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

• Mineral chemistry classify the Jabal Al-Hassir ring complex as alkaline granite.
• Low oxygen fugacity (ƒO2) indicate that the granite formed under reduced conditions.
• ɛHf(t) values (+7.0 to +10.3) consistent with melting of juvenile crustal protoliths.
• Single zircon grains show mantle-like to weakly elevated δ18O values (+5.8 to +7.4‰).
• Hf-crustal ages (0.71–0.94 Ga) show no evidence of pre-Neoproterozoic protolith.

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 ɛHf(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-tNC) of 0.71–0.94 Ga indicate minor contribution from an older continental crust in the formation of the Jabal Al-Hassir granitic rocks (crystallization age = 620 ±3 Ma), but any such component is likely to be Neoproterozoic in age. Temperature and oxygen fugacity (ƒO2) estimates suggested that the Jabal Al-Hassir A-type granite magma was generated at high temperature (820–1050 °C) and low ƒO2. Geochemical characteristics (e.g., low ƒO2), 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.

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