Abstract

The Himalayan region has a record of several phases of igneous activity which indicate a continued N—S orientated tensional regime since at least late Precambrian time. Though the earlier volcanic eruptions were restricted both in time and space, the Late Carboniferous-Triassic activity was widespread in terms of both the time interval and area covered.

The Abor Volcanic activity in the Eastern Syntaxial Bend of the Himalayan fold belt is coeval with the initiation of Late Carboniferous—Triassic Panjal Trap volcanism of the Western Syntaxial Bend and eruption of the Sikkim lavas. Chemical characters of the Abor Volcanics indicate (1) a within-plate rift tectonic setting of the lava eruptions, (2) a transitional nature between tholeiitic and alkalic compositions, (3) relative enrichment in incompatible elements, and (4) high Fe-Ti values similar to many such basalts erupted in the regions of active lithospheric rifting. The olivine-free nature, a linear trend for MgO and low Co contents of the studied rocks are suggestive of high pressure olivine fractionation for the generation of the bulk chemistry. However, large variations in the incompatible element abundances and ratios of rocks at a similar fractionation stage discount the evolution of the lava chemistry exclusively by simple fractionation or partial melting processes. Instead, these characters together with the low Zr/Nb ratio and within-plate rift tectonic setting suggests the modification of an upper mantle source region, possibly by fluid phase CO2 immediately before the partial melting and lithospheric rifting. Combining geology of the region with the geochemistry and inferred tectonic setting of these rocks, the lava eruptions are suggested to have been associated with the 'rift valley stage' of the Tethys Ocean formation in the present Himalayan region.

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