GEOCHEMISTRY, PETROGENESIS AND TECTONIC SIGNIFICANCE OF THE BASIC VOLCANIC UNITS OF THE ZILDAT OPHIOLITIC MELANGE, INDUS SUTURE ZONE, EASTERN LADAKH (INDIA)

Basic volcanic rocks within the Zildat ophiolitic melange of Indus sut ure zone in eastern Ladakh are medium to fine grained with partially p reserved primary texture and mineralogy. These rocks are predominantly alkaline basalt with high Nb/Y and enriched incompatible trace elemen t characteristics, similar to those of the oceanic island basalt (GIB) . The minor sub-alkaline basaltic rocks resemble N-type mid ocean ridg e basalt (N-MORB) but with much lower abundances oi incompatible trace element including REE. The alkaline rocks probably generated through variable, bur low degrees of partial melting of enriched mantle source and evolved through high pressure olivine and clinopyroxene fractiona tion. Low pressure plagioclase and Fe- Ti oxide do not appear to be ma jor fractionating phases. Limited data on the sub-alkaline rocks sugge st that their parental melts were derived from mantle sources some wha t similar to that of N- MORB. Significant role of added cumulates of o livine, clinopyroxene and Fe- Ti oxides is also indicated in their gen esis. Ophiolitic melanges all along the Indus suture zone appear to ha ve formed due to thr accumulation of melange material in the upper par t of the subduction zone where they suffered glancophanitic (blueschis t) metamorphism and retrograded partially to greenschist grade as thes e were subsequently obducted to its present position probably during t he Cenozoic Himalayan orogeny due to collision of Indian and Eurasian plates.