIB). As OIBtypemagmas are also enriched in incompatible elements, the depletionin LREE and LILE in Kohistan compared to the bulk continental crustwould also bemitigated by the addition of a fewpercent of OIBmaterial.

Alternatively, the HFSE enrichment of the continental crust relativeto our estimate of the Kohistan arc could also result from an enrichmentof heavy minerals such as zircon, rutile, sphene, apatite etc. in the sedi-mentary rocks (shales and loess) that have been used to constrain thebulk composition of the upper continental crust. Such heavy mineralsare more resistant to weathering than most silicates, are an importantcarrier for HFSE and REE and have been shown to become enrichedwith respect to their source region by sedimentary processes. Yet, it isthought that composite shale analyses are better estimates for the com-position of the source region (e.g., Gromet et al., 1984).

7.5. Secondary reworking processes

A long standing debate circles around the role of secondary refiningprocesses which have been proposed to transform putative basaltic oce-anic arc crust into andesitic continental crust. For example partial melt-ing of hydrated basaltic rocks in the lower crust was proposed to be animportant mechanism for crustal evolution (Tatsumi et al., 2008). Ithas also been postulated that significant refining mechanisms occurafter arcs are accreted to the continental lithosphere. In this model, col-lision and accretion result in an increase in crustal thickness, enablingdifferent melt fractionation paths compared to a shallower oceanic arcsystem (e.g. Lee et al., 2007). Increasing crustal depth will also facilitatepartial melting of basaltic compositions (Vielzeuf and Schmidt, 2001).Crustal thickening related to collisionmay lead tometamorphic densifi-cation resulting in eclogite formation and thus trigger the foundering ofdense eclogitic roots (e.g., Kay andKay, 1993). Nevertheless, subtractingbasaltic eclogites from a putative basaltic arc does not alter the arc's bulkcomposition. In this scenario, only a combination of partial meltingwitheclogitization may lead to a net evolution of the arc's composition.




The petrology and chemistry of the mafic and ultramafic rocks inthe deeper part of the arc is in accordance with a cumulate formationmechanism but conflicts with a restitic origin of these rocks. Detailedfield, petrological and geochemical studies have shown that instead ofpartial melting, hydrous high to medium pressure fractionation, wasthe dominant granitoid forming mechanism in Kohistan (Jagoutz et al.,2009; Jagoutz, 2010; Jagoutz et al., 2011). Arc volcanics with primitivecharacteristics (in terms of Ni, Cr and XMg) in Kohistan are dominantlybasaltic, only one high XMg andesite has been reported (Bignold et al.,2006). Therefore, we assume that the Kohistan sequence wasdominantly formed from basaltic liquids. To evolve toward an andesiticbulk composition significant volumes of mafic and ultramafic cumulatesmust fractionate and are effectively now missing in the Kohistan se-quence, implying a return flow from the site of crust generation to themantle. Petrological density modeling of cumulates shows that thoseformed from hydrous fractional crystallization have a much larger po-tential to delaminate than those cumulates formed by a drier fraction-ation sequence (Jagoutz et al., 2011). Over which time period(s) themafic and ultramafic cumulates were lost from the Kohistan and whya sequence of them are preserved in the Southern Plutonic Complexneeds to be investigated further. Ar–Ar cooling ages in the Jijal sequenceare ~85–83 Ma, i.e. contemporaneous with the extension related intru-sion of the Chilas Complex into the Southern Plutonic Complex(Treloar et al., 1989). The presence of only garnet-free lower crustal xe-noliths in the Chilas Complex could be used to speculate thatmost of thegarnet-bearing granulites andmafic–ultramafic cumulates of the South-ern Plutonic Complex had already been delaminated when the ChilasComplex intruded. Our bulk composition is characterized by slightly de-pleted incompatible elements compared to the bulk continental crustand a pronounced Sr/Nd anomaly, typical for arc magmas. The high Sr/Nd is not reflected in the BCC and indicates that some additional processnot reflected in the bulk Kohistan arc contributes to the bulk compositionof the continental crust.We speculate thatweathering could be an impor-tant process to explain this mismatch (Lee et al., 2008) as significant por-tion of the bulk continental crust composition is derived from sediments,whereas the Kohistan arc bulk composition is derived from igneous rocksonly. Also the effect of the post-collisional granites on the trace elementsystematics of the Kohistan bulk needs to be further investigated.

8. Concluding remarks

The bulk composition of the Kohistan arc, which represents a mod-ern intra-oceanic environment, is andesitic. We thus demonstrate thatintra-oceanic magmatic processes are capable to generate a juvenileevolved crust. This conclusion is based on the geochemical analysis ofa complete cross section through the arc, which encompasses the com-plete range from subaerial volcanic deposits to the uppermost portion ofthe subarc mantle. Bulk crust differentiation from a primitive basalticparent composition occurs through foundering of ultramafic cumulates.

Secondly, the juvenile Kohistan arc crust is very similar to the bulkcontinental crust. This suggests thatmajor reworking or refining process-es, in particular further magmatic differentiation, possibly related tocrustal thickening,were not necessary in Kohistan to derive the presentlypreserved continental crust composition. The commonly proposed ideathat oceanic arcs are generally basaltic in composition needs to be revis-ited. Processes intrinsic to oceanic supra-subduction arcs can producecontinental crust composition. Secondary reworking mechanism werenot essential in Kohistan to explain the major element bulk compositionbut could overall be important to e.g., explain certain characteristics ofthe trace element concentrations and need to be further investigated.

Acknowledgments

We thank H. Dawood and S. Hussain for many years of collaborationregarding field work in the Kohistan, without their help, none of thiswould have been possible. We thank J.P. Burg, O. Müntener and A.


Enggist for sharing thoughts and data, and L. Zehnder and L. Ramalingamfor analytical support. C.T. Lee is thanked for a thought provoking review.The editorial handling and the review of K. Mezger is acknowledged. R.Rudnick is thanked for a thorough pre-publication review. OJ's workwas in part supported by a NSF grant (EAR 0910644). M.W.S. acknowl-edges ETH for generous funding.

Appendix A. Supplementary data

Supplementary data to this article can be found online at doi:10.1016/j.chemgeo.2011.10.022.

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