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

AbstractThe 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 interact...