Continuous vs. discontinuous garnet growth in mylonitic micaschists from northeastern Sardinia, Italy: Evidence from LA-ICPMS trace element mapping

Garnet with complex, discontinuous zoning is a common occurrence in metamorphic terrains, and the rela tionship between major and trace element zoning can provide insight into the metamorphic evolution of the host rock. Mylonitic micaschists along the Posada-Asinara Shear Zone in the Axial Zone of the Sardinia Variscan chain contain garnet porphyroblasts, enveloped by the S2 schistosity, with distinct core and rim domains. A large garnet porphyroblasts was investigated by laser ablation-inductively coupled plasma-mass spectrometry (LA ICPMS) mapping. The major element compositional variation follows a bell-shaped zoning, with Ca and Mn contents progressively decreasing, and Fe and Mg increasing, from the core to the outer rim. LA-ICPMS mapping revealed a thin and sharp annular enrichment zone in Y, Sc, Dy, Ho, Er, Tm at the mantle-rim boundary. The trace element (TE) compositional profiles show a central enrichment area for HREE (Tm, Yb, Lu). This enrich ment decreases progressively, as a function of atomic number, for Er, Ho and Dy. Elements with even lower atomic number (Tb, Gd, Eu and Sm), are depleted in this central domain, but their content increases in broad shoulders towards the garnet rim. The position of these lateral shoulders migrates progressively rimwards with decreasing atomic number. The REE distribution, trend and behavior in the growth zones of the garnet is an example of TE control during a continuous growth ruled by diffusion-limited REE uptake. The Y + HREE annular enrichment zone, interpreted as resulting from a decrease in the garnet growth rate, reflects a short-lived episode in the garnet growth history

Multilayer corona textures in the high-pressure ultrabasic amphibolite of Mt. Nieddu, NE Sardinia (Italy): equilibrium versus disequilibrium

Rocks with coronitic textures around igneous relics of olivine and plagioclase were sampled from the ultrabasic amphibolite of Mt. Nieddu being part of the so-called Migmatite Complex of northeast Sardinia. These textures are characterized by layers of orthopyroxene and clinopyroxene around olivine, and a symplectite of clinopyroxene+spinel and garnet around plagioclase; all these minerals were overgrown by amphibole. We applied conventional geothermobarometry and pressure-temperature (P-T) pseudosection modelling of microdomains in order to constrain the main steps of the evolution of the sampled rocks. The igneous crystallization occurred at 0.2-0.5 GPa and minimum T of 780-850 °C. Garnet is the last coronitic phase formed at estimated pressures of 1.3-1.7 GPa at 680-730 °C. As similar conditions were previously determined for the pressure peak on an adjacent rock, we conclude that during corona formation chemical equilibrium was reached probably over a much wider range than of mm-sized microdomains. Slow reaction kinetics is responsible for the partial preservation of the igneous phases

Geology of the Zicavo Metamorphic Complex, southern Corsica (France)

In this study, we investigated the Zicavo Metamorphic Complex (southern Corsica), which ispart of the innermost Axial Zone of the Corsica-Sardinia Variscan belt. To better evaluate itsgeological and structural outline, a 1:5000 geological map, coupled with new structural/microstructural and petrographic data, is presented. The complex is formed by threetectonic units, from bottom to top: (i) an Orthogneiss Unit, (ii) a Leptyno-Amphibolite Unit,and (iii) a Micaschist Unit. They are separated by ductile shear zones with a top-to-the-SEsense of shear. They underwent a polyphase deformation and polymetamorphic history,with a shortening stage in the amphibolite facies, responsible for the main structures andshearing, followed by an exhumation phase

Geological and archaeological heritage in the Mediterranean coasts: proposal and quantitative assessment of new geosites in SW Sardinia (Italy)

The south-western Sardinian coast (Italy), in the sector between the small village of Chia (Domus de Maria) and the Piscinnì locality (Teulada), is one of the most interesting of the island, being characterized by a wide geodiversity, a complex history testified by several archaeological sites, rich ecosystems, and beautiful landscapes. This area is already famous as a seaside resort, but its rich geological heritage is totally unrecognized and the educational and touristic potential of its archaeological sites is widely undervalued. In this paper, we propose an itinerary in seven stages, each of which selected considering mainly its geology (geodiversity, scientific relevance of the geological features, and educational potential), and also the additional values as the presence of archaeological sites and/or of ecological diversity and/or touristic attractions. The aim of this work is to propose the study area as a geotrail since it represents a valuable geological (and not only) heritage to be protected, preserved, and valorized in the broadest meaning of the word. The following sites have been selected: Chia tower hill, showing interesting geological features and the Punic-Roman archaeological site of Bithia; Campana and Su Giudeu beaches with their dune system and the adjacent wetlands; Settiballas, near to the Chia-Laguna resort displaying a significant geological contact; Capo Spartivento rias and cliffs; Tuerredda beach and sand dunes; Capo Malfatano, a panoramic promontory made up of 460 million years old metamorphic rocks; and Piscinnì area, where a beautiful beach and a wide Punic/Roman sandstone quarry can be observed. In addition, the most panoramic points are overlooked by Spanish towers built during the 16th to 17th century. The study area is described from geological and archaeological points of view, after a detailed field survey, the petrographic characterization of the main rocks, and an overview of the scientific literature. The quantitative assessment of the geosite values has been also performed, revealing the high potential for scientific, educational, and touristic purposes. The main concerns related to the preservation and valorization of this geosite are also addressed. Finally, a discussion on the geosite assessment methods is proposed to contribute to the scientific literature on the geoheritage. The site-by-site description is reported at the end of the paper to be used as a field guide

Garnet‐rich veins in an ultrabasic amphibolite from NE Sardinia, Italy: An example of vein mineralogical re‐equilibration during the exhumation of a granulite terrane

A complex system of mono‐ and polymineralic centimeter‐thick veins occurs within the ultrabasic amphibolites of Montigiu Nieddu hill in northeastern Sardinia, and they are filled with garnet, amphibole, chlorite, and epidote. Some garnet‐rich veins are margined by an amphibole layer at the interface with the host rock and/or show replacement of epidote concentrated in the vein core. Together with homogeneous matrix garnet (Grt1), millimetric, euhedral, and strongly zoned garnet porphyroblasts occur within these veins. The estimated pressure–temperature conditions (P = 1.0–1.7 GPa, T = 650–750 °C) for the formation of Grt1 match the metamorphic peak and early exhumation derived previously for the host rocks and confirm that the garnet veins also formed under high‐pressure (HP) conditions. The igneous protolith of the host rocks experienced HP metamorphism in a subduction zone and underwent exhumation in an exhumation channel. The vein system in the ultrabasic amphibolites formed by cyclic hydrofracturing as rapid and transient events such as crack‐seal veining. The growth of multiple vein‐filling mineral assemblages indicates the formation of separate vein‐producing cycles

Cities and climate change from a EU perspective

The paper analyzes the legal basis of the European Policy on climate change