Petrogenesis of the Solanas gabbro-granodiorite intrusion, Sàrrabus (southeastern Sardinia, Italy): implications for Late Variscan magmatism

Abstract

The igneous complex of Solanas is a small, composite calc-alkaline intrusion emplaced at ~ 300 Ma in the Sàrrabus region (southeastern Sardinia), and consists of olivine gabbronorites, amphibole gabbros, microgabbros, quartz diorites, tonalites, amphibole granodiorites, and biotite granodiorites. Thermobarometry calculations indicate that the Solanas rocks were emplaced at mid-to-upper crustal levels (0.6–4.0 kbar). The intermediate and silicic rocks are metaluminous to weakly peraluminous and are enriched in large ion lithophile elements. The range in the initial Sr and Nd isotopic compositions is small throughout the intrusion despite a large range in silica contents (46.3–73.6 wt% SiO₂). The isotopic signatures, mineralogy, and geochemistry suggest that the quartz diorites, tonalites, and granodiorites derived from fractional crystallisation and crustal contamination processes starting from different mafic parental magmas. The origin of tonalites and granodiorites is compatible with removal of plagioclase, hornblende, biotite, apatite and zircon starting from a quartz dioritic magma. The mafic rocks range in composition from primitive to relatively evolved (Mg# 49–70). The olivine gabbronorites and amphibole gabbros have petrographic and geochemical features of arc cumulates derived from different basaltic magmas. The microgabbros have geochemical characteristics similar to high-alumina basalts with fractionated rare-earth element patterns (La_N/Yb_N = 4.3–6.0), enrichment in large ion lithophile elements (e.g., Rb, Ba, U, and K) and depletion in Nb and Ta compared with the primitive mantle. These characteristics are consistent with partial melting of a mantle source that was enriched by subduction-related fluids.

Appendix 1: Analytical techniques

The collected Solanas samples were processed and analysed for petrochemical characterisation at the Dipartimento di Scienze della Terra, dell’Ambiente e delle Risorse (DiSTAR), Università degli Studi di Napoli Federico II. Samples were first cut with a diamond blade saw and then ground in a steel jaw crusher. Rock slabs were used for the preparation of thin sections subjected to petrographic investigations at the polarising microscope. Modal analysis was performed on rock samples by point counting using the Leica QwinPlus software image analysis (1500 points for each thin section). Rock chips were washed in distilled water, hand-picked under a binocular microscope to remove any sign of either alteration or presence of xenolithic material, and powdered in an ultrapure agate mill. Four grams of rock powder for each sample (mixed with 1 ml of Polyvinyl alcohol solution) were used to prepare pressed powder pellets (at 20 tons/cm² for 20 s), and analysed for major and trace elements concentrations with an Axios Panalytical X-ray fluorescence (XRF) spectrometer at DiSTAR. The spectrometer is equipped with six analyser crystals, three primary collimators, and two detectors (flow counter and scintillator), operating at different kV and mA for each analyte. Analytical uncertainties are in the order of 1–2% for major elements and 5–10% for trace elements. The weight loss on ignition (LOI) was obtained with the standard thermogravimetric techniques, firing at 1000 °C small aliquots of powders pre-dried at 110 °C overnight.

Additional whole-rock compositional data on a subset of samples were obtained through Inductively Coupled Optical Emission Spectrometry (ICP-OES) and Inductively Coupled Plasma Mass Spectrometry (ICP–MS) at Actlabs (Canada). Samples were mixed with a flux of lithium metaborate and lithium tetraborate and fused in an induction furnace. The melt was immediately poured into a solution of 5% nitric acid-containing an internal standard and mixed continuously until completely dissolved (~ 30 min). The samples were analysed for major oxides and selected trace elements (Ba, Be, Sc, Sr, V, Y, and Zr) by Thermo Jarrell-Ash ENVIRO II or a Varian Vista 735 ICP optical spectrometer. Calibration was performed using seven prepared USGS and CANMET certified reference materials. Fused samples were diluted and analysed by Perkin Elmer Sciex ELAN 6000, 6100 or 9000 ICP–MS for the other trace elements (Cr, Co, Ni, Cu, Zn, Ga, Ge, As, Rb, Nb, Mo, Ag, In, Sn, Sb, Cs, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Hf, Ta, W, Tl, Pb, Bi, Th, and U). Three blanks and five standards (three before the sample group and two after) were analysed per group of samples. Duplicates were fused and analysed every 15 samples.

Representative analyses of the mineral phases (on polished thin sections) were obtained using a microanalysis unit equipped with an INCA X-act detector and a JEOL JSM-5310 Scanning Electron Microscope (SEM) in Energy-Dispersive Spectrometry (EDS) at DiSTAR. The standard operating conditions included a primary beam voltage of 15 kV, filament current of 50–100 µA, and variable spot size from 30,000 to 200,000 × magnification, 20 mm WD. Measurements were taken with an INCA X-stream pulse processor and elaborated with the Energy^® software by Jeol. Energy^® uses the XPP matrix correction scheme developed by Pouchou and Pichoir (1988) and the pulse pile-up correction. The quant optimization is carried out using cobalt (FWHM-full width at half maximum peak height-of the strobed zero = 60–65 eV). The following standards were used for the calibration: diopside (Ca), San Carlos olivine (Mg), anorthoclase (Al, Si), albite (Na), rutile (Ti), fayalite (Fe), Cr₂O₃ (Cr), rhodonite (Mn), orthoclase (K), apatite (P), fluorite (F), barite (Ba), strontianite (Sr), zircon (Zr, Hf), synthetic Smithsonian orthophosphates (REE, Y, Sc), pure vanadium, niobium and tantalum (V, Nb, Ta), Corning glass (Th and U), sphalerite (S, Zn), galena (Pb), sodium chloride (Cl), and pollucite (Cs). The Kα, Lα, or Mα lines were used for calibration, according to the element. Back-scattered electron (BSE) images were obtained with the same instrument.

Whole-rock Sr and Nd isotope analyses were determined at the Geochronological Research Center of the University of São Paulo using the conventional ion exchange chromatography combined with thermal ionisation mass spectrometry (TIMS) following the analytical procedures published in Souza (2009) and Petronilho (2009). The Sr and Nd isotope ratios were normalised to ⁸⁶Sr/⁸⁸Sr = 0.1194 and ¹⁴⁶Nd/¹⁴⁴Nd = 0.7219, respectively, for in-run isotopic fractionation correction. The blanks for Sr are 110 pg. The blanks for Nd are 150 pg. The accuracy of measurements was checked against the NBS987 standard for Sr isotopic ratios ⁽⁸⁷Sr/⁸⁶Sr = 0.710236 ± 0.000020, n = 20), and JNdi-1 standard for Nd isotopic ratios (¹⁴³Nd/¹⁴⁴Nd = 0.512090 ± 0.000008, n = 24).

The errors of age-corrected Sr and Nd isotope ratios were evaluated by an error propagation method applied to Sr and Nd isotopes, Rb/Sr, Sm/Nd, and age data. With an age uncertainty between ± 5 Ma (299.6 ± 0.3 Ma, the Rb–Sr age of the Solanas quartz diorites), the calculation for ⁸⁷Sr/⁸⁶Sr data indicates errors to five significant figures in the rocks with Rb/Sr up to 0.13 (Table 2) and to four significant figures in the rocks analysed with Rb/Sr between 0.22 and 0.73 (Table 2). For the Sm/Nd ranges (0.16–0.25), the errors on age-corrected Nd isotope ratios remain on the fifth/sixth figures.