Forsterit-serpentinový mramor s klinohumitem a spinelem u Chotěboře (moldanubikum; Český masiv); příklad retrográdní metamorfózy dolomitického mramoru

Abstract

A body of dolomite marble near Chotěboř was found in migmatized biotite paragneiss of the Moldanubicum directly on the border with migmatites/orthogneisses of the Kutná Hora Crystalline Complex. The area underwent significant tectonic reworking of rocks along the Přibyslav mylonite zone. Therefore, only small relics of early prograde HT/LP metamorphism at T > 650 - 700 °C were preserved in this marble. They are represented by the assemblage dolomite+calcite+forsterite+Zn-spinel+clinohumite ±Ba-phlogopite and rare baddeleyite. This mineral assemblage underwent strong low-temperature retrograde metamorphism (< 400 - 300 °C) under the conditions of high H2O activity (XCO2 < 0.10 - 0.15), which was manifested by almost complete serpentinization of forsterite. Partial alteration of clinohumite produced serpentine with numerous inclusions of a TiO2-mineral and rare sphalerite. The examined serpentine marble (ophicalcite) is lithologically and mineralogically similar to the dolomite marbles found in the Strážek Moldanubicum.