Abstract
Kyanite eclogites from the Pohorje Mountains, Slovenia, are providing the first evidence of ultrahigh-pressure Eo-Alpine metamorphism in the Eastern Alps. Polycrystalline quartz inclusions in garnet, omphacite and kyanite are surrounded by radial fractures and exhibit microtextures diagnostic for the recovery after coesite breakdown. The non-stoichiometric supersilicic omphacites found in Pohorje eclogites contain up to 5 mol % of Ca-Eskola molecule. Such clinopyroxenes are known to be stable exclusively at high-pressure conditions exceeding 3 GPa. Their breakdown during decompression resulted in exolution of quartz rods and needles that are oriented parallel to omphacite c-axis. The absence of coesite is a consequence of near-isothermal decompression during the first stages of exhumation. Pressure and temperature conditions for the formation of the peak metamorphic mineral assemblages have been assessed through a consideration of a) Fe2+-Mg partitioning between garnet and omphacite pairs, based on different calibrations; b) the equilibrium between garnet + clinopyroxene + phengite ± kyanite ± quartz/coesite assemblage. Estimated peak pressure and temperature conditions of 3.0-3.1 GPa and 750-783 °C are well within the coesite, i.e. the ultrahigh-pressure stability field.
Highlights
Ultrahigh-pressure (UHP) metamorphism is an important type of orogenic metamorphism that has been recognized in many Phanerozoic collision belts (e.g. LIOU et al, 1998; CHOPIN, 2003, and references therein)
The dominant rock type is weakly retrograded eclogite, which occurrs in bands, lenses and boudins within the surrounding continental crustal rocks
A direct mineral indicator of UHP conditions, were not found but its existence is clearly revealed from: (1) radial fractures around quartz inclusions within robust host minerals that were caused by expansion ensuing the transformation of high-pressure coesite to its low-pressure polymorph; and (2) polycrystalline appearance of these inclusions interpreted as a pseudomorphs after former coesite due to their distinctive Polycrystalline polygonal quartz (PPQ) and multicrystalline polygonal quartz (MPQ) microtextures (WAIN et al, 2000)
Summary
Ultrahigh-pressure (UHP) metamorphism is an important type of orogenic metamorphism that has been recognized in many Phanerozoic collision belts (e.g. LIOU et al, 1998; CHOPIN, 2003, and references therein). Well investigated intracratonic collisional orogens that exhibit scattered effects of subsolidus UHP recrystallization include the Quinling-Dabie-Sulu belt of east-central China, the Kokchetav Complex of northern Kazahstan, the Dora Maira massif of the Western Alps, and the Western Gneiss Region (WGR) of Norway (LIOU et al, 1994; COLEMAN & WANG, 1995; ERNST et al, 1995). This four classic and several other UHP terranes (Figure 1) share common structural and lithological characteristics (LIOU, 2000). From the recurrent occurrences both in time and space, since late Proterozoic, it is clear that UHP metamorphism is a common process, inherent to continental collision
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