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Cretaceous continental subduction and exhumation in the HP/UHP terrane of the eastern Alps, Pohorje Mts., Slovenia: petrological and age constraints.

Författare och institution:
Cees-Jan de Hoog (Institutionen för geovetenskaper); M. Janak (-); David H. Cornell (Institutionen för geovetenskaper); N Froizheim (-)
Publicerad i:
MSG research in progress meeting, Cambridge UK, March 2008, Abstracts s. 7
Konferensbidrag, övrigt
Sammanfattning (abstract):
The metamorphic grade of Cretaceous, so-called “Eo-Alpine” metamorphism in the Eastern Alps reached the highest metamorphic conditions in its southeastern parts, in the Pohorje Mts in Slovenia. The area belongs to the Austroalpine nappe system consisting mostly of medium to high-grade metamorphic rocks of the continental crust. An ultramafic complex comprising serpentinized harzburgites and locally garnet peridotites/pyroxenites occupies the southeastern part of the Pohorje. Eclogites are embedded as lenses in both the continental crust and ultramafic (mantle) rocks. Evidence for UHP metamorphism comes from kyanite-bearing eclogites and garnet peridotites (Janák et al., 2004, 2006). Eclogites can be devided into three groups, namely quartz eclogites, zoisite eclogites and kyanite eclogites. Quartz eclogites contain Grt + Omp + Qz + Rut + Zo, and are interpreted to be metabasalts, having a typical N-MORB composition. Zoisite eclogites contain Omp + Grt + Zo + Phn + Rut + Qz + Ky, whereas kyanite eclogites contain the peak metamorphic assemblage Omp + Grt + Ky + Rut + Zo + Qz. Omphacite commonly contains rods of quartz + amphibole as exsolutions. The content of the Ca-Eskola component is up to 7-8 mol %. Polycrystalline quartz possibly represents former coesite. Geothermobarometry on the peak metamorphic assemblage yields 3.0-3.1 GPa and 760-820 °C, within the stability field of coesite. Relict minerals from a prograde metamorphic stage, with exception of minor amphibole as inclusions in garnet, are not preserved. Retrograde phases involving amphibole, plagioclase, sapphirine, corundum and spinel formed during exhumation. Estimated P-T conditions for sapphirine formation are 0.11-0.12 GPa and 740-780oC, indicative of nearly isothermal exhumation. The protoliths of zoisite and kyanite eclogites were gabbros with compositions similar to those found at mid-oceanic ridges. Kyanite eclogites are found within or in the near vicinity of the ultramafic complex only, and are strongly depleted in LREE (LaN < 0.5, LaN/LuN between 0.05 and 0.3). Possibly they suffered small amounts of partial melting during decompression, following the reaction zo + qz = ky + gt + melt, or alternatively they represent strongly depleted cumulate dikes formed within depleted mantle. Serpentinized peridotites occur within an ultramafic body of 5x1 km in the southeast corner of Pohorje. The body is dominantly composed of cr-spinel-bearing harzburgites, but due to intense serpentinization only few remnants of olivine, orthopyroxene and Cr-spinel are preserved. Large orthopyroxene grains often display oriented clinopyroxene exsolutions and are variably replaced by pargasitic amphibole. From the presence of large (up to 1 cm) bastites we infer that the rocks were coarse-grained. The majority of the rock mass is replaced by serpentine minerals, chlorite, tremolite, magnesite and talc. Rare earth element patterns and high Cr# of spinels (up to 52) are indicative of large degrees of melting similar to those of peridotites found near spreading ridges. Garnet peridotites including garnet lherzolite, garnet websterite and garnetite are found within the ultramafic body and as a small single body further east. They show a polystage evolution. Stage I as spinel peridotite stage is inferred from the presence of Cr- spinel and aluminous pyroxenes. Stage II is a UHPM stage defined by the assemblage Grt + Ol + low-Al Opx + Cpx + Cr-Spi. Garnet formed as exsolutions from high-Al Cpx, coronas around Crspinel, and porphyroblasts. Stage III is a decompression stage with formation of kelyphitic assemblage of high-Al Opx, Al- Spi, Cpx and pargasite. Tremolitie, chlorite, serpentine and talc have been formed during stage IV. Geothermobarometry on stage II assemblage yields PT conditions of around 900 oC and 4 GPa. Continental crustal rocks, which host the above (ultra)mafic rocks, consist of amphibolites, ortho- and paragneisses and schists. They were strongly retrogressed during late-stage fluid introduction at low pressure and high temperature (ca. 700-600 °C and 0.5-1.2 GPa), which led to consumption of phengite and garnet, and the development of a secondary matrix consisting of muscovite, biotite, plagioclase, K-feldspar and occasionally sillimanite and staurolite. Cathodoluminescence images as well as zircon chemistry reveal cores rimmed by newly-formed metamorphic zircon. U-Pb dating (Cameca 1270, Nordsim facilities, Sweden) of zircon cores yielded Permian (286±10, 258±7 Ma) to Triassic (238±7 Ma) concordia ages, which are interpreted to be detrital. Rims of the zircon gave Cretaceous concordant ages between 90 and 94 Ma, with the weighted mean of 91.9±0.5 Ma. A few spots have slightly lower ages averaging to 86.6±1.0 Ma. The zircon rims all exhibit very low Th/U ratio (<0.02) typical of a metamorphic origin. The zircon rims are believed to record the Cretaceous (92-91 Ma) HP/UHP metamorphism, as they have nearly flat HREE patterns (LuN/GdN=1-4) and only small negative Eu anomalies, which indicate growth in the presence of garnet and absence of plagioclase, and is corroborated by the presence of garnet inclusions in the zircon rims. In contrast, older core domains have much steeper REE patterns and variable negative Eu anomalies, consistent with growth at low-pressure conditions. According to pseudosection modelling, the main growth of zircon occurred during the prograde path close to the P-T peak and continued during nearly isothermal decompression down to ca. 15 kbars, as indicated by pyrope-rich garnet overgrown by the zircon and instability of plagioclase above 15 kbars. Younger ages, down to 77 Ma, probably resulted from lead loss. The 91.9±0.5 Ma zircon age obtained in this study is nearly identical with that of peak metamorphism determined in the Pohorje eclogites using U/Pb ID-TIMS zircon (90.7±1.0 Ma) and Sm-Nd garnet-whole rock (90.7±3.9 and 90.1±2.0 Ma) dating (Miller et al. 2005). Hence, the eclogites with their country rocks were subducted and exhumed together as a coherent piece of continental crust, together with the Lower Austroalpine units from the Koralpe and Saualpe in the Austrian Eastern Alps of similar composition and age (Thöni and Miller, 1996). There is no evidence for a mélange-like rock assemblage that followed different P-T-t paths or several subduction/exhumation cycles as proposed for some other HP/UHP terranes. Sinking of the mantle and lower crustal wedge initially overlying the UHP rocks into the
Ämne (baseras på Högskoleverkets indelning av forskningsämnen):
Geovetenskap och miljövetenskap
eclogite, exhumation, pohorje
Postens nummer:
Posten skapad:
2009-12-04 17:44

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