Petrogenesis of chromite and associated minerals in the upper mantle peridotite of the northern Semail ophiolite (UAE).

摘要

Chromite bodies occur in the upper mantle peridotite of the Semail ophiolite in the Al'Hel, Siji and Zikt areas. Siji is not considered in detail because it is similar to Al'Hel, and this study is focussed in the Al'Hel and Zikt areas that are divided into three major zones: Al'Hel, E. Zikt, and S. Zikt. The Al'Hel zone is close to the mantle-crust transition, and is characterized by less depleted peridotites than those in Zikt, and by lowgrade, Al-rich chromites. In contrast, the Zikt zone is located deeper in the mantle sequence, and contains high-grade, Cr-rich chromite. E. Zikt is between that of Al'Hel and S. Zikt.; Chromite is associated with olivine (and sometimes pyroxene) and textures largely reflect fabrics developed during pre-, syn-, and post-emplacement deformation. However some original magmatic textures such as rhythmic layering are preserved. The chromite/olivine ratio varies from 1 to zero, forming rocks from massive chromitite to dunite.; A magmatic origin for the formation of chromite and associated minerals is indicated by whole-rocks and mineral geochemistry. The geochemical characteristics of the Al'Hel bodies are different from those of Zikt. Disequilibrium interaction between rising melts and the wall rocks was the main driving mechanism for chromite precipitation, particularly the massive chromitites. This process leads to Mg and Si enrichment in the melts by dissolution of some peridotitic pyroxene. Increasing silica content increases polymerization and decreases Cr solubility in the melts, and thus triggers chromite precipitation.; The genesis of chromite bodies in the UAE Semail ophiolite involves three stages. First, partial melting at a MOR spreading center occurred during drifting of Afro-Arabia from Eurasia. No significant chromite accumulation occurred during this phase. Second, island arc magmatism occurred above an intra-oceanic subduction zone that formed due to northeastward movement of Arabia toward Asia. At this stage, the Zikt chromite orefields originated by interaction of calc-alkaline melts with upper mantle harzburgites. Third, back-arc (marginal basin) spreading began, and the Al'Hel orefields formed through interaction of MORB-like melts with the relatively less depleted peridotites.