Contact metamorphism in Middle Ordovician arc rocks (SW Sardinia, Italy): New paleogeographic constraints

    Contact metamorphism in Middle Ordovician arc rocks (SW Sardinia, Italy): New paleogeographic constraints Luca Giacomo Costamagna, Franco Marco Elter, Laura Gaggero, Federico Mantovani PII: DOI: Reference:

S0024-4937(16)30302-4 doi:10.1016/j.lithos.2016.09.014 LITHOS 4075

To appear in:

LITHOS

Received date: Accepted date:

14 May 2016 11 September 2016

Please cite this article as: Costamagna, Luca Giacomo, Elter, Franco Marco, Gaggero, Laura, Mantovani, Federico, Contact metamorphism in Middle Ordovician arc rocks (SW Sardinia, Italy): New paleogeographic constraints, LITHOS (2016), doi:10.1016/j.lithos.2016.09.014

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ACCEPTED MANUSCRIPT CONTACT METAMORPHISM IN MIDDLE ORDOVICIAN ARC ROCKS (SW SARDINIA, ITALY): NEW PALEOGEOGRAPHIC CONSTRAINTS

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Luca Giacomo Costamagna1, Franco Marco Elter2, Laura Gaggero2*, Federico

Department of Chemical and Geological Sciences (DSCG), University of Cagliari, via

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Trentino 51, I-09127, Cagliari, Italy, [email protected] 2

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Mantovani2

Department of Earth, Environment and Life Sciences (DISTAV), University of Genoa,

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Corso Europa 26, I-16132, Genoa, Italy, [email protected], [email protected] *Corresponding Author (e-mail: [email protected])

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Abstract

In the early Cambrian Bithia Formation in the Variscan foreland of Sardinia, a Middle

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Ordovician granitic intrusion (478–457 Ma) is hosted by marly metasedimentary rocks that were affected by high-temperature (HT) metamorphism. A detailed structural–petrographical transect was conducted through the granitic intrusion and its host rocks. Field data and

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relationships between HT/low-pressure (LP) mineral assemblages in the metasedimentary rocks (Grt + Wo + Ves in carbonate lenses and And in pelite) 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. In the Canigò or Canigou Massif (Eastern Pyrenees), the Somail Massif (Montagne Noire), and the Ruitor Massif (Internal Massifs, NW Alps), Middle Ordovician orthogneiss with relict igneous textures are deciphered despite being overprinted by Variscan amphibolite-to-granulite-facies metamorphism and subsequent Alpine low-grade metamorphism. Comparisons of associated igneous and metasedimentary rocks in the 1

ACCEPTED MANUSCRIPT Sardinia foreland with the High-Grade Metamorphic Complex in the Variscan Axial Zone and the Canigou Massif indicate a convergent Middle Ordovician evolution that was overprinted

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by HT Variscan metamorphism.

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Keywords

Ordovician contact metamorphism; geothermometry; pseudosection; mineral chemistry;

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metasedimentary rocks

1. Introduction

In continental successions of Paleozoic terranes, the study of the interplay of

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sedimentary, igneous, and tectono-metamorphic events is the primary tool for unravelling the geological history of, and understanding links between, similar tectonic units. A few

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basement rocks record an exceptional convergence of events, such as at Mt. Settiballas in SW Sardinia. The present study aims to re-appraise the characteristics and structural evolution of the Southern Sulcis Complex, with particular attention to the composition of the

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pre-existing protoliths, their geotectonic relationships, and their evolution since the early Paleozoic. Close associations between high-grade metasedimentary rocks and foliated granitic intrusions have resulted in multiple interpretations of previous authors, with different geodynamic significance. We conducted lithological and structural mapping to study the main exposed lithologies, to identify the major lithofacies, and to select the most reliable locations for collecting samples and making structural measurements. Mineral compositions of significant assemblages analyzed under polarized light optical microscopy and Scanning Electron Microscopy, coupled with Energy Dispersive Spectrometry (SEM-EDS) allowed microtextures to be linked to a quantified pressure–temperature (P–T) evolution. Following 2

ACCEPTED MANUSCRIPT the work of Elter and Palmeri (1992) and by making detailed field investigations of stratigraphic and structural constraints, we develop a new interpretation of the intrusions and their associated metasedimentary rocks, representing an important contribution to

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understanding the pre-Variscan and collisional history of this Gondwana-derived Variscide

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segment.

2. Geological setting

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The Variscan basement of Sardinia, Italy, has been subdivided into four NW–SE-trending metamorphic zones (Elter et al., 2004, 2010), which show an increase in metamorphic grade

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toward the NE (Franceschelli et al., 2005). From SW to NE, the four zones are the External Zone (foreland), the Nappe Zone (subdivided into the External and Internal Nappe Zones), the Condensed Isograds Zone (Posada–Asinara Shear Zone; Elter et al., 2010), and the

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Axial Zone or High-Grade Metamorphic Complex (HGMC; Elter et al., 2010) (Fig. 1). The External Zone represents the foreland of the Variscan chain and consists mainly of

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Cambrian–Lower Ordovician terrigenous sequences and carbonate platform successions. The Nappe Zone is characterized by Cambrian–Ordovician metasedimentary strata, Ordovician meta-volcanic rocks, and Silurian–Devonian metasedimentary sequences, which

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are stacked in a NW–SE-trending nappe with a main vergence toward the SW. The foreland and Nappe Zones are also characterized by a Middle Ordovician angular unconformity (the Sardic Unconformity; Carmignani et al., 1992, 1994 and references therein) that is also recognized in the Eastern Iberian Plate (Casas et al., 2010; Navidad et al., 2010). Beneath the Sardic Unconformity, the Gondwanian Sardinia basement records a Cambrian–Early Ordovician volcanic event ascribed to an incipient rift geodynamic environment (Oggiano et al., 2010; Gaggero et al., 2012), as suggested by the younging ages of felsic volcanic rocks along a S-to-N transect (491 ± 3.5 and 486 ± 1.2 Ma in the S and to 479 ± 2.1 Ma in the N). Subsequent widespread Middle Ordovician bimodal calc-alkaline volcanism is stratigraphically constrained between the Sardic Unconformity and the transgressive 3

ACCEPTED MANUSCRIPT sedimentary cover of the volcanic suite, which is considered to be Late Ordovician (Katian– Hirnantian) based on biostratigraphy. Radiometric ages of the volcanic rocks bounded by the unconformities yielded consistent ages: Giacomini et al. (2006) obtained an age of 460 ± 1

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Ma from a meta-rhyolite in the External Nappe Zone; Helbing and Tiepolo (2005) determined an age of 456 ± 14 Ma for the Lodè orthogneiss in the Internal Nappe Zone; and Oggiano et

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al. (2010) obtained a U–Pb age of 465.0 ± 1.4 Ma for a dacite dike that cross-cuts the Cambrian–Lower Ordovician meta-sandstone basement of the External Nappe Zone.

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In the Condensed Isograds Zone (Elter et al., 2010; Padovano et al., 2014), an increase in metamorphic grade from greenschist to granulite facies is recorded from SW to NE along

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the Posada–Asinara Line (Elter et al., 1990). The High-Grade Metamorphic Complex is divided into two high-temperature (HT) complexes (Elter et al., 2010): the high-grade Old Gneiss Complex (amphibolite to granulite facies), and the New Gneiss Complex

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(greenschist to amphibolite facies), which was sheared during retrograde metamorphism. 2.1. Geology of the Sardinia foreland

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In SW Sardinia, the Sulcis–Iglesiente Complex forms part of the External Zone of the Sardinian Variscan chain (Carmignani et al., 1994). In the southernmost Sulcis (Fig. 2A and

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B), a medium-grade metamorphic complex (Mt. Filau Core Complex, Carmignani et al., 1992, 1994; Southern Sulcis Complex, Carosi et al., 1998) is composed of orthogneiss and “micaschist” Auct..

The Southern Sulcis Complex is surrounded and superposed by the Bithia Formation, which consists of epimetamorphic fine-grained siliciclastic rocks, rare coarsegrained siliciclastic rocks, minor carbonate beds, and scarce volcanic rocks (Fig. 3). The Bithia Formation is poorly understood, and its age and spatial extent have only recently been constrained. Paleontological and radiometric dating conducted on numerous samples have yielded disparate results, ranging from early Cambrian (Bechstädt and Boni, 1994) to Late Ordovician (Pavanetto et al., 2012). The latter study hypothesized that the entire Mt. Filau– 4

ACCEPTED MANUSCRIPT Settiballas–Bithia block was an Ordovician allochthonous synformal klippe that was transported westwards over the autochthonous Sulcis Cambrian–Ordovician succession during Variscan orogenesis.

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Carmignani et al. (1992, 1994) interpreted the Mt. Filau Core Complex as a portion of lower crust that was superposed by the low-grade metamorphic crust rocks of the Bithia

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Formation by means of low-angle extensional faults, creating a mylonitic shear belt. Carosi et al. (1995, 1998) described three deformation events that affected the Southern Sulcis

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Complex and the enclosing Bithia Formation: early deformation that was probably of Cadomian age (Costamagna, 2014), a second deformation event related to WSW–ENE

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Variscan compression, and subsequent deformation that may have resulted from extensional uplift.

The Southern Sulcis Complex (Fig. 2A and B) is composed of the Mt. Filau Orthogneiss

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and the Settiballas Micaschists, whose mutual relationships remain controversial (Mazzoli and Visonà, 1992 and references therein). The orthogneiss is considered to have been

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derived from Middle Ordovician granitic intrusions that were metamorphosed during Variscan orogenesis (Delaperriere and Lancelot, 1989; Ludwig and Turi, 1989; Mazzoli and Visonà, 1992). A crystallization age of 457 ± 0.17 Ma was determined for the granitic

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protolith using the chemical abrasion thermal ionization mass spectrometry (CA-TIMS) U–Pb method on zircon separates (Pavanetto et al., 2012). The Settiballas Micaschists are composed of Precambrian siliciclastic strata with a polymetamorphic history and relict amphibolite-facies assemblages, and the strata have been interpreted to represent either older basement (Mazzoli and Visonà, 1992) or distinct thermo-metamorphic facies of the Bithia Formation (Carosi et al., 1998). The Southern Sulcis Complex is overlain by the Bithia Formation (BF; Fig. 3), which is a 500-m-thick, epimetamorphic, mainly terrigenous succession of siltstone–sandstone containing rare calc-alkaline pyroclastic rocks and lavas and minor 1-m-thick carbonate beds (Costamagna, 2014). The contact between the BF and the Southern Sulcis Complex is 5

ACCEPTED MANUSCRIPT tectonic as a result of subsequent Variscan events, but it is also a pristine intrusive contact in places. The upper BF unconformable boundary with the Matoppa Formation (Pillola, 1990) is marked by the Malfatano Metaconglomerate (Costamagna, 2014), a coarse-

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grained, poly- to monogenic unit measuring up to 100 m thick. This unit probably represents a Cadomian tectonic phase and the beginning of the early Cambrian–Early Ordovician

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tectono-sedimentary cycle of SW Sardinia (Bechstädt and Boni, 1994 and references therein).

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An undetermined Archeaocyatha assemblage that was recently discovered in the BF yielded an early Cambrian age (Costamagna, 2014), confirming that the BF forms the

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autochthonous basement of the Cambrian–Ordovician Sulcis–Iglesiente sequence. The Variscan stacking phase then produced thrusts and gentle folds in the Ordovician sediments

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and meta-volcanic rocks (Pavanetto et al., 2012), as well as in the early Cambrian BF.

3. Geology of the study area

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The study area contains two lithotypes: the Mt. Filau Granite (FG; Mt. Filau Orthogneiss Auct.), which was dated at 478 ± 16 Ma by Delaperriere and Lancelot (1989) and at 457 ± 0.17 Ma by Pavanetto et al. (2012); and the Settiballas Spotted Schists (SSS; Settiballas

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Micaschists Auct.) (Figs. 2A and B, 4, and 5). The FG is white to gray in color (Fig. 4A) and exhibits increasing deformation intensity from core to edge, with decreasing grain size closer to the contact with the SSS, where the FG becomes mylonitic (Mylonitized Mt. Filau Granite, MFG; Fig. 4B and C). The thickness of the mylonitic interval ranges from a few centimeters to 10 m (Fig. 4). The SSS are dark colored and exhibit ellipse-shaped, mm- to cm-sized bands defined by andalusite (And; Fig. 4E) that decrease in diameter with increasing distance from the contact with the FG and MFG. The And bands were locally gently folded by a later tectonic phase (Fig. 5D and E). The main protolith of the SSS was pelitic, with scarce meta-siliciclastic, meta-volcanic, and marble horizons (Fig. 4H). In the SSS, rare dm- to m-thick marble lenses 6

ACCEPTED MANUSCRIPT (ML; Fig. 4F) characterized by cm-sized Grt, Ves, and Wo have been stretched parallel to the schistosity, and are interpreted to have had a marly protolith. The FG, MFG, and SSS are in tectonic contact with the overlying anchimetamorphic BF.

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A Variscan leucogranite cross-cuts the And-bearing banding in the SSS (Fig. 4D). Subsequent tectonic events are represented by extensional quartz veins in the BF thrust and

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by aplite veins that cross-cut the SSS and FG.

Near Cogoni Beach, the FG, MFG, and SSS are in contact with a late Carboniferous

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granodiorite that was foliated in a visco-plastic regime (Fig. 4G; Arburese granitic intrusions, 304–287 Ma; Boni et al., 2003; Naitza et al., 2015). A thin hornfels band represents the

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thermo-metamorphic aureole in the SSS. To the east, one of the ML developed a randomly oriented, green, epidote-rich alteration patch replacing the earlier Wo + Grt + Ves assemblage as a static thermo-metamorphic overprint (Fig. 4). The ML are also

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characterized by the growth of epidote sprays in extensional fractures. Stretching and drag folds of limestone bands occur close to the contact (Fig. 4H).

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At Chia Beach, an additional late Carboniferous granitic intrusion is in contact with the anchimetamorphic BF. Randomly oriented, mm-sized And spots and folds in marble

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represent the localized thermal overprint (Fig. 4H).

4. Structural features A pervasive secondary foliation, defined as SX, lacks clear geometrical superimposition relationships and is continuously developed in the SSS and MFG. Sx is expressed by highstrain quartz (Qtz) and K-feldspar (Kfs; Fig. 4B) in the MFG (Fig. 5C), by the preferred orientation of And in the SSS, and by Wo, Ves, and stretched Grt (Fig. 4F) in the ML. Sx becomes less recognizable with decreasing distance from the interior of the unit (Fig. 4A). In the SSS, ML, and MFG, the main mesoscale kinematic indicators on the Sx–XZ plane are: i) And, Kfs, and Grt σ-type structures, stepped slickenlines, and quarter mats; ii) And and Kfs domino porphyroclasts and synthetic microfaults; iii) fold asymmetry; and iv) S–C 7

ACCEPTED MANUSCRIPT planes with N-type flanking folds (Passchier and Trouw, 1996). All kinematic indicators show a general top-to-the-SE sense of shear, but a top-to-the-NW sense of shear is recorded locally.

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Sx strikes generally NW–SE and dips toward the NE in the SSS and MFG (Fig. 6A). On the Sx surface, a well-defined And mineral lineation is developed in the SSS, whereas rods

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of Kfs + Qtz are developed in the MFG. The And lineation (Fig. 5F) and the Kfs + Qtz rods (Fig. 5G) strike NW–SE and dip to the NW (Fig. 6B). The And spots are ellipse shaped,

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generally with the maximum axis (X) oriented parallel to Sx (pure shear; Fig. 5A), but in places also exhibiting evidence of simple shear (Fig. 5B).

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In the SSS, Sx is overprinted by two tectonic events following the And stretching and preceding the emplacement of granitic intrusions from 304 to 287 Ma. The events are therefore interpreted as Variscan in age. The first event produced NE–SW-trending fold axes

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(Fig. 6B) without axial plane anisotropy (Fig. 5D), and the second event formed tight to open folds with NW–SE-trending axes (Figs. 5E, 6B).

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In the SSS, ML, and MFG, the strain on the XZSx surface in the assemblage Qtz + Kfs + And was compared with that in the Grt + Kfs assemblage (Fig. 7). In spite of different rheological behaviors and host rocks, And strain values in the SSS are similar to Grt values

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in the ML, and to Qtz + Kfs values in the MFG. In general, the X major axis intersects the reference surface (Sx) at 0°, indicating pure shear. The strain data have significant implications for the evolution of the FG.

5. Analytical methods Compositions of major, trace, and rare earth elements (REEs) were determined for sample FM26 at Activation Laboratories Ltd. (Actlabs) in Canada, using the lithium metaborate/tetraborate fusion inductively coupled plasma–mass spectrometry (ICP–MS) method. The mineral phases were analyzed using a Tescan Vega 3 LM Scanning Electron 8

ACCEPTED MANUSCRIPT Microscope (SEM) equipped with an Apollo X detector and a Microanalysis TEAM Electron Dispersive Spectroscopy (EDS) System with operating conditions of 15 kV accelerating voltage and 2.1 nA of beam current on graphite-coated samples. Reference standards for

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the elements (shown in parentheses) were jadeite (Na), forsterite (Mg), albite (Al), augite (Si, Ca), microcline (K), ilmenite (Ti), chromite (Cr), rhodonite (Mn), and fayalite (Fe). Other

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elements were below detection limits.

The pyroxene atomic proportions were calculated assuming stoichiometry and charge

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balance. The nomenclature followed the scheme of Morimoto (1988) and Rock (1990). Garnet atomic proportions were calculated using 5 cations and 12 oxygens. Andalusite

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compositions are based on two cations and five oxygens. Epidote was normalized to 12.5 oxygens. Vesuvianite atomic proportions are based on 16 cations, 36.5 oxygens, and 10 OH molecules. Micas were normalized to 11 oxygens with a total cation charge (K + Na + Ca) of

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6. Mineral abbreviations follow Kretz (1983).

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6. Petrography

Field relationships and microtextures of the SSS and ML suggest that these units shared the first HT overprint during the Middle-Late Ordovician emplacement of the FG at 457 ±

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0.17 Ma (Pavanetto et al., 2012).

6.1. Mt. Settiballas Spotted Schists (SSS) The Mt. Settiballas metasedimentary strata are meta-siltstones that are composed of biotite (Bt), muscovite (Ms), Qtz, and feldspars, all of which occur as pristine clastic minerals. The mineral assemblages range from being Bt-rich (40–50 vol.%) to Qtz-rich (35– 45 vol.%). The psammitic texture is characterized by homogeneous grain size. The first HT event resulted in a grain-size coarsening of Bt and Ms (Fig. 4E). The second HT event produced peak metamorphic conditions in the SSS in contact with the MFG and FG, characterized by the development of coarse-grained And that decreases in size with increasing distance from the FG (Fig. 8), suggesting a thermal gradient. 9

ACCEPTED MANUSCRIPT Sx is characterized by the mineral assemblage And + Ms + Bt + Qtz ± oxides. The And grew at the expense of Ms and is deformed and partially replaced by fine-grained Ms ± Qtz (Fig. 8A–F).

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6.2. Marble lenses (ML)

The ML associated with the SSS have marly compositions and are characterized by the

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mineral assemblage Wo + Grt ± Di ± En ± Cal ± Ves ± Qtz ± Ap, which defines Sx. Within the ML, nematoblastic Wo1 and Di and granoblastic Grt1 are the most abundant phases (Fig.

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9A–F).

Backscattered electron images of garnet (Fig. 9B and E) support static recrystallization

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associated with compositional change (Fig. 9E). Microtextures suggest that Wo2, Di, Grt2, and Ves grew under strain-free, static conditions, forming fine-grained crystal aggregates at the expense of Wo1, Grt1, and Di.

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6.3. Mylonitic Mt. Filau Granite (MFG)

In thin sections of oriented samples, the mineral lineation corresponds to the X-axis, and

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Sx is defined by Qtz + feldspars + white mica. There are three types of ribbon Qtz, which are parallel to Sx and are interpreted as syn-kinematic: B-polycrystalline ribbons (type 1); Amonocrystalline ribbons with elongated, domino-like crystals (type 2); and tabular ribbons,

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characterized by coarse and irregularly shaped, tabular grains (type 4; Boullier and Bouchez, 1978).

6.4. Mt. Filau Granite (FG; 457 ± 0.17 Ma) The FG contains Bt porphyroclasts with inclusions of Qtz + Kfs (microcline) and Kfs porphyroclasts (Fig. 10A–C). The porphyroclastic texture formed by the Kfs within a finegrained Qtz + Bt + Pl matrix that changes gradationally to a granoblastic texture with a coarser grain size with increasing distance from the contact with the SSS. At the microscale, a subtle secondary foliation is defined by Qtz + Bt + oxides, and occurs only near the contact with the SSS (MFG).

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ACCEPTED MANUSCRIPT 6.5. Mt. Cogoni granodiorite and Chia Beach granitic intrusion Late Carboniferous intrusions cross-cut the FG–SSS contact. The Mt. Cogoni granodiorite is a 1-km-wide sub-intrusive body (Fig. 2A and B) with porphyritic texture and

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contains abundant fine-grained basic enclaves (Fig. 4G). The granodiorite cross-cuts the And bands in the SSS at a high angle (Fig. 4D). The Chia Beach granitic intrusion is a sub-

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intrusive body that exhibits clear intrusive contacts with the BF. A subtle syn- and postemplacement thermal overprint is recorded in the host metasedimentary rocks, including the

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phyllite and limestone of the BF thrust and the SSS.

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7. Mineral chemistry

And in the SSS is almost pure Al2SiO5, with a negligible Fe content of 0.01 atoms per formula unit (a.p.f.u.; Table 1).

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The brown mica that defines Sx is Bt, which has a K + Na + Ca value of 1, contains no

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AlIV in tetrahedral coordination, and lacks trioctahedral substitution. Cr occurs as a trace

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component (0.22 wt%; Table 1).

The white mica that defines Sx in the SSS is pure Ms, with Si:Al ≤ 3 (Table 1). Calcic clinopyroxene in the ML (samples FM28A and FM26) has a homogeneous

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composition (Wo52En40–47Fs0–8). In sample FM26, one analysis with an enstatitic composition (Wo2–5 En91Fs 5–8) is consistent with the stability of ferromagnesian pyroxene in the pseudosection model (Table 2). Wo has a somewhat variable composition of Wo99.42–100En0Fs0–0.58. Wo1 has a composition of Wo99.42–99.49En0–0.26Fs0.25–0.58. The composition of recrystallized Wo2 is Wo100En0Fs0. The depleted En and Fs contents of Wo2 suggest that Mg and Fe were preferentially partitioned into Ves and Grt with increasing temperature (Table 2). In the ML close to the FG (sample FM26; Fig. 9E), Grt1 has a composition of Alm1.11– 1.34Py0.30–0.52Grs91.12–83.08Sps0.13–0.00Adr5.52–11.35.

Grt2 has a composition of Alm1.47–2.02Py0.42– 11

ACCEPTED MANUSCRIPT 0.65Grs54.84–67.90Sps1.37–0.15Adr24.63-40.09.

The uvarovite content ranges from 0.21 to 0.55 mol.%.

The proportion of CaTi varies between 1.52 and 4.40 mol.%, suggesting that Grt was a sink for Ti, consistent with the scarcity of Ti-bearing minerals in the ML (Table 3).

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Neoblastic Grt2 is interstitial to Wo and Di, and has a homogeneous composition of Alm0.34–1.04Py1.02–0.98Grs67.83–69.97Sps0.31–0.44Adr24.63–27.38.

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Ves contains 0.6–1.09 wt% TiO2, 0.18–0.23 wt% Mn, and 2.47–2.54 wt% Fe (Table 4).

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8. Thermobarometry

In the ML, the assemblage Grt1 + Di1 + Wo1 re-equilibrating to Grt2 + Wo2 + Ves has

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been constrained by microtextural evidence revealed by SEM–BSE imaging coupled with EDS analyses (Fig. 10A–F) and by analogous data from previous studies. A P–T pseudosection was calculated for sample FM26 from the ML in the calc-silicate

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system over a temperature range of 400–700°C and a pressure range of 0.1–0.4 GPa (Fig. 11). The P–T conditions of the first Middle-Late Ordovician HT event are estimated to be

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520–620°C at xCO2 = 0.1 and 0.2–0.4 GPa. A temperature increase is needed to attain an assemblage of Grt2 + Wo2 + Ves + Di at temperatures of 600–670°C with xCO2 = 0.1 at 0.2–0.4 GPa. Casillas et al. (2011) modeled the assemblage Ves + Wo + Di + Grt + Cal at

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temperatures exceeding 600°C at low pressures and XCO2 = 0.1. The P–T estimates obtained for contact metamorphism in the ML are consistent with those of previous studies of analogous rocks elsewhere (Table 5). In the SSS, the mineralogical association of And + Ms + Bt + Qtz suggests conditions of 0.35–0.40 GPa and 500–600°C, in accordance with Likhanov et al. (2001) and Kamvong and Zaw (2009).

9. Discussion The new geological and petrographical data allow us to revise the interpretation of the Settiballas–Mt. Filau area. Previously, the Mt. Filau Orthogneiss Auct. and Settiballas Micaschists Auct. were interpreted to represent an independent lower-crustal complex (Mt. 12

ACCEPTED MANUSCRIPT Filau Core Complex, Carmignani et al., 1992, 1994; Southern Sulcis Complex, Carosi et al., 1998) tectonically overlain by upper-crustal cover rocks with a lower metamorphic grade. Our investigations suggest that the so-called “complex” forms an integral part of the SW

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Sardinia Cambrian–Ordovician succession, which was intruded and metamorphosed by a Middle – Late Ordovician granite, and is unlikely to represent the lower part of the early

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Cambrian succession of the BF. The Settiballas Micaschists Auct. are characterized by deformed and stretched And and underwent P–T conditions of 550–660°C and 0.1–0.4 GPa;

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therefore, they are more correctly defined as Settiballas Spotted Schists (SSS). These micaschists contain rare ML. The contact relationship between the SSS and the Mt. Filau

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Granite (Orthogneiss Auct.) is characterized by a transition through mylonitic Filau Granite (MFG), which exhibits the same secondary foliation (Sx) that occurs in the SSS, and the same orientation of Qtz + Kfs lineations. Sx gradually disappears closer to the inner part of

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the Mt. Filau Granite, where igneous structures remain preserved through the cataclastic overprint. Given these textural elements, we prefer to define the Mt. Filau Orthogneiss Auct.

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as the Mt. Filau Granite (FG). The strain conditions indicated by And, Grt, Kfs, and Qtz on XZ planes are consistent in the SSS, ML, and MFG. Thus, the planar and linear structural fabrics allow us to assume that the tectonic histories of the SSS, ML, and MFG are closely

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associated. However, in the SSS, evidence of two subsequent deformation events under very-low-grade metamorphic conditions, which transposed Sx without axial plane anisotropies, supports a Variscan age for the two syn-kinematic events in the surrounding BF. The P–T values obtained for the SSS and ML are not comparable with the surrounding very-low-grade P–T conditions of the anchimetamorphic BF. Conversely, the P–T conditions derived from the mineralogical assemblages of SSS and ML (Fig. 10) suggest a depth of 3– 12 km. On the basis of the Middle – Late Ordovician age of the granitic intrusion, we infer that the FG was part of an extensive arc magmatic event that occurred concurrently in some tectonic 13

ACCEPTED MANUSCRIPT units in Sardinia (Porfidi Auct.). Considering that the ages for the FG range from 478 ± 16 Ma (Delaperriere and Lancelot, 1989) to 457 ± 0.17 Ma (Pavanetto et al., 2012), and based on textural evidence, the stretched And blasts may record the syn-emplacement thermo-

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metamorphic aureole of the FG.

This is consistent with two further considerations: i) the protolith of SSS was a pelitic

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sequence with carbonate lenses, and ii) the rocks surrounding the FG must be older than the FG itself. The FG-surrounding pelitic lithotypes belong to the early Cambrian BF, which

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contains alternating intervals of phyllite, meta-sandstone, meta-volcanic rocks, and lenses of impure marble. Consequently, the BF could have been the protolith of the SSS whereas the

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FG was emplaced within the BF, then metamorphosed to SSS during the Middle – Late Ordovician. 9.1 Regional correlations

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In the High-Grade Metamorphic Complex of NE Sardinia (Tamarispa area; Fig. 11), Elter and Palmeri (1992) highlighted the presence of a contact aureole with calc-silicate lenses

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containing Wo, dm-sized Grt, Di, and Cal that are in direct contact with the Middle – Late Ordovician San Lorenzo-Tanaunella Orthogneisses. The orthogneisses have been dated at 453 ± 10 Ma (Giacomini et al., 2006; Rossi et al., 2009). The calc-silicate lenses and the

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San Lorenzo-Tanaunella Orthogneisses are within the Brunella migmatitic gneiss, which is located in the Variscan Bt + Ms isograd zone and has been dated at 344 Ma (Franceschelli et al., 2005). As the migmatitic gneiss also contains abundant calc-silicate nodules, its protolith could be analogous to that of the BF. Furthermore, the FG and San LorenzoTanaunella Orthogneisses have similar emplacement ages. In the Settiballas–Mt. Filau area, the host rocks are affected by Variscan anchimetamorphic to incipient greenschist-facies conditions, whereas the Brunella area attained amphibolite-facies conditions. Thus, the pre-Variscan history of southern Sardinia is preserved because the HT/low-pressure (LP) metamorphism of the SSS is of higher grade than that in the adjacent BF. However, in northern Sardinia, the same geological framework 14

ACCEPTED MANUSCRIPT was overprinted by the Variscan event under amphibolite-facies conditions. Elter and Palmeri (1992) measured diameters of garnet crystals in calc-silicate lenses in the Tamarispa area of 10 to 30 cm, which is distinct from other areas in the High-Grade

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Metamorphic Complex, and inferred that garnet growth did not result from the Variscan orogenic overprint. In the Settiballas–Mt. Filau ML, which contain Wo + Di + Grt, garnet

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growth is more clearly related to older contact metamorphism that resulted from the emplacement of the Ordovician FG. Subsequent static recrystallization occurred after

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regional Variscan metamorphism, which was related to the emplacement of Carboniferous granitoids. Garnet grew over a period of approximately 150 Myr, coinciding with the interval

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between the two intrusive events.

Middle – Late Ordovician granitic intrusions are well established in peri-Gondwanian terranes (Guillot et al., 2002; Giacomini et al., 2006; von Raumer and Stampfli, 2008; von

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Raumer et al., 2009, 2013, 2014; Oggiano et al., 2010; Stampfli et al., 2011, 2013; Padovano et al., 2014), but their contact metamorphic aureoles are rarely described. Middle

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Ordovician granitic intrusions with thermal aureoles have been studied in the Eastern Pyrenees, where Deloule et al. (2002) and Casas et al. (2010) described a similar Middle Ordovician framework, characterized by an older Proterozoic–early Cambrian magmatic

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event and a younger Early–Middle Ordovician igneous event. In particular, in the Canigò (or Canigou) Massif, the younger igneous event is characterized by orthogneiss with emplacement ages of 471 ± 4 Ma (Deloule et al., 2002) and 453.0 ± 4.4 Ma (Casas et al., 2010). Deloule et al. (2002) also determined that the Canigou laccolith is surrounded by a contact metamorphic aureole that overprinted the early Cambrian Canaveilles metasedimentary succession, pre-dating Variscan tectonism and metamorphism. The HT/LP Variscan deformation then overprinted pristine igneous features. The Canigou orthogneiss displays a mylonitic fabric along its contact, but the interior is less deformed. The mylonitic fabric is interpreted to have formed during laccolith emplacement. Similar features also occur in the Montagne Noire area (Somail orthogneiss; Cocherie et 15

ACCEPTED MANUSCRIPT al., 2005). The differences between the Canigou and Somail laccoliths and the granitic intrusions in Sardinia must be attributed to different Variscan metamorphic overprints. In the Pyrenees and Montagne Noire areas, the Variscan metamorphic grade is HT/LP, whereas

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metamorphism in the study area occurred at very low P–T conditions (0.2–0.3 GPa, T < 400°C; Franceschelli et al., 2005). However, in the High-Grade Metamorphic Complex of the

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Axial zone in Sardinia, the Ordovician San Lorenzo-Tanaunella orthogneiss is overprinted by HT/LP Variscan metamorphism, as in the Pyrenees and Montagne Noire areas. Our data

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indicate that the SSS are not, as previously supposed, Precambrian rocks that were removed from the lower crust due to the rise of the Mt. Filau Granite, analogous to the

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interpretation of Deloule et al. (2002) for the Canigou Massif. Conversely, the SSS represent the thermally metamorphosed BF protolith, forming a discontinuous belt around the Mt. Filau Granite. The host rocks then underwent very-low-grade regional Variscan metamorphism

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(Franceschelli et al., 2005), as demonstrated by the weakly metamorphosed volcanic beds and calc-silicate lenses in the SSS, which are typical stratigraphic features of the BF, and by

10. Conclusions

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the transition from the SSS to the BF.

(Fig. 12):

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The sequence of events in the Settiballas–Mt. Filau area can be summarized as follows

1) Early Cambrian: deposition of the Bithia Formation. 2) Middle Ordovician: intrusion of the Mt. Filau Granite (457 ± 0.17 Ma) into the Bithia Formation. 3) Middle Ordovician: the emplacement of the Mt. Filau Granite resulted in a contact metamorphic aureole in the Bithia Formation, forming the And-bearing Settiballas Spotted Schists and the Wo + Grt + Di-bearing marble lenses. 4) Early to late Carboniferous: Variscan orogenesis occurred under low temperatures (<400°C) and pressures (<0.25 GPa). Folds and thrusts deformed the Bithia Formation and 16

ACCEPTED MANUSCRIPT produced folds that locally transposed the And-bearing bands in the Settiballas Spotted Schists. The folding preceded the emplacement of late Carboniferous granitic intrusions. The Mylonitic Filau Granite and the Filau Granite were unaffected by Variscan deformation,

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whereas the tectonic contact between the Settiballas Spotted Schists and the Bithia Formation is attributed to Variscan orogenesis.

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5) Late Carboniferous–Permian: emplacement of the Mt. Cogoni and Chia Beach granitic intrusions, with the development of contact aureoles in the Bithia Formation. The contact

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aureole contains randomly oriented, fine-grained And neoblasts. In the ML of the Settiballas–Mt. Filau area, sprays of epidote overgrow the earlier HT assemblage Wo + Ves

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+ Grt.

The Middle Ordovician to Carboniferous history preserved in the Sulcis area of the Sardinia foreland allows the following inferences to be made:

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A) The well-preserved Middle Ordovician contact metamorphic aureole pre-dates Variscan tectonics and metamorphism in Sardinia. Therefore, the granitic

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intrusions and associated metasedimentary strata cannot be interpreted as Cadomian basement overlain by lower Paleozoic cover rocks. B) We suggest that the Bithia Formation is very similar in age, stratigraphy, and

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composition to the protolith of the HT rocks that crop out in the High-Grade Metamorphic Complex of northern Sardinia. C) Taking into account the calc-alkaline affinity of the Ordovician granitic intrusions (Fig. 13), the data provide additional evidence that Sardinia, the Pyrenees, and Montagne Noire formed parts of Gondwanaland, in agreement with Guillot et al. (2002), Nysæther et al. (2002), Lefebvre and Fakta (2003), Cocherie et al. (2005), von Raumer and Stampfli (2008), Stampfli et al. (2011, 2013), and von Raumer et al. (2009, 2013, 2014).

17

ACCEPTED MANUSCRIPT Acknowledgments This work was conducted as part of the PRA 2012 project of the University of Genoa (Laura Gaggero) and PRID (FBS) 2013 project of the University of Cagliari (Luca G.

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Costamagna). The authors thank Giovanni Montesi (UNICA) for his help in the field, Laura Negretti (UNIGE) for her assistance with SEM–EDS analyses, and Nicola Campomenosi for

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assistance with pseudosection modeling. We warmly acknowledge revisions by an

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anonymous referee and by Jurgen von Raumer.

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Figure Captions

Fig. 1. (a) Simplified geological map of the Variscan orogen in Sardinia, modified after Stampfli et al. (2011). Sd, Sardinia; Co, Corsica; BM, Bohemian Massif; MDz, Moldanubicum Zone; STz, Saxothuringian Zone; Rhz, Rheno-Hercynian Zone; AA, Austro-Alpine; Pe, Penninic; Br, Briançonnais; Hv, Helvetic; MC, Massif Central; Am, Armorica; Lz, Lizard; Py, Pyrenees; Cz, Cantabrian Zone; Ib, Iberian; OM, Ossa Morena; Sp, South Portuguese Zone. (b) Tectonic framework of the Variscan orogen in Sardinia. (c) Location of the study area. Fig. 2. (a) Geological map of the Southern Sulcis Complex. (b) Detailed geological map of the Mt. Settiballas area. (c) Bedding. (d) Schistosity. 1, Punta Su Putzu; 2, Mt. Settiballas; 3, Mt. Cogoni; 4, Antoni Orrù. 22

ACCEPTED MANUSCRIPT Fig. 3. Stratigraphic column for the Bithia Formation and the lower Matoppa Formation. Fig. 4. Photographs and sketches of lithotypes cropping out in the study area (the central

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sketch is not to scale). (a) The Mt. Filau Granite (FG; 457 ± 0.17 Ma) viewed from the south.

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(b) The Mylonitic Mt. Filau Granite (MFG) viewed from the southwest. (c) The dark Mylonitic

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Mt. Filau Granite (MFG) in the Su Putzu area, viewed from the east. (d) Angular discordance between the upper Mt. Cogoni granodiorite and And-bearing bands in the Settiballas Spotted Schists (SSS), viewed from the east. (e) And-bearing Settiballas Spotted Schists (SSS)

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viewed from the south. (f) A small lens of Wo + Grt-bearing marble (ML) embedded in the Settiballas Spotted Schists (SSS), viewed from the south. (g) Late Carboniferous

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granodiorite with microgranular basic enclaves in the Mt. Cogoni Beach area, viewed from the southwest. (h) Drag folds in the BF, close to the contact with the Chia granitoid in the

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Chia Beach area, viewed from the south.

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Fig. 5. Photographs and sketches of planar and linear structural elements (the sketch is not to scale). (a) Deformed sigmoidal And with X-axes oriented parallel to Sx in the SSS,

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indicating pure shear (viewed from the south). (b) σ-type And deformed by Sx in the SSS, indicating simple shear with a top-to-the-SE shear sense (viewed from the southwest). (c)

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Dextral S-C planes in the MFG (viewed from the southwest). (d) The first Variscan deformation event, which formed similar folds in the SSS (viewed from the southeast). (e) The second Variscan deformation event, which formed tight to open folds in the SSS (viewed from the southeast). (f) XYSx plane: And mineral lineation in the SSS (viewed from the southwest). (g) XYSx plane: mineral lineation defined by feldspars + Qtz in the MFG (viewed from the southwest). Fig. 6. Planar and linear structural fabrics plotted on a lower-hemisphere equal-area net. The center of the net is indicated by a cross symbol. (a) Circle: pole to the Sx plane in the MFG; square: pole to the Sx plane in the SSS. (b) Square: orientation of Kfs in the MFG; circle: pole to the plane of And in the SSS. 23

ACCEPTED MANUSCRIPT Fig. 7. Diagram of X+1 versus Z+1, representative of the strain on the XZSx plane. Circle: Qtz in the MFG; closed circle: Grt in the ML; triangle: Kfs in the MFG; square, diamond,

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cross: And in the SSS.

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Fig. 8. (a–d) Transmitted light optical microscopy photomicrographs of the SSS, showing an

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overall decrease in the grain size of And with increasing distance from the FG contact. (a) Coarse-grained And porphyroclast overgrowing Ms and replaced by white mica 2 (crosspolarized light; scale bar 0.1 mm). (b) Medium-grained And porphyroclast overgrowing white

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mica 1 (cross-polarized light; scale bar 0.05 mm). (c) and (d) Fine-grained And overgrowing Sx in the SSS, viewed in plane-polarized light (c) and cross-polarized light (d), with a 0.01

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mm scale bar. (e) and (f) SEM–BSE images of the SSS. An And porphyroclast with Qtz inclusions is rimmed by bands of coarser-grained, recrystallized Bt + Ms + Mag + Qtz and

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fine-grained Bt + Ms + Qtz.

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Fig. 9. SEM–BSE images of significant microtextural relationships and assemblages from calc-silicate lenses. (a) Wo2 + Di assemblage overgrowing Wo1 + Cal. (b) Di + Wo + Ves +

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Grt2 overgrowing Wo1 + Grt1. (c) Granoblastic texture exhibited by Wo + Ves. (d) Grt2 overgrowing Wo1. (e) Transect across Grt1 (Alm 1.11–1.34 Py 0.30–0.52 Grs 91.12–83.08 Sps 0.13–0.00

40.09).

(f) Czo.

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Adr 5.52–11.35) to recrystallized Grt2 (Alm 1.47–2.02 Py 0.42–0.65 Grs 54.84–67.90 Sps 1.37–0.15 Adr 24.63–

Fig. 10. P–T–t diagram for the Mt. Settiballas area, based on the pseudosection that was modeled using Perplex (Connolly, 2009) for the calc-silicate sample FM26 in the CFMASH system (CaO: 40.97; Fe2O3tot: 3.05; MgO: 1.7; Al2O3: 5.46; SiO2: 47.12; LOI: 0.75 wt%) in the P–T range of 0.1–0.4 GPa and 500–700°C and using phase equilibria for meta-siltstone. Legend: And–Sil–Kyanite triple point (Holdaway, 1971); 1: Ms + Qtz = Kfs + Al2SiO5 + H2O; 2: Ms = Kfs + Al2O3 + H2O; 3: wet granite solidus; M1, M2: Middle Ordovician metamorphic events. Fig. 11. Wo + Grt + Di-bearing marble in the High-Grade Metamorphic Complex of NE 24

ACCEPTED MANUSCRIPT Sardinia (Tamarispa), modified after Elter and Palmeri (1992). Site A indicates the location of the Tamarispa outcrop, whereas site B corresponds to the Antoni Orrù ML outcrop in the

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study area.

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Fig. 12. Proposed time-integrated sketch illustrating the early Cambrian to Permian

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geodynamic evolution in the study area. 1: Early Cambrian deposition of the Bithia Formation (BF); 2, 3: Ordovician intrusion of the Mt. Filau Granite (457 ± 0.17 Ma) into the BF, and the development of the thermal metamorphic aureole that is composed of the

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Settiballas Spotted Schists (SSS), with Wo + Grt + Di-bearing marble lenses (ML); 4: Early to late Carboniferous Variscan orogenesis results in low-temperature (≈300°C) and low-

MA

pressure (>0.25 GPa) metamorphism and the development of folds and thrusts in the BF. The Mt. Filau cataclastic granitoid (MF) and the Settiballas Spotted Schists (SSS) with

D

marble lenses (ML) become affected by increasing metamorphic grade. 5: The emplacement

TE

of Carboniferous granitoids (VG, Mt. Cogoni, and Chia Beach), with the development of contact metamorphic aureoles in the BF but with no effect on the Settiballas–Mt. Filau area

CE P

(MF–SSS). The contact aureole contains randomly oriented And blasts. Fig. 13. Paleogeographic reconstruction at 460 ± 10 Ma, modified after von Raumer et al.

AC

(2013), showing the relative positions of Sardinia (Sard), Montaigne Noire (Mn), Co (Corsica), and the Pyrenees (Py). MOR: Mid-Ocean Ridge.

Table Captions Table 1. Representative EDS electron microprobe analyses of andalusite and brown and white micas in the SSS. Table 2. Representative EDS electron microprobe analyses of Wo and Cpx in the ML. Table 3. Representative EDS electron microprobe analyses of Grt from the ML. Table 4. Representative EDS electron microprobe analyses of Ves from the ML. Table 5. Reactions and mineral assemblages recorded for the calc-silicate composition of marble lenses in

25

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the P–T range of 460–650°C and 0.1–3.3 GPa.

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39

ACCEPTED MANUSCRIPT Table 1

MC14

MC12

T

Sample MC14 P1A1

MC14 P1A1 MC14 P1A2 MC14 P1A3 MC12 P1A1 MC12 FA1

Site

coex

Phase

Muscovite 1 Andalusite

MC12 FA1 MC12 P1A1 MC12 P2A1

CR

IP

Analysis

Muscovite 1

Biotite

33.860

43.830

44.170

1.770

0.960

21.490

US

Wmica II

Oxide wt%

Muscovite

Biotite

Andalusite

45.780

35.510

36.070

0.680

0.600

2.320

0.190

35.230

35.610

36.110

19.410

63.370

43.240

35.700

43.210

43.310

TiO2

0.400

-

0.510

0.730

Al2O3

36.010

62.220

35.460

35.500

Cr2O3

-

-

-

-

-

-

-

-

0.220

-

MgO

0.290

-

0.850

0.430

8.360

0.890

0.580

0.780

9.040

-

FeO

0.680

0.360

1.520

0.860

17.000

1.140

1.250

1.000

17.540

0.370

MnO

-

-

-

-

0.180

-

-

-

0.220

-

CaO

0.100

-

0.100

-

0.150

-

-

0.100

-

-

Na2O

0.370

-

0.320

0.730

0.140

0.510

0.650

0.600

0.140

-

K2O

9.940

-

9.960

9.580

8.610

9.590

9.590

10.160

8.470

-

Total

91.030

98.280

91.940

91.140

91.560

92.150

92.530

95.130

92.870

100.000

AC

CE P

TE D

MA N

SiO2

40

ACCEPTED MANUSCRIPT

Si

3.000

1.179

3.001

3.003

2.966

3.008

3.000

2.737

1.171

Ti

0.021

-

0.027

0.038

0.117

0.050

0.035

0.030

0.135

-

Al

2.944

2.421

2.902

2.901

2.219

2.850

2.870

2.789

1.763

2.425

Fe3+

0.000

-

0.000

0.000

0.000

0.000

0.000

0.055

0.000

-

Fe2+

0.039

0.010

0.088

0.050

1.245

0.065

0.071

0.000

1.131

0.010

Mn

0.000

-

0.000

0.000

0.013

0.000

0.000

0.000

0.014

-

Mg

0.030

-

0.030

0.044

1.092

0.091

0.059

0.076

1.039

-

Ca

0.007

-

0.007

0.000

0.014

0.000

0.000

0.007

0.000

-

Na

0.050

-

0.050

0.098

0.024

0.068

0.068

0.076

0.021

-

K

0.880

-

0.881

0.847

0.962

0.840

0.837

0.849

0.833

-

H

2.000

-

2.000

2.000

2.000

2.000

2.000

2.000

2.000

-

Total

8.972

3.610

8.986

8.981

10.652

8.971

8.961

8.882

9.673

3.607

CR

US

MA N

TE D

CE P

AC

T

3.021

IP

Cations (a.p.f.u.)

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ACCEPTED MANUSCRIPT Table 2

FM26 P2A1

FM28A

FM28

T

FM26 P2A1

FM26 P2A2

FM26 P2A2

Wo2

Wo1

Wo1

Wo2

50.93

51.54

51.39

53.51

52.62

-

-

0.23

0.11

0.26

1.70

-

-

0.09

-

17.28

14.11

0.21

0.35

0.15

0.28

2.12

6.22

-

-

0.17

0.15

0.21

0.27

50.63

47.29

46.91

47.35

47.58

26.45

25.50

-

-

-

-

-

-

-

56.85

98.65

98.19

99.54

99.52

99.96

100.49

Granoblastic Wo 1

Oxide wt%

FM26AP1A1 FM28AP2A1 FM28AP2A1 FM28AP5A1

Diopside

50.39

50.65

50.19

6.22

Al2O3

-

0.11

-

-

MgO

-

-

-

-

FeO

0.22

0.21

0.31

MnO

-

0.16

-

CaO

45.40

47.26

46.71

Na2O

-

-

-

Total

96.01

98.39

97.21

Si

2.031

1.992

1.998

0.411

2.006

2.007

2.003

1.998

1.946

1.941

Al

0.000

0.005

0.000

0.000

0.000

0.000

0.011

0.005

0.011

0.074

Fe3+

0.000

0.007

0.005

0.000

0.000

0.000

0.000

0.000

0.064

0.043

AC

TE D

MA N

SiO2

-

51.16

IP

Phase

FM26AP2A1

CR

FM26 P2A1

FM26A

US

Analysis

FM26

CE P

Sample

Cations (a.p.f.u.)

42

ACCEPTED MANUSCRIPT 0.007

0.000

0.006

0.000

0.007

0.012

0.005

0.009

0.000

0.149

Mn

0.000

0.005

0.000

0.000

0.000

0.000

0.006

0.005

0.006

0.008

Mg

0.000

0.000

0.000

0.000

0.000

0.000

0.005

0.000

0.937

0.776

Ca

1.961

1.991

1.992

3.589

1.987

1.981

1.971

1.982

1.031

1.008

Wo

99.62

100.00

99.72

100.00

99.65

99.42

99.49

99.54

52.38

52.15

En

0.00

0.00

0.00

0.00

0.00

0.00

0.26

0.00

47.62

40.15

Fs

0.38

0.00

0.28

0.00

0.58

0.25

0.46

0.00

7.70

MA N

US

CR

IP

T

Fe2+

AC

CE P

TE D

0.35

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ACCEPTED MANUSCRIPT Table 3 FM28A

FM26AP2A1 FM26AP2A2 FM26AP2A3 FM26AP2A3 FM26AP2A3 FM26AP2A3 FM28AP1A1 FM28AP3A1 FM28AP3A2

Garnet

CR

Phase

IP

Analysis

FM26A

T

Sample

US

Oxide wt%

39.39

36.83

37.42

37.08

1.52

0.55

0.44

0.45

1.08

14.95

20.98

9.20

11.05

10.13

0.07

0.08

0.10

0.11

0.10

0.34

0.11

0.17

0.08

0.16

-

0.26

13.17

8.59

2.32

16.41

14.07

14.46

0.63

0.07

0.06

0.15

0.66

0.53

36.46

35.06

35.55

36.58

34.36

34.10

34.60

97.54

100.51

100.13

98.98

100.07

97.650

97.850

98.470

2.979

2.970

2.950

3.000

2.971

2.975

2.976

3.002

2.963

0.032

0.045

0.066

0.032

0.089

0.031

0.027

0.027

0.065

37.61

38.94

38.42

TiO2

0.53

0.76

1.15

0.55

Al2O3

14.81

15.53

19.29

12.12

Cr2O3

0.51

0.25

0.19

MgO

0.26

0.25

0.14

FeO

8.58

8.18

4.35

MnO

0.14

0.20

0.00

CaO

34.42

34.76

Total

96.73

Si Ti

38.05

MA N

37.48

AC

CE P

TE D

SiO2

Cations (a.p.f.u.)

44

1.387

1.445

1.722

1.115

1.376

1.868

0.876

1.045

0.954

Cr

0.032

0.016

0.011

0.004

0.005

0.006

0.007

0.006

0.021

Fe3+

0.560

0.509

0.235

0.815

0.499

0.113

T

1.111

0.891

0.968

Fe2+

0.010

0.031

0.040

0.045

0.062

0.033

0.000

0.053

0.000

Mn

0.009

0.013

0.000

0.042

0.005

0.004

0.010

0.045

0.036

Mg

0.031

0.029

0.016

0.013

0.020

0.019

0.000

0.031

Ca

2.960

2.941

2.959

2.933

US

0.009

2.974

2.960

2.975

2.931

2.963

Almandine

0.34

1.04

1.34

1.47

2.02

1.11

0.00

1.76

0.00

Pyrope

1.02

0.98

0.52

0.42

0.65

0.30

0.64

0.00

1.02

Grossular

67.83

69.97

83.08

54.84

67.90

91.12

42.96

51.34

46.48

Spessartine

0.31

0.44

0.00

1.37

0.15

0.13

0.34

1.48

1.18

Uvarovite

1.57

0.76

0.55

0.21

0.24

0.29

0.34

0.31

1.05

Andradite

27.38

24.63

11.35

40.09

24.63

5.52

54.45

43.77

47.14

Ca-Ti Grt

1.55

2.18

3.16

1.59

4.40

1.52

1.31

1.33

3.16

CR

MA N

TE D CE P

IP

Al

AC

ACCEPTED MANUSCRIPT

45

ACCEPTED MANUSCRIPT Table 4 FM26

Phase

Vesuvianite

FM26 P2A4

US

37.14

35.44

1.09

0.60

1.09

15.05

13.87

13.93

14.37

-

TE D

-

-

-

1.64

3.07

0.16

2.97

3.34

4.02

5.15

10.47

4.69

-

0.15

-

-

0.23

0.18

35.48

34.60

35.39

34.46

34.66

34.39

34.73

Na2O

0.39

0.34

AC

Oxide wt%

FM26 P2A3

IP

FM28AP1A1 FM28AP1A1 FM28AP3A1 FM28AP3A1 FM26 P2A3

CR

Analysis

FM28A

T

Sample

0.27

0.36

-

-

-

Cl

0.09

0.20

-

0.31

-

-

-

F

3.19

2.91

1.79

2.33

2.54

-

2.47

99.53

97.25

96.90

97.19

95.70

96.92

95.94

36.30

35.47

36.33

35.69

TiO2

1.71

3.00

0.41

3.32

Al2O3

16.40

14.87

16.41

Cr2O3

-

-

0.14

MgO

1.93

1.90

2.67

FeO

4.04

3.96

MnO

-

CaO

Total

35.32

CE P

MA N

SiO2

46

ACCEPTED MANUSCRIPT Cations (a.p.f.u.) 1.477

1.466

1.484

Ti

0.052

0.093

0.013

0.103

0.034

Al

0.779

0.724

0.787

0.729

0.687

Cr

-

-

0.005

-

0.136

0.137

0.114

0.138

Mn

-

-

0.005

-

Mg

0.116

0.117

0.162

Ca

1.533

1.531

Na

0.030

Total

4.110

1.480 0.034

0.675

0.707

-

-

-

0.181

0.360

0.164

-

0.008

0.006

0.100

0.192

0.010

0.185

1.542

1.517

1.560

1.516

1.554

0.027

0.021

0.029

-

-

-

4.094

4.125

4.081

4.138

4.116

4.132

CR

MA N

CE P

IP

0.019

AC

Fe 3+

1.528

T

1.465

US

1.464

TE D

Si

47

ACCEPTED MANUSCRIPT Table 5

615°±13°

0.22 ± 0.02

6 Cal + 4 Di + 11 Grs + 9H2O = 2 Ves + 5 Wo + 6CO2

~610°

0.2

Qtz + Cal = Wo + CO2

~ 640°

Chatterjee (1966)

CR

US

MA N

El Khalile et al. (2014)

Cal + Qtz + Ep = Grt + An + CO2 + H2O

Duzs-Moore et al. (2003)

Wo in

Ves + Wo

CE P

Watters (1985) Bebout and Carlson (1996)

Ves + Grt + Wo Ves + Grs + Wo + Cal

AC

Patel (2007) Casillas et al. (2011)

XCO2 < 0.15

Di + Grt + Cal = Ves + Wo

Burianek and Dollnicek (2011)

Ahmed-Said and Leake (1996)

[P, GPa]

550°-650°

IP

Grammatikopoulos and Clark (2006) Di + Grt + Cal = Ves + Wo Johnson et al. (2000)

Assemblages

[T, °C]

TE D

Reactions

Reaction

T

Reference

458°-528°

0.1

663°

PCO2 = 0.1

570°-660°

~ 0.1

600°

0.2

660°-620° 3.3 < P < 0.25

Ves + Grt + Wo + Di + Cal

580°

XCO2 = 0.10

Ves + Grt + Wo + Di + Cal

640±20°

0.33 ± 0.03

48

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IP

T

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Graphical abstract

49

ACCEPTED MANUSCRIPT Highlights 1. A Middle-Late Ordovician well-preserved contact aureole pre-dates the Variscan orogeny in Sardinia. 2. The

protolith

are

siliciclastic

sediments

(Andalusite)

limestones

T

(Grt+Di+Wo+Ves).

and

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3. The same rock association and HT overprint occurs in Pyrenees and Massif

AC

CE P

TE

D

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SC R

Central basements.

50