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 (Rtinc) commonly show higher SiO2 and FeO contents and lower Nb2O3 and ZrO2 contents if compared with rutile in the matrix of the garnet-rich layer (Rtmat). Cr2O3 concentrations are quite similar in both types of rutile. Rtmat commonly shows a greater variability in minor elements, especially Nb2O3 (0.049–0.284 wt.%) and SiO2 (0.019 - 0.193 wt.%) whereas Rtinc 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.
Highlights
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)
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
Earth and Planetary Sciences Geochemistry and petrology Table Chart Figure Field survey, sample collection, thin section and epoxy mounts Scanning Electron Microscope (SEM), FEI Quanta 200 coupled to Thermo ScientificTM UltraDry EDS Detector Electron MicroProbe (EMP), CAMECA SX100 193 nm ArF excimer laser ablation, GeoLas200Q-Microlas coupled to HR-ICP-MS, Thermo Finnigan Element I Software: Glitter Raw and Analyzed Data Several samples of a Variscan metabasite were collected and the most representative one was chosen for these analyses
Summary
Chemical analyses of “major” oxides of in-situ rutile were determined by EMP on petrographic thin sections, allowing us to distinguish between rutile in the rock matrix (hereafter Rtmat) and as inclusions in other minerals (Rtinc) (Table 1 and 2, respectively, and Fig. 1). Part of these data (mainly Rtinc) belongs to the supplementary material of [1]. FeO, the main oxide in rutile other than TiO2, shows quite variable contents with a wider compositional range for Rtinc (0.468 - 1.281 wt.%) than for Rtmat (0.389–0.915 wt.%). In the Tera-Wasserburg plot [4] (Fig. 2) the Pb uncorrected rutile data define a lower intercept at 276 ± 13 Ma
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