Meso–Proterozoic tectonic evolution of Chhotanagpur Gneissic Complex (CGC): Existence of an either way subduction within Central Indian Tectonic Zone (CITZ)

The present study investigates the tectonic evolution of the western extremity of Chhotanagpur Gneissic Complex (CGC), within the eastern part of E-W trending Central Indian Tectonic Zone (CITZ). The study has been done along a N-S stretch (mostly within Chhattisgarh, India) extending from Sanawal in the north to Pali in the South. The reportedly Meso- to Neo-Proterozoic litho-package of the area, comprise metasedimentaries, metavolcanics and younger granite gneiss-granitoids, is mapped in details and records five episodes of deformation (named D₁ to D₅). The D₁ is manifested by rarely preserved recumbent Class-2 folds (F₁), while D₂ is represented by E-W trending, low-plunging, upright to inclined and Class-1A, 1B, 1C geometry F₂ folds. The D₃ exhibits ESE-WNW trending, reclined to near vertical, Class-2 geometry F₃ folds and D₄ is brittle-ductile shear zone. D₅ related F₅ is a N-S cross warping. We prove that the first (D₁), third (D₃) and fourth (D₄) episodes of deformation in CGC are due to thrust movement, which is manifested by development of shear zones and related folds. In one such northerly dipping thrust zone, named Balangi-Sanawal thrust zone (BSTZ, considered as a splay of the Son-Narmada South Fault, SNSF), enderbite and khondalite (of the Makrohar Granulite belt, MGB) are seen to be present as discrete bodies within the CGC granitoids. These granulite occurrences and adjacent CGC has been geologically mapped, which show that the enderbites of MGB have been thrusted over the D₂ related F₂ folds developed within the amphibolites of CGC. In the proximal zones of all these shear zones sheared porphyroclastic augen syenogranite is emplaced along numerous narrow channels as lensoidal bodies. These sheared syenogranite, along with the associated alternate amphibolite layers, exhibit D₃ related near-vertical F₃ folds.

Existing literatures from the southern part of CITZ, reveal granulites (Balaghat-Bhandara granulites, BBG and Chhatuabhavna granulites, CBG) occur with the greenschist-amphibolite facies CGC rocks against southerly dipping Central Indian Shear (CIS) zone along with emplacement of aforementioned syn-tectonic porphyroclastic augen syenogranite during ~1.62-1.42 Ga (Bhowmik et al., 2011). The field evidences suggest that this CIS thrusting event also marks regional D₃ episode of shearing in CGC. Glancing through these geological evidences, we correlate the northerly dipping BSTZ in the northern part of CITZ to be at ~1.62-1.42 Ga. In the central part of CITZ, CGC rocks got juxtaposed with Ramakona-Katangi granulites (RKG) along northerly dipping Gavilgarh-Tan shear zone (GTSZ) placed at ~1.04-0.93 Ga (Chattopadhyay et al., 2020). We propose that this GTSZ, represents D₄ episode of deformation within the CGC. This has produced a peculiar ‘Ramp and Flat’ geometry with imprints of brittle-ductile shearing in the study area. It is interesting to note that at ~1.62-1.42 Ga, during the D₃ deformation phase, both northerly and southerly subduction of the Central Indian block (CIB) (now preserved as CGC) occurred along the northern and southern boundary of CIB respectively. This prompts us to think of a unique either way subduction accompanied by thrusting of deep-crustal granulites through hanging wall block over the Central Indian continental landmass.