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  

作　　者：Gaurav Hazarika Pallabi Basumatary Tribujjal Prakash Hiredya Chauhan Bibhuti Gogoi 

机构地区：[1]Department of Geology,Cotton University,Guwahati,Assam 781001,India [2]Inter University Accelerator Centre,New Delhi 110067,India

出　　处：《Acta Geochimica》2022年第3期453-469,共17页地球化学学报（英文）

基　　金：the DST-SERB grant vide Project No.CRG/2020/002635;CSIRJRF fellowship No.09/1236(0005)/2019-EMR-I;through DST-FIST funding (SR/FST/ ESI-152/2016)。

摘　　要：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 homo

关 键 词：Mafic magmatic enclaves Hybrid rocks Orthoclase-microcline transformation Shillong Group Kathalguri pluton 

分 类 号：P588.1[天文地球—岩石学] P542[天文地球—地质学]