Peraluminous potassium-rich granitoids in the Semail Ophiolite

Abstract

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. New chemical analyses show that the biotite granites lie on the same evolutionary trend as the quartz diorites and tonalites, occurring as late intrusives in the gabboro and mantle sequence. Based on mineral chemistry, the P-T conditions of andalusite-cordierite bearing pegmatoidal granites were calculated, giving pressures of 1.8–2.2 kbar and a solidus within the stability field of andalusite. Hydrothermal experiments on five natural granite and tonalite compositions gave minimum melting between 650 and 675° C at 2 kbar. At 700° 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 fractional crystallization in the late intrusive series. Granitic melts can also be produced by a two-stage process, in which fluids escaping from the underlying metamorphic sole trigger partial melting of the more felsic members in the ophiolite series. Two-mica granites and pegmatites in the underlying metamorphic sole, possibly melting products of amphibolites and biotite gneisses, are contemporaneous. These escaping fluids could also explain the Pb and Sr isotopic compositions of the leucocratic dikes, indicating variable crustal contamination.