Structural map of Variscan northern Sardinia (Italy)

In this paper, we present a geological structural map (1:100,000 scale, ∼2300 km2 surface area) of the Variscan basement of northern Sardinia. The map integrates field structural analysis, extensive gamma-ray spectrometry, and high-resolution ELA-ICP-MS U/Th-Pb zircon and monazite dating. A set of 10 samples of granitic rocks collected from different plutons were characterized for their crystallization age. This provided an accurate timing of magmatic events related to the development of the Corsica-Sardinia Batholith. The structural map, complemented with geochronological results represents a benchmark for future studies on Variscan geodynamics.


Remote sensing
Image analysis provided a preliminary inspection of the geometry of faults and principal magmatic contacts in northern Sardinia. A digital elevation model (DEM) of northern Sardinia was created from a mosaic of 1:10,000 topographic maps. The DEM was integrated with georeferenced high-resolution aerial orthophotos (property of Regione Autonoma della Sardegna) and IKONOS 2005 satellite images. Detailed information about the spatial resolution and accuracy of the images can be found in (Virdis, Oggiano, & Disperati, 2012). The results were processed using Matlab to extract significant geomorphological features related to tectonic structures, such as major faults, joints networks, and lithologic contacts within granitic outcrops. The algorithm consists of a pre-processing routine that converts true color RGB images into grayscale images by calculating the luminance (Y) component as the weighted sum of the three color channels (R, G and B), as Y ¼ 0.299R + 0.587G + 0.114B. User-defined features were identified using a semi-automatic pattern recognition routine based on the Hit-or-Miss Transform method for grayscale images (Soille, 1999).

Field structural analysis
The geometry of plutons and their emplacement-related fabric were investigated by structural analysis (Vernon, Johnson, & Melis, 2004). The magmatic flow trajectories have been constrained measuring the shape preferred orientation of various flow markers, such as: (i) metamorphic xenoliths, (ii) large, idiomorphic k-feldspar phenocrysts, (iii) micro-granular mafic enclaves, (iv) micro-granitoid batches, (v) schlieren of biotite, and (vi) pegmatite or aplite veins. Except for fine-grained Permian leuco-monzogranites, that lack any consistent flow marker, several measurements have been performed in all plutons to portray the fine detail of the magmatic structure with an optimal statistical coverage.

Gamma-ray spectrometry measurements
Magmatic rocks and metamorphic high-grade rocks have been characterized for their U, Th and K composition (Puccini et al., 2014). The portable gamma-ray spectrometer adopted for this study consists of an NaI(Tl) single crystal with a cubic shape (10.2 × 10.2 × 10.2 cm 3 ) and energy resolution of 7.3% at 662 keV ( 137 Cs) and 5.2% at 1172 and 1332 keV ( 60 Co). The crystal is optically coupled with an integrated photomultiplier tube consisting of bias supply, preamplifier and digital multichannel analyzer. The device is controlled by a portable computer that allows processing of the raw spectrum in real time, determining the concentration of both 235,238 U 232 Th [ppm], and 40 K [%] directly in the field using the jRadview software package (Caciolli et al., 2012). The analysis is performed over the full spectrum using a non-negative least squares (FSA-NNLS) constraint; each spectrum is reconstructed from a linear combination of the standard spectra for 238 U, 232 Th, 40 K, 137 Cs and the background radiation. The concentration of 235, 238 U and 232 Th are estimated by 214 Bi and 208 Tl decay, under the assumption that the uranium and thorium decay series are in secular equilibrium. In 396 measurements on different Variscan rocks of North Sardinia, statistical uncertainties were below 1.5% for 40 K and about 3% for 235,238 U and 232 Th, respectively (Figure 2).

Geochronology
In order to implement the geochronological constraints of the granites history, 10 representative samples of magmatic and metamorphic rocks exposed in northern Sardinia were selected for insitu U-Pb zircon dating (Table 1). Mineral separation was carried out using conventional methods (vibrating table, magnetic separation and heavy liquids). Crystals without fractures, visible inclusions, and compositional zonations were identified by backscattered electron (BSE) and cathodoluminescence (CL) analysis with a Scanning Electron Microscope (SEM-JEOL JXA840). Suitable zircon grains were hand-picked, mounted in epoxy resin and polished using 0.25 mm diamond paste. The 202 Hg, 204 (Pb + Hg), 206,207,208 Pb,232 Th and 238 U isotopic compositions were determined with excimer laser ablation (ELA)-ICP-MS at the CNR -IGG -Unità di Pavia. The laser ablation instrument consists of an ArFexcimer laser microprobe at 193 nm (Geolas200Q-Microlas) and a high-resolution sector field ICP-MS. The instrumental and laser-induced U-Pb fractionations were corrected adopting the 1065 Ma 91500 zircon (Wiedenbeck et al., 1995) (Del Moro et al., 1996) of the standards was propagated to all determinations according to the equation of Horstwood, Foster, Parrish, Noble, and Nowell (2003); after this procedure, analyses were considered accurate within quoted errors. A 295 Ma 02123 zircon (Ketchum, Jackson, Culshaw, & Barr, 2001) was analyzed during each run for quality control, yielding 1.7% (2 sigma) mean accuracy. The isotopic ages were calculated using the concordia method (Wetherill, 1956).

Structure and geochronology of the Corsica-Sardinia Batholith
The structural mapping of northern Sardinia shows fragments of a Variscan high-grade, mainly migmatitic, metamorphic basement intruded by the Corsica-Sardinia Batholith (C-SB). Anatexis and southward thrusting of UMC onto the LMC occurred in Late Devonian -Early Visean (351.7 + 2.9 Ma, Table 1), typical of crustal melting in the Moldanubian domain of central Europe (Faure et al., 2010). The oldest magmatic complexes in the Sardinian part of C-SB are indicated in the map as Barrabisa (BBR) and Isola di S. Maria (ISM) plutons. They consist of dyke-shaped, S-to I-type, strongly foliated peraluminous monzogranites and granodiorites emplaced between 15 -20 km of depth within E-W and NW-SE ductile shear zones rooted in a granulitic lower crust (Casini, 2012;Casini et al., 2012Casini et al., , 2013. The early U2 plutons show systematically moderate to absent grain-size reduction towards the migmatitic host rock and preserve a well-developed magmatic foliation almost parallel to the fabric of migmatites ( Figure 3). All these features indicate that the contacts between the host migmatites and these plutons were plastic at the time of emplacement , which is consistent with the inferred emplacement depths (Figure 3). U/Th-Pb zircon and monazite dating provided two similar crystallization ages of 321.2 + 8.3 Ma (ISM, zircon) and 313.4 + 5.1 Ma (BBR, monazite), respectively (Table 1). The early granodiorites were successively intruded by larger, sill-shaped, metaluminous to slightly peraluminous monzogranitic plutons of Early Pennsylvanian to Early Permian age (320-290 Ma), indicated on the map as Tempio (TMP a , 316.8 + 2.3 Ma), Arzachena (AZN a , 320 + 10; AZN b , 311 + 5 Ma; AZN d , 308.1 + 3.8), S. Teresa (STR b , 307 + 4 Ma), La Maddalena (MDN a , 294 + 4 Ma), Porto Rotondo (RTN, 290 + 3 Ma), Trinità d'Agultu (TDA, 300.1 + 6.1 Ma) and Aglientu (AGL, undated). In the mapped area, the fabric of these plutons is overall flat or gently dipping to the NE in contrast to the vertical structure of the BBR and ISM plutons ( Figure 3). This difference indicates a transition from strike-slip tectonics, typical of Variscides in Pennsylvanian times, to distributed horizontal flow probably related to sudden injection of large volumes of melt . The younger magmatic complex ( 290 -270 Ma) is labeled on the map as the Early Permian Igneous Complex (EPIC). It consists of several compositionally heterogeneous plutons and subvolcanic complexes emplaced within NE-SW tensional fractures that collectively form large E-W belts (Figure 1). The relatively more mafic terms (EPIC a ) such as gabbro and granodiorite gave consistent ages ranging from about 290 to about 285 Ma (sample SANT3: 288.3 + 2.5 Ma; sample BO2: 285.8 + 1.9 Ma). EPIC is formed by the mixing of mafic, poorly evolved, subalkaline to alkaline melts and crustal melts (Gaggero et al., 2007;Renna, Tribuzio, & Tiepolo, 2006). Brittle contacts decorated by magmatic breccias can be observed around the mafic bodies ( Figure 3); therefore, it is likely that part of the crustal component derived from re-melting of the pre-Sakmarian plutons. The results of gamma-ray analysis support this interpretation as Late Carboniferous plutons have a distinctive Th/U ratio mostly in the 3 to 4 range, whereas Early Permian granites, including EPIC, show a much larger spread of values ( Figure 2).
An Early Permian continental basin, indicated as the Lu Falzu Basin on the map (LFB), is distinguished in the uppermost section of EPIC, beneath a rhyolitic volcanic sequence dated at 288 + 11 Ma by the Rb/Sr whole rock isochron method (Del Moro, Di Pisa, & Oggiano, 1996). This is a narrow transtensional basin filled by a thick ( 700 mt) sequence of conglomerates, muddy siltites and black quartzites. The sediments were accumulated along a NNW-SSE fault active since at 294 Ma, as indicated by the youngest ages of detrital zircons in the basal conglomerate (Table 1). The lower part of LFB in contact with a gabbroic complex associated with EPIC recorded HT-LP contact metamorphism at about 285 Ma. Finally, all lithostratigraphic units of post-Permian age, including Tethysian carbonate platforms and Eocene to Quaternary volcanic-sedimentary cover, have been grouped together for simplicity.

Post-Variscan brittle tectonics
The map shows several mainly NW-SE dextral and NE-SW sinistral strike-slip faults cutting across northern Sardinia. These structures displace the pre-Carboniferous metamorphic units and all plutons of the C-SB, including the youngest Permian complexes collectively indicated as EPIC. Therefore, in the absence of high-resolution geochronologic constraints, faulting is inferred to be Late-Carboniferous to Early Permian and possibly related to the large clockwise rotation of the Corsica-Sardinia block in late Variscan times (Edel, Casini, Oggiano, Rossi, & Schulmann, 2014). The faults characterized by sinistral kinematic and NE to ENE direction entangle the Mesozoic-Cenozoic sedimentary and volcanic sequences, generating either positive flower structures or strike-slip basins corresponding to restraining or releasing bends respectively (Carmignani et al., 1994). The age of syntectonic sediments indicates that most of these late Variscan faults were reactivated in Oligocene to Miocene times (Carmignani et al., 1995), during the collision between the Corsica-Sardinia block and Adria (Carmignani, Funedda, Oggiano, & Pasci, 2004;Oggiano, Funedda, Carmignani, & Pasci, 2009). The major structures, such as the Cannigione and Olbia faults (Figure 1), are traceable for several tens of kilometers and show up to 5 -7 km of horizontal offset, bridging different plutons and generating a complex mosaic pattern.

Conclusions
This paper presents the first detailed structural map of the Variscan basement of north Sardinia (Italy), including the southern part of the Corsica-Sardinia Batholith. High-resolution zircon dating of 10 selected samples of late Variscan plutons, together with published geochronological constraints, evidence two principal magmatic domains. The first consists of pre-Sakmarian peraluminous to metaluminous S-to I-type granites injected within E-W and NNW-SSE shear zones. The younger plutons ( 290 -275 Ma) are generally A-to S-type leuco-monzogranites emplaced within NE-SW dilatant zones. These latter massifs are systematically associated with sub-alkaline to alkaline mafic complexes ( 288 -284 Ma) composed of gabbros and diorites.

Software
The geological map was digitized using Esri ArcGIS 9.3. Google Earth and Matlab were used for remote sensing analysis. Field spectrometric analysis was performed using jRadview (Caciolli et al., 2012). The isotopic composition of zircon and monazite grains was analyzed using the Glitter package (Van Achterberg et al., 2001).