Geochemistry and petrogenesis of the Triassic volcanic rocks in the east-central segment of Tethyan Himalaya.

The Triassic volcanic rocks are distributed in the east-central Tethyan Himalaya as interbeds, and are paleogeographically belonged to the northeastern margin of Greater India. Major and trace element compositions and whole-rock isotopes for these volcanic rocks are systematically analyzed by atomic absorption spectrophotometry, ICP-MS and isotopic dilution using Finnigan MAT 262, respectively, to constrain their compositional variations with time and to infer their petrogenesis. The Triassic volcanic rocks are belonging to basalt based on the petrographical and geochemical indicators. Low contents of MgO (4.27% similar to 7.72%) for these basalts indicate an evolved magma. High TiO2, TFeO and P2O5 contents of the Early and Middle Triassic basalts, together with marked fractionation between LREE and HREE, enriched HFSE (e. g. Nb, Zr), additionally high ratios of Ti/V, Ti/Y and Zr/Y, indicating a similarity with Emeishan high-Ti basalts and a difference with Hawaiian alkaline basalts for their higher La, Ce, Zr and Hf abundances. These basalts belonged to alkaline basalt with OIB-type features that formed in a rift setting of continental margin. However, the Late Triassic basalts, which belonged to tholeiitic basalt with E-MORB features that formed in the rift setting of continental margin, have low TiO2, TFeO and P2O5 contents, together with slight fractionation between LREE and HREE, low concentrations of HFSE ( e. g. Nb, Zr). Nd isotopic compositions of the Triassic basalts show a continuous change from enriched compositions to depleted compositions with time ( e. g. averaging epsilon(Nd)(t) = - 1.2 -> -0.5 -> 4.4 for the Early, Middle and Late Triassic basalts respectively), indicating that magmatic source changed from enriched to depleted with time. Trace element indicators (e. g. La/Sm, La/Ta, (Th/Ta)(PM), (La/Nb) (PM)) show that the Early and Middle Triassic basalts were experienced variable contamination of lower crust while the Late Triassic basalts were not contaminated by crust. Trace element signatures and Nd isotopic compositions for the Early, Middle Triassic basalts provide the discernable contributions from both the OIB-type source and the lithospheric mantle source, in which the Early, Middle Triassic basalts are related to the low-degree partial melting of garnet lherzolite, and for the Late Triassic basalts are not only characterized by E-MORB, but also are associated with the larger-degree partial melting of spinel lherzolite. The Triassic basalts are interpreted as the products of variable degree of partial melting of the magmas that upwelling asthenosphere-derived melts mixed with lithospheric mantle-derived components at extensional settings with different extents. The Triassic rifting in Tethyan Himalaya is most probably related to the active rifting involving in the upwelling of asthenosphere, and the less Triassic melts could be attributed to the mantle rising beneath the rift may cool by conduction, or to the incubating plume beneath eastern Gondwana lithosphere at that time.