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

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.

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.
© 2020 The Authors. Published by Elsevier Inc. This is an open access article under the CC BY license.
( http://creativecommons.org/licenses/by/4.0/ ) Specifications Raw and Analyzed Data Parameters for data collection Several samples of a Variscan metabasite were collected and the most representative one was chosen for these analyses. Thin sections were studied by optical and electron microscopy. Rutile was obtained by magnetic and heavy liquid separation after crushing the sample. The single grains were hand-picked and mounted in epoxy resin. Description of data collection SEM imaging was performed on polished and C-coated thin section Minerochemical data were acquired on polished and C-coated thin section by EMP U/Pb dating was performed by HR-ICP-MS-LA on rutile mounted in epoxy resin Data

Value of the data
• Rutile provides constraints on the genesis, metamorphism and age of the host rock [ 2 , 3 ] • Petrologists dealing with the evolution of the Variscan chain can found interesting information on rutile chemistry and ages • Regional-scale studies could benefit from these data for comparison with similar rocks • Few data about rutile in Variscan metabasites are available in literature • The petrological interest for Ti-bearing mineral phases is growing in the last years [2] Fig. 1. Box and whisker plots of major oxides (other than TiO 2 ) analyzed in rutile either in the matrix (empty boxes) or as inclusions in other minerals (full boxes). FeO is displayed in a separate plot due to the different scale of the Y axis.

Chemical analyses
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 Rt mat ) and as inclusions in other minerals (Rt inc ) ( Table 1 and 2 , respectively, and Fig. 1 ). Part of these data (mainly Rt inc ) belongs to the supplementary material of [1] . TiO 2 content in Rt inc ranges between 98.423 and 99.279 wt.% (avg. 98.908 wt.%), slightly lower than that of Rt mat ranging 98.722 -99.390 wt.% (avg. 99.104 wt.%). FeO, the main oxide in rutile other than TiO 2 , shows quite variable contents with a wider compositional range for Rt inc (0.468 -1.281 wt.%) than for Rt mat (0.389-0.915 wt.%). The interquartile range (IQR) better shows the different FeO concentration in the two rutile populations; Rt inc are commonly richer in iron (IQR = 0.72-0.96 wt.%) than Rt mat (IQR = 0.48-0.64 wt.%). Silica contents in Rt inc and Rt mat are comparable considering the IQR (0.062-0.090 and 0.047-0.082, respectively) whereas, looking at the whole range, a high data dispersion is observed with values up to 0.193 wt.% in Rt mat . Additional analyses of trace elements were determined by LA-ICP-MS, on rutile in a resin mount ( ex-situ rutile , Table 3 ). The concentrations of Nb (510-1360 ppm), Zr (422-560 ppm) and Cr (164-369 ppm) of such rutile, converted to wt.% of oxides (Nb 2 O 3 0.073-0.195; ZrO 2 0.057-0.076; Cr 2 O 3 0.024-0.054 wt.%), are well comparable to those determined on in-situ rutile by EMP, testifying the reliability of LA-ICP-MS analyses. In particular, Zr and Nb suggest that exsitu rutile was hosted in the rock matrix (instead of being inclusions in other minerals) since their concentration match quite well those determined on Rt mat . LA-ICP-MS analyses also reveal that V is the most abundant trace element, being in almost all analyzed grains > 10 0 0 ppm. Subordinate trace elements are Zn ranging between 2.6 and 21.7 ppm, Sr 2.8-3.8 ppm, Ta 14-42 ppm, Hf 12-16 ppm and U 9-19 ppm. Many of the other trace elements including Mn, Ni, Rb and all REE (light, medium and heavy) are below detection limit for most of the analyzed rutiles. Table 1 EMP chemical analyses of rutile grains in the rock matrix (Rt mat ) from thin section ( in-situ ). TiO 2 was determined as difference from 100%.

U/Pb radiometric dating
U/Pb analyses were performed on rutile embedded in an epoxy resin mount. Thus, is not possible to assess whether or not Rt mat and Rt inc record different radiometric ages. Analyzed rutile is commonly homogeneous, free of inclusions and euhedral with shallow to pervasive fractures. The shape is mainly elongated and the size ranges between 100 and 150 μm in length and 40-  All data from this study. bdl = below detection limit.

Fig. 2.
Tera-Wasserburg diagram [4] showing isotopic data and the lower intercept age of the analyzed rutile.
60 μm in width. Subhedral and rounded grains are also found, probably resulting from intense fracturing. Rare ilmenite exsolutions, as thin needles, are found in rutile. 13 points in as many rutile grains were analyzed but 4 analyses were rejected due to a low U signal; results are listed in Table 4 .

Geological setting
The Sardinian metamorphic basement is a segment of the Variscan orogenic belt, showing an almost complete transect of the chain, from the Foreland in the southwest to the Inner (or Axial) Zone in the north-northeast [5] . Two different metamorphic complexes, separated by a WNW-ESE-trending shear zone named Posada Asinara Line (PAL) [6] , have been distinguished in the Inner Zone: the High-Grade Metamorphic Complex (HGMC, or Migmatite Complex) north of the PAL and the Medium-Grade Metamorphic Complex (MGMC), south of the PAL ( Fig. 3 a).
The HGMC is mainly made up by gneiss and migmatites [ 7 , 8 , 9 ] that record a multistage deformation history in which at least three main phases have been recognized [ 10 , 11 ]. The occurrence of marble [12] , calcsilicate rocks and especially metabasite lenses and bodies [ 13 , 14 ] is quite common. Metabasites are very interesting because, although retrogressed to amphibolite/greenschist-facies conditions, they still preserve relics of the granulite and eclogite facies [ 1 , 14 , 15 ].
The largest metabasite body in the HGMC crops out NE of Olbia in the Montiggiu Nieddu locality (hereafter Mt. Nieddu) and consists of a 1.5 km long and 350 to 500 m wide body roughly elongated in SW-NE direction. The main lithology consists of a banded amphibolite formed by alternating centimeter-to decimeter-thick whitish plagioclase-rich bands and dark  green amphibole-rich bands; in addition, epidote-rich veins and centimeter-thick levels hosting abundant garnet grains, up to 1 cm in size, are locally found within the banded amphibolite [ 1 , 16 ] ( Fig. 3 b,c). On the southern side of Mt. Nieddu, a smaller body of metabasite, derived from an ultrabasic igneous protolith, occurs; it preserves relics of igneous structures and minerals, as well as relics of the granulite facies [17] . The here presented data come from a sample of a garnet-rich layer collected in the banded amphibolite; such layers have been interpreted as the less retrogressed rocks of the Mt. Nieddu complex and thus are suitable to reconstruct the early metamorphic stages.

Petrography
The garnet-rich layer is characterized by abundant (up to 30% vol.) sub-centimetric garnet porphyroblasts in a matrix consisting of amphibole, plagioclase, quartz andclinopyroxene + plagioclase symplectite. Garnet is rimmed by sub-millimetric coronas of plagio- clase + amphibole and hosts inclusions of amphibole, plagioclase, quartz, clinopyroxene, rutile and ilmenite ( Fig. 4 a). Accessory minerals are rutile, titanite, zircon, K-feldspar, chlorite, Feoxides, monazite, and biotite. Rutile grains occur in both garnet and matrix and are mainly free of inclusions. These grains are euhedral with elongated shape and length ranging between 80-130 μm for Rt mat and 20-30 μm for Rt inc . Subhedral and rounded grains are also found, probably resulting from intense fracturing. Ilmenite replacement and thin titanite rims rarely occur ( Fig. 4 b,c).

Materials and methods
Polished thin sections were prepared from a selected sample of the garnet-rich layer. The preliminary observation, under a polarization microscope, was followed by BSE imaging performed at CeSAR (Centro Servizi d'Ateneo per la Ricerca, Università di Cagliari) with a scanning electron microscope FEI Quanta 200, equipped with a Thermo Scientific TM UltraDry EDS Detector for qualitative minero-chemical analyses. The quantitative chemical analyses of the here reported rutile were performed with a CAMECA SX100 electron microprobe (EMP) equipped with five wave-length dispersive (WD) spectrometers at the (former) Institut für Mineralogie und Kristallchemie, Universität Stuttgart. The operative conditions were 15 kV acceleration voltage, 200 nA beam current and 5 μm spot size. For the description of the used standards, counting times and analytical errors see [18,19] . SEM and EMP analyses were performed on thin sections (i.e. insitu ) allowing us to distinguish between rutile inclusions in other minerals and rutile in the rock matrix.
On the contrary, ex-situ rutile embedded in a resin mount was used for U/Pb radiometric dating performed at the CNR-Istituto di Geoscienze e Georisorse (Pavia, Italy) by ablation with a 193 nm ArF excimer laser (GeoLas200Q-Microlas) coupled to an Agilent 8900 quadrupole mass spectrometer. About 4 kg sample material was carefully crushed. Rutile crystals were separated by magnetic and heavy liquid separation. Finally, the single grains were hand-picked and mounted in epoxy resin. The rutile-bearing mount was then polished to reach the core of the grains. The external reproducibility was calculated from the standard Sugluk-4 and PCA-S207 [20] analyzed during the analytical session whereas the errors were propagated according to [21] .

Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships which have, or could be perceived to have, influenced the work reported in this article.