Sm–Nd isotope systematics in garnet from different lithologies (Eastern Alps): age results, and an evaluation of potential problems for garnet Sm–Nd chronometry

Abstract

Interpretation of Sm–Nd garnet ages is frequently impaired by one of the following restrictions: (a) high-LREE inclusions, (b) isotopic disequilibrium, and (c) the uncertainty about the closure temperature. These issues are addressed by way of an evaluation of garnet Sm–Nd data from different rock types of the Austroalpine basement units, Eastern Alps, including metabasic eclogites, mica schist and paragneiss, metapegmatite and metagranite. Nd concentration in handpicked garnet varies between 0.021 and 23.1 ppm in metabasites, 0.49 and 17.4 ppm in metapelites and between 0.024 and 4.6 ppm in metapegmatites and metagranites. The overall range of 147Sm 144Nd is 0.15–2.5 in garnet from metabasites, 0.12–3.03 in metapelite garnet and 0.66–7.21 in Mn-rich garnet from metapegmatites and metagranites. A clear negative correlation between Nd concentration and SmNd is observed in garnets from all these lithologies. Therefrom, it is concluded that even optically “clean” garnet separates may contain high-LREE microinclusions, such as epidote-allanite, zoisite, apatite, sphene, monazite or zircon. However, very low Nd concentrations correlated with low SmNd as well as high Nd concentrations (>5 ppm) correlated with fairly high Sm–Nd ratios (0.8) have also been observed. Apart from replicate analyses within as well as between samples with a common PT-history, leaching experiments are a useful technique to elucidate any distorting influence of unequilibrated inclusions on the garnet age, especially if the observed Sm–Nd ratio is low (<0.5). Leaching of garnet separates with HCl (2.5, 5.8 M) produces no obvious element fractionation, but may improve SmNd, and hence age precision, considerably. Isotopic disequilibrium between garnet and other matrix minerals is observed preferentially in basic eclogites, derived from gabbroic precursors. Sm–Nd garnet analysis allows the recognition of several distinct garnet-forming events in the Eastern Alps. (a) A Variscan high-P event is documented in metabasites from the northern–central Ötztal basement around 360–350 Ma, whereas garnet from sillimanite-bearing gneisses dates the Variscan thermal peak in the western part of the same subunit around 345–330 Ma. (b) A long-lived, Permian to Triassic event (285–225 Ma), correlated with crustal extension and low-P metamorphism, is documented by spessartine-rich garnet from metapegmatites as well as almandine-rich garnet cores from mica schist. (c) Age data of garnet from eo-Alpine (Cretaceous) deeply subducted rocks of the southern/eastern Austroalpine units are related to near-peak PT, eclogite- to amphibolite-facies metamorphic conditions (peak: 2 GPa/685 °C), and/or incipient isothermal decompression, due to fast, tectonically driven exhumation (110/100–85 Ma). At cooling rates of 20–30 °C/Ma (exhumation rates: 3–5 km/Ma), the Sm–Nd closure temperature ( T c) for mm-sized garnet in these rocks is estimated at 650–680 °C.