Single zircon Hf-O isotope constraints on the origin of A-type granites from the Jabal Al-Hassir ring complex, Saudi Arabia

The Jabal Al-Hassir ring complex in the southern Arabian Shield is an alkaline granite complex comprising an inner core of biotite granite that outwardly becomes a porphyritic sodic-calcic amphibole (ferrobarroisite-katophorite) granite. A combined study of mineral chemistry and single zircon Hf-O zircon isotope analyses was carried out to infer the magma sources of the Neoproterozoic post-collisional A-type granitoids in Saudi Arabia. The granitic rocks show high positive initial e{open}Hf(t) values of +7.0 to +10.3 and δ¹⁸O values of +5.8‰ to +7.4‰ that are consistent with melting of a juvenile crustal protolith that was formed during the Neoproterozoic assembly of the Arabian-Nubian Shield (ANS). Crustal-model ages (Hf-t_NC) of 0.71-0.94Ga indicate minor contribution from an older continental crust in the formation of the Jabal Al-Hassir granitic rocks (crystallization age=620±3Ma), but any such component is likely to be Neoproterozoic in age. Temperature and oxygen fugacity (f{hook}O₂) estimates suggested that the Jabal Al-Hassir A-type granite magma was generated at high temperature (820-1050°C) and low f{hook}O₂. Geochemical characteristics (e.g., low f{hook}O₂), geochronological data, and Hf and O isotope compositions, indicate that the magmas of the Neoproterozoic A-type granites of the Jabal Al-Hassir ring complex were likely generated by crustal partial melting of a juvenile Neoproterozoic lower crustal tholeiitic rocks, following collision between East and West Gondwana in the final stages of the evolution of the Arabian Shield.