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. For permission to copy, contact [email protected] Geological Society of America 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. 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. 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 A Figure 1. (A) Simplified tec- tonic map of the Aegean re- gion showing the main tectonic zones above the Hellenic sub- duction zone (modified after Jolivet et al., 2010). (B) Simpli- fied geological map of Cyclades and ages of high-pressure metamorphism and retrogres- sion, 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). B 94 Geological Society of America Bulletin, v. 129, no. 1/2 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 in crustal thinning, tectonic unroofing, high Structure of Naxos M2 metamorphism