Zirconolite and Baddeleyite in an Ultramafic Suite from Southern India: Early Ordovician Carbonatite‐Type Melts Associated with Extensional Collapse of the Gondwana Crust

We report here the occurrence of rare zirconium-bearing minerals, zirconolite (CaZrTi2O7) and baddeleyite (ZrO2), from an unusual ultramafic suite within the Achankovil Shear Zone (ACSZ) in southern India. Zirconolite occurs as inclusions within spinel in phlogopite-graphite spinellite and shows characteristic development of radial cracks. Baddeleyite is commonly observed as an included phase within phlogopite from phlogopite dunite and graphite-spinel glimmerite. The mineral also occurs less commonly within spinel and graphite from graphite-spinel glimmerite. The composition of zirconolite is characterized by an enrichment of U and Th over rare earth elements. Baddeleyite shows abundance of Zr with minor Hf, Ti, and U. The mode of occurrence along with the chemical composition of these minerals implies their formation as early-stage crystallization products from a silica-undersaturated melt that was enriched in "carbonatite-phile elements" such as Ca, Zr, Ti, and volatiles CO2 and H2O. We report U-Pb chemical ages from the zirconolite that show a mean of 469 ± 11 Ma. We correlate this age with the timing of emplacement and consolidation of the ultramafic suite within ACSZ, and it is considerably younger than the late Neoproterozoic-Cambrian ages reported from this zone. Our data suggest early Ordovician carbonatite-type melts emplaced within ACSZ, which we identify as a mantle-rooted zone. We infer that deep-seated extension along the ACSZ probably triggered the generation of such melts, related to the extensional collapse of the orogen following the collisional assembly of the Gondwana supercontinent. © 2006 by The University of Chicago. All rights reserved