Abstract: A large number of Early Cretaceous mafic rocks are widely exposed in the Tethyan Himalaya, southern Tibet, China. These rocks provide an important record of the tectonic-magmatic evolution of the north margin of the Indian continent before the Cenozoic Indian-Eurasian collision. Their genesis is critical for reconstructing the evolution history of the Indian passive margin. This paper presents systematical petrographic and geochemical data of the Early Cretaceous metabasites from the Ramba area, eastern Tethyan Himalaya. The Ramba metabasites include amphibolitie-amphibolitic gneisses, which occurred as dikes and lenses in the low-medium grade metasedimentary rocks of the Ramba dome. These Ramba metabasites display tholeiitic affinity and low contents of SiO₂ (44.78-47.42 wt%), relatively high contents of MgO (7.31-9.60 wt%) and FeOt (9.68-15.87 wt%) with variable TiO₂ contents (0.73-2.16 wt%) and medium to high Mg# values (46.4-63.9). The radiogenic Nd isotopic compositions of the Ramba metabasites (ε_Nd(t) values range from 5.7 to 6.5) are comparable with that of the Indian Oceanic MORB. However, the REEs distribution patterns of the Ramba metabasites are similar to that of the E-MORB, indicating involvement of enriched components in their origin. It is worth noting that the Ramba metabasites do not show depletion in high-field-strength elements (HFSE), and their Nb/U ratios are similar to that of the mantle-derived rocks. Therefore, it can be inferred that the continental crust contamination is not significant in the formation of Ramba metabasites and their enrichment components should be mainly attributed to an enriched mantle source. Consequently, it is speculated that the Ramba metabasites were products of the interaction between magma derived from a depleted asthenospheric mantle and an enriched continental lithospheric mantle. They were generated during the early stage of the breakup of eastern Gondwana (~140 Ma) and were originated from the asthenosphere upwelling that was triggered by early activity of the Kerguelen mantle plume.

Key words: Tethyan Himalaya, metabasite, depleted asthenospheric mantle, the enriched continental lithospheric mantle