ND ISOTOPIC CONSTRAINTS ON THE ORIGIN OF POST-OROGENIC A-TYPE GRANITES OF THE ARABIAN SHIELD, KINGDOM OF SAUDI ARABIA
These post-orogenic granites were emplaced throughout the Arabian Shield, a collage of arc and continental terranes that were accreted in Pan-African time. The western terranes of Midyan, Hijaz, and Asir are composed of Neoproterozoic oceanic arcs, whereas the eastern terranes of Affif, Ar Rayn, Abas, and Al-Mahfid have continental affinity as indicated by radiogenic Pb isotopic compositions, early Proterozoic and Archean Nd model ages, and several Proterozoic U-Pb ages. The oceanic and continental terranes are joined along the 680-640 Ma Nabitah suture.
We have obtained Sm-Nd data on 24 samples of post-orogenic granite from the Hijaz, Asir and Afif terranes in Saudi Arabia. This suite exhibits subtle geographic variations that may reflect the differing character of the crust into which they were emplaced: the eastern granites have slightly more evolved Rb/Sr and K/Rb ratios and include strongly peraluminous granites whereas the western granites include a greater proportion of peralkaline compositions and syenitic granites. There is no strong geographic or geochemical control on initial Nd isotopic compositions of the granites. All are more radiogenic than CHUR, and all but two samples have initial epsilon Nd between +2.7 to +5.1. Only samples from the >715 m.y. Taif province of the Asir terrane have distinct Nd isotopic compositions of +10.1 and +10.7 at their assumed intrusive age of 570 Ma. These results are interpreted to suggest that the source(s) of the A-type granites lie mainly at depth beneath the exposed upper crust. Emplacement appears to accompany transcurrent faulting and associated mild local compression and extension.