Geology of late-Variscan Sàrrabus pluton (south-eastern Sardinia, Italy)

ABSTRACT This paper deals with the geological mapping of the late-Variscan Sàrrabus pluton, (south-eastern Sardinia), a shallow multiple and composite igneous complex dominated by several generations of granodiorites, metaluminous and peraluminous granites and repeated pulses of mantle-derived mafic magmas. The map has been compiled based on geological surveys at 1:10,000 and 1:5,000 scales, assisted by in situ gamma-ray spectrometry and detailed petrographic investigations. Granite-related ore deposits have been also reported. The emplacement age of the pluton can be constrained by U/Pb dating on zircons of Cala Regina granodiorite, yielding an age of 286 ± 9 Ma. The resulting scenario documents a bimodal magmatism controlled by an EW trending shear zone, followed by the shallower emplacement of several pulses of independent granite magmas.

According to Conte et al. (2017), the southern part of the Sardinia batholith grew during two main magmatic peaks (Naitza et al., 2015), clustered around 305 Ma (Old Magmatic Peak, OMP) and 285 Ma (Young Magmatic Peak, YMP). A complex succession of tectonic and magmatic events marks these two stages of batholith growth and, at present, no single and generally accepted model exists to account petrogenesis and spatial/chronological relationships among its intrusive sequences.
Plutons emplaced within different tectono-metamorphic portions of the Sardinian basement show different geological features in terms of emplacement style and mantle/crust contribution. In the axial zone (N Sardinia), pre-300 Ma calcalkaline plutons are dominated by monzogranites; between 300 and 285 Ma, granodiorite volume increases. Conversely, the pre-300 Ma intrusives occurring in the Nappe Zone are dominated by granodiorites, while leucogranites represent the younger intrusions (Conte et al., 2017, and references therein).
Recent papers emphasized the control of lithospheric shear zones on the emplacement of major plutons outcropping in northern Sardinia (Casini et al., 2012;Casini, Cuccuru, Maino, et al., 2015;Cuccuru et al., 2018;Edel et al., 2014), and the involvement of meta-igneous and meta-pelitic crustal sources with subordinate mantle contribution (Bralia et al., 1982;Cocherie et al., 1994;Conte et al., 2017;Di Vincenzo et al., 1996Poli et al., 1989;Rossi et al., 2015;Tommasini et al., 1995). When referred to Frost and Frost (2011) petrochemical discrimination diagrams, the compositional shift from production of prevailingly magnesian magmas to ferroan, crustal-derived F-bearing magmas, marks the transition between the OMP and YMP. Overall, large areas of the batholith lack detailed geological and structural field mapping, along with dating on the single intrusions. In addition, mafic and acidic dike swarms spread at regional scale are still awaiting to be framed in the different pre-, syn-and post-magmatic stages of batholith growth. This paper deals with the Sàrrabus igneous massif, which is a wide (400 km 2 ) multiple and composite pluton located in south-easternmost part of the batholith. It shows a complex architecture, including uncommon suites (i.e. hastingsite granites) and, compared to other massifs at the scale of the Variscan belt, a large incidence of mafic dikes. Previous studies Conte et al., 2017;Poli & Tommasini, 1999;Ronca et al., 1999;Secchi & Lorrai, 2001) evidenced petrogenetic processes involving repeated production of felsic magmas from meta-igneous sources at different crustal levels, and production of mantle-derived magmas. A wide network of fluorite-barite and Pb-Ag sulfide hydrothermal veins, mostly hosted in the metamorphic basement close to the contact with the intrusives is also spatially and genetically associated with the Sàrrabus pluton.
Within this scenario, the geological map represents a mandatory basic tool for disclosing the tectono-magmatic history of the massif. In this regard, some unsolved issues mainly concern the discrimination within different intrusive units of similar composition and internal petrographic and structural features. Mapping 40 K, 232 Th and 238 U abundances through a portable, high-sensitive gamma-ray spectrometer have been successful in discriminating granite pulses in northern Sardinia (Kaçeli Xhixha et al., 2016;Puccini et al., 2014) and was employed to improve the map detail. The geostatistical processing of more than 100 in situ measurements better allowed the distinction among units and was useful in the reconstruction of the architecture of the Sàrrabus igneous massif, providing a geological-structural map at 1:50,000 full scale realized with a multi-methodological approach.

Petrographic studies
Geological field surveys were coupled with a detailed petrographic study on more than 500 thin sections.

Remote sensing
A Digital Elevation Model (DEM) was created from a mosaic of 1:10,000 topographic maps. The DEM was integrated with geo-referenced high-resolution aerial orthophotos (property of Regione Autonoma della Sardegna) and IKONOS 2005 (Regione Autonoma Sardegna, 2005) satellite images. Detailed information about correlated errors, spatial resolution and image accuracy, are reported in Virdis et al. (2012). Results were processed to boost geomorphological features related to dikes, major faults, joints networks, and lithological contacts within the composite pluton, deduced by different fracture patterns as well as different weathering styles. Data were finally processed in GIS environment, overlayed and integrated with geological data and converted into digital maps.

Structural data
Magmatic flow-trajectories in granodiorites were deduced by the preferred orientation of microgranular dark enclaves, feldspar megacrysts in the inequigranular facies as well as aplo-pegmatitic veins, stretching lineation was deducted by major axis of enclaves and K-feldspar orientation. Poles to foliation were projected in conventional stereoplots to document the statistical distribution of magmatic foliation and to constrain the resulting geometry of the pluton and the evolution of emplacement mechanisms for various intrusive units.

Gamma-ray spectrometry
The occurrence of different coalescing intrusions in the narrow monzogranite-leucogranite compositional range, and the abundance of colluvium covers and regolith required an improvement of geological surveys by in situ gamma ray. Specific activities of 40 K, 238 U and 232 Th were measured using a portable gamma-ray spectrometer equipped with a 1-liter NaI (Tl) scintillator, in more than 100 representative outcrops (Table 1) of magmatic and surrounding metamorphic rocks, according to the procedure reported in Puccini et al. (2014) andKaçeli Xhixha et al. (2016). The uncertainty of the method is estimated to be 1.5%, 3% and 3% for 40 K, 238 U and 232 Th, respectively (Caciolli et al., 2012). Data were interpolated in GIS environment with the Inverse Distance Weighting algorithm in order to realize three radionuclide maps.

Geochronology
A representative sample of Cala Regina Unit granodiorite from Capo Carbonara area was selected for in situ U/Pb zircon dating to compare U/Pb ages with previously published chronological data obtained in the same locality by K/Ar and Rb/Sr internal isochrones by Nicoletti et al. (1982).
Zircons were analyzed both by 100-μm thin section thick or extracted from 3 to 4 kg of sample using conventional separation techniques. Final handpicked crystals were mounted in epoxy resin and examined with cathodoluminescence imaging using a Nova Nano SEM 450 SEMFEG equipped with a CLD KE stand-alone Centaurus detector. U/Pb isotopes were determined using a quadropole ICP-MS X Series II (Thermo Fisher Scientific) coupled to a 213 nm Nd:YAG laser ablation system (New Wave Research™) at the laboratories of the Centro Interdipartimentale Grandi Strumenti of the Università di Modena e Reggio Emilia following the analytical protocol and data processing by Giovanardi et al. (2018). U-Pb ratios were corrected for laser-induced elemental fractionation using zircon reference material TEMORA-2 (average 206 Pb/ 238 U of 0.66719 ± 0.00348 for n = 12; literature accepted value of 0.66710).
U/Pb obtained data have been compared to a Rb/Sr isochron obtained for selected granodiorite samples from Cala Regina Unit. Sr isotopic compositions were carried out at IGAG-CNR laboratories of University of Rome 'La Sapienza' (Rome, Italy) using a FINNI-GAN MAT 262RPQ multi-collector mass spectrometer with W single and Re double filaments in static mode. Rb and Sr elemental concentrations were determined at Activation Laboratories, Ancaster, Ontario, Canada by ICP-MS techniques with accuracy of ±5%.

Field relationships
Eight intrusive units were recognized ( Figure 1); from older to younger, the emplacement succession is reported as follows, reconstructed on the basis of common geological indicators (intersections, roof pendents, chilled margins, inclusion trains): Burcèi Unit (gabbrotonalites; BU) → Monte Cresia Unit (granodiorites grading to monzogranites; MC) + Monte Nai Unit (granodiorites grading to monzogranites; MN) → Cala Regina Unit (granodiorites; CR) → Monte Maria Unit (peraluminous leucogranites + Bruncu Nicola Bove Unit (monzogranites grading to leucogranites; BNB) → San Priamo Unit (leucogranites; SP) → Monte Sette Fratelli Unit (monzogranites grading to leucogranites; SF). Figure 2 documents some outcrop features and relevant emplacement relationships recognized in the field, as the Monte Maria/Cala Regina intrusive contact (Figure 2(a)) and the close mafic/felsic implication in the Cala Regina Unit. Figure 3 displays the sharp macroscopic textural variations of mapped rock-units. Moreover, the whole data set of macroscopic features and petrographic data of mapped rock-units is summarized in Table 2.
In the following, the architecture of the pluton and the relationships between the mapped rock-units will be discussed first. Schematically, the pluton is made up of granodiorites (Cala Regina, Monte Nai and Monte Cresia Units) intruded by EW trending monzogranites and leucogranites (Bruncu Nicola Bove and San Priamo Units) with a granodiorite/granite 1/1 field ratio; Monte Cresia and S. Priamo Units are in turn intruded by Monte Sette Fratelli Unit granite, which represents the last intrusive event ( Figure 4).
Looking at the granodiorites, the Monte Cresia Unit outcrops discontinuously at the northern edge of the pluton, with flat contacts with the basement. It outcrops southward as a continuous belt over the Bruncu Nicola Bove Unit. The Monte Cresia/Cala Regina Units relationships are less visible, but the local occurrence in the Cala Regina Unit of chilled margins is an evidence of its later emplacement. The Monte Nai Unit does not show univocal emplacement relationships with the Monte Cresia/Cala Regina Units, making it difficult to establish their intrusion sequence. Overall, the granodioritic units display definite heterogeneities in geological style and petrographic features (Table 2), further confirmed by gamma-Ray measurements ( Table 1). The Cala Regina Unit granodiorites exhibit a general flat trend evidenced both by magmatic foliation and by pronounced internal grain size variations, from medium-grained foliated rocks with a high incidence of flattened dark enclaves, to fine-grained porphyritic facies with pegmatitic pods indicating the top of the intrusion, further defined by small roof pendants of hornfelsed siliciclastic rocks. K-feldspar megacrysts granodiorites prevail in the south-eastern sector. Conversely, the Monte Cresia Unit commonly displays homogeneous equigranular textures, with rare and rounded dark enclaves. The Monte Nai Unit shows textural and compositional internal variations, ranging from granodiorite to monzo-and leucogranite. Except for Burcèi gabbrotonalites which based on geochronological constraints (see below) predates the growth of the pluton, mafic intrusions consist of stretched and dismembered synplutonic dikes and stocks of gabbroic rocks (Solànas complex in the map; SO) that crop out for more than 10 km along a narrow EW mingled belt within the Cala Regina granodiorite Secchi & Lorrai, 2001; Figure 2(b,c)). Na-rich fluid flows conveyed along this belt favored metasomatic effects in the cooling granodiorite, locally producing Na-metasomatized episyenitic rocks (S in the map) occurring as homogeneous, small, isolated bodies. Episyenites also form roughly layered bodies at the top of the Cala Regina intrusion and are present, more locally, in the Monte Cresia Unit. The whole pluton is crosscut by several generations of late mafic and felsic dikes. According to Conte et al. (2017) the incidence and thickness of mafic varieties strongly decrease from granodiorites to leucogranites. Felsic dikes include earlier NE trending peraluminous garnet-bearing two-micas granites (Figure 2(d)) crosscut by NNW trending mafic dikes (mostly spessartites), frequently associated with metaluminous microgranite dikes. Remarkably, the latest intrusive units (San Priamo and Sette Fratelli) are crosscut by olivine-bearing plagiophyric dikes ('tholeiitic dolerites ', in Ronca et al., 1999). Finally, NNW trending granophyres and rhyolitic dikes and stocks crosscut the whole pluton from Punta Molentis to Burcei and represent the end of Variscan magmatism in the region.  Table 1 and plotted in the density maps ( Figure 6) confirm the granites/granodiorites areal distribution, depicting a sharp difference between a much more radiogenic northern part of the pluton, dominated by leucogranites, and a less radiogenic southern part, in which granodioritic rock-units prevail. Likewise, the more radiogenic nature of the granodioritic/monzogranitic Monte Nai Unit allows to discriminate it with respect to the granodioritic Cala Regina and Monte Cresia
Units, while the peraluminous satellite intrusions of the Monte Maria Unit emerge as higher radiogenic spots within the southern low radiogenic part of the pluton.

Petrography and geochronology
Main petrographic features of Sàrrabus igneous units are outlined in , Conte et al. (2017),  and summarized in Figure 7 and Table 2. Overall textural and mineralogical features, as well as chemical signatures, confirm the discrimination of different intrusive units recognized in the field and was used to support mapping. Petrographic differences observed in the granodioritic rock-units regard mainly the increasing (a) size and content of dark enclaves (from tonalitic to hornblende quartz gabbros), (b) color index (from 14% to 20%), as well as (c) incidence of primary hornblende (1%-4%) from Monte Cresia and Monte Nai Units to Cala Regina Unit, respectively. Plagioclase with relic calcic cores commonly observed in granodioritic rocks are locally documented in Nonte Nai granodiorites (Figure 6(a)).  Conversely, a wide range of petrographic variations is observed for felsic rock-types in terms of mafic mineralogy, feldspar composition and accessory phases (Figures 6(b-d)). Bruncu Nicola Bove Unit ranges from biotite monzogranite to leucogranite; these rock-types are characterized by a normal oligo-albitic plagioclase (An 30-15 ). The San Priamo Unit is made up of biotite leucogranite showing a more calcic plagioclase (An 40-26 ; Conte et al., 2017) and large allanite grains as typical accessory mineral. The Monte Maria Unit granites are garnet-bearing two-mica varieties, whereas the Monte Sette Fratelli Unit is characterized by the unusual early Fe-hastingsitic amphibole, as well as by annitic dark mica, fluorite and fayalite as late-stage phases (Conte et al., 2017;Secchi & Lorrai, 2001).
Mafic intrusions and mafic dikes show similar petrographic fingerprints (Figures 7(e,f)). According to Brotzu et al. (1993), Burcèi Unit ranges from mesocratic gabbroic rocks to tonalites characterized by the common occurrence of ortho-and clinopyroxene, reddish biotite and only ilmenite as Fe-oxide phase (Figure 7(g)). Conversely, dismembered mafic masses in Cala Regina Unit (SO complex) are hornblende gabbroic rocks with relics of orthopyroxene followed by clinopyroxene, grading to hornblende quartz gabbros. The SO complex also comprises olivine-bearing gabbronorites and leuco-gabbros with cumulate textures Secchi & Lorrai, 2001). Remarkably, an orthopyroxene → clinopyroxene succession replaced by amphibole, is also frequently observed in the spessartitic mafic dikes.
Geochronological data obtained with different systematics indicate coherent ages. New U/Pb zircon dating of the Cala Regina Unit granodiorite sampled in Capo Carbonara yield an age of 286 ± 9 Ma ( Figure  8(a)), which is undistinguishable from a Rb/Sr isochron obtained from selected Cala Regina Unit granodiorite samples, which yield an age of 292 ± 17 Ma (Figure 8(b)). Remarkably, the obtained values are undistinguishable from those found by Nicoletti et al. (1982) by two conventional K/Ar internal isochrons for granodiorites from Capo Carbonara, which gave ages of 285 ± 11 and 286 ± 5 Ma.
More regional constraints are provided by the Rb/ Sr geochronological data reported in Ronca et al. (1999) for garnet-bearing peraluminous dikes, giving ages in the range of 281-293 ± 3 Ma. Based on the whole available data set, the U/Pb value assume the role of emplacement age of the entire pluton.
In this reconstruction, the Burcèi Unit belongs to an earlier ilmenite rock-series predating the growth of Sàrrabus igneous massif, also recognized in the Arbus igneous complex of southern Sardinia (Secchi et al., 1991). Besides the compositional affinities, a further constrain in favor of a single early mafic event framed into the OMP of southern Sardinia (Conte et al., 2017), is provided by the Rb/Sr age of 311 ± 9 Ma obtained for Burcèi gabbrotonalites (Brotzu et al., 1993), which is undistinguishable, inside the correlated error, from the age of Arbus intrusives for which a U/Pb value of 304 ± 1 Ma is reported .

Structural field evidence
The Sàrrabus pluton emplaced in lower Cambrian to Silurian anchimetamorphic metasediments at very shallow crustal levels in the frontal part of Variscan Nappe Zone (Carmignani et al., 1994) of the Sardinian basement. Shallow crustal emplacement (about 6 km) is constrained by: (1) the occurrence of a narrow (less than 200 m wide) andalusite-cordierite-dark mica   Table 2. aureole in the host basement along with the intrusive contact; (2) the Al-in hornblende barometer, which yields values close to 2 kb in the Cala Regina granodiorite, and up to 1 kb in the Monte Sette Fratelli granite Conte et al., 2017;. The contacts with the country rocks are widely exposed along to the northern boundary of the pluton; small remnants of the basement rocks locally occur along the Southern coastline (Geremeas) and may testify a possible southern closure of the pluton. Different emplacement trends characterize the different units: three units, Monte Cresia S. Priamo and Bruncu Nicola Bove show reciprocal intrusive contacts parallel to the contact between the older intrusion (i.e. the Monte Cresia granodiorite) and the anchimetamorphic Palaeozoic metasediments.
A switch towards EW direction characterizes the steep contact between Cala Regina and Bruncu Nicola Bove units. The Cala Regina granodiorite in its central part exhibits a marked planar anisotropy defined by dark enclaves and dark micas. Along the southern coastline, this foliation abruptly steepens along an EW trending belt strongly enriched in mafic-intermediatein place highly stretchedmicrogranular enclaves (Figure 2(c)), synplutonic mafic blobs of different size, stretched at different degrees, which are encircled by elliptical to ribbon-like -straight to convolute - (Figure 2(b)) mafic dispersions. These features are suggestive of a shear enhanced mingling between different magmas as commonly occurs in calcalkaline plutons (Barbarin & Didier, 1992;Valle Aguado et al., 2017;Zorpi et al., 1991).
The association of mingled mafic rocks and highly strained mafic enclaves along a 2 km-wide belt are evidence for an important EW trending synplutonic shear zone; hereafter South Sarrabus Shear zone (SSSZ). This shear belt hosts episyenite bodies, which displays deformational microtextures such as quartz subgrains (Pirinu et al., 1996), and ectometric slices of monzogranitic granodiorites with strongly foliated fabrics suggestive of sub-magmatic to solid state flow. The long axis of the stretched enclaves and the lineations defined by the K-feldspar megachrists in the monzogranitic granodiorite systematically plunge down deep (Figure 2(d)). These features are suggestive of a vertical flow along a dilatant shear zone where the Cala Regina granodiorite is possibly rooted at depth as a product of mixing processes sustained by a thermal input able to lower the viscosity contrast between felsic and mafic magmas (Georgiev et al., 2009;Prabhakar et al., 2009). At shallow levels, the temperature lowering resulted in mingling and localized hybridization generating tonalitic sheets. The intrusion inflation exceeding the dilation rate was mainly accommodated by upwards and lateral emplacement at upper crustal levels within a fractured crust. This accounts for flat foliation away from SSSZ. The occurrence of repeated, variably mingled, mafic injections observed Cala Regina Unit, indicates that the SSSZ acted as the feeder of this unit.
Changes in stress coordinates, are constrained during the complete cooling and partially uplifted igneous massif (Martinez-Poza and Druguet, 2016), which control the late emplacement of shallower peraluminous intrusions (i.e. Monte Maria Unit) and the later bimodal dike swarm.

Ore deposits
In the mapped region, granite-related ore deposits and occurrences are hosted in low-grade metamorphics of the Variscan basement and, in minor amount, in the northernmost intrusive units of Sàrrabus igneous massif. They may be grouped (Funedda et al., 2018;Naitza et al., 2015) as: (a) Zn-Cu-Pb sulfide skarns and (b) F-Ba ± Pb-Ag-Cu-Zn vein systems. Only the (b)-type ores are reported in the map and here examined, as they appear closely involved with the petrogenetic processes of the granitoid suites of the Sàrrabus massif (Conte et al., 2017). The fluorite-barite ± Pb-Ag-Cu-Zn sulfide hydrothermal vein systems develop along the EW and NNW structural patterns (Valera, 1974). EW silver-rich polymetallic veins form a narrow swarm (the so-called 'Sàrrabus silver lode') that runs alongside the northern borders of the igneous massif, up to 4 km from intrusive contact; they mainly follow major fault lineaments in the basement and locally crosscut the Monte Cresia Unit granodiorites. The NNW trending sulfide-poor and fluorite-barite rich veins widely occur outside the northern edge of the pluton (Valera, 1974), but a noticeable cluster is recognized in the western part of the area, forming a mineralized NW belt that runs for over 4 km from the Sette Fratelli Unit intrusion to the Monte Genis granite and, still further, towards the Silius mine district, which represents one of the largest fluorite vein deposit in Europe. The closefield association and the geochemical affinity of the ores with the fluorite-bearing granites that form the northern part of the Sàrrabus igneous massif (i.e. San Priamo and Monte Sette Fratelli Units) clearly indicate a genetic linkage (Castorina et al., 2020;Conte et al., 2017). The whole southern sector of the Sàrrabus pluton is virtually devoid of ore mineralization, as expected by the lack of F-bearing granites (Main Map).

Conclusions
The exposed geological map at 1:50,000 scale portrays a circumscribed multiple and composite Variscan pluton, characterized by a decreasing interaction between mafic and felsic magmas, resulting in the emplacement of several intrusive units at shallow crustal levels. The whole data set is consistent with fast growth of the pluton during the granodioritic stage (i.e. Monte Cresia, Monte Nai and Cala Regina Units). In this stage, granodiorites are generated either as magmas of possible lower crustal anatectic origin (Monte Cresia, Monte Nai) and as magma related to mantle/crustal interaction (Cala Regina Unit), suggesting a zoned reservoir at depth. After the emplacement of the Cala Regina unit, the felsic activity increases, as testified by Bruncu Nicola Bove and S.Priamo granites ( Figure  9). The growth stage ended with a voluminous mafic and felsic activity represented by swarms of NNW dikes, whose frequency and thickness are higher in the Cala Regina Unit. F-bearing granites, such as Monte Sette Fratelli unit, seal up the -N70 trending contact between Monte e Cresia and S.Priamo units. These intrusives represent the late stage of the pluton growth. The major hydrothermal vein systems and ore deposits of the region may be related to this late stage.
Ascribing the sequential growth of the Sàrrabus granitic massif to a well-defined mechanism, in the lack of well-exposed boundaries with the host Variscan basement, should be largely hypothetical. Nevertheless, considering that in the early Permian the crustal sector of Sardinia was involved in the intra-Pangea megshear system during its early Permian rotation (Aubele et al., 2014;Edel et al., 2014), several mechanisms related to this strike slip dynamic can account for the generation of dilational zones bounded by normal fault and normal shears. Among these, the emplacement at dilational step over (El Desouky et al., 1996) or at termination of strike slip faults  Nicoletti et al. (1982) and Poli and Tommasini (1999); open symbols after . Plotted data include 2σ decay constant errors.   Valle Aguado et al., 2017) are compatible with the late Carboniferous-early Permian wrench faults network (e.g. the dextral NS trending Quirra Fault) that affects the Sardinian Variscan Basement.

Software
Overlaying of maps, geostatistical interpolations and digitalization of the final maps were carried out using Esri Arc GIS 9. Field spectrometric analysis was performed using jRadview. The isotopic composition of monazite grains was analyzed using the Glitter package.