Petrology of ultrahigh-pressure metamorphic rocks from Pohorje Mountains : ph. d. thesis
Vrabec, Mirijam
Ultrahigh-pressure metamorphism has been recorded in Eo-Alpine eclogites and garnet peridotites from the Pohorje Mts., Slovenia, belonging to the Eastern Alps. Eclogite bodies are hosted in ...continental crustal rocks, and are closely associated with meta- ultrabasic rocks, predominantly serpentinized dunites and harzburgites with garnet peridotite remnants. The eclogites contain a peak metamorphic assemblage of garnet, omphacite, kyanite and phengite. Pyrope-rich garnet is unzoned and almost free of inclusions. The non-stoichiometric supersilicic omphacites contain up to 5 mol% of Ca- Eskola molecule. Such clinopyroxenes are known to be stable exclusively at high- pressure conditions exceeding 3.o GPa. Their breakdown during decompression resulted in exsolution of quartz rods and needles that are oriented parallel to omphacite c-axis. Phengite contains up to 3.5 Si pfu. Polycrystalline quartz inclusions in primary minerals are surrounded by radial fractures and often exhibit microtextures diagnostic of the former presence of coesite. The absence of coesite is a consequence of near-isothermal decompression during the first stages of exhumation. Eclogites are rich in Mg0 and have Ca0and A1203 contents consistent with mafic magmatic rocks. They are more depleted than MORB with positive Sr and Pb anomalies, and with Cs, Rb, Ba enrichments. Precursor rocks were plagioclase-bearing (gabbroic) cumulates from depleted mantle- derived melts, which most probably formed in the Permiantime. Garnet peridotites show signs of a complex four-stage metamorphic history. The protolith stage is represented by a low-P high-T assemblage of olivine + Al-rich orthopyroxene + Al-rich clinopyroxene + Cr-spinel. Due to metamorphism, primary clinopyroxene shows exsolutions of garnet, orthopyroxene, amphibole, Cr-spinel and ilmenite. The UHP metamorphicstage is defined by the assemblage garnet + olivine + Al-poor orthopyroxene + clinopyroxene + Cr-spinel. Subsequent decompression and final retrogression stage resulted in formation of kelyphitic rims around garnet and crystallization of tremolite, chlorite, serpentine and talc. The garnet peridotites are relatively rich in A1203, Ca0 and Sr compared to primitive mantle, where as REE commonly show depleted patterns with positive Eu anomalies. Most probably garnet peridotites represent depleted mantle rocks that became refertilized within the plagioclase stability field before tectonic incorporation in the subducting continental slab, or, alternatively, they might carry the signature of metasomatism by melts derived from a plagioclase-bearing source, possibly even the eclogites. P-T estimates for eclogites and garnet peridotites indicate that the peak of metamorphism reached over 3 GPa and 800 °C in the eclogites and close to 4 GPa and 900 °C in the garnet peridotites, well within the UHP stability field. Eclogites and garnet peridotites did not share the same P-T path until the incorporation of peridotite from the mantle wedge into the subducting crust. The, rocks of the Pohorje Mts. record the highest-pressure conditions of all Eo-Alpine metamorphism in the Alps. Petrographic and geochemical evidence shows that these conditions were reached during deep subduction (at least 100 km) of continental slab. The southward increase inpeak pressure from approximately 2 GPa in the Koralpe and Saualpe to 3 or 4 GPa in the Pohorje suggests the south to south-eastern dip of the Cretaceous intra-Alpine subduction zone. Ultrahigh-pressure rocks of Pohorje were probably exhumed by ,slab extraction", that is, the downward removal of the mantle and lower crustal wedge overlying the subducted rocks.
Type of material - dissertation
Publication and manufacture - Ljubljana : [M. Vrabec], dec. 2007
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