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High-precision 232Th-208Pb dates have been obtained from allanite porphyroblasts that show unambiguous microstructural relationships to fabrics in a major syn-metamorphic fold in the SE Tauern Window, Austria. Three porphyroblasts were analysed from a single garnet mica schist from the Peripheral Schieferhülle in the core of the Ankogel Synform, one of a series of folds which developed shortly before the thermal peak of Alpine epidote-amphibolite facies metamorphism: allanite grain 1 provided two analyses with a combined age of 27.7 ± 0.7 Ma; grain 2, which was slightly bent and fractured during crenulation, provided two analyses with a combined age of 27.7 ± 0.4 Ma; a single analysis from grain 3, which overgrew an already crenulated fabric, gave an age of 28.0 ± 1.4 Ma. The five 232Th-208Pb ages agree within error and define an isochron with an age of 27.71 ± 0.36 Ma (95% confidence level; MSWD = 0.46). The results imply that the crenulation event was in progress in a short interval (<1 Ma) c. 28 Ma, and that the Ankogel Synform was forming at this time. The thermal peak of regional metamorphism in the SE Tauern Window was probably attained shortly after 28 Ma, only c. 5 Ma after eclogite facies metamorphism in the central Tauern Window. Metasediment may contain allanite porphyroblasts with clear-cut microstructural relationships to fabric development and metamorphic crystallization; for such rocks, 232Th-208Pb dating on microsamples offers a powerful geochronological tool. [ABSTRACT FROM AUTHOR]

The Leo Pargil dome, northwest India, is a 30 km-wide, northeast-trending structure that is cored by gneiss and mantled by amphibolite facies metamorphic rocks that are intruded by a leucogranite injection complex. Oppositely dipping, normal-sense shear zones that accommodated orogen-parallel extension within a convergent orogen bound the dome. The broadly distributed Leo Pargil shear zone defines the southwest flank of the dome and separates the dome from the metasedimentary and sedimentary rocks in the hanging wall to the west and south. Thermobarometry and in-situ U-Th-Pb monazite geochronology were conducted on metamorphic rocks from within the dome and in the hanging wall. These data were combined with U-Th-Pb monazite geochronology of leucogranites from the injection complex to evaluate the relationship between metamorphism, crustal melting, and the onset of exhumation. Rocks within the dome and in the hanging wall contain garnet, kyanite, and staurolite porphyroblasts that record prograde Barrovian metamorphism during crustal thickening that reached ∼530-630 °C and ∼7-8 kbar, ending by c. 30 Ma. Cordierite and sillimanite overgrowths on Barrovian assemblages within the dome record dominantly top-down-to-the-west shearing during near-isothermal decompression of the footwall rocks to ∼4 kbar by 23 Ma during an exhumation rate of 1.3 mm year−1. Monazite growth accompanied Barrovian metamorphism and decompression. The leucogranite injection complex within the dome initiated at 23 Ma and continued to 18 Ma. These data show that orogen-parallel extension in this part of the Himalaya occurred earlier than previously documented (>16 Ma). Contemporaneous onset of near-isothermal decompression, top-down-to-the-west shearing, and injection of the decompression-driven leucogranite complex suggests that early crustal melting may have created a weakened crust that was proceeded by localization of strain and shear zone development. Exhumation along the shear zone accommodated decompression by 23 Ma in a kinematic setting that favoured orogen-parallel extension. [ABSTRACT FROM AUTHOR]