New insights and constraints on the late Neoproterozoic post-collisional mafic magmatism in the Arabian Shield, Saudi Arabia

We present new field evidences, mineralogical and geochemical data on the late Neoproterozoic Khamal gabbro-diorite complex (KGDC), Western Arabian Shield, Saudi Arabia. The KGDC represents a large post-collisional layered gabbro-diorite complex covering about 100 km2. It has an elliptical shape striking NW-SE and tilting to the north with the lower layers (olivine gabbro and gabbronorite) exposed to the south and the upper layers (hornblende gabbro, anorthosite and diorites) exposed to the north. The complex intrudes subduction-related island arc assemblages, ophiolitic rocks and syn-tectonic granitoids, and in turn it is intruded by post-tectonic alkaline granites and is covered by Cenozoic basalts. Few xenoliths of schist, gneiss and syn-tectonic granodiorite are observed within the KGDC, confirming a relative age younger than these rocks. The lithologies of KGDC show rhythmic layering with disseminated and massive ore deposits including nelsonite and FeTi oxides. Green spinel (pleonaste) is recorded in the KGDC rocks; a characteristic of layered intrusions. The KGDC is neither metamorphosed nor deformed, indicating its post-collisional tectonic setting, although it underwent deuteric alteration (uralitization) only along the contacts with alkaline granite intrusion. Based on mineralogical and geochemical affinities, the KGDC rocks show tholeiitic/calc-alkaline character. The olivine has forsterite contents (Fo = 0.79–0.84), a diagnostic of layered mafic intrusions. The KGDC is a well-differentiated pluton and is characterized by high Mg# in the olivine gabbro, which systematically decreased toward the top of the intrusion. The rocks of KGDC display parallel to sub-parallel patterns of REEs indicating that they are comagmatic (derived from a common magma source). The parent magma was generated by partial melting of the Arabian Shield lower mafic crust and linked to lithospheric delamination. The upwelling of asthenospheric mantle material supplied heat to melt the juvenile crust of the Arabian Shield to produce Al-rich basaltic magma. The latter magma was evolved through fractional crystallization and emplaced in successive phases. The earliest phase produced anorthosite, which had been intruded by gabbro and dioritic rocks. The early pulse includes a diapir rich in floated plagioclase (anorthosite), which hand been intruded by the second phase of differentiated magma that formed olivine gabbro, gabbronorite and hornblende gabbro. The last pulse is represented by differentiated diorite and quartz-diorite. Immiscibility of Fe-Ti-P-rich melt from silicate melt was controlled the nelsonite formation that occurred during the interaction of lithosphere with upwelling asthenospheric melts. The temperature estimates calculated by several geothermometers yielded crystallization temperatures ranging from 900 to 1000 °C for the gabbros, and from 750 to 900 °C for the diorites, at pressure below 4 kbar, reflecting depth of rock solidification <20 km. A subduction-related signature in the KGBC rocks is inherited from geochemical tendencies of a previous subduction phase during the maturation of an island arc system. The present study indicates that the KGDC does not a massif-type anorthosite as previously thought, but it represents a mafic layered intrusion because the gabbros and diorites are the dominant lithologies, while anorthosite is less common.