Unusual tiny inclusions and accessory minerals: evidence of ultra-deep origin and low-pressure modification of the Luobusa podiform chromitites

Abstract:

Podiform chromitites in Luobusa ophiolite, southern Tibet, yield a wide variety of unusual minerals, including UHP minerals (micro-diamond, coesite, moissanite), highly reduced minerals (native elements, carbide, nitride, alloys), and crustal minerals (quartz, kyanite, zircon etc.). We report unusual silicate exsolution lamellae (coesite, clinopyroxene and MgSiO3 phase) in chromite (Yamamoto et al., 2009), and unusual ancient crustal zircons from chromitites (Yamamoto et al., 2013). To deduce long history of mantle convection, including both upward mantle-upwelling and horizontal movement, micro-inclusions preserved in refractory minerals can provide potential mineralogical and geochemical evidence. Coesite as an exsolution in chromite is diagnostic UHP evidence and clinopyroxene exsolution in chromite is possible evidence of former CF-type high-pressure polymorph of chromite (>380 km deep). More importantly, we found that silicate exsolutions in chromites are restricted in the nodular- and massive-type chromites, and that disseminated-type chromites have no exsolutions. Petrographically, nodular-type chromites with abundant silicate-exsolutions are gradually modified into disseminated-type chromites in their morphology and exsolution abundance. The characteristic of the disseminated-type chromite, such as their interstitial distribution, euhedral to subhedral morphology and absence of silicate-exsolutions, suggest their formation under the low-pressure magmatic conditions. Therefore, we propose that the podiform chromitites at the Luobusa retain evidence of their multi-stage development from ultrahigh-pressure environment to low-pressure magmatic processes, and tiny inclusions shielded by refractory minerals, such as chromite, diamond and zircon, record their prolonged history in the mantle.