Genesis and tectonic setting of podiform chromitites in the Hegenshan and Solonker ophiolites, Inner Mongolia, southeastern Central Asian Orogenic Belt

Ophiolite is the remnants of ancient oceanic crust and mantle, which can reveal the tectonic evolution of paleo-oceanic basins. Podiform chromite deposit in ophiolites can retain the original information of petrogenesis and mineralization during the later deformation and metamorphism process, and it is a key object that can be used to decipher the origin and tectonic setting of ophiolites and the evolution of paleo-oceanic basins. Ophiolite suites are widely developed in Hegenshan and Solonker tectonic belts, the Inner Mongolia segment of the southern Central Asian Orogenic Belt. However, the genesis and tectonic environment of ophiolite and associated podiform chromitites remain debated, which restrict the understanding of the tectonic evolution and metallogenic background of the orogenic belt. Here, we conducted a detailed study of field, petrography, and mineral chemistry on the podiform chromitites in the Hegenshan and Solonker ophiolites in Inner Mongolia to explore their origin and tectonic environment. Petrographic results show that the Hegenshan chromites contain abundant high-pressure, hydrous mineral inclusions of sodic amphibole, white mica, and clinopyroxene, along with previously reported ultra-high pressure minerals (e.g., diamond); whereas the Solonker chromite contains minor white mica inclusions. Mineral chemical analysis shows that the Hegenshan ophiolite is dominated by high-Al type spinels with subordinate high-Cr type spinels; whereas the Solonker ophiolite mainly contains High-Cr type spinels. Accordingly, we suggest that the Hegenshan chromitites formed initially in a mid-ocean ridge (MOR) setting of a backarc ocean basin, then experienced modification in a suprasubduction zone (SSZ) setting, with deep mantle recycling and two stages of melt-peridotite interactions due to backarc subduction initiation; and the Solonker chromitites formed by boninitic melt-peridotite reaction in the SSZ forearc setting probably due to slab roll-back or subduction re-initiation following ridge subduction. These findings provide important constraints on the petrogenesis of chromites/ophiolites, regional tectonic evolution and mineralization background of chromitites in the Inner Mongolia segment of the Central Asian Orogenic belt.