The formation of giant podiform chromitite by asthenospheric melts in supra-subduction zone environments

Podiform chromitite hosted in supra-subduction zone (SSZ) ophiolite accounts for a substantial proportion of the global chromium supply market. However, there is no consensus regarding the specific processes involved in the source and formation of this chromium. It seems unlikely that fractional crystallization of basaltic melt or the boninitic melt–mantle harzburgite reaction could provide such huge amounts of chromium given the constraints of Cr mass balance. Here we identify two specific melts responsible for the formation of the typical ophiolite-related Luobusa chromite deposit in the Yarlung–Zangbo Suture Zone in Tibet, China. One is Cr-rich melt derived from the deep asthenosphere, and the other is boninitic melt generated by hydrous melting of previously depleted peridotites. We propose that the Luobusa podiform chromitite was produced through mixing of these two melts, of which the primitive asthenospheric Cr-rich melt provided huge amounts of Cr, and the introduction of boninitic magma triggered the crystallization of chromite. The findings of this study are important for understanding the genesis of global podiform chromite deposits hosted in SSZ ophiolite.