PLAGIOGRANITES AND GABBROIC ROCKS FROM THE MINGORA OPHIOLITIC MELANGE, SWAT VALLEY, NW FRONTIER PROVINCE, PAKISTAN

Major, trace element composition and Sr isotopic data were collected f or gabbroic rocks, plagiogranites and albitites in the ophiolite assem blage from Swat Valley (NW Frontier Province, Pakistan). Petrographic study revealed that these rocks were subjected to important structural and mineralogical modifications due to greenschist-epidote-amphibolit e facies sub-sea-floor metamorphism and to brecciation. On the other h and, the examination of whole rock chemical composition and of chemica l trends showed that these rocks were affected by some chemical modifi cations, concerning especially Na2O, K2O and Rb. The very low contents of HFS (high field strength) and RE elements found in gabbroic rocks and plagiogranites were considered to be a primary magmatic feature po inting in part to their cumulitic nature and in part to an origin from a refractory parental magma. The Sr isotopic data indicate that gabbr oic rocks and plagiogranites were subjected to exchange with sea water . The particular chemical features shared by gabbroic rocks and plagio granites suggested that fractional crystallization was a possible evol ution process. In contrast, albitites are characterized by anomalously high contents in HFSE and LREE and by values of the Sr-87/Sr-86 ratio very close to sea water. These features suggest a more complex origin with respect to gabbroic rocks and plagiogranites.