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

R. Islam ¹, K. K. Purohit ¹, V. C. Thakur ¹

Affiliations
1 Wadia Institute of Himalayan Geology, Dehra Dun - 248 001, IN

Abstract

On the basis of geochemical study, two granitic plutons are identified in the Bhilangna valley of the Garhwal Himalaya, viz. the Chaili porphyritic granite gneiss (CPGG) and the Ingedinala granite gneiss (IGG). Both, CPGG and IGG rocks fall in the quartz rich zone (QRZ) of chemical-mineralogical typology of Dcbon and Le Fort, 1982. CPGG and IGG are strongly peraluminous, S-type granites and enriched in Si and K and depleted in Ca and Sr. The unusually high K₂O suggests introduction of LILE during metasomatic processes. Both, CPGG and IGG are crustally derived and produced due to the partial melting of psammo-pelitic sediments. Probably the source materials for CPGG and IGG are same, attested by the geochemical similarities. The Q-Ab-Or plots of CPGG and IGG revealed that the temperature higher than that of the 'minimum' melt was attained during anatexis. During the Himalayan orogeny, intracontinental subduction took place along MCT and provided channels for the metasomatic fluids which induced K and Rb metasomatism and subsequently modified the nature of CPGG and IGG.

Keywords

Anatexis, Granitic Pluton, Garhwal Himalaya, Subduction, Main Central Thrust, Metasomatism.

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R. Islam ¹, K. S. Bist ¹, B. K. Chaudhuri ¹, B. S. Rawat ¹, K. K. Purohit ¹

Affiliations
1 Wadia Institute of Himalayan Geology, Dehra Dun-248001, IN

Abstract

A maiden attempt has been made to study the far-flung Hanuman Tibba Granites of Kullu valley, Himachal Pradesh. The granite has intruded the low grade Haimanta (-Lahaul) Formation which tectonically overlies the Rohtang Gneissic Complex. Integrated field, petrographic and geochemical studies have revealed that it is peraluminous S- type granite derived from sedimentary protolith. Compositional difference in terms of depletion of Fe, Mg, Ti, Ca, Sr, Zr and enhancement of Sit K and Rb from margin to core of the granite body is consistent with fractional crystallization process.

The emplacement of Hanuman Tibba Granite on the same tectonic level as Jispa and Ratilaman granite (500±ma) in adjacent Lahaul-Spiti region suggests a Cambro-Ordovician age for the Hanuman Tibba Granite.

Keywords

Petrology. Geochemistry, Hanuman Tibba Granite (HTG), Haimanta Formation, Kullu Valley, Himalaya.

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Subhajit Sinha ¹, Sumit K. Ghosh ¹, Rohtash Kumar ¹, R. Islam ¹, P. Sanyal ², S. J. Sangode ³

Affiliations
1 Sedimentology Group, Wadia Institute of Himalayan Geology, Dehradun - 248 001, IN
2 Department of Geology and Geophysics, Indian Institute of Technology, Kharagpur - 721 302, IN
3 Department of Geology, University of Pune, Ganeshkhind, Pune - 411 007, IN

Abstract

The Neogene Siwahk fluvial succession (12 77-4 48 Ma) of the NW Himalayan foreland basin, east of Ravi River, are deposited under varied Tectono-Climatic conditions. Petrography and geochemistry of the sediments were carried out to understand the relative roles of tectonics and climate in this basin.

The sandstones are classified as sub-litharenites with subordinate arkosic-graywacke. The dental components particularly the rock fragments are consistent with the inferred source area presently disposed towards the north of the depocenter and suggest that the Higher, Lesser and Lower Tertiary formations supplied detntus since 12 77 Ma. The Th/Sc vs. Zr/Sc ratio indicates sediment recycling. The Cr and Ni, negative Eu anomaly, LREE enrichment and moderately flat HREE in the mudstones collectively suggest dominant contribution from felsic igneous rocks beside contribution from basic, sedimentary/metamorphic hithology. Near consistent nature of the dental modes and geochemical parameters through time suggest unvarying source since 12 77 Ma.The present study also infers that the source areas uplift owing to tectonic activity of Chad Thrust at least by 12 77 Ma with a perceptible activity of MBT after 8 Ma.

The 5¹⁸0 variations in soil carbonates reveals ongoing intense monsoon system since 12 77 Ma followed by a phase of aridity at 91 Ma. Presence of fresh and weathered feldspar, limestone, basic volcamcs and mica, both in humid and arid phase indicate rapid deposition and preservation. Despite the climatic turnover from humid to arid, distinctive changes in the framework mineral compositions over time is not discernable.

The petrographic and geochemical data reveals that the near similar source area, relatively rapid transport, moderate chemical weathering and sediment recycling controlled the composition of the sediments of the HFB in response to tectonics and climate.

Keywords

Climate, Provenance, Tectonics, Himalayan Foreland Basin, Siwahk, Kangra.

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K. H. Sachan ¹, R. J. Bodnar ², R. Islam ¹, Cs. Szabo ², R. D. Lawi ²

Affiliations
1 Wadia Institute of Himalayan Geology, Dehra Dun-248001, IN
2 Department of Geological Sciences, Virginia Polytechnic Institute and State University, Blacksburg, VA-24061, US

Abstract

Eclogites from the Tso-Morari Crystalline Complex in Ladakh, India, preserve evidence for high P, low T metamorphism of subducted Indian continental crust in the presence of a moderate salinity aqueous fluid phase. The phengite barometry estimates a pressure range of  11-14 kbar for eclogitization whereas garnet-omphacite geothermometry reveals a temperature range of 530°-580°C. Following eclogitization at 530°-580°C at a minimum pressure of 14 kbar, the eclogites were tectonically exhumed along a nearly isothermal P-T path that brought them to within 10-30 kilometers of the surface. Final transport to the surface followed a more normal continental geotherm.

Keywords

Tso-Morati, Eclogite, Fluid Inclusion, Mineral Geothermobarometry, Ladakh Himalaya.

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R. Islam ¹, Sumit K. Ghosh ¹, Himanshu K. Sachan ¹

Affiliations
1 Wadia Institute of Himalayan Geology, 33 General Mahadeo Singh Road, Dehra Dun - 248 001, IN

Abstract

The shallow marine Nagthat siliciclastics of the Jaunsar Group show extensive development of medium to thickly bedded arenites and wackes, and represent the terminal Proterozoic siliciclastic episodes of the Lesser Himalaya. Chemical variation between arenites and wackes of Nagthat siliciclastics is related to the elemental distribution within the different mineralogical phases. Inverse correlation of most of the major oxides and trace elements (Ni, Cr, Co and Nb) with SiO₂, indicate quartz dilution effect. The majority of arenites show the common presence of plagioclase, K-feldspar and clay minerals, but lack ferromagnesium minerals and therefore indicate minimal decomposition of felspar and greater decomposition of ferromagnesian minerals. This phenomenon is well documented by the positive correlation of Fe₂O₂^(t) Ni, CO, K₂O, Rb, Sr, Th, Nb and Y with Al₂O₃ hereby suggesting that elemental distributions are mainly controlled by clay minerals. A moderate correlation exists between Al₂O₃ and La and Yb suggesting that REEs are similarly controlled in part by clays or micas. The plots of Nagthat siliciclastics in A-CN-K diagram, along with their enriched K₂O contents suggest variable degree of post-depositional K-metasomatism. Presence of higher amounts of monocrystalline quartz than the polycrystalline quartz, characteristic major and trace element contents, and enriched LREE pattern with negative Eu anomaly indicate a source of granite or granitic gneiss. High Eu/Eu ratio, low Ni concentration and low La/Co, Th/Co and Cr/Th ratios corroborate the acidic source. Granite or granitic gneiss source signature, along with indicative palaeocurrent directions, suggest that the detritus for Nagthat siliciclastics may have been derived from the southerly direction where Banded Gneissic Complex (BGC) of Aravalli-Bundelkhand craton is widely developed.

Keywords

Neoproterozoic, Nagthat Siliciclastics, Source Rock, Geochemistry, Banded Gneissic Complex (BGC), Kumaun Lesser Himalaya.

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