In situ garnet U-Pb dating of granulitized eclogites from NE Sardinia, Italy

Gabriele Cruciani¹, Lorenzo Dulcetta¹, Marcello Franceschelli¹, Aratz Beranoaguirre^2,3, Leo J. Millonig³ & Axel Gerdes^3
1Dipartimento di Scienze Chimiche e Geologiche, Università degli Studi di Cagliari – S.S. 554 Cittadella Universitaria, 09042, Monserrato (CA), Italy.
²Institut für Angewandte Geowissenschaften, Karlsruhe Institute of Technology (KIT), Karlsruhe, Germany.
³Frankfurt Isotope and Element Research Center (FIERCE), Goethe-University Frankfurt, Frankfurt, Germany.
Corresponding author e-mail: francmar@unica.it

A sample from the Punta de li Tulchi granulitized eclogite, cropping out within the Variscan migmatites of NE Sardinia, was investigated by LA-ICPMS garnet U–Pb age dating. The majority of U-Pb analyses define a regression line with a lower intercept date of 395.1 ± 9.8 / 12.4 Ma. In addition, nine data points define a second regression line with a lower intercept date of 355.0 ± 18.1 / 19.4 Ma. The ~395 Ma age is interpreted to represent initial garnet growth during eclogite facies metamorphism. This age is another indication for an extensive subduction event at 380-400 Ma during the Variscan orogeny, as reported from other areas of the European Variscides. The ~355 Ma age is interpreted to indicate partial Pb-loss or fluid-mediated recrystallization of the garnet during the amphibolite to granulite facies metamorphism. This work demonstrates that in situ garnet U-Pb dating by LA-ICPMS is a powerful tool for deciphering the evolution of metamorphic rocks and that this method can provide valuable information and context for the interpretation of geochronological U-Pb data obtained from other geochronometers such as zircon.

garnet dating, U-Pb geochronology, metamorphism, eclogite, Variscan Sardinia.