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Article

The Relationships between the Internal Nappe Zone and the Regional Mylonitic Complex in the NE Variscan Sardinia (Italy): Insight from a New Possible Regional Interpretation?

by
Franco Marco Elter
* and
Federico Mantovani
*
Department of Earth, Environment and Life Science (DISTAV), University of Genova, 16132 Genova, Italy
*
Authors to whom correspondence should be addressed.
Geosciences 2024, 14(10), 260; https://doi.org/10.3390/geosciences14100260
Submission received: 28 July 2024 / Revised: 24 September 2024 / Accepted: 27 September 2024 / Published: 28 September 2024
(This article belongs to the Special Issue Metamorphism and Tectonic Evolution of Metamorphic Belts)
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Abstract

:
This study presents an updated interpretation of geological data collected between 1984 and 2022. The area under consideration holds significant regional importance as it is located between the Internal Nappe Zone (INZ) and the Regional Mylonitic Complex (RMC). Re-evaluation of the geological data has highlighted a more intricate structural framework than what is currently documented in the existing literature. This paper aims to illustrate, through structural analysis, that the Posada Valley Shear Zone (PVSZ) does not serve as the transitional boundary between the Inner Nappe Zone and the Regional Mylonitic Complex or High-Grade Metamorphic Complex (HGMC) as traditionally thought. Instead, the authors’ findings indicate that the transition boundary is confined to a shear band with a variable thickness ranging from 10 to 70 m at its widest points. The development of the Posada Valley Shear Zone is characterized by a series of transitions from mylonite I S-C to mylonite II S-C, extending over approximately 5 km. The formation of the Posada Valley Shear Zone is chronologically confined between the development of the East Variscan Shear Zone (EVSZ) and the emplacement of the Late Variscan granites. The differing orientations of Sm and S3 observed in the mylonitic events of the Posada Valley Shear Zone and the Regional Mylonitic Complex, respectively, are likely attributable to an anticlockwise rotation of the shortening directions during the upper Carboniferous period. Furthermore, this study proposes that the Condensed Isogrades Zone (CIZ), despite its unclear formation mechanism, should be recognized as the true transition zone between the Inner Nappe Zone and the Regional Mylonitic Complex or High-Grade Metamorphic Complex. This new interpretation challenges the previously accepted notion of increasing Variscan metamorphic zonation toward the northeast. This conclusion is supported by the identification of the same NE–SW orientation of the D2 tectonic event in both the Old Gneiss Complex (OGC in the Regional Mylonitic Complex) and the lithologies of the Inner Nappe Zone and the Condensed Isogrades Zone. The comprehensive analysis and new insights provided in this paper contribute to a refined understanding of the geological relationships and processes within this region, offering significant implications for future geological studies and interpretations.

1. Introduction

The Variscan Belt, also known as the Variscides, formed during the Devonian–Carboniferous period due to the convergence of the supercontinents Laurussia to the north and Gondwana to the south [1,2,3]. This collision led to the closure of Paleozoic oceans, such as the Rheic Ocean, and the opening of the Paleotethys, resulting in complex tectonic structures like nappes, shear zones, and metamorphic complexes [4]. The intracontinental East Variscan Shear Zone (EVSZ) played a crucial role in this tectonic scenario, contributing significantly to crustal deformation during and after the orogeny [5,6,7].
The geodynamic evolution of the Variscides and its geological implications can be summarized as follows:
  • Devonian to early Carboniferous: subduction and collisional events led to significant nappe stacking and crustal thickening, characterized by intense tectonic activity, including the subduction of oceanic lithosphere and continental collision [3]. In the early stages, the subduction of oceanic plates resulted in bimodal magmatism associated with volcanic arc systems, as evidenced by early Paleozoic [8,9]. Later, the collision between continental blocks led to the accretion of terranes to southern Europe, progressively suturing the European and Gondwanan plates [2,10].
  • Late Carboniferous to early Permian: dextral wrenching, driven by the oblique convergence between Gondwana and Laurussia, resulted in the reactivation and formation of large-scale strike–slip faults and shear zones. This period also saw the emplacement of large volumes of granitoids, the formation of narrow intracontinental basins with bimodal magmatism, and the reconfiguration of the existing structural framework to accommodate significant tectonic displacements [3,11].
  • Middle Permian: generalized extension marked the beginning of the opening of the Neotethys Ocean, leading to the development of expanding sedimentary basins, the onset of the Alpine sedimentary cycle, significant lithospheric thinning, and the initiation of rifting processes that would eventually form new oceanic basins [12,13,14,15].
The orogeny left a lasting imprint on Europe’s geological formations. Nappe structures and the external massifs of the Alps are prominent examples of Variscan tectonic evolution [16,17]. Shear zones, such as the Atlas-East Variscan Shear System, influenced the crustal architecture of Europe and extended the effects of the Variscan orogeny into the Triassic, contributing to the opening of the Alpine Tethys [6].
In this study, we re-evaluate the role and the timing of the Posada Valley Shear Zone (PVSZ; sensu [18]) within this broader geodynamic framework. Traditional interpretations have considered the Posada Valley Shear Zone as a part of the Posada Asinara Shear Zone (PASZ [7,11,19,20,21,22,23,24,25]) and as the transitional boundary between the Internal Nappe Zone (INZ) and the High Grade Metamorphic Complex (HGMC [11,26,27,28,29,30,31]) or the Regional Mylonitic Complex (RMC [32]). The Posada Asinara Shear Zone is considered 6 km thick in [7] and 10–15 km thick in [19]. However, our findings suggest that the Posada Valley Shear Zone is confined to a narrower shear band characterized by transitions from mylonite I S-C to mylonite II S-C [33,34,35] developed temporally between the East Variscan Shear Zone (EVSZ [36]) and the emplacement of Late Variscan granites [32,37,38].
Furthermore, we propose that the Condensed Isogrades Zone (CIZ; [39,40,41]) should be recognized as the actual transition zone between the Internal Nappe Zone and the Regional Mylonitic Complex. This new interpretation calls into question the previous assumption of increasing Variscan metamorphic zonation toward the NE [11,20,28,29,30,31,42,43]. This conclusion is supported by the identification of the same NE–SW orientation of the D2 tectonic event in both the Regional Mylonitic Complex and the lithologies of the Internal Nappe Zone and the Condensed Isogrades Zone.
Overall, this research aims to provide a refined understanding of the geological processes and structural relationships within this region. By integrating detailed structural analysis with a comprehensive review of geological data, we offer insights into the tectonic evolution of the Variscan orogeny and its implications for the broader geodynamic context.

2. Materials and Methods

The study was conducted using both historical and newly acquired data over the past 30 years. Fieldwork involved gathering structural and petrographic data from various outcrops in the transition zone between the Internal Nappe Zone and the Regional Mylonitic Complex or High-Grade Metamorphic Complex of Sardinia with a focus on detailed observations and measurements of deformation features and metamorphic overprints. The newly acquired data were correlated with the existing literature to provide a comprehensive understanding of the geological framework. Petrographic analyses were performed using thin sections (46 mm × 27 mm in dimension and 30 µm thick) of oriented samples prepared on the XZ plane of the finite strain ellipsoid with X parallel to the lineation. They were examined under optical microscopy to identify mineralogical compositions and textural relationships. The mineral abbreviations are the same as those used by Warr (2021) [44] except for oligoclase (Olg). Overall, 879 structural data (Table 1) were plotted on hemispherical projections (equatorial equal area, lower hemisphere). Planar structures (473 data) were plotted in a rose diagram (bin size 10°, 50% of total value of perimeter), while linear structural elements (406 data) were plotted in a contour plot (1% area contouring, 1% contour interval, 1% counting area, interval of 2, significance level of 3, without Terzaghi correction).
The classification of quartz ribbons follows [45,46], while the classification of S-C mylonites follows [33,34,35]. The classification of microstructures follows [47].
Specialized software were used: “Stereonet” software by Richard W. Allmendinger © 2020–2022 v. 11.5.4 was employed for processing and interpreting structural data, “QGis™” software v. 3.34.7-Prizren was used for the creation of geological maps, and “Inkscape: Open Source Scalable Vector Graphics Editor™” v. 1.3 and “Vectary™” software v. 3.0 were utilized to produce high-quality visual representations of the data.

3. Geological Outlines of Sardinia

The Variscan region of Sardinia is categorized into four NW-SE trending tectono-metamorphic zones (Figure 1) with metamorphic grade intensifying toward the northeast [27,30,48,49,50,51,52].
Progressing from the southwest to the northeast, these zones include the External Zone (foreland [11,53,54]), the Nappe Zone (subdivided into External and Internal Nappe Zones [55,56]), the Condensed Isogrades Zone [28,40,51,57] and the Regional Mylonitic Complex [32] or the Axial Zone/High-Grade Metamorphic Complex [21,27,29,31,51,57,58] in the northern part of the island. The transition from the Condensed Isogrades Zone to the Regional Mylonitic Complex is characterized by an abrupt increase in metamorphic grade, reflecting intense tectonothermal processes. The metamorphic evolution of the Variscan Sardinian region follows a typical clockwise pressure–temperature–time (P-T-t) path indicative of continental collision dynamics [27,59,60,61,62,63,64].

3.1. The Variscan Shear Zone

Since 1990, shear zones have been identified on a regional scale within the Variscan orogeny, including notable examples like the East Variscan Shear Zone [5,18,25,32,65,66,67] and its branches: the Monte Grighini Shear Zone (MGSZ; [65,68]), the Posada Asinara Shear Zone [7,11,19,20,21,22,23,24,25] and the Posada Valley Shear Zone [18]. The East Variscan Shear Zone is a major transpressional structure that developed during the late stages of the Variscan orogeny, accommodating significant crustal deformation across the southern European Variscan belt. It is preserved today in fragmented segments across regions like Sardinia, Corsica, the Maures-Tanneron Massif, and the Alpine External Crystalline Massifs [5,25,32,65,66,67]. It formed during the collision of Gondwana and Laurussia (380–320 Ma [69,70,71,72]) and its shearing was active mainly between 340 and 300 Ma [5,65,66,67].
The Monte Grighini Shear Zone formed within the Monte Grighini dome in central Sardinia around 315 million years ago. It is characterized by dextral transpressive deformation, with low- and high-strain areas, and experienced high-temperature (~625 °C) and low-pressure (~0.4–0.6 GPa) conditions [65].
The Posada–Asinara Shear Zone is a steeply dipping km thick major dextral transpressional structure forming the boundary between the High-Grade Metamorphic Complex and the Inner Nappe Zone in northern Sardinia [19,29]. This shear zone extends for approximately 150 km and is recognizable along the Posada Valley and the Asinara Island [7,19,20,30,50]. It developed during the late Variscan orogeny, around 325 to 300 Ma [7,19,20,25,28,29,50,69,73,74], under progressively decreasing temperature conditions, transitioning from amphibolite to greenschist facies [7,19,20,25,27,29,30,75,76].
The Posada Valley Shear Zone (sensu Elter et al., 1990 [18]) is the easternmost part of the Posada Asinara Shear Zone and is geographically limited to the Posada Valley area.

3.2. The External Zone

The External Zone represents the foreland of the Variscan chain in Sardinia and is composed by Cambrian–Early Ordovician terrigenous sequences and platform carbonate successions [53,75,77,78,79,80]. The structural framework is characterized by (i) E–W trending open folds contemporaneous with early nappe emplacement in the nearby nappe zone; (ii) recumbent, quasi-isoclinal folds with axial plane foliation and widespread “top-to-SW” penetrative shearing; (iii) N–S trending folds with axial plane foliation, contemporaneous with late nappe emplacement; (iv) back thrusts and related asymmetrical folds developed during the final stages of shortening, postdating foreland-verging structures [54,81].
In the Early Cambrian Bithia Formation, a Middle Ordovician granitic intrusion (478 ± 16–457 ± 0.17 Ma; [82,83]) is hosted by marly metasedimentary rocks that were affected by high-temperature (HT) metamorphism [57,84,85]. Field data and relationships between HT/LP mineral assemblages in the metasedimentary rocks (Grt + Wo + Ves in carbonate lenses and And in pelites) demonstrate that the study area was affected by a polyphase HT overprint (I: T = 520–620 °C at XCO2 = 0.1, P: 0.2–0.4 GPa; and II: T = 600–670 °C at XCO2 = 0.1, P = 0.2–0.4 GPa) that pre-dates the Variscan tectonic, metamorphic, and igneous phases [57].

3.3. The Nappe Zone

The Nappe Zone of Sardinia is composed of several tectonic units, each with distinct lithostratigraphic successions that have equilibrated under greenschist facies conditions. These units are imbricated and were emplaced with a top-to-SW transport [9]. The lithostratigraphic succession is characterized by Cambrian–Lower Ordovician metasedimentary and Ordovician volcanic rocks (491.7–479.9 Ma). These include intermediate and felsic transitional volcanic rocks, which were dated at 465 Ma for the calc-alkalic rhyodacites and 440 Ma for the alkalic metaepiclastites. These rocks are stacked in a NW–SE-trending nappe displaying a predominant SW vergence resulting from NW–SE trending thrust [81,86,87]. The Nappe Zone can be divided into two subzones based on the increase in metamorphic grade: the lower metamorphic External Nappe Zone, mainly deformed under syn-tectonic regional greenschist-facies metamorphism (with the exception of the Monte Grighini Unit, which exhibits amphibolite-facies conditions [55,65]) and the higher metamorphic Internal Nappe Zone [88], which formed under a polyphase deformation from greenschist to amphibolite facies metamorphism [55]).
A polyphase evolution has been pointed out by different authors [75,88,89,90,91] for the Nappe Zone. The structural framework is characterized by an early deformation phase (D1), generally observed far from the tectonic contact, a syn-nappe ductile deformation linked to the Barbagia Thrust activity with a top-to-S/SW sense of shear (D2) and a large-scale nappe refolding (D3). A late extensional stage (D4), with the development of collapse folds, marks the end of the orogenic cycle [22,55,56].

3.4. The Condensed Isogrades Zone (CIZ)

The Condensed Isogrades Zone [28,40,51,80] exhibits a sharp increase in metamorphic grade from the SW to the NE within a narrow band, transitioning from the Bt zone, through the Grt zone, then the St + Bt zone and the Ky + Bt zone to the Sil zone [27] (Figure 2).
The Bt zone is defined by the first appearance of Bt; the mineral assemblage is Qz, Ab, Ms, Bt and Chl. The Grt zone is defined by the first appearance of Alm-rich Grt and the coexistence of Grt, Bt and Chl; the mineralogy is Qz, Ab, Olg, Grt, Ms, Bt, Chl, Ilm and Spl [92]. The St + Bt zone is defined by the first appearance of the St + Bt association observed on contact between metapelites and granodioritic orthogneiss/augen gneiss in the Piano S. Anna of Siniscola [27]. The Ky + Bt zone is characterized by the mineral assemblage Ky, Bt, Grt, Ms and Pl [27].
Author [69], using various geothermobarometers, calculated temperatures of 497–560 °C and pressures of 0.7–1.1 GPa for the Grt zone. For the St + Bt zone, authors [27,42] calculated temperatures of 610 °C and pressures of 0.8–1.0 GPa; author [69] reported temperatures of 588–624 °C and pressures of 0.6–0.9 GPa. Regarding the Ky + Bt zone, temperatures of 650–700 °C and pressures of 0.7–0.9 GPa are estimated by [23].
The Condensed Isogrades Zone is also the area where the Posada Valley Shear Zone [18,93], and the Posada Asinara Shear Zone, outcrops [7,11,19,20,21,22,23,24,25].

3.5. The Regional Mylonitic Complex (RMC) or High Grade Metamorphic Complex (HGMC)

The High-Grade Metamorphic Complex is characterized by intense metamorphism, ranging from high amphibolite to granulite facies conditions [27]. It is mainly made by migmatite, migmatized orthogneiss, calc-silicate nodule, micaschist, paragneiss and metabasite lens [7,19,27,64]. Several authors [21,27,28,30,31,64,94,95,96] have interpreted it as part of the axial zone. However, Mantovani and Elter (2024) [32] proposed that the High-Grade Metamorphic Complex [97,98,99] should not to be considered as the Axial Zone of the Sardinian Variscan chain but rather as a Regional Mylonitic Complex associated with the top-right East Variscan Shear Zones [7,21,25,36,73,100]).
In this paper, the authors adopt this interpretation and follow the chronological framework of events proposed by Mantovani and Elter (2024) [32] and summarized in Table 2.
The Regional Mylonitic Complex appears to be composed of two metamorphic complexes: an older one, the Old Gneiss Complex (OGC), embedded within the subsequent New Gneiss Complex (NGC). The Old Gneiss Complex consists of a wide variety of lithotypes, including orthoderivates, paraderivates, rare marbles, metabasites, and minor migmatitic gneiss with stromatitic structure [101]. The proportion of leucosomes within these lithotypes has been estimated to be about 3% in orthoderivates and 5% in paraderivates. Some authors [57] suggest that the Bithia Formation (External Zone) is very similar in age, stratigraphy, and composition to the protolith of the high-temperature (HT) rocks that are exposed in the Old Gneiss Complex. The structural characteristics of the Old Gneiss Complex are marked by a significant deformational event (D2), which is easily recognizable in these lithotypes. The syn-tectonic M2 metamorphic event is delimited between the granulite stage (T = 650–750 °C and P = 0.8–1.2 GPa; [27]), dated at 359 ± 4 Ma ([40]), and the amphibolite stage (T = 550–740 °C and P = 0.3–0.7 GPa) around 350–344 Ma ± 7 Ma ([27,102]), resulting in the development of a pervasive regional S2 foliation that is consistently oriented in a NE–SW direction with a notable dip toward the SE.
The New Gneiss Complex is primarily distinguished by the presence of Bt-bearing mylonitic gneiss (Cat’s Eyed Facies, CEF, [58]), along with syn-tectonic peraluminous granites. These Bt-bearing gneiss showcase a notable augen structure, characterized by millimetric to centimetric porphyroclasts, which contribute to their distinct appearance and texture. Additionally, dispersed throughout the complex are pods of various sizes, ranging from centimeters to decameters, containing lithotypes associated with the Old Gneiss Complex, including amphibolitized eclogites, amphibolites, orthogneiss, quartzites, and calcsilicate nodules. The D3 event is prominently manifested within the New Gneiss Complex. This event coincides with the emplacement of the NW–SE-oriented Capo Ferro syn-tectonic Barrabisa granite, which is precisely dated at 325 ± 1.3 Ma [40,103,104]. The synkinematic M3 metamorphic event associated with the D3 event aligns with the greenschist stage (T = 300–400 °C and P = 0.2–0.3 GPa; [40]), leading to the development of a pervasive schistosity (S3).

3.6. The Sardinian Variscan Granite Complex

The intricate structure of this late-Variscan batholith arose from the merging of multiple magmatic events [105], which were concentrated in Sardinia into three primary peaks [40,103,105].
The initial phase of magmatic activity, involving peraluminous foliated leucocratic granitoids, occurred around 320–315 Ma and largely coincided with regional strike–slip deformation facilitated by NW–SE and E–W crustal-scale shear zones (e.g., East Variscan Shear Zone; [32,40,103]).
Notably, the beginning of the second phase (~300 Ma) aligned with the Late Variscan drifting and the significant clockwise rotation (>90°) observed in the Corsica–Sardinia block [37,106,107,108,109,110]. This period also corresponds to the widespread development of ferroan granites in southern Sardinia, indicating a shift in the magmatic processes [111,112].
The third phase of post-collisional late Variscan magmatism in the Sardinia–Corsica batholith peaked around 290 Ma. In southern Sardinia, within the frontal section of the Variscan orogenic wedge, this magmatism is represented by three suites of slightly peraluminous and fluorine-bearing granitoids, reflecting a complex magmatic evolution influenced by the late stages of Variscan tectonics [113].

4. Results

The authors present the data categorized according to the various complexes identified and analyzed, which are further divided into two major categories: the complexes belonging to the Internal Nappe Zone and the complexes belonging to the Condensed Isogrades Zone. The data are presented to highlight the transition between the Internal Nappe Zone to the south and the Regional Mylonitic Complex to the north, traversing the Condensed Isogrades Zone.
The transition between the Internal Nappe Zone and the Regional Mylonitic Complex is delineated in the region encompassing Santa Lucia, Lula, Lodè, Mamone, and the northern side of the Posada River Valley (Figure 3). This transition is especially evident between the Piano di S. Anna and Punta Orvili.

4.1. The Complexes of the Inner Nappe Zone (INZ)

In this section, the complexes belonging to the Internal Nappe Zone are discussed, specifically, the S. Lucia complex and the Ordovician orthoderivates, as well as the Lodè-Mamone-Rio Mannu complex, the Ordovician orthoderivates, and their associated contact aureoles.

4.1.1. The S. Lucia Complex

The lithological composition consists of mica schists (Figure 4A), Grt-bearing mica schists, occasionally porphyroblastic paragneiss, pale quartzites, gray marbles, acidic metavolcanites (e.g., porphyroids Figure 4B), and rare black schists. The mica schists are light-colored, sometimes oxidized, shiny, and contain garnets visible to the naked eye. The paragneiss exhibit a brownish hue, sometimes featuring a fine-grained porphyroblastic texture. The quartzites are light in color and occur either in metric-thick layers or as lenses within the mica schists. Although rare, there are also occurrences of black shale layers.
This complex exhibits a secondary penetrative regional foliation (S2; [27]). This foliation is characterized by the following mineralogical associations: (i) Qz + Pl + Bt + Ms + Ox; (ii) Qz + Pl + Grt + Bt + Ms + Ox; (iii) Qz + Pl + Cld ± Bt ± Grt ± Ms + Accessories + Ox. The S2 foliation show a consistent NE–SW strike (Figure 5A) and dips toward the SE. The mineralogical lineation (L2) identified on the S2 surface (Chl and Ms) plunges from horizontal to oblique (Figure 5B).
Related to S2, rare isoclinal folds with B2 axes are recognizable (Figure 6A), which are sub-orthogonal to L2, and their trend is E–W/NW–SE. Additionally, kinematic indicators associated with S2 can be identified, such as a top-to-right quartz sigma object. The S2 foliation was subsequently deformed by a later crenulation cleavage/open fold (Figure 6B,C) with axes trending in the E–W direction.

4.1.2. The Lodè-Mamone–Rio Mannu Complex

Lithologically, the area comprises the Rio Mannu orthogneiss, the Lodè-Mamone leucocratic granodioritic orthogneiss/meta aplite, and the La Caletta-Siniscola augen gneiss. The Lodè-Mamone orthogneiss are dated to 458 ± 31 Ma [114,115,116] and 456 ± 14 Ma [11,115,116], while the age of the augen gneiss is 441 ± 33 Ma [114,115,116].
The La Caletta-Siniscola Ordovician augen gneiss has a light color and small Kfs porphyroclasts varying in size from a few millimeters to a few centimeters (Figure 7A). Structurally, the augen gneiss exhibit a marked regional penetrative schistosity (S2; Figure 7B). The most common S2 mineralogical association includes Qz, Pl, Kfs, Bt, Ms, Chl, accessory minerals, and oxides (Figure 7C). Microstructural analysis reveals that the Kfs porphyroblasts (Figure 7C) are surrounded by an oriented micaceous feldspathic Qz structure and, locally, crystal-lepidoblastic structures in more tectonized lithotypes. Geochemical analyses reveal a calc-alkaline trend for these rocks [11].
The Ordovician Lodè-Mamone leucocratic granodioritic orthogneiss and its dyke system of meta aplites shows a white–grayish color (Figure 8A) and a higher Bt content than the La Caletta-Siniscola augen gneiss. Structurally, the leucocratic orthogneiss and its NW–SE dyke system of meta aplites exhibit the same marked regional penetrative schistosity (S2) observed in the la Caletta-Siniscola augen gneiss. The most common S2 mineralogical association includes Qz, Pl, Kfs, Bt, Chl, Ms (Figure 8B) and Grt (in the meta aplites) along with accessory minerals (Ap, Aln and Ep) and oxides. Geochemical analyses reveal a calc-alkaline suite for these rocks [11].
The Rio Mannu Ordovician (Figure 9A) granodioritic orthogneiss is distinctly different from the earlier complex due to its high concentration of various types of xenoliths (Figure 9B) and enclaves (Figure 9C). The structural framework of the Rio Mannu orthogneiss is marked by the presence of two regional foliations, along with remnants of a rare and discontinuous foliation, which might be classified as a primary surface associated with the emplacement of the Ordovician batholith. This primary foliation, observed along the Rio Mannu, is transposed by the regional schistosity S2, as indicated by the preferential orientation of both basic and acidic magmatic enclaves (Figure 9B–D). Moving northwards (toward the Posada Valley), the first signs of a mylonitic event begin to appear in the river. This mylonitic event is characterized by shear bands of variable thickness, from 1 m to several meters (Figure 9E).
The shear zones are marked by a penetrative schistosity Sm with a W–E strike direction and vertical to oblique dip (Figure 10A). Several kinematic indicators are visible. The Lm shows an SE trend with a plunge to the SE (Figure 10B).
The kinematic indicators identified include drag folds (Figure 11A) and σ-type porphyroblasts visible at the macro/micro-scale (Figure 11B–D). Most kinematic indicators identify a top-to-right sense of shear (Figure 11B–D), while locally, a top-to-left sense of shear is also identified (Figure 11A). Rare sheath folds are observable in the middle part of the Rio Mannu (Figure 11E). These folds are associated with Sm but are only visible in two dimensions; hence, they cannot be used as kinematic indicators [117].
The extent of the area over which an eye pattern can be observed in a cross-section perpendicular to the shear direction depends on the initial slip surface configuration [118].
The contact between the Ordovician orthogneiss and the paraderivatives outcropping to the north, near the Posada Valley, is characterized by the presence of marbles/calcsilicates and hornfels rocks. Two types of aureoles are notable: one associated with the Late Variscan Punta Tepilora granite, with its own static mineralogical associations, and another where the minerals are deformed and folded (Figure 12).
At a macroscopic level, the Grt in both the marble/calc-silicate lenses and the And in the paraderivates are clearly visible (Figure 13A–C). In the northern marble/calc-silicate lenses, the garnets are deformed into isoclinal folds with an axial surface (S2; Figure 13B).
The southern marble/calc-silicate lenses are embedded within St-bearing micaschists and in proximity to two magmatic bodies: the Lodè-Mamone-Rio Mannu Orthogneiss (Ordovician; pre-Variscan) and the Punta Tepilora Granite (upper Carboniferous-Permian; late Variscan) (Figure 12). The marble/calc-silicate lenses can be classified as impure, given whether the amount of calcite and/or dolomite always less than 80% of the rock volume, and the presence of Grt and Ves, which confirm the presence of clay content. The mineralogical association consists of Wo + Grt ± Di ± Cal ± Qz ± Ves, which suggests that the protolith was a carbonate lens embedded in a pelitic sequence with scarce meta-siliciclastic and meta-volcanic horizons. The Grt (94% grossular) are slightly zoned. The Ves shows growth at the expense of the Grt.
The northern marble/calc-silicate lenses present a geological framework very similar to the southern one. At the mesoscale, the samples appear identical, but optical microscope observation shows clear differences in structural and mineralogical aspects. The main differences are the intense fracturing, the presence of numerous microcrystalline phases unresolved by optical microscopy, and the apparent absence of Ves. Nevertheless, it was possible to identify the Wo + Grt ± Di ± Cal ± Qz association, which also in this case would indicate the protolith as a carbonate lens embedded in a pelitic sequence with scarce meta-siliciclastic, meta-volcanic horizons.

4.2. The Complexes of the Condensed Isogrades Zone (CIZ)

In this section, we discuss the complexes belonging to the Condensed Isogrades Zone, specifically focusing on the Punta Gortomedda, Fruncu Nieddu, Punta Figliacoro and Punta Orvili complexes. The Punta Figliacoro complex is further subdivided into three minor complexes: Punta Figliacoro sensu stricto, Pedra su Gattu, and Sedda Eneas.

4.2.1. The Punta Gortomedda Complex

The Punta Gortomedda complex is characterized by micaschists (Figure 14A), paragneiss, quartzites, and gray marbles (Figure 14B). Micaschists are the most frequent lithotype; they appear shiny with both Grt and St centimetric in size.
The structural framework is characterized by two different secondary foliations: regional S2, with superimposed Sm mylonitic schistosity, already observed in the Ordovician orthogneiss complexes. The S2 foliation strikes in the NE–SW/E–W direction (Figure 15A) and dips toward SE/S. The L2 observed trend reveals a consistent NW–SE orientation with a plunge toward the SE (Figure 15B). The strike of Sm foliation ranges from NW–SE to E–W with vertical dip (Figure 15C). The Lm trend observed reveals a NW–SE orientation with plunge to the SE (Figure 15D). The relationships between S2 and Sm can be classified as I S-C mylonites [33,34,35]: on a macroscopic scale, the overlapping geometric characteristics and the top-to-right shear component associated with Sm are clearly observable.
At the microscopic scale, the mineral assemblage of S2 consists of Qz + Olg + Grt + St + Ms ± Bt + Tur + Ox. In contrast, the mineralogical association of Sm is characterized by Qz + Ms + Chl ± Ab + Ox. The relationships between S2 and Sm are clearly observable even at the microscale: kinematic indicators such as S-C planes and mica fish are the most common (Figure 16A,B). Other recognizable microstructures include boudins of Grt and Qz ribbons (Figure 16C).
Polycrystalline ribbons [45,47] may result from the transposition of pre-existing Qz banding or from the deformation and segregation of quartz relative to other minerals in the rocks. The most common quartz ribbons are type B1 (assemblage of deformed but variable-sized grains), type B2a (mosaic of elongated grains), type B2b (grains of constant size), and the rare type B3 (mosaic of grains elongated parallel to the ribbon wall) [45,46]. Type B2a and B2b ribbons develop under metamorphic conditions ranging from greenschist facies to amphibolite facies. Type B3 ribbons form under higher-grade metamorphic conditions [46]. Type B3 ribbon Qz wraps the ductile deformed garnet (Figure 16C). Considering that Grt exhibits ductile behavior at temperatures around 700 °C [119], this association indicates granulite facies conditions during the development of S2.

4.2.2. The Fruncu Nieddu Complex

The Fruncu Nieddu complex consists of Ky-Grt-bearing paragneiss (Figure 17A,B), mica schists, white and green quartzites (Figure 17C), gray marbles, and rare calcsilicate nodules.
The structural framework is characterized by two distinct secondary foliations: the regional S2, with superimposed Sm mylonitic schistosity, is already observed in the Punta Gortomedda complex. The S2 foliation strikes from E–W to NE–SW (Figure 18A), dipping toward the S/SE. The L2 mostly trends NW–SE and plunges toward the SE (Figure 18B). The Sm foliation show a consistent strike in the E–W direction (Figure 18C) and has a steep dip toward S. The Lm slickenlines trends mostly NW–SE, plunging to the SE (Figure 18D). The relationships between S2 and Sm can be classified as type II S–C mylonites [33,34]. On a macroscopic scale, the Sm takes on the role of the main foliation, partially obliterating the S2.
At a microscopic scale, the mineral assemblage of S2 consists of Qz + Olg + Grt + Ky + Ms + Bt + Tur + Ox, while the mineralogical association of Sm is characterized by Qz + Ms + Chl + Ab + Ox. The macroscopic relationships between S2 and Sm are more difficult to identify than those observed in the Punta Gortomedda complex, as the Sm foliation becomes much more penetrative, obliterating S2, which appears only as mineralogical relics embedded in a microcrystalline matrix (Figure 19A). Several kinematic indicators can be identified, such as top-to-right Ky fish (Figure 19B,C) and type B 2a/b Qz ribbon [45,120].

4.2.3. The Punta Figliacoro Complex (Sensu Stricto)

The Punta Figliacoro complex (sensu stricto) comprises HP-MORB affinity/enriched-type MORB amphibolites [29,121,122,123] (Figure 20A) in contact with paragneiss, laminated orthoderivates (Figure 20B), and Ky-bearing mica schists [93]. The authors [31,95] identify the S2 deformation under amphibolitic conditions.
In the Punta Figliacoro complex (sensu stricto), the same planar and linear structures observed in the two previous complexes are present. The S2 strikes in an E–W direction (Figure 21A) with dips to the S. The L2 trend is NW–SE oriented, plunging toward the SE (Figure 21B). The Sm foliation shows an E–W/NW-SE strike (Figure 21C) and has a slight plunge toward the S. The Lm slickenlines trends NW–SE with a slightly plunge toward the SE/E (Figure 21D).

4.2.4. The Pedra su Gattu Complex

The Pedra su Gattu complex is characterized by the presence of a particular mylonitic lithotype, the phyllonites, which exhibit a dark green color and sometimes an augen structure (Figure 22A). Its thickness ranges between 50 and 70 m. The mineralogical association of this foliation consists of Qz + Ab + Ms + Chl + Ep + Ox ± Bt. Authors [31] report the same lithological body in Anglona with similar thicknesses ranging between 20 and 70 m. This phyllonitic level is located in the same geometric position as that of Pedra su Gattu, dividing the Condensed Isogrades Zone from the Regional Mylonitic Complex.
Structurally, it exhibits only the secondary penetrative foliation (Sm) associated with the Posada Valley Shear Zone, which is characterized by rare slickenlines mineralogical lineations. This foliation deforms the Ab porphyroblasts through simple shear, indicating a top-to-right sense of shear (Figure 22B).
In the Pedra su Gattu complex, only the Sm mylonitic schistosity is present and strikes E–W (Figure 23A), dipping to the S. The Lm trends NW–SE/E–W with a moderate to minor plunge toward the SE/E (Figure 23B).

4.2.5. The Sedda Eneas Complex

The Sedda Eneas Complex is characterized by the presence of Sil-Ms-bearing paragneiss (Figure 24A) and mica schists (Figure 24B).
The tectonic framework becomes more complicated due to the appearance of the S3 foliations (Figure 24A). The S2 foliation is typically found in the Old Gneiss Complex, while the S3 foliation is characteristic of the New Gneiss Complex. The S2 strikes NE–SW dipping toward the SE, while S3 shows a NW–SE strike and a steep dip toward the SW (Figure 25A). The L2 trend in mainly NW–SE oriented and plunges to the SE (Figure 25B). The Sm shows a mostly E–W/slightly NW–SE strike (Figure 25C). The Lm slickenlines trends NW–SE with plunge toward the SE (Figure 25D). Locally, the relationships between S2, S3 and Sm foliation can be observed, providing insights into the sequential development of structural features in the complex. Authors [32] investigate these relationships by analyzing the tectonic framework and providing an interpretation.

4.2.6. The Punta Orvili Complex

The Punta Orvili area marks the southernmost exposure of the Regional Mylonitic Complex [32] and belongs to the Sil + Ms zone [27]. This complex comprises mylonitic gneiss, referred to as the Cat’s Eye Facies (CEF, [58]), along with remnants of amphibolites, rare amphibolitized eclogites, gneiss with stromatitic structure [101] and Sil-Ms-bearing gneiss (Old Gneiss Complex, [32,58]).
The structural framework of the area is distinguished by the presence of the regional mylonitic foliation S3 that strikes NW–SE and transposes pods of high-temperature (HT) rocks (belonging to Old Gneiss Complex) that contain the regional S2 foliation that strikes NE–SW (see Figures 9–11 in [32]). At this location, the S3 foliation is cut and displaced by the E–W-oriented Sm foliation: this interaction is evident where thin shear zones (Sm) cut across the S3 foliation [43] (see Figure 12 in [32]).

5. Discussion

Although the Posada Valley has been studied since 1987 [93] and numerous works have been published on its structural framework [5,18,20,23,24,30,31,32,41,58,73,76,124,125] and metamorphic aspects [25,29,43,64,126,127,128], as well as on geodynamic correlations [7,18,39,41,123,127], authors propose a different regional interpretation.
In order of their geometric superposition from S to N, the complexes belonging to the Internal Nappe Zone are (i) the Santa Lucia complex and (ii) the Lodè-Mamone-Rio Mannu Ordovician meta orthoderivates complex. The latter can be divided into three sub-complexes, which, in their geometric order of overlap from S to No, are La Caletta-Siniscola rhyolitic augen gneiss, the Lodè-Mamone leucocratic orthogneiss with its dike system of meta-aplites, and the Rio Mannu orthogneiss.
Belonging to the Condensed Isogrades Zone are the Punta Gortomedda complex, the Fruncu Nieddu complex, and the Punta Figliacoro complex. The latter can be divided into three sub-complexes, which, in geometric order of superposition from S to N, are the Punta Figliacoro complex sensu stricto, the Pedra su Gattu complex, and the Sedda Eneas complex.
Even though it does not belong to the Condensed Isogrades Zone, the southernmost of the complexes belonging to the Regional Mylonitic Complex was considered: the Punta Orvili complex.
All the complexes present similar lithotypes (micaschists, paragneiss, quartzites, marbles except for the porphyroids that are only present in the Santa Lucia complex) but exhibit different metamorphic degrees. The complexes belonging to the Internal Nappe Zone are confined to the upper Grt Zone [27], while those belonging to the Condensed Isogrades Zone are in the St + Bt Zone, Ky + Bt Zone and Sil Zone [27].
In the Ordovician orthogneiss complex, a new lithotype was recognized for the first time: the Rio Mannu orthogneiss, which differ from other orthoderivates due to the marked presence of both magmatic and metamorphic enclaves and xenoliths. For the first time, the presence of remnants of thermometamorphic aureoles is also recognized, which is indicated by Grt-Wo-Ves-bearing hornfels/marbles/calc-silicate.
The Punta Figliacoro complex is the key complex between the Inner Nappe Zone and the Regional Mylonitic Complex. The three recognized sub-complexes are characterized of being confined in a mylonitic zone that involves different lithotypes (amphibolites, orthoderivates, micaschists, and paragneiss, as well as lithotypes belonging to the Condensed Isogrades Zone). The most important of these sub-complexes is Pedra su Gattu, an ultramylonite (phyllonite) in greenschist facies, which is in contact with the first lithotypes belonging to the Regional Mylonitic Complex (Sedda Eneas complex). The last complex, Punta Orvili, exhibits the mylonitic features of the Regional Mylonitic Complex previously described and, locally, the effects of the transition zone are still recognized.
Structurally, the observations indicate that the framework varies according to the specific complex under consideration:
  • In the Santa Lucia complex, there is a NE–SW regional foliation (S2) which transposes an older regional foliation (S1) into the Lula area. Such relationships disappear as we move toward the other complexes.
  • In the Ordovician orthoderivates complex, the NE–SW-oriented S2 is consistently present, but locally, an E–W trending mylonitic Sm foliation is also observed. The Sm foliation can transpose the S2 into mylonitic shear bands with a top-to-right sense of shear. A top-to-left sense of shear is also locally visible, but direct relationships that could define their chronological order have not been observed.
  • In the Punta Gortomedda complex, mylonites with S-C structures are identifiable [33,34,35]. The relationships between S2 (NE–SW oriented) and Sm (E–W oriented) indicate a top-to-right sense of shear component.
  • In the Fruncu Nieddu complex, the relationships between S2 and Sm change, with Sm (E–W oriented) becoming pervasive, and S2 (NE–SW trending) preserved only in isolated pods with type II S-C relationships [33,34,35], always with a top-to-right sense of shear.
  • In the Punta Figliacoro complex sensu strictu, the regional Sm foliation (E–W trending) becomes pervasive with S2 appearing only in isolated pods.
  • In the Pedra su Gattu complex, the Sm (E–W trending) completely transposes both the lithologies involved and the previous structures. Kinematic indicators associated with Sm still indicate a top-to-right sense of shear.
  • In the Sedda Eneas complex (belonging to the Regional Mylonitic Complex [32]), the structural complexity increase, which is due to the first appearance of lithotypes in the Sil Zone [27]. In this complex, the following are identified in order of geometric overlap: Sm (E–W trending), the Regional Mylonitic Complex mylonitic schistosity S3, and relics of S2 (belonging to the Old Gneiss Complex; [32,58]).
  • The Punta Orvili complex is considered the southernmost complex of the Regional Mylonitic Complex. The authors [129] discuss the complex metamorphic evolution of the Punta Orvili metabasite, highlighting the significance of the NW–SE-oriented S3 foliation in the deformation history. This structural configuration is similar to that previously described for the Sedda Eneas complex; however, a significant difference is that in Sedda Eneas, the Sm appears as a regional foliation transposing the S3, whereas in Punta Orvili, the Sm is confined to narrow isolated shear bands.
Regarding the contact aureoles, it is necessary to distinguish the St-bearing micaschists from the marble/calc-silicate lenses. Several studies on the P/T conditions of metamorphism exist for the schists (summarized by [30]), which indicate that these rocks have undergone regional metamorphism characterized by the presence of St + Bt (Amphibolite Facies). The P-T conditions provide values of P greater than 0.7–0.8 GPa and T of 600 °C. However, this contrasts with the stability field of Ves (T = 400–700 °C and P = 0.05–0.25 GPa; [130]). Additionally, the microstructures of the micaschist samples reveal the presence of two generations of And, one of which crystallized under static conditions. The hypothesis is that the marbles and micaschists have undergone at least three metamorphic events:
  • Pre-Variscan (Ordovician): emplacement of the Lodè-Mamone granite, which led to the crystallization of the typical mineral assemblage associated with contact metamorphism in the marbles and the crystallization of And in the schists.
  • Variscan: the orogeny enhanced the growth of Grt in the marbles, resulting in their zoning, and triggered the crystallization of St in the micaschists.
  • Late Variscan (upper Carboniferous-Permian): emplacement of the Punta Tepilora Granite, which led the crystallization of the Ves in the marbles and the static And in the micaschists.
This complex geological framework suggests a different interpretation of what was previously known and published about this area. We do not agree, in the current state of knowledge, with the hypothesis that the Posada Valley could be considered part of the South European Hercynian Suture Zone [121] or as a subduction zone [29,43,64].
Furthermore, the literature defining the Variscan metamorphic zoneography with an increase in grade toward the NE is also called into question. If the increase in metamorphic grade is linked with the regional S2 foliation, which shows a NE–SW orientation throughout northern Sardinia [32], then the zoneography should increase toward the NW rather than parallel to S2. However, we agree with the model proposed by [103], which associates the formation of the Sardinian batholith complex with late-Carboniferous magmatism. This model suggests that heat sources were primarily concentrated in the crust affected by long-lived shear zones. These shear zones likely accumulated strain in discrete increments rather than continuously, creating a significant positive feedback loop between shear heating, temperature, and partial melting during the late Carboniferous.

6. Conclusions

The geological framework of this area is presented (Figure 26) starting from the southernmost outcrops belonging to the Inner Nappe Zone (Santa Lucia Complex) up to the northern outcrops of the Regional Mylonitic Complex (Sedda Eneas and Punta Orvili complex).
From the geological and structural data, it is clear that the relationships between the Inner Nappe Zone and the Regional Mylonitic Complex are very complex. The structural framework varies according to the complexes involved:
  • In the Santa Lucia complex, two regional foliations (S1 and S2) are present but can only be observed in the southernmost part of the complex (e.g., Lula). Moving northward S2, characterized by a NE–SW orientation, becomes pervasive.
  • In the Lodè-Mamone-Rio Mannu, Punta Gortomedda and Fruncu Nieddu complexes, Sm mylonitic foliation consistently exhibits a top-to-right sense of shear. The Sm gradually becomes more pervasive, moving northwards toward the Punta Figliacoro metamorphic complex. In the Lodè-Mamone-Rio Mannu and Punta Gortomedda complexes, the relationships between S2 (NE–SW oriented) and Sm (E–W oriented) are characterized by type I S-C mylonites. However, in the Fruncu Nieddu metamorphic complex, these relationships evolve: Sm becomes more pervasive and the structure shifts to type II S-C mylonites. Notably, the Rio Mannu orthogneiss is distinguished for the first time, exhibiting unique textural features compared to the other two Ordovician orthoderivates due to its high concentration of enclaves and xenoliths.
  • In the Punta Figliacoro complex sensu stricto, only Sm is observable, while the S2 foliation is completely absent.
  • In the Sedda Eneas complex, the relationships between the transition zone and the Regional Mylonitic Complex are highlighted for the first time. The Sm mylonitic foliation intersects the mylonitic S3 belonging to the Regional Mylonitic Complex with a top-to-right sense of shear.
  • In the Punta Orvili, the southernmost outcrop of the Regional Mylonitic Complex, the relationshi between Sm and S3 is less distinct, with Sm appearing only as thin shear bands.
From this perspective, it is evident that the oldest structure (S2) identified in the transition zone has the same NE–SW strike as the S2 highlighted in the Old Gneiss Complex, which is incorporated into the Regional Mylonitic Complex. The Sm mylonitic schistosity transposes and is thus more recent than the S3 mylonitic schistosity observed in the Regional Mylonitic Complex.
Based on these new findings, it appears that the Posada Valley Shear Zone cannot be considered the transition zone between the Inner Nappe Zone and the Regional Mylonitic Complex or the High-Grade Metamorphic Complex. Instead, the Posada Valley represents a regional shear zone predominantly confined to its maximum development within the Pedra su Gattu complex. The Sm mylonitic schistosity developed in greenschist conditions and associated with the Posada Valley Shear Zone deforms the mineralogical associations related to the M2 event, which is synkinematic to the D2 event [27]. The Posada Valley Shear Zone thus appears to be a late shear zone, post-East Variscan Shear Zone, which is likely related to an anticlockwise rotation of the late Carboniferous shortening direction from NW–SE to NE–SW. The relationships between Sm and S3 were established later and before the intrusion of the late-orogenic granites of Punta Tepilora. This could be consistent with Carosi et al. (2022; [25]), highlighting that the East Variscan Shear Zone (EVSZ) is not simply a late Variscan (or even Permian) strike–slip shear zone that developed synchronously along all its strands. Instead, it represents a crustal-scale system of transpressive shear zones characterized by a complex and progressive evolution over time. A detailed investigation of other shear zones potentially linked to the EVSZ could lead to the identification of additional generations of shear zones and more intricate architectures.
The actual transition zone between the Internal Nappe Zone and the Regional Mylonitic Complex is defined by the Condensed Isogrades Zone, though the mechanism responsible for this condensation phenomenon remains unclear. The understanding of the relationships between the Old Gneiss Complex and the Internal Nappe Zone remains unresolved due to their complete transposition by both the East Variscan Shear Zone and the subsequent PVSZ.
Considering the position of the Sardinia–Corsica–Calabria block during the upper Carboniferous [32,37,106,107,108,109,110,131], this period was characterized by a widespread episode of fracturing that affected not only the Variscan fold-belt but also the adjacent platforms. This tectonic event occurred later than, and independently of, the main Variscan tectogenesis [132].
We hypothesize an anticlockwise rotation of the shortening directions between 315 Ma and 300 Ma, shifting from NW–SE (East Variscan Shear Zone) to NE–SW (Posada Valley Shear Zone).

Author Contributions

Conceptualization, F.M. and F.M.E.; methodology, F.M. and F.M.E.; software, F.M.; validation, F.M.E.; formal analysis, F.M.; investigation, F.M. and F.M.E.; resources, F.M.E.; data curation, F.M. and F.M.E.; writing—original draft preparation, F.M. and F.M.E.; writing—review and editing, F.M. and F.M.E.; visualization, F.M. and F.M.E.; supervision, F.M.E.; project administration, F.M.E.; funding acquisition, F.M. and F.M.E. All authors have read and agreed to the published version of the manuscript.

Funding

Funding was provided by 100022-2022-ALTRIPOSTL-Mantovani-XXXVII ciclo-10% per attività di ricerca in Italia e all’estero-MANTOVANI.

Data Availability Statement

All the data are presented in the paper.

Acknowledgments

The authors thank Laura Negretti (UNIGE) for her assistance with SEM–EDS analyses and Anastasia Cella, Barbara Loretti, Milis Cella and Grappa Loretti Elter for the emotional support. We warmly acknowledge revisions by referees who allowed for the improvement of the early version of the manuscript with their very constructive comments.

Conflicts of Interest

The authors declare no conflicts of interest. The funders had no role in the design of the study, in the collection, analyses, or interpretation of data, in the writing of the manuscript or in the decision to publish the results.

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Figure 1. Sketch map of the Variscan Belt in Sardinia and its zones.
Figure 1. Sketch map of the Variscan Belt in Sardinia and its zones.
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Figure 2. Simplified map showing the metamorphic zonation of the studied area (red rectangle) with the locations of sampling and field investigations (modified from [27]).
Figure 2. Simplified map showing the metamorphic zonation of the studied area (red rectangle) with the locations of sampling and field investigations (modified from [27]).
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Figure 3. Geological map of the study area showing the locations mentioned in this paper; modified after [28].
Figure 3. Geological map of the study area showing the locations mentioned in this paper; modified after [28].
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Figure 4. S. Lucia complex: (A) Grt-Ab-Olg-bearing micaschist (seen from SW); (B) contact between porphyroids and paragneiss (seen from E).
Figure 4. S. Lucia complex: (A) Grt-Ab-Olg-bearing micaschist (seen from SW); (B) contact between porphyroids and paragneiss (seen from E).
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Figure 5. Stereonet (equal area, lower hemisphere) of the S. Lucia complex: (A) rose diagram of 52 S2 planes; (B) contours calculation of 40 mineralogical lineations (L2) on the S2 surface.
Figure 5. Stereonet (equal area, lower hemisphere) of the S. Lucia complex: (A) rose diagram of 52 S2 planes; (B) contours calculation of 40 mineralogical lineations (L2) on the S2 surface.
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Figure 6. S. Lucia complex: (A) rare S1 relics transposed by S2 folds (seen from SW); (B) crenulation Cleavage (seen from W); (C) open fold in porphyroid (seen from SW).
Figure 6. S. Lucia complex: (A) rare S1 relics transposed by S2 folds (seen from SW); (B) crenulation Cleavage (seen from W); (C) open fold in porphyroid (seen from SW).
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Figure 7. La Caletta-Siniscola augen gneiss: (A) augen gneiss (seen from N); (B) mylonite I S-C [33,34,35] top-to-right sense of shear on the XZ plane (seen from S); (C) thin section of mylonitic I S-C [33,34,35] with a clast showing top-to-right sense of shear (oriented sample, seen from S).
Figure 7. La Caletta-Siniscola augen gneiss: (A) augen gneiss (seen from N); (B) mylonite I S-C [33,34,35] top-to-right sense of shear on the XZ plane (seen from S); (C) thin section of mylonitic I S-C [33,34,35] with a clast showing top-to-right sense of shear (oriented sample, seen from S).
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Figure 8. The Lodè-Mamone leucocratic orthogneiss: (A) leucocratic orthogneiss (seen from S); (B) thin section (oriented sample; seen from S).
Figure 8. The Lodè-Mamone leucocratic orthogneiss: (A) leucocratic orthogneiss (seen from S); (B) thin section (oriented sample; seen from S).
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Figure 9. Rio Mannu granodioritic orthogneiss: (A) orthogneiss (seen from E); (B) xenolith embedded in the granodioritic orthogneiss (seen from SE); (C) basic enclave embedded in the granodioritic orthogneiss (seen from W); (D) aplite enclave embedded in the granodioritic orthogneiss (seen from W); (E) shear band in the granodioritic orthogneiss (seen from SE).
Figure 9. Rio Mannu granodioritic orthogneiss: (A) orthogneiss (seen from E); (B) xenolith embedded in the granodioritic orthogneiss (seen from SE); (C) basic enclave embedded in the granodioritic orthogneiss (seen from W); (D) aplite enclave embedded in the granodioritic orthogneiss (seen from W); (E) shear band in the granodioritic orthogneiss (seen from SE).
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Figure 10. Stereonet (equal area, lower hemisphere) of the Rio Mannu granodioritic orthogneiss: (A) rose diagram of 52 Sm planes; (B) contour calculation of 41 mylonitic lineations (Lm) on the Sm surface.
Figure 10. Stereonet (equal area, lower hemisphere) of the Rio Mannu granodioritic orthogneiss: (A) rose diagram of 52 Sm planes; (B) contour calculation of 41 mylonitic lineations (Lm) on the Sm surface.
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Figure 11. The Rio Mannu granodioritic orthogneiss: (A) drag folds with a top-to-left sense of shear on the XZ plane (seen from SE); (B) σ-type Kfs porphyroclasts showing a top-to right sense of shear (seen from S); (C) thin section of a σ-type Qz porphyroclasts showing a top-to right sense of shear (oriented sample; seen from S); (D) σ-type Kfs porphyroclasts showing a top-to right sense of shear (seen from S); (E) rare sheath fold (seen from S).
Figure 11. The Rio Mannu granodioritic orthogneiss: (A) drag folds with a top-to-left sense of shear on the XZ plane (seen from SE); (B) σ-type Kfs porphyroclasts showing a top-to right sense of shear (seen from S); (C) thin section of a σ-type Qz porphyroclasts showing a top-to right sense of shear (oriented sample; seen from S); (D) σ-type Kfs porphyroclasts showing a top-to right sense of shear (seen from S); (E) rare sheath fold (seen from S).
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Figure 12. Simplified geological map of the area highlighting marble and calc-silicate lenses.
Figure 12. Simplified geological map of the area highlighting marble and calc-silicate lenses.
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Figure 13. The contact aureoles: (A) static Grt + Ves-bearing marble related to the late Variscan Punta Tepilora granite contact aureole (seen from SW); (B) contact (dotted black line) between blue marble and the deformed Grt-Bearing calc-silicate related to the Ordovician Rio Mannu granodioritic orthogneiss (seen from S); (C) static And-bearing hornfel related to the late Variscan Punta Tepilora granite contact aureole (seen from SW).
Figure 13. The contact aureoles: (A) static Grt + Ves-bearing marble related to the late Variscan Punta Tepilora granite contact aureole (seen from SW); (B) contact (dotted black line) between blue marble and the deformed Grt-Bearing calc-silicate related to the Ordovician Rio Mannu granodioritic orthogneiss (seen from S); (C) static And-bearing hornfel related to the late Variscan Punta Tepilora granite contact aureole (seen from SW).
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Figure 14. Punta Gortomedda complex: (A) St-Grt-bearing micaschist (seen from S); (B) dark gray marble (seen from SE).
Figure 14. Punta Gortomedda complex: (A) St-Grt-bearing micaschist (seen from S); (B) dark gray marble (seen from SE).
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Figure 15. Stereonet (equal area, lower hemisphere) of the Punta Gortomedda complex: (A) rose diagram of 43 S2 planes; (B) contour calculation of 38 mineralogical lineations (L2) on the S2 surface; (C) rose diagram of 48 Sm planes; (D) contour calculation of 35 milonitic lineations (Lm) on the Sm surface.
Figure 15. Stereonet (equal area, lower hemisphere) of the Punta Gortomedda complex: (A) rose diagram of 43 S2 planes; (B) contour calculation of 38 mineralogical lineations (L2) on the S2 surface; (C) rose diagram of 48 Sm planes; (D) contour calculation of 35 milonitic lineations (Lm) on the Sm surface.
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Figure 16. Punta Gortomedda complex: (A) thin section of a mica fish showing a top-to-right sense of shear in the type I S-C mylonite [33,34,35] (oriented sample; seen from S); (B) thin section of a mica fish showing a top-to-right sense of shear in the I S-C mylonite [33,34,35] (oriented sample; seen from SW); (C) thin section of a deformed Grt wrapped by type B3 ribbon Qz [45,46] (oriented sample; seen from SE).
Figure 16. Punta Gortomedda complex: (A) thin section of a mica fish showing a top-to-right sense of shear in the type I S-C mylonite [33,34,35] (oriented sample; seen from S); (B) thin section of a mica fish showing a top-to-right sense of shear in the I S-C mylonite [33,34,35] (oriented sample; seen from SW); (C) thin section of a deformed Grt wrapped by type B3 ribbon Qz [45,46] (oriented sample; seen from SE).
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Figure 17. The Fruncu Nieddu complex: (A) Ky-bearing micaschist (seen from SE); (B) Ky-Grt-bearing micaschist; (C) Qz ribbon in the I S-C mylonite [33,34,35] paragneiss (seen from S).
Figure 17. The Fruncu Nieddu complex: (A) Ky-bearing micaschist (seen from SE); (B) Ky-Grt-bearing micaschist; (C) Qz ribbon in the I S-C mylonite [33,34,35] paragneiss (seen from S).
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Figure 18. Stereonet (equal area, lower hemisphere) of the Fruncu Nieddu complex: (A) rose diagram of 33 S2 planes; (B) contour calculation of 39 mineralogical lineations (L2) on the S2 surface; (C) rose diagram of 52 Sm planes; (D) contour calculation of 44 mylonitic lineations (Lm) on the Sm surface.
Figure 18. Stereonet (equal area, lower hemisphere) of the Fruncu Nieddu complex: (A) rose diagram of 33 S2 planes; (B) contour calculation of 39 mineralogical lineations (L2) on the S2 surface; (C) rose diagram of 52 Sm planes; (D) contour calculation of 44 mylonitic lineations (Lm) on the Sm surface.
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Figure 19. Fruncu Nieddu complex: (A) Thin section showing a top-to-right Ky fish with Ms rim and Grt with snowball structures (oriented sample; seen from S); (B) thin section of II S–C mylonite [33,34,35] showing a top-to-right Ky fish with Ms rim (oriented sample; seen from S); (C) thin section of II S–C [33,34,35] mylonite showing relics of Ky, Ms and Grt (oriented sample; seen from S).
Figure 19. Fruncu Nieddu complex: (A) Thin section showing a top-to-right Ky fish with Ms rim and Grt with snowball structures (oriented sample; seen from S); (B) thin section of II S–C mylonite [33,34,35] showing a top-to-right Ky fish with Ms rim (oriented sample; seen from S); (C) thin section of II S–C [33,34,35] mylonite showing relics of Ky, Ms and Grt (oriented sample; seen from S).
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Figure 20. Punta Figliacoro complex (sensu stricto): (A) thin section of laminated amphibolite (oriented sample; seen from S); (B) laminated orthoderivates with σ-type Kfs porphyroclasts (seen from S).
Figure 20. Punta Figliacoro complex (sensu stricto): (A) thin section of laminated amphibolite (oriented sample; seen from S); (B) laminated orthoderivates with σ-type Kfs porphyroclasts (seen from S).
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Figure 21. Stereonet (equal area, lower hemisphere) of the Punta Figliacoro complex sensu stricto: (A) rose diagram of 28 S2 planes; (B) contour calculation of 37 mineralogical lineations (L2) on the S2 surface; (C) rose diagram of 27 Sm planes; (D) contour calculation of 25 mylonitic lineations (Lm) on the Sm surface.
Figure 21. Stereonet (equal area, lower hemisphere) of the Punta Figliacoro complex sensu stricto: (A) rose diagram of 28 S2 planes; (B) contour calculation of 37 mineralogical lineations (L2) on the S2 surface; (C) rose diagram of 27 Sm planes; (D) contour calculation of 25 mylonitic lineations (Lm) on the Sm surface.
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Figure 22. The Pedra su Gattu complex: (A) phyllonite with Ab porphyroclasts (seen from E); (B) thin section of the phyllonite with a σ-type Ab porphyroclasts deformed by Sm with a top-to-right sense of shear (oriented sample; seen from S).
Figure 22. The Pedra su Gattu complex: (A) phyllonite with Ab porphyroclasts (seen from E); (B) thin section of the phyllonite with a σ-type Ab porphyroclasts deformed by Sm with a top-to-right sense of shear (oriented sample; seen from S).
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Figure 23. Stereonet (equal area, lower hemisphere) of the Pedra su Gattu complex: (A) rose diagram of 24 Sm planes; (B) contours calculation of 29 mylonitic lineations (Lm) on the Sm surface.
Figure 23. Stereonet (equal area, lower hemisphere) of the Pedra su Gattu complex: (A) rose diagram of 24 Sm planes; (B) contours calculation of 29 mylonitic lineations (Lm) on the Sm surface.
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Figure 24. The Sedda Eneas complex: (A) paragneiss with a leucosome (Qz + Pl) ribbon (seen from S); (B) Sil-Ms-bearing gneiss with stromatitic structure [101] (seen from S).
Figure 24. The Sedda Eneas complex: (A) paragneiss with a leucosome (Qz + Pl) ribbon (seen from S); (B) Sil-Ms-bearing gneiss with stromatitic structure [101] (seen from S).
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Figure 25. Stereonet (equal area, lower hemisphere) of the Sedda Eneas complex: (A) rose diagram of 39 S2 planes (green petals) and 34 S3 planes (pink petals); (B) contours calculation of 42 mineralogical lineations (L2) on the S2 surface; (C) rose diagram of 41 Sm planes; (D) contours calculation of 36 mylonitic lineations (Lm) on the Sm surface.
Figure 25. Stereonet (equal area, lower hemisphere) of the Sedda Eneas complex: (A) rose diagram of 39 S2 planes (green petals) and 34 S3 planes (pink petals); (B) contours calculation of 42 mineralogical lineations (L2) on the S2 surface; (C) rose diagram of 41 Sm planes; (D) contours calculation of 36 mylonitic lineations (Lm) on the Sm surface.
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Figure 26. Hypothetical sketch of the studied area and a simplified scheme showing structural relationship between the Inner Nappe Zone (INZ), the Condensed Isogrades Zone (CIZ), the Posada Valley Shear Zone (PVSZ) and the Regional Mylonitic Complex (RMC); both seen from above.
Figure 26. Hypothetical sketch of the studied area and a simplified scheme showing structural relationship between the Inner Nappe Zone (INZ), the Condensed Isogrades Zone (CIZ), the Posada Valley Shear Zone (PVSZ) and the Regional Mylonitic Complex (RMC); both seen from above.
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Table 1. Number of data points collected during the survey campaign, divided into planar and linear elements for each location. The locations are categorized according to their respective metamorphic zones. Abbreviation: (CIZ) Condensed Isogrades Zone; (INZ) Internal Nappe Zone; (L2) Lineation; (Lm) Mylonitic lineation; (S2 and S3) Schistosity; (Sm) Mylonitic schistosity.
Table 1. Number of data points collected during the survey campaign, divided into planar and linear elements for each location. The locations are categorized according to their respective metamorphic zones. Abbreviation: (CIZ) Condensed Isogrades Zone; (INZ) Internal Nappe Zone; (L2) Lineation; (Lm) Mylonitic lineation; (S2 and S3) Schistosity; (Sm) Mylonitic schistosity.
LocalitiesPlanar AnisotropyLineation
S2S3SmL2Lm
INZSanta Lucia52 40
Lodè-Mamone-Rio Mannu 52 41
CIZP.ta Gortomedda43 483835
Fruncu Nieddu33 523944
P.ta Figliacoro28 273725
Pedra su Gattu 24 29
Sedda Eneas3934414236
Sub-total 19534244196210
Total 473406
Table 2. Chronological framework of events that affected the High-Grade Metamorphic Complex; modified from [32]. Abbreviation: (EVSZ) East Variscan Shear Zone; (L2) Lineation; (Lm) Mylonitic lineation; (M2 and M3) Metamorphic events; (S2 and S3) Schistosity; (Sm) Mylonitic schistosity; (PVSZ) Posada Valley Shear Zone.
Table 2. Chronological framework of events that affected the High-Grade Metamorphic Complex; modified from [32]. Abbreviation: (EVSZ) East Variscan Shear Zone; (L2) Lineation; (Lm) Mylonitic lineation; (M2 and M3) Metamorphic events; (S2 and S3) Schistosity; (Sm) Mylonitic schistosity; (PVSZ) Posada Valley Shear Zone.
AgeTectonic FrameMetamorphismPlanar AnisotropyLineationP-T Condition
300PVSZ (dextral strike-slip top-to-E)Greenschist stage (M3)Sm in the PVSZLm in the PVSZ300–400 °C; 0.2–0.3 GPa
310
315EVSZ (dextral strike–slip top-to-SE)S3L3
320
330
340Laurussia-Gondwana transpressional event (top to SE)Granulitic to amphibolitic stage (M2)S2L2650–750 °C and 0.8–1.2 GPa; 550–740 °C and 0.3–0.7 GPa
350
360
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Elter, F.M.; Mantovani, F. The Relationships between the Internal Nappe Zone and the Regional Mylonitic Complex in the NE Variscan Sardinia (Italy): Insight from a New Possible Regional Interpretation? Geosciences 2024, 14, 260. https://doi.org/10.3390/geosciences14100260

AMA Style

Elter FM, Mantovani F. The Relationships between the Internal Nappe Zone and the Regional Mylonitic Complex in the NE Variscan Sardinia (Italy): Insight from a New Possible Regional Interpretation? Geosciences. 2024; 14(10):260. https://doi.org/10.3390/geosciences14100260

Chicago/Turabian Style

Elter, Franco Marco, and Federico Mantovani. 2024. "The Relationships between the Internal Nappe Zone and the Regional Mylonitic Complex in the NE Variscan Sardinia (Italy): Insight from a New Possible Regional Interpretation?" Geosciences 14, no. 10: 260. https://doi.org/10.3390/geosciences14100260

APA Style

Elter, F. M., & Mantovani, F. (2024). The Relationships between the Internal Nappe Zone and the Regional Mylonitic Complex in the NE Variscan Sardinia (Italy): Insight from a New Possible Regional Interpretation? Geosciences, 14(10), 260. https://doi.org/10.3390/geosciences14100260

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