PERALUMINOUS, POTASSIUM-RICH GRANITOIDS IN THE SEMAIL OPHIOLITE

Potassic granites are found as dikes and small stocks in the uppermost part of the mantle sequence of the Semail ophiolite, predominantly in the northern and less frequently in the central and southern part. Ne w chemical analyses show that the biotite granites lie on the same evo lutionary trend as the quartz diorites and tonalites, occurring as lat e intrusives in the gabbro and mantle sequence. Based on mineral chemi stry, the P-T conditions of andalusite-cordierite bearing pegmatoidal granites were calculated, giving pressures of 1.8-2.2 kbar and a solid us within the stability field of andalusite. Hydrothermal experiments on five natural granite and tonalite compositions gave minimum melting between 650 and 675 degrees C at 2 kbar. At 700 degrees C, excess H2O , there is already 70-80% melt from the granitic starting compositions . Potassic granites in the ophiolite can be the result of extreme frac tional crystallization in the late intrusive series. Granitic melts ca n also be produced by a two-stage process, in which fluids escaping fr om the underlying metamorphic sole trigger partial melting of the more felsic members in the ophiolite series. Two-mica granites and pegmati tes in the underlying metamorphic sole, possibly melting products of a mphibolites and biotite gneisses, are contemporaneous. These escaping fluids could also explain the Pb and Sr isotopic compositions of the l eucocratic dikes, indicating variable crustal contamination.