GRANULITE XENOLITHS FROM WESTERN SAUDI-ARABIA - THE LOWER CRUST OF THE LATE PRECAMBRIAN ARABIAN-NUBIAN SHIELD

Mafic and intermediate granulite xenoliths, collected from Cenozoic al kali basalts, provide samples of the lower crust in western Saudi Arab ia. The xenoliths are metaigneous two-pyroxene and garnet granulites. Mineral and whole rock compositions are inconsistent with origin from Red Sea rift-related basalts, and are compatible with origin from isla nd arc calc-alkaline and low-potassium tholeiitic basalts. Most of the samples are either cumulates from mafic magmas or are restites remain ing after partial melting of intermediate rocks and extraction of a fe lsic liquid. Initial Sr-87/Sr-86 ratios are less than 0.7032, except f or two samples at 0.7049. The Sm - Nd data yield T(DM) model ages of 0 .64 to 1.02 Ga, similar to typical Arabian-Nubian Shield upper contine ntal crust. The isotopic data indicate that the granulites formed from mantle-derived magmas with little or no contamination by older contin ent crust. Calculated temperatures and pressures of last reequilibrati on of the xenoliths show that they are derived from the lower crust. C alculated depths of origin and calculated seismic velocities for the x enoliths are in excellent agreement with the crustal structure model o f Gettings et al. (1986) based on geophysical data from western Saudi Arabia. Estimation of mean lower crustal composition, using the granul ite xenoliths and the Gettings et al. (1986) crustal model, suggests a re markably homogeneous mafic lower crust, and an andesite or basaltic andesite bulk composition for Pan-African juvenile continental crust.