Mineral chemistry and K–Ar ages of plutons across the Karakoram fault in the Shyok-Nubra confluence of northern Ladakh Himalaya, India

Gondwana Research 17 (2010) 180–188

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Mineral chemistry and K–Ar ages of plutons across the Karakoram fault in the Shyok-Nubra confluence of northern Ladakh Himalaya, India N.X. Thanh a,e,⁎, T. Itaya a, T. Ahmad b, S. Kojima c, T. Ohtani c, M. Ehiro d a

Research Institute of Natural Sciences, Okayama University of Science, 1-1 Ridai-cho, Okayama 700-0005, Japan Department of Geology, University of Delhi, Delhi-110007, India Department of Civil Engineering, Gifu University, Gifu 501-1193, Japan d The Tohoku University Museum, Aoba 6-3, Aramaki, Aoba-ku, Sendai 980-8578, Japan e Hanoi University of Mining and Geology, Dong Ngac, Tu Liem, Hanoi, Vietnam b c

a r t i c l e

i n f o

Article history: Received 14 August 2008 Received in revised form 5 August 2009 Accepted 22 August 2009 Available online 2 September 2009 Keywords: K–Ar geochronology Ladakh arc Hundar plutons Karakoram granite

a b s t r a c t Petrographic analyses of plutons across the Karakoram fault in the Shyok-Nubra confluence of northern Ladakh area revealed that the rocks consist of diorite, granodiorite, granite and leucogranite. Pyroxene and biotite chemistries by EPMA indicate that most of the diorite, granodiorite and granite are calc-alkaline and formed in an arc-related tectonic setting. These calc-alkaline plutons yield K–Ar biotite ages from 49.3 ± 1.1 to 51.2 ± 1.1 for Ladakh batholith in Khardung La and from 60.8 ± 1.3 to 65.8 ± 1.4 Ma for granites and diorites in Hundar area. The K–Ar ages of the Hundar igneous complex are within the age range of 46–70 Ma of the Ladakh batholith near Leh town and the neighbor sites, indicating the Hundar igneous complex as a part of the Ladakh arc. Their ages are also coincident with the Khardung volcanics (67.4 and 60.5 Ma). The calc-alkaline granites from Panamik of Karakoram block yield K–Ar biotite ages of 95.7 ± 2.1 and 96.7 ± 2.1 Ma, suggesting that the granites are member of the plutons (90–120 Ma) occurring in northern Pakistan, Tangtse in Ladakh and western Tibet that emplaced during subduction of the Neo-Tethyan oceanic plate under the Karakoram block. The leucogranite and granodiorite in the Panamik region yield K–Ar biotite and muscovite ages ranging from 9.18 ± 0.21 to 9.45 ± 0.21 Ma. The leucogranites are the S-type related to post-collision tectonic setting and are considered to have emplaced in relating to activation of the Karakoram fault. © 2009 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.

1. Introduction The Himalaya mountain chain defines the orogenic belt between Indian and Eurasian continents, where consumption of Neo-Tethyan Ocean and subsequent continental collision took place during Cenozoic (e.g. Rao et al., 2006; Goscombe and Gray, 2009; Bhattacharyya and Mitra, 2009). Many workers have studied the Ladakh Himalaya in order to elucidate the geologic and tectonic developments of the suture zones (Rowley, 1996; Weinberg and Dunlap, 2000; Weinberg et al., 2000; Jain et al., 2002; Upadhyay et al., 2004) and the pre- and post-collisional magmatism (Honegger et al., 1982; Debon et al., 1987; Parrish and Tirrul, 1989, Debon and Khan, 1996; Ahmad et al., 1996, 1998, 2008; Singh et al., 2003). These studies have revealed that the Ladakh arc is an plutono-volcanic arc developed in Cretaceous to Eocene time, and is composed of huge mass of granitic rocks with minor amounts of rhyolitic to andesitic volcanic rocks known as Khardung volcanics (Honegger et al., 1982; Debon et al.,

⁎ Corresponding author. Research Institute of Natural Sciences, Okayama University of Science, 1-1 Ridai-cho, Okayama 700-0005, Japan. E-mail address: [email protected] (N.X. Thanh).

1987; Parrish and Tirrul, 1989, Ahmad et al., 1996, 1998; Debon and Khan, 1996; Dunlap and Wysoczanski, 2002). The Karakoram block consisting of high-grade metamorphic rocks and granitic rocks was the southern margin of the Eurasian continent during the Late Mesozoic to early Tertiary time (Petterson and Windley, 1985). The ages of pre- and post-collision magmatism are well established by several authors (e.g. Debon et al., 1987; Ahmad et al., 1996; Weinberg and Searle, 1998; Weinberg and Dunlap, 2000). However, the age range and distribution of the plutons in the Shyok suture zone between Ladakh arc and Karakoram block are not fully determined yet. We, a group of Japanese, Indian and Vietnamese geologists, jointly studied the Indus and Shyok suture zones, Ladakh Himalaya, northern India, from 2004 to 2006 field seasons. During the field research, we discovered the Hundar granite–diorite complex that intruded into the sedimentary rock sequence in the Shyok suture zone and in turn are intruded by Ladakh batholith. Here, we present detailed petrography, mineral chemistry, K–Ar biotite and muscovite ages of the Hundar granite–diorite complex in the Nubra-Shyok confluence and have comparison with rocks in Khardung La of Ladakh arc and the pre- and post-collision plutons in Panamik of Karakoram block. We also discuss Cretaceous–Cenozoic igneous activity in Ladakh region of Himalaya.

1342-937X/$ – see front matter © 2009 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.gr.2009.08.002

N.X. Thanh et al. / Gondwana Research 17 (2010) 180–188

2. Outline of geology The Ladakh Himalaya is composed of five geotectonic units (Fig. 1): Karakoram block, Shyok suture zone, Ladakh arc, Indus suture zone and Tethys Himalayas, from north to south (Thakur and Rawat, 1992). The Karakoram block consists of high-grade metamorphic rocks and granitic rocks. The granites occur as a batholith with dimension extending about 700 km and have intruded into the Palaeozoic to Triassic sedimentary sequences of southern edge of the Eurasian Plate. They are interpreted to represent subductionrelated magmatism (Debon et al., 1987; Crawford and Searle, 1992), suggesting a subduction zone along the southern margin of Asian continent (Debon et al., 1987). The magmatic bodies are composed of predominant biotite granite with subordinate two-mica and hornblende-biotite granites. They give ages ranging 130 to 50 Ma (Srimal et al., 1986; Searle, 1991; Debon and Khan, 1996; Sinha et al., 1997; Weinberg and Searle, 1998). In the southern Karakoram Range, post-collisional leucogranites occur widely. They are correlated to Baltoro granite (21–26 Ma, Parrish and Tirrul, 1989) in Kohistan and Tangtse granite (17 Ma, Schärer et al., 1990; Searle

181

et al., 1998) in Ladakh. In Tangtse, leucogranites yield Ar–Ar biotite ages of 11.9 Ma and pegmatite dykes intruding into the Pangong metamorphic complex give 9.7 Ma (Dunlap et al., 1998). In the northeastern side of the Shyok-Nubra confluence, Srimal (1986) reported Ar–Ar biotite and muscovite ages (9.7–13 Ma) of leucogranites and Weinberg et al. (2000) presented a crystallized age (15 Ma) of leucogranite in Tirit. The Shyok suture zone between the Ladakh arc and the Karakoram block is composed of mafic-ultramafic rocks, sedimentary rocks and metamorphic rocks. The sedimentary unit is intruded by maficultramafic rocks on the southwestern margin, and cut by the dextral Karakoram fault in the northeast. Recently, Ehiro et al. (2007) divided the sedimentary unit into the lower Tsoltak Formation and the upper Shyok Formation. The Tsoltak Formation consists mainly of mudstone with minor amounts of sandstone and alternating sandstone and mudstone, and yields Callovian (Middle Jurassic) ammonoids such as Macrocephalites sp. and Jeanneticeras sp. (Ehiro et al., 2007). The Shyok Formation is subdivided into the Lower and Upper Members; the former is composed of sandstone, conglomerate and minor limestone, whereas the latter is volcaniclastic rocks. Limestone of the

Fig. 1. Geological map of Ladakh Himalaya, NW India showing the sample localities (modified after Thakur and Rawat, 1992).

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Lower Member includes Albian (upper part of Lower Cretaceous) orbitolinid foraminifera (Matsumaru et al., 2006). The Tsoltak and Shyok Formations are composed of mudstone, sandstone, conglomerate, limestone and volcaniclastic rocks, and are lacking in chert and melanges characteristic of subduction/accretion complexes, indicating that the formations were accumulated in a shallow marine shelf environment. Since the conglomerate includes granitic rocks, provenance of these formations is most probably mature continent and/or volcanic arc. The back arc sediments have been reported in the Shyok suture zone of the Kohistan and Ladakh areas (Crawford and Searle, 1992; Khan et al., 1997; Burg et al., 1998; Robertson and Collins, 2002). But, the fore arc sediments and accretion complexes are not observed so far. The Ladakh arc is composed of huge masses of batholithic rocks with minor amounts of rhyolitic to andesitic rocks that formed by an Andean-type calc-alkaline magmatism related to the subduction of the Neo-Tethys oceanic plate under the Eurasian continental plate during Early Cretaceous to Late Eocene (Sharma and Choubey, 1983; Jain et al., 2002). This type of batholithic rocks is distributed widely in Himalaya and named the Kohistan batholith in Pakistan, the Ladakh batholith in India, Kailas and Gangdese plutonic complex in South Tibet, Lohit batholith in Arunachal Prasdesh. The Ladakh batholith constitutes an important part of the Trans-Himalaya zone, which is about 600 km long and 30–80 km wide (Fig. 1). It extends from Astor, Deosai and Skardu in the northwest to Demchuk in the southeast through Leh, Upshi and Lyoma, and comprises mainly biotitehornblende-bearing granodiorite and granite, with minor occurrences of noritic gabbro and diorite (Ahmad et al., 1998; Rajeev et al., 2007). The Ladakh plutonic complex is an I-type granitoid and appears to have been emplaced between 100 and 40 Ma with dominant phase around 66–50 Ma (Honegger et al., 1982). Weinberg and Dunlap (2000) and Dunlap and Wysoczanski (2002) have presented SHRIMP zircon ages of the Khardung volcanics and Gyamsa migmatic complex near Leh where migmatites occur in the western side of the complex and granites, in the eastern side, and reconfirmed the dominant phases (50–70 Ma) of the Ladakh arc around Leh. The collision between the Kohistan–Ladakh arc complex and the Eurasian plate occurred at about 85–100 Ma (Brookfield and Reynolds, 1981; Petterson and Windley, 1985; Hanson, 1989; Searle et al., 1999). The Indus suture zone is the site of demise of the Neo-Tethyan oceanic plate and also the site of collision between the Asian and Indian plates from ~ 55–50 Ma (Patriat and Achache, 1984; Achache et al., 1984). In Ladakh, the suture zone consists of ophiolite and ophiolitic mélanges of Cretaceous to Tertiary periods (Ahmad et al., 1996, 2008; Upadhyay and Sinha, 1998; Robertson, 2000; Kojima et al., 2001). The Tethys Himalayas are composed of mainly the fossiliferous sediments of Precambrian to Cretaceous age that were accumulated on the northern margin of the Indian subcontinent (Rao et al., 2006). Its northern part has been metamorphosed under the ultra-highpressure metamorphic condition (Sachan et al., 2001, 2004; Gouzu et al., 2006).

Fig. 2. Geological route map of Hundar river area showing the locations of samples studied in detail. K–Ar ages are also shown.

4. Petrography and mineral chemistry 3. Sampling During field studies (between 2004 and 2006), we mapped the Hundar pluton (Fig. 2). The map shows that the Hundar granite– diorite complex has intruded into the sedimentary unit that has experienced thermal effects to form hornfels. Samples were collected systematically along the Hundar river section (Fig. 2). We also collected three samples of granite and granodiorite from the site of Khardung La in the Ladakh batholith and five samples of granite, leucogranite and granodiorite from the site of the Karakoram block in Panamik by Nubra valley (Fig. 1).

The details of the samples collected are described in Table 1, where sample number, rock type, GPS position, altitude of sampling site and mineral assemblage are tabulated. Photomicrographs of representative samples are shown in Fig. 3. Electron microprobe (EMP) analyses of minerals were carried out at Okayama University of Science using a JEOL JXA-8900R. The quantitative analyses of mineral chemistry were performed with 15 kV accelerating voltage, 12 nA beam current and 3 µm beam size. Natural and synthetic silicates and oxides were used for calibration. The ZAF method (oxide basis) was employed for matrix corrections. The mineral composition data are from Thanh and

N.X. Thanh et al. / Gondwana Research 17 (2010) 180–188 Table 1 Descriptions of the samples studied showing sample number, rock type, GPS position, altitude of sampling site and mineral assemblage. Sample no.

Altitude (m)

Khardung La 201 5154

Rock type

GPS

Mineral assemblage

Coarse-grained granite

N34°15′71.9″ E77°38′07.7″

Qtz, Kfs, Pl, Bt. Accessories: Hem, Mag, Sp, Zrn. Qtz, Kfs, Pl, Bt. Accessories: Mag, Ilm, Ap, Zrn. Pl, Qtz, Am, Bt. Accessories: Mag, Ilm, Sp, Zrn, Ap.

202A

5356

Coarse-grained granite

N34°16′74.3″ E77°36′35.4″

202B

5412

Coarse-grained granodiorite

N34°17′00.1″ E77°37′32.4″

Hornfels

Hundar river 301 3231 303

3268

Coarse-grained diorite

N34°35′10.8″ E77°27′53.4″ N34°34′65.7″ E77°27′45.2″

304B

3294

Fine-grained diorite

N34°34′56.6″ E77°27′42.3″

304X(D)

3294

Fine-grained diorite

N34°34′56.6″ E77°27′42.3″

304X(G)

3294

Coarse-grained granite

N34°34′56.6″ E77°27′42.3″

305A

3308

Coarse-grained diorite

N34°34′41.4″ E77°27′42.1″

305B

3308

Coarse-grained diorite

N34°34′41.4″ E77°27′42.1″

307A

3325

Medium-grained granite

N34°34′08.5″ E77°27′44.7″

401

3370

Medium-grained granite

N34°33′31.4″ E77°26′97.0″

Panamik area 502A 3224 502B

3224

502C

3224

602c

3172

602f

3172

Coarse-grained granodiorite Coarse-grained leucogranite Coarse 2-mica leucogranite Coarse-grained granite Fine-grained granite

N34°45′48.6″ E77°33′41.4″ N34°45′48.6″ E77°33′41.4″ N34°45′48.6″ E77°33′41.4″ N34°40′40.0″ E77°33′70.0″ N34°40′40.0″ E77°33′70.0″

Qtz, Bt, Pl, Kfs, and opq Pl, Cpx, Opx, Am, Bt, Qtz. Accessories: Ap, Ilm, Zrn, Mag. Pl, Am, Bt, Qtz. Accessories: Mag, Ilm, Ap. Pl, Am, Bt, Qtz, Accessories: Zrn, Ilm, Mag, Sp. Qtz, Pl, Kfs, Bt. Accessories: Zrn, Mag, Sp. Pl, Cpx, Bt, Am, Kfs, Qtz. Accessories: Mag, Ilm, Py, Ccp, Gn. Pl, Cpx, Opx, Bt, Am, Kfs, Qtz. Accessories: Mag, Sp, Zrn. Qtz, Kfs, Pl, Bt. Accessories: Ap, Zrn, Mag, Sp. Qtz, Pl, Kfs, Bt. Accessories: Mag, Ap, Zrn.

Pl, Qtz, Bt, Am, Kfs. Accessories: Mag, Zrn. Qtz, Kfs, Bt. Accessories: Ccp, Zrn. Kfs, Qtz, Pl, Bt, Ms. Accessories: Mag, Zrn. Kfs, Qtz, Pl, Bt. Accessories: Ccp, Zrn. Kfs, Qtz, Pl, Bt. Accessories: Ccp, Zrn.

Qtz: quartz, Kfs: alkaline-feldspar, Pl: plagioclase, Bt: biotite, Ms: muscovite, Hem: hematite, Mag: magnetite, Sp: sphalerite, Zrn: zircon, ilm: illimenite, Ap: apatite, Py: pyrite, Gn: galena, opq: opaque mineral.

Itaya (2007) and their representative data are listed in Appendices 1, 2, 3 and 4. 4.1. Panamik Five samples were collected along the roadside from Sumur to Panamik, northeastern side of Nubra valley. Among them, coarsegrained (602c) and fine-grained (602f) granites were collected from the site near La Bame Tso. Coarse-grained granodiorite (502A), leucogranite (502B) and two-mica leucogranite (502C) were collected from the site 5 km southeast of Panamik Monastery. The granodiorite (502A) consists of plagioclase, quartz, biotite and amphibole with magnetite and zircon as accessories. Plagioclases

183

occur as euhedral to subhedral crystals with twining and plot in the andesine to labradorite fields (An37–58) (Fig. 4a). Biotites are pale brown in color and slightly altered to chlorite along cleavages and have Fe2+/(Fe2++ Mg) ratio of 0.5, Al(a.p.f.u) of 2.6–2.8 (Fig. 5). Amphiboles occur as subhedral and medium-grained crystal. They are of calcic amphibole group and fall in the tschermakite-hornblende to magnesio-hornblende with Mg/(Mg + Fe2+) ratio of 0.5–0.6 (Fig. 6). The granites (602c,f) and leucogranites (502B,C) contain Kfeldspar, quartz, plagioclase, biotite ± muscovite and accessories of chalcopyrite and galena. The leucogranites are rich in K-feldspar and very poor in plagioclase. Sample 502C contains both biotite and muscovite. The granite samples (602c,f) are weakly deformed and contain porphyroblastic K-feldspar with perthite texture. Most plagioclases of granitic rocks occur as subhedral crystals with fine twining texture and plot in oligoclase fields (An11–28) (Fig. 4a). Plagioclases are sometimes altered to fine-grained sericite in the core. Biotite is pale green to light brown in the leucogranites and brown to dark brown in granite. Biotite of leucogranites have high Al content (3.1–3.3), while biotite of granites present quite high Fe2+/(Fe2++ Mg) ratio of about 0.8 and low Al content (2.5–2.7) (Fig. 5). 4.2. Khardung La Coarse-grained granites (201, 202A) and granodiorite (202B) were collected from Khardung La. Plagioclase occurs as subhedral crystals with twining and their core parts are slightly replaced by fine-grained sericite, chlorite, epidote. Plagioclase is oligoclase (An15–23) in granite and oligoclase-andesine (An13–37) in granodiorite (Fig. 4b). Amphibole in granodiorite has about 5% in modal abundance and is homogeneous. They belong to calcic group with very low Na in Bsite (0–0.3 a.p.f.u), plotting in the magnesio-hornblende field (Fig. 5). Biotite is more abundant and has the higher Fe2+/(Fe2+ + Mg) ratio in granodiorite (0.45 to 0.55) than granite (0.38 to 0.4) (Fig. 5). Some coarse-grained biotites are altered to chlorite in rim and along cleavages. 4.3. Hundar Detailed mapping of the Hundar igneous complex revealed two types of plutons: diorite and granite. The diorites consist of fine- and coarse-grained types. The latter is exposed widely in the northern part of the Hundar river section (Fig. 2). Granites are composed of coarseand medium-grained types. The former occur in the northern part of the section and the later, in southern part. Minor late-magmatic leucogranite, andesitic and rhyolitic dykes intruding into the Hundar plutons also occur in the study area (Fig. 2). In the Hundar river section we also have recognized a magma mingling structure (Barbarin and Didier, 1992) in the site 304 between fine-grained diorite and coarse-grained granite (Fig. 2). Contacts between the diorite and granite are sharp. Chilled margins or fining of grain size towards the contact between the diorite and the granite are extremely rare (Fig. 3a and b). Coarse-grained granites (303, 304X(G)) in the northern part and medium-grained granites (307A, 401) in the southern part of the Hundar area were collected (Fig. 2). Their photomicrographs are shown in Fig. 3c and d. They consist of quartz, alkali-feldspar, biotite, plagioclase and accessories of zircon, magnetite and pyrite (Table 1). Plagioclase is more abundant in medium-grained granite than coarsegrained granite. Most plagioclases occur as subhedral crystals with twining texture. Some grains have core parts altered to fine-grained sericite. Plagioclase falls in the oligoclase fields (An11–34) (Fig. 4c). Most K-feldspars represent a microperthitic texture. Some K-feldspars in the medium-grained granite are microcline. Biotite is more abundant in the medium-grained granites than the coarse-grained granite. Some biotites alter to chlorite along the cleavages. Biotite of

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Fig. 3. (a) Photograph showing the occurrence of magma mingling structure between coarse-grained granite and fine-grained diorite in the Hundar area (sample 304). Rectangular area shows sampling part for K–Ar dating. (b) Photomicrograph of the part between coarse-grained granite and fine-grained diorite (304X). (c) Coarse-grained granite (304XG). (d) Medium-grained granite (307). (e) Coarse-grained diorite (305A). (f) Fine-grained diorite (304B). (Qtz: quartz, Kfs: alkaline-feldspar, Pl: plagioclase, Bt: biotite, Cpx: clinopyroxene, Opx: orthopyroxene, Am: amphibole).

Fig. 4. Plagioclase chemistries plotted in the Or–Ab–An diagram of Deer et al. (1966).

N.X. Thanh et al. / Gondwana Research 17 (2010) 180–188

185

Fig. 7. Pyroxene chemistries of Hundar coarse-grained diorite plotted in the En–Wo–Fs diagram of Morimoto (1988).

Fig. 5. Biotite chemistries plotted in the Al vs. (Fe2+/Fe2++Mg) diagram of Deer et al. (1966).

coarse-grained granite is lower in Al than that of medium-grained granite, which is 2.2–2.4 and 2.4–2.7, respectively (Fig. 5). Coarse-grained diorites (303, 305A, B) and fine-grained diorites (304B, 304X(D)) were also collected in the northern part of the Hundar area (Fig. 2). Their photomicrographs are shown in Fig. 3e and f. They consist of plagioclase, amphibole, biotite ± pyroxene, little quartz and K-feldspar, and accessories of magnetite, illmenite, pyrite, chalcopyrite and galena. The coarse-grained diorite seems to be more basic because it is rich in amphibole and contains pyroxene. Plagioclase occurs as subhedral crystal showing twining. Some grains are altered to finegrained sericite and epidote in core parts. Plagioclase of both coarse- and fine-grained diorites plot in andesine to bytownite fields (An30–90) (Fig. 4c). Biotite is abundant in coarse-grained diorite. Few biotite grains are altered to chlorite along cleavages. Biotites has Fe2+/(Fe2++Mg) ratios of 0.4–0.7 in coarse-grained diorite and of 0.5–0.6 in fine-grained diorite (Fig. 5). Amphibole occurs as euhedral to subhedral crystals. Most amphiboles in coarse-grained diorite are subhedral. Amphiboles form sometimes around pyroxenes. All amphiboles of both diorites are of calcic group and range from magnesio-hornblende to actinolitehornblende. Amphibole rims and amphiboles around pyroxene in coarse-grained diorite fall in actinolite fields (Fig. 6), suggesting secondary phases. Pyroxene of coarse-grained diorite is mainly clinopyroxene with some minor amount of orthopyroxene. Clinopyroxene occurs as subhedral crystals with twining texture. It forms sometimes around orthopyroxenes. Pyroxenes are often altered to amphibole and chlorite in the rims and along the cleavages. Clinopyroxene plots in the diopside and augite fields, and orthopyroxene in the enstatite field (Fig. 7).

5. K–Ar analyses The samples were crushed with a jaw crusher and then sieved. Micas were separated from the 100–150, 150–200 or 200–250 mesh size fractions of the samples. We chose the best fraction for the abundance and purity and then carried out K and Ar analyses. Analysis of potassium and argon of biotite and muscovite separates, calculations of ages and errors were carried out following the methods described by Nagao et al. (1984) and Itaya et al. (1991). Potassium was analyzed by flame photometry using a 2000 ppm Cs buffer with an analytical error within 2% at a 2-σ confidence level. Argon was analyzed on a 15 cm radius sector type mass spectrometer with a single collector system using the isotopic dilution method and 38Ar spike. Multiple runs of the standard (JG-1 biotite, 91 Ma) indicate that the error of argon analysis is about 1% at a 2-σ confidence level (Itaya et al., 1991). The decay constants of 40K to 40Ar, 40Ca, and 40K content in potassium used in the age calculations are 0.581 × 10− 10/year, 4.962 × 10− 10/year and 0.0001167, respectively (Steiger and Jäger, 1977). The age results are shown in Table 2. Excess argon has been reported in some ultra-high and high pressure metamorphic rocks (e.g. Itaya et al., 2005; Gouzu et al., 2006) by 40Ar /39Ar analysis method (Hyodo, 2008). However, the biotite and muscovite minerals in plutonic rocks may contain small quantities of excess argon giving no significant effect on the K–Ar result. 5.1. Panamik Biotites of the coarse- and fine-grained granites (602c and 602f) from the site near La Bame Tso yield ages of 96.7 ± 2.1 and 95.7 ± 2.1 Ma, respectively. Biotites from the coarse-grained granodiorite (502A) and leucogranite (502B) yield 9.45 ± 0.21 Ma and 9.18 ± 0.21 Ma, respectively. The two-mica leucogranite (502C) gives 9.91 ± 0.23 Ma for muscovite and 9.34 ± 0.24 Ma for biotite. The former is older than the later beyond uncertainty between two sigma errors. This could be due to the difference of the closure temperatures of muscovite (500 °C) and biotite (450 °C) (Villa, 1998; Itaya et al., 2009) as described later. 5.2. Khardung La Biotites from two granites (201 and 202A) yield ages of 49.3 ± 1.1 and 50.9 ± 1.1 Ma, and one granodiorite (202B) yield an age of 51.2 ± 1.1 Ma. They are effectively the same age within the error. 5.3. Hundar

Fig. 6. Amphibole chemistries plotted in the Si vs. (Mg/Mg + Fe2+) diagram of Leake (1978).

Three coarse-grained diorite samples (303, 305A, 305B) from the Hundar area yield ages of 64.4 ± 1.4, 65.8 ± 1.4 and 65.8 ± 1.4 Ma. Two fine-grained diorite samples (304B, 304X(D)) from the centeral part of Hundar route yield ages of 64.0 ± 1.4 and 64.4 ± 1.4 Ma. The samples having magma mingling structure (Fig. 3a) give the same age

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Table 2 K–Ar age data of igneous rocks and hornfels from Khardung La, Hundar river and Panamik area (analytical error is 2% at a 2 confidence level). 40

Ar (10− 8ccSTP/g)

K–Ar age (Ma)

Non Rad.

No of specimen

Rock type

Mineral for K–Ar analyses

K (wt.%)

Rad.

Khardung La 201 202A 202B

Coarse-grained granite Coarse-grained granite Coarse-grained granodiorite

Biotite Biotite Biotite

6.409 ± 0.128 5.955 ± 0.119 6.504 ± 0.130

1247 ± 12 1193 ± 11 1312 ± 13

49.3 ± 1.1 50.9 ± 1.1 51.2 ± 1.1

2.3 1.8 2.2

Hundar river 301A 303 304B 304X(D) 304X(G) 305A 305B 307A 401

Hornfels Coarse-grained diorite Fine-grained diorite Fine-grained diorite Coarse-grained granite Coarse-grained diorite Coarse-grained diorite Medium-grained granite Medium-grained granite

Biotite Biotite Biotite Biotite Biotite Biotite Biotite Biotite Biotite

7.014 ± 0.140 3.841 ± 0.077 5.790 ± 0.116 6.175 ± 0.123 5.118 ± 0.102 3.821 ± 0.076 5.785 ± 0.116 2.975 ± 0.059 6.982 ± 0.140

1767 ± 17 977 ± 10 1464 ± 14 1570 ± 15 1293 ± 12 993 ± 10 1560 ± 14 709 ± 7.0 1750 ± 16

63.8 ± 1.4 64.4 ± 1.4 64.0 ± 1.4 64.4 ± 1.4 63.9 ± 1.4 65.8 ± 1.4 65.8 ± 1.4 60.8 ± 1.3 61.8 ± 1.3

2.2 5.2 3.2 2.9 2.8 4.3 3.6 7.8 2.1

Panamik area 502A 502B 502C 502C 602c 602f

Coarse-grained granodiorite Coarse-grained leucogranite Coarse-grained leucogranite Coarse-grained leucogranite Coarse-grained granite Fine-grained granite

Biotite Biotite Biotite Muscovite Biotite Biotite

7.234 ± 0.145 7.389 ± 0.118 5.224 ± 0.107 8.652 ± 0.173 6.161 ± 0.123 5.841 ± 0.117

266.3 ± 2.8 264.0 ± 3.2 193.7 ± 3.0 333.5 ± 4.0 2374 ± 22 2226 ± 21

9.45 ± 0.21 9.18 ± 0.21 9.34 ± 0.24 9.91 ± 0.23 96.7 ± 2.1 95.7 ± 2.1

8.8 10.9 31.1 18.3 1.9 1.8

of 63.9 ± 1.4 Ma for the fine-grained diorite (304X(D)) and 64.4 ± 1.4 Ma for coarse-grained granite (304X(G)). The former diorite is also the same age as that of the fine-grained diorite (304B) (64.4 ± 1.4 Ma). The hornfels (301A) in contact with the coarse-grained granite, north of the Hundar area, yields 63.8 ± 1.4 Ma that is similar to the coarse-grained granite (304X(G)). The medium-grained granites (307A and 401) occurring in the south of Hundar area give ages of 60.8 ± 1.3 and 61.8 ± 1.3 Ma that are younger than the others (Fig. 2). 6. Magma type and tectonic setting Many workers have studied the chemistry of biotites and clinopyroxenes to estimate magma types and original tectonic environments (Kushiro, 1960; Le Bas, 1962; Nisbet and Pear, 1977; Pearce et al., 1984; Abdel Rahman, 1994). Abdel Rahman (1994) showed that the diagram of MgO vs. Al2O3 for biotite can discriminate three compositionally distinct fields (Fig. 8): (A) anorogenic alkaline suites formed in association with extensional tectonic environments, (P) peraluminous suites, including collisional S-type granites, (C) calc-alkaline orogenic suites, including the I-type suites, formed within subduction-related environments. Most biotites of rocks from Khardung La, Hundar and Panamik areas fall on calc-alkaline fields associated with subduction-related environments. The younger leucogranites in Panamik area plot on the peraluminous suites presenting S-type granite related to post-collision tectonic setting in the Karakoram block (Fig. 8). Kushiro (1960) and Le Bas (1962) pointed out that the compositions of clinopyroxenes depend upon the physical and chemical conditions of the magma from which they crystallized. The Al2O3– TiO2 diagram can be used to clarify magma types if physical conditions are comparable. The pyroxenes from diorites in the Hundar area plot on the calc-alkalic series, indicating that they were formed in a subduction-related environment (Fig. 9).

40

Ar (%)

based on the zircon U/Pb and hornblende 40Ar /39Ar chronological data (c.f. Reiners, 2002). The two-mica leucogranite (502C) from the Panamic area gives 9.9 Ma for muscovite K–Ar age and 9.3 Ma for biotite K–Ar age (Table 2). The difference of the closure temperatures of muscovite (500 °C) and biotite (450 °C) by Villa (1998) gives a cooling rate of 83 °C/Ma. These cooling rates from both plutonic rocks are consistent with each other, suggesting it takes few million years for the cooling from the zircon crystalline temperature to the biotite closure temperature. Based on the K-feldspar 40Ar/39Ar ages and zircon SHRIMP U/Pb ages of mylonitic gneiss in Digar of Ladakh batholith, Weinberg and Dunlap (2000) have proposed a cooling rate about 56 °C/Ma from crystallization to lower greenschists facies temperatures for the Ladakh batholith around Leh and the neighbor sites. This means it is possible to use the K–Ar biotite ages of the granites, granodiorites and diorites in the present study as a timing of igneous activity within the few million years difference. The granites and diorites in the Hundar area yield K–Ar biotite ages from 61 to 66 Ma that are similar between their errors, indicating they are one phase of igneous products. The ages are significantly older than the ages of granites and granodiorite from the site of Khardung La, suggesting they formed in the different phase of igneous activities. However, both the ages are within the age ranges from 46 to 70 Ma from the Ladakh batholith near Leh town and the neighbor sites

7. Igneous activity in Ladakh Himalaya Plutonic rocks cool down quickly in the first stage just after the emplacement into the crust on the analogy of thermal diffusion model (c.f. Carslaw and Jaeger, 1959). The exact cooling rate has been also estimated to be 120 °C/Ma for the Speel River pluton in SE Alaska

Fig. 8. MgO vs. Al2O3 diagram of biotites. C: calc-alkaline subduction-related suites, P: peraluminous suites, A: anorogenic alkaline suites (after Pearce et al., 1984).

N.X. Thanh et al. / Gondwana Research 17 (2010) 180–188

187

the leucogranites from the Panamik area indicate the S-type related to post-collision tectonic setting. These types of igneous rocks are considered to have emplaced in relating to activation of the Karakoram fault. Acknowledgments

Fig. 9. TiO2 vs. Al2O3 diagram of clinopyroxene from Hundar diorites (after Le Bas, 1962).

(Honegger et al., 1982; Weinberg and Dunlap, 2000). Dunlap and Wysoczanski (2002) determined the extrusion ages of 67.4 and 60.5 Ma for the Khardung rhyolite and porphyritic sill, respectively. The ages are similar to the K–Ar ages of granite and diorite in the Hundar area, suggesting the Hundar plutons are contemporaneous with the Khardung volcanics. The petrographic analyses and the mineral chemistries by EPMA of rocks from the Khardung La and Hundar regions revealed that the diorite, granodiorite and granite are calc-alkaline and formed in an arc-related tectonic setting, indicating the rocks belong to the Ladakh arc that is an plutono-volcanic arc developed in Cretaceous to Eocene time (Honegger et al., 1982; Debon et al., 1987; Parrish and Tirrul, 1989, Ahmad et al., 1996, 1998; Debon and Khan, 1996; Dunlap and Wysoczanski, 2002). Slightly older plutons (75 Ma) in Kohistan and Tirit granitoids (68–74 Ma) in northern Ladakh (Petterson and Windley, 1985; Weinberg et al., 2000; Upadhyay, 2008) are also likely of the Ladakh arc. The calc-alkaline granites in the Panamik area give the K–Ar biotite ages of 97–96 Ma significantly older than the Tirit granitoids (68– 74 Ma) in the Nubra-Shyok river confluence. The ages coincide with the SHRIMP zircon core age (106 Ma) from the gneissic granodiorite in the Pangong complex (Searle et al., 1998) and Rb–Sr biotite age (115 ± 18 Ma) from the calc-alkaline tonalite in the Skyangpoche, about 30 km northwest of Panamik (Ravikant, 2006). These plutons can be spatially correlated with the K2 gneiss (115–120 Ma) in the Baltoro region of Karakoram and Hunza plutons (106 Ma) in northern Pakistan (e.g. Fraser et al., 2001; Ravikant, 2006). Similar ages have been also obtained from the foliated granite of Rutog in western Tibet (Ravikant, 2006). These age data suggest that the middle Cretaceous calc-alkaline magmatism took place in the wide area from north Pakistan to western Tibet through northern Shyok-Nubra valley and Pangong area in the Ladakh, India, suggesting one magmatic phase emplaced during subduction of Neo-Tethyan oceanic plate under the Karakoram block. The leucogranites and granodiorite in the Panamik area give the K– Ar biotite ages from 9.2 to 9.5 Ma. These ages are consistent with the Ar–Ar biotite and muscovite ages (9.7–13 Ma) from the leucogranites and pegmatites occurring in the southern part of the Karakoram block of the Ladakh (Srimal, 1986; Dunlap et al., 1998). The SHRIMP zircon ages are also reported to be 15 Ma for the Tirit leucogranite (Weinberg et al., 2000) and 17 Ma for the Tangtse leucogranite (Searle et al., 1998, Dunlap et al., 1998). The difference between the mica argon ages and zircon U/Pb ages is due to that between the closure temperatures of the mica-argon and zircon U/Pb systems. On the other hand, the leucogranite known as the Baltoro granite in the Kohistan yields U–Pb monazite age of 21–26 Ma (Parrish and Tirrul, 1989), suggesting the earlier magmatic phase than the leucogranites of the Ladakh. The leucogranites occurring widely in the southern part of the Karakoram block including those in the Panamik area could be formed in a series of magmatism though there is some age variation. The petrographical analyses and the mineral chemistries by EPMA of

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