Timing of high-pressure metamorphism and exhumation of the eclogite type-locality (Kupplerbrunn-Prickler Halt, Saualpe, south-eastern Austria): constraints from correlations of the Sm-Nd, Lu-Hf, U-Pb and Rb-Sr isotopic systems

摘要

Sm-Nd, Lu-Hf, Rb-Sr and SIMS U-Pb data are presented for meta-gabbroic eclogites from the eclogite type-locality (Hauy, 1822) Kupplerbrunn-Prickler Halt and other areas of the Saualpe (SE Austria) and Pohorje Mountains (Slovenia). Mg-rich eclogites derived from early gabbroic cumulates are kyanite- and zoisite rich, whereas eclogites with lower Mg contents contain clinozoisite +/- kyanite. Calculated P-T conditions at the final stages of high-pressure metamorphism are 2.2 +/- 0.2 GPa at 630-740 degrees C. Kyanite-rich eclogites did not yield geologically meaningful Sm-Nd ages due to incomplete Nd isotope equilibration, whereas Sm-Nd multifraction garnet-omphacite regression for a low-Mg eclogite from Kupplerbrunn yields an age of 91.1 +/- 1.3 Ma. The Sm-Nd age of 94.1 +/- 0.8 Ma obtained from the Fe-rich core fraction of this garnet dates the initial stages of garnet growth. Zircon that also crystallized at eclogite facies conditions gives a weighted mean U-Pb SIMS age of 88.4 +/- 8.1 Ma. Lu-Hf isotope analysis of a kyanite-eclogite from Kupplerbrunn yields 88.4 +/- 4.7 Ma for the garnet-omphacite pair. Two low-Mg eclogites from the Gertrusk locality of the Saualpe yield a multimineral Sm-Nd age of 90.6 +/- 1.0 Ma. A low-Mg eclogite from the Pohorje Mountains (70 km to the SE) gives a garnet-whole-rock Lu-Hf age of 93.3 +/- 2.8 Ma. These new age data and published Sm-Nd ages of metasedimentary host rocks constrain the final stages of the eo-Alpine high-pressure event in the Saualpe-Pohorje part of the south-easternmost Austroalpine nappe system suggesting that garnet growth in the high-pressure assemblages started at c. 95-94 Ma and ceased at c. 90-88 Ma, probably at the final pressure peak. Zircon and amphibole crystallization was still possible during incipient isothermal decompression. Rapid exhumation of the high-pressure rocks was induced by collision of the northern Apulian plate with parts of the Austroalpine microplate, following Jurassic closure of the Permo-Triassic Meliata back-arc basin.