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Dating a single garnet crystal with very high Sm/Nd ratios (Campo basement unit, Eastern Alps)

Institut für Geologie, University of Vienna, Althanstr. 14 / UZA2-Geozentrum, A-1090 Vienna, Austria

^* e-mail (corresponding author): Helmuth.Soelva{at}univie.ac.at

The Sm-Nd isotope systematics of a single large garnet crystal have been analyzed, applying both standard and leaching separation techniques. The sample containing the garnet is a coarse-grained pegmatite from the Campo basement unit (northern Italy), part of the Austroalpine realm. The results revealed Sm/Nd elemental ratios among the highest ever reported worldwide (> 13, equivalent to a ¹⁴⁷Sm/¹⁴⁷Nd ratio of up to 7.9). This reflects strong REE fractionation in the pegmatite garnet, which, together with the inclusion-free nature of the crystal, points to the high suitability of such garnets for Sm-Nd dating.

The core and the rim fraction of the magmatic garnet give Sm-Nd ages of 255.4±2.8 Ma and 250.4±2.7 Ma, respectively, calculated with K-feldspar as the low-Sm/Nd phase. These ages are interpreted to reflect the time of magmatic crystallization of the garnet. The corresponding Nd (t) value of -11.7 is typical for crustal-derived rocks. An unleached garnet separate and three leachates deviate slightly, but systematically from the K-feldspar-garnet Sm-Nd isochron, due to the presence of secondary metamorphic minerals (fine-grained white mica and chlorite). Coarse-grained white mica and K-feldspar from the same sample yield a Rb-Sr date of 261.7±2.6 Ma. The surprisingly low ⁸⁷Sr/⁸⁶Srinitial ratio of 0.7032±0.0012 and the higher age value relative to the Sm-Nd age of the garnet point to a slight secondary, most probably metamorphic perturbation of the Rb-Sr system. The results emphasize the presence of a Permian pegmatite-forming event in the Campo basement unit. The crustal-derived pegmatites were subsequently affected by Alpine metamorphism and polyphase deformation.

Key-words: Austroalpine Campo basement, pegmatite garnet, Sm-Nd dating, REE fractionation, Permian event.

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