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Websteritic and rare garnet orthopyroxenite and clinopyroxenite dykes, crosscutting a harzburgite, were dated at 252±2.4 Ma on zircon (a clinopyroxenite dyke, s. ST-8 ; U/Pb SIMS). This kilometre-size mantle fragment is associated with the HP metamorphosed continental crust fragment of an Austroalpine (Sieggraben) structural complex (Putiš et al., 2002) in the Eastern Alps between Steinbach and Gschorrholz in Burgenland, Austria. The eclogitized continental crust fragment, composed of eclogites, marbles, gneisses, is crosscut by Permian (256±2.8 Ma, s. GS-8 ; zircon SIMS U/Pb age) gneisseous granite veins. The Cretaceous subduction was dated on zircon from an Ordovician gabbro (468.8±5.4 Ma, s. S- 202-3) transformed to an eclogite lens (s. SW-1) at 89.8±4.2 Ma of this complex typically located between Sieggraben and Schwarzenbach (ca. 80 km south of Vienna). The ultramafic dykes contain various mineral breakdown textures. The breakdown phases were the tschermakite-rich clinopyroxene, orthopyroxene, spinel and Mg-hornblende. The Ti-spinel snd ilmenite exsolutions indicate nearly isobaric cooling in a mantle wedge (D0 event). The garnet exsolution, corona and Cpx-Grt symplectite textures from Ts-rich Cpx, or Grt exsolutions and coronas of Opx and Mg-Hbl, and Spl replacement by Grt suggest a compression (D1, subduction) event. In contrast, Grt replacement by Prg–Spl symplectite in clinopyroxenite, Opx–Spl symplectite in orthopyroxenite, and Ol–Spl or Opx–Spl symplectites replacing Cr-rich Spl in websterite, indicate a high-temperature decompression (D2, exhumation) event. The Perple_X pseudosection calculation from a clinopyroxenite dyke yielded peak D1 burial conditions at ~825–850 °C and 25.5–27.5 kbar. The D1 peak assemblage of Grt (or Cr-Spl), Cpx, Ol, Ky, Zo, Rt, Mgs/Dol, ±Prg-Amp and the estimated P– T conditions suggest that the mantle wedge fragment was dragged by the subducting continental slab and underwent eclogite-facies metamorphism. Garnet (or Cr-Spl) replacement by Spl-bearing symplectites during the D2 exhumation were estimated at ~800–650 °C and 17–13 kbar. Zircon dating at ca. 90 Ma most likely fix this event. The D3 exhumation event in mostly amphibolite facies (Tr, Ep, Mg-Chl, Ctl, Mag) occurred due to ductile normal faulting and erosion within the Cretaceous accretionary wedge. |
