Journal of Geological Society of India (Online archive from Vol 1 to Vol 78)

Sandeep Singh ¹, A. K. Jain ¹

Affiliations
1 Department of Earth Sciences, University of Roorkee, Roorkee - 247 667, IN

Abstract

A newly identified Chor Thrust demarcates the southern margin of the Proterozoic Chor granitoid massif within the Jutogh Group metamorphics of the Lesser Himalaya in Himachal Pradesh. Numerous shear criteria reveal a topto- SW overthrust-type Intense ductile strain along this margin and are consistent with the southward propagating nappes.

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Sandeep Singh ¹, Biswajayee A. Patra ¹, A. R. Vijan ², A. K. Jain ³

Affiliations
1 Department of Earth Sciences, Indian Institute of Technology Roorkee, Roorkee - 247 667, IN
2 Geochronology Division, KDMIPE, ONGC, Kaulagarh Road, Dehradun - 248 001, IN
3 Department of Earth Sciences, Indian Institute of Technology Roorkee, Roorkee - 247 667

Abstract

Biotite and K-feldspar Rb-Sr ages from three granitoid intrusives within the Karakoram Metamorphic Belt (KMB), eastern Ladakh yield indistinguishable ages between 12-10 Ma. Since the closure temperature of biotite for Sr diffusion is 300±50°C, we interpret these ages due to exhumation at 10 to 7 km depth within the crust. This implies that the Pangong Metamorphic Belt of the KMB, located at the southern termination of the Eurasian Plate, was undergoing a fast exhumation in a transpressional regime during Middle Miocene. These data also indicates that the southern edge of the Eurasian Plate has exhumed rapidly in comparison to the Higher Himalayan Crystallines within the Indian Plate segment.

Keywords

Rb-Sr Ages, Granitoids, Karakoram Complex, Eastern Ladakh.

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Sandeep Singh ¹, Stefan Claesson ², A. K. Jain ¹, David G. Gee ³, P. G. Andreasson ⁴, R. M. Manickavasagam ⁵

Affiliations
1 Department of Earth Sciences, Indian Institute of Technology Roorkee, Roorkee - 247 667, IN
2 Laboratory for Isotope Geology, Swedish Museum of Natural History, Box 50007, SE- 104 05 Stockholm, SE
3 Department of Earth Sciences, Uppsala University, Villavagen 16, SE-752 36, Uppsala, SE
4 Department of Lithosphere and Biosphere Science, Solvegatan 12, SE-223 62, Lund, SE
5 Institute Instrumentation Centre, Indian Institute of Technology Roorkee, Roorkee 247 667, IN

Abstract

Along the Sutlej valley, the lower package of the Higher Himalayan Crystallines (HHC) exposes a small concordant to discordant intrusive grey granite -The Maglad Khad Granite, within garnet mica schist/Banded gneiss of the Jeori Formation. This body is fine grained and foliated along the margins, whereas the central part is relatively undeformed. This body along with aplites and pegmatites intrudes the country rock during early to syn-D₁ deformation. This is later affected by the most pervasive D₂-deformation producing gneissosity within the granite. U-Pb dating of zircons by conventional isotopic dilution technique yield an upper intercept age of 2068±5 Ma (2σ) from 6 zircon-Fractions with MSWD=0.93, constraining the age of crystallization in the basal parts of the HHC during Early Proterozoic as well as the constraining pre-Himalayan fabric development.

Keywords

Higher Himalayan Crystallines, Maglad Khad Granite, Geochronology, U-Pb Zircon Dating, Sutlej Valley, Himachal Pradesh.

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Sandeep Singh ¹

Affiliations
1 Department of Earth Sciences, Indian Institute of Technology, Roorkee - 247 667, IN

Abstract

Conventional zircon U-Pb analyses of optically clear multi-grain fractions from the Chor Granitoid sample - CH 1/1:provide a comp1ex pattern with eight points yielding a reference line with an upper intercept age of c. 910 Ma and large scatter. Three zircon fractions from the more deformed sample CH 4/11 of this body define a-discordia line with an imprecise upper and lower intercept ages of 859±¹¹⁹₂₇ Ma and 460±264 Ma respectively. On the other hand, SHRIMP analyses of carefully selected core and rim of CH 1/1 zircon give magmatic rim/grain age of 823± 5 Ma with zircon cores having more than 5% discordant ²⁰⁷Pb/²⁰⁶Pb ages ranging from ∼900 Ma to ∼1560 Ma. The presence of older cores indicate that 0.9 Ga age obtained from conventional TlMS U-Pb technique is probably a mixed age of magmatic and inherited zircons.

Keywords

U-Pb dating, Zircons, TIMS, SHRIMP, Chor granitoid, Himachal Himalaya.

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Priyom Roy ¹, A. K. Jain ², Sandeep Singh ²

Affiliations
1 Department of Earth Sciences, Indian Institute of Technology Roorkee, Roorkee - 247 667, IN
2 Department of Earth Sciences, Indian Institute of Technology Roorkee, Roorkee – 247 667, IN

Abstract

The Karakoram Shear Zone is a northwest-southeast trending dextral ductile shear zone, which has affected the granitic and granodioritic bodies of the southern Asian Plate margin in three distinct episodes. The ductile shearing of the granitic bodies at Tangste and Darbuk has resulted in the development of mylonites with mylonitic foliation and stretching lineation. More intense deformation is noted in the Tangste granite grading up to orthomylonite, as compared to the Darbuk granite. Kinematic indicators include S-C foliation, synthetic C' and C" antithetic shear bands, Type A s-mantled porphyroclasts, oblique quartz foliation, micro-shears with bookshelf gliding, mineral fishes including Group 2 mica fishes, and Type 1 and 2a pull-apart microstructures, and exhibit strong dextral sense of ductile shearing towards southeast. The textural features of the minerals, especially that of quartz and feldspar, indicate temperature of mylonitisation ranging between 300 and 500°C in the upper greenschist facies, and appear to have been evolved during exhumation as a consequence of oblique strike-slip movements along the Karakoram shear zone.

Keywords

Karakoram Shear Zone, Mylonites, Texture, Kinematic Indicators, Karakoram, Uttarakhand.

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A. K. Choudhary ¹, A. K. Jain ², Sandeep Singh ², R. M. Manickavasagam ¹, K. Chandra ¹

Affiliations
1 Institute Instrumentation Centre, Indian Institute of Technology Roorkee, Roorkee - 247 667, IN
2 Department of Earth Sciences, Indian Institute of Technology Roorkee, Roorkee - 247 667, IN

Abstract

This work provides unequivocal evidence of the existence of Mesoarchean granulite facies metamorphic event in the Palghat-Cauvery Shear Zone (PCSZ) of South India. Charnockite samples from two prominent hills at Kollaimalai (KM) and Pachchaimalai (PM) as well as from two quarries within the Bhavani Shear Zone (BSZ) have been analyzed for their Sm-Nd and Rb-Sr ages to investigate the existence or otherwise of the Archean granulite facies events within the PCSZ. The Rb-Sr whole-rock isochron ages for massive charnockites from both the hills appear to be contemporaneous at 2.9 Ga with the initial Sr isotopic ratios of 0.7012 and 0.7014, respectively. However, the Rb-Sr data for whole-rock samples of basic granulites from one of the quarries within the BSZ indicate open system behavior, while the charnockites from the other quarry have insufficient spread in ⁸⁷Rb/⁸⁶Sr ratios and do not yield any isochron. The Sm-Nd data, on the other hand, do not distinguish between the massive charnockite and the lowland charnockite and yield Depleted Mantle model ages in the range 2.98±0.3 Ga for all of them. The ε^T _CHUR for all of these rocks are highly positive. Both the Sr isotopic ratios and positive ε^T _CHUR values for these rocks strongly suggest a mantle source for all of them. An upper age limit of ∼3.28 Ga may be assigned to the crustal accretion of the protolith of all these rocks on the basis of their Nd model ages. The Rb-Sr isochron ages of 2.9 Ga for the two massifs could be the age of granulite facies metamorphism. Thus, the metamorphism in the KM and PM Hills took place within ∼100 Ma of crustal accretion of these rocks and probably was part of the same geological event of crust formation and metamorphism. The open system behavior with respect to Rb-Sr isotopes in the basic granulite from Bhavani is possibly due to the migration of Sr isotopes, triggered during the later shearing of these rocks.

Keywords

Rb-Sr and Sm-Nd Geochronology, Granulite Facies Metamorphism, Southern Granulite Terrain.

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