Sm-Nd isotope systematics of high-REE accessory minerals and major phases:: ID-TIMS, LA-ICP-MS and EPMA data constrain multiple Permian-Triassic pegmatite emplacement in the Koralpe, Eastern Alps

Detailed Sm-Nd mineral ID-TIMS, EMPA and LA-ICP-MS analysis of meta-pegmatite minerals from the Koralpe-Stubalpe area. Austria, involving single crystal separation, step-wise dissolution and acid leaching techniques, yielded (i) information about the partitioning of REE between accessory and major mineral phases in meta-pegmatites, and (ii) age constraints on the long-lasting Permian-Triassic event in the Austroalpine basement units of the Eastern Alps. Internal Sm-Nd mineral isochrons performed on porphyroclast domains from the three localities Rostockbach, Kuppergrund and Wildbachgraben, including garnet, xenotime, apatite, monazite and feldspar indicate multiple emplacement of pegmatoid melts between 273 +/- 2 and 258 +/- 3 Ma. Other localities yielded Younger ages, indicating that magmatic garnet crystallisation persisted from 251 +/- 7 to c. 230 Ma. Ages calculated between high-REE accessories (monazite, apatite, xenotime) and the low-REE major phases feldspar and garnet are largely consistent. Zircon, however, plots clearly off the internal regression line. H2SO4 leaching experiments on meta-pegmatite garnet rich in micron-sized xenotime and apatite inclusions, locally affected by dynamic recrystallisation, yielded information about the behaviour of the Sm-Nd system during strong tectono-metamorphic overprinting. The data suggest multiple injections of pegmatite melts, at distinctly different times, during protracted crustal thinning and heating in the Permian-Triassic. Ky-paramorphs after andalusite, and micro-inclusions of kyanite, staurolite and sillimanite in magmatic garnet indicate crystallisation of the garnet at moderately high, but variable pressure conditions. During subsequent eclogite-facies overprinting (peak PT c. 700 degrees C/2.2 GPa) and intense deformation in Cretaceous time, the magmatic chemical and isotopic signatures of porphyroclasts were largely preserved, except for localized within-grain micro-shear zones and outermost rim domains, whereas part of the feldspar-rich matrix recrystallised during pervasive mylonitisation along the eclogite- to amphibolite-facies-grade exhumation path. (C) 2008 Elsevier B.V. All rights reserved.