Abstract
The study areas (Bulghah and Hamimuah) are located in the Afif terrane between the Halaban-Zarghat fault zone and Ar Rika fault zone. They consist of many gabbroic to granitic I-type intrusions emplaced into Neoproterozoic volcanosedimentary rocks and are intruded by Neoproterozoic A-type granites. The studied plutonic rocks are I-type magmatic rocks, calk-alkaline, metaluminous to slightly peraluminous (A/CNK > 1.2), formed in a volcanic arc setting. On a primitive mantle-normalized spider diagrams, almost all rocks show a significant Nb-Ta-Ti depletions relative to K and La, which is typical of magmatism from a subduction zone tectonic setting. Geochemical features of the mafic intrusion (gabbro and diorite) are comparable to those of the arc-metavolcanic calc-alkaline rocks of the Arabian Shield, which were produced by partial melting of plagioclase- or spinel-peridotite in the upper most mantle <80 km deep in an intra-oceanic island arc. This suggests that the mafic intrusive rocks of Bulghah and Humaymah represent the plutonic equivalents of the Arabian Shield arc metavolcanic calc-alkaline rocks. The compositional variations from granodiorite to monzogranite of Bulghah and Humaymah suggest various degree of fractional crystallization of feldspar, biotite and amphibole. Y/Nb with Th/Nb, Th/Ta and Ce/Pb relationships indicate that the granodiorite and monzogranite were generated by a mafic parental magma contaminated with crustal materials, and controlled by fractional crystallization. Zircon U-Pb dating indicates that the mafic intrusive rocks from Bulghah and Humaymah, Saudi Arabia were formed at similar to 670 Ma, whereas the granitoid I-type intrusions were formed between 661 +/- 5 and 643 +/- 4 Ma, confirming the importance of the 640-700 Ma crust forming event in Saudi Arabia.