Reaction textures and fluid behaviour in very high pressure calc‐silicate rocks of the Münchberg gneiss complex, Bavaria, Germany

Abstract

Reaction textures and fluid behaviour in very highABSTRACT Calc-silicate rocks occur as elliptical bands and boudins intimately interlayered with eclogites and high-pressure gneisses in the Munchberg gneiss complex of NE Bavaria. Core assemblages of the boudins consist of grossular-rich garnet, diopside, quartz, zoisite, clinozoisite, calcite, rutile and titanite. The polygonal granoblastic texture commonly displays mineral relics and reaction textures such as postkinematic grossular-rich garnet coronas. Reactions between these mineral phases have been modelled in the CaO-Al 2 0 3 -Si02-C02-H 2 0 system with an internally consistent thermodynamic data base. High-pressure metamorphism in the calc-silicate rocks has been estimated at a minimum pressure of 31 kbar at a temperature of 630°C with X^oSQ.Gi. Small volumes of a C0 2 -N 2 -rich fluid whose composition was buffered on a local scale were present at peak-metamorphic conditions. The P-T conditions for the onset of the amphibolite facies overprint are about 10 kbar at the same temperature. A'co., of the H 2 0-rich fluid phase is regarded to have been <0.03 during amphibolite facies conditions. These P-T estimates are interpreted as representing different stages of recrystallization during isothermal decompression. The presence of multiple generations of mineral phases and the preservation of very high-pressure relics in single thin sections preclude pervasive post-peak metamorphic fluid flow as a cause of a re-equilibration within the calc-silicates. The preservation of eclogite facies, very high-pressure relics as well as amphibolite facies reactions textures in the presence of a fluid phase is in agreement with fast, tectonically driven unroofing of these rocks.

Grenvillian age decompression of eclogites in the Glenelg–Attadale Inlier, NW Scotland

“…Framboidaltextured titanites are always developed around a prismatic rod of rutile or ilmenite and composed of numerous tiny subhedral prisms of titanite (Fig. 2b), this type of texture being a common decompression texture in high-P rocks (Klemd et al 1994;Frost et al 2000;Bingen et al 2001). This distinctive texture is heterogeneously developed and primarily reflects the distribution of host rutile or ilmenite crystals.…”

The early exhumation history of the Western TianshanUHPmetamorphic belt, China: New constraints from titanite U–Pb geochronology and thermobarometry

“…Studies of other HP-UHP belts revealed that some metasedimentary rocks also experienced HP to UHP metamorphism and shared a common metamorphic history with the enclosed/interlayered eclogites (e.g. Compagnoni, Rolfo, & Castelli, 2012;Gross, Burchard, Schertl, & Maresch, 2008;Klemd, Matthes, & Okrusch, 1991;Klemd, Matthes, & Sch€ ussler, 1994;Klemd et al,1991;Spear & Franz, 1986).…”

Monazite in a Variscan mylonitic paragneiss from the Münchberg Metamorphic Complex (NE Bavaria) records Cadomian protolith ages

“…The MMC is regarded as a nappe pile transported on top of autochthonous, anchi-metamorphic Palaeozoic sequences of the Thuringian lithofacies, as exposed in the southeastern Frankenwald, in the Lower Carboniferous (Franke, 1984;Klemd et al, 1994;Klemd, 2010). This nappe pile is subdivided from the bottom to the top into the following units: Prasinit-Phyllit, Rand-Amphibolit, Liegendserie, and Hangendserie (e.g.…”

“…The peak P-T conditions of these eclogites were estimated by several studies (Franz et al, 1986;Klemd, 1989;Klemd et al, 1991Klemd et al, , 1994O'Brien, 1993;Liebscher et al, 2007). These conditions cluster in the P-T range of 20-34 kbar and 550-750°C.…”