Major and trace element chemistry and Sm–Nd age correlation of magmatic pegmatite garnet overprinted by eclogite-facies metamorphism

Abstract

Meta-pegmatites are widespread in the polymetamorphic Koralpe–Saualpe basement, an Austroalpine unit with eo-Alpine high-pressure overprint. Major and trace element data of coarse-grained garnet and equilibrium relationships with other pegmatite minerals indicate that garnet crystallized during pegmatite emplacement. Whole rock and garnet separates from the meta-pegmatites yield Sm–Nd isochrons with Late Permian–Early Triassic ages of 254 ± 2–239 ± 2 Ma. Surprisingly, the Cretaceous eclogite facies metamorphic overprint did not produce pervasive material exchange between garnet and matrix. Only the outermost 10–50 μm thick rims of cm-sized garnet crystals show diffusive major element re-equilibration. Detailed Sm–Nd investigations, using different separation techniques demonstrate isotopic equilibration of garnet, inclusions and the whole rock. High-precision age information is provided by garnet with extremely high 147Sm/ 144Nd ratios of 2−10, best-fit isochrons of multiple garnet magnetic fractions and reproducible results. However, age variations between different samples and between different crystals separated from single samples range outside analytical uncertainties. The Sm–Nd age results, covering a time-span of 18 Ma, are interpreted as reflecting long-term, though continuous garnet crystallization during pegmatite-emplacement. This was supposedly related to low pressure metamorphism with widespread formation of metamorphic andalusite in the metapelitic pegmatite host-rock. Protracted magmatic and metamorphic processes are in line with a tectonic scenario of crustal scale extension and initial rifting causing highly perturbed geothermal gradients, partial melting of metapelitic rocks and enhanced magmatic activity as postulated for the Austroalpine basement in Permian–Triassic time.