A translithospheric suture in the vanished 1-Ga lithospheric root of South India: Evidence from contrasting lithosphere sections in the Dharwar Craton

Garnet xenocrysts and eclogite xenoliths from 15 kimberlites (1.0–1.1 Ga in age) have been used to map the composition and structure of the lithospheric mantle along an 80-km traverse across the eastern margin of the Closepet Granite in Andhra Pradesh. The SCLM at the SW end of the traverse is more depleted (abundant harzburgites, mean XMg of olivine ≈ 93.5, mean whole-rock Al2O3 ≈ 1.5%) than that at the NE end (fewer harzburgites, XMg ≈ 92, Al2O3 > 2%). The depleted layer is ca 195 km thick in the SW, and ca 170 km in the NE, though geotherms are similarly low (ca 35–37 mW/m2). The middle of the traverse is underlain by a strongly refertilised SCLM with a higher geotherm (ca 40 mW/m2) and extensive evidence of metasomatism. At the SW end of the traverse, abundant eclogites are tightly concentrated in a layer from 175 to 190 km depth, coinciding with a zone of melt-related metasomatism. In the central part, eclogites are distributed through the highly metasomatised section from 90 to 160 km depth. These data suggest that the kimberlites at either end of the traverse sampled two distinct lithospheric blocks, perhaps coinciding with the Eastern and Western Dharwar Cratons. The zone of refertilised SCLM between them is interpreted as the cratonic suture, metasomatised by mafic melts (now eclogites). If this suture dips 70–80° to the east, its surface outcrop lies within the Closepet Granite. The “tilt” of the proposed cratonic suture may reflect overthrusting of the Eastern Dharwar Craton crust up to 100 km to the west. Recent geophysical data (seismic, MT) suggest that the depleted lithospheric root beneath the Dharwar Craton that was sampled by the 1.1 Ga kimberlites is no longer present. The removal or major modification of this root could have occurred during the breakup of Gondwanaland, and may help to explain Indiaʹs rapid northward drift. India thus joins the North China Craton as an example of the destruction of an Archean continental keel.