Publication Type:Book Chapter
Source:Geological Society of America, 2009 annual meeting, Geological Society of America (GSA), Boulder, CO, United States, Volume 41, p.587 (2009)
Keywords:Carpathians, Central Europe, crust, eclogite, Europe, exhumation, extrusive rocks, high-grade metamorphism, Metamorphic rocks, metamorphism, middle crust, migmatites, p-t-t paths, phase equilibria, Slovakia, Tatra Mountains
The Tatric crystalline unit of the western Carpathians represents the northeastern exposure of the Variscan Orogen in Europe which marks the collision of Laurasia with Gondwana during the Paleozoic. The basement core of the Western Tatra Mountains displays an inverted metamorphic sequence where high-grade migmatites and orthogneisses are overlying lower-grade mica schist. Enclosed in the migmatites are lenses and bands of eclogite-bearing amphibolites, and the entire suite is intruded by late granites. Conventional geothermobarometry coupled with isochemical modeling constrained P-T paths that show contrasting metamorphic histories for rock units that are now heterogeneously interleaved. Relict eclogite facies assemblages with occasionally preserved omphacite yield peak pressure estimate at 17-18 kbar followed by near isothermal decompression at 750 degrees C leading to intensive re-equilibration of eclogites at high-pressure granulite facies conditions and development of Cpx+Plg symplectite textures. Granulite facies migmatites that host the eclogite boudins record lower peak metamorphic conditions of 12 kbar at approximately 750 degrees C and a similar retrograde P-T path. New ID-TIMS Sm-Nd dating of garnet separated from the omphacite-bearing eclogite yields a whole rock-garnet isochron age of 337 + or - 10 Ma, with an epsilon Nd isotopic composition of +8.3. Major element profiles across the garnet display little variation; trace element distribution shows a typical HREE enrichment profile, with LREE and MREE concentrations higher in the cores and higher HREE in the rims. Monazite from the migmatites surrounding the eclogite boudins yields two populations of U-Pb ages. The predominant 340 + or - 11 Ma monazite age is indistinguishable from our garnet geochronology, and a younger monazite population at 300 + or - 16 Ma is consistent with our Ar-Ar thermochronometric cooling ages of ca. 310 Ma that likely reflects the late I-type magmatism in the Tatric unit. We interpret the 340 Ma age to represent the cooling and exhumation of the eclogitic root of the Variscan Orogen into the mid-crust that was coeval with widespread anatexis. This exhumation was tectonically forced by the action of a rigid indentor which compelled the weak lower crust to be heterogeneously extruded at mid-crustal levels, similar to nappe stacking, where it equilibrated and was further reworked.
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