The P–T evolution of ultra high temperature garnet‐bearing ultramafic rocks from the Saxonian Granulitgebirge Core Complex, Bohemian Massif

Abstract

Garnet‐bearing ultramafic rocks (GBUR) enclosed in granulite or high‐grade gneiss are rare, yet typical constituents of alpine‐type collisional orogens. The Bohemian Massif of the European Variscides is exceptional for the occurrence of a large variety of mantle‐derived rocks, including GBUR (garnet peridotite and garnet pyroxenite). GBUR occur in several metamorphic units belonging to both the Saxothuringian and the Moldanubian zones of the Bohemian Massif. The northernmost outcrops of GBUR in the Bohemian Massif are situated in the Saxonian Granulitgebirge Core Complex in the Saxothuringian zone and are the subject of this study. Thermobarometric results and exsolution textures imply that the Granulitgebirge GBUR belong to the ultra high temperature group of peridotites. They experienced a decompression‐cooling path being constrained by the following four stages: (i) ∼1300–1400 °C and 32 kbar, (ii) 1000–1050 °C and 26 kbar, (iii) 900–940 °C and 22 kbar, and (iv) 860 °C and 12–13 kbar. Occasional layers of garnet pyroxenite within GBUR lenses are interpreted as high pressure cumulates that crystallized at 32–36 kbar by cooling below 1400 °C. The GBUR were most probably derived from upwelling asthenosphere and came in contact with crustal granulite at ∼60 km depth. Slab break‐off is suggested here as the most probable cause for: (i) asthenosphere upwelling and cooling of the latter as well as (ii) ultra high temperature granulite facies metamorphism of the crustal host rocks. The Granulitgebirge‐type peridotite is very similar to the Mohelno‐type peridotite from the Gföhl unit, Moldanubian zone, in the southern part of the Bohemian Massif. In contrast, peridotite from the adjacent Erzgebirge (also within the Saxothuringian zone) is derived from the subcontinental mantle and much resembles the Nove Dvory‐type peridotite from the Gföhl unit (Moldanubian zone). The fact that the Saxothuringian and Moldanubian zones host the same types of mantle rocks (asthenospheric and lithospheric) of the same metamorphic ages suggests that the classic distinction into the Saxothuringian and Moldanubian zones cannot be supported, at least as far as high‐grade units hosting GBUR are concerned.

Orogenic, Ophiolitic, and Abyssal Peridotites

“…Then, their upwelling would be triggered by slab break off or lithospheric delamination Maruyama et al, 1996;Medaris et al, 1998;Schmaedicke et al, 2010;Van der Wal and Vissers, 1993). Typical examples of 'UHT' peridotites are found in the Bohemian Massif of the European Variscides, including the Mohelno-type peridotite from the Gfö hl unit in the Moldanubian Zone Medaris et al, 1995Medaris et al, , 1998 and garnet peridotites from the Granulitgebirge Core Complex in the Saxothuringian Zone (Schmaedicke et al, 2010). Certain garnet peridotites from the Sulu UHP terrane, Eastern China, record an early HT stage of decompression melting in the spinel stability field and are therefore akin to this group (e.g., the Zhimafang peridotite - Song et al, 2009a,b).…”

“…Examples of HT metamorphism are observed in the country rocks of the Beni Bousera (northern Morocco) and Ronda (southern Spain) massifs (Loomis, 1972), as well as in the limestones hosting the Pyrenean lherzolites (Goldberg and Leyreloup, 1990). However, several orogenic peridotites are associated with HP to UHP terranes, notably in the Central Alps (Alpe Arami, Cima di Gagnone, and Ulten peridotites), China (Dabie-Sulu, Qinling, and North Qaidam-Altyn-Tagh UHP metamorphic belts), the Western Gneiss Region (WGR) in Scandinavian Caledonides, and the Bohemian Massif in Central Europe Ernst, 1981;Ernst and Liou, 2008;Kotkova et al, 2011;Liou et al, 2002aLiou et al, ,b, 2009Liu et al, 2010;Reverdatto and Selyatitskii, 2005;Schmaedicke et al, 2010;Terry et al, 2000;van Roermund, 2009b;Zhang et al, 2005).…”

Timescales of exhumation and cooling inferred by kinetic modeling: An example using a lamellar garnet pyroxenite from the Variscan Granulitgebirge, Germany

“…This metamorphic complex is a suitable example for two reasons: (1) the geological setting is simple enough to place solid constraints on the geometry, different lithologies, and tectonic setting ( Fig. 1 and Reinhardt and Kleemann 1994) and (2) there are many studies that focused on the P-T evolution using exchange reactions and thermodynamic calculations Massonne and Bautsch 2002;Roetzler et al 2004Roetzler et al , 2008Hagen et al 2008;Schmaedicke et al 2010) in addition to geochronological data that provide independent estimates for the timescale of the exhumation process (von Quadt 1993;Romer and Roetzler 2001;Roetzler and Romer 2010). It is shown here that a combination of equilibrium phase petrology and modeling of mass and heat transport can improve the reconstructed P-T-t exhumation path.…”

Ductile deformation and rheology of sub-continental mantle in a hot collisional orogeny: Example from the Bohemian Massif

“…Caledonian Western Gneiss Region in Norway; Brueckner, 1998;Brueckner et al, 2002). Lack of such studies in peridotites is due to high degree of serpentinisation of these rocks allowing only petrological and geochemical analyses of few well preserved mineral assemblages (Medaris et al, 1990(Medaris et al, , 2005Schmädicke and Evans, 1997;Schmädicke et al, 2010).…”