Compressional origin of the Naxos metamorphic core complex, Greece Compressional origin of the Naxos metamorphic core complex, Greece: Structure, petrography, and thermobarometry Thomas N. Lamont1,†, Michael P. Searle1, David J. Waters1, Nick M.W. Roberts2, Richard M. Palin3, Andrew Smye4, Brendan Dyck5, Phillip Gopon1, Owen M. Weller6, and Marc R. St-Onge7 1Department Earth Sciences, University of Oxford, South Parks Road, Oxford, OX1 3AN, UK 2Geochronology and Tracers Facility, British Geological Survey, Environmental Science Centre, Nottingham, NG12 5GG, UK 3Department of Geology and Geological Engineering, Colorado School of Mines, Golden, Colorado 80401, USA 4Department of Geosciences, The Pennsylvania State University, University Park, Pennsylvania 16802, USA 5Department of Earth Sciences, Simon Fraser University, 8888 University Drive, Burnaby, British Columbia, V5A 1S6, Canada 6Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge, CB2 3EQ, UK 7Geological Survey of Canada, 601 Booth Street, Ottawa, Ontario, K1A 0E8, Canada ABSTRACT morphism associated with kyanite-grade Whitney, 2002). However, the pre-extensional conditions of ~10 kbar and 600–670 °C. With histories of many such examples worldwide The island of Naxos, Greece, has been continued shortening, the deepest structural are poorly understood, including those on the previously considered to represent a Cordi- levels underwent kyanite-grade hydrous Cycladic islands, Greece. MCCs occur in a va- lleran-style metamorphic core complex that melting at ~8–10 kbar and 680–750 °C, fol- riety of tectonic settings, ranging from wholly formed during Cenozoic extension of the lowed by isothermal decompression through extensional (e.g., Basin and Range Province; Aegean Sea. Although lithospheric exten- the muscovite dehydration melting reaction Coney, 1980; Armstrong, 1982; Flecher and sion has undoubtedly occurred in the region to sillimanite-grade conditions of ~5–6 kbar Hallet, 1983; Wernicke, 1985; Yin, 1991) to since 10 Ma, the geodynamic history of older, and 730 °C. This decompression process was purely compressional (e.g., North Himalayan regional-scale, kyanite- and sillimanite- associated with top-to-the-NNE shearing gneiss domes; e.g., Burg et al., 1984; Lee et al., grade metamorphic rocks exposed within along passive-roof faults that formed because 2004, 2006) regimes, making it unlikely that the core of the Naxos dome is controversial. of SW-directed extrusion. These shear zones they share a common mechanism of formation Specifically, little is known about the pre- predated crustal extension, because they are (Platt et al., 2014). Although the exhumation of extensional prograde evolution and the rela- folded around the migmatite dome and are deep-crustal metamorphic rocks by footwall un- tive timing of peak metamorphism in relation crosscut by leucogranites and low-angle nor- roofing beneath an extensional detachment fault to the onset of extension. In this work, new mal faults. The migmatite dome formed at is now largely accepted (Wernicke, 1981; Axen, structural mapping is presented and inte- lower-pressure conditions under horizontal 2007), there remains a limited understanding grated with petrographic analyses and phase constriction that caused vertical boudinage of the relationship between the timing of meta- equilibrium modeling of blueschists, kyanite and upright isoclinal folds. The switch from morphism and the onset of extensional motion gneisses, and anatectic sillimanite migma- compression to extension occurred immedi- (Platt et al., 2014). Furthermore, the processes tites. The kyanite-sillimanite–grade rocks ately following doming and was associated driving the initiation of detachment faulting, and within the core complex record a complex with NNE-SSW horizontal boudinage and whether these are associated with lithospheric- history of burial and compression and did top-to-the-NNE brittle-ductile normal faults scale faulting or localized extension in a com- not form under crustal extension. Deforma- that truncate the internal shear zones and pressional environment (e.g., Searle, 2010), are tion and metamorphism were diachronous earlier collisional features. The Naxos meta- still debated (Fig. 1). and advanced down the structural section, morphic core complex is interpreted to have The transition from compression to extension resulting in the juxtaposition of several dis- formed via crustal thickening, regional meta- is commonly recorded by the development of tinct tectono-stratigraphic nappes that expe- morphism, and partial melting in a compres- characteristic extensional structures and meta- rienced contrasting metamorphic histories. sional setting, here termed the Aegean orog- morphic assemblages that overprint older col- The Cycladic Blueschists attained ~14.5 kbar eny, and it was exhumed from the midcrust lisional features (Platt, 1986; Vanderhaeghe and and 470 °C during attempted northeast-di- due to the switch from compression to exten- Teyssier, 2001). Examples include overprinting rected subduction of the continental margin. sion at ca. 15 Ma. of suture zones by low-angle extensional detach- These were subsequently thrusted onto the ments, and the replacement of high-pressure– more proximal continental margin, result- INTRODUCTION low-temperature metamorphic mineral assem- ing in crustal thickening and regional meta- blages by high-temperature–medium-pressure Metamorphic core complexes (MCCs) are assemblages, often due to an increased basal heat generally believed to form during lithospheric flow associated with crustal thinning, e.g., Betics †tnl1@st -andrews .ac .uk extension (e.g., Lister et al., 1984; Teyssier and Rif, Spain (e.g., Platt et al., 2013). Never the less, GSA Bulletin; Month/Month 2019; v. 131; no. X/X; p. 1–49; https://doi.org/10.1130/B31978.1; 23 figures; 5 tables; Data Repository item 2019167.; published online XX Month 2016. Geological Society of America Bulletin, v. 1XX, no. XX/XX 1 © 2018 The Authors. Gold Open Access: This paper is published under the terms of the CC-BY license. Downloaded from https://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/doi/10.1130/B31978.1/4719075/b31978.pdf by guest on 05 June 2019 Lamont et al. A B C Figure 1. Schematic cartoons illustrating various tectonic models and pressure-temperature-time (P-T-t) paths associated with the forma- tion of metamorphic core complexes (MCCs) (after Weller et al., 2013). The cogenetic suite of P-T-t paths is shown for three samples (A, B, and C), where the P-T loci of their respective T-max positions define the metamorphic field gradient, which is typically concave to theT -axis, polychronic, and at a steep angle to the P-T paths of an individual sample (England and Richardson, 1977; England and Thompson, 1984; Spear, 1993): (A) Classical cordilleran-style MCC formed by simple shear extension of the entire continental lithosphere and unroofing under a low-angle normal fault (e.g., Basin and Range). Predicted P-T-t path dominantly follows an isobaric heating excursion at lower pressure due to increased basal heating. BDT—brittle-ductile transition. (B) Compressional-type MCCs formed by doming above a thrust ramp at depth coeval with exhumation under a passive-roof normal fault (e.g., North Himalayan gneiss domes). These record a classical Barrovian type P-T-t path with heating to peak temperatures due to radiogenic heating. (C) Transitional MCC, where compression and crustal thickening cause metamorphism and early synorogenic extrusion followed by the switch to regional extension responsible for the last phase of heating and exhumation. This type is characterized by a classical Barrovian-type clockwise P-T-t path followed by an isobaric heating excursion upon the onset of extension. Barrovian metamorphic overprints and MCCs both overall crustal shortening and extension, the top of the GHS beneath the South Tibetan are characteristic features in many active oro- and they record the relative uplift and exhuma- Detachment System (STDS); Law et al., 2006; gens, such as the Lepontine Dome and Tauern tion of material, rather than a specific tectonic Searle et al., 2006); or (3) normal faulting during Window in the Alps (e.g., Smye et al., 2011, and stress regime. Extensional microstructures typi- crustal extension and rifting (e.g., Lister et al., references therein) and the Greater Himalayan cally form via one of three geodynamic mecha- 1984; Jolivet et al., 2010; Teyssier and Whitney, Series (GHS) in the Himalayan Range (e.g., Jes- nisms: (1) buoyancy-driven extrusion of crustal 2002; Wernicke, 1985). Mechanisms 1 and 2 sup et al., 2006; Searle et al., 2006). Notably, a material transported to depth within a subduction occur in compressional tectonic settings, while thickened crust that formed in response to conti- zone, which occurs under a passive-roof normal mechanism 3 occurs in crustal or lithospheric nental collision would also equilibrate along a fault on the interface between the subducting extensional settings. All three processes are re- hotter geothermal gradient, resulting in wide- slab and overlying mantle wedge, in conditions ported herein to have occurred on Naxos. spread regional metamorphism driven largely by responsible for creating extensional fabrics in The Cycladic islands in the Aegean Sea the conductive relaxation of perturbed isotherms blueschists and eclogites (e.g., England and (Fig. 2), and the island of Naxos in particular (Bickle et al., 1975; England and Richardson, Holland, 1979; Hacker