Redox Condition, Nature and Tectono-magmatic Environment of Granitoids and Granite gneisses from the Karbi Anglong Hills, Northeast India: Constraints from Magnetic Susceptibility and Biotite Geochemistry

Abstract

The Karbi Anglong hills (erstwhile Mikir hills) in northeast India are detached and separated from the Meghalaya plateau by a NW-SE trending Kopili rift. The Karbi Anglong hills granitoids (KAHG) and its granite gneissic variants belong to Cambrian plutons formed during Pan-African orogenic cycle, which commonly intrude the basement granite gneisses and Shillong Group metasediments. The KAHG can be broadly classified into three major granitoid facies viz., coarse grained porphyritic granitoid, medium grained massive non-porphyritic granitoid, and granite gneiss, which share a common mineral assemblage of plagioclase-K-feldspar-quartz-biotite±hornblende-apatite-titanite-zircon-magnetite but differ greatly in mineral proportion and texture. Modal mineralogy of KAHG, granite gneiss and basement granite gneiss largely represents monzogranite and syenogranite. The magnetic susceptibility (MS) of the KAHG, granite gneiss and basement granite gneiss varies widely between 0.11×10−3 and 43.144×10−3 SI units, corresponding to ilmenite series (<3×10−3 SI; reduced type) and magnetite series (>3×10−3 SI; oxidized type) of granitoids respectively. The observed MS variations are most likely intrinsic to heterogeneous source regions, modal variations of orthomagnetic and ferromagnetic minerals, and tectonothermal and deformational processes that acted upon these rocks. The primary and re-equilibrated compositions of biotites from the KAHG, granite gneiss and basement granite gneiss suggest calc-alkaline, metaluminous (I-type) nature of felsic host magma formed in a subduction or post-collisional to peraluminous (S-type) host magma originated in syn-collisional tectonic settings, which were evolved and stabilized between FMQ and NNO buffers typically corresponding to reducing and oxidising magma environments respectively.

Geochemistry of Proterozoic and Cambrian granites from Meghalaya Plateau, north‐east India: Implication on petrogenesis of post‐collisional, transitional from I‐type to A‐type felsic magmatism

“…Tectonically, the Meghalaya Plateau represents a pop‐up structure (uplifted block) bounded by the Dauki Fault in the south, Brahmaputra Fault in the north, Jamuna Fault in the west, and the Kopili rift in the east (Figure 1) (e.g., Adlakha, Patel, & Lal, 2011; Anettsungla, Rino & Kumar, 2018; Bilham & England, 2001; Karunakaran, 1974; Sengupta & Agarwal, 1998). The Meghalaya Plateau, including the Mikir Hills, covers an area of ca.…”

“…Based on the limited number ( N = 4) of geochemical analysis on Proterozoic granite gneisses from Rongjeng, Sonsak, Longavalli, and Guwahati, they are characterized as calc‐alkaline metaluminous to peraluminous, that formed in volcanic arc and post‐collisional environments (Kumar, Rino, et al, 2017). Redox series assessment of Cambrian South Khasi (Kumar, Pieru, & Rino, 2005), Kyrdem (Kumar & Singh, 2008) and Karbi Anglong (Rino & Kumar, 2018) plutons is carried out based on magnetic susceptibility mapping and biotite chemistry. Mafic to hybrid microgranular enclaves (MEs) of various shapes and sizes are reported from Cambrian South Khasi (Kumar, Pieru, et al, 2017; Kumar, Pieru, Rino, & Lyngdoh, 2005) and Nongpoh (Sadiq et al, 2018) plutons, which are likely to have formed by mixing of lithospheric mantle‐derived melt with lower crust melt during the Pan‐African Orogeny.…”

Redox series assessment, petrogenetic, and geodynamic appraisal of Neoarchean granites from the Bundelkhand Craton, Central India: Constraints from phase petrology and bulk rock geochemistry

“…In terms of (Al IV + Fe +3 )-Mg-(Fe +2 + Mn) components (Foster, 1960), biotites of FMEG show affinity with Mgbiotite, whereas the CGG biotites are transitional between Mg-and Febiotites, and MBG biotites represent ferri-biotite (Figure 5d). Mgenriched biotites are commonly crystallized in a lesser felsic, intermediate magma as compared to Fe +2 biotites hosting felsic melt (e.g., Anettsungla, Rino, & Kumar, 2018;Bora & Kumar, 2015;.…”