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The roles of flux- and decompression melting and their respective fractionation linesfor continental crust formation: Evidence from the Kohistan arc

Oliver Jagoutz a,⁎, Othmar Müntener b, Max W. Schmidt c, Jean-Pierre Burg c

a Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, USAb Institute of Mineralogy and Geochemistry, University of Lausanne, Switzerlandc Department of Earth Science, ETH Zurich, Switzerland

a b s t r a c ta r t i c l e i n f o

Article history:Received 3 May 2010Received in revised form 3 December 2010Accepted 6 December 2010Available online 31 January 2011

Editor: R.W. Carlson

Keywords:continental crust formationsubduction zonesfractionationliquid lines of descent

Delamination and foundering of the lower continental crust (LCC) into the mantle is part of the crust-formingmechanism. However, knowledge of the composition and mineralogy of the preserved or delaminated LCCover geological timescales remains scarce. We provide a synopsis of recent research within the Kohistan arc(Pakistan) and demonstrate that hydrous and less hydrous liquid lines of descent related to flux assisted anddecompression mantle melting, respectively, produce compositionally different lower crustal rocks. Theargument refers to two lower crustal sections exposed in Kohistan, the older Southern Plutonic Complex(SPC) and the younger Chilas Complex.The SPC typifies a hydrous, high-pressure fractionation sequence of olivine–pyroxenes–garnet–Fe/Ti-oxide–amphibole–plagioclase. The Chilas Complex illustrates a less hydrous fractionation sequence of olivine–clinopyroxene–orthopyroxene–plagioclase–amphibole. Despite the similarity of the Chilas Complex rocks toproposed lower crust compositions, the less hydrous fractionation results in unrealistically small volumes ofsilica-rich rocks, precluding the Chilas Complex gabbros to represent the magmatic complement to the uppercrust. The composition of the SPC lower crust differs markedly from bulk lower crust estimates, but iscomplementary to silica-rich rocks exposed along this section and in the Kohistan batholith.These observations inspire a composite model for the formation of continental crust (CC) where thenegatively buoyant delaminated and the buoyant preserved lower continental crusts (LCC) differ in genesis,mineralogy, and composition. We propose that the upper, non-sedimentary CC is dominantly formed byhydrous high-pressure fractionation with subsequent removal of the complementary, negatively buoyantgarnet–pyroxene–amphibole-plagioclase-rich cumulates. In contrast, the LCC, which is buoyant andpreserved over geological timescales, is formed by less hydrous parental mantle melts. We suggest that thebulk continental crust composition is related to mixing of these petrologically not directly related endmembers.

Published by Elsevier B.V.

1. Introduction

Even though a broad consensus exists that subduction-relatedprocesses are essential for continental crust (CC) formation (Barthet al., 2000; Rudnick and Fountain, 1995), the cause(s) for the majorCC characteristics are still in discussion. For example, the silica-richandesitic bulk composition and the low Mg# (=molar MgO/(FeO+MgO)) of the CC, ultimately derived from dominantly basaltic highMg# mantle melts, puzzled earth scientists for decades. Similarly, theinternal compositional stratification of the CC into a granitic upperand basaltic lower crust is poorly understood. Traditionally, the lowercontinental crust is regarded as the residue after partial melting or thecumulate fraction of crystallization processes resulting in the upper

continental crust (Taylor and McLennan, 1985). However, thisassumption is in conflict with e.g., the lack of sufficient Eu enrichment(Rudnick, 1992) as well as the primitive island arc basalt-likeincompatible element content of the bulk lower crust (e.g., K andRb, Rudnick and Gao, 2003), making the latter an inappropriatecomplement to the upper crust. Furthermore, upper crustal granitoidsof continental and oceanic arcs are generally too low inMg# at a givenSiO2 compared to the bulk continental crust (Jagoutz et al., 2009;Kelemen, 1995). Multiple processes have thus been combined toexplain the andesitic composition of the bulk continental crust. Theemerging model is complex and involves formation of a basalticproto-continental crust in subduction zones and subsequent refiningof the proto-crust by partial melting processes (Tatsumi et al., 2008).The latter is regarded as the main mechanism producing a graniticupper crust (Hildreth andMoorbath, 1988; Tamura et al., 2010;Whiteand Chappell, 1983), although alternative views advocate that crystalfractionation is the dominant mechanism (Cawthorn and O'Hara,

Earth and Planetary Science Letters 303 (2011) 25–36

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

0012-821X/$ – see front matter. Published by Elsevier B.V.doi:10.1016/j.epsl.2010.12.017

Contents lists available at ScienceDirect

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1976; Davidson et al., 2007; Jagoutz, 2010; Müntener et al., 2001).Finally, delamination of garnet-rich mafic residues drives the bulkcrust towards andesitic compositions. This model however, falls shortin accounting for the low Mg# of continental granitoids comparedwith the bulk continental crust. The compositional discrepanciesbetween the presently produced crust in subduction zones and thebulk continental crust imply either that estimated bulk continentalcrust compositions are wrong (too SiO2-rich) or that continental crustdoes not result from a single petrogenetic process but from multipleprocessing.

In this study, we document the preservation of higher-pressure“hydrous” and lower-pressure “dry” (in the sense of “less hydrous”)igneous fractionation in the Kohistan arc, related to flux assisted anddecompression melting in the mantle, respectively. The hydroushigher-pressure fractionation produces derivative liquids that explainthe mid- to upper crustal Kohistan batholith (Jagoutz, 2010; Jagoutzet al., 2009). The less hydrous fractionation produces rocks similar tolower continental crust (Jagoutz et al., 2006) but volumetrically veryminor granitic rocks. Based on these observations we propose a newcomposite model for continental crust formation.

2. Geological setting of the Kohistan arc

2.1. General introduction

The Kohistan arc (NW Pakistan) is separated from Eurasia by theKarakoram–Kohistan arc Suture Zone, and from the Indian Plate bythe Indus Suture Zone (Fig. 1, and electronic Appendix). The Kohistanarc is generally regarded as a fossil Jurassic to Cretaceous island arcwedged between the Indian and Asian plates during collision (Bard,1983; Burg et al., 1998; Tahirkheli, 1979). The intraoceanic arcoriginated above a north-dipping subduction zone in the equatorialarea of the Tethys Ocean, possibly in the vicinity of the Eurasiancontinent (Bignold and Treloar, 2003; Coward et al., 1987; Khan et al.,1993). Owing to intra-crustal extension around ~85 Ma (Burg et al.,2006) and obduction on the Indian lithosphere around 50 Ma(Garzanti et al., 1987), a complete crustal section ranging from themantle to the uppermost volcanic and sedimentary sequences isexposed (Coward et al., 1986; Searle et al., 1999; Treloar et al., 1996).

The southern part of the Kohistan arc is composed of the ultramaficto mafic Jijal Complex and the Southern Amphibolites representingthe upper mantle and the lower to middle crust of the island arc (Janand Howie, 1981; Jan and Windley, 1990; Ringuette et al., 1999). Thispart is also collectively referred to as the Southern Plutonic Complex(SPC, Burg et al., 2005). The contact between the ultramafic mantlerocks and the mafic lower crustal rocks is interpreted as the sub-arcMoho (Miller and Christensen, 1994). The SPC is separated from the

Gilgit Complex, which comprises the Kohistan batholith and itsvolcano-sedimentary cover (Jagoutz et al., 2009) by the ChilasComplex, a large ultramafic–mafic intrusion attributed to intra- andback-arc rifting (Jagoutz et al., 2006; Khan et al., 1989, 1997). Thesouthern and central Kohistan batholith is dominantly composed ofplutonic rocks with screens of volcanic and sedimentary rocks andtheir metamorphosed equivalents (Petterson and Windley, 1985;Pudsey et al., 1985). The northern limit of the batholith is covered byvarious volcanic sequences (Chalt, Shamran, and Teru) and sediments(Yasin).

Despite the large size of the Chilas Complex U–Pb zircon agesdocument a rather limited time span of pluton emplacement at~85 Ma (Zeitler, 1985; Schaltegger et al., 2004). In contrast, themagmatic activity is significantly longer in both the SPC (120–70 Ma,Anczkiewicz and Vance, 2000; Schaltegger et al., 2004; Yamamoto andNakamura, 1996, 2000; Yamamoto et al., 2005) and the Kohistanbatholith (110–40 Ma, (Jagoutz et al., 2009 and ref within). Pressureestimates in the lower crustal Jijal section vary significantly,nevertheless the average of well-constrained geobarometers appliedto mineral rim compositions yields ~1.5 GPa for the Moho (Ringuetteet al., 1999; Yoshino and Okudaira, 2004; Yoshino et al., 1998).

2.2. Detailed field relationships

Field mapping has revealed that the structure of the Kohistan arccomprises two different lower crustal sections: the Southern PlutonicComplex at the structural bottom and the Chilas Complex. The SouthernPlutonic Complex represents the “older” section, which was tiltednorthwards and uplifted during arc extension at ~85 Ma (Burg et al.,2006) (Fig. 1and seeelectronicAppendix). It is characterizedbyabundantmagmatic amphibole and represents a cumulate sequence formed by“hydrous” medium- to high-pressure fractionation evolving along thegeneral simplified fractionation sequence ol→pyroxenes→amph/grt→plag→qz. The SPC includes ultramafics at its base composed ofclinopyroxene-rich lithologies (wherlite, olivine–clinopyroxenite and(garnet) websterite) with subordinate dunite. Amphibole becomesincreasingly abundant up section, and pyroxene-rich ultramafic rocksgrade into hornblende-, clinopyroxene- and garnet-rich ultramaficcumulates. The ultramafic rocks are overlain by granulitic garnet-gabbrosof the Jijal Complex and then by gabbros, diorites to tonalites andmetavolcanics of the Southern Amphibolites on top (Burg et al., 2005).The recordedpressures range from~1.5 GPanear the base of the gabbroicsequence to ~0.7 GPa at the top of the section (Jan and Howie, 1981;Ringuette et al., 1999; Yamamoto, 1993; Yoshino et al., 1998). TheKohistan batholith, which has dominantly preserved its originalorientation, can be related to the SPC by a common fractionationmechanism (Jagoutz, 2010; Jagoutz et al., 2009).

Fig. 1. Geological cross section through the Kohistan arc (modified after Burg et al., 2006). Indicated are the pressure ranges of the three different building blocks of the Kohistan arc.Pressure ranges after (Enggist, 2007; Ringuette et al., 1999; Yamamoto, 1993; Yoshino and Okudaira, 2004; Yoshino et al., 1998).

26 O. Jagoutz et al. / Earth and Planetary Science Letters 303 (2011) 25–36


95% of the Chilas Complex are composed of gabbro-norite andminor (quartz-) diorite. Volumetrically minor ultramafic rocks aredominated by dunite with minor ol-websterite and pyroxenite (fordetails see Appendix). Contrary to the SPC, in the Chilas Complexamphibole is generally absent or a late magmatic phase (Jagoutz et al.,2007). The Chilas Complex is thus considered as a “dry” lower crustalfractionation sequence of ol→pyroxenes→plag→amph→qz. Theintrusive Chilas Complex formed during intra-arc extension, whichalso produced an intra-arc basin. A likely volcanic equivalent are thebasaltic rocks within the Dir volcanics (Shah and Shervais, 1999)which have trace and major element characteristics very similar tothose of the Chilas gabbronorite (Jagoutz et al., 2006).

Subsequently, we compare the petrological and geochemicalcharacteristics of the two different crustal sections. In the Appendix,we describe in more detail the rock associations of the SPC and ChilasComplex. The interested reader is also directed to previous, morecomprehensive descriptions of the SPC (Arbaret et al., 2000; Burget al., 2005; Jagoutz, 2010) and the Chilas Complex (Burg et al., 2006;Jagoutz et al., 2006, 2007; Jan and Howie, 1980).

3. Results

We present 58 new whole rock analyses from the SPC (eTable 1,analytical details in the electronic Appendix) and compile all availablewhole rock data for the SPC and the Chilas Complex. The entire datasetincludes 551 whole rock analyses (electronic supplement).

A general problem in interpreting mafic or more evolved plutonicrocks relates to the fact that they may represent true liquid or cumulatecompositions. FollowingKelemenet al. (2003), cumulates are definedasrocks that formed by accumulation of minerals while some remainingmelt left the system. This definition includes rocks that have variableamounts of trapped liquid and/or interacted with unrelated migratingmelts. Although not all volcanic rocks necessarily represent true meltcompositions we refer to plutonic rock that chemically resemblevolcanic rocks as liquids. For the Chilas Complex, detailed trace elementmodeling has shown that some mafic plutons resemble liquidcomposition (see below) whereas others are cumulates with variableamount of trapped liquid (Jagoutz et al., 2006, 2007). Such aquantitativemodel is not available for the SPC and is beyond the scope of this paper.In general plutonic rock with SiO2N56–57 resembles volcanic rocksfrom higher levels in the arc in terms of Sr/Nd, Eu/Sm and Al/Si. Wecharacterize granitoids as cumulative if the Sr/Nd or Eu/Sm are higherthan the values observed in volcanics (i.e., Sr/NdN50 and/orEu/SmN0.46). For rock sample where no trace element data were

available, we used the criteria of SiO2b50 wt.% and/or Si/Alb2.9 toidentify cumulates.With the exception of the small exposure of residualmantle in the Sapat area (Bouilhol et al., 2009), ultramafic rocks inKohistan are generally cumulates.

Whole rock compositions from the two units generally follow acalc-alkaline fractionation trend, although most cumulates fall intothe tholeiitic field (Fig. 2A). Incompatible trace elements are enrichedcompared to averagemid-ocean ridge basalts (MORB) and depleted inhigh field strength elements (HFSE) compared to large ion lithophileelements (LILE) (Fig. 2B) as typical for subduction zone magmatism.Whole rock geochemical compositions of the individual units of thedifferent sections have been discussed in detail earlier (Dhuime et al.,2007, 2009; Garrido et al., 2006; Jagoutz, 2010; Jagoutz et al., 2006,2009; Jan and Howie, 1980, 1981; Yamamoto and Yoshino, 1998). Inthe following, we compare the geochemical characteristics of theplutonic rocks from the SPC to those from the Chilas Complex (Fig. 3).In terms of major elements the SiO2–Mg# variability illustrates bestthe role of cumulates whereas compositional differences betweenderivative liquids are most distinct in silica vs. incompatible elementdiagrams (Fig. 3). Additional geochemical diagrams illustrating thedifferences and similarities of the two sections can be found in theelectronic Appendix.

For liquid compositions the most prominent difference betweenthe two plutonic series is the amount of silica-enrichment over a givenincrease in highly incompatible elements (e.g., Rb, Hf, and K2O)(Fig. 3D,E). The Chilas plutonics are characterized by a significantenrichment in K2O (from 0.36 to 2.8 wt.% for 95% of all measuredsamples) with a limited increase in SiO2 (50–57 wt.%). Contrasting,the trend from the SPC is characterized by a larger increase in SiO2

(50–76 wt.%) for a comparable K2O enrichment (0.32 to 2.9 wt.% for95% of all samples). The observed similar amount of highlyincompatible element enrichment reflects similar degrees of fraction-ation assuming comparable parental magma compositions for bothseries. As average primitive arc melts show a limited range of K2Ocontents (0.3–1.2 wt.%, the majority being ~1 wt.%, Kelemen et al.,2003), the strong difference in silica enrichment between the ChilasComplex (ΔSiO2~7 wt.%) and the SPC (ΔSiO2~24 wt.%) series shouldstem from fundamentally different fractionation processes and thusrepresent two different fractionation lines. In accordance with theminor silica enrichment in the Chilas Complex and the importance ofgarnet and hornblende-bearing cumulates for the silica enrichment(Alonso-Perez et al., 2009; Davidson et al., 2007; Jagoutz, 2010), thesetwo minerals and their cumulates are generally absent in the ChilasComplex while they are prominent in the SPC.

Fig. 2. (A) AFM diagram showing the overall calc-alkaline character of the whole rock compositions of liquid-type plutonic rocks from the Southern Plutonic and the Chilas Complex.(B) Averaged primitive mantle normalized trace element concentrations of the bulk Southern Plutonic and the Chilas Complex (data in eTable 1). Note the general enrichedincompatible element composition of the rock in Kohistan relative to MORB (Hofmann, 1988) and the close correspondence of the bulk Chilas Complex to the bulk lower continentalcrust of Rudnick and Gao (2003).

27O. Jagoutz et al. / Earth and Planetary Science Letters 303 (2011) 25–36


The ultramafic cumulates present in both sections also differ intheir geochemistry, which influences their derivative liquids and sothe liquid line of descent. The main difference already begins with theFeO and MgO evolution of dunitic rocks (Fig. 3A,B). Dunites of theChilas Complex tend to lower Mg# (0.88→0.78) than dunites of theJijal ultramafics (~0.90) illustrating the larger interval of olivinefractionation in the Chilas sequence (Jagoutz et al., 2006). Similarly,Jijal pyroxenites have generally higher Mg# (~0.90–0.88) and higherCr contents (2000–4000 ppm) than Chilas pyroxenites (Mg#~0.80,Crb2000 ppm), and most Jijal pyroxenites could have crystallizedfrom a primitive melt. In the Jijal ultramafics, TiO2 is initiallyincompatible but becomes compatible with the early appearance ofhornblende and FeTi-oxides in the cumulate sequence (Jagoutz, 2010;Jagoutz et al., 2009) (electronic Appendix). Accumulation of oxidesand amphibole in rocks from the SPC is common whereas theseminerals are generally absent in the Chilas gabbros. Furthermore,ultramafics in the SPC have generally higher Fe/Mg at a given SiO2

than in the Chilas Complex. In the Chilas Complex, TiO2 is generallyincompatible resulting in significantly higher TiO2 concentrations inrocks with SiO2N52 wt.% than in the SPC (electronic Appendix). At agiven silica content, gabbroic rocks with SiO2N50 wt.% from the SPC

sequence are generally poorer in MgO, NiO, and Na2O than in theChilas Complex.

4. Discussion

4.1. Fractional crystallization sequence of the SPC

Thewhole rock geochemistry and petrographic observations of theSPC can be interpreted in terms of cumulate and melt compositionsbelonging to one common liquid line of descent exemplarilyillustrated in Mg# vs. SiO2 (Fig 3A): primitive cumulates with highNi, Cr and generally high Mg# have variable SiO2 concentrations of39–50 wt.% reflecting modal abundances of olivine and pyroxene. Thegenerally high Mg# of the Jijal dunites indicates that olivinefractionation had only a limited control on their Mg# evolution. TheMg# of ultramafic rocks decreases with the appearance of Cr-rich(clino-)pyroxene indicating a limited interval of olivine-only frac-tionation and an early appearance of clinopyroxene on the liquidus.These cumulates define the first segment of the z-shaped trend inFigure 3A. Up-section, the Jijal rocks are characterized by a modalincrease of silica-poor minerals such as garnet, Fe/Ti-oxides, and

Fig. 3. Harker diagrams illustrating the different fractionation trends for the Southern Plutonic (A,D) and the Chilas Complex (B,E). The black, red, and orange stars indicate thecomposition of the lower, bulk and upper continental crust from Rudnick and Gao (2003). The black and red lines correspond to a liquid line of descent modeled for the Kohistan(Jagoutz, 2010). The black line is a pure fractional crystallization model, and the red line includes few percent of assimilation. By field occurrence, the Kohistan volcanics cannot beattributed to the “hydrous” or “dry” liquid lines of descent (detailed petrological descriptions lacking), but two trends in agreement with those in the plutonics could be recognized.(C) Experimentally derived adcumulate compositions from hydrous and anhydrous equilibrium and fractional crystallization experiments (Müntener and Ulmer, 2006). Cumulatecompositions from the hydrous experiments are color-coded following the scheme from the SPC whereas anhydrous experiments are color-coded following the Chilas Complexscheme. The yellow triangles are residues to fluid-absent amphibolite melting calculated from the experiments of Wolf and Wyllie (1994). (F) Experimentally derived LLD fromhydrous (blue) and anhydrous (red) fractional crystallization experiments (Villiger et al., 2004 and Alonso-Perez et al., 2009, respectively). To compensate for differences in thestarting composition the change of SiO2 is shown in relative % and K2O is normalized over the initial K2O in the parental melt. All data and further element variations are supplied inthe electronic Appendix.)



hornblende in cumulates, and also An-rich plagioclase in gabbroiccompositions. Accordingly, with increasing modal proportions ofthese minerals, whole rock compositions become progressively lowerin SiO2 and Mg#, leading to the characteristic middle limb of the z-shaped trend (Fig. 3A). Magmatic accumulation leads for both themiddle and first segments to an increase in CaO, TiO2, Na2O, and Al2O3

content with decreasing Mg#. The third limb begins with gabbroic todioritic rocks higher up in the section, characterized by a low butrather constant Mg# near 0.45 and decreasing TiO2 over an interval of52–60 wt.% SiO2. Tonalite, granodiorite and minor granite and quartz-monzonite in the top part of the section display decreasingMg# (from0.4 to 0.1) at 55–77 wt.% SiO2, accompanied by a similar decrease inTiO2.

It should be mentioned, that the original mineralogy of the garnet-bearing gabbro is under discussion (Jagoutz, 2010): Yamamoto andYoshino (1998) proposed a prograde metamorphic origin whileGarrido et al. (2006) interpreted them as restites of partial melting.The latter interpretation is in conflict with field observations (Arbaretet al., 2000; Burg et al., 2005), textural and mineralogical data, whichfavor a magmatic high-pressure cumulate origin of these rocks(Ringuette et al., 1999). The characteristic whole rock trend illustratedin Figure 3A can be understood andmodeled in terms of accumulationprocesses of olivine→pyroxenes→garnet+hornblende+Fe–Ti oxi-des→plagioclase (Jagoutz, 2010). The geochemical evolution of theSPC is in line with medium- to high-pressure fractional crystallizationexperiments on H2O-bearing undersaturated liquids (Alonso-Perezet al., 2009; Müntener et al., 2001; Fig. 3C,F). Cumulate compositionscalculated from experimentally fractionating phases reproduce the z-shaped trends (Fig. 3C). On the contrary, restite compositions ofpartial melting experiments (Wolf and Wyllie, 1994) calculated bymass balance display an overall negative correlation between Mg#and SiO2 in disagreement with the observed trend from the SPCcomplex (Fig. 3C).

The agreement between observed cumulates and crystallizationexperiments supports that the SPC represents a hydrous liquid line ofdescent resulting from high-to medium-pressure fractionation. Start-ing from a primitive basaltic, mantle-derived melt, granitoid compo-sitions and volumes in the upper part of the SPC and in the Kohistanbatholith can be modeled by fractionation of cumulates present in thelower part of the Kohistan arc (Jagoutz, 2010; Jagoutz et al., 2009).Therefore, we adopt the interpretation that the mid- to upper-crustalKohistan batholith is also related to this hydrous fractionationmechanism. This does not imply that the entire sequence resultsfrom a single common parental magma. Field observations, geochro-nological and isotopic data imply multiple intrusion events withdifferent sources stretching over tens of Ma (Schaltegger et al., 2004;Yamamoto et al., 2005; Dhuime et al., 2007).

4.2. Crystallization regime of the SPC

The most characteristic features of the SPC are the limited controlof olivine-only fractionation on the Mg# evolution and the earlyappearance of amphibole and garnet in the fractionation sequence.Amphibole and garnet require significant H2O contents in the primaryliquids (Alonso-Perez, 2006; Cawthorn and O'Hara, 1976; Münteneret al., 2001). In hydrousmelts the role of H2O for garnet crystallizationis indirect, suppressing early plagioclase and thus allowing forAl-enrichment in the melt. The increase in silica at near constantMg# (~0.89–0.88) and the limited olivine-only fractionation interval inthe ultramafic cumulates could have several reasons, none of which ismutually exclusive: 1) The parental melt is characterized by relativelyhigh Fe3+/Fe2+ due to high fO2, which favors the fractionation ofpyroxene over olivine (Alonso-Perez, 2006, Kägi et al. 2005). ElevatedfO2 is characteristic formanyarcmagmas (Ballhaus, 1993;Parkinson andArculus, 1999). 2) Fractionation initiated close to the last multiplesaturationwithaperidotitic assemblage. Theolivine-only crystallization

interval is smallest at pressures close to the olivine–orthopyroxene–clinopyroxene multiple saturation point of a given primitive composi-tion (Müntener et al., 2001). 3) Pyroxenites may be formed via the wellknown peritectic reaction ol+melt1→pyx+melt2. This is supportedby field observations (Burg et al., 1998) indicating that somepyroxenites were formed at the expense of dunites.

We consider a combination of these threemechanisms responsiblefor the observed early clinopyroxene saturation and conclude that theSPC was formed by hydrous melts last equilibrated with mantleassemblages just below the petrological Moho. The petrological andgeochemical observations, together with the general hydrous natureof the section, leads to the interpretation that the SPC was formed byhydrousmagmas dominantly originating from flux assistedmelting inthe mantle wedge (e.g. Grove et al., 2002).

4.3. Fractionation sequence of the Chilas Complex

The whole rock major element evolution of the Chilas Complex isillustrated in Figure 3B,E. The dunitic rocks document a significantvariation inMg# (0.88–0.80)with little variation in SiO2 implying thatolivine-only fractionation had an important control on the initial Mg#evolution. Accordingly, pyroxene fractionation started late andwehrlites and ol-websterites in Chilas have significantly lower Mg#(of 0.80) compared to similar rocks in the SPC (Mg# of 0.88). Withincreasing pyroxene content the Mg# of these cumulates remainsfairly constant. Pyroxene formation thus had only a limited influenceon the Mg#melt evolution documenting the importance of melt-rockreactions also in the Chilas ultramafics (Jagoutz et al., 2006, 2007).With the exception of rare high Mg# olivine-bearing gabbros, Chilascumulate mafic rocks generally have higher silica contents thancumulate gabbros from the SPC. This is a result of silica-poor mineralsbeing absent at this stage of the fractionation sequence, amphiboleappears only as a late magmatic phase. More evolved rocks havingliquid compositions are not strongly enriched in silica and aredominantly basaltic–andesitic in composition.

Silica rich tonalitic rocks occur occasionally in the upper part ofthe Chilas section, in particular in the Dir Kalam area. However, mostare younger (75–42 Ma; Jagoutz et al., 2009) than the Chilasintrusion and trace element patterns document involvement ofgarnet. Therefore, we consider most tonalites to be unrelated to theChilas Complex.

The Chilas Complex ultramafic and gabbronorite sequences stemfrom a common parental magma but evolved along different mineralfractionation trends (Jagoutz et al., 2006, 2007). The ultramafic rocksare dominantly formed by fractional crystallization whereby clino-pyroxene precedes plagioclase. In the gabbronorites, plagioclaseprecedes clinopyroxene and the trace element signature of themafic sequence is best explained by in-situ crystallization (Jagoutzet al., 2006). These two fractionation sequences have been explainedby vertical uplift of the ultramafic bodies with respect to the maficgabbronorite complex, thus juxtaposing units that record differentfractionation depths at the same exposure levels.

The inferred liquid line of descent with the late appearance ofamphibole together with the relatively early appearance of plagioclaseis typical for intermediate pressure fractionation (b0.7 GPa) of parentalmelts with initial low H2O contents (≤2 wt.%; Jagoutz et al., 2007). Thelack of silica-rich rocks in the Chilas section is consistent with theabsence of silica-poor low Mg# cumulates inferred to be important forsilica enrichment in the derivative liquids as observed in the SPC. Thisinterpretation is in linewith experimental results documenting that drygabbro fractionation produces very limited volumes of silica-rich rocks(Villiger et al., 2004). We conclude that the liquid line of descent of theChilas Complex did not produce any significant rock volumes of uppercrustal compositionwithSiO2N57 wt.%. Akeyobservation is thatMg#ofChilas mafic rocks at a given silica content are significantly higher thanthose of the SPC (Fig. 3A,B). The trace andmajor element composition of



the bulk Chilas Complex is strikingly similar to bulk lower crustestimates (Fig. 2B, Jagoutz et al., 2006).

4.4. Melting regime for the Chilas Complex

Petrological observations indicate that: (I) olivine fractionationhad a strong control on the Mg# evolution of the Chilas Complex,and (II) amphibole occurred only as a late magmatic phase after 50–80% crystallization of anhydrous phases. We thus propose that theChilas parental magmas had significantly lower initial watercontents than the SPC parental magmas. Olivine fractionation canbe enhanced by 1) addition of a hydrous component, whichincreases the olivine-only crystallization volume; 2) low fO2,which reduces the Fe3+/Fe2+ and favors fractionation of olivineover pyroxene (Botcharnikov et al., 2008); and 3) significantdecompression during or before olivine fractionation as meltsmultiply saturated with a mantle assemblage saturate in olivine-only with decreasing pressures (Kelemen, 1990).

Mechanism (1) is unlikely because of the late appearance ofamphibole in the fractionation sequence. Low oxygen fugacities(mechanism 2) are a viable explanation as fO2 in arcs supposedlycorrelates with a hydrous subduction component (Arculus, 1994;Wood et al., 1990) which is less important in the Chilas Complex. Thelatter intruded during intra-arc rifting (Burg et al., 2006) andextension was probably accompanied by decompression meltingrendering mechanisms 2 and 3 viable. Decompression melting is alsoconsistent with the high degree of partial mantle melting (~20%)inferred from trace element modeling (Jagoutz et al., 2007) and thelarge volumes of melt emplaced over a relative short time period(Khan et al., 1989).

We conclude that the Chilas Complex was formed from consider-ably less hydrous parental magmas than the SPC, these magmas beingformed from decompression melting associated with or triggered byintra-arc extension. In summary, flux assisted melting was compar-atively less important in the Chilas complex than in the SouthernPlutonic Complex.

5. Comparison to other plutonic arc sections

In the following, we compare the Kohistan plutonic trends to(A) the plutonic Talkeetna section (NE Alaska), (B) lower crustalxenolith suites such as those compiled by Rudnick and Presper(1990), and (C) melt inclusions from arc volcanoes in order to presentevidence for a more widespread occurrence of similar fractionationlines from other arc systems.

(A) Whole rock geochemical data from the Talkeetna ultramafic andgabbroic plutonic rocks (DeBari and Sleep, 1991; Greene et al.,2006; Kelemen et al., 2003;) predominantly follow the Z-shapedSPC trend, whereas fewer samples follow the “dry” ChilasComplex trend(Fig. 4A). The “dry”Talkeetna trend, in accordancewith our observations from the Chilas Complex, does not extendto rocks with higher silica content. Silica-rich rocks fromTalkeetna rather follow the hydrous SPC trend. Low Mg# andlow silica Talkeetna rocks (mainly basal gabbronorites, Greeneet al., 2006) have primary magmatic amphibole, similar to theSPC.More primitive gabbroic rocks with slightly higherMg# andSiO2 are very oxide-rich (up to 20%, Greene et al. 2006) and aregenerally void of magmatic amphibole or garnet. This differencebetween the Talkeetna and Kohistan arcs could be related to ahigher fo2 in the Talkeetna melts enhancing oxide fractionation(Sisson and Grove, 1993).

(B) Mafic granulite xenoliths from the lower crust as compiled byRudnick and Presper (1990) are generally characterized by acorrelation betweenMg# and SiO2 similar to the trend found inthe SPC (Fig. 4B), indicating that partial melting of mafic rocksto produce so-called I-type granites (White and Chappell,1983) is unlikely to be a widespread mechanism in the lowercrust. Some of these xenoliths have been interpreted ascomplementary to batholithic rocks (Ducea and Saleeby,1998) and could thus represent the equivalent of the SPCcumulates and gabbros.

(C) We compiled mineral hosted melt inclusions from tholeiitic/calc-alkaline volcanic rocks from 15 arcs from the georock

Fig. 4. The respective trends of the two different fractionation series manifested in the Southern Plutonic (blue) and the Chilas Complex (red) compared to the geochemicalcharacteristics of (A) plutonic rocks of the Talkeetna arc (DeBari and Sleep, 1991; Kelemen et al., 2003; Plafker et al., 1989 and Kelemen pers. com; Greene et al., 2006). Similar to theKohistan arc, rocks from the Talkeetna follow both the SPC and the Chilas fractionation trend. Color coding as in Figure 3. (B) The world wide lower crustal mafic granulite xenolithcompilation of Rudnick and Presper (1990) compared to the fractionation path of the SPC and Chilas Complex. Most mafic xenoliths are close to the second (mafic) segment of the“hydrous” SPC trend. (C) Mineral hosted melt inclusion (green circles; olivine, brown diamond: clinopyroxene) from tholeiitic/calc-alkaline volcanic rocks from 15 arcs as compiledfrom georock (http://georoc.mpch-mainz.de.gwdg.de/georoc/). Olivine hosted melt inclusion trend to high K2O concentration at limited SiO2 enrichment whereas clinopyroxenehosted melt inclusion trend to significantly higher SiO2 content, both at K2O concentrations similar to the trends observed in liquid compositions in the SPC (blue arrow) and theChilas Complex (red). All data are supplied in eTable 1.



database (http://georoc.mpch-mainz.de.gwdg.de/georoc/, fordetails see electronic Appendix). Olivine hostedmelt inclusionstrend to high K2O concentrations over a limited span of SiO2

enrichment, whereas clinopyroxene hosted melt inclusionstrend to significantly higher SiO2 contents at comparable K2Oconcentrations (Fig. 4C) mirroring the trends observed in theliquid compositions of the SPC and Chilas Complex (Fig. 3A,B).

Overall our comparison documents that two liquid lines of descentas present in the Kohistan arc appear to be common in subduction-related magmas but their identification is hampered by magmamixing and assimilation processes. The presence of two liquid lines ofdescent in several arcs implies that the two principal modes ofprimarymagma formation (Grove et al. 2002) may also apply to manyother arcs, albeit possibly obfuscated by their juxtaposition in timeand space.

6. Dry and hydrous fractionation sequences contributing tocontinental crust formation

As detailed above, members of the ‘dry’ fractionation sequencehave, for a given K2O content, significantly lower SiO2 contents thanthose of the ‘hydrous’ fractionation sequence. Moreover, intermediatecompositions related to the hydrous fractionation trend are scarce inthe SPC and have comparably low K2O contents (b1 wt.%). Neverthe-less, the scarceness of intermediate compositions in the lower crustalSPC is counterbalanced by the mid- to upper crustal quartz-diorite togranodiorite suite of the volumetrically dominant Kohistan batholith,leaving no gap of intermediate compositions in the hydrousfractionation trend. In the following we discuss the possibility thatthe two fractionation trends ultimately lead to two different crustalcompositions, both at least partially preserved upon geologicaltimescales: hydrous fractionation produces upper crustal composi-tions with high SiO2 and K2O contents, whereas dry fractionationproduces compositions akin to bulk lower crust. In that respect it isnoteworthy that the Kohistan batholith and the Chilas Complex arethe twomajor building blocks of the Kohistan arc, while the SPC and inparticular its ultramafic and garnet-rich part is volumetrically minor(Fig. 1 and electronic Appendix).

6.1. Upper and lower crust from different fractionation trends

The two fractionation trends of the Kohistan arc are different fromthe trend defined by the array formed by the average upper, lower andbulk continental crust compositions of Rudnick and Gao (2003)(Fig. 3). However, the lower continental crust is similar in composi-tion to the low-SiO2, low-K2O Chilas Complex (Jagoutz et al., 2006)

resulting from “dry” fractionation, whereas the upper continentalcrust is similar to the high SiO2, high K2O Kohistan batholith resultingfrom “hydrous” fractionation of SPC-type cumulates (Jagoutz, 2010)(Fig. 5A,B). The different K2O contents of the two crustal compositionscould be attributed to slight differences in parental magma compo-sition, in particular as these would be amplified during fractionalcrystallization. The lower K2O content of the dry fractionationsequence could reflect average parental magmas slightly lower inK2O than parental magmas of the hydrous fractionation sequence.This interpretation is consistent with a covariance of the incompatibleelements derived from a hydrous slab component, e.g. K2O, H2O, andO2 (Grove et al., 2002; Jagoutz et al., 2007). Alternatively because ofthe extended temperature interval of fractionation of hydrous meltscompared to anhydrous melts (e.g., Sisson et al. 1996), the higher K2Ocontent of the hydrous fractionation sequence could indicate moreextreme fractionation of hydrous melts (see below).

We conclude that the two different liquid lines of descentprincipally form the two main reservoirs upper and lower crust byhydrous and dry fractionation, respectively, and that the bulk crustcomposition represents a mixture between these two endmembers.

6.2. The liquid lines of descent in the Kohistan arc and the formation ofthe continental crust

To elucidate how the two different main units of the Kohistan arcrelate to the average continental crust,we calculated amixing “Kohistanarc array” between the average Chilas Complex including 5% ofultramafics (Jagoutz et al., 2006) and the bulk Kohistan batholithcomposition derived in a companion paper (eTable 2). The comparisonof themixing “Kohistan array” to the “continental crust array” of upper,middle, lower and bulk crust compositions from Rudnick and Gao(2003) reveals the following (Fig. 5 and electronic Appendix):

(1) Major element characteristics are generally similar and mixingbetween the Kohistan batholith and the Chilas Complex bulkcompositions can explain the high Mg# andesitic compositionof the bulk continental crust. In detail, for any given silicacontent, the Kohistan arc array is slightly lower in TiO2, FeO,Na2O and higher in Al2O3 than the continental crust array. Thebulk continental crust composition is best reproduced by a~50:50 mix between Kohistan batholith and Chilas Complex inagreement with upper and lower continental crust reservoirs ofsimilar size (Rudnick and Gao, 2003).

(2) Trends for HFS elements (Zr in Fig. 5, but also other HFSE) aresimilar for both arrays, but with the exception of Zr and Hf theHFSE are generally depleted in the Kohistan compared to thecrustal array (see Appendix). This finding is in agreement with

Fig. 5. Calculated mixing arrays of the bulk Chilas Complex and the bulk Kohistan batholith composition (see electronic supplement) compared to the composition of the upper,lower and bulk continental crust composition of Rudnick and Gao (2003). Mixing between these two products of the “hydrous” fractionation trend (Kohistan batholith) and the “dry”fractionation trend (Chilas Complex), respectively, explains the geochemical composition of the bulk continental crust for most majors, transition metals and trace elements.



the idea that a few percent of the bulk continental crust isformed in non-subduction zone setting (Barth et al., 2000;Rudnick, 1995).

(3) Most of the trace element concentrations in the Kohistan arrayare comparable but consistently lower than the continentalcrust array. The mismatch is however, generally less than20–30% (see Appendix).

(4) The compatible transition metals (e.g. Ni, Fig. 5D) arecongruent for both trends even so the Chilas Complex andthe Kohistan batholith have higher respectively lower Niconcentrations than the bulk continental crust. This differencecould result from not considering volcanic rocks for the bulkKohistan batholith.

7. A composite model for continental crust formation in arcs

7.1. The continental crust as a mixing array of “dry” fractionation lowerand “hydrous” fractionation upper crust

Based on our observations we propose a composite model for theformation of continental crust in subduction zones: the bulkcontinental crust results from mixing of two principal components,which do not stem from the same fractionation trends, i.e. a “dry”mafic lower crust and a “hydrous” granitoid upper crust. This is inaccordance with previous ideas that the bulk continental crust ifformed by physical mixing (magmamixing, assimilation) and tectonicjuxtaposition of a granitic upper crust and a mafic lower crust (Groveet al., 1982; McBirney et al., 1987). In our model the two endmembersare derived from primitive melts with initially different H2O contentsand ultimately from different melting regimes active in the subarcmantle: the lower crustal rocks, exemplified in Kohistan by the ChilasComplex, develop from low-H2O melts formed by decompressionmelting in the upwelling convective mantle wedge, which, in the caseof Chilas, is accompanied by intra-arc extension. Due to the low watercontent, the stabilities of low silica minerals such as amphibole andgarnet are reduced and crystallizing plagioclase is less anorthitic, thusfractionation produces less silica-rich derivative liquids. The uppercrustal rocks, i.e. the Kohistan batholith, are formed by fractionation ofmore hydrous melts dominantly produced by fluxing the mantle

wedge with a hydrous slab component. These melts evolve along amedium- to high-pressure hydrous liquid line of descent, character-ized by early fractionation of silica-poor minerals (garnet, amphibole,Fe–Ti-oxide, and An-rich plagioclase) and a consequent silicaenrichment of the derivative liquids. Our proposed model relies(A) on a filtering effect of H2O-rich derivative melts from low-H2Omelts at mid crustal levels and (B) preferred delamination of garnet–amphibole-rich cumulates complementary to the H2O-rich graniticliquids.

7.2. Temperature and density filtering of “dry” vs. “hydrous” melts

Sisson et al. (1996) proposed that density and temperaturedifferences of high and low water content mafic melts might resultin different intrusion levels. Based on calculated melt densities, theyconcluded that low-H2O arc basalts would be neutrally or negativelybuoyant relative to a solidified granodiorite–granite batholith in themid- to upper crust. This negative buoyancy would increase if thegranitoids are partly or completely molten, the only positivelybuoyant mafic liquids being those with high H2O contents. Sissonet al. (1996) consider H2O-controlled density filtering as the mainmechanism to explain the low volume of mafic intrusions at shallowcrustal levels and their characteristically hydrous character (horn-blende gabbro/diorite, as opposed to pyroxene dominant). Using theMelts algorithm (Asimow and Ghiorso, 1998; Ghiorso and Sack, 1995)we calculated melt densities at variable pressure, H2O-contents andappropriate liquidus temperatures (Fig. 6) for an average primitiveKohistan arc basalt and for melt compositions derived from the latterby fractionating 30% and 60% of cumulates (Jagoutz, 2010). Using thesame method, we recalculated the densities of the melt compositionsof Sisson and Layne (1993). We then compare melt densities tocalculated densities of an average granodiorite and leucotonalite fromthe Kohistan batholith using Perplex (Connolly, 2005) (Fig. 6). Due tothe molar volume of H2O, melt densities decrease with increasingwater content at a given pressure. However, in our calculations evendry melts are generally less dense than solidified intermediateKohistan granitoids at crustal pressures (Fig. 6) questioning theeffectivity of density filtering in the mid- to upper crust in Kohistan.Nevertheless, the calculated and measured (Chroston and Simmons,

Fig. 6. (A) Density of arc melts as a function of their water content (calculated using Melts, Asimow and Ghiorso, 1998), compared to the density of average granitoids of the Kohistanbatholith (calculated using Perplex, Connolly, 2005). Shown is the effect of increasing water content for a primitive mantle derived melt (PM) and a melt that fractionated 30% and60% of cumulates (compositions from Jagoutz 2010) illustrating the limited effect of fractionation on the density of basaltic to basaltic–andesitic liquids compared to the dominanteffect of increasing water content. The density of the calculated liquid has been calculated at 0.5 GPa and 1.0 GPa. Also shown are mafic liquids with measured preeruptive watercontents (Sisson and Layne, 1993, black circles) calculated at 0.5 GPa and fO2 at QFM. (B) Pressure dependence of the density of dry liquids (primitive melt and 30% fractionatedmelt) compared to the density of the average Kohistan batholith. Our results indicate that basaltic to basaltic andesitic liquids are generally less dense than the average Kohistanbatholith granitoid, but that they might be negatively buoyant with respect to leuco-granites.



1989) densities of the preponderant Kohistan batholith granitoids(~2.75–2.85 g/cm3) differ significantly from the densities inferred bySisson et al. (2005) for the Sierra Nevada batholith (2.65–2.75 g/cm3)which are similar to our calculated densities of a granitic composition(Fig. 6). This may reflect the fact that the Sierra Nevada batholith is onaverage more SiO2 and K2O rich than the Kohistan batholith.Alternatively, hydrous melts generally have N100 °C lower liquidustemperatures compared to dry melts. Thus, water rich melts willretain a higher melt fraction during ascent through the lower crustwhereas more dry liquids are forced to crystallize at depth (Sissonet al., 1996). Based on the density consideration we consider the lattermechanism to be more effective in the Kohistan arc.

7.3. Temperature and density driven separation of garnet–amphibolerich cumulates

To establish a preferred delamination of cumulates related to thehydrous fractionation sequence over that of the dry sequence wecompare the density of a variety of representative gabbros andcumulates from both sequences with the density of an averageresidual subarc harzburgite as exposed in the Kohistan (Bouilhol et al.,2009). We used Perplex to calculate the densities of averagegabbronorite and a primitive ol-gabbro cumulate from the ChilasComplex and of average garnet-gabbro and a garnet–hornblenditecumulate from the SPC. Densities are calculated at the quartz–fayalite–magnetite (qfm) buffer and plotted as density contrast tosubarc harzburgite (Fig. 7). The gabbronorites intruded at 0.6–0.7 GParemain generally buoyant compared to subarc mantle and onlybecome negatively buoyant during eclogite facies conditions. Theirolivine bearing gabbroic cumulates are positive buoyant duringintrusion but become negative buoyant upon cooling (Fig. 7A,B)

once overstepping the reaction olivine+plagioclase=pyroxenes+spinel (1) and opx+plagioclase=garnet+cpx+quartz (2) (Kushiroand Yoder, 1966). Contrasting, the garnet-gabbros and garnet–hornblendite cumulates have a strong negative buoyancy whenforming at ~0.8–1.5 GPa (Fig. 7C,D) in accordance with direct densitymeasurements of rocks from the Kohistan suite (Miller andChristensen, 1994). The garnet-gabbros become only buoyant whenre-equilibrating at b0.8 GPa into a garnet-free assemblage throughthe reaction garnet+cpx+quartz=opx+plagioclase (Yamamotoand Yoshino, 1998). At lower arc crust pressures (i.e. 0.8–1.2 GPa) thegranulite facies mineral assemblages of the hydrous fractionationsequence will generally be denser than typical sub arc mantleharzburgite, whereas gabbros from the dry fractionation sequenceare not (Fig. 7).

Increasing bulk Fe/Mg ratios and decreasing silica activity will shiftthe above garnet-forming reaction (2) to lower pressures, whereasthe former reaction (1) is restricted to olivine-bearing silica-undersaturated compositions. An important effect of H2O-richfractionation is to delay plagioclase crystallization at the expense ofclinopyroxene thereby reducing the stability field of olivine+plagioclase in gabbroic rocks (Müntener et al., 2001). Additionally,the water dependence of the plagioclase KD

Ca–Na (=(Ca/Na)plag/Ca/Na)melt) (Sisson and Grove, 1993) yields more An-rich cumulateswith lower SiO2 contents for higher magmatic H2O. Finally, hydrousmelts and elevated fO2 favor igneous amphibole and at greaterpressure (≥0.8 GPa) igneous garnet (Alonso-Perez et al., 2009), andstabilize Fe–Ti oxides (Sisson and Grove, 1993). These combinedeffects result in higher Fe/Mg and lower SiO2 content of gabbroiccumulates derived from hydrous fractionation compared to dryfractionation as indicated by the whole rock compositions (Figs 2, 3and Appendix). As a result densification due to reactions (1) and (2)

Fig. 7. Density contrast of crustal gabbros and cumulates and average harzburgite (ρsample−ρharz) for the bulk Chilas and Southern Plutonic Complexes, isopleths are labeled inkg/m3. (A) Average gabbronorite and (B) primitive ol-gabbro cumulate from the Chilas Complex, and (C) average garnet-gabbro and (D) average garnet–hornblendite cumulatefrom the SPC. The density difference between crustal rocks and harzburgite is mainly controlled by the garnet mode of the former. Due to the difference in Al2O3, Fe/Mg and SiO2

content, rocks of the SPC sequence have higher garnet modes and higher delamination potential at lower crustal pressures than rocks from the Chilas Complex. Note thatapproximate formation pressures of the Chilas Complex are 0.6–0.7 GPa and of the garnet-gabbros and garnet–hornblendites from the SPC are approximately 1.5 GPa.



causes a preferred removal of cumulates formed by hydrousfractionation. At lower arc pressures (i.e., b1.2 GPa) only the primitiveol-bearing cumulates within the dry fractionation sequence have thepotential to become more dense than underlying mantle due toreactions (1) and (2). We emphasize that the crustal level at whichdelamination is likely, coincides with the average depth range of theMoho in stable continental regions, i.e. 41±6 km (Christensen andMooney, 1995).

If plagioclase is absent in the cumulate sequence, the resultingrock-suite has densities and seismic properties comparable to themantle and could be present in significant amounts below thegeophysical Moho (Miller and Christensen, 1994; Müntener andUlmer, 2006). Nevertheless, hornblendite and garnetite units presentin the lower SPC are rare in the geological record, and our densitycalculations (Fig. 7) indicate that these rocks are generallymore densethan residual upper mantle allowing these units to efficientlydelaminate back into the mantle.

8. Conclusions

Different liquid lines of descent are preserved in the Kohistan arcsequence: a more hydrous medium- to high-pressure fractionationsequence that formed the SPC and Kohistan batholith and a lesshydrous, almost dry medium-pressure fractionation sequence thatformed the Chilas Complex. We infer that the different H2O contentsare related to different melting regimes: the hydrous fractionationsequence is derived from parentalmagmas dominantly formed by fluxassisted melting whereas the less hydrous melts are dominantlyformed by decompression melting (Fig. 8).

We speculate that the bulk continental crust composition is amixture between two reservoirs conserving those parts that have(a) gravitationally the highest preservation potential and (b) thelower liquidus temperature allowing for extensive fractionalcrystallization: the upper crust derives by hydrous fractionationwhereas the complementary cumulates delaminate (Fig. 8). Thelower crust derives from less hydrous fractionation whereby melts“freeze” in the lower arc crust without reaching siliceouscompositions. We propose that the “crustal array”, and thereforethe bulk continental crust is the result of mixing (physical andmathematical) and reworking of the two reservoirs due to tectono-magmatic intra crustal processes. As an aggregate bulk, thiscomposition has no direct genetic relevance concerning continen-tal crust formation.

Supplementarymaterials related to this article can be found onlineat doi:10.1016/j.epsl.2010.12.017.

Acknowledgments

We like to thank S. Hussain and H Dawood for logistic supportduring field work in Pakistan. The reviews by Sue DeBari and ananonymous reviewer significantly improved the paper. OJ's work wasin part supported by NSF EAR 0910644.

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A CB D

isotherms

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Author's personal copyeaps.mit.edu/faculty/jagoutz/Publications_files/Earth and Planetary … 2011 Jagoutz.pdf2007). The Chilas Complex is thus considered as a dry lower crustal fractionation