Tonian shoshonitic to ultrapotassic granitoids from Chhotanagpur Gneissic Complex, Eastern Indian Shield: Age, origin and tectonic implications

Abstract

This work reports petrogenesis of an ultrapotassic granitoid pluton emplaced in the Tonian (949.4 ± 2.3 Ma; new LA-ICPMS zircon U–Pb dating) along a regional shear zone during the post-collisional stage of the Grenvillian Satpura orogeny in Eastern India. The hypidiomorphic granitoids comprise dominantly perthite, microcline (BaO up to 5.85 wt.%), quartz, albite and subordinate amphibole ± diopside ± epidote, allanite, titanite, magnetite ± ilmenite ± biotite ± calcite. Preservation of magmatic epidotes and resorbed boundaries indicates rapid ascent of the granitoid magma. Mylonitic deformation overprinted the southern part of the E-W trending pluton. Magmatic epidote with resorbed boundaries suggests rapid magma ascent. The metaluminous granitoids display affinities with shoshonitic rocks, i.e., enrichment of K₂O (5.79–11.41 wt.%), large ion lithophile elements (Ba 461.5–7004.8 ppm; Sr 151.3–3548.3 ppm), light rare earth elements (LREE 111.2–1317.7 ppm) and high K₂O/Na₂O (1.77–11.35) and La_CN/Yb_CN (11.7–82.48) ratios with both negative and positive Eu-anomalies (Eu/Eu* = 0.58–1.43; average 0.89). Trace element characteristics of zircons demonstrate their magmatic origin. Pseudosection modeling displays high temperature (∼800°C), high fO₂ (ΔNNO +0.8 to +2.6), and CO₂ activity (0.9) of the magma that intruded at shallow crustal depth (∼300 MPa). Biotite remains unstable at this physicochemical condition of the shoshonitic magma. Metaluminous nature, high (La/Yb)_CN (11.7–82.48) and Sr/Y (6.46–277.21) ratios, and Nb/U (avg. 7.4), Ce/Pb (avg. 6.8), Nb/Ta (avg. 11.9), Zr/Hf (avg. 31.61), and low Rb/Sr (0.09–1.39) ratios of these rocks indicate the derivation of the magma from partial melting of the mafic lower crust. Batch melting modeling shows the granitoid magma originated from 5 to 30 % batch melting of K–Ba–Sr-rich shoshonitic mafic (hornblende granulite) source. The study proposes new (Ba + Sr)–Ti–P and Ba–Sr–Ti triangular diagrams for distinguishing mantle vs. crustal sources of post-collisional granitoids.