Petrology and geochemistry of ultramafic rocks in the Mogok belt, Myanmar: Cumulates from high-pressure crystallization of hydrous arc melts

The Mogok metamorphic belt (MMB) in central Myanmar is well known for its complex tectonics, magmatism, and metamorphism in the framework of Tethyan subduction and India-Asia collision. It is also world renowned due to the gemstone-class rubies and sapphires. Identification and discrimination of those petrological units in this region therefore are important for understanding the gemstone mineralization. Ultramafic massifs also crop out together with the gemstone-bearing metamorphic rocks in the MMB. It is still poorly constrained about their origin (i.e., mantle peridotites of Jurassic-Cretaceous ophiolites or cumulate rocks) and their relationship with the gemstone generation. Here, we report petrological and geochemical data for the Mogok ultramafic rocks. Petrographic observations show that they have typical cumulate textures, with a crystallization order of olivine/spinel-orthopyroxene-clinopyroxene. They have low whole-rock Al2O3 and CaO, and extremely high spinel Cr# (=Cr/[Cr + Al]; up to 0.86). Clinopyroxenes of these rocks show light rare earth element enriched patterns when normalized to CI chondrite. The results suggest that the Mogok ultramafic rocks have different compositions, both whole-rock and mineral, from mantle peridotites of Myanmar ophiolites, such as the Kalaymyo and Myitkyina ophiolites, but resemble typical Alaskan-type ultramafic cumulates. Therefore, the Mogok ultramafic rocks are cumulates generated by high-pressure crystallization of hydrous arc melts, probably during the subduction of the Tethyan oceanic slab. We argue that the emplacement of these arc melts may have provided additional heat for the earlier magmatic rocks and regional high-temperature metamorphism, although there is a need to further constrain the ages of these rocks. This, along with magmatism and metamorphism during post-collisional extension, probably collectively contributed to the generation of world-class coloured gemstone mineralization.