Neoproterozoic highly fractionated Ⅰ-type granitoids of Shillong Plateau, Meghalaya, Northeast India: geochemical constraints on their petrogenesis

作者：Debjani Choudhury,M.Faruque Hussain

摘要：There are several Pan-African granitoid plutons widely distributed in Shillong Plateau,NE India.Nongpoh(506.7±7 Ma)and Mylliem(480-430 Ma)plutons were chosen for the petrological and geochemical study to constrain their petrogenesis.Nongpoh pluton consists of coarse-grained porphyritic quartz-monzogranite(NQM)and Mylliem pluton consists of medium to coarse-grained porphyritic granite(MG).The constituent minerals are K-feldspar,plagioclase,quartz,and biotite in both granitoids with accessory minerals of hornblende,zircon,sphene,and ilmenite.Both NQM and MG are metaluminous to weakly peraluminous(A/CNK=0.82-1.06),exhibit varied ranges of SiO2(NQM:58.4-64.9 wt%;MG:66.9-69.9 wt%),and display a clear compositional gap in the Harker variation diagram.NQM contains higher abundances of CaO,MgO and Fe2O3t and similar K2O and total alkali contents compared to MG.They have distinctive geochemical features typical of highly fractionated I-type granitoids such as higher abundances of K2O,Al2O3,MgO,CaO,Al2O3+CaO>15 wt%and A/CNK<1.1,low P2O5 content,enrichment in LILE,depletion in HFSE and HREE and highly fractionated REE patterns with moderate Eu anomalies,implying magma generation in a post-collisional extension setting and thinning induced asthenosphere upwelling,accompanied by the partial melting of the overlying enriched lithospheric mantle.The multi-element diagram of both NQM and MG shows pronounced negative anomalies at Ba,Nb,Sr,P,Zr,and Ti which implies a major role of crystal fractionation in their petrogenesis.High concentrations of Th,U,and Pb in the granitoid types point to additional involvement of crustal components in their generation.However,MGs with more pronounced positive spikes at U,Th,and Pb compared to NQM in multi-element diagram suggests the involvement of more felsic crustal material.The observed geochemical features of the granitoid types thus suggest that they are genetically unrelated to each other and their parental magmas were modified during magmatic differentiation processes.We suggest that the

發文機構：Department of Earth Science

關鍵詞：Ⅰ-typegranitoidsShillongplateauGEOCHEMISTRYPETROGENESIS

分類號： P58[天文地球—巖石學]