Thermobaric structure of the Himalayan Metamorphic Belt in Kaghan Valley, Pakistan

Journal of Asian Earth Sciences 29 (2007) 390–406 www.elsevier.com/locate/jaes

Thermobaric structure of the Himalayan Metamorphic Belt in Kaghan Valley, Pakistan HaWz Ur Rehman a,¤, Hiroshi Yamamoto a, Yoshiyuki Kaneko b, Allah Bakhsh Kausar c, Mamoru Murata d, Hiroaki Ozawa d b

a Department of Earth and Environmental Sciences, Kagoshima University, Kagoshima 890-0065, Japan Graduate School of Environment and Information Sciences, Yokohama National University, Yokohama 240-8501, Japan c Geological Survey of Pakistan, Plot No. 84, Street No. 03, Sector H-8/1, Islamabad, Pakistan d Departments of Geosciences, Faculty of Science, Naruto University of Education, Naruto, Tokushima 772-8502, Japan

Received 9 December 2004; accepted 18 June 2006

Abstract The thermobaric structure of the Himalayan Metamorphic Belt (HMB) has been constructed along the Kaghan Valley transect, Pakistan. The HMB in this valley represents mainly the Lesser Himalayan Sequence (LHS) and Higher Himalayan Crystallines (HHC). Mineral parageneses of 474 samples, from an approximately, 80-km traverse from southwest to northeast, were examined. Microprobe analyses were carried out to quantify the mineral composition. To determine the pressure–temperature (P–T) conditions, 65 thin sections (7 pelites from LHS and 25 pelites, 9 maWc rocks/amphibolites and 19 eclogites from HHC) were selected. Based on Weld observations and mineral paragenesis, low-grade to high-grade metapelites, show Barrovian-type progressive metamorphic sequence, with chlorite, biotite, garnet and staurolite zones in LHS and staurolite, kyanite and sillimanite zones in HHC. By using well-calibrated geothermobarometers, P–T conditions for pelitic and maWc rocks are estimated. P–T estimates for pelitic rocks from the garnet zone indicate a condition of 534 § 17 °C at 7.6 § 1.2 kbar. P–T estimates for rocks from the staurolite and kyanite zones indicate average conditions of 526 § 17 °C at 9.4 § 1.2 kbar and 657 § 54 °C at 10 § 1.6 kbar, respectively. P–T conditions for maWc rocks (amphibolites) and eclogites from HHC are estimated as 645 § 54 °C at 10.3 § 2 kbar and 746 § 59 °C at 15.5 § 2.1 kbar, respectively. The coesite-bearing ultrahigh-pressure (UHP) eclogites record a peak P–T condition of 757–786 °C at 28.6 § 0.4 kbar and retrograde P–T conditions of 825 § 59 °C at 18.1 § 1.7 kbar. These results suggest that HMB show a gradual increase in metamorphic grade from southwest to northeast. The P–T conditions from Pelitic and adjacent maWc rocks having identical peak conditions in the same metamorphic zone, while the structural middle in HHC reached the highest P–T condition upto the UHP grade. © 2006 Elsevier Ltd. All rights reserved. Keywords: Thermobaric structure; Kaghan Valley; P–T conditions; Coesite-bearing eclogites

1. Introduction The Kaghan Valley in Pakistan is uniquely important to an understanding of the metamorphic history and tectonic evolution of the Himalayas. In this valley, a complete section of the Himalayan metamorphic belt (HMB) is exposed, ranging from feebly metamorphosed rocks to *

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ultrahigh-pressure (UHP) rocks. Regional geology (Fig. 1), stratigraphy, metamorphism, geochronology, and collision tectonics in the area have been investigated by a number of earth scientists (Ghazanfar and Chaudhry, 1986; Chaudhry and Ghazanfar, 1987; Greco et al., 1989; Treloar et al., 1989; Greco and Spencer, 1993; Tonarini et al., 1993; Spencer et al., 1995; Burg et al., 1996; O’Brien, 2001; O’Brien et al., 2001; Foster et al., 2002; Kaneko et al., 2003). Although metamorphic pressure–temperature conditions have been reported from various localities in the Kaghan

H.U. Rehman et al. / Journal of Asian Earth Sciences 29 (2007) 390–406

391

Legend

China

Granitic intrusive bodies Kohistan arc Zanskar shelf

Afghanistan

Higher Himalaya sequence Lesser Himalayan sequence Siwalik molasse

30˚N

Fig.2

Himalayan Range

Pakistan India

Nepal

25˚N

Arabian Sea

70˚E

80˚E

90˚E

Fig. 1. Regional map of the Indo-Pakistan subcontinent showing the main tectonic units. Rectangular block at upper left shows the present research area in the western HMB (modiWed after Gansser, 1964; Searle et al., 1999; Kaneko et al., 2003).

Valley (Greco, 1989; Pognante and Spencer, 1991; Smith et al., 1995; Treloar, 1997; Lombardo and Rolfo, 2000), the thermobaric structure of the HMB in this area has not been disscussed in detail. In this paper, we describe the textural features and mineral chemistry of pelitic schists/gneisses and maWc rocks, and in particular, the coesite-bearing eclogites. A comparative study of pelitic and maWc rocks in the Kaghan Valley can provide us with a better understand-

ing of the thermobaric structure of the HMB and its metamorphic evolution. 2. Geological setting The Kaghan Valley is located to the southwest of Nanga-Parbat in the northwestern region of the Himalayas, N. Pakistan (Fig. 2). This area in the Himalayan chain consists of Proterozoic to Tertiary rocks of the Indian Plate,

Fig. 2. Regional geological map of the western Himalaya, showing its main tectonic units, along with the Kohistan Island arc and Asian plate units. Block denoting Fig. 3 shows the location of the Kaghan Valley (modiWed after Searle et al., 1999; Kaneko et al., 2003). Abbreviations used for major faults are explained in the text except for MKT: Main Karakoram Thrust; ZSZ: Zanskar Shear Zone.

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village. Detailed stratigraphy of the Kaghan Valley is described in a number of papers by Ghazanfar and Chaudhry (1985), Chaudhry et al. (1997), Greco and Spencer (1993) and Spencer (1993). The main lithological unit is the Oligocene–Miocene Murree Formation of the Rawalpindi Group (Najman et al., 2001). To the north, it is in tectonic contact with LHS at MBT. MBT is a north-dipping reverse fault carrying molasse sediments of the Siwaliks and the Murree Formation on its footwall and northeast-plunging LHS schists and gneisses, on its hanging wall (Bossart et al., 1988).

with adjacent metamorphosed Tethyan sediments, suture zones, Kohistan Island Arc and Trans-Himalayan granitic batholiths (Tahirkheli, 1979; Chaudhry and Ghazanfar, 1987; Bossart et al., 1988; Greco et al., 1989; Papritz and Rey, 1989; Treloar et al., 1989; Pognante and Spencer, 1991; Greco and Spencer, 1993; Searle et al., 1999). In this complex geological setting, the lowest exposed structural lithologies are Tertiary foreland molasse sediments with no evidence of metamorphism. Tectonically these sediments occupy the core of Hazara–Kashmir syntaxis and have been called the ‘foreland’ by Wadia (1934) or the ‘Sub-Himalaya’ by Gansser (1964). This tectonic block is separated from LHS to the north by the Main Boundary Thrust (MBT). The MBT cuts through the Murree Formation of the Sub-Himalaya, and brings with it the northeast dipping LHS schists and gneisses on its hanging wall (Bossart and Ottiger, 1989). The lower part of LHS, the Panjal Unit, consists of four tectonic slices. The structural base of the Panjal Unit is formed by a tectonic mélange of Permian to Eocene rocks while, the upper three units represent the upper Paleozoic to lower Mesozoic rocks. The Panjal Unit thrusts over the Salkhala Series (Wadia, 1934) or the Salkhala Formation (Calkins et al., 1975). The rocks of the LHS are comprised of unmetamorphosed Proterozoic to Eocene sediments, low-grade metamorphic rocks of the chlorite and biotite zones (Lower greenschist facies) in the south. The Metamorphic grade increases to the garnet zone (Upper greenschist facies) to the north near the Main Central Thrust (MCT) which is also termed the ‘Mylonite zone’ (Greco et al., 1989; Chaudhry and Ghazanfar, 1990). The HHC lies to the north of MCT, showing peak metamorphism up to eclogite facies.

The LHS consists of strongly folded and sheared rocks of Precambrian to Eocene age. It is subdivided into four tectonic units with diVerent stratigraphical properties and metamorphic grades. These units form imbricated slices in a typical duplex structure with the MBT as a Xoor- and the Panjal Thrust as a roof-thrust. In the eastern limb of the Hazara-Kashmir Syntaxis, the Salkhala Formation represents the uppermost Himalayan unit. This Formation is composed mainly of low-grade metapelites (carbonaceous, graphitic and quartzitic). The most abundant lithologies in LHS are Wne-grained pelitic and psammitic schists preserving primary sedimentary structures (e.g., ripple marks, cross bedding, and current lamination). Marble bands are sporadically exposed in places ranging in thickness from a few centimeters to a few meters. Some maWc rock layers of greenschist-facies are intercalated with schists. The metamorphic index minerals change from chlorite in the southwest, to biotite, garnet, and staurolite to the northeast.

2.1. Sub-Himalaya

2.3. Higher Himalayan crystallines (HHC)

The rocks of the Sub-Himalaya are exposed in the southern most part of the Kaghan Valley near Balakot

The HHC lie in the uppermost reaches of the Kaghan Valley (Fig. 3) and are separated from the LHS to the

2.2. Lesser Himalayan sequence (LHS)

Fig. 3. Geological map of the Kaghan Valley along the Kaghan-Babusar road section showing main lithological and tectonic units with metamorphic mineral assemblage distribution in apparent Barrovian-type metamorphic sequence (modiWed after Kaneko et al., 2003). (A–D) Indicates a projection line of the cross-section shown in Fig. 12.

H.U. Rehman et al. / Journal of Asian Earth Sciences 29 (2007) 390–406

south by the MCT, and from the rocks of the Kohistan sequence by the Main Mantle Thrust (MMT) to the north. HHC are subdivided into ‘basement’ and ‘cover’ sequences (Greco et al., 1989; Greco and Spencer, 1993). In this paper, we refer to three tectonic units, Units I, II, and III, metamorphosed under diVerent P–T conditions (also see Kaneko et al., 2003) from the structural base to the top. Unit I of the HHC, previously called the ‘basement’ is composed mainly of granitoids, which were intruded into the psammitic, pelitic and calcareous metasediments. They show well-developed intrusive features, and the presence of xenoliths and homogeneous mineralogy. In most localities, the aplite and pegmatite dikes cross-cut the schistosity planes discordantly. Deformed basic dikes and schistose garnet–amphibolite sheets also occur in some localities. Unit II corresponds to the middle-lower cover of the HHC and is the major UHP unit, comprising mainly felsic gneisses, marbles and amphibolites, with locally abundant eclogite lenses or layers. Felsic gneisses are mostly coarse-grained and garnetiferrous and include the amphibolite and eclogite bodies. Amphibolites appear as lenses or thin layers from a few centimeters to 2 m in width, extending for a few tens to several hundred meters (Fig. 4a). These rocks are derived from the equivalents of Permian Panjal trap volcanics (Honegger et al., 1982; Ghazanfar et al., 1987; Greco et al., 1989; Papritz and Rey, 1989; Greco and Spencer, 1993). Eclogites are exposed in a few localities as small lenticular bodies of a few centimeters thickness (Fig. 4b), interlayered with felsic gneisses of UHP grade (Kaneko et al., 2003) and with marbles. Coesite-bearing eclogite is found to the northwest of the Gittidas village as an isolated block, less than 2 m in diameter (Fig. 4c). Unit III consists of siliceous schists, pelitic/psammitic gneisses, marbles, amphibolites, and amphibolitized eclogites. These rocks are overlain tectonically by the Kohistan sequence along the MMT, and partly by low-grade Tethyan metasediments. 2.4. Tethyan Tectonic metasediments (TTs) This unit of the low-grade metamorphic sequence is exposed locally in the Babusar pass area (Fig. 3) on top of Unit III. This metasedimentary unit lies unconformably above the HHC and occurs in greenschist facies grade as thin layers parallel to MMT. 2.5. The Indus Suture/Main Mantle Thrust A subduction boundary called the Indus Suture or Main Mantle Thrust (MMT) separates Indian plate rocks from the overlying maWc and ultramaWc rocks of the Kohistan Island arc sequence along the northern Xank of the Kaghan Valley (Fig. 2). On the footwall of this thrust lie strongly deformed, dark graphitic, staurolite–kyanite, and garnetbearing phyllitic schists. On the hanging wall sequence,

393

W

E

Marble Mafic rock layer

Eclogite-bearing part

Pelitic-felsic gneisses

Marble

a

Pelitic gneisses

Eclogite lense

Eclogite lense

b SW

NE

Coesite-bearing eclogite body

Pelitic gneisses

c Fig. 4. Field photographs taken near the Lulusar Lake, showing eclogite bodies within layered maWc rocks. The country rocks are mainly pelitic–psammitic gneisses and minor marble bands. (a) Layered type eclogite-bearing maWc rock; (b) Lenticular eclogites 30–70 cm wide and 1.5–3 m long, located northwest of Gittidas; (c) Coesite-bearing lens-like eclogite body from Unit II of HHC surrounded by pelitic gneisses.

retrograde metamorphism of maWc and ultramaWc rocks increases towards the thrust plane. 3. Petrography Mineralogical and textural characteristics of the rock samples collected along the Kaghan Valley have been examined in 474 thin sections. Abbreviations of minerals used in the text and Wgures are adopted after

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Kretz (1983) except Coesite D Cs, Phengite D Phn, Amphibole D Amp, and Barroisite D Bar. 3.1. Lesser Himalayan sequence (LHS) The LHS metapelites have a main mineral assemblage of Bt + Chl § Czo + Grt + Ms + Pl + Qtz. The foliation, as well as ductile S–C fabrics, is deWned by biotite and muscovite. Chlorite appears as Wne-grained matrix-mineral, along with epidote and quartz (Fig. 5a). Garnet contains synkinematic inclusions of quartz, epidote, and biotite. Some of the garnet porphyroblasts show rotation trails, and are altered to chlorite at the rims (Fig. 5b). MaWc rocks of the LHS show a greenschist facies mineral assemblage of Act + Bt + Chl + Ep + Ms + Pl + Qtz. Accessory minerals include ilmenite, rutile, graphite, sulWdes, and traces of zircon. 3.2. Higher Himalayan crystallines (HHC) HHC metapelites from Unit I contain mineral assemblages of Grt + Bt + Ms + Pl § Ky § Chl + Qtz. Accessory minerals include tourmaline, rutile, zircon and apatite. Garnets are euhedral and pre- to synkinematic with respect to the main schistosity. They occasionally contain inclusions of quartz, biotite, and muscovite. Snowball garnets with rotated trails of Wne-grained minerals are common (Fig. 5c). Partial replacement of garnet by green biotite and chlorite shows successive retrograde evolution (Fig. 5d). Metapelites from Unit II contain mineral assemblages of Grt + Bt + Ms + Pl + Ky § Sil § St + Qtz. Prismatic staurolite oriented parallel to the dominant foliation (Fig. 5e) is considered to be broadly syn-tectonic. In pelitic gneisses, two generations of kyanite-bearing assemblages are observed. The earlier generation are large prismatic kyanite crystals and deWning a penetrative S1 schistosity, together with biotite and muscovite (Fig. 5f). The later generation are Wne-grained kyanite, intergrown with quartz (Fig. 5g). Sillimanite occurs as small Wbers (Fig. 5h) constituting the schistosity, but some are oblique to the schistosity. Unit III is similar in mineralogy and texture to Unit II, but has a diVerent metamorphic grade. Eclogites exposed in Purbi Nar are composed mainly of fresh garnet and retrograde amphibole, with cores of omphacite, quartz, epidote, and accessory rutile (Fig. 6a). Amphibole–albite–quartz symplectite and reaction zones around omphacite are common (Fig. 6b). Eclogites from Gittidas have a mineral assemblage of Grt + Omp + Qtz/ Cs + Phn + Ep + Rt with secondary biotite, sphene, zircon and rare carbonates. Garnet in eclogites from Gittidas shows an inclusion-rich xenomorphic domain in the core, and an inclusion-poor idioblastic outer rim. Inclusions are quartz, rutile, epidote, omphacite, phengite, and ilmenite. Rare zircon is present in these eclogites. Eclogites from northwest of Gittidas contain coesite inclusions in omphacite displaying radial cracks and palisade texture (Fig. 6c).

4. Mineral chemistry Major minerals were analyzed using a JEOL JXA 8600SX electron microprobe analyzer at Kagoshima University, with 15 kV accelerating voltage, 12 nA beam current and a 2–20m probe diameter. X-ray intensities were reduced using ZAF matrix correction. 4.1. Garnet The chemical composition and end members of garnet are calculated on the basis of 16 cations and 24 oxygen atoms (Table 1) by charge balance constraints (Rickwood, 1968; Droop, 1987; Knowles, 1987; Deer et al., 1992). Compositional variation among end members for garnets from the HHC pelites and eclogites, is shown in a Ternary diagram (Fig. 7) in comparison with previous studies. Garnets from LHS pelites are Alm rich (51–77%) with a decrease in Alm content from the core to the rim, and Prppoor (3–6%) showing irregular zoning. Garnets from the HHC pelites are also Alm-rich (55–82%) and Prp-poor (12–19%). They show normal zoning, with increasing Grs and Prp and decreasing Alm and Sps toward the rim. The zoning pattern in garnets from pelitic rocks, maWc rocks and UHP eclogites from HHC is shown in Fig. 8a–e, for comparison. The elemental X-ray map (Mg and Ca) of garnet from HHC pelites show clear zonation from rim to core (Fig. 9). Garnets from eclogites show minor zonation that involves a uniform decrease in Grs and Sps and an increase in Prp indicating prograde growth zoning, similar to that reported by Massonne and O’Brien (2003). The outermost rims of garnet have compositions compatible with the highest temperature attained during garnet growth (i.e., peak garnet). 4.2. Clinopyroxene The chemical composition of omphacite, with end members, is calculated on the basis of 4 cations and 6 oxygen atoms (Table 2). All Fe as Fe2+ and Fe 3+ are calculated by charge-balanced constraint after Ryburn et al. (1976) and Droop (1987). Omphacite chemistry on Wollastonite–Enstatite–Ferrosilite (WEF), Jadeite (JD), and Aegirine (AE) triangular plot is shown in Fig. 10. Omphacite from eclogites from Purbi Nar has low- jadeite (8.1–11.9%) and aegirine (8.3–11.7%) while quadrilateral pyroxene is high (79.8–81.42%). It is mainly omphacitic and surrounded by symplectites of low jadeite–acmite content. In contrast omphacite from UHP eclogite (Gittidas area) has jadeite (30–42%), aegirine (1.2–18%), and quadrilateral pyroxene (50–57%). This variation is due to patchy zoning in the matrix grains. Some large patches with sharp boundaries consist of areas poor in Na and Fe and rich in Mg. Omphacite inclusions within garnet overlap in composition with the matrix grains.

H.U. Rehman et al. / Journal of Asian Earth Sciences 29 (2007) 390–406

395

Bt Ep/Zo Ms

Qtz

Chl Chl Grt Chl

Qtz

a

0 100

500μm

b

0

500um

100 200

Chl Cal

Bt

Qtz

Qtz

Chl

Ms Ms

Grt Grt Bt

c

0100 300 500μm

d

0100 300 500μm

Qtz

Rotated Ky Ti/Ilm

St

e

Qtz

Qtz

0 100

Bt

Opq

Ms

Rt

200μm

500

f

0

100 200μm

500

Chl Ky Accicular Ky

Bt Sil

Ms

Bt

Grt Opq

Plg

Plg

Ms St

g

0

100 200μm

500

h

Grt 0

50

100μm

Fig. 5. Photomicrographs from pelitic schists and gneisses of the HMB, showing prograde metamorphism: (a) low-grade chlorite zone; (b) sygmoidal garnet with rotation trails; (c) snow ball garnet with inclusions of quartz, muscovite and chlorite; (d) garnet replaced by biotite and chlorite due to retrogression; (e) staurolite euhederal prism having titanite/ilmenite at rim, indicating rapid cooling; (f) coarse-grained prismatic kyanite showing rotation and penetrative schistosity; (g) Wne-grained acicular kyanite, associated with garnet and plagioclase, and (h) sillimanite Wbers displaying main foliation along with staurolite, muscovite, and biotite.

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Ep/Zo

Amp

Grt

Omp

a

100

0

250μm

Fig. 7. Comparative ternary plot of garnet end-members from gneisses and eclogites, with previous work (Treloar, 1995; Lombardo and Rolfo, 2000; O’Brien et al., 2001; Kaneko et al., 2003).

symplectite

Qtz Grt

Amp

b

0

50

100

150 200μm 250

Symplectite

Ep

Amp

Grt

blende, tremolite, edenite, and barroisite (Fig. 11). In eclogites, amphibole composition (Si D 6.2–7.4, Na(B) D 0.04–0.85, Mg(C) D 2.05–2.87) represents barroisite to tschermakite and edenite. The tshermakitic and edenite components decrease from the core to the rim. The symplectic amphibole has a low glaucophane component and lower Fe/(Fe + Mg) ratio (0.32) as compared to porphyroblastic amphibole (0.42). Hornblende-plagioclase symplectites replace the garnet in eclogites (Fig. 6b) and are similar in optical continuity to the amphibole formed from omphacite. 4.4. White mica

Cs Omp

Rt Ilm

c

0

50

100

150

250μm

Fig. 6. Photomicrographs from HP and UHP eclogites of the Kaghan Valley: (a) eclogite from Purbi Nar showing omphacite cores and retrograde amphibole rims; (b) replacement of omphacite to symplectitic augite, amphibole and quartz. Garnet at lower right is also replaced by hornblende-plagioclase symplectites; (c) coesite inclusion in omphacite showing radial cracks and palisade texture.

4.3. Amphibole The chemical composition of amphibole is shown in Table 3, on the basis of 15 cations and 23 oxygen atoms. Amphibole in low-grade maWc rocks is mainly Tr-Act. These are SiO2-rich (>47%) and Al2O3-poor (<8%). Amphibole from HHC maWc rocks displays a wide range of compositions (Si D 6.32–7.44, Na(B) D 0.02–0.20, Mg(C) D 2.03–3.0) that correspond to magnesiohorn-

White mica is mainly muscovite in low-grade pelitic schists, whereas it is phengitic in high-grade gneisses and in eclogites. Phengite also occurs as inclusions in garnet and kyanite in HHC pelites. Phengitic mica from gneisses surrounding the eclogites is characterized by a lower celadonite component and a lower Fe/(Fe + Mg) ratio than that in the eclogites. Si content in phengite from gneisses ranges from 6.13–6.60 while from eclogites the range is 6.49–6.90. The chemical composition of white micas is shown in Table 4. 4.5. Feldspar Feldspar is fairly abundant in pelitic and maWc rocks of the Kaghan Valley. Plagioclase from LHS is mainly albite (85–88%) with minor orthoclase (0.2–0.4%) while from HHC pelites, it is 56–63% albite and <0.5% orthoclase component. In eclogites, plagioclase occurs as symplectic intergrowths after omphacite. It contains an anorthite component, ranging from 0.04% to 0.23%. The chemical composition and end members content of Pl are shown in Table 4.

H.U. Rehman et al. / Journal of Asian Earth Sciences 29 (2007) 390–406

397

Table 1 Chemical composition of Garnet (Grt) from selected metapelites and eclogites LHS pelites

HHC pelites

HP–UHP eclogites

Sample:

Ph66-191 Ph114-228 Ph160-237 Ph160-240 Ph181-26 Ph181-27 Ph350-7 Ph312-24 Ph312-25 Ph407-101 Ph423-6 Ph423-8 Ph425-14 425-16

Mineral:

Grt

Grt core

Grt rim

Grt rim

Grt core Grt

SiO2 TiO2 Al2O3 Cr2O3 FeO MnO MgO CaO

36.48 0.13 21.34 0.02 27.07 9.69 0.89 5.15

38.01 0.07 21.41 bdl 29.02 3.68 1.26 7.42

37.80 0.11 20.63 0.07 34.00 1.02 0.78 6.63

37.96 bdl 21.35 bdl 35.40 0.35 1.45 5.46

39.28 bdl 22.17 0.03 26.41 0.14 4.99 8.81

39.20 0.23 21.82 bdl 27.14 0.14 3.25 10.05

38.44 0.04 22.91 bdl 30.21 1.06 3.29 3.54

39.40 0.08 21.21 29.54 0.20 0.50 4.43 7.21

38.78 0.09 21.03 29.68 0.23 0.43 4.06 7.85

38.80 0.01 21.23 25.52 0.03 0.44 5.35 9.71

40.26 0.08 22.63 21.90 0.01 0.18 8.45 8.77

39.94 0.05 22.13 21.07 bdl 0.17 8.87 9.24

39.66 0.00 22.04 21.39 0.22 0.55 7.94 10.10

39.45 0.06 21.66 22.34 0.19 0.44 7.45 9.97

Total

100.77

100.86

101.03

101.95

101.83

101.84

99.49

102.57

102.13

101.09

102.28

101.48

101.90

101.56

TSi TAl AlVI Fe3 Ti Fe2 Mg Mn Ca FeO calc Fe2O3 calc Alm And Grs Prp Sps Uvr

5.871 0.129 3.916 0.182 0.016 3.462 0.214 1.320 0.888 25.72 1.50 51.9 5.4 12.1 4.2 26.2 0.1

Grt

6.051 0 4.013 0.193 0.008 3.670 0.299 0.496 1.265 27.57 1.61 64.1 5.1 17.0 5.2 8.7 0

6.058 0 3.894 0.228 0.014 4.329 0.185 0.138 1.139 32.30 1.89 74.8 5.9 13.5 3.2 2.4 0.2

6.010 0 3.980 0.234 0 4.452 0.341 0.046 0.926 33.63 1.97 77.2 6.1 10.0 5.9 0.8 0

6.015 0 3.998 0.169 0 3.213 1.139 0.018 1.445 25.09 1.47 55.3 4.4 20.4 19.6 0.3 0.1

6.054 0 3.969 0.175 0.027 3.330 0.748 0.019 1.664 25.79 1.51 57.8 4.6 24.3 13.0 0.3 0

6.132 0 4.303 0.201 0.005 3.828 0.783 0.143 0.606 28.70 1.68 71.4 5.6 5.7 14.6 2.7 0

Grt core Grt rim

6.053 0 3.838 0.190 0.010 3.607 1.015 0.064 1.187 28.07 1.64 61.4 4.8 14.8 17.3 1.1 0.6

5.993 0.007 3.820 0.192 0.011 3.644 0.935 0.056 1.299 28.19 1.65 61.4 4.8 16.3 15.8 0.9 0.7

Grt

5.974 0.026 3.824 0.164 0.002 3.122 1.227 0.057 1.601 24.25 1.42 42.1 4.9 27.1 24.6 1.2 0.1

Grt rim Grt core Grt rim

5.994 0.006 3.963 0.136 0.009 2.591 1.875 0.023 1.400 20.81 1.22 44.0 3.5 20.3 31.8 0.4 0

5.974 0.026 3.872 0.132 0.006 2.504 1.978 0.022 1.481 20.02 1.17 33.0 3.8 24.7 38.1 0.4 0

5.940 0.060 3.827 0.134 0 2.545 1.772 0.070 1.621 20.32 1.19 33.5 3.9 26.5 34.0 1.3 0.7

Grt core

5.955 0.045 3.804 0.141 0.007 2.679 1.676 0.056 1.612 21.22 1.24 34.8 4.1 26.6 32.7 1.1 0.7

Cations and end members are calculated on the basis of 16 cations and 24 oxygen atoms (after Droop, 1987; Knowles, 1987). bdl means below detection limit.

4.6. Other minerals Other major minerals of importance for use in geothermobarometry include chlorite, with a Fe/(Fe + Mg) ratio ranging from 0.51–0.59, biotite, Fe/(Fe + Mg) ratio ranging from 0.38–0.57 and epidote/zoisite Fe/(Fe + Al3+) ratio ranging from 0.002–0.1. The chemical compositions, with calculated cations of representative samples are shown in Table 4. 5. Geothermobarometry In order to obtain quantitative constraints on the pressure–temperature evolution of the Himalayan Metamorphic Belt (HMB) in the Kaghan Valley, the mineral chemistry of 7 samples from LHS pelites, 25 samples from HHC pelites, 9 samples from HHC maWc rocks, and 19 samples from eclogites, have been utilized. Samples characterized by textures suggesting peak equilibrium conditions were selected for geothermobarometry. Several well-calibrated geothermobarometers were applied to the garnet-bearing assemblages from pelitic rocks of the Kaghan Valley section. Garnet–biotite (Berman, 1990) and garnet–phengite (Krogh and Raheim, 1978) Fe–Mg exchange thermometers and garnet–plagioclase– muscovite–biotite and garnet–plagioclase–muscovite–

quartz geobarometers (Hodges and Crowley, 1985; Powell and Holland, 1988; Hoisch, 1991) were used to calculate average P–T conditions by using THERMOCALC ver. 3.2 with its internally consistent data set (Powell and Holland, 1994; Holland and Powell, 1998). Uncertainties and standard deviation for pressures and temperatures were calculated to the minimum error. The signiWcance Wtness was kept to a conWdence level of 95% or more. Mineral compositions were processed with Holland T.J.B. Win Ax software to calculate the end-member properties. These properties were used in THERMOCALC for further P–T calculations. End-members with large uncertainties were removed when calculating the reactions. From maWc rocks and eclogites intercalated within pelitic rocks, representative samples were used for calculations with garnet–clinopyroxene (Krogh, 1988) and garnet–phengite (Green and Helman, 1982) Fe–Mg exchange geothermometers. Mineral compositions were selected from locations where the relevant mineral phases shared sharp contacts. For geothermobarometry with the rim composition of minerals, compositions were measured along the contacts. The results of geothermobarometry from pelitic rocks and eclogites are summarized in Figs. 12 and 13, and Tables 5 and 6. All P–T estimates for the HHC plot within the kyanite P–T stability Weld, in good agreement with the presence of kyanite in pelitic rocks. These results from pelitic rocks

398

H.U. Rehman et al. / Journal of Asian Earth Sciences 29 (2007) 390–406 Grt end members plot from mafic rocks of HHC (Sample Ph-236)

a

End-member (%)

80 Alm

60

Grs

40

Prp Sps

20 0

0

125

End-member (%)

80 Grs

40

Prp

0

25

50 75 Distance (μm)

100

120

Grt end members plot from HHC Pelites (Sample Ph-366) 80

40 20 0

Alm

End-member (%)

60

0

Grs Prp Sps

0.2

0.4

0.6 0.8 1.0 1.2 Distance (mm)

1.4

1.6

Grt end members plot from UHP eclogites (Sample Ph-423)

d

5.1. Metamorphic conditions

Sps

20

c End-member (%)

Alm

60

0

End-member (%)

500

Grt end members plot from eclogites (Sample Ph-280)

b

80 Alm

60

Grs

40

Prp

Regional metamorphism has been constrained in much detail from various parts of western Himalaya (e.g. Swat nappe); kyanite-bearing rocks were metamorphosed at 625 § 50 °C and 9 § 2 kbar (DiPietro and Lawrence, 1991). For Hazara nappe, staurolite-grade rocks were metamorphosed at 480–620 °C and 5–10 kbar, and kyanite- and sillimanite-bearing rocks at 600–740 °C and 7– 12 kbar (Treloar, 1997). For the immediate footwall of MMT in the northernmost part of the Kaghan Valley, greenschist-facies rocks were estimated at 500 § 50 °C at 8 § 1 kbar (Chamberlain et al., 1991). P–T estimates for eclogites by Pognante and Spencer (1991) were 650 § 50 °C and 14.5 § 2.5 kbar, far lower than the original peak P–T conditions of eclogites in the Kaghan Valley as these rocks reached the coesite stability Weld (e.g. >27 kbar and >720 °C). Our estimated P–T conditions from the pelitic rocks of the garnet zone (493–574 °C, 6.4– 8.3 kbar), staurolite zone (520–560 °C, 8.6–10.9 kbar), and kyanite zone (610–740 °C, 6.1–12.8 kbar) show a gradual increase in metamorphic P–T conditions toward the

Sps

20 0

Mg 0

e End-member (%)

250 325 Distance (μm)

show Barrovian-type metamorphism in the Kaghan Valley. The P–T estimate provided by samples collected at a similar structural level of the HHC in pelites and in maWc rocks from the same unit yielded similar results. In this study, the estimated P–T data from pelitic rocks from the garnet zone (493–574 °C, 6.4–8.3 kbar), the staurolite zone (520–560 °C, 8.6–10.9 kbar), and the kyanite zone (610–740 °C, 6.1–12.8 kbar) show a gradual increase in metamorphism. This P–T data is consistent with those calculated by Treloar (1997). P–T data calculated from the LHS maWc rocks show 590 °C, 7.7 kbar, while HHC maWc rocks were estimated at 553–712 °C, 8.3–12.3 kbar. This P–T data from maWc rocks is consistent with data from adjacent pelitic rocks, showing a gradual increase in metamorphic grade in this section (Fig. 12).

100

200 300 Distance (μm)

400

Grt end members plot from UHP eclogites (Sample Ph-425) 80 Alm

60

Grs

40

Prp Sps

20 0

0

100

200 300 Distance (μm)

400

Fig. 8. Garnet end members plot from pelitic rocks, maWc rocks, and eclogites showing weak zonation pattern.

Mg

Ca

500μm Ca

Fig. 9. Garnet X-ray map from HHC pelites showing normal zonation for Mg and Ca.

H.U. Rehman et al. / Journal of Asian Earth Sciences 29 (2007) 390–406

399

Table 2 Chemical composition of Omphacite (Omp) from selected samples of HP–UHP eclogites Sample:

Ph285-51

Mineral: Omp

Ph285-53 Ph312-9 Ph312-27 Ph312-34 Ph407-104 Ph423-2 Ph423-11 Ph423-13 Ph423-14 Ph423-60

Ph425-13 Ph425-36

Omp

Omp

Omp

Omp

Omp

Omp

Omp

Omp

Omp

Omp

Omp

Omp

SiO2 TiO2 Al2O3 FeO Cr2O3 MgO CaO Na2O

56.64 bdl 8.60 6.84 bdl 7.85 15.77 4.59

51.56 0.22 6.22 10.95 bdl 8.99 17.59 3.73

52.31 0.09 3.34 10.89 0.22 10.19 19.80 2.66

52.63 0.06 3.53 10.57 0.25 10.42 20.21 2.51

52.78 0.09 3.02 10.44 0.18 10.42 18.56 2.73

53.82 0.14 8.79 8.57 bdl 7.94 13.86 6.34

55.07 0.14 12.18 7.76 0.01 7.87 12.74 6.56

55.77 0.13 9.91 7.43 0.04 8.32 13.05 6.61

54.97 0.15 10.08 8.07 0.01 7.87 12.29 6.12

55.55 0.08 9.43 8.09 bdl 7.89 12.79 6.88

54.58 0.12 9.94 8.08 0.01 7.85 12.54 6.93

54.83 0.21 9.86 7.26 0.22 7.58 13.61 6.26

54.76 0.09 9.86 7.76 0.18 7.56 12.93 6.62

Total

100.29

99.26

99.48

100.18

98.22

99.46

102.32

101.26

99.57

100.70

100.04

99.83

99.76

1.914 0.086 0.186 0.006 0.155 0.155 0 0.497 0.030 0.002 0.700 0.268 45.5 32.3 22.2 72.1 15.2 12.7

1.948 0.052 0.095 0.003 0.137 0.184 0.006 0.566 0.018 0 0.790 0.192 46.6 33.4 20.0 80.2 8.1 11.7

1.986 0.014 0.119 0.003 0.083 0.194 0.005 0.584 0.051 0.001 0.748 0.199 45.0 35.2 19.8 79.8 11.9 8.3

1.949 0.051 0.323 0.004 0.165 0.080 0 0.428 0.015 0.002 0.538 0.445 43.8 34.9 21.3 54.4 30.2 15.4

1.974 0.026 0.392 0.006 0.053 0.128 0.006 0.407 0.038 0 0.525 0.437 45.6 35.4 19.0 55.7 39.0 5.3

1.967 0.033 0.384 0.002 0.099 0.095 0.005 0.405 0.040 0.002 0.498 0.461 43.8 35.6 20.7 53.0 37.4 9.6

TSi TAl M1Al M1Ti M1Fe3 M1Fe2 M1Cr M1Mg M2Fe2 M2Mn M2Ca M2Na Wo En Fs WEF Jd Ae

2.059 0 0.368 0 0.000 0.205 0 0.425 0.003 0 0.614 0.323 49.2 34.1 16.7 65.9 34.1 0.0

1.944 0.056 0.098 0.002 0.127 0.181 0.007 0.574 0.018 0.002 0.800 0.180 47.0 33.7 19.3 81.4 8.1 10.5

1.929 0.071 0.431 0.004 0.077 0.074 0 0.411 0.077 0 0.478 0.445 42.8 36.8 20.4 53.9 39.2 7.0

1.974 0.026 0.387 0.003 0.083 0.085 0.001 0.439 0.052 0 0.495 0.453 42.9 38.1 19.0 54.1 37.7 8.1

1.989 0.011 0.419 0.004 0.012 0.138 0 0.425 0.094 0 0.476 0.429 41.6 37.1 21.4 56.9 41.9 1.2

1.978 0.022 0.374 0.002 0.118 0.086 0 0.419 0.037 0 0.488 0.475 42.5 36.5 21.0 52.0 36.5 11.5

1.953 0.047 0.372 0.003 0.150 0.056 0 0.419 0.036 0.001 0.481 0.481 42.1 36.6 21.3 50.8 35.1 14.1

Cations and end members are calculated on the basis of 4 cations and 6 oxygen atoms. bdl means below detection limit. WEF

Lombardo and Rolfo 2000; O`Brien et al. 2001

1.0

Bar

Win

Eclogite Ph-423

Kaneko et al. 2003

LHS Amph Ph-37

Na(M4)

Eclogites this study Ph-285 Ph-312 Ph-423 (UHP)

Ph-312 Ph-285

TTS Amph Py-10

0.5

0

Ts/Prg

Hbl/Ed

Tr

0

0.5

1.0 Al(T)

1.5

2.0

Fig. 11. Compositional variation of amphibole from eclogites and maWc rocks of LHS and TTs metasediments. Bar: Barroisite; win: winchite, and others after Kretz (1983). JD 70

AE 70

Fig. 10. Clinopyroxene composition plot of eclogites on ternary diagram comparing with previous workers (Lombardo and Rolfo, 2000; O’Brien et al., 2001; Kaneko et al., 2003).

tern to those of the pelitic rocks, showing a gradual increase through the mineral zones. 5.2. P–T–time path

northeast, and are consistent with those calculated by Treloar (1997). The P–T conditions estimated from LHS maWc rocks showed slightly higher temperature ranges (»590 °C) than those of LHS pelitic rocks while HHC maWc rocks were estimated at 553–712 °C, 8.3–12.3 kbar. The P–T conditions of maWc rocks show a consistent pat-

A metamorphic pressure–temperature–time path in the Kaghan Valley can be reconstructed, based on petrographic observations and geothermobarometric data from both pelitic rocks and eclogites. The growth zonation in garnet from metapelites shows an increase in pressure from

400

H.U. Rehman et al. / Journal of Asian Earth Sciences 29 (2007) 390–406

Table 3 Chemical composition of Amphiboles (Amp) from selected maWc rocks and eclogites LHS maWc rocks Sample:

Ph37-37

HHC maWc rocks

HP–UHP eclogites

Ph84-89

Ph155-57

Ph166-251

Ph236-87

Ph280-61

Ph285-113

Ph312-29

Ph407-8

Ph421-13

Ph423-4

Ph425-2

Mineral:

Amp

Amp

Amp

Amp

Amp

Amp

Amp

Amp

Amp

Amp

Amp

Amp

SiO2 TiO2 Al2O3 FeO Cr2O3 MnO MgO CaO Na2O K 2O

48.86 0.24 7.23 19.75 bdl 0.17 9.94 11.54 1.19 0.18

41.31 0.39 15.61 19.02 bdl 0.32 7.01 10.76 1.85 0.64

43.57 0.52 16.33 15.00 0.01 0.01 9.80 11.19 1.57 0.48

43.55 0.44 14.61 16.10 0.02 0.13 9.93 10.42 2.20 0.26

45.14 0.92 13.46 14.74 0.02 0.04 10.91 11.64 1.27 0.55

42.76 0.93 11.48 17.94 0.01 0.05 10.78 11.00 1.77 1.33

51.48 0.14 5.17 11.17 0.03 0.01 9.60 20.14 2.51 bdl

42.27 1.40 12.23 17.01 0.22 0.06 9.52 10.88 3.01 0.24

42.22 1.44 13.14 14.83 0.18 0.02 10.42 11.94 1.91 0.15

47.86 0.27 13.04 12.90 0.04 0.11 11.86 6.91 6.17 0.62

45.09 0.16 15.19 13.96 0.01 0.05 10.94 8.99 3.81 0.60

46.83 0.22 11.79 11.37 0.28 0.08 13.18 8.37 3.53 0.88

Total

99.08

96.91

98.48

97.65

98.69

98.05

100.25

96.85

96.26

99.79

98.78

96.52

TSi TAl CAl CCr CTi CMg CFe2 CMn CCa BFe2 BMn BCa BNa ACa ANa AK Fe_FeMg Sum_cat

7.206 0.794 0.461 0 0.026 2.184 2.328 0 0 0.108 0.021 1.823 0.048 0 0.291 0.034 0.53 15.33

6.285 1.715 1.082 0 0.045 1.591 2.282 0 0 0.138 0.041 1.753 0.068 0 0.478 0.124 0.60 15.60

6.350 1.650 1.153 0.001 0.056 2.131 1.659 0 0 0.170 0.001 1.748 0.080 0 0.364 0.090 0.46 15.45

6.443 1.557 0.989 0.002 0.049 2.190 1.770 0 0 0.222 0.016 1.651 0.110 0 0.520 0.050 0.48 15.57

6.565 1.435 0.870 0.002 0.100 2.365 1.663 0 0 0.129 0.005 1.814 0.052 0 0.305 0.103 0.43 15.41

6.440 1.560 0.476 0.002 0.106 2.422 1.996 0 0 0.264 0.006 1.730 0 0.046 0.517 0.256 0.48 15.82

6.408 1.592 0.590 0.027 0.160 2.150 2.073 0 0 0.084 0.008 1.767 0.141 0 0.744 0.046 0.50 15.79

6.357 1.643 0.687 0.022 0.164 2.339 1.789 0 0 0.079 0.002 1.919 0 0.006 0.558 0.030 0.44 15.59

6.813 1.187 0.999 0.005 0.029 2.517 1.450 0 0 0.085 0.013 1.053 0.848 0 0.856 0.112 0.38 15.97

6.522 1.478 1.110 0.001 0.017 2.358 1.514 0 0 0.174 0.006 1.393 0.427 0 0.642 0.111 0.42 15.75

6.846 1.154 0.876 0.033 0.024 2.872 1.196 0 0 0.194 0.010 1.310 0.486 0 0.514 0.164 0.33 15.68

7.400 0.600 0.275 0.003 0.015 2.058 1.342 0.001 1.306 0 0 1.795 0.205 0 0.495 0 0.39 15.50

Cations and end members are calculated on the basis of 15 cations and 23 oxygen atoms using MinPet geological software. bdl means below detection limit.

core to rim (8.6 § 1.1 to 10.9 § 1.4 kbar), during prograde metamorphism. The same prograde results were obtained from the core to rim data from the sample PH181 (9.9 § 1.2 to 12.8 § 1.7 kbar). Garnets from eclogites of the Purbi Nar area, gave core to rim pressure conditions of 16.6 § 2.5 to 19.8 § 2.6 kbar (Table 6). The UHP eclogites from the Gittidas area showed peak metamorphic P–T conditions of 27–32 kbar and 727–799 °C (O’Brien et al., 2001; Kaneko et al., 2003). Based upon core to rim compositions in garnet, the rare zonation indicates an insigniWcant change in pressure, giving same estimate of 28.6 § 0.4 kbar. Meanwhile, retrograde phases in UHP eclogites involving Grt + Ep § Amp § albite § jadeite symplectite had a pressure of 18.1 § 1.7 kbar. Geochronological studies provide a record of the cooling histories and of metamorphic episodes in the area. Ar– Ar cooling age for hornblende and mica from gneisses record ca. 43 and ca. 25 Ma (Chamberlain et al., 1991). This cooling history suggests a rapid unrooWng of the HHC rocks. The Sm/Nd (garnet–omphacite), Rb/Sr (phengite), and U/Pb (rutile) methods revealed that the eclogite facies event took place at 49 § 6 Ma (Tonarini et al., 1993).

The Peak UHP event in the Kaghan Valley took place at 46 Ma using U/Pb method on zircon containing coesite inclusions, from felsic/pelitic gneisses surrounding the coesite-bearing eclogites (Kaneko et al., 2003). 5.3. Thermobaric structure The presence of UHP metamorphic rocks in HHC proves that Indian plate rocks were subducted deeply in the collision with the Kohistan arc. Tectonic models, collision histories, subduction rates and angles, and exhumation processes have been proposed by a number of earth scientists (e.g., Molnar and Tapponnier, 1977; England and Thompson, 1984; Rowley, 1996; Treloar, 1997; Searle et al., 1999; O’Brien et al., 2001; Guillot et al., 2003, 2004; Kaneko et al., 2003; Treloar et al., 2003; Leech et al., 2005 and references there in). P–T conditions using various mineral assemblages from pelitic and maWc rocks of diVerent metamorphic grades and zones have also been estimated by many workers (e.g., Greco, 1989; Pognante and Spencer, 1991; Smith et al., 1995; Treloar, 1997; Lombardo and Rolfo, 2000; O’Brien et al., 2001; Kaneko et al., 2003; Treloar et al., 2003) but a comprehensive thermobaric structure has not been presented to date.

H.U. Rehman et al. / Journal of Asian Earth Sciences 29 (2007) 390–406

401

Table 4 Chemical composition of Chlorite (Chl), Biotite (Bt), Muscovite (Ms), Epidote (Ep) and Plagioclase (Pl). Cations for Chl, Bt, Ms, Ep and Pl are calculated on the basis of 14, 22, 22, 12.5, and 8 oxygen atoms, respectively Sample:

Ph37-38 Ph114-2 Ph161-13 Ph114-4 Ph155-55 Ph366-17 Ph66-195 Ph366-12 Ph423-16 Ph382-4 Ph423-5 Ph181-23 Ph236-85

Mineral:

Chl

Chl

Chl

Bt

Bt

Bt

Ms

Ms

Ms

Ep

Ep

Pl

Pl

SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2 O

25.66 0.07 20.81 30.53 0.19 11.80 0.04 0.01 bdl

25.81 0.34 21.89 26.79 0.20 13.04 bdl bdl 0.83

24.74 0.09 22.72 26.71 0.09 14.57 0.05 0.03 bdl

36.18 2.09 18.51 21.29 0.10 10.59 0.05 0.04 9.30

39.07 1.57 18.38 15.01 0.01 13.69 0.01 0.11 9.08

37.08 1.39 19.82 17.51 0.04 12.77 0.05 0.27 7.78

46.15 0.16 34.89 1.83 0.03 0.74 0.02 1.39 9.74

49.36 1.36 35.93 0.02 0.02 0.04 0.04 1.25 10.59

53.15 0.52 25.87 2.95 bdl 4.46 bdl 0.49 11.02

38.79 0.16 24.76 11.95 0.09 bdl 23.40 bdl bdl

38.97 0.13 26.75 8.85 bdl 0.15 23.41 bdl bdl

58.99 bdl 26.20 0.11 0.02 0.01 8.03 6.64 0.08

59.65 bdl 25.89 bdl bdl bdl 7.63 7.25 0.04

Total

89.11

88.91

88.99

98.14

96.92

96.71

94.96

98.61

98.46

99.14

98.26

100.08

100.46

Si AlIV AlVI Ti Fe2 Mn Mg Ca Na K Fe_FeMg

4.247 3.753 0.303 0.009 4.225 0.027 2.912 0.007 0.003 0 0.59

4.212 3.788 0.420 0.042 3.657 0.028 3.173 0 0 0.172 0.54

4.019 3.981 0.367 0.011 3.630 0.012 3.529 0.009 0.010 0 0.51

5.622 2.378 1.010 0.244 2.767 0.013 2.453 0.008 0.012 1.844 0.53

5.917 2.083 1.195 0.178 1.901 0.002 3.092 0.001 0.031 1.754 0.38

5.672 2.328 1.242 0.159 2.241 0.005 2.912 0.009 0.079 1.519 0.57

6.171 1.829 3.666 0.016 0.204 0.007 0.147 0.003 0.361 1.662 0.58

6.209 1.791 3.532 0.128 0.127 0 0.005 0 1.700 13.992 0.60

6.901 1.099 2.856 0.051 0.320 0 0.863 0 0.123 1.825 0.27

3.412 0 2.565 0.879 0.006 0 2.205 0 0 0.010

3.313 0 2.678 0.019 0.629 0 2.132 0.001 0 0.009

5.253 2.747 0.008 0.002 0.001 0.766 1.146 0.009 59.7 39.9 0.5

5.294 2.706 0 0 0 0.726 1.248 0.005 63.1 36.7 0.3

Si Al Fe2 Mn Mg Ca Na K Ab An Or

bdl means below detection limit. Cations and end member names given to the right of the table are for Pl analysis only.

o

T ( C)

800 700 600 500

P (kbar)

400 30

20

10

0

Fig. 12. Comprehensive thermobaric structural proWle along the Kaghan valley section. P–T estimates from selected samples from pelitic rocks, maWc rocks and eclogites are shown. Red Wlled circles show P–T estimates from pelitic rocks while blue Wlled circles indicate eclogites and other maWc rocks using garnet rim composition; half shaded and open circles show P–T estimates using garnet core composition. The core to rim trend is indicated by small arrow heads. Green circle represents TTs metasediments. Cross-section at the top is constructed based on projections of the structures from Fig. 3 (modiWed after Kaneko et al., 2003). Position of projection line (A–D) is shown in Fig. 3.

Taking advantage of Weld, petrological and structural data available for the HMB, an attempt has been made for the Wrst time, to construct the thermobaric structure repre-

senting the whole Kaghan Valley section (Fig. 12). The estimated average P–T conditions are plotted in a proWle along the geological section of the Kaghan Valley (Fig. 12).

H.U. Rehman et al. / Journal of Asian Earth Sciences 29 (2007) 390–406

35

Previous study Kaneko et al. 2003

-1

40

5 Co km

402

Treloar, 1995 This study Gittidas eclogites

30

DEC

Dia p Gr

Purbi Nar eclogites

25

Cs Qtz

ZEC

Pressure (kbar)

LEC 20

AEC BS

15

Qtz Jd+ b A

HGR

Ky Sil

10 EA -1

GS 20o C km

5

Am

Ky And

0 300

400

LGR

Sil And 500

600

700

800

o

Temperature ( C ) Fig. 13. P–T condition of UHP eclogites (Gittidas) and Purbi Nar eclogites. Retrograde path in UHP eclogites is shown with a red (solid and dashed) line; for Purbi Nar eclogites are shown with a (dotted) line with arrow heads. The grey (solid and short dashed) line shows the path from Kaneko et al. (2003). Solid star and circles show the average P–T intersection points for UHP eclogites and Purbi Nar eclogites, respectively. The P–T conditions for the Kaghan gneisses (Treloar, 1995; Kaneko et al., 2003) are shown for comparison. Metamorphic facies boundaries and reaction curves are modiWed from Holdaway (1971), Bundy (1980), Holland (1980), Bohlen and Boettcher (1982), Maruyama et al. (1996), Oh and Liou (1998). Abbreviations: AM, amphibolite facies; BS, blueschist facies; EA, epidote-amphibolite facies; GS, greenschist facies; HGR, high-pressure granulite subfacies; LGR, low-pressure granulite subfacies; AEC, amphibole–eclogite subfacies; DEC, dry-eclogite subfacies; LEC, lawsonite–eclogite subfacies; ZEC, zoisite–eclogite subfacies.

The P–T estimates from pelitic rocks as well as maWc rocks indicate an increase in metamorphic grade towards higher structural levels. These results are consistent with the structure and tectonics of the area. Grt, Ms, Chl, Bt, and Pl compositions from the garnet zone in pelitic rocks south of MCT, yield temperature and pressure ranges of 493–574 °C and 6.4–8.3 kbar, showing a gradual increase in temperature and pressure towards the northeast. In St zone, P–T estimates from the garnet core and rim were 522 § 14 °C at 8.6 § 1.1 kbar and 520 § 17 °C at 10.9 § 1.4 kbar, respectively. A local increase in the temperature shown by LHS rocks near the MCT indicate prograde metamorphism, while retrograde metamorphosed HHC rocks close to this tectonic contact gives average P–T estimates of 553 § 25 °C at 10.1 § 1.2 kbar. The kyanite zone yielded higher P–T conditions of 692 § 89 °C at 9.3 § 1.6 kbar. In the kyanite zone with secondary St, the average P–T conditions were 664 § 16 °C at 7.2 § 0.9 kbar (Table 5). Eclogites from Purbi Nar indicate lower P–T conditions than those from Gittidas localities close to the UHP

dome (Fig. 4). The coesite-bearing lithology yields a temperature range of 757–786 °C at 28.6–28-8 kbar using the garnet–clinopyroxene–kyanite–phengite–coesite/quartz geothermobarometer (Krogh and Terry, 2004). The retrograde phases (symplectic amphibole–augite–albite) in these UHP eclogites yielded P–T values of 18.1 § 1.7 and 825 § 59 °C. Based on these results, it can be concluded that the P–T conditions for pelitic and maWc rocks from the same tectonic unit in HMB are identical in each tectonic unit (LHS and Units I–III of the HHC). The overall metamorphic temperature shows a maximum in the central part of HHC with minor breaks along MCT in the south and along greenschist facies rocks (TTs) in the north. The peak pressure conditions within the HHC indicate that Unit II underwent subduction at a deep level while Units I and Unit III were probably situated at a shallower level. The path of subduction and exhumation is well constrained by the coesite inclusions in zircon from the felsic and pelitic gneisses of the HHC Unit II (Kaneko et al., 2003) and the

H.U. Rehman et al. / Journal of Asian Earth Sciences 29 (2007) 390–406

403

Table 5 Estimated average P–T conditions from garnet-, staurolite-, and kyanite zones of the pelitic rocks from the Kaghan Valley Sample No.

Mineral assemblage

Rim/core

LHS pelites Garnet zone Ph66 Ph73 Ph84 Ph114

Ave T (°C)

Ave P (kbar)

Grt + Bt + Ms + Pl + Chl Grt + Bt + Ms + Pl + Chl Grt + Bt + Ms + Pl §Chl Grt + Bt + Ms + Pl §Chl

Rim Rim Rim Rim

506 § 20 493 § 10 574 § 15 566 § 22

6.4 § 1.9 7.4 § 0.7 8.2 § 0.8 8.3 § 1.4

Staurolite zone Ph160R Ph160C Ph161

Grt + Bt + Ms + Pl §Chl Grt + Bt + Ms + Pl §Chl Grt + Bt + Ms + Pl §Chl

Rim Core Rim

520 § 17 522 § 14 538 § 21

10.9 § 1.4 8.6 § 1.1 8.6 § 1.2

HHC Pelites Kyanite zone Ph178 Ph181R Ph181C Ph183 Ph212 Ph232 Ph350 Ph366

Grt + Bt + Ms + Pl + Ky Grt + Bt + Ms + Pl §Chl Grt + Bt + Ms + Pl §Chl Grt + Bt + Ms + Pl + Ky § Ep Grt + Bt + Ms + Pl §Chl Grt + Bt + Ms + Pl §Ep Grt + Bt + Ms + Pl Grt + Bt + Ms + Pl + St § Chl

Rim Rim Core Rim Rim Rim Rim Rim

692 § 89 722 § 53 610 § 43 669 § 29 645 § 175 585 § 24 606 § 47 664 § 16

9.3 § 1.6 12.8 § 1.7 9.9 § 1.2 10.5 § 1.0 6.2 § 2.0 6.1 § 1.3 11.5 § 2.4 7.2 § 0.9

Average P–T conditions are calculated using Grt rim and core. Composition of all mineral phases utilized for P–T calculations are treated with Win Ax software of Holland T.J.B. and obtained end members properties were used in THERMOCALC ver.3.2. End-members with greater uncertainties are removed from calculations. P–T data yielded the best Wt conditions of 95% conWdence level or closer are presented here. Table 6 Estimated average P–T conditions from maWc rocks and eclogites of the Kaghan Valley. Sample No.

Mineral assemblage

Rim/core

LHS maWc rocks Ph37 Ph155

Bt + Pl + Amp + Ep + Chl Grt + Bt + Pl + Amp

HHC maWc rocks Ph166 Ph209 Ph236 Ph280 HP Eclogites Ph285 Ph312 Ph312 Ph382 UHP Eclogites Ph407 Ph421 Ph423 Ph423 Ph423 Ph425 Ph447 Tethyan metasediments Py 10

Ave T (°C)

Ave P (kbar)

– Rim

593 § 61 591 § 18

3.3 § 1.7 7.7 § 0.8

Grt + Bt + Pl + Amp + Ep § Chl Grt + Ms + Bt + Pl + Amp Grt + Bt + Ms + Pl + Amp Grt + Bt + Ms + Amp + Ep

Rim Rim Rim Rim

553 § 25 550 § 57 673 § 55 712 § 49

10.1 § 1.2 12.3 § 2.2 8.3 § 1.3 10.4 § 2.5

Grt + Cpx + Pl + Amp + Ep Grt + Cpx + Phn + Amp Grt + Cpx + Phn + Amp Grt + Bt + Pl + Amp + Ep

Rim Rim Core Rim

843 § 57 784 § 61 785 § 73 646 § 45

13.7 § 1.7 19.8 § 2.6 16.6 § 2.5 12 § 1.5

Grt + Cpx + Phn + Ky + Qtz Grt + Cpx + Ms + Amp + Ky Grt + Cpx + Phn + Ky + Ep + Cs + Amp Grt + Cpx + Phn + Amp + Ep + Cs Grt + Cpx + Phn + Amp + Ep § Pl § Bt Grt + Cpx + Phn + Amp + Cs Grt + Cpx + Amp + Ep

Rim Rim Rim Core Ret Rim Rim

810 § 139 667 § 49 762 § 46 757 § 64 825 § 59 786 § 65 747 § 43

23.3 § 2.7 21.1 § 1.0 28.6 § 0.4 28.6 § 0.4 18.1 § 1.7 28.8 § 0.5 20.1 § 1.6

Pl + Amp + Ep § Chl

–

551 § 47

9.1 § 1.9

Mineral abbreviations are adopted after Kretz (1983) except Amp, amphibole; Cs, coesite. Ret represent retrograde.

thermobaric structure presented in this study. The formation of the UHP eclogite and gneiss in Unit II indicate that the leading edge of the Indian Plate subducted to a depth of at least 90–100 km and UHP rocks were extruded between the overlying Unit III and the underlying Unit I.

6. Discussion In the Kaghan Valley, Indian plate sediments suVered regional metamorphism in synchroneity with ductile shearing, during Tertiary collision. Metamorphism in this area followed a path of increasing pressure during subduction.

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The UHP metamorphism and the structure of the Kaghan Valley indicate that in parts of the subduction front the Indian plate sediments reached deeper levels with continued underthrusting. This phenomenon resulted in early-Eocene regional metamorphism (Tonarini et al., 1993; Treloar, 1997) with deep subduction and UHP metamorphism (O’Brien et al., 2001; Rehman et al., 2004). The minimum depth of subduction reached by these rocks is well constrained by the presence of coesite inclusions in omphacite from eclogites. The multistage growth history of zircon from the felsic and the pelitic gneisses with an inner detrital core, overgrown by a non-UHP metamorphic zone and outer rim of UHP metamorphic domain (Kaneko et al., 2003) proves that the depth exceeding »100 km. The continent–continent collision in this region of the Himalayan chain, thrust the Kohistan Island arc over the leading edge of the Indian Plate along the MMT (Tahirkheli, 1979) resulting in intense deformation in a ductile shear regime (Treloar, 1997) and in regional-scale extension-related folding (Burg et al., 1996). The slight increase in pressure to the north shows that the metamorphic grade increased during active subduction of the leading edge of the Indian Plate beneath the Kohistan Arc, synchronous with deformation. The geochronological results show that the eclogite-forming event in the HHC postdated blueschist formation in the MMT suture zone, and predated regional Barrovian-type metamorphism in the Himalayan chain (O’Brien, 2001; O’Brien et al., 2001). The apparent Barrovian metamorphism in this area comprises an older (»30 Ma) kyanite-bearing stage, overprinted by a younger (13–20 Ma), lower pressure, but higher temperature, sillimanite-bearing stage (Honegger et al., 1982; O’Brien, 2001 and references therein). Peak UHP metamorphism in the Kaghan Valley (46 Ma) (Kaneko et al., 2003) took place no later than »19 m.y. after the initiation of the collision between the Indian Plate and the Kohistan Arc (65 Ma) (Beck et al., 1995). The Ar–Ar cooling ages of ca. 43 and ca. 25 Ma from hornblende and mica from the gneisses (Chamberlain et al., 1991) indicate the end of the regional metamorphism, together with rapid unrooWng of the Indian plate rocks. The same phenomenon of rapid cooling and retrogression is evidenced in the eclogite samples, which have abundant rutile and amphibole–augite–albite symplectites (Fig. 6). The decrease in pressure from 28.6 § 0.4 (coesite present phase) to 18.1 § 1.7 kbar (retrograde phase of symplectic amphibole–augite–albite) in UHP eclogites (Table 6) is evidence of the fast cooling and rapid exhumation of the UHP rocks. The fabric-porphyroblast relationship and strain-related rotation in garnet and kyanite from pelitic rocks (Fig. 5b, c, and f) demonstrates that the regional metamorphism in Indian plate rocks was synchronous with ductile deformation associated with the early stages of collision. With the subsequent subduction of Indian plate rocks, the metamorphic piles were disrupted to form a south vergent thrust stack, followed by the overthrusting and folding of the HHC. This structural model is consistent with an increase in metamorphic grade towards

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