Abstract

In the Eastern Alps (Central Europe), about two dozen spodumene pegmatite bodies extend heterogeneously over the Austroalpine Unit. They are spatially associated with leucogranites and simple pegmatites formed during the Permian extensional event. The pegmatites and leucogranites were overprinted during the Alpine orogeny with different intensities, leading to various structural and chemical changes to the magmatic features. The magmatic assemblage consists of K-feldspar, quartz, plagioclase, muscovite, garnet, and tourmaline, with additional spodumene in the pegmatites. Beryl is rarely present and other accessory phases such as apatite, cassiterite, or Nb-Ta phases are exceptional. Mineral compositions indicate that the magmatic garnet is rich in Mn and Fe and has Ca-rich and Mn-poor Alpine metamorphic overgrowths. Sm-Nd ages of carefully separated crystals of magmatic garnet from the leucogranites and spodumene pegmatites fall in the range of 245–280 Ma. These Permian-Early Triassic ages are in good agreement with previously published results from associated simple pegmatites. Tourmalines from spodumene pegmatites belong to the elbaite–schorl series, with an unusual but significant dravite component. The occurrence of tourmaline and the absence of biotite may be due to the low Ti content (<0.06 wt.% TiO2) in the melt of both rock types. Trace-element compositions (Rb, Li, Cs, Tl, or Ba) of magmatic muscovite show that the leucogranites and spodumene pegmatites belong to the same fractionation trend, with more fractionated signatures in the spodumene pegmatites. Whole-rock geochemical compositions indicate that both lithologies have a common peraluminous granitic geochemistry, similar minor- and trace-element distribution patterns, and share a fractionation trend of trace elements including Be, Li, Rb, Cs, Sn, Ge, Ba, and REE. Positive and negative Eu anomalies in analyzed leucogranites and spodumene pegmatites, respectively, are interpreted to be due to the extraction of Eu in plagioclase crystallizing in leucogranite during the melt evolution. Values of ε(t)Nd (approximately –8) and initial 87Sr/86Sr ratios (0.7098–0.7353) indicate a crustal origin for the primary melts. Using the Permian metamorphic conditions of the country rock as a proxy, the spodumene pegmatites intruded at more than 10 km depth at somewhat higher structural levels than the leucogranites. This set of field and petrographic observations as well as geochemical and geochronological data suggest that the leucogranites and spodumene pegmatites of the Austroalpine Unit are contemporaneous and cogenetic, whereby the spodumene pegmatites show a higher degree of chemical evolution and concentration of rare metals.

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