JOURNAL GEOLOGICAL SOCIETY OF INDIA Vol.74, September 2009, pp.363-374

Petrogenesis of the Granitoid Rocks from Askot Crystallines, Kumaun Himalaya D. RAMESHWAR RAO and RAJESH SHARMA Wadia Institute of Himalayan Geology, 33 General Mahadeo Singh Road, Dehradun - 248 001 Email: [email protected]; [email protected] Abstract: The Askot crystallines form a doubly plunging synformal belt and occurs as a detached crystalline belt or klippen in the vast sedimentary terrain lying between Central crystallines towards north and the Almora crystallines to the south. It is dominated by granite gneiss and augen gneiss, and also comprise of metapelites, migmatites and basic intrusives. In this paper, the geochemical studies of the granite gneiss and augen gneiss from the Askot crystallines, Kumaun Himalaya were carried out in order to understand their origin and evolution. The granite gneiss is generally foliated, with less foliated and porphyritic variety seen in the core part. The K-feldspar shows Carlsbad twinning, while plagioclases show complex twinning. They show euhedral zircon and apatite along with titanite as accessory minerals. The granite gneiss is moderately evolved (Mg# ~50) and has granodiorite composition with metaluminous, calc-alkaline trends. They show higher concentration of Ti, Ca, Mg and low abundance of 6REE (~165 ppm) in comparison to augen gneiss. They show volcanic arc signatures and compare well with Lateorogenic granites of Proterozoic times distributed world wide. These calc-alkaline granites appear derived from a Paleoproterozoic mafic/intermediate lower-crust reservoir probably involving arc magma underplating. Granite gneiss is also peraluminous with molar A/CNK>1.1, and the heterogeneity of granite gneiss can be explained with the precursor melts, experiencing assimilation during up-rise through crust or contamination of source itself involving sediments from the subduction zone. The augen gneiss is more evolved (Mg# ~18) and show granite composition. They show megacrysts of perthites in a fine-to medium-grained matrix of feldspars and micas. The REE pattern of the augen gneiss shows much wide compositional variation (6REE ~171 ppm) than granite gneiss. It shows syn- to post-orogenic environment and derivation from the partial melting of an upper crustal source. Existing Rb-Sr isotopic data suggest that the granite gneiss defines an isochron age of ~1700-1800 Ma with a Sri ratio of ~0.71, while the augen gneiss defines an age of ~1300 Ma with much evolved Sri ratio (~1.65). The dominance of granite gneiss in the eastern Kumaun region suggests the production of heterogeneous granitic melts similar to those of Askot crystallines as an important event of crustal growth during Late Paleoproterozoic period in the region. Keywords: Granites, Augen gneiss, Petrogenesis, Askot crystallines, Kumaun Himalaya.

INTRODUCTION

The Wilson Cycle recognizes that there have been cycles of ocean creation and destruction during Earth’s history, where in periodically, oceans open and close. As a result material is added to continental margins and continents grow through time. Granites provide the main evidence about the growth and evolution of continents through time Myers (1997). Some of the granites are generated in zones of rifted continental or oceanic crust, but most of the granites are generated in zones of collision between continents and oceanic crust, where continents were amalgamated (cf. Myers, 1997). Emphasis has been made here on to the development of granites in a volcanic arc setting during

Proterozoic, widespread world over, for e.g., Sierra Nevada batholith (Rogers et al. 1980; Rogers and Greenberg, 1981), Ben Ghenma batholith (Roger et al. 1980), Proterozoic of Sweden (Wilson, 1980), Bundelkhand granite (e.g. Rahman and Zainuddin, 1993). The understanding of magma genesis in tectonic zones is difficult, as it involves contamination in mantle source area by fluids, subducted sediments, and contamination of mantle derived magmas by assimilation of lithospheric mantle or crustal rocks during ascent and emplacement (e.g. Davidson, 1996). However, Roddick (1983) is of the opinion that contribution of our understanding will lead to a more comprehensive picture of what in these enigmatic

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D. RAMESHWAR RAO AND RAJESH SHARMA

terranes remains unsatisfactorily explained. Agreeing to Roddick (1983) view point, the authors try to project their understanding of the Palaeoproterozoic granitoid rocks from the Askot crystallines of eastern Kumaun region, and in this paper the geochemistry of the granite gneiss and augen gneiss is presented, and their origin discussed. GEOLOGY OF THE REGION

The Kumaun Himalaya lies between the borders of Nepal and Garhwal regions. It is dominated by crystalline nappe rocks and Precambrian to Palaeozoic sedimentary rocks. Auden (1935) and Heim and Gansser (1939) were among the earlier workers who studied the stratigraphy and structure of this part of the Himalaya. Valdiya (1980) described the regional stratigraphy and structural synthesis of Kumaun Himalaya. He distinguished four major lithounits; the autochthonous unit of the Damtha and Tejam, the Krol nappe, the Ramgarh nappe, and the Almora nappe. He described the root of the Almora nappe to be the Munsiari Formation constituting the base of the Great

Himalaya. There are number of klippens between the root and the Almora nappe: the Chiplakot, Askot-Thal, Daramghar, and Bageshwar-Baijnath-Nandprayag, which were generally considered to be the erosional remnants of the thrust sheet. However, Mehdi et al. (1972) suggested autochthonous nature for the Almora, Baijanath and Askot crystallines. Similarly, Saxena and Rao (1975) considered that the Almora nappe does not exist; instead they consider the Almora crystallines to be of autochthonous nature overlying Nagthat Formation. The Baijnath, Askot and Chiplakot klippen have been earlier described by Heim and Gansser (1939), Valdiya (1962), Pande and Verma (1970), and Powar (1972). More recently, the significant lithounits and structural features of the crystalline rocks between the Ramganga and Kali Rivers are given by Paul (1998), while Chakraborty and Malaviya (1996) have described the geology of the Almora Group of Kumaun Himalaya. Kumar et al. (1995) studying the petrogenesis of CambroOrdovician granitoids of Lesser Himalayan rocks of Kumaun Himalaya, showed that they are characterized predominantly by peraluminous (S-type) characters mixed

JOSHIMATH NANDA DEVI

Ka

Ladakh sh

mi

N

25645

MAIN CENTRAL T.

r

TRISHUL H.

P.

23360 MUNSIARI

Ku ma un

DELHI

ga

R.

MUNSIARI T.

R.

a

CHHIPLAKOT NANDPRAYAG

R.

an

un

NE PA L

li

G

Ka

Ya m

DHARAMGHAR



BAIJNATH GWALDOM

N

SANDEV ASKOT

Ka

E li

R.

P A

BERINAG

L

DUDHATOLI BINSAR KAUSANI

PITHORAGARH RAMESHWAR

INDEX DWARAHAT

1

NORTH ALMORA T.

ALMORA

2

SOUTH ALMORA T. CHAMPAWAT

3 4 KOIDAL

Thrust

RAMGARH T.

NAINITAL

Fault



Sample location

DEBGURU

RAMGARH

0

5

10

15 Km

KATHGODAM

MAIN BOUNDARY T.

Fig.1. Geological map representing part of eastern Kumaun region (after Valdiya, 1980), showing sample location. Index: 1. Vaikrita Group of rocks, 2. Almora-Jutogh-Munsiari nappe with augen gneiss and granite-granodiorite rocks, 3. Ramgarh-Chail nappe and quartz porphyry rocks, and 4. Krol-Jaunsar-Berinag nappes and autochthonous zone rocks. JOUR.GEOL.SOC.INDIA, VOL.74, SEPT. 2009

PETROGENESIS OF THE GRANITOID ROCKS FROM ASKOT CRYSTALLINES, KUMAUN HIMALAYA

and co-mingled with a small amount of mantle-derived mafic magma, commonly represented by modified mafic magmatic enclaves. A syn-collisional tectonic environment is attributed by Rashid and Zainuddin (1995) to Lower Palaeozoic granites from Ranikhet, Kumaun Himalaya. Kumar and Patel (2004), working on the deformation mechanisms in the Chiplakot crystalline belt along KaliGori valley, have reported four phases of deformation, with early phases representing the pre-Himalayan deformation. A synthesis of granitic activity in the northwestern Himalaya is given by Islam et al. (2005). The granitoid rocks (granite gneiss and augen gneiss), the rocks under study, form a part of doubly plunging synformal belt of Askot crystallines. It is a detached crystalline belt occurring in the vast sedimentary terrain of quartzites and dolomitic rocks, lying between the Central crystallines towards north and the Almora crystallines to the south (Fig. 1). The synform is composed of derivatives of pelite, semi-pelite and psammite, migmatites and basic intrusives. The schists contain some bands of augen gneiss, with the core of the synform occupied by granitoids, while good exposures of augen gneiss are seen near Didihat-Askot. The granite gneiss show intrusive contacts with schistose rocks and augen gneiss. The granite body is in general foliated with less foliated and porphyritic variety seen in the core part. Ghose (1972) gave a generalized tectonic sequence in and around Askot region, and grouped the rocks into the Garhwal Group and the Askot Group, separated by a Thrust. The Garhwal Group has a calcareous base overlain by quartzite. The Askot Group has schists with intrusive granites along with lenticular bands of biotite bearing augengneiss. Amphibolites occupy the contact zone between the quartzite of the Garhwal Group and tectonically overlying schists of Askot Group (ibid). Crystalline rocks occurring as klippens were reported to be of Palaeoproterozoic age; for example, Trivedi et al. (1984) gave whole rock Rb-Sr isochron age of 1795+30 Ma with a Sri ratio of 0.7090+15 for gneissic rocks from Dharamghar-klippen, while Powell et al. (1979) and Bhanot et al. (1980) have reported whole rock Rb-Sr isochron ages of 1620±90 Ma and 1983±80 Ma respectively for Almora and Askot crystalline rocks. Structural details of the region are given by Bhanot et al. 1977; Gairola, 1967; Ghose, 1972; Misra and Sharma, 1973; Valdiya, 1980. PETROGRAPHY

The granite gneiss of Askot klippen are medium to coarse grained foliated biotite gneisses, and are well exposed between Sandev-Chouwati. Foliation is defined by planar JOUR.GEOL.SOC.INDIA, VOL.74, SEPT. 2009

365

blebs and streaks of biotite. The main constituents are quartz, K-feldspar, plagioclase and biotite. Mylonitization is prominently occurring along the contact of the country rocks. Quartz occurs as fresh grains with sharp outlines, having inclusions of biotite, muscovite, chlorite, sericite, euhedral zircon and opaques. Quartz also occurs as inclusions within the K-feldspar, plagioclase, and mica. K-feldspar is a late crystallizing mineral phase. Quartz, biotite, plagioclase, muscovite, chlorite and euhedral zircon occur as inclusions within the K-feldspar, which is altered to sericite and kaolinite. K-feldspar occurs as orthoclase and perthites, and shows Carlsbad twinning (Fig.2a). Plagioclase is mostly subhedral and euhedral (Fig.2a), and commonly shows albite, albite-carlsbad and albite-pericline complex twinning (Figs.2a-c). The optically determined composition of plagioclase ranges from albite to andesine (An 6-32 ). Inclusions of quartz, biotite, muscovite, sericite and zircon are observed within plagioclase. Plagioclase is altered to chlorite, sericite and zoisite. Plagioclase also occurs as small grains and as inclusions within biotite. Biotite is a dominant mica mineral in these rocks. It occurs as massive grains and also as flakes, the latter occurs as small yellowish brown flakes. Biotite with green colour is observed in a few thin samples, and exhibit poikilitic relation with plagioclase and K-feldspar. Muscovite is sometime found in association with biotite. Chlorite is observed interleaved within biotite flakes and along margins. Zircon, apatite, magnetite and titanite are the minor accessories, while epidote and chlorite occur as products of retrogression. Euhedral zircon and apatite (Fig. 2d) occur as inclusions within quartz, K-feldspar, plagioclase and biotite minerals. The augen gneiss of Askot crystallines has heterogeneous composition, consisting of coarse texture K-feldspar megacrystic microperthites (Fig.2e), in a fine- to mediumgrained granoblastic intergrowth of quartz, K-feldspar, plagioclase and biotite. The microperthite is subhedral to anhedral, randomly oriented and hosts smaller grains of quartz, plagioclase, and biotite (Fig.2e). The grain boundaries of these inclusions are diffused. The plagioclase occurring as inclusions within K-feldspar, shows altered cores. The plagioclase generally shows polysynthetic twinning and at places oscillatory zoning. Quartz occurs as anhedral grains, and as interstitial polycrystalline quartz at some places stretching into quartz ribbons and wrapping around the microperthite augens. Biotite clots are irregularly shaped and unevenly distributed in the rock and occur as irregular dark clusters. Muscovite also occurs in the matrix, sometime inter-grown with biotite, and as inclusions in plagioclase. Myrmekitic intergrowths of plagioclase with quartz are common. The other accessory minerals include

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D. RAMESHWAR RAO AND RAJESH SHARMA

Fig.2. Granite gneiss showing (a) Carlsbad twinning in orthoclase along with euhedral plagioclase, (b-c) complex zoning in plagioclase, and (d) euhedral zircon and apatite. Augen gneiss showing (e) inclusions of plagioclase, quartz and biotite in subhedral perthite, and (f) rare tourmaline.

epidote, titanite, zircon, and opaque minerals, however, sample RR-99 shows the presence of rare tourmaline (Fig.2f). GEOCHEMISTRY

Samples of granite gneiss and of augen gneiss from the Askot crystallines of eastern Kumaun Himalaya were

selected for bulk analyses using the XRF (SIEMENS SRS 3000) and ICP-MS (Perkin-Elmer SCIEX) instruments at the Wadia Institute of Himalayan Geology, Dehradun, India. The data of the analyzed samples are summarized in Table 1 and 2. Analytical accuracy on XRF is better than 5% and 12% for major oxides and trace elements respectively, and the precision in terms of maximum observed standard deviation on repeated measurements is better than 2% (Saini JOUR.GEOL.SOC.INDIA, VOL.74, SEPT. 2009

PETROGENESIS OF THE GRANITOID ROCKS FROM ASKOT CRYSTALLINES, KUMAUN HIMALAYA

367

Table 1. Geochemical data of granite gneiss from Askot crystallines, eastern Kumaun RR91

RR92

RR93

RR94

RR95

RR96

RR97

RR98

RR100

RR101

RR102

RR103

SiO2 TiO2 Al2O3 FeOt MnO MgO CaO Na2O K2O P2O5 Total

64.18 0.48 16.75 4.51 0.04 3.45 1.7 3.06 4.03 0.13 98.33

70.65 0.34 15.79 2.98 0.02 2.34 0.97 4.45 2.53 0.1 100.17

66.97 0.47 16.04 3.95 0.05 2.09 2.35 3.15 4.15 0.13 99.35

66.29 0.46 15.66 3.97 0.05 2.32 2.54 3.07 3.89 0.15 98.40

65.1 0.61 14.37 5.09 0.06 3.93 3.35 2.36 3.52 0.13 98.52

71.34 0.28 16.7 2.44 0.02 1.11 1.6 5.49 1.89 0.08 100.95

70.65 0.35 14.65 3.19 0.04 1.74 2.1 3.9 2.89 0.08 99.59

70.37 0.34 15.49 2.97 0.03 1.65 1.96 3.61 4.02 0.07 100.51

69.4 0.44 15.65 3.58 0.04 1.57 2.41 3.66 2.73 0.16 99.64

67.74 0.5 15.65 3.86 0.05 1.89 2.95 4.05 2.02 0.13 98.84

66.43 0.56 15.54 4.07 0.05 1.97 2.77 4.42 1.94 0.14 97.89

68.09 0.41 15.17 3.53 0.04 2 2.38 3.28 3.85 0.11 98.86

Rb Sr Zr Nb Ni Y La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu 6REE

123 113 126 6 18 27 28.4 55.2 5.7 23.4 4.5 0.8 3.4 0.6 4.1 0.9 2.1 0.3 2.7 0.4 132

158 106 101 14 36 28 44.2 85.2 8.8 36.7 6.5 1.1 4.7 0.7 4.7 1.0 2.2 0.3 2.4 0.3 199

188 276 122 13 43 39 33.6 64.6 6.7 28.2 5.6 0.9 4.4 0.8 6.0 1.3 3.0 0.5 3.3 0.5 159

135 268 127 14 49 15 32.3 62.7 6.6 27.6 5.1 0.9 3.6 0.5 3.0 0.5 1.1 0.2 1.2 0.2 145

149 294 130 21 96 25 40.0 76.7 7.9 32.8 5.9 1.0 4.2 0.7 4.3 0.9 2.0 0.3 2.2 0.3 179

61 284 131 9 19 7 na na na na na na na na na na na na na na na

77 249 115 7 31 25 37.1 70.1 7.2 29.8 5.4 1.0 3.9 0.6 4.1 0.9 1.9 0.3 2.2 0.3 165

100 245 104 7 26 29 37.7 74.5 7.8 32.7 6.2 0.8 4.5 0.7 4.6 1.0 2.2 0.4 2.8 0.4 176

86 316 122 7 26 20 31.6 60.1 6.2 25.1 4.5 0.9 3.3 0.5 3.4 0.7 1.5 0.2 1.7 0.2 140

109 372 139 4 31 14 36.4 69.8 7.2 30.0 5.4 0.9 3.8 0.6 3.1 0.5 1.0 0.1 1.0 0.1 160

80 365 142 9 35 22 40.0 75.7 7.9 32.5 5.6 1.0 4.0 0.6 3.7 0.7 1.7 0.3 1.9 0.3 176

132 260 115 7 39 24 38.7 75.9 7.9 32.6 6.0 0.9 4.4 0.7 4.1 0.8 1.9 0.3 2.3 0.3 177

1.6 58 1.3

1.3 58 0.6

1.2 49 1.3

1.1 51 1.3

1.0 58 1.5

1.2 45 0.3

1.1 49 0.7

1.1 50 1.1

1.2 44 0.7

1.1 47 0.5

1.1 46 0.4

1.1 50 1.2

A/CNK* Mg# K2O/Na2O

A/CNK* = Molar Al2O3/(CaO+Na2O+K2O); Mg# = Mg/(Mg+Fet); n.a. = not analyzed

et al. 1998). The relative standard deviation (RSD) for most REE elements analyzed on ICP-MS is better than 10% (Khanna et al. 2009). The augen gneiss and granite gneiss of Askot crystallines are peraluminous, and show calc-alkaline character (Table 1 and 2). The augen gneiss show highly evolved character (mg# range from 14-28; mean value ~18) and have granite composition, while the granite gneiss are less evolved (mg# range from 44-58; mean value ~50) and have granodiorite composition (Fig.3). They have distinct chemistry, the granite gneiss in comparison to augen gneiss has higher concentration of Ti, Al, Fe, Mg, Ca, Na, Sr, Ni, and low abundance of Si, K, Rb, Nb, 6REE and K2O/Na2O ratio (Table 1 and 2). Some of these variations are shown JOUR.GEOL.SOC.INDIA, VOL.74, SEPT. 2009

on the Harker’s variation diagram (Fig. 4). The augen gneiss because of their different origin and high silica content is seen distinctly plotted as a separate group on variation diagrams. However, the trends shown by major and trace elements of granite gneiss against SiO2 indicate characteristic of fractional crystallization involving fractionation of plagioclase, K-feldspar and biotite, and derivation of rocks from igneous protoliths. The fractionation is confirmed by an increase in Rb/Sr ratio with decreasing Sr content. The REE patterns of the augen gneiss show much wider compositional variation than granite gneiss (Fig. 5a). The total REE of augen gneiss ranges between 128 to 241 ppm, with a mean value of around 171 ppm. LREE of augen gneiss

368

D. RAMESHWAR RAO AND RAJESH SHARMA Table 2. Geochemical data of augen gneiss from Askot crystallines, eastern Kumaun RR67

RR68

RR69

RR70

RR71

RR72

RR73

RR74

RR75

RR76

RR77

RR78

RR99

SiO2 TiO2 Al2O3 FeOt MnO MgO CaO Na2O K2O P2O5 Total

74.96 0.28 12.84 3.33 0.02 0.71 0.70 2.34 4.38 0.13 99.70

75.73 0.26 12.49 3.25 0.03 0.59 0.68 2.31 4.35 0.12 99.80

73.49 0.17 14.31 2.74 0.02 0.26 0.69 2.68 5.02 0.13 99.50

73.19 0.17 14.47 2.66 0.02 0.24 0.68 2.45 5.44 0.12 99.45

76.06 0.20 12.78 2.72 0.02 0.29 0.76 2.47 4.57 0.12 99.99

74.25 0.18 14.00 2.80 0.02 0.29 0.85 2.61 5.44 0.13 100.58

74.08 0.18 14.30 2.73 0.02 0.28 0.85 2.84 5.15 0.12 100.57

75.75 0.40 12.12 3.80 0.03 0.72 0.68 2.64 3.32 0.15 99.60

73.01 0.33 13.26 3.66 0.03 0.48 1.13 2.43 4.85 0.13 99.31

72.74 0.15 15.13 2.23 0.01 0.23 0.77 3.07 6.19 0.13 100.64

75.80 0.18 12.97 2.64 0.02 0.28 0.70 2.76 5.33 0.13 100.80

70.70 0.49 13.31 4.94 0.04 0.68 1.61 2.62 4.24 0.16 98.78

74.81 0.08 14.26 1.61 0.02 0.25 0.74 3.25 5.18 0.13 100.3

Rb Sr Zr Nb Ni Y La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu 6REE

314 34 142 14 11 27 na na na na na na na na na na na na na na na

299 48 130 13 11 23 41.4 87.5 9.0 35.9 6.0 1.2 6.1 1.0 4.7 0.8 2.1 0.3 1.5 0.2 198

527 32 96 14 1 17 na na na na na na na na na na na na na na na

470 36 106 14 4 14 na na na na na na na na na na na na na na na

443 35 109 13 6 20 30.21 62.81 6.42 25.66 5.43 1.04 5.53 0.93 5.32 0.92 2.41 0.31 1.53 0.20 149

495 43 89 12 4 17 na na na na na na na na na na na na na na na

489 37 97 13 1 18 29.6 60.8 6.3 24.5 4.4 0.9 4.5 0.8 4.4 0.8 2.0 0.3 1.3 0.2 140

275 54 177 17 13 21 na na na na na na na na na na na na na na na

408 55 170 15 10 23 na na na na na na na na na na na na na na na

453 50 85 11 1 10 27.4 55.3 5.7 23.1 4.4 0.9 4.2 0.7 3.3 0.5 1.3 0.2 0.7 0.1 128

363 31 92 10 5 22 na na na na na na na na na na na na na na na

393 65 195 17 14 36 46.1 105.3 11.2 45.5 7.2 1.4 7.4 1.2 6.8 1.3 3.8 0.5 3.0 0.4 241

631 23 52 14 12 23 na na na na na na na na na na na na na na na

A/CNK* 1.30 Mg# 28 K2O/Na2O 1.87

1.28 14 1.88

1.29 15 1.87

1.30 14 2.22

1.23 16 1.85

1.19 16 2.08

1.21 15 1.81

1.32 25 1.26

1.17 19 2.00

1.15 16 2.01

1.12 16 1.93

1.13 20 1.62

1.15 22 1.59

A/CNK* = Molar Al2O3/(CaO+Na2O+K2O); Mg# = Mg/(Mg+Fet); n.a. = not analyzed

are enriched by 80 to 110 times, while HREE are enriched by 2.7 to 15 times to chondrite, and show fractionation trends with a (La/Lu)N ratio of around ~16. On the other hand, the granite gneiss shows relatively smaller variation in their total REE concentration (132 and 199 ppm with a mean value of around 165 ppm). LREE of granite gneiss are enriched by 90 to 105 times, while HREE are enriched by 5 to 20 times, and (La/Lu)N ratio is around ~13. The REE and primordial mantle-normalized patterns of granite gneiss (Figs. 5b and c) compare well with the Late-orogenic granites data (Rogers and Greenberg, 1990). On the FeOt/(FeOt+MgO), tectonic discrimination diagrams after Maniar and Piccoli (1989) the augen gneiss occupy the post-orogenic granite field, i.e. crystallizing during the last phase of orogeny, while

the granite gneiss falls in orogenic granite fields (Fig. 6). The contrast in the chemical behaviour of granite gneiss and augen gneiss is also well reflected on the tectonic discrimination diagram of (Y+Nb) vs Rb, wherein the granite gneiss plots in the volcanic arc granite field (VAG), and the augen gneiss plots in syn-collision field (Fig. 7). PETROGENESIS

It is apparent from the foregoing geochemical discussion of granitoids rocks from Askot crystallines that the augen gneiss and granite rocks have distinct geochemical characteristics, and differ in their origin. The granite gneiss has moderately evolved granodiorite JOUR.GEOL.SOC.INDIA, VOL.74, SEPT. 2009

PETROGENESIS OF THE GRANITOID ROCKS FROM ASKOT CRYSTALLINES, KUMAUN HIMALAYA

1000

lite

R2

a Ton

500

-

+ + +++ + Granodiorite + + ++ ++ X

X X X XX XXXXX X

Granite

-

-

-

-

0

-

Alkaline Granite

1500

2000

2500

3000

R1 Fig.3. The classification of granite gneiss and augen gneiss from Askot crystallines on R1=4Si+11(Na+K)-2(Fe+Ti) vs R2=(6Ca+2Mg+Al) diagram after de la Roche (1980). Symbols : augen gneiss is represented by cross (x) and granite gneiss by plus (+).

composition with calc-alkaline trends, and has higher concentration of Ti, Ca, Mg, and moderately fractionated REE pattern (Fig. 5a). The granite gneiss shows volcanic arc signatures suggesting their igneous origin (Fig. 7). The calc-alkaline I-type nature of these rocks also gets support from (i) the euhedral to subhedral zircons, apatite (Fig. 2d) and titanite, (ii) orthoclase showing Carlsbad twinning (Fig. 2a), and (iii) plagioclase showing albite to andesine composition, showing euhedral grains, showing complex albite, albite-carlsbad, albite-pericline twinning (Figs.2a-c). However, because of their high alumina content these rocks also show peraluminous nature with mol. A/CNK ratio >1.1 and normative corundum values >1 wt%. The enriched LREE and LILE patterns of these rocks resemble with upper crustal rocks, while their HREE and HFSE patterns are related with lower crustal rocks. The petrochemistry of the rock thus point out to a heterogeneous source composition for their origin. Pitcher (1982) has recognized S- and Itypes in mobile belts as old as 1700 Ma, where their presence has been taken to indicate the operation of plate tectonic processes. The primitive nature of the granitic magma that produced the granodiorite is evident from low Rb/Sr (~ 0.54) and Nb (~10 ppm). The negative Sr anomaly of these rocks explains a distinctive mantle source chemistry or plagioclase fractionation. The rocks commonly show geochemical attributes of orogenic related magmatism, such as high La/ JOUR.GEOL.SOC.INDIA, VOL.74, SEPT. 2009

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Nb ratios (1.9 to 8.6) along with negative Nb and Ti anomalies (Fig. 5c). The Nb provides an important clue in the identification of emplacement environment, e.g. negative Nb anomaly occurs either in the within plate continental flood basalts (Kent, 1995) or in the arc related rocks (Tarney et al. 1981). Volcanic arc granite affinity (Fig. 7) and the high LILE, and low HFSE of samples also suggest a subduction related post-collisional arc setting. The observed geochemical features like, calc-alkaline nature, higher CaO, MgO, Al2O3, Sr and lower total FeO, K2O and total alkalies, and fractionated REE pattern correlate the granite gniess with Late-orogenic granites as defined by Harris et al. (1986). The average REE and compatible and incompatible elements of granite gneiss (Figs. 5b and c) compare well with the Late-orogenic granites (Rogers and Greenberg, 1990). The source characterization of the granite gneiss, the Y-depleted and fractionated HREE trend are consistent with hydrous partial melting of an amphibole and/or garnetbearing mafic source (cf. Sheraton et al. 1985). The higher Ce/Yb (ca 300) and the low Y (ca. 10 ppm) are consistent with the presence of garnet in the source region. According to Rogers and Greenberg (1990), the late-orogenic granites with inclined REE patterns and Sr abundances can be explained by fractionation of amphibole, clinopyroxene, and plagioclase from calc-alkaline magmas, leaving a small amount of residual granitic liquid, and suggested their derivation from the mafic/intermediate magmatic products of continental-margin subduction. The foregoing discussion suggests that the primary melts of the granodiorite are derived from vapour-saturated partial melting around ~1800 Ma of a Paleoproterozoic mafic/ intermediate lower-crust reservoir due to crustal thickening associated with arc magma underplating (Figs.5-7). However, it is also observed that the granite gneiss shows peraluminous nature, enriched LREE patterns, variable enrichment of the LIL elements with Rb and K showing enriched abundances, which together correlate well with upper crustal values and hence suggest a heterogeneous source characterization in the generation of granite gneiss. The heterogeneous nature can be explained with the melts experiencing assimilation during up-rise through crust or contamination of source itself involving sediments from the subduction zone, resulting in peraluminous character for the calc-alkaline granitoids. It is to be noted that the phenomena of generation of granite rocks in subduction related process is observed world over in Precambrian times (e.g. Rogers et al. 1980; Rogers and Greenberg, 1981; Bentor, 1985). Although not much reports for subduction-related magmatism during Paleoproterozoic times are described in Himalaya, the Bundelkhand granites are considered to

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17 16

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Fig.4. Harker’s diagram showing SiO2 variation of granite gneiss and augen gneiss from Askot crystallines against Ti, Al, Mg, Ca, Rb, Sr, Ni, Mg# and K2O/Na2O. Symbols : augen gneiss is represented by cross (x) and granite gneiss by plus (+).

continue into the Lesser Himalayan gneissic basement (cf. Sharma, 1998) and are interpreted to as subduction related magmatism (Rahman and Zainuddin, 1993), while Naqvi et al. (1974) have considered the accretion of Indian plate from several micro-plates during Precambrian. On the other hand, the augen gneisses of the Askot crystallines are more evolved felsic rocks of granite composition. They show typical peraluminous (S-type) characters and have lower concentration of Ti, Ca, Mg and higher Rb/Sr and Nb values (Table 1 and 2), and fractionated

REE pattern in comparison to associated granite gneiss. Augen gneiss occupies syn- to post-orogenic field in contrast to volcanic arc nature of granite gneiss (Figs. 6-7). From the petrochemistry of the augen gneiss partial melting of upper crustal source can be envisaged for their origin. Further, some degree of fractionation trends of granite gneiss to augen gneiss (Fig. 4) and the overlapping composition of REE elements (Fig. 5a) may also point that the early formed granodiorite of the region could be a probable source for the generation of augen gneiss. The granite gneiss and augen JOUR.GEOL.SOC.INDIA, VOL.74, SEPT. 2009

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Fig.5. (a) Chondrite normalized (values after Haskin et al. 1968) REE pattern of granite gneiss and augen gneiss from Askot crystallines. (b) Average REE abundances of granite gneiss compared with patterns of Late-Orogenic granites, PostOrogenic granites, Anorthosite-Rapakivi Granites and with granites in Alkaline Ring Complex (data from Rogers and Greenberg, 1990 and references there in). The dotted region in the figure represents REE variation of granite gneiss from Askot region. (c) Average granite gneiss spidergram normalized to Primordial Mantle (values after Wood et al., 1979), and compared with the average Late-Orogenic granites (data from Rogers and Greenberg, 1990).

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(Bhanot et al. 1977; Powell et al. 1979; Bhanot et al. 1980; Trivedi et al. 1984). However, in view of present geochemical study the authors are of the opinion that the age reported for augen gneiss is quite confusing. This is largely because earlier workers have presumed the augen gneiss and granite gneiss to be of same age, and tried to fit them in one isochron. For example, the four samples of augen

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gneiss thus have different origin, and are probably formed at different times. Geochronological data of granite gneiss and augen gneiss from eastern Kumaun region provide a general conclusive age of granite gneisses around 1800 Ma. JOUR.GEOL.SOC.INDIA, VOL.74, SEPT. 2009

Fig.7. Tectonic discrimination of granite gneiss and augen gneiss from Askot Crystallines on (Y+Nb) ppm versus Rb ppm. Fields after Pearce et al. (1984). Symbols : augen gneiss is represented by cross (x) and granite gneiss by plus (+).

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D. RAMESHWAR RAO AND RAJESH SHARMA 6.70 Augen Gneiss

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4.70 Age : 1375+38Ma Sri : 1.649+79 MSWD: 14.01

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Fig.8. Re-interpretation of Rb-Sr isotopic data of Askot crystallines given by Trivedi et al. (1984). Granite gneiss includes sample nos. AS-9 to 13 and Augen Gneiss includes sample nos. AS-3, AS-4, AS-6 and AS-7 (data is from Table 2; JGSI, v.25, p.645).

gneiss from Dhaun of the Almora nappe as reported by Trivedi et al. (1984) do not fit well into the isochron defined for 1800 Ma. Similarly augen gneiss from Askot and Dharamghar klippen also do not fall in 1800 Ma. Further, it is also interesting to note that the four samples of Askot augen gneiss reported by Trivedi et al. (1984) when plotted separately define an isochron age of around 1300 Ma with a much evolved Sri ratio (Fig. 8). Pandey et al. (1981) also gave ~1300 Ma for rocks from Baijnath and Gwaldam. Even it seems that the age of ~1800 Ma for augen gneisses is either absent or unknown from the adjacent Nepal Himalaya. Further, the five sample data of granite gneiss reported by Trivedi et al. (1984) define an age of around 1700 Ma

(Fig. 8). So, in the Askot crystallines it is proposed that the origin of augen gneiss and granite gneiss is independent to each other, and most like the augen gneiss can be attributed to a latter orogeny. CONCLUSIONS

There is clear discrimination between potential source materials involved in the genesis of augen gneiss and granite gneiss of Askot crystallines. The genesis of augen gneiss gets support for their derivation from the partial melting of upper crustal source. The granite gneiss on the other hand, involves heterogeneous melts derived from a Paleoproterozoic mafic/intermediate lower-crust reservoir probably involving arc magma underplating, that has experienced contamination during up-rise through crust or involving sediments from the subduction zone. The re-interpretation of existing Rb-Sr isotopic data of the region further suggests that the augen gneiss is much younger (~1300 Ma) and is unrelated to granodiorite rocks (~1800 Ma). The dominance of granite gneiss in Askot crystallines and in other parts of eastern Kumaun suggests an important event of crustal growth that took place during the Late Paleoproterozoic involving the production of large volume of heterogeneous granitic melts in the region. Thus, more focus on the regional studies of eastern Kumaun crystalline rocks of Almora nappe, Chiplakot and Munsiari Formation is required for better understanding of Late Paleoproterozoic crustal evolution of the region. Acknowledgements: The authors are grateful to Prof. B.R. Arora, Director, Wadia Institute of Himalayan Geology, for providing facilities and giving permission to publish this paper. We are thankful to Prof. Santosh Kumar for critical and constructive comments that benefited greatly to improve the manuscript. We thank Dr. P.P. Khanna, Dr. N.K. Saini and Sh. Chandra Sekhar for their help in generating the geochemical data on XRF and ICP-MS instruments.

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(Received: 16 June 2008; Revised form accepted: 29 April 2009)

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