The Pressure–Temperature Path and the Origin of Phlogopite in Spinel–Garnet Peridotites from the Blanský Les Massif of the Moldanubian Zone, Czech Republic

Abstract

A new lithotype of peridotite, phlogopite- and apatite-bearing spinel–garnet peridotite, associated with leucocratic granulite, has been recognized at the Plešovice quarry in the Gföhl Unit within the Moldanubian Zone of the Bohemian Massif, Czech Republic. There are three equilibrium stages in the Plešovice peridotite. The existence of Stage I, the precursor spinel ± garnet peridotite stage, is supported by the presence of an aluminous (Al2O3 ∼ 3·0 wt %) orthopyroxene megacryst in the matrix. The minimum temperature of Stage I was estimated to be 1020 ± 15°C. Stage II is defined by the cores of relatively large (<3 mm long) grains of olivine, low-Al orthopyroxene (Al2O3 ∼ 1·3–1·7 wt %), clinopyroxene, and chromian spinel [Cr/(Cr + Al) = 0·50–0·57], along with relatively small (<1 mm long) Ba-rich phlogopite (BaO = 1·0–4·0 wt %), Sr-rich apatite (SrO ∼1·7 wt %) and rare potassic (K2O ∼0·9–1·2 wt %) amphibole. Garnet generally occurs as large spheroidal grains (up to 20 mm in diameter). It contains inclusions of olivine, orthopyroxene, chromian spinel, and phlogopite, all of which have similar compositions to their matrix counterparts. Therefore, garnet appears to be in equilibrium with the matrix phases at Stage II. Application of appropriate geothermobarometers to the assemblage at Stage II yielded temperatures of 850–1030°C and pressures of 2·3–3·5 GPa. Stage III is defined by aluminous orthopyroxene (Al2O3 ∼ 2·1–4·0 wt %), aluminous clinopyroxene and aluminous spinel along with pargasitic amphibole and Ba-rich phlogopite in kelyphite; temperature conditions at this stage were estimated to be 730–770 (± 27)°C at 0·8–1·5 GPa. Multiphase solid inclusions, mainly composed of phlogopite, dolomite, apatite and calcite with minor amounts of chlorite and magnesiohornblende, are present only within large grains of chromian spinel, which are surrounded by kelyphites. The idiomorphic outline of the multiphase solid inclusions suggests that frozen remnants of carbonatite melts or supercritical fluids were trapped in the spinel. The mineral assemblage in the multiphase solid inclusions suggests relatively low-P and low-T conditions (T < 750°C; P < 1·6 GPa) for its crystallization. Furthermore, the timing of the crystallization of the multiphase solid inclusions appears to predate Stage II, as most multiphase solid inclusions are completely surrounded by the host chromian spinel. These data suggest that the Plešovice peridotite experienced cooling after Stage I and was then transformed to spinel–garnet peridotite by subsequent subduction processes.