The Lesser Himalayan belt is an integral part of the Paleoproterozoic northernmost Indian Block (NIB), which is composed of granite, granite gneiss, meta-mafics, and massive pink and grey quartzites.They are also exposed in the outer Kumaun Lesser Himalaya.A peculiar highly chloritized lithological unit occurs in and around Laugar close proximity to these lithounits and appears similar to the fine to porphyritic mafic volcanics, which contain pronounced xenocrysts of feldspars and quartz embedded in a fine-grained matrix.In the published records, the same lithological unit is termed as porphyroid or keratophyre and remains elusive in terms of its nomenclature and stratigraphic position.Does the so-called porphyroid or keratophyre represent assimilation and mixing?This paper addresses this question based on field relation, petrographic features, and zircon U-Pb-Lu-Hf-O isotopic investigations.The field and petrographic observation suggest that volcanic rocks exposed in the outer Kumaun Lesser Himalaya are a composite suite of mafic volcanics, intermediate trachyte porphyry (porphyroid or keratophyre), and ignimbrites, which may be genetically related.They show mixing, assimilation and devolatilization-related petrographic features.The chronological records of assimilated zircons (1.81 Ga) in the trachyte porphyry are identical to the primary zircon ages (1.88-1.85Ga) of Amritpur granites, which underline the assimilation and mixing process.This is consistent with the field and petrographic evidence.The assimilation leads to profuse devolatilization forming the barren silicic and pegmatite veins which are evident in the field.Chondritic to sub-chondritic zircon εHf t values of the Amritpur granite (-0.5 to -4.9) and trachyte porphyry (+2.9 to -3.2) with T DM (2.6 to 2.3 Ga), zircon inheritance (2.2, 2.3 and 2.4 Ga), and higher zircon ∂ 18 O ‰ values (8.51 ± 0.50) also support our hypothesis.The isotopic results also narrate a long crustal evolutionary history of NIB.It is suggested that 1.9 to 1.8 Ga bimodal magmatism is produced by the reworking of older crustal components, accompanied by mantle input.The development of successive marginal rift basins allows the penecontemporaneous deposition of continent-derived and mafic volcanogenic sediments.Assimilation and mixing played a dominant role in 1.8-1.9Ga bimodal magmatism and crustal evolution of NIB.
Quartz reefs and veins of variable thickness have been intruded in host Paleoproterozoic Malanjkhand granites in the mine area and are primarily restricted to phyllic as well as potassic alteration zones. They are mainly of two types: mineralized and barren. Fluid inclusion petrography depicts mainly five types of inclusion which are aqueous biphase, monophase, monocarbonic, H2O-CO2 and polyphase (L+V+H). They are called here Type I, Type II, Type III, Type IV, and Type V, respectively, and are present in both mineralized and barren quartz veins/reefs. All types of inclusion are common except type V, which appears rare in both. However, the sizes of type II and IV are unexpectedly small. The micro thermometry results imply a relatively high temperature (209.4-376.4oC) of fluid entrapment in the mineralized counterpart. However, it is considerably lower (133.9-182.2oC) for the barren counterpart. Although the salinity of fluid appears low for mineralized quartz veins/reef (0.63-0.87 wt.% NaCl equivalent), while for barren counterpart, it is considerably higher (0.92-0.98 wt.% NaCl equivalent). The observed textural and microthermometry results advocate that the Malanjkhand hydrothermal system has resemblances with the porphyry system and indicates probable genetic linkage between barren and mineralized quartz veins/reef. Keywords: Fluid Inclusions, Quartz Veins/Reef, Aqueous Inclusions
The Proterozoic felsic and mafic magmatism in India in varied tectonic settings is reviewed and discussed based on available geological, geochemical, and geochronological constraints.Neoarchean-Paleoproterozoic magmatism, as discrete volcanoplutonic complexes and Large Igneous Provinces (LIPs) in the Bastar, Singhbhum and Dharwar cratons and associated mobile belts are also included.Paleoproterozoic magmatism also contributed to the geodynamics of Himalaya.Meso-to Neoproterozoic magmatism comprises kimberlites and lamproites in the Bastar and Dharwar cratons.Neoproterozoic magmatic rocks chiefly constitute the Aravalli-Delhi mobile belts.Mantle-derived magmas, mainly as mafic to hybrid enclaves and syn-plutonic dykes, have contributed significantly in the evolution of calc-alkaline, metaluminous (I-type) to peraluminous (S-type) granites formed in subduction to post-collision tectonic zones.Anorogenic (A-type) granites are commonly reported in post-collision to rift environments.As a whole Proterozoic mafic-felsic magmatism contributed greatly in the crustal architecture and evolution of Indian subcontinent that are correlatable with the construction and break-up of Columbia and Rodinia supercontinents.
The early Paleoproterozoic Malanjkhand granitoids (2.45-2.5Ga)host Cu (±Mo ± Au) deposit.The MG lie in the south of Central Indian Tectonic Zone (CITZ), forming an integral part of the Bastar Craton.The MG bear moderately high Sr/Y and (La/Yb) N similar to as adakite-like melt which is supposed to be peculiar for Phanerozoic Porphyry style Cu-deposits.It is unique in a sense that it holds Precambrian Cu-deposit which somewhat a rare occurrence.Numerous attempts have been done so far to decode the Physio-chemical environment of its mineralization.However, less known for Physio-chemical environment and nature of magmatism of host MG.The present work is an comprehensive attempt to quantification of Physio-chemical environment, nature of magmatism and mineralization of Paleoproterozoic Malanjkhand Cu-deposit.Electron probe micro analysis (EPMA) of major and accessory minerals constituting the mineralized and unmineralized MG has been carried out.The chemistry of amphiboles, plagioclase, biotite, sphene, magnetite and chlorite has been done to deduce the chemical evolution, elemental substitution, nature and physical condition of evolving host magma and mineralization fluid.The mineralized MG exhibit coarse-grained hypidiomorphic textures with abundant secondary minerals.They bear rockforming Pl-Kfs + Qz + Bt + Hbl ± Ap ± Zrn ± Mag ± Spn ± Chl and ore-forming Ccp+Py±Sp±Cv±Po assemblages.The unmineralized MG are relatively fresh, unaltered, and bear Pl-Kfs + Qz + Bt+ Hbl ± Ap± Zrn ± Mag ± Spn assemblage.The hydrous ferromagnesian minerals (Hbl-Bt) from MG are altered to chlorite.Fe-Ti oxides (Mag-Ilm) are commonly found associated with biotite and amphiboles.The results suggest that MG crystallized at very high 'O' fugacity condition with log fO 2 ranges from -12 to -13 at narrow temperature range (700-850 0 C) and emplaced at shallow level (1.5-3kb).However, 'O' fugacity decreases with decrease of temperature.The magma contains very high amount of water (4.5-7 wt.%) which supress the crystallisation of plagioclase relative to amphibole and provide peculiar adakitic character to MG.The later produce hydrothermal system marks the ore precipitation.The ores were precipitated by CO 2 -bearing H 2 O rich brine hydrothermal solutions at low to high temperatures (93-342 0 C).
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