Geodynamic significance of the Archean A-type granites exposed along the western margin of a Proterozoic orogenic belt: Insights on the final docking of the Eastern Ghats Belt with the Indian subcontinent

The Proterozoic orogenic belts incorporated in and around the present-day continents preserve complex magmatic, metamorphic, and geophysical signatures of the ancient supercontinents. One such orogenic belt, the Eastern Ghats Belt (EGB) is amalgamated with the Archean cratons of India along a crustal-scale suture zone known as the Terrane Boundary Shear Zone (TBSZ). The continental margin – orogenic belt interfaces, such as the TBSZ, are the black boxes of ancient tectonic processes, since they are rheologically weakened crustal discontinuities that undergo intense deformation and metamorphism recording the complete orogenic history. There have been two schools of thought on the age of final amalgamation of the EGB with the Bastar craton, as the TBSZ records two major tectonothermal events at ~950Ma and ~550Ma, coeval with the formation of supercontinents Rodinia and Gondwana, respectively. The age and mechanism of this amalgamation have implication on the crustal architecture of the Proterozoic supercontinents.

Recent studies confirmed the presence of felsic and mafic granulites of Archean Sm-Nd model ages (3.3 – 3.1 Ga) from the TBSZ that have undergone high-pressure granulite facies metamorphism. It is speculated that these rocks are of Bastar craton in origin and the underthrusting of the Bastar craton beneath the EGB, during the final collision, led to the high-pressure metamorphic conditions. In this communication, we have carried out a comparative petrological and geochemical investigation of the Archean felsic rocks (Grt-bearing charnockites) from the TBSZ and the Hbl-Bt granites from the adjacent regions of the Bastar craton to understand origin and tectonic significance of the charnockites. The garnet-bearing charnockites from the TBSZ are characterised by coarse grained Grt + Opx + Pl + Qz + Kfs + Hbl + Bt ± Ilm. The Hbl-Bt granites of the Bastar craton, adjacent to the TBSZ, are characterized by coarse grained Hbl + Bt + Qz + Kfs + Pl, with small Opx grains forming around Hbl in few places at the interface. The Grt-bearing charnockites and the Hbl-Bt granites are both ferroan and metaluminous to slightly peraluminous in nature. The high concentrations of trace elements, high Y/Nb (>1.2) ratio and pronounced negative anomalies of Eu, Sr and Ti in both the rocks are characteristic of A₂-type within plate granitoids, similar to the other reported granitoids from the Bastar craton. The strong similarity in the geochemistry of Grt-bearing charnockites and Hbl-Bt granites along with the available Archean model ages of the charnockites indicate that the Grt-bearing charnockites of the TBSZ are granulite-facies equivalents of the Hbl-Bt granites and hence represent the remnants of cratonic margin in the TBSZ. This geochemical study along with the Tonian ages (~950 Ma) from monazite cores and inclusions in garnet within the co-exposed metapelites in the suture zone indicate that the Bastar craton underthrusted beneath the EGB during the formation of Rodinia. The ~500 Ma ages reported from the strongly recrystallized monazite rims might represent the reactivation of the intracontinental suture zone due to the far-field stress from the Kuunga orogeny (~530 – 490 Ma) during the formation of East Gondwana.