Geochemical and geochronological dataset of rutile from a Variscan metabasite in Sardinia, Italy

Abstract

A c. 500 m wide and 1.5 km long body consisting of basic to ultrabasic rocks, metamorphosed up to granulite-facies and retrogressed to amphibolite-facies conditions during the Variscan orogeny, crops out near Olbia (NE Sardinia, Italy). Among abundant samples, one, collected from a garnet-rich centimetric layer, was chosen for a detailed analysis of rutile; chemical analyses of rutile were performed with the electron microprobe on petrographic thin sections, whereas U/Pb ages were determined by LA-ICP-MS on rutile mounted in epoxy resin. Chemical analyses show that rutile included in other minerals (Rt inc ) commonly show higher SiO 2 and FeO contents and lower Nb 2 O 3 and ZrO 2 contents if compared with rutile in the matrix of the garnet-rich layer (Rt mat ). Cr 2 O 3 concentrations are quite similar in both types of rutile. Rt mat commonly shows a greater variability in minor elements, especially Nb 2 O 3 (0.049–0.284 wt.%) and SiO 2 (0.019 - 0.193 wt.%) whereas Rt inc compositions are more homogeneous except for FeO (0.251–0.562 wt.%). The U-Pb isotopic data provided discordant ages and defined a lower intercept in the Tera-Wasserburg diagram of 273 ± 13 Ma. Few compilations of geochemical and geochronological data on rutile in Variscan metabasites can be found in literature, thus these data represent a new insight on a mineral phase the significance and scientific interest of which are rising in the last years. Future studies on the origin and ages of emplacement and metamorphism (either prograde or retrograde) of this kind of rock, widespread in the Variscan chain, will benefit from these data as a term of comparison.

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

“…Nieddu formed during retrograde metamorphism, evolving trough different cycles of fluid flux during the exhumation of the ultrabasic amphibolites at the end of the Variscan orogeny. The only existing geochronological data on the studied rocks are the U-Pb isotopic data on matrix rutile from ultrabasic amphibolite, giving a lower intercept in the Tera-Wasserburg diagram of 273 ± 13 Ma [62]. The considered geodynamic scenario of the studied rocks is reported in [53] for the adjacent amphibolites that host the ultrabasic amphibolites.…”