Pressure–temperature–deformation–time of the ductile Alpine shearing in Corsica: From orogenic construction to collapse

Lithos 218–219 (2015) 99–116

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Pressure–temperature–deformation–time of the ductile Alpine shearing in Corsica: From orogenic construction to collapse Federico Rossetti a,⁎, Johannes Glodny b, Thomas Theye c, Matteo Maggi a a b c

Dipartimento di Scienze, Sez. Scienze Geologiche, Università Roma Tre, 00146 Roma, Italy Deutsches GeoForschungsZentrum GFZ, Telegrafenberg, D-14473 Potsdam, Germany Institut für Mineralogie und Kristallchemie, Universität Stuttgart, Stuttgart, Germany

a r t i c l e

i n f o

Article history: Received 6 October 2014 Accepted 13 January 2015 Available online 23 January 2015 Keywords: Shear zones Subduction zone metamorphism Rb–Sr tectonochronology Alpine orogen Mediterranean region Corsica

a b s t r a c t Definition of the Tertiary tectono-metamorphic history of Alpine Corsica is a key task to decipher the space–time linkage between the Alpine and Apennine subduction systems in the Mediterranean region. Alpine Corsica exposes a nappe stack of oceanic- and continental-derived units, structurally juxtaposed onto the former European continental margin (Hercynian Corsica). Still uncertain is the timing of involvement of the continental-derived units in orogenic construction and shift to regional extension. This paper focuses on reconstruction of the pressure–temperature–deformation–time evolution of selected ductile shear zones activated during transition from the tectonic underplating to the extensional reworking stages. New Rb–Sr mineral age data, integrated with structural and thermobarometric investigations constrain the waning stages of the high-pressure (from blueschist to greenschist facies metamorphic conditions) top-to-the-W thrusting of the HP, oceanic-derived realm (Schistes Lustrés Complex) onto the Hercynian Corsica along the East Tenda Shear Zone in the early Oligocene (from ~32 to ~27 Ma). This early compressional evolution is overprinted by a major phase of retrogressive, syn-greenschist top-to-the-E extensional shearing in the Schistes Lustrés Complex with the last episode of deformation-related ductile recrystallization recorded during the early Miocene at ~20– 21 Ma, in a continuum transition from ductile to brittle shearing. The same early Miocene Rb–Sr deformation ages are recovered from the ductile-to-brittle top-to-the-E reactivation domains within the East Tenda Shear Zone, documenting that transition from compression to extension in Alpine Corsica occurred during the late Oligocene–early Miocene time lapse. Implications of these data are discussed in the broader context of the Tertiary geodynamic evolution of the Central Mediterranean region. © 2015 Elsevier B.V. All rights reserved.

1. Introduction The Central Mediterranean region (Fig. 1) is a key area for studying the range of chemical-physical processes associated with crustal assembly and differentiation during orogenic construction and subsequent tectonic reworking. In particular, Tertiary post-orogenic extension led to exhumation of the deep seated roots of the Alpine orogen (e.g., Dewey, 1988; Faccenna et al., 2004; Jolivet et al., 1998, 2003; Platt et al., 2003; Rosenbaum et al., 2002; Van Hinsbergen et al., 2014; Vignaroli et al., 2009), allowing to study in detail structures and metamorphic regimes active at depth during subduction underplating, orogenic construction, and collapse. In this context, the tectonometamorphic histories reconstructed from the exhumed roots of the Alpine chain in Corsica (Figs. 1–2) have been the source of controversy over the last years, with different paleotectonic models proposed so far ⁎ Corresponding author at: Dipartimento di Scienze, Sezione Scienze Geologiche, Università Roma Tre, Largo S. L. Murialdo, 1, 00146 Roma, Italy. Tel.: +39 0657338043; fax: +39 0657338201. E-mail address: [email protected] (F. Rossetti).

http://dx.doi.org/10.1016/j.lithos.2015.01.011 0024-4937/© 2015 Elsevier B.V. All rights reserved.

(Daniel et al., 1996; Fournier et al., 1991; Gueydan et al., 2003; Jolivet et al., 1990, 1998; Maggi et al., 2012; Malavieille et al., 1998; Molli and Malavieille, 2010; Molli et al., 2006; Principi and Treves, 1984; Vitale Brovarone and Herwartz, 2013). This is largely due to the contrasting tectonic/kinematic models proposed and the paucity of age constraints on the timing of Alpine deformation and metamorphism, particularly for what concerns the East Tenda Shear Zone (ETSZ; Jolivet et al., 1990), the Alpine shear zone boundary between the continental margin of the European plate and the oceanic neo-Tethyan (Ligurian– Piemontese branch) realm of the Schistes Lustrés Complex (Brunet et al., 2000; Gibbons and Horak, 1984; Gueydan et al., 2003; Jolivet et al., 1990; Maggi et al., 2012, 2014; Molli et al., 2006). Recent geochronological data have been made available for the high-pressure Alpine metamorphism in both the oceanic-derived (Martin et al., 2011; Vitale Brovarone and Herwartz, 2013) and the ETSZ (Maggi et al., 2012) units, which collectively constrain tectonic underplating and peak of subduction zone metamorphism to Eocene times. Nevertheless, still unsolved is (i) the timing of involvement of the European continental margin in the Alpine orogeny (i.e. synchronicity or diachronicity with subduction zone metamorphism in the oceanic-derived units);

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Fig. 1. The Mediterranean region with the major geological structures indicated. The compressional and extensional shear senses as derived from the exhumed roots of the Alpine– Apennine orogen are also indicated. (Modified and readapted after Vignaroli et al., 2009 and references therein).

and (ii) the precise timing of the extensional reworking of the nappe edifice, with impact on the regional paleogeographic configuration of the active margin during the Tertiary evolution of the Alpine-Apennine orogeny (cf. Vitale Brovarone and Herwartz, 2013). In this paper, by presenting new Rb–Sr mineral age data, integrated with structural and thermobarometric investigations on ductile shear zones studied along a structural transect across the ETSZ and selected greenschist-facies retrogressive domains from the Schistes Lustrés Complex (Figs. 2 and 3), we shed light on the regional tectonometamorphic evolution, in the transition from tectonic underplating and nappe stacking to orogenic collapse. Our data set document synorogenic exhumation along the ETSZ in the early Oligocene (between ~32 and ~27 Ma), overprinted by a major phase of ductile extensional reactivation that culminated during the early Miocene, at ~21–20 Ma. Implications of these data are discussed in the broader context of Alps-Apennine tectonic linkage and framed within the scenario of subduction, accretion and back-arc extension during the Tertiary geodynamic evolution of the Central Mediterranean region. 2. Geological background Alpine Corsica is made up by a nappe stack of oceanic- and continental-derived units, structurally juxtaposed onto the former European continental margin represented by the Hercynian Corsica basement (mainly Carboniferous to Permian magmatic and volcanic rock successions) with a discontinuous Mesozoic to middle Eocene sedimentary cover (Dallan and Puccinelli, 1995;

Durand-Delga, 1984; Durand-Delga et al., 2001; Faure et al., 2014; Malavieille et al., 1998; Vitale Brovarone et al., 2013). From bottom to top, the nappe stack consists of (Figs. 2 and 3): (i) European-derived continental crust units (Tenda units, including the ETSZ, and the external continental units) that were tectonically reworked during the Alpine orogenic cycle; (ii) the Schistes Lustrés nappe Complex, an oceanicderived high-pressure/low-temperature (HP/LT) metamorphic domain arranged to form the major antiformal structure of Cape Corse; and (iii) the upper nappe system (Balagne, Nebbio and Macinaggio units), composed of non-metamorphic or slightly metamorphosed ophiolite units. The structural architecture is a result of two major tectonic/ geodynamic events: (i) early orogenic construction and nappe stacking during convergence/subduction-related consumption and overthrusting of the Ligurian–Piemontese oceanic realm onto the European continental margin; and (ii) subsequent collapse of the orogenic chain during regional back-arc extension (Liguro-Provençal and Tyrrhenian rifting) in the upper-plate of the Apennine subduction system (Brunet et al., 2000; Carminati et al., 2012; Daniel et al., 1996; Gueydan et al., 2003; Jolivet et al., 1990, 1998; Maggi et al., 2012, 2014; Molli and Malavieille, 2010; Molli et al., 2006; Vitale Brovarone et al., 2013). The Alpine evolution occurred concurrently with major rigid block rotations that affected the Sardinia-Corsica block relative to Eurasia, first during Eocene times, prior to the Oligocene (at ~ 30 Ma) onset of extension in the Liguro-Provençal back-arc basin (Advokaat et al., 2014) and then during its rifting/drifting history in Miocene times (Gattacceca et al., 2007; Speranza et al., 2002).

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Fig. 2. (A) Geological–structural map of Alpine Corsica showing the main lithotectonic complexes (modified and re-adapted after Vitale Brovarone et al., 2013 and references therein), with indication of the associated HP/LT and retrogressive shear senses (after Jolivet et al., 1990; Daniel et al., 1996; Molli et al., 2006; Molli and Malavieille, 2010; Maggi et al., 2012, 2014). Location of the ER1 and ER2 samples from the Schistes Lustrés Complex are also indicated. (B) Geological–structural map (modified and re-adapted after Maggi et al. (2014) of the study area within the ETSZ and location of the studied samples (T5c, T3, Tc1, HRVO, T8)).

The kinematics of the two tectonic events in Alpine Corsica is characterized by nearly sub-parallel but opposite sense of tectonic transports, with early, dominantly top-to-the-W compressional (equilibrated under HP/LT, blueschist-to-eclogite facies peak metamorphic conditions) and late top-to-the-E extensional (equilibrated under retrogressive greenschist facies metamorphic conditions) shear senses, respectively (Daniel et al., 1996; Gibbons and Horak, 1984; Jolivet et al., 1990, 1998; Maggi et al., 2012, 2014; Mattauer et al., 1981; Molli et al., 2006) (Figs. 2–3). Shear strain partitioning is documented in the ETSZ during the top-to-the-W thrusting event, with shear strain localization

occurring along mica-rich blueschist mylonitic bands that wrap around massive bodies with gneissic texture (Maggi et al., 2014; Molli et al., 2006). Post-orogenic extension operated in the Schistes Lustrés Complex during a general progression from ductile-to-brittle deformation conditions, with tectonic reactivation of the early structured ETSZ as a major extensional shear zone (Daniel et al., 1996; Gueydan et al., 2003; Maggi et al., 2014). Structurally-controlled fluid-rock interaction and rock metasomatism accompanied formation of the polyphase topto-the-W shear fabrics along the ETSZ, with the ultimate stage of deformation/alteration being typified by formation of variably-

Fig. 3. Interpretative geological cross-section (see Fig. 2A for location; modified and re-interpreted after Jolivet et al. (1990), Molli and Malavieille (2010) and Vitale Brovarone et al. (2013). Not to scale, location of structures is only indicative) with the main structures indicated and localization of the studied samples. For lithological symbols refer to Fig. 2A.

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developed phyllonitic horizons. The phyllonitic horizons constitute the weakest structural elements within the ETSZ that also corresponds to the preferred sites of tectonic reactivation during the subsequent topto-the-E crustal extension (Fig. 3; Maggi et al., 2014). Metamorphic climax during the Alpine nappe construction in Corsica occurred under suppressed paleo-geothermal conditions. Tectonic underplating of the European margin (ETSZ units) was attained under blueschist facies metamorphic conditions (1.0–1.2 GPa and ~ 350 °C–400 °C; Maggi et al., 2012; Molli et al., 2006; Tribuzio and Giacomini, 2002; Vitale Brovarone et al., 2013). Subduction zone metamorphism in the oceanic-derived Schistes Lustrés Complex was equilibrated under lawsonite blueschist- to-eclogite-facies peak metamorphic conditions (1.5-2.4 GPa and 400–550 °C; Fournier et al., 1991; Jolivet et al., 1998; Lahondère, 1996; Ravna et al., 2010; Vitale Brovarone et al., 2011, 2013). Alpine metamorphism within the European margin units is attributed to the early Eocene, based on U–Pb rutile (48 ± 18 Ma) and acmite–phengite (54 ± 8 Ma) TIMS geochronology on syn-shearing mineral assemblages (Maggi et al., 2012). Available 40Ar/39Ar phengite geochronology documents plateau ages between ~39 and 25 Ma. The generally discordant Ar release spectra, spanning from ~45 Ma to 22–25 Ma, the Ar–Ar data attest to polyphase phengite crystallization during the ETSZ development (Brunet et al., 2000). Timing of regional thrusting in the external domains of the Alpine Corsica orogen is constrained to be post-Bartonian (middle Eocene; 37–41 Ma) in age, based on the youngest stratigraphic ages of the sedimentary cover of the Hercynian basement units (European continent-derived external units) involved in the nappe stacking and metamorphism (Bezert and Caby, 1988; Egal, 1992; Malasoma and Marroni, 2007). A late Eocene–early Oligocene timing is also in line with the isotopic age data obtained for the climax of subduction zone metamorphism in the Schistes Lustrés, as provided by U–Pb zircon dating (34.4 ± 0.8 Ma; Martin et al., 2011) and Lu–Hf garnet and lawsonite ages, dating prograde to peak subduction zone metamorphism at 35–34 Ma in the lawsonite eclogite units and 37.5 ± 1.3 Ma in the lawsonite blueschist units, respectively (Vitale Brovarone and Herwartz, 2013). On the other hand, the 40Ar/39Ar dating of HP phengites from blueschist and eclogitic metamorphic assemblages from the Schistes Lustrés provided discordant age spectra ranging from 65 to 35 Ma, which at least in part may reflect presence of excess Ar (Brunet et al., 2000), a phenomenon well known from HP/LT terranes worldwide (e.g., Sherlock et al., 1999). Age of the retrogressive, syn-greenschist shearing and exhumation in the Schistes Lustrés is up to now poorly constrained, but attributed to a regional switch from orogenic compression to crustal extension during Oligocene–Miocene regional extension at the back of the eastward migrating Apennine slab (Brunet et al., 2000; Jolivet et al., 1998). Zircon and apatite fission-track thermochronology attests for a continuous cooling of the Hercynian and Schistes Lustrés units between late Eocene and Miocene times (between c. 40 and 15 Ma; Cavazza et al., 2001; Fellin et al., 2006; Zarki-Jakni et al., 2004). In particular, fission

track ages and the U–Th/He data on apatite indicate a major cooling/ exhumation event starting in the early Miocene (at ~20 Ma), commonly related to back-arc extension during the Liguro Provençal and Tyrrhenian rifting (Cavazza et al., 2001; Fellin et al., 2005, 2006; Zarki-Jakni et al., 2004). 3. Methods To constrain the timing of ductile deformation in Alpine Corsica, we investigated Rb–Sr multimineral isotope systematics in selected S-L tectonites. Five representative samples were chosen for the Rb–Sr geochronological study in order to investigate the complete inventory of the ductile shear strain across the ETSZ (Figs. 2–3) in the transition from top-to-the-W compressional to top-to-the-E extensional shearing. Two additional samples were collected from a prominent, greenschist facies mylonitic shear zone from the Schistes Lustrés Complex (Erbalunga area; Figs. 2–3), already described in Daniel et al. (1996), in order to frame the deformation history of the ETSZ into the polyphase tectonic evolution of Alpine Corsica. Sample selection was based on the structural architecture at outcrop scale and on meso- and micro-scale textural and mineralogical features. Samples are shown in their structural context in Figs. 2 and 3 and listed in Table 1, where their location, fabrics, and constituent mineralogy are detailed. Electron microprobe analyses were used to define compositions of the constituent mineral assemblages. These compositional data are used for quantitative thermobarometry as obtained from P–T–X (forward) pseudosection modeling by applying the Perple_X_07 software (Connolly, 2005; http://www.perplex.ethz.ch, v. 6.6.9) starting from whole rock chemical data, with the aim to assess the pressure– temperature environment of shear zone enucleation and development. Details on the analytical methods and protocols adopted in this study are provided in the Appendix. In the following, mineral abbreviations are after Whitney and Evans (2010). 4. Field relations and sample description 4.1. ETSZ samples Development of the ETSZ corresponds to the progressive transformation of an isotropic magmatic Hercynian protolith (Casta granodiorite) into sheared equivalents, including massive gneisses, mylonitic shear zones and phyllonites when approaching contact with the overlying Schistes Lustrés (Maggi et al., 2014; Molli et al., 2006). These textural changes were associated with diffuse rock alteration and metasomatism during structurally controlled fluid-rock interaction, with the phyllonites corresponding to the ultimate product of shear strain localization (Maggi et al., 2014). Main shear foliation dips shallowly eastward and syn-kinematic stretching lineations trend WSW.

Table 1 Sample location with constituent mineralogy and textural characteristics. Sample

GPS position

Structural fabric

Mineralogy(1)

Latitude

Longitude

4722539N 4720900N 4722747N 4723001N 4726223N

521957E 521708E 522307E 522389E 522405E

Strongly foliated blueschist facies mylonite Fine-to-medium grained, blueschist facies mylonitic gneiss Fine-grained, strongly-foliated blueschit facies mylonite Fine-grained phyllonitic mylonite Fine-to-medium grained, greenschist facies phyllonite

Qz + Na-Amp + Ph + Ab/Mc, Mg/Hem, Ap, Ttn Qz + Na-Amp + Ph + Ab/Mc, Mg/Hem, Ap, Ttn Ph + Acm + Qz + Rt + Ab ± Na-Amp ± Bt, Mc, Ap, Mag/Hem Ph + Qz + Ab + Mc, Ap, Zrc Ph + Qz + Ab ± Mc, Ap, Zrc

Schistes Lustrés ER1 4735794N ER2 4735794N

538883E 538880E

Fine grained greenschist facies mylonite Fine grained greenschist facies mylonite

Ph + Pg + Qz + Chl + Ab + Cal/Ank + Dol + Gr Ph + Pg + Qz + Chl + Ab + Cal/Ank + Dol + Gr

ETSZ Tc1 T3 T5 HRVO T8

(1)

Mineral abreviations after Whitney and Evans (2010); Accessory phases in italics.

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4.1.1. Sample T5c The sample is from the phyllonitic, Acm-Rt shear zone described in Maggi et al. (2012). This shear zone retains the climax of the Alpine metamorphism associated with the top-to-the-W ductile shearing within the ETSZ. The mineral assemblage consists of Ph (up to 60% vol.; Si-content per formula unit (p.f.u.) of 3.6–3.7)–Acm–Qz–Rt–Ab, with accessory Mic–Ap–Mag/Hem. Acm usually host Na-Amp inclusions and is rimmed by retrogressive Bt and, locally, Chl overgrowth. The peak mineral assemblage defines a P–T range of crystallization of 1.0–1.2 GPa and ~400 °C (Maggi et al., 2012). 4.1.2. Sample T3 and Tc1 These samples are from decimeter-to-meter thick top-to-the-W, blueschist-facies mylonitic shear zones that occur within and wrap around the gneissic host rocks of the ETSZ (Figs. 2B and 3). Synkinematic mineralogy consists of Qz–Na-Amp–Ph–Ab/Mic, with accessory Mg/Hem, Ap, Ttn. Transition from gneissic textures to finegrained mylonitic shear bands is gradational in the field and the mylonitic horizons show anastomose geometries Kinematic indicators in both the gneissic host and the mylonitic shear zones systematically document top-to-the-W shear sense (Fig. 4A–B). The amphibole grains occur along the main shear foliation and co-define the main stretching direction; Na-Amp also occurs in asymmetric pressure shadows surrounding Fsp and Qz porpyroclasts flowing within the shear matrix (Figs. 4B–D and 5A). Mylonitic shear fabrics are systematically overprinted by a pervasive and partitioned crenulation cleavage, which results in a composite planar fabric. This crenulation cleavage affects exclusively the “softer” mica-rich shear zone domains, being absent in the gneissic host (Figs. 4E–F and 5B). In both samples Na-Amp is zoned. Commonly, magnesioriebeckite/ferro-glaucophane cores and glaucophane rims are observed. Often, a thin outer rim, showing ferroglaucophane/magnesio-riebeckite compositions also occurs. This corresponds to wide range of XFe3+ (with XFe3+ [=Fe3+ ⁄ (Fe3+ + AlVI)]) from 0.20 to 0.80 (Fig. 6; Table 2a). Both samples also contain high Si-phengite, ranging from 3.38 to 3.55 p.f.u. (Table 2a). These compositions plot off the Tschermak exchange line between muscovite and celadonite in a Si vs. Altot diagram (not shown) and display elevated Mg + Fe, likely caused by the presence of ferric iron substituting for Al. BSE images of phengite grains from the crenulation domains show patchy chemical zoning as documented by dark and lighter domains that correspond to higher and lower phengitic composition, respectively (Fig. 7A–B). 4.1.3. Sample HRVO and T8 These samples come from two phyllonitic shear zones within the ETSZ, located at about 300 m from the contact with the Schistes Lustrés (sample HRVO; distance measured perpendicular to the main foliation) and at the top of the ETSZ, respectively (sample T8; Fig. 3). Sample HRVO corresponds to fine-grained, up-to-meter thick mylonitic horizons showing polyphase kinematics. Early shear fabrics developed during penetrative ductile top-the-W shearing (Fig. 5C–D). Synkinematic mineralogy consists of modally abundant (up to 60% vol.), high-Si phengite that shows a rather restricted compositional variation (Si = 3.48–3.60 p.f.u; Table 2b) in association with Qz–Fsp with accessory Ap and Zrc. Most of the phengite composition analyses plot near the muscovite-celadonite substitution line, although some have elevated Mg + Fe, pointing to the presence of ferric iron (Fig. 7C). Notably, feldspar (Ab-Mic) neoblastesis occurs during the shear fabric development (Fig. 5C–D). This early shear fabric is locally overprinted by semi-brittleto-brittle, spaced C′-type shear bands with top-to-the-E kinematics (Fig. 8A). No systematic phengite recrystallization occurs along the late-stage, E-verging shear bands. Sample T8 comes from a large, pluridecameter thick phyllonitic shear zone that marks the contact with the overlying Schistes Lustrés. The shear fabric is dominated by transpositive, top-to-the-E, C′-type shear bands with a progressive evolution from ductile-to-brittle

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deformation conditions (Fig. 8B). Similarly to sample HRVO, the synkinematic mineralogy consists of Ph–Qz–Ab ± Mic (Fig. 5E). Notably, however, the modal abundance of phengite is higher (up to 75 vol.%) and its composition shows a lower phengitic substitution (Si = 3.38–3.43 p.f.u.; Table 2b). These compositions systematically diverge from the Tschermak substitution line, again evidencing elevated Mg + Fe contents (Fig. 7C) 4.2. Schistes Lustrés samples In the Schistes Lustrés Complex exposed all along the Tyrrhenian side of the Alpine Corsica (Fig. 3), the early high-pressure (S-L) fabrics are overprinted by retrogressive deformation, synkinematic relative to a pervasive greenschist facies metamorphic overprint. Greenschist facies deformation is partitioned between domains of coaxial and noncoaxial shearing, with a typical progression from ductile to brittle deformation conditions, under a constant top-to-the-E sense of shear (Daniel et al., 1996). Samples ER1 and ER2 come from a retrogressive greenschist facies shear zone that cuts across the Schistes Lustrés metasediments (calcschists) exposed along the eastern margin of the Cape Corse antiform, along the Erbalunga coastline (Figs. 2A–3). Taking into consideration the peak temperature estimates (500–550 °C) based on Raman spectroscopy of carbonaceous material thermometry (RSCM-T), the studied metasediments are considered as a part of the lawsoniteeclogite facies rock domain of the Schistes Lustrés Complex (Vitale Brovarone et al., 2013). The shear zone fabrics consist of a gently E-dipping greenschistfacies S-L tectonite, consisting of the mineral assemblage Ph–Pg–Qz– Chl–Ab–Cal/Ank–Dol–Gr. Aligned in the greenschist shear foliation, mm sized carbonate knots occur, which contain box-shaped structures up to 0.5 mm in size. These are interpreted as pseudomorphs after lawsonite, now consisting mainly of white mica–carbonate aggregates (Fig. 5F). The shear foliation strikes N50° and the stretching lineation, as provided by Qz–Ph–Chl aggregates, trends N135° (Fig. 9A–B). Mylonitic foliation is stretched apart by SE-dipping C′-type shear bands and cut across by subparallel, SE-dipping high-angle extensional faults (Fig. 9B). Ductile kinematic indicators, as dominantly provided by S-C and C′-type fabrics at the outcrop- and thin-section scale (shear band cleavage in Passchier and Trouw, 2005), systematically provide a top-to-the-SE sense of shear (Figs. 5F and 9B–C). A southeastward asymmetric extension was also reconstructed from the brittle shear fabrics as provided by fault kinematic analyses of the late-stage extensional faulting (Fig. 9B). Both samples contain compositionally heterogeneous phengite compositions. Chemical zoning is always characterized by a sudden decrease in celadonite content in the rims of larger phengite grains (Fig. 7D). Core compositions are Si rich (3.36–3.40 p.f.u), while rim and matrix phengite have lower Si contents (3.13–3.23 p.f.u.) and higher Na, with the paragonite component ([Na/(Na + K)]) reaching up to 0.11. Chlorite shows a rather restricted compositional variation with XMg [Mg/(Mg + Fetot)] ranging 0.45–0.46. Albite contains An b 1 mol% (Table 2c). 5. Thermobarometry For a quantitative approach, P–T pseudosections have been calculated with the Perple_X_07 software (Connolly, 2005; http://www. perplex.ethz.ch, v. 6.6.9) in order to provide first-order constrains to the P–T environment of the composite shear zone fabric development along the examined transect. 5.1. ETSZ samples Evaluation of the P–T evolution of the ETSZ lithologies is difficult because shear zones mineralogy is dominated by high variance mineral

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Fig. 4. Structural characteristics of the blueschist shear zones preserved within the ETSZ at the mesoscale. (A) Eastward-dipping cm-to-dm thick blueschist shear bands in gneissic host rocks. Outcrop oriented parallel to the stretching lineations and orthogonal to the mylonitic foliation (Tc1 sampling site). (B) Detail showing transition from the gneissic host rocks and the blueschist shear zone, typified by increasing modal abundance of Na-amphibole (blue colored). (C), (D) Top-to-the-SW kinematic indicators synkinematic relative to the blueschist metamorphism, as typified by S-C fabrics and sigma-type quartz–feldspar porphyroclasts (white colored). (E), (F) Crenulated blueschist shear bands. Note the crenulation domains vanishing when entering the massive geissic lithology. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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Fig. 5. Microscale fabrics. (A) Oblique foliation defined by a fine-grained recrystallized quartz matrix in equilibrium with porphyroblastic plagioclase (Ab); shear foliation is defined by elongated Na-amphibole–phengite aggregates. Shear sense is top-to-the-WSW (crossed polars; thin section cut orthogonal to mylonitic foliation and parallel to the stretching lineations; sample Tc1). (B) Crenulation cleavage reworking an early S1 foliation defined by phengite–Na-amphibole. Note the syn-kinematic growth of Na-amphibole along the crenulation foliation (natural light). (C) S-C fabrics in phyllonites defined by fine-grained recrystallized quartz and syn-kinematic phengite. Sense of shear is top-to-the-WSW as defined by S-C fabrics, oblique foliation and micafish (crossed polars; thin section cut orthogonal to mylonitic foliation and parallel to the stretching lineations; sample HRVO). (D) Porphyroblastic albite in equilibrium with an oblique foliation made of recrystallized quartz and phengite grains. Sense of shear is top-to-the-WSW (crossed polars; thin section cut orthogonal to mylonitic foliation and parallel to the stretching lineations; sample HRVO). (E) Top-to-the-NE S-C fabrics reworking early top-to-the-SW shear fabrics in phyllonites (crossed polars; thin section cut orthogonal to mylonitic foliation and parallel to the stretching lineations; sample T8). (F) Top-to-the-ESE shear fabrics in the Schistes Lustrés as defined by elongated phengite–chlorite aggregates in an oblique foliation made of stretched and ribbon quartz grains. Kinematic indicators are provided by S-C fabrics, micafish and oblique foliation (crossed polars; thin section cut orthogonal to mylonitic foliation and parallel to the stretching lineations; sample ER1).

assemblages resulting from intensive, structurally-controlled fluid-rock interaction (Maggi et al., 2014). In the most common case, only Qz, Ab, K-Fsp and Ph are present as the main constituents. All Fe2 + and Mg (=FM) is contained in phengite. The Si content in phengite is therefore determined both by the whole-rock FM content and by the P–T conditions defining a maximum Tschermak substitution. Variations in the phengite composition in individual samples (Fig. 7A–C) are consequently related to non isochemical reactions as due to variations in the composition of an externally controlled fluid phase, combined with a change of the thermobaric environments. Without interaction with an external fluid, decrease of the Si content of phengite as result of decreasing pressure should result in the appearance of an additional FM mineral such as chlorite or biotite. This is generally not observed in the ETSZ samples, suggesting that the Si content of phengite is close to maximum at given P–T conditions, as a result of intensive interaction with the fluid phase. The Na-Amp and Acm bearing domains, occasionally occurring in the ETSZ, are characterized by rock compositions with a higher Fe3+ content. Generally, the FM and Si contents of phengite in

the ETSZ samples are relatively high, indicating HP conditions during shearing. Calculations were focused on the phyllonite samples containing the assemblage Ph + Kfs + Ab + Qz (Samples HRVO and T8). The P–T pseudosections were calculated in the system NKFMASH (Na2O–K2O– FeO–MgO–Al2O3–SiO2–H2O) as obtained from bulk rock composition (see Appendix A). The H2O content was high enough to maintain water-saturated conditions. Because the Pheng(HP) mixing model does not consider Fe3+, total iron was considered as FeO. Furthermore, the small Ca contents were not considered because this element can be combined with small amounts of Ti to form Ttn as excess phase. The following solid solution mixing models offered by Perple_X (details in solut.dat; PERPLEX_X 07; database: hp04ver.dat) were taken into account: Pheng(HP) for Ph, TiBio(HP) for Bt, Chl(HP) for Chl, and feldspar for Fsp, respectively. Calculations resulted in a pattern of the Si isopleths for phengite that can serve as a quite robust indicator for (minimum) pressure. Appearance of Bt or Chl at low pressure is just a consequence of the closed system assumption adopted for the calculation (Fig. 10A).

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Fig. 6. Left: Binary plot (Leake et al., 2004) showing the Na-amphibole composition in the blueschist shear zones (blue colors; Glaucophane/Fe-glaucophane; green color: Mg-riebeckite). Right: Back scattered electron (BSE) image showing phengite–Na-amphibole aggregates along the shear foliation in sample T3. The inset shows the Na-amphibole core-rim compositions based on the microprobe point analyses numbered in the image. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

For sample HRVO, the calculated maximum phengites Si content of ~3.6 p.f.u. is close to the analyzed one and defines a lower limit for the pressure. An upper pressure limit is indicated by the absence of Jd. A maximum temperature in the range of 400 °C is indicated by lack of crystal plastic deformation fabrics in feldspar grains (Maggi et al., 2014). The available RSCM-T data from the metasedimentary cover of the Tenda unit provided peak temperatures of ~ 350 °C (Vitale Brovarone et al., 2013). Accordingly, a peak pressure of ~ 0.9 GPa can be suggested for the HRVO sample. Likewise, presence of Fe3 + rich Na–Cpx (Acm) in sample T5c suggests minimum pressure of 1.2 GPa for the same temperature window (Maggi et al., 2012). The phyllonite sample T8, characterized by transpositive top-to-the-E shearing, has a lower Si content of phengite (Si = ~3.4 p.f.u). It is thus suggested that, compared to HRVO, sample T8 represents lower P–T conditions (below 0.5 GPa), accounting for the metamorphic re-equilibration during the top-to-the-E ETSZ reactivation (Fig. 10A). The FM component liberated during decrease of maximum Si content in phengite was removed by the fluid phase. Modeling of phase relations for the Na-Amp bearing Tc1 and T3 samples was not successful with the available mixing models offered by the PERPLEX_07 data set. Apart from poor knowledge of thermodynamic data for the mixing and end-member properties of the analyzed amphibole compositions, ferric iron in phengite was not considered in the Pheng(HP) mixing model. Therefore, equilibrium relations involving Fe3+ cannot be properly considered and evaluated. Probably, an increase of celadonite and Fe3+ content in phengite is related to a decrease of blue amphibole content and a concurrent shift in the composition of these minerals. 5.2. Schistes Lustrés samples In contrast to the ETSZ samples, phengite compositions of the ER1–2 samples are bimodal, and the FM phases Chl and Dol/Ank also occur. Calculations were performed in the system CNKFMASHC (CaO–Na2O– K2O–FeO–MgO–Al2O3–SiO2–H2O–CO2) based on the analyzed bulk rock composition (see Appendix A). The H2O content was sufficiently high to ensure water-saturated conditions. Fe3+ was not considered because the presence of graphite in the shear matrix attests to low oxygen fugacity. The following solution mixing models in PERPLEX_X (details in solut.dat; PERPLEX_X 07; database: hp04ver.dat) were chosen: oCcM(HP) for Cal, Chl(HP) for Chl, Do(HP) for Dol, Pheng(HP) for Ph, TiBio(HP) for Bt, feldspar for Fsp, respectively. An early HP stage, equilibrated under blueschist-facies metamorphic conditions at ~1.0–

1.2 GPa and ~ 350–400 °C, can be inferred from the Si-rich cores of phengite (Si = ~ 3.4 p.f.u) and from the presence of pseudomorphs after lawsonite (i.e. absence of clinozoisite in the peak assemblage). Pressure release to below ~ 0.7 GPa is indicated by the breakdown of lawsonite and by Si-poorer rims of phengite (Si = ~ 3.1–3.2 p.f.u), synkinematic relative to the penetrative, top-to-the-E greenschist shear fabric development (Fig. 10B). To sum up, an early stage of HP blueschist metamorphism (minimum peak thermobaric estimates of ~ 1.0–1.2 GPa/400 °C) can be derived for the studied ETSZ and the Schistes Lustrés metasedimentary samples. A distinct major late greenschist facies overprint, however, is clearly discernible only in the Schistes Lustrés Complex and, possibly, along the tectonically reactivated topmost phyllonite levels of the ETSZ (T8 sample). 6. Age of Alpine shearing: Rb–Sr tectonochronology Rb–Sr internal mineral isochron data are particularly well suited to directly date ductile deformation in metamorphic, white mica-bearing rocks. Deformation-induced recrystallization of white mica together with associated phases (like albite or apatite) will commonly lead to complete Sr-isotopic reequilibration and reset of ages, even at temperatures lower than 350 °C (Inger and Cliff, 1994; Müller et al., 1999). Such reset ages date the last recrystallization-inducing process, i.e., the waning stages of deformation (Freeman et al., 1998), provided that no later thermal-diffusive or retrogressive reactive overprint occurred. Purely diffusional resetting of the Rb–Sr system in white mica is activated only at very high temperatures of near 600 °C and above (cf. Glodny et al., 2008a; Villa, 1998). In our sample set, peak metamorphic temperatures were generally lower (see above), so that post-crystallization diffusional reset of isotopic signatures can largely be ruled out. Whenever feasible we analyzed white mica in different grain size fractions, to check for possible presence of mixed mica populations, i.e. of out-ofequilibrium, detrital, pre- or early-deformational white mica relics (cf Müller et al., 1999). For samples showing incomplete Sr-isotopic reequilibration, derivation of geologically meaningful age information is less straightforward. A common pattern here is a grain size vs apparent age correlation for mica grain size fractions, with younger apparent ages for smaller grain size fractions. This observation is consistent with strain partitioning into trails of fine-grained material in mylonites (Lister and Snoke, 1984; Rutter and Brodie, 1988) and with the fact that several ductile deformation mechanisms are grain size sensitive (Platt and Behr, 2011). Strain partitioning into fine-grained domains

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Table 2 Representative chemical composition of constituent mineral phases: (a) blueschist-facies shear zones of the ETSZ; (b) phyllonitic shear zones of the ETSZ; (c) greenschist-facies shear zones in the Schistes Lustrés Complex. a Sample

Tc1

T3 a

Phengite

b

Feldspar

Mineral

Amphibole

Position

Rim

Rim

Core

Gln

Gln

Fe-Gln

SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O Total

55.67 0.13 8.33 16.96 0.20 7.89 0.61 6.68 0.04 96.51

55.55 0.04 9.32 16.69 0.24 7.45 0.37 6.79 0.02 96.47

55.66 0.17 8.88 17.54 0.28 7.32 0.54 6.60 0.05 97.04

51.16 0.08 23.15 3.71 0.07 4.37 0.00 0.05 11.51 94.10

51.11 0.03 23.60 3.54 0.00 4.21 0.00 0.08 11.56 94.13

49.15 0.13 22.22 6.64 0.05 4.51 0.04 0.09 11.08 93.91

69.56 0.05 19.47 0.07 0.03 0.01 0.05 11.53 0.04 100.81

Si Ti Al Fe3+ Fe2+ Mn Mg Ca Na K XMg

7.93 0.01 1.40 0.55 1.48 0.02 1.68 0.09 1.85 0.01 0.53

7.91 0.01 1.56 0.48 1.50 0.03 1.58 0.06 1.87 0.00 0.51

7.92 0.02 1.49 0.44 1.64 0.04 1.55 0.08 1.82 0.01 0.49

3.51 0.00 1.87 – 0.21 0.00 0.45 0.00 0.01 1.01 0.68

3.50 0.00 1.90

3.44 0.01 1.83 – 0.39 0.00 0.47 0.00 0.01 0.99 0.55

3.01 0.00 0.99 0.00 – 0.00 0.00 0.00 0.97 0.00

Core

Core

0.20 0.00 0.43 0.00 0.01 1.01 0.68

Rim

Ab

c

Amphibolea Kfs

Phengiteb

Core

Inner rim

Outer rim

Mg-Rbk

Gln

Mg-Rbk

64.65 0.01 18.02 0.07 0.00 0.00 0.00 0.22 16.97 99.94

56.50 0.04 5.82 18.20 0.34 9.09 0.65 7.03 0.03 97.70

58.62 0.07 10.18 8.75 0.41 11.83 0.17 7.65 0.00 97.69

3.00 0.00 0.99 0.00 – 0.00 0.00 0.00 0.02 1.00

7.95 0.00 0.97 1.02 1.12 0.04 1.91 0.10 1.92 0.01 0.63

7.94 0.01 1.63 0.42 0.57 0.05 2.39 0.02 2.01 0.00 0.81

Feldsparc

Core

Core

Rim

Ab

Ab

55.39 0.02 4.01 17.48 0.38 10.52 2.04 6.04 0.02 95.89

52.06 0.19 22.74 3.80 0.05 4.10 0.00 0.10 11.21 94.24

51.71 0.02 25.11 3.40 0.09 3.17 0.03 0.26 10.87 94.64

48.70 0.11 25.10 4.64 0.06 3.34 0.01 0.17 11.21 93.33

67.48 0.00 19.77 0.25 0.00 0.13 0.01 11.30 0.34 99.29

68.98 0.00 19.51 0.24 0.00 0.00 0.03 11.43 0.04 100.23

7.94 0.00 0.68 1.14 0.95 0.05 2.25 0.31 1.68 0.00 0.70

3.55 0.01 1.83 – 0.22 0.00 0.42 0.00 0.01 0.98 0.66

3.50 0.00 2.00 – 0.19 0.00 0.32 0.00 0.03 0.94 0.62

3.38 0.01 2.05 – 0.27 0.00 0.35 0.00 0.02 0.99 0.56

2.97 0.00 1.03 0.01

3.00 0.00 1.00 0.01

0.00 0.00 0.00 0.97 0.02

0.00 0.00 0.00 0.96 0.00

b Sample mineral position

HRVO

T8

Phengited

Feldspare

Phengited

Feldspare

Core

Core

Rim

Core

Core

Rim

Ab

Kfs

Kfs

Core

Core

Rim

Rim

Ab

Kfs

SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O Total

54.45 0.05 22.65 4.40 0.06 4.18 0.00 0.05 11.43 97.27

53.86 0.01 23.63 4.29 0.05 3.90 0.00 0.14 10.92 96.80

52.68 0.01 25.48 3.53 0.12 3.53 0.01 0.03 11.17 96.56

53.55 0.01 23.94 3.80 0.05 3.80 0.04 0.05 11.31 96.53

52.06 0.19 22.74 3.80 0.05 4.10 0.00 0.10 11.21 94.24

51.71 0.02 25.11 3.40 0.09 3.17 0.03 0.26 10.87 94.64

67.35 0.00 19.18 0.04 0.00 0.00 0.03 11.69 0.10 98.40

63.60 0.01 18.36 0.00 0.00 0.01 0.00 0.24 17.01 99.24

63.85 0.00 18.51 0.00 0.00 0.00 0.00 0.25 17.22 99.84

49.62 0.07 25.55 3.86 0.02 3.08 0.00 0.13 11.57 93.90

49.88 0.13 25.67 4.28 0.06 2.89 0.00 0.15 11.46 94.52

48.70 0.11 25.10 4.64 0.06 3.34 0.01 0.17 11.21 93.33

49.53 0.12 26.47 4.11 0.11 2.81 0.03 0.13 11.34 94.67

68.17 0.00 19.28 0.06 0.00 0.00 0.02 11.65 0.03 99.21

64.26 0.01 17.96 0.00 0.00 0.00 0.01 0.30 16.74 99.28

Si Ti Al Fe3+ Fe2+ Mn Mg Ca Na K XMg

3.60 0.00 1.77 – 0.24 0.00 0.41 0.00 0.01 0.96 0.63

3.57 0.00 1.84

3.49 0.00 1.99

3.56 0.00 1.87

0.24 0.00 0.38 0.00 0.02 0.92 0.62

0.20 0.01 0.35 0.00 0.00 0.94 0.64

0.21 0.00 0.38 0.00 0.01 0.96 0.64

3.55 0.01 1.83 – 0.22 0.00 0.42 0.00 0.01 0.98 0.66

3.50 0.00 2.00 – 0.19 0.00 0.32 0.00 0.03 0.94 0.62

2.99 0.00 1.00 0.00 – 0.00 0.00 0.00 1.01 0.01

2.98 0.00 1.01 0.00 – 0.00 0.00 0.00 0.02 1.02

2.98 0.00 1.01 0.00 – 0.00 0.00 0.00 0.02 1.01

3.42 0.01 2.08 – 0.22 0.00 0.32 0.00 0.02 1.02 0.59

3.40 0.01 2.09 – 0.25 0.00 0.30 0.01 0.02 0.98 0.55

3.38 0.01 2.05 – 0.27 0.00 0.35 0.00 0.02 0.99 0.56

3.38 0.01 2.13 – 0.23 0.01 0.29 0.00 0.02 0.99 0.55

3.00 0.00 1.00 0.00 – 0.00 0.00 0.00 0.99 0.00

3.00 0.00 0.99 0.00 0.00 0.00 0.00 0.00 0.03 1.00

c Sample mineral position

ERBA1–2 Phengitef

SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O Total

Paragonitef

Chloriteg

Core/early

Core/early

Late/rim

Late/rim

Shear matrix

Shear matrix

Shear matrix

Shear matrix

Shear matrix

Shear matrix

51.66 0.12 28.53 2.35 0.00 2.99 0.01 0.46 10.69 96.79

52.08 0.17 28.27 2.76 0.02 2.83 0.00 0.35 10.48 96.97

47.65 0.17 35.18 0.87 0.00 0.94 0.02 0.80 9.89 95.52

48.01 0.08 34.88 1.01 0.00 1.06 0.02 0.84 9.85 95.75

47.67 0.11 39.77 0.38 0.00 0.23 0.03 6.72 0.68 95.60

48.70 0.04 39.59 0.43 0.02 0.22 0.03 6.42 1.20 96.67

48.83 0.02 40.01 0.20 0.00 0.29 0.02 7.05 0.22 96.65

25.45 0.04 22.31 26.76 0.17 13.15 0.00 0.00 0.00 87.87

24.82 0.01 22.32 27.38 0.14 13.26 0.00 0.00 0.00 87.92

26.79 0.03 23.52 25.76 0.13 12.52 0.00 0.00 0.00 88.75

(continued on next page)

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Table 2 (continued) c Sample mineral position

ERBA1–2 Phengitef

Si Ti Al Fe3+ Fe2+ Mn Mg Ca Na K XMg a b c d e f g

Paragonitef

Chloriteg

Core/early

Core/early

Late/rim

Late/rim

Shear matrix

Shear matrix

Shear matrix

Shear matrix

Shear matrix

Shear matrix

3.38 0.01 2.20 – 0.13 0.00 0.29 0.00 0.06 0.89 0.69

3.40 0.01 2.17 – 0.15 0.00 0.27 0.00 0.04 0.87 0.65

3.13 0.01 2.72 – 0.05 0.00 0.09 0.00 0.10 0.83 0.66

3.15 0.00 2.69 – 0.06 0.00 0.10 0.00 0.11 0.82 0.65

3.01 2.96 0.01 – 0.02 0.00 0.02 0.00 0.82 0.05 0.52

3.04 2.91 0.00 – 0.02 0.00 0.02 0.00 0.78 0.10 0.48

3.03 2.93 0.00 – 0.01 0.00 0.03 0.00 0.85 0.02 0.72

2.69 0.00 2.78 – 2.36 0.01 2.07 0.00 0.00 0.00 0.47

2.63 0.00 2.79 – 2.43 0.01 2.10 0.00 0.00 0.00 0.46

2.77 0.00 2.86 – 2.23 0.01 1.93 0.00 0.00 0.00 0.46

Cations on the basis of a total of 15 cations (excluding K), 23 oxygen ions. Cations based on 22 oxygen. Cations based on 8 oxygens. Cations based on 22 oxygen. Cations based on 8 oxygens. Cations based on 22 oxygen. cations based on 14 oxygens.

during ductile deformation will result in more efficient chemical and isotopic re-equilibration of small grains compared to larger grains. In consequence, the smallest mica crystals in a given rock will commonly be most easily affected by dynamic recrystallization processes and show isotopic signatures most closely associated to the last event of ductile deformation. Apparent ages for small grain size mica fractions can thus considered as maximum age estimates for the end of ductile deformation in a given sample. Results are shown on Fig. 11 as isochron plot and detailed in Table 3.

6.1. ETSZ samples T5c-Microtexture shows a cm-scale layering of phengite-rich and “dark” Acm–Amp rich domains. The analyzed ‘dark’ domain is fine grained, and preparation of pure mineral separates of Acm, Amp, Ab and Ap was hampered by ubiquitous intergrowth of all other phases with abundant phengite. Rb–Sr multimineral analysis shows that all minerals retain unusually high 87Rb/86Sr ratios (N1000 for two white mica fractions), combined with very high ‘initial’ (Eocene–Oligocene) 87 Sr/86Sr ratios (N0.84 for Ap, Ab and Na-Amp). This signature is combined with drastic Sr-isotopic disequilibria between Acm and the other low-Rb/Sr phases (Fig. 11). Taken together, isotopic disequilibria and the Rb-enriched, Sr-depleted signature of the rock points to a strong metasomatic imprint during crystallization and ductile deformation of the minerals of this rock, probably by fluids that were previously involved in breakdown of high-87Sr/86Sr, high-Rb/Sr phases. We hypothesize that these fluids were released by and/or obtained their signature from syn-shearing alteration and metasomatism of the Hercynian basement in the footwall of the ETSZ (see also Maggi et al., 2014). Despite the Sr-isotopic intermineral disequilibria, the high Rb/Sr ratios of the phengite fractions facilitate to derive an age estimate for the end of ductile deformation in this sample. Regression of all data results in an age value of 30.8 ± 4.2 Ma. The Rb–Sr data for Acm plot markedly below the regression line (Fig. 11), which may indicate that Acm was formed early in the deformation history and persisted as an

isotopic and petrologic relic in the assemblage during progressive rock deformation, alteration and metasomatism. Exclusion of the Acm data results in an age estimate of 29.1 ± 1.5 Ma (stippled line in Fig. 11) for the last increments of ductile shear in this sample.

6.1.1. T3 and Tc1 Rb–Sr mineral data for both samples show very similar features, namely Sr-isotopic disequilibrium between Na-Amp and Fsp, combined with slight but significant phengite apparent age vs. grain size correlations (Fig. 11). Sr-isotopic disequilibria between the Na-Amp and the quartz–feldspar separates most likely reflect the presence of small proportions of relic, pre-HP feldspar porphyroclasts. The homogeneous high-Si phengite compositions indicates that no pre-metamorphic white mica relics are present. Instead, the correlation between apparent ages and grain sizes probably reflects a protracted HP deformation history, with only incomplete Sr-isotopic reequilibration of ‘big’ crystals during the last deformation increments. Regression of all mineral data yields identical apparent ages of 31.5 ± 2.5 Ma for sample T3 and of 31.8 ± 1.7 Ma for sample Tc1, respectively. Maximum ages for the end of ductile deformation in both samples, calculated from the data for Na-amphibole and the most fine-grained phengite fractions, are 29.3 ± 0.5 Ma for sample T3 and 30.9 ± 0.6 Ma for sample Tc1, respectively (Fig. 11).

6.1.2. HRVO and T8 The phillonite, top-to-the-W sample HRVO yields a well defined Rb/Sr mineral isochron age of 27.1 ± 0.3 Ma (Fig. 11). Instead, sample T8, with its strong top-to-the E deformational overprint, yields an apparent age of 20.2 ± 2.2 Ma. This early Miocene age signal is in marked contrast to the Oligocene deformation ages obtained for all the samples recording top-W thrusting (see above). The phengite population in this sample shows heterogeneous Rb–Sr signatures, again with higher apparent ages for bigger grains. A maximum age for the end of ductile shearing, calculated from feldspar combined with the 125 to 90 μm phengite fraction data, is 19.4 ± 0.3 Ma (Fig. 11).

Fig. 7. (A) Cumulative (Mg + Fetotal) vs Si plot of white mica (phengites) from the studied blueschist shear zone samples showing indications of non-Tschermak substitution in white mica. (B) Left: BSE image showing crenulation cleavage affecting a composite phengite–Na-amphibole shear foliation. Post-kinematic Na-amphibole–phengite growth is also observed. The Naamphibole grains show core-to-rim chemical zonation from glaucophane (Gl) to riebeckite (Rb) compositions. The phengite grains instead show a patchy chemical zoning as evidenced by the dark and light colored domains distribution. Right: (Mg + Fetotal) vs Si plot of phengites based on the microprobe point analyzed numbered in the image. Composition varies with the structural position of the analyzed grain, with higher celadonite substitution preserved at the fold hinge and lower celadonite contents measured in post-kinematic grains or in grains along the axial plane foliation. (C) Cumulative (Mg + Fetotal) vs Si plot of white mica (phengites) from the studied phyllonite samples showing influence of other non-Tschermak substitutions. (D) Representative qualitative compositional cation map (left) and (Mg + Fetotal) vs Si plot of white mica (phengites) from the studied Schistes Lustrés ER1–2 samples. Note the core-torim chemical zoning typified by a systematic decrease of the celadonite substitution.

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Fig. 8. Composite shear fabrics in the phyllonitic shear zones. (A) Spaced top-to-the-E semibrittle shear surface reworking early top-to-the-W ductile fabric (HRVO sampling locality; outcrop oriented parallel to the stretching direction and normal to shear foliation). (B) Transpositive top-to-the-E shear fabrics in phyllonites close to the contact with the overlying Schistes Lustrés Complex (T8 sampling locality; outcrop oriented parallel to the stretching direction and normal to shear foliation). The stereoplot (Schmidt net, lower hemisphere projection) show the attitude of the C′-type, top-to-the-E shear bands in phyllonites (arrows indicating the hanging wall movement).

6.2. Schistes Lustrés samples 6.2.1. ER1 and ER2 The Rb–Sr data for carbonate (Fe-bearing calcite), feldspar and white mica (Ph/Pg) of the two samples yield well defined isochron correlations, with nearly identical, early Miocene ages (20.6 ± 0.5 Ma for sample ER1 and 21.3 ± 0.3 Ma for sample ER2, respectively) (Fig. 11).

7. Discussion The tectono-metamorphic data presented above documents polyphase shear fabrics development during the Alpine construction of Corsica. The early shear fabrics reconstructed within the ETSZ (the basal thrust of the Alpine orogen) is composite and developed under HP blueschist-facies conditions at the expenses of the Hercynian continental basement of Corsica (European continent-derived external units). Along the ETSZ, the HP stage is documented by highly substituted, syn-kinematic phengites (with Si up to 3.6-3.7 p.f.u.), locally associated with Na-Amp and Cpx-Rt assemblages. The corresponding tectonic fabric is dominated by shear strain partitioning (gneissic and phyllonitic shear zones) and attests for progressive deformation during a general top-to-the-W compressional sense of shear. In particular, development of crenulations in ductile shear zones attests for tectonic exhumation of the ETSZ during the progressive localization of the top-to-the-W shearing. When approaching contact with the overlying Schistes Lustrés Complex, the early top-to-the-W composite shear fabrics of the ETSZ

are progressively reworked and transposed into top-to-the-E extensional shear bands. These late-stage shear domains are characterized by phengite recrystallization, as attested by a marked decrease of the celadonitic content (Si down to 3.4 p.f.u. in sample T8). Pseudosection modeling of the phyllonitic shear zones indicates that the top-to-theW, high-pressure shearing associated with orogenic construction took place at pressure around 0.9-1.2 GPa for temperatures not in excess of 400 °C. The top-to-the-E extensional reactivation occurred at shallower crustal levels, likely at pressure conditions well below 0.5 GPa (Fig. 10A), in a semi-brittle-to-brittle dominated deformation regime (Maggi et al., 2014). Preservation of highly substituted phengites in both types (topto-the-W and top-to-the-E) of shear zones samples suggests general cooling during exhumation of the ETSZ, along a path nearly parallel to the phengite isopleths (see also Maggi et al., 2012) (Fig. 10A). Likewise, in the Schistes Lustrés Complex, the top-to-the-E shear senses are associated with greenschist retrogressive metamorphism, under the control of progressive, ductile-to-brittle top-to-the-E extensional shearing (Fig. 9). The evidence of lawsonite pseudomorphs preserved within the extensional shear lenses can be attributed to the early prograde HP-LT metamorphism that affected the oceanic domains during oceanic subduction (Fournier et al., 1991; Ravna et al., 2010; Vitale Brovarone et al., 2011, 2013). Pseudosection modeling places the HP stage at c. 1.0-1.2 GPa for temperature below c. 400 °C. Taking into consideration the peak temperature estimates proposed in Vitale Brovarone et al. (2013) for the studied samples (RSCM-T of ~500 °C), the HP (blueschist-facies) stage reconstructed in this study can be possibly regarded as an intermediate stage of metamorphic re-equilibration during exhumation of the Schistes Lustrés Complex. The final

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Fig. 9. (A) Prominent, syn-greenschist linear fabric made up by stretched quartz in calcschists of the Schistes Lustrés Complex exposed in the Erbalunga area (ER1–2 sampling locality; plain view). (B) Top: Ductile-to-brittle top-to-the-SE shearing in the Erbalunga area (exposure parallel to the NW-SE-trending stretching lineations and normal to mylonitic foliation). The syn-greenschist facies S-L mylonitic fabric is progressively overprinted by SE-dipping brittle extensional faults. Bottom: The stereoplots (Schmidt net, lower hemisphere projection) show the attitude of the syn-greenschist S-L fabrics and the brittle extensional faulting. (C) Detail shown in (B) showing C′-type shear bands pointing to top-to-the-SE shearing.

syn-greenschist retrogression is constrained at pressures below ~ 0.50.7 GPa, likely achieved during a nearly isothermal exhumation path (Fig. 10B), in a scenario dominated by extensional unroofing.

7.1. Age of Alpine shearing The Rb–Sr systematics obtained from the shear zones samples studied along the geological transect presented in this study may help to elucidate the major phases of nappe construction and subsequent

extensional reworking of the Alpine orogen in Corsica. In particular, the following key points can be taken into account: (i) The early Oligocene ages (from ~32 Ma to ~29 Ma) obtained from the ETSZ blueschist samples (T5c, Tc1 and T3) are interpreted to closely date the waning stages of the high-pressure top-to-theW thrusting along the ETSZ. This is in line with results obtained from the top-to-the-W phyllonite sample HRVO, which provides a late Oligocene deformation age (~27 Ma). Given the fact that this latter age value is defined by high-Si phengite which did

Fig. 10. (A) NCKFMASH P–T pseudosection for a representative bulk rock composition (wt%) Na2O 3.35, K2O 4.61, FeO 1.71, MgO 1.22, Al2O3 13.90, SiO2 73.0 (H2O saturated) of a phyllonitic shear zone (sample HRVO). The fields are contoured with isopleths of phengite (Si p.f.u.) compositions. Equilibrium thermobaric conditions during shearing are constrained by the observed mineral assemblage and phengites compositions (see text for further details. (B) CNKFMASHC P–T pseudosection for a representative bulk rock composition (wt.%) CaO 16.40, Na2O 0.69, K2O 2.32, FeO 6.44, MgO 3.01, Al2O3 13.9, SiO2 40.70, CO2 13.50 (H2O saturated) of a greenschist-facies shear zone in the Schistes Lustrés samples (Sample ER1). The fields are contoured with isopleths of phengite (Si p.f.u.) compositions. Occurrence of lawsonite pseudomorphs and absence of epidote constrain the overall thermobaric evolution. Mineral abbreviations after Whitney and Evans (2010).

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Fig. 11. Mineral data and Rb–Sr age results for samples from the ETSZ and the Schistes Lustrés. Analytical data are given in Table 3. Grain size is indicated when different grain size fractions were analyzed. Mineral abbreviations after Whitney and Evans (2010).

not recrystallise during later top-to-the-E semi-brittle shearing, we conclude that this age value is close to the latest phase of top-to-the-W ductile deformation in Alpine Corsica. (ii) No high-pressure, top-to-the-W shearing prior to the Early Oligocene is recorded in the Rb–Sr data set. Nevertheless, earlier (pre-Oligocene) increments of HP deformation or recrystallization cannot be ruled out using the Rb–Sr dataset. It is also worth noting that the Oligocene ages are derived from texturally overprinted (crenulated) top-to-the-W tectonites, indicating that the main rock fabrics were acquired within a general scenario of rock exhumation. (iii) As documented by the phyllonite samples from the ETSZ (sample T8), switch from compressional top-to-the-W to extensional top-to-the-E shearing regime is bracketed between

~ 27 and ~ 20 Ma. The late Oligocene–early Miocene thus corresponds to time of the extensional tectonic reactivation of the ETSZ. (iv) The top-to-the-E ductile retrogressive greenschist facies shearing in the Schistes Lustrés culminated during the early Miocene (at ~21 Ma), as documented by the samples ER1and ER2. Top-tothe-E shearing proceeded in the brittle regime post ~21 Ma. 7.2. Tectonic and geodynamic implications The P–T–deformation–time histories reconstructed from the ETSZ samples places the waning stage of the top-to-the-W, high-pressure (ductile) Alpine compressional shear during the Oligocene, at ~ 3227 Ma. This timing partly overlaps the late Eocene–Oligocene

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Table 3 Rb–Sr isotopic data. Sample no.

Materiala

Rb (ppm)

Sr (ppm)

Analysis no. ER1 (20.6 ± 0.5 Ma, MSWD = 2.3, Sri = 0.712735 ± 0.000025) — Schistes Lustrés PS1762 Cal 11.2 315 PS1763 Qz–Fsp 4.11 143 PS1764 Ph N250 μm 200 104 PS1765 Ph 250–160 μm 182 102 PS1766 Ph 160–125 μm 182 103 PS1767 Ph 125–90 μm 160 109 ER2 (21.3 ± 0.3 Ma, MSWD = 1.7, Sri = 0.712733 ± 0.000025) — Schistes Lustrés PS1768 Qz–Fsp 1.23 59.6 PS1769 Cb 4.59 283 PS1770 Ph 160–90 μm 251 78.0 PS1771 Ph N160 μm 317 63.7 T8 (20.2 ± 2.2 Ma, MSWD = 15, Sri = 0.7232 ± 0.0024) — ETSZ PS1837 Qz–Fsp 5.67 4.51 PS1839 Ph 355–250 μm 418 13.2 PS1840 Ph 250–160 μm 424 12.8 PS1841 Ph 160–125 μm 423 15.1 PS1842 Ph 125–90 μm 421 16.3 HRVO (27.1 ± 0.3 Ma, MSWD = 2.6, Sri = 0.723281 ± 0.000076) — ETSZ PS1844 Qz–Fsp 39.5 13.1 PS1845 Ph N 250 μm 431 24.5 PS1846 Ph 250–160 μm 445 16.4 PS1847 Ph 160–125 μm 445 11.3 T5c (30.8 ± 4.2 Ma (all data); 29.1 ± 1.5 Ma (excl. Acm), MSWD = 14312, Sri = 0.817 ± 0.037) — ETSZ PS1933 Ph 200–160 μm 1030 2.68 1068 2.25 PS1866 Ph 250–160 μm PS1864 Ph 160–63 μm (a) 699 4.88 PS1931 Ph 160–63 μm (b) 691 5.14 PS1929 Ap (a) 54.7 174 PS1862 Ap (b) 51.3 196 PS1928 Ab 29 88.4 PS1927 Na-Amp 134 5.9 PS1863 Acm (a) 90.5 7.46 PS1932 Acm (b) 90.2 7.16 T3 (31.5 ± 2.5 Ma, MSWD = 2378, Sri = 0.7152 ± 0.0062) — ETSZ PS1772 Na-Amp 33.6 8.51 PS1773 Qz–Fsp 6.24 25.5 PS1774 Ph 355–250 μm 400 5.02 PS1775 Ph 250–160 μm 402 5.4 PS1776 Ph 160–125 μm 396 5.44 PS1777 Ph 125–90 μm 387 5.92 Tc1 (31.8 ± 1.7 Ma, MSWD = 19, Sri = 0.72318 ± 0.00079) — ETSZ PS1832 Na-Amp (a) 53.2 13.7 PS1833 Qz–Fsp 9.52 3.69 PS1834 Ph 160–125 μm 323 21.4 PS1835 Ph N 160 μm 330 18.2 PS1836 Ph 125–90 μm 330 20.5 PS1921 Na-Amp (b) 61.0 20.3 a

40

87

Rb/86Sr

87

Sr/86Sr

87

Sr/86Sr 2σm (%)

0.1 0.08 5.56 5.19 5.15 4.24

0.712751 0.712776 0.714325 0.714288 0.714258 0.713949

0.0013 0.0006 0.0018 0.0009 0.0010 0.0011

0.06 0.05 9.30 14.4

0.712733 0.712761 0.715570 0.717057

0.0013 0.0014 0.0018 0.0026

3.64 92.3 96.5 81.3 75.3

0.724509 0.750422 0.751462 0.745848 0.744214

0.0080 0.0120 0.0071 0.0412 0.0050

8.78 51.1 78.8 115

0.726651 0.743212 0.753541 0.767095

0.0028 0.0060 0.0026 0.0116

1182 1471 426 400 0.92 0.77 0.96 66.9 35.3 36.7

1.341043 1.453281 1.007237 0.998492 0.846313 0.846167 0.846205 0.886769 0.762297 0.766080

0.0030 0.0031 0.0024 0.0038 0.0011 0.0007 0.0024 0.0032 0.0040 0.0026

11.4 0.71 233 217 213 191

0.723647 0.712957 0.822814 0.813053 0.808613 0.798278

0.0016 0.0012 0.0026 0.0055 0.0023 0.0024

11.3 7.48 43.9 52.7 46.8 8.69

0.728415 0.726363 0.742591 0.747569 0.744004 0.727294

0.0018 0.0100 0.0018 0.0076 0.0032 0.0019

Mineral Abreviations after Whitney and Evans (2010).

Ar/ 39 Ar phengite dating of the ETSZ presented by Brunet et al. (2000) and is compatible with post-Bartonian (40–37 Ma) thrusting in the external domain of Alpine Corsica (Bezert and Caby, 1988; Egal, 1992; Malasoma and Marroni, 2007), which collectively constrain the syn-orogenic (i.e.syn-thrusting) exhumation of the Alpine Corsica Domain during the early Oligocene. This time lapse is also compatible with the late Eocene ages recently proposed for the peak of subduction zone metamorphism of the oceanic-derived Schistes Lustrés Complex (Martin et al., 2011; Vitale Brovarone and Herwartz, 2013). This evidence suggests a scenario of late increments of (highpressure) compressional shear during the early Oligocene and, therefore, poses the issue on significance of the early Eocene ages derived from TIMS U–Pb dating of the T5c Acm-Rt-bearing sample from the ETSZ (Maggi et al., 2012). It is worth noting, however, that a prolonged early Eocene to early Oligocene activity of the ETSZ as a compressional shear zone is compatible with post-early Eocene to pre-Oligocene (~ 50-30 Ma) ~ 45° counterclockwise rotation of Sardinia–Corsica relative to Eurasia (Advokaat et al., 2014). The Eocene rotation of Sardinia–Corsica was synchronous with and likely responsible for

documented N-S shortening in the Provence (Lacombe and Jolivet, 2005) and the incorporation of the Briançonnais continental domain, likely connected to Corsica, into the western Alps (Advokaat et al., 2014). Indeed, an early Eocene age for the ETSZ is in line with the maximum 40Ar/39Ar phengite ages presented by Brunet et al. (2000) for the ETSZ and, more in general, with the Eocene involvement of the European continental margin units (Briançonnais Domain) in the subduction processes of the internal zones of the Alpine chain (Berger and Bousquet, 2008; Handy et al., 2010; Villa et al., 2014). Our study also documents that transition from compression to extension in Alpine Corsica occurred during the late Oligocene–early Miocene time lapse (post ~ 27 Ma), with the last episode of deformationrelated ductile recrystallization being recorded at ~ 21-20 Ma both within the exhuming Schistes Lustrés Complex and during the major extensional reactivation of the ETSZ. The early Miocene ductile extension and the continuous transition to brittle extensional faulting documented in this study reinforce the significance of the early Miocene rapid exhumation event already detected in the Alpine Corsica region (Cavazza et al., 2001; Fellin et al., 2005; Zarki-Jakni et al., 2004). In this

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view, conclusive evidence is provided to assert that deposition of the early Miocene (Burdigalian-Langhian) Saint Florent basin (Cavazza et al., 2007; Ferrandini et al., 1998) on top of the exhuming HP units (Fig. 3) occurred during the activity of the ETSZ as an extensional shear zone (Daniel et al., 1996; Gueydan et al., 2003; Jolivet et al., 1998). Finally, our study demonstrates that the extensional collapse of the Alpine orogen in Corsica culminated 7–8 Ma after the last top-tothe-W compressional shear increments along the ETSZ, concurrently with the early Miocene transition from rifting to drifting in the LiguroProvençal basin (Gattacceca et al., 2007; Speranza et al., 2002). The tectono-metamorphic and Rb–Sr geochronological data presented in this study correspond to a time window (early Oligocene–early Miocene) central in reconstruction of the Alps-Apennine relationships, with different geodynamic scenarios proposed so far (e.g., Advokaat et al., 2014; Argnani, 2012; Carminati et al., 2012; Doglioni et al., 1998; Dumont et al., 2011; Faccenna et al., 2001, 2004; Handy et al., 2010; Lacombe and Jolivet, 2005; Malusà et al., 2011; Molli and Malavieille, 2010; Turco et al., 2012; van Hinsbergen et al., 2014; Vignaroli et al., 2008). On this regard, integration of the results presented in this study with the paleomagnetic evidence presented in Advokaat et al. (2014) indicate that the major stages of Alpine orogenic construction and extensional reworking in Corsica occurred concurrently with two major rotation episodes of the Sardinia-Corsica block with respect to Eurasia, during Eocene and early Miocene, respectively. This evidence supports a paleotectonic configuration that places (i) development of the Alpine Corsica orogenic edifice during underthrusting of the European margin (Briançonnais realm) in the frame of the Alpine (east-dipping) subduction during the Eocene, and (ii) the extensional collapse of the orogenic edifice as a consequence of the back-arc extensional regime generated in the upper-plate of the Apennine (west-dipping) subduction. This reconstruction suggests that both subduction systems (Alpine and Apennine) were active during Eocene–Oligocene times, likely accommodated along a zone of subduction polarity reversal (cf. Advokaat et al., 2014; Malusà et al., 2011; Vignaroli et al., 2008). Within the frame of the continuous northward motion of the Adria plate, from Middle Eocene onward (e.g., Jolivet and Faccenna, 2000; Malusà et al., 2011; Rosenbaum et al., 2002; van Hinsbergen et al., 2014), we then tentatively propose that the late Eocene–early Oligocene stage of orogenic construction in Alpine Corsica was the consequence of the northward propagation of the Apennine subduction front within the Ligurian–Piemontese oceanic realm, in the retroside of the Alpine chain. Later on, when the Apennine subduction was mature enough to roll back, Alpine Corsica entered in the upper-plate of the Apennine subduction with the consequent collapse of the Alpine orogenic domain from late Oligocene onward.

8. Conclusions The structural and metamorphic scenario, together with the Rb– Sr geochronological data presented in this study document that the terminal orogenic construction of Alpine Corsica occurred during Eocene–Oligocene times (~ 27-32 Ma). The shift to crustal-scale ductile-to-brittle extension (orogenic collapse) occurred after ~27 Ma, with the concurrent tectonic reactivation of the early-structured thrust-related fabrics along the ETSZ and exhumation of the Schistes Lustrés Complex that culminated during the Early Miocene (~ 2021 Ma). This corresponds to the time when the exhuming ductile HP domain of Alpine Corsica entered in the brittle crust. These results indicate that the major stages of Alpine orogenic construction and extensional reworking of the nappe stack in Corsica occurred concurrently with the major rotation episodes of the Sardinia-Corsica block with respect to Eurasia, first during the Eocene as a part of the Briançonnais Domain and then during the early Miocene as a part the back-arc domain of the Apennine subduction.

Acknowledgments This paper is dedicated to the memory of Christophe Brunet. Stimulating discussion held with the participants to the CorseAlp2011 meeting and with G. Molli, C. Faccenna, M. Malusà and H.-J. Massonne are acknowledged. Constructive reviews and comments by A. Vitale Brovarone and L. Jolivet contributed to improve the paper. Appendix A. Analytical details Mineral chemistry Mineral compositional data were obtained using a Cameca SX100 electron microprobe at the Institut für Mineralogie und Kristallchemie, Universität Stuttgart. Operating conditions were 15 kV and a 10 to 15 nA beam current (WDS mode). Mineral compositions were determined relative to natural and synthetic standards. Spot sizes were 1–10 μm depending on the phases analyzed. Concentration maps for major elements (Ca, Fe, Mn, Mg and Al or Na) were also produced by stepwise movements of the thin sections under the electron beam; counting times per step were 100 ms. BSE imaging was obtained by using the same electron microprobe. Classification of amphibole with the general formula AB2C5T8O22(OH)2 has been made according to the IMA recommendations (Leake et al., 2004), by means of the WINAMPH software of Yavuz (2007). Other mineral structural formulae were calculated through the software CalcMin_32 (Brandelik, 2009). Whole rock geochemistry The whole rock chemical compositions of the studied samples were determined at Activation Laboratories (Ontario, Canada), through ICP emission (major and some trace elements). For major elements the precision is estimated better than 2% for values higher than 5 wt.% and better than 5% in the range 0.1–5 wt.%. The CO2 content in sample ER1 was measured with a LECO H-C element analyzer at the Institut für Mineralogie und Kristallchemie, Universität Stuttgart. Rb–Sr isotope analyses For mineral separation work, we used small samples (less than 100 g), texturally recording high-strain ductile deformation, and containing white mica as a high Rb/Sr phase. Isotopic data were generated at GFZ Potsdam using a Thermo Scientific TRITON thermal ionization mass spectrometer. Sr isotopic composition was measured in dynamic multicollection mode. Rb isotope dilution analysis was done in static multicollection mode. The value obtained for 87Sr/86Sr in the NIST SRM 987 isotopic standards during the period of analytical work was 0.710255 ± 0.000005 (n = 23). For age calculation, standard errors of ±0.005% for 87Sr/86Sr and of ±1.5% for 87Rb/86Sr ratios (as derived from replicate analyses of spiked white mica samples) were assigned to the results, provided that individual analytical uncertainties were smaller than these values. Otherwise, individual analytical uncertainties were used. Handling of mineral separates and analytical procedures are described in more detail in Glodny et al. (2008b). Uncertainties of isotope and age data are quoted at 2σ in this work. The program ISOPLOT/ EX 3.71 (Ludwig, 2009) was used to calculate regression lines. The 87Rb decay constant is used as recommended by Steiger and Jäger (1977). References Advokaat, E.L., van Hinsbergen, D.J.J., Maffione, M., Langereis, C.L., Vissers, R.L.M., Cherchi, A., Schroeder, R., Madani, H., Columbu, S., 2014. Eocene rotation of Sardinia, and the paleogeography of the western Mediterranean region. Earth and Planetary Science Letters 401, 183–195. Argnani, A., 2012. Plate motion and the evolution of Alpine Corsica and Northern Apennines. Tectonophysics 579, 207–219. http://dx.doi.org/10.1016/j.tecto.2012.06.010. Berger, A., Bousquet, R., 2008. Subduction-related metamorphism in the Alps: review of isotopic ages based on petrology and their geodynamic consequences. In:

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