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Petrogenesis of the Borjuri diorite pluton in the Mikir Massif of Northeast India:implications for post-collisional intermediate magmatism during the Pan-African orogeny

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Acta Geochimica 2023年第4期42卷 747-764页

作者： Tribujjal Prakash Ashima Saikia Pallabi Basumatary Bibhuti Gogoi Department of Geology Cotton UniversityGuwahatiAssam 781001India Department of Geology University of DelhiDelhi 110007India

The Assam-Meghalaya Gneissic Complex(AMGC)of northeast India contains numerous Pan-African granitic bodies that have been attributed to post-collisional rift-related magmatism.The present study is concerned with the first appraisal of intermediate magmatism(diorite,monzonite,and monzodiorite)found in the Borjuri pluton of Mikir Massif,which is the eastern extension of AMGC.The diorites are strongly metaluminous and exhibit enriched LREE([La/Yb]N=1.63–7.37)with respect to HREE([Gd/Yb]N=1.95–2.27).The studied rocks do not show any mineralogical or textural indication of metamorphism.Tectonic discrimination diagrams indicate that these rocks originated in a within-plate tectonic setting.The lower Mg#(33.49–38.69),low Cr(below detection limit),and Ni(27–41 ppm)contents along with elemental ratios such as Rb/Sr(0.32–0.95),La/Nb(0.49–4.21),and Nb/Ce(0.11–0.64)suggest a crustal source for the diorites.Discrimination diagrams coupled with elemental ratios suggest that these rocks originated due to partial melting of mafic components in the crust with possible contribution from mantle materials.The P–T conditions of diorite emplacement(7.4 kbar,688℃)were calculated using the amphibole-plagioclase geothermobarometer.Geochemical and geochronological data of the Pan-African felsic plutons reported from the AMGC indicate that these rocks were emplaced in a post-collisional extensional regime.The Borjuri pluton is in close proximity with the kathalguri pluton,which has been reported as a product of PanAfrican magmatism.In view of the numerous extensional Pan-African felsic magmatism reported from the AMGC and based on the close vicinity of the Borjuri diorites with the kathalguri granites,we speculate that the Borjuri diorites are products of the Pan-African post-collisional magmatism.

关键词： Granites Rift-related magmatism Intermediate magmatism Within-plate tectonic setting kathalguri pluton Assam-Meghalaya Gneissic Complex

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Magma mixing dynamics in a vertically zoned granitic magma chamber inferred from feldspar disequilibrium assemblage and biotite composition: a case study from the Mikir Massif,eastern India

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Acta Geochimica 2022年第3期41卷 453-469页

作者： Gaurav Hazarika Pallabi Basumatary Tribujjal Prakash Hiredya Chauhan Bibhuti Gogoi Department of Geology Cotton UniversityGuwahatiAssam 781001India Inter University Accelerator Centre New Delhi 110067India

The kathalguri pluton is a granitic pluton confined within the Palaeo-Mesoproterozoic Shillong Group of rocks in the Mikir Massif, eastern India. The pluton is vertically zoned characterized by lower medium-to coarsegrained equigranular to porphyritic granite and upper finegrained equigranular granite. Small to large mafic magmatic enclaves(MME) are distributed within the lower granite, while homogeneous grey-coloured hybrid rocks dominate the upper granite. From field relationships, textural features, and mineral chemical analyses we infer that the kathalguri pluton was a vertically zoned felsic magma chamber that was intruded by mafic magma during its evolution. The lower portion of the magma chamber was occupied by crystal mush, while the upper portion was dominated by melt when mafic magma intruded it. The occurrence of upper and lower zonations was probably brought about by fractional crystallization. The presence of such zonations within the felsic reservoir caused the mafic magma to interact with the two distinct zones differently,forming MME in the lower granite and homogeneous hybrid rocks in the upper portion. The plagioclase compositions of the larger MME, smaller MME, hybrid rocks,and granitic host rocks vary between An;–An;, An;–An;, An;–An;,and An;–An;respectively. The alkali feldspar compositions of the hybrid rocks range from Or;to Or;, while that of the host granite varies from Or;to Or;. The biotite is eastonitic and siderophyllitic in composition, while the pyroxene is diopside. The apparent pressure range of biotite crystallization in the granitic rocks was calculated at 1.93 to 2.28 kbar, while the biotite crystallization temperature for the different rock types is in the range of 628 and 759 ℃. Oxygen fugacity estimates from the biotite suggest that the kathalguri magmas crystallized at fO;conditions above the nickel-nickel oxide(NNO) buffer. Distinct disequilibrium textures indicating mixing and mingling between the mafic and felsic magmas are preserved in the smaller MME and homogeneous hy

关键词： Mafic magmatic enclaves Hybrid rocks Orthoclase-microcline transformation Shillong Group kathalguri pluton

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