Ultrahigh‐pressure metamorphism and exhumation of garnet peridotite in Pohorje, Eastern Alps

Abstract

New evidence for ultrahigh-pressure metamorphism (UHPM) in the Eastern Alps is reported from garnet-bearing ultramafic rocks from the Pohorje Mountains in Slovenia. The garnet peridotites are closely associated with UHP kyanite eclogites. These rocks belong to the Lower Central Austroalpine basement unit of the Eastern Alps, exposed in the proximity of the Periadriatic fault. Ultramafic rocks have experienced a complex metamorphic history. On the basis of petrochemical data, garnet peridotites could have been derived from depleted mantle rocks that were subsequently metasomatized by melts and/or fluids either in the plagioclase-peridotite or the spinel-peridotite field. At least four stages of recrystallization have been identified in the garnet peridotites based on an analysis of reaction textures and mineral compositions. Stage I was most probably a spinel peridotite stage, as inferred from the presence of chromian spinel and aluminous pyroxenes. Stage II is a UHPM stage defined by the assemblage garnet + olivine + low-Al orthopyroxene + clinopyroxene + Cr-spinel. Garnet formed as exsolutions from clinopyroxene, coronas around Cr-spinel, and porphyroblasts. Stage III is a decompression stage, manifested by the formation of kelyphitic rims of high-Al orthopyroxene, aluminous spinel, diopside and pargasitic hornblende replacing garnet. Stage IV is represented by the formation of tremolitic amphibole, chlorite, serpentine and talc. Geothermobarometric calculations using (i) garnet-olivine and garnet-orthopyroxene Fe-Mg exchange thermometers and (ii) the Al-inorthopyroxene barometer indicate that the peak of metamorphism (stage II) occurred at conditions of around 900°C and 4 GPa. These results suggest that garnet peridotites in the Pohorje Mountains experienced UHPM during the Cretaceous orogeny. We propose that UHPM resulted from deep subduction of continental crust, which incorporated mantle peridotites from the upper plate, in an intracontinental subduction zone. Sinking of the overlying mantle and lower crustal wedge into the asthenosphere (slab extraction) caused the main stage of unroofing of the UHP rocks during the Upper Cretaceous. Final exhumation was achieved by Miocene extensional core complex formation.

Tectonic evolution of the southeastern part of the Pohorje Mountains (Eastern Alps, Slovenia)

“…Janák et al (2006) documented that garnet peridotites in Pohorje reached UHP metamorphic conditions of up to 900 °C and 4 GPa during Cretaceous intracontinental subduction. They proposed that garnet peridotites were derived from depleted mantle rocks which were subsequently metasomatized in the plagioclase-peridotite or the spinel-peridotite stability field.…”

“…In this paper we show the field relations between ultramafic mantle rocks, including garnet peridotite, and crustal gneisses in the Pohorje Mountains of the Eastern Alps which experienced HP to UHP metamorphism (Hinterlechner-Ravnik 1987;Hinterlechner-Ravnik et al 1991;Janák et al 2004Janák et al , 2006Janák et al , 2009Sassi et al 2004;Vrabec 2004Vrabec , 2007Miller et al 2005;De Hoog et al 2009). Janák et al (2006) documented that garnet peridotites in Pohorje reached UHP metamorphic conditions of up to 900 °C and 4 GPa during Cretaceous intracontinental subduction.…”

Temperature-initiated structural changes in FeS2 pyrite from Pohorje, Eastern Alps, North-Eastern Slovenia

“…The two latter nappes represent the Upper Central Austroalpine. 9,10 The entire nappe stack is overlain by Early Miocene sediments that belong to the svn-rift basin fill of the Pannonian Basin. 11 The central part of Pohorje is occupied by granodioritic to tonalitic intrusion of Miocene age (18)(19).…”

Rare-element granitic pegmatite of Miocene age emplaced in UHP rocks from Visole, Pohorje Mountains (Eastern Alps, Slovenia): accessory minerals, monazite and uraninite chemical dating

“…An 8 ×1 km body of metaultrabasic rocks is located in the vicinity of Slovenska Bistrica (the Slovenska Bistrica Ultramafic Complex, SBUC; Janák et al 2006) composed of serpentinized harzburgite and rarely garnet peridotite (Hinterlechner-Ravnik et al 1991;Janák et al 2006;De Hoog et al 2009. All these rocks experienced high-to ultrahigh-pressure metamorphism (Hinterlechner-Ravnik et al 1991;Janák et al 2004Janák et al , 2006Janák et al , 2009Sassi et al 2004;Miller et al 2005;Vrabec et al 2012) resulting from deep subduction of continental crust Stüwe & Schuster 2010). The timing of HP/UHP metamorphism is Cretaceous, ca.…”

“…Wenger & Armbruster 1991;Pezzotta 2000;Alexandrov et al 2001;Aurisicchio et al 2001Aurisicchio et al , 2002Guastoni et al 2008;Guastoni 2012). (2000), Schmid et al (2004) and Janák et al (2006). SAM-Southern Border of Alpine Metamorphism after Hoinkes et al (1999).…”