Superposition of replacements in the mafic granulites of the Jijal complex of the Kohistan arc, northern Pakistan: dehydration and rehydration within deep arc crust

Lithos 43 Ž1998. 219–234

Superposition of replacements in the mafic granulites of the Jijal complex of the Kohistan arc, northern Pakistan: dehydration and rehydration within deep arc crust Hiroshi Yamamoto a

a,)

, Takashi Yoshino

b

Department of Earth and EnÕironmental Sciences, Faculty of Science, Kagoshima UniÕersity, Korimoto 1-21-35, Kagoshima 890-0065, Japan b Geological Institute, Graduate School of Science, UniÕersity of Tokyo, Hongo 7-3-1, Tokyo 113-033, Japan Received 21 October 1997; revised 19 April 1998; accepted 19 April 1998

Abstract A deep-level crustal section of the Cretaceous Kohistan arc is exposed in the northern part of the Jijal complex. The occurrence of mafic to ultramafic granulite-facies rocks exhibits the nature and metamorphic evolution of the lower crust. Mafic granulites are divided into two rock types: two-pyroxene granulite Žorthopyroxeneq clinopyroxeneq plagioclase" quartz w1x.; and garnet–clinopyroxene granulite Žgarnetq clinopyroxeneq plagioclaseq quartz w2x.. Two-pyroxene granulite occurs in the northeastern part of the Jijal complex as a relict host rock of garnet–clinopyroxene granulite, where the orthopyroxene-rich host is transected by elongated patches and bands of garnet–clinopyroxene granulite. Garnet–clinopyroxene granulite, together with two-pyroxene granulite, has been partly replaced by amphibolite Žhornblende" garnetq plagioclaseq quartz w3x.. The garnet-bearing assemblage w2x is expressed by a compression–dehydration reaction: hornblendeq orthopyroxeneq plagioclases garnet q clinopyroxeneq quartzq H 2 O≠. Subsequent amphibolitization to form the assemblage w3x is expressed by two hydration reactions: garnet q clinopyroxeneq plagioclaseq H 2 O s hornblendeq quartz and plagioclaseq hornblendeq H 2 O s zoisite q chloriteq quartz. The mafic granulites include pod- and lens-shaped bodies of ultramafic granulites which consist of garnet hornblendite Žgarnet q hornblendeq clinopyroxene w4x. associated with garnet clinopyroxenite, garnetite, and hornblendite. Field relation and comparisons in modal–chemical compositions between the mafic and ultramafic granulites indicate that the ultramafic granulites were originally intrusive rocks which dissected the protoliths of the mafic granulites and then have been metamorphosed simultaneously with the formation of garnet–clinopyroxene granulite. The results combined with isotopic ages reported elsewhere give the following tectonic constraints: Ž1. crustal thickening through the development of the Kohistan arc and the subsequent Kohistan–Asia collision caused the high-pressure granulite-facies metamorphism in the Jijal complex; Ž2. local amphibolitization of the mafic granulites occurred after the collision. q 1998 Elsevier Science B.V. All rights reserved. Keywords: Petrogenesis; Granulite; Lower crust; Kohistan; Himalaya

)

Corresponding author. Fax: q81-99-2594720; E-mail: [email protected]

0024-4937r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 0 2 4 - 4 9 3 7 Ž 9 8 . 0 0 0 1 4 - 0

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H. Yamamoto, T. Yoshinor Lithos 43 (1998) 219–234

1. Introduction In the western extension of the Himalayas, an exotic tectonic block separates the Asian and Indian plates. The block, called ‘the Kohistan sequence’, consists of basic to acidic volcanic rocks, sedimentary rocks, gabbroic to granitic intrusive rocks, and high-grade basic metamorphic rocks. The sequence is considered to be a dissected island arc, which is called the Kohistan arc ŽTahirkheli et al., 1979; Bard et al., 1980; Bard, 1983.. The Kohistan arc is now sandwiched by the Main Mantle Thrust on the south and the Shyok Suture on the north between the Asian and Indian continental crust ŽCoward et al., 1982, 1986; Pudsey, 1986; Treloar et al., 1996.. The collision between the Asian continent and the Kohistan arc was inferred to have taken place between 102 to 85 Ma, based on Rb–Sr and K–Ar ages of pre- or syn-collisional tonalitertrondhjemite and post-collisional basic dikes in the footwall of the Shyok suture ŽPetterson and Windley, 1985; Treloar et al., 1989.. The Kohistan arc is one of the most complete crustal sections exposed in the world, extends approximately 400 km in length and 170 km width ŽFig. 1.. The Jijal complex is situated in the southern part of the Kohistan arc and includes mafic to ultra-

mafic granulites. The granulites are regarded as components of the lower crust of the Kohistan arc Že.g., Coward et al., 1987; Yamamoto, 1993.. Unlike most of the deeply exhumed terrains, the Jijal complex belongs to the relatively young ŽCretaceous. island– arc sequence and is situated in a simple plate-tectonic setting. Thus the Jijal complex itself have peculiar importance for the study of the lower crust. The evolution of the Kohistan arc as a whole has been expressed elsewhere Že.g., Coward et al., 1987; Khan et al., 1993; Treloar et al., 1996. and some petrological studies on the granulites of the Jijal complex have been performed Že.g., Jan and Howie, 1981; Jan and Jabeen, 1991; Yamamoto, 1993., however, the granulite-facies metamorphism in the Jijal complex has not been well integrated into the tectonic history of the Kohistan arc. In this paper, we present new observations on field occurrence and petrography of the granulites in order to attempt to reconstruct petrogenetic processes relating to the genesis of the Jijal complex. 2. Geological setting The northeastern part of the Jijal complex is dominated by garnet-bearing granulites with the

Fig. 1. Regional setting of the Kohistan arc. Simplified from Treloar et al. Ž1996..

H. Yamamoto, T. Yoshinor Lithos 43 (1998) 219–234

southwestern part comprising peridotite and pyroxenite. In this paper, the northeastern part is referred to as the ‘Patan granulite body’ and the southwestern part is referred to as the ‘Jijal ultramafic body’ ŽFig. 2.. The rocks of the Patan granulite body exhibit variable mode of occurrence and can be divided broadly into plagioclase-rich and plagioclase-absent lithologies respectively Žcf. Jan and Howie, 1981.. The plagioclase-rich rocks include garnet–clinopyroxene granulite, two-pyroxene granulite, and amphibolite. The plagioclase-absent rocks include garnet hornblendite, garnet clinopyroxenite, garnetite, hornblendite, and rare peridotite. Most of the Patan granulite body is occupied by garnet–clinopyroxene granulite. The pressure–temperature conditions of garnet–clinopyroxene granulite determined by

221

geothermobarometry range from 10.2 kbar at 7358C to 17.0 kbar at 9498C ŽYamamoto, 1993.. Garnet hornblendite and garnet pyroxenite of the Patan granulite body have interfingered contacts with pyroxenite of the Jijal ultramafic body at the east of Sandar ŽQureshi and Jan, 1977.. The Jijal ultramafic body is composed of pyroxenite, diopsidite, websterite, dunite, and peridotite ŽJan and Howie, 1981; Jan and Windley, 1990.. Petrography and mineral compositional data suggest that the rocks are ultramafic cumulates derived from an arc-related high-Mg tholeiitic magma, possibly at pressure in excess of 8 kbar ŽJan and Windley, 1990.. The southern margin of the ultramafic body is bounded by the Main Mantle Thrust where the Kohistan arc is separated form the Indian continental crust.

Fig. 2. Lithological map of the southern part of the Indus Kohistan. Open box marks limits of the route map ŽFig. 3..

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H. Yamamoto, T. Yoshinor Lithos 43 (1998) 219–234

3. Petrography and field relations The mode of occurrence of the Patan granulite body is highly complicated. We describe the petrography and field relations of the major rock types of the Patan granulite body from Patan southwestwards towards Sandar along the Karakoram Highway ŽFig. 3.. 3.1. Plagioclase-rich rocks 3.1.1. Two-pyroxene granulite The two-pyroxene granulite consists of 1–2 mmsized granoblastic grains of orthopyroxene, clinopyroxene, plagioclase, "quartz, irregular shaped opaque minerals Žilmenite and magnetite., and rare hornblende. The mineralogy may represent a relict gabbronorite assemblage. Orthopyroxene and clinopyroxene grains have moderately elongated prismatic shape Žup to 1:3 in aspect ratio.. Plagioclase is generally the most dominant mineral in the two-pyroxene granulite, although mafic layers Žpyroxene-rich, plagioclase-poor. up to several tens of centimeters thick occur in places. We interpret the local mafic layering as magmatic in origin. Two-pyroxene granulite is locally replaced by garnet–clinopyroxene granulite. Garnet–clinopyroxene granulite forms elongated patches, lenses, or bands separated by narrow reaction zones from the host rock ŽFig. 4a, b.. Garnetiferous veins with thickness of 1–5 mm are generally located in the middle of the reaction zones. The outlines of the garnet–clinopyroxene granulite are commonly symmetric against the garnetiferous veins. Some bands and lenses of garnet–clinopyroxene granulite overprint earlier bands and lenses ŽFig. 4a.. The mafic layers in two-pyroxene granulite are preserved as garnet–clinopyroxene-rich and plagioclase-poor layers after garnet crystallization. The mafic layers continue through the boundary between garnet-absent and garnet-bearing domains without any change in their thickness and trend ŽFig. 4b.. These observations indicate that the formation of garnet–clinopyroxene granulite is strongly related to fluid infiltration into two-pyroxene granulite along the garnetiferous veins.

3.1.2. Garnet–clinopyroxene granulite The road-side sections near the northeastern side of the route map ŽFig. 3. are predominated by the two-pyroxene granulite. The area occupied by garnet–clinopyroxene granulite in outcrop surfaces increases toward the southwest. Blobs of relict two-pyroxene granulite disappear at about 500 m from Patan, where the outcrop surface is fully occupied by the coarse-grained garnet–clinopyroxene granulite. The garnet–clinopyroxene granulite consists of garnet, clinopyroxene, plagioclase, quartz, rutile, opaque minerals Žilmenite and magnetite., minor hornblende, epidote, and rare scapolite. Equidimensional 1–2 mm-sized grains of plagioclase form a good granoblastic texture. Clinopyroxene has subrounded or short–prismatic shapes with long axes of about 1 mm. Quartz forms irregular shaped polycrystalline aggregates which consist of small Žup to 0.5 mm. polygonal grains. Rutile has an elongated-bipyramidal habit with a long axis measuring about 0.3 mm. Garnet occurs both as poikiloblastic grains Žup to 5 mm in diameter. and polygonal grains Ž0.5–1 mm. in the matrix. Common inclusion minerals in garnet grains are clinopyroxene, rutile, quartz and opaque oxide. These inclusions have rounded shapes with diameters of 0.01–0.05 mm except quartz which has bipyramidal habit and is smaller than the other inclusions. The mineral assemblages of inclusions in each grain of the porphyroblasts are clinopyroxene q quartz q rutile, clinopyroxeneq quartz q opaque minerals, and rutile q opaque minerals. Symplectic intergrowth of skeletal garnet and elongated quartz is observed in places between clinopyroxene and plagioclase, although clinopyroxene and plagioclase are commonly in direct contact. The symplectic garnet– quartz is about 0.1 mm wide and less than 0.5 mm long. The boundary zone between the two-pyroxene granulite and the garnet–clinopyroxene granulite is up to 2-cm-wide with a transitional mineral assemblage. The quantity of garnet increases and that of orthopyroxene and plagioclase decreases away from the two-pyroxene granulite domain to the garnet– clinopyroxene granulite domain. It is only in the boundary zones that orthopyroxene and garnet are observed together, however, there is no direct contact between them. These minerals are separated by thin films of quartz or clinopyroxene.

H. Yamamoto, T. Yoshinor Lithos 43 (1998) 219–234

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Fig. 3. Route map through mafic complex along the Karakoram Highway. Ž). Two-pyroxene granulite including bands and lenses of garnet–clinopyroxene granulite which are too small to be represented. Ž)). Garnet hornblendite is dominant and associated with garnet clinopyroxenite, garnetite, and hornblendite. Ž))). Pyroxenite including small patches of the garnet clinopyroxenite and garnet hornblendite Žsee Fig. 6..

3.1.3. Amphibolite Garnet–clinopyroxene granulite together with two-pyroxene granulite is locally intersected by narrow bands of amphibolite ŽFig. 4c, d.. Amphibolite consists of hornblende, plagioclase, epidote, quartz, rutile, chlorite, rare muscovite, with or without gar-

net. The attitude of amphibolitized zones is discordant to that of garnet–clinopyroxene granulite patches or lenses. Leucocratic veins Žup to 1 cm thick. which consist of plagioclase and hornblende are commonly developed in the middle of the amphibolitized zones. The amphibolitized zones which run

224 H. Yamamoto, T. Yoshinor Lithos 43 (1998) 219–234 Fig. 4. Photographs of road sections. See Fig. 3 for locations. Ža. Occurrence of the two-pyroxene granulite and the garnet–clinopyroxene granulite near Patan village. Žb. Compositional layering ŽN88W 788W. in the two-pyroxene granulite Žgrayish domain. continues through the garnet–clinopyroxene granulite Žreddish domain.. Žc. A feldspathic vein associated with an amphibolite band obliquely cuts the garnet–clinopyroxene granulite. Inset indicates outlines of lithological boundaries on the outcrop surface. Žd. Close up of the central part of Žc., where amphibolitization overprints the two-pyroxene granulite and the garnet–clinopyroxene granulite.

H. Yamamoto, T. Yoshinor Lithos 43 (1998) 219–234 225

Fig. 5. Photographs of road sections. Ža. Lens-shaped bodies ŽN108E 328W. of the garnet hornblendite set in the garnet–clinopyroxene granulite. See Fig. 3 for location. Žb. Close up of the lower-middle part of Ža.. The compositional layering in the garnet–clinopyroxene granulite Žhost rock. is discontinued at the border with the garnet hornblendite. Žc. Garnetiferous domains surrounded by hornblende-rich domain. See Fig. 3 for location. Žd. Localized patchy occurrence of the garnet hornblendite in the pyroxenite. See Fig. 6 for location.

226

H. Yamamoto, T. Yoshinor Lithos 43 (1998) 219–234

across the area of two-pyroxene granulite are composed of hornblende, plagioclase, epidote, and opaque minerals. Within a narrow Žabout 1–2 cm. reaction zone along the rim of the amphibolitized zone, relics of orthopyroxene and clinopyroxene occur in cores of some hornblende grains. Plagioclase grains in the amphibolitized zone are partly or totally replaced by intergrown epidote–quartz. These replacing textures indicate retrograde hydration of the granulites due to infiltration of external H 2 O-rich fluid. In places, the amphibolitized zones overprint bands or lenses of the garnet–clinopyroxene granulite, where garnet is preserved after amphibolitization ŽFig. 4d.. To the southwest of Patan, the amphibolitized zones are sporadically distributed in garnet–clinopyroxene granulite. 3.2. Plagioclase-absent rocks Pod- and lens-shaped bodies of plagioclase-absent rocks are distributed in garnet–clinopyroxene granulite ŽFigs. 3 and 5a.. The pods and lenses are up to several tens of meters wide except a circa 500 m

wide body at 3 km southwest of Patan and scattered throughout the garnet–clinopyroxene granulite from around Ganbir to the southwest ŽFig. 3.. Garnet hornblendite is the most common rock type among the plagioclase-absent rocks and is associated with varieties of garnet clinopyroxenite, garnetite, and hornblendite. Garnet hornblendite consists of hornblende, garnet, clinopyroxene, rutile, opaque minerals, rare scapolite. The modal mineral composition of the garnet hornblendite much varies particularly in the modal garnet content even within a single pod or lens. The pods and lenses of garnet hornblendite commonly have garnet-rich rims Žseveral centimeters wide. and hornblende-rich cores. Some rims and cores are monomineralic and can be called garnetite and hornblendite respectively. The boundary between garnet–clinopyroxene granulite Žhost rock. and garnet hornblendite Žreplacement rock. is marked by aligned large garnet grains Žup to 3 cm in diameter. which are discordant to the compositional layering present in the host rock ŽFig. 5b.. The compositional layering in garnet–clinopyroxene granulite abruptly

Fig. 6. Route map along Karakoram highway near Sandar village. Ž). Patches consist of garnet, hornblende, clinopyroxene, rutile, and opaque minerals.

H. Yamamoto, T. Yoshinor Lithos 43 (1998) 219–234

terminates at the boundary. This indicates that the formation of protoliths of the plagioclase-absent rocks postdates the protolith of the garnet–clinopyroxene granulite. In places, bands and patches of garnet clinopyroxenite occur within the hornblende rich matrix of the garnet hornblendite ŽFig. 5c.. These bands and patches contain abundant garnet, much clinopyroxene and rare hornblende. The plagioclase-absent rocks include several rock types with a variety of modal mineral compositions as stated above, however, microscopic appearances of the mineral grains Žshapes and sizes. in the rocks are common to all rock types. Hornblende displays elongated-polygonal or rhomboidal shapes with long axes measuring circa 5 mm. Garnet occurs in this rock as porphyroblasts Žup to 3–8 mm. and less commonly as smaller polygonal grains between clinopyroxene and hornblende. The grains of garnet porphyroblasts have polygonal to subrounded irregular outlines. Clinopyroxene displays subrounded to short prismatic crystal shapes with long axes measuring about 3 mm. Rutile displays elongated-bipyramidal or subrounded-oval shapes with long axes of about 0.5 mm. Common inclusion minerals in garnet porphyroblasts are clinopyroxene, hornblende, rutile, and opaque minerals. These inclusions have rounded shapes with diameters of 0.01–0.05 mm. To the east of Sandar, the garnet hornblendite of the Patan granulite body is interfingered with pyroxenite of the Jijal ultramafic body. The pyroxenite generally contains clinopyroxene, orthopyroxene, "olivine, and opaque minerals. Only sample J032 contains small amount of garnet, which are always accompanied by opaque minerals. Pyroxenite hosts patches of mineral aggregates which are petrographically similar to garnet hornblendite and garnet clinopyroxenite of the Patan granulite body ŽFig. 5d.. Along the road-side sections near Sandar, the abundance of garnet-bearing patches in pyroxenite gradually decreases toward the west ŽFig. 6.. The patches disappear at about 250 m from J032, and do not occur to the west.

4. Whole-rock chemical and modal analyses Whole-rock chemical and modal compositions of 24 samples are analyzed in order to make compar-

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isons between the host rocks and the replacement rocks. Concentrations of the major elements Si, Ti, Al, Fe, Mn, Mg, Ca, Na, K, and P were determined by X-ray fluorescence spectrometer ŽPHILLIPS PW1480. at Geological Institute, University of Tokyo. Modal abundances of minerals were measured by counting 2000 points covering the whole of one section from each sample. The results are listed in Table 1 and petrographical classification of samples are indicated in Table 2. The diagrams oxides vs. SiO 2 are shown in Fig. 7.

4.1. Patan granulite body The SiO 2 content of 14 samples from the plagioclase-rich rocks Žtwo-pyroxene granulite, garnet– clinopyroxene granulite, and amphibolite. ranges from 48.73 to 54.72 wt.%. The rocks have basaltic to basaltic andesitic compositions with TiO 2 levels of 0.57–0.85%, high Al 2 O 3 content Ž17.21–19.03%., and relatively low MgO Ž4.33–5.88%. and CaO Ž8.69–10.58%. contents. Alkali contents are relatively high and variable. In Fig. 7, each rock type of the plagioclase-rich rocks has similar compositional domains except for total alkalis. Total alkalis in two-pyroxene granulite is higher than 2.7%, which differentiates compositional sub-domain of two-pyroxene granulite from the other plagioclase-rich rocks Žgarnet–clinopyroxene granulite and amphibolite.. Compositions of five samples ŽP015, P027, P103, P033 and P035. from the plagioclase-absent rocks are distinct from the plagioclase-rich rocks ŽFig. 7.. The SiO 2 content of the plagioclase-absent rocks ranges from 39.86 to 44.79% and they are compositionally foiditic to picro-basaltic. The rocks have between 0.28–1.89% TiO 2 , 12.62–16.59% Al 2 O 3 , 9.90–14.94% FeO total , 10.76–14.00% MgO, and 10.95–16.53% CaO. Wide ranges in chemical compositions reflect a diversity of modal mineral compositions. Hornblende-rich samples ŽP015, P027 and P103. are far richer in TiO 2 and alkalis than garnetrich and clinopyroxene-rich samples ŽP033 and P035 respectively.. The garnet-rich sample ŽP033. has the least content of TiO 2 and the clinopyroxene-rich sample ŽP035. is the richest in CaO among the plagioclase-absent rocks.

H. Yamamoto, T. Yoshinor Lithos 43 (1998) 219–234

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Table 1 Major-oxide composition and mode of granulites and pyroxenite Rock type

Two-pyroxene granulite

Garnet–clinopyroxene granulite

Plagioclase-rich rocks Patan granulite body Sample no.

P002 a

P101b

P111c

KU65d

P003a

P102 b

P112 c

KU66 d

P014e

P025

P122

Oxide (wt.%) SiO 2 TiO 2 Al 2 O 3 FeO total MnO MgO CaO Na 2 O K 2O P2 O5 total

51.18 0.85 18.11 9.74 0.19 5.74 10.49 3.30 0.28 0.08 99.96

49.65 0.84 17.53 9.85 0.19 5.52 10.27 3.43 0.30 0.12 97.70

50.26 0.72 17.95 9.52 0.20 5.37 10.14 2.85 0.28 0.07 97.36

51.65 0.74 18.65 9.11 0.19 5.30 10.12 2.52 0.26 0.10 98.64

51.41 0.85 18.25 9.89 0.20 5.83 10.56 1.74 0.29 0.09 99.11

50.92 0.74 18.04 9.96 0.20 5.71 10.25 1.91 0.24 0.11 98.08

50.77 0.78 18.79 10.02 0.21 5.61 10.53 1.68 0.18 0.09 98.66

50.54 0.81 19.03 9.12 0.17 5.76 11.30 2.33 0.17 0.04 99.27

54.72 0.72 17.74 8.17 0.16 4.67 9.85 2.47 0.33 0.07 98.90

54.67 0.85 17.21 9.39 0.19 4.33 8.69 2.63 0.15 0.13 98.24

49.38 0.57 18.71 9.18 0.19 6.16 11.13 2.42 0.10 0.05 97.89

– – 13.1 15.5 8.2 59.9 – 1.3 – 2.1 –

– – 14.4 14.8 4.0 63.7 – 0.9 – 2.2 –

– – 12.3 8.9 3.5 74.2 – 0.2 – 1.1 –

– – 16.6 12.0 0.3 63.4 – 5.7 – 2.0 –

39.3 – – 17.9 0.6 35.0 0.4 6.3 0.6 – –

35.1 – – 20.8 0.4 36.3 – 6.6 0.5 0.5 –

32.3 – – 21.2 – 42.1 – 3.9 0.6 – –

33.2 – – 21.4 0.8 36.2 – 7.7 0.6 0.1 –

29.8 – – 21.6 0.2 34.4 – 13.2 0.7 0.1 –

30.4 – – 12.6 – 41.5 – 14.1 0.5 1.0 –

34.7 – – 25.9 – 33.3 0.2 5.2 0.9 – –

Mode (Õol %) Grt Ol Opx Cpx Hbl Pl Ep Qtz Rt Opaque other minerals

Sample locations are shown in Figs. 2, 3 and 6. FeO total : total Fe as FeO. Abbreviations of minerals refer to Kretz Ž1983.. Pairs of samples taken from every single outcrop are annotated by a , b, c , d , and

4.2. Jijal ultramafic body Compositions of five pyroxenite samples ŽJ030, J032, J112, J117, and J118. from the Jijal ultramafic body are distinct from the rocks of the Patan granulite body ŽFig. 7.. The rocks have between 46.26 to 53.54% SiO 2 , 0.07–0.33% TiO 2 , 1.35–4.66% Al 2 O 3 , 4.94–9.49% FeO total , 18.44–24.21% MgO, and 14.01–19.41% CaO. The high MgO and CaO, and the low Al 2 O 3 and alkalis chemistry reflects pyroxene-rich modal compositions. The variations in chemical compositions are principally due to differ-

e

on the sample numbers.

ent proportions of the three dominant minerals Žclinopyroxene, orthopyroxene, and olivine. as already reported by Jan and Howie Ž1981..

5. Comparisons between hosts and replacements 5.1. Two-pyroxene granulite Õs. garnet–clinopyroxene granulite Each pair of samples P002–P003, P101–P102, P111–P112 and KU65–KU66 was taken from a

H. Yamamoto, T. Yoshinor Lithos 43 (1998) 219–234

Amphibolite

Garnet hornblended

Garnet pyroxenite

229

Pyroxenite

Plagioclase-absent rocks Jijal ultramafic body P008

P013

P028

P015e

P027

P103

P033

P035

J030

J032

J112

J117

J118

50.62 0.71 18.00 9.44 0.19 5.88 10.58 1.72 0.16 0.06 97.36

51.53 0.68 17.86 9.53 0.20 5.29 10.25 1.77 0.12 0.09 97.32

48.73 0.79 19.01 10.33 0.20 5.10 10.45 2.30 0.07 0.11 97.09

41.24 1.86 15.74 14.94 0.18 10.93 10.95 1.57 0.28 0.04 97.73

39.86 1.62 15.26 14.12 0.15 11.36 11.42 2.32 0.27 0.04 96.42

40.50 1.89 14.35 14.22 0.13 10.76 11.90 1.68 0.25 0.04 95.72

42.08 0.28 16.59 13.03 0.44 14.00 11.61 0.11 0.00 0.03 98.17

44.79 0.82 12.62 9.90 0.17 12.77 16.53 0.97 0.00 0.03 98.60

46.26 0.33 4.34 9.49 0.16 21.25 16.61 0.00 0.00 0.03 98.47

48.61 0.32 4.66 8.28 0.18 18.44 15.99 0.10 0.00 0.03 96.61

53.54 007 1.36 4.99 0.15 24.21 14.01 0.00 0.00 0.02 98.35

50.51 0.09 1.35 4.94 0.13 21.03 19.41 0.05 0.00 0.03 97.54

51.79 0.13 2.10 5.89 0.17 19.67 17.96 0.03 0.00 0.03 97.77

28.8 – – – 31.1 14.6 11.7 9.4 0.4 – Chl

20.0 – – – 36.6 10.9 20.0 9.2 0.4 – Chl

29.1 – – – 17.9 7.5 27.8 9.8 0.6 – Chl q Ms

22.1 – – 11.6 60.3 – – – 1.7 3.0 Scp

15.2 – – 1.6 81.3 – – – – 2.0 –

25.7 – – 24.9 44.1 – – – 0.5 1.4 Scp

64.7 – – 26.6 7.8 – – – – 0.5 "Scp

27.9 – – 67.4 4.3 – – – 0.2 0.3 –

– 6.4 20.1 – 10.1 35.7 54.5 52.9 – 1.7 – – – – – – – – 3.3 3.3 Chl q Sp –

– 1.2 50.1 45.4 – – – – – 0.4 Chl q Sp

– 5.8 20.0 73.2 – – – – – 1.1 –

– – 31.6 65.5 – – – – – 1.2 Chl q Sp

single outcrop. The former of each pair represents the host Žtwo-pyroxene granulite. and the latter represents the replacement Žgarnet–clinopyroxene granulite. in each outcrop. The oxide concentration ratio of associated hostrreplacement is shown in Fig. 8. There are little differences in SiO 2 , TiO 2 , FeO total , MgO, MnO, and CaO contents between the host and the coexisting replacement. Na 2 O contents of replacements are lower than those of the host rocks except that the ratio KU66rKU65 shows a slight difference. K 2 O contents of replacement rocks are lower than those of the host rocks except that the ratio P003rP002 shows a slight difference. In all cases, total alkalis of replacement rocks are lower

than the host rocks. The loss in alkali content indicates that a certain extent of metasomatic alteration accompanied the two-pyroxene granulite to garnet– clinopyroxene granulite transformation but it did not much change chemical composition of the host rocks as a whole. There are some differences in P2 O5 also, however, those are insignificant in the following discussions because of the very small content of P2 O5 in the samples Žless than 0.2 wt.%.. The modes of the host rocks are much different from that of the replacement rocks. Garnet and rutile are absent from the host rocks ŽP002, P101, P111 and KU65., whereas they are common in the replacement rocks ŽP003, P102, P112 and KU66..

230

H. Yamamoto, T. Yoshinor Lithos 43 (1998) 219–234

Fig. 7. Oxide vs. SiO 2 diagrams of two-pyroxene granulite garnet–clinopyroxene granulite, amphibolite, garnet pyroxenite, garnet hornblendite, and pyroxenite.

Orthopyroxene is absent from the replacement rocks, whereas it is common in the host rocks. Clinopyroxene, plagioclase, quartz, and opaque minerals Žincluding ilmenite. are present in both of them. Modal contents of clinopyroxene and quartz increase after transformation to the garnet–clinopyroxene granulite, whilst the plagioclase modal content decreases halves. Thus, the two-pyroxene granulite to the gar-

net–clinopyroxene granulite transformation involves the breakdown of orthopyroxene and plagioclase to form garnet and clinopyroxene, and the breakdown of ilmenite to form rutile. The host rocks ŽP002, P101, P111 and KU65. include variable amounts of hornblende Ž0.3 to 8.2 vol %., whereas their respective replacement rocks ŽP102, P003, KU66, and P112. include little horn-

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Fig. 8. Oxide concentration ratio of associated replacementrhost. For concentration ratio calculation, the results of XRF analysis are normalized to 100%.

blende Ž0 to 0.6%.. Except the pair KU65–KU66 that the host rock has little hornblende Ž0.3%., abundance of hornblende in the hosts remarkably decreases after transformation to the garnet–clinopyroxene granulite. Thus garnet formation were probably accompanied by a dehydration reaction Žbreak down of hornblende.. The garnet formation, together with loss of H 2 O in vapor phase, approximates the following reaction hornblendeq orthopyroxeneq plagioclase s garnet q clinopyroxeneq quartz q H 2 O≠.

Ž 1. The reaction Ž1. is principally pressure-dependent and suggests compression through the formation of garnet–clinopyroxene granulite.

5.2. Garnet–clinopyroxene granulite Õs. garnet hornblendite The samples P015 and P014 were taken from a lens-shaped body of garnet hornblendite and its host rock Žgarnet–clinopyroxene granulite. respectively ŽFig. 5a and b.. The replacement rock ŽP015. has an ultrabasic composition that is significantly different from the host rock ŽP014. and is far richer in mafic component. The replacement rock has considerably higher TiO 2 , FeO total , MgO, and lower SiO 2 contents than those of the host rock. This reflects hornblende-dominated modal compositions of the replacement rock. There is also much difference in the mode between the host rock and replacement rock. Plagioclase and quartz are common minerals in the host

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rock, whereas they are absent from the replacement rock. Hornblende is minor component of the host rock Ž0.2 vol %., whereas it is most abundant in the replacement rock Ž60.3%.. The chemical and modal differences between P014 and P015 cannot be accommodated by any isochemical reactions. Garnet hornblendite could be formed either intrusion or intense metasomatism. The origin of the plagioclase-absent rocks is discussed later. 5.3. Garnet–clinopyroxene granulite Õs. amphibolite Three samples of amphibolite ŽP008, P013, and P028. were taken from amphibolitized zones which are hosted by the garnet–clinopyroxene granulite. Because we have not obtained amphibolite and its host rock samples from a single outcrop, the chemical compositions and mode of the amphibolite samples are compared with the garnet–clinopyroxene granulite samples which were taken from the other places ŽFig. 3.. The chemical compositions of amphibolite are similar to those of the garnet–clinopyroxene granulite ŽTable 1 and Fig. 7.. The differences in modal compositions are as follows: Clinopyroxene is a common mineral in the garnet– clinopyroxene granulite, whereas it is absent from the amphibolite. Garnet–clinopyroxene granulite contains little or no hornblende, epidote, and chlorite, whereas the amphibolite includes abundant hornblende, epidote and a little chlorite. The garnet content Ž20.0 to 29.1 vol %. and the plagioclase content Ž7.5 to 14.6%. of the amphibolite are smaller than those of the garnet–clinopyroxene granulite Ž29.8 to 39.3 and 33.3 to 42.1% respectively.. The samples of amphibolite have larger amounts of quartz than those of the garnet–clinopyroxene granulite when comparisons are made between the samples which have the closest SiO 2 contents, such as P008 vs. KU66, P013 vs. P003, and P028 vs. P122. This evidence indicates that the garnet–clinopyroxene granulite to amphibolite transformation involves the breakdown of clinopyroxene, garnet, and plagioclase to form hornblende, epidote, and quartz. The reactions can be approximated as: garnet q clinopyroxeneq plagioclaseq H 2 O s hornblendeq quartz

Ž 2.

plagioclaseq hornblendeq H 2 O s zoisite q chloriteq quartz.

Ž 3.

Since the host rock Žgarnet–clinopyroxene granulite. contains few hydrous minerals, the source of H 2 O has to be external to the host. Probably, H 2 O has been supplied through veins in the middle of the amphibolitized zones.

6. Evolution of the mafic granulites of the Jijal complex The mutual relationships of the rocks of the northern part of the Jijal complex are summarized in Fig. 9. We suggest that the earliest granulite mineral assemblage is theorthopyroxeneq clinopyroxeneq plagioclase" quartz assemblage of the two-pyroxene granulite. This assemblage is replaced by the garnet q clinopyroxeneq plagioclaseq quartz assemblage of the garnet–clinopyroxene granulite. The transformation was essentially isochemical process on major elements since the compositional layering in the two-pyroxene granulite is preserved in the garnet– clinopyroxene granulite and the major-oxide composition of the replacement rock is much the same as the host rock. The reaction Ž1. and the above-mentioned field observations indicate that increasing pressure along with infiltration of H 2 O-poor and probably CO 2-rich fluid is responsible for the replacement. The garnet–clinopyroxene granulite and the two-pyroxene granulite underwent retrograde hydration along feldspathic veins to form garnet-bearing and garnet-absent amphibolite respectively. The development of a hornblende–epidote-bearing assemblage from an anhydrous host mineral assemblage implies the introduction of an aqueous fluid phase into the granulites. The pods and lenses of plagioclase-absent rocks could be formed by metasomatic transformation of the host rocks or magmatic intrusion. The intrusive origin is preferable to the metasomatic origin, because: Ž1. the petrographically similar pod- and lens-shaped bodies are distributed in the two different kinds of host rocks Žgarnet–clinopyroxene granulite and pyroxenite.; Ž2. the compositional layering

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Fig. 9. Schematic evolution through the Patan granulite body of the Jijal complex.

in garnet–clinopyroxene granulite is abruptly broken off at the host-replacement boundary, and Ž3. no petrographical relict of the host rock is found within the replacement rock. The other indirect evidence also implies intrusive origin rather than metasomatism. If the plagioclase-absent rocks replaced the gabbroic host rock by metasomatic reactions, removal of significant amount of SiO 2 was one of essential processes of the metasomatism. However, there are no traces of outgoing SiO 2 Že.g., quartz veins or zones of silica-enrichment. in and around the bodies of the plagioclase-absent rocks. Intrusion of the protoliths of the plagioclase-absent rocks into the gabbroic host rocks Žprotoliths of two-pyroxene granulite and garnet–clinopyroxene granulite. could be earlier or later than the granulite-facies metamorphism. We suggest earlier intrusion, because the estimated pressure–temperature conditions of the plagioclase-absent rocks are comparable to those of garnet–clinopyroxene granulite Žcf. Jan and Howie, 1981.. The results of this study lead to the following conclusions: Ž1. The garnet–clinopyroxene granulite in the northern part of the Jijal complex is a nearly isochemical metamorphic replacement for two-py-

roxene granulite. Ž2. Increasing pressure and infiltration of H 2 O-poor fluid caused the compression–dehydration reaction to form garnet–clinopyroxene granulite. Ž 3. The pods and lenses of the plagioclase-absent rocks were originally intrusive rocks which dissected the protoliths of the plagioclase-rich rocks and then have probably been metamorphosed simultaneously with the formation of garnet–clinopyroxene granulite. Ž4. Amphibolitization owing to infiltration of H 2 O-rich fluid followed the high-pressure granulite-facies metamorphism. The above conclusions combined with isotopic ages reported elsewhere give constraints on the tectonic process in the lower crust of the Kohistan arc. A 91.0 " 6.3 Ma Sm–Nd isochron age of garnet– clinopyroxene granulite at Patan is regarded as dating the cooling of the granulites ŽYamamoto and Nakamura, 1996. and the collision between Asia and Kohistan arc is dated between 102 and 85 Ma ŽPetterson and Windley, 1985; Treloar et al., 1989.. Thus the formation of the garnet–clinopyroxene granulite is earlier than or coincide with the collision. The high-pressure granulite-facies event must have resulted from magmatic loading during arc development and the following arc–continent colli-

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sion ŽYamamoto, 1993; Yoshino et al., 1998.. Treloar et al. Ž1989. reports two similar hornblende Ar–Ar ages Ž83 " 1 Ma. of amphibolite samples from near to the north of the study area. The isotopic ages suggest that the amphibolitization of granulites had occurred after the arc–continent collision Žcirca 80 Ma.. Thus the amphibolitization event was probably related to the relaxation of compression after the formation of the Shyok suture. Acknowledgements This paper has benefited from critical reading on manuscript by M.Q. Jan, M.G. Petterson, and P. Le Fort. Discussions on petrography with M. Yamamoto are acknowledged. We thank S. Yamane and Khanzada for helpful assistance during our stay in Pakistan. Field work was supported by a grant from Fukada Geological Institute. References Bard, J.P., 1983. Metamorphism of an obducted island arc: example of the Kohistan sequence ŽPakistan. in the Himalayan collided range. Earth Planet. Sci. Lett. 65, 133–144. Bard, J.P., Maluski, H., Matte, P., Proust, F., 1980. The Kohistan sequence: crust and mantle of an obducted island arc. Geol. Bull., Univ. Peshawar 13, 87–93, Spec. issue. Coward, M.P., Jan, M.Q., Rex, D.C., Tarney, J., Thirlwall, M., Windley, B.F., 1982. Geo-tectonic framework of the Himalaya of N. Pakistan. J. Geol. Soc., London 139, 299–308. Coward, M.P., Windley, B.F., Broughton, R.D., Luff, I.W., Petterson, M.G., Pudsey, C.J., Rex, D.C., Khan, M.A., 1986. Collision tectonics in the NW Himalayas. In: Coward, M.P., Ries, A.C. ŽEds.., Collision Tectonics. Geol. Soc., London, Spec. Pub. 19, 203–219. Coward, M.P., Butler, R.W.H., Khan, M.A., Knipe, R.J., 1987. The tectonic history of Kohistan and its implications for Himalayan structure. J. Geol. Soc., London 144, 377–391. Khan, M.A., Jan, M.Q., Weaver, B.L., 1993. Evolution of the lower arc crust in Kohistan, N. Pakistan: temporal arc magma-

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