Evolution of the lithospheric mantle beneath Eastern Dharwar craton of Southern India: constraints from peridotite xenoliths from P3 kimberlite pipe of the Wajrakarur

Even though the southern Indian Dharwar craton hosts several kimberlites, lamproite, and lamprophyre fields of the Mesoproterozoic age, mantle-derived peridotitic xenoliths are very rare and are often highly altered and poorly preserved. Due to these constraints, xenolith-based direct mantle investigations have been limited beneath the Indian cratons. In this study, we report extensive geochemical analyses on peridotite xenoliths from the P3 kimberlite pipe of the Wajrakarur kimberlite field from the Eastern Dharwar craton (EDC). With the help of major and trace element compositions of the garnets and clinopyroxenes, this study aims to characterize the mantle below EDC and to comment on its evolution.

During this study, 57 peridotite xenoliths were identified. P-T estimates were carried out using garnet compositions. Based on the vertical distribution of garnets on a projected depth, it is observed that the upper part of the lithosphere is composed mostly of lherzolites(G9) with few harzburgites (G10), whereas the base of the lithosphere is dominated by Ti-Metasomatized garnets(G11).

Garnet compositions show an anomaly in the TiO₂ content, which is marked by a sudden increase in TiO₂ at ~160 km of depth. This depth coincides with an increased concentration of G11 garnets. Zr/Hf vs Ti/Eu plot for garnets shows that carbonatitic and kimberlitic fluids are involved in metasomatizing the SCLM. The Mg# and Cr# values suggest that the lithosphere gets more depleted with increasing depth. Clinopyroxene compositions show the presence of two types. Type 1 is enriched in LREE than the Type 2 clinopyroxenes showing the metasomatic enrichment.

The depth range of the studied peridotite xenoliths indicates sampling of the mantle from ~170 to 190 km of depth, indicating a 190 km thick LAB at 1.1 Ga. However, geophysical studies show a present-day estimate of a ~110 to 120 km thick lithosphere. This further indicates about 70-80 km of delamination of the lithospheric keel in post-Mesoproterozoic times. Such large-scale delamination of the lithosphere might be possible due to the increased frequency of mantle plumes, convective erosion, and the heavily metasomatized nature of the SCLM.