29. PETROLOGY OF BASALTS OF HOLES 447A, 449, AND 450,

Basalts studied represent different stages of basement formation of the Philippine Sea plate, including the West Philippine Basin (middle Oligocene—Hole 447A) and the Parece Vela Basin (upper Oligocene to middle Miocene-Holes 449 and 450 and Dredge Site 1398, on the seventeenth cruise of the Dmitry Mendeleev). All of these basalts display compositions similar to that of normal mid-ocean ridge (MOR) tholeiites, and despite the age differences, they seem to be differentiation products of an initial tholeiitic melt of essentially the same composition. According to the suggested petrologic scheme, this initial liquid is derived from dry melting of mantle lherzolite at 10 kb and temperatures about 1270°C. Basalts of Hole 447A were formed after a rapid magma injection to shallow depths and cotectic olivine-plagioclase crystallization at 1 atm, whereas Parece Vela basalts were derived from anchieutectic clinopyroxene-plagioclase-olivine crystallization at intermediate pressures (from 5 kb-1 atm). In each case, extrusion and crystallization of the magma were preceded by fractionation of excessive olivine from primary tholeiitic liquid. The petrologic and geochemical features of these basalts do not indicate any influence of subduction zone activity. Within tholeiitic compositional limits, there is a slight progressive increase of K, Ti, P, and Ba in the basalts and volcanic glasses from the West Philippine Basin to the Parece Vela Basin and thence to the Mariana Trough; this may indicate slightly increasing depths of origin of the primary tholeiitic liquids from west to east along the South Philippine Sea transect