Structure of the deep crust beneath the Central Indian Tectonic Zone: An integration of geophysical and xenolith data

Abstract

Xenoliths of garnet granulite and "crustal" eclogites in alkaline dykes from the northern margin of the Dharwar craton suggest that the deep crust beneath this region is dominated by mafic granulite. The granulites consist primarily of clinopyroxene, orthopyroxene, garnet and plagioclase. Clinopyroxene from most garnet-bearing lithologies is eclogitic with Jd/Ts more than 0.5; the Jd component varies from more than 5 to 8 percent. Garnets from the granulites are similar to those in crustal eclogites whereas the ones from pyroxenites are akin to those in kimberlites. Composite xenoliths are indicative of interstratification of the lower crust by ultramafic rocks represented by pyroxenites and websterites that display prominent, high density reflectors in deep seismic sounding profiles. They represent cumulates in relict magma chambers and are expressed as conductors in magnetotelluric transects. Thermobarometric estimates for granulite xenoliths are 740-1094 degrees C, 0.71-1.3 GPa equivalent to depths of 20-40 km, whereas those for pyroxenite xenoliths are 726-1042 degrees C, 0.57-1.05 GPa. These provide for the first time a spatial context to these rock types, suggesting derivation from lower crustal depths. The lower crustal rocks beneath the Central Indian Tectonic Zone are near to their melting point as is evident from the anomalously high geothermal gradient due to which they show lower V(p)s than those characteristic of typical garnet granulites. The deep crust has undergone thickening both due to crustal inflation and under- and intra-plating beyond its original pre-rift thickness, overcompensating for the crustal thinning due to extension. Lateral flow of the lower crust has also contributed to crustal thickening in addition to under- and intra-plating. Transport of midcrustal rocks to the surface along ductile shear zones has caused additional crustal modification.