Olivine-bearing symplectites in fractured garnet from eclogite, Moldanubian Zone (Bohemian Massif) - a short-lived, granulite facies event
Abstrakt
Recent petrological studies on high-pressure (HP)-ultrahigh-pressure (UHP) metamorphic rocks in the Moldanubian Zone, mainly utilizing compositional zoning and solid phase inclusions in garnet from a variety of lithologies, have established a prograde history involving subduction and subsequent granulite facies metamorphism during the Variscan Orogeny. Two temporally separate metamorphic events are developed rather than a single P-T loop for the HP-UHP metamorphism and amphibolite-granulite facies overprint in the Moldanubian Zone.
Here further evidence is presented that the granulite facies metamorphism occurred after the HP-UHP rocks had been exhumed to different levels of the middle or upper crust. A medium-temperature eclogite that is part of a series of tectonic blocks and lenses within migmatites contains a well-preserved eclogite facies assemblage with omphacite and prograde zoned garnet.
Omphacite is partly replaced by a symplectite of diopside + plagioclase + amphibole. Garnet and omphacite equilibria and pseudosection calculations indicate that the HP metamorphism occurred at relatively low temperature conditions of similar to 600 degrees C at 2.0-2.2GPa.
The striking feature of the rocks is the presence of garnet porphyroblasts with veins filled by a granulite facies assemblage of olivine, spinel and Ca-rich plagioclase. These minerals occur as a symplectite forming symmetric zones, a central zone rich in olivine that is separated from the host garnet by two marginal zones consisting of plagioclase with small amounts of spinel.
Mineral textures in the veins show that they were first filled mostly by calcic amphibole, which was later transformed into granulite facies assemblages. The olivine-spinel equilibria and pseudosection calculations indicate temperatures of similar to 850-900 degrees C at pressure below 0.7GPa.
The preservation of eclogite facies assemblages implies that the granulite facies overprint was a short-lived process.