Preliminary garnet dating in retrogressed eclogites from Punta de Li Tulchi, NE Sardinia, Italy

The Punta de li Tulchi retrogressed eclogite crops out 50 km south from Olbia as an E–W oriented, 150 m long 20–40 m thick lens embedded within the Variscan migmatites of NE Sardinia. The lens is parallel to the E–W-oriented schistosity of the migmatite. The eclogite consists of an alternation of garnet-pyroxene-rich and amphibole-plagioclase-rich layers, striking E–W and dipping 50°N. The amphibole, plagioclase-rich layers show an EW-oriented foliation crosscut by a third retrograde S3 foliation defined by the occurrence of millimetric white pods consisting of plagioclase–amphibole kelyphites. Locally the S3 is crosscutted by centimetric to decimetric late shear zones. A pre-eclogite stage is documented by the occurrence of tschermakite and zoisite relics within garnet porphyroblasts. Four main metamorphic stages have been distinguished in the eclogite evolution: (1) eclogite stage, revealed by the occurrence of omphacite relics within garnet porphyroblasts; (2) granulite stage, producing orthopyroxene/clinopyroxene–plagioclase symplectites replacing omphacite; (3) amphibolite stage, leading to the formation of amphibole–plagioclase kelyphites between garnet and symplectite and to the growth of cummingtonite on orthopyroxene; (4) greenschist to sub-greenschist stage, defined by the appearance of actinolite, chlorite, and epidote. The P-T path is clockwise, with T =660-700 °C at the peak of pressure (1.7–2.1 GPa) and peak of temperature close to 800 °C at P=1.0–1.3 GPa, in the HP granulite facies. Palmeri et al. (2004, Neues J. Mineral. Monat. 6, 275–288) in the eclogite of Punta de li Tulchi found U–Pb zircon ages giving three weighted means of 453 ± 14, 400 ± 10 and 327 ± 7 Ma. The first one was interpreted as the gabbroid protolith age, the second was considered as the likely age of the HP eclogitic event or the result of Pb loss during the main Variscan event, while the third mean was referred to the final retrogression to amphibolite facies. With the aim to better define the ages of the different steps of the metamorphic evolution, an eclogite sample was selected and prepared to be investigated by LA-ICPMS U–Pb age dating on garnet. Although the majority of the analyses contain low uranium, an age of 380 ± 9.9/10.4 Ma was obtained based on 31 garnet spots. Besides, a less precise age of 340.9 ± 18.2/18.4 Ma was also calculated from 10 analysed points. The older age corresponds to the eclogite stage event, whereas we tentatively interpret the second age (340 Ma) as the possible age of the granulite event. The age of ca. 340 Ma is coeval to the 335-355 Ma high-temperature event recorded in other parts of the Variscan massif (e.g. Plešovice zircon, Slama et al., 2008 Chem. Geol. 249, 1–35). However, it cannot be excluded that such age, which was obtained with a limited number of spots, could be related to the Pb-loss of some areas of the garnet during the garnet breakdown that led to the corona/ kelyphite formation.