TECTONOMAGMATIC EVOLUTION OF THE THARWAH SEGMENT OF THE NEOPROTEROZOIC BIR UMQ-NAKASIB SUTURE ZONE, ARABIAN-NUBIAN SHIELD, WESTERN SAUDI ARABIA

The Arabian-Nubian Shield (ANS) was an integral component of the Gondwana supercontinent and it was the locus of most, if not all, juvenile crust development during Neoproterozoic time. The formation of a number of juvenile arc terranes resulted from subduction of the Mozambique Ocean, which separated E and W Gondwana. Subduction and arc formation culminated in the accretion of the arc terranes to form the ANS Superterrane ca. 800-700 Ma, and was followed by terminal collision between E and W Gondwana ca. 600 Ma that produced the East African Orogen (EAO). The Tharwah segment in Arabia lies in the central portion of the 600-km-long Bir Umq-Nakasib suture zone, the oldest and best-preserved Neoproterozoic suture in the ANS. The northern EAO, which includes the ANS, contains thick, well-preserved volcanic sequences (some of which contain boninites), calc-alkaline magmatic suites, and abundant ultramafic suites interpreted as ophiolitic nappes.

Previous studies of the lateral equivalent to the Tharwah segment in Sudan have resulted in several controversies surrounding the evolution of the arc terranes and nature of the suture zones separating them. Some workers have suggested that the suture records a complete Wilson cycle, with initial rift-related volcanism, seafloor spreading, subduction, and final arc collision and suturing. However, other workers suggest that the suture records a more "Cordilleran"-style of terrane accretion not associated with rifting. Work in the Tharwah segment is testing these hypotheses by constraining the tectonic setting of the volcanic and magmatic units through their trace- and rare-earth-element geochemistry, by constraining their ages through robust U-Pb zircon isotopic studies, and by investigating the structural relationships between tectonic units that include one of the most complete Penrose ophiolite sequences in the ANS.

Juvenile Neoproterozoic volcanic rocks along the suture are well-preserved, typically weakly metamorphosed, and include basaltic to rhyolitic lavas, basaltic pillows and hyaloclastites, and mafic and felsic ash-fall and felsic welded ash-flow tuffs. Geochemical results from lithologies such as these will be presented in conjunction with field evidence supporting one of the above hypotheses for the Tharwah segment of the Bir Umq-Nakasib suture zone.