Petrology of Mafic Ultramafic Rocks around Bangriposi and Kuliana Orissa Eastern India Implications for Paleotectonic setting
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The eastern margin of the Paleoarchean Singhbhum Craton of eastern Indian hosts a number of mafic-ultramafic rock suites. Among them, this research work has been focused on a unique group of mafic-ultramafic rocks, found within polydeformed, metasedimentary North Singhbhum Mobile Belt assemblage, in proximity of the Bangriposi Shear Zone. Two dismembered rock units has been identified in the area of study- (i) a layered gabbroic body near the town of Kuliana (Orissa state), and (ii) a serpentinized wehrlite body near the town of Bangriposi (Orissa state). The gabbroic rocks are found within multiply deformed and folded Banded Magnetite Quartzite rocks, and the N-S trending, elongated wehrlite body is found within high-grade (Kyanite±Staurolite±Chloritoid bearing) schistose rocks. No evidence of emplacement in a liquid state (contact metamorphism, country rock xenoliths, apophytic tongues, chilled margins, feeder dykes etc.) has been found in either of the rock units. The gabbro unit of Kuliana shows excellent mineral laminations in the field and its modal layering is defined by variable abundance of plagioclase, orthopyroxene and clinopyroxene. Crystal Size Distribution analysis of plagioclase grains has identified the involvement of textural coarsening mechanism in textural evolution of these rocks. Their crystallization history was controlled by the transitional interplay between adcumulus growth and textural coarsening mechanism. The internal differentiation and chemical evolution was punctuated by an event of magma chamber replenishment. The wehrlite rocks of Bangriposi contain serpentinized olivine, clinopyroxene, orthopyroxene, Cr-spinel, phlogopite, apatite and numerous accessory minerals formed during serpentinization. Their petrographical and mineralogical characteristics indicate a deeper level metasomatic episode, apart from the shallow level serpentinization event. Reaction textures defined by secondary clinopyroxene, phlogopite, orthopyroxene and occurrence of secondary apatite constitute modal metasomatic effects, whereas elevated LREE contents in whole rocks and clinopyroxene REE patterns constitute the effects of cryptic metasomatism. The gabbroic rocks, representing a fragment of the original static magma chamber in which they formed, display affinities towards gabbroic rocks from oceanic crustal magma chambers in terms of major, trace and clinopyroxene chemistry. Trace elemental composition of melts in equilibrium with these gabbroic rocks were modelled using the Equilibrium Distribution Method. These calculated equilibrium melts have enriched MORB characteristics and the residual melts of the gabbroic rocks share similarities with volcanic rocks from Supra Subduction Zone ophiolites. Their pressure-temperature calculation has yielded conditions of approximately 990 °C and 1.3 GPa. The detailed mineralogical and geochemical study of the wehrlite rocks has revealed a complex picture of the deeper level metasomatic episode and identified the possible infiltrating melt as an alkali- carbonatite. Relict clinopyroxene grains (Cr-diopside) from these rocks show affinities towards mantle clinopyroxenes in terms of mineral chemistry. These grains also yield ~ 1100 °C as the lower limit of temperature, and ~ 3.5 GPa as the upper limits of pressure. Protolith reconstruction of the wehrlite rocks shows that prior to the deeper level metasomatic episode, the original lithology was possibly a depleted MORB type mantle unit. Finally, to establish the genetic link between these individual dismembered mafic ultramafic units a two-stage petrological modelling involving selected immobile elements has been implemented. The results show that ~ 35% partial melting of the wehrlite rocks produced basaltic melts, which subsequently underwent ~ 30% fractionation of early formed olivine, pyroxene, plagioclase and spinel to generate melts which are compositionally identical to the equilibrium melts of the layered gabbroic rocks. The major implication of this inference is that the gabbro and wehrlite rocks are co-genetic and they represent fragments of the central part and the underlying lithospheric mantle of the original oceanic magma chamber respectively. These oceanic crustal and mantle fragments found within mobile belt assemblage near a high-strain zone in the eastern margin of Singhbhum Craton, serve as evidence for a tectonic event involving closure of an ocean basin. It is hoped that with further isotopic and geochronological studies, more details of this event can be obtained and regional and global implications explored.
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Igneous geochemistry, Precambrian oceanic crust, Layered gabbro, Metasomatism, Trace element modelling