Av. Mcguire et Rj. Stern, GRANULITE XENOLITHS FROM WESTERN SAUDI-ARABIA - THE LOWER CRUST OF THE LATE PRECAMBRIAN ARABIAN-NUBIAN SHIELD, Contributions to Mineralogy and Petrology, 114(3), 1993, pp. 395-408
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.