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Geochemical modelling of the Chilas Complex in the Kohistan Terrane, northern Pakistan
Journal of Asian Earth Sciences 29 (2007) 336–349 www.elsevier.com/locate/jaes
Geochemical modelling of the Chilas Complex in the Kohistan Terrane, northern Pakistan Yutaka Takahashi a,¤, Masumi U. Mikoshiba a, Yuhei Takahashi a, Allah Bakhsh Kausar b, Tahseenullah Khan b, Kazuya Kubo a b
a Geological Survey of Japan, AIST, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8567, Japan Geoscience Laboratory, Geological Survey of Pakistan, Chak Shazad, P.O. Box 1461, Islamabad, Pakistan
Received 4 January 2005; accepted 14 April 2006
Abstract The Chilas Complex in the Kohistan Terrane, Pakistan, is a huge basic intrusion, about 300 km long and up to 40 km wide, which is regarded as tilted island-arc type crust. It has been interpreted as the magma chamber root zone of the Kohistan Island Arc. The Chilas Complex is composed mainly of gabbronorite (main facies) and several masses of ultramaWc–maWc–anorthosite (UMA) association. The UMA association consists mainly of olivine-dominant cumulate (dunite, wehrlite, lherzolite) and plagioclase-dominant cumulate (troctolite, olivine gabbro, gabbronorite, anorthosite), with minor amount of pyroxene-dominant cumulate (clinopyroxenite, websterite). The major element geochemistry of the gabbronorite (main facies) and rocks of the UMA association, plotted on Harker diagrams, are explained by a cumulate and a non-cumulate model, respectively. Namely, the UMA association is explained as variable crystal cumulates from a primary magma and the gabbronorite of the main facies is explained as due to the fractionation of the residual melt. Chemical variations of major, trace and rare earth elements for the gabbronorite of the main facies in the Chilas Complex are explained by fractional crystallization and accumulation of plagioclase, orthopyroxene and clinopyroxene from the residual melt of the primary magma. © 2006 Elsevier Ltd. All rights reserved. Keywords: Chilas Complex; Kohistan; Pakistan; Cumulate; Fractional crystallization
1. Introduction The Asian and Indian plates are bounded by the IndusTsangpo Suture Zone (Gansser, 1964), but in the western Himalaya, the Kohistan and Ladakh terranes are sandwiched between the two plates (Tahirkheli et al., 1979) (Fig. 1). Based on Rb-Sr and K-Ar ages of pre- or syncollisional deformed leucogranites, and undeformed postcollisional basic dykes intruded into the northern part of the Kohistan Terrane the collision between the Asian continent and the Kohistan Terrane was inferred to have taken place between 102 and 75 Ma (Peterson and Windley,
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1985). The main collision between the Asia-Kohistan Block and the Indian Plate was initiated at about 50 Ma (Patriat and Achache, 1984) with the obduction of the Kohistan Block onto the Indian Plate. The Kohistan Terrane is regarded as island arc-type crust, which was tilted during the collision, hence deeper facies are located in the south and shallower facies in the north (Coward et al., 1982; Bard, 1983) (Fig. 1). The Kohistan Island Arc is bounded to the south by the Main Mantle Thrust (MMT), and to the north by the Northern Suture (or Main Karakoram Thrust, MKT). Southern Kohistan includes the Jijal Complex, which is composed of peridotite and basic granulite (Jan and Howie, 1981), the Kamila Amphibolites and the Chilas Complex (Fig. 1). Northern Kohistan is dominated by numerous large-scale granitic plutons of the Kohistan Batholith, a sequence of arc-type
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Fig. 1. SimpliWed geological map of the Kohistan terrane (modiWed from Treloar et al., 1990).
volcanic rocks called the Chalt Volcanics (Petterson and Windley, 1985, 1991), and a sequence of slates, turbidites and shallow marine Tethyan limestones of the Mid-Cretaceous Yasin Group (Pudsey, 1986) (Fig. 1). In addition, there are medium pressure/temperature type metamorphic rocks (Gilgit Formation) overlain by meta-volcanics (Gashu-ConXuence Volcanics) to the south of Gilgit, which have been correlated to the Chalt Volcanics (Khan et al., 1994). The Chilas Complex is a huge basic intrusion up to 40 km wide and stretches for 300 km along the entire length of the terrane (Jan et al., 1984). It has been interpreted as a magma chamber root zone of the Kohistan Island Arc (Khan et al., 1989), emplaced during intra-arc rifting (Khan et al., 1994). The intrusion of the Chilas Complex was inferred to have taken place at around 111 § 24 Ma, which was estimated by Rb-Sr whole rock isochron age for the gabbronorite of the main facies (Mikoshiba et al., 1999). However, U-Pb zircon dating has yielded 84–85 Ma ages for the gabbronorites from the Swat Valley (Zeitler et al., 1980; Schaltegger et al., 2002). The cooling age of the gabbronorite through 500 °C, estimated by K-Ar and 40 Ar–39Ar ages of hornblende, was around 80 Ma (Treloar et al., 1990). In this paper, we present geochemical modelling of the Chilas Complex based on detailed geological and petrographical studies. 2. Geology of the Chilas Complex 2.1. General geology The Chilas Complex is composed mainly of gabbronorite with minor amount of pyroxene quartz diorite (main gabbronorite) and several masses of the ultramaWc–maWc–
anorthosite (UMA) association (Khan et al., 1989; Khan and Jan, 1992). The gabbronorite of the Chilas Complex is bounded on the south by the sheared Kamila Amphibolites (Treloar et al., 1990). In the northern part of the complex, quartz diorite to tonalite of the Kohistan Batholith is intruded into the Chilas Complex (Fig. 2). At lower reaches of the Kinner River, northeast of Chilas, a xenoblock of garnet-biotite gneiss occurs in the main gabbronorite, and at upper reaches of the Hodda River, which is a right hand branch of the Indus River coming from the north, a xenoblock of garnet-biotite gneiss occurs in foliated quartz diorite of the Kohistan Batholith (Fig. 2). The gabbronorite and quartz diorite of the main facies are composed of orthopyroxene, clinopyroxene and plagioclase with small amount of hornblende, biotite, quartz, magnetite and ilmenite, exhibiting granoblastic textures. The rocks of the Chilas Complex are more or less deformed, exhibiting foliation. An E-W trending antiform and synform are observed, indicating that the Chilas Complex has a folded structure (Fig. 2). 2.2. The ultramaWc–maWc–anorthosite (UMA) association The UMA association occurs mostly in eastern part of the Chilas Complex (Fig. 2). An UMA body just to the east of Chilas has excellent outcrops (Figs. 2, 3A and B, 4). It is composed mainly of olivine-dominant cumulate (dunite, wehrlite, lherzolite) (Fig. 3E) and plagioclasedominant cumulate (troctolite, olivine gabbro, gabbronorite, anorthosite) (Fig. 3D) with minor amount of pyroxenedominant cumulate (clinopyroxenite, websterite) (Fig. 3F). The plagioclase-dominant cumulate often develops a layered structure on the olivine-dominant part, with sedimentary structures such as graded bedding and trough
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Fig. 2. Geological map of eastern part of the Chilas Complex. The large area left blank is the outcrop of the gabbronorite of the main facies.
banding etc. (Figs. 3A and B, 4) (Jan et al., 1984; Khan and Jan, 1992; Takahashi et al., 2003). Within each gabbroic band of the plagioclase-dominant cumulate, the eastern side exhibits an upper sequence, grading from a Wne-layered bottom part into a massive part (Fig. 4). The main gabbronorite of the Chilas Complex is intruded into the plagioclase-dominant cumulate and partly cuts the layered structure (Fig. 3A). Also, the main gabbronorite intrudes into the olivine-dominant cumulate of the UMA association in some places (Fig. 4). Under the microscope, the well-layered part of the plagioclase-dominant cumulate in the UMA association has an accumulate texture (Fig. 3D), whereas, the main gabbronorite, consisting of orthopyroxene, clinopyroxene and plagioclase, with small amounts of hornblende, biotite, magnetite and ilmenite, has a granoblastic texture (Fig. 3C).
lites correspond to the Kamila Amphibolites. Furthermore, there are many amphibolites which look like xenoblocks within the Chilas Complex (Fig. 2). The Kamila Amphibolites are classiWed into meta-volcanic and meta-plutonic amphibolites based on their lithofacies (Treloar et al., 1990) and geochemistry (Khan et al., 1993). Amphibolite xenoliths in the main gabbronorite of the Chilas Complex in the Thor River may be meta-volcanic amphibolite, and amphibolites looking like xenoblocks in the Chilas Complex may be hydrated and recrystallized products derived from the main gabbronorite. The Chilas Complex is a very large intrusive body emplaced into the Kohistan arc meta-volcanic amphibolites and metasediments.
2.3. Relation to the Kamila Amphibolites
The geochemistry of the Chilas Complex has already been reported by Khan et al. (1989) and a tectonic model of the Kohistan Island Arc was presented, based on the geochemistry (Khan et al., 1993, 1997). The following description is based upon our original data, generated during the present study.
Some parts of the Kamila Amphibolites are interpreted to have been derived from the Chilas Complex (Treloar et al., 1990). Accordingly, the northern part of the Kamila Amphibolites is interpreted as a hydrated and recrystallized product derived from the gabbronorite in the Chilas Complex. However, in the upper reaches of the Thor River (Fig. 2), the main gabbronorite of the Chilas Complex intrudes into amphibolite and includes many xenoliths of amphibolite. These amphibo-
3. Geochemistry of the Chilas Complex
3.1. Whole rock chemistry Major elements of whole rock samples were analyzed by XRF (RIGAKU XRF-3370E) at Geoscience Laboratory,
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Fig. 3. (A) Well developed layered structure of plagioclase-dominant cumulate of the UMA association. Layered structure developed in olivine-dominant cumulate, grading into massive olivine gabbro. Gabbronorite of the main facies intrudes into the plagioclase-dominant cumulate, partly cutting the layered structure. (B) Close up view of the rhythmic layering of the plagioclase-dominant cumulate. Graded bedding is clear, indicating upward younging. (C) Photomicrograph of the main gabbronorite, showing a granoblastic texture. Crossed polars. (D) Photomicrograph of the plagioclase-dominant cumulate (anorthosite) of the UMA association, showing an adcumulate texture. Crossed polars. (E) Photomicrograph of olivine-dominant cumulate (dunite–wehrlite) of the UMA association. (F) Photomicrograph of pyroxene-dominant cumulate (clinopyroxenite) of the UMA association. Pl: plagioclase, Opx: orthopyroxene, and Cpx: clinopyroxene.
Geological Survey of Pakistan. Trace and rare earth elements were analyzed by ICP–MS at Sumiko Consultants Ltd. and the Geological Survey of Japan following the method of Imai (1990) and Ujiie and Imai (1995). Representative whole rock chemical compositions (major, trace and rare earth elements) for the main gabbronorite and rocks of the UMA association in the Chilas Complex are presented in Table 1.
3.1.1. Major elements Bulk chemical compositions for constituent rocks of the Chilas Complex are plotted on the AFM diagram, along with those of the Kohistan Batholith (Fig. 5). Also shown are the Welds of island arc non-cumulate and island arc maWc–ultramaWc cumulate by Beard (1986). In this diagram, chemical compositions for the main gabbronorite of
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Table 1 Representative whole rock chemical compositions (major, trace and rare earth elements) of the main gabbronorite and rocks of UMA association in the Chilas Complex Unit
Main facies (gabbronorite)
Sample no.
92CH-18
92CH-20
92CH-43
92CH-63
92CH-82
92CH-87
92CH-91
92CH-99
92CH-100
92CH-114
Locality
East of Chilas
Thak River
West of Chilas
Thor River
Khanbari River
Khanbari River
Kiner River
Kiner River
Kiner River
Hoda River
Rock name
Gabbronorite
Gabbronorite
Gabbronorite
Gabbronorite
px.-qtz diorite
Gabbronorite
Gabbronorite
Gabbronorite
Gabbronorite
Gabbronorite
Total a
MgO/(MgO+Fe2O3 ) (ppm) Cr Ni Rb Ba Th Nb Sr Hf Zr Y La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
52.54 0.35 18.57 8.03 0.13 7.74 9.57 2.77 0.27 0.03
51.60 0.40 20.34 7.59 0.12 5.91 10.61 3.04 0.18 0.01
52.74 0.40 19.00 7.36 0.12 6.26 9.34 2.71 0.43 0.05
51.37 0.82 18.82 9.68 0.16 5.46 9.90 3.25 0.36 0.10
58.09 0.68 18.29 7.37 0.13 3.13 7.91 3.71 0.50 0.17
55.05 0.88 17.31 8.82 0.14 4.86 8.51 3.23 1.06 0.14
54.06 0.86 18.65 8.97 0.14 4.36 8.59 3.55 0.62 0.18
58.14 0.93 16.28 7.72 0.12 4.33 7.29 3.32 1.43 0.20
51.95 0.84 19.62 8.92 0.14 4.81 9.87 3.41 0.28 0.12
54.43 0.97 17.47 8.88 0.14 4.98 8.69 3.50 0.72 0.22
100.00
99.80
98.41
99.92
99.98
100.00
99.98
99.76
99.96
100.00
0.49 249 89 0.7 55.6 — 0.3 376 — 20 7.0 2.02 4.17 0.60 2.86 0.85 0.49 1.15 0.20 1.32 0.26 0.81 0.11 0.78 0.12
0.44 143 44 0.5 44.7 — 0.1 423 — 16 5.0 1.23 2.61 0.38 1.82 0.53 0.48 0.86 0.15 0.85 0.19 0.56 0.09 0.58 0.09
0.46 159 83 4.16a 89.7 0.2 0.7 363.0 — 33.0 9.0 3.35 7.05 1.01 4.52 1.36 0.58 21.64 0.26 1.86 0.37 1.11 0.17 1.19 0.17
0.36 119 33 1.8 98.1 0.2 0.9 404.0 — 27.0 20.0 4.34 8.85 1.32 5.97 1.80 1.05 2.48 0.42 2.89 0.59 1.78 0.24 1.58 0.25
0.30 89 8 1.9 208.4 0.0 3.3 490 — 121 27.0 9.28 21.19 3.17 15.54 4.32 1.15 5.13 0.79 4.80 1.06 2.94 0.44 2.95 0.42
0.36 105 42 14.53a 156.4 0.7 3.4 334 — 95 21.0 9.80 20.59 2.69 12.19 3.16 0.96 3.80 0.62 4.00 0.81 2.36 0.32 2.15 0.34
0.33 71 25 6.0 180.0 1.0 2.0 350 2.0 33 16.0 7.0 15.0 1.9 8.0 2.2 1.0 2.0 0.4 2.6 0.5 1.3 0.2 1.5 0.2
0.36 157 38 33.66a 285.9 1.0 3.8 362.1 — 163.0 23.0 12.67 26.70 3.70 15.52 3.93 1.00 4.17 0.66 4.10 0.83 2.27 0.38 2.20 0.34
0.35 92 28 3.0 150.0 1.0 2.0 320 2.0 3.0 10.0 1.0 8.0 1.0 5.0 1.6 0.8 1.4 0.2 1.8 0.4 1.0 0.2 1.1 0.1
0.36 136 39 6.0 180.0 1.0 2.0 340 2.0 72 18.0 7.0 20.0 2.4 11.0 2.6 0.9 2.3 0.4 2.7 0.6 1.3 0.3 1.3 0.2
Y. Takahashi et al. / Journal of Asian Earth Sciences 29 (2007) 336–349
SiO2 TiO2 Al2O3 Fe2O3a MnO MgO CaO Na2O K2O P2O5
Total a
MgO/(MgO+Fe2O3 ) (ppm) Cr Ni Rb Ba Th Nb Sr Hf Zr Y La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
92CH-75
92CH-77
92CH-116
92CH-7
92CH-9
92CH-59
East of Chilas Lherzolite
Thurli River Dunite
Thurli River Wehrlite
Thurli River Wehrlite
Khanbari River Websterite
Khanbari River Lherzolite
Thak River pl-lherzolite
Thak River Anorthothite
East of Chilas Anorthothite
Thor River hbl. gabbro
38.13 0.20 2.85 18.60 0.15 35.04 3.89 — 0.02 0.01
38.04 0.04 0.93 19.54 0.22 41.41 0.65 — — —
42.46 0.20 1.95 14.08 0.18 30.52 9.23 — — —
39.25 0.06 1.41 15.50 0.18 42.43 1.07 — 0.04 0.01
45.94 0.21 5.81 13.29 0.13 29.53 3.95 0.13 0.03 0.01
38.05 0.05 1.82 20.61 0.23 37.35 1.60 0.05 0.03 0.01
41.37 0.08 7.07 15.92 0.19 28.39 6.96 0.09 — —
44.06 0.04 27.42 3.98 0.06 6.30 15.55 0.62 — —
45.78 0.11 22.25 4.20 0.06 8.86 17.23 0.70 0.04 0.01
54.68 0.84 17.77 8.93 0.16 4.59 9.37 3.03 0.31 0.14
98.88
100.83
98.62
99.95
99.02
99.79
100.07
98.03
99.24
99.81
0.65 3600 820 2.0 10.0 1.0 2.0 10 2.0 3.0 4.0 <1.0 1.0 0.2 1.0 0.2 0.1 0.3 0.1 0.4 0.1 0.1 <0.1 0.2 <0.1
0.68 700 1700 2.0 — 1.0 — 12 — — — <1.0 2.0 <5.0 <5.0 <0.1 <0.5 <50.0 0.8 <1.0 <1.0 <20.0 <1.0 0.1 0.1
0.68 2500 1200 1.0 40.0 1.0 2.0 16 2.0 3.0 6.0 <1.0 2.0 0.3 2.0 0.5 0.3 0.5 0.1 0.7 0.2 0.4 0.1 0.4 —
0.73 1000 1880 2.0 10.0 1.0 2.0 7 2.0 3.0 2.0 <1.0 1.0 0.2 1.0 0.2 0.1 0.1 <0.1 0.3 <0.2 0.1 <0.1 0.2 <0.1
0.69 9000 1000 1.0 10.0 1.0 2.0 10 2.0 3.0 8.0 <1.0 <2.0 0.1 1.0 0.2 0.1 0.5 0.1 0.4 0.1 0.2 <0.1 0.3 <0.1
0.64 700 1500 2.0 10.0 1.0 — 16 2.0 3.0 4.0 <1.0 1.0 0.2 1.0 0.1 0.1 0.3 0.1 0.3 <0.2 0.1 <0.1 0.1 <0.1
0.61 222 102 0.13a 21.0 — 0.3 406 — 29.0 3.2 0.36 0.65 0.11 0.56 0.15 0.19 0.20 0.04 0.22 0.04 0.13 0.02 0.10 0.01
0.68 300 166 0.3 16.4 — 0.4 357 — 11.0 2.0 0.48 1.04 0.19 1.03 0.37 0.21 0.51 0.10 0.59 0.12 0.33 0.05 0.25 0.04
0.34 81 12 4.0 160 1.0 4.0 310 2.0 27.0 16.0 5.0 13.0 1.6 8.0 1.9 1.1 1.7 0.3 2.3 0.6 1.1 0.2 1.6 0.2
0.64 — — 2.0 — 1.0 — 110 — — — <1.0 <2.0 <5.0 <5.0 0.2 <0.05 <50.0 0.2 <1.0 <1.0 <20.0 <1.0 0.1 <0.1
Y. Takahashi et al. / Journal of Asian Earth Sciences 29 (2007) 336–349
SiO2 TiO2 Al2O3 Fe2O3a MnO MgO CaO Na2O K2O P 2O 5
UltramaWc–maWc–anorthothite (UMA) association 92CH-5 92CH-44 92CH-47 92CH-55
Total Fe as Fe2O3; —, below detection limit. px.: pyroxene, qtz.: quartz, Riv.: river. a Determined by isotope dilution method.
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cumulate (anorthosite, gabbronorite) is plotted near the cluster of plagioclase crystals. 3.1.2. Trace elements Trace element compositions for the main gabbronorite and the UMA association of the Chilas Complex are shown on primitive mantle-normalized spidergrams (Figs. 7A and B). In the spidergram for the main gabbronorite (Fig. 7A), the large ion lithoWle (LIL) elements are enriched, and Nb and Sr show negative and positive anomalies, respectively. These are geochemical characteristics of island arc-type basalts (Wilson, 1989). The concentrations of trace elements are related to SiO2 contents of the main gabbronorite (Fig. 7A). Namely, basic compositions (SiO2 < 52 wt%) have the lowest, and acidic rocks (SiO2 > 58 wt%) the highest amount of trace elements. The intermediate rocks (52 wt% < SiO2 < 58 wt%) have intermediate amounts of trace elements. In the case of UMA association, the content of trace elements is relatively low, and high Weld strength (HFS) elements such as P and Ti are relatively depleted (Fig. 7B). These features were also reported by Khan et al. (1989) and are attributed to their cumulate characters. 3.1.3. Rare earth elements Chondrite-normalized rare earth elements (REE) patterns for the main gabbronorite and UMA association of the Chilas Complex are shown in Fig. 8. In the diagram for the main gabbronorite (Fig. 8A), light rare earth elements (LREE) show enriched patterns, which are typical for island arc-type basalt (Wilson, 1989). The relatively basic rocks (SiO2 < 52 wt%) of the main gabbronorite have relatively depleted REE contents and positive Eu anomalies. On the other hand, relatively acidic rocks (SiO2 > 58 wt%)
Fig. 4. Columnar section for the UMA association east of Chilas Town (modiWed from Kubo, 2004).
the Chilas Complex are plotted mostly in the Weld of island arc non-cumulate and those for the rocks of UMA association are plotted in the Weld of island arc maWc–ultramaWc cumulate. The analyses of the Kohistan Batholith are plotted mostly in the alkali-rich Weld, compared with the main gabbronorite of the Chilas Complex. Representative major element (Al2O3, FeOt, MgO, and CaO) compositions for the rocks of the Chilas Complex along with constituent mineral compositions are plotted on Harker diagrams (Fig. 6). Data from granitic rocks of the Kohistan Batholith are also plotted for comparison. In these diagrams, the plots of the main gabbronorite cluster along a straight trend. But rocks of the UMA association are plotted separately from the trend. Olivine-dominant cumulate (dunite, wehrlite, lherzolite) is plotted close to the cluster of olivine crystals. And the plagioclase-dominant
Fig. 5. Major element geochemistry of the Chilas Complex plotted on the AFM (Na2O + K2O – FeOt – MgO) diagram. Compositional Welds of island arc non-cumulate and island arc maWc–ultramaWc cumulate by Beard (1986) are indicated by solid and broken lines, respectively. The boundary between tholeiite and calc-alkaline rocks after Barker and Arth (1976) are also shown. Data from the Kohistan Batholith are plotted for comparison.
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of the main gabbronorite are relatively enriched in REE contents and have negative Eu anomalies. Most of the rocks of intermediate gabbronorite (52 wt% < SiO2 < 58 wt%) have intermediate REE contents, without apparent Eu anomalies. In the case of UMA association (Fig. 8B), contents of REE are relatively low. This is concordant to their cumulate character, because REE are basically incompatible, especially in the case of basic magma (Hanson, 1980).
gabbronorite (MgO D 13.1–14.5 wt%, FeOt D 7.4–9.9 wt%) (Fig. 6, Table 2). On Harker diagrams, the chemical compositions of clinopyroxene in the UMA association plot in restricted areas (Fig. 6). The mineral chemistry described above supports the petrographic and geochemical subdivision of the Chilas Complex into two groups (the main gabbronorite and UMA association) as previously noted by Jan et al. (1984) and Khan et al. (1989).
3.2. Mineral chemistry
4. Geochemical modelling of the Chilas Complex
Chemical compositions of constituent minerals in the main gabbronorite and rocks of the UMA association were analyzed using EPMA at Geological Survey of Japan, Shimane University and the Geoscience Laboratory, Geological Survey of Pakistan. The accelerating voltage, specimen current and beam diameter were kept at 15 kV, 12 nA and 2 m, respectively. Representative chemical compositions for constituent minerals (orthopyroxene, clinopyroxene, olivine, plagioclase) of the main gabbronorite and UMA association in the Chilas Complex are presented in Table 2.
4.1. Major element modelling
3.2.1. Plagioclase The chemical compositions of plagioclase in the main gabbronorite and UMA association are plotted on Harker diagrams, together with olivine, orthopyroxene and clinopyroxene (Fig. 6, Table 2). Plagioclases in the UMA association have high anorthite contents (An84-94), but anorthite contents of plagioclase in the main gabbronorite are relatively low (An44-74). Accordingly, plagioclases in the main gabbronorite have higher SiO2 (51.3–57.4 wt%) and lower CaO (8.6–14.9 wt%) contents, compared with those of the UMA association (SiO2: 40.5–47.1 wt%, CaO: 13.9– 20.2 wt%). 3.2.2. Olivine The forsterite contents of olivine in the UMA association (dunite, wehrlite, lherzolite, troctolite) are Fo75 to Fo83. On Harker diagrams, chemical compositions of olivine are plotted in restricted areas, which have SiO2 contents of 36.7–41.7 wt% and MgO contents of 38.4–53.1 wt% (Fig. 6). 3.2.3. Orthopyroxene Chemical compositions of orthopyroxene in the UMA association (lherzolite, olivine gabbro, websterite) are relatively rich in MgO (25.5–30.8 wt%) and poor in FeOt (11.0– 15.6 wt%) compared with the main gabbronorite (MgO D 17.2–23.7 wt%, FeOt D 20.4–24.9 wt%) (Fig. 6, Table 2). On Harker diagrams, the chemical composition of orthopyroxene plots in restricted areas (Fig. 6). 3.2.4. Clinopyroxene Chemical compositions of clinopyroxene in the UMA association (wehrlite, olivine gabbro, clinopyroxenite, websterite) are relatively rich in MgO (15.2–20.0 wt%) and poor in FeOt (2.7–7.8 wt%) compared with those of the main
As mentioned earlier, the chemical composition of the main gabbronorite of the Chilas Complex mostly plots in the Weld of island arc non-cumulate, and those of the UMA association in the Weld of island arc maWc–ultramaWc cumulate on the AFM diagram (Fig. 5). This is consistent with the geological, petrographical characters that the rocks of the UMA association have accumulative structures (Figs. 3A and B) and textures (Figs. 3D–F). On Harker diagrams (Fig. 6), the chemical compositions of the gabbronorite and rocks of the UMA association are explained by a cumulate and a non-cumulate model (Fig. 6). At Wrst the chemical composition of the main gabbronorite, which can coexist with olivine in the UMA association, is assumed to be the melt composition of the primary magma. The olivine-liquid Fe/Mg partition coeYcient KD(Fe/Mg)ol-liq D (Fe/Mg)ol/(Fe/Mg)liq is nearly constant (around 0.3) in basaltic magma (Takahashi and Kushiro, 1983). In the case of the intermediate gabbronorite (CH-43, SiO2 D 52.74 wt%), KD(Fe/Mg)ol-liq D 0.309, which can coexist with olivine in the UMA association and suitable for the melt composition of the primary magma. The chemical composition of the primary magma should be in the area among olivine, orthopyroxene, clinopyroxene and plagioclase in the UMA association, and the inferred melt (CH-43, SiO2 D 52.7 wt%) on the Harker diagram (Fig. 6). The composition of the primary magma can be calculated from the ratio of the subtracted phases (olivine, orthopyroxene, clinopyroxene, plagioclase) and melt. However, the ratio of the subtracted phases and melt is unknown. Hence, the composition of the primary magma is calculated based on an assumption that the primary magma is composed of olivine, orthopyroxene, clinopyroxene and plagioclase, and melt with a ratio of 37:1:4:8:50. The ratio of subtracted phases (olivine, orthopyroxene, clinopyroxene, plagioclase) is given, based on the proportion of these minerals in the UMA association near Chilas (Fig. 4). The volume ratio of the olivine-dominant cumulate, plagioclase-dominant cumulate, websterite and clinopyroxenite in the UMA association is about 78 (olivine 74, clinopyroxene 4):20 (plagioclase 16, orthopyroxene 1.5, clinopyroxene 2.5):1 (orthopyroxene 0.5, clinopyroxene 0.5):1 (clinopyroxene 1) (Fig. 4). Accordingly, the ratio of subtracted phases (olivine, orthopyroxene, clinopyroxene,
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Table 2 Representative chemical compositions of olivine, orthopyroxene, clinopyroxene and plagioclase in the main gabbronorite and UMA association in the Chilas Complex Unit
Main facies
UltramaWc–maWc–anorthothite (UMA) association
Sample
91CH-14
91CH-34
CH-122
91CH-24B
91CH-32B
91CH-45
91CH-32B
Locality
Chilas
Chilas
West of Thor River conXuence
Chilas
Chilas
Chilas
Chilas
Rock name
Gabbronorite
Gabbronorite
Layered gabbronorite
Websterite
ol. gabbro
Dunite
ol. gabbro
Mineral
Opx
Opx
Opx
Opx
Cpx
Opx
Cpx
Ol
Ol
SiO2 TiO2 Al2O3 Cr2O3 FeO MnO MgO CaO Na2O K2O
53.62 0.03 1.30 0.02 23.79 0.80 19.62 0.49 — 0.02
50.48 0.34 2.36 0.02 9.46 0.32 13.75 22.75 0.42 0.02
52.85 0.18 1.62 0.05 22.72 0.50 21.56 0.57 — 0.03
50.93 0.48 2.94 0.00 9.93 0.11 13.40 20.93 0.50 0.01
53.67 0.1 1.64 0.04 21.24 0.41 22.05 0.44 — 0.04
52.75 0 2.64 0.05 7.95 0.16 13.65 21.99 0.48 0.09
58.53 0.01 0.86 0.07 12.61 0.62 25.47 0.43 0.15 —
57.68 0.23 2.89 0.27 6.57 0.28 20.01 11.34 0.54 0.05
51.82 0.01 6.6 0.02 12.39 0.2 27.9 0.24 — 0.02
54.55 — 2.35 — 3.83 0.08 15.81 23.49 0.38 0.02
39.31 0.02 — 0.07 14.96 0.38 45.80 0.01 0.03 —
39.69 — 0.02 — 18.58 0.29 41.07 0.02 0.03 —
Total
99.70
99.91
100.06
99.23
99.63
99.76
98.74
99.86
99.2
100.51
100.59
99.69
Cpx
Cpx
Cpx
Cations per 6 oxygens Si Al Ti Cr Fe Mn Mg Ca Na K Total
2.011 0.057 0.001 0.001 0.746 0.026 1.097 0.020 — 0.001
1.903 0.105 0.010 0.001 0.298 0.010 0.772 0.919 0.031 0.001
3.959
4.050
Cations per 4 oxygens 1.969 0.071 0.005 0.001 0.708 0.016 1.197 0.023
1.921 0.141 0.019 — 0.303 0.004 0.754 0.846 0.036
— 0.001
1.989 0.072 0.003 0.001 0.658 0.013 1.217 0.017
—
3.991
4.023
1.961 0.116
0.002
0.001 0.247 0.005 0.756 0.875 0.035 0.004
2.087 0.036 — 0.002 0.376 0.186 1.353 0.016 0.010 —
2.043 0.121 0.006 0.008 0.200 0.009 1.056 0.431 0.037 0.002
3.973
4.000
4.066
3.911
—
—
1.854 0.278 — 0.001 0.371 0.006 1.487 0.009
1.976 0.100 — —
0.001
0.116 0.002 0.853 0.911 0.027 0.001
4.007
3.987
—
Unit
Main facies
SiO2 TiO2 Al2O3 FeO* MnO MgO CaO Na2O K2O Cr2O3
CH-14 Chilas Gabbronorite Plagioclase 56.13 57.36 0.01 — 28.65 28.58 0.12 0.15 — 0.08 — 0.02 9.31 8.58 4.65 5.65 0.37 0.40 — —
CH-34 Chilas Gabbronorite Plagioclase 55.28 53.96 — 0.04 28.36 29.29 0.01 0.10 0.08 — — 0.04 12.64 13.84 4.95 3.85 0.18 0.16 0.03 0.05
CH-122-1 West of Thor River conXuence Layered gabbronorite Plagioclase 51.27 51.42 0.03 — 31.82 31.83 0.24 0.16 0.02 0.01 0.02 — 14.84 14.87 2.58 3.11 0.39 0.36 0.04 0.01
CH-31 Chilas Ol gabbro Plagioclase 46.09 45.57 — — 32.39 33.33 — 0.04 — — 0.00 0.04 19.22 19.14 0.72 0.81 0.01 0.04 0.05
UMA association Ch32B Chilas Ol gabbro Plagioclase 46.48 44.54 0.10 0.05 34.53 34.73 0.13 0.04 — — 0.01 0.02 18.25 18.82 0.68 0.72 0.02 0.01 — 0.05
Total
99.23
101.52
101.25
98.47
98.96
100.20
0.714 0.270 0.020
2.159 1.788 — 0.001 — — — 0.964 0.065 0.000
2.126 1.832 — 0.002 0.001 — 0.003 0.956 0.073 0.000
4.999 71.1 26.9 2.0
4.9786 93.7 6.3 0.0
4.99349 92.9 7.1 0.0
100.81
101.33
101.77
98.97
Cations per 8 oxygens Si Al Ti Cr Fe Mn Mg Ca Na K
2.527 1.520 — — 0.004 — — 0.449 0.405 0.021
Total An (%) Ab (%) Or (%)
4.926 51.3 46.3 2.4
2.544 1.494 — —
2.468 1.492
2.417 1.546 0.001 0.002 0.004
0.604 0.428 0.010
0.003 0.664 0.335 0.009
2.307 1.687 0.001 0.001 0.009 0.001 0.001 0.715 0.225 0.022
5.005 58.0 41.1 1.0
4.980 65.9 33.2 0.9
4.971 74.3 23.4 2.3
— 0.001 0.006 0.003 0.001 0.408 0.485 0.023
4.963 44.5 53.0 2.5
— 0.003 —
Total Fe as FeO; —, below detection limit.
—
2.306 1.682 — — 0.006 — —
2.131 1.866 0.003
2.077 1.909 0.002 0.002 0.001
— 0.005 — —
— 0.896 0.061 0.001
0.001 0.940 0.065 0.000
4.964 93.5 6.3 0.1
4.998 93.5 6.4 0.0
0.984 — — 0.001 0.313 0.008 1.707 0.000 0.001 — 3.016
1.015 0.001 — — 0.397 0.006 1.564 0.001 0.002 — 2.985
Y. Takahashi et al. / Journal of Asian Earth Sciences 29 (2007) 336–349
plagioclase) is 74:2:8:16 D 37:1:4:8. Calculated primary magma (SiO2 D 47.5 wt%, Al2O3 D 12.4 wt%, FeOt D 10.5 wt%, MgO D 19.2 wt% and CaO D 7.1 wt%) is shown in the Harker diagrams (Fig. 6). Chemical compositions of olivine, orthopyroxene, clinopyroxene and plagioclase of the olivine gabbro (91CH-32B) in the UMA association were used in this calculation. Consequently, the olivine-dominant cumulate (dunite, wehrlite, lherzolite) and plagioclase-dominant cumulate (troctolite, olivine gabbro, gabbronorite, anorthosite) are explained by the accumulation of variable ratios of olivine, plagioclase, orthopyroxene and clinopyroxene crystals from the primary magma. For example, if olivine crystals are predominantly extracted from the primary magma and accumulated, the resulting cumulate rocks (dunite, wehrlite, lherzolite) will be poorer in SiO2 and CaO, compared with the primary magma. On the other hand, plagioclase-dominant cumulates (anorthosite) would be poorer in SiO2 and richer in CaO when compared with the primary magma. Consequently, the composition of the residual melt, which is assumed to be the initial magma of the main gabbronorite, will be richer in SiO2 compared with the primary
345
magma. Other major elements such as Al2O3, FeOt and MgO can also be explained in the same way. Chemical variations of the main gabbronorite are explained by the fractional crystallization and accumulation of plagioclase, orthopyroxene and clinopyroxene from the residual melt of the primary magma, which is the initial magma of the main gabbronorite. The fractional crystallization model for the gabbronorite of the main facies is examined with a least-squares mass balance calculation for the major elements. The results of this calculation are given in Table 3. For the process of the mass balance calculation, we use CH-43 (SiO2 D 52.74 wt%) as the initial magma, and CH-99 (SiO2 D 58.14 wt%, the most enriched sample) as the Wnal magma. The mineral compositions of the phases subtracted in the modelling are taken to be equivalent to the orthopyroxene, clinopyroxene and plagioclase in CH-122 (Table 3). Fractional crystallization of orthopyroxene, clinopyroxene, and plagioclase in the weight proportions of 30.01:5.27:64.72 is compatible with the derivation of the Wnal magma (CH-99) after 65.51% crystallization of the initial magma (CH-43).
Fig. 6. Variation diagrams of representative major elements (CaO, Al2O3, MgO, and FeOt) versus SiO2 for the rocks and constituent minerals (olivine, orthopyroxene, clinopyroxene, plagioclase) of the Chilas Complex. Total Fe is calculated as FeO. Red arrows indicate the change in composition of the main gabbronorite by fractional crystallization and accumulation of orthopyroxene, clinopyroxene and plagioclase from the initial magma.
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Table 3 Least-squares fractional crystallization model using major elements for the main gabbronorite of the Chilas Complex Initial magma
Final magma
Subtracted phase
CH-43
CH-99
Plagioclase
Clinopyroxene
Orthopyroxene
51.27 0.03 31.82 0.27 0.02 14.84 42.40% 64.72%
52.75 0.00 2.64 8.82 13.65 21.99 3.45% 5.27%
53.67 0.10 1.64 23.58 22.05 0.44 19.66% 30.01%
52.74 58.14 SiO2 0.40 0.93 TiO2 19.00 16.28 Al2O3 FeO 7.36 7.72 MgO 6.26 4.33 CaO 9.34 7.29 Calculated result (degree of crystallization D 65.51) Fraction properties Sum of squares D 0.04
This fractionation model for the main gabbronorite is examined in the following trace element and REE modelling. 4.2. Trace element modelling of the main gabbronorite As mentioned earlier, the trace element content of the gabbronorite of the main facies is related to the SiO2 content (Fig. 7). For geochemical modelling, the chemical composition of the main gabbronorite with intermediate SiO2 content (92CH-43, SiO2 D 52.7 wt%) is assumed to be the initial magma composition for the main gabbronorite of the Chilas Complex following the major element modelling. In order to explain the chemical variations of the trace elements (Fig. 7A), numerical calculations for the fractional crystallization of orthopyroxene, clinopyroxene and plagioclase (Opx/Cpx/Pl D 30:5:65) from the intermediate initial magma was carried out. Numerical calculations of fractional crystallization follow the Rayleigh fractionation law
Calculated melt
58.22 1.10 16.28 7.27 4.69 7.13 34.49%
(Hanson, 1978, 1980). Partition coeYcients used in these calculations are after Rollinson (1993). The trace element compositions for the main gabbronorite of the Chilas Complex lie mostly in the shaded area between the calculated compositions of 65% fractionated melt from the initial magma (92CH-43) and solid phases (orthopyroxene, cinopyroxene, plagioclase) coexisting with the melt. 4.3. Examination of the model by rare earth elements The content of rare earth elements for the main gabbronorite is also related to the SiO2 content. For geochemical modelling, the chemical composition for gabbronorite of intermediate SiO2 content (92CH-43, SiO2 D 52.74 wt%) is also assumed to be the initial magma composition for the main gabbronorite of the Chilas Complex. Numerical calculations for fractional crystallization of orthopyroxene, clinopyroxene and plagioclase (Opx/Cpx/Pl D 30:5:65) from the intermediate initial magma using rare earth elements
Fig. 7. (A) Spidergrams for the main gabbronorite of the Chilas Complex. An area between calculated compositions of 65% fractionated melt and solid phases (orthopyroxene, clinopyroxene, plagioclase) coexisting with the melt is shown as shaded pattern. (B) Spidergrams for the UMA association in the Chilas Complex. Compositional range for the main gabbronorite in the Chilas Complex are represented by the shaded area. Normalizing values of the primitive mantle are taken from Sun and McDonough (1989).
Y. Takahashi et al. / Journal of Asian Earth Sciences 29 (2007) 336–349
347
Fig. 8. (A) Chondrite-normalized REE compositions for the main gabbronorite of the Chilas Complex. An area between calculated compositions of 65% fractionated melt and solid phases (orthopyroxene, clinopyroxene, plagioclase) coexisting with the melt is shown as shaded pattern. (B) Chondrite-normalized REE composition for the UMA association in the Chilas Complex. Compositional range for the main gabbronorite of the Chilas Complex are shown by shaded area. Normalizing values of chondrite are taken from Sun and McDonough (1989).
was also carried out. Calculated compositions of 65% fractionated melt from the initial magma (92CH-43) and solid phases (orthopyroxene, clinopyroxene, plagioclase) coexisting with the melt shown in Fig. 8A. The Rayleigh fractionation law (Hanson, 1978, 1980) and partition coeYcients after Rollinson (1993) were used in these calculations. The rare earth element composition for the main gabbronorite is mostly in the shaded area between the calculated 65% fractionated melt from the initial magma (92CH-43) and the solid phases (orthopyroxene, clinopyroxene, plagioclase) coexisting with the melt. 5. Discussion The genetic relationship between the main gabbronorite and UMA association of the Chilas Complex is still not conWrmed. Jan et al. (1984) proposed a genetic link between the two groups of rocks and mentioned the possibility of the UMA being a cumulate from the main gabbronorite. Khan et al. (1989, 1993) interpreted the calc-alkaline main gabbronorite of the Chilas Complex as crystallizing in a subarc magma chamber and suggested that the original magma of the UMA association was a more primitive magma batch emplaced into the base of the gabbronorite magma chamber. They also proposed that the complex was generated from a rising diapir of magma formed during intra-arc rifting. On the other hand, Burg et al. (1998) suggested that the UMA association represents the apices of intra-arc mantle diapirs that served as porous Xow conduits to feed the gabbronorite. However, as described in this paper, rocks of the UMA association have characteristic accumulative structures and textures, which indicate they are cumulate in origin. The main gabbronorite was apparently intruded into the rocks of UMA association, cutting the layered structure of the plagioclase-dominant cumulate. However, around the contact zone between the main gab-
bronorite and UMA association, evidence of thermal eVects or of quenching have not been recognized. Therefore, the main gabbronorite and UMA association are considered to have been generated in a magma chamber contemporaneously. Mikoshiba et al. (2004) suggested that the REE composition of melt coexisting with the UMA association was similar to that of the main gabbronorite, based on REE composition of the clinopyroxene and plagioclase in the wehrlite, websterite and clinopyroxenite of the UMA association. This supports our cumulate and non-cumulate model of the Chilas Complex. A schematic scenario for the development of the Chilas Complex may be described as follows: 1. Intrusion of the primary magma (mantle diapir ?) into the root zone of the Kohistan Island Arc. 2. Crystallization and subsequent accumulation of olivine, orthopyroxene, clinopyroxene and plagioclase on the bottom of the magma chamber. 3. Fractionation and accumulation of the residual melt (initial magma of the main gabbronorite). 4. Intrusion of the fractionated initial magma into the cumulate of the primary magma at the bottom of the magma chamber. 6. Summary 1. The Chilas Complex is a huge basic intrusion, which forms the main part of the lower crust in the Kohistan Island Arc. It is composed mainly of gabbronorite and several masses of a ultramaWc–maWc–anorthosite (UMA) association. The UMA association is intruded by the main gabbronorite and is composed mainly of olivine-dominant cumulate (dunite, wehrlite, lherzolite) and plagioclase-dominant cumulate (troctolite, olivine
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gabbro, gabbronorite, anorthosite), with minor amount of pyroxene-dominant cumulate (websterite, clinopyroxenite). 2. On the AFM diagram, the chemical composition of the main gabbronorite plots mostly in the island arc noncumulate Weld, and those of the UMA associations plot in island arc cumulate Weld. On Harker diagrams, the geochemistry of the UMA association and the main gabbronorite are explained by the cumulate and non-cumulate models. Namely, olivine-dominant cumulate (dunite, wehrlite, lherzolite) and plagioclase-dominant cumulate (troctolite, olivine gabbro, gabbronorite, anorthosite) are generated by the accumulation of variable ratios of crystals of olivine, plagioclase, orthopyroxene and clinopyroxene from a primary magma. 3. The chemical composition of trace elements and REE in the main gabbronorite of the Chilas Complex are explained by fractional crystallization with the accumulation of plagioclase, orthopyroxene and clinopyroxene from an initial magma, which is derived from the primary magma after extraction of accumulative minerals (olivine, orthopyroxene, clinopyroxene, plagioclase) of the UMA association. Acknowledgements This study was carried out as collaborative work between the Geological Survey of Japan and the Geological Survey of Pakistan. The authors thank Dr. T. Shirahase, of the Geological Survey of Japan and Mr. S. H. Gauhar, of Geological Survey of Pakistan for their constructive discussions and support throughout the course of the collaborative work. We would like to acknowledge Prof. M. Q. Jan and anonymous reviewers for detailed and constructive reviews. Prof. K. Arita and A. Barber are acknowledged for improvement of the manuscript. Chemical analyses were carried out by Mr. A. Aziz, and Mr. Nasheem, using XRF at the Geoscience Laboratory, Geological Survey of Pakistan. Mineral chemistry was analyzed by Mr. I. H. Khan, using EPMA at Geoscience Laboratory, Geological Survey of Japan and Shimane University. Thin sections used in this study were made by Messrs Y. Sato, T. Nogami and A. Owada of Geological Survey of Japan. We sincerely thank them for their assistance. References Bard, J.P., 1983. Metamorphism of an obducted island arc: example of the Kohistan sequence (Pakistan) in the Himalayan collided range. Earth and Planetary Science Letters 65, 133–144. Barker, F., Arth, J.G., 1976. Generation of trondhjemitic tonalitic liquids and Archean bimodal trondhjemite-basalt suites. Geology 4, 596–600. Beard, J.S., 1986. Characteristic mineralogy of arc-related cumulate gabbros: implications for the tectonic setting of gabbroic plutons and for andesite genesis. Geology 14, 848–851. Burg, J.P., Bodinier, S., Chaudry, S., Dawood, H., 1998. Infra-arc mantlecrust transition and intra-arc mantle diapirs in the Kohistan Complex (Pakistan Himalaya): petro-structural evidence. Terra Nova 10, 74–80.
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Y. Takahashi et al. / Journal of Asian Earth Sciences 29 (2007) 336–349 Saunders, A.D., Norry, M.J. (Eds.), Magmatism in the Ocean Basins, 42. Geological Society, London Special Publication, pp. 313–345. Tahirkheli, R.A.K., Mattauer, M., Proust, F., Tapponnier, P., 1979. The India–Eurasia suture zone in northern Pakistan: synthesis and interpretation of recent data at plate scale. In: Farah, A., DeJong, K.A. (Eds.), Geodynamics of Pakistan. Geological Survey of Pakistan, Quetta, pp. 125–130. Takahashi, E., Kushiro, I., 1983. Melting of a dry peridotite at high pressures and basalt magma genesis. American Mineralogist 68, 859–879. Takahashi, Yut., Mikoshiba, M., Takahashi, Yuh., Kausar, A.B., Khan, T., 2003. Geology and geochemistry of the Chilas complex in the Kohistan Island Arc, northern Pakistan. Xinjiang Geology 21, 57–64.
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Geochemical modelling of the Chilas Complex in the Kohistan Terrane, northern Pakistan Yutaka Takahashi a,¤, Masumi U. Mikoshiba a, Yuhei Takahashi a, Allah Bakhsh Kausar b, Tahseenullah Khan b, Kazuya Kubo a b
a Geological Survey of Japan, AIST, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8567, Japan Geoscience Laboratory, Geological Survey of Pakistan, Chak Shazad, P.O. Box 1461, Islamabad, Pakistan
Received 4 January 2005; accepted 14 April 2006
Abstract The Chilas Complex in the Kohistan Terrane, Pakistan, is a huge basic intrusion, about 300 km long and up to 40 km wide, which is regarded as tilted island-arc type crust. It has been interpreted as the magma chamber root zone of the Kohistan Island Arc. The Chilas Complex is composed mainly of gabbronorite (main facies) and several masses of ultramaWc–maWc–anorthosite (UMA) association. The UMA association consists mainly of olivine-dominant cumulate (dunite, wehrlite, lherzolite) and plagioclase-dominant cumulate (troctolite, olivine gabbro, gabbronorite, anorthosite), with minor amount of pyroxene-dominant cumulate (clinopyroxenite, websterite). The major element geochemistry of the gabbronorite (main facies) and rocks of the UMA association, plotted on Harker diagrams, are explained by a cumulate and a non-cumulate model, respectively. Namely, the UMA association is explained as variable crystal cumulates from a primary magma and the gabbronorite of the main facies is explained as due to the fractionation of the residual melt. Chemical variations of major, trace and rare earth elements for the gabbronorite of the main facies in the Chilas Complex are explained by fractional crystallization and accumulation of plagioclase, orthopyroxene and clinopyroxene from the residual melt of the primary magma. © 2006 Elsevier Ltd. All rights reserved. Keywords: Chilas Complex; Kohistan; Pakistan; Cumulate; Fractional crystallization
1. Introduction The Asian and Indian plates are bounded by the IndusTsangpo Suture Zone (Gansser, 1964), but in the western Himalaya, the Kohistan and Ladakh terranes are sandwiched between the two plates (Tahirkheli et al., 1979) (Fig. 1). Based on Rb-Sr and K-Ar ages of pre- or syncollisional deformed leucogranites, and undeformed postcollisional basic dykes intruded into the northern part of the Kohistan Terrane the collision between the Asian continent and the Kohistan Terrane was inferred to have taken place between 102 and 75 Ma (Peterson and Windley,
*
Corresponding author. Tel.: +81 29 861 3655; fax: +81 29 861 3653. E-mail address: [email protected] (Y. Takahashi).
1367-9120/$ - see front matter © 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.jseaes.2006.04.007
1985). The main collision between the Asia-Kohistan Block and the Indian Plate was initiated at about 50 Ma (Patriat and Achache, 1984) with the obduction of the Kohistan Block onto the Indian Plate. The Kohistan Terrane is regarded as island arc-type crust, which was tilted during the collision, hence deeper facies are located in the south and shallower facies in the north (Coward et al., 1982; Bard, 1983) (Fig. 1). The Kohistan Island Arc is bounded to the south by the Main Mantle Thrust (MMT), and to the north by the Northern Suture (or Main Karakoram Thrust, MKT). Southern Kohistan includes the Jijal Complex, which is composed of peridotite and basic granulite (Jan and Howie, 1981), the Kamila Amphibolites and the Chilas Complex (Fig. 1). Northern Kohistan is dominated by numerous large-scale granitic plutons of the Kohistan Batholith, a sequence of arc-type
Y. Takahashi et al. / Journal of Asian Earth Sciences 29 (2007) 336–349
337
Fig. 1. SimpliWed geological map of the Kohistan terrane (modiWed from Treloar et al., 1990).
volcanic rocks called the Chalt Volcanics (Petterson and Windley, 1985, 1991), and a sequence of slates, turbidites and shallow marine Tethyan limestones of the Mid-Cretaceous Yasin Group (Pudsey, 1986) (Fig. 1). In addition, there are medium pressure/temperature type metamorphic rocks (Gilgit Formation) overlain by meta-volcanics (Gashu-ConXuence Volcanics) to the south of Gilgit, which have been correlated to the Chalt Volcanics (Khan et al., 1994). The Chilas Complex is a huge basic intrusion up to 40 km wide and stretches for 300 km along the entire length of the terrane (Jan et al., 1984). It has been interpreted as a magma chamber root zone of the Kohistan Island Arc (Khan et al., 1989), emplaced during intra-arc rifting (Khan et al., 1994). The intrusion of the Chilas Complex was inferred to have taken place at around 111 § 24 Ma, which was estimated by Rb-Sr whole rock isochron age for the gabbronorite of the main facies (Mikoshiba et al., 1999). However, U-Pb zircon dating has yielded 84–85 Ma ages for the gabbronorites from the Swat Valley (Zeitler et al., 1980; Schaltegger et al., 2002). The cooling age of the gabbronorite through 500 °C, estimated by K-Ar and 40 Ar–39Ar ages of hornblende, was around 80 Ma (Treloar et al., 1990). In this paper, we present geochemical modelling of the Chilas Complex based on detailed geological and petrographical studies. 2. Geology of the Chilas Complex 2.1. General geology The Chilas Complex is composed mainly of gabbronorite with minor amount of pyroxene quartz diorite (main gabbronorite) and several masses of the ultramaWc–maWc–
anorthosite (UMA) association (Khan et al., 1989; Khan and Jan, 1992). The gabbronorite of the Chilas Complex is bounded on the south by the sheared Kamila Amphibolites (Treloar et al., 1990). In the northern part of the complex, quartz diorite to tonalite of the Kohistan Batholith is intruded into the Chilas Complex (Fig. 2). At lower reaches of the Kinner River, northeast of Chilas, a xenoblock of garnet-biotite gneiss occurs in the main gabbronorite, and at upper reaches of the Hodda River, which is a right hand branch of the Indus River coming from the north, a xenoblock of garnet-biotite gneiss occurs in foliated quartz diorite of the Kohistan Batholith (Fig. 2). The gabbronorite and quartz diorite of the main facies are composed of orthopyroxene, clinopyroxene and plagioclase with small amount of hornblende, biotite, quartz, magnetite and ilmenite, exhibiting granoblastic textures. The rocks of the Chilas Complex are more or less deformed, exhibiting foliation. An E-W trending antiform and synform are observed, indicating that the Chilas Complex has a folded structure (Fig. 2). 2.2. The ultramaWc–maWc–anorthosite (UMA) association The UMA association occurs mostly in eastern part of the Chilas Complex (Fig. 2). An UMA body just to the east of Chilas has excellent outcrops (Figs. 2, 3A and B, 4). It is composed mainly of olivine-dominant cumulate (dunite, wehrlite, lherzolite) (Fig. 3E) and plagioclasedominant cumulate (troctolite, olivine gabbro, gabbronorite, anorthosite) (Fig. 3D) with minor amount of pyroxenedominant cumulate (clinopyroxenite, websterite) (Fig. 3F). The plagioclase-dominant cumulate often develops a layered structure on the olivine-dominant part, with sedimentary structures such as graded bedding and trough
338
Y. Takahashi et al. / Journal of Asian Earth Sciences 29 (2007) 336–349
Fig. 2. Geological map of eastern part of the Chilas Complex. The large area left blank is the outcrop of the gabbronorite of the main facies.
banding etc. (Figs. 3A and B, 4) (Jan et al., 1984; Khan and Jan, 1992; Takahashi et al., 2003). Within each gabbroic band of the plagioclase-dominant cumulate, the eastern side exhibits an upper sequence, grading from a Wne-layered bottom part into a massive part (Fig. 4). The main gabbronorite of the Chilas Complex is intruded into the plagioclase-dominant cumulate and partly cuts the layered structure (Fig. 3A). Also, the main gabbronorite intrudes into the olivine-dominant cumulate of the UMA association in some places (Fig. 4). Under the microscope, the well-layered part of the plagioclase-dominant cumulate in the UMA association has an accumulate texture (Fig. 3D), whereas, the main gabbronorite, consisting of orthopyroxene, clinopyroxene and plagioclase, with small amounts of hornblende, biotite, magnetite and ilmenite, has a granoblastic texture (Fig. 3C).
lites correspond to the Kamila Amphibolites. Furthermore, there are many amphibolites which look like xenoblocks within the Chilas Complex (Fig. 2). The Kamila Amphibolites are classiWed into meta-volcanic and meta-plutonic amphibolites based on their lithofacies (Treloar et al., 1990) and geochemistry (Khan et al., 1993). Amphibolite xenoliths in the main gabbronorite of the Chilas Complex in the Thor River may be meta-volcanic amphibolite, and amphibolites looking like xenoblocks in the Chilas Complex may be hydrated and recrystallized products derived from the main gabbronorite. The Chilas Complex is a very large intrusive body emplaced into the Kohistan arc meta-volcanic amphibolites and metasediments.
2.3. Relation to the Kamila Amphibolites
The geochemistry of the Chilas Complex has already been reported by Khan et al. (1989) and a tectonic model of the Kohistan Island Arc was presented, based on the geochemistry (Khan et al., 1993, 1997). The following description is based upon our original data, generated during the present study.
Some parts of the Kamila Amphibolites are interpreted to have been derived from the Chilas Complex (Treloar et al., 1990). Accordingly, the northern part of the Kamila Amphibolites is interpreted as a hydrated and recrystallized product derived from the gabbronorite in the Chilas Complex. However, in the upper reaches of the Thor River (Fig. 2), the main gabbronorite of the Chilas Complex intrudes into amphibolite and includes many xenoliths of amphibolite. These amphibo-
3. Geochemistry of the Chilas Complex
3.1. Whole rock chemistry Major elements of whole rock samples were analyzed by XRF (RIGAKU XRF-3370E) at Geoscience Laboratory,
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339
Fig. 3. (A) Well developed layered structure of plagioclase-dominant cumulate of the UMA association. Layered structure developed in olivine-dominant cumulate, grading into massive olivine gabbro. Gabbronorite of the main facies intrudes into the plagioclase-dominant cumulate, partly cutting the layered structure. (B) Close up view of the rhythmic layering of the plagioclase-dominant cumulate. Graded bedding is clear, indicating upward younging. (C) Photomicrograph of the main gabbronorite, showing a granoblastic texture. Crossed polars. (D) Photomicrograph of the plagioclase-dominant cumulate (anorthosite) of the UMA association, showing an adcumulate texture. Crossed polars. (E) Photomicrograph of olivine-dominant cumulate (dunite–wehrlite) of the UMA association. (F) Photomicrograph of pyroxene-dominant cumulate (clinopyroxenite) of the UMA association. Pl: plagioclase, Opx: orthopyroxene, and Cpx: clinopyroxene.
Geological Survey of Pakistan. Trace and rare earth elements were analyzed by ICP–MS at Sumiko Consultants Ltd. and the Geological Survey of Japan following the method of Imai (1990) and Ujiie and Imai (1995). Representative whole rock chemical compositions (major, trace and rare earth elements) for the main gabbronorite and rocks of the UMA association in the Chilas Complex are presented in Table 1.
3.1.1. Major elements Bulk chemical compositions for constituent rocks of the Chilas Complex are plotted on the AFM diagram, along with those of the Kohistan Batholith (Fig. 5). Also shown are the Welds of island arc non-cumulate and island arc maWc–ultramaWc cumulate by Beard (1986). In this diagram, chemical compositions for the main gabbronorite of
340
Table 1 Representative whole rock chemical compositions (major, trace and rare earth elements) of the main gabbronorite and rocks of UMA association in the Chilas Complex Unit
Main facies (gabbronorite)
Sample no.
92CH-18
92CH-20
92CH-43
92CH-63
92CH-82
92CH-87
92CH-91
92CH-99
92CH-100
92CH-114
Locality
East of Chilas
Thak River
West of Chilas
Thor River
Khanbari River
Khanbari River
Kiner River
Kiner River
Kiner River
Hoda River
Rock name
Gabbronorite
Gabbronorite
Gabbronorite
Gabbronorite
px.-qtz diorite
Gabbronorite
Gabbronorite
Gabbronorite
Gabbronorite
Gabbronorite
Total a
MgO/(MgO+Fe2O3 ) (ppm) Cr Ni Rb Ba Th Nb Sr Hf Zr Y La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
52.54 0.35 18.57 8.03 0.13 7.74 9.57 2.77 0.27 0.03
51.60 0.40 20.34 7.59 0.12 5.91 10.61 3.04 0.18 0.01
52.74 0.40 19.00 7.36 0.12 6.26 9.34 2.71 0.43 0.05
51.37 0.82 18.82 9.68 0.16 5.46 9.90 3.25 0.36 0.10
58.09 0.68 18.29 7.37 0.13 3.13 7.91 3.71 0.50 0.17
55.05 0.88 17.31 8.82 0.14 4.86 8.51 3.23 1.06 0.14
54.06 0.86 18.65 8.97 0.14 4.36 8.59 3.55 0.62 0.18
58.14 0.93 16.28 7.72 0.12 4.33 7.29 3.32 1.43 0.20
51.95 0.84 19.62 8.92 0.14 4.81 9.87 3.41 0.28 0.12
54.43 0.97 17.47 8.88 0.14 4.98 8.69 3.50 0.72 0.22
100.00
99.80
98.41
99.92
99.98
100.00
99.98
99.76
99.96
100.00
0.49 249 89 0.7 55.6 — 0.3 376 — 20 7.0 2.02 4.17 0.60 2.86 0.85 0.49 1.15 0.20 1.32 0.26 0.81 0.11 0.78 0.12
0.44 143 44 0.5 44.7 — 0.1 423 — 16 5.0 1.23 2.61 0.38 1.82 0.53 0.48 0.86 0.15 0.85 0.19 0.56 0.09 0.58 0.09
0.46 159 83 4.16a 89.7 0.2 0.7 363.0 — 33.0 9.0 3.35 7.05 1.01 4.52 1.36 0.58 21.64 0.26 1.86 0.37 1.11 0.17 1.19 0.17
0.36 119 33 1.8 98.1 0.2 0.9 404.0 — 27.0 20.0 4.34 8.85 1.32 5.97 1.80 1.05 2.48 0.42 2.89 0.59 1.78 0.24 1.58 0.25
0.30 89 8 1.9 208.4 0.0 3.3 490 — 121 27.0 9.28 21.19 3.17 15.54 4.32 1.15 5.13 0.79 4.80 1.06 2.94 0.44 2.95 0.42
0.36 105 42 14.53a 156.4 0.7 3.4 334 — 95 21.0 9.80 20.59 2.69 12.19 3.16 0.96 3.80 0.62 4.00 0.81 2.36 0.32 2.15 0.34
0.33 71 25 6.0 180.0 1.0 2.0 350 2.0 33 16.0 7.0 15.0 1.9 8.0 2.2 1.0 2.0 0.4 2.6 0.5 1.3 0.2 1.5 0.2
0.36 157 38 33.66a 285.9 1.0 3.8 362.1 — 163.0 23.0 12.67 26.70 3.70 15.52 3.93 1.00 4.17 0.66 4.10 0.83 2.27 0.38 2.20 0.34
0.35 92 28 3.0 150.0 1.0 2.0 320 2.0 3.0 10.0 1.0 8.0 1.0 5.0 1.6 0.8 1.4 0.2 1.8 0.4 1.0 0.2 1.1 0.1
0.36 136 39 6.0 180.0 1.0 2.0 340 2.0 72 18.0 7.0 20.0 2.4 11.0 2.6 0.9 2.3 0.4 2.7 0.6 1.3 0.3 1.3 0.2
Y. Takahashi et al. / Journal of Asian Earth Sciences 29 (2007) 336–349
SiO2 TiO2 Al2O3 Fe2O3a MnO MgO CaO Na2O K2O P2O5
Total a
MgO/(MgO+Fe2O3 ) (ppm) Cr Ni Rb Ba Th Nb Sr Hf Zr Y La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
92CH-75
92CH-77
92CH-116
92CH-7
92CH-9
92CH-59
East of Chilas Lherzolite
Thurli River Dunite
Thurli River Wehrlite
Thurli River Wehrlite
Khanbari River Websterite
Khanbari River Lherzolite
Thak River pl-lherzolite
Thak River Anorthothite
East of Chilas Anorthothite
Thor River hbl. gabbro
38.13 0.20 2.85 18.60 0.15 35.04 3.89 — 0.02 0.01
38.04 0.04 0.93 19.54 0.22 41.41 0.65 — — —
42.46 0.20 1.95 14.08 0.18 30.52 9.23 — — —
39.25 0.06 1.41 15.50 0.18 42.43 1.07 — 0.04 0.01
45.94 0.21 5.81 13.29 0.13 29.53 3.95 0.13 0.03 0.01
38.05 0.05 1.82 20.61 0.23 37.35 1.60 0.05 0.03 0.01
41.37 0.08 7.07 15.92 0.19 28.39 6.96 0.09 — —
44.06 0.04 27.42 3.98 0.06 6.30 15.55 0.62 — —
45.78 0.11 22.25 4.20 0.06 8.86 17.23 0.70 0.04 0.01
54.68 0.84 17.77 8.93 0.16 4.59 9.37 3.03 0.31 0.14
98.88
100.83
98.62
99.95
99.02
99.79
100.07
98.03
99.24
99.81
0.65 3600 820 2.0 10.0 1.0 2.0 10 2.0 3.0 4.0 <1.0 1.0 0.2 1.0 0.2 0.1 0.3 0.1 0.4 0.1 0.1 <0.1 0.2 <0.1
0.68 700 1700 2.0 — 1.0 — 12 — — — <1.0 2.0 <5.0 <5.0 <0.1 <0.5 <50.0 0.8 <1.0 <1.0 <20.0 <1.0 0.1 0.1
0.68 2500 1200 1.0 40.0 1.0 2.0 16 2.0 3.0 6.0 <1.0 2.0 0.3 2.0 0.5 0.3 0.5 0.1 0.7 0.2 0.4 0.1 0.4 —
0.73 1000 1880 2.0 10.0 1.0 2.0 7 2.0 3.0 2.0 <1.0 1.0 0.2 1.0 0.2 0.1 0.1 <0.1 0.3 <0.2 0.1 <0.1 0.2 <0.1
0.69 9000 1000 1.0 10.0 1.0 2.0 10 2.0 3.0 8.0 <1.0 <2.0 0.1 1.0 0.2 0.1 0.5 0.1 0.4 0.1 0.2 <0.1 0.3 <0.1
0.64 700 1500 2.0 10.0 1.0 — 16 2.0 3.0 4.0 <1.0 1.0 0.2 1.0 0.1 0.1 0.3 0.1 0.3 <0.2 0.1 <0.1 0.1 <0.1
0.61 222 102 0.13a 21.0 — 0.3 406 — 29.0 3.2 0.36 0.65 0.11 0.56 0.15 0.19 0.20 0.04 0.22 0.04 0.13 0.02 0.10 0.01
0.68 300 166 0.3 16.4 — 0.4 357 — 11.0 2.0 0.48 1.04 0.19 1.03 0.37 0.21 0.51 0.10 0.59 0.12 0.33 0.05 0.25 0.04
0.34 81 12 4.0 160 1.0 4.0 310 2.0 27.0 16.0 5.0 13.0 1.6 8.0 1.9 1.1 1.7 0.3 2.3 0.6 1.1 0.2 1.6 0.2
0.64 — — 2.0 — 1.0 — 110 — — — <1.0 <2.0 <5.0 <5.0 0.2 <0.05 <50.0 0.2 <1.0 <1.0 <20.0 <1.0 0.1 <0.1
Y. Takahashi et al. / Journal of Asian Earth Sciences 29 (2007) 336–349
SiO2 TiO2 Al2O3 Fe2O3a MnO MgO CaO Na2O K2O P 2O 5
UltramaWc–maWc–anorthothite (UMA) association 92CH-5 92CH-44 92CH-47 92CH-55
Total Fe as Fe2O3; —, below detection limit. px.: pyroxene, qtz.: quartz, Riv.: river. a Determined by isotope dilution method.
341
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Y. Takahashi et al. / Journal of Asian Earth Sciences 29 (2007) 336–349
cumulate (anorthosite, gabbronorite) is plotted near the cluster of plagioclase crystals. 3.1.2. Trace elements Trace element compositions for the main gabbronorite and the UMA association of the Chilas Complex are shown on primitive mantle-normalized spidergrams (Figs. 7A and B). In the spidergram for the main gabbronorite (Fig. 7A), the large ion lithoWle (LIL) elements are enriched, and Nb and Sr show negative and positive anomalies, respectively. These are geochemical characteristics of island arc-type basalts (Wilson, 1989). The concentrations of trace elements are related to SiO2 contents of the main gabbronorite (Fig. 7A). Namely, basic compositions (SiO2 < 52 wt%) have the lowest, and acidic rocks (SiO2 > 58 wt%) the highest amount of trace elements. The intermediate rocks (52 wt% < SiO2 < 58 wt%) have intermediate amounts of trace elements. In the case of UMA association, the content of trace elements is relatively low, and high Weld strength (HFS) elements such as P and Ti are relatively depleted (Fig. 7B). These features were also reported by Khan et al. (1989) and are attributed to their cumulate characters. 3.1.3. Rare earth elements Chondrite-normalized rare earth elements (REE) patterns for the main gabbronorite and UMA association of the Chilas Complex are shown in Fig. 8. In the diagram for the main gabbronorite (Fig. 8A), light rare earth elements (LREE) show enriched patterns, which are typical for island arc-type basalt (Wilson, 1989). The relatively basic rocks (SiO2 < 52 wt%) of the main gabbronorite have relatively depleted REE contents and positive Eu anomalies. On the other hand, relatively acidic rocks (SiO2 > 58 wt%)
Fig. 4. Columnar section for the UMA association east of Chilas Town (modiWed from Kubo, 2004).
the Chilas Complex are plotted mostly in the Weld of island arc non-cumulate and those for the rocks of UMA association are plotted in the Weld of island arc maWc–ultramaWc cumulate. The analyses of the Kohistan Batholith are plotted mostly in the alkali-rich Weld, compared with the main gabbronorite of the Chilas Complex. Representative major element (Al2O3, FeOt, MgO, and CaO) compositions for the rocks of the Chilas Complex along with constituent mineral compositions are plotted on Harker diagrams (Fig. 6). Data from granitic rocks of the Kohistan Batholith are also plotted for comparison. In these diagrams, the plots of the main gabbronorite cluster along a straight trend. But rocks of the UMA association are plotted separately from the trend. Olivine-dominant cumulate (dunite, wehrlite, lherzolite) is plotted close to the cluster of olivine crystals. And the plagioclase-dominant
Fig. 5. Major element geochemistry of the Chilas Complex plotted on the AFM (Na2O + K2O – FeOt – MgO) diagram. Compositional Welds of island arc non-cumulate and island arc maWc–ultramaWc cumulate by Beard (1986) are indicated by solid and broken lines, respectively. The boundary between tholeiite and calc-alkaline rocks after Barker and Arth (1976) are also shown. Data from the Kohistan Batholith are plotted for comparison.
Y. Takahashi et al. / Journal of Asian Earth Sciences 29 (2007) 336–349
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of the main gabbronorite are relatively enriched in REE contents and have negative Eu anomalies. Most of the rocks of intermediate gabbronorite (52 wt% < SiO2 < 58 wt%) have intermediate REE contents, without apparent Eu anomalies. In the case of UMA association (Fig. 8B), contents of REE are relatively low. This is concordant to their cumulate character, because REE are basically incompatible, especially in the case of basic magma (Hanson, 1980).
gabbronorite (MgO D 13.1–14.5 wt%, FeOt D 7.4–9.9 wt%) (Fig. 6, Table 2). On Harker diagrams, the chemical compositions of clinopyroxene in the UMA association plot in restricted areas (Fig. 6). The mineral chemistry described above supports the petrographic and geochemical subdivision of the Chilas Complex into two groups (the main gabbronorite and UMA association) as previously noted by Jan et al. (1984) and Khan et al. (1989).
3.2. Mineral chemistry
4. Geochemical modelling of the Chilas Complex
Chemical compositions of constituent minerals in the main gabbronorite and rocks of the UMA association were analyzed using EPMA at Geological Survey of Japan, Shimane University and the Geoscience Laboratory, Geological Survey of Pakistan. The accelerating voltage, specimen current and beam diameter were kept at 15 kV, 12 nA and 2 m, respectively. Representative chemical compositions for constituent minerals (orthopyroxene, clinopyroxene, olivine, plagioclase) of the main gabbronorite and UMA association in the Chilas Complex are presented in Table 2.
4.1. Major element modelling
3.2.1. Plagioclase The chemical compositions of plagioclase in the main gabbronorite and UMA association are plotted on Harker diagrams, together with olivine, orthopyroxene and clinopyroxene (Fig. 6, Table 2). Plagioclases in the UMA association have high anorthite contents (An84-94), but anorthite contents of plagioclase in the main gabbronorite are relatively low (An44-74). Accordingly, plagioclases in the main gabbronorite have higher SiO2 (51.3–57.4 wt%) and lower CaO (8.6–14.9 wt%) contents, compared with those of the UMA association (SiO2: 40.5–47.1 wt%, CaO: 13.9– 20.2 wt%). 3.2.2. Olivine The forsterite contents of olivine in the UMA association (dunite, wehrlite, lherzolite, troctolite) are Fo75 to Fo83. On Harker diagrams, chemical compositions of olivine are plotted in restricted areas, which have SiO2 contents of 36.7–41.7 wt% and MgO contents of 38.4–53.1 wt% (Fig. 6). 3.2.3. Orthopyroxene Chemical compositions of orthopyroxene in the UMA association (lherzolite, olivine gabbro, websterite) are relatively rich in MgO (25.5–30.8 wt%) and poor in FeOt (11.0– 15.6 wt%) compared with the main gabbronorite (MgO D 17.2–23.7 wt%, FeOt D 20.4–24.9 wt%) (Fig. 6, Table 2). On Harker diagrams, the chemical composition of orthopyroxene plots in restricted areas (Fig. 6). 3.2.4. Clinopyroxene Chemical compositions of clinopyroxene in the UMA association (wehrlite, olivine gabbro, clinopyroxenite, websterite) are relatively rich in MgO (15.2–20.0 wt%) and poor in FeOt (2.7–7.8 wt%) compared with those of the main
As mentioned earlier, the chemical composition of the main gabbronorite of the Chilas Complex mostly plots in the Weld of island arc non-cumulate, and those of the UMA association in the Weld of island arc maWc–ultramaWc cumulate on the AFM diagram (Fig. 5). This is consistent with the geological, petrographical characters that the rocks of the UMA association have accumulative structures (Figs. 3A and B) and textures (Figs. 3D–F). On Harker diagrams (Fig. 6), the chemical compositions of the gabbronorite and rocks of the UMA association are explained by a cumulate and a non-cumulate model (Fig. 6). At Wrst the chemical composition of the main gabbronorite, which can coexist with olivine in the UMA association, is assumed to be the melt composition of the primary magma. The olivine-liquid Fe/Mg partition coeYcient KD(Fe/Mg)ol-liq D (Fe/Mg)ol/(Fe/Mg)liq is nearly constant (around 0.3) in basaltic magma (Takahashi and Kushiro, 1983). In the case of the intermediate gabbronorite (CH-43, SiO2 D 52.74 wt%), KD(Fe/Mg)ol-liq D 0.309, which can coexist with olivine in the UMA association and suitable for the melt composition of the primary magma. The chemical composition of the primary magma should be in the area among olivine, orthopyroxene, clinopyroxene and plagioclase in the UMA association, and the inferred melt (CH-43, SiO2 D 52.7 wt%) on the Harker diagram (Fig. 6). The composition of the primary magma can be calculated from the ratio of the subtracted phases (olivine, orthopyroxene, clinopyroxene, plagioclase) and melt. However, the ratio of the subtracted phases and melt is unknown. Hence, the composition of the primary magma is calculated based on an assumption that the primary magma is composed of olivine, orthopyroxene, clinopyroxene and plagioclase, and melt with a ratio of 37:1:4:8:50. The ratio of subtracted phases (olivine, orthopyroxene, clinopyroxene, plagioclase) is given, based on the proportion of these minerals in the UMA association near Chilas (Fig. 4). The volume ratio of the olivine-dominant cumulate, plagioclase-dominant cumulate, websterite and clinopyroxenite in the UMA association is about 78 (olivine 74, clinopyroxene 4):20 (plagioclase 16, orthopyroxene 1.5, clinopyroxene 2.5):1 (orthopyroxene 0.5, clinopyroxene 0.5):1 (clinopyroxene 1) (Fig. 4). Accordingly, the ratio of subtracted phases (olivine, orthopyroxene, clinopyroxene,
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Y. Takahashi et al. / Journal of Asian Earth Sciences 29 (2007) 336–349
Table 2 Representative chemical compositions of olivine, orthopyroxene, clinopyroxene and plagioclase in the main gabbronorite and UMA association in the Chilas Complex Unit
Main facies
UltramaWc–maWc–anorthothite (UMA) association
Sample
91CH-14
91CH-34
CH-122
91CH-24B
91CH-32B
91CH-45
91CH-32B
Locality
Chilas
Chilas
West of Thor River conXuence
Chilas
Chilas
Chilas
Chilas
Rock name
Gabbronorite
Gabbronorite
Layered gabbronorite
Websterite
ol. gabbro
Dunite
ol. gabbro
Mineral
Opx
Opx
Opx
Opx
Cpx
Opx
Cpx
Ol
Ol
SiO2 TiO2 Al2O3 Cr2O3 FeO MnO MgO CaO Na2O K2O
53.62 0.03 1.30 0.02 23.79 0.80 19.62 0.49 — 0.02
50.48 0.34 2.36 0.02 9.46 0.32 13.75 22.75 0.42 0.02
52.85 0.18 1.62 0.05 22.72 0.50 21.56 0.57 — 0.03
50.93 0.48 2.94 0.00 9.93 0.11 13.40 20.93 0.50 0.01
53.67 0.1 1.64 0.04 21.24 0.41 22.05 0.44 — 0.04
52.75 0 2.64 0.05 7.95 0.16 13.65 21.99 0.48 0.09
58.53 0.01 0.86 0.07 12.61 0.62 25.47 0.43 0.15 —
57.68 0.23 2.89 0.27 6.57 0.28 20.01 11.34 0.54 0.05
51.82 0.01 6.6 0.02 12.39 0.2 27.9 0.24 — 0.02
54.55 — 2.35 — 3.83 0.08 15.81 23.49 0.38 0.02
39.31 0.02 — 0.07 14.96 0.38 45.80 0.01 0.03 —
39.69 — 0.02 — 18.58 0.29 41.07 0.02 0.03 —
Total
99.70
99.91
100.06
99.23
99.63
99.76
98.74
99.86
99.2
100.51
100.59
99.69
Cpx
Cpx
Cpx
Cations per 6 oxygens Si Al Ti Cr Fe Mn Mg Ca Na K Total
2.011 0.057 0.001 0.001 0.746 0.026 1.097 0.020 — 0.001
1.903 0.105 0.010 0.001 0.298 0.010 0.772 0.919 0.031 0.001
3.959
4.050
Cations per 4 oxygens 1.969 0.071 0.005 0.001 0.708 0.016 1.197 0.023
1.921 0.141 0.019 — 0.303 0.004 0.754 0.846 0.036
— 0.001
1.989 0.072 0.003 0.001 0.658 0.013 1.217 0.017
—
3.991
4.023
1.961 0.116
0.002
0.001 0.247 0.005 0.756 0.875 0.035 0.004
2.087 0.036 — 0.002 0.376 0.186 1.353 0.016 0.010 —
2.043 0.121 0.006 0.008 0.200 0.009 1.056 0.431 0.037 0.002
3.973
4.000
4.066
3.911
—
—
1.854 0.278 — 0.001 0.371 0.006 1.487 0.009
1.976 0.100 — —
0.001
0.116 0.002 0.853 0.911 0.027 0.001
4.007
3.987
—
Unit
Main facies
SiO2 TiO2 Al2O3 FeO* MnO MgO CaO Na2O K2O Cr2O3
CH-14 Chilas Gabbronorite Plagioclase 56.13 57.36 0.01 — 28.65 28.58 0.12 0.15 — 0.08 — 0.02 9.31 8.58 4.65 5.65 0.37 0.40 — —
CH-34 Chilas Gabbronorite Plagioclase 55.28 53.96 — 0.04 28.36 29.29 0.01 0.10 0.08 — — 0.04 12.64 13.84 4.95 3.85 0.18 0.16 0.03 0.05
CH-122-1 West of Thor River conXuence Layered gabbronorite Plagioclase 51.27 51.42 0.03 — 31.82 31.83 0.24 0.16 0.02 0.01 0.02 — 14.84 14.87 2.58 3.11 0.39 0.36 0.04 0.01
CH-31 Chilas Ol gabbro Plagioclase 46.09 45.57 — — 32.39 33.33 — 0.04 — — 0.00 0.04 19.22 19.14 0.72 0.81 0.01 0.04 0.05
UMA association Ch32B Chilas Ol gabbro Plagioclase 46.48 44.54 0.10 0.05 34.53 34.73 0.13 0.04 — — 0.01 0.02 18.25 18.82 0.68 0.72 0.02 0.01 — 0.05
Total
99.23
101.52
101.25
98.47
98.96
100.20
0.714 0.270 0.020
2.159 1.788 — 0.001 — — — 0.964 0.065 0.000
2.126 1.832 — 0.002 0.001 — 0.003 0.956 0.073 0.000
4.999 71.1 26.9 2.0
4.9786 93.7 6.3 0.0
4.99349 92.9 7.1 0.0
100.81
101.33
101.77
98.97
Cations per 8 oxygens Si Al Ti Cr Fe Mn Mg Ca Na K
2.527 1.520 — — 0.004 — — 0.449 0.405 0.021
Total An (%) Ab (%) Or (%)
4.926 51.3 46.3 2.4
2.544 1.494 — —
2.468 1.492
2.417 1.546 0.001 0.002 0.004
0.604 0.428 0.010
0.003 0.664 0.335 0.009
2.307 1.687 0.001 0.001 0.009 0.001 0.001 0.715 0.225 0.022
5.005 58.0 41.1 1.0
4.980 65.9 33.2 0.9
4.971 74.3 23.4 2.3
— 0.001 0.006 0.003 0.001 0.408 0.485 0.023
4.963 44.5 53.0 2.5
— 0.003 —
Total Fe as FeO; —, below detection limit.
—
2.306 1.682 — — 0.006 — —
2.131 1.866 0.003
2.077 1.909 0.002 0.002 0.001
— 0.005 — —
— 0.896 0.061 0.001
0.001 0.940 0.065 0.000
4.964 93.5 6.3 0.1
4.998 93.5 6.4 0.0
0.984 — — 0.001 0.313 0.008 1.707 0.000 0.001 — 3.016
1.015 0.001 — — 0.397 0.006 1.564 0.001 0.002 — 2.985
Y. Takahashi et al. / Journal of Asian Earth Sciences 29 (2007) 336–349
plagioclase) is 74:2:8:16 D 37:1:4:8. Calculated primary magma (SiO2 D 47.5 wt%, Al2O3 D 12.4 wt%, FeOt D 10.5 wt%, MgO D 19.2 wt% and CaO D 7.1 wt%) is shown in the Harker diagrams (Fig. 6). Chemical compositions of olivine, orthopyroxene, clinopyroxene and plagioclase of the olivine gabbro (91CH-32B) in the UMA association were used in this calculation. Consequently, the olivine-dominant cumulate (dunite, wehrlite, lherzolite) and plagioclase-dominant cumulate (troctolite, olivine gabbro, gabbronorite, anorthosite) are explained by the accumulation of variable ratios of olivine, plagioclase, orthopyroxene and clinopyroxene crystals from the primary magma. For example, if olivine crystals are predominantly extracted from the primary magma and accumulated, the resulting cumulate rocks (dunite, wehrlite, lherzolite) will be poorer in SiO2 and CaO, compared with the primary magma. On the other hand, plagioclase-dominant cumulates (anorthosite) would be poorer in SiO2 and richer in CaO when compared with the primary magma. Consequently, the composition of the residual melt, which is assumed to be the initial magma of the main gabbronorite, will be richer in SiO2 compared with the primary
345
magma. Other major elements such as Al2O3, FeOt and MgO can also be explained in the same way. Chemical variations of the main gabbronorite are explained by the fractional crystallization and accumulation of plagioclase, orthopyroxene and clinopyroxene from the residual melt of the primary magma, which is the initial magma of the main gabbronorite. The fractional crystallization model for the gabbronorite of the main facies is examined with a least-squares mass balance calculation for the major elements. The results of this calculation are given in Table 3. For the process of the mass balance calculation, we use CH-43 (SiO2 D 52.74 wt%) as the initial magma, and CH-99 (SiO2 D 58.14 wt%, the most enriched sample) as the Wnal magma. The mineral compositions of the phases subtracted in the modelling are taken to be equivalent to the orthopyroxene, clinopyroxene and plagioclase in CH-122 (Table 3). Fractional crystallization of orthopyroxene, clinopyroxene, and plagioclase in the weight proportions of 30.01:5.27:64.72 is compatible with the derivation of the Wnal magma (CH-99) after 65.51% crystallization of the initial magma (CH-43).
Fig. 6. Variation diagrams of representative major elements (CaO, Al2O3, MgO, and FeOt) versus SiO2 for the rocks and constituent minerals (olivine, orthopyroxene, clinopyroxene, plagioclase) of the Chilas Complex. Total Fe is calculated as FeO. Red arrows indicate the change in composition of the main gabbronorite by fractional crystallization and accumulation of orthopyroxene, clinopyroxene and plagioclase from the initial magma.
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Table 3 Least-squares fractional crystallization model using major elements for the main gabbronorite of the Chilas Complex Initial magma
Final magma
Subtracted phase
CH-43
CH-99
Plagioclase
Clinopyroxene
Orthopyroxene
51.27 0.03 31.82 0.27 0.02 14.84 42.40% 64.72%
52.75 0.00 2.64 8.82 13.65 21.99 3.45% 5.27%
53.67 0.10 1.64 23.58 22.05 0.44 19.66% 30.01%
52.74 58.14 SiO2 0.40 0.93 TiO2 19.00 16.28 Al2O3 FeO 7.36 7.72 MgO 6.26 4.33 CaO 9.34 7.29 Calculated result (degree of crystallization D 65.51) Fraction properties Sum of squares D 0.04
This fractionation model for the main gabbronorite is examined in the following trace element and REE modelling. 4.2. Trace element modelling of the main gabbronorite As mentioned earlier, the trace element content of the gabbronorite of the main facies is related to the SiO2 content (Fig. 7). For geochemical modelling, the chemical composition of the main gabbronorite with intermediate SiO2 content (92CH-43, SiO2 D 52.7 wt%) is assumed to be the initial magma composition for the main gabbronorite of the Chilas Complex following the major element modelling. In order to explain the chemical variations of the trace elements (Fig. 7A), numerical calculations for the fractional crystallization of orthopyroxene, clinopyroxene and plagioclase (Opx/Cpx/Pl D 30:5:65) from the intermediate initial magma was carried out. Numerical calculations of fractional crystallization follow the Rayleigh fractionation law
Calculated melt
58.22 1.10 16.28 7.27 4.69 7.13 34.49%
(Hanson, 1978, 1980). Partition coeYcients used in these calculations are after Rollinson (1993). The trace element compositions for the main gabbronorite of the Chilas Complex lie mostly in the shaded area between the calculated compositions of 65% fractionated melt from the initial magma (92CH-43) and solid phases (orthopyroxene, cinopyroxene, plagioclase) coexisting with the melt. 4.3. Examination of the model by rare earth elements The content of rare earth elements for the main gabbronorite is also related to the SiO2 content. For geochemical modelling, the chemical composition for gabbronorite of intermediate SiO2 content (92CH-43, SiO2 D 52.74 wt%) is also assumed to be the initial magma composition for the main gabbronorite of the Chilas Complex. Numerical calculations for fractional crystallization of orthopyroxene, clinopyroxene and plagioclase (Opx/Cpx/Pl D 30:5:65) from the intermediate initial magma using rare earth elements
Fig. 7. (A) Spidergrams for the main gabbronorite of the Chilas Complex. An area between calculated compositions of 65% fractionated melt and solid phases (orthopyroxene, clinopyroxene, plagioclase) coexisting with the melt is shown as shaded pattern. (B) Spidergrams for the UMA association in the Chilas Complex. Compositional range for the main gabbronorite in the Chilas Complex are represented by the shaded area. Normalizing values of the primitive mantle are taken from Sun and McDonough (1989).
Y. Takahashi et al. / Journal of Asian Earth Sciences 29 (2007) 336–349
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Fig. 8. (A) Chondrite-normalized REE compositions for the main gabbronorite of the Chilas Complex. An area between calculated compositions of 65% fractionated melt and solid phases (orthopyroxene, clinopyroxene, plagioclase) coexisting with the melt is shown as shaded pattern. (B) Chondrite-normalized REE composition for the UMA association in the Chilas Complex. Compositional range for the main gabbronorite of the Chilas Complex are shown by shaded area. Normalizing values of chondrite are taken from Sun and McDonough (1989).
was also carried out. Calculated compositions of 65% fractionated melt from the initial magma (92CH-43) and solid phases (orthopyroxene, clinopyroxene, plagioclase) coexisting with the melt shown in Fig. 8A. The Rayleigh fractionation law (Hanson, 1978, 1980) and partition coeYcients after Rollinson (1993) were used in these calculations. The rare earth element composition for the main gabbronorite is mostly in the shaded area between the calculated 65% fractionated melt from the initial magma (92CH-43) and the solid phases (orthopyroxene, clinopyroxene, plagioclase) coexisting with the melt. 5. Discussion The genetic relationship between the main gabbronorite and UMA association of the Chilas Complex is still not conWrmed. Jan et al. (1984) proposed a genetic link between the two groups of rocks and mentioned the possibility of the UMA being a cumulate from the main gabbronorite. Khan et al. (1989, 1993) interpreted the calc-alkaline main gabbronorite of the Chilas Complex as crystallizing in a subarc magma chamber and suggested that the original magma of the UMA association was a more primitive magma batch emplaced into the base of the gabbronorite magma chamber. They also proposed that the complex was generated from a rising diapir of magma formed during intra-arc rifting. On the other hand, Burg et al. (1998) suggested that the UMA association represents the apices of intra-arc mantle diapirs that served as porous Xow conduits to feed the gabbronorite. However, as described in this paper, rocks of the UMA association have characteristic accumulative structures and textures, which indicate they are cumulate in origin. The main gabbronorite was apparently intruded into the rocks of UMA association, cutting the layered structure of the plagioclase-dominant cumulate. However, around the contact zone between the main gab-
bronorite and UMA association, evidence of thermal eVects or of quenching have not been recognized. Therefore, the main gabbronorite and UMA association are considered to have been generated in a magma chamber contemporaneously. Mikoshiba et al. (2004) suggested that the REE composition of melt coexisting with the UMA association was similar to that of the main gabbronorite, based on REE composition of the clinopyroxene and plagioclase in the wehrlite, websterite and clinopyroxenite of the UMA association. This supports our cumulate and non-cumulate model of the Chilas Complex. A schematic scenario for the development of the Chilas Complex may be described as follows: 1. Intrusion of the primary magma (mantle diapir ?) into the root zone of the Kohistan Island Arc. 2. Crystallization and subsequent accumulation of olivine, orthopyroxene, clinopyroxene and plagioclase on the bottom of the magma chamber. 3. Fractionation and accumulation of the residual melt (initial magma of the main gabbronorite). 4. Intrusion of the fractionated initial magma into the cumulate of the primary magma at the bottom of the magma chamber. 6. Summary 1. The Chilas Complex is a huge basic intrusion, which forms the main part of the lower crust in the Kohistan Island Arc. It is composed mainly of gabbronorite and several masses of a ultramaWc–maWc–anorthosite (UMA) association. The UMA association is intruded by the main gabbronorite and is composed mainly of olivine-dominant cumulate (dunite, wehrlite, lherzolite) and plagioclase-dominant cumulate (troctolite, olivine
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gabbro, gabbronorite, anorthosite), with minor amount of pyroxene-dominant cumulate (websterite, clinopyroxenite). 2. On the AFM diagram, the chemical composition of the main gabbronorite plots mostly in the island arc noncumulate Weld, and those of the UMA associations plot in island arc cumulate Weld. On Harker diagrams, the geochemistry of the UMA association and the main gabbronorite are explained by the cumulate and non-cumulate models. Namely, olivine-dominant cumulate (dunite, wehrlite, lherzolite) and plagioclase-dominant cumulate (troctolite, olivine gabbro, gabbronorite, anorthosite) are generated by the accumulation of variable ratios of crystals of olivine, plagioclase, orthopyroxene and clinopyroxene from a primary magma. 3. The chemical composition of trace elements and REE in the main gabbronorite of the Chilas Complex are explained by fractional crystallization with the accumulation of plagioclase, orthopyroxene and clinopyroxene from an initial magma, which is derived from the primary magma after extraction of accumulative minerals (olivine, orthopyroxene, clinopyroxene, plagioclase) of the UMA association. Acknowledgements This study was carried out as collaborative work between the Geological Survey of Japan and the Geological Survey of Pakistan. The authors thank Dr. T. Shirahase, of the Geological Survey of Japan and Mr. S. H. Gauhar, of Geological Survey of Pakistan for their constructive discussions and support throughout the course of the collaborative work. We would like to acknowledge Prof. M. Q. Jan and anonymous reviewers for detailed and constructive reviews. Prof. K. Arita and A. Barber are acknowledged for improvement of the manuscript. Chemical analyses were carried out by Mr. A. Aziz, and Mr. Nasheem, using XRF at the Geoscience Laboratory, Geological Survey of Pakistan. Mineral chemistry was analyzed by Mr. I. H. Khan, using EPMA at Geoscience Laboratory, Geological Survey of Japan and Shimane University. Thin sections used in this study were made by Messrs Y. Sato, T. Nogami and A. Owada of Geological Survey of Japan. We sincerely thank them for their assistance. References Bard, J.P., 1983. Metamorphism of an obducted island arc: example of the Kohistan sequence (Pakistan) in the Himalayan collided range. Earth and Planetary Science Letters 65, 133–144. Barker, F., Arth, J.G., 1976. Generation of trondhjemitic tonalitic liquids and Archean bimodal trondhjemite-basalt suites. Geology 4, 596–600. Beard, J.S., 1986. Characteristic mineralogy of arc-related cumulate gabbros: implications for the tectonic setting of gabbroic plutons and for andesite genesis. Geology 14, 848–851. Burg, J.P., Bodinier, S., Chaudry, S., Dawood, H., 1998. Infra-arc mantlecrust transition and intra-arc mantle diapirs in the Kohistan Complex (Pakistan Himalaya): petro-structural evidence. Terra Nova 10, 74–80.
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