Accessory priderite and burbankite in multiphase solid inclusions in the orogenic garnet peridotite from the Bohemian Massif, Czech Republic

Abstract

This paper reports on priderite (potassium titanate) and burbankite (alkali Sr–Ca–REE–Ba carbonate) from an orogenic garnet peridotite body enclosed in high–pressure garnet–kyanite–bearing quartzo–feldspathic Gföhl granulite in the Bohemian Massif of the Variscan belt. The garnet peridotite contains ubiquitous phlogopite and was interpreted to be derived from the mantle wedge formed at the convergent plate margin. The earliest generation of chromian spinel, surrounded by kelyphitized garnet, ubiquitously contains multiphase solid inclusions (MSIs), which are mainly composed of phlogopite, dolomite, calcite, apatite, graphite, monazite, thorianite, and sulfides, and priderite and burbankite are newly identified as rare accessory minerals in such MSIs. Most of these MSIs contained significant amounts of carbonates. The presence of peculiar accessory minerals in MSIs characterizes the nature of parental melts. The formation of priderite requires an ultrapotassic condition, which is usually defined by K₂O >3 wt% and K₂O/Na₂O >2 in bulk composition, and high Cr₂O₃ content in priderite (15–18 wt%) suggests that it was formed as a reaction product between a melt inclusion and a host chromite. Burbankite contains significant amounts of Na₂O and K₂O (~ 3 wt%) and REE concentration (>31 wt%). The formation of burbankite requires a per–alkaline condition —K₂O + Na₂O > Al₂O₃ in mol— and requires more sodic composition. The presence of priderite and burbankite in MSIs suggests that some of them crystallized from ultrapotassic melts, whereas others crystallized from sodic peralkaline melts. Such alkali–carbonate melts could be present in the mantle wedge peridotite before its’ incorporation into the granulite.