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

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-K2O metaluminous (I-type) granitoids. Linear to near-linear variations of chemical elements against SiO2 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 207Pb/206Pb 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.

Cambrian magmatism in the Tethys Himalaya and implications for the evolution of the Proto‐Tethys along the northern Gondwana margin: A case study and overview

“…In the north‐eastern Indian Craton, Kumar, Pieru, et al () reported zircon U‐Pb ages of I‐type granites (520 Ma) and associated mafic microgranular enclaves (MMEs; 515 Ma) from the Meghalaya plateau, which is part of the Pan‐African‐Braziliano Orogeny associated with Gondwana amalgamation. Kumar, Rino, et al () suggested that the Palaeozoic granitoids (535‐507 Ma) formed during the late stage of the eastern Gondwana amalgamation during the Pan‐African event (Chatterjee, Bhattacharya, Duarah, & Mazumdar, ). The A2‐type granite (515 Ma) exposed in the Khasi Hills was attributed as a product of extension in the post‐orogenic setting of the Pan‐African event (Majumdar & Dutta, ).…”

“…B. Gou et al, 2015;Gu et al, 2013;Guynn, Kapp, Gehrels, & Ding, 2012;Hu, Li, Su, Li, & Yu, 2010;Hu, Li, Wang, Xie, & Wu, 2013;Hu et al, 2015;Johnson, Oliver, Parrish, & Johnson, 2001;Ji et al, 2009;J. King, Harris, Argles, Parrish, & Zhang, 2011;Kumar, Pieru, Rino, & Hayasaka, 2017;Lee & Whitehouse, 2007;C. Li, Xie, Sha, & Dong, 2008;G.…”

“…B. Gou et al, ; Gu et al, ; Guynn, Kapp, Gehrels, & Ding, ; Hu, Li, Su, Li, & Yu, ; Hu, Li, Wang, Xie, & Wu, ; Hu et al, ; Johnson, Oliver, Parrish, & Johnson, ; Ji et al, ; J. King, Harris, Argles, Parrish, & Zhang, ; Kumar, Pieru, Rino, & Hayasaka, ; Kumar, Rino, et al, ; Lee & Whitehouse, ; C. Li, Xie, Sha, & Dong, ; G. J. Li, Wang, Huang, Chen, & Dong, ; G. J. Li, Wang, Huang, Gao, & Yu, ; Lin, Cong, Gao, & Zou, ; Y. Liu, Siebel, Massonne, & Xiao, ; S. Liu et al, ; Q. S. Liu, Ye, & Wu, ; Y. M. Liu, Li, Xie, et al, ; Z. C. Liu, Wu, Ding, et al, ; Miller et al, ; Pan et al, ; Peng et al, ; Pullen, Kapp, Gehrels, Ding, & Zhang, ; Quigley et al, ; Shi et al, ; Shi et al, ; Song et al, ; Song, Niu, Wei, Ji, & Su, ; Spencer, Harris, & Dorais, ; Visonà, Rubatto, & Villa, ; Wang, Zhang, Yang, & Zhang, ; Wang et al, ; Wang et al, ; Wang et al, ; Wang et al, , ; Z. H. Wu, Ye, Wu, & Zhao, ; Xie et al, ; Xie, Li, Su, Wu, & Xie, ; Xie, Li, Fan, & Su, ; Xing et al, ; Xu et al, ; Xu et al, ; Yan et al, ; Yang et al, ; Zhang, Wang, Shen, & Shi, ; Zhang et al, ; Zhang et al, ; Zhang, Xiang, et al, ; S. W. Zhao, Lai, Gao, Qin, & Zhu, ; S. W. Zhao, Lai, Qin, & Zhu, ; Zhu et al, ) [Colour figure can be viewed at wileyonlinelibrary.com]…”

Magma chamber processes and geodynamic implications of the Pithora pluton, Bastar Craton, Central India

“…These features are reported from different parts of the Bastar Craton (Kumar, Rino, & Pal, and 2004b; Kumar & Rino, ; Pandit & Panigrahi, ; Elangovan, Krishna, Vishwakarma, Hari, & Ram Mohan, ). MEs are also reported in postcollisional granitoids from Mahakoshal belt (Bora, Kumar, Yi, Kim, & Lee, ) and related to Pan‐African Orogeny in south Khasi Hills of Meghalaya Plateau (Kumar & Pieru, ; Kumar, Pieru, Rino, & Hayasaka, ; Kumar, Pieru, Rino, & Lyngdoh, ). In this study, we have systematically documented the field and petrographic features, provided the mineralogical and whole‐rock geochemical data for the ME, syn‐plutonic mafic dyke, mixed zones, and host rock from the Pithora pluton to understand the extent and nature of interaction between the felsic and mafic magmas to quantify the mixing of interacting magmas.…”

The age and geochemistry of the Bardkish syenite, northwest Iran: Syenite formation during Neo‐Tethyan subduction

“…Evidence of magma interactions (Figure 3) favors the coupling of Abbreviations for minerals are based on Whitney and Evans (2010); Amp, amphibole; Bt, biotite; Cpx, clinopyroxene; Grt, garnet; Kfs, Kfeldspar; Pl, plagioclase mixing and fractional crystallization processes leading to simultaneous mixing-fractionation of coeval mafic and felsic magmas (Bora, Kumar, Yi, Kim, & Lee, 2013;Kumar, Pieru, Rino, & Hayasaka, 2017). Positive…”