Titanite-bearing omphacitite from the Jade Tract, Myanmar: Interpretation from mineral and trace element compositions

Abstract

Jadeitite is a rare rock composed predominantly of jadeite, which typically occurs in association with tectonic blocks of high-pressure/low-temperature metabasaltic rocks such as eclogite or blueschist, often as a matrix in exhumed serpentinite mélanges. Omphacitite are far less common occurring together with jadeitite, such as those in the “Jade Tract” of Hpakan area in Myanmar. The omphacitite in this locality is mostly composed of omphacite and jadeite, with minor titanite, ilmenite, epidote and zircon. The jadeite formed after omphacite shows a lower Jd-content than that in the neighboring white jadeitite. The omphacite shows significant variation in Jd-content and is associated with aegirine augite. Both rocks show relatively linear and upslope pattern from LREE to HREE, and a slight enrichment of Ba, Th, U, Zr and Hf relative to chondrites, in the absence of Eu-anomaly. The titanite occurs in two groups: one as discrete islands replacing ilmenite, and the other as precipitation within jadeite veinlet. Titanite grains show convex patterns from LREE to HREE, and depletion of Sr, Zr and Hf with no Eu-anomaly. Chemical characteristics of the titanite and ilmenite alteration around titanite suggest that the omphacitite is of secondary origin, likely derived from pyroxenite through the replacement of pyroxene by jadeite. Based on the previous findings of jadeitization of chromitite, serpentine, and rodingite, it is suggested that protoliths such as plagiogranite or gabbro trapped within serpentinite mélange might have undergone jadeitization.