Geochemistry and Tectonic Evolution of the Orogenic Granitoids Associated with the Andean-Type Siham Arc, Central Arabian Shield

Abstract

The syntectonic granitoids and the Al Bara batholith trondhjemite- granite suite exposed in the Afif quadrangle, Saudi Arabia, are the major plutonic components of the Andean-type Siham arc, developed during the period from about 750 to 700 Ma along the western margin of the Afif terrane. These plutonic rocks have a wide range of lithologies, from diorite to leucogranite. They are characterized by multiple periods of plutonism and rejuvenation during Pan-African orogeny (c. 900-550 Ma). The syntectonic granitoids and the Al Bara batholith rocks are calc-alkaline, I-type and metaluminous to marginally peraluminous. They are enriched in Rb, Ba, K, La, Ce and Sr relative to Nb, Zr and Ti, suggesting their emplacement along an active continental margin. The Al Bara batholith rocks, with the largest variations in lithology, texture and incompatible elements’ patterns, indicate the greater effects of Pan-African rejuvenation. The post-tectonic Dariyah batholith granites were intruded at 585 ± 8 Ma in the late Proterozoic (650-615 Ma) Murdama group, a back-arc basin filled up with clastic sedimentary rocks with minor limestone and volcanic rocks. The batholith ranges in composition from co-magmatic granodiorite to syenogranite. Biotite granite is the abundant rock type. All lithologies of the batholith are in part porphyritic with lack of chilled contacts. The porphyritic varieties are modeled as mixtures of phenocrysts (presumably restite) and minimum melts. The higher levels of Rb, Na, La, Ce and Sm, the marked depletion of Ba, Sr, and Ti and the strong Eu negative anomaly are all indicative of in-situ fractionation of the Dariyah rocks after intrusion into higher levels of the crust.