Granitoids of the Jonnagiri greenstone belt of eastern Dharwar craton: Constraints from geochemistry and U-Pb zircon geochronology.

The Jonnagriri greenstone belt (JGB) of eastern Dharwar craton is one of the unique and economically important greenstone belts of southern India. This greenstone sequence consists of metabasalts, meta-felsic volcanics and schistose meta-tuffs surrounded by granitoid intrusives. These intrusives are the only known granitoid in India that hosts orogenic type gold deposit in the Dona sector of JGB. In our present work, we are focusing on the granitoids emplaced surrounding the JGB that are broadly classified into two main groups; (i) The Pagadarayi granodiorite (PGD) and (ii) The Chennampalli granites (CGR). The present study is to examine the geochemical evolution, decipher the tectonic settings and determine the timing of emplacements of the granitoids using whole rock geochemistry and U-Pb zircon dating. This will also help us to understand the crustal architecture of the part of the eastern Dharwar Carton in the southern Indian peninsula. The PGD compositionally ranges from tonalite to granodiorite suits and the adjacent CGR ranges from granite to syenogranite suits of rocks. The petrogenetic evolution of these granitoid clearly showed a transition from calc-alkaline to a more fractionated potassic rich granite. The major oxide and trace element behavior showed that they are metaluminous to peraluminous in nature where the PGD are geochemically linked to the TTGs while the potassic rich CGR phase majorly shares its genetic link with hybrid and two-mica granites. The source of PGD suits of rocks is inferred to be an outcome of partial melting of TTG crust and the CGR suits are ferromagnesian deficit phase with an increase in K₂O/Na₂O due to crustal contamination, which indicates a result of crustally reworked magmatic phase. Therefore, an enrichment of LREE and LILE relative to HREE and HFSE with depletion of Nb-Ta-Ti notably is observed, which represents a convergent setting signature for these granitoids. The PGD suit of rocks dominantly showed I-type characteristics affirmed by their ASI index, and with the typical SiO₂, CaO and FeO contents, indicating that they are part of volcanic arc to island arc granitoid settings. The CGR showed distinctive characteristics of A₂ type granites due to its Y/Nb value ranging from 1.2 to 1.83 depicting a shallow depth subduction regime which are formed within the crust, indicating a within plate post-colliotional settings. Zircon U-Pb dating by LA-ICP-MS reveals that the first emplacement of PGD recorded a concordant emplacement age of 2651±14 Ma and the CGR yielded concordant ages of 2557±10 Ma and 2532±22 Ma respectively. This implies that the Jonnagiri granitoid represents an orogenic type tectonic settings where diversification in the mode of granitoid intrusion showed a change in volcanic-arc magmatism to within plate magmatism, from the older phase of tonalite-granodiorite emplacement to the successive younger potassic rich syenogranite emplacement.