Naxos, Cyclades, Greece

Retrogression of a high-temperature metamorphic core complex Low-grade retrogression of a high-temperature metamorphic core complex: Naxos, Cyclades, Greece

Shuyun Cao1,2,†, Franz Neubauer1, Manfred Bernroider1, Johann Genser1, Junlai Liu3, and Gertrude Friedl1 1Department of Geography and Geology, University of Salzburg, Hellbrunnerstrasse 34, A-5020 Salzburg, Austria 2State Key Laboratory of Geological Processes and Mineral Resources, School of Earth Sciences, China University of Geosciences, Wuhan 430074, China 3State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Beijing 100083, China

ABSTRACT metapelites (at temperatures of ~350–130 °C) sion of metamorphic complexes along the in the metamorphic core complex core re- upper margins, particularly close to the brittle- Retrogressive deformation and metamor- sulted mainly from late-stage E-W shorten- ductile boundary (e.g., Siebenaller et al., 2013; phism are often reported from the main ing and folding. Late-stage flow of hydrous Whitney et al., 2013; Gébelin et al., 2014; low-angle shear zones and detachments of fluids resulted in resetting of fabrics and Methner et al., 2015, and references therein). metamorphic core complexes, but their im- enhancement of ductile deformation. The Low-temperature retrogressive deformation portance is not sufficiently emphasized for middle–late Miocene retrogression events and metamorphism of high-grade metamorphic the footwall interior. In order to contribute are also reflected by a similarly aged tectonic fabrics are often reported from detachments of to a better understanding of exhumation- collapse basin in the hanging-wall unit above metamorphic core complexes (e.g., Mehl et al., related retrogression processes within and at the detachment. The wide temporal range of 2007; Harigane et al., 2008; Hetzel et al., 2013; the top of metamorphic core complexes, an retrogression within the Naxos metamorphic Whitney et al., 2013). The detachment fault is integrated detailed microstructural, textural, core complex coincides in age with retrogres- characterized by retrogressive shear fabrics 40Ar/39Ar geochronological, and thermobaro- sive deformation within other metamorphic and forms under decreasing temperature-pres- metric study on the Naxos metamorphic core core complexes of the Aegean Sea. We inter- sure conditions from usually ductile defor- complex within the Aegean Sea is presented pret the long temporal range of retrogression mation within amphibolite-facies conditions that provides a new perspective on low-grade to reflect outward, southwestward retreat of (>500 °C) through the brittle-ductile transition retrogression during exhumation through the subduction and sequential activation of (250–400 °C) to purely brittle conditions (e.g., shallow ductile levels. We found variable major detachment zones. Whitney et al., 2013). This is expressed by chlo- retrogressive deformation within the Naxos ritization of mafic minerals, sericitization of metamorphic core complex, which even per- INTRODUCTION feldspars, and formation of chlorite breccia at vasively affected significant portions of the the top by pervasive fluid flow (e.g., Cathelineau migmatite-grade metamorphic core and rem- In tectonic reconstructions, the recogni- and Nieva, 1985; Cathelineau, 1988; Kirschner nant high-pressure areas of the metamorphic tion of exhumed crust is critically important et al., 1996; Dunlap, 1997; Reddy and Potts, core complex, where retrogression led to because such rocks provide information on 1999). In some metamorphic core complexes, pervasive formation of new fabrics within the tectono-thermal history of the crust. This low-grade retrogression may also occur in the greenschist-facies metamorphic conditions is particularly the case for metamorphic core interior during exhumation, and several studies during brittle-ductile transition. Within a complexes with plastically deformed rocks have reported local retrogressive shear zones continuum of retrogression, 40Ar/39Ar white exhumed from middle- to lower-crustal levels (e.g., Urai et al., 1990; Urai and Feenstra, 2001; mica dating allowed us to deduce three retro to the surface (e.g., Whitney et al., 2013, and Parra et al., 2002). Although previous workers gressive ages at 16.52 ± 0.39 Ma (within the references therein; Platt et al., 2015, and ref- have already noted this retrograde deforma- Naxos metamorphic core complex), 12.6 ± erences therein). Many details are known tion in the footwall of metamorphic core com- 0.28 Ma (Moutsounas detachment shear zone concerning the exhumation history and struc- plexes, they have not sufficiently emphasized on the eastern boundary of the metamor- tures related to exhumation, juxtaposing the its importance. The importance of widespread phic core complex), and 10.43 ± 0.44 Ma to typically high-temperature metamorphic core retrogression in the interior of metamorphic 8.40 ± 0.76 Ma (last ductile activity along the complex against older metamorphic and sedi- core complexes and its temporal and structural Naxos-Paros shear zone to the north of the mentary upper-plate rocks. Deformation stages relationships to detachments are the focus of metamorphic core complex). A further stage within metamorphic core complexes are the this study. of retrogression at 12–11 Ma occurred along result of various superimposed processes, In this study, based on structural field work, distinct low-angle normal faults within the which range from initial viscous deformation we completed detailed microstructural and middle Miocene Naxos Granite. Retrogres- to brittle deformation and also include syn- textural investigations, thermobarometric calcu- sive microstructures, low-temperature cal- kinematic fluid flow (e.g., Lister and Davis, lations, and 40Ar/39Ar white mica dating of retro cite fabrics in marbles, and chloritization in 1989; Verdel et al., 2007; Kargaranbafghi et al., gressive fabrics to reveal the significance of low- 2012). Exhumation can channelize the flow of grade retrogression of high- and medium-grade †shuyun.cao@sbg.ac.at hydrous fluids and limit pervasive retrogres- metamorphic and granitic rocks. These results,

GSA Bulletin; January/February 2017; v. 129; no. 1/2; p. 93–117; doi: 10.1130/B31502.1; 13 figures; 1 table; Data Repository item 2016294; published online 31 August 2016.

GeologicalFor permission Society to of copy, America contact [email protected] Bulletin, v. 129, no. 1/2 93 © 2016 The Authors. Gold Open Access: This paper is published under the terms of the CC-BY license.

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combined with previous studies, provide new metamorphic core complexes along the North and Faccenna, 2000; Huet et al., 2009, 2011; insights into the regional retrogression associ- Cycladic (e.g., Andros, Tinos, Mykonos, and Jolivet et al., 2010; Ring et al., 2010; Grase- ated with ductile to ductile-brittle deformation Ikaria), Central Cycladic (including the Naxos- mann et al., 2012). and cooling of the Naxos metamorphic core Paros detachment), and West Cycladic (Sifnos, The Attic-Cycladic Belt includes two units, complex. The new data also allow fresh insights Serifos, Kea) detachments (Fig. 1; Jolivet et al., the lower Cycladic basement unit (mainly gran- into the mode and history of tectonic extension 2010; Grasemann et al., 2012). The develop- itoids, and/or migmatite, and gneiss overlain in the Aegean region during Neogene times, and ment of these metamorphic core complexes by a thin layer of mica schist) and the middle the methodology could possibly be transferred has been linked to N-S extension of the Aegean Cycladic Blueschist unit (e.g., Ring et al., 2001, to other metamorphic core complexes. lithosphere in the backarc domain of the Hel- 2007, 2010; Huet et al., 2009, 2011; Royden lenic subduction zone (Lister et al., 1984; and Papanikolaou, 2011; Jolivet et al., 2013). GEOLOGY OF THE SOUTHERN Jolivet et al., 2003; Krohe et al., 2010, and ref- They form the footwall units within the meta- AEGEAN SEA REGION erences therein) in the early Neogene (ca. 23 morphic core complexes. The upper plate con- Ma; Gautier et al., 1993; Tirel et al., 2008; Ring tains nearly unmetamorphosed ophiolites and One of the most striking features within the et al., 2007, 2010). Extension was and is trig- Miocene sedimentary formations (for details, south-central Aegean region is the sequen- gered by the southward retreat of the African see following). Extension associated with tial north to south formation of numerous slab (e.g., Le Pichon and Angelier, 1981; Jolivet exhumation of the Attic-Cycladic Belt resulted

Figure 1. (A) Simplified tectonic map of the Aegean region showing the main tectonic zones above the Hellenic subduction zone (modified after Jolivet et al., 2010). (B) Simpli- fied geological map of Cyclades and ages of high-pressure metamorphism and retrogression, mainly based on results of 40Ar/39Ar dating. Data sources: Wijbrans and Mcdougall (1986, 1988); Wijbrans et al. (1990); Bröcker et al. (1993, 2004, 2013); Huet et al. (2015); Cossette et al. (2015).

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in crustal thinning, tectonic unroofing, high Structure of Naxos M2 metamorphism was accompanied by intense heat flow and, in some islands, emplacement foliation development and isoclinal folding (D1; of Miocene magmatic arc–related I- and S-type The classic Naxos metamorphic core Urai et al., 1990; Vanderhaeghe et al., 2007; granites within the metamorphic core com- complex lies in the Cyclades in the western Kruckenberg et al., 2011). plexes (e.g., Pe-Piper et al., 1997; Bolhar et al., Aegean Sea and is associated with the Naxos- After initial attempts with K-Ar ages giving 2010; Kruckenberg et al., 2011) and finally in Paros ductile low-angle fault (e.g., Lister results of ca. 15 Ma (hornblende) to ca. 11 Ma surface exposure of the lower Cycladic base- et al., 1984; Gautier et al., 1993) along the (mica; Jansen and Schuiling, 1976; Jansen, ment unit and the middle Cycladic Blueschist northern margins of Naxos and Paros, which 1977), the timing of peak Barrovian-type M2 unit (Ring et al., 2003). Tectonic unroofing is part of the Central Cycladic detachment metamorphism is dated now by Rb/Sr mica and continued in places until latest Miocene times, system (Fig. 1; Jolivet et al., 2010). The main garnet ages (Duchêne et al., 2006) and U-Pb associated with a gradient in retrogression of structure of the Naxos metamorphic core zircon rim ages to range from 19 to 14 Ma the high-pressure–low-temperature (HP-LT) complex is a migmatite-cored gneiss dome (Keay, 1998; Keay et al., 2001; Martin et al., parageneses (Parra et al., 2002), as recorded with marbles, calc-schists, phengite-schists, 2006, 2008). An age of 20–25 Ma for the M2 by apatite fission-track cooling ages (e.g., Hejl orthogneisses, migmatites, and amphibolites, greenschist-facies retrogression has been pro- et al., 2002; Brichau et al., 2010). The Alpine and this is structurally overlain by a low-angle posed (Andriessen et al., 1979; Andriessen, metamorphic history of Attic-Cycladic Belt normal fault in the east and a steep strike-slip– 1991), whereas Wijbrans and McDougall (1988) includes Meso-Hellenic high-pressure meta- type fault zone in the west (Fig. 2). The west- reported 40Ar/39Ar white mica ages of 27 and morphism (M1) mainly at ca. 45 Ma (and ern part of Naxos exposes the Miocene Naxos 19.9 Ma, which they interpreted to possibly ages as old as 116 Ma; Huet et al., 2015) and Granite (e.g., Pe‑Piper et al., 1997; Koukou relate to partial retrogression after M1 metamor- Neo-Hellenic Barrovian-type metamorphism velas and Kokkalas, 2003). The hanging-wall phism. Duchêne et al. (2006) dated recrystallized (M2) at greenschist facies to local amphibolite unit of the Naxos metamorphic core complex phengitic white mica in a sample from zone III facies (on Naxos) overprinting high-pressure and Naxos Granite consists of remnants of (Fig. 2) and interpreted the 29.3 Ma age to rep- rocks. M1 metamorphism formed due to crustal unmetamorphosed Mesozoic ophiolites and resent the retrogressive evolution from peak thickening during Hellenic subduction in the overlying Miocene and Pliocene sedimentary M1 to M2 recrystallization, which is dated at Cyclades (e.g., Jacobshagen, 1986). The tim- rocks (Kuhlemann et al., 2004), which form a 22.7–19.8 Ma by K/Ar, 40Ar/39Ar, and Rb/Sr on ing of the M2 metamorphic overprint is largely synform between the Naxos metamorphic core muscovite. constrained between 20 and 12 Ma (e.g., complex and the Naxos Granite. Zircon fission-track ages range from 25.2 ± Altherr et al., 1982; Andriessen et al., 1987; 3.8 Ma to 9.3 ± 2.8 Ma (with youngest ages Wijbrans and McDougall, 1988; Okrusch and Metamorphic Stages and within the migmatite dome), and apatite fission- Bröcker, 1990; Avigad, 1998; Keay et al., 2001; Subsequent Cooling track ages range from 11.2 ± 1.6 MA to 8.2 ± Tomaschek et al., 2003; Kumerics et al., 2005). 1.2 Ma (Seward et al., 2009). Apatite fission- Ages of retrogression after M1 and M2 meta- Two main successive Alpine metamorphic track ages, however, are progressively younger morphism scatter over a wide range between 30 events affected the metamorphic zoning within from south to north, varying from 12.9 to 9.0 Ma and 8 Ma (Fig. 1B). the Naxos metamorphic core complex (e.g., (Seward et al., 2009). Brichau et al. (2006) The upper units above the detachments con- Andriessen et al., 1979; Urai et al., 1990; Keay presented zircon fission-track ages ranging sist of the weakly to nonmetamorphic com- et al., 2001). The earlier HP-LT Eocene meta- between 11.8 ± 0.8 Ma and 9.7 ± 0.8 Ma and posite Cycladic ophiolite nappe and latest Oli- morphism (M1) phase is recorded by relics apatite fission-track ages from 11.2 ± 1.6 Ma to gocene to Pliocene sediments (Böger, 1983; of blueschist-facies assemblages preserved in 8.2 ± 1.2 Ma. A single (U-Th)/He apatite age of Lister et al., 1984; Avigad and Garfunkel, 1991; metamorphic rocks in the southern part of the 10.9 Ma is available from the migmatite core Lee and Lister, 1992) interpreted as infill of island of Naxos (~12 kbar, 470 °C, ~13 °C/km; of the Naxos metamorphic core complex (Ver- collapse-type basins resting on the upper unit Wijbrans and McDougall, 1986; Avigad and meesch et al., 2007). (e.g., Sanchez-Gomez et al., 2002; Bargnesi Garfunkel, 1991; Avigad, 1998). No field gra- Recently, Siebenaller et al. (2013) stud- et al., 2013). dient is known because of overprint of the M1 ied fluids from quartz veins, mainly from the phase by M2 metamorphism in central and southern part of the M2 migmatite core. They 40 39 GEOLOGY AND EXHUMATION northern Naxos. The Ar/ Ar white mica ages distinguished immiscible CO2-rich fluid and a HISTORY OF THE NAXOS of ca. 45 ± 5 Ma document an Eocene age for high-salinity brine generated by metamorphic METAMORPHIC CORE COMPLEX blueschist metamorphism on Naxos (Andries- reactions and magma crystallization, respec- sen et al., 1979; Wijbrans and McDougall, 1986, tively. Fluids were initially trapped at 625 °C Since its recognition as a metamorphic core 1988). Martin et al. (2006) presented evidence and 400 MPa and then remobilized during duc- complex (Lister et al., 1984), extensive research for M1 high-pressure metamorphic conditions tile deformation at ~350 °C and 140 MPa. Brit- on the classic Naxos metamorphic core com- within the high-grade rocks affected by M2 tle microcracks contain aqueous meteoric fluids plex has provided a wealth of information on its metamorphism. trapped at 250 °C and 80 MPa. This peculiar structure, thermal and geomorphologic evolu- Based on NE-SW–trending mineral zones, retrograde pressure-temperature (P-T ) pathway tion, and tectonic implications (e.g., Jansen and which delineate a metamorphic dome (Fig. 2A), implies that the brittle-ductile transition zone Schuiling, 1976; Duchêne et al., 2006; Schenk M2 metamorphism grades from greenschist- separated two fluid reservoirs, namely (1) the et al., 2007; Vanderhaeghe et al., 2007; Seward facies conditions in SE Naxos to amphibolite- ductile crust, into which fluids originating from et al., 2009; Kruckenberg et al., 2011; Cao et al., facies conditions in northern central Naxos and crystallizing magmas and fluids in equilibrium 2013a; Siebenaller et al., 2013). Here, we focus reached conditions conducive to migmatization with metamorphic rocks circulated at a geother- on a summary of the main points needed for the in the dome core (650–700 °C, 6–7 kbar; Jansen mal gradient of 30 °C/km at lithostatic pressure, purpose of this study. and Schuiling, 1976; Buick and Holland, 1989). and (2) the brittle upper crust, through which

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B A B′

A′ D

F A′

G B′

Figure 2. (A) Geological map of the Naxos metamorphic core complex in the Cyclades Islands, Greece, showing lithologies, metamorphic isograds, and faults with arrows for kinematics after John and Howard (1995), Keay et al. (2001), and Koukouvelas and Kokkalas (2003), and sample locations. (B) Detailed geological map of the Melanes area in the northwestern central sectors of Naxos island with sample locations (modified after Jansen, 1977; Vanderhaeghe et al., 2007). (C) Detail of the eastern sectors of Naxos Island with sample locations (modified after Jansen, 1977). (D) Lower-hemisphere, equal-area stereoplot of foliation planes and stretching lineation of the Melanes shear zone. (E) Lower-hemisphere, equal-area stereoplot of foliation planes and stretching lineation of the Naxos metamorphic core complex. (F) E-W cross section across Naxos showing the general structure of the Naxos metamorphic core complex dome in the east and the Naxos Granite dome in the west (Jansen, 1973; Vanderhaeghe, 2004). (G) N-S cross section across Naxos (modified after Jansen, 1973; Vanderhaeghe, 2004).

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meteoric fluids percolated along a high geother- on “Crystallographic Preferred Orientations ophiolites are juxtaposed to the west (Fig. 2B). mal gradient of 55 °C/km at hydrostatic pres- Related to Retrogressive Fabrics”). The defor- The Melanes shear zone is characterized by sure (Siebenaller et al., 2013). mation and metamorphism conditions were mylonitic rocks (amphibolite, paragneiss, and estimated using the microstructure, texture, and marble) with a gently W-dipping foliation and a Miocene Plutonism and Cooling mineral thermometry (section on “Conditions penetrative subhorizontal N-S–trending stretch- of Metamorphism during Retrogression”) fol- ing lineation. The microboudins and late exten- The I-type Naxos Granite, with mostly lowed by white mica 40Ar/39Ar dating results sional veins (quartz, chlorite) and microfaults granite and subordinate granodiorite mainly (“White Mica 40Ar/39Ar Dating” section). are filled with chlorite (Figs. 4A and 4B). at margins, is now in tectonic contact with the Miocene clastic succession in northern Naxos. Structures of the Internal Naxos Naxos Granite The Naxos Granite body was emplaced at ca. Metamorphic Core Complex 12–14 Ma according to U/Pb dating of zircons At the southern end, the contact between the Although high-pressure mineral relics are pre- (e.g., Henjes-Kunst et al., 1988; Pe-Piper et al., hornblende-biotite granite and schists in served in the M1-dominated zone, our detailed 1997; Keay et al., 2001; Bolhar et al., 2010). the Naxos metamorphic core complex is exposed ductile structural analysis, particularly of calcite Rapid cooling of the batholith is indicated by (Fig. 4C), with the foliation of the country rocks marbles and phyllitic rocks, reveals that there the ca. 12.5 Ma 40Ar/39Ar ages from various predating granodiorite intrusion. There, the is no structural difference between these two minerals (Wijbrans and McDougall, 1988), a granite and mafic enclaves exhibit a magmatic M1- and M2-dominated domains. Both units are K-Ar biotite age of ca. 10 Ma (Pe-Piper et al., and/or high-temperature N-S–striking solid- pervasively affected by retrogression and show 1997), and pseudotachylyte formation at ca. state foliation and a stretching lineation defined similarly oriented ductile fabrics, including the 10 Ma (Andriessen et al., 1979). Zircon and by elongated feldspars, amphibole, and biotite NNE-trending stretching lineation. All rocks have apatite fission-track ages of the Naxos Granite (Figs. 4D–4G). The stretching lineation plunges a pervasive foliation (S) and a similarly oriented range from 13.7 ± 2.2 Ma to 12.2 ± 1.4 Ma and moderately to the south. Internally, undeformed NNE- or SSW-plunging stretching lineation (Fig. 12.9 ± 4.4 to 9.0 ± 2.6 Ma, respectively (Altherr granite dikes postdate the foliation. Along its 2E). Bookshelf domino-type structures (Fig. 3A) et al., 1982; Hejl et al., 2003; Brichau et al., northern and eastern margin, the granodiorite indicate top-to-the-NNE transport. Boudinage 2006; Seward et al., 2009), which is coeval with shows abundant signs of retrogression, mainly parallel to the NNE-SSW–trending stretching top-to-the-north, low-angle normal faulting. The chloritization along shear zones and faults and lineation of rheologically stiff layers within cal- (U-Th)/He ages range from 10.4 ± 0.4 Ma to chloritized protocataclasites, with some ductile cite marbles is a common phenomenon (Fig. 3B). 9.2 ± 0.3 and 10.7 ± 1.0 Ma to 8.9 ± 0.6 Ma for overprint during retrogression at the eastern mar- In the center of the migmatite dome, S-type gran- zircon and apatite, respectively (Brichau et al., gin (Figs. 4D and 4E), as well as formation of ites intruded, as well as aplite and granitic dikes. 2006). Together, the mentioned geochrono abundant partially deformed quartz veins. These, as well as the penetrative foliation, are logical data from the Naxos metamorphic core folded around N-S–trending fold axes (Figs. 3C RETROGRESSIVE MICROFABRICS complex and Naxos Granite were used to deter- and 3D) and therefore postdate peak conditions mine slip rates of 6.4 (+6.8/–2.2) mm/yr to 13.2 of S and L formation. The folded aplitic dikes More than 50 samples from different sections (+9.4/–4.5) mm/yr over the temperature range and country rocks often reveal a steep approxi- across Naxos were analyzed (Table 1; for loca- from ~300 °C to ~40 °C for the brittle phase of mately N-S–striking axial plane foliation (Fig. tions, see Fig. 2). All thin sections of samples northward slip along the detachment (Brichau 3E), which is less intense in the south. were cut parallel to the kinematic x-z section et al., 2010). Mostly subvertical granite dikes are variably (that is, parallel to the stretching lineation and deformed and folded (see previous) or postdate normal to the foliation). Here, we distinguish MAIN RETROGRESSIVE the penetrative foliation (Fig. 3F). In the lat- retrogressive microfabrics within (1) the relict DEFORMATION STRUCTURES ter case, these dikes often strike N-S and bear high-pressure rocks in the southeastern and WITHIN THE NAXOS METAMORPHIC a weak subvertical foliation similar in strike to the interior parts of the Naxos metamorphic CORE COMPLEX the axial plane foliation. Subvertical, mostly core complex, (2) the Melanes shear zone, and E-W–striking, calcite-filled veinlets and exten- (3) the Naxos Granite. In this section, we describe the retrogressive sional veins filled with coarse epitaxial (rim) deformation structures within the Naxos meta- and isometric calcite crystals occur within the Retrogressive Microfabrics within Relict morphic core complex from field observations marble sometimes (Figs. 3G–3I). They docu- High-Pressure Rocks of the Southeastern and macroscopic structural analysis. All micro- ment approximately N-S stretching and fluid Region and in the Internal Naxos structural details from samples in the interior and flow during late stages of the exhumation of the Metamorphic Core Complex boundary shear zones of the Naxos metamorphic Naxos metamorphic core complex. core complex and the Naxos Granite are com- At least two deformation episodes are well piled in Table 1 and the section titled “Retro- Western Boundary Shear Zone at Melanes preserved in the mica schists of SE Naxos. Early gressive Microfabrics.” In the field, zones of deformation is characterized by elongated garnet retrogression are characterized by the presence In the surroundings of Melanes, at the west- and feldspar grains and quartz grain aggregates, of quartz and calcite veins and variable chloriti- ern border of the Naxos metamorphic core which imply deformation at relatively high-tem- zation of otherwise chlorite-free lithologies. complex, a subvertical belt of fine-grained perature conditions (Figs. 5A–5D). These rocks Mineral deformation features and rock N-S–striking mylonites is exposed (here termed also underwent high-pressure metamorphism microstructures were characterized with special the Melanes shear zone; Figs. 2B, 2D, and 4), (e.g., detailed white mica chemistry can be found reference to feldspar, quartz, white mica, bio- and these form the western, structurally upper in Cao et al., 2013a). Retrogression, during a tite, calcite, and chlorite. Texture measurements boundary of the Naxos metamorphic core subsequent stage of shearing, is superimposed were performed on quartz and calcite (section complex, against which the unmetamorphic on the earlier high-temperature microstructures.

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Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/129/1-2/93/3414614/93.pdf by guest on 01 October 2021 Cao et al. ) Ar-Ar dating Wm— continue d fine grains ( hl * Chl Chl Chl* Chl* Chl* Chl* Chl*, Wm Microprobe pe IC pe I pe II * Cal ype I ype I ype I ype I ype I ype II ype II ype I* ype I* ype II* ype II* ype II* ype II * ype II * T T T T Ty Ty T T T Ty T T exture type pe II T pe II I pe II *T Qtz ype II ype II* ype III* ype III* T Ty Ty T Ty T T AND METHODS TED SAMPLES AMORPHIC CORE COMPLEX THE INVESTIGA THE NAXOS MET Main microstructure of retrogression/overprinted deformation thin twins reduction, thick and thin twin s discrete microshear zone and clear shear sense coarse Chl+Wm grains in microlithon s cross twin s zone s and Chl; aggregates Qt: elongated and undulose extinction; Wm: grain-size reduction along discrete microshear zone; Cal: elongated coarse grains with grain-size reduction Qtz+Wm: grain-size reduction along discrete microshear zones Cal: grain-size reduction Qtz: oriented/elongated with discrete zones of grain-size reduction; Wm: grain-size reduction along discrete microshear zone Cal: coarse grains with irregular boundaries, thick twins overprinted by Cal: fine grained Cal: intense grain-size reduction Cal: grain-size reduction Qtz+Wm: S-C fabrics with intensive grain-size reduction; Pl: elongated and parallel porphyroclasts with fractures filled by Cal; Cal: elongated coarse grains with grain-size reductio n Cal: grain-size reduction; Chl: sheared fine-grained aggregates with clear shear sens e Cal: extremely fine graine dT Cal: coarse grains with irregular boundaries and bulging; grain-size Cal: extremely fine grained, less twin sT Cal: grain-size reduction and extremely fine-grained aggregates along Qtz: grain-size reduction; Chl: sheared fine-grained aggregates within discrete microshear zones Qtz: slight elongation; Wm: grain-size reduction; Ser: along discrete microshear zone Cal: fine-grained aggregates ; Cal: coarse grains with grain-size reduction; Wm+Chl: intense grain-size reduction within discrete microshear zone; Cal: fine grained Cal: grain-size reduction with core-mantle structure, thin twin sT Pl: elongated; Qtz: fine-grained aggregates ; Wm+Chl: intensive grain-size reduction within discrete microshear zone ; Cal: grain-size reductio n Cal: intense grain-size reductio nT Cal: coarse elongated and oriented grains with grain-size reduction, Qtz: elongated, undulose extinction and grain-size reduction; Wm: intensive grain-size reduction; fine-grained along discrete shear Pl: elongated and oriented ; Qtz: weakly elongated in aggregates; Wm: discrete shear zones with grain-size reduction; Cal: elongated coarse grains with grain-size reductio n Cal: coarse grains with irregular boundaries, thick and thin twin s Grt: weakly elongated and fracture filled with irregular mica, calcite, Qtz Pl: elongated; Qtz: ribbon with intensive grain-size reduction; phengite and paragonite Pl: elongated and oriented ; Qtz: ribbons with undulose extinction and weakly elongated grains ; Wm: discrete shear zones with grain-size reductio n CHARACTERISTICS OF SAMPLES FROM phyllite Mylonitic Mylonitic Mylonitic Mylonitic Mylonitic Mylonitic Mylonitic Rock type mica schist mica schist mica schist mica schist mica schist mica schist quartz phyllite Calcite marble quartz-bearing banded marble Banded marble Mylonitic marble Mylonitic marble Mylonitic marble Mylonitic marble Mylonitic marble Mylonitic marble Mylonitic marble Mylonitic marble Mylonitic marble Mylonitic marble Mylonitic marble Mylonitic marble Mylonitic phyllite sericite quartzite Mylonitic phyllitic Mylonitic quartzit e garnet-mica schist mylonitic quartzitic Mylonitic quartzitic INDIVIDUAL Mylonitic banded marble TO ″ ′ AND MICROSTRUCTURAL 17.9 ″ 30.0 ″ 46.3 ″ 26.1 ″ 26.9 ″ 30.3 ″ ′ ′ ′ 29.8 (°E) APPLIED Longitude 25°35 25°35 ′ 20.9 ″ 25°35 ′ 20.9 ″ 25°34 ′ 35.5 ″ 25°35 25°35.154 ′ 25°34.712 ′ 25°33 ′ 25°35.154 25°3 5 ′ 25°2 8 ′ 25°3 3 ′ 01.7 ″ 25°2 8 ′ 25°2 0 ″ ′ 00.7 ″ 43.0 ″ 28.5 ″ 16.4 3 ″ 06.1 ″ ′ ′ 07.0 (°N) Latitud e ABLE 1. MACRO- 6°57 ′ 48.3 ″ 6°56 ′ T 37°05 37°04 ′ 46.8 ″ 37°04 ′ 46.8 ″ 37°05 ′ 00.7 ″ 37°05 ′ 24.5 ″ 37°04.858 ′ 37°05.001 ′ 36°59 ′ 37°04.858 36°5 8 ′ 36°5 6 ′ 36°5 6 1A 1B 1C N5 N48 N2 83 N1A N7A N9 A3 N1B N2B N2E N9 B N7B N1C N6C N1D N7C N9C N1 N1 N69A N29A N30A N1 N69B N29B N30B N10C N29C Sample number East Location East East East East East East East East East East Southeast, high-pressure belt East East East Southeast, high-pressure belt Southeast, high-pressure belt Southeast, high-pressure belt Southeast, high-pressure belt South, high-pressure belt South, high-pressure belt Naxos core Southeast, high-pressure belt Southeast, high-pressure belt South, high-pressure belt South, high-pressure belt South, high-pressure belt

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Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/129/1-2/93/3414614/93.pdf by guest on 01 October 2021 Retrogression of a high-temperature metamorphic core complex Wm– Ar-Ar Wm– dating fine grain s fine grains m Ch l Chl Amp+Pl Microprobe pe I pe I pe I pe I pe I pe I Cal ype I ype I ype IW Ty Ty Ty T Ty Ty Ty T T exture typ e pe I pe I pe II pe II pe II T Qtz ype II ype II ype I* ype II I ype II* Ty Ty Ty Ty Ty T T T T T APPLIED ) AND METHODS continued Am p—amphibole . TED SAMPLES THE INVESTIGA AMORPHIC CORE COMPLEX ( Main microstructure of retrogression/overprinted deformation with grain-size reductio n distributed shear zones with grain-size reduction of Qtz and Bt distributed shear zones with grain-size reduction of Qtz and Bt reduction, thick and thin twin s recrystallization, cross thick and thin twin s recrystallization along discrete shear zones; cross thick and thin twin s shear zones with grain-size reduction; along discrete shear zone typical core-mantle structure ; recrystallization, cross thick and thin twin s reduction reduction; recrystallization; Pl+Kfs: elongated coarse grains with myrmekite ; Hb: strong elongated and sheared; rare distributed microshear zones Qtz: ribbons, serrated high-angle and straight low-angle boundaries; Cal: grain-size reductio n Pl+Kfs: elongated coarse grains with myrmekite, S-C fabrics; Chl: sheared Qtz and Bt: zones of grain-size reduction; Pl: porphyroclasts Qtz: ribbons, serrated high-angle and straight low-angle boundaries; Cal: elongated coarse grain with irregular boundaries and grain-size Cal: large coarse grain with lots of thin twins, locally bulging Qtz: weakly elongated ribbons or aggregates, locally bulging Wm: intensive fine grain along discrete shear zone s Cal: coarse grain with irregular boundaries and bulging recrystallization, QTZ: weakly elongated grain aggregates; Wm: sheared fine-grained aggregates along discrete shear zone Qtz: ribbons or aggregates ; Pl: elongated coarse grains with bulging recrystallization; Wm: sheared coarse grains and dynamic recrystallization; discrete Chl: sheared in fractured Pl and tails. Cal: coarse grains with irregular boundaries and grain-size reduction Qtz+Bt: homogeneous grain-size reduction; Amp: elongated amphibol e Cal: dynamically recrystallized grains and elongated porphyroclasts with Chl: aggregates along discrete shear zone and vein s Qtz+Bt: extreme and homogeneous grain-size reductio n Qtz+Bt: extreme and homogeneous grain-size reductio n Qtz: elongated grains with grain-size reduction; Wm: coarse grains sheared along discrete shear zone Pl: elongated; Qtz: distributed grain-size reduction; Wm: sheared fine-grained aggregates along discrete shear zone Cal: elongated coarse grains with irregular boundaries and thin twin s Cal: coarse grains with irregular boundaries and locally bulging Cal: coarse elongated and preferred oriented grains with grain-size Qtz: fine grains; Wm: fine-grained aggregates along discrete shear zone QTZ: weakly elongated grain ribbons or aggregates with grain-size Wm: sheared fine-grained aggregates along discrete shear zone s Qtz: elongated or ribboned grained aggregates with grain-size reduction; Wm: fine-grained aggregates along discrete shear zone Qtz: grain-size reduction, elongated coarse grains with bulging Wm: intensive grain-size reduction along discrete shear zone THE NAXOS MET CHARACTERISTICS OF quartzite Mylonitic Mylonitic Mylonitic Rock type mica schist mica schist mica schist mica schist amphibolite Orthogneiss Granodiorite aplite gneiss Ultramylonitic Ultramylonitic Ultramylonitic Coarse marble Mylonitic marble Mylonitic sericite SAMPLES FROM Mylonitic quartzite Mylonitic quartzite Mylonitic quartzitic Mylonitic Bt-gneiss Mylonitic Bt-gneiss Mylonitic phyllonitic Mylonitic mica schist Mylonitic mica schist recrystallized marble Mylonitic orthogneiss Coarse-grained marble chlorite-bearing marble Coarse-grained marble Chloritized granodiorite Medium-grained marble Mylonitic banded marble ″ ′ ′ ′ ′ ′ ′ ′ ′ 14.4 ″ 16.0 ″ 26.8 37.7 ″ 53.4 ″ ′ ′ ′ (°E) INDIVIDUAL AND MICROSTRUCTURAL Longitude TO 25°23.04 2 25°33.32 6 25°24.35 4 ′ 25°32.12 5 25°28.00 2 ′ 25°29.73 3 25°30.87 4 25°28.82 6 ′ 25°29.04 0 ′ 25°29.04 0 ′ 25°26 25°27 ′ 59.6 ″ 25°26 25°29.25 5 ′ 25°30.37 1 25°31.30 0 ′ 25°31.43 9 25°28 ′ 25°28 ′ 25°31.169 ′ 25°28 25°28.104 ″ ′ ′ ′ ′ ′ ′ ′ ′ ′ 43.4 ″ 16.2 52.7 ″ 13.6 ″ ′ ′ .356 1.63 5 ′ (°N) Latitud e 7°09.673 7°05.395 ′ 7°08.716 7°05.395 ′ 7°00.940 ′ 7° 11 37°06.56 5 37°05.23 4 37°05.47 8 37°00.94 9 ′ 37°00.94 9 ′ 37°05 37°28 ′ 31.0 ″ 37°05 ′ 50.1 ″ 37°10.09 6 ′ 37°02.68 9 37°1 36°57 ′ 36°57 ′ 36°59.796 36°58 36°56.105 ABLE 1. MACRO- T , Qtz—quartz, Wm—white mica, Bt—biotite, Chl—chlorite, Cal—calcite, Grt—garnet, Ser—sericite, 71 52 54 72 A3 53 B3 N6 63 N6 7 N6 53 N6 23 N6 4 N6 03 N24 N32 N59 N58 N57A N17A N16A N56A N33A N63B N57B N17B N16B N33B N31C Sample number al. (2013a). Pl—plagioclase, Kfs—K-feldspar st, granodiorit eN st, granodiorit eN *Data from Cao et Note : estern, Melanes shear zone est, Melanes shear zone est, Melanes shear zone est, Melanes shear zone We Center Center We Center Center–migmatite dome Center–migmatite dome Center–dom e Center Center Center W W W Near migmatite dome W Nort hN Nort hN Nort hN Center South, high-pressure belt South, high-pressure belt South, high-pressure belt South, high-pressure belt Location South, high-pressure belt South, high-pressure belt

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A foliation B domino-style boudins axial plane foliation

boudin

Figure 3. Field photographs of structures formed during retrogression in the interior of L2 axial plane foliation the Naxos metamorphic core C D complex. (A) Domino-style S2 S2 boudinage within marble. granite (B) Foliated calc-schist with a dike foliated calcite marble layer with dolomite marble lenses. (C) Open folds within schists. (D) Folded granitic dike within schist and axial plane foliation crosscutting the schist and gra- 30cm nitic dike. (E) Large-scale fold hinges with gently E-dipping axial surfaces on the ridge east E F of Mount Zas. (F) Subvertical, internally deformed and foli- orthogneiss granite dike ated late-stage granite dike cut- folded foliation ting the main foliation S1 of a marble. (G–I) Dikes and veins marble within the Naxos metamorphic core complex. (G) Subvertical granite dike within the succession of paragneiss and marble. Some of dikes bear a weak N-S– paragneiss trending subvertical internal foliation. (H) Subvertical cal-

G marble H I cite-filled veinlets oriented perpendicular to the main foliation within marble. (I) Subvertical late-stage E-W–striking exten- granite foliation dike sional vein filled with coarse isometric epitaxial (rim) and elongated calcite crystals (east

paragneiss calcite-filled of Mount Zas).

foliation veinlets vein filled with coarse calcite marble

foliation

15cm

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A B quartz vein

mylonitic foliation

chloritic shear zone

S N chlorite-filled micro-boudins C micro-faults

granodiorite

chlorite veins mylonitic foliation

S N D E

chloritic shear zone schist cross- granodiorite cutting foliation chlorite veinlets

F G quartz-filled vein foliated foliated mafic enclave mafic enclave

protocataclastic granodiorite foliated granite granodiorite dike

Figure 4. Field photographs of structures within Melanes shear zone (A–B) and the Naxos Granite (C–G). (A) Mylonitic paragneiss with numerous quartz veins crosscutting the foliation. (B) Mylonitic paragneiss with microboudins, chlorite veins, and subvertical microfaults filled by chlorite. (C) Intrusive contact between the Naxos Granite and schists of the Naxos metamorphic core complex in southwestern Naxos. (D) Chloritic shear zone within partly retrogressed Naxos Granite to the east of the town of Naxos. (E) Foliated granodiorite and chloritic shear zone within partly retrogressed Naxos Granite, showing brittle faults crosscutting chlorite veinlets. (F) Granite dike within foliated Naxos Granite. The foliated granitic dike postdates foliation within granodiorite and mafic enclaves, suggesting high ductility of mafic enclave during granite dike emplacement. (G) Protocata clastic granodiorite and extensional vein filled with epitaxial euhedral quartz growing into an open gash.

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Figure 5. Retrogressive micro- A fabrics formed by overprinting B high-temperature deformation in relict high-pressure metamorphic zone in southern Naxos Island (A–E) and in the interior Chl Calc of the Naxos (F–H). (A) Two Grt stages of metamorphism pre- Grt served in the garnet–mica Chl Chl schist. Elongated garnet (Grt) surrounded by sheared white mica aggregates. Retrogression is especially characterized by N30A 100um N30A 100um fractured garnet and infill by chlorite (Chl) in strain shad- C DD ows of garnet during shearing. Qtz (B) Chlorite (Chl) flakes fill the Qtz fracture of the garnet (Grt) and WM NG wrap around the boundary of the garnet. (C) Small dynamically recrystallized grains, sub- Qtz grains, and elongated quartz NG (Qtz) aggregates with relatively WM irregular boundaries in the WM mica schist. Small subgrains NG (NG) are developed along the boundary of quartz mainly by 100um 100um N28 N30B bulge recrystallization at low- temperature conditions. Image E F Mus also shows a new discontinuous foliation formed by recrystal- graphite lized white mica (WM) aggregates and opaque minerals. weakly deformed (D) New discontinuous, anas- Qtz tomosing foliation formed by fine-grained white mica aggregates and opaque minerals. (E) Irregular or serrated grain boundaries of calcite with elongation oriented parallel to the 500um shear plane. Weakly deformed N31C N66 500um calcite grains are locally preserved. (F) A new discontinu- G thin twins thin twins H ous foliation formed by the recrystallized white mica ag- thick twins gregates, graphite, and opaque minerals. Bulged recrystallized Re fine grains occur at the boundary of coarse quartz grain. Re (G) Thin twins overprinting coarse-grained calcites. Coarse thick twins thin twins calcite grains show serrated grain boundaries. (H) Coarse- grained calcite grains with N62 500um N57B 500um bulged recrystallized fine grains along boundaries.

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NAX16BA B

Kfs Amph

200um 200um N16B N17B C E

Plag Chl-filled vei Chl

500um Kfs N17A C

D C shear band

Chl Calc S

N17A 500um N71 1000um

Figure 6. Retrogressive microstructure within the Melanes shear zone (A–E) and within the Naxos Granite (F–G). (A–B) Extremely fine-grained amphibole (Amph), mica, and quartz grains, K-feldspar (Kfs), and a few relict elongated amphibole grains in the ultramylonitic amphibolite and mica schist. (C) Chlorite (Chl)–filled vein crosscutting the foliated marble. (D) Coarse- and fine-grained chlorite in the mylonitic marble. (E) S-C fabric in the mylonitic granodiorite, where S plane is defined by grain aggregates of two feldspars (K-feldspar [Kfs] and plagioclase [Plag]), quartz, and chlorite, and C plane is defined by strongly elongated grains of two feldspars. (G) Sheared chlorite indicating shear sense.

which are ascribed to coaxial strain (Passchier terns record a significant component of nonco- and sample locations are presented in Fig- and Trouw, 2005). Type III fabrics are character- axial top-to-the-NNE sense of shear deformation ures 8 and 9. ized by asymmetric single girdle fabrics passing (Fig. 7), consistent with information presented Calcite microstructures of these samples through the y-direction, and the concentrations of by Krabbendam et al. (2002). from the interior of the Naxos metamorphic c-axes are close to the z axis (N66, N28, N11A, core complex show protomylonitic fabrics with N1A). The single girdle patterns show clockwise Calcite CPOs coarse calcite porphyroclasts surrounded by rotation with respect to the x-z plane, suggesting later recrystallized smaller grains. In most sam- dominant activation of basal slip at lower Calcite CPOs analysis was conducted on 24 ples, coarse grains show serrated and irregular temperatures, possibly within lower-greenschist- mylonitic marbles from the Naxos metamor- boundaries and are elongated slightly oblique facies conditions. The slightly asymmetric pat- phic core complex. All calcite c-axis textures to the foliation. The CPO is characterized by

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N66 Z0 L:10/12 N24 Z0 c<0001> of quartz L:356/17 X0

X0 Z0 N65 L:15/16 N52 Z0 L:11/7

+ kyanit X0 N66 N11A e Z0

t L:196/23 N64 Z0 N52 i

L:187/15 n a

k + melt phase melt +

+ N65 N24 N65 X0 X0

o NC1 45 Z0 o Z0 L:196/12N11A,C Z0 N57 20 NA1 L:202/2 L:214/18 Naxos Melanes N64 N11 Stelida X0 N1 N1AX0 X0

*sillimalite

Z0 NC7 <0001> Z0 N60 L:193/13 m

X0 X0 N57 + corundu L:174/19 N60 L:185/18 35 o Z0 Z0 L:18/13 N9C Z0 o N33A N33B 0 N7 7 L:187/22

N33 X0 X0 N9 X0

N32 Z0 N28 N29 N32 Z0

N28 N58

X0 Z0 N58 X0 L:202/12 N29A Z0 L:lineation L:183/11 L:224/35

X0 X0 L:183/7

Figure 7. Quartz-c axis textures from partially retrogressed rocks (measured by backscattered electron diffraction). Equal-area, lower- hemisphere projections are contoured at uniform interval distributions. Foliation is horizontal, and lineation is in plane in the E-W direction. The locations of samples are shown in Figure 2. Samples N1C, N1A, N11A, and N9C are from eastern Moutsounas shear zone, N24 is from near a migmatite dome, and N33A is from a high-pressure belt for comparison (taken from Cao et al., 2013a).

a broad c-axis <0001> maximum close to the (dominated by c slip) and e-twinning under recrystallized microstructure. The small calcite z plane and a weak girdle in y-z, which is sub- low-temperature conditions (Cao et al., 2013a). grains from most samples show strong CPOs normal to the foliation plane. The axis is In calcite, the c-axis maximum rotated against with a single c-axis maximum subperpendicular distributed within a girdle in the x-y shear plane, the shear sense with respect to the main shear to the foliation. The a planes form a girdle sub- and the e planes are relatively weakly distrib- plane observed in porphyroclasts and recrystal- parallel to the foliation, and e planes exhibit a uted on a similarly oriented axis. This sup- lized grains. The asymmetry indicates top-to- weak single maximum similar to the c axis and ports the fact that the calcite texture presented the-N–directed shear. Most samples from the are orientated nearly perpendicular to the folia- in this paper developed by intracrystalline slip eastern boundary are ultramylonites with a fully tion plane.

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Z0 N54 L:3/17 N48 c<0001> of calcite Z0 L:20/3 N5C Z0 L:17/1 N6C Z0 L:181/11 X0

Z0 N53B L:16/20 X0 X0 X0 N54 N53

Z0 N62 X0 L:183/23 + kyanit

e Z0 L:204/16 N11A Z0

+ X0 X0 N63 L:360/15 Z0

N69B Z0 Z0 N2E X0 L:179/13 + staurolite e

N1+ 1biotit X0 X0 17 N63 N56A N5 Z0 N62 N48 L:199/17 N69 N2 N6 *sillimalite N1 Z0 Z0 L:181/19 Z0 N1C N1D X0

X0 X0

N57B N56 Z0 + corundum L:196/12 L:214/18 N57 Z0 L:185/15 N7A Z0 N10C

X0 N7 X0 N59 X0 N10C Z0 N30B Z0 L:178/17 L:183/9 N33 N9B N7C N7B Z0 L:193/1 N67 N29 N28 N32 X0 N31 X0 Z0N29A N58 N30 N9B Z0 L:174/9 Z0 L:183/11 Z0 N67 N9C L:183/13

X0 X0

L:174/4 L:187/22 L-lineation L:1352/6

Figure 9. Calcite c-axis textures from partially retrogressed rocks measured with backscattered electron diffraction. Equal-area, lower-hemisphere projections are contoured at uniform interval distributions. Foliation is horizontal, and lineation is in plane in the E-W direction. The locations of samples are shown in Figure 2. Samples N1D, N7A, N11C, N7C, N9B, N5C, N6C, N7B, N1C, N11A, N9C, and N10C, mainly from eastern Moutsounas shear zone, are, for comparison, taken from an earlier paper (Cao et al., 2013a).

CONDITIONS OF METAMORPHISM blende-plagioclase thermometry) in a paragen- N72 from a granodiorite of the Naxos Granite DURING RETROGRESSION esis also containing quartz and K-feldspar to are given in Table DR1.1 Based on hornblende constrain the pressure and temperature during solid-solution models and well-constrained Hornblende-Plagioclase Thermometry the crystallization of an intermediate magma or natural and experimental systems, the tem- of Granodiorite (Naxos Granite) subsequent metamorphism. Chemical composi- perature was calculated by application of the tions of plagioclase and hornblende of sample hornblende-plagioclase geothermometer of The Al content in hornblende (aluminum-in- hornblende barometry) is commonly used with 1GSA Data Repository item 2016294, Tables DR1–DR5, is available at http://www.geosociety.org/pubs coexisting hornblende and plagioclase (horn- /ft2016.htm or by request to editing@geosociety.org.

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Holland and Blundy (1994) and the pressure by was done using microprobe analyses of two Chlorite Thermometry–Related the Al-in-hornblende geobarometer of Schmidt coexisting feldspars according to the method of Microfabric Types (1992), Anderson (1996), and Ridolfi and Ren- Perchuk et al. (1991). In the case of the Naxos zulli (2012). The calculated temperature is in the Granite, we used the pressure during intrusion High-resolution backscatter electron (BSE) range from 661 °C to 583 °C, and the average is of ~1.5 kbar to calculate the disequilibration of analyses allow better definition of the details 621 ± 19 °C. The calculated pressures mainly temperature (Table DR2 [see footnote 1]). For of microfabrics in various types of chlorite- range from 1.70 to 1.24 kbar, and the average is sample N71, with coarse-grained colorless rich sheared rocks, discussed earlier (Table 1). 1.5 ± 0.2 kbar, representing the lithostatic pres- plagioclase and flesh-colored K-feldspar, we Two deformed microfabric types were observed sure during emplacement of the granodiorites of calculated a temperature range from 539 °C to for the occurrences of chlorite and white mica the Naxos Granite (Gautier et al., 1993). 238 °C, which implies reequilibration under vari- from the samples (Fig. 10), (1) type I sheared,

able temperatures after solidification (Table DR2 coarse-grained chlorite and white mica (D1); Two-Feldspar Thermometry [see footnote 1]). Samples N61 and N53 are and (2) type II sheared, fine-grained chlorite

orthogneisses from the core of the Naxos meta- and white mica in the microshear zones (D2). A few samples from the Naxos metamor- morphic core complex, for which the calculated The temperature of the shear zone paragen- phic core complex and foliated Naxos Granite equilibration temperature ranges from 438 °C to eses can be estimated from the location of the contain both sodic and potassic feldspar. The 258 °C (N61), and from 334 °C to 214 °C (N53), following equilibrium (e.g., Cathelineau and thermometry for samples N71, N53, and N61 suggesting low-temperature mineral equilibration. Nieva, 1985): (clinochlore + sudoite)chl =

A o B 312C 335Co Type II o 299C o o 304C o Plag 308C 296C o o 298C 300C 305Co Type I Chl o o 296C 317Co 323 C Chl o Chl o Type I Chl 333 C 317 C Kfs o 335 C Kfs Plag Kfs Plag Plag N71 500um N71 500um Figure 10. Backscattered electron microscopy images of mi- C D crofabrics containing chlorite: (A–B) Images showing the two Calc 313Co Calc types of chlorite in sheared Naxos Granite; see text for

o explanation. (C–D) Images 317C Kfs showing the two types of chlo- o 324C o rite in sheared marbles within 317C Type I Qtz Chl 215oC Melanes shear zone. (E–F) Im- 332oC Type I Qtz Chl ages showing the two types of Type II chlorite in sheared mica schists within eastern sectors of Naxos Island. Chl—chlorite; Kfs— N17B 200um N17B 200um K-feldspar; Plag—plagioclase; Calc—Calcite; WM—white o o 279C mica; Qtz—Quartz. E 282C o F 270C o Type I 284C o 283C o o 313 C o Chl+WM 278C 319 C 306oC Calc o Kfs o 273C Type II 241C Chl o o 316 C Type II 280C o o o o 294C 304 C 298 C 281C 287Co 302oC

N11C 500um N11C 500um

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(Mg-amesite)chl + quartz + H2O. Tempera- note 1). The analysis of white mica from the Sample N1A is a mylonitic quartzite close ture calculations were done using the chlorite four samples indicates the compositions of vari- to the upper boundary of the Moutsounas shear solid-solution model and thermodynamic data ous types of white mica reflect diverse origins zone along the eastern boundary of the Naxos from Vidal et al. (2001) and Vidal et al. (2005) and times of growth. White micas in sample metamorphic core complex. In this sample, mica assuming activity of water equal to unity. N33B have a dominance of low Si at ~6.08– grains are fully recrystallized and mostly fine Conventional chlorite thermometry based on 6.17 a.p.f.u. and, therefore, muscovite compo- grained (<100 µm), and almost no porphyro- the Fe-Mg ratio of chlorite (Cathelineau and sition. One grain presented a high Si value at clasts are present (Figs. 12C and 12D). Polycrys- Nieva, 1985) was also done for comparison. ~6.69 a.p.f.u. with phengitic composition. White talline and elongated quartz aggregates are com- Four chlorite-bearing samples from the middle micas from samples N1A and N1B yielded a mon due to recrystallization. The recrystallized (N17B), the eastern part (N11C), the northern wide range of compositions of Si from ~6.23 quartz and white mica aggregates with opaque region (N24), and one sample (N71) from the to 6.40 a.p.f.u. and have, therefore, muscovite minerals form a continuous or discontinuous Naxos Granite were selected for chlorite ther- composition with a low Si a.p.f.u. and phengite foliation and stretching lineation. The argon mometry analysis (Table DR3 [see footnote 1]; composition with a low Si a.p.f.u., respectively release pattern of the white mica concentrate of Fig. 10). Three different chlorite geothermome- Coarse-grained white micas in sample N1C rare larger grains results in a pattern with two age ters were applied in this work: the geothermom- yielded a dominance of high Si at ~6.55–6.97 groups. A plateau age of 14.34 ± 0.36 Ma (steps eter of Cathelineau (1988) (T1), based on the a.p.f.u. and have, therefore, phengite composi- 2–6 including 65.1% of 39Ar released; Fig. 12C) number of tetrahedral Al, and the geothermom- tion. Some phengitic compositions with high-Si can be juxtaposed with another mean age of eters of Kranidiotis and MacLean (1987) (T2) white micas also occur in the sheared zones. 20.1 ± 1.4 Ma (steps 7–11 including 30% of 39Ar and Jowett (1991) (T3), based on the number Fine-grained white micas in sample N24B released). Isotope inversion of all steps resulted of tetrahedral Al and the Fe/(Fe + Mg) value. yielded a dominance of low Si at ~6.14–6.18 in an age of 12.6 ± 0.28 Ma (with a 40Ar/36Ar ini- The Cathelineau (1988) (T1) equation yields a.p.f.u. and, therefore, muscovite composition. tial value of 713 ± 240 Ma; Fig. 12D), showing results very similar to the methods presented We note that phengite is, therefore, inherited significant excess argon. We think that the sam- in Kranidiotis and MacLean (1987) and Jowett from high-pressure metamorphism, and it was ple consists of micas of two different age popula- (1991) (Table DR3 [see footnote 1]; results of only rarely reset to low-pressure muscovite dur- tions, although we cannot exclude the presence geothermometers T1, T2, and T3; for more ing shearing and exhumation. The muscovite of excess Ar in the older population. We consider details, see Cao et al., 2013a, 2013b). Chlorite grains with a low Si value at ~6.2 a.p.f.u. (Table that the to plateau age of 14.34 ± 0.36 Ma of the shows Si contents in the range of 3–2.5 a.p.f.u. DR4 [see footnote 1]) yielded mainly low- low-temperature steps approximately dates Ar (atoms per formula unit). pressure conditions (<2.5 kbar). loss of older grains during shearing. In most cases, the calculated temperatures In the thin section of sample N1C, we yielded the following ranges: ~312–348 °C WHITE MICA 40Ar/39Ar DATING observed that the mica grains are substantially

(D1, T1) and ~274–303 °C (D2, T1) for grano- more affected by recrystallization than micas diorite sample (N71) from the western part, White mica concentrates were prepared from from adjacent rocks (e.g., sample N1A). The

~306–332 °C (D1, T1) and ~215–298 °C (D2, four samples from the southern relict high-pres- age spectrum of sample N1C shows a staircase T1) for a marble sample (N17B) from the sure rocks (sample N33B), the eastern bound- pattern ranging from an age of ca. 5 Ma to a central part of the Naxos metamorphic core ary (samples N1A and N1C), and the northern maximum of 50 Ma, all with high analytical 40 39 complex, ~293–321 °C (D2, T1) and ~240– part (sample N24). The Ar/ Ar analytical uncertainties because of the low gas yield due

294 °C (D2, T1) for chlorite-bearing marble technique is described in Appendix A3, and the to the small grain size (Fig. 12E). We interpret samples (N11A, N11C) from the eastern part, analytical results are given in Table DR5 (see this pattern as mixture between two or three age

and ~330–350 °C (D1, T1) for mica schist footnote 1) and Figure 12. The compositions components, including a possibly untrustworthy from the northern part (N21). The chlorite of white micas in dated samples were also ana- young age component at 5–14 Ma (with a too temperatures from sample N24 range from lyzed with electron microprobe (see previous low radiogenic component of <12% 40Ar*), one

132 °C to 140 °C (D3, T1). In the microfabric sections). at ca. 50 Ma, and a third that could be deduced

type II (D2), the thermometry results from the Sample N33B from the southern relict from a mean age of 30.7 ± 3.5 Ma from the last sheared fine-grained chlorite grains yielded a high-pressure belt, south of the corundum-in four steps. To assess potential age components, temperature ranging from 321 °C to 215 °C, isograd of M2 metamorphism, is a deformed we performed single-grain total fusion experi- suggesting sub-greenschist-facies metamor- fine-grained mica schist (Figs. 12A and 12B). ments on several further single grains (with only phic conditions well below the temperatures Small, newly formed white mica grains (<250 µm) one or five steps because of the low gas yield usually considered to represent the brittle- that crystallized during retrogressive deformation due to the small grain size and young age). The ductile transition zone (~300 °C). were sampled to investigate the extent of reset- experiments N1C-exp1 and N1C-exp2 (Figs. ting of the Ar isotopic system in the preexisting 12F and 12G) show staircase patterns, and ages White Mica Compositions and Barometry white mica. The Ar release pattern resulted in as low as ca. 13.9 ± 0.9 Ma and 11.9 ± 0.8 Ma a plateau age of 16.52 ± 0.39 Ma (steps 5–10 in the initial steps of the experiments and well- According to microstructures based on including 87% of 39Ar released; Fig. 12A). Iso- constrained ages of 24.5 ± 2.0 Ma and 19.3 ± BSE analysis (Fig. 11), intensely recrystal- tope inversion resulted in an age of 16.3 ± 1.2 Ma 1.0 Ma. Finally, a total fusion experiment lized fine-grained white micas were observed (with a 40Ar/36Ar initial value of 324 ± 120 Ma; (experiment 3) on a small single grain yielded along discrete microshear zones in our dated Fig. 12B). We therefore consider the plateau age an age of 38.3 ± 4.2 Ma. The staircase patterns samples (see next section). The compositional of 16.52 ± 0.39 Ma to be geologically significant clearly show Ar loss of older mica grains, which results of representative analysis of white mica and to date the formation age during retrogres- possibly formed during M2 or M1 (experiment from the 39Ar/40Ar dated samples (N33B, N1A, sion within greenschist-facies conditions after 3) metamorphism (Figs. 12E and 12F) during N1C, N24) are given in Table DR4 (see foot- peak M2 metamorphic conditions. final shearing at elevated temperatures. This

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A B

White mica mica White

N33B 100um N33B 100um C D

White mica

White mica White White mica White

White mica

100um N1A 100um N1A E White mica F

White mica

White mica White White mica White

N1C 100um N1C 100um G H

White mica White mica

250um N24 200um N24

Figure 11. Microfabrics of strongly retrogressed samples containing white mica used for 40Ar/39Ar dating.

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Figure 12. 40Ar/39Ar release patterns and iso- 32 N33B N33B topic inversion diagrams for white mica sep- 0.003

31.78±1.53 Ma r arates from strongly retrogressed samples. Age=16.3±1.2 Ma

28 40 Initial 40Ar/A36 r=324±120

MSWD—mean square of weighted deviates. Ar /A MSWD=0.68 36 24 24.05±0.68 Ma 0.002

20 Plateau age=16.52±0.39 Ma

Age (Ma ) MSWD=0.59 0.001 means that this sample includes a variety of age 16 populations with old ages and two younger populations, which is the same as the isochrones of 12 B 0.000 the nearby sample N1A at 12.6 ± 0.28 Ma and A 0.00.2 0.40.6 0.81.0 20.1 ± 1.4 Ma. Together, the experiments show a 8 34 mixture of inherited high-pressure grains (38.3 ± 0.0024 N1A Mean age=20.1±1.4 Ma 30 Age=12.60±0.28 Ma 4.2 Ma) and of newly recrystallized grains dur- MSWD=0.89 40 36 0.0020 Initial Ar/Ar=713±240 Ar

40 MSWD=0.67 ing shearing. The age of the latter is reasonably 26

constrained by Ar loss between 13.9 ± 0.9 Ma 0.0016Ar / 36 22 and 11.9 ± 0.8 Ma. Plateau age=14.34±0.36 Ma

Age (Ma ) MSWD=0.37 0.0012 Sample N24 is a mylonitic aplite gneiss from 18 Age=12.9±2.6 Ma Initial 40Ar/A36 r=588±250 MSWD=0.18 the northern part close the Naxos-Paros detach- 0.0008 14 ment. Microstructures show strong plastic Age=19.5±7.8 Ma 0.0004 40 36 deformation of the main mineral phases (e.g., 10 Initial Ar/Ar=384±230 D MSWD=0.05 quartz, white mica, feldspar) in the sample. 6 0.0000 C 0.00.2 0.40.6 0.81.0 1.2 Sheared porphyroclasts and recrystallized white 39Ar/A40 r mica are surround feldspar porphyroclasts. Two 2 60 30 experiments on multiple grains were performed. N1C N1C-exp1 49.68±1.70 Ma single grain Mean age: White mica from sample N24-exp1 gave a mean 50 30.7±3.5 Ma 20 F age of 6.71 ± 0.34 Ma for steps 2–4 and a plateau MSWD=1.2

39 Age (Ma ) age of 9.37 ± 0.37 Ma (steps 5–8, 50% of Ar 40 29.90±3.91 Ma released; Fig. 12H). Isotope inversion resulted 10 28.05±1.08 Ma 40 36 in an age of 9.2 ± 4.1 Ma (with a Ar/ Ar initial 30 5 0.00.2 30 0.40.6 0.8 Age (Ma ) value of 330 ± 310 Ma; Fig. 12I). We consider N1C-exp2 20 single grain the plateau age of 9.37 ± 0.37 Ma to be geologi- 20

cally significant and to date the time of shear- 12.51±2.53 Ma ing. The largest white mica grains were selected 10 Age (Ma ) 10 G to perform another experiment on sample N24. 4.47±2.53 Ma E 0 0 0.00.2 0.40.6 0.81.0 The experiment N24-exp2 gave a somewhat Cumulative 39Ar Fraction scattered Ar release pattern, which allowed the 20 N24 experiment 1 N24 experiment 1 calculation of a mean age of 8.40 ± 0.76 Ma 0.003 Age=9.2±4.1 Ma (steps 2–6) and a plateau age of 10.43 ± 0.44 Ma 40 36 16 Initial Ar/Ar=330±310 MSWD=0.00

(steps 7–12; Fig. 12J). Isotope inversion resulted Ar 40 in an age of 11.56 ± 0.82 Ma (with a 40Ar/36Ar Plateau type age=9.37±0.37 Ma 0.002 12 MSWD=0.060 Ar / initial value of 140 ± 58 Ma; Fig. 12K). We 36 think that the results of both experiments are geologically meaningful, showing shearing, Age (Ma ) 8 0.001 which started at 10.43 ± 0.44 Ma and possibly lasted until 6.71 ± 0.34 Ma. 4 Mean age=6.71±0.34 Ma I MSWD=0.22 H 0.000 0.004 DISCUSSION 0 N24 experiment 2

N24 experiment 2 16 Impact of Retrogression on Microfabrics Plateau age=10.43 ± 0.44 Ma 0.003 MSWD=1.4

As described already, in the Naxos meta- 12 Age=11.56±0.82 Ma 40 36 0.002 Initial Ar/Ar=140±58

morphic core complex, retrogression has been Age (Ma ) MSWD=2.6 documented along some local shear zones 8 (e.g., Urai et al., 1990), but regional retrogression is still rarely discussed in more detail. It is 0.001 especially noteworthy that the presence of low- 4 Mean age=8.40 ± 0.76 Ma K temperature overprinted fabrics in these locali- MSWD=0.22 J 0 0.000 ties indicates that retrogression was not only 0.00.2 0.40.6 0.81.0 1.21.4 0.00.2 0.40.6 0.8 39 40 restricted to the boundary shear zone of Naxos Cumulative 39Ar Fraction Ar/Ar

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(e.g., Cao et al., 2013a), but it also affected large blages with hydrous fluids, possibly hydrother- trolled by the relative importance of micro- domains of the entire metamorphic sequence mal fluids along the zone of shearing and ret- structure development, including grain-size of the Naxos metamorphic core complex. The rogression during brittle-ductile transition. This reduction (e.g., Mulch and Cosca, 2004). White metamorphic rocks show abundant microstruc- is believed to reset the chemical composition of mica can grow over a variety of time scales tures that can be readily related to retrograde feldspar, chlorite, and white mica. We therefore under a large range of metamorphic conditions processes. These processes include develop- take the temperatures of the chlorite thermom- (e.g., Mulch and Cosca, 2004). Retrogressive ment of fine quartz grains by bulging recrystal- etry as approximate values of the final stages of mylonitization within the Naxos metamorphic lization, brittle deformation of feldspar, coarse retrogression, and these values are consistent core complex occurred under greenschist-facies calcite grains with irregular boundaries and with temperatures found in the fluid inclusion conditions during exhumation. The new data thin mechanical twins, homogeneous grain-size study of Siebenaller et al. (2013). The data show from fine-grained sheared rocks also reveal reduction of biotite, and the occurrence of dis- that earlier microfabrics such as those in micro- that some earlier-formed, phengite-rich white crete microshear zones with fine-grained white lithons (type 1) yield higher temperatures than mica grains possibly remained compositionally mica, which relate to the regional deformation/ the subsequent sheared chlorites (type 2). If the stable, although subsequent shearing and other exhumation. Displacement and/or pull-apart of temperatures are approximately correct, this muscovite-rich grains formed new microstruc- fractured feldspar porphyroclasts in the quartz would suggest that ductile shear in phyllosili- tures. These fine-grained white micas were well and biotite matrix indicate that shearing con- cate-rich rocks and marbles possibly occurred at developed in the mylonitic foliation and in shear tinued under low-temperature (lower green- sub-greenschist-facies metamorphic conditions bands during the retrogressive deformation schist-facies) conditions. Textures of quartz- (at temperature ranging from 320 °C to 215 °C). (post-M2) evidenced by microstructural and ofeldspathic mylonitic gneisses and marbles petrographic observations. are consistent with extensive late retrogressive Retrogression and Geological Significance Phengites of the M1 metamorphic stage activity. The compositions of muscovite and of the Melanes Shear Zone underwent deformation during the interval chlorite reflect a diverse origin and time of between the M1 and M2 metamorphic stages. growth. Conspicuously, inherited grains from The W-dipping Melanes zone is a retrogres- However, the effect of deformation on the Ar M1 and M2 stages of metamorphism are often sive shear zone, along which amphibolite-grade isotopic ages seems to have been different in preserved in mylonites. The presence of sec- metamorphic rocks were retrogressed during different samples. Instead, crystallization of ondary chlorite and albite in boudins and shear NNE-directed shearing. The ductile deforma- muscovite during the M2 metamorphism at bands, and the absence of annealing of the high- tion of the metamorphic rocks in the Melanes the expense of phengite appears to be the most pressure mineralogy indicate that later stages of shear zone was accompanied by extreme grain- important process for resetting of the argon iso- deformation also occurred at relatively low P-T size reduction of minerals during the retro- topic system. From the results of white mica conditions. The orthogneisses and aplitic dikes gressive event. An estimate of the temperature 40Ar/39Ar dating, we found three reasonably may be explained by a sequence in which early conditions during retrogressive deformation constrained ages associated with retrogressive high-temperature mylonitic fabrics and micro- can be inferred by the features of the deformed growth of white mica within the Naxos meta- structures were overprinted by semibrittle to behavior of the minerals in shear zones (Simp- morphic core complex: (1) A plateau age of brittle adjustments during exhumation through son, 1985). The temperature is also estimated, 16.52 ± 0.39 Ma in a shear zone in the south- the ductile-brittle transition. Shear sense crite- by the chlorite thermometry, at low-grade condi- ern part is interpreted as the formation age of ria in these rocks indicate top-to-the-N sense tions within marble (300–400 °C). The Melanes a retrogressive shear zone overprinting earlier of motion, as documented in other parts of the shear zone forms the eastern limb of a synform fabrics. (2) Another plateau age of 14.34 ± island in relation to extension (Urai et al.,1990; between the Naxos metamorphic core complex 0.36 Ma and evidence for Ar loss of older grains Buick, 1991a, 1991b), but late shear bands in the east and the Naxos Granite in the west, between 13.9 ± 0.9 Ma and 11.9 ± 0.8 Ma indi- showing top-to-the-S sense of shear are also in which the bedding of Lower Miocene clastic cate motion along in the Moutsounas shear zone well developed (Avigad, 1998). rocks is in part in a nearly subvertical position. and directly date shearing at the upper margin of The Naxos metamorphic core complex can Consequently, the Naxos-Paros detachment the Naxos metamorphic core complex, although be interpreted to have developed finally under fault, including the Melanes shear zone, seems some uncertainty remains. This is in agreement retrogressive conditions so that the zone of to have been folded after deposition and after with the stratigraphic evidence given by over- strongest shearing narrowed toward the mar- intrusion of the Naxos Granite by a major phase lying sediments from Naxos and Paros, which gins of the Naxos Granite, suggesting that strain of E-W shortening, as already mentioned. For range in age between 11 and 5 Ma (Bargnesi localization occurred during cooling of the gra- a few Cycladic islands, such late-stage E-W et al., 2013). (3) Finally, very young plateau nitic rocks. Within the Naxos metamorphic core shortening was also previously postulated by ages of 10.43 ± 0.44 Ma to 8.40 ± 0.76 Ma were complex, the interior is also strongly affected by Urai et al. (1990), Avigad et al. (2001), and Mar- found in the northern part of the Naxos meta- retrogression. Retrogression in the migmatite sellos et al. (2010). morphic core complex, which is consistent with core occurred by inflow of meteoric hydrous the youngest time of ductile shear deformation fluids along approximately E-W–trending Timing of Deformation with Respect along the Naxos-Paros ductile normal fault. quartz veins, for which Siebenaller et al. (2013) to Retrogression within the Naxos The age is close to cooling ages also recorded found P-T conditions at 250 °C and 80 MPa. Metamorphic Core Complex in nearby (U-Th)/He zircon and apatite fission- Chlorites within mylonitic phyllites are found to track ages of ca. 12 Ma and ca. 9 Ma (Seward lose stability even within sub-greenschist-facies The relative succession recorded by micro- et al., 2009; Brichau et al., 2006). This implies metamorphic conditions during shearing, and structures and fabrics and the 40Ar/39Ar white rapid cooling and possibly localized shear heat- a succession of chlorite formation from higher mica ages allow dating of these events for the ing affecting white mica during recrystalliza- to lower temperatures can be found. A reason whole metamorphic core complex. Isotopic tion by thermally resetting the argon isotopic could be the interaction of older mineral assem- exchange in recrystallized minerals is con- system through loss of radiogenic argon. A high

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geothermal gradient of 55 °C/km at hydrostatic overlying unit during exhumation. A rolling zone (e.g., Michibayashi and Masuda, 1993), pressure was assumed for this final stage of hinge top-to-the-N displacement was proposed although retrogression in much deeper structural exhumation (Siebenaller et al., 2013), support- in recent models (Brichau et al., 2006; Jolivet levels could occur, e.g., the amphibolitization of ing the assumption of rapid cooling. et al., 2010). Interestingly, we can distinguish water-free granulite- or eclogite-facies mineral two stages of tilting. In the south, the intru- assemblages (e.g., El-Shazly and Sisson, 1999; Retrogression within the Granodiorite sional relationships between granodiorite of the Xu et al., 2009). At the brittle-ductile transi- (Naxos Granite) Naxos Granite body and the Naxos metamor- tion, retrogressed sheared rocks are most com- phic core complex are exposed, which formed mon, and fluid flow is proven by precipitation Shear deformation within the Naxos meta- during intrusion of the granodiorite between 13 of minerals such as chlorite along the overlying morphic core complex continued after emplace- and 11 Ma, and, in the north, a synform with cataclastic carapace. This also involves leach- ment of bodies of the Naxos granodiorite to the Lower Miocene sedimentary rocks is observed. ing of elements and minerals mostly from the west (e.g., Faure et al., 1991; John and Howard, Consequently, the southern part is from a deeper footwall. The sources of hydrous fluids could be 1995). The data presented herein suggest that level, implying tilting to the north. This is in from three basic sources, which could be pos- the penetrative microstructural and fabric devel- contrast to cooling ages of Brichau et al. (2006), sibly distinguished by fluid inclusion and stable opment in the granodiorite at eastern margins which imply an earlier age of cooling between isotope (hydrogen, oxygen) studies (Siebenaller of the Naxos Granite was associated with early ca. 11 and 8 Ma in the south and consequently et al., 2013). These sources include: (1) descend- N-directed shearing at relatively high-grade top-to-the-S tilting according to their rolling ing meteoric water (and possibly seawater) from temperature conditions. The calculated temper- hinge model. sedimentary basins or the surface (e.g., Yardley ature of sample N72 is in the range from 661 °C et al., 2000), which infiltrates along brittle nor- to 583 °C, and the average is 621 ± 19 °C Southward Shift of Retrogression in mal faults to depth, and (2) ascending magmatic- (Table DR1 [see footnote 1]). Since these the Attic-Cycladic Belt induced hydrous fluid flow during exhumation granitic rocks were not metamorphosed, this of metamorphic rocks, e.g., from synexten- deformation phase could have developed dur- Retrogression, generally overprinting high- sional granites (e.g., Zong et al., 2010). (3) The ing cooling of the granodiorite. Ductile-brittle pressure rocks, is a common phenomenon in the higher-than-normal heat flow also might trigger reactivation, associated with low-temperature Attic-Cycladic belt (Fig. 1B). For retrogression, metamorphic dehydration reactions at depth, hydrothermal alteration and formation of per- we also recognize a trend from older ages of like decomposition of chlorite, muscovite, or vasive chlorite, appears to have occurred late 29–21 Ma in the north to 11–8 Ma in the south biotite, which then yields a significant amount in the deformation history of the granodiorite, (Fig. 1B). Again, Naxos has in intermediate posi- of uprising hydrous metamorphic fluids. The indicating that the deformation occurred within tion but shows a wide temporal range of retro flow of hydrous fluids would trigger a number the greenschist facies, as supported by results gression within the Naxos metamorphic core of processes along the zone of shearing and of the chlorite thermometry calculations. Con- complex, consistent with retrogressive deforma- retrogression during ductile-brittle transition, sequently, microstructures reflect cooling of the tion within other metamorphic core complexes among which the lowering of the shear stress granodioritic rocks from the ductile to semiduc- of the Aegean Sea (Fig. 1B). We interpret these of rock-constituting minerals like quartz (Tullis tile (ductile-brittle) regime (e.g., Sibson, 1977; long temporal ranges of retrogression to reflect and Yund, 1989; Chernak et al., 2009), sericitiza- Shimamoto, 1989) during retrogression. outward southwestward retreat of subduction tion of feldspar, and chloritization are the most Keay et al. (2001) and Bolhar et al. (2010) and, therefore, sequential activation of major important ones. In the case of the Naxos meta- found that the I-type Naxos Granite body crys- detachment zones (Ring et al., 2010). morphic core complex and the Naxos Granite, tallized episodically between 13 and 11 Ma. fluids could be expected to have derived from Note that the ages of retrogression within the Generalized Model for Retrogression: all three sources (see also Siebenaller et al., Naxos Granite are, therefore, significantly Implications for Exhumation 2013): metamorphic dehydration reactions at younger than the age of motion along the Mout- depth, magmatic fluids from granite intrusions, sounas shear zone. This observation was attrib- An understanding of the retrogression history and meteoric fluids from the hanging-wall unit. uted to a protracted history involving initial of the Naxos metamorphic core complex has Because of the long duration of the metamorphic partial melting at deeper crustal levels, followed helped in deciphering the metamorphic scheme, processes, some continuity of retrogression can by crystallization and cooling at progressively including the P-T path and exhumation probe expected between ca. 16 and 9 Ma. shallower crustal levels. All zircon and apatite cesses of metamorphic rocks. This reequilibra- In Figure 13, we describe a generalized fission-track ages of the Naxos metamorphic tion is facilitated by information from fluid-rock model for retrogression related to late-stage core complex and the Naxos Granite range interaction, and new mineral assemblages that ductile-brittle transition, when the metamorphic from 11.8 ± 0.8 to 9.7 ± 0.8 Ma and 11.2 ± 1.6 are often developed in discrete zones of exten- core complex reached shallow crustal levels to 8.2 ± 1.2 Ma, respectively, and consistently sive retrogression during ductile-brittle transi- during exhumation. The model is based on the decrease northward in the direction of hanging- tion (e.g., Kreulen, 1980; Cartwright and Buick, geodynamic, structural, and petrologic condi- wall transport (Hejl et al., 2002; Brichau et al., 1995; Buick and Holland, 1989; Cao et al., tions of the Naxos metamorphic core complex, 2006; Andriessen, 1991), but Seward et al. 2013a, 2013b) and inflow through subvertical but it could be applied to other geodynamic set- (2009) could not find this trend. Brichau et al. veins (Siebenaller et al., 2013). During low-tem- tings in which extension prevails. The cooling (2006) interpreted their results to support the perature retrogression, microshear zones appear of the exhuming metamorphic core complex scenario of an identical fault dip and a constant to have been zones of more focused and discrete results in ascent of the brittle-ductile transition or slightly accelerating slip rate of ~6–8 km/m.y. deformation and fluid-rock interaction in com- zone to higher structural levels by cooling and on the Naxos-Paros extensional fault system parison to earlier high-temperature retrogres- exhumation of formerly deeply buried rocks. across the brittle-ductile transition, which sepa- sion. In most structural settings, dynamic retro- In these settings, subvertical gashes and veins rated the metamorphic core complex from the gression is related to the brittle-ductile transition can be found, and some of these veins are filled

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meteoric descending sedimentary (SEM) at the China University of Geosciences, Bei- hydrous fluids basin jing (Liu et al., 2012). Detailed optical microstructural late stage veins observations and CPO analysis were conducted on thin sections. All thin sections were cut parallel to the brittle-ductile transition kinematic x-z plane (where x is parallel to lineation, extension zone of shearing y is parallel to foliation and normal to lineation, and z direction and retrogression is normal to foliation). Highly polished thin sections ascending were prepared without cover and polished using a col- H O-producing magmatic fluids 2 loidal silica suspension with a particle size of 500 nm reaction HO2 -producing reaction t granite metamorphic for 1–3 h. Thin sections were put in the SEM chamber water gabbro at a 70° tilt angle with the rock lineation (structural x zone of melting reference direction) parallel to the SEM x axis. The er detachmen in the lower crust ast BSE patterns were acquired at a low acceleration m voltage of 15 kV and a beam working distance of ~18–20 mm. Conducting resin tapes attached to the Moho overheated Moho mafic sample surface surrounding the measurement area magma were used to reduce charging effects. The BSED data mantle lithosphere mantle lithosphere acquisition was done using both point scan and map- ping modes. Indexing was accepted when at least six detected Kikuchi bands matched with those in the standard reflector file for the analyzed mineral phases, and indexed points with a mean angular deviation (MAD) larger than 1.2 (between detached and simu- Figure 13. General tectonic model of retrogression related to late-stage ductile-brittle transi- lated patterns) were eliminated to avoid suspicious tion, when the metamorphic core complex reached shallow crustal levels during exhumation. indexing. BSED analysis was completed using the HKL Channel 5 software package. Pole figures were plotted in lower-hemisphere equal-area stereographic by minerals precipitated from ascending and tions during the exhumation of the Naxos meta- diagrams with the trace of the mylonitic foliation (S) descending fluids. It is generally at the level of morphic core complex. and the stretching lineation (L) as reference directions. the veins where ascending fluids are overheated (2) Detailed analysis of the microstructure, Systematic misindexing was noted in automated orientation maps, and such data were replaced by zero 40 39 with respect to the temperature of the country composition, and Ar/ Ar dating of synkine- solution pixels. A detailed description of the BSED rocks, representing, therefore, hydrothermal matic white micas constrains retrogressive technique can be found in, e.g., Prior et al. (1996). fluids (Scott et al., 2015). In such cases, these deformation conditions and change of fabrics Electron Microprobe Analysis (EPMA) fluids can also transform the country rocks by within and at the top of the Naxos metamorphic Methodology static retrogression. This may be generally the core complex and Naxos Granite. The thermo- case when these veinlets and veins are gener- chronologic data record retrogression in late EMPA of the mineral chemistry of minerals (white ally undeformed and occur in a late stage of Miocene times (ca. 16.52 ± 0.39 Ma to 8.40 ± mica, two feldspars, chlorite, garnet, biotite) were carried out on a JEOL electron microprobe (JXA-8600) the structural succession, as in the case of the 0.76 Ma), consistent with final exhumation of at the Department Geography and Geology, Univer- Mykonos (Menant et al., 2013) and Sifnos the Naxos metamorphic core complex. sity of Salzburg, using a wavelength dispersive sys- metamorphic core complexes (Neubauer, 2005). (3) Retrogressive microstructures, low-tem- tem. We used an acceleration voltage of 15 kV and a Note that such veins are often also mineralized perature calcite fabrics in marbles, and chloriti- sample current of 40 nA. Natural and synthetic mineral standards were used to calibrate the microprobe, with ore minerals. zation in metapelites (at temperatures of ~350– and raw data were reduced using standard ZAF cor- In summary, all these minerals lower the dif- 130 °C) in the Naxos metamorphic core complex rection. The detection limits (2s) for the elements are: ferential stress along the zone of shearing, retro core mainly developed during late-stage E-W Si = 0.06 wt%, Al = 0.04 wt%, and Na, K, Mg, Mn, gression, and faults and lead finally to shear shortening and folding. The lower-temperature and Fe = 0.025 wt%. concentration within such zones at the ductile- group could be tentatively interpreted to be the 40Ar/39Ar Method brittle transition. This represents and provides result either of a late-stage hydrothermal over- a positive feedback mechanism for faulting print related to granite/granodiorite intrusions or The 40Ar/39Ar analytical techniques largely follow and stabilization of retrogression immediately inflow of meteoric water. The flow of hydrous descriptions given in Handler et al. (2004) and Rieser et al. (2006). Preparation of the samples before and below the brittle-ductile transition zone during fluids resulted in resetting of fabrics. after irradiation, 40Ar/39Ar analyses, and age calcula- exhumation of metamorphic core complexes. (4) The wide temporal range of retrogression tions were carried out at the ARGONAUT Laboratory within the Naxos metamorphic core complex of the Geology Division at the University of Salzburg. CONCLUSIONS may be consistent with retrogressive deforma- The mineral separates were obtained by crushing hand-sized samples and sieving through 200–355 µm tion within other metamorphic core complexes fractions. They were further purified by washing with The following major conclusions can be of the Aegean Sea. We interpret this long tem- deionized water. Mineral concentrates were packed in drawn from this study: poral range to reflect outward southwestward aluminum foil and placed in quartz vials. For calcu- (1) Microstructural and textural analysis of retreat of subduction and therefore sequential lation of the J-values, flux monitors were placed be- samples and field observations reveals that there activation of major detachment zones. tween each 4 or 5 unknown samples. The sealed quartz vials were irradiated in the Rez reactor (Prague, Czech is variable retrogression within the detachment Republic) for 16 h. Correction factors for interfering APPENDIX: ANALYTICAL METHODS shear zone. Retrogression even pervasively isotopes were calculated from 10 analyses of two affected significant portions of the migmatite- Backscattered Electron Diffraction Method Ca‑glass samples and 22 analyses of two pure K-glass 36 37 grade metamorphic core and remnant high-pres- samples, and they are: Ar/ Ar(Ca) = 0.00022500, Crystallographic preferred orientations (CPOs) 39Ar/37Ar(Ca) = 0.00061400, and 40Ar/39Ar(K) = sure areas. There, retrogression led to pervasive were measured using a backscattered electron diffrac- 0.026600. Variation in the flux of neutrons was moni- formation of new fabrics within greenschist- tion (BSED) detector mounted on a tungsten filament tored with the DRA1 sanidine standard, for which a and sub-greenschist-facies metamorphic condi- Hitachi S-3400N-II scanning electron microscope 40Ar/39Ar plateau age of 25.03 ± 0.05 Ma was origi-

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