The Kathalguri Pluton is a granitic pluton con-fined within the Palaeo-Mesoproterozoic Shillong Group of rocks in the Mikir Massif,eastern India.The pluton is vertically zoned characterized by lower medium-to coarse-grained equigranular to porphyritic granite and upper fine-grained equigranular granite.Small to large mafic mag-matic enclaves(MME)are distributed within the lower granite,while homogeneous grey-coloured hybrid rocks dominate the upper granite.From field relationships,tex-tural 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 com-positions of the larger MME,smaller MME,hybrid rocks,and granitic host rocks vary between An41-An47,An34-An44,An02-An52,and An03-An29 respectively.The alkali feldspar compositions of the hybrid rocks range from Or84 to Or97,while that of the host granite varies from Or92 to Or96.The biotite is eastonitic and siderophyllitic in com-position,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 crys-tallized at fO2 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 hybrid rocks.These textures are mainly found in plagio-clase crystals,which include resorbed grains,oscillatory zoned plagioclase,boxy-cellular morphology,and over-growth texture.Some common magma mingling and mixing textures like quartz ocelli and back-veining are also preserved in the smaller MME.An interesting feature observed in the homogeneous hybrid rocks of our study area is mantled alkali feldspar in which orthoclase is mantled by microcline.We propose that this overgrowth texture formed due to epitaxial crystallization of microcline on orthoclase owing to mixing between the felsic and mafic magmas.Such magma mixing event produces a heteroge-neous system that is in an extreme state of disequilibrium and facilitates the epitaxial growth of one feldspar on another.On the other hand,the larger MME of our study domain shows limited interaction with the felsic host as indicated by the replacement of clinopyroxene crystals by amphibole and biotite.

Gaurav Hazarika;Pallabi Basumatary;Tribujjal Prakash;Hiredya Chauhan;Bibhuti Gogoi

Department of Geology,Cotton University,Guwahati,Assam 781001,India;Inter University Accelerator Centre,New Delhi 110067,India

地球化学学报（英文版）

[1]Gaurav Hazarika;Pallabi Basumatary;Tribujjal Prakash;Hiredya Chauhan;Bibhuti Gogoi-.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)[J].地球化学学报（英文版）,2022(03):453-469

zoned,Mikir,Kathalguri,Pluton,Palaeo,Shillong,equigranular,zonations,An41,An47,An34,An44,An02,An52,An03,An29,Or84,Or97,Or92,Or96,eastonitic,siderophyllitic,plagio,clase,resorbed,boxy,ocelli,mantled,microcline

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