Metamorphic evolution and P–T path of the Posada Valley amphibolites: new insights on the Variscan high pressure metamorphism in NE Sardinia, Italy

Abstract

The Posada Valley amphibolites in the inner zone of Variscan Sardinia are characterized by garnet porphyroblasts containing epidote, titanite, quartz, clinopyroxene, amphibole, and plagioclase concentrated in the garnet core. The matrix is made up of amphibole and plagioclase and finegrained clinopyroxene and plagioclase. The amphibolites recorded an HP stage, a metamorphic re-equilibration, and a third stage under the greenschist facies. P -T conditions of stage I (530-650°C/0.9-1.3 GPa for garnet core and 570-690°C/1.0-1.4 GPa for garnet rim) were obtained by P -T pseudosection modelling. The P -T conditions for stage 2 (T = 600-700°C/P = 0.5-0.8 GPa) were obtained by applying the same approach to the most retrogressed samples. The Posada Valley amphibolites underwent a cooler evolution, with a thermal peak lower than that recorded by the northward eclogites from the high-grade metamorphic complex of Sardinia.The proposed geodynamic scenario starts with the subduction of oceanic crust under the peri-Gondwanan terrane in Upper Devonian. The Posada Valley amphibolites were subducted up to maximum depths of 50 km. Subsequently, they reached the subduction channel or were detached from the slab before the eclogite facies conditions were reached.

Spatial variation of pure and simple shear across transpressive shear zones: flow kinematics map of the Posada-Asinara shear zone (NE Sardinia, Italy)

“…It is important to note that all the kinematic vorticity data have been derived with the same methods, i.e., the C' shear band, but from different lithotypes. In fact, the investigated area by Carosi et al (2020) is characterized by micaschist and paragneiss from the garnet to the sillimanite + white mica zones and granodioritic and granitic augen orthogneiss and amphibolite lenses within the kyanite-bearing micaschist (Cruciani et al, 2013, 2022, Scodina et al, 2021. Despite most of the investigated lithotypes are associated with similar rheological behaviour due to the presence of a similar rock matrix made by white mica and biotite, a detailed rheological characterization of the present lithotypes is out from the aim of this work.…”

Constraining the Timing of Evolution of Shear Zones in Two Collisional Orogens: Fusing Structural Geology and Geochronology

“…Transpression affected the boundary between the HGMC and L-MGMC after collision-related deformation and metamorphism (D1 phase) [67,79]. An earlier HP event occurred in the HGMC ( [83,84] with references), in metabasites of the PASZ [85] and in metapelites a dozen kilometers south of the PASZ [83]. The PASZ developed under decreasing temperature and pressure, from amphibolite facies conditions down to greenschist facies conditions [82][83][84] in the time span of ~25 Ma [66,67].…”

Deciphering the nature and age of the protoliths and peakP−Tconditions in retrogressed mafic eclogites from the Maures-Tannneron Massif (SE France) and implications for the southern European Variscides

“…However in large and hot orogens, such as the European Variscan belt, eclogites can be severely transformed and re-equilibrated under granulite and/or amphibolite-facies conditions, making it challenging to decipher natural and protolith ages, as well as eclogite peak P-T conditions (Dufour et al 1985;Piboule & Briand, 1985;O'Brien, 1997O'Brien, , 2000Medaris et al, 1995;Klápová et al, 1998;Faryad et al, 2010;Lardeaux, 2014;Štípská et al, 2014;Cruciani et al, 2015;Scodina et al, 2021). This is particularly the case for the eclogites from the Maures-Tanneron Massif, which is located in southeast France, for which available geochemical, petrologic are equivocal while modern geochronologic constraints are lacking (see Schneider et al, 2014;Oliot et al, 2015).…”