OROGENIC RECORDS OF ARC-CONTINENT COLLISION ZONES OF SOUTHERN INDIA: IMPLICATIONS FOR CONTRASTING STYLES OF GEODYNAMICS AND CRUSTAL EVOLUTION IN THE PRECAMBRIAN

The location and mechanism of initial mantle extraction producing primitive crust and its transformation into a compositionally evolved continental crust remain enigmatic. Accretionary orogens are identified as one of the viable locations, where manufacturing and reworking of the continental crust occur. Thus, granitoid magmas emplaced in the accretionary orogens can provide crucial evidence on the origin and evolution of the continental crust. Here, petrogenesis of crustal-derived granitoids of southern India has been discussed and to identify the source area processes in relation to the geodyamics.

The Kerala Khondalite Belt (KKB) in southern Indian granulite terrain (SGT) contains granitoid complex consisting interspersed distribution of metatonalites and metagranites formed during arc-continent collision. The zircon crystallization ages reported for these granitoids show a prolonged time gap of 210 Ma between the formation of metatonalites (ca. 2.1 Ga) and metagranites (ca. 1.89 and 1.85 Ga) indicating multiple stages of melting events in the KKB. The ε_Hf(t) values (−0.4 to +3.1) obtained on zircons from metatonalites point to an isotopically heterogeneous protolith and demand the presence of a depleted mantle component mixed with early formed crust in the source region. Further, geochemistry of the metatonalites also indicate their origin in relation to partial melting of a thickened oceanic-arc crust composed of Archaean mafic source rocks (i.e., eclogite or garnet amphibolite) with a garnet amphibolite residue. The negative ε_Hf(t) values (−6.1 to −9.2) obtained on zircons of the metagranites and whole-rock high Rb/Sr (avg. 1.80) and Ba/Sr (> 6) ratios indicate crustal reworking in their genesis, suggesting remelting of a quartzo-feldspathic (TTG) source, within the plagioclase stability fields. Thus, the multiple tectono-magmatic records of the metagranitoids of KKB suggest building up of an over-thickened oceanic-arc leading to initial mantle extraction forming the primitive crust. Subsequent melting of this primary crust by basaltic underplating and finally by arc-continent collision caused intra-crustal melting and differentiation of the continental crust leading to its compositional evolution, characterizing the changeovers of geodynamic styles in the Precambrian.