Tectono-metamorphic evolution of the KarakorumMetamorphic complex (Dassu±Askole area, NE Pakistan):exhumation of mid-crustal HT±MP gneisses ina convergent context

Y. ROLLAND,1 G. MAHEÂ O,2 S. GUILLOT2 AND A. PECHER1

1LGCA, UPRES-A5025 CNRS ± Universite J. Fourier, Maison des GeÂosciences, BP 53, 38041 Grenoble, Franceyann.rolland@uif-grenoble.fr2Laboratoire de Dynamique de la LithospheÁre, CNRS UMR 5570, Universite Lyon 1 et Ecole Normale SupeÂrieure de Lyon,27 bd du 11 Novembre 1918, 69622 Villeurbanne, France

ABSTRACT The South Karakorum margin, east of the Himalayan syntaxis, consist of an E±W elongated zone ofyoung (10±3 Ma) high-grade metamorphic rocks (M2) and related migmatitic domes. This late tectono-metamorphic event post-dates the Palaeogene (55±37 Ma) phase of thickening of the belt featured byNW±SE structures and associated M1 amphibolite facies metamorphism (0.7 GPa, 700 uC). This M2metamorphism is characterised by low-pressure, high-temperature conditions coeval with migmatiteformation in response to a thermal increase of c. 150 uC compared to M1, culminating at a temperatureof c. 770 uC and a pressure of 0.5±0.6 GPa. Rapid exhumation of migmatitic domes, at a rate of5 mm yrx1, was accommodated by vertical extrusion, in the core of E±W crustal-scale folds. Thesecrustal-scale folds formed in response to N±S syn-collisional shortening and were enhanced by thermalweakening of the migmatised continental crust.

M2 metamorphism is spatially and temporarily associated with granitoids showing a mantle af®nity,®rmly suggesting that this could be the advective heat source for the granite and syenite generation andthe subsequent migmatisation of the mid-crustal level. Such relationships between a mantle-relatedmagmatism and a high-temperature metamorphism in a convergent shortening context are suggestiveof the breakoff of the subducted Indian slab since 20 Ma.

Key words: doming; heat advection; Karakorum; medium-pressure high-temperature gneisses; slab breakoff.

INTRODUCTION

High temperature (HT ) metamorphism and relatedpartial melting of the continental crust are generallyinterpreted as processes caused by thermal relaxationat the end of an orogenic cycle. In this context, post-orogenic extension is the principal mechanism involvedin the formation and exhumation of migmatites andHT±MP gneisses during doming events (e.g. Davis& Coney, 1979). Our recent investigations in theKarakorum Range (Shigar and Braldu valleys, Northof Skardu, NE Pakistan), building on previous studiesby Bertrand et al. (1988), Searle et al. (1989), Allen &Chamberlain (1991) and Lemennicier et al. (1996),have established a broad zone of young (Miocene toPliocene) medium-pressure high temperature (MP-HT)gneisses in Southern Karakorum (Fig. 1). This paperfocuses on this zone, which presents several particu-larities as compared to other Himalayan areas withMP±HT gneisses. Most notably, this zone has theyoungest metamorphism in the whole of the Himalayanbelt, with HT metamorphism being as young as

7±6 Ma (U±Pb on monazite, Smith, 1993). Suchrecent ages have been found only in the NangaParbat area, SW of the investigated area (Schneideret al., 1999), and in the Eastern Syntaxis, at the otherextremity of the Himalayan belt (Burg et al., 1997).The young metamorphic zoning pattern is alignedE±W, oblique to the general NW±SE orientation ofthe Karakorum belt and overprints an older meta-morphic pattern that is linked to the main SW-vergingstacking and associated crustal thickening. These7±6 Ma, HT±MP gneisses have been exhumed in ashortening context, during a period of continuingIndia-Asia convergence. In addition, this zone presentsclear temporal and spatial relationships between melt-ing, HT±MP metamorphism of mid-crustal rocksand doming in an active collisional context. The aimof this paper is to describe the tectonic and meta-morphic evolution of two domal structures (Dassuand Askole domes, Fig. 1) in the Skardu area (NEPakistan) and then to discuss the possible heat sourceof high-T metamorphism of the continental crustduring convergence.

J. metamorphic Geol., 2001, 19, 717±737

# Blackwell Science Inc., 0263-4929/01/$15.00 717Journal of Metamorphic Geology, Volume 19, Number 6, 2001

GEOLOGICAL SETTING

The main regional units

The NW part of the Himalaya±Karakorum belt ischaracterised by three main crustal-scale tectonicunits: (i) the Karakorum margin of Asia (ii) theKohistan±Ladakh volcanic arc terranes and (iii) theNanga Parbat spur of Indian crust. The studied areais located along the southern Karakorum margin,between the Karakorum Batholith and the ShyokSuture Zone.

The Kohistan±Ladakh volcanic arc, formed duringthe Middle Cretaceous, was accreted to the Karakorummargin during the Upper Cretaceous (Petterson &Windley, 1985; Weinberg et al. 2000; Rolland et al.

2000). Between the Kohistan±Ladakh arc and theKarakorum margin, the Shyok Suture Zone was laterreactivated by a steep reverse fault, the MKT, classi-cally considered as the cartographic northern bound-ary of the Kohistan±Ladakh Terrane (Coward et al.,1986). Since the 50 Ma collision between India andAsia, the Kohistan±Ladakh arc has been pinchedout between the two blocks and obducted onto theIndian margin along the Main Mantle Thrust (MMT),or Southern Suture, which is actually the westernprolongation of the Indus±Tsangpo suture zone. In thecore of the Western Syntaxis, Indian crust outcropsin the N±S elongated Nanga Parbat±Haramosh (NPH)spur. The NPH spur is interpreted as a crustal-scale,transverse anticline re¯ecting deformation partitioning(shortening parallel to the belt strike) in the oblique

Kohistan

Karakoram

MKT

MCT

MBT

Himalaya

SSZ

Indus

Ladakh

NEPAL

Hun

za

Hunza

Indus

Indus

Indus

TurmikStak

Askor

e

Thalle

Shyok

Indus

Braldu

MM

T

MM

T

Gl. Baltoro

Hushe

Ho

Lun

gma

Panm

ah V

alle

y

K2 (8611m)

Broad Peak(8051m)

Gasherbrum IV(7925m)Gasherbrum I-II

(8035-8060m)Masherbrum (7821m)

Rakaposhi(7788m)

Haramosh(7390m)

Spantik(7027m)

(7452m)

74°30'E 75°00'E

75°30'E

75°00'E

35°30'N

36°00'N

36°30'N

36°00'N

36°30'N

74°30'E

75°30'E

76°00'E

76°00'E

35°30'N

MMTMMT

LEH

INDIA

Chalt

Gilgit

Skardu

Dassu

DarchanJutial

Hemasil

Iskere

Bar

Raikot

Sassi

Askole

Hushe

Muchilu

Aliabad

Kohistan

Ladakh

Nanga Parbat-HaramoshMassif

(Indian Plate)

0 50 km

Zone of 8000 m peaks

MKT

domes

Mango Gusorgranite

Shigar

Pre-M1 to Syn-M1 granites and granitoids( Axial Batholith, Cretaceous to early Tertiary)

Early M2 granites and monzonites(Baltoro Batholith, 21 Ma )

Late M2 Granitoids(Jutial: 10 Ma, Hemasil & Sumayar: 9 Ma)

Granites:

Sumayar

Himalaya

NangaParbat

Shyok Suture

Astor valley

Shigar Valley

Karakoram (Asian Plate)

Karakoram

Fault

MCT

MBT

PAKISTAN

CHINANEPAL

Zone of Karakoram Mio-PlioceneHT–MP gneisses and migmatites

Zone of Himalayan Mio-PlioceneHT–MP gneisses and migmatites

Rai

kot F

ault

Sil

Ky-St-GrtSt-Grt

Bt-Ms-Cld-Grt

KySt-Grt

Bt-Ms

Smectite-Chl

Bt-Ms

Thrust fault

Normal fault

Baltoro batholith

Braldu

Ho L

ungma

Gl. Chogo Gl. Biafo

Hispar gl .

Fig. 1. Geological and metamorphic sketch map of the Hunza and Hushe valleys, Northern Pakistan (modi®ed and completedfrom PeÃcher & Le Fort, 1999). Main metamorphic zones from Searle et al. (1999) in the Hunza valley, Lemennicier et al. (1996)in the Chogo Lungma glacier±Hemasil area, Hanson (1989) in the Shigar valley, and our own data, mainly in Shigar±Hushe area.Plain triangles: peaks above 8000 m. The frame (right part of the ®gure) corresponds to the maps shown in Figs 2 and 5. Insetlocation map: main tectonic contacts are the Main Boundary Thrust (MBT), the Main Central Thrust (MCT), the Main MantleThrust or Southern Suture (MMT), the Shyok Suture Zone or Northern Suture (SSZ), reactivated by the Main Karakorum Thrust(MKT), west of the Shigar fault.

718 ROLLAND ET AL.

India-Asia convergence (Seeber & PeÃcher, 1998).North of the Shyok Suture, syn-convergent continuousthickening gives rise to the high Karakorum range. Itsbackbone is made by the large NW±SE elongatedcomposite Karakorum batholith. The core of thebatholith (the Hunza granodiorite) has a calc-alkalinesignature, and is related to the Tethyan subduction(ages ranging from 120±80 Ma, Debon et al., 1987;Searle et al., 1989). In the southeast Karakorum, thesouthern contact of the Hunza granodioritic batholithis crosscut by the W±E elongated Baltoro leucogranite,dated at 21.0¡0.5 Ma (U/Pb on zircon, by Parrish &Tirrul, 1989). The youngest granites (9±7 Ma) alsocrosscut the southern edge of the Karakorum batho-lith. South of the Karakorum batholith, the SouthernKarakorum margin, made of Precambrian basementand Cambro±Ordovician rocks (Rolland et al., inpress) is affected by a NW±SE trending tectono-M1metamorphic belt. In this area, called the KarakorumMetamorphic Complex (KMC) the metamorphicgrade increases from the lower part of the structuralpile (SW) to the upper part (NE). In some places,the M1 metamorphic foliation which is parallel to theregional SE±NW metamorphic zoning, appears tobe axial plane to kilometre-scale recumbent folds,formed during SW±NE shortening.

The domes

North of Skardu city, in the Dassu±Askole area, theM1 metamorphic foliation trend is irregular (Fig. 2)

and re¯ects reactivation of the foliation in kilometre-scale, non-cylindrical (roughly conical) folds. Thisdeformation generates kilometre-scale domes, eithersubcircular (Askole area) or NW±SE elongated(Arandu and Dassu areas) closer to the MKT. Thedomal structures are also underlined by a strongstretching lineation pattern, with a radial pattern insubcircular domes, and parallel to the elongation ofdomes in the NE±SW elongated ones (Fig. 2). Thiszone of domes is also characterised by a secondaryHT metamorphic imprint, here after referred to asthe M2 metamorphism. The M2 metamorphism ismainly recorded in the core of the dome zone by latesillimanite, de®ning a L2 mineral lineation, and bypatches of migmatitic leucosomes developed parallel tothe S1±S2 foliation. The limbs of the domes locallydisplay shear zones and small-scale folds featuringupward movement of the domes relative to the sur-rounding rocks (Fig. 3). This upward movement,associated with the development of the domes, is alsoevidenced by the attitude of the cross-cutting leuco-some (Fig. 4a) and of the dykes (Fig. 4b) feedinglate granitic sills. They are symmetrically tilted to theNE or SW on the NE and SW limbs of the dome,respectively. Recent mapping (PeÃcher & Le Fort, 1999)shows that the Dassu±Askole domes belong to aseries of domes which are aligned in an E±W direc-tion. This domes lineament is a very recent structure,which can be followed for more than 100 km from theBaltoro glacier to the N135u dextral strike slip ShigarFault (Fig. 1).

Fig. 2. Map of foliation and ofsyn-kinematic M2 mineral lineationtrajectories, north of Skardu. Note thedisruption of the regional NW±SE trendof foliation in the Dassu±Askole area.Here, the foliation is deformed in domalstructures which are subcircular in theAskole area, and NW±SE elongated inthe Dassu area.

MKT

Foliation trajectories

Zone of late sillimanite crystallization

Mineral lineation trajectories

5609

6459 m

4695

GreatTower

5800

Hornbor peak6328 m

Cathedral Peak5944

Mitre

Masherbrum Peak7821

7163

5500

MangoGusor

6288

5084

5642 Hushe

KandeKussomik

Namka Peak

Koser Gunge(6400)

Dassu

Ganchen6463

Bi afo g l ac i e r

Hemasil

Turmik River

Shigar

SKARDU

Shyok River

Indus

Basha

Shigar River

Askole Korophon

0 5 1 0 1 5 20 km

TECTONO-METAMORPHIC EVOLUTION OF THE KARAKORUM METAMORPHIC COMPLEX 719

METAMORPHISM IN THE DASSU±ASKOLE AREA

Aim and procedures

The main goal of this section is to estimate and constrain the pressureand temperature (P±T ) conditions of the M2 metamorphism and, to

a lesser extent, the P±T conditions of the M1 metamorphic event.We have studied more than 50 metapelite samples, 15 of whichwere selected for mineral analysis. Analyses (Tables 1 & 2) wereobtained on the Cameca Camebax SX100 electron microprobe ofthe Blaise Pascal University in Clermont-Ferrand.

Al-rich metapelites are widespread throughout the studied area andhave been selected to study its metamorphic evolution. In thesemetapelites, MnO and TiO2 are scarce and only enriched in garnetand rutile/ilmenite, and plagioclase is the only CaO-rich mineral.Thus metapelites can be described in the KFMASH system (Spear,1993). To determine P±T conditions the metamorphic equilibriumand metamorphic reactions, deduced from petrographic texturalrelationships, were ®rst projected on the KFMASH petrogeneticgrids de®ned by Spear & Cheney (1989). The grids were completedfor MP±HT conditions with the KFMASH stability curves givenby Vielzeuf (1984), Vielzeuf & Holloway (1988) and Le Breton& Thompson (1988). Complementary P±T estimates have beenobtained using cationic exchange reactions between garnet and otherminerals. The choice between the numerous existing calibrationsis very critical. Actually Chipera & Perkins (1988) and Spear (1993)showed that the most precise and accurate results are not system-atically given by the most developed calibrations. Moreover, in highgrade metamorphic conditions, it is dif®cult to obtain accurate P±Testimates of the peak metamorphic conditions, calculated tempera-tures usually correspond to the closure temperature of the Fe-Mgexchange of crystal pair such as garnet and biotite (Spear, 1993). Incontrast, calculated pressure rely upon the net transfer of Ca2+, andare not easily affected by secondary diffusion (Spear, 1993). Thus,P±T estimates on the same sample by thermobarometry do notnecessarily provide temperatures that can be directly linked to the

Fig. 3. The Panmah dome. View looking east from the westernbank of the Biafo glacier (photo Y. Rolland). The domeshape is underlined by the trend of metamorphic foliationand migmatitic segregation. To the left, some minor foldsemphasise the extrusion movement of the dome core.

Fig. 4. Late dykes and leucosomes tilted on the limbsof the Dassu dome. (a) Syn migmatitic way-up criteria(Burg & Vanderhaeghe, 1993) of M2 segregation, eastern edgeof the dome (photo S. Guillot, looking to NW; scale given bythe compass edge, 10 cm). (b) Dykes and sills, products of theM2 partial melting, assumed to be more or less vertical atthe time of their emplacement, are tilted during growth of thedome (photo S. Guillot, looking to the north; scale given bythe man, bottom left corner of the photo).

720 ROLLAND ET AL.

calculated pressure. We have used thermobarometers currentlyapplied for MP±HT gneisses, and calculated the uncertainties foreach sample taking into account the sample heterogeneity and theanalytical imprecision (e.g. Kohn & Spear et al., 1991). Uncertaintiesfor garnet-biotite thermometry correspond to the standard deviation(at the 2 s-level) calculated for each selected calibration on eachsample. Uncertainties in pressure, also corresponding to the standarddeviation (at the 2 s-level), were calculated for each selected calibra-tion with the previously calculated average temperature interval forthe same biotite-garnet pair. For the garnet-biotite thermometer theFerry & Spear (1978) calibration was used for upper estimates of thetemperature, and the main thermometer including correction for

the grossular component in garnet (GARB, Hodges & Spear, 1982;Ganguly & Saxena, 1984; Perchuk & Lavrent'eva, 1983) as most ofthe analysed garnet has a relatively high grossular content. In thesame way, we used the barometers including this correction, based onthe net transfer reactions computed by Spear et al. (1991): Grt-Pl-Sil/Ky-Qtz (GASP; Hodges & Spear, 1982; Hodges & Crowley, 1985;Koziol & Newton, 1988), Grt-Pl-Ms-Bt (GMBP; Ghent &Stout, 1981; Hodges & Crowley, 1985; Hoisch, 1990), Grt-Pl-Ms-Qtz (GMQP; Hodges & Crowley, 1985; Powell & Holland, 1988)and Grt-Pl-Qtz (GPQ, Hoisch, 1990). A P±T path was calculated bythermobarometry in one sample, using the method of St-Onge (1987).The earlier P±T conditions, were determined using the compositions

Table 1. Representative compositions of M1 related minerals Standards used were natural silicates; the acceleration voltage was15 kV, with a beam current of 20 nA and a counting time of 10 s.

(a) GARNET

Sample

core

L162

rim

L162

core

L195

rim

L195

rim

SK26

rim

SK29

core

SK29

SiO2 36.41 36.68 36.78 36.53 36.89 36.97 36.39

TiO2 ± 0.01 ± ± ± ± 0.12

Al2O3 21.11 20.98 20.72 20.79 21.21 21.47 21.30

FeO 37.72 37.46 35.86 37.07 36.32 34.69 34.47

MnO 1.64 1.27 0.88 0.68 1.19 1.00 1.85

MgO 1.70 1.83 2.85 2.60 2.41 3.02 1.76

CaO 2.09 1.82 3.05 1.95 2.60 3.51 4.21

Total 100.67 100.05 100.14 99.62 100.62 100.66 100.10

No. Ox. 24 24 24 24 24 24 24

Site T

Si 5.91 5.96 5.94 5.95 5.92 5.88 5.87

Al 0.09 0.04 0.06 0.05 0.08 0.12 0.13

Site O

Al 3.95 3.99 3.89 3.94 3.93 3.91 3.92

Ti ± ± ± ± ± ± 0.01

Site A

Mg 0.41 0.44 0.69 0.63 0.58 0.72 0.42

Fe2+ 5.12 5.09 4.85 5.05 4.87 4.62 4.65

Mn 0.23 0.17 0.12 0.09 0.16 0.13 0.25

Ca 0.36 0.32 0.53 0.34 0.45 0.60 0.73

XAlm 83.66 84.48 78.41 82.56 80.42 76.12 76.80

XGrs 5.94 5.26 8.54 5.57 7.39 9.87 12.03

XPyr 6.72 7.36 11.10 10.33 9.52 11.79 7.00

XSps 3.68 2.90 1.95 1.53 2.68 2.22 4.17

XFe 0.93 0.92 0.88 0.89 0.89 0.87 0.92

Sample

(b) PLAGIOCLASE

Core

L194

Core

L30

Core

SK26

Core

SK31

SiO2 68.46 62.80 60.90 63.94

TiO2 ± ± ± 0.02

Al2O3 19.29 23.68 24.15 22.72

FeO 0.25 ± ± 0.11

MnO 0.07 ± ± ±

MgO 0.02 ± ± 0.02

CaO 0.29 4.68 5.84 3.49

Na2O 11.89 9.34 8.35 9.95

K2O 0.06 0.18 0.24 0.24

Total 100.19 100.68 99.48 100.49

No. Ox. 8 8 8 8

Site T

Si 6.01 5.54 5.45 5.64

Al 1.99 2.46 2.55 2.36

Site A

Mg ± ± ± ±

Fe 0.02 ± ± ±

Mn ± ± ± ±

Ti ± ± ± ±

Ca 0.03 0.44 0.56 0.33

Na 2.02 1.60 1.45 1.70

K ± 0.02 0.03 0.03

% Or 0.33 0.99 1.36 1.33

% Alb 98.34 77.54 71.12 82.64

% An 1.33 21.47 27.52 16.03

Sample

(c) BIOTITE

rim

L162

core

SK26

Inc. in Ky

SK26

rim

SK29

(d) MUSCOVITE

rim

L162

inc. in Grt

L162

core

SK26

inc. in ky

SK26

core

SK31

(e) CHLORITE

Core

L162

Core

L195

SiO2 35.66 34.21 35.52 34.75 47.72 49.146 46.47 45.52 46.11 25.89 25.57

TiO2 2.10 2.34 3.18 2.69 0.55 0.05 0.86 0.57 0.99 0.18 0.10

Al2O3 21.57 20.19 19.31 20.34 32.61 26.699 34.27 36.34 34.99 18.56 21.23

FeO 19.96 23.21 20.42 19.29 3.41 4.514 1.50 1.03 1.26 30.91 26.33

MnO 0.40 0.08 0.09 0.19 0.05 0.028 0.01 ± ± 0.02 0.03

MgO 6.85 5.89 7.35 7.96 1.50 3.28 1.02 0.52 0.82 9.35 11.95

CaO 0.05 0.11 ± 0.05 0.38 ± 0.05 0.04 ± 0.25 0.44

Na2O 0.41 0.23 0.31 0.19 0.58 0.151 0.69 0.87 0.69 0.01 0.08

K2O 9.00 8.96 9.21 9.36 9.55 11.43 10.20 10.38 10.32 0.37 0.37

Total 96.00 95.22 95.39 94.82 96.35 95.30 95.07 95.27 95.18 85.54 86.10

No. Ox. 22 22 22 22 22 22 22 22 22 36 36

Si 5.37 5.38 5.48 5.37 6.34 6.69 6.19 6.05 6.13 7.46 7.14

AlIV 2.63 2.62 2.52 2.63 1.66 1.31 1.81 1.95 1.87 0.54 0.86

AlIV 1.20 1.12 0.99 1.08 3.44 2.97 3.57 3.74 3.61 5.77 6.12

Fe2+ 2.52 2.71 2.34 2.22 0.38 0.51 0.02 0.01 0.02 7.45 6.14

Mg 1.54 1.38 1.69 1.84 0.30 0.67 0.20 0.10 0.16 4.02 4.97

Ti 0.24 0.28 0.37 0.31 0.05 0.01 0.09 0.06 0.10 0.04 0.02

Mn 0.05 0.01 0.01 0.03 0.01 ± ± ± ± 0.01 0.01

K 1.73 1.80 1.81 1.85 1.62 1.98 1.73 1.76 1.75 0.08 0.08

Na 0.12 0.07 0.09 0.06 0.15 0.04 0.18 0.22 0.18 ± 0.02

XFe 0.62 0.67 0.58 0.55 0.56 0.43 0.43 0.50 0.45 0.65 0.55

TECTONO-METAMORPHIC EVOLUTION OF THE KARAKORUM METAMORPHIC COMPLEX 721

of biotite (muscovite) and plagioclase inclusions in the garnet cores.When there are no inclusions in garnet, the garnet core compositionand selected compositions from the cores of biotite, plagioclase andmuscovite in the matrix close to (at the cm scale) the studied garnet,but not directly in contact with it were used. If relict kyanite ispresent, it is used in the GASP reaction. To evaluate P±T conditionsduring the development of the main foliation, rim compositionsof minerals in textural contact were used. As sillimanite is the

only aluminosilicate stable in the foliation, it was used in theGASP reaction.

Syn-stacking metamorphism (M1) outside the dome area

At a regional scale, from the lower Hunza Valley tothe Hushe Valley, isograds of M1 metamorphism are

Table 2. Representative compositions of M2 related minerals. Standards used were natural silicates; the acceleration voltage was15 kV, with a beam current of 20 nA and a counting time of 10 s.

Zone

Sample

(a) GARNET

TZ

core

L527

TZ

rim

L527

TZ

core

L545

TZ

rim

L545

TZ

core

SK60

TZ

rim

SK60

HGZ

core

L523

HGZ

rim

L523

HGZ

core

L525

HGZ

rim I

L525

HGZ

rim II

L525

HGZ

core

SK18

HGZ

rim

SK18

HGZ

rim

SK46

HGZ

core

SK46

HGZ

core

SK57

HGZ

rim

SK57

SiO2 36.88 35.60 37.58 36.93 36.60 36.09 37.11 36.63 36.59 36.15 35.69 36.99 37.04 37.40 36.24 35.79 36.12

TiO2 0.05 0.02 ± ± 0.03 0.02 0.04 ± ± 0.02 ± 0.04 0.02 0.07 0.03 0.22 ±

Al2O3 20.90 21.41 21.80 21.69 20.72 20.47 21.42 21.74 21.47 20.91 20.50 21.18 21.20 21.16 21.30 21.17 20.97

FeO 37.52 38.63 34.39 33.58 33.28 32.47 33.27 33.22 33.83 33.92 33.28 33.65 33.12 34.86 36.59 32.89 32.51

MnO 0.09 0.07 1.08 0.82 7.45 8.31 3.66 4.00 4.38 4.36 5.80 6.35 8.18 4.67 2.36 6.71 7.15

MgO 3.77 3.52 3.95 3.94 1.63 1.48 3.83 3.39 3.54 3.15 2.11 2.49 2.28 1.96 2.60 1.90 1.75

CaO 0.74 0.62 2.82 3.01 1.11 1.12 1.49 2.01 1.33 1.13 1.21 1.01 0.72 1.28 1.35 1.17 1.29

Total 99.95 99.87 101.62 99.97 100.82 99.96 100.82 100.99 101.14 99.64 98.59 101.71 102.56 101.40 100.47 99.85 99.79

Nb. Ox. 24 24 24 24 24 24 24 24 24 24 24 24 24 24 24 24 24.00

Site T

Si 5.95 5.80 5.92 5.90 5.93 5.90 5.92 5.86 5.86 5.89 5.91 5.90 5.88 5.99 5.84 5.83 5.89

Al 0.05 0.20 0.08 0.10 0.07 0.10 0.08 0.14 0.14 0.11 0.09 0.10 0.12 0.01 0.16 0.17 0.11

Site O

Al 3.93 3.91 3.97 3.99 3.88 3.85 3.95 3.96 3.91 3.91 3.91 3.88 3.84 3.99 3.88 3.89 3.92

Ti 0.01 ± ± ± ± ± 0.01 ± ± ± ± ± ± 0.01 ± 0.03 ±

Site A

Mg 0.91 0.86 0.93 0.94 0.39 0.36 0.91 0.81 0.84 0.76 0.52 0.59 0.54 0.47 0.62 0.46 0.42

Fe2+ 5.06 5.26 4.53 4.49 4.51 4.44 4.44 4.44 4.53 4.62 4.61 4.49 4.39 4.67 4.93 4.48 4.43

Mn 0.01 0.01 0.14 0.11 1.02 1.15 0.49 0.54 0.59 0.60 0.81 0.86 1.10 0.63 0.32 0.93 0.99

Ca 0.13 0.11 0.48 0.52 0.19 0.20 0.25 0.34 0.23 0.20 0.22 0.17 0.12 0.22 0.23 0.20 0.23

XAlm 82.85 84.40 74.53 74.16 73.70 72.24 72.78 72.39 73.12 74.74 74.84 73.46 71.39 77.93 80.70 73.78 73.03

XGrs 2.09 1.74 7.84 8.51 3.15 3.18 4.17 5.61 3.67 3.18 3.49 2.82 2.00 3.67 3.82 3.37 3.71

XPyr 14.85 13.72 15.25 15.49 6.44 5.86 14.95 13.18 13.62 12.36 8.46 9.67 8.77 7.82 10.20 7.60 6.99

XSps 0.21 0.15 2.37 1.84 16.71 18.72 8.11 8.82 9.59 9.72 13.21 14.04 17.85 10.58 5.27 15.25 16.26

XFe 0.86 0.85 0.83 0.83 0.92 0.93 0.83 0.85 0.84 0.86 0.90 0.88 0.89 0.91 0.89 0.91 0.91

Zone

Sample

(b) K-FELDSPAR

HGZ

Core

SK17

HGZ

Core

SK57

(c) PLAGIOCLASE

TZ

Core

L526

TZ

Core

SK60

HGZ

Core

L523

HGZ

Core

SK18

HGZ

Core

SK46

SiO2 65.37 63.58 59.59 62.15 58.21 63.76 63.16

TiO2 ± ± ± ± ± ± 0.01

Al2O3 18.79 18.87 24.94 24.30 25.63 23.17 22.56

FeO ± ± 0.03 0.36 0.06 0.21 0.18

MnO ± ± 0.01 0.01 0.02 0.03 ±

MgO ± ± 0.01 ± 0.01 0.02 ±

CaO 0.03 ± 6.91 5.34 7.90 3.86 3.54

Na2O 1.87 1.23 7.76 8.39 7.26 9.83 9.97

K2O 14.65 15.13 0.33 0.20 0.04 0.22 0.13

Total 100.71 98.81 99.58 100.75 99.13 101.10 99.55

Nb. Ox. 8 8 8 8 8 8 8

Site T

Si 5.98 5.93 5.36 5.48 5.27 5.60 5.63

Al 2.02 2.07 2.64 2.52 2.73 2.40 2.37

Site A

Mg ± ± ± ± ± ± ±

Fe ± 0.04 ± 0.03 ± 0.02 0.01

Mn ± ± ± ± ± ± ±

Ti ± ± ± ± ± ± ±

Ca ± ± 0.67 0.50 0.77 0.36 0.34

Na 0.33 0.22 1.35 1.43 1.27 1.67 1.72

K 1.71 1.80 0.04 0.02 ± 0.02 0.01

% Or 83.60 88.99 1.83 1.16 0.21 1.19 0.71

% Alb 16.25 11.01 65.79 73.14 62.29 81.20 83.00

% An 0.15 ± 32.38 25.70 37.50 17.62 16.30

Zone

Sample

(d) BIOTITE

TZ

rim

L526

TZ

core

SK60

TZ

rim

SK60

HGZ

core

L523

HGZ

core

SK18

HGZ

rim

SK57

SiO2 34.25 34.03 34.24 35.15 35.02 34.19

TiO2 2.68 3.58 2.37 3.62 2.99 2.72

Al2O3 18.32 19.23 20.54 18.81 19.78 20.50

FeO 19.23 23.08 22.83 19.08 21.76 21.78

MnO ± 0.34 0.18 ± 0.35 0.26

MgO 8.38 5.14 5.43 8.20 6.35 5.79

CaO 0.02 0.04 0.02 0.12 0.07 0.05

Na2O 0.27 0.19 0.16 0.14 0.19 0.17

K2O 8.68 9.61 9.11 9.71 9.46 9.47

Total 91.83 95.24 94.88 94.83 95.97 94.93

Nb. Ox. 22 22 22 22 22 22

Si 5.41 5.38 5.39 5.39 5.43 5.37

AlIV 2.59 2.62 2.61 2.61 2.57 2.63

AlIV 0.82 0.96 1.20 0.79 1.04 1.16

Fe2+ 2.54 2.71 2.67 2.45 2.51 2.54

Mg 1.97 1.21 1.27 1.87 1.47 1.35

Ti 0.32 0.43 0.28 0.42 0.35 0.32

Mn ± 0.04 0.02 ± 0.05 0.03

K 1.75 1.94 1.83 1.90 1.87 1.89

Na 0.08 0.06 0.05 0.04 0.06 0.05

XFe 0.56 0.69 0.68 0.57 0.63 0.66

722 ROLLAND ET AL.

parallel to the major NW±SE trending regionalfoliation (Fig. 1). The M1 peak conditions in thehighest-grade zone (kyanite+biotite+garnet) wereestimated at c. 1.0 GPa and 700 uC (Bertrand et al.,1988; Searle & Rex, 1989; Allen & Chamberlain, 1991).The data collected in this study allow us to constrainthe metamorphic conditions to the north (Ho Lungmavalley) and to the south (from Shigar valley to Hushevalley) of the Askole±Dassu domes.

In the Shigar±Hushe area, metamorphic gradeincreases from SW to NE, i.e. from the lower to theupper part of the structural pile (Fig. 5). In the loweststructural part, metamorphic conditions are lower thandescribed elsewhere in the Southern Karakorum, andOrdovician fossiliferous calcschists are interlayeredwith low-grade metapelites. In the calc-schists, themetamorphic paragenesis is de®ned by an assemblageof smectite+Fe-rich chlorite, whereas the metapelitesare characterised by a Ms±Bt±Kfs¡Cld assemblage(Fig. 5). The stability of smectite and absence of biotitesuggest a temperature lower than 400 uC and pressureslower than 0.5 GPa. Farther to the NE (higher in thetectonic pile), the occurrence of garnet and destabilisa-tion of chloritoid indicate increasing metamorphicgrade. From SW to NE, the following assemblagesare found: Ms±Bt±Alb±Chl¡Grt¡Cld, then Ms±Bt±Grt¡St, and ®nally Ms±Bt±Grt¡Ky (Fig. 5). In allthese assemblages (samples L162 & L195, Figs 5 & 6a,Table 1), garnet with normal zoning of increasingalmandine and decreasing grossular from core to rim,suggest a temperature increase during growth (Spear,1988). Plagioclase is albite-oligoclase with no signi®-cant core to rim variations. All biotite has elevated Altot

and XFe. Muscovite has variable XFe and Si rangingfrom 6.32 p.f.u. in the matrix to 6.70 p.f.u. in garnetinclusions, suggesting different generations of musco-vite, growing at different pressures (Massonne &Schreyer, 1987). Muscovite included in the garnetcould have grown at relatively high pressure duringM1 metamorphism, while lower pressure, matrix-muscovite could have been re-equilibrated later during

Table 2. (Cont'd).

Zone

Sample

(e) MUSCOVITE

TZ

rim

L527

TZ

rim

SK60

HGZ

rim

L523

HGZ

rim

L525

HGZ

core

SK46

HGZ

core

SK57

SiO2 46.50 45.66 45.31 46.28 44.37 44.78

TiO2 0.60 0.31 1.54 0.24 0.29 0.15

Al2O3 35.76 35.24 33.84 36.52 36.58 36.54

FeO 1.45 1.53 1.25 1.34 0.85 1.02

MnO ± 0.02 ± ± ± 0.03

MgO 0.69 0.66 0.83 0.76 0.43 0.56

CaO ± 0.03 ± 0.03 ± 0.02

Na2O 1.35 0.41 0.42 0.53 0.63 0.72

K2O 9.55 10.84 11.29 10.14 10.61 10.48

Total 95.90 94.70 94.48 95.84 93.76 94.30

Nb. Ox. 22 22 22 22 22 22

Si 6.12 6.12 6.13 6.11 6.00 6.02

AlIV 1.88 1.88 1.87 1.89 2.00 1.98

AlIV 3.67 3.69 3.52 3.79 3.83 3.81

Fe2+ 0.02 0.02 0.14 0.15 0.01 0.01

Mg 0.14 0.13 0.17 0.15 0.09 0.11

Ti 0.06 0.03 0.16 0.02 0.03 0.02

Mn ± ± ± ± ± ±

K 1.60 1.85 1.95 1.71 1.83 1.80

Na 0.34 0.11 0.11 0.14 0.16 0.19

XFe 0.52 0.55 0.45 0.50 0.50 0.48

Fig. 5. Metamorphic maps, SouthernKarakorum and Shyok Suture Zone,Skardu area. M1 NW±SE structures arewell preserved in the south-eastern partof the area (Thalle and Hushe valleys).In contrast, M1 isograds strike E±Wbetween Shigar and Braldu rivers,as they have been folded during theM2 event. Notice that the two M2high-grade zones are centred on thecore of the Dassu and Askole-Panmahdomes (see Fig. 2).

TECTONO-METAMORPHIC EVOLUTION OF THE KARAKORUM METAMORPHIC COMPLEX 723

M2 metamorphism. As compared to chlorite in thesmectite-chlorite zone, chlorite in the biotite-chloritezone is more magnesian.

Thermobarometric estimates (Fig. 7) for lower gradegarnet-bearing sample L162, yield 531¡66 uC and0.4¡0.1 GPa using garnet core compositions, whilerims give 545¡60 uC and 0.3¡0.1 GPa. SampleL195 yields similar conditions of 535¡55 uC and0.5¡0.2 GPa for core composition, and slightly highervalues of 613¡54 uC and 0.7¡0.2 GPa, for rimcompositions. Based on textural features of L162 andL195 garnet (which shows two generations of growth)we believe that only the garnet cores relate to the M1event. The rim result indicate an increase in tempera-ture for sample L195, which is closer to the domes area,but a constant temperature for sample L162, located tothe south in the Shyok Suture Zone. The conditionsof garnet rim growth could thus either be due tore-equilibration, or to a second stage of M2 growth.

In the Ho Lungma valley, north of the Dassu±Askoledomes, high-grade kyanite-sillimanite-bearing rocks(samples SK26, SK29 & SK31, Fig. 5) are found in aprolongation of the sillimanite zone known farther tothe NW and related to the SW-verging nappe stacking(PeÃcher & Le Fort, 1999). This M1 kyanite-sillimanitezone is separated from the sillimanite bearing domes(M2 related metamorphism) by a zone of sillimanite-free metapelites. The Ho Lungma rocks are mainlymetapelitic gneisses, with Sil+Bt layers de®ning theM1 foliation and granoblastic leucocratic layers, char-acterised by K-feldspar and numerous centimetre-sizedgarnet porphyroblasts containing inclusions of rutile.Relict kyanite with inclusions of rutile is texturally

associated with relict muscovite and biotite, and theyare all surrounded by the Bt+Sil M1 foliation(Fig. 6b). Late andalusite porphyroblasts also crystal-lized in sample SK26 (Fig. 6c). No staurolite wasobserved in this area, but it was found further west, inthe Chogo Lungma area (Lemennicier et al., 1996) andfurther east, south of the Askole dome. Garnet doesnot have any signi®cant compositional zoning, and isalmandine-rich and spessartine-poor. The crystalliza-tion of Fe-rich biotite is associated with sillimanite,formed at the expense of garnet and muscovite(Fig. 6d). Relict muscovite in equilibrium with relictkyanite is slightly Si- and Mg-poor compared tomuscovite in the foliation. Plagioclase is albite-richand never presents core-rim variations. These associa-tions (sample SK26, Fig. 6d) can be linked at thetransition from kyanite to sillimanite to the reaction(Vielzeuf, 1984; Vielzeuf & Holloway, 1988):

Grt+MspSil+Bt (1)

In the KFMASH grid (Fig. 8), this reaction implies apressure drop associated with a slight temperatureincrease in the P±T space 600±750 uC and 0.6±0.7 GPa(Vielzeuf, 1984; Vielzeuf & Holloway, 1988). P±Testimates based on cationic exchange in the assemblageGrt±Bt±Ky±Pl (sample SK26), are 683¡55 uC and0.7¡0.2 GPa, using the garnet core composition. Forthe assemblage Grt±Bt±Sil-Pl, using the garnet rimcomposition, P±T estimates are 633¡36 uC and0.6¡0.1 GPa. These estimates are in good agreementwith petrological observations. The end of the retro-grade P±T path is constrained by the late occurrence

Fig. 6. Photomicrographs of the M1 assemblages, all taken inplane polarised light. Scale bar: 0.5 mm (a) Sample L162,Shyok Suture Zone. Garnet zoning in a Grt±Ms±Btassemblage. (b) Sample SK26, Ho Lungma. Ky±Ms±Rt relictassemblage in a Bt±Sil gneiss (c) Sample SK26, Ho Lungma.A porphyroblast of late and crosscuts the Bt±Sil association(d) Sample SK26, Ho Lungma. Replacement of Grt and Msby Bt and Sil.

724 ROLLAND ET AL.

of andalusite and by the absence of cordierite, whichimply retrogressive conditions of c. 500 uC at pressures>0.3 GPa (Fig. 8). Consequently, these results implya retrograde cooling path after M1 (Fig. 8). Contraryto the samples located south of the M2 HT meta-morphism, sample SK26 shows no evidence of atemperature increase linked to M2 in areas far fromthe dome area.

Syn-doming metamorphism (M2)

In the studied area, the M2 temperature increase isspatially characterised by prograde destabilisation ofM1 assemblages. We have separated two zones,according to the importance of the M2 thermal imprint(Fig. 5): a transition zone (external part of the domes),where metapelites have still well-preserved M1 assem-blages, and are only partially re-equilibrated underMP±HT conditions during M2, and a high-grade zone(core of the domes), with almost totally recrystallizedand partly melted rocks in MP±HT conditionsduring M2.

Petrography and mineralogy of metapelites fromthe transition zone

In the Askole area (Fig. 5), metapelites from thetransition zone are characterised by disequilibriumtextures, with the presence of Ky+Grt+Ms+Pl¡Kfs¡St¡Sil mineral associations, with rutile,ilmenite, graphite, zircon and apatite as accessoryminerals. In the Dassu area (sample SK60, Fig. 5), thesame assemblages are observed, but without anystaurolite relics. When the S1 foliation is not com-pletely transposed by the S2 foliation, rutile andstaurolite are often associated (sample L526). Theyform relics included into ilmenite and kyanite-biotiteassemblages, respectively (Fig. 9a). Kyanite andmuscovite form large crystals (1±3 mm), generallyirregular, with rim recrystallization into K-feldsparand sillimanite. Kyanite and muscovite are roughlyoriented in the S1 foliation transposed by the S2foliation. Granoblastic intergrowths of biotite and®brolitic sillimanite de®ne the S2 foliation (samplesL522, L526, L545, SK60). They generally embed largegarnet porphyroblasts and relics of staurolite and

L195, garnet core

0

0.2

0.4

0.6

0.8

1

1.2

400 500 600 700

4 3 1 24

3

2

P (GPa)

T (T ˚C)

SK26, garnet core

0

0.2

0.4

0.6

0.8

1

1.2

500 600 700 800

41 2 3 1&2

34

P (GPa)

T (T ˚C)

SK26, garnet rim

0

0.2

0.4

0.6

0.8

1

1.2

500 600 700

1 4 2&3

1

2

34

P (GPa)

T (T ˚C)800

L195, garnet rim

0

0.4

0.6

0.8

1

1.2

400 500 600 700

1 4 3 2

4

2

3

P (GPa)

T (T ˚C)

L162, garnet core

0

0.2

0.4

0.6

0.8

1

1.2

400 500 600

4 3 21

4

3

2

P (GPa)

T (T ˚C)

L162, garnet rim

0

0.2

0.4

0.6

0.8

1

1.2

400 500 600

4 3 1 2

4

3

2

P (GPa)

T (T ˚C)

Fig. 7. Thermobarometric estimates of M1 conditions. Calibrations of the GARB thermometer are: 1, Perchuk & Lavrent'eva,1983); 2, Ganguly & Saxena (1984); 3, Hodges & Spear (1982); 4, Ferry & Spear (1978). Calibrations of the barometers are: solidlines (GMBP barometer): 1, Hoisch (1990); 2, Powell & Holland, 1988); 3, Ghent & Stout (1981); 4, Hodges & Crowley (1985);spaced broken lines (GASP barometer): 1, Koziol & Newton (1988); 2, Hodges & Crowley (1985); 3, Hodges & Spear (1982);4, Ganguly & Saxena (1984); unspaced broken lines (GMQP barometer): Hodges & Crowley (1985); Bold solid lines(GPQ barometer): Hoisch (1990).

TECTONO-METAMORPHIC EVOLUTION OF THE KARAKORUM METAMORPHIC COMPLEX 725

muscovite (Fig. 9b). Garnet of the transition zoneforms generally large idioblastic grains (1±10 mm,Fig. 9c), showing two growth generations. An earlycore (remnant of the M1 garnet) contains inclusions ofstaurolite (generally at the core of the grain), quartz,muscovite and biotite, while the rim, crystallized duringM2, contains fewer inclusions of quartz and muscovite.In the matrix of sample L552, garnet is smaller, andwithout inclusions of staurolite, suggesting that theygrew only during M2 metamorphism. As illustratedby the microprobe pro®le on sample L526 (Fig. 10),garnet cores show relatively high grossular andspessartine and low pyrope and almandine contents,whereas garnet rims have lower grossular and spessar-tine and higher pyrope and almandine contents. Thiszoning can be interpreted as growth during increasingtemperature in the transition zone (Spear, 1988). Thepro®le is not symmetrical, probably due to the left sideof the pro®le, Fig. 10, being in contact with biotite,while the other side is in contact with muscovite, whichmay have prevented garnet growth (Spear & Peacock,1989). Sample L552 provides a more symmetricalzoning, showing a typical bell-shaped spessartinepro®le and a U-shaped almandine pro®le. Increase ofalmandine and pyrope at the expense of spessartine andgrossular, from core to rim provides evidence that itgrew only during M2. Biotite ranges from low Fe andAl contents (L545) to high Fe and Al contents (SK60)(Table 2). As all the samples come from the same area

and show the same paragenesis, variability of biotitecomposition probably mainly re¯ects different initialwhole-rock compositions. Muscovite shows homo-geneous and low Si contents, but different Fe contents.Plagioclase ranges from oligoclase to andesine.

Petrography and mineralogy of metapelites fromthe high-grade zone

High-grade metapelites from the Dassu±Askole domesare generally migmatitic (Fig. 4a), and have a foliationdominated by biotite-sillimanite (Fig. 9d), locallytransformed into muscovite. Most sillimanite is ®bro-litic, but prismatic sillimanite also occurs in the innercore of the migmatitic domes (Fig. 9e). RareM1-related relics of kyanite are found in the Askoledome, as very small grains mantled by garnet orbiotite+sillimanite, suggesting that these rocks alsorecorded earlier M1 metamorphic conditions. Stauro-lite relics are no longer observed. Leucocratic layers(Fig. 9f) are rich in quartz and plagioclase porphyro-clasts. K-feldspar, often perthitic (Fig. 9g), withalbitic antiperthites, mainly occurs in leucosomes ofmigmatites. K-feldspar is associated with myrmekiticquartz and plagioclase and surrounded by biotite+sillimanite (Fig. 9h). Leucosomes are characterised bytabular mosaic textures, elongated parallel to the S2foliation. Garnet forms euhedral porphyroblasts (radii>1 mm), with numerous inclusions of biotite, quartz,plagioclase, graphite, ilmenite and sometimes rutile.The main accessory minerals are tourmaline, apatite,zircon, graphite, magnetite and ilmenite. In the innercore of the domes, K-feldspar neosomes are rich inbiotite, plagioclase, quartz and skeletal muscoviteinclusions. They are sometimes associated with ®broliteand small (<1 mm) rounded garnet (Fig. 9f ).

The garnet of the high-grade zone shows lowercontents of grossular and spessartine, and generallyhigher contents of almandine and pyrope comparedto those of the Transition zone. As shown in the pro-®les (Fig. 10) of samples L525, L527, SK46 and SK57(location in Fig. 5), these garnet do not showsigni®cant compositional zoning. The depletion inalmandine and pyrope contents of the core of garnetSK57 can be explained by local re-equilibration withbiotite inclusions. The rather ¯at garnet pro®lescommonly observed in the other samples (Fig. 10)could indicate either HT re-equilibration (Spear, 1988)or diffusive re-homogenisation during retrogression(Yardley, 1977). Cooling rates for the area of70±110 uC . Max1 (Villa et al., 1996), seem too high toallow complete diffusive garnet re-homogenisationduring retrogression, even for small garnet (e.g. Spear,1988); This ¯at zoning seems due to HT re-equilibra-tion and thus, the Fe/Mg ratio should also preserve HTconditions. Nevertheless, almandine depletion at thegarnet rims of samples SK46 and SK57, and spessartineenrichment at the rims of sample L525 (Fig. 10),suggest that retrogressive diffusion also affected the

P (GPa)

T(°C)

1

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

400 600 700500

Ms

Kfs+Sil+H

2O

Bt+Cd

St

Grt+Bt+Ky

Ms+Bt

Grt+Kfs+Melt

Ms

Kfs+Sil+Melt

KFMASHXH2O=1.0Quartz in excess

Ky

And

Ky

Sil

SilAnd

Grt+Sil+H2OCd

Bt+Sil

Grt+Kfs+Melt

Ms+ Grt

Bt+ Sil

0.9

0.8

0.7

0.6

H2Osatured

metapelitesolidus

Grt+Kfs+H

2O

Fig. 8. P±T path of the M1 metamorphic event in the upperHo Lungma valley. KFMASH petrogenetic grid for XH2O

=1and quartz in excess. Dotted lines are almandine isopleths. TheH2O-saturated metapelite solidus after Thompson (1982),Grt+Ms and Grt+Kfs producing reactions from Vielzeuf(1984) and Vielzeuf & Holloway (1988), Ms to Kfs+Sildestabilisation from Le Breton & Thompson (1988). Otherreactions from Spear & Cheney (1989). Mineral abbreviationsfollowing Kretz (1983).

726 ROLLAND ET AL.

Fig. 9. Photomicrographs of the M2 assemblages. Scale bar: 1 mm(a) Sample L559, transition zone. Staurolite destabilisation into M2assemblage Ky+Bt. (b) Sample L545, transition zone. Destabilisation ofMs+Grt to Bt+Sil M2 decompression assemblages. (c) Sample L526,transition zone. A large M2 porphyroblast of Grt displaying inclusions ofBt and St. (d) Sample, SK18, high-grade zone. Crystallization of Bt ± Sillassemblage replacing of Grt±Ms assemblage. (e) Sample L523, high-gradezone, showing the typical Grt+Kfs+Bt+Sil (prismatic crystals) assemblageof the felsic granulites. (f ) Sample SK46, high-grade zone. Leucosomeshowing migmatitic round Grt and Kfs surrounded by restitic Bt and Sil(g) Sample SK44, high-grade zone. Retrograde destabilisation of Sil andperthitic Kfs into Ms and Qtz.

L526

7

11

15

0

2

1

0 1 2 3

76

78

80

distance (mm)

%6

4

0

2

0

3

0.6 1.2 1.8 2.4 3.0

distance (mm)

80

82

84

L552

10

12

Transition Zone

High Grade Zone

1

distance (mm) distance (mm)

82

84

14

16

1 .0

2 .0

0 1 2 3 4 5 70

L527

6

distance (mm)

72

74

76L525

8

10

12

14

0 1 2 3 42 .9

3 .3

3 .7

distance (mm)

High Grade Zone1, Askole dome

2 Dass dome

%

distance (mm)

74

76

78

80

82SK46

3

3.75

4.5

0 0.2 0.4 0.6

.

.4

8

12

16

distance (mm)

71

72

73

74SK 57

2

3

4

0 0.2 0.4 0.6

6

7

8

14161820

distance (mm)

2, Dassu dome

%

Fig. 10. Representative compositional pro®les of garnet in thetransition zone (samples L526 & L552) and in the high-gradezone (samples L527, L525, SK57 & SK46). Note the zoningand the marked core-rim composition differences observed intransition zone garnet, as compared to the more homogeneouscompositions found in garnet of the high-grade zone.&=Grossular, m=Pyrope, r=Almandine, *=Spessartine.

TECTONO-METAMORPHIC EVOLUTION OF THE KARAKORUM METAMORPHIC COMPLEX 727

outermost rims of garnet. As the matrix mineralsare completely re-equilibrated during the M2 meta-morphism we have used core analyses of the largegarnet (radii >0.5 mm) to estimate peak M2 condi-tions, and rim compositions to estimate P±T conditionsat the beginning of the retrogressive evolution.

Three groups of biotite can be distinguished: (i)included in garnet (ii) isolated in leucocratic layers and(iii) biotite-rich layers de®ning the foliation. Thesebiotite have the same compositions (Table 2): relativelyrich in Fe and Al, implying that all (even the garnetinclusions) have been completely homogenised duringthe HT event, which is in good agreement with thegarnet chemical pro®les. Thus, the garnet±biotitecompositions can be used in exchange thermometryto estimate the M2 peak conditions. Muscovite haslow Si4+ contents, and low XFe, which characteriserelatively low P±T conditions of crystallization(Massonne & Schreyer, 1987), suggesting late retro-gressive equilibration. All K-feldspar is albite-poor(Xab<0.2) and shows numerous antiperthites of albiticcomposition (Xab>0.75). Plagioclase crystals haveandesine tolabradorite compositions with a slightlyalbitised rim.

P±T ESTIMATES FOR THE M2 METAMORPHISM

P±T estimates of transition zone samples

In samples close to the High Grade Zone (L526), rutileis rimmed by ilmenite at the proximity of kyanite andgarnet, suggesting the following net transfer reaction(Fig. 11):

Grt+RtpIlm+Ky+Qtz (2)

The position of this reaction, which is mainly pressure-dependent, is ®xed by the almandine content of garnet(Bohlen et al., 1983), and as the garnet core in thetransition zone was not re-equilibrated during the M2metamorphism, the core composition was used toconstrain the metamorphic pressure before M2. Wededuce a minimum peak pressure of 0.8 GPa fromsample L526, which preserved an initial garnet corecomposition (with almandine content of 0.76, Fig. 10).

In all the transition zone, relics of staurolite, found atthe contact with granoblastic kyanite and biotite(Fig. 9a), suggest the prograde reaction (Fig. 11):

St+ChlpBt+Ky (3)

For sample L526, the calibration of Spear & Cheney(1989) gives a temperature estimate of c. 620 uC for apressure of 0.8 GPa, ®xed by reaction (2).

The replacement of garnet by the biotite+sillimaniteassemblage is associated with muscovite consumption.Thus, crystallization of biotite+sillimanite assemblage

must be a product of the garnet+muscovite destabili-sation, according to the reaction (Fig. 11):

Ms+GrtpSil+Bt+Qtz (4)

This reaction is mainly pressure dependent (Vielzeuf,1984; Vielzeuf & Holloway, 1988), the sillimanite+biotite assemblage being stable below 0.6 GPa, andtakes place at temperatures between 650 and 725 uC.Biotite+sillimanite can also be produced by destabi-lisation of garnet+K-feldspar. However, K-feldsparand sillimanite are always stable together.

P±T evolution in the transition zone

P±T estimates for the transition zone show aprogressive temperatures increase towards the coreof the Askole and Dassu domes. Far from the cores ofthe domes (i.e. far from the K-feldspar±in/kyanite±out isograd), thermobarometric estimates yield c.575±600 uC at 0.35±0.45 GPa. The garnet core com-positions of sample L545 gave a similar temperatures of575¡20 uC at 0.37¡0.1 GPa. Core-rim thermobaro-metric variations are within the range of uncertainties.Estimates using garnet rim compositions of L545 are550 uC¡30 uC at 0.48¡0.1 GPa. Towards the innerpart of the transition zone (sample L552), temperatureare slightly higher at 620¡50 uC and 0.4¡0.2 GPa(garnet core) and 585¡45 uC and 0.33¡0.2 GPa(garnet rim).

P (GPa)

T(°C)

1

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

400 600 700500

Ms

Kfs+Sil+H

2OSil

And

Ms

Kfs+Sil+

Melt

KFMASHXH2O=1.0Quartz in excess

Ms+Grt

Bt+Sil

Grt(Alm 0,76

)+Bt

Ilm+Ky

St+

Chl

Bt+

Als

Ct+

Cd

Bt+

Ky

Ms+GrtBt+Ky

Grt+Chl

St+

Bt

Ky

Sil

Ky

And

0.9

0.8

0.6

0.7

H2O satured

metapel i te solidus

Fig. 11. The M2 P±T path in the transition zone close to theHigh Grade Zone of Dassu±Askole domes. This path isbased on the textural observations and thermobarometriccalculations on sample L526. KFMASH petrogenetic grid forXH2O

=1.0 and quartz in excess, same equilibria and referencesas in Fig. 8.

728 ROLLAND ET AL.

Close to the High Grade Zone (samples SK60& L526), the M1 assemblage, characterised byrutile+staurolite relics, de®nes a pre-M2 P±T ®eldbounded by reactions (2) and (3) at P>0.8 GPa andT<625 uC (Fig. 11). Subsequent destabilisation intobiotite+kyanite implies a temperature increase in thekyanite stability ®eld according to the reaction (3)(T>600 uC, P>0.6 GPa). This step is followed bycrystallization of biotite+sillimanite, symptomatic ofa pressure decrease to below 0.6 GPa during M2. Theabsence of K-feldspar at this stage implies thattemperatures never exceeded 700 uC, i.e. the M2thermal increase after the pressure peak must be<80 uC in the transition zone (Fig. 10). This P±Tevolution deduced from the textural observationmade on sample L526 and SK60 is also documentedby thermobarometry. Garnet±biotite geothermometrygives temperature estimates close to those deducedfrom net transfer reactions (Fig. 12). For sample SK60,the garnet core gave 641¡60 uC and 0.57¡0.2 Gpa,while the rim gave 547¡50 uC and 0.30¡0.2 GPa.Sample L526 gives a higher temperature for the rimthan the core compositions. This observation re¯ectsthe increase of the pyrope in the garnet compositionalpro®les (Fig. 10), and shows that garnet of sampleL526 grew during increasing temperature. For sampleL526 minimal P±T conditions for M1 of 625¡50 uCand 0.70¡0.1 GPa were obtained using garnetcore compositions in the presence of kyanite. Garnetrim compositions, in presence of sillimanite, yield M2P±T conditions of 685¡50 uC and 0.60¡0.2 GPa.These estimates suggest that, close to the high gradezone, samples from the transition zone recorded asmall, but signi®cant, thermal increase (60 uC) fromM1 to M2.

P±T estimates from the high-grade zone samples

This zone is characterised by a migmatitic imprint,and premigmatisation P±T estimates are impossible.Textural observations show that the biotite+sillima-nite association crystallized before the migmatisationbut no relics predating the biotite+sillimanite associ-ation are preserved. However, as no structural break isobserved between the transition zone and the high-grade zone (these zones being in geological continuity),we assume that pre-M2 conditions were roughly similarand consequently, the P±T conditions before thecrystallization of biotite+sillimanite in the core ofthe domes could be <725 uC and >0.6 GPa accordingto reactions (3) and (4) (Fig. 13).

K-feldspar crystallization and dehydration meltingreactions

In all the high-grade zone, migmatisation is common,and K-feldspar is in the leucosomes of both peliticgneisses and orthogneisses. K-feldspar is rich in biotiteand plagioclase inclusions and is often associated with

garnet and sillimanite both in the leucosomes andneosomes (Fig. 9f). The K-feldspar+garnet associ-ation is limited by three divariant reactions:

Bt+Ms+Qtz¡PlpGrt+Kfs+H2O (5)

Bt+Sil+Qtz¡PlpGrt+Kfs+H2O (6)

Bt+Cd+QtzpGrt+Kfs+H2O (7)

These reactions are essentially pressure dependent and,at 750 uC, fall at >0.6 GPa (reaction 5), 0.3 GPa<P<0.6 GPa (reaction 6) and P<0.3 GPa (reaction 7),respectively (Thompson, 1976; Vielzeuf, 1984; Vielzeuf& Holloway, 1988; Le Breton & Thompson, 1988).Combining these reactions with the H2O-saturatedmetapelite solidus (Thompson, 1982), a P±T domain isde®ned, in which H2O liberation enhances melting(Fig. 13). Reaction (6) has been identi®ed in all samplesfrom the high-grade zone, where poikilitic K-feldspar,including relics of biotite and sillimanite, is associatedwith round to idioblastic garnet in migmatitic leuco-somes. Field and thin-section observations in this zonesuggest that the development of K-feldspar+garnet inthe leucosomes occurred in presence of sillimanite.In the absence of cordierite, which gives a lowerpressure boundary (reaction 7), the development ofK-feldspar+garnet according to reaction (6) shouldhave occurred at temperatures higher than 750 uCfor pressures between 0.3 and 0.6 GPa. P±T estimatesbased on cationic exchange geothermometers are ingood agreement with these estimates. In the Askoledome, the GARB calibrations of Ferry & Spear (1978),Hodges & Spear (1982) and Ganguly & Saxena (1984)applied to sample L523, from the inner part of theHigh Grade Zone, with core compositions (Figs 5 &12) yield peak conditions of 776¡10 uC and 0.46¡0.1 GPa. Sample L525, from the outer part of the HighGrade Zone yields a lower temperature, also for garnetcore composition, of 670¡50 uC (P=0.5¡0.2 GPa).This lower estimate re¯ects both heterogeneous HTequilibration in the dome core and retrogressivere-equilibration of L525 garnet rims (see Fig. 10).

Farther west and closer to the MKT/Shigar Fault,in the migmatitic Dassu dome, sample SK46 (Fig. 5),also yields high P±T conditions of 735 uC¡15 uC and0.92¡0.2 GPa using the garnet core composition.Sample SK18, located in the outer part of the high-grade zone, yields 677¡10 uC at 0.58¡0.15 GPa,using garnet core compositions. These estimatesare slightly below the temperature peak deducedfrom mineral associations (>750 uC) and probablyre¯ect post-HT peak re-equilibration. Such thermalre-equilibration are classically observed in HimalayanMP±HT gneisses (MacFarlane, 1999; Guillot, 1999).

To summarise, the temperature peak in the innerpart of the High Grade Zone did not exceed 800 uC asno pyroxene has been observed in the metapelites, andwas probably close to 770¡10 uC, as recorded by

TECTONO-METAMORPHIC EVOLUTION OF THE KARAKORUM METAMORPHIC COMPLEX 729

assemblages and mineral chemistry in the Askole dome.In the Dassu dome, lower temperatures of 735¡15 uCare recorded. However, this may only re¯ect the lack ofsampling in the very steep mountain zone of the innerpart of the Dassu dome.

Late evolution

Late poikilitic muscovite is observed in all K-feldsparbearing samples, at the contact between sillimaniteand K-feldspar (Fig. 9g), and suggests the following

L552, garnet coreP (GPa)P

T (T ˚C)

0

0.2

0.4

0.6

0.8

1

1.2

400 500 700 800

1&4 2 3

21

1

L552, garnet rimP (GPa)P

T (T ˚C)

0

0.2

0.4

0.6

0.8

1

1.2

400 500 600 700 800

4 1&2 3

2

11

L545, garnet rim

T (T ˚C)

P (GPa)P

0

0,2

0,4

0,6

0,8

1

1,2

400 500 700 800

4 1 2&3

2

1

1

L545, garnet core

T (T ˚C)

P (GPa)P

0

0.2

0.4

0.6

0.8

1

1.2

400 500 700 800

4 12 3

1122

1

T (T ˚C)

P (GPa)P L526, garnet core

0.2

0.4

0.6

0.8

1

1.2

0

400 500 600 700 800

1&4 3 222

1

21

P (GPa)P

T (T ˚C)

L526, garnet rim

0

0.2

0.4

0.6

0.8

1

1.2

400 500 600 700 800

1 4 2&3

1

221

SK60, garnet coreP (GPa)P

T ˚C)

0

0.2

0.4

0.6

0.8

1

1.2

400 500 600 700 800

14 3 22

1

2

1

SK60, garnet rimP (GPa)P

T (T ˚C)

0.2

0.4

0.6

0.8

1

1.2

400 500 600 700 800

4 3 1 212

P (GPa)P

T (T ˚C)

SK18, garnet rim

0

0.2

0.4

0.6

0.8

1

1.2

400 500 600 700 800

4 3 2 1

21

P (GPa)P

T (T ˚C)

SK18, garnet core

0

0.2

0.4

0.6

0.8

1

1.2

400 500 600 700 800

1

1

2

4 3 2

L523, garnet coreP (GPa)P

T (T ˚C)

0

0.2

0.4

0.6

0.8

1

1.2

400 500 600 700 800

2 3&4

1

2

13

L523, garnet rimP (GPa)P

T (T ˚C)

0

0.2

0.4

0.6

0.8

1

1.2

400 500 600 700 800

4 3 2

1

2

13

P (GPa)P

T (T ˚C)

L525, garnet rim

0

0.2

0.4

0.6

0.8

1

1.2

400 500 600 700 800

4 2 3

122

4

1

P (GPa)P

T (T ˚C)

L525, garnet core

0

0.2

0.4

0.6

0.8

1

1.2

400 500 600 700 800

1&41111

2

3&4 2

P (GPa)P

T (T ˚C)

SK46, garnet core

0

0.2

0.4

0.6

0.8

1

1.2

400 500 600 700 800

4 2 3 1

233

1

Fig. 12. P±T thermobarometric estimates of M2 conditions. Thermometers (GARB), steep dP/dT lines. Barometers: solid lines(GMPB), spaced broken lines (GASP), unspaced broken lines (GMQP), Bold solid lines (GPQ). For references, see Fig. 7.

730 ROLLAND ET AL.

reaction (Fig. 13), which implies the availability ofwater during retrogression:

Kfs+Sil+H2OpMs+Qtz (8)

The retrograde P±T path is also constrained by theabsence of cordierite, and from Fig. 13, implies initialconditions close to 600±650 uC, at >0.3 GPa, in boththe Dassu and Askole domes. Using garnet rimanalyses gave P±T estimates of 720¡60 uC and0.50¡0.1 GPa for L523, and a much lower tempera-ture for L525 of 550¡50 uC (P=0.25¡0.25 GPa), inthe Askole dome. In Dassu dome, the garnet rims P±Tgive 586¡40 uC at 0.40¡0.15 GPa for SK18 and586¡45 uC, 0.4 for SK46. These results imply a strongcooling after the thermal peak (Fig. 13) in both Dassuand Askole domes.

DISCUSSION

The metamorphic evolution of the southernKarakorum margin has been variously interpreted;on one hand, the highest temperature conditions havebeen attributed to M1 (Searle et al., 1989; Allen &Chamberlain, 1991; Smith, 1993), resulting fromnappe-stage inversion of a crustal section (M1 stage),and decompression (M2 stage) associated with possibledome-stage heating (M3). Alternatively, it has beenproposed that a late metamorphic event (referred to asM3), followed a continuous evolution from nappe-style

deformation to doming, and was due to contactmetamorphism around the Baltoro granite (Searleet al., 1989; Smith, 1993). In contrast, Bertrand et al.(1988) and Lemennicier et al. (1996) suggested that aperiod of latency is required to explain the M2 heating.Bertrand et al. (1988) proposed that the late thermalincrease and doming could result from heating duringintrusion of the Baltoro granite (25±21 Ma). However,Lemennicier et al. (1996) and Villa et al. (1996) haveshown that heating and doming occurred long after theBaltoro intrusion, as indicated by young crystallizationand cooling ages recorded in the Dassu dome and inthe Hemasil syenite area, and also proposed that M1could be younger, and coeval with the Baltoro graniteintrusion. Thus, in their interpretation, the M2 meta-morphism re¯ects thermal relaxation during decom-pression, which shortly post-dates the M1 pressurepeak. Following PeÃcher & Le Fort (1999), we arguethat a distinct and young (Mio-Pliocene) M2 meta-morphic history occurs in the Karakorum margincontemporaneously to the NPH formation.

The M1 metamorphism is characterised by upperamphibolitic conditions (650±700 uC, 0.6±0.8 GPa)for the high grade rocks (Ho Lungma Valley). Theretrogression of these rocks occurred along a coolingP±T path, featured by a high dP/dT gradient (Fig. 9).As discussed previously, this tectono-metamorphicevent is related to the earlier stacking and thickeningof the India-Asia collision zone. SW-verging thrust-ing and stacking could have started already in theUpper Cretaceous times in the Shyok suture zone asde®ned by dykes and plutons cross-cutting SW-vergingthrusts along the northern boundary of Kohistan andLadakh (Petterson & Windley, 1985; Weinberg et al.,2000). SW-verging thrusting continued after theinitial India±Asia contact (Fig. 14a), and is locallysealed by the 37.0¡0.8 Ma Mango Gusor granite(Fig. 1) (Searle et al., 1989). Thus, the main metamor-phic evolution could have started before 80 Ma,and lasted to at least 35 Ma.

Two E±W tectonometamorphic and magmaticfeatures, the Baltoro granite to the north, and thealignment of domal structures to the south (Fig. 1)obliquely crosscut the M1 structural pattern, parallel tothe belt. The domes zone coincides with the occurrenceof young (<10 Ma) M2 MP-HT gneisses and migma-tites. The E±W Baltoro granite emplacement occurredin a similar context. Thus, the discordant M2 eventmay have initiated as early as 25 Ma in the Baltoroarea, and could have developed southward into thedomes area at <10 Ma.

Our metamorphic study shows that rocks affectedby the M2 metamorphism have equilibrated at hightemperatures (max. 770 uC) and medium to lowpressures (0.4±0.6 GPa). At the outer margin of thedomes, in a transition zone, the M2 metamorphic eventpartially overprints the M1 mineral association. Here,the P±T path is characterised by a small temperatureincrease, up to 650 uC at 0.5±0.6 GPa, followed by

P (GPa)

T(°C)

1

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

600 700 800500

KFMASHXH2O=1.0Quartz in excess

Melting

domain

Doming

domainKy

Ky

Sil

SilAnd

Ms 2O

Bt+

Cd

Bt+

Sil

Grt+Kfs+Melt

Ms+Grt

Bt+Sil

Ms+

Bt

Grt+Kfs+Melt

St

Grt+Bt+

Ky

Ms

Kfs+Sil+

Melt

Bt+Sil

Cd+Kfs+

H O

And

0.9

0.8

0.7

0.6

H2O

satu

red

metapelite solid

us

Grt+Kfs+H

2O

Kfs+Sil+H

2

Fig. 13. P±T path of M2 metamorphic event in the high-gradezone of the domes. This path is based on the texturalobservations and thermobarometric calculation performed onsamples L523 and SK46. KFMASH petrogenetic grid forXH2O

=1.0 and quartz in excess, same curves references as inFig. 8. Partial melting occurs along the path of increasingtemperature, by muscovite- and biotite-dehydration melting.Doming in decreasing temperature path, as indicated by ®eldrelations.

TECTONO-METAMORPHIC EVOLUTION OF THE KARAKORUM METAMORPHIC COMPLEX 731

pressure and temperature decrease under low amphi-bolitic facies conditions (Fig. 11). In the high-gradezone, the M1 mineral association is completely erased,and thus it is dif®cult to evaluate the peak pressureconditions. Considering ®eld continuity from the outerto the inner part of the domes, we assume the same initialP±T conditions (Fig. 13). Combining early M2 condi-tions and M2 thermal peak conditions (around 770 uCand 0.5±0.6 GPa), a P±T path for the inner part of thedomes zone is characterised by (i) a temperature increaseof about 150 uC associated with a slight pressure drop ofc. 0.2 GPa; this temperature increase is responsible forpartial melting of metapelites and orthogneisses, byand biotite dehydration melting. (ii) The retrogressivecooling path initiated at c. 0.5 GPa (Fig. 13).

The combination of published thermochronologicaldata with the estimated P±T path allows calculationof exhumation rates for the core of Dassu dome,before and during the M2 tectonometamorphic events.Exhumation rates (Vz) have been calculated usingthe equation Vz=dz/dt=(1/rg)3(dT/dt)3(dP/dT )(DucheÃne et al., 1997) where z=depth, g=accelerationof gravity and r=2800 kg mx3. The dT/dt valuewas obtained using the closure temperatures of450 uC¡50 uC for Ar/Ar in biotite (MacDougall &Harrison, 1988) and 735¡15 uC for U±Pb in monazite(Copeland et al., 1988), and was directly extracted fromthe obtained P±T path. The age of the M1 event is chosenat 37 Ma (Parrish & Tirrul, 1989), and the maximumpressure for M1 ranges between 1 GPa (Lemennicieret al., 1996) and 0.7 GPa (Fig. 8). The age of the M2temperature peak is ®xed at 6.7¡0.5 Ma in the coreof the Dassu dome, from U±Pb monazite dating(Smith, 1993) and the pressure is estimated at 0.4 GPa.The age of cooling to below 450¡50 uC, at 0.1 GPa, isestimated from K/Ar ages on biotite at 4.7¡0.2 Ma(Searle et al., 1989). Subsequently, the exhumation ratededuced from these data for the period between M1 toM2 is low, ranging between 0.82¡0.01 mm yx1 and0.47¡0.01 mm yx1 (Fig. 15), suggesting that during

this period, exhumation is controlled mainly byerosional processes. In contrast, after the M2 tempera-ture peak (during the formation of the domes), theexhumation rate was high at 5.47¡0.20 mm yx1,similar to rates calculated for the Nanga±Parbatsyntaxis during the same period (Winslow et al.,1994). The viscosity decrease induced by the partialmelting of the mid-crustal rocks is probably responsiblefor the increase of the exhumation rate after M2.

These two distinct periods of exhumation con®rmthat the metamorphic evolution from M1 to M2 is notcontinuous, and show that simple thermal relaxation ofa previously thickened crust cannot account for theHT metamorphism recorded during M2 (Fig. 15).

The E±W alignment of domal structures suggests aN±S direction of shortening, and has been interpretedas the ampli®cation of crustal-scale folds (PeÃcher & LeFort, 1999). This interpretation is also supported by a

INDIAN PLATEMoho

a : 55-37 Ma

Himalaya

Karakoram

axialbatholithLa-Ko

SE NW

Moho MHTINDIAN PLATEMoho

b : c. 22 Ma

BaltorobatholithSE NW

Moho

MCT

MHT

c : Present day

MBT

domeszoneSE NW

MohoMoho

INDIAN PLATE

MHT

DetachedIndian plate

Fig. 14. Schematic evolution of the NW Himalaya-Karakorum convergent zone. (a) The early M1 evolution is characterised bysouth-verging stacking on both sides of the Ladakh-Kohistan arc (La-Ko), related to the subduction of the Indian Plate. In thecentral Karakorum, the main part of the axial batholith is being emplaced. (b) Early Miocene emplacement of the Baltoro graniterelated to slab-breakoff of the Indian Plate. This process leads to the uprise of asthenospheric mantle and to melting of theKarakorum crust and lithosphere by underplating (e.g. Davies & Blanckenburg, 1995). (c) After slab-breakoff the Indiancontinental lithosphere is stacked below the Karakorum lithosphere. The buckling of the Karakorum plate and of the thermallyweakened mid-crustal HT±MP rocks leads to the development of crustal-scale folds in the southern part of the Karakorum.

1.2

1.0

0.8

0.6

0.4

0.2

00 5 10 15 20 25 30 35 40

Age (Ma)

5.47 mm.y-1 +/- 0.20

0.82 mm.y-1 +/- 0.01

0.47 mm.y-1 +/- 0.01U/Pb Mz (2)

K/Ar Bt (3)

U/Pb Mz (1)

M1

P (GPa)

Fig. 15. Pressure-time evolution of the core of the Dassudome, between the M1 and M2 events. U/Pb ages on monaziteare from (1) Parrish & Tirrul (1989) and (2) Smith (1993).K/Ar ages on biotite are from (3) Searle et al. (1989).

732 ROLLAND ET AL.

large E±W-trending negative Bouguer anomaly, trend-ing from 8000 m peaks of the Karakorum (top ofBaltoro glacier, see Fig. 1) to the north of Haramoshspur (Caporali, 2000). This author interpreted thisBouguer anomaly as a response to buckling (folding) ofthe lithosphere with a wavelength of 300 km andsecondary buckling of the 60 km thick Karakorumcrust with a wavelength of 80 km along a N±S direc-tion. Field relationships show that melting startedbefore the completion of the doming, as in the core ofthe Himalayan syntaxes (Schneider et al., 1997; Burget al., 1997), which is interpreted as crustal-scale foldampli®cation following the thermal weakening of apreviously thickened crust (Burg & Podladchikov,1999). However, in those two areas, the domes strikeN±S, accounting for E±W shortening. The presence oftwo dome systems (E±W & N±S) without evidence ofinterference implies temporal and spatial partitioningof the deformation in a transpressive regime alongtwo nearly orthogonal directions of N±S and E±Wshortening (Seeber & PeÃcher, 1998). High-temperaturemetamorphism in collisional zones is classicallyinterpreted as the result of thermal relaxation of thethickened continental crust, exhumed by erosional ortectonic mechanisms (England & Thompson, 1984).In the case of exhumation by a combination of erosionand tectonic processes, as observed in the MCT zone,the retrograde P±T path follows the relaxed geotherm(Fig. 16). In contrast, in the case of rapid, dominantlytectonic exhumation (>4 mm yx1) as observed in thesyntaxes (Winslow et al., 1994), the retrograde path isnearly adiabatic and crosscuts the relaxed geotherm(Fig. 16). In this case, melting occurs by decompres-sional anatexis (Zeitler et al., 1993; Whittington et al.,1998). Nevertheless, thermal models of exhumation byextrusion processes in continually convergent orogenshave been shown to produce only a limited temperatureincrease (Batt & Brawn, 1997; Thompson et al., 1997).In the case of the Himalayan syntaxes, the high rate ofexhumation linked to a fold ampli®cation mechanismseems unlikely to produce any thermal increase duringexhumation. Actually, the P±T path evolutionobserved in the Dassu±Askole area differs from thecases previously discussed by the occurrence of a strongtemperature increase that occurred before doming, andby crosscuting the relaxed geotherm (Fig. 16). As theexhumation in the dome was rapid (5.5 mm yx1), laterthan the thermal peak and associated with a strongcooling, the heat transfer in the Dassu±Askole areacannot be explained by the same mechanism as thatsuggested for the syntaxes. To explain such a strongtemperature increase before the rapid period ofexhumation, an additional heat input must beadvocated (De Yoreo et al., 1991).

A ®rst possibility for an additional heat source couldbe shear heating, as the domes lie in the vicinity of theMKT. However, models based on shear heating(Treloar, 1997; Leloup et al., 1999) indicate that evenin the most favourable conditions this mechanism

cannot result in temperatures up to 700 uC forpressures of 0.6 GPa. Moreover, it is noted thattemperatures reached along the MKT, west of theDassu dome, are lower than farther to the east, in theAskole dome, far from any major fault. Anothertectonic setting for which the dT/dP gradient is evenhigher than that observed in the Dassu±Askole domesis a late-orogenic extensional setting, in which heatadvection is related to asthenospheric upwellingcombined with radioactive heat production in thecrust (Gardien et al., 1997; Vanderhaeghe et al., 1999).But, in the case of the Dassu±Askole domes, the stillactive convergent context excludes any heat inputrelated to post-orogenic extension.

An alternative hypothesis for additional heat input inthe Dassu±Askole domes is supported by the thermalgradient record across the domes, and the chemistry ofrelated granitoids. A cross-section from the outer part(transition zone) to the internal part (high-grade zone)of the dome belt shows that temperature increases from575¡20 uC to 770¡10 uC, whereas estimated pres-sures at the thermal peak increase from 0.4¡0.2 GPato 0.5¡0.1 GPa. The pressure difference between theexternal part of the transition zone to the inner partof the high-grade zone suggests that the high-gradezone records a vertical exhumation of about 3±4 kmrelative to the transition zone during the doming event

0 200 400 600 800 T°C

10

20

30

40

50

Zkm

1.4

1.2

1.0

0.8

0.6

0.4

0.2relaxed geotherm

stable geotherm

M1

Pe

rturb

ed

ge

oth

erm

0 200 400 600 800 T°C

10

20

30

40

50

Zkm

PGPa

Pe

rturb

ed

ge

oth

erm

post orogenic domes

HHC

Nan

gaP

arba

t

M2 dom

es

Fig. 16. Stable, perturbed and relaxed geotherms compared toP±T paths obtained during this study (black lines), and someother paths (grey lines): for the Himalayan Nanga Parbat,Pliocene-exhumed, gneisses (Whittington et al., 1998), for theMiocene syn-stacking metamorphism in the High HimalayanCrystallines (Guillot, 1999), and for the Hercynianpost-orogenic metamorphism in the French Central Massif(Gardien et al., 1997).

TECTONO-METAMORPHIC EVOLUTION OF THE KARAKORUM METAMORPHIC COMPLEX 733

(Fig. 17a). Assuming a vertical motion of 3±4 kmand a temperature difference of 200 uC between thetransition zone and the core of the high-grade zoneleads to a geothermal gradient within the domes struc-ture of 60¡10 uC/km. The geothermal gradient abovethe transition zone is evaluated to be 30¡10 uC/km(Fig. 17b). The fact that the geothermal gradient ishigh within the dome zone and that it is lower abovethe domes can be interpreted in terms of differenttypes of heat transport. Above the domes the estimatedgeothermal gradient is close to "normal" continentalgeothermal conditions, suggesting that heat conduc-tion prevails over heat advection (e.g. Turcotte &Schubert, 1985). In contrast, within the dome, thehigh estimated geothermal gradient suggests thatheat advection becomes more ef®cient (Turcotte &Schubert, 1985). The occurrence of numerous pegma-titic dykes cross-cutting the earlier M2 foliation, tiltedduring doming, and the presence of granitic andsyenitic plutons exhumed at the present-day surfacelevel in the domes, suggest that the potential heatsource to explain such a high thermal gradient couldbe pluton emplacement below the domes (Fig. 17b). Aminimal temperature of 770±800 uC for the underlyingmagma is required to maintain anatectic productionin the dome core (Scaillet et al., 1995). Moreover themantle signature of the 20 Ma lamprophyre andshoshonite in the Baltoro and K2 area (Rex et al.,1988; Searle et al., 1989; Pognante, 1990) and of the8 Ma Hemasil syenite in the Dassu area (Lemennicier,1996) imply that a mantle-related Neogenous magma-tism occurred in south Karakorum. This mantle-relatedmagmatism suggests a mantle-derived advective heatsource causing HT±MP metamorphism of the mid-crustal levels now exhumed in the Dassu±Askole domesarea.

Such relationships between a mantle-related mag-matism and a HT metamorphism in a collisionalcontext have already been observed, and interpreted asresulting from slab breakoff (Davies & Blanckenburg,1995). Such a slab breakoff process could have allowedasthenosphere to rise up in the narrow E±W-trendinggap produced in the detachment zone and triggered themelting of metasomatised Indian lithospheric mantle.The magmas produced in this way, migrating through-out the Karakorum thickened crust, could then have

advected mantle heat to the crust and have caused theM2 overprint. This hypothesis of slab breakoff forthe Himalaya±Karakorum belt is supported by recenttomographic data (Van der Voo et al., 1999), reinter-preted by Chemenda et al. (2000) for the CentralHimalaya, which suggest that at c. 25±20 Ma the sub-ducted Indian lithospheric mantle detached (Fig 14b,c).The detachment of the subducting lithosphere is alsosupported by the analysis of the distribution of thefocal depth of earthquake NW of the Kohistan arc,which shows a seismicity gap along the Indian slab(Chatelain et al., 1980).

CONCLUSIONS

Mid-crustal migmatitic rocks exhumed in the southernKarakorum margin provide evidence of a young(25±5 Ma) high-temperature medium pressure meta-morphism associated with doming in a shorteningcontext. These rocks record high M2 peak tempera-tures (750±770 uC), and a strong temperature increase(c. 150 uC) at 0.4±0.6 GPa, overprinting a previousMP±MT (M1) event. The exhumation and furthercooling of these HT rocks occurred in a convergentcontext. The temperature increase, due to M2metamorphism, has caused muscovite and biotitedehydration melting and thus, triggered migmatisationof mid-crustal rocks. These rocks, weakened by theanatexis, are exhumed in the core of an E±W fold bya mechanism of crustal-scale folding during N±Sshortening. In this context, the domes may be due tosecond-order ampli®cation of the molten core ofthe fold. Relationships between young (<22 Ma)magmatism with mantle af®nities, doming and rapidexhumation of HT±MP rocks in the Karakorum couldbe related to a process of Indian slab breakoff.

The domes of the Karakorum provide a caseexample of very young migmatites and HT±MProcks, formed and exhumed in a convergent, short-ening, tectonic setting by processes implying addi-tional mantle heating and crustal softening. Similarmechanisms may have to be considered as a possiblealternative for the formation of HT±MP rocks in oldorogens, classically interpreted as the result of post-orogenic extension.

10 km

770°C 0.5GPa

575°C 0.4 GPa

620°C 0.5GPa

southnorth

a: Present day

770°C

575°C15 km

18 km

0 0°C

30°/km

65°/km

dome zone

upper crust

> 770°C

b: 6-7 Mafoliation

Fig. 17. (a) Schematic present-dayspatial distribution of P and T duringM2, from the inner part to the outerpart of the dome area. (b) Palaeo-temperature (6 Ma) distribution in thedome and above the dome, deducedfrom (a). The higher geothermalgradient in the dome compared to thatof above the dome suggests theoccurrence of magmatic intrusionsbelow the domes.

734 ROLLAND ET AL.

ACKNOWLEDGEMENTS

This study was supported by Centre National de laRecherche Scienti®que and by the French ForeignMinistry. The Geosciences Laboratory of Islamabad(Pakistan) is thanked for ®eld assistance. We thankJ.-M. Lardeaux for discussions and suggestions.Careful review and suggestions of an earlier draft byP. O'Brien were much appreciated, and constructivereviews of O. Vanderhaeghe, B. Lombrado andD. Robinson have much improved the manuscript.

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Received 10 December 2000; revision accepted 18 June 2001.

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