Deciphering protoliths nature, protoliths age and peak P-T conditions in retrogressed mafic eclogites: An attempt in the metabasites from Maures-Tannneron Massif (SE France) and consequences for the southern European Variscides

Abstract

We present new constraints on age, nature and tectonic setting of mafic eclogites protoliths from the Maures-Tanneron Massif, southern Variscan belt. Whole-rock major and trace elements geochemistry was combined with 206Pb/238U zircon dating to improve the understanding of this key-target of the European Southern Variscides. Geochemical data show that protoliths of the mafic eclogites are typical MORBs, while REE and HFSE patterns suggest an E-MORB affinity. However, the geochemical study shows several signs of crustal contamination, increasing with degree of retrogression. A comparison with Sardinian eclogites, which belong to the same Variscan micro-plate “MECS” (Maures-Estérel-Corsica-Sardinia), demonstrate that the eclogites included in migmatites, which is the case of the studied samples, are the most contaminated. Anyway, the Maures-Tanneron mafic eclogites represent the witness of oceanic basaltic crust. Zircon cores display homogeneous Th/U ratios (0.3-0.4), consistent with a magmatic origin, and define an age peak at ca. 500 Ma interpreted as the most plausible emplacement age of the basaltic protolith. This age suggests that this protolith was part of an oceanic floor older than the Rheic Ocean and located north of the Gondwana active continental margin as predicted by recent unified full plates reconstruction models. Although the eclogites studied are retrogressed the study of frozen mineral inclusions trapped in garnets combined with thermodynamic modelling yields a P-T range of 16.8-18.5 kbar for 630-660°C, coherent with standard subduction paleo-geotherm. These new data suggest that mafic eclogites recognized in the “MECS” Variscan micro-plate represent the closure of oceanic domains of different ages.