Structure of a Variscan migmatite-granite transition zone (N Sardinia, Italy)

ABSTRACT This paper presents the structural map of the Barrabisa Complex, an E-W ribbon-like migmatitic massif exposed in the northern part of Sardinia. The migmatites consist of metatexites and diatexites with variable melt proportion, grading in the central part of the complex to a foliated peraluminous granodiorite. Xenotime/monazite dating indicates that the foliated granodiorite pluton emplaced in upper Carboniferous times (≈307–313 Ma) within a narrow E-W ductile top-to-the east shear zone cutting through a Variscan high-grade metamorphic basement composed of orthogneisses, metasedimentary-derived migmatites and subordinate amphibolites. Field geological mapping, coupled with structural and micro-structural analysis, allowed us to distinguish the granodioritic pluton and three metamorphic units characterized by variable melt to protholith ratio, that represent a continuous transition from metatexite to diatexite with high melt concentration, to foliated granodiorite.


Introduction
Despite a few recent papers about the geodynamic evolution of the Corsica-Sardinia Batholith (Cocherie et al., 2005;Casini et al., 2012;Cuccuru et al., 2012;Casini et al., 2015b;Conte et al., 2017;Secchi et al., 2022), the architecture and internal geometry of most plutons is still largely unknown except in a few areas where detailed field geological mapping and geophysical imaging have been carried out (Casini et al., 2012(Casini et al., , 2015a;;Cuccuru et al., 2018Cuccuru et al., , 2016;;Secchi et al., 2022;De Luca et al., 2022).This paper focuses on the geometry and structural setting of the Barrabisa Complex, an E-W migmatite-granite transition zone exposed in the northern part of Sardinia (Figure 1; Figure Main Map).The complex consists of several migmatite types ranging from metatexite to diatexite (Casini et al., 2022).Migmatites wrap around a foliated dyke-shaped, 12-13 Km-long, peraluminous granodioritic intrusion, which forms the core of the Barrabisa Complex.Xenotime and monazite dating consistently indicate that the pluton formed in the upper Carboniferous , before the emplacement of the calcalkaline plutons precursor of the Corsica-Sardinia Batholith (Rossi & Cocherie, 1991;Paquette et al., 2003;Casini et al., 2015aCasini et al., , 2015b)).The geological map (scale 1:25,000) shows the fine details of the magmatic structure of the pluton, as well as the geometry of the transition zone between the granodiorite and its source zone, represented by the surrounding migmatitic complex.The information provided by field mapping have been integrated with structural analysis at the outcrop scale, microstructural analysis.The results presented in this paper implement the cartographic database of the Variscan crust of Sardinia, providing information on a key intrusion that represents one of the oldest precursors of the Corsica-Sardinia Batholith.

Geological setting
The Variscan belt is a Devonian-Permian orogen resulting from the subduction of the Rheic Ocean followed by oblique continental collision of Laurussia and a set of Gondwana-derived microplates (Stampfli et al., 2002;Von Raumer et al., 2003).The remnants of Variscan crust are discontinuously exposed throughout most of the central and southern Europe, from the Bohemian Massif to the Vosges-Black Forest, French Massif Central, the Alps and the Iberian Massif (Figure 1a).The southern branch of the orogen includes several small metamorphic massifs spread from the Pyrenees to the Maures-Esterel and the Corsica-Sardinia block, a microplate detached from continental Europe in the Oligocene (Alvarez, 1972).The Variscan crust of northern Sardinia (Figure 1b) consists mainly of amphibolite-facies orthogneiss and migmatites occasionally preserving relic metamorphic assemblages testifying a prograde high pressure (HP) granulitic evolution followed by exhumation and decompression-related melting (Giacomini et al., 2006(Giacomini et al., , 2008;;Casini & Oggiano, 2008;Cruciani et al., 2008Cruciani et al., , 2019)).According to recent studies, anatexis begun in lower Carboniferous in the kyanite stability field (0.9-1.1 GPa; Cruciani et al., 2019;Giacomini et al., 2006) and continues during decompression until the Carboniferous-Permian transition (Casini et al., 2022).The onset of anatexis is coeval with the development of a steep, apparently N-S to NNW-SSE-striking, S1 fabric marked by trondhjemitic to granodioritic quartz + plagioclase ± K-feldspar ± biotite leucosomes, axial planar of rootless F1 folds rarely preserved at the outcrop scale.This early migmatitic fabric is intensely deformed and almost completely transposed during a second melting stage at hightemperature and low pressure (LP) conditions (0.3-0.4 GPa, 720-750 °C; Casini et al., 2022), being still recognizable only within a few large, melt-poor and relatively strong, metatexitic orthogneiss domains such as those in the western tip of the Barrabisa Shear Zone (Figure 1b).The second melting stage is associated to the development of tight E-W to WNW-ESE-striking F2 folds associated to a locally pervasive moderately to steeply dipping S2 axial planar foliation and shear zones, frequently injected by quartz + plagioclase + K-feldspar + biotite ± garnet ± cordierite (Casini et al., 2022).Finally, the migmatitegranodiorite complex records a last deformation event evidenced by local development of N-S striking upright open F3 folds that refold the previous structures.The F3 folds are associated to the development of a weak S3 crenulation cleavage marked by retrograde muscovite + chlorite assemblages (i.e.Casini et al., 2022).The metamorphic basement is intruded by the Corsica-Sardinia Batholith (C-SB), a large magmatic-volcanic province formed from middle Carboniferous to late Permian in response to post collisional shearing and exhumation of the chain (Rossi & Cocherie, 1991;Paquette et al., 2003;Casini & Funedda, 2014;Edel et al., 2014;Casini et al., 2015b;Secchi et al., 2022).Most plutons exposed in northern Sardinia consist of either slightly peraluminous calcalkaline U2 granites and granodiorites, or metaluminous sub-alkaline gabbro-leucogranite sub-volcanic complexes (Ferré & Leake, 2001;Paquette et al., 2003;Cuccuru et al., 2012;Casini et al., 2015b).

Field structural analysis
Field structural analysis and detailed field mapping using an high-resolution digital elevation model (Database Geotopografico alla scala 1:10.000DBGT10k, Regione Autonoma della Sardegna) allowed us to distinguish three high-grade metamorphic units within the migmatitic complex and one magmatic unit, which represents the magmatic endmember of the Barrabisa Complex.The metamorphic units are characterized on the basis of their melt to protholith ratio (M, vol.%) as metatexites (M < 0.3 vol.%), relatively melt-poor diatexite (0.3 < M < 0.5) and, at higher melt proportion, massive melt-rich diatexite (0.5 < M < 0.9).The orientation of the metamorphic foliations S1 and S2, as well as that of F1, F2 fold axes recorded by melt-poor migmatites (i.e.sedimentary-derived metatexites and metatexitic orthogneiss) were then projected in stereoplots to show information about the statistical distribution of the pre-magmatic deformation structures.The shape preferred orientation of plagioclase phenocrystals and that of elongated metamorphic xenoliths is also projected in stereoplots to represent the pattern of magmatic flow trajectories within the granodioritic magma batch.Post-Variscan faults were first detected by remote sensing, then verified on the field to constrain the kinematics and the principal offsets accommodated by the fault network.

Microstructural analysis
A set of 73 polished thin sections were cut from 17 samples of granodiorite, 4 samples of metatexites and 15 samples of diatexites evenly distributed within the study area (Figure 1b).Given the general parallelism between the L2 mineralogical stretching lineation and the F2 fold axes, all thin sections are oriented parallel to the observable XZ plane of the finite strain ellipsoid defined by the macroscopic foliation S2 and F2 fold axes.Microstructural observations were determined through optical and scanning electron microscopy.

Cartographic units
All post-Variscan covers, mainly represented by Quaternary continental sedimentary sequences and Pliocene marine deposits, are poorly exposed in the study area and were grouped in a single lithostratigraphic unit (pvc).As the map focuses on the transition zone between the metatexite-diatexite migmatitic Barrabisa Complex and the central granodioritic pluton, the younger (post-310 Ma; Casini et al., 2015a) late Variscan granites exposed close to the study area (Figure 1b) have been distinguished into five intrusions without providing much details of their internal geometry and compositional variability, such as: (i) Arzachena pluton (Azn), (ii) Aglientu pluton (Agn), (iii) S. Teresa pluton (ST), (iv) Punta Falcone pluton, and (v) La Maddalena pluton (MDN).The Aglientu pluton and the Punta Falcone pluton are further distinguished into internal and relatively homogeneous magmatic units (Agn a and pf a , respectively) and marginal units characterized by either porphyry and magmatic breccias, indicated as Agn b and pf b , respectively.The main compositional and structural features of these plutons, as well as their U-Pb zircon ages, are described in Casini et al. (2012Casini et al. ( , 2015aCasini et al. ( , 2015b)).The main magmatic body of the Barrabisa Complex, identified as the central Granodioritic Unit (b4), is a rather homogeneous garnet + cordierite ± biotite-bearing fine-grained, foliated anatectic granodiorite with a high proportion of melt to protholith fragments (M > 0.9).The dominant characteristic recognized in the field is a pervasive foliation marked by the shape preferred orientation of biotite + muscovite and by incipient quartz ribbons (Figure 2a).This foliation is roughly parallel or slightly oblique respect the orientation of the pluton in map view, and the regional S2 foliation observed in diatexites.The two Diatexite units (b3, b2) are migmatites with high, though variable, melt to protholith proportion (0.9 < M < 0.3).These rocks show a range of compositions and have quite heterogeneous macroscopic fabric, also on a local scale (Figure 2b).Diatexites with higher M values (b3, 0.9 > M > 0.5), in fact, are compositionally homogeneous and show a pervasive S2 foliation parallel to the sub-magmatic foliation observed in granodiorites (Figure 2c).These characteristics make b3 diatexites quite similar to the granodiorite unit exposed in the central part of the massif, except for the more frequent occurrence, at places, of metatexite fragments (Figure 2b).On the other hand, at lower M values the S2 fabric is weakly developed and the composition changes depending on the dominant type of protholith fragments, being more felsic (quartz + feldspar + plagioclase ± biotite) where most of them are sourced from the orthogneiss, and relatively more femic (quartz + plagioclase ± garnet ± biotite ± cordierite) where the majority of fragments are provided by metasedimentary-derived migmatites and amphibolites.Diatexites with relatively low melt proportion (b2, 0.3 < M < 0.5) are compositionally similar to b3 diatexites; yet the S2 fabric is more heterogeneously developed varying from pervasive in places with relatively high melt proportion, to weak or not developed in melt-poor sites (M = 0.3 vol.%).In places, where the S2 fabric is less developed, the degree of preservation of the earlier S1 fabric is generally higher (Figure 2d).Finally, migmatites with low melt to protholith ratio M < 0.3 have been defined as metatexites (b1).These rocks are extremely variable ranging from fine-grained sedimentary-derived migmatites to migmatitic orthogneisses.Metatexites derived from sedimentary protholiths generally show thin (1-10 cm-thick) stromatic leucosomes alternating to 0.1-1 cm-thick biotite-rich melanosomes and thicker (5-50 cm-thick) mesosomes (Figure 2e).The migmatitic orthogneiss is a coarse-grained migmatite composed of 0.5-1 mm-thick quartz and feldsparrich layers alternating to up to 1 mm-thick biotite selvedges (Figure 2f).The contacts between granodiorites (b4) and the Melt-rich Diatexites (b3) are generally ductile and gradational, ranging in width from a few meters up to some tens of meters.Therefore, their position on the map should be considered as indicative and subject to about 10-50 m of uncertainty.Conversely, the contacts between metatexites and diatexites with lower melt proportion, as well as the contacts between the younger Variscan granites and the Barrabisa Massif are generally sharp.Regardless of their effective width, all contacts were marked by black continuous lines on the map (Figure 6).

Structure and deformation features
The Central Granodioritic Pluton (b4) and the meltrich b3 diatexites show a pervasive well oriented fabric defined by a magmatic to solid-state S2 foliation marked by the strong preferential orientation of biotite + garnet/cordierite schlieren, metamorphic xenoliths, and incipient quartz + plagioclase ribbons (Figures 3a and 4b).The poles to S2 foliation show two clusters about NNE and SW consistent with the mostly steep orientation of the fabric (Figure 4b).The L2 lineation marked by the preferred orientation of plagioclase phenocrysts, elongated metamorphic xenoliths, and A2 axis of highly non-cylindrical F2 folds, is nearly horizontal to gently dipping to ESE or W (Figure 4c-e).Therefore, the geometry of the magmatic to solid-state fabric in the main granodiorite body and diatexites is overall steep, suggesting a dyke-like shape of the melt-rich zone.Melt-rich diatexites (b3) show only a few remnants of a disrupted pervasive S1 fabric, which is preserved only within orthogneiss and metatexite fragments (Figure 3b).Conversely, this earlier S1 fabric is well developed in the marginal part of the Barrabisa Complex where the dominant rock types are melt-poor metatexitic orthogneisses and metatexites derived from sedimentary protholiths (Figure 3c).The orientation of S1 is highly variable changing from NE-SW nearly subvertical in the Punta Bianca locality, to NW-SE dipping toward NE at Cala Capra (Figure 6), in the eastern part of the study area (Figure 4c).The main brittle structures consist of either synmagmatic Variscan or post-Variscan faults.The first are thin pseudotachylite-bearing faults formed after S1 and before the S2 fabric (Figure 3d), as indicated by field relationships and geochronological constraints (Casini et al., 2022).The latter consist mainly of either NNW-SSE or NE-SW strike-slip faults and N-S to NE-SW steeply dipping normal faults (Figure 4h).The strike-slip faults formed in the Oligocene-Miocene accommodating the counter clockwise rotation of the Corsica-Sardinia block during the opening of the Ligurian-Provençal basin (Alvarez, 1972;Funedda et al., 2000;Oggiano et al., 2009).However, the systematic parallelism between faults and late Permian dykes dated between about 285 and 250 Ma indicates that Tertiary extension likely reactivated late Variscan structures (Gaggero et al., 2007;Casini et al., 2015a).A huge early Permian E-W normal fault running along the southern margin of the Barrabisa complex has been interpreted as related to post-orogenic extension of the Variscan crust (Casini et al., 2012).

Microstructure
Overall, metatexites have a gneissic-granoblastic microstructure.Metatexites derived from sedimentary protholiths are characterized by 1-5 cm-thick quartz + plagioclase + K-feldspar stromatic leucosomes alternating to 2-15 cm-thick biotite-rich mesosomes.Both the leucosomes and the mesosomes are roughly parallel to the regional S1 foliation recognized in the field.The boundaries between stromatic leucosomes and mesosomes are generally marked by a local increase of biotite yielding to the development of thin biotite + sillimanite/cordieritemelanosomes and biotite selvedges (Figure 5a).Common accessory minerals include ilmenite, apatite, zircon, monazite and titanite (Figure 5b).Metatexites derived from magmatic protholiths, namely orthogneisses, consist of thick (0.5-3 cm) granoblastic layers composed of quartz + plagioclase + K-feldspar and thin (< 1 mm) biotiterich bands.Large quartz and feldspar crystals show several intracrystalline deformation features such as subgrains with serrated to lobate boundaries, deformation lamellae, stress twins and kink folds (Figure 5c).Most big quartz crystals exhibit undulose extinction defining the typical chessboard pattern characteristic of high temperature (HT) crystal-plastic deformation (Kruhl, 1996).Thin recrystallized pseudotachylite veins are commonly observed within the orthogneiss.They mostly consist of polycrystalline small fragments of the orthogneiss set within a very fine-grained matrix formed by quartz + K-feldspar + plagioclase ± biotite (Figure 5d).Injection veins and dilatational step overs connecting adjacent pseudotachylites are frequently composed of large biotite crystal oriented at high angle to the fracture walls (Figure 5e).
Diatexites are more heterogeneous than metatexites and granodiorites.These melt-rich migmatites are composed of a fineto medium-grained quartz magmatic matrix (mean grain size 0.5-2 mm) and fragments derived from metatexites.The most common mineral assemblage consists of quartz + plagioclase + K-feldspar + biotite + garnet ± cordierite (Figure 5f).Accessory minerals include zircon + monazite ± xenotime ± titanite.The shape preferred orientation of idiomorphic biotite crystals marks a locally welldeveloped magmatic flow foliation parallel to the regional S2 observed in the field (Figure 6).Plagioclase and K-feldspar are generally sub-idiomorphic to interstitial showing oscillatory zoning and Carlsbad twinning (Figure 5f).Quartz is usually interstitial and shows extensive evidence for HT crystalplastic deformation such as chessboard pattern, deformation lamellae and subgrains, whereas all other minerals show only limited evidence for solid-state deformation.

Age of the pluton
Previous geochronological studies using different mineral phases such as xenotime, monazite and zircon show that the Barrabisa pluton (b4) formed almost coeval with diatexites units with high melt proportion (b3, b2).Xenotime dated by ID-TIMS indicates an age of 307.6 ± 1.4 Ma (MSWD = 0.18), which has been interpreted as the final crystallization of the pluton (Casini et al., 2015a).Two concordant monazite crystals separated from the same sample provided a slightly older age of 313.4 ± 5.1 Ma (MSWD = 0.015), which is interpreted as the onset of pluton assembly (Casini et al., 2015a(Casini et al., , p. 2015b;;Casini et al., 2022).The monazite age is undistinguishable, within errors, with the 302 ± 8 Ma LA-ICP-MS zircon age recently obtained from diatexites with highest melt proportion in the western tip of the Barrabisa shear zone (Casini et al., 2022).The ages provided by magmatic xenotime, monazite and zircon crystals are mostly overlapping  within errors, indicating that the emplacement age of the pluton is bracketed between about 306 and 318 Ma.

Conclusions
Field mapping allowed drawing an accurate geological and structural map of the transition zone between a migmatitic complex composed of melt-poor metatexites to melt-rich diatexites grading to a foliated granodiorite in the zones of higher melt concentration.The structural analysis of the granodiorite fabric and the metamorphic foliations preserved within migmatites provided robust constraints on the HT sub-magmatic flow trajectories and the relevant geometry of the pluton, which is comparable to a dyke-shaped body.On the other hand, the analysis of post-Variscan faults allow reconstructing the brittle deformation that modified the pluton shape during the post-Variscan tectonic evolution.

Software
The map was digitized using the software ArcGis v10.8.The geological sections have been drawn using the software FreeHand 10.0, and the stereonets have been plotted using the software Stereonet v.11 by Cardozo and Allmendinger (2013).

Figure 1 .
Figure 1.Geological setting: (a) schematic map of Variscan Europe (modified from Martínez Catalán et al., 2021, Cocco et al., 2023); CIZ -Central Iberian Zone, BM -Bohemian Massif, MC -French Massif Central, ME -Maures-Esterel, C-S -Corsica-Sardinia Massif, Ca -Calabria Massif, ECM -External Crystalline Massifs of the Alps, MGCR -Mid-German Crystalline Rise; the position of the study area is indicated by the black rectangle, (b) Structural scheme of the Barrabisa Complex showing the trend of sub-magmatic S2 foliation (red) and the pattern of magmatic foliation (blue) in the surrounding plutons.

Figure 2 .
Figure 2. Representative outcrops of the main cartographic units: (a) sub-magmatic flow foliation in foliated Barrabisa granodiorite (b4) unit, about 1.2 cm-wide pencil for scale; the white arrow marks an incipient quartz-feldspar ribbon, (b) contact zone between the two diatexite units (b3,b2) showing the local compositional variability of these rock units; the white arrows indicate fragments of metatexitic orthogneiss embedded within b3 diatexites, (c) melt-rich Diatexite Unit (b3) characterized by high melt proportion (M̴ 0.8); the dashed line marks the S2 foliation parallel to the sub-magmatic foliation of the foliated Barrabisa granodiorite (b4), (d) melt-poor Diatexite Unit (b2) showing pervasive S1 fabric, (e) F1 folds in leucosomes of a sedimentary-derived metatexite (b1); the white arrow marks a 2 cm-thick quartz-plagioclase + k-feldspar leucosome, (f) detail of the compositional banding in metatexitic orthogneiss, folded by an asymmetric F2 fold; the white arrow marks a quartz + plagioclase leucosome.

Figure 4 .
Figure 4. Orientation of structural data, lower hemisphere projection, Schmidt diagram: (a) contour plot of poles to S1 foliation, (b) contour plot of poles to S2 foliation, (c) L2 lineation resulting from the shape preferred orientation of magmatic plagioclase in b4 granodiorite unit, (d) L2 lineation resulting from the shape preferred orientation of metamorphic xenoliths in b4 granodiorite, (e) contour plot of F2 fold axes, (f) contour plot of F3 fold axes, (g) contour plot of poles to magmatic foliation in the plutons adjacent to the Barrabisa Complex, (h) post-Variscan faults.

Figure 5 .
Figure 5. Microstructure: (a) detail of melanosome in metasedimentary-derived metatexite (b1) showing a large cordierite crystal including thin needles of prismatic sillimanite which define the S2 foliation (dashed white line), plane polarized light (ppl), (b) close-up of a thin biotite selvedge from the S2 foliation of b1 metasedimentary-derived metatexite, including apatite and monazite as accessory phases (ppl), (c) close-up of a kink fold in a large plagioclase crystal, crossed polarizers (xpl), (d) electron backscattered image of a pseudotachylyte vein; the white dashed lines marks the boundaries of the pseudotachylyte vein, the solid white lines indicate two fragments of metatexitic orthogneiss, (e) detail of a thin biotite-rich pseudotachylyte vein, the white arrow indicates an injection vein (ppl), (f) microstructure of melt-rich b3 diatexite showing the typical mineralogical association of quartz (qtz) + biotite (bt)+ plagioclase (pl) + k-feldspar (kfs) + garnet (grt).