The Carboniferous history of the Variscan chain, exposed in several basement units throughout western and central Europe, is related to the evolution of the Gondwana and Laurussia continental margins after their collision in Devonian times. One of the geodynamic scenarios that seem to acquire a growing consensus implies the south-eastward thrusting of the Laurussia continental margin under Gondwana, a geodynamic model also referred to as Himalayan-style collision. The Himalayan model implies the subduction of oceanic crust and subsequent continent-continent collision accompanied by underthrusting leading to Barrovian metamorphism and crustal anatexis. This is the preferred model for different sectors of the Variscides such as the Argentera Massif, (Rubatto et al., 2010) and the Bohemian Massif (Massonne and O’Brien, 2003, but for an alternative view see also Schulmann et al., 2009). Here we present petrologic and geochronologic data on high-pressure metapelites occurring in two different sectors of the eastern Variscan chain: the Mid-German Crystalline High, a metamorphic region adjacent to the southern margin of Laurussia, and the Ulten Zone, a crystalline basement unit with Gondwanan affinity enclosed in the Eastern Alps. The study of these two units highlights the similar evolution of different sectors of the Variscan chain and provides important data for the Early Carboniferous evolution of the Variscan orogen. The Spessart Crystalline Complex of the Mid-German Crystalline High is composed of several medium-grade metamorphic units comprising orthogneisses, several kinds of metasediments and small intrusive bodies. These units extend in NE-SW directions and seem to have formed a nappe stack. The detailed petrological investigation of garnet-bearing mica-schists from the northern part of the Spessart Crystalline Complex (Geiselbach and Mömbris Formations) yielded pressure(P)-temperature(T) paths characterised by a pressure peak of 1.2-1.5 GPa at 500-540°C that was followed by a thermal maximum at 560-620°C at 1.1-0.9 GPa. The early retrograde metamorphism resulted, for instance, in the formation of staurolite and plagioclase porphyroblasts in mica-schists from the Mömbris Formation at P ≤ 0.7 GPa. Monazite ages, determined with the electron microprobe, range from early Carboniferous to early Permian and provide the timing of the studied mica-schists from the Spessart Crystalline Complex. Geochronologic results on different monazite domains allowed us to identify the following age peaks and relate them to specific events: (1) high-pressure metamorphism occurred at 346 Ma; (2) the thermal peak and the retrograde event under amphibolite-facies conditions (staurolite-grade) was at 329 and 316 Ma; (3) a late metamorphic event accompanied by infiltration of hydrous fluids into the broadly exhumed metapelitic Geiselbach Formation occurred at 295 Ma. In the Austroalpine region of the Italian Eastern Alps, the Ulten Zone represents a basement unit of Variscan continental crust that survived the Late Cretaceous-Paleogene Alpine metamorphic overprint. The Ulten Zone is generally correlated to the Moldanubian region on the basis of similar rock assemblage (Godard et al., 1996). For instance, both in the Ulten Zone and in the Gföhl Unit in the southern Bohemian Massif garnet peridotites and pyroxenites are enclosed in high-pressure migmatitic gneisses. The Ulten Zone is a deep-seated continental crust that comprises kyanite-grade metapelitic gneisses with a metamorphic evolution similar to the mica-schists of the Spessart Crystalline Complex, i.e. the peak pressure of 1.0-1.8 GPa at 600-650°C (Braga et al., 2007) occurred before the temperature climax around 750°C between 0.9-1.3 GPa (Braga and Massonne, 2008). The late metamorphic stage is marked by the onset of small staurolite idioblasts. U-Th-Pb isotopic ages recently acquired on large matrix monazite from unmelted metapelitic gneiss of the Ulten Zone indicate that the me...
Braga R., Massonne H.-J., Langone A., Tiepolo M. (2012). The Carboniferous evolution of high-pressure metapelites from the Spessart Crystalline Complex, Germany, and the Ulten Zone, Italian Alps.
The Carboniferous evolution of high-pressure metapelites from the Spessart Crystalline Complex, Germany, and the Ulten Zone, Italian Alps
BRAGA, ROBERTO;
2012
Abstract
The Carboniferous history of the Variscan chain, exposed in several basement units throughout western and central Europe, is related to the evolution of the Gondwana and Laurussia continental margins after their collision in Devonian times. One of the geodynamic scenarios that seem to acquire a growing consensus implies the south-eastward thrusting of the Laurussia continental margin under Gondwana, a geodynamic model also referred to as Himalayan-style collision. The Himalayan model implies the subduction of oceanic crust and subsequent continent-continent collision accompanied by underthrusting leading to Barrovian metamorphism and crustal anatexis. This is the preferred model for different sectors of the Variscides such as the Argentera Massif, (Rubatto et al., 2010) and the Bohemian Massif (Massonne and O’Brien, 2003, but for an alternative view see also Schulmann et al., 2009). Here we present petrologic and geochronologic data on high-pressure metapelites occurring in two different sectors of the eastern Variscan chain: the Mid-German Crystalline High, a metamorphic region adjacent to the southern margin of Laurussia, and the Ulten Zone, a crystalline basement unit with Gondwanan affinity enclosed in the Eastern Alps. The study of these two units highlights the similar evolution of different sectors of the Variscan chain and provides important data for the Early Carboniferous evolution of the Variscan orogen. The Spessart Crystalline Complex of the Mid-German Crystalline High is composed of several medium-grade metamorphic units comprising orthogneisses, several kinds of metasediments and small intrusive bodies. These units extend in NE-SW directions and seem to have formed a nappe stack. The detailed petrological investigation of garnet-bearing mica-schists from the northern part of the Spessart Crystalline Complex (Geiselbach and Mömbris Formations) yielded pressure(P)-temperature(T) paths characterised by a pressure peak of 1.2-1.5 GPa at 500-540°C that was followed by a thermal maximum at 560-620°C at 1.1-0.9 GPa. The early retrograde metamorphism resulted, for instance, in the formation of staurolite and plagioclase porphyroblasts in mica-schists from the Mömbris Formation at P ≤ 0.7 GPa. Monazite ages, determined with the electron microprobe, range from early Carboniferous to early Permian and provide the timing of the studied mica-schists from the Spessart Crystalline Complex. Geochronologic results on different monazite domains allowed us to identify the following age peaks and relate them to specific events: (1) high-pressure metamorphism occurred at 346 Ma; (2) the thermal peak and the retrograde event under amphibolite-facies conditions (staurolite-grade) was at 329 and 316 Ma; (3) a late metamorphic event accompanied by infiltration of hydrous fluids into the broadly exhumed metapelitic Geiselbach Formation occurred at 295 Ma. In the Austroalpine region of the Italian Eastern Alps, the Ulten Zone represents a basement unit of Variscan continental crust that survived the Late Cretaceous-Paleogene Alpine metamorphic overprint. The Ulten Zone is generally correlated to the Moldanubian region on the basis of similar rock assemblage (Godard et al., 1996). For instance, both in the Ulten Zone and in the Gföhl Unit in the southern Bohemian Massif garnet peridotites and pyroxenites are enclosed in high-pressure migmatitic gneisses. The Ulten Zone is a deep-seated continental crust that comprises kyanite-grade metapelitic gneisses with a metamorphic evolution similar to the mica-schists of the Spessart Crystalline Complex, i.e. the peak pressure of 1.0-1.8 GPa at 600-650°C (Braga et al., 2007) occurred before the temperature climax around 750°C between 0.9-1.3 GPa (Braga and Massonne, 2008). The late metamorphic stage is marked by the onset of small staurolite idioblasts. U-Th-Pb isotopic ages recently acquired on large matrix monazite from unmelted metapelitic gneiss of the Ulten Zone indicate that the me...I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
