干旱区生态环境知识资源中心

This paper details the mode of occurrence, petrography, and chemistry of mineral inclusions hosted in chromian spinets of the Neoproterozoic chromitites in the Southern Eastern Desert of Egypt. Neoproterozoic podiform chromitites from the Arais, Balamhindit, and Abu Dahr areas, in the Southern Eastern Desert, can be texturally and chemically classified into two main types: primary high-Al (spinet Cr# < 0.67) and high-Cr (spinel Cr# > 0.75) chromitites. The former, being free of primary-mineral inclusions, was crystallized mainly from the MORB-like tholetitic melt generated during proto-forearc spreading at the initiation of subduction, whereas the latter was formed from boninitic melts resulting from the high-degree melting of the sub-arc depleted mantle in the presence of slab-derived fluids at a mature-arc stage. The primary mineral inclusions, such as Na- and K-phlogopites, pargasite-edenite and olivine with subordinate pyroxenes, millerite, and laurite, were trapped within the chromian spinet during the magmatic precipitation of the chromitites. The Abu Dahr chromitites are free of primary hydrous inclusions; on the other hand, Arais and Balamhindit high-Cr chromitites are enriched in Na- and K-phlogopites, respectively, as a result of a difference in the K/Na ratio of the magma responsible for chromitite crystallization at different mantle depths. This difference in the K/Na ratio can possibly be attributed to fractionation of the upward-migrating hydrous fluids/melts by the crystallization of K- or Na-rich minerals. The Balamhindit complex, where the chromitite showed K-phlogopite inclusions within the chromian spinel, was probably derived from a deeper part of the mantle than the other areas, where the chromitite shows inclusions of Na-rich hydrous phases. Both K- and Na-phlogopites were possibly formed from alkali-rich hydrous fluids/melts trapped within the chromian spinets during the chromitite formation at different mantle depths, where the K/Na ratio decreases upward through the incorporation of Na from the peridotite wall-rock, combined with the precipitation of K-rich phases at deeper depths. The chemistry of both primary mineral inclusions and chromian spinels suggests an arc-related tectonic setting for our chromitites that were crystallized at 1000 degrees C-1300 degrees C under pressures <3 GPa. The host peridotites were generated during the proto-forearc extension and emplaced as a result of the collision of the intra-oceanic arc with the Bitan gneiss. The chromitite parental magmas are rich in K, Na, LREE, B, Cs, Pb, Sr, Li, Rb and U relative to HREE and HFSE (Nb, Ta, Th), reflecting the alkalis fluids/melts that prevailed during the chromitite genesis. (C) 2015 Elsevier B.V. All rights reserved.