Genesis and Petrology of Postcollisional Rare-Metal-Bearing Granites in the Arabian Shield: A Case Study of Aja Ring Complex, Northern Saudi Arabia

Abstract

The Jabal Aja Ring Complex (ARC) is a late Ediacaran composite pluton of postcollisional A-type granites in the northeastern part of the Arabian Shield. It is an elliptical body with discontinuous ring-shaped outcrops due to later faulting. Field relationships enable the recognition of two main phases of magmatic activity in the ARC. The early phase represents the core of the intrusion (monzogranite, syenogranite, granophyre, and alkali feldspar granite) surrounded by the last phase (alkali volcanics and alkaline/peralkaline granites). The contacts between two phases are gradational and/or sharp, indicating their emplacement within a very short time period before the complete crystallization of the earlier phase. The ARC is alkaline to peralkaline rare-metal granites with the common geochemical characteristics of postcollisional intraplate A-type rocks. The pronounced negative Eu anomalies (Eu/Eu*=0.14−0.25) reflect extreme magmatic fractionation and perhaps the effects of late fluid-rock interaction. All A-type rocks of the ARC retain a positive Nb-Ta anomaly that increases from the early phase to the most evolved alkaline/peralkaline granites. They define geochemical signatures reflecting their derivation from the same magma sources that evolved through fractional crystallization with crustal contamination. The geochemical characteristics of the A-type granites of ARC reflect remelting of previously formed arc material. The mafic microgranular enclaves represent metamorphic rocks derived from or in the deeper part of the crust and represent the source rocks from which the granitic magma was generated by partial melting. The early phase of magmatism of the ARC represents melts derived from partial melting of lower or middle crustal sources during the postcollisional stage that fractionated to give the various granitic rocks. The absence of mafic xenoliths in the late stage of the ARC indicates that the contribution of crustal sources to magma generation is absent in the late stage.