• journal_title：European Journal of Mineralogy
• Contributor：Helmuth SÖLVA ; Martin THÖNI ; Gerlinde HABLER
• Publisher：E. Schweizerbart'sche Verlagsbuchhandlung Science Publishers
• Date：2003-
• Format：text/html
• Language：en
• Identifier：10.1127/0935-1221/2003/0001-0035
• journal_abbrev：Eur J Mineral
• issn：0935-1221
• volume：15
• issue：1
• firstpage：35
• section：Articles

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 ϵNd (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.