The Ediacaran post-collisional Khamal gabbro-anorthosite complex from the Arabian Shield and its Fe-Ti-P ore: An analogy to Proterozoic massif-type anorthosites

Abstract

The Khamal complex is an Ediacaran post-collisional gabbro-anorthosite igneous complex that intruded Tonian and Cryogenian subduction-related island arc and ophiolitic rocks and syn-tectonic granitic rocks. The complex is reminiscent of massif-type anorthosites. It has an elliptical shape and is unmetamorphosed and undeformed. The Khamal complex is differentiated into four lithological units, marginal gabbro, anorthosite, central gabbro and northern gabbro. Anorthosite is the most dominant unit and is surrounded by marginal high-Al (Al2O3 >16 wt%) gabbro and gabbronorite showing a positive Eu anomaly. Iron-rich gabbroic rocks associated with nelsonite (Fe-Ti-P ore) and magnetite-ilmenite (FeTi ore) are exposed in the central portion of the anorthosite. Towards the north, less differentiated massive gabbronorite is exposed, which is characterized by its high Al2O3 contents in addition to the MgO contents. The association between anorthosite and AlFe mafic melts is recorded in several Proterozoic massif-type anorthosite complexes. The formation of the Khamal gabbro-anorthosite complex was most likely related to the waning stage of the East African orogeny. After the assembly of the Tonian-Cryogenian island arc terranes, slab break-off and lithospheric delamination induced asthenospheric upwelling and ponding of the mafic melt at the crust-mantle boundary. The resulting mantle-derived or lower crustal melt evolved, which led to the formation of floated plagioclase. Then, the plagioclase-rich diapir ascended to form the anorthosite and high-Al marginal gabbroic rocks. The less differentiated high-Al gabbronorite of the northern unit intruding the anorthosite mimics its parental magma but does not directly represent the magma from which anorthosite crystallized. The central gabbroic unit is characterized by an enrichment in iron. This unit might represent the residual melt that was extracted and segregated into a small chamber from the anorthosite, which was emplaced in a dynamic environment. In this chamber the melt underwent further crystal fractionation and accumulation processes. The compositional gap between the nelsonite and magnetite-ilmenite ores and the enclosing gabbroic rocks as well as the similarity between these ores and the host gabbro and the discordant nature of the FeTi ore favor the immiscibility origin of the ore.