Domain structure and texture in fine grained symplectite from garnet breakdown in peridotite xenoliths (Zinst, Bavaria, Bohemian Massif)
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Almandine
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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.
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Phlogopite
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Melt inclusions
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Peridotite
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Abstract The pressures required for diamond and coesite formation far exceed conditions reached by even the deepest present-day orogenic crustal roots. Therefore the occurrence of metamorphosed continental crust containing these minerals requires processes other than crustal thickening to have operated in the past. Here we report the first in situ finding of diamond and coesite, characterized by micro-Raman spectroscopy, in high-pressure granulites otherwise indistinguishable from granulites found associated with garnet peridotite throughout the European Variscides. Our discovery confirms the provenance of Europe's first reliable diamond, the “Bohemian diamond,” found in A.D. 1870, and also represents the first robust evidence for ultrahigh-pressure conditions in a major Variscan crustal rock type. A process of deep continental subduction is required to explain the metamorphic pressures and the granulite–garnet peridotite association, and thus tectonometamorphic models for these rocks involving a deep orogenic crustal root need to be significantly modified.
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