Metamorphic evolution and P–T path of the Posada Valley amphibolites: new insights on the Variscan high pressure metamorphism in NE Sardinia, Italy

The Posada Valley amphibolites in the inner zone of Variscan Sardinia are characterized by garnet porphyroblasts containing epidote, titanite, quartz, clinopyroxene, amphibole, and plagioclase concentrated in the garnet core. The matrix is made up of amphibole and plagioclase and finegrained clinopyroxene and plagioclase. The amphibolites recorded an HP stage, a metamorphic re-equilibration, and a third stage under the greenschist facies. P–T conditions of stage I (530– 650 °C/0.9–1.3 GPa for garnet core and 570–690 °C/1.0–1.4 GPa for garnet rim) were obtained by P–T pseudosection modelling. The P–T conditions for stage 2 (T = 600–700 °C/P = 0.5–0.8 GPa) were obtained by applying the same approach to the most retrogressed samples. The Posada Valley amphibolites underwent a cooler evolution, with a thermal peak lower than that recorded by the northward eclogites from the high-grade metamorphic complex of Sardinia. The proposed geodynamic scenario starts with the subduction of oceanic crust under the periGondwanan terrane in Upper Devonian. The Posada Valley amphibolites were subducted up to maximum depths of 50 km. Subsequently, they reached the subduction channel or were detached from the slab before the eclogite facies conditions were reached.


Introduction
Retrogressed eclogites are widespread in the internal zone of the Variscan chain of Sardinia within the low-to medium-grade metamorphic complex (L-MGMC) and in the high-grade metamorphic complex (HGMC) [also known as migmatite complex; Carmignani et al., 2001].
In the high-grade metamorphic complex hectometre-size lenses of eclogites crop out at Golfo Aranci, Punta Tittinosu, and Punta de li Tulchi (Figure 1(a)).Eclogites of the HGMC are characterized by the occurrence of omphacite relics.The eclogite bodies from the L-MGMC occur as decametric-long lenses inside gneisses and mylonites of the Asinara Island, Giuncana, and the Posada Valley (Figure 1(a)).However, only the metabasite rocks from Giuncana [Cortesogno et al., 2004, Cruciani et al., 2015a] preserve true relics of eclogite facies assemblages (i.e., omphacite).Although the shape of the P -T path is similar between the L-MGMC and HGMC eclogites, a significant difference in temperature (and pressure) has been observed [Cruciani et al., 2012].The other metabasites from the L-MGMC are still considered eclogites, as well, due to their position along the Posada-Asinara shear zone (Figure 1(a)).The protoliths of Sardinian eclogites are mostly midocean ridge basalt (MORB)-type tholeiites [Cappelli et al., 1992], with the exception of the metabasite from Punta Orvili that exhibits an alkaline affinity [Cruciani et al., 2010], considered by many authors as part of the oceanic crust subducting under the peri-Gondwanan terranes (previously accreted to Laurussia) during the Variscan orogeny.
Thus, the northern part of the medium-grade metamorphic complex is featured by the occurrence of metabasite lenses commonly defined as retrograde eclogites but never studied in depth from a petrographic and metamorphic (P -T evolution) point of view.In this paper we present a detailed petrological investigation of the Posada Valley amphibolites in order to reconstruct their metamorphic history and P -T path through P -T pseudosection modelling.The results will be compared with metamorphic rocks of the inner zone of the Sardinian basement and finally discussed in the geodynamic scenario of the late-Palaeozoic Variscan continent-continent collision.

Geological setting
The Sardinian Variscan basement consists of metaigneous to metasedimentary sequences ranging in age from the Cambrian to the lower Carboniferous [Carmignani et al., 2001].This basement was only slightly affected by the subsequent Alpine Orogeny, and therefore preserves Variscan deformation and metamorphism.
The Sardinian chain has been subdivided into a southern external zone, a central Nappe zone, and an inner (or axial) zone in the northernmost part of the island, which extends to southern Corsica [Faure et al., 2014, Massonne et al., 2018, Cruciani et al., 2021].For the geological, palaeogeographic reconstruction and the correlation of the Variscan Corsica-Sardinia block with the Maures-Tanneron Massif, the reader is referred to Rossi et al. [2009], Faure et al. [2014], Schneider et al. [2014], and Gerbault et al. [2018].In the external and Nappe zones, the basement rocks consist of metasedimentary sequences with middle to late Ordovician volcanics [Cruciani et al., 2013a[Cruciani et al., , 2016]].The arrangement of rock units from the Nappe zone derived from a southward thrusting with km-scale isoclinal folding and syntectonic metamorphism [Carmignani et al., 1994].The northern part of the inner zone (HGMC) mainly consists of polydeformed sedimentary-derived gneisses and migmatites [Massonne et al., 2013, Cruciani et al., 2014, Fancello et al., 2018], which reached an high metamorphic grade up to the sillimanite + Kfeldspar zone [Franceschelli et al., 2005].Within the HGMC, subordinate rocks like orthogneisses, calcsilicate nodules and metabasite lenses (basic and ultrabasic rocks with eclogite-and granulite facies relics, Franceschelli et al., 2005, 2007, Cruciani et al., 2011, 2015a,b, Scodina et al., 2019, 2020) also occur.The southern L-MGMC is made up of micaschists and paragneisses locally with relics of HP assemblages [Cruciani et al., 2013b] and with sporadic quartzite and metabasite bodies [Cappelli et al., 1992, Cruciani et al., 2010, 2015a].The metasedimentary rocks reached greenschist-to amphibolite facies conditions and are characterized by an increasing metamorphic grade from south to north [Franceschelli et al., 1982, 1989, Cruciani et al., 2015b].Near the contact between the HGMC and L-MGMC, there is a sharp increase in metamorphic grade, and the Barrovian isograds are very tight [Carosi et al., 2005].
The boundary between HGMC and L-MGMC, well exposed in the Posada Valley, southern Gallura, and Asinara island, is marked by a dextral transpressional shear zone known as the Posada-Asinara shear zone (PASZ), (Figure 1(a)).U-(Th)-Pb analyses on monazite from the PASZ indicate that the shear zone has been active at ∼325-300 Ma in a transpressive setting, in agreement with the ages of the other dextral transpressive shear zones in the southern Variscan belt [Carosi et al., 2020].Similar ages (∼323 Ma) obtained by monazite petrochronology, mark the onset of transpression of the Cavalaire fault in the neighbouring Maures-Tanneron Massif [Simonetti et al., 2020].The PASZ has been correlated to the Grimaud fault in the Maures-Tanneron massif [Edel et al., 2018 and references therein].
The inner zone of northern Sardinia underwent a polyphase ductile deformation [Carosi et al., 2005, Elter et al., 2010].The oldest deformation phase (D1), dated at ∼345-340 Ma by Di Vincenzo et al. [2004], is well preserved in some areas of the L-MGMC and is related to the collisional and thickening stage [Carmignani et al., 1994], which resulted in HP metamorphism, with metapelites reaching pressures of 1.7 GPa [Cruciani et al., 2013b].Di Vincenzo et al. [2004] also found apparent 40 Ar-39 Ar ages of 320-315 Ma for most syn-D2 white mica from the garnet zone of the MGMC.In the HGMC, the D1 deformation is sporadically documented by the transposition of centimetre-sized leucosomes in the migmatites.The D2 deformation phase, which progressively transpose the S1 foliation towards the HGMC, is related to the development of NE verging folds and orogen-parallel dextral shear zones in the field [Carosi and Palmeri, 2002].The resulting transpressional regime was responsible for most of the exhumation of high-grade metamorphic rock units with initial increasing temperature followed by decompression and cooling [Carosi et al., 2009, Di Vincenzo et al., 2004].In the HGMC, two opposite senses of shear (top to the NW and top to the SE/NE) on the S2 foliation have been interpreted by Corsi and Elter [2006] as being associated with the end of compression/crustal thickening (the first, top to the NW) and with tectonic inversion during the exhumation of the metamorphic basement (top to the SE/NE).D3 deformation is characterized mainly by metric chevron and/or asymmetric folds associated with an S3 axial plane crenulation cleavage.The D4 tectonic phase forms metric to decametric-scale folds with subhorizontal axial planes.It is not associated with the development of foliations and object lineations.The metamorphic basement of Sardinia was finally intruded by the Permo-Carboniferous batholith emplaced between ∼320 and 280 Ma [Carmignani et al., 2001, Casini et al., 2015, andreferences therein].

Field geology
The study area is located in the Posada Valley, in the northernmost part of the L-MGMC, along the PASZ (Figure 1).In this area, from Punta Figliacoro to Pedra su Gattu, the metamorphic complex consists of metabasite, orthogneiss, paragneiss, mylonite, and micaschist (Figure 1 Elter, 1989], outcrop as bodies and/or lenses ca.300 m long and 100 m wide within the mylonites.These lenses follow a W-E direction (Figure 1(b)) and corresponds to small hills inside the valley, due to their higher resistance to erosion as compared to the surrounding mylonites.The geological and structural relationships between the different rocks cropping out in the Posada Valley had been described since Elter et al. [1990, their Figure 7, p. 398].The metabasites at Pedra su Gattu are slightly foliated rocks characterized by an irregular and discontinuous alternance of garnet-rich and garnet-poor layers (Figure 2).The most foliated portions display dark-green, amphibole-rich layers alternating with whitish, plagioclase-rich ones.Garnet porphyroblasts (up to a maximum of 1-2 cm in size) are observed in both layers (Figure 2 The mylonitized schist and gneiss that surround the metabasite bodies derive from igneous and sedimentary protoliths which underwent intense deformation and shearing.They show a prominent planar anisotropy that results in a penetrative schistosity in which the only visible mineral is muscovite or, more rarely, biotite.

Analytical methods
An FEI Quanta 200 scanning electron microscope (SEM), equipped with a nitrogen free Thermofisher EDS-X-ray detector at Centro Servizi d'Ateneo per la Ricerca (CeSAR) of Cagliari University, was used for microstructural investigation.Mineral chemistry was determined with a fully automated JEOL 8200 Super Probe at Dipartimento di Scienze della Terra "Ardito Desio", Università di Milano.Operating conditions were 15 kV accelerating voltage, beam current of 15 nA, and 5-10 µm variable spot size.Natural and synthetic wollastonite, olivine, corundum, magnetite, rutile, orthoclase, jadeite, pure Mn, pure Cr, fluorophlogopite, and barite were used as standards.Selected samples were cut in thin slices and then pulverized for major elements whole-rock analyses, which were performed at ALS-Laboratories (Seville) by X-ray fluorescence (XRF) after lithium-borate fusion.P -T pseudosections were calculated using the software package PERPLE_X [Connolly, 1990[Connolly, , 2009] ] and the thermodynamic data set of Holland andPowell [1998, updated 2011] for H 2 O and minerals.

Petrography
Four samples from Pedra su Gattu area were selected: two garnet-rich amphibolites (U1 and U4), and two garnet-poor amphibolites (U5 and U33), the latter two representing retrogressed domains.Garnet-rich samples (U1, U4) are characterized by the presence of subhedral to euhedral porphyroblastic garnet (Figures 2(c), 3(a)).The garnet porphyroblasts are occasionally concentrated along layers parallel to the main foliation in the amphibolites.Garnet contains several inclusions of epidote (Ep 1a ), titanite (Ttn 1a ), quartz, and euhedral grains of amphibole (Amp 1a ) and plagioclase (Pl 1a ) (Figures 3(b, c)), mostly concentrated in the garnet core (Figure 3(a)).Trace amounts of clinopyroxene (Cpx 1a ) have also been found as inclusions in garnet.Chlorite can occur as a filling phase mineral inside the cracks.Garnet porphyroblasts are surrounded by a sub-millimetric thin rim of plagioclase (Pl 2a ), commonly sericitized, and minor amphibole (Amp 2a ) (Figure 3(d)).The rock matrix is mainly made up of anhedral crystals of amphibole (Amp 2b ) and plagioclase (Pl 2b ) and small flakes or trails of fine-grained clinopyroxene (Cpx 1b ) and plagioclase (Pl 1b ) (±amphibole; Amp 1b ), elongated parallel to the regional schistosity (Figures 3(e,  f)).Plagioclase from the matrix (Pl 2b ) can be arranged in trails of coarse-grained crystals along the S2 foliation.Amphibole from the matrix (Amp 2b ) is locally and partially replaced with a late-stage amphibole (actinolite, Amp 3 ).In the proximity of garnet, the elongation of amphibole follows the direction of garnet edges.The rock matrix contains abundant titanite (Ttn 2b , up to 3 mm in size), occasionally in association with ilmenite, and, more rarely, with albite.Other minerals in the matrix are epidote, quartz, chlorite, apatite, prehnite, biotite, and trace amounts of rutile and K-feldspar.Zircon is accessory mineral.The garnet-poor samples (U5 and U33) are characterized by abundant amphibole and plagioclase oriented parallel to the regional S2 schistosity.Rare garnet relics are replaced by amphibole and chlorite.Other minerals observed are epidote, titanite, quartz, very rare biotite, and ilmenite.
The metamorphic evolution of the Posada Valley amphibolites is summarized in Figure 4.The early history recorded by the amphibolites is linked to the growth of garnet porphyroblasts and its inclusions (stage 1a) and the formation of the Cpx 1b + Pl 1b (±Amp 1b ) mineral assemblage (stage 1b).The minerals around garnet (Amp 2a + Pl 2a ; stage 2a) and the mineral assemblage in the rock matrix (mostly Amp 2b + Pl 2b ; stage 2b) represent the subsequent retrograde re-equilibration.The last metamorphic stage (stage 3) is related to the local growth of late retrograde phases such as actinolite, chlorite, albite, titanite.Prehnite is ascribed to sub-greenschists facies stage.

Mineral chemistry
Selected microprobe analyses of garnet, clinopyroxene, amphibole, plagioclase, epidote, and titanite from samples U1 and U4 and from retrogressed samples U5 and U33 of the Posada Valley amphibolites are reported in Tables 1-3 and S1 of Supplementary Material.
The garnet composition, in terms of molar fractions, is Alm 0.49-0.54Grs 0.34-0.42Prp 0.07-0.11Sps 0.01-0.05(with components Alm: almandine, Grs: grossular [+andradite], Prp: pyrope, Sps: spessartine).Compositional X-ray maps (Mg, Fe, Ca, Mn) of two selected garnet crystals from samples U1 and U4 are shown in Figure 5. Garnet porphyroblasts show a weak compositional zoning, graphically represented in the compositional rim-corerim profiles reported in Figure 6.By using the molar content of Ca (garnet core = X Ca < 0.40; garnet mantle = X Ca > 0.40; garnet rim = X Ca < 0.40) as a reference, we distinguished three compositional garnet domains: a core, a mantle, and a rim (Figures 5, 6).The representative garnet core composition is Alm 0.51-0.53Grs ∼0.35 Prp ∼0.10 Sps 0.03 .From core to mantle almandine decreases down to 49 mol% and then slightly increases towards the rim (51 mol%).Grossular shows a moderate increase from core to mantle (up to 41-42 mol%) and a faint decrease down to 38-39 mol% in the rim.
Pyrope keeps an almost constant concentration through the rim-core-rim profile with a very slight decrease in the mantle (down to 7 mol%).Spessartine very slightly decreases to 1-2 mol% in the rim.Garnet compositional zoning of the Posada Val- ley amphibolites resemble that of the retrogressed eclogites from the L-MGMC [Cruciani et al., 2015c] and differs from that of the HGMC eclogites [Cruciani et al., 2019].
Epidote in garnet (Ep 1a ) is characterized by a variable trivalent iron content ranging from 0.15 to 1.00 apfu.Titanite inclusions and matrix titanite do not show significant differences in composition.

Isochemical phase diagram
The metamorphic history of the Posada Valley amphibolites has been reconstructed by P -T pseudosections in the NCKFMASHO+Ti+Mn system calculated in the range 400-800 °C, 0.2-1.8GPa for     to 100% (Table 4).The resulting P -T pseudosection for sample U1 is shown in Figure 9(a).The main stage 1 phases (garnet, clinopyroxene, amphibole, and plagioclase) are stable in the multivariant P -T fields at <1.0-1.1 GPa, <600 °C.Further constrains on stage I have been obtained by garnet core and garnet rim composition.Figure 9(b) shows the intersection of almandine (X Fe = 0.53), grossular (X Ca = 0.33), pyrope (X Mg = 0.10), and spessartine (X Mn = 0.03) for garnet core composition, and of X Ca [=Ca/(Ca+Na)]= 0.12-0.14for Pl 1a .In addition, the isomodes 15 and 20 vol.% for garnet were drawn, which represent the maximum amount observed in the sample.All these isopleths and isomodes cross or come close together at conditions of P = 0.9-1.2GPa and T = 530-620 °C (Figure 9(b)).The fields with plagioclase + garnet + biotite + clinopyroxene + amphibole + chlorite ± titanite ± rutile ± ilmenite ± epidote ± quartz matches the mineralogical composition of the sample.P -T conditions obtained with the same approach for sample U4 are 1.0-1.3GPa and 550-650 °C (Figure S1a, b, in Supplementary Material).
For the P -T conditions of the garnet rim, an effective composition was obtained by subtracting the garnet core and mantle from the bulk-rock composition after considering the garnet mode in the rock (15%) and the volume area of the core and mantle estimated from the X-ray maps (by using an image analysis software) and EMP analyses (core+mantle around 60% in both U1 and U4 samples).The resulting P -T pseudosection is shown in Figure 9(c).The selected isomodes (15-20 vol.%) and isopleths of garnet (X Mg = 0.10), plagioclase (Pl 1a /Pl 1b X Ca = 0.14-16), clinopyroxene (Cpx 1b X Mg = 0.66-68) constrain P -T conditions at 1.0-1.4GPa and 590-690 °C (Figure 9(d)).Garnet isomode trend evidences a slight, prograde increase in temperature and pressure with garnet growth.The corresponding stability fields (clinopyroxene + plagioclase + amphibole + garnet + biotite ± ilmenite ± chlorite ± rutile) match the mineral assemblage observed in the rock samples.Similar P -T conditions have been obtained with the same approach for sample U4 (1.0-1.3GPa and 570-680 °C; Figure S1c, d).
In order to gain P -T conditions for metamorphic stage 2, the composition of the most retrogressed samples at Pedra su Gattu locality (U5 and U33, Table 4), with abundant amphibole and plagioclase (i.e., Amp 2b and Pl 2b ) was used.The resulting P -T pseudosection for sample U5 is shown in Figure 9(e).Amphibole and plagioclase display a large stability field at high temperatures and low pressures (Figure 9(e)).Figure 9(f) shows the intersection, for sample U5, of isopleths for the Si content (Si = 7.0-7.1 apfu) and X Mg (0.62) in Amp 2b with X Ca = 0.28 of Pl 2b .Lower temperatures of ca.50 °C are obtained by calculations with all divalent iron (dotted green line in Figure 9).The resulting P -T conditions are P = 0.5-0.8GPa and T = 600-700 °C (marked by the ellipse in Figure 9(f )).These condi-Figure 9. P -T pseudosection (a) and selected compositional isopleths for garnet and plagioclase (b) for the bulk composition of sample U1; P -T pseudosection (c) and selected compositional isopleths for garnet and plagioclase (d) for the bulk composition of sample U1 minus the composition of garnet core+mantle; P -T pseudosection (e) and selected compositional isopleths for amphibole and plagioclase (f ) for the bulk composition of re-equilibrated sample U5.Dotted green lines represent amphibole Si (apfu) isopleths calculated with all divalent iron in the bulk composition.Bulk core: composition for modelling the garnet core obtained by XRF analyses corrected for apatite and normalized to 100%.Bulk rim: effective composition for modelling the garnet rim obtained by subtracting the garnet core and mantle from the bulk-rock composition.
tions overlap two stability P -T fields which differ for the presence/absence of chlorite, ilmenite, and orthopyroxene (clinopyroxene + plagioclase + amphibole + garnet + biotite ± ilmenite ± chlorite ± orthopyroxene).Except for titanite (observed in the rock instead) and orthopyroxene (not present), the assemblage corresponds to what is observed in the sample.Similar results have been obtained with the same method on sample U33 (Figure S1e, f).Temperature estimates on the same amphibole-plagioclase pairs by the Holland and Blundy [1994] and Blundy and Holland [1990] geothermometers are in the range 580-640 °C and 560-650 °C for samples U5 and U33, respectively.

P -T evolution of the Posada Valley amphibolites
The Posada Valley amphibolites recorded three stages of metamorphic evolution (Figure 10): (1) prograde evolution from 0.9-1.2GPa and T = 530-620 °C to HP conditions at 1.0-1.4GPa and 590-690 °C; (2) a retrograde stage documented by matrix phases, characterized by an almost isothermal decompression from stage I (with a very slight increase in temperature) down to P = 0.5-0.8GPa and T = 600-700 °C; and (3) the local growth of late phases such as actinolite, chlorite, and epidote which overgrew or replaced minerals of the previous stages (about P = 0.2-0.3GPa and T = 330-350 °C), as already documented for several metabasites from NE Sardinia [Franceschelli et al., 2007, Cruciani et al., 2012].Further re-equilibration in sub-greenschist facies is suggested by the occurrence of prehnite.The resulting P -T path is clockwise (Figure 10).The Posada Valley amphibolites reached peakpressure conditions up to 1.4 GPa, at transitional P -T conditions between amphibolite, HP granulite, and amphibole-eclogite facies fields.These rocks never fully reached eclogite facies conditions, according to the following two observations: (1) in the large amount of observed samples, no omphacite relics were found, neither as inclusion in garnet porphyroblasts; (2) plagioclase occurs in every stage of the metamorphic evolution, primarily as inclusion in garnet as documented in the petrographic description and in Figures 3(b, c).
The P -T path is consistent with those of other metabasites and metamorphic rocks from the inner zone of the Sardinian basement, with the exception of the Mt.Nieddu banded and ultrabasic amphibolites [Scodina et al., 2019, 2020, Cruciani et al., 2020] which recorded instead an anticlockwise P -T evolution.
For a thorough comparison of the P -T history and evolution of metamorphic rocks belonging to the northern part of the medium-grade metamorphic complex, nearby the PASZ, such as the Giuncana eclogites [Cruciani et al., 2015a] and retrogressed eclogites of the Migmatite complex the reader is referred to Cruciani et al. [2011Cruciani et al. [ , 2015c]].The Giuncana eclogites, belonging to the medium-grade metamorphic complex, experienced a clockwise P -T path divided in four metamorphic stages: from an HP stage at 580 < T < 660 °C, 1.3 < P < 1.8 GPa, the eclogite underwent pressure decrease and temperature increase, leading to the breakdown of omphacite at ∼1.25-1.40GPa and 650-710 °C.The subsequent amphibolite facies re-equilibration occurred at T = 600-670 °C, P = 0.65-0.95GPa, and subsequently, in the range of greenschist facies conditions.The shape of the resulting P -T trajectory [Figure 11, p. 810 of Cruciani et al., 2015c] is similar to the path reconstructed for the here studied Posada Valley amphibolites.However, the pressure peak is fairly different (lower) and this resulted in different petrographic features.
Retrogressed eclogites of the Migmatite complex (Punta de Li Tulchi, Golfo Aranci, Punta Tittinosu) experienced an eclogite facies stage at 1.6-2.1 GPa, 660-700 °C, followed by granulite facies equilibration with temperature in excess of 800 °C and pressure in the range of 1.0-1.3.These stages were in turn followed by re-equilibration into low-P amphibolite facies and greenschist facies.The studied amphibolites from Posada Valley, similarly to the other metabasites from the L-MGMC, underwent a cooler evolution, predating the Barrovian stage, with a thermal peak lower than that recorded by the eclogites embedded in HGMC.

Geodynamic scenario
The Variscan collisional geodynamic scenario for southern Corsica and northern Sardinia has been depicted in a couple of recent papers [Scodina et al., 2019, Cruciani et al., 2021].This scenario comprises the presence of metamorphic rocks with clockwise and anticlockwise P -T path coupled together within the inner zone of the Variscan chain in northern Sardinia and southern Corsica [Massonne et al., 2018, Scodina et al., 2019, 2020, Cruciani et al., 2021].The geodynamic models proposed by these authors provide the presence of rock slices from different crustal levels of both upper and lower continental plates that were involved in collisional tectonics and subsequently brought adjacent to each other during the uplift in an exhumation channel [Massonne, 2016].According to Massonne [2012], an exhumation channel is an environment in which, in spite of an upwards-directed component, mass flow can be assumed to be more or less subhorizontal at depth, in contrast to the mass flow in the subduction channel, which follows the dip of the subduction zone.The occurrence of an exhumation channel in the Corsica-Sardinia Variscan belt has been hypothesized since by Massonne et al. [2018] and Scodina et al. [2019].
The geodynamic scenario proposed in this paper (Figure 11) follows that conceived by Massonne et al. [2018] and revised by Scodina et al. [2019] and Cruciani et al. [2021].It intends to explain the different P -T path and peak pressure reached by the eclogites belonging to the medium-grade metamorphic complex and those of the Migmatite complex.The geochemical signature of the different eclogitic lenses cropping out in Sardinia points to MORB and N-MORB protoliths [Cappelli et al., 1992, Cortesogno et al., 2004, Cruciani et al., 2015a] consistent with derivation from oceanic crust.
The model that we propose starts with the subduction of oceanic crust under the peri-Gondwanan terrane which was previously accreted to Laurussia, in the Silurian-Devonian (Figure 11).The Posada Valley amphibolites were part of the oceanic crust of the subducting slab and reached maximum depths of 50 km (corresponding to the maximum of 1.4 GPa documented by P -T pseudosection modelling), while the eclogites of the HGMC reached greater depths of 65-70 km (Figure 11(a)).Convergence and subduction of oceanic crust in Silurian-Devonian times testified by eclogitic relics were also reported from the neighbouring terrains of the external crystalline Massifs (Western Alps) and from the Maures-Tanneron Massif in southern France [Guillot and Ménot, 2009 and references therein].At this point, the Posada Valley amphibolites likely, early reached the subduction channel or somehow were detached from the slab, probably by tectonic or subduction erosion before the eclogite facies conditions were reached and prior to the beginning of the collisional phase.After the continental collision, the slab break-off event occurred (Figure 11(b)), together with mantle upwelling (red arrow in Figure 11(b)) which caused heating of the HP slices of northern Sardinia and southern Corsica that were transported below the upper plate [Cruciani et al., 2021].
Subsequently, different slices of oceanic crust and continental crust from both upper and lower plates were brought together and tectonically mixed in an exhumation channel where they continued the uplift during the early and middle Carboniferous.The Posada Valley amphibolites emerged between the metamorphic complexes of NE Sardinia, while the eclogites further north were embedded between the rock units of the Migmatite complex (Figure 11(c)).

Concluding remarks
The Posada Valley amphibolites from the mediumgrade metamorphic complex of NE Sardinia experienced a complex metamorphic history which resulted in a clockwise P -T evolution.These rocks record a prograde segment followed by an almost isothermal decompression from the metamorphic peak of 0.9-1.4GPa at T = 530-690 °C (stage I), recorded by garnet composition, its inclusions and the Cpx+Pl assemblage, to a retrograde reequilibration at amphibole stage conditions (stage II; P = 0.5-0.8GPa and T = 620-720 °C), and a final stage (stage III) at greenschist facies conditions.The reconstructed P -T trajectory and the petrological features of the Posada Valley amphibolites (i.e., the absence of omphacite and the occurrence of plagioclase in every stage of the metamorphic evolution) exclude the full attainment of eclogite facies.These rocks were part of the oceanic crust of the subducting slab involved in the Variscan continental collision and subsequently exhumed together with continental slices from both upper and lower plates in an exhumation channel.

Figure 1 .
Figure 1.Geological sketch map of (a) northern Sardinia Variscan chain (simplified tectonic sketch map of the Variscan Sardinian chain in the inset), (b) the studied area between P. Figliacoro and Pedra su Gattu localities, in the Posada Valley.PASZ: Posada-Asinara shear zone.

Figure 2 .
Figure 2. Amphibolites from Pedra su Gattu locality: (a) field aspect of the amphibolites; at the outcrop scale, the rocks show an S2 regional foliation (dotted line); (b) detail of a garnet-rich layer in the amphibolites; (c) cut and polished hand sample from garnet-rich layer; Grt = garnet; Pl = plagioclase; Cpx = clinopyroxene; Amp = amphibole.
(b)).Locally, inside the amphibole-rich bands, discontinuous millimetric to centimetric-thick layers with a yellowish clinopyroxene + plagioclase assemblage can be observed.These layers, as well as the amphibole-and plagioclase-rich bands, are parallel to the S2 regional foliation (Figure 2(c)).

Figure 3 .
Figure 3. Photomicrographs showing the most relevant microstructural features of the amphibolites from Pedra su Gattu.(a) Overview of a garnet porphyroblast with several inclusions concentrated in the core, plane polarized light; sample U1; (b) BSE image of euhedral amphibole in garnet (sample U1); (c) BSE image of plagioclase inclusion in garnet.The plagioclase inclusion in turn contains titanite and quartz nanoinclusions (sample U4); (d) BSE image of the Pl 2a + Amp 2a rim around garnet; sample U1; (e) overview of the matrix containing elongated flakes of Cpx 1b + Pl 1b ; plane polarized light; sample U4; (f ) BSE image showing a detailed view of Cpx 1b + Pl 1b (±Amp 1b ), sample U1.

Figure 5 .
Figure 5. Mg, Fe, Ca, and Mn concentration maps of a selected garnet crystal from (a) sample U1 and (b) sample U4.The scales for the colour code on the right-hand side of each image indicate counts of specific X-ray radiation per time unit.

Figure 6 .
Figure 6.Rim-core-rim compositional profiles of the garnet crystals from samples (a) U1 and (b) U4, along the lines A-A and B-B shown in Figure 5. Element concentrations are expressed in molar fractions.

Figure 11 .
Figure 11.Geodynamic scenario reconstructed for the retrogressed eclogites and amphibolites from NE Sardinia [modified after Cruciani et al., 2021].(a) Subduction of oceanic crust under a peri-Gondwanan terrane, previously accreted to Laurussia; Silurian-Devonian; (b) slab break-off and mantle upwelling (red arrow) after continental collision; late Devonian-early Carboniferous; (c) crustal thickening at the end of collision and exhumation of the Posada Valley amphibolites in the MGMC of the Corsica-Sardinia metamorphic belt; early-middle Carboniferous.

Table 1 .
Representative microprobe analyses of garnet and clinopyroxene from samples U1, U4

Table 4 .
Bulk-rock analyses (wt%) of samples U1, U4, U5, and U33 determined by X-ray fluorescence (XRF) spectrometry and modified compositions for the calculation of P -T pseudosections (see text for details)