HR: 1340h
AN: V43A-1564    [Abstracts]
TI: Ultrahigh-Pressure Metamorphism Recorded in the Pohorje Mts., Eastern Alps, Slovenia
AU: * Vrabec, M
EM: mirijam.vrabec@ntfgeo.uni-lj.si
AF: University of Ljubljana, Deptartment of Geology, Askerceva 12, Ljubljana, SI-1000 Slovenia
AU: Janák, M
EM: marian.janak@savba.sk
AF: Geological Institute, Slovak Academy of Sciences, Dúbravská 9, POB 106, Bratislava 45, 840 05 Slovakia (Slovak Republic)
AU: Froitzheim, N
EM: niko.froitzheim@uni-bonn.de
AF: Geologisches Institut, Universität Bonn, Nussallee 8, Bonn, D-53115 Germany
AU: De Hoog, J
EM: cees-jan@gvc.gu.se
AF: Institute for Earth Sciences, Göteborg University, Guldhedsgatan 5A, Göteborg, 413 20 Sweden
AB: Ultrahigh-pressure (UHP) metamorphism in the Eastern Alps has been recorded in eo-Alpine rocks from the Pohorje Mts. in Slovenia. Eclogite bodies are hosted in amphibolites, gneisses and micaschists, and are closely associated with meta-ultrabasic rocks, predominantly serpentinised dunites and harzburgites with garnet peridotite remnants. The eclogites contain a peak metamorphism assemblage of garnet, omphacite, kyanite and phengite. Pyrope-rich garnet is unzoned and almost free of inclusions. Omphacite contains exsolved quartz rods parallel to the c-axis, indicative of a supersilicic precursor. Phengite contains up to 3.5 Si pfu. Quartz inclusions in primary minerals often exhibit microtextures diagnostic of the former presence of coesite. Garnet peridotites show signs of a complex metamorphic history. The protolith stage is represented by a low-P high-T assemblage of olivine+Al-rich Opx+Al-rich Cpx+Cr-spinel. Primary Cpx shows exsolutions of garnet, Opx, Amp, Cr-spinel and ilmenite, due to metamorphism. The UHP stage is defined by the assemblage garnet+olivine+Al-poor Opx+Cpx+Cr-spinel. Subsequent decompression and hydation resulted in the formation of kelyphitic rims around garnet and formation of tremolite, chlorite, serpentine and talc. The garnet peridotites are relatively rich in Al₂O₃, CaO and Sr compared to primitive mantle, wheras REE show depleted patterns commonly with positive Eu anomalies. This suggests a plagioclase-bearing precursor of garnet peridotites, either a cumulate formed from a mantle-derived magma intruded in the crust, or plagioclase-bearing peridotite. A peculiarity is the presence of a negative Ce anomaly, which indicates some contamination with deep sea sediments. P-T estimates for eclogites and garnet peridotites indicate that the peak of metamorphism reached 3 GPa and 800° C in the eclogites and 4 GPa and 900° C in the garnet peridotites, well within the UHP field. The rocks in the Pohorje Mts record the highest-pressure conditions of all Eo-Alpine metamorphism during the Cretaceous orogeny in the Alps, and the petrographic and geochemical evidence shows that the rocks experienced these conditions during deep subduction of a continental slab.
DE: 3600 MINERALOGY AND PETROLOGY
DE: 3620 Mineral and crystal chemistry (1042)
DE: 3660 Metamorphic petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005