High pressure experimental studies on the mineralogical constitution of the lower crust

Diorite and gabbroic anorthosite have been proposed as two possible overall compositions constituting the lower crust. The mineral assemblages stable in these compositions under anhydrous conditions at temperatures of 900-1200 °C and pressures of up to 36 kb have been determined. The low pressure mineralogy is dominated by plagioclase, with subordinate pyroxene and minor quartz. With increasing pressure garnet appears, and garnet, quartz and clinopyroxene form at the expense of plagioclase. Finally at pressures greater than 20 kb at 900-1200 °C plagioclase disappears and the high pressure assemblage consists of clinopyroxene+quartz(coesite)+garnet+K-feldspar ± kyanite(?). Extrapolating the experimental results to P-T conditions predicted for a stable, anhydrous lower crust and calculation of compressional wave velocities for these compositions supports the models of a lower crust composed of diorite or gabbroic anorthosite, where the mineralogy of these compositions consists of clinopyroxene, sodic plagioclase and subordinate quartz, garnet.