Petrogenesis of chromitites and its records of Ti metasomatism in crust-mantle transition zone, Bulqiza ophiolite massif, Albania

Podiform chromitites are one of the important sources of the key metal chromium. A lot of progress has been made on the research of podiform chromitites, but little referred to the genesis of the chromitites located in crust-mantle transition zone of ophiolite. The Cerruja podiform chromitites and dunite wall-rocks were intruded by pyroxenite dikes in the crust-mantle transition zone of the Bulqiza ophiolite massif, Albania. Highly brecciated spinel and Ti-bearing minerals such as rutile, ilmenite and titanite were found in pyroxenite dikes and in the interaction zone between pyroxenite dikes and chromitites. Such characteristics make them an ideal subject for the study of the chromitites in the crust-mantle transition zone. Cerruja podiform chromitites are high-Al variety with Cr# of chromitites ore varying from 0.56 to 0.58 and of dunite wall-rocks varying from 0.52 to 0.55. Spinels in the interaction zone between pyroxenite dikes and chromitites and in the pyroxenite dikes are characterized by the obviously higher Cr# value (0.57 to 0.67 and 0.72 to 0.83, respectively) than chromitites ores. Contents of Ti、V、Mn、Sc、Co、Zn and Ga of spinels in the interaction zone are higher with the closer distance to the pyroxenite dikes. According to the texture characteristics of spinel and the variations of mineral chemical composition, we propose that the chromitites in the crust-mantle transition zone of Bulqiza massif is the result of a multi-stage process:First, high-Al chromitites were produced by the reaction between peridotites and the transition melts which has the geochemical properties both of MORB-like and boninitic, formed during the evolution of initial subduction of the Mirdita-Pindos ocean basin (~165 Ma); and then, Ti-Fe-rich residual melts were produced by intercumulus crystal fractionation of the MORB-like melt in a crystal-melt mush, metasomatizing and transforming the surrounding high-Al chromite into high-Cr chromite, and also crystallizing Ti-rich minerals such as rutile, ilmenite and titanite.