Petrogenesis of the post-collisional porphyritic granitoids from Jhalida, Chhotanagpur Gneissic Complex, eastern India

Abstract

The Jhalida porphyritic granitoid pluton is exposed in a regional shear zone belonging to the Chhotanagpur Gneissic Complex of the Satpura Orogen (c. 1.0 Ga), regarded as the collisional suture between the South and North Indian blocks. The pluton intruded the migmatitic gneisses, metapelites, calc-silicate rocks and amphibolites belonging to the amphibolite facies. The mineral assemblage indicates the calc-alkaline nature of the granitoids. Mafic (Pl–Qz–Bt±Hbl) schists occur as xenoliths within the pluton. The granitoids are classified as alkali-calcic to alkalic, dominantly magnesian grading to ferroan, metaluminous to slightly peraluminous, and shoshonitic to ultrapotassic. Geochemically, the granitoids are enriched in large-ion lithophile elements (LILE), particularly K, and light rare earth elements (LREE), but are comparatively depleted in Nb, Ta, and heavy rare earth elements (HREE). The strong negative correlation between SiO2 and P2O5, metaluminous to weakly peraluminous character, high liquidus temperature (798–891°C) and high fO2 (ΔQFM +0.8 to +1.6) of the melt suggest their I-type nature. Field relations and tectonic discrimination diagrams imply their post-collisional emplacement. Low Nb/U (average 8.5), Ce/Pb (average 9.0), and Al2O3/(Al2O3 + FeO(t) + MgO + TiO2) ratios and relatively low Mg number (average 0.15) of these granitoids indicate a crustal mafic source. Batch melting (at 825–950°C) of 10–20% of an old, incompatible elements-rich high-K high-alumina hornblende granulite can generate the porphyritic granite melt. The heat source for melting was an upwelling of the asthenospheric mantle in the post-collisional set-up. Textural and chemical characteristics of the mafic xenoliths show that invading porphyritic granitoid magma metasomatized the amphibolite protoliths.

Discovery of Early Tonian Calc‐alkaline and Shoshonitic Metamafic Rocks from the North Purulia Shear Zone, Chhotanagpur Gneissic Complex, Eastern India: Implications of Proterozoic Sub‐continental Lithospheric Mantle

“…Several granitic plutons of the shoshonitic series of around 1000 Ma age were recorded from the NPSZ of the CGC (Goswami and Bhattacharyya, 2014;Roy et al, 2021). The shoshonitic mafic rocks of the present study show a clockwise P-T-t path during 990-940 Ma (cf., Karmakar et al, 2011), indicating the fossil suture zone between the North and South Indian blocks.…”

“…(2) As calc-alkaline to shoshonitic metamafic rocks are restricted within the E-W trending NPSZ, such a restricted structural control of magmatic activity is more compatible with the slab break-off model proposed by Davies and von Blanckenburg (1995). The slab break-off model for the genesis of porphyritic granite batholiths occurring in the NPSZ has previously been suggested (Goswami and Bhattacharyya, 2014;Roy et al, 2021). Several authors have applied the slab break-off model in order to explain the post-collisional magmatic genesis in some wellstudied orogens, such as the Alps (Davies and von Blanckenburg, 1995), the Himalayas (Stearns et al, 2013), the Caledonides (Atherton and Ghani, 2002), as well as the East African Orogen (Goodenough et al, 2010) and the Tianshan belt (Deng et al, 2015).…”

“…The early Tonian mafic and felsic magmatism in the study area occurred along extensional shear zones of the North Purulia Shear Zone, implying that tectonic structures controlled the emplacement and elongated lenticular shape of the plutons Bhattacharyya, 2010, 2014;Das et al, 2020;Roy et al, 2021). The foliated granite plutons following the foliation of the shear zone indicate that granite emplacement was coeval with shearing (Goswami and Bhattacharyya, 2014;Goswami et al, 2018;Das et al, 2020).…”

“…The mantle upwelling was related to the generation of shoshonitic rocks, high-grade metamorphism and exhumation of the crust along the NPSZ during 990-940 Ma (Karmakar et al, 2011). Additionally, the emplacement of several batholiths of post-collisional porphyritic granite (~1000 Ma) took place in the NPSZ during this time (Goswami and Bhattacharyya, 2014;Das et al, 2020;Roy et al, 2021) (Supp. Fig.…”