Grossular-bearing jadeite omphacite rock in the Myanmar jadeite area: a kind of jadeitized rodingite?

摘要

A new rock type in the Hpakant jadeitite area, Myanmar is identified as jadeitized rodingite. The rock consists of omphacite, garnet and jadeite, with minor allanite-(La), phlogopite, zeolite, chlorite, and a barian mineral. The garnet contains more than 80 mol% of the grossular end-member. Omphacite has variable composition and is Mg-rich (X_(Mg-aver) = 90), while jadeite contains 82-92 mol% of jadeite end-member. Petrography shows that the garnet formed before omphacite, and many omphacite grains have jadeite rims or occur as island surrounded by replacement jadeite, indicating that a jadeitization event occurred subsequent to grossular and omphacite formation. Considering the geochemical interpretation that the protolith of rodingite (gabbro, plagiogranite or pyroxenite) has considerable Na_2O (mostly >2.0 wt% bulk), whereas rodingite has low Na_2O (mostly<1.0 wt% bulk), serpentinization-related rodingitization could locally yield Na-rich fluids, which could be an additional source for the fluids for jadeitite formation, and may play an important role in fluid-rock interactions in subduction zones.