Prograde eclogite in the Gföhl Nappe, Czech Republic: new evidence on Variscan high‐pressure metamorphism

Abstract

A layer of relict, high-temperature, prograde eclogite has been discovered within felsic granulite of the Gfö hl Nappe, which is the uppermost tectonic unit in the Moldanubian Zone of the Bohemian Massif, the easternmost of the European Variscan massifs. Pressure-temperature conditions for eclogite (≥890°C, 18.0 kbar) and felsic granulite (c. 1000°C, 16 kbar) place early metamorphism of the polymetamorphic Gfö hl crustal rocks within the eclogite facies, and preservation of prograde compositional zoning in small garnet grains in high-temperature eclogite requires very rapid heating, as well as cooling. Mantle-derived garnet and spinel-garnet peridotites are associated with the high temperature-high pressure crustal rocks in the Gfö hl Nappe, and this distinctive lithological suite appears to be unique among European Phanerozoic orogenic belts, implying that tectonic processes during the late stages in evolution of the Variscan belt were different from those in the Caledonian and Alpine belts. The unusually high temperatures and pressures in Gfö hl crustal rocks, mineralogical evidence for rapid heating and cooling, juxtaposition of lithospheric and asthenospheric mantle with crustal rocks, and widespread production of latestage granites indicate that culmination of the Variscan Orogeny may have been driven by lithospheric delamination and asthenospheric upwelling.

High-pressure metabasic rocks from the Kutná Hora Complex: geological position and petrology of exotic lithologies along the segmented Moldanubian margin, Bohemian Massif

“…8). The results are rather exceptional for the KHC core, but generally correspond to the data for the Běstvina Unit published, for instance, by Medaris et al (1998Medaris et al ( , 2005.…”

“…In this work, the Běstvina Unit is discussed as an integral part of the inner part of the KHC. Eclogites from Úhrov and Spačice in the Běstvina Unit and from Dobešovice and Bečváry in the inner KHC were studied in detail by Medaris et al (1998). Their thermobarometric data for the peak metamorphic event correspond to 1030-1200 °C and 17-22 GPa for kyanite eclogite from Úhrov (Medaris et al 2006), 800-850 °C and 1.7-1.9 GPa for Spačice eclogite, 1160-1180 °C a 1.7-1.9 GPa for the quartz-rich eclogite, and 960 °C a 1.9 GPa for other eclogites.…”

“…Faryad (2009) determined peak conditions of kyanite eclogite and associated felsic granulite from Spačice at 3.4 GPa and 900-960 °C. Lower peak conditions were inferred for eclogite associated with serpentinite from Borek south of the contact between the Běstvina Unit and the adjacent Moldanubian Zone (650 °C and 1.4-1.5 GPa; Medaris et al 1998).…”

An ultra-high-pressure metamorphic condition and isothermal decompression path obtained from a garnet-pyroxenite in the Horní Bory granulite of the Bohemian Massif

“…Some UHPM evidence was found from minerals/rocks such as pyrope/orthopyroxene -bearing Mg -Al rich metasedimentary rock (Kotková et al, 1997), clinopyroxene with K -feldspar lamellae in Lower Austria (Becker and Altherr, 1992), garnet with small but significant amounts of Na 2 O (= 0.18 wt%) and TiO 2 (= 0.5 wt%) in the Gföhl granulite (Vrána and Frýda, 2003), and SiO 2 inclusions in zircons with 521 cm −1 Raman spectrum extracted from the Gföhl gneiss (Kobayashi et al, 2008). However, the UHPM evidence has not been found either in the Gföhl granulite (Carswell and O'Brien, 1993) or the mafic rocks directly enclosed within the Gföhl granulite (e.g., Medaris et al, 1998;Ackerman et al, 2008). The studied Horní Bory granulite, located in the eastern part of the Czech Republic (western Moravia; Fig.…”

Determination of SiO2 Raman spectrum indicating the transformation from coesite to quartz in Gföhl migmatitic gneisses in the Moldanubian Zone, Czech Republic

“…Peridotite bodies sporadically occur both in the Gföhl granulite and Gföhl migmatitic gneiss. Eclogites in the Gföhl Unit are associated with the peridotite bodies, except for one locality near Spačice (Medaris et al, 1998). Previous studies show that the Gföhl granulites have experienced a clockwise P -T path with peak conditions of approximately 1.6 -2.0 GPa and 900 -1100 °C, followed by nearly isothermal decompression to medium -P and high -T conditions (0.6 -0.8 GPa, 700 -800 °C) (Vrána, 1989;Carswell and O'Brien, 1993;Cooke, 2000;Cooke and O'Brien, 2001).…”