Geochemistry and U–Pb SHRIMP zircon geochronology of microgranular enclaves and host granitoids from the South Khasi Hills of the Meghalaya Plateau, NE India: evidence of synchronous mafic–felsic magma mixing–fractionation and diffusion in a post-collision tectonic environment during the Pan-African orogenic cycle

Abstract

Abstract Felsic magmatism in the South Khasi Hills of the Meghalaya Plateau, NE India, referred herein as the South Khasi granitoids (SKG: 519.5±9.7 Ma), invariably contains rounded to elongate, fine- to medium-grained, mafic to porphyritic microgranular enclaves (ME: 515±13 Ma) showing sharp to crenulate contacts with the host SKG. Compositions of plagioclase, amphibole and biotite in the ME are slightly distinct or similar to those of the host SKG, which appear re-equilibrated through diffusion mechanisms during partial liquid (semi-solid) conditions prior to the complete solidification of the mafic–felsic interacting system maximum at shallow continental crustal depths of approximately 9.5 km ( c. 250 MPa) under oxidizing conditions. Although the ME are chemically modified, both the ME and SKG exhibit a wide chemical variation as high-K 2 O metaluminous (I-type) granitoids. Linear to near-linear variations of chemical elements against SiO 2 may suggest the origin of the ME as the result of the mixing of crystal-charged mafic and felsic magmas in various proportions. However, the data scatter and ill-defined chemical variations can be attributed to chaotic chemical mixing, diffusion and, to some extent, mechanical sorting of the crystals. The identical trace element patterns of the ME and the respective SKG have strengthened the idea of chemical re-equilibration at varying levels between them through diffusion during synchronous mixing–fractionation and mingling. Mean zircon 207 Pb/ 206 Pb ages from the ME (515±13 Ma) and SKG (519.5±9.7 Ma) underline the co-existence of Cambrian mafic and felsic magmas formed during the later stages of the assembly of East Gondwanaland as an integral part of the Pan-Indian–African–Brasiliano orogenic cycle. The ME in SKG thus represent mingled, undercooled, heterogeneous hybrid magma globules formed by linear to chaotic mixing that was synchronous with fractionation of coeval crystal-charged mafic (enclave) and felsic (SKG) magmas, which experienced differential degrees of chemical exchange through diffusion with the surrounding felsic host in an open magma system.