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 °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.