Compressional Origin of the Naxos Metamorphic Core Complex, Greece

Home , Andros, Filoti, Hydra (island), Kos (regional unit), Milos (regional unit), Naxos

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). Nevertheless,

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.

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 formation 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 et al., 2013); (2) exhu- (Fig. 3), expose a range of geological features 1977; England and Thompson, 1984). mation and extrusion of high-grade migmatites that document the transition from a compres- Although normal-sense extensional shear fab- and gneisses from a deep crustal root by coeval sional to an extensional tectonic regime (Jan- rics indeed record localized extension on their movement of a thrust at the base and an exten- sen and Schuiling, 1976; Buick and Holland, associated shear zone, they occur in zones of sional synorogenic normal fault at the top (e.g., 1989; Urai et al., 1990). Naxos also exposes the

2 Geological Society of America Bulletin, v. 1XX, no. XX/XX

Tectonic Map of the Aegean

20 E 21 E 22 E 23 E 24 E 25 E 26 E 27 E 28 E 29 E Key Meso Rhodope Massif 41 N North Aegean North Anatolian Fault 41 N Active Faults -Hellenic Basin V adar Trough Istanbul Marmaros Normal Faults Sea Thrust Faults Strike Slip Faults 40 N Hellenides 40 N Inactive Faults

Turkey Detachment Faults 39 N 39 N Thrust Faults Greece Gulf of Tectonic Units

Evi Gulf of a Miocene/ Quaternary Sediments Co NCDS rinth and Ophiolites 38 N Andros 38 N Samos Eocene Forearc Basin Athens Tinos WCDS Mykonos Menderes Massif Kea Syros Ikaria Rhodope-Sakarva Block Kos Peloponnese Sifnos Naxos Paros NPDS Vadar-Izmir Oceanic unit Serifos 37 N Amorgos 37 N Cyclades Ios Menderes cover and Basement Santorini Pelagonian-Lycian Block Cretan Sea Rhodes Pindos Oceanic unit including 36 N 36 N Cycladic Blueschists

Crete Cycladic Basement

35 N 35 N Aegean Granitoids

Tripolitza Block

34 N Mediterranean Ridge Accretionary Complex 34 N Ionian Block

Hellenic Subduction Zone Mediterranean Ridge Accretionary Complex 33 N Scale 33 N African Lithosphere African Lithosphere 100 Km 32 N 32 N Aegean Volcanics 20 E 21 E 22 E 23 E 24 E 25 E 26 E 27 E 28 E 29 E Figure 2. Simplified tectonic map of the Aegean region, after Jolivet et al. (2013), showing the major tectonic structures and rock types. Naxos lies in the Cycladic-Attic Massif in the central Cyclades. NCDS—North Cycladic detachment system; NPDS—Naxos-Paros detachment system; WCDS—West Cycladic Detachment System; yellow triangles—current Hellenic volcanic arc.

deepest structural levels of the Attic-Cycladic GEOLOGICAL SETTING Many studies have documented extension af- Massif, which experienced high-grade meta- fecting the area from the Miocene until present morphism, anataxis, and leucogranite formation Regional Geology day, and some estimate southward propagation during the Miocene. This study documents the of the Hellenic subduction zone on the order of pre-extensional, prograde, and retrograde his- The MCCs of the central Aegean region 1000 km over this time period (John and How- tory of the Naxos core complex and links the form part of the Attic-Cycladic Massif (Durr ard, 1995; Seward et al., 2009). It is estimated timing of kyanite- and sillimanite-grade Bar- et al., 1978), which is a belt of thinned conti- that NE-SW–oriented crustal extension rates rovian-type metamorphism to the prevailing nental crust located to the north of the Hellenic within the last 15 m.y. may be up to twice as fast tectonic regime. A future study will present the subduction zone, where the Nubian plate is as present-day rates (Urai et al., 1990; Jolivet absolute ages of both compressional and exten- subducting northward beneath Eurasia (Wortel et al., 2001, 2004). In response to extension, sional microstructures, which were constrained and Spakman, 2000; Jolivet and Brun, 2010). three bivergent, crustal-scale, low-angle normal via in situ U-Th-Pb isotope geochronology per- Within this subduction setting, the region can fault systems developed. The North Cycladic formed on zircon, monazite, allanite, xenotime, be subdivided into the forearc (Crete, Kar- Detachment System (NCDS), exposed on the is- and rutile. Finally, we assessed various tectonic pathos, Rhodes), the Hellenic arc (Santorini, lands of Andros, Tinos, and Mykonos, displays scenarios to explain the history of Naxos prior Milos), and the back arc (Cyclades), which is top-to-the-NE sense of shear (Jolivet et al., to extension, and we conclude that the core reported to have experienced lithospheric exten- 2010). The Naxos-Paros Detachment System complex formed due to prolonged compression in response to retreat of the Hellenic slab (NPDS), exposed on Naxos and Paros, displays sion, crustal thickening, and heating prior to the (Le Pichon and Anglier, 1979, 1981; Le Pichon top-to-the-NNE shear fabrics (Buick, 1991a, switch to extension in the late Miocene. et al., 2002; Jolivet et al., 2010, 2013, 2015). 1991b; Cao et al., 2013, 2017; Urai et al., 1990;

Geological Society of America Bulletin, v. 1XX, no. XX/XX 3

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. rads e ) gmatites) , - ′ mi yanit ne (KSZ )

+K Zo cladic Blueschists ) tion Key ult (NPDS) otics , B–B ne (ZSZ) ′ Cy Metamorphic isog Fa Schuiling s gmatites and orthogneiss y/ Sill grade gneisses) mi t and Sill-grade cation Ky Lo Zas Shear Zo Koronos Shear (Moutsana Detachment TLN49 ammitic and Hb-Bt gneiss (Basement) ault os I-Type Granodiorite ps Sample otwall Units Thrust y±Sill±Kfs±Ms s Fo phibolite grade K m urm / Ms+Bt±Gr tensional Detachment +Bt+Ms±Sill±Kfs schists and gneisses 4k Major Normal Sensed Shear Zone (Reactivated Thrust) Normal F Relict Ex (Am stern Nax s t+Bt±K t+ To t+ Ky ench melange with marble ex t+Bt+Chl+Ms Schists and Calc reenschists retrogressed from Serpentinites and pillow basalt Continental sediments (Miocene-Pleistocene) Tr S-Type leucogranite dikes and sills: We Mylonitized Silici ed cher t Gr Calcite marble with amphibolite horizons (Upper amphibolite Gr Ultrama c lenses Amphibolites Dolomitic and calcite marble Gr Gr Dolomitic Marbles m (G Hanging-wall Units (Upper Cycladic Nappe) os Map and Cross Sec 2k e ults Nax Fa m Core Unit: Koronos Unit: Igneous rock Scal Naxos-Paros Detachment System (NPDS ) 0k Metamorphic Rock Zas Unit: N 37 ’ 03‘ 00’ N 37 ’ 02‘ 00’ N 37 ’ 01‘ 00’ N 37 ’ 00‘ 00’ N 36 ’ 59’‘ 00 N 37 ’ 13 00’ N 37 ’ 12 00’ N 37 ’ 11 00’ N 37 ’ 10 00’ N 37 ’ 09‘ 00’ N 37 ’ 08‘ 00’ N 37 ’ 07‘ 00’ N 37 ’ 06‘ 00’ N 37 ’ 05‘ 00’ N 37 ’ 04‘ 00’ N 36 ’ 58’‘ 00 N 36 ’ 57’‘ 00 N 36 ’ 56’‘ 00 N 36 ’ 55’‘ 00 37 ’ 00’ E 25 ’ E

′

S d N A

itoi iotit

te 09 09 or

E 25 ’ 36 00’ +B W E 25 ’ 36 00’

-C 11 11 Moutsana +Stauroli 20 20 20 20 E 25 ’ 35 00’ E 25 ’ 35 00’ 32 32 30 30 14 14 15 15 08 08 25 25 30 30

16 16

15 15 20 20 ZSZ 40 40 E 25 ’ 34 00’ E 25 ’ 34 00’ 48 48 60 60 15 15 20 20 18 18 75 40 40 38 38 46 46 25 25 18 18 20 20 Apollonas 18 yanite 18 E 25 ’ 33 00’ E 25 ’ 33 00’ 45 45 25 15 25 15 48

+K 48 37 37 45 45

55 55 42 e 42

70 70 17TL110 30 30

40 40 B 35 35 +Sillimanit 40 40 10 10 09 09 58 58 41 41 TLN16 35 35 65 65 40 40 50 50 60 60 25 25 40 40 24 24 50 50 61 61 42 42 45 45 60 60 21 21 42 E 25 ’ 32 00’ E 25 ’ 32 00’ 42 62 62 37 37 45 18 45 18 80 80 38 38 30 30 46 46 46 46 58 58 23 23 75 75 56 40 20 56 40 20 36 36 80 80 69 69 Koronos 27 27 55 55 39 39 58 20 58 20 45 45 40 40 60 60 41 41 42 40 42 40 TLN22 50 50 20 20 44 44 40 40 72 72 os 60 60 20 20 25 25 40 40 63 63 20 20 40 40 35 24 35 24 17 17 17TL75 60 60 30 30 52 52 30 30 45 45 52 52 25 25 49 42 42 49 TLN21 38 38 22 22 28 49 28 49 45 45 TLN13 35 35 33 33 48 48 43 43 30 30 58 24 58 24 16 16 60 37 60 37 E 25 ’ 31 00’ E 25 ’ 31 00’ 42 42 40 40 65 65 45 45 58 58 40 40 40 40 56 56 39 39 TLN30 22 22 70 70 20 20 61 61 49 49 49 49 28 28 42 42 20 20 89 89 30 30 74 74 70 70 47 90 47 90 31 31 25 25 28 20 28 20 43 43

38 38 65 65

12 12 51 51

15 15 25 25 73 73 22 22 45 45 30 30

25 25

47 47 42 42 28 28 TL67 75 42 75 42 68 68 Z TLN20 KS 30 60 40 40 30 60

30 30 45 26 25 45 26 25 37 22 37 22 Apiranthos TLN47 50 50 55 60 55 60 40 36 40 36 46 30 68 46 30 68 ZSZ 40 40 80 80 35

35 50 50 28 28 34 34 45 29 45 29 10 10 31 31 28 28

68 68 60 60 50 50 +Sillimanite 38 35 25 38 35 25 32 32 20 20 76 76 70 70 TLN25 57 57 29 29 42 42 50 50 24 36 24 36 35 TLN35 35 10 10 57 57 44 44 67 67 70 70 14 21 21 14 80 80 Mt Zas 1004m E 25 ’ 30 00’ 61 12 61 12 25 25 73 73 51 51 E 25 ’ 30 00’ 46 51 46 51 61 61 39 39 40 30 40 30 70 70 30 30 38 12 38 12 38 38 54 54 56 56 a 40 40 loti 50 39 50 39 64 64 38 38 53 53 38 20 38 20 61 61 40 40 48 30 48 30 57 57 23 23 69 69 70 70 37 72 37 72 35 20 15 35 20 15 76 76 57 57 70 70 Fi 62 62 34 34 TLN34 s 37 37 60 60 59 43 43 85 85 76 76 70 15 39 70 15 39 50 50 47 47 42 42 38 38 44 44 28 28 60 60 TLN18 42 42 39 39

45 45 67 Aigi 67 72 72 31 70 31 70 15 10 15 10 43 43

ro 55 55 TLN54 74 10 74 10 70 50 87 70 50 87 08 08

Mt Koronos 990 m e

0 0

63 31

2 2 50 25 50 25 30 30 32 32 42 42

33 33 42

42 60 67 60 67 84 84 +Melt 76 76

70 70 Chal ki E 25 ’ 29 00’ E 25 ’ 29 00’ 85 85 33 33 39 39 30 30 23 23

75 75 80 73 80 73 79 yanit 75 49 45 79 75 49 45

30 30

82 82 63 63 75 75 68 71 71 47 30 47 30 52 52

60 e 20 20 72 72 42 19 45 45 80 80 +K 42 42 70 70 74 74 83 83 70 70 76 76 64 64 latados 36 36 72 72 52 52 27 50 40 27 50 67 67 29 29 28 28 30 30 42 76 42 76 78 78 50 50 74 TL15 74 60 30 28 60 30 53 53 Kinida 37 69 69 47 47 63 40 63 40 41 41 76 76 81 81 40 46 40 46 76 76 TLN26 56 56 42 42 69 69 TLN49 75 75 79 79 20 20 75 75 52 52 52 52 85 85 Ka 46 46 87 87 76 60 35 76 60 52 52 35 35 55 55 83 83 35 63 63 20 74 70 70 74 77 77

51 51 88 35 88 65 65 TL66 56 56 61 61 40 40 88

88 76 76 48 48 65 65 35 35 78 55 55 78 29 35 29

Melt 30 35 35

78 78 68 45 78 68 45 80 77 80 77

60 60 57 20 64 64 60 80 80 40 E 25 ’ 28 00’ E 25 ’ 28 00’ 40 17 17

30 64 + 64 77 77 84 +Sillimanit 84 20 20

30 30 30 40 40 90 90

′ 77 77 14 14

30 30 30 30

70 70

60 40 60 40

80 80

65 65

20 20

76 76

66 66 69 69

C C- ion ct 40 40 43 43 e 23 31 23 31 12 12 S 21 21 Z KS 3 3

25 29 25 29

71 71

45 45

Line of Line 74 74 24 24 e 50 28 40 28 40 38 38 27 73 27 73 35 35 43 43 43 43 72 72 70 70 27 27 25 25 68 45 68 45 57 57 39 C 39 30 30 20 20 66 66 70 70 45 45 36 36 E 25 ’ 27 00’ 38 57 38 57 72 72 D’ 37 38 E 25 ’ 27 00’ 37 38 27 27 24 24 32 32 55 55 74 74 64 64 55 55 50 50 54 54 17 45 45 29 29 26 26 63 28 55 28 55 40 40 43 43 45 45 63 87 87 D 29 TL59 27 27 37 37 10 10 21 21 50 50 35 40 40 35 41 NPDS 41 78 78 42 60 60 49 49

30 30 68 68 81 81 55 20 55

25 08 35 08 35 25 44 +Staurolit 50 50 44 11 11 70 46 70 46 60 69 69 ZSZ 36 36 65 65 44 44 66 66 65 24 65

27 27 55 50 55 50 38 38

41 41 41 41 70 35 70 35 e

3 3 43 43 78 40 E 25 ’ 26 00’ 40 35 35 17 64 17 64

2 2 41 41 29 29 30 30 69 29 69 29 E 25 ’ 26 00’ 33 33 25 25 70 70 81 81

65 65 50 50 26 26 18 18 83 83 82 82

70 70 D 40 40 21 21 53 53 19 19 it 28 +Biot 28 72 72 23 23

76 75 76 75

35 35 Geological Map of Nax

42 42 42 42 46 46 hloritoid 51 -C 51 61 61 47 61 47 61 42 42 69 69 80 80 81 81 08 08 30 30 60 60 60 12 60 53 53 40 40 52 52 B’ 18 18 B’ 33 33 05 05 60 60 35 35 85 85 45 45 are presented in Figures 4 and 7. Mineral abbreviations follow Whitney and Evans (2010). Whitney and Evans follow and 7. Mineral abbreviations in Figures 4 presented are 70 70

19 19 cladic Nappe E 25 ’ 00’ E 25 ’ 00’ 45 45 ′ ′

54 30 54 30 75 75 73 73 30 30 20 20

17 ′ 17 30 28 28 30 B

10 -B 10

40 40 B

50 50

n 51 tio 51 70 70 ′

26 26

vlos

C Sec f

Vi o

E 25 ’ 24 00’ E 25 ’ 24 00’

Upper Cy Line Glinado 75 i , and D–D E 25 ’ 23 00’ E 25 ’ 23 00’ ′ 18 18

rgak ′

Pi -A

wn A E 25 ’ 22 00’ E 25 ’ 22 00’ Figure 3. New geological map of Naxos metamorphic core complex, following previous mapping after Jansen (1973), Jansen and (1973), Jansen mapping after Jansen previous Figure 3. complex, following core New geological map of Naxos metamorphic (2011). Major tectonic units within the meta et al. (2004), and Kruckenberg Vanderhaeghe (1990), (1976), Buick (1991a, 1991b), Urai et al. Unit (amphibolite-facies shelf carbonates), and Core Cycladic Blueschists), Koronos include the Zas Unit (retrogressed footwall morphic A–A Lines of sections discussion and interpretation. more gneisses and migmatites). See text for Unit (upper-amphibolite-facies C–C To gl a os Vi i 22

a Section Section kr Nax E 25 ’ 21 00’ E 25 ’ 21 00’ 19 Mi

Stelid ine of of ine

A L ’ 58 00’ N 37 ’ 12 00’ N 37 ’ 11 00’ N 37 ’ 10 00’ N 37 ’ 09 00’ N 37 ’ 08 00’ N 37 ’ 07 00’ N 37 ’ 06 00’ N 37 ’ 05 00’ N 37 ’ 04 00’ N 37 ’ 03 00’ N 37 ’ 02 00’ N 37 ’ 01 00’ N 37 ’ 00 00’ N 36 ’ 59 00’ N 37 ’ 13 00’ N 36 ’ 57 00’ N 36 ’ 56 00’ N 36 ’ 55 00’ N 36

4 Geological Society of America Bulletin, v. 1XX, no. XX/XX

Kruckenberg et al., 2010, 2011). The West Cy- Buick, 1991a, 1991b; Urai et al., 1990), reach- include diapirism (Jansen and Schuiling, 1976; cladic Detachment System occurs on the islands ing upper-greenschist facies in the east (biotite Vanderhaeghe, 2004), exhumation during re- of Kea, Kythnos, and Serifos and shows top-to- zone: ~5 kbar and ~400 °C) and overprint- gional extension (Gautier and Brun, 1994b),

the-SSW kinematics (Grasemann et al., 2012). ing relict (M1) blueschist assemblages along and superimposed folding from E-W shorten- Despite extensional deformation affecting the southeast coastline (Avigad, 1998; Peillod ing followed by top-to-the-NNE shearing (Urai all structural levels, little is known about the et al., 2017), and reaching upper-amphibolite et al., 1990; Buick, 1991a, 1991b), as well as

geological evolution of the area prior to the (M3) anatectic conditions in the core (sillimanite a combination of buoyancy- and isostasy-driven initiation of extensional tectonics. Structural zone: ~7 kbar and ~700 °C). In the west, an flow during crustal extension (Kruckenberg and metamorphic evidence from several is- I-type granodiorite pluton with a U-Pb age of et al., 2011). This study presents a systematic lands suggests that the Cyclades experienced a ca. 12.2 Ma intrudes the metamorphic sequence characterization of the tectonic and metamor- complete cycle of mountain building (Ring and and developed a narrow metamorphic aureole phic evolution of Naxos that addresses the role

Layer, 2003; Ring et al., 2007a, 2007b; Peillod (M4; Keay et al., 2001; Koukouvelas and Kok- of compressional and extensional tectonics in et al., 2017). Remnant ophiolites crop out at the kalas, 2003; Jansen and Schuiling, 1976). formation of the core complex. highest level on several islands, in the hanging The core region consists of highly strained

walls of the low-angle normal faults (Stouraiti M3 sillimanite-bearing migmatites and gneisses, FIELD RELATIONSHIPS

et al., 2017). These are underlain by (M1) Eo- with intercalated marbles and leucogranites, cene high-pressure eclogite- to blueschist-facies which are separated from a lower-grade de- Geological mapping was carried out (Figs. 3 rocks of the Cycladic Blueschists in the foot- formed sequence of Mesozoic shelf carbon- and 4), building on previous detailed work walls of the detachments, dated at 52.2–51.4 Ma ates by a major ductile shear zone, forming the (e.g., Jansen and Schuiling, 1976; Buick, by U-Pb zircon and Lu-Hf garnet geochronol- carapace (the attenuated metamorphic sequence 1991a, 1991b; Buick and Holland, 1989; Urai ogy from Syros (Tomaschek et al., 2003; Lagos structurally above the migmatite core) to the et al., 1990; Vanderhaeghe, 2004; Krucken- et al., 2007) and at ca. 46 Ma from Sifnos eclo migmatite dome. Top-to-the-NNE kinematic berg et al., 2011), but with new and important gites (Dragovic et al., 2012). The area then ex- indicators are associated with this ductile shear structural interpretations (Figs. 4–10). In this perienced regional Barrovian metamorphism zone, which formed during exhumation of the section, we describe each tectono-stratigraphic

(M2) during the Oligocene to early Miocene core complex. Due to this complex deforma- unit in descending structural order and discuss (40–18 Ma), followed by a sillimanite-grade tion and metamorphic history, the development the major fabrics, folds, and mineral growth

event (M3) at ca. 16 Ma that reached anatectic of most of the footwall structures and fabrics is (Figs. 4–10) in relation to the deformation. conditions on Naxos and Ios (Jansen and Schuil- poorly understood, and there is confusion as to ing, 1976; Buick and Holland, 1989; Keay et al., whether they formed during crustal extension Tectono-Stratigraphy 2001). Within the high-grade dome of the Naxos or predate the extensional movements and were core complex, there is evidence for NNE-SSW reactivated by them (Ring et al., 2007b). Re- Zas Unit (Retrogressed Cycladic Blueschists)

extensional fabrics coeval with M3 conditions cent work has shown that the blueschist rocks Retrogressed blueschists exposed in the SE (Ring et al., 2007b), indicating a switch in tec- on southeast Naxos experienced a contrasting corner of the island (Avigad, 1998) represent tonic regime during this period. Understanding metamorphic history to the underlying higher- the highest structural level of the metamorphic

the timing of this switch from compression to grade rocks (Peillod et al., 2017), and there has carapace. These are composed of (M1) blue-

extension is fundamental to understanding the been confusion about whether the entire island schist- and (M2) greenschist-facies rocks de-

formation of the Aegean MCCs. experienced (M1) blueschist-facies conditions or rived from proximal to distal shelf and slope whether there are discrete tectono-stratigraphic metasediments including turbidites, dolomitic

Geology of Naxos units. Furthermore, the cause of regional M2 and marbles, pelites, meta-conglomerates, and meta

M3 kyanite-sillimanite–grade metamorphism, volcanics. The existence of deeper-sea clays is Naxos is the largest island in the Cyclades and the pre-extensional, prograde evolution of the indicated by piemontite-bearing schists, which shows evidence for having experienced multiple core complex, and the relative timing of peak together have similarities to the Cycladic Blue- tectono-metamorphic events (see Figs. 3 and 4). metamorphism in relation to the onset of exten- schist Unit exposed on Syros, Ios, Andros, and It is composed of a NNE-SSW–elongated dome sion have remained controversial. Two contrast- Tinos. Additional exposures of blueschist-facies in the exhumed footwall of a major low-angle ing models have been proposed for the initiation assemblages can be found toward the top of the normal fault, the NPDS, which developed in re- of extension. Wijbrans et al. (1993) and Parra sequence, spanning the entire eastern coastline sponse to regional Aegean extension (Cao et al., et al. (2002) argued that extension commenced (see Figs. 3 and 4). Within this unit, there are 2013, 2017). The hanging wall mainly consists at 30 Ma (see also Jolivet et al., 2003) due to two types of top-to-the-NNE extensional shear

of ophiolitic rocks with a mélange of disrupted slab roll-back, followed by M2 heating of the fabrics. S1 is defined by blueschist-facies assem- Miocene–Pliocene sediments (Jansen, 1973), Cycladic blueschists (Bröcker and Franz, 1998). blages including glaucophane, phengitic mica, which are possibly synextensional (Urai et al., Alternatively, others argued for the concept of and paragonite that are preserved within only 1990; Gautier and Brun, 1994a, 1994b). Upper synorogenic extension during underthrusting partially retrogressed high-pressure boudins, Pliocene conglomerates containing pebbles of and compression in the Cyclades until 21 Ma, bearing many similarities to the fabric seen footwall material were deposited on the flanks followed by postorogenic extension and crustal throughout the Cycladic Blueschist Unit. This of the dome during unroofing (Lister et al., thinning (Peillod et al., 2017; Ring and Layer, extensional fabric is associated with synoro- 1984; Buick, 1991a, 199b; Gautier and Brun, 2003; Ring et al., 2007a, 2007b, 2010). genic extrusion from a NE-dipping subduc- 1994a, 19914b). The lower unit represents a Competing models have been proposed to tion zone during the Eocene by synorogenic Barrovian-type metamorphic sequence that de- explain the decompression of migmatites and passive-roof normal faults such as the Vari de-

veloped during M2 (Jansen, 1973; Jansen and the formation of the doubly plunging migma- tachment on Syros and Sifnos (e.g., Ring et al., Schuiling, 1976; Buick and Holland, 1989; tite dome making up the core of Naxos. These 2003; Roche et al., 2016; Laurent et al., 2016).

Geological Society of America Bulletin, v. 1XX, no. XX/XX 5

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. - - thern Subdome Nor Figure 4. Three-dimensional models of the Naxos metamorphic core complex illustrating structural and crosscutting relationships, pro relationships, complex illustrating structural and crosscutting Figure 4. core of the Naxos metamorphic models Three-dimensional geological units as in Fig section, scale 1:1, using same color scheme for with superimposed cross duced using the QGIS plugin 2threejs shear zone. detachment system; ZSZ—Zas shear zone; KSZ—Koronos NPDS—Naxos-Paros ure 3.

6 Geological Society of America Bulletin, v. 1XX, no. XX/XX

Figure 5. Panoramic photos of folding styles on Naxos (color scheme: A B green—schist, blue—marble, pink— gneiss, tan—migmatite, purple—amphibolite). (A) Google Earth imagery using three-dimensional (3-D) view

mode to show an F1 NE-SW–verging recumbent isoclinal fold within the Koronos Unit north of the town of Filoti (37.066816°N, 25.507647°E).

(B) F1 and F2 isoclinal and sheath folding on the Zas shear zone C D under the western flank of Mount Zas (37.031232°N, 25.497227°E). E (C) Google Earth 3-D view of in-

tense F1 and F2 folding to the east

of Filoti, illustrating the F2 isoclinal sheath-type folds superimposed on

early F1 NE-SW–trending isoclinal recumbent folds (37.057519°N, 25.504086°E). (D) Annotated and overlaid field sketch of the interior of the migmatite dome and the mantling Koronos shear zone (KSZ) on the eastern margin of the migmatite dome, showing the repetition of marble bands due to intense shearing (37.092747°N, 25.493531°E). (E) Panoramic photograph and annotated photograph of the folding styles within the Koronos Unit and

their contrast to the underlying F3 folds within the Core Unit looking north (37.042451°N, 25.498678°E). F (F) Panoramic photograph and annotated photograph from the top of Apano Kastro (37.066894°N, 25.459924°E), showing the interior structure of the migmatite dome, including the Core high-strain zone (CHSZ), marked by vertically dipping marble bands representing tight

upright F3 folds at the center of the migmatite dome.

Geological Society of America Bulletin, v. 1XX, no. XX/XX 7

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 D

Principal

Figure 6 (on this and following page).

8 Geological Society of America Bulletin, v. 1XX, no. XX/XX

Poles to foliations of the migmatite dome of F G

HIJK

L M NO

Figure 6 (continued). Mesoscale annotated photographs of the folding geometries within the metamorphic carapace and schematic

model to explain their structural relationships. (A) Boudinaged F1 isoclinal fold closures of marble bands within the Zas shear zone (ZSZ) under the western flank of Mount Zas (37.035727°N, 25.495403°E). (B) Overview of folding geometries crosscutting Fanari

(863 m; N37.107506°N, E25.527892°E), showing late upright F3 folds superimposed on early recumbent and isoclinal folds. (C) Pan-

orama looking north along strike of the ZSZ, showing tight F2 isoclinal folds overprinting the limbs of F1 folds, with the western face of Mount Zas to the right of the image (37.013864°N, 25.480907°E). (D) Internal isoclinal folding within dolomitic marble of the Zas Unit near Kalatados Bay (36.935074°N, 25.468323°E), the limbs of which are detached following ductile shearing. (E) Schematic cartoon

showing the development of F1, F2, and F3 folds on Naxos. Stage 1—NE-SW–verging isoclinal folds form during the prograde evolu-

tion. Stage 2—Non-coaxial top-to-the-NNE general shearing initiates and produces F2 folds that overprint the limbs of F1 folds, and sheath-type structures develop with “cat’s-eyes.” Stage 3—Rotation of principal stress axes causes late-stage E-W constriction to produce E-W–trending open to tight folds. KSZ—Koronos shear zone. See text for more discussion. (F–K) Lower-hemisphere equal-area stereonets of structural data from the Zas, Koronos, and Core Units: (F) Foliations of the migmatite southern subdome demonstrating a symmetric morphology to the dome. (G) Poles to foliations of the migmatite dome southern subdome, demonstrating its upright sym-

metrical and doubling plunging nature along a NNE-SSW axis, and poles to foliations of an F3 fold within the Koronos Unit, highlighting its upright and isoclinal nature with a fold axes trending: 189°, 83°. (H) Lineations on the ZSZ, with mean lineation: 195.4°, plunge 17.4°. (I) Poles to foliations on ZSZ, with best-fit great circle: 020°, 78°. (J) Lineations on the Zas anticline on the western flank of Mount Zas, with mean lineation: 201.8°, 17.4°. (K) Poles to foliations on the Zas anticline, with best-fit axial plane: 022°, 78°. (L) Poles to foliation for Koronos Unit, with best-fit great circle: 120.1°, 66.9°. (M) Poles to foliations for Core Unit from KSZ, with best-fit great circle: 094.1°, 84.1°. (N) Lineations on the northern segment of the KSZ, with mean lineation: 014.7°, 13.4° plunge. (O) Poles to S-type granite dikes and sills emanating from the migmatite dome. On the eastern side of the Core Unit, granite dikes dip to the west and crosscut the KSZ shear fabric, and in the center of the Core Unit, dikes dip vertically, whereas on the western side of the dome, they rotate into alignment with the Naxos-Paros detachment system foliation and strike subparallel with the lineation.

Geological Society of America Bulletin, v. 1XX, no. XX/XX 9

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.

Pelitic horizons have highly strained contacts Figure 7 (on following page). Representative outcrop photographs of key features from the with dolomitic marbles that preserve internal Upper Cycladic Nappe and Naxos-Paros detachment system (NPDS), with small-scale cross folding and centimeter-scale crenulation cleav- section through the NPDS. (A) Silicified and altered pillow basalt from the trench mélange within the Upper Cycladic Nappe (37.075998 N, 25. 416638 E). (B) Exotic marble cataclasite age (S2a). These are overprinted and truncated ° ° by a subparallel, highly localized extensional within the trench mélange, marked by pink coloring, diagnostic of piemontite (manganese epidote) representing interlayered distal oceanic oozes (37.077357 N, 25.420516 E). (C) Out- S-C′ shearing fabric (S3), exclusively defined by ° ° greenschist-facies assemblages. The intensity crop of serpentine cataclasite and fault breccia and blocks from the brittle Moutsana detach- of shearing increases toward the boundaries of ment of the NPDS (37.071605°N, 25.416424°E). (D) Ultramylonitized dolomitic marble of the the unit, where major normal-sense shear zones, Zas Unit directly under the Moutsana detachment on west Naxos (37.070526°N, 25.415716°E), the Moutsana detachment and Zas shear zone 50 m from photo C. (E) Brittle cataclasite and fault gouge of dolomitic marble from the (ZSZ) (Fig. 7), have developed. Along these Zas Unit against brecciated serpentinites of the Upper Cycladic Nappe, separated by the shear zones, some discontinuous horizons of Moutsana detachment (37.079374°N, 25.428907°E). (F) S‑type leucogranite sills intruding serpentinite enclaves crop out (Katzir et al., syntectonically into the NPDS ductile shear zone on NW Naxos (37.183128°N, 25.506907°E). 1999, 2002, 2007), which are also affected by (G) Simplified cross section through the NPDS on western Naxos representing extreme telescoped isograds with metamorphic field gradients of up to 700 C km–1. Outcrop photos are the (S3) extensional foliation. These are inter- ° preted as fragments of serpentinized mantle projected into the line of section. (H) Lineations of mylonites from the Moutsana detachment, from the overlying ophiolitic thrust sheet that with mean lineation: 011.4°, 10.1° plunge. (I) Rose diagram of slickenslides along normal where incorporated at low temperature during faults in the hanging wall of the Moutsana detachment, with most faults slipping on ESE- WNW–trending or ENE-WSW–trending planes. (J) Poles to foliations of mylonites from the extensional shearing (S3). Moutsana detachment, western Naxos, with best-fit great circle, representing a synform with Koronos Unit (Kyanite-Grade Mesozoic its hinge line striking 012° and plunging at 14° to the NNE, which formed due to late-stage Shelf Carbonates) E-W shortening. The Koronos Unit is a 4-km-thick metasedimentary sequence of intercalated marble and schists that represents a proximal Mesozoic shelf carbonate cover to the Cycladic basement unit increases toward the base, where the Koro- alternating quartzofeldspathic and biotite layers (Durr et al., 1978; Andriessen et al., 1979). This nos shear zone (KSZ) separates this unit from with distinct leucosomes and melanosomes (see

unit is characterized by M2 amphibolite-facies the underlying migmatites. also Kruckenberg et al., 2011). Leucosomes Barrovian metamorphic rocks, in contrast to the occur along the foliation marked by biotite, overlying Zas Unit. In coarse-grained dolomitic Core Unit (Kyanite- to Sillimanite- muscovite, and kyanite in aluminum-rich do- marbles at the top of the unit, corundum (emery) Grade Migmatites) mains, increase in abundance with depth, and

and magnetite horizons represent paleosols that The Core Unit is composed of M2 kyanite- become concentrated in fold hinges, where they

formed during sedimentation and subaerial grade to M3 sillimanite-grade migmatites, display magmatic textures. Leucosomes also exposure of the continental margin during the gneisses, and leucogranites, which make up the occur in boudin necks and in shear bands that

Jurassic (Altherr et al., 1982). Within the suc- deepest levels of the core complex. The first- crosscut compositional layering (S0). At deeper cession, meter-scale amphibolite bodies trend order dome structure contains second-order structural levels, both coarse quartzofeldspathic parallel to the regional foliation and represent subdomes (Kruckenberg et al., 2011), which horizons and pockets of melt are ptygmatically doleritic sills that intruded along bedding prior to are separated by a high-strain zone (Core High- folded, with melt distributed along fold hinges,

burial and metamorphism. M2 kyanite gneisses, Strain Zone [CHSZ]) of tight upright and isocli- indicating it accumulated in dilatant struc- marbles, and schists crop out at the deepest lev- nally folded marbles, amphibolites, and migma- tural sites during compression (Brown, 2002; els, and the metamorphic grade dramatically tites that have been subsequently boudinaged. Sawyer, 2001; Holness, 2008). Where evidence decreases up structural section to staurolite and The Core Unit is separated from the overlying of melting becomes locally more extensive, garnet grade. At the base of this unit, slices of carapace by the KSZ along the dome margins particularly in pelitic horizons, diatexites are continental basement are exposed and are struc- (see below; Figs. 4 and 5). Within the anatec- characterized by magmatic textures containing turally repeated in a sequence overlying semi- tic core, the extent of partial melting varies enclaves of the host rocks, and they are hetero- pelitic schists and marbles of the shelf carbon- with bulk composition, as well as with increas- geneous at outcrop and thin section scales. They ate sequence. This indicates structural repetition ing metamorphic grade. There is lithological include schollen, and schlieren-structured vari- of the shelf sequence, which could only be variation throughout the migmatite zone, with eties (Fig. 9) with a magmatic foliation defined achieved by thick-skinned thrusting of the shelf banded leucocratic gneisses representing conti- by schlieren of biotite or alignment of schollen and basement over more-proximal continental nental basement commonly interleaved between (see also Kruckenberg et al., 2011). Although shelf metasediments. Garnet- and clinopyrox- pelitic and semipelitic protoliths and marbles of these features can be locally extensive, our ob- ene-bearing amphibolites outcrop 100 m below the shelf carbonate cover sequence. This struc- servations and data reveal they are not as wide- this horizon, within the metasedimentary se- tural repetition in lithology again provides fur- spread as Kruckenberg et al. (2011) implied. quence, as a semicontinuous layer along strike. ther evidence for thick-skinned ductile thrusting We suggest that most of the gneisses previously These enclose partially serpentinized peridotite and imbrication of the shelf-basement contact described as diatexites are instead granitic base- lenses (Katzir et al., 2002, 2007) that remained during crustal thickening. Stromatic migmatites ment with Variscan metamorphic ages (pre- stronger during deformation as the amphibolites (Fig. 9), representing the first appearance of sented in a future study). Although some diatex- flowed and deformed around them during top- melt, outcrop along the perimeter of the dome or ites do locally occur, they are confined to fertile to-the-NNE ductile shearing (see Fig. 7). The adjacent to metasedimentary rafts and are char- lithologies upon crossing the K-feldspar isograd intensity of top-to-the-NNE shearing within the acterized by a continuous foliation marked by at the deepest levels of the migmatite dome.

10 Geological Society of America Bulletin, v. 1XX, no. XX/XX

A B

C D

Brecciated Serpentinites (Talc and asbestos) Fault Gouge

Heavily brecciated and gouged dolomitic marble (Zas Unit)

Staurolite in hanging wall Staurolite out

Ultra Mylonites

hanging wall detachment H I J

Figure 7.

Geological Society of America Bulletin, v. 1XX, no. XX/XX 11

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.

Figure 8. Representative field A B outcrop photographs from intermediate levels of the Naxos metamorphic core complex. (A) Cat’s-eye sheath fold within the Zas shear zone (37.037370°N, 25.492597°E) indicative of non- coaxial general shear with fold hinge plunging to the NNE.

(B) S2 crenulation cleavages within micaceous schist from the

Koronos Unit with S3 C′ planes

overprinting S2 cleavage domains (36.932450°N, 25.429476°E). (C) Ultramafic partially serpen- C D tinized pod within host foliated amphibolite from the Main Ul- tramafic Horizon (Katzir et al., 1999, 2002, 2007), directly structurally above the Koronos shear zone (KSZ), also characterized by top-to-the-NNE shear fabrics (37.104116°N, 25.510806°E). (D) Blastomylonite basement gneiss from the KSZ (37.047572°N, 25.455538°E) displaying diagnostically S-C and S-C′ top-to-the-NNE kinematic indicators and quartz rods that E F form σ and δ porphyroclasts with consistent shear senses. Pl—plagioclase. (E) Foliated kyanite-grade stromatic migmatite from the perimeter of the migmatite dome (37.060807°N, 25.452099°E) showing coarsening textures and representing the first appearance of partial melt. (F) Diatexite migmatite marked by extensive melting destroy- ing all previous fabrics, characteristic of fertile lithologies and locally extensive anatexis G H (37.090701°N, 25.460878°E). Dike (G) Ptygmatically folded partial melt indicative of contractional deformation in the presence of extensive melting (37.091540°N, 25.459035°E). (H) Garnet + tourmaline + biotite leucogranite sill crosscutting the extensional top-

to-the-NNE S3 shear fabric at the perimeter of the migmatite dome (37.067065°N, 25.447091°E). Ptygmatic folding Mineral abbreviations follow Whitney and Evans (2010).

12 Geological Society of America Bulletin, v. 1XX, no. XX/XX

A B

Figure 9. Outcrop photographs of characteristic deformation from deep levels of the core com-

plex. (A) F2 fold closure within Compass clinometer graphitic schist of the Koronos Unit looking NNE (37.100354°E,

25.517359°E). (B) F3 upright 30 cm isoclinal fold closure trending NNE-SSW within the migmatite dome (37.095943°N, C D 25.468734°E). (C) Deformation within the Core high-strain zone (CHSZ; 37.106237°N, 25.482500°E), with subvertical meter-scale S-C fabrics in amphibolites that have been later boudinaged both vertically and horizontally in the N-S direction. (D) Horizontally boudinaged amphibolites and leucogranites from the Core high strain zone (37.107807°N, 25.482510°E), showing internal centimeter- scale isoclinal folds. (E) Up- E F right folded amphibolites along NNE-SSW axes, with vertically boudinaged amphibolites and also a leucogranite dike diag dike nostic of horizontal constriction (37.108102°N, 25.483013°E) within the center of the Naxos migmatite dome. (F) Horizon- tal extension and boudinage indicated by amphibolites show ~50% pure shear and brittle simple shear in the amphibolites, with counterclockwise rotation. (G) S-C extensional G H fabrics overprinting F3 folds of amphibolites, aligned axial planar with the vertical fold. (H) Extensional axial planar boudinage and shearing fabrics

overprinting F3 intensely and isoclinally folded amphibolites (37.107807°N, 25.482510E).

Geological Society of America Bulletin, v. 1XX, no. XX/XX 13

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 BC

Figure 10. Core High-Strain Zone (CHSZ) at the center of the migmatite dome (37.108102°N, 25.483013°E). (A) Overview photograph showing the three- D dimensional exposure of the Kinidaros Quarry and deformation in the CHSZ. (B) Vertical ductile boudinage of amphibolites, with a domino-style more brittle overprint. (C) Upright E F E-W–trending F3 fold with vertical boudinage overprinting its limbs. (D) Pinch-and- swell boudinage affecting limbs

of an E-W–trending F3 fold. (E) Stereonets of structural data displaying poles to foliations, axial planes of folds, and extensional lineations. Note that the intersections of axial planes of folds represent an overall vertically orientated sheath fold with bimodal vertically and NNW-SSE–orientated stretching lineations and boudinage. (F) Three-dimensional map of deformation within the CHSZ, highlighting the struc- G tural superposition of three fold generations in relation to the intrusion of a leucogranite dike and vertical and horizontal stretching. Note that dike intrusion postdates all folding and predates shearing and vertical and horizontal boudinage. (G) Three-dimensional block model of deformation structures within the CHSZ, where

F1 and F2 isoclinal folding is

overprinted by F3 tight-isoclinal upright folds along NNE-SSW– trending axes.

Structures within the Metamorphic a series of normal-sense shear zones (Jansen, F1 and F2 Folds Core Complex 1973; Buick and Holland, 1989; Urai et al., Isoclinal, recumbent, and noncylindrical

1990; Buick, 1991a, 1991b). Three-dimensional (F1) folds occur mainly within the metamor- The large-scale structure of Naxos consists of models, panoramic sketches, and accompany- phic carapace (Zas and Koronos Units), up to a complex superimposition of three generations ing interpretations are presented in Figures 4–6, kilometer scale, and trend along NNE-SSW fold of kilometer- to meter-scale isoclinal and sheath while structural analyses of these folds and axes. They are characterized by subhorizontal folds, which are overprinted and truncated by shear zones are displayed in Figures 6 and 7. hinge lines that strike parallel to the fold axes

14 Geological Society of America Bulletin, v. 1XX, no. XX/XX

and display a SW vergence direction and hang- F3 Folds under greenschist-facies conditions, as rotation

ing-wall anticline geometries above discrete de- Upright isoclinal folds (F3; Figs. 4–10) also into alignment with top-to-the-NNE shearing

tachments (e.g., on the western flank of Mount occur along NNE-SSW axes parallel to the would cause flattening. Because F3 structures Zas; Figs. 5 and 6). The limbs of these folds are NNE-SSW lineation and have been noted by sev- fold ductile mylonite fabrics, have NNE-SSW refolded by smaller tight to isoclinal, also NNE- eral workers on Naxos (Urai et al., 1990; Buick, boudinaged limbs (see Fig. 10), and have up-

SSW–trending (F2) folds and sometimes pro- 1991a, 1991b) and other Aegean islands, includ- right fold axes that are truncated by the NPDS, duce an overall sheath fold–type geometry with ing Tinos, Andros, Syros, Paros, and Mykonos we suggest they must have formed due to sig- their noses trending NNE-SSW, parallel to the (e.g., Virgo et al., 2018, and references therein). nificant E-W shortening that predated NNE- lineation and shear direction, and these are par- These folds outcrop across all units despite spa- SSW crustal extension. ticularly well developed along the ZSZ (Figs. 5, tially contrasting in style. Within the migmatitic 6, and 8A). Sheath folds form by progressive core, an earlier generation of upright, E-W– Core High-Strain Zone rotation of fold hinges and are defined by curvi- trending isoclinal folds is refolded by orthog Three-dimensional relationships of poly- linear fold axes with more than 90° of curvature onal to NNE-SSW–trending upright structures. phase folding can be spectacularly observed in (Ramsay and Huber, 1987). They initially form These structures are defined by vertically dip- amphibolite bands within marble in the CHSZ as isoclinal folds with axes aligned orthogonal ping marble bands and pelitic horizons that of the Boulibas and Kinidaros Quarries, where to shear and rotate toward the transport direc- sometimes enclose boudinaged amphibolites, ductile pinching and boudinage of tight upright

tion under progressive non-coaxial, high-strain which have experienced polyphase micro- and F3 antiforms and synforms crop out (Figs. 9 and deformation (e.g., Cobbold and Quinquis, 1980; macroscale folding. Although meter-scale folds 10). These amphibolite horizons act as strain Ramsay and Huber, 1987; Alsop and Holds in the migmatites do occur, they do not have a markers and show evidence for superimposition

worth, 2006). Sheath fold formation involves consistent axial trend and were probably formed of multiple F3 fold geometries, vertical boudi- (1) initial buckling of layers, largely controlled by flow of partial melts (Jansen and Schuiling, nage (i.e., boudinage with subhorizontal neck by lithological contrasts such as between mar- 1976). Within the migmatite dome, our obser- lines), horizontal NNE-SSW boudinage (i.e.,

ble and schist; and (2) amplification and rota- vations suggest that F3 folds are completely boudinage with neck lines orientated subverti- tion of fold axes as shear strain increases (Alsop discordant to the overlying structures and are cally), flattening, brittle fracture, and S-C shear and Holdsworth, 2006). It has been shown that truncated by normal faults and the NPDS that fabrics with opposing shear senses on opposite sheath folds can form under both constrictional bounds the metamorphic footwall. Within mig- limbs of folds. Boudinage and shear bands (e.g., Ez, 2000) and “nonconstrictional” regimes matite, the leucosomes and kyanite blades are with opposing shear senses are orientated in (e.g., Cobbold and Quinquis, 1980; Alsop and folded with the foliation (Figs. 8, 13B, and 13C) the vertical and NNE-SSW horizontal planes,

Holdsworth, 2006). Under constriction, a bull’s- by F3 folds. This differs from the overlying cara- symmetrically arranged on the limbs of F3 folds eye sheath fold pattern is predicted with ellip- pace, where these structures range from open to and aligned with their axial surfaces (Fig. 9 and

ticity of the innermost ring less than that of the tight and refold the limbs of F1 and F2 folds, but 10). These indicate directions of maximum ex- outermost ring, whereas under general shear, a also have their fold axes trending parallel to the tension in the vertical and NNE-SSW horizon-

cat’s-eye geometry is predicted with innermost lineation (NNE-SSW; Fig. 6H). Therefore, F3 tal planes. F3 fold interference patterns suggest

ring ellipticity greater than that of the outer ring. folds must postdate peak M2 kyanite-grade con- that E-W–trending upright isoclinal folds are

During simple shear, innermost and outermost ditions and F1 and F2 folding, yet they must have refolded by tight to isoclinal NNE-SSW–trend- rings have similar ellipticities, producing an formed in the presence of melt and simultane- ing folds with vertically plunging hinge lines, analogous eye pattern. ous to doming as they wrapped around the core which in turn are refolded by several scales

F2 tight to isoclinal fold closures commonly (Buick, 1991a, 1991b). of upright parasitic folds with N-S–trending do not show eye patterns. However, along the The origin of these folds has been the hinge lines. Intersection of these three fold axes ZSZ, cat’s-eye geometries occur, indicative of subject of considerable debate. Some work- about a vertical point (Fig. 10E) suggests this

general shear (a combination of simple shear ers have suggested that F3 folds formed dur- structure is a Ramsay type-2 fold interference and pure shear flattening). Along the perimeter ing E-W compression responsible for gentle feature. Vertical boudinage affects the limbs of of the migmatite dome, these folds strongly ro- island-scale postmetamorphic E-W doming these folds and vertically intruded pegmatites

tate into alignment with the strong (S3) top-to- of the isograds (Buick and Holland, 1989; and indicates the minimum principal stress (σ3) the-NNE shearing fabric in the KSZ, indicating Urai et al., 1990; Buick, 19911, 1991b), while was vertically orientated and much less than the the structures formed during top-to-the-NNE other studies inferred their origin to be a re- horizontal stresses. These observations lead us shearing (Alsop and Holdsworth, 2006). These sult of extensional shearing, as similar fea- to suggest the maximum and intermediate prin-

folds preserve S2 crenulation cleavages in their tures are present in other extensional terrains cipal stresses (σ1 and σ2) must have been almost limbs with axial planar kyanite, indicating that (Hodges et al., 1987). Rey et al. (2011, 2017) equal and orientated in the horizontal plane at

these structures formed during M2 conditions. and Kruckenberg et al. (2011) argued that folds the time of F3 folding and shortly thereafter.

Kyanite is also aligned with the pervasive S3 in the migmatite dome developed during con- This indicates that bulk horizontal shortening NNE-SSW stretching lineation, and there- vergent flow and the viscous collision of two and constriction coincided with sillimanite- fore we interpret that the folds formed in two weak channels of the lower crust during re- grade partial melting. During this constrictional

stages: (1) F1 isoclinal folds developed during gional NE-SW crustal extension. However, this regime, the maximum principal stress σ1 must prograde burial and thickening. (2) This was mechanism cannot explain the earlier isoclinal have been initially oriented N-S to produce followed by non-coaxial top-to-the-NNE gen- folding, top-to-the-NNE–related sheath fold- E-W–trending folds and was subsequently ro-

eral shear during exhumation, synchronous with ing, and upright F3 fold geometries within the tated about a vertical axis into an E-W orienta-

(M2) Barrovian conditions forming F2 folds that metamorphic carapace. Alternatively, Buick tion to produce NNE-SSW–trending fold axes.

produce the sheath-type geometries as shown in (1991a, 1991b) proposed F3 folds within the Finally, NNW-SSE horizontal stretching lin- Figure 6E. carapace originated in their current orientation eations, horizontal boudinage, and horizontal

Geological Society of America Bulletin, v. 1XX, no. XX/XX 15

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.

fracture, indicative of a horizontal NNE-SSW– Extensional Shear Zones low; Fig. 6P). These dikes appear to have been

oriented σ3, postdated the folding and vertical rotated by ~45° due to later doming of all fabrics.

boudinage and occurred while the amphibolite Although extensional (S3) top-to-the-NNE Domains of less-strained material unaffected by

was starting to behave in a brittle manner dur- shearing is penetrative throughout the en- penetrative (S3) S–C′ fabrics reveal earlier fold-

ing cooling. These NNE-SSW horizontally tire metamorphic carapace (Zas and Koronos ing, i.e., (S2a) crenulation cleavages and (S2b) boudinaged amphibolites indicate NNE-SSW Units), there is petrological and geochronologi- cleavage domains that display top-to-sigma extensional strains of ~50% by dominantly cal evidence that the fabrics formed at different porphyroclasts with opposing top-to-the-SW pure shear (Fig. 9F), with a component of sim- times and at different conditions across Naxos. shear senses (see Fig. 8), signifying structural ple shear as demonstrated by domino boudins Discrete shear zones, represented by mylonites reactivation at high-grade conditions. Structural (Fig. 9D). The boudins show opposing shear and show microstructures displayed in Fig- imbrication and preservation of relict thrust fea-

senses on opposite limbs of F3 folds, suggest- ures 11–13, separate each tectono-stratigraphic tures are inferred from thin marble bands along ing the shearing postdated folding (Fig. 9G; unit and are truncated by the geometry of the the KSZ that display tight to isoclinal internal Virgo et al., 2018). Based on this evidence, we NPDS. These are now described in chronologi- folding bounded by highly strained intercalated propose that NNE-SSW stretching, which is cal order from structurally deep to shallow. schists that have localized the deformation due definitively related to horizontal crustal exten- to their competency contrast with the stronger

sion, must have postdated upright F3 folding Koronos Shear Zone marbles (see Fig. 8). Imbricated slices of the and horizontal constriction, but it commenced The KSZ is a normal-sense NNE-SSW–trend- same marble horizon can be identified by an while the rocks were still hot enough to de- ing shear zone that wraps around and is folded abrupt change in foliation across strike, and they form in a plastic manner shortly following peak by the underlying migmatite dome. It repre- splay off the main detachment horizon. anatectic conditions, but at temperatures when sents the basal contact of the highly strained Slices of orthogneisses basement underlying amphibolite starts to deform in a brittle manner metamorphic carapace mantling the migmatite the shelf sequence are structurally repeated and (e.g., <640 °C at 6.5 kbar; Cao et al., 2017). dome and places the kyanite-grade Koronos overlie metasediments and amphibolites at the These interpretations agree with Virgo et al. Unit against relatively less sheared migmatites upper levels of the shear zone. This, combined (2018) and Von Hagke et al. (2018), who dem- of the Core Unit. Immediately around the dome with microstructural evidence, suggests that the onstrated that there are five generations of margins, the migmatitic foliation is concordant KSZ possibly represents a relict SW-verging boudinage and that the first vertical boudins with the overlying carapace; however, less than thick-skinned thrust that was one of presum- are overprinted by later NNE-SSW horizontal 100 m down structural section, this foliation be- ably many thrusts responsible for thickening of

boudins. Vertical boudinage includes two gen- comes highly discordant, where F3 upright folds the continental margin. These structures imply erations of pinch and-swell boudins, the first and kilometer-scale subdomes can be traced crustal shortening and thickening was extensive with a longer and the second with a shorter along the migmatitic foliation (Kruckenberg across Naxos and possibly led to regional Bar- wavelength. These features are followed by et al., 2011). This abrupt truncation in foliation rovian metamorphism. The compressional fea- NNE-SSW horizontal domino boudins, torn and its gradual rotation into alignment with the tures and thrusts were then reactivated as nor- boudins, and hairline veins reflecting embrittle- enveloping Koronos Unit suggest the presence mal-sense shear zones during the exhumation of

ment of the amphibolite layers associated with of a major tectonic boundary along the migma- the migmatites from (M2) kyanite-grade to (M3) retrograde cooling and exhumation. These stud- tite-gneiss contact, characterized by extreme sillimanite-grade conditions in the footwall. ies also demonstrated that outcrop-scale para- grain-size reduction. High-temperature blasto- sitic asymmetric folds predate torn boudins and mylonites, feldspar plasticity, grain boundary Zas Shear Zone hairline veins, whereas domino boudins indicate migration, and chessboard extinction of quartz A NE-SW–trending greenschist-facies shear locally deviating shear sense on opposite limbs indicate deformation temperatures exceeding zone cuts through the flank of Mount Zas, juxta-

of F3 folds (Fig. 9G). Virgo et al. (2018) and Von 600 °C (Figs. 8 and 12). Strain analysis of kine posing the greenschist-facies (relict blueschists) Hagke et al. (2018) concluded that long-wave- matic indicators implies overall general shear Zas Unit against the underlying kyanite-grade length pinch-and-swell boudins (i.e., vertically (a combination of NNE-SSW simple shear and Koronos Unit, and it can be traced over 20 km orientated boudinage) are consistent with syn- E-W pure shear), suggesting a large component along strike (Figs. 3 and 4). This structure is

migmatitic flow and 3F folding in the surround- of flattening perpendicular to this shear zone associated with an increase in mylonitization ing rocks and noted that static recrystallization coeval with definitively top-to-the-NNE kine- over a 500 m zone, overprinting and truncat-

in the amphibolite must have postdated verti- matics on all dome margins in both northern ing F1 and F2 structures. These folds rotate cal boudinage. Based on this reasoning and the and southern Naxos. This strongly indicates this into alignment with the shear zone, producing

synchronicity among upright F3 folding, vertical shear zone is folded by the doming migmatites, sheath folds with their noses pointing parallel boudinage, and migmatization, we also agree and therefore its movement predates migmatite to the lineation (NE-SW), in agreement with that static recrystallization must have postdated dome formation. Buick (1991a, 1991b). This structure could rep-

upright F3 folding and horizontal constriction at Along the east and west margins of the resent a reactivated thrust originally placing the the center of the migmatite dome, but annealing migmatite dome, leucogranitic sills and dikes Cycladic Blueschist Unit onto the underlying, was due to high temperatures outlasting con- increase in abundance and emanate from the more-proximal sedimentary cover (Koronos

tractional deformation during M3, and prior to underlying migmatites (Vanderhaeghe, 2004). Unit), as relict (F1) hanging-wall anticlines, S2 NNE-SSW extensional deformation. We there- These sills and dikes commonly truncate the crenulation cleavages, and relict top-to-the-SW

fore propose that the CHSZ documents a major S3 mylonite foliation, suggesting they intruded S-C fabrics are preserved in micaceous domains rotation in principal stress axes from bulk hori- synchronously with or postdate high-tempera- and within greenschist-facies porphyroblasts zontal shortening and constriction to NNE-SSW ture shearing on the KSZ. However, on western (see Figs. 6 and 11). S-C′ mylonites are associ-

extension during the close of the M3 event in the Naxos, they rotated about a NNE-SSW horizon- ated with top-to-the-NNE (S3) shearing and ex- presence of partial melt. tal axis into alignment with the NPDS (see be- clusively affect greenschist-facies assemblages.

16 Geological Society of America Bulletin, v. 1XX, no. XX/XX

Figure 11. Representative Czo photomicrographs and back- A B S3 C Ms scattered electron images of S2 Zas Unit microstructures and b S3 C’ S2 mineral chemistry. Mineral ab- a breviations follow Whitney and Evans (2010). (A–C) TLN54 Qtz (meta-tuff) glaucophane phen- Phg gite schist displaying well-pre- 1 mm served glaucophanes aligned S3 C Rt needles C with S1 top-to-the-NNE shear- Qtz Gln ing fabrics, but also affected by S3 C Phg Rt needles S3 C’ S top-to-the-NNE S-C fabrics 3 ′ S2 Phg b S2 that affect both M and M as- a Ank Gln 1 2 Czo semblages. In lower-strain

domains, S2a and S2b crenulation cleavages can be identi- Chl fied by folded bands of rutile 500 um 1 mm

needles, titanite phengite, and S2b D E S2b F plagioclase, suggesting S2 post- S2a S2a dated high-pressure condi- Qtz Pl S2 tions. (D–F) Quartz mica schist a S2b

displaying well-developed S2 Ms microstructures including milli Qtz Ms meter-scale isoclinal folds and Ms

S2a crenulation cleavages, with Rt needles

no evidence for S3. This greenschist assemblage implies that 500 um 1 mm 500 um S2 fabrics also developed dur-

ing M2. (G) TLN25 (Zas Unit) G H I Pl backscattered image showing Chl Sph S2 calc-schist, where epidote traps a S2a Ep

earlier S2a fabric, and micro- Ep Sph Sph

folding is defined by phengitic Act Pl S1 mica, actinolite, and titanite. Chl Gl Pg Gl S1 Inclusions of glaucophane and Rt Chl Pg paragonite in epidote repre- Pl S2a sent M1 conditions and outline S fabrics. S top-to-the-NNE Gl Ep 1 3 1 mm 100 um 200 um extensional S-C′ fabrics affect Gln Glaucophane vs Total edenite (Na+K) Gln Glaucophane vs Total tschermakite All Naxos White Mica matrix assemblages including 2 2 1 TLN54 Amphiboles TLN54 Amphiboles Mg Zas actinolite, chlorite, and plagio- J K 0.9 L Fe2+ Zas TLN25 Amphiboles TLN25 Amphiboles Na Zas 0.8 Mg Koronos clase. (H) TLN25 backscattered TLN54 Peak TC Output TLN54 Peak TC 1.5 1.5 Output Fe2+ Koronos 0.7 Na Koronos image showing S2 microfolded unit a Mg Core fabric that is defined by grains ul 0.6 Fe2+ Core

Tar rm n Bar Kat Kat Na Core Unit n fo Gl 1 1 0.5 Win Win r of titanite that formed after Tar Gl Bar pe

s 0.4

peak M1 conditions and predate on 0.3 greenschist-facies shearing. 0.5 0.5 Ca 0.2 This implies that S2 crenula- Ed Prg 0.1 tion cleavages formed at upper- Hbl Act Prg Ed Hbl 0 0 0 -0.1 00.1 0.20.3 0.40.5 0.60.7 0.80.9 1 -0.1 00Act .1 0.20.3 0.40.5 0.60.7 0.80.9 1 greenschist-facies conditions on Ts 33.1 3.23.3 3.43.5 Ed(Na+K) Si caons the retrograde part of the pres- Tsch

sure-temperature loop. (I) TLN25 close-up backscattered image of M1 inclusions in epidote defined by glaucophane and paragonite, which

preserve S1 S-C shear fabric. Small inclusions of sphene are also included in epidote, although coarse sphene grows externally. (J) Glauco- phane vs. total edenite for Zas Unit samples plotting solely in the bottom left-hand corner diagnostic of low-temperature–high-pressure amphibole. TC output—Thermocalc output. (K) Glaucophane vs. total tschermakite for Zas Unit samples TLN54 and TLN25 showing the range from actinolite to glaucophane compositions, with TLN54 modeled glaucophane composition for comparison. (L) All Naxos white mica; note the distinctive trimodal population of data, where high Si (>3.4 pfu) represents high-pressure phengite solely within the Zas Unit, high Na (>0.9 pfu) is characteristic of paragonite, again only within the Zas Unit, and where Koronos and Core Units represent one mica population

characteristic of Barrovian M2 conditions.

Geological Society of America Bulletin, v. 1XX, no. XX/XX 17

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.

Figure 12. Collection of photo A B C S2 b S2 micrographs from the Koro- Ky Ky a S2 Ms nos Unit to demonstrate key a Ms

microstructures and fabrics S2b

during prograde and peak M2 conditions. Mineral abbre- Ky S2 viations follow Whitney and a Evans (2010). (A–C) 17TL110, Ms a kyanite muscovite schist with coarse millimeter-sized 1 mm 1 mm 1 mm kyanite blades that overgrow D E S2 the S2a crenulation cleavages Bt b defined by muscovite and are S3 Bt aligned subparallel to the sub- Ms sequent S foliation. (D) TL67 2b S2a (Koronos Unit) peak M2 Grt- Qtz S2b Ky-Rt-Ilm-Ms-Pl enclave that Grt

is pretectonic with respect to ilm S3, which deforms around it. 500 um Garnet is posttectonic to S2b, Rt Pl which developed along cleavage F Qtz domains of previous S2a crenu- Ky lations, seen as inclusion trails Bt S3 through garnet. Ky and Rt S2 grew simultaneously with S , b 2b Ky as they are aligned parallel to Grt the fabric, but discordant to the Ms

external S3 fabric. (E) TLN47, Qtz a kyanite mica schist preserv- 1 mm 1 mm ing S2a crenulations. (F) Garnet porphyroclast showing sinstral G H S3 top-to-NE shearing fabric (S3), S3 which is subparallel to the in- Bt Grt clusion trails that define the 2bS Ky Pl foliation. (G) TLN13 (Koronos Bt Unit), plane-polarized light Ky S2 S2b (PPL) image showing enclaves b of pretectonic Ky + Ms break- ing down to Bt, which is affected Ms

by S3 S-C′ fabric. Small garnets (<100 µm) are found as inclu-

sions within kyanite, preserving 1 mm 500 um peak assemblage Grt-Ky-Bt- S3 I S3 J Ms-Plag-Qtz. (H) TLN22 (Ko- Post tectonic Grt WRT S ronos Unit) showing early 3 Qtz kyanite enclaves that preserve

S1 and S2, which have been rotated counterclockwise associ-

ated with development of S3. Small posttectonic garnets have idioblastic faces and crosscut CO2 incs in Grt the S3 fabric, and they show 1 mm 500 um fluid inclusion bubbles, many of

which are dark, indicating a mixed CO2-H2O fluid during garnet growth after top-to-the-NNE shearing. (I) TLN22 clearly showing small

postkinematic garnets that crosscut the S3 fabrics, implying high temperatures outlasted deformation and were possibly associated with

hydration at M3 sillimanite-grade conditions. WRT—With respect to. (J) TL67 blastomylonite quartz microstructures showing sweeping subgrain boundaries consistent with grain boundary migration followed by static recrystallization and coarsening.

18 Geological Society of America Bulletin, v. 1XX, no. XX/XX

Figure 13. Representative A B photomicrographs and backscattered-electron images and Bt mineral data from Core Unit samples. Mineral abbrevia- Pl F fold tions follow Whitney and Evans 3 (2010). (A) TL15 garnet kyanite Bt Ky migmatite showing prograde garnet with a rotational in- Grt ilm S2 clusion trail that rotates into a alignment with the external Pl S2b matrix fabric, indicating that garnet formed coeval with

S2a and S2b. (B) TL15 kyanite

folded with the S2b foliation by 1 mm 500 um an upright F fold, clearly im- 3 Ky S2 C D a S2b E plying that kyanite predates Grt Pl F3 folding. (C) TL15, showing Bt another example of an F3 fold Qtz Pl closure affecting biotite kyanite Secondary Ms F fold muscovite folia, with some pri- 3

mary prismatic muscovite pre- Bt Pl served. (D) TLN34 showing Primary Ms well-developed S2b foliations Sil with S2a crenulations preserved by the interlocking habit of 500 um 1 mm 1 mm biotite. (E–F) TL66 garnet Kfs sillimanite migmatite showing F G Qtz H Pl coarse crosscutting secondary Sil muscovite in both leucosome Pl S2 and melanosomes, with atoll Secondary Ms b Bt garnets. Sillimanite-biotite Grt Qtz F3 fold Pl intergrowths show decussate Kfs Bt Sil textures diagnostic of kyanite Ms to sillimanite transformation and coarse crosscutting sec- Qtz ondary muscovite indicative of re-equilibration and back re- 1 mm 500 um 1 mm action through the muscovite I Core Unit Migmate White Mica dehydration melting reaction. Leucosome 0.40 0.20 Pl J Mg Core unit (G) TLN34 sillimanite garnet t 0.36 0.18

uni Fe2+ Core Unit K-feldspar migmatite with Kfs a 0.32 0.16 ul

peritectic K-feldspar adjacent rm 0.28 Na Core Unit 0.14 fo r to biotite and sillimanite, in- Ti Core Unit t Bt pe 0.24 0.12 a uni

N dicative of incongruent melting. S2 la b +, 0.2 0.10 mu It also shows a range of defor- e2 or F

Melanosome f g, 0.16 0.08 er

mation microstructures from M Sil p brittle fracture to dynamic 0.12 0.06 Ti

recrystallization and exten- Mz Caons 0.08 0.04 sive grain-size reduction with 0.04 0.02 1 mm chessboard extinction in quartz 0 0.00 following anatexis, indicative of 33.053.1 3.15 3.23.25 deformation temperatures from Si caons

700 °C to 350 °C. (H) TLN20A biotite sillimanite migmatite with peritectic K-feldspar and well-developed S2b fabrics, where there is a very small amount of primary muscovite preserved. (I) TLN34 showing inclusions of monazite within biotite, and sillimanite quartz intergrowths within melanosomes, which will be dated in a future paper. (J) Core Unit white mica data, showing the spectrum of muscovite compositions, where Si <3.10 pfu and Ti >0.06 pfu are diagnostic of secondary muscovite.

Geological Society of America Bulletin, v. 1XX, no. XX/XX 19

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.

These structures signify extensional reactivation mation, indicative of deformation temperatures matrix bear many similarities to the ophiolitic

during the close of regional (M2) conditions at less than ~500 °C (Stipp et al., 2002a). Over mélanges of the Oman–United Arab Emirates temperatures of 280–450 °C, as demonstrated 95% of samples demonstrate top-to-the-NNE ophiolite (Searle and Cox, 1999, 2002). We by subgrain rotation and bulging of quartz grain kinematic indicators by asymmetrical alpha and tentatively suggest that this sequence represents microstructures (Stipp et al., 2002b). delta porphyroclasts of biotite, kyanite, plagio- a tectonic mélange formed in a trench environ- clase, and garnet, in agreement with Urai et al. ment and was deformed during SW obduction Naxos-Paros Detachment System (1990) and Buick (1991a, 1991b). We interpret of an ophiolite prior to Eocene subduction of The NPDS records the evolution of a duc- this brittle-ductile shear zone to be related to the continental margin. Although scattered re- tile shear zone from solely brittle deformation crustal extension and exhumation, as brittle nor- mains of upper ophiolitic material are distrib- at the top to completely ductile deformation mal faults root into it, and it clearly postdates, uted across the Cyclades at high structural levels beneath a few hundred meters (Buick, 1991a, is discordant to, and truncates all compressional (Katzir et al., 1996; Hinsken et al., 2017), ophio 1991b). This detachment horizon truncates all and metamorphic features. The west Naxos I- lite stratigraphy, including mantle peridotite, compressional structures (Fig. 3) and is gently type granodiorite is mylonitized on its eastern gabbro, plagiogranite, and a metamorphic sole, folded along a NNE-SSW axis, bounding the margin, indicating the fault was active dur- is exposed on Tinos (Lamont, 2018), implying entire island. The brittle fault (Moutsana detach- ing crystallization at ca. 12.2 Ma (Keay et al., that the ophiolite may have been widespread as ment; Cao et al., 2013, 2017), and its associated 2001). The NPDS presumably represents a sim- the highest structural nappe across the entire Cy- deformation, is best exposed in the NW and ilar structure to the North Cycladic Detachment clades. The Miocene to Pleistocene sedimentary central Naxos (near the towns of Melanes and System on Mykonos, Tinos, and Andros (e.g., successions are unconformably laid on top of Galanado; Fig. 3) and on the eastern coastline Jolivet et al., 2010; Jolivet and Brun, 2010) in this mélange, and they are composed of fluvial at Moutsana peninsula. Numerous brittle high- accommodating regional crustal extension by conglomerates, in some cases with boulders and angle faults root into the fault horizon, which top-to-the-NNE shearing along a localized zone clasts of a variety of ophiolitic and metamorphic is associated with a 20 m cataclasite and fault on the brittle-ductile transition. material derived from the footwall. The sedi- breccia zone, including pseudotachylytes, that In contrast to the NPDS, the extensional fab- mentary rocks dip at ~30° away from the core grades into ductile mylonites. The fault surface rics in the ZSZ and the KSZ and condensed complex, which is inconsistent with the dip pre- is commonly steeply dipping at ~40° perpen- right-way-up high-grade metamorphic isograds dicted by normal faulting. Clearly, these sedi dicular to the transport direction and is associ- are purely ductile features that are concordant ments must have been rotated by gentle doming ated with a shallowly NNE-plunging lineation, to the island-scale foliation. The structural dis- of the island during the Pliocene–Pleistocene which is oblique to the foliation, consistent with cordance between these internal shear zones and and therefore indicate a significant component a lateral ramp geometry. In central Naxos, the the brittle-ductile NPDS (Fig. 3) suggests that of E-W shortening after the core complex was detachment is folded into a gently northerly movement on the KSZ and ZSZ predated move- exhumed. The I-type granodiorite has aplitic plunging synform (see Fig. 7G), causing the ment on the NPDS and therefore records the sills that are folded along an N-S–trending axis nonmetamorphic hanging wall to be exposed in early stages of exhumation in the mid- to lower (see Data Repository Item1), also suggesting a graben-type structure that is bounded by steep crust prior to migmatite doming and regional significant E-W shortening during and follow- late crosscutting E-W–trending normal and NE- NNE-SSW Aegean extension. ing crystallization at ca. 12.2 Ma. SW– and NW-SE–trending strike-slip faults (see Fig. 3), juxtaposing the granodiorite to the Structures Outside the Metamorphic PETROGRAPHY AND MINERAL west and the metamorphic sequence to the east Core Complex CHEMISTRY (see Figs. 3 and 4). This doming also affects the Pliocene–Pleistocene sedimentary successions Upper Cycladic Nappe (Trench Mélange) Blueschist, gneiss, and migmatite samples within the hanging wall of the NPDS and re- In western central Naxos, a largely dismem- used in this study were collected at various struc- quires a component of E-W shortening during bered sequence of ophiolitic material composed tural levels of the Naxos MCC to investigate and after movement on this structure, which was of hydrothermally altered pillow basalts, ser- spatial variations in the pressure-temperature also responsible for folding the isograds and de- pentinites, and oceanic sediments, including (P-T) conditions of peak metamorphism. Previ- velopment of minor brittle thrust faults. cherts with manganese nodules, outcrops in the ous thermobarometric P-T results from studies Beneath this low-angle normal fault, right- hanging wall of the NPDS that transects central of the Naxos MCC are presented in Table 1. The way-up regional metamorphic isograds are Naxos (Jansen and Schuiling, 1976; Vander- analytical techniques and procedures utilized to telescoped and affected by flattening, indicating haeghe, 2004). Structurally above this, there is collect mineral composition data are outlined in the fault cut through metamorphic stratigraphy a disturbed sequence of Nummulite-bearing do- the Data Repository Item (see footnote 1). Min- (Fig. 7G). On west Naxos, brittle deformed cata- lomitic limestones/marbles, chloritized schists, eral abbreviations follow the guidelines of Whit- clasites and sediments are juxtaposed against and randomly orientated exotic blocks of mar- ney and Evans (2010), and migmatite terminol- upper-amphibolite-grade migmatites, a transi- ble and volcanic debris (Fig. 7). These field rela- ogy is after Ashworth (1975). Anhydrous phase tion of ~700 °C within 1000 m across strike, re- tions have been previously attributed to a “km- compositions were calculated to standard num- vealing frozen-in metamorphic field gradients of scale gravity slide” derived from the footwall of bers of oxygen per formula unit (pfu; Deer et al., –1 up to ~700 °C km . Although S-C′ fabrics (S3) this detachment system (Vanderhaeghe, 2004; 1992), micas were recalculated to 11 oxygens, affect all structural levels of the footwall, there Kruckenberg et al., 2011). Although this could and chlorite was recalculated to 28 oxygens.

is a larger intensity of shearing fabric develop- explain the wide variety of rock types, randomly Where present, H2O content was assumed to oc- ment on this structure. Quartz microstructures orientated metamorphic boulders, and evidence 1GSA Data Repository item 2019167, Additional indicate dynamic recrystallization occurred via for paleo-karstification, the extreme folding and field and petrological observations and data, is avail- a spectrum of deformation mechanisms, from entrainment of a variety of ophiolitic and distal able at http://www .geosociety .org /datarepository subgrain rotation and bulging to brittle defor- oceanic material as blocks within a serpentine /2019 or by request to editing@geosociety.org.

20 Geological Society of America Bulletin, v. 1XX, no. XX/XX

cur in stoichiometric amounts. The proportion cloudy plagioclase are deformed in crenulation 3+ 6) 6) of Fe /Fetotal was calculated using AX (Hol- cleavages (S2) that form a continuous fabric as 97 97 ) ) ) land, 2009), and amphibole exchange vectors inclusions throughout epidotes, with preserved 989) 989) 17 17 17 were calculated using the method of Holland microlithons (S2a) and cleavage domains (S2b; 999) (2006 ) (2006 ) 985) (20 (20 (20 998 ) (1 and Blundy (1994). Representative photomicro- Figs. 11B and 11D–11H). These grains are graphs, backscattered electron images, and min- rimmed by retrograde plagioclase and chlorite, ad (1 Study OS

vig eral compositions are shown for the Zas Unit in indicating they are most likely associated with eenstra (1 A atzir et al. F eillod et al. eillod et al. eillod et al. k and Holland (1 k and Holland (1 Figure 11, the Koronos Unit in Figure 12, and decompression from high-pressure conditions. P P P Deuchêne et al. Deuchêne et al. Buic Buic the Core Unit in Figure 13. A summary of the S2a is also associated with centimeter-scale iso- ansen and Schuiling (1 ansen and Schuiling (1 relative timings of fabric development is dis- clinal folding, with well-developed microlithons played in Figure 14, and garnet maps and line that act as parasitic folds on the limbs of larger-

profiles from samples TL67, TL15, and TL66 scale folds (see Fig. 11F). S2a and S2b presum- yJ yJ AMORPHISM ON NAX are presented in Figure 15, Tables 2–4, and the ably formed by shortening along two orthogonal -T -T -T -T

yK Data Repository Item (see footnote 1). Sum- axes at upper-greenschist-facies conditions but 988) 988) ometr ometr mary P-T results of all samples are presented predate the characteristic lower-pressure retro P P erage P erage P erage P erage P AND MET undum in Table 5. grade M2 greenschist assemblage. We there- AS AS mometr mobar mobar Av Av Av Av orr fore relate S2 fabrics to overthrusting and duc- TION e-C ALC ALC ALC ALC Method t-Bt, G t-Bt, G Petrography of the Zas Unit tile stacking of high-pressure nappes at crustal Gr Gr -Cpx ther depths that postdated high-pressure conditions. entional ther entional ther well and Holland (1 well and Holland (1 Diaspor Opx All samples in this unit display microstruc- S2 fabrics contrast to the more localized top- nv nv Po Po THERMOC THERMOC THERMOC THERMOC tural evidence for M2 greenschist-facies over- to-the-NNE S-C′ shearing fabric (S3), which Co Co

printing of older M1 blueschist-facies mineral purely affected the greenschist-facies assem-

GES OF DEFORMA assemblages and fabrics, with replacement spa- blages, including quartz, mica, plagioclase,

TA tially concentrated along zones of fluid altera- and chlorite–rich domains. S mylonite fabrics

tainty 3

6 tion. M1 high-pressure precursors occur as pro- are heterogeneous and localized along discrete

, °C) grade inclusions or as relict matrix phases that shear zones. S3 is particularly developed in sam- ±50 .1, ±30 ed uncer ARIOUS S ±2, ±50 ±2, ±50 ±2, ±50 ±2, ±50 ±2, ±50 ±2, ±50 have been pseudomorphed or trapped inside late ples taken from the ZSZ, in a parallel orientation ±1 ±0.5, ±1 ±0.9, ±32 (kbar ±1–2, ±50 ±1–2, ±50 lower-grade minerals. Glaucophane is unzoned to the fold axial planes, and appears to utilize

Associat and is preserved within the cores of epidote S2b cleavage domains. Microstructurally, S3

TES FOR V grains, with phengitic and paragonitic white deforms the quartz and fine-grained mica-rich mica, clinozoisite, and no plagioclase, and also matrix around pretectonic greenschist-facies TIMA

6 9 along unaltered microstructural domains and fo- porphyroclasts, including clinozoisite, epidote, 600 °C) ) ES

, 670 , 700 liations (Figs. 11A, 11C, 11G, and 11I). and late crosscutting muscovites, creating strain , 470 2, 450 .5, 57 .5, 61 0, 500 –7 –7 –8, 690 , 400–670 12 3.8, 384 >1

15 16 All mineral compositional trends are dis- shadows, asymmetrical boudinage, and rota- (kba r, .0–9. 7, –7 –, 700–800

P - T conditions played in Figures 11J, 11K, and 11L and high- tion of the porphyroclasts (Figs. 11A, 11B,

TURE ( P - T light the high Na and low Al contents of primary and 11C). Dynamic recrystallization via sub- amphibole, high Si pfu of mica, indicative of grain rotation and bulging is persistent within phengitic compositions, and paragonite that is quartz-rich domains, indicating deformation e6 e6 e6 e8 characteristic of blueschist-facies conditions. temperatures must not have exceeded 450 °C ni t as as as as as onos or or or or nos-Za s1

or A subsidiary population of actinolite is con- (Stipp et al., 2002a, 2002b). Due to these rela- onos-Za s5 ro or fined to the matrix, and it characterizes the M tionships, we interpret S as a purely retrograde Ko 2 3

greenschist-facies overprint. Late crosscutting feature postdating (M1) high-pressure and (M2) ). greenschist-facies minerals are well developed greenschist-facies conditions, and it was related 10 in lesser-strained domains within the preexist- to exhumation of the core complex through the sZ sZ sZ sZ sZ dom eC dom eC dom eC dom eC ing foliation, presumably from the breakdown brittle-ductile transition (Buick and Holland, ans (20

rapace of large porphyroblasts such as garnet to form 1989; Urai et al., 1990; Avigad, 1998). ocatio nU arapac eK arapac eK ast Na xo Ca SE Na xo SE Na xo SE Na xo SE Na xo epidote, plagioclase, chlorite, tremolite, and ac- y and Ev Migmati te Migmati te Migmati te Migmati te tinolite (Figs. 11G and 11H). Sodic amphiboles Petrography of the Koronos Unit (Kyanite- ION OF PREVIOUS PRESSURE-TEMPERA itne occur mainly as small prismatic inclusions in Grade Gneisses) Wh e cC e eC e e epidote and commonly display decussate-shear e eE w textures that could be associated with top- M2 kyanite-grade schists reveal that this unit ollo COMPIL AT .

etabai te etabai te to-the-NE shearing during exhumation from attained a contrasting metamorphic history to etapelit etapelit etapelit etapelit etabasit Diaspor Litholog yL Metabait Ultrama fi Metapeli te Metapeli te blueschist facies (S1; Figs. 11A, 11C, 11G, and the overlying Zas Unit, with no petrographic viations f

ABLE 1 11I). This is aligned subparallel to the pervasive evidence for (M1) blueschist-facies conditions T greenschist-facies top-to-the-NNE extensional (Figs. 12 and 15). High-grade deformational

fabric (S3), which solely affects the surrounding textures are present throughout this unit, with quartz-mica–rich matrix. In low-strain domains, evidence for prograde, peak, and retrograde Mineral abb re

: blueschist- to greenschist-facies assemblages microstructural fabrics that developed over a

ograde M2 containing phengitic muscovite, paragonite, large temperature range from lower- to upper- tr ak M 2M ak M 2M ak M 2M ak M 2M ak M2 ak M 1M ak M1 ak M 1M ak M 2M No te ograde M2 ograde M2 Pe Pe Pr Pe Pr Pe Pe Pe Pe Pe Pe Re Stage biotite, plagioclase, green amphibole, and amphibolite-facies conditions (Buick and Hol-

Geological Society of America Bulletin, v. 1XX, no. XX/XX 21

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.

North

Naxos-Paros Detachment Upper Cycladic Nappe System (NPDS)

Zas Unit A Zas Shear Zone E Koronos Unit top-to-NNE shearing C F1 Folds

D Core Unit B F2 Folds Koronos Shear Zone S2a Koronos Shear Zone Crenulation F3 Folds top-to-NNE shearing Zas Unit S1 Gln incs Cleavage A SSW NNE Unit Mineral S1 S2 S3 F1 F2 F3 Ep Zas Unit Gln Ep Czo Pg Bt Ms Ab 1mm Chl S3 top to NNE S-C’ Koronos Unit: M2 Ky-grade fabrics Grt B SSW NNE Koronos Unit Ky S3 Bt S2a S2b Sil Grt Ky Core Grt Ky 1mm Unit M2 Grt post-S2b pre-S3 Ky Parallel with S2b Bt Sil Koronos Unit: M3 Sil-grade fabrics SSW NNE Kfs C S2b Ky Core Unit: M3 Sil grade Quartz Recrystallization Fabrics Ms Dehydration Melting Unit Bulging SGR GBM Chess SSW NNE Grt E 280–400 400–500 550–650 >650

Bt S3 NPDS Sil Post-Tectonic Peritectic Kfs Grts Oblique Qtz 1mm Zas CPO fabric Unit Core Unit: M2 Ky-grade Hydrous Melting Leucosome ZSZ D W E Koronos Sil Unit Bt Grt Melanosome KSZ Ky Core Grt Unit Melanosome Leucosome 1mm 1mm F3 Fold Ky folded post-dating with foliation Ky-grade fabrics Figure 14. Schematic illustration of the deformation macrostructures and microstructures on Naxos, showing fabrics, order of mineral growth, deformation temperatures, and sequence of shearing and simplified sketches of microstructures. Mineral abbreviations follow Whitney and Evans (2010). GBM—grain boundary migration; SGR—subgrain rotation; KSZ—Koronos shear zone; ZSZ—Zas shear zone; CPO—Crystal preferred orientation. See text for discussion.

22 Geological Society of America Bulletin, v. 1XX, no. XX/XX

X X’ A TL67 Ca TL67 Mg 0.76 Figure 15. Representative fully e) TL67 0.74 B quantitative electron micro- X Fe probe maps and line profiles of tion (F 0.72 0.70 garnet for samples TL15, TL67, Rim Core Rim 0.68 and TL66 showing cation mole Mn in ection

ation mole frac 0.66 Mn in ection in outer rim Syn S2b Core fractions of grossular, alman- in outer rim 0.16 dine, pyrope and spessartine Ca 1 mm 1 mm )C 0.14

(see text and Data Repository , Ca TL67 Mn TL67 BSE 0.12 Item [text footnote 1] for dis- X Grt n, Mg cussion). Gaps in the profiles 0.10 0.08 Mg Growth zoning tion (M increasing exist from crossing fractures S2b 0.06 Mg & decreasing and inclusions. Mineral abbre- Rt Ca towards rim viations follow Whitney and 0.04 Mn ilm 0.02 Mn excursion Evans (2010). (A) TL67 maps = Fracture X’ Cation mole frac 0 of Ca, Mg, Mn, Y, and Yb show- Resorption 0100 200300 400500 600700 800900 1000 1100 1200 1300 1400 1500 1600 at rim 1 mm X’ 1 mm ing prograde growth zoning Distance across prole (µm) TL67 Y TL67 Yb Y Y’ and a backscattered-electron 0.78

e) D TL15 (BSE) image showing line pro- 0.76

file X–X′ across the internal tion (F 0.74 Rim Core Rim Fe S2b fabric. Note brighter color 0.72

corresponds to larger element 0.70 Mn in ection Mn in ection abundances. (B) TL67 profile 0.68

Cation mole frac in outer rim in outer rim X–X′ showing prograde growth 0.16 Mg 1 mm 1 mm zoning with increasing Mg from ) 0.14 , Ca core to rim and decreasing Ca. Ca TL15 Mg 0.12 Growth zoning C TL15 increasing Ca

n, Mg Pre S Core An Mn inflection ~120 µm from Mg & decreasing 2 0.10 Ca towards rim the garnet rim is indicative of tion (M 0.08 Mn partial modification of the outer 0.06

rim by diffusion. (C) TL15 ele- 0.04 ment maps and backscattered- 0.02 electron image (BSE) of a Cation mole frac 0 0100 200300 400500 600700 800900 1000 1100 1200 1300 1400 garnet with line Y–Y′ crosscut- 1 mm 1 mm Distance across prole (µm)

ting the S rotational inclu- TL15 Mn TL15 BSE e) Z Z’ 2a Y sion trails. (D) TL15 profile 0.78 S2a E TL66

Grt tion (F 0.76 Y–Y′ showing a slight increase Fe in Mg and an abrupt stepwise 0.74 decrease in Ca from core to rim 0.72

ation mole frac Rim Core Rim indicative of a two-stage garnet 0.18 Grt resorption growth history. An inflection in Resorption 0.16 Mg at rim Y’ )C

Mn indicates only the outer- 1 mm 1 mm , Ca 0.14 most 100 µm rim is somewhat TL15 Y TL15 Yb n, Mg 0.12 Diusional zoning modified by diffusion. (E) TL66 decreasing Mg 0.10 & Ca towards rim profile Z–Z showing diffusional tion (M ′ Ca zoning with a decrease in Mg 0.08 from core to rim and generally 0.06 decreasing Ca with an enrich- 0.04

Cation mole frac Mn Mn increasing ment in Mn toward the rim (see 0.02 towards rim = diusion Data Repository Item [text foot- 1 mm 1 mm 0 0 100200 300400 500600 700800 9001000 note 1] for trace of line profile). Distance across prole (µm)

land, 1989; Urai et al., 1990). In metapelitic matrix separated into quartzofeldspathic domains This unit also shows contrasting microstruc-

lithologies, major phases include quartz, plagio- and biotite and muscovite folia (see Figs. 12A– tural fabrics to the overlying Zas Unit. S1 is clase, Ti-rich biotite, muscovite, garnet alumino- 12D). The rare occurrence of staurolite is attrib- aligned parallel to bedding and is preserved silicate, and minor K-feldspar (Figs. 13A–13J), uted to the semipelitic nature of the protoliths, as inclusions within kyanite and garnet cores,

and accessory minerals include tourmaline, apa- with only relict fragments remaining following whereas crenulation cleavages (S2a and S2b) tite, chlorite, rutile, ilmenite, magnetite, mona- breakdown to produce garnet and kyanite: can be found in less-strained enclaves and as zite, and zircon. In aluminum-rich bands, up to inclusions in garnet. Isoclinal folding occurs centimeter-scale kyanite blades and euhedral staurolite + muscovite + quartz → on a millimeter scale and is associated with

garnet grains occur in close proximity, with the biotite + kyanite + garnet + H2O. (1) microlithons (S2a) and utilizes compositional

Geological Society of America Bulletin, v. 1XX, no. XX/XX 23

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.

010 000 layering between quartzofeldspathic and pelitic .08 .004 Pl 0.008 0.011 0.00 0.33 4.988 0.07 0.00 0.05 0.18 2.980 0.000 1.011 0. 0. 0.002 0.022 0.008 0.951 0 68 19.59 99.50 11.20 TLN26 domains (see Figs. 12A–12C and 12E). Cleav-

age domains (S2b) are superimposed on S2a and .992 Prg 28 1.31 1.13 4.17 9.56 1.13 0.28 0.06 0.15 3.089 0.014 2.573 0.179 0.072 0.003 0.110 0.011 0.143 0.798 0.604 71.3 47.20 33.35 9 TLN26 in places completely overprint the microlithons

56 to produce a strong spaced foliation defined by Ms 12 0.0 2.86 2.67 6.96 9.84 0.50 0.63 0.15 0.00 3.386 0.033 2.166 0.000 0.165 0.009 0.274 0.000 0.067 0.865 0.624 biotite-muscovite foliation (Figs. 12D and 12E). 49.12 26.66 92.43 TLN26 Large kyanite porphyroblasts are aligned coaxial

Ph with the foliations (S and S ) and sometimes 11 0.0 3.56 2.55 6.954 0.00 0.00 0.14 0.00 3.498 0.000 2.040 0.000 0.204 0.008 0.261 0.000 0.000 0.943 0.561 2a 2b 50.96 25.21 93.19 10.77 TLN26 crosscut them, indicating these are prograde features. In numerous samples (e.g., TL67), euhe- Ph 11 0.0 3.64 2.69 6.952 0.01 0.15 0.00 0.11 3.475 0.008 2.054 0.000 0.209 0.000 0.275 0.008 0.001 0.922 0.568 50.64 25.39 93.16 10.53 TLN26 dral garnets overgrow kyanite-bearing S2b fabrics preserved in aluminum-rich domains, whereas Chl 41 0.0 9.971 0.01 0.11 0.23 0.00 0.01 2.716 0.019 2.613 0.000 2.578 0.000 2.020 0.001 0.023 0.001 0.439 24.84 20.27 28.20 12.40 86.07 TLN26 in other samples (e.g., TLN22), garnets are enclosed by large kyanite porphyroblasts, suggest- Pl 0.008 81 0.45 0.17 4.983 0.05 0.07 0.00 0.18 2.993 0.002 0.967 0.030 0.016 0.000 0.011 0.008 0.953 0.003 –0.034 68.59 18.80 99.58 11.27

TLN54 ing simultaneous prograde growth of kyanite at

peak M2 conditions. In sample TLN21, garnets Prg 0.14 0.63 6.966 0.40 7.33 0.10 0.06 0.07 3.027 0.000 2.911 0.046 0.030 0.000 0.000 0.008 0.897 0.047 0.000 are up to several millimeters in diameter and dis- 11 60.5 48.22 37.73 94.68 TLN54 tributed uniquely along tight isoclinally folded

Ph pelitic layers (S ), whereas in TL67, euhedral 3.41 1.86 6.949 0.46 0.27 0.00 0.01 3.464 0.004 2.045 0.048 0.165 0.003 0.287 0.008 0.027 0.898 0.635 2a 11 22.5 49.01 27.02 92.41 10.37 TLN54 garnet porphyroblasts show straight faces trun-

cating both S2a and S2b foliations, indicating Ph 3.53 1.89 6.949 0.36 0.52 0.00 0.14 3.408 0.027 2.124 0.047 0.201 0.000 0.192 0.010 0.047 0.893 0.489 11 19.0 50.10 26.49 93.32 10.29 TLN54 garnet growth in this sample postdated initial thrusting and folding (Fig. 12D). Within some Chl 2.7 9.944 0.06 0.03 0.07 0.32 0.00 2.776 0.006 2.490 0.075 2.709 0.029 1.845 0.000 0.006 0.008 0.405 14 25.63 19.50 29.91 11.43 86.95 TLN54 of these garnet porphyroblasts, dark fluid inclusions indicate garnet growth in a mixed saline 5.2 6.98 0.00 7.06 0.11 0.06 0.22 7.998 0.012 1.817 0.090 1.642 0.008 1.469 0.034 1.933 0.000 0.472 Amp CO /H O fluid, as noted previously (Baker and 23.00 56.66 10.92 14.67 15.003 96.70 TLN54 2 2 Matthews, 1994; Baker et al., 1989; Bickle and 6.85 9.3 0.05 7.25 0.08 0.00 0.19 8.018 0.008 1.805 0.159 1.547 0.000 1.435 0.028 1.977 0.009 0.481

Amp Baker, 1990; Kruelen, 1980). Kyanite has com- 23.00 57.04 10.89 14.51 14.985 96.85 TLN54 monly broken down to biotite and is confined to pelitic domains with garnet. These domains form 5.2 0.03 7.03 0.13 0.07 6.65 0.24 7.975 0.014 1.800 0.096 1.750 0.008 1.412 0.037 1.942 0.005 0.447 Amp 23.00 55.94 10.71 15.48 15.038 96.27 TLN54 enclaves that are partially recrystallized as an extensional blastomylonitic top-to-the-NNE shear 4.7 0.00 0.96 6.89 0.03 1.51 0.00 7.939 0.004 0.249 0.059 1.183 0.000 3.584 1.758 0.259 0.000 0.752 Amp 23.00 10.64 17.23 11.75 15.033 99.01 TLN25 YSES OF ZAS UNIT SAMPLES DERIVED FROM ELECTRON MICROPROBE fabric (S3) affecting the matrix and deforming

55 around stronger kyanite-biotite-muscovite- mp 3 0.00 7.05 0.08 8.48 0.00 0.38 8.061 0.008 1.352 0.583 0.912 0.000 2.068 0.055 1.847 0.000 0.694 39.0 59.6 13.23 10.26 14.886 99.13 TLN25 plagioclase-garnet–rich enclaves, indicating TIVE ANAL these M2 kyanite-grade assemblages are pre- TA 32 0.07 7.99 0.07 9.77 0.17 8.91 0.31 8.039 0.007 1.578 0.229 1.246 0.019 1.819 0.045 2.124 0.013 0.593 Am pA tectonic with respect to S (Figs. 12D, 12G, and 15.5 58.66 12.87 15.119 98.81 TLN25 3

12H). S3 was accommodated via grain boundary Ta 12 0.00 0.32 3.05 0.20 0.25 6.83 0.00 0.14 3.802 0.009 0.018 0.399 0.344 0.000 2.381 0.009 0.037 0.000 6.999 0.874 migration of quartz and ductile deformation of 53.7 26.50 97.29 TLN25 TIVE QUANTI plagioclase associated with temperatures ex- 86

Ta ceeding 600 °C (Stipp and Kunze, 2008). This 11 0.43 0.12 0.07 0.70 7.02 0.00 0.18 3.769 0.003 0.050 0.398 0.353 0.000 2.396 0.012 0.014 0.033 7.028 0.872 53.0 62.6 26.73 97.93 TLN25 was followed by coarsening and Ostwald ripen- ing of grain boundaries, revealing static recrys- Bt 11 8.82 0.20 0.78 0.18 0.34 2.749 0.040 1.254 0.446 0.812 0.010 1.634 0.025 0.026 0.758 7.754 0.668 35.5 40.82 15.80 14.42 16.28 97.64 TLN25 tallization. Annealing postdated deformation at

BLE 2. REPRESEN TA close to peak conditions (Fig. 13J). Pl TA 0.278 0.059 81 0.10 8.06 0.09 0.00 2.43 0.45 5.65 2.653 0.000 1.041 0.282 0.087 0.003 0.029 0.258 0.665 0.005 5.023 62.30 20.73 99.81 TLN25 Toward the base of this unit along the KSZ, many samples display evidence for a thermal and Ep 0.07 0.09 0.28 0.00 8.53 0.25 2.922 0.000 2.266 0.502 0.541 0.018 0.028 1.908 0.013 0.007 8.205 0.049 static, lower-pressure overprint on a blastomy- 12.5 48.1 38.53 25.34 23.48 96.57 TLN25

lonitic S3 fabric. This is demonstrated by post- Ms 1 0.36 0.00 0.17 3.98 2.72 0.22 3.149 0.008 2.057 0.434 0.218 0.000 0.266 0.015 0.046 0.851 7.044 0.550 tectonic growth of (M3) sillimanite and garnet 66.6 10.17 48.01 26.60 92.23 TLN25 (e.g., TLN21 and TLN22), which truncate the

.00 top-to-the-NNE S fabric, indicating that high Ms 3 11 0.70 0.32 3.63 2.98 0.00 3.208 0.016 1.976 0.430 0.197 0.000 0.288 0.000 0.088 0.830 7.033 0.594 68.6 10.02 49.40 25.81 92.86 TLN25 temperatures outlasted deformation. Peak (M2)

rg kyanite-bearing assemblages are overprinted by 11 0.86 7.41 0.25 0.62 0.00 0.08 0.31 2.843 0.011 2.789 0.393 0.031 0.000 0.007 0.020 0.854 0.065 7.013 0.184 92.7 47.82 39.79 97.14 TLN25 (M3) sillimanite, and biotite intergrowths occur with plagioclase and quartz in dilational strain Pr gP 11 1.69 7.10 0.28 0.66 0.03 0.56 0.21 2.898 0.012 2.685 0.390 0.033 0.001 0.049 0.013 0.812 0.127 7.02 0.598 92.2 49.12 38.60 98.25 TLN25 zones, demonstrating that the transition from

s1 kyanite to sillimanite occurred via a Carmicheal- 3 % 2 O

2 type reaction scheme (Carmicheal, 1969) that O 2 Note: Mineral abbreviations follow Whitney and Evans (2010). 3+ 2+ 3+ O 2 O O 2 Mg Ca otals Mineral: Si Ti Al FeO MnO MgO CaO Na K T Oxygen Ti Al Fe Fe Mn Mg Ca Na K Sum X Fe Alm% Sps% Prp% Grs% X Pl Al/Si Si Sample: postdated shearing (Figs. 12H and 12I).

24 Geological Society of America Bulletin, v. 1XX, no. XX/XX

7 Within metapelitic samples, garnet shows Pl 0.00 0.00 0.00 5.04 8.68 0.45 0.243 0.245 2.748 0.000 1.243 0.017 0.000 0.000 0.000 0.240 0.747 0.000 4.995 0.00 61.94 23.7 99.87 Matrix TLN22 a variety of compositional profiles that cor-

718 001 272 020 000 006 009 232 744 011 013 relate directly with their microstructural rela- .95 .19 Pl 0.13 0.02 0.17 4.86 8.61 2. 0. 1. 0. 0. 0. 0. 0. 0. 0. 5. 0.54 0.235 0.275 88 0.20 60 24 99.67 Matrix TLN22 tionships (Fig. 15; Data Repository Item [see footnote 1]). Garnet in TL67 preserves pro- Bt 1.95 0.17 0.55 0.00 2.710 0.109 1.706 0.000 1.092 0.011 1.234 0.044 0.000 0.842 7.748 0.531 8.89 11.16 17.60 36.52 19.50 11 96.33 Matrix

TLN22 grade growth zoning associated with a smooth transition from core to rim, increasing pyrope 60 Ms 1.30 0.00 0.61 0.00 1.12 3.079 0.058 2.717 0.000 0.074 0.008 0.074 0.009 0.132 0.838 6.989 0.500 0.95 9.81 content, and homogeneous to slightly decreas- 47.45 35.35 11 96. Matrix TLN22 ing grossular content, as shown in Figure 15B. Ms 1.07 0.44 1.09 0.00 1.13 3.091 0.045 2.737 0.000 0.078 0.021 0.048 0.000 0.186 0.767 6.973 0.381 0.72 000 9.37 An inflection in the spessartine content on the 46.06 35.45 11 95.33 Matrix TLN22 outermost rim is indicative of some garnet re- .1 Int Grt 0.00 3.16 4.25 3.60 0.20 2.991 0.000 1.921 0.128 1.916 0.211 0.499 0.304 0.031 0.000 8.001 0.207 0.00 7. 0 sorption following peak conditions. In contrast, 31.03 10.1 37.97 20.69 66.2 16.6 12 TLN22 100.90 TLN22 has small garnets that crosscut the S3 fabric and in places are engulfed by kyanite. Grt 0.18 6.28 2.02 1.50 0.01 3.129 0.011 1.853 0.000 2.139 0.427 0.242 0.129 0.002 0.002 7.934 0.102 4.3 8.1 00 0.02 rim Outer 31.83 38.94 19.56 73.4 12 14.2 TLN22 100.34 These garnets show either a general decrease .9 .1 .3 in the grossular component from core to rim, Grt 0.05 4.14 3.83 2.80 0.26 2.976 0.003 1.916 0.166 1.928 0.279 0.453 0.238 0.040 0.000 7.999 0.190 0.00 rim Inner 31.53 37.46 20.46 67.7 15.1 12 TLN22

100.54 or the pyrope content slightly increases toward the inner rim followed by a decrease toward Grt 0.00 2.76 3.32 4.27 0.00 3.060 0.000 1.967 0.000 1.989 0.185 0.393 0.363 0.000 0.001 7.958 0.165 00 2. 17 0.01 6. 29 Core 29.98 38.57 21.03 68.6 13.1 12 99.94 TLN22 the outer rim, which is indicative of late garnet growth at lower pressure and temperature or Pl 0.08 0.06 0.00 5.23 8.96 2.734 0.003 1.247 0.000 0.011 0.002 0.000 0.250 0.776 0.008 5.032 0.30 0.242 0.252 0.14 61.23 23.69 99.71 Matrix TLN21 homogeneous garnet profiles due to re-equilibration at lower pressures. Ms 1.28 0.00 1.19 0.08 0.75 3.106 0.039 2.741 0.000 0.059 0.000 0.060 0.000 0.098 0.859 6.962 0.504 1.52 46.75 34.11 11 96.58 10.89 Along the KSZ, there is a band of ultramafic Matrix TLN21 rock that is enclosed within foliated garnet-bear- Bt 3.99 0.22 8.13 0.00 0.19 2.707 0.228 1.636 0.000 1.250 0.014 0.922 0.000 0.028 0.947 7.732 0.424 9.75 ing amphibolites that are semicontinuous along 19.64 35.56 18.23 11 95.70 Matrix TLN21 strike and wrap around the dome. The meta- Bt 3.74 0.00 8.09 0.24 0.07 2.720 0.212 1.696 0.000 1.259 0.000 0.908 0.019 0.010 0.800 7.624 0.419 000 8.32 peridotite enclaves enclosed within the amphib- 19.98 36.11 19.10 11 95.65 Matrix TLN21 olites show partial serpentinization to anthopyl-

.1 lite, talc, and actinolite, but they preserve in situ Grt 0.13 6.12 2.57 2.08 0.59 2.954 0.008 1.961 0.213 1.862 0.416 0.307 0.179 0.092 0.007 7.999 0.142 6. 01 0.07 rim Outer 30.93 36.82 20.73 12 69.9 10.2 13.9 TLN21 100.05 olivine and orthopyroxene. Katzir et al. (1999)

.6 provided a detailed description of the petrogra- Grt 0.32 2.20 3.47 3.37 0.07 3.005 0.019 1.943 0.019 2.154 0.149 0.412 0.288 0.011 0.000 8.000 0.161 00 5.0 0.00 rim Inner 32.60 37.70 20.67 12 71.7 13.7 TLN21

100.38 phy and geochemistry of these lenses and noted

rt bimodal temperatures of ~700 °C and 1200 °C .1 .4 0.00 3.27 2.98 2.59 0.24 2.952 0.000 1.966 0.169 2.077 0.222 0.355 0.222 0.037 0.001 8.001 0.146 7. 49 0.01 YSES FROM THE KORONOS UNIT SAMPLES DERIVED ELECTRON MICROPROBE Core 33.56 36.90 20.84 12 73.4 11.8 TLN21

100.41 from pyroxene thermometry, suggesting they remained hot as they were emplaced into the shelf .8 Int Gr tG 0.28 1.78 4.16 3.43 0.13 3.028 0.017 1.928 0.002 2.091 0.121 0.498 0.295 0.020 0.000 8.000 0.192 00 4. 07 0.00 31.17 37.72 20.37 12 69.5 16.6 99.05

TLN21 carbonate sequence. Within these amphibolites,

there is some degree of hydrous M2 migmatiza- atrix 0.09 0.02 0.00 0.01 0.00 0.982 0.002 2.007 0.000 0.018 0.000 0.000 0.000 0.000 0.005 3.015 0.77 0.14 yanite TL67 36.03 62.45 99.51 tion but no petrographic evidence whatsoever

TIVE MINERAL ANAL to suggest they attained M1 blueschist-facies Kf sK 0.18 0.00 0.00 0.00 0.40 2.987 0.006 1.005 0.000 0.000 0.000 0.000 0.000 0.035 0.975 5.008 0.01 0.000 0.005

TL67 conditions. 65.34 18.64 16.71 with Gr tM 101.28 . Pl 0.00 0.01 0.00 5.58 8.34 2.728 0.000 1.274 0.001 0.000 0.000 0.000 0.266 0.718 0.012 4.999 0.03 0.267 0.273 0.22 Petrography of the Core Unit TL67 61.41 24.33 99.91 Matrix

TA TIVE QUANTI Samples from the migmatite dome preserve a Ms 1.29 0.00 0.61 0.00 0.60 3.107 0.064 2.691 0.000 0.091 0.000 0.060 0.000 0.077 0.865 6.955 0.397 1.65 1885 TL67 46.88 34.44 1 95.70 10.23 with Ky petrologic record of kyanite- to sillimanite-grade

(M2–M3) partial melting, and they display no Ms 1.05 0.00 0.60 0.06 0.85 3.073 0.052 2.722 0.000 0.103 0.000 0.059 0.004 0.109 0.892 7.014 0.364 1.85 TL67 46.30 34.79 11 96.03 10.54 Matrix evidence for having experienced Eocene M1 con-

BLE 3. REPRESEN ditions. Careful textural examination revealed Bt 3.40 0.16 8.09 0.05 0.31 9.64 2.678 0.195 1.653 0.000 1.346 0.010 0.921 0.004 0.046 0.939 7.792 0.406 TL67 TA 35.07 18.36 21.08 11 96.17

with Ky that hydrous melting was the dominant melting process occurring within the Naxos dome, capa Bt 3.27 0.23 9.70 0.00 0.05 9.84 2.679 0.182 1.700 0.000 1.181 0.014 1.067 0.000 0.007 0.927 7.757 0.475 0000 TL67 36.29 19.54 19.13 11 98.05 Matrix ble of producing between 1% and 7% volume melt based on reaction stoichiometry. However, rt .7 Int 0.20 2.10 3.28 5.07 0.07 0.00 2.978 0.012 1.910 0.120 2.000 0.142 0.391 0.435 0.011 0.000 7.999 0.164 0.1 TL67 37.20 20.24 31.66 12 67.7 13.0 14. 59 99.82 at the deepest levels in pelitic lithologies, incongruent muscovite dehydration melting was the Gr tG 0.08 4.73 2.71 5.72 0.00 0.03 2.970 0.010 1.990 0.060 1.850 0.320 0.320 0.490 0.000 0.000 8.001 0.148 Core TL67 major melting process associated with growth 10. 64 37.51 21.29 28.77 12 62.5 10.7 16.2 100.84 of peritectic K-feldspar, removal of muscovite, rt 0.10 2.22 3.57 4.83 0.01 0.00 3.010 0.006 1.939 0.031 2.032 0.149 0.422 0.410 0.002 0.000 8.001 0.172 00 5.0 rim and melt segregation from larger melt fractions TL67 Inner 37.97 20.75 31.11 12 67.3 14.1 13.7 100.56

:G (>20% melt), which eventually pooled to form Mineral abbreviations follow Whitney and Evans (2010). Int—interior s i S-type leucogranites that crosscut and intruded 3 % 2 O 2 O m 2 O Note: 3+ 2+ 3+ O O O 2 O 2 Mg Ca otals into the mantling carapace. Sample: Minerals Location: Si Ti Al Fe MnO MgO Ca Na K T Oxygen Si Ti Al Fe Fe Mn Mg Ca Na K Su X Fe Alm% Sps% Prp% Grs% X Pl Al/S

Geological Society of America Bulletin, v. 1XX, no. XX/XX 25

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.

There is evidence for a heterogeneous de-

Sill gree of melting throughout the Core Unit, due 0.00 0.24 0.00 0.00 0.13 0.986 0.000 1.999 0.000 0.014 0.006 0.000 0.004 0.000 0.008 3.018 0.27 0.62 5 99.58 36.16 62.16 TLN3 4 to lithological and bulk compositional controls Melanosome because of repetition of infertile orthogneissic

Pl basement along with more fertile metasedi- 0.14 0.00 0.00 2.20 2.909 0.000 1.091 0.000 0.000 0.007 0.000 0.104 0.860 0.008 4.979 0.19 0.00 0.107 0.091 Matrix TLN34 10.03 99.29 65.79 20.94 mentary horizons, due to prograde thrusting. y Samples TL66 and TL15 outcrop as stromatic

Ms migmatites with centimeter-scale folded leuco- 0.44 1.57 0.66 0.00 3.075 0.078 2.728 0.000 0.041 0.000 0.065 0.000 0.057 0.934 6.978 0.00 0.73 0.613 00 18 TLN34 11.03 95.63 46.34 34.86

Secondar some and melanosome banding, whereas samples TLN18 and TLN20A represent diatexites. Kfs 1.40 0.47 0.00 0.00 2.973 0.016 1.023 0.000 0.000 0.000 0.000 0.000 0.124 0.850 4.986 0.00 0.00 0 0.023 81 Melanosome foliations are defined mostly by 14.59 65.12 19.01 Matrix TLN34 100.59 muscovite, biotite, fibrolite (fibrous sillimanite), relict kyanite, and in some samples peritectic K- Grt 0.00 0.13 4.58 0.45 2.965 0.008 1.945 0.111 1.887 0.500 0.544 0.038 0.000 0.002 8.000 7.41 0.02 1.3 0.224 0.1 Core 37.23 20.71 63.9 18.1 30.00 16.7 12 TLN34 feldspar (Figs. 13G–13I). Leucosome segrega- 100.53 tions contain around equal proportions of quartz Grt 0.00 0.00 3.07 0.55 2.989 0.000 1.969 0.054 1.837 0.736 0.369 0.047 0.000 0.000 8.001 0.00 1.6 0.167 0 and plagioclase, with lesser amounts of biotite, Rim 10.78 37.10 20.73 61.6 12.3 28.07 24.5 12 TLN34 100.30 muscovite, fibrolite, garnet, and ilmenite, while K-feldspar occurs in other samples (e.g., TL59, l TLN18, TLN34, TLN35). Accessory minerals 7.66 0.10 0.00 0.00 6.23 2.715 0.003 1.291 0.001 0.000 0.000 0.000 0.294 0.654 0.010 4.968 0.18 0.04 0.307 0.289 TL66 61.66 24.87 100.73

Leucosome include tourmaline, apatite, magnetite, pyrite, zircon, and monazite. Leucosome segrega-

lP tions comprise ~10%–20% of the migmatite 6.80 0.00 0.00 0.00 6.64 2.521 0.000 1.558 0.003 0.000 0.000 0.000 0.315 0.584 0.020 5.001 0.36 0.07 0.343 0.517 88 TL66 56.95 29.84

100.66 sequence in outcrop. Diatexite only occurs at Melanosome the deepest levels and has foliations defined by schollen of biotite and some relict muscovite, tP 0.31 3.40 0.16 8.09 0.05 2.678 0.195 1.653 0.000 1.346 0.010 0.921 0.004 0.046 0.939 7.792 9.64 0.406

TL66 with leucosomes characterizing larger volumes 96.17 35.07 18.36 21.08 11

Melanosome of the rock (>20%).

y Garnet occurs in both the leucosome and sB melanosomes as small idioblastic porphyro 0.44 1.78 1.20 0.01 0.51 0.02 3.076 0.060 2.732 0.000 0.100 0.001 0.051 0.001 0.057 0.893 6.971 0.338 00 TL66 11 95.00 45.95 34.62 10.46

Secondar blasts <1 mm in diameter, displaying subhedral to anhedral faces (Figs. 13A, 13E, and 13F). In Int Gr tM 0.00 6.6 0.00 4.22 3.02 2.31 2.989 0.000 1.958 0.065 2.135 0.288 0.362 0.199 0.000 0.002 7.998 9.6 0.02 0.145 sample TL15, leucosome garnet preserves rota- TL66 71.7 12.1 37.12 20.63 32.68 12 YSES FROM CORE UNIT SAMPLES DERIVED ELECTRON MICROPROBE 100.00 tional inclusion trails, which define the 2aS fabric, that rotate into alignment with the external rt 0.00 6.4 0.20 1.77 3.78 2.24 3.003 0.012 1.931 0.037 2.247 0.121 0.454 0.193 0.000 0.000 7.998 4.0 0.00 0.168 00 Core TL66 99.49 74.4 15.1 37.27 20.33 33.90 12 matrix S2b fabric toward its rims (Figs. 14A and 14J), indicating garnet grew synkinematically with the development of these prograde features. Gr tG 0.13 6.5 0.00 4.03 2.94 2.25 2.985 0.000 1.951 0.111 2.103 0.274 0.352 0.194 0.020 0.010 8.000 9.1 0.10 0.143 0.1 Rim TL66 72.7 11.7 37.12 20.58 32.92 12 TIVE MINERAL ANAL 100.06 However, many of these porphyroblasts in silli- TA manite-grade samples are fractured and filled by Pl 8.72 0.26 0.00 0.00 0.00 5.08 2.766 0.000 1.224 0.010 0.000 0.000 0.000 0.241 0.749 0.005 4.995 0.242 0.226 0.08

TL15 biotite and muscovite intergrowths (Fig. 13F). 62.45 23.45 Matrix 100.040 In samples TLN34 and TL66, garnet is almost TIVE QUANTI

A compositionally homogeneous (Fig. 15F; Data Ms 0.44 1.54 1.37 0.06 0.88 0.00 3.134 0.069 2.643 0.000 0.086 0.003 0.087 0.000 0.057 0.854 6.933 0.503 18 TL15 95.096 47.07 33.67 10.06

Prima ry Repository Item [see footnote 1]). Muscovite in sample TL59 and TL66 occurs similarly as laths with associated fibrolite or as symplec- Bt 0.32 3.29 0.08 8.49 0.10 2.722 0.185 1.662 0.000 1.271 0.005 0.949 0.008 0.047 0.872 7.721 0.427 9.12 11 TL15 96.777 36.31 18.80 20.28 tic intergrowths with quartz (Figs. 13E–13H). Melanosome BLE 4. REPRESENT These intergrowths are often associated with TA either biotite-quartz symplectite or myrmekite rt Int 21 0.00 0.07 1.68 3.47 3.83 3.015 0.004 1.944 0.021 2.159 0.114 0.414 0.328 0.000 0.002 8.001 0.161 3.8 0.02 000 TL15 99.901 37.67 20.60 32.56 71.5 13.8 10.9 (plagioclase-quartz intergrowths). Secondary muscovite in leucosomes is oriented randomly relative to the foliation and fibrolitic sillimanite Gr tG 0.00 7.1 0.03 2.48 3.45 2.48 3.017 0.002 1.920 0.043 2.223 0.169 0.413 0.213 0.000 0.000 8 0.157 5.6 0.00 21 TL15 99.991 37.55 20.27 33.73 73.5 13.8

Outer rim in the matrix, and on the margins of leucosomes, it is typically surrounded by coarse-grained, rt 0.31 0.00 1.27 4.49 3.55 3.067 0.000 1.878 0.059 2.006 0.085 0.531 0.302 0.048 0.023 7.999 0.209 2.8 00 0.23 21 crosscutting muscovite (Figs. 14E–14F; Spear TL15 99.677 38.64 20.07 31.12 69.4 17.7 10.1 Inner rim et al., 1990). Biotite occurs as individual crys- tals in the leucosome, as part of the melanosome Gr tG 21 0.00 0.00 1.85 2.60 6.09 2.969 0.000 1.945 0.131 1.982 0.125 0.310 0.523 0.000 0.014 7.999 0.135 4.2 0.1 0.14 Core TL15 99.910 37.08 20.60 31.56 68.1 10.3 17.4 foliation, and also as symplectic intergrowths

s1 with quartz, but it shows no significant com- 3 %

2 positional variation between these petrographic O 2 O 2 Note: Mineral abbreviations follow Whitney and Evans (2010). Int—intermediate . 3+ 2+ 3+ O 2 O O 2 Mg Ca otals Alm% Mineral: Location: Si Ti Al FeO MnO MgO CaO Na K T Oxygen Si Ti Al Fe Fe Mn Mg Ca Na K Sum X Fe Sps% Prp% Grs% X Pl Al/Si Sample: positions with Mg/(Mg + Fe). In kyanite-grade

26 Geological Society of America Bulletin, v. 1XX, no. XX/XX

Downloaded from https://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/doi/10.1130/B31978.1/4719075/b31978.pdf by guest on 05 June 2019 Compressional origin of the Naxos metamorphic core complex, Greece – – – (°C) ± 1 S.D. 470 ± 30 670 ± 20 600 ± 15 720 ± 25 725 ± 25 670 ± 20 710 ± 20 690 ± 25 690 ± 15 T –– –– –– –– –– –– –– Pseudosection (kbar ) ± 1 S.D. 6.1 ± 0.5 5.5 ± 0.6 6.0 ± 0.6 8.0 ± 0.4 7.5 ± 0.3 7.7 ± 0.5 P 14.5 ± 0.5 10.0 ± 0.5 10.0 ± 0.5 –– – –– –– (°C) i-in-Bt T 578–740 570–712 586–713 578–740 586–713 616–731 616–732 651–751 651–751 458–699 616–733 589–690 589–690 612–674 615–709 temp. range albite in albite in peak M2 Peak M2 Peak M2 Peak M3 Peak M2 TION FOR ALL NAXOS SAMPLES Prograde- of melting interpretation sillimanite- Retrograde M1 albite in M2 kyanite- equilibration equilibration Prograde M2 T Postpeak M3 Postpeak M1 Postpeak M1 Prograde M2 Prograde M2 Postpeak M3 grade melting grade melting grade melting retrograde re- Postpeak M2- Postpeak M2- retrograde re- - Peak M2 onset high temp (M3) high temp (M3) onset of melting Postpeak M2 re- Peak M2 kyanite- P equilibration (M3) Cor r 0.61 0.98 0.988 0.984 0.951 0.979 0.986 0.985 0.819 0.686 0.778 0.812 0.854 0.770 0.517 0.774 0.983 0.693 0.673 .3.40i and dataset62 (Powell Holland, 1988; Holland Powell, (kbar ) 7.2 ± 2.1 5.5 ± 1.3 7.6 ± 1.4 7.3 ± 2.5 6.1 ± 1.0 8.9 ± 1.1 9.4 ± 1.3 7.5 ± 1.2 9.3 ± 1.3 8.1 ± 1.3 7.8 ± 1.1 6.2 ± 1.0 9.7 ± 1.1 7.7 ± 1.1 11.6 ± 2.2 12.6 ± 0.8 10.1 ± 1.9 11.4 ± 1.0 10.8 ± 1.6 avP ± 1 S.D. -PT AND PSEUDOSECTION CALCULA (°C) vPT 534 ± 134 598 ± 66 473 ± 35 695 ± 67 483 ± 13 465 ± 34 678 ± 122 625 ± 48 703 ± 69 682 ± 38 622 ± 68 742 ± 39 675 ± 83 607 ± 67 684 ± 118 726 ± 37 629 ± 47 710 ± 31 675 ± 32 avT ± 1 S.D. O 2 osection results using Thermocalc v Thermocalc A -H 2 -fact- -ts-daph- -cc-rt-sph -cc-rt-sph Pseud 2 2 y. O-CO O-CO 2 2 ts-parg-rt-q-an-ab ms-cel-q-ky-rt-ilm mu-ky-sill-q-rt-ilm mu-ky-sill-q-rt-ilm prp-alm-grs-tr Y AND THERMOCALC DS-62 AV fcel-phl-ann-an-ky-q prp-grs-alm-mu-cel- phl-ann-east-an-ky-q prp-alm-phl-ann-east- prp-alm-phl-ann-east- prp-alm-phl-ann-east- ilm-sph-rt-cc-CO cel-fcel-ky-sill-q-rt-ilm mu-cel-fcel-pa-phl-ab- ms-cel-fcel-an-q-rt-ilm End members included ab-q-H ab-q-H mu-cel-fcel-an-ky-ilm-r t gln-fgln-rieb-tr ames-ep-ta-fta-tats-clin- prp-grs-alm-mu-cel-fcel- cel-fcel-an-q-ky-sill-rt-ilm cel-fcel-an-q-ky-sill-rt-ilm ms-cel-fcel-an-q-ky-rt-ilm prp-grs-alm-phl-ann-east- prp-grs-alm-phl-ann-east- prp-alm-phl-ann-east-mu- prp-grs-alm-phl-ann-east- prp-grs-alm-phl-ann-east- prp-grs-alm-phl-ann-east- prp-grs-alm-phl-ann-east- prp-grs-phl-ms-cel-an-q-ky mu-cel-fcel-an-ky-sill-ilm-r t mu-cel-fcel-an-ky-sill-ilm-r t mu-cel-fcel-an-ky-sill-ilm-r t gln-fgln-ms-cel-fcel-pa-clin- prp-grs-alm-phl-ann-east-ms- prp-grs-alm-phl-ann-east-ms- O 2 H i-in-biotite thermometr 0.99 1.00 1.00 0.99 0.99 1.00 1.00 0.90 0.50 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 0.50–1.00 0.50–1.00 Note sX Grt int Grt int Grt int Grt rim Grt rim Grt rim Grt rim Grt rim Grt rim Grt rim Grt rim Grt rim Grt rim with ab with ab with ab Grt core S FROM THERMOBAROMETR Grt int-rim Grt core-int T ) RESU LT - y et al. (2005) for details of T P Location 37.036781°N , 25.493024°E 37.100794°N , 25.515643°E 37.16236°N , 25.50238°E 37.125787°N , 25.518516°E 37.125993°N , 25.517824°E 36.935434°N , 25.475521°E 36.999229°N , 25.486564°E 37.08808°N , 25.49797°E 37.08880°N , 25.47022°E 37.127084°N , 25.524576 37.16228°N , 25.50452°E 37.08022°N , 25.47802°E 37.06824°N , 25.44170°E TURE ( ), ilm ilm r r, r, Ms Ms , Czo, . , Bt, Ms, , Sill, Bt, Ms, , Bt, Ms, Kfs, , Bt, Ms, Kfs, ilm, Ap, Chl Pl, ilm, Qtz Mineralogy r, Qtz, Rt, Ilm Qtz, Chl, Rt Grt, Sill, Bt, ilm, Qtz, Tu seconda ry seconda ry Grt, Bt, Pl, Ms, Grt, Bt, Kfs, Pl, Grt, Bt, Kfs, Pl, Kfs, Pl, Rt, Qtz, Grt, Ky Rt, Sph, Qtz, Pl Pl, ilm, Qtz, Kf s Grt, Hb, Pl, ilm, Ep, Bt, Chl, Cal, Ms(seconda ry Tu Gln, Act, Tc results Act, Czo, Ms, Phg, Cal, Rt, Sph, Bt, Pl Pl, Rt, Qtz, Tu Pl, Rt, Qtz, Tu Qtz, ilm, Ap, minor Qtz, ilm, Ap, minor Grt, Sill, Bt, Kfs, Pl, Grt, Ky Grt, Ky Grt, Ky Qtz, Ms, Pg, Ep, Chl, T PRESSURE-TEMPERA Gln, Czo, Qtz, Ms, Phg, Pg, Chl, Cal, Rt, Sph, Pl t P - t s Summa ry schis t gneis s Garnet Kyanite - Kyanite - Rock type migmatite migmatite migmatite migmatite Muscovite - Sillimanite - Sillimanite - rutile schis t amphibolite dehydration Glaucophane Biotite-schis t Epidote-calc- Garnet-biotite Kyanite schis Kyanite gneis Mica-calc-schis sillimanite-gneis s ABLE 5. SUMMA RY T TL67 TL66 TL15 TL59 TLN54 TLN16 TLN21 TLN34 TLN26 TLN25 TLN22 TLN35 Sample 17TL75 Note : Mineral abbreviations follow Whitney and Evans (2010). See Henr Zas Unit Koronos Unit 2011), see text for further details on activity models used Koronos Unit Koronos Unit Koronos Unit Core Unit Zas Unit Unit Core Unit Zas Unit Core Unit Koronos Unit Core Unit Core Unit

Geological Society of America Bulletin, v. 1XX, no. XX/XX 27

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.

migmatites (e.g., TL15), kyanite is associated blasts containing sillimanite, or as symplec- support for high-grade metamorphic conditions,

with the S2 foliation, which is folded by F3 folds tic intergrowths with quartz (Figs. 13E–13H; as inferred from compositional and textural

(Figs. 14B–14D), indicating upright F3 folding Ashworth, 1975, 1979; Tippett, 1980). Textural characteristics in muscovite. Several authors postdated kyanite growth and occurred after the features in sample TL66 include muscovite laths have demonstrated that garnet compositional rocks attained their peak pressures. containing numerous fibrolite inclusions and zoning profiles form principally as a function of muscovite-quartz symplectite and intergrowths changes in pressure and temperature conditions Textures of Sillimanite-Grade Migmatites of sillimanite-quartz “faserkiesel,” which are during growth (e.g., Tracy et al., 1976; Spear and At midcrustal pressures in the upper am- typical of retrograde replacement of K-feldspar Selverstone, 1983; St-Onge, 1987). During pro- phibolite facies, the first appearance of anatectic at sillimanite-grade conditions (Figs. 13E–13G; grade metamorphism in a pelitic bulk composi- melt is usually triggered by the intersection of Evans and Guidotti, 1966; Ashworth, 1975; Tip- tion, garnet preferentially fractionates Mn into the wet granite or pelite solidus with destabili- pett 1980; Brown, 2002), i.e., the reversal of the core regions, and therefore spessartine content zation of prograde muscovite through the fol- muscovite dehydration melting reaction. decreases toward rims as a result of progressive lowing hydrous melting reactions (Pëto, 1976; Although peritectic K-feldspar is commonly Mn depletion in the reacting matrix (Hollister, St-Onge, 1984; Patiño Douce and Harris, 1998; lacking in the Naxos migmatites due to hydrous 1966). However, in the Naxos migmatites, gar- Weinberg and Hasalová, 2015): melting textures and retrograde replacement by net contains spessartine profiles that are homo- secondary muscovite, leucosomes in migmatites geneous or concave upward, with increasing Mn muscovite + quartz + plagioclase + from deep levels are volumetrically abundant in content from core to rim. The length scale of Mn

K-feldspar + H2O → melt, (2) outcrop, representing ~15%–25% volume pro- (and Mg, Fe, and Ca) zonation within the gar- portion of rock. This volume of melt greatly nets is less than the diffusional length scale (up muscovite + quartz + plagioclase + exceeds the proposed melt connectivity thresh- to 3 mm) suggested by >5 m.y. residence time

H2O → aluminosilicate + melt. (3) old of 7%–10% that allows melt extraction in at peak temperatures >650 °C (Caddick et al., such lithologies (Rosenberg and Handy, 2005). 2010). These factors indicate that preservation However, these reactions produce very small However, it has been shown that the observed of the prograde growth history will be modified volumes of melt, unless a substantial external leucocratic component of migmatites does not somewhat by diffusion, whereas peak conditions supply of fluid is available (Thompson, 1982; directly correlate with actual quantities of melt- will be preserved by thermal re-equilibration Pattison and Tracy, 1991). Nevertheless, volu- ing. At shallow dome levels, rocks that have (Woodsworth, 1977; Spear, 1993). Neverthe- metrically significant melting can occur with been described as leucosomes, leucogneiss, and less, the shape and trends of garnet profiles will continued prograde metamorphism above the diatexites are interpreted here as orthogneiss be somewhat preserved if peak conditions were wet solidus via the fluid-absent muscovite de- basement that coarsened upon experiencing attained for only a geologically brief time, and hydration melting reaction (Eq. 4). Due to the small degrees of partial melting (B. Dyck, 2016, therefore insight into the prograde metamorphic positive P-T slope of this reaction, at lower personal commun.). This has major implications history of the Core Unit may be obtained. There- temperatures and pressures, K-feldspar is also for the proposed mechanisms that drove migma- fore, in the following, we take peak pressure es- produced by its vapor-present equivalent reac- tite doming, as the weakening effect depending timates as a minimum depth of burial. tion (Eq. 5) at subsolidus temperatures. In the on the extent of partial melting may have been field, these reactions are often referred to as extremely overestimated. At structurally high Textures of Kyanite-Grade Migmatites the “upper sillimanite” or “second sillimanite levels of the dome, the stability of primary Kyanite-grade migmatites (e.g., TL15) oc- isograd” (Evans and Guidotti, 1966) due to the muscovite within the melanosomes supports cur at higher structural levels of the dome and formation of a K-rich neosome (García-Casco this interpretation, as they did not experience record the earlier prograde history. Texturally, et al., 2003): muscovite dehydration melting (the first major these are stromatic migmatites with distinct melt-producing reaction). Although diatexites leucosomes highlighting centimeter-scale ptyg- muscovite + quartz → K-feldspar + occur on Naxos, they are confined to fertile lith matic flow folding during anatectic conditions.

aluminosilicate + melt, (4) ologies at deeper levels upon crossing the K- In these samples, melanosomes preserve the S2a

feldspar isograd. crenulation cleavages and S2b spaced foliations muscovite + quartz → K-feldspar + Some inferences about the temperature of along cleavage domains similar to the overlying

aluminosilicate + H2O. (5) mineral growth can be made using the compo- Koronos Unit (Figs. 13C, 13D, and 13H). Kya-

sitional characteristics of different mineral gen- nite is bladed and lies in the folded S2 foliation, Several authors have observed that K-feld- erations. Prograde muscovite commonly con- indicating that it grew in Al-rich bands prior to spar is commonly absent from sillimanite- tains higher paragonite content than retrograde anatectic deformation associated with upright

grade migmatites despite being present at peak muscovite that has replaced K-feldspar (Evans F3 folding (Figs. 13B–13C). In contrast, some conditions, due to replacement by secondary and Guidotti, 1966; Ashworth, 1975). Gener- biotite and secondary muscovite in the melano- muscovite in the back reaction during cooling ally, low Si contents (<3.10 pfu per 11 oxygens) some are orientated axial planar to and crosscut (Ashworth, 1975; Tippett, 1980). By identify- combined with high Ti contents (up to 0.08 pfu) the folding, indicating they postdate deforma- ing different generations of muscovite through are characteristic of high-grade and magmatic tion and are mainly retrogressive features asso- various petrographic characteristics, sec muscovite (Miller et al., 1981; Guidotti, 1984). ciated with hydration contemporaneous with re- ondary muscovite can be distinguished. Tyler In all analyzed samples, leucosome muscovite gional NNE-SSW extensional exhumation. This and Ashworth (1982) suggested that prograde is low in Si and contains a moderate to high Ti specimen does not exhibit any of the K-feldspar (primary) muscovite that survives the partial content (~0.04–0.10; Fig. 15), indicating that replacement features described above, suggest- melting process often occurs as flakes orien- they formed during melt crystallization or retro- ing that its leucosomes were never of a granitic tated in the melanosome foliation. Retrograde gressively while still at high temperature. Com- composition, and consequently their formation muscovite occurs as large lath-like porphyro positional zoning profiles in garnet give further must be a result of a K-feldspar–absent melting

28 Geological Society of America Bulletin, v. 1XX, no. XX/XX

reaction. When H2O is in excess, K-feldspar– PRESSURE-TEMPERATURE Modeling of metasediments TL67, TL15, and

absent reactions extend to lower temperatures CONDITIONS OF METAMORPHISM TL66 was performed in the MnO–Na2O–CaO–

(García-Casco et al., 2001, 2003; Cruciani K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–O et al., 2008); increasing pressure also favors Several forms of thermobarometry were used (MnNCKFMASHTO) system using the follow- fluid-saturated melting reactions before reach- in this work to constrain the P-T conditions of ing activity-composition relations: silicate melt ing conditions that favor dehydration melting metamorphism on Naxos, including the Ti-in- (White et al., 2007), cordierite (Mahar et al., during typical prograde metamorphism (Cru- biotite thermometer of Henry et al. (2005), the 1997; Holland and Powell, 1998), garnet and ciani et al., 2008). Due to an inverse correla- garnet–aluminum silicate–plagioclase–quartz ilmenite (White et al., 2005), orthopyroxene tion between pressure and plagioclase stability, (GASP) barometer, the garnet-biotite thermo (White et al., 2002), chlorite (Mahar et al., 1997; an increase in the plagioclase/muscovite ratio barometer (Spear, 1993; Bhattacharya et al., Holland et al., 1998), Ti-bearing biotite (White consumed in melting reactions with increas- 1992; Holdaway, 2000), and AvPT (Powell et al., 2007), muscovite (Coggon and Hol- ing pressure (Patiño Douce and Harris, 1998), et al., 1998) using THERMOCALC version land, 2002), K-feldspar and plagioclase (Hol-

and increasing H2O activity lead to preferential 3.40i, which was employed using characteristic land and Powell, 2003), and magnetite (White solution of plagioclase components by lower- end members for each sample. The results are et al., 2002). Modeling of metabasic lithol ogies ing the plagioclase + quartz solidus (Conrad displayed in Figure 16. Activities of solid-solu- in TLN54 was performed in the MnO-absent et al., 1988). These factors result in a significant tion end members were calculated (Table 5) us- NCKFMASHTO system, with the additional

shift of products of H2O-fluxed melting toward ing AX (Holland, 2009). The results systemati- usage of the omphacitic clinopyroxene and tonalitic/trondhjemitic compositions and are cally demonstrate that there are two distinct P-T amphibole activity-composition relations of

typical of fluid-rich environments at high-pres- populations on Naxos. M1 blueschist-facies P-T Green et al. (2016). The pure phases andalusite,

sure and low-temperature conditions. conditions are exclusively confined to the Zas kyanite, sillimanite, rutile, quartz, and H2O were Garnet within leucosome segregations in Unit (Fig. 16A, blue circles) and give typical considered in all cases. The bulk-rock suprasoli- sample TL15 often demonstrates idioblastic P-T results of 12.3 ± 0.8 kbar and 483 ± 13 °C dus water content of migmatitic units was fixed faces (Fig. 13J), which, as a microstructural fea- (TLN54). In contrast, an upper-amphibolite- on an individual basis in order to allow minimal

ture in migmatites, is usually interpreted to rep- facies M2-M3 population is recorded within the fluid saturation at the wet solidus, here defined

resent growth in the presence of melt (Whitney Koronos and Core Units (Fig. 16A, green and as ~1 mol% free H2O. and Irving, 1994; Guilmette et al., 2011). This red circles, respectively). These rocks typically Detailed investigation into the prograde evo- inference is further supported by different core record increasing pressure and temperature con- lution and P-T conditions leading up to peak and rim compositions in large porphyroblasts, ditions from garnet core to rim. Kyanite-bearing metamorphism was achieved by investigating

and evidence for prograde growth zoning during M2 assemblages record higher pressures, up compositional isopleths for pyrope and gros-

development of S2 (Figs. 13A, 15C, and 15D), to 11.4 ± 1.0 kbar and 682 ± 38 °C (TL67). sular content in garnet. Since these isopleths

suggesting that garnet growth occurred in two Sillimanite-bearing M3 assemblages record re- vary to first order with changes in pressure and stages, each at different P-T conditions (Spear, equilibration at lower-pressure conditions, i.e., temperature and commonly intersect at high 1993). In the case of almandine and grossular 7.5 ± 1.2 kbar and 742 ± 39 °C (TL66). Within angles, they specify unique intersection points content in the measured profile, rim composi- these units, prograde garnet compositions show with a high degree of confidence for tracking tions are separated from core compositions increases in both pressure and temperature garnet composition evolution in P-T space. The by an abrupt step and only sometimes show a from core to rim diagnostic of crustal thicken- intersections of isopleths representing measured smooth transition (Figs. 15C and 15D), with ing and do not represent isobaric heating, as compositions are represented by shaded boxes the latter interpreted as a diffusion profile that previously inferred (Buick and Holland, 1989; indicating uncertainties at the 1σ level calcu- formed between two initially distinct composi- Buick, 1991a, 1991b). However, the nature of lated by THERMOCALC. tional domains (e.g., Caddick et al., 2010). This the P-T path leading up to and during peak ana- two-stage growth scenario is further supported tectic conditions is poorly constrained using this TLN54: Retrogressed Blueschist

by zonation in both Y and Yb (Fig. 15C), which method. This is because conventional thermo (M1 Zas Unit) show a stepwise decrease from core to rim, and barometry is largely ineffective at constraining small leucosome garnet grains that have similar peak P-T conditions in high-grade and/or par- Figure 17 shows the calculated P-T pseudo- compositions to rims of large porphyroblasts, tially melted rocks (e.g., Halpin et al., 2007; section for blueschist-facies sample TLN54. Us- indicating growth in the presence of melt. Diffu- Indares et al., 2008; Palin et al., 2013) owing ing the methods described in the Data Reposi- sion has only modified the outermost ~100 µm to changing mineral compositions due to retro- tory Item (see footnote 1), and the absence of of the rim, as demonstrated by an inflection in grade diffusion (Kohn and Spear, 2000; Pattison carbonate phases for the modeled area, the fluid

spessartine content, suggesting core–inner rim and Begin, 1994, 2003), and it has large asso- was assumed to have pure H2O in excess. The

zoning was prograde (e.g., Caddick et al., 2010). ciated uncertainties (typically ±50 °C and ±1 observed peak M1 assemblage Gl-Ms-Chl-Rt- For these reasons, we conclude that sample kbar at 1 standard deviation; Powell and Hol- Sph-Qtz was calculated to be stable at ~11–15 TL15 best records the prograde evolution of the land, 2008; Palin et al., 2016). As such, phase kbar and ~400–520 °C. Glaucophane isopleths Core Unit and the initial stages of partial melt- diagram modeling (pseudosection construction) of z(gl) = 0.96, f(gl) = 0.02–0.03, and a(gl) =

ing at higher-pressure M2 conditions, through was employed herein as the main investigative 0.05–0.06 suggest that peak M1 P-T conditions the following K-feldspar–absent hydrous reac- tool to harness P-T information, which has sig- reached at least 14.5 ± 0.5 kbar and 470 ± 20 °C. tion as proposed by García-Casco et al. (2001) nificantly smaller uncertainties. Model amphibole compositions match well with for similar kyanite-bearing pelitic migmatites: Pseudosections were constructed using observed sodic amphibole analytical data from THERMOCALC version 3.40i and internally both samples TLN54 and TLN25 (Fig. 15). biotite + muscovite + plagioclase + consistent thermodynamic data set ds-62 (Powell Ranges of AvPT results from samples TLN54,

quartz + H2O → garnet + kyanite + melt. (6) and Holland, 1988; Holland and Powell, 2011). TLN25, and TLN26 from the Zas Unit record

Geological Society of America Bulletin, v. 1XX, no. XX/XX 29

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.

Naxos average P-T results minimum P-T conditions of around 12 kbar and Ti in Bt (Henry et al., 2005) 450 °C, i.e., a similar result to peak metamor- 14 A M1 Zas Unit M2 Koronos Unit phic conditions of ~13 kbar and ~450 °C sug- Grt rim gested by Avigad (1998) for a jadeite-bearing peak M2 12 e anit blueschist within the Zas Unit, and comparable Ky e to the recently published results of 15.5 kbar and 10 Sillimanit 576 °C by Peillod et al. (2017).

bar) 8 TL67: Kyanite Gneiss (M2 Koronos Unit) Grt core

essure(k M3 Core Unit

Pr Sample TL67 reveals the prograde and peak 6 M2 Zas Unit P-T conditions for the Koronos Unit and is Figure 16. Visualization of shown in Figure 18. Using a pure H2O aqueous thermobarometry results 4 fluid, the predicted assemblages are incompati- from samples analyzed in Zas Unit ble with the observed textures and assemblages, this study. (A) Av-PT results 2 Koronos Unit particularly, the lack of evidence for melting using THERMOCALC Core Unit and subsolidus K-feldspar. Upon careful obser- version 3.40i for all samples 0 vation, the existence of graphite and dark CO2 with overlaid Ti-in-biotite 400 500 600 700 800 900 fluid inclusions in garnet and quartz indicate thermometry results (see Temperature (oC) circulation of CO2 fluids during peak metamor- Table 5; see Data Reposi- Grt-Als-Bt-Pl thermobarometry results phism. A reduction in water activity, a(H2O), tory Item [text footnote 1] causes the dehydration equilibria that underlie 14 B M2 peak Ky for more discussion). (Grt innner rim) prograde processes in metasedimentary rocks (B) Results from Grt-Ky/ to be displaced to lower temperature. Although 12 Sill-Plag-Qtz geobarom- e estimating the actual value of water activity anit etry (Spear, 1993) com- Ky e in a rock system is difficult, some constraints bined with Grt-Bt Fe/Mg 10 Sillimanit can be placed if the rock contains graphite. exchange thermometry The coexistence of solid carbon with a C-H-O

bar) (Bhattacharya et al., 1992) 8 fluid has three degrees of freedom, so that one showing an increase in both Grt Core further constraint, such as oxygen fugacity, po- TLN22 pressure and temperature essure(k

Pr tentially allows the concentration of all C-H-O 6 from garnet core to rim; TLN21 fluid species to be calculated at any given P and M3 Sill TLN34 and TL66 record TL67 T. Therefore, an investigation of the effect of (Core Unit) lower pressures due to dif- 4 water activity on the calculated phase relations TL15 fusional homogenization was conducted (Fig. 18B). Using this method, of garnet compositions. TL66 2 a reduced a(H2O) = 0.5 was chosen to repre- (C) Ti-in-biotite thermom- TLN34 sent fluids during peak metamorphism, which etry (Henry et al., 2005) 0 corresponds to the findings of previous studies results for Naxos high- 400500 600700 800900 of metamorphic fluids on the island (Kreulen, grade samples (Koronos Temperature (oC) 1980; Baker et al., 1989). This has been shown and Core Unit); note the Ti in Biotite thermometry 0.7 to shift the solidus to higher temperatures (Pëto, range in temperatures due 80 C 0 1976; Peterson and Newton, 1989; Stevens and o TL66 C to thermal re-equilibration TL15 Clemens, 1993; Weinberg and Hasalová, 2015), of particularly high-grade 0.6 TL67 stabilizing K-feldspar at subsolidus tempera- rocks during retrogression. TLN22 TLN34 tures as shown in Figures 18B and 18C (New- Mineral abbreviations fol- 0.5 TLN21 ton, 1989). TLN16 low Whitney and Evans 700 °C For a localized Al-rich domain, composi- (2010). TL59 ) TLN35 tional isopleths representative of garnet core do- 0.4 TLN20A mains (~10% pyrope and ~11% grossular) inter- 22 (O sect at ~6.5 kbar and 550 °C in the assemblage pe r

Ti 0.3 field Grt–Ms–St–Bt–Pl–Ilm–Qtz–H2O, and 600 °C those representative of the inner rim (14%–15% pyrope and 13% grossular) intersect at 10.5 0.2 500 °C kbar and 675 °C in the assemblage field Grt–

Ms–Bt–Pl–Ilm–Rt–Qtz–H2O. This is consis- 4 0.1 00 °C tent with temperature estimates of 630–710 °C calculated from the Ti concentration of biotite 0 (Henry et al., 2005) and similar to pressures cal- 0.20.3 0.40.5 0.60.7 0.80.9 1 culated by Duchene et al. (2006); therefore, we X(Mg) take these estimates to represent peak M2 P-T

30 Geological Society of America Bulletin, v. 1XX, no. XX/XX

conditions. Garnet outermost rims (13%–14% TLN54 Zas Unit Retrogressed Blueschist NCKFMASHTO a(H2O) = 1.0 A SiO2 TiO2 AL2O3 FeO MgO CaO Na2O K2O O pyrope and 10% grossular) have isopleth inter- 60.101 4.158 13.027 7.878 6.409 1.087 4.534 2.675 0.132 All Assemblages + Qtz + Excess H2O 16.0 sections at 7.5 kbar and 655 °C and cross the 5 6 TLN54 Gl Law Chl = 0.06 -Sph muscovite dehydration reaction to produce K- 15.0 Ms Rt feldspar. This is associated with an increase in a(gl) .07 a(gl) = 0.05 0 24 Gl Jd Law Chl Ms Rt f(gl) = 0.02 25 garnet mode, which is consistent with garnet 14.0 Gl Ms Rt Sph rims crosscutting the S kyanite-grade fabric. 4 a(gl) = Gl Jd Ms 2 +Gl -Bt 13.0 f(gl) = 0.03 TLN54 -Chl Ab Rt 16 Garnet-rim upward inflections in Fe/(Fe + Mg) Gl peak Q 95 . +Jd 17 and spessartine content are also characteristic -Sph 0 15 12.0 Gl Jd Ms = 0.96 26 l) = 0.94

features of retrograde replacement by biotite z(gl) = 0.97 -Ab bar) Gl Chl g Ab Rt z(gl) = ( z(gl) = 0.93 z z(gl) (Kohn and Spear, 2000), and they suggest some 11.0 +Jd Ms Rt Sph rt

z(gl) = 0.98 Sph +G garnet was resorbed during retrogression. Jd Chl Ms 14

essure (k essure 18

Pr 10.0 3 Gl Rt Sph -Ab Ep Ms Bt Rt Ab 19 TL15: Kyanite Migmatite (Core Unit): 2 8 13 1 11 Ep Ms Bt Rt Sph Ab 9.0 Ab Ms Bt Peak (M2) Kyanite-Grade Conditions 7 9 12 Ms Bt -Bt Gl Ms Chl Rt 10 Rt Sph 20 Pl Ab +ilm Sph Ab 8.0 -Chl Rt Sample TL15 records the prograde evolu- +Gl Ms Bt Pl Ab Ep Ms Rt ilm tion of the Naxos migmatites, and the corre- 7.0 Bt Rt Sph sponding pseudosection is shown in Figure 19. Chl Ms Bt Rt Sph Ab Chl Ab Ep Ms Bt Rt Chl Ab Isopleths for garnet core compositions (10% 6.0 21 Chl Ms Bt Pl Ab Rt 23 pyrope and 13%–16% grossular) indicate pro- 5.0 22 grade metamorphism through the Grt–Ms– 350400 450 500 550 600

Bt–Pl–Ilm–Chl–Qtz–H2O assemblage field at Temperature (oC) ~6 kbar and ~550 °C. During prograde meta- B 16.0 morphism, significant garnet is predicted to 5 6 TLN54 Gl Law Chl Ms Rt Q -Sph have formed via staurolite breakdown (e.g., 15.0 Eq. 1). Garnet compositions just within the in- TLN54 24 Gl Jd Law Chl Ms Rt 25 ner diffusion-affected porphyroblast rims pro- 14.0 Gl peak Gl Ms Rt Sph vide minimum constraints on the conditions 4 Gl Jd Ms-B t +Gl 13.0 -Sph AvPT TLN54 -Chl Ab Rt 16 of peak metamorphism (Caddick et al., 2010). +Jd 17 Isopleths for 17% pyrope and 9–10% grossular Gl Chl Ms Rt Q Sphh 15

12.0 Gl Jd Ms ) bar -Ab intersect at ~9.5 kbar and 700 °C in the Grt– AvPT TLN26 Ab Rt 26 Ms–Bt–Pl–Ilm–Qtz–Liq–H O–Ky assemblage +Jd Sph t 2 11.0 Gr Jd Chl Ms + field, indicating that prograde metamorphism (k essure AvPT TLN25 14

Pr Gl Rt 18 10.0 3 Sph occurred along a heating path that crossed the -Ab Ep Ms Bt Rt Ab 19 water-saturated solidus (Eq. 6) at a minimum of 2 8 131 1 11 Ep Ms Bt Rt Sph Ab 9.0 Ab Ms Bt 8 kbar and 680 °C. High-pressure melting is in 7 9 122 Ms Bt -Bt Gl Ms Chl Rt 10 Rt Sph 20 Pl Ab +ilm agreement with the presence of prograde mus- Sph Ab 8.0 -Chl Rt covite, bladed kyanites, and small proportions +Gl Ep Ms Ms Bt Pl Ab (1%–4%) of melt distributed along rounded 7.0 Chl Ms Bt Rt Sph Ab Bt Rt Sph Rt ilm grain boundaries, although peak pressures may Chl Ab Ep Ms Bt Rt Chl 6.0 21 Chl Ms Bt Pl have been higher due to diffusional re-equili- Ab Ab Rt 23 bration (Caddick et al., 2010). As this rock 5.0 22 lies within the same unit as sillimanite-grade 350400 450 500 550 600 migmatites, it is likely that this rock experi- Temperature (oC) enced the same prograde path as the underly- TLN54 Phase Field Assemblages ing sillimanite-bearing migmatites experienced 1 Gl Ms Chl Rt Ab Jd Sph Qtz 10 Gl Ms Bt Chl Rt Qtz Ab 19 Grt Ms Bt Rt Qtz Ab Pl before obtaining higher temperatures. Due to 2 Gl Ms Bt Chl Rt Ab Jd Sph Qtz 11 Gl Ms Sph Rt Qtz Ab 20 Ms Bt Ep Rt Qtz Ab Pl the lack of sillimanite in this rock, but locations 3 Gl Ms Chl Rt Law Jd Sph Qtz 12 Jd Ms Bt Sph Rt Qtz Ab 21 Chl Ms Bt Ep Rt Qtz Ab Pl within tens to hundreds of meters of the silli- 4 Gl Ms Chl Rt Law Sph Qtz 13 Gl Jd Ms Bt Sph Rt Qtz Ab 22 Chl Ms Bt Ep Rt Sph Qtz Ab Pl 5 Gl Ms Rt Law Qtz 14 Gl Jd Ms Sph Rt Qtz Ab 23 Chl Ms Bt Rt ilm Qtz Ab Pl manite isograd, this rock may have crossed the 6 Gl Ms Rt Law Sph Qtz 15 Jd Ms Bt Sph Rt Qtz Ab 24 Gl Ms Rt Qtz kyanite-sillimanite transition at peak conditions 7 Gl Ms Rt Qtz Ab 16 Jd Ms Rt Qtz Ab 25 Gl Jd Ms Rt Qtz via an indirect Carmichael reaction scheme, 8 Gl Ms Bt Rt Qtz Ab 17 Jd Ms Bt Ep Rt Qtz Ab 26 Grt Ms Bt Ep Rt Qtz Ab but it preserves no textural evidence owing to 9 Gl Ms Bt Sph Rt Qtz Ab 18 Grt Ms Bt Ep Rt Qtz Pl Ab its sluggish reaction kinetics. Overall, peak P-T conditions of ~730 ± 20 °C and 9.5 ± 0.8 kbar Figure 17. Pressure-temperature (P-T) pseudosection for sample TLN54 from the Zas Unit, obtained from sample TL15 are consistent with using the observed bulk composition, where blue arrows and dashed lines present suggested peak conditions for sample TL59 (see Data P-T paths taken by this rock. (A) Glaucophane compositional isopleths of f(gl), z(gl), and a(gl) Repository Item [see footnote 1]). Therefore, intersect at peak conditions of ~14.5 kbar and 470 °C. (B) Overlain Av-PT results from this burial of the Core Unit occurred under kyanite- and other samples from the Zas Unit. Mineral abbreviations follow Whitney and Evans (2010).

Geological Society of America Bulletin, v. 1XX, no. XX/XX 31

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.

Figure 18 (on following page). Calculated pressure-temperature (P-T) pseudosections for grade (M2) conditions of 680–730 °C and minimum pressures of 8–10 kbar, similar to condi- sample TL67 from the Koronos Unit, using the observed bulk compositions, where compo- tions of 670 ± 31 °C and 10 ± 0.5 kbar in the sitional isopleths of grossular (Grs) and pyrope (Prp) for garnet are blue and red, respec- overlying Koronos Unit. This should be taken tively. (A) Calculated pseudosection for TL67 in the presence of pure water, predicting peak as a minimum pressure, and the apparent pres- conditions suprasolidus. (B) Temperature vs. water activity plots at 9 and 10 kbar showing sure inversion between the Core Unit and the the predicted change in assemblages in response to water activity, where water activity of Koronos Unit could be explained by diffusional 0.5 relates to the occurrence of Ms + Ky + Kfs at reasonable temperatures from independent homogenization between two contrasting gar- Ti-in-biotite thermometry. (C) P-T pseudosection of TL67 for a water activity of 0.5 showing net compositions at high-grade conditions. The key phase fields. (D) Compositional isopleths for garnet showing core compositions inter- textural relationships of kyanite aligned and secting at ~6 kbar and 560 °C, whereas inner rim compositions representing peak conditions within the upright folded migmatitic foliation intersect at ~10 kbar and 680 °C. (E) Garnet mode isopleths showing garnet growth with both increasing pressure and temperature, but upon crossing the muscovite dehydration suggest that upright (F3) folding along a N-S axis occurred immediately following peak reaction, garnet mode isopleths are almost isothermal, indicating garnet growth by isobaric heating proposed for small posttectonic garnets, e.g., TLTN22. Mineral abbreviations follow M2 kyanite-bearing conditions. Therefore, the E-W–directed compressional deformation that Whitney and Evans (2010). produced these folds took place at lower pres-

sures in the sillimanite stability field (M3), for which the P-T conditions were constrained using garnet rim compositions affected by retro- At subsolidus conditions, both models pre- biotite + sillimanite + quartz + grade equilibration. These compositions (13% dict low proportions of garnet, consistent with plagioclase → garnet + muscovite. (7) pyrope and 6% grossular) suggest P-T condi- the sample, and a garnet core composition (14% tions of ~7 kbar and 710 °C (Fig. 18), signi- pyrope, 9% grossular) plotting within the Grt– In this case, 11% volume K-feldspar could

fying an almost isothermal decompression P-T Ms–Bt–Ky–Ilm–Pl–Qtz–H2O field at 6.0 kbar be attained with just 14% volume muscovite path during exhumation, cooling at ~3 °C km–1. and 610 °C. This is within 0.3 kbar of the garnet- in a simple 1:1 molar ratio. Production of sec producing reaction, probably reflecting delayed ondary muscovite through this reaction would TL66: Sillimanite Migmatite (Core Unit): garnet nucleation. The model predicts a prograde also account for less sillimanite and biotite ob-

Peak (M3) Sillimanite-Grade Conditions P-T path through the kyanite stability field, ac- served than predicted (8% observed compared counting for kyanite inclusions in muscovite. to 10% predicted) by production of secondary TL66 was collected from the deepest parts of At suprasolidus conditions, the externally open garnet and muscovite. However, due to diffu-

the dome across the K-feldspar isograd, and bulk system H2O model predicts K-feldspar absence sional equilibration at high temperature, it is compositions were calculated from the observed in the presence of high melt fractions, consistent unlikely that garnet rim compositions represent mineral proportions combining both leucosome with faserkiesel textures and diatexites. Garnet peak temperatures. Ti-in-biotite thermometry and melanosome. Leucocratic networks within rim compositions (15% pyrope, 6% grossular) also predicted most biotite homogenized at tem- and radiating from the Naxos high-grade core plot at 4.9 kbar and 705 °C with 33% melt. It peratures of 650–700 °C, which is unlikely to combined with the partial preservation of gar- is unlikely this rock experienced such extensive represent peak temperatures. However, AvPT net suggest that some melt was lost from the melting, and the presence of kyanite preserved predicted garnet innermost rims with represen- rock system during its P-T evolution (White and as inclusions within muscovite suggests little tative matrix minerals compositions of 7.5 ± 1.2 Powell, 2002). Therefore, this bulk composition fluid infiltration above the solidus. kbar and 742 ± 39 °C (Table 5), in agreement is appropriate for modeling P-T conditions after For the closed system case, less melt is with predicted garnet modal proportions. Based the loss of that melt fraction (i.e., it is valid for predicted (20%), and the garnet rim composi- on these lines of reasoning, we are confident

assessment of peak/near-peak and suprasolidus tions plot at higher pressure and temperature these results represent peak M3 sillimanite- retrograde metamorphism, but not the prograde (6.0 kbar, 720 °C), in the stability field of the grade conditions. subsolidus evolution prior to melt loss). Phase Grt–Kfs–Bt–Sil–Pl–Ilm–Qtz–Liq assemblage

equilibria with excess H2O were calculated for beyond the muscovite dehydration melting re- Melt Reintegration (Prograde Evolution) subsolidus and suprasolidus conditions, rep- action. This is consistent with the abundance of To account for leucogranites that emanate resenting an external flux of water to produce secondary muscovite produced through the back from and throughout the Core Unit, it is likely maximum melting. The resulting pseudosec- reaction by eliminating all K-feldspar shown in that the Naxos migmatites have experienced tions are presented in Figures 19D and 19F, re- Equation 4. Assuming a molar 1:1 ratio between some degree of melt loss. This occurs because vealing complex subsolidus topologies, whereas K-feldspar and muscovite, it was calculated that once melt accumulation reaches a critical the suprasolidus phase relationships are simple, 18% by volume K-feldspar would be required threshold, a portion may be lost, creating an with large areas of P-T space without a change to produce all secondary muscovite (24% by open-system environment (Vanderhaege, 2004). in variance. A closed system model, more real- volume). However, only 11% K-feldspar is pre- This melt composition is dependent on the P-T istic melting, was calculated by analyzing the dicted to coincide with garnet rim compositions, conditions of formation (White and Powell, water content at the hydrous solidus. It was de- suggesting secondary muscovite formed by a 2002), and melt extraction events may occur in

termined that 7.6% H2O was required to ensure more complicated process. Release of fluids an interrupted or continuous process (Sawyer,

all H2O enters melt at the solidus, leaving water during crystallization through the solidus rehy- 1996). This open-system behavior, combined undersaturated at suprasolidus conditions. The drates the rock associated with the assemblage with the uncertainty concerning the proportion calculated bulk compositions are shown in the Grt–Bt–Ms–Pl–Qtz–Ilm and may account for and composition of melt lost, makes recovering Data Repository (see footnote 1), along with as- more muscovite production through retrograde a bulk composition of the protolith very unlikely sociated pseudosections. water-conserving reactions such as: (Palin et al., 2013). However, White and Powell

32 Geological Society of America Bulletin, v. 1XX, no. XX/XX

TL67 Koronos Unit MnNCKFMASHTO SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O O H2O 60.930 2.127 12.349 7.415 0.337 3.504 2.086 1.568 2.821 0.361 6.500 A TL67 a(H2O) = 1.0 Pseudosection All Assemblages +Qtz B 12.0 T - a(H2O) diagram at 9 Kbars T - a(H2O) diagram at 10 Kbars Grt Bt Ms Rt 18 20 Grt Ms ilm Rt 850 +Melt s 19 TL67 Pl H2O 17 Gr Pl Kfs Liq TL67 TL67 .16 s Grt Ky kfs Rt Grt Bt Ms Rt 0 G Grt Bt Ms +Melt +Melt 11.0 16 5 s ilm Pl Liq Grt Ky Kfs Pl So Pl ilm Liq 0.15 Gr4 G Rt Pl ilm Solidu Grs ilm Rt H2O lidu 0.14 Gr rs Kfs p 0.13 G Soli .12 s Liq 800 Grt Ky Kfs Pl ilm Rt H2O s Grt Bt Ms Rt s p 0 Gr p 1 rp rpp 1 Grt Ky Bt Ppr Pr P 1 14 dus 10.0 Pl ilm H2O 0.110.0 Pr 2 Pr 0. 1 rs kfs Rt ilm .13 rp G Grt Sill kfs Rt 0.10 0.0 0 P rp 10 P 0. Pl Liq ilm Pl Liq -Bt 0.14 5 +Kfs 0.1 C) -Bt Grt Bt Ky 9.0 Grt Sill Bt kfs Rt O 1 ( Kfs Pl ilm Grt Ky Bt ilm Pl Liq 750 Grt Bt Ky p

bar) 13 Rt H2O Pr Ms Rt Pl 12 Kfs Pl 6 ilm Liq .1 ilm Rt 0 21 Grt Bt lting H2O 8.0 0.15 24 Ms Ep 8 Pr p Grt Sill Bt +Kfs sure (k sure 7 26 22 Me -Ms Pa Pl 2 0.14 Pr p Ms Pl +Ky es 6 Rt ilm 23 ilm Liq 700

mperature Te Pr 3 25 Gr t Ky Bt Ms Pl 7.0 H2O dration 0.13 Prp Grt Sill Bt kfs ilm Pl Liq Rt ilm H2O Grt Sill Bt hy De Grt Bt Ms Pl Ms Pl 4 Grt St Bt Ms 0.12 Prp Ms -Ms ilm Rt H2O 6.0 Grt Bt Grt St ilm Pl ilm H2O 15 Grt Ky Ms Ep Bt Ms H2O 650 Grt Bt Ms Pl ilm Rt H2O Pa Pl Ep Pl Bt Ms +Ky ilm ilm 9 11 Grt Sill Pl ilm 5.0 H2O H2O kfs ilm Rt Pl Liq H2O Grt Ky Ms Kfs Pl Grt Ky Ms Kfs Pl ilm Rt H2O 10 ilm Rt H2O 4.0 5 600 600 650 700 750 800 850 000.25 0.50 .75 1.00 000.25 0.50 .75 1.00

O a(H2O) p p a(H2O) Temperature ( C) r Pr

C TL67 a(H2O) = 0.5 Pseudosection All Assemblages +Qtz +H2O D TL67 a(H2O) = 0.5 Pseudosection 15 P 12.0 12.0 0.14 0. Grt Ms Bt Pl s 4 Grt Peak p r Grt Ms Pa r rp TL67 TL67 P G Rt 18 P 2 3 Bt Pl Rt Grt Kfs Ky Pl ilm Rt 13 .1 +Ky 0 0.1 0. s 11.0 Grt Ms Pa Bt 11.0 p p rs Pr G Ep Rt -Bt Pr p 12 Gr 19 r .17 GGrsGr s P 0 6 Gr 0. rs 1 5 G 5 -Ms +Melt 0.10 0.1 .15 s 0.09 11 0 s 3 s 10.0 10.0 0. 14 rsr 0.11 6 Grt Ms Bt Pl ilm Rt Grt Kfs Ky 0.14 GrG Grt Kfs Sill 3 10 Gr rs Bt Pl ilm Rt 0.13 0. G Pl ilm Rt 09 0. 9.0 s 9.0 Grs

08 )

ar +Kfs 0. b Grt Ms Grt Ms Grt Kfs Sill bar) Grt Intermediate 7 Grs Grt Ky Ms Bt Solidu Pa Ep Pa Ep +Rt Bt Pl ilm Rt 0.0 idus Bt ilm Pl ilm Rt ol 8.0 Bt Pl S

ure (k ure 8.0 Grt Kfs (k ure Rt +Ky 21 6 Grs ilm Rt 7 20

ess Grt Core 0.0

Sill Pl ess

2 rs Pr ilm + Melt Pr 0.08 Pr s Grs 7.0 13 7.0 s 05 + Melt 12 0.12 G 0. 8 Grt Ky Ms Bt p 0.11 Gr 0.07 0.10 Gr Grs Grt outer rim

Pl ilm Pr 0.09 -Ms p 6.0 +Ky 6.0 +Rt Grt St Ms Grt Kfs Sill Bt Pl ilm -Bt 0.08 Grs 9 14 22

Bt Pl ilm 0 0

0. 0

.18 .

. 1

0 0 0 9 9

23 0.0

Grt Kfs Crd 0.07 Grs 1 17

. 5

. .

5.0 . 1

Grt Sill 5.0 4

1 1 10 1

2 1 P P Sill Pl ilm Pr

rp r

r P

Ms Bt r

P P

Grs P p

1 15 P p

+Kfs r p

r r r

p p 11 p

Pl ilm 0.06 p 4.0 16 17 24 4.0 450 500 550 600 650 700 750 800 850 900 450 500 550 600 650 700 750 800 850 900 Temperature (OC) Temperature (OC)

E TL67 a(H2O) = 0.5 Pseudosection TL67 a(H2O) = 1.0 Phase Field Assemblages 12.0 Grt Peak TL67 1 Grt Bt Ms Pa Rt ilm Pl Qtz H2O 14 Grt Bt Ky Kfs Ms Rt ilm Pl Qtz Liq 2 Grt St Bt Ms Ep Rt ilm Pl Qtz H2O 15 Grt Bt Sill Ms ilm Pl Qtz Liq H2O 11.0 3 Grt St Bt Ms Ep ilm Pl Qtz H2O 16 Grt Bt Ms Rt ilm Pl Qtz Liq H2O 4 Grt St Bt Ms Ep Pa ilm Pl Qtz H2O 17 Grt Bt Ms Rt Pl Qtz H2O 10.0 5 Grt Bt Ms Pa ilm Pl Qtz H2O 18 Grt Bt Ms Rt Pl Qtz H2O 6 Grt St Bt Ms ilm Pl Qtz H2O 19 Grt Ky Kfs Ms Rt ilm Pl Qtz Liq 9.0 7 Grt St Bt Ms Rt ilm Pl Qtz H2O 20 Grt Bt Ms Rt Pl Qtz H2O

s bar) Grt Intermediate 8 Grt Bt Ms Rt Pl Qtz H2O 21 Grt Bt Ky Ms Rt ilm Pl Qtz Liq H2O

0.10 Gr

0. 9 22 8.0 t Solidu Grt Sill St Bt Ms ilm Pl Qtz H2O Grt Bt Sill Ms Rt ilm Pl Qtz Liq H2O

ure (k ure t t 09 Gr 0

.08 Gr Grt Core 09 Gr.08 Gr rt 10 Grt Bt Sill Kfs Ms ilm Pl Qtz H2O 23 Grt Bt Sill Ms Rt ilm Pl Qtz H2O ess . G 0 0 t

.10 Gr t Pr 0 0.07 rt t 11 Grt Bt Sill Kfs Ms ilm Pl Qtz Liq 24 r t + Melt Grt Bt Sill Ms Rt ilm Pl Qtz Liq 7.0 0.06 Gr 0.07

0.06 G05 G 0.05 Gr 0. rt 0. 12 Grt Bt Sill Ms Rt ilm Pl Qtz Liq 25 Grt Ky St Bt Ms ilm Pl Qtz H2O G Gr Post-tectonic 04 t t garnets 0.04 t

Gr 0.03 Gr 13 Grt Bt Sill Kfs Ms Rt ilm Pl Qtz Liq 26 Grt Ky St Bt Ms Rt ilm Pl Qtz H2O

rt t 6.0 G 0.03 Gr 0

.02 0.02 0.0 rt TL67 a(H2O) = 0.5 Phase Field Assemblages G t 1 Gr

01 Gr

. 0. t

5.0 t 1 Grt Bt Ms Rt Pl Qtz H2O 13 Grt Bt Ms St Ky Rt ilm Pl Qtz H2O 2 Grt Bt Ms Pa Ep Chl Rt ilm Qtz H2O 14 Grt Bt Ms St Ky ilm Pl Qtz H2O 4.0 3 Grt Bt Ms Pa Ep Chl Rt Qtz H2O 15 Grt Bt Ms St Sill ilm Pl Qtz H2O 450 500 550 600 650 700 750 800 850 900 4 Grt Bt Ms Pa Rt Qtz H2O 16 Grt Bt Ms St And ilm Pl Qtz H2O O Temperature ( C) 5 Grt Bt Ms Ep Rt Qtz H2O 17 Grt Bt Ms And ilm Pl Qtz H2O 6 Grt Bt Ms Pa Pl ilm Rt Qtz H2O 18 Grt Bt Ms Kfs Rt ilm Pl Qtz H2O 7 Grt Bt Ms Ep ilm Rt Qtz H2O 19 Grt Bt Ms Ky Kfs Rt ilm Pl Qtz H2O 8 Grt Bt Ms Ep ilm Rt Pl Qtz H2O 20 Grt Bt Ms Ky Kfs ilm Pl Qtz H2O 9 Grt Bt Ms Pa Ep ilm Pl Qtz H2O 21 Grt Bt Ky Kfs ilm Pl Qtz H2O 10 Grt Bt Ms Ep ilm Pl Qtz H2O 22 Grt Bt Ms Sill Kfs ilm Pl Qtz H2O 11 Grt Bt Ms St Ep ilm Pl Qtz H2O 23 Grt Bt Sill Kfs ilm Crd Pl Qtz H2O Figure 18. 12 Grt Bt Ms St Rt ilm Pl Qtz H2O 24 Grt Bt Kfs ilm Crd Pl Qtz H2O

Geological Society of America Bulletin, v. 1XX, no. XX/XX 33

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.

(2002) showed that various melt-loss scenarios Figure 19 (on following page). Calculated pressure-temperature (P-T) pseudosections for do not change the major subsolidus and supra- samples TL15 and TL66 from the Core Unit, with garnet compositional isopleths for pyrope solidus topologies or compositions of peritec- (Prp) in blue and for grossular (Grs) in pink and modal proportion of garnet plotted in tic phases. It is therefore possible to calculate yellow. (A) TL15 pseudosection using the observed assemblage and pure water activity an approximate bulk composition suitable for showing key phase fields. (B) Isopleths for garnet compositions show a clockwise P-T loop prograde modeling. To reduce uncertainty, and suggest that the P-T path never crossed the muscovite dehydration melting reaction. the melt loss was modeled as a single extrac- (C) Isopleths for garnet mode and proportions of melt, note the rapid increase in partial tion event, representing a simple end-member melt upon crossing the muscovite dehydration melting reaction. (D) TL66 pseudosection case. A melt-reintegrated bulk composition was using the observed assemblage and pure water activity showing phase relations. (E) TL66 therefore calculated by the addition of a propor- isopleths for garnet compositions only showing the retrograde part of the P-T path through tion of melt to the observed bulk composition to the sillimanite stability field due to garnet homogenization and resorption. (F) Garnet mode and melt fraction isopleths for TL66; note isothermal decompression through the muscovite displace the solidus down-temperature to H2O- saturated conditions, maximizing the amount dehydration melting reaction is expected to decrease garnet mode and cause resorption. of mica present prior to melting following the Mineral abbreviations follow Whitney and Evans (2010). method of White et al. (2004). The composition of reintegrated melt was calculated at 7.75 kbar and 680 °C by using the solidus of the original bulk composition in the presence of kyanite. of only ~1.5% garnet, we conclude temperatures and 470 °C followed only by retrograde green- Although higher pressure than final melt crys- did not increase significantly upon passing this schist-facies conditions at <450 °C. In contrast, tallization, this P-T value was chosen because melting reaction, and thus it is likely that cross- at deeper structural levels (Koronos and Core

it accounts for the presence of kyanite-bearing ing the effective solidus was largely related to Units), (M1) high-pressure–low-temperature con migmatites and inclusions in secondary musco- decompression at peak metamorphic conditions. ditions are not recorded, and instead the rocks

vite at higher structural levels (TL15), allowing experienced a clockwise P-T path involving (M2) for a clockwise P-T evolution. It was deter- Postpeak Suprasolidus Evolution prograde burial and heating along a Barrovian mined that 10% melt should be added to make Crystallization of the final melt fraction upon metamorphic thermal gradient of ~25 °C km–1 to

the solidus water-saturated. crossing the solidus is thought to mark the fi- peak (M2) P-T conditions of ~10 kbar and 600– Contours for modal proportion of silicate melt nal retrograde textural evolution of an anatectic 730 °C (TL15, TLN22, TL67). At the deepest are presented in the Data Repository material rock, if no further episodes of deformation or levels (Core Unit), the rocks crossed the water- (see footnote 1), and they show that melt pro- fluid influx occur (e.g., White and Powell, 2002; saturated solidus and then experienced hydrous

duction across the wet solidus in garnet-bearing Indares et al., 2008; Palin et al., 2013). Nonethe- (M2) kyanite-grade melting (Eq. 6; TL15). They assemblages is negligible (~1%). It is not until less, postpeak compositional changes may still then recorded near-isothermal decompression to

fluid-absent muscovite dehydration occurs that occur via intracrystalline diffusion, as seen in (M3) sillimanite-grade conditions of 5–6 kbar melt generation reaches significant volumes, garnet porphyroblast outer rims in contact with and 700–730 °C (TL66, TLN34, TLN20A). increasing from 4% to 12% over a very small matrix biotite, which have grossular and pyrope During this process, the hottest and deepest temperature interval. Contours for calculated contents of ~6% and 15%, respectively. These rocks crossed the muscovite dehydration melt- modal proportion of garnet allow interpretation are lower than the concentrations recorded in ing reaction to produce larger melt volumes and of probable conditions of garnet core growth core regions, consistent with equilibration at peritectic K-feldspar. Textural evidence sug- with respect to the melting history. Although lower temperature and pressure (Spear, 1993). gests that most melt remained in communica- garnet cores lack prograde compositional zon- Isopleths for the calculated modal proportions tion with the rock, and therefore crystallization ing, making the early stages of the P-T evolu- of garnet and melt are presented in Figure 19, through the solidus at ~5 kbar and 690 °C re- tion poorly constrained, certain inferences can and they show that both phases are produced leased fluids, causing metasomatism and growth be made. The presence of muscovite inclusions at higher temperature due to the breakdown of of secondary muscovite crosscutting the earlier

within garnet suggests garnet nucleated prior to muscovite. However, retrograde decompression extensional fabric (S3). Juxtaposition of the hot the muscovite dehydration melting reaction in a from our calculated peak P-T conditions to the migmatites against the overlying Koronos Unit

muscovite-present field. The presence of kyanite solidus would have involved a reduction in the caused a thermal (M3) sillimanite-grade over- relicts within secondary muscovite suggests that modal proportion of garnet to form biotite and print and posttectonic garnet growth that trun-

garnet grew when kyanite was stable. A possible sillimanite, consistent with observed textures. cated the S3 top-to-the-shear fabrics and earlier

interpretation involves a P-T evolution along an Taking these factors into consideration reveals (M2) kyanite-grade assemblage (e.g., TLN13, initial continental geotherm of 25–30 °C km–1. that the Naxos migmatites remained at supra- TLN21, TLN22). The rocks then experienced This allows initial garnet growth to occur at solidus conditions during deep crustal exhuma- a kink in the P-T path associated with cool- ~600 °C and 6 kbar in an assemblage contain- tion and crystallized through the water-saturated ing at <5 kbar pressure, causing resorption of

ing Bt–Ms–Pl–Ilm–Qtz–H2O. Contours for solidus at ~4.9 kbar and 690 °C. garnet and cooling along a thermal gradient of modal proportion of garnet reveal that growth in ~40 °C km–1. muscovite-bearing assemblages was minor, and Summary of P-T Results significant growth of garnet was delayed until K- PALINSPASTIC RESTORATION OF THE feldspar was produced as a result of muscovite The P-T data clearly demonstrate that two NAXOS CORE COMPLEX dehydration along the effective solidus where the distinct and contrasting clockwise P-T loops proportion of garnet increases significantly with are recorded within the Naxos core complex Although balanced and restored cross sec- increasing temperature (see Data Repository ma- (Figs. 16–19). At high structural levels, the tions cannot be applied to intensely deformed

terial [footnote 1]). However, due to the presence Zas Unit records M1 conditions of ~14.5 kbar rocks under ductile conditions, we attempted

34 Geological Society of America Bulletin, v. 1XX, no. XX/XX

TL15 Core Unit MnNCKFMASHTO a(H2O) = 1.0 TL66 Core Unit MnNCKFMASHTO a(H2O) =1.0 A SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O O H2O All Assemblages D SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O O H2O All Assemblages 63.561 0.612 11.366 6.224 0.129 3.603 2.259 2.689 2.640 0.419 6.50 +Qtz 62.51 0.62 14.69 4.20 0.17 3.18 1.75 1.65 3.19 0.44 7.60 +Qtz 19 10.0 Grt Ky ilm 10.0

18 TL66 9 Grt Bt Kfs +

TL15 + Grt Ky ilm Ms Kfs Bt Pl Ky Pl ilm M M 7

Bt Pl Liq 6 Grt Bt Ms Ky e

Grt Ms Bt Pl ilm e Liq Grt Bt Ms Pg Liq l

l t 9.0 Grt Ms Pl Bt ilm Ep H2O H2O t 20 Chl Zo ilm Pl ilm Liq 28 17 9.0 H2O 10 27 Solidus Grt Sill ilm Kfs

+G Grt Bt Ms Ky Soli Bt Pl Liq 8 Pl ilm H2O

8.0 r dus Grt Ky Ms t 11 29 Grt Bt Kfs Sill Pl Grt Ms Pl Bt Bt Pl ilm 8.0

) ilm Liq ilm Ep Chl H2O 21 Bt Ms Pg Chl Zo Grt Bt 7.0 H2O Grt Sill 12 +Grt ilm Ms ilm H2O Ms Sill Ms Pl Grt St Grt 13 ing 4 Pl ilm t 7 Grt Sill Bt Pl l Bt ilm Ms Bt Pl 7.0 Bt Liq 8 Ms Bt 16 Liq Ms Dehydration Melting Ep Chl ilm H2O 5 Ms Chl +G on Me 6.0 Pl ilm H2O 26 i H2O Sill Grt Sill ilm Pl ilm 16 rt essure (kbar) Grt Bt

Pressure (kbar

6 r ilm Ms Kfs Crd Pl Liq H2O 5 P Ms Sill

Bt Pl 15 +M ehydrat 6.0 3 Pl ilm H2O Liq22 14

5.0 9 2 Bt Ms Chl Zo e 14 lt Ms Pl Bt Ms Pl Bt Sill Grt ilm Kfs Ms D 4 Pl ilm H2O 17 +Grt ilm Chl H2O ilm H2O Sill Kfs Bt 23 Crd Pl Liq 2 19 Bt Ms Chl Pl Grt ilm Kfs Bt 5.0 18 25 4.0 3 Crd Pl Liq ilm H2O Grt Crd Sill Kfs 1 20 Bt Ms Sill Pl ilm Ms Pl Bt And 30 Pl ilm Liq 11 15 24 Sill Bt Kfs H2O 32 1 ilm H2O 13 23 31 10 25 Crd Pl ilm Liq 21 24 3.0 4.0 22 500 550 600 12650 700 750 800 850 450 500 550 600 650 700 750 800 850

B Temperature (°C) E Temperature (°C) +

+ 10.0 0.20 10.0 M M 0.18 Prp

Pr ng e TL15 e TL66 p i

t 0.19 Pr l

ll l t Grt Inner Rim (Peak) t G 10 Grs 0.10 0.17 Prp p

0.18 Pr us Solid Grs 0.09 Grs on Me 0 9.0 p i 1 s 0.07 Grs t 0. 0.16 Prp 9.0 0.17 0.06 Grs .09 Gr 0.08 Grs Prp 0

Grt intermediate p s r

0.15155 Prp +G P

Dehydra 8 Gr

s Solidu

2 0.0 8.0 r .

rs t Ms 0 G 0.14 Pr p 8.0 0.1 0.11 0.15 6 0.05 Grs P s

) rp P p Gr

Grt Coree 0.13 rp Prp Pr

7.0

+Grt 0.10 1 p 0.07 9

s r

1 P

2 P p

r 0.

0.06 Grs 8

s P

r 1

p p p

p 0.07 Gr .

r r

0. Gr 7

0.110 P 7.0 0

P P

14 1

Ms Dehydration Melting .

6 6

Pr Grs s 0.08 0 0.06 Grs 1

+G 1 .

6.0 rp p .10 Gr . Grt intermediate

0 0 Garnet 0 0 Grt Rim r 0.09 t p

Pressure (kbar) No Garnet r

(Resorbed)outer rim P

Pressure (kbar

+ + + +

Grt Rim (Resorbed) 6.0 No Garnet s . 0.05 Grs Me Gr 0

5.0 07

t t 0. rs t 0.13 P G 0.1 rp rt +G .06 8 0

Prp r

PPr

5.0 3 p

r Resorbed Rim 131 4.0 P

0.0 0.16 Pr 0.12 Pr 0.13 Pr 0.1 0.1 0.17 Pr 14 Grt Rim

4 5 Pr 0.

p p p p p p 3.0 4.0 500 550 600 650 700 750 800 850 450 500 550 600 650 700 750 800 850 Temperature (°C) Temperature (°C) C F 10.0 10.0 lt lt t t t lt elt TL15 TL66 elt el el e el Grt Inner Rim (Peak) .02 Melt lt Melt M M 1 Melt 0 M M Melt t Melt 3 l 0 07 M 8 0.0 0.02 Melt 0.05 Melt 0.07 Melt . 04 M 06 0.03 Melt 0.04 Me 0.06 Me .30 0. Melting 0 0. 0.05 0. 0.08 0 t Melt t 0.2 0.29

0.30 Me 9.0 045 Gr el 27 0.29 Melt .26 Me . 0. 0.28 Melt 0 0 0.27 Melt 9.0 M 0.26 Melt t + 5

4 Gr M 25 Melt 0.045 Gr .2 .0 ydration 0 0 0. t Grt intermediate e Melting t 0.04 Gr 2 Gr Gr l + t

t .0 l 35 0.035 Gr 0

.0 G e

+ s 0 t Solidu 8.0 rt M

t M 03 Gr 0.03 Gr Ms Deh 4 0. rt t 2

5 G t e 8.0 l r elt 0. 0.

.02 0.24 Melt G t 0 t 5 M rt 0 01 G 0.23 Melt 0. 25 Gr 0.02 t Grt Core rt s Dehydratio0.02 Gr n G 0 0.23 7.0 15 M +Grt 0.0 .015 Gr t t

Grt 1 Gr Soli 0.01 0.01 Gr kbar) 0.0

t ( lt

7.0 d 0.21 Melt t

rt us Me 0.005 G Grt +G 0.22 Melt t 05 0 0.005 Gr 22 6.0 0.0 Grt intermediate0 .0 0 0. .0

Garnet . r 2 0 3 t

25 Pressure (kbar) G 0.015 Gr G outer rim 0.01 r r

No Garnet Pressure t G t

t +M 0.005 r

Grt Rim (Resorbed) 6.0 No Garnet t Gr

5.0 e t t

t t G rt r +G t 5.0 4.0 Resorbed Rim Grt Rim

3.0 4.0 500 550 600 650 700 750 800 850 450 500 550 600 650 700 750 800 850 Temperature (°C) Temperature (°C) TL15 Phase Field Assemblages TL66 Phase Field Assemblages Grt Ms Bt Pl ilm Ky Qtz Liq H2O 1 Ms Bt Pl ilm Chl And Qtz H2O 9 Ms Bt Pl ilm St Qtz H2O 17 1 Ms Chl Zo Pl ilm Qtz H2O 11 Grt Bt Ms Ky Zo Pl ilm Qtz H2O 22 Bt Ms Chl St And ilm Pl Qtz H2O 18 Grt Ms Bt Pl ilm Qtz Liq H2O 23 Bt Ms St Chl Sill ilm Pl Qtz H2O 2 Ms Bt Pl ilm Chl St Qtz H2O 10 Ms Bt Kfs Pl ilm Qtz And H2O 2 Ms Chl Pa Zo Pl ilm Qtz H2O 12 Grt Bt Ms Ky Chl Pl ilm Qtz H2O 19 Grt Ms Bt Pl ilm Qtz Liq 24 Bt Ms And Pl ilm Qtz H2O 3 Ms Bt Pl ilm St And Qtz H2O 11 Ms Bt Kfs Pl ilm Qtz Sill H2O 3 Bt Ms Chl Pg Zo Pl ilm Qtz H2O 13 Grt Bt Ms Ky Chl St Pl ilm Qtz H2O 20 Grt Ky Ms Bt Kfs Pl ilm Qtz Liq 25 Bt Ms Sill ilm Pl Qtz Liq H2O 4 Ms Bt Pl ilm St Sill Qtz H2O 12 Bt Kfs Pl ilm Qtz And H2O 4 Grt Bt Ms Chl Pg Zo Pl ilm Qtz H2O 14 Grt Bt Ms Chl St Pl ilm Qtz H2O 21 Grt Sill Ms Bt Kfs Pl ilm Qtz Liq 26 Grt Bt Ms Sill ilm Pl Qtz Liq H2O 5 Grt Ms Bt Pl ilm Chl St Qtz H2O 13 Bt Kfs Pl ilm Qtz Sill H2O 5 Grt Bt Ms Chl Zo Pl ilm Qtz H2O 15 Grt Bt Ms St Pl ilm Qtz H2O 22 Sill Ms Bt Kfs Pl ilm Qtz Liq 27 Grt Bt Ms Ky ilm Pl Qtz Liq H2O 6 Grt Ms Bt Pl ilm St Sill Qtz H2O 14 Ms Bt Pl ilm Sill Qtz Liq H2O 6 Grt Bt Ms Zo Pg ilm Qtz H2O 16 Grt Bt Ms Ky St ilm Pl Qtz H2O 23 Grt Sill Bt Kfs Crd Pl ilm Qtz Liq 28 Grt Bt Ms Ky Kfs Pl ilm Qtz Liq 7 Grt Ms Bt Pl ilm St Ky Qtz H2O 15 Bt Kfs Pl ilm Qtz Sill Liq H2O Grt Bt Ms Zo Pg Pl ilm Qtz H2O 17 Bt Ms St Pl ilm Qtz H2O 24 Sill Bt Kfs Crd Pl ilm Qtz Liq 7 29 Grt Bt Ms Sill Kfs Pl ilm Qtz Liq 8 Grt Ms Bt Pl ilm Chl Qtz H2O 16 Grt Ms Bt Pl ilm Sill Qtz Liq H2O 18 Bt Ms St Chl Pl ilm Qtz H2O 25 Sill Bt Kfs Crd Pl ilm Qtz Liq H2O 8 Grt Bt Ms Zo Pg Pl ilm Qtz H2O 30 Bt Ms Kfs Sill Pl ilm Qtz Liq 9 19 Bt Ms St Ky ilm Pl Qtz H2O Grt Bt Ms Ky Zo Pg Pl ilm Qtz H2O 31 Bt Kfs Sill Pl ilm Qtz Liq 10 Grt Bt Ms Ky Pg Pl ilm Qtz H2O 20 Bt Ms Sill St Pl ilm Qtz H2O 32 Grt Bt Crd Sill Kfs Pl ilm Qtz Liq Figure 19. 21 Bt Ms Chl And Pl ilm Qtz H2O

Geological Society of America Bulletin, v. 1XX, no. XX/XX 35

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 palinspastic restoration of the metasedimen- continental margin sequence in a similar style folding of the NPDS, the sedimentary units in tary and basement units to place minimum to the Helvetic Nappes in the Alps and Greater the upper Cycladic Nappe, and the metamorphic constraints on crustal shortening. A simple Himalayan Series (Fig. 20, stage 3). The results sequence. Although the origin of Miocene E-W restoration of the Naxos MCC was performed show that the restored continental margin was shortening remains unclear, there are several to estimate minimum amounts of shortening intruded by numerous basaltic and doleritic possible mechanisms that are discussed below.

during the leadup to peak M3 conditions and sills, with intercalated tuffaceous, siliciclastic, Due to extreme attenuation, missing meta- postmetamorphic doming, by considering all and carbonate material. morphic stratigraphy, and many unknowns, structural, petrographic, pressure-temperature- An ~13-km-wide section today restores to such as the original dip of the NPDS, restora- deformation (P-T-D), and U-Th-Pb data pre- a minimum of 60 km. Approximately 400% tion of the extensional features and therefore the sented herein. U-Th-Pb results will be presented ENE-WSW shortening across the Naxos MCC full amount of extension cannot be accurately in a future study. A simple two-dimensional is required to explain the folding and thicken- determined. However, NNE-SSW crustal exten- area- and line-balance restoration along section ing during its prograde evolution. Although sion is the last phase of deformation, and it is A–A′ provided insight into the way in which accurate section balancing is precluded in ter- pervasive, overprinting and truncating all short- the MCC formed, and how nappes of differ- rains with ductile deformation such as the ening structures. It is frozen in the metamorphic ing lithologies attained different P-T conditions Naxos MCC, an absolute minimum shortening stratigraphy and the migmatite dome. Assum- at different times and were juxtaposed against of 47 km cannot be rectified. Although several ing the doming occurred at ~5 kbar (17.5 km each other. The restoration assumes that, in gen- assumptions were applied that may be over- depth), and the NPDS maintained its original eral, deformation propagated down-structural simplistic and violated in ductile environments, dip of ~10° to the north and was responsible for section with time, in agreement with structural, this exercise illustrates that the folding of high- the entire exhumation of Naxos, the throw on petrographic, and P-T-D data. The restoration grade rocks forming the core complex could this single fault would be >100 km. This fault process involved the following steps: not have formed by purely extensional mecha- must have been active in its present-day shal- (1) The late-stage E-W doming and normal nisms. This magnitude of shortening within a low geometry and could not have been rotated faults were restored prior to pre-Miocene exten- continental margin sequence is commonly seen from steeper to shallower angles, as the mig- sion and granodiorite intrusion using the Gala- across many recent mountain ranges that expe- matite dome and metamorphic footwall cannot nado normal fault as a pinning point (pinning rienced shortening and underthrusting of rocks have been rotated by over 40°. It seems likely, point 5; Fig. 20, stage 1). to kyanite-grade conditions over time scales of however, that the NPDS was not responsible for (2) Three units (Zas Unit, Koronos Unit, and tens of millions of years, such as the Himalaya the entire exhumation on Naxos, but rather rep- Core Unit) were identified within the metamor- (Corfield and Searle, 2000) and accretionary resents the brittle-ductile transition, mechani- phic footwall and treated separately; each unit is wedges such as those in Oman (Searle et al., cally uncoupling the upper- and midcrust, and separated by a major extensional shear zone that 2004). Therefore, we believe that most of the it was exhumed in its present-day geometry by

cannot be restored due to lack of suitable pin- structures (F1 and F2 folding) and prograde and uplift on several younger crosscutting brittle

ning points (see text for more discussion). peak (M2) Barrovian conditions of the Naxos normal faults. The simplistic relationships and (3) Zas Unit (Cycladic Blueschists) was as- MCC occurred under a compressional tectonic restoration for the geometry of the Naxos-Paros

sumed to behave in a brittle-ductile manner, regime, and not under crustal extension. Up- detachment and earlier peak M2 and M1 geom-

dominantly by thrusting, and was restored first right (F3) folding within migmatites occurred etries based on field relations andP -T-D data

(Fig. 20, stage 2). at M3 sillimanite-grade conditions and invokes are presented in Figure 21. (4) Koronos Unit thrusts were restored to a horizontal contractional stress regime. What-

align the F1 and F2 folds. These folds were then ever the cause for this constriction, it highlights DISCUSSION restored using area and line-length balancing a significant space problem requiring lateral and assuming there are only three bands of displacement of material from the sides, and Integration of Structural, Petrographic, marble structurally repeated. Basement-cover therefore it must have formed when the maxi- and P-T-D Data

repetitions were assumed to represent thick- mum principal stresses (σ1) were orientated in a skinned thrusts and were restored to their origi- horizontal plane. Deformation as a result of this Crustal Thickening and Prograde and nal geometry. contraction must have become localized within Peak Metamorphism (5) The Core Unit deformation was restored the rheologically weaker migmatites upon expe- The field, petrological, and P-T-D data from in two steps. The KSZ was restored by line- riencing muscovite dehydration melting, caus- this study imply a complicated tectonic evo- length balancing of the imbricate stack that uti- ing them to intensely fold, thicken, and dome lution that cannot be explained by extension lized basement material as a relict thick-skinned the overlying units. alone, with juxtaposition of several nappes that thrust. Ductile deformation and isoclinal folding A significant component of E-W shortening is experienced differing metamorphic histories, as were restored using line-length and area balanc- necessary to form the structures on Naxos, and it summarized by Figure 22. Previous models of

ing as above (Fig. 20, stage 2). This represents is suggested that some E-W shortening is occur- Naxos, whereby all rocks experienced M1 high- a minimum amount of shortening, because the ring throughout the Aegean today (McClusky pressure conditions followed by isobaric heating thick-skinned thrusts structurally repeating the et al., 2000), accommodated on active conju- and Barrovian metamorphism during crustal ex- basement over shelf are laterally discontinuous, gate ENE-WSW– and WNW-ESE–trending tension (e.g., Buick and Holland, 1989; Buick, overprinted, and reworked by upright folding strike-slip faults like those that crosscut Naxos 1991a, 1991b; Urai et al., 1990; Avigad, 1998; and ductile flow. (see Fig. 2). This could be possibly related to a Vanderhaeghe, 2004; Kruckenberg et al., 2010), (6) Finally, a restoration of the continen- 20° counterclockwise rotation of Naxos about a cannot explain the clockwise Barrovian P-T-D- tal margin was attempted in stage 3, based on vertical axis since the mid-Miocene (Kissel and time (t) paths, which involve prograde increas- structural mapping in this study. This suggests Laj, 1988; Avigad et al., 1998; Malandri et al., ing pressure and temperature in the Koronos and that Naxos records a thrusted and thickened 2016), and it could explain the late-stage gentle Core Units.

36 Geological Society of America Bulletin, v. 1XX, no. XX/XX

Naxos Geological Cross-Section Palinspastic Pseudo-restoration Prior to Extension

Pinning Point 1

Western Naxos Shear Zone East A′ Hanging wall Galanado Fault Late Miocene-Paleogene Normal/Strike-Slip Faulting overprint Koronos West A Pinning Point 5 Shear Zone Moutsana Detachment 1500 Hanging-wall Trench Melange 1000 Footwall Moutsana overlying ophiolite Hanging-wall Anticline Continental Fluvial 500 Conglomerates Stelida/ Aiga Propokios Vertical 0 Zas Unit Cycaldic Blueschists depth (m) Retrogressed to Greenschits –500 Ky Migmatites Zas Shear Zone 8–10 Kbar 730°C Czo + Bt +Chl Schists West Naxos I-type Hbl Bt Granodiorite –1000 Grt, two mica/ Grt tourmaline –1500 S-type Leucogranites Koronos Grt + Sill + Kfs Migmatites Unit Grt + Ky + Kfs Gneisses 10 Kbar 675 0C –2000 5–6 Kbars 730°C

Pinning Point 2 Stage 1: Unfolding the Naxos dome prior to extensional shear zone reactivation 0 Km 1 Km 2 Km 3 Km 4 Km Inferred structures from down plunge projection from North and South Late strike slip fault trace

Cycladic Blueschists (~ 45–50 Ma) (Retrogressed to Greenschists) Pinning Point 1 Pinning Point 2 4 Km

Zas Shear zone 3 Km Pinning Point Arc from unfolding Pinning Point 3 2 Km Koronos Unit (10 kbars, 675°C at ~ 22–18 Ma)

1 Km Pinning Point 5

Koronos Shear Zone 0 Km

Core Unit sillimanite-grade migmatites with interbanded marbles (5–6 kbars, 730°C)

Stage 2: Restoring all internal folding and thrusting Naxos Geological Pseudo-restored Cross-Section Tsiknias Ophiolite Tsiknias Thrust (Tinos) Zas Unit (Cycladic Blueschists Retrogressed to Greenschists) (Seen on Tinos) Koronos Unit Distal shelf facies, conglomerates meta-volcanics and dolomites (14–15 kbars 470°C ~ 49–46 Ma) (Kyanite Grade) Pinning Point 1 Pinning Point 3 Pinning Point 2 4 Km Proximal shelf facies B C D 2 Km 9 Kyanite Grade Gneisses (Basement to the Mesozoic Shelf) Zas Shear Pinning (10 kbars 675°C ~ 22–18 Ma) 0 Km Point 5 Zone Core Unit Migmatites (Sillimanite Predicted thrust propagation due to later folding 0 Km 2 Km 4 Km Grade 5–6 Kbars 730°C ~15–16 Ma) Koronos Shear Zone

Stage 3: Restored Passive Margin at ~ 70 Ma

Core Unit Koronos Unit Zas Unit (Cycladic Blueschists) Deep Oceanic NE 0 Km SW Proximal Shelf Facies Distal Shelf Facies Tsiknias Thrust (Tinos) Tsiknias Ophiolite (Tinos) 5 Km South Cycladic Thrust (Ios) Metamorphic Sole (Tinos) Cycladic Basement to the Mesozoic Shelf Trench Melange 10 Km Approximate Scale: 40 Km (West Naxos)

Passive roof 80 Ma Basalts normal fault (Eclogites on Syros (Vari Detachment) and Tinos) Figure 20. Palinspastic pseudorestoration of the Naxos metamorphic core complex, using the same color scheme as the maps and cross sections along line of section A–A′. Deformation was restored to that prior to late doming and I-type intrusion at 12.2 Ma, and then each unit was restored individually with deformation propagating down structural section with time, finally to the undeformed restored continental margin at ca. 70 Ma. See text for more discussion and details on each stage of the restoration. Mineral abbreviations follow Whitney and Evans (2010).

Geological Society of America Bulletin, v. 1XX, no. XX/XX 37

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.

Zas Unit Koronos Unit Line of section B–B′ Core Unit Late Stage Normal and transtensional Truncation of compressional structures strike slip faults Naxos-Paros Detachment System and shear zones by the NPDS SSW B’ NNE B 3 Trench Mélange 3 2 Ophiolite 2 ZSZ 1 KSZ 1 P Height 0 0 (km) –1 –1 Koronos Unit Core Unit –2 –2 Migmatites –3 –3 Southern Subdome Pinched Synform Northern Subdome 01234 5 6 Horizontal Distance (Km) at the Center of the Dome T Stage 1: Restoring the Naxos Paros Detachment System to M3 conditions and Truncation of KSZ and ZSZ by the NPDS 0 Migmatite Dome formation Trench Mélange responsible for nal exhumation of core complex Ophiolite 5 Moutsana Detachment Depth Naxos-Paros Detachment System (NPDS) (Km) 10 Zas Unit (Cycladic Blueschists) D P 15 Rotation of KSZ and ZSZ and leucogranites C due to formation of migmatite ZSZ 20 TL66 A B dome at M3 conditions +Sill KSZ M3 Core Unit 5–6 Kbars, 720°C Stage 2: Restoring the migmatite dome to synorogenic extrusion geometry T 0 Trench Mélange

5 Ophiolite NPDS Projection 10 Zas Unit Intrusion of S-type TLN54 D (Cycladic Blueschists) P Depth 15 SE Naxos C′ leucogranites prior to (Km) NPDS Projection 20 ZSZ dome formation +Sill B′ +Melt 680 +Sill C 25 A′ oC TL66 +Kfs 30 Cycladic Basement +Sill +Sill T (Ios) Koronos Unit ? +Ky +Ky +Ky Synorogenic extrusion of Core 35 KSZ B SCT (Ios)? Core Unit and Koronos Units by top to NNE +Kfs A 40 TL15 shearing on ZSZ and KSZ producing S3 shear fabrics and sheath folds Stage 3: Restoring the Koronos and Zas Shear Zones to M2 conditions prior to extrusion 0 Trench Mélange Ophiolite 5 NPDS Projection 10 P Zas Unit D (Cycladic Blueschists) 15 TLN54 SE Naxos Depth NPDS Projection (Km) 20 +St 550oC 25 C ZSZ KSZ T 30 Cycladic Basement +Ky 600oC Koronos Unit Underthrusting of Koronos (Ios) Lorem ipsum0 ? 10 kbar 670 C and Core Units to M2 Kyanite o 35 +Melt 680 C TL67 B SCT (Ios)? Core Unit grade conditions producing S2 40 TL15 A 10 kbars 7300C M2 crenulation cleavages

Stage 4: Restoring to peak M1 HP-LT conditions in Zas Unit Not to Scale Trench Mélange 0 C Core Unit 5 Koronos Unit Ophiolite A 10 B P 15 Cycladic Basement Depth (Ios) (Km) 20

30 Zas Unit (Cycladic T Blueschists) 14.5 kbar 470oC 35

40 D

45 Extrusion of Blueschists TLN54 from subduction zone 50 Eclogites producing S1 Top to NNE (Syros Tinos Sifnos) shear fabric in Zas Unit

Figure 21. Schematic restoration of the Naxos metamorphic core complex along line of section B–B′, highlighting that the Naxos-Paros detachment system (NPDS) must truncate and postdate movement on the Zas shear zone (ZSZ) and Koronos shear zone (KSZ). Schematic predicted pressure-temperature-time (P-T-t) paths for each unit are displayed on the right. Stage 1—Restoration prior to extension, placing the core complex at ~18–20 km depth during migmatite formation. Stage 2—Restoration of the migmatites prior to dome formation.

Stage 3—Restoration of the core complex prior to extrusion at peak M2 kyanite-grade conditions representing ~35 km depth. Stage 4—Res-

toration prior to prograde burial of the Koronos and Core Units during peak M1 conditions in the Zas Unit. See text for more discussion. Mineral abbreviations follow Whitney and Evans (2010). HP-LT—high-pressure–low-temperature; SCT—South Cycladic Thrust.

38 Geological Society of America Bulletin, v. 1XX, no. XX/XX

Naxos P-T-t Paths Summary 15.0 Peillod, 2017 Naxos Field Gradient t3 (M1, SE Naxos) t2 50–45 Ma t1 t4 14.0 P Avigad, 1998 M1 Blueschist peak metamorphism Naxos Metamorphic (M1, SE Naxos) TLN54 ~14.5 Kbars 470 °C ~ 45–50 Ma (U-Pb Allanite) +Mel t5 +H2O 13.0 t Evolving Geotherm S1 M2 Crustal thickening M2 Overthrusting T 12.0 and Kyanite Nappes Solidu Field Gradient TL67 Grt Rim: ~10 Kbars 675 °C of Kyanite Nappes ~22–18 Ma (U-Pb Mnz) and burial of core unit to TL15 Kyanite Migmatite s 8–10 Kbars, 730 °C 11.0 –1 1 Buick and Holland, ~18.5–17.5 Ma (U-Pb Mnz) – (1989) M2 Ky Cycladic Blueschists (Core Unit)

r) (Zas Unit) o C Km

b C Km 10.0 o 15 20–18 Ma dration Meltinge

( Decompression hy anit Ky e 10 during SW extrusion e r from subduction

s zone Ms De s 9.0 18.5–17 Ma Sillimanit

r 22–20 Ma

P } S2 S3 +Ms +Kfs ~18–16 Ma Ideal 8.0 Buick and Holland, Migmatites prograde path through NNE-SSW simple (1989) M3 Sill 40 Ma Kyanite eld–1 (M2) shear causing (Core Unit) decompression 30–20 Ma –1 ?Early M1 conditions o C Km through Ms dehydration of Koronos Unit? }melting reaction 7.0 o 16–15 Ma S2 Nucleation20 of Garnet (TL67) C Kme 16–15 Ma 25 anit e 40–30 Ma Ky anit ~16–15 Ma Non-ideal Sillim E-W pure shear 30 Ma } F3 folds and vertical 6.0 –1 M3 isobaric heating, S1 extrusion o post-tectonic garnets M2 (Biotite Zone) C Km 30 ~15–16 Ma Xeno ~ 30–20 Ma M3 anatexis and

20 Ma 14–15 Ma } decompression melting 5.0 Migmatites TL66 Sillimanite Migmatite –1 anite o Onset of crustal Grt Rim: ~5–6 Kbars 730 °C C Km TL15 M2 Ky 35 extension ~15 Ma with ~ 20% melt (Core Unit) S3 e ~17.5–15 Ma (U-Pb Mnz) Kyanit Andalusite12.7 Ma 4.0 +Melt 450 500 550 600 650700 750 800 Te mperature (°C)

Juxtaposition of Koronos and Core Units Metasomatism ~14–15 Ma followed by rapid exhumation and cooling via normal faulting on NPDS

Zas Unit Koronos Unit Core Unit Figure 22. Summary pressure-temperature-time-deformation (P-T-t-D) paths for all units from the Naxos metamorphic core complex based on the results of this study. Shaded black hashed boxes represent P-T conditions estimated previously by Buick and Holland (1989), Avigad (1998), and Peillod et al. (2017; shown as Peillod 2017 in figure). Colored polygons represent estimated peak metamorphic conditions presented in this study, with labeled colored arrows that represent suggested P-T paths discussed in the text. U-Pb data will be presented in a future study. NPDS—Naxos-Paros detachment system. Mineral abbreviations follow Whitney and Evans (2010).

The rocks of southeast Naxos (Zas Unit or understood how meta-peridotites occur as en- ate sequence. (3) They could have been doleritic Cycladic Blueschists) underwent a completely claves within meta-gabbro (amphibolites) and or gabbroic sills that intruded the continental different metamorphic evolution than the under are associated with both the Koronos and Zas margin prior to deformation, and the ultramafic lying higher-grade units. Contrary to previous shear zones (Katzir et al., 1999, 2002), several component represents cumulates that formed suggestions that the whole island experienced mechanisms have been proposed: (1) Katzir during fractional crystallization. (4) They could

(M1) high-pressure conditions (Avigad, 1998; et al. (1999) suggested that the ultramafics were be slivers of serpentinite that were incorporated Martin et al., 2006), we find no evidence in emplaced during Eocene subduction of the con- from a structurally higher ophiolitic thrust sheet, the petrography or geochronology to sug- tinental margin. (2) They could have also been although this requires over several kilometers of gest that either the Koronos or the Core Units emplaced by thrusting of lithospheric mantle structural movement. To explain the thermom- experienced this event. Although it is poorly and meta-gabbros into the Naxos shelf carbon- etry data of Katzir et al. (2002), minor composi-

Geological Society of America Bulletin, v. 1XX, no. XX/XX 39

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.

tional layering within the host amphibolites, and to the positive buoyancy of continental margin Following thick-skinned thrusting, conduc- the absence of high-pressure relicts (see 17TL75 material (England and Holland, 1979). Although tive relaxation of the footwall geotherm would in the Data Repository material [footnote 1]), this thrust is not convincingly exposed on Naxos have heated and driven partial melting over a we suggest they represent doleritic-gabbroic due to extensional reworking, there is a strong time span of several million years (England

sills that intruded the continental margin prior record of thrust-related fabrics (S2a and S2b), and 1978; England and Thompson, 1984). Footwall to deformation. This is because the host garnet- it is possible that the South Cycladic thrust on rocks first underwent prograde hydrous melt- bearing meta-gabbros show no evidence of a Ios (Huet et al., 2009; Mizera and Behrmann, ing in the kyanite field, producing kyanite- and high-pressure history, and this is the most likely 2015) could be this structure or a related struc- garnet-bearing microstructural domains and

rock composition to preserve evidence for blue- ture. This would produce S1 top-to-the-NE shear leucosomes (Eq. 6). Upon experiencing melt schist- or eclogite-facies metamorphism (Hein- fabrics in the Zas Unit and the rest of the Cy- weakening, the rocks then record decompres- rich, 1982). Furthermore, the mantle wedge in cladic Blueschists and would adequately explain sion through the muscovite dehydration melt-

a subduction setting is cold, and this disagrees S2 crenulation cleavages and thrust fabrics under ing reaction to sillimanite-grade conditions.

with two-pyroxene thermometry temperatures M2 greenschist-facies conditions. Overthrusting This decompression was associated with non-

exceeding 1000 °C (Katzir et al., 2002); it is of this high-pressure unit onto the more-proxi- coaxial (S3) top-to-the-NNE shearing, flatten-

more likely to be associated with hot intrusion. mal Cycladic continental margin, along with ing, and the development of F2 sheath folds on If the Koronos and Core Units did experience continued crustal shortening, would have led to the Koronos and Zas shear zones. This reaction

M1 conditions, they must have been reincor- the generation of a nappe pile (Dixon and Rob- is the first major melt-producing reaction and porated into another prograde burial Barrovian ertson, 1984). The proximal sedimentary cover would allow deformation to become localized tectono-metamorphic cycle that obliterated all of the continental margin (Koronos Unit) would along weakened zones of melt segregation and trace of this previous metamorphic event. have subsequently experienced burial, polyphase facilitate the development of upright folding. Eocene zircon ages of ca. 40 Ma have been folding, and thick-skinned thrusting, as exempli- Kyanite-grade fabrics are folded with this folia- reported from amphibolites from northern fied by structural repetition of basement material tion, placing tight constraints on the timing of

Naxos and within the Koronos Unit (Mar- overlying metasedimentary protoliths during M2 upright (F3) folding after (M2) kyanite-grade

tin et al., 2006; Bolhar et al., 2017), and these kyanite-grade conditions. This would have led to conditions and following decompression to (M3)

could represent two possibilities: (1) The ages crustal thickening and (M2) regional metamor- sillimanite-grade conditions.

represent zircon growth during (M1) high-pres- phism during the Oligocene to Miocene (40– sure–low-temperature conditions but not in the 15 Ma). A crustal shortening mechanism would Partial Melting, Decompression, and

presence of garnet (Martin et al., 2006), and the explain isoclinal F1 folding during prograde (M2) Formation of the Migmatite Dome unit was extruded to the midcrust before being Barrovian metamorphism and development of Several models for isothermal decompression

reburied and incorporated as part of a separate the (S2a) crenulation cleavages (S2b) and top-to- and migmatite dome formation have been sug- metamorphic cycle. In this scenario, prograde the-SW thrust fabrics under amphibolite-facies gested, but many fail to explain all structural,

and peak M2 conditions would occur ca. 25 Ma conditions, preserved in domains unaffected by petrological, and P-T-D constraints. In a subse-

after experiencing high-pressure conditions, like late crosscutting S3 extensional fabrics. Burial quent study, it will be shown that decompres-

the Lepontine metamorphism in the central Alps of the continental margin to (M2) kyanite-grade sion of migmatites from ~10 to 5 kbar occurred (e.g., Wiederkehr et al., 2008). (2) Alternatively, pressures of ~10 kbar (~35 km depth) was asso in less than 2.5 m.y., a time scale much shorter

the ages do not represent M1 conditions, which ciated with the development of these shortening than previously thought. From the structural and occurred on Naxos between 49 and 46 Ma and structures. Growth of garnet porphyroblasts oc- petrological features described in this paper, a regionally at 54–45 Ma (Tomaschek et al., 2003; curred syn- to posttectonically with localized relative chronology and framework for the se-

Lagos et al., 2007; Dragovic et al., 2012). In the (S2b) thrust fabrics along cleavage domains at the quence of deformation events leading to doming absence of high-pressure relicts, the ca. 40 Ma base of the Koronos Unit, indicating a synchro can be firmly established, and therefore the vari-

zircon ages could be interpreted to represent nicity between peak (M2) kyanite-grade condi- ous dome forming mechanisms can be assessed.

the earliest M2 Barrovian-type age due to over- tions and thick-skinned thrusting. Crosscutting relationships on the KSZ

thrusting of the blueschists (Zas Unit). The latter Contrasting peak P-T conditions between the bounding the migmatites demonstrate that (S3) of these two suggestions is our preferred choice Koronos and Core Units indicate these pack- top-to-the-NNE shearing predated doming and agrees with the petrological record on ages equilibrated along an evolving geotherm and leucogranite intrusion. This is because the Naxos and new U-Pb data that will be presented at slightly different times (U-Pb data will be shear zones are folded around the dome with in a future study. provided in a future study). The most likely ex- consistent top-to-the-NNE kinematics on all To account for contrasting P-T-D paths re- planation in accordance with field evidence is margins. This normal-sense shearing must have corded at different structural levels on Naxos, our that thick-skinned thrusting was responsible for been associated with SW-directed exhumation model features subduction of the distal continen- burying the Core Unit under the Koronos Unit. of the footwall Core Unit relative to the over- tal margin (Zas Unit) down a NE-dipping sub- Repetition of basement rocks interleaved with lying Koronos Unit. S-type leucogranite dikes

duction zone to (M1) high-pressure–low-temper- metasedimentary schists and marbles along the and sills clearly postdate shearing, because they

ature conditions that reached blueschist-facies base of the Koronos Unit and within the Core crosscut the (S3) mylonite fabrics on the KSZ conditions on Naxos, but also eclogite-facies Unit is consistent with this idea. Further evi- (refer to Figs. 7–8; see also Data Repository conditions as seen on the islands of Syros, Tinos, dence for a southwestward-verging thrust also material [footnote 1]), and they show emanat- and Sifnos. This was followed by SW expul- occurs on the island of Ios as the South Cy- ing geometries from the migmatite core (see sion of high-pressure rocks under a passive-roof cladic thrust, which places a thin package of Figs. 6O and 8H). The crosscutting dikes dip normal fault (Vari detachment on Syros; Roche amphibolite-grade metasedimentary cover over at ~45° toward the center of the dome and in-

et al., 2016; Laurent et al., 2016) at the top of the Cycladic basement (Huet et al., 2009; Mizera trude perpendicular to the S3 foliation, particu- subduction channel and a thrust at the base due and Behrmann, 2015). larly on the eastern margin. Because these dikes

40 Geological Society of America Bulletin, v. 1XX, no. XX/XX

and the shearing foliation are tilted around the crossing the muscovite dehydration melting re- including the Greater Himalayan Series, the dome, we infer that both the dikes and shear action. Although lower-crustal isostasy-driven Lepontine dome, and the Tauern Window in zones have experienced rotations of ~45° about flow during crustal extension has been sug- the Alps. There is an abundance of evidence a NNE-SSW horizontal axis from their original gested as a mechanism to produce such upright for SSW-verging thick-skinned thrusting and subhorizontal orientation. The leucogranites contractional features at the center of migma- compression across Naxos, and it seems likely dikes probably intruded vertically when the tite domes (Kruckenberg et al., 2011; Rey et al., that more thick-skinned thrusts are preserved KSZ and migmatites where subhorizontal and 2011, 2017), the predicted sequence of defor- at depth under the migmatites. were passively rotated as the dome formed. mation is inconsistent with our observations. We propose that normal-sense shearing on the Because leucogranites are the product of melt Isostasy-driven flow predicts that decompres- Koronos and Zas shear zones occurred due to ex- extraction, and this only occurs upon reaching sion is caused by convergent flow of a partially trusion in an overall crustal thickening scenario the critical melt threshold (7%–10%; Rosen- molten lower crust under a horizontally extend- based on five key reasons: (1) If the crust was berg and Handy, 2005), this volume of melting ing upper crust. In this scenario, we would ex- extending, we would not expect horizontal con- only occurred upon decompression through the pect high overall melt fractions and doming to strictional strains associated with doming. NNE- muscovite dehydration melting reaction (Eq. 4), occur simultaneously with shearing and oppos- SSW crustal extension clearly produced NNE-

at lower-pressure (M3) sillimanite-grade con- ing and divergent shear senses on all margins of SSW pure shear boudinage under M3 conditions,

ditions, also coeval with the timing of upright the dome due to a mechanical decoupling of the which postdated upright F3 folding in the mig-

folding. Upright F3 folds in the CHSZ have three migmatites with the overlying rocks. Instead, matite dome. Because F3 folding predated NNE- fold axis populations that intersect around a ver- we suggest the doming, upright folding, verti- SSW extension but postdated movement on the tical point, representing a type-2 Ramsey fold cal boudinage, and constriction are late features KSZ, extrusion and top-to-the-NNE shearing on interference pattern. Vertical boudinage of folds that formed under compression, and they post- the KSZ and ZSZ must also have predated exten- and leucogranite dikes (see Figs. 8–10) suggests date decompression and top-to-the-NNE shear- sion. (2) The KSZ and ZSZ record only top-to-

overall horizontal constriction while also at M3 ing from peak M2 pressures. The migmatite the-NNE kinematics on all margins of the dome sillimanite-grade conditions. The intense up- dome folds the isograds, intruding S-type gran- and were therefore active prior to doming. They right E-W–trending and N-S–trending isoclinal ite dikes and the entire overlying metamorphic are not multidivergent shear zones as predicted folds in the center of the dome require that the sequence, including the top-to-the-NNE shear by extensional dome models (Kruckenberg

maximum principal stresses σ1 and σ2 must have zones (KSZ and ZSZ), and it was therefore et al., 2010, 2011; Rey et al., 2011, 2017). (3) been almost equal and orientated E-W and N-S not mechanically decoupled from the overly- The KSZ and ZSZ are completely discordant to in the horizontal planes, whereas the minimum ing rocks at the time of its formation. Doming the NPDS at a higher structural level, which trun-

principal stress, σ3, must have been orientated therefore represents a separate process to the cates them (see Figs. 3 and 4), and the S3 folia- vertically. This horizontal contraction scenario decompression of rocks from kyanite- to silli- tion associated with the Zas and Koronos shear

can be easily explained by overall shortening of manite-grade conditions. To explain the high- zones is refolded by F3 folds, which in turn are the crust, and although diapirism would predict temperature N-S boudinage overprinting the cut by the NPDS. Therefore, these shear zones similar features (Ramberg, 1980), the low over- upright folds and vertical boudinage, we sug- formed earlier than and are separate structures all melt fractions in migmatites (see Fig. 19) are gest that early stages of doming resulted from from the NPDS, which exhibits both brittle and inconsistent with diapirism being the dominant constrictional strain localization in the midcrust ductile deformation features. (4) If movement doming process. Similar features have been associated with the rheological weakening that on the KSZ represents the deep lower-crustal documented in Archean granite-greenstone ter- occurred upon crossing the muscovite dehydra- signature of a detachment fault associated with ranes, such as the Pilbara North Pole Dome in tion melting reaction, whereas the latter stages crustal extension, then we would expect rapid Australia (e.g., Nijman et al., 2017). Diapirism of doming coincided with a change in bound- cooling and migration of the brittle-ductile tran- or crustal turnover has been proposed as a mech- ary conditions, most notably, a decrease in the sition (England and Jackson, 1987). There is no anism to explain these tonalite-trondhjemite- N-S principal stress, presumably associated the evidence to suggest this occurred. Instead, iso- granodiorite (TTG)–cored domes (e.g., Collins transition from overall compressional to exten- thermal decompression of migmatites to ~5 kbar et al., 1998), which are associated with larger sional tectonics (see Fig. 23). led to cooling by <30 °C upon their exhumation melt fractions than Naxos migmatites. The pat- to the midcrust. This isothermal decompression tern of doming and basins with other observed Origin of Top-to-the-NNE Fabrics: P-T path would be predicted if the KSZ and ZSZ shortening phenomena indicates the importance Synorogenic Extrusion vs. Crustal Extension represent stretching faults or ductile shear zones of regional compression during the formation Isothermal decompression of migmatites that progressed up structural section with time of these domes, which may have caused a first and gneisses from 10 to 5 kbar (~35–17.5 km (Means, 1989). (5) Hydrous melting is the major

Raleigh instability, leading to an egg-box pat- depth) was rapid and associated with S3 top- anatectic process for the Naxos migmatites that tern of sinking greenstones around the TTG to-the-NNE shearing on the normal-sense was responsible for initial melting and weaken-

domes (Nijman et al., 2017). N-S horizontal Koronos and Zas shear zones. If this normal- ing at M2 kyanite-grade conditions. This hydra- boudinage, and a range of brittle deformation sense shearing occurred synchronously with tion reaction acts as a large sink for water. To features overprinting these folds, suggests the SW-directed thrusting within or under the balance the net water budget of the deep crust, principal stress axes rotated to a NNE-SSW– migmatites, it would result in synorogenic calculations require a large external supply of

orientated σ3 and E-W–oriented σ1 direction ductile extrusion in an overall compressional water. A potential water source that could drive shortly after these folds formed. setting (Law et al., 2006; Searle et al., 2006, the partial melting process could be the result Migmatite doming must have therefore oc- 2010; Searle, 2010). Ductile extrusion can of prograde dehydration reactions from under-

curred as a late feature at low-pressure (M3) explain the decompression P-T path and ex- thrusted crust. Underthrusted rocks would have conditions (~5–6 kbar and 700–730 °C) after tensional fabrics recoded in several other Bar- been experiencing prograde metamorphism and top-to-the-NNE shearing on the KSZ, and upon rovian metamorphic terranes in compression, dehydration reactions at the time of migmatiza-

Geological Society of America Bulletin, v. 1XX, no. XX/XX 41

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.

tion in the overlying units. This could alleviate Figure 23 (on following page). Extrusion model to explain and visualize the sequence of this conundrum because liberated water could deformation events that created S3 top-to-the-NNE shear fabrics on the Koronos shear zone migrate into the overlying migmatites and act (KSZ) and Zas shear zone (ZSZ) and the decompression of high-grade rocks on Naxos from as a source to drive anatexis. In contrast, during 10 to 5 kbar followed by migmatite doming. U-Pb age data will be presented in a future crustal extension, underthrusting of hydrated study. Samples TL15, TL66, TL67, and TLN54 are approximately located, and their spatial rocks would not occur, and we would expect evolution is illustrated as the model progresses. Stage 1—Pre–18 Ma: Prograde burial and dry granulite-facies rocks to underlie Naxos metamorphism of the Koronos and Core Units forming F1 isoclinal folds. Stage 2—Peak migmatites, which would be strong and less M2 kyanite-grade metamorphism and partial melting of the Core Unit. Stage 3—Synoro- likely to flow or act as a source of water to drive genic extrusion of the Koronos and Core Units under passive-roof normal faults. Stage 4— anatexis. Rotation of principal stress axes causing constriction at M3 sillimanite-grade conditions. If indeed the Koronos and Zas shear zones Stage 5—Onset of extensional tectonics and initiation of Naxos-Paros detachment system, represent passive-roof stretching normal faults which truncates and is discordant to all earlier structures. Mineral abbreviations follow in an overall compressive and E-W horizontal Whitney and Evans (2010). constrictional regime, these faults would form at the top of the SW-extruding wedge, as material was returned via ductile flow toward the foreland, and by possible erosion due to melt Exhumation of the Core Complex and isostatic footwall rebound alone. It is unclear weakening and the buoyancy and viscosity con- Regional Extension what the origin of this shortening was during the trasts of migmatites. Together, these shear zones Regional extension subsequent to 15 Ma Miocene, as it would have predated movement preserve a sequence of right-way-up metamor- (Fig. 23, stage 3; a future paper will present the on the North Anatolian fault, which commenced phic isograds, similar to the South Tibetan De- U-Th-Pb ages) was accommodated by normal at ca. 11 Ma (e.g., Menant et al., 2013). Conver- tachment System (STDS) in the Himalaya, by faulting and horizontal NNE-SSW–directed gent E-W lower-crustal flow during NNE-SSW a process of general shear (simple shear plus pure shear ductile boudinage. In the upper crust, extension (Kruckenberg et al., 2011) could pos- pure shear flattening) that telescoped the iso- normal faults sole into and crosscut a low-angle sibly explain E-W shortening in the middle of the grads (Jessup et al., 2006; Cottle et al., 2007; detachment horizon (NPDS). This structure, migmatite dome, but this should have been si- Law et al., 2006; Searle et al., 2006, 2010). Top- which is discordant to, and truncates, the fold- multaneous with NNE-SSW extensional boudi-

to-NNE shearing (S3; Fig. 23, stage 1) first de- ing, migmatite dome, and metamorphic stratig- nage and would exclusively affect migmatites, in

veloped along the rheological boundary mark- raphy within the core complex, caused extreme disagreement with our observations of F3 folding ing the solidus. Deformation temperatures in telescoping of previously frozen-in isograds also affecting the entire carapace. Furthermore, the KSZ exceeded 650 °C, and it was therefore beneath it, and rapid footwall cooling at rates many islands in the Cyclades document a signifi- responsible for the earliest and deepest stages of 60–90 °C m.y.–1 along a peak thermal gra- cant component of E-W shortening (e.g., Virgo of exhumation. With subsequent ductile flow, dient of 40 °C km–1. The onset of extension is et al., 2018, and references therein), indicating it sheath folds developed under general shear due recorded by the kink in the P-T path while the was a regional compressive stress and not related

to high non-coaxial shear strain followed by migmatites were at the close of the M3 event at to ductile convergent flow of migmatites under E-W constriction (Fig. 23, stage 2). ~5 kbar and 690 °C (Fig. 22). The Naxos I-type crustal extension (Kruckenberg et al., 2011). It Emplacement of hot migmatites at mid- granodiorite intruded into the metamorphic se- is possible that Miocene E-W shortening was crustal levels would have provided the thermal quence at ca. 12.2 Ma, and it is cut by the NPDS, related to the westward escape of Anatolia into perturbation in the hanging wall of the KSZ to with brittle-ductile deformation features includ- the Cyclades (e.g., Phillipon et al., 2014). This

nucleate posttectonic garnets that crosscut S2 ing pseudotachylytes, suggesting the arrival of would have been accommodated via dextral

kyanite-grade and S3 top-to-the-NNE shearing the core complex at the brittle-ductile transition strike-slip reactivation of the eastward Vardar su-

fabrics and M3 sillimanite-biotite intergrowths at this time. In contrast, in the ductile crust, ex- ture zone to the north of the Cyclades, and pos- with strong annealing textures (Fig. 23, stage 2). tensional strain during the latter stages of the sibly related to strike-slip movement on the Mid- This thermal pulse would also explain the intru- sillimanite-grade event was accommodated by Cycladic Lineament (e.g., Malandri et al., 2016), sion of S-type leucogranites into the Koronos mainly NNE-SSW–directed pure shear with ex- prior to rollback on the Hellenic subduction

Unit that crosscut the S3 blastomylonite fabric. tensional strains of up to ~50%, resulting in hori- zone. This scenario could produce horizontal All structures were then gently domed about a zontal boudinage and stretching that overprinted constriction-type conditions due to shortening in N-S axis that resulted in the frozen bull’s-eye earlier compressional features (e.g., Figs. 9 and all directions, like those recorded in the Naxos isograd pattern. 10) and that formed the elliptical dome structure. migmatite dome. This would have been followed Following extrusion of migmatites to shal- by NNE-SSW extension following a change in

lower crustal levels and F3 upright folding, Significance of E-W Shortening boundary stresses, either due to the onset of slab initiation of NNE-SSW regional extension This study and several others highlight a rollback on the Hellenic subduction zone or produced retrograde greenschist-facies, top-to- significant component of E-W shortening on removal of lithospheric mantle during the mid-

the-NNE (S3) mylonite fabrics associated with Naxos (e.g., Virgo et al., 2018). This shorten- Miocene. Doming of Pleistocene sediments on the NPDS, which truncated, reactivated, and ing occurred during migmatite dome formation the margins of Naxos is probably related to a

utilized subparallel (S2a) crenulation cleavages and F3 folding and occurred prior to and syn- component of recent E-W transpression, which

and top-to-the-SW (S2b) fabrics. NNE-SSW chronous with regional NNE-SSW extension. possibly resulted in a 20° counterclockwise rota- boudinage of amphibolites in the CHSZ indicate All structures, including the NPDS, aplite sills tion about a vertical axis (Kissel and Laj, 1988) a rapid switch from crustal thickening to NNE- within the 12.2 Ma granodiorite, and even Plio- and the series of conjugate NNE-SSW–trending SSW regional extension during the closure of cene–Pleistocene sediments, are folded or tilted and NNW-SSE–trending strike-slip faults that

the M3 event. around the island, which cannot be explained by crosscut the island.

42 Geological Society of America Bulletin, v. 1XX, no. XX/XX

1. Pre-18 Ma Burial and underplating of continental margin to peak M2 Kyanite-grade conditions SSW NNE 20 Km

25 Km

Koronos Shear Zone 30 Km Relict Thrust Key 35 Km Zas Unit

? South Cycladic Thrust M2 Kyanite-grade gneisses Koronos Unit Ios (Basement) 10 kbar, 670 °C 2. 18–17 Ma M2 Kyanite-grade hydrous melting and NNE-SSW ideal Core Unit simple shear extrusion 20 Km S-Type Leucogranites

25 Km Initiation of Variscan Granitic Basement Koronos Shear Zone

30 Km TLN54 Gln-Blueschist TL67 Ky-gneiss 35 Km TL15 Ky-migmatite ? South Cycladic Thrust Underthrusting of Ios (Basement) Cycladic basement M2 Kyanite Migmatites 8–10 kbar, 730 °C TL66 Sil-migmatite 3. 17–16 Ma NNE-SSW ideal simple shear extrusion, decompression from M2 Kyanite -M3 Sillimanite-grade conditions and muscovite dehydration melting 20 Km S-type leucogranite intrusions 25 Km ZSZ Top to NNE shearing migrated Sillimanite Migmatites to higher structural levels with time 30 Km 6–7 kbar, 730 °C

35 Km KSZ

Underthrusting of ? South Cycladic Thrust Cycladic basement Ios (Basement) 4. 15–16 Ma Rotation of stress axes, non-ideal E-W pure shear, M3 Sillimanite-grade upright folding, constriction, vertical extrusion and NNE-SSW extensional shearing

Rotation of S-type leucogranites around the dome Folding of Koronos and Zas Shear Zones 20 Km Koronos Shear Zone Sillimanite Migmatites 5–6 kbar, 730 °C Zas Shear Zone Vertical boudinage 25 Km and extrusion

30 Km KSZ ZSZ

35 Km

? South Cycladic Thrust Ios (Basement)

5. Post-15 Ma Initiation of crustal extension, development of a “buckling instability” in migmatites, N-S boudinage and rapid exhumation and cooling 10 Km Elliptical N-S stretching of dome Zas Shear Zone Sillimanite Migmatites <5 kbar, 690 °C ZSZ 15 Km Naxos-Paros Detachment System

20 Km KSZ Base of Upper Cycladic Nappe ? 25 Km

? South Cycladic Thrust Ios (Basement)

Figure 23. Preservation of Kyanite-grade synorogenic top-to-NNE shear fabrics on Koronos and Zas Shear Zones

Geological Society of America Bulletin, v. 1XX, no. XX/XX 43

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.

Possible Comparisons to Other exhumation of footwall material beneath a fixed tile extensional shear zones that coincided with

Mountain Belts hanging wall in an overall compressional set- peak M2–M3 Barrovian conditions at the deepest Naxos records the entire evolution of an orog- ting, and they preserve a sequence of right-way- levels. This resulted in telescoping of previously eny, from ophiolite obduction to postorogenic up metamorphic isograds that are strikingly frozen-in isograds during extrusion of footwall collapse, and it displays similarities to many similar to the Koronos and Zas shear zones on rocks, similar to the normal-sense shear zone well-studied geological areas, including the Naxos. Naxos has gone one stage further than of the STDS bounding the top of the Greater following: the Himalaya and represents a collapsed orogen Himalayan Series. Deformation migrated from (1) Oman/United Arab Emirates: The rela- that beautifully documents the transition from deep to shallow structural levels with time and tively unmetamorphosed hanging wall of the an overall compressional to extensional envi- was associated with general shear. These shear NPDS displays an ophiolitic mélange similar to ronment that coincided with the latter stages of zones predated doming and are discordant to, the Haybi oceanic unit, directly underlying the migmatite dome formation. and predated, normal faulting related to crustal sole of the Oman/United Arab Emirates ophio- (4) Betic-Rif, Spain: This structure records extension. In contrast, the low-angle normal lite (Searle and Cox, 2002). This unit is inter- the transition from overall compressional to faults truncate all structures and metamorphic preted to represent off-axis alkali seamounts extensional tectonics. Eocene to Oligocene stratigraphy in their footwall. with carbonate caps that formed on the lower south-directed subduction was associated with (3) The hanging wall of the NPDS domi- plate and were scraped off into a mélange dur- high-pressure metamorphism of the Iberian con- nantly consists of trench mélange formed along ing the subduction/obduction process. Oman tinental margin and was followed by significant the base of the obducting ophiolite. This may also records blueschist- and eclogite-facies crustal thinning and coeval heating during the record further evidence for a relict ophiolite rocks that are bounded by normal-sense exten- early stages of arc formation (e.g., Platt et al., obduction event, as seen on Tinos (Lamont,

sional shear zones within the continental mar- 2013). The mechanisms that caused extension 2018), which preceded M1 high-pressure meta- gin sequence under the ophiolite. These formed within the hinterland of this orogen are debated, morphism in the Cyclades. Ophiolitic rocks are ~15 m.y. after the initiation of obduction as the but removal of lithospheric mantle seems to be exposed at the highest structural level across the leading edge of the Arabian continental margin the most likely scenario and would result in up- Cyclades. Miocene–Pliocene fluvial sediments attempted to subduct. A comparable ophiolite welling of the asthenosphere, causing heating, a and conglomerates were deposited unconform- obduction scenario could also have taken place rise in topography, crustal extension, and rapid ably on top of this, suggesting that topography in the Cyclades prior to high-pressure metamor- westward rollback on the West Mediterranean was much higher during the Miocene.

phism of the Cycladic Blueschists, which also subduction zone (Platt et al., 2013, and refer- (4) At the deepest levels, M2 kyanite-grade represent the leading edge of the Cycladic con- ences therein). Comparable processes could hydrous melting (Eq. 6) was the dominant melt- tinental margin. have caused extension within the Aegean during process and was followed by isothermal de- (2) The Tauern Window, Austrian Alps: The ing the late Miocene. An important difference compression from ~10 kbar and 680–750 °C to metamorphic footwall of the Naxos MCC re- between Naxos and the Betics is that there is 5–6 kbar and 700–730 °C. With small degrees cords Barrovian metamorphism due to the stack- substantial kyanite-grade metamorphism and no of partial melting, the migmatites weakened ing of various tectonic slices. Many compari- subcontinental lithospheric mantle exhumed in and decompressed under a passive-roof normal sons can be drawn to the Tauern Window in the the Cyclades, as the Moho lies at ~25 km depth fault (KSZ). The hottest and most deeply buried Austrian Alps, one of the most intensely studied and deepens to the north (Cossette et al., 2016). rocks decompressed through the muscovite de- overthrust terranes on the planet (e.g., Schmid hydration melting reaction (Eq. 4), locally pro- et al., 2013; Smye et al., 2011, and references CONCLUSIONS ducing up to 20%–25% melt. This advective therein). It is composed of an amphibolite-facies process may have acted as substantial source of metamorphic core that underlies discrete slices The tectonothermal evolution of the Naxos heat applicable to many Barrovian metamorphic of eclogite- and blueschist-facies thrust sheets. metamorphic core complex is more complicated terranes. Barrovian metamorphism was a result of over- than can be explained purely by crustal exten- (5) The migmatite dome is a late feature, and thrusting Austroalpine nappes derived from the sion. The data presented in this study demon- it was formed in a constrictive stress regime at

Adriatic continental margin (upper plate), in strate that Naxos records a prolonged history of lower-pressure M3 sillimanite-grade conditions. some respects like the Upper Cycladic Nappe compression, resulting in crustal thickening and This postdated top-to-the-NNE shearing on the (Jolivet et al., 2013) and Cycladic Blueschists. regional metamorphism, followed by extension. Koronos and Zas shear zones and folded the (3) The Greater Himalayan Series (GHS) Eight key points can be made: entire metamorphic sequence. Structural analy- in the Himalaya: The metamorphic core of the (1) The footwall of the Naxos MCC rep- sis of polyphase deformation within the center Himalaya records an almost identical P-T-D resents three tectono-metamorphic units that of the dome indicates a major change in stress path to that observed on Naxos, with kyanite- each record contrasting P-T-D-t paths, and each regime from NNE-SSW compression to domi- grade conditions followed by isothermal de- represents a series of nappes that were stacked nantly E-W contraction and later NNE-SSW compression through the muscovite dehydration due to prolonged crustal thickening. During this extension, coinciding with doming. melting reaction to lower-pressure sillimanite- process, rocks were buried and metamorphosed (6) Naxos records the transition from com- grade conditions and S-type leucogranite for- diachronously as the geothermal gradient con- pression to extension in the Cyclades. The mation. Decompression occurred during SW- ductively increased over time, and eventually formation of the sillimanite-grade migmatites directed extrusion of the GHS in an overall resulted in Barrovian metamorphism and partial marks the climax of high-grade metamorphism

compressional setting, bounded by the Main melting. The sequence records M1 blueschist- in the Cyclades, and no evidence for crustal

Central thrust at the base and the South Tibetan facies conditions at the top to M2–M3 upper- thickening postdates this event. Within the Detachment System at the top, which were ac- amphibolite-facies migmatites at the base. dome, N-S pure shear horizontal extensional

tive at the same time. The extensional top-to- (2) These nappes were juxtaposed against one boudinage occurred during the end of the M3 the-NE shear fabrics on the STDS formed due to another by top-to-the-NNE normal-sense duc- sillimanite-grade event and overprints earlier

44 Geological Society of America Bulletin, v. 1XX, no. XX/XX

Koronos Unit—A kyanite-grade amphibolite- upright F3 isoclinal folds and vertical boudi- We build on this structural and metamorphic nage. This corresponds to the final stage of mig- framework and show that crustal extension was facies unit that represents the more-proximal conti nental margin, forming the deeper carapace to the matite dome formation and marks the transition associated with rapid cooling and brittle-ductile metamorphic core complex. from regional compression to extension in the normal faulting and was responsible for the final Core Unit—The kyanite- to sillimanite-grade core Cyclades. stages of core complex exhumation, which post- of the island, with migmatites, orthogneisses, and (7) Regional extension and crustal thinning dated peak metamorphism, partial melting, and structurally repeated and uprightly folded and boudi- were accommodated by brittle-ductile nor- synorogenic extrusion. naged marbles and amphibolites that form a doubly plunging migmatite dome. mal faulting that crosscut and is discordant to ACKNOWLEDGMENTS Main ultramafic horizon—A semicontinuous band all footwall structures. It attenuated and tele- of amphibolites directly above the Koronos shear zone scoped the frozen-in metamorphic isograds to This work forms part of T.N. Lamont’s doctoral that wraps around the Core Unit, locally containing produce metamorphic field gradients of up to project, which was funded by the Natural Environment enclaves of ultramafic lithologies. 700 °C km–1, particularly in west Naxos. In the Research Council (NERC; grant NE/L0021612/1). ZSZ—Zas shear zone, the top-to-the-NE greenschist-facies shear zone separating blueschist-facies ductile crust, horizontal N-S pure shear boudi- Analytical work at the NERC Isotope Geosciences Laboratory was funded by NERC Isotope Geosci- rocks of the Zas Unit (on top) from amphibolite-facies nage accommodated extensional strains of up ences Facilities Steering Committee grant IP-1597- rocks of the Koronos Unit (below). to 50%. These structures postdate all earlier 1115. We thank Philip England, Anthony Watts, Dave KSZ—Koronos shear zone, the top-to-the-NE, collisional features, and brittle-ductile normal Wallis, Lars Hansen, and Tyler Ambrose for thought- upper-amphibolite-facies shear zone bounding the faulting was responsible for the final stage of provoking and critical discussion. We extend thanks migmatite dome (Core Unit), which is folded and cut to Callum Higgins, William Nash, and Anna Bidgood by S-type leucogranites. exhumation from 5 kbar (~17.5 km depth) to the for assistance in the field, Adrian Wood for assistance M1—High-pressure–low-temperature blueschist- surface along an apparent geothermal gradient with mineral separation, Owen Green and Jonathan facies metamorphism that locally reached eclogite- of 40 °C km–1. Wells for thin section preparation, and Gren Turner, facies conditions on Syros, Tinos, and Sifnos. (8) Three types of extensional fabrics are re- Jeremy Rushton, and Jon Wade for analytical assis- M2—Regional metamorphism that reached green- corded on Naxos: (i) SW extrusion from the sub- tance. Chris Ballhaus and Christoph Von Hagkne are schist-facies conditions in the Cycladic Blueschists thanked for thorough reviews, which greatly improved (Zas Unit) and kyanite-grade conditions in the Koro- duction zone, as recorded in the Zas Unit with the quality of the manuscript, and Nick Timms, Chris nos and Core Units.

top-to-the-NE fabrics, (ii) synorogenic SSW ex- Kirkland, and Aaron Cavosie are thanked for handling M3—Regional metamorphism that is associated trusion of migmatites and high-grade gneisses the manuscript and providing further comments. We with sillimanite-grade conditions, partial melting, and under a passive-roof fault (Koronos and Zas finally thank University College, Oxford for travel S-type leucogranite intrusion at the deepest levels of bursaries covering fieldwork expenses and the Uni- shear zones), with top-to-the-NNE kinematic Naxos in the Core Unit. versity of Oxford and NERC for generously covering P-T-D-(t) paths—Pressure, temperature, deforma- indicators, and (iii) top-to-NNE extensional fab- publication and open access costs. tion, and time paths.

rics due to late brittle-ductile crosscutting nor- F1—First generation of folds, which show isoclinal mal faults related to crustal extension and cool- APPENDIX: GLOSSARY to recumbent geometries and developed during burial ing (NPDS). This movement was superimposed and SW-directed thrusting. MCC—metamorphic core complex. F —Second generation of folds to form, which on earlier fabrics along the western side of the 2 Moutsana detachment—The brittle low-angle nor- display sheath and isoclinal geometries that refold the core complex. In particular, extensional fabrics mal fault that was partly responsible for exhuming the F1 folds, and that developed during top-to-the-NNE on the Koronos and Zas shear zones within the island. shearing, during exhumation. NCDS—North Cycladic Detachment System, a metamorphic footwall were responsible for syn- F3—Last generation of folds that formed under brittle-ductile low-angle normal fault and series of lower-pressure conditions. These are upright folds orogenic extrusion of kyanite- to sillimanite- shear zones that display top-to-the-NE kinematics that trend along NNE-SSW axes across all units and grade gneisses and migmatites that predate exposed on the north coast of Mykonos, Tinos, and are boudinaged both vertically and horizontally in the Andros, juxtaposing I-type granites and blueschists in extension. These are truncated and crosscut by Core High-Strain Zone. later S-type granite dikes and lower-grade fab- the footwall from relatively less metamorphosed ophio litic rocks and sediments in the hanging wall. REFERENCES CITED rics and structures related to crustal extension. NPDS—Naxos-Paros Detachment System, the Because the timing constraints and rates of brittle-ductile low-angle normal-sense extensional Alsop, G.I., and Holdsworth, R.E., 2006, Sheath folds as deformation, metamorphism, and partial melt- shear zone that cuts all previous metamorphic stra- discriminators of bulk strain type: Journal of Structural ing are crucial for resolving the complete P-T- tigraphy and produces telescoping of isograds on the Geology, v. 28, p. 1588–1606, https://doi.org/10.1016 D-t history of Naxos to a high resolution, we western side of the island. /j.jsg.2006.05.005. Carapace—the metasedimentary units that over- Andriessen, P.A.M., Boelrijk, N.A.I.M., Hebeda, E.H., will provide new U-Th-Pb age data in a future lie the migmatite dome, including the Zas Unit and Priem, H.N.A., Verdurmen, E.A.T., and Verschure, paper. We also present a fully integrated tectonic Koronos Unit. R.H., 1979, Dating the events of metamorphism and model for the development and exhumation of CHSZ—Core high-strain zone, a zone of upright granitic magmatism in the Alpine orogen of Naxos folded marbles and amphibolites that are affected by (Cyclades, Greece): Contributions to Mineralogy and the Naxos MCC and show that Naxos docu- Petrology, v. 69, p. 215–225, https://doi.org/10.1007 vertical and horizontal boudinage at the center of the ments the entire evolution of what we term the /BF00372323. migmatite dome, best exposed in the Boulibas and Armstrong, R.L., 1982, Cordilleran metamorphic core “Aegean orogeny.” This mountain belt records a Kinadaros quarries. complexes from Arizona to southern Canada: Annual full orogenic cycle from ophiolite obduction, to Upper Cycladic Nappe—The relatively unmeta- Review of Earth and Planetary Sciences, v. 10, p. 129– high-pressure metamorphism, through subduc- morphosed, ophiolitic and sedimentary hanging wall 154, https://doi .org /10 .1146 /annurev .ea .10 .050182 tion-related exhumation, crustal thickening, and of the Naxos metamorphic core complex and the .001021. hanging wall of the North Cycladic detachment sys- Ashworth, J.R., 1975, The sillimanite zones of the Huntly regional metamorphism to postorogenic exten- tem on Mykonos, Tinos, and Andros. Portsoy area in the north-east Dalradian, Scotland: sion. We demonstrate that the transition from GHS—Greater Himalayan Series. Geological Magazine, v. 112, p. 113–136, https://doi STDS—South Tibetan Detachment System. .org/10.1017/S0016756800045817. compression to extension on Naxos coincided Ashworth, J.R., 1979, Textural and mineralogical evolution with the latter stages of migmatite formation TTG—Tonalite-trondhjemite-granodiorite. of migmatites: Geological Society of London, Special Zas Unit—A blueschist-facies unit that can be re- at ca. 15 Ma, corresponding with the timing of Publications, v. 8, no. 1, p. 357–361, https://doi.org/10 gionally correlated with the Cycladic Blueschist Unit, .1144/GSL.SP.1979.008.01.40. a major slowdown in the convergence rate be- representing the distal Cycladic (Adriatic) continental Avigad, D., 1998, High-pressure metamorphism and cooling tween Nubia and Eurasia (DeMets et al., 2015). margin. on SE Naxos (Cyclades, Greece): European Journal

Geological Society of America Bulletin, v. 1XX, no. XX/XX 45

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.

of Mineralogy, v. 10, p. 1309–1320, https://doi.org/10 Cao, S., Neubauer, F., Bernroider, M., and Liu, J., 2013, The subduction: An example from Sifnos, Greece: Chemi- .1127/ejm/10/6/1309. lateral boundary of a metamorphic core complex: The cal Geology, v. 314, p. 9–22, https://doi.org/10.1016/j Avigad, D., Baer, G., and Heimann, A., 1998, Block rotations Moutsounas shear zone on Naxos, Cyclades, Greece: .chemgeo.2012.04.016. and continental extension in the central Aegean Sea: Journal of Structural Geology, v. 54, p. 103–128, Duchêne, S., Aïssa, R., and Vanderhaeghe, O., 2006, Pres- Palaeomagnetic and structural evidence from Tinos https://doi.org/10.1016/j.jsg.2013.07.002. sure-temperature-time evolution of metamorphic rocks and Mykonos: Earth and Planetary Science Letters, Cao, S., Neubauer, F., Bernroider, M., Genser, J., Liu, J., from Naxos (Cyclades, Greece): Constraints from v. 157, p. 23–40, https://doi.org/10.1016/S0012-821X and Friedl, G., 2017, Low-grade retrogression of a thermobarometry and Rb/Sr dating: Geodinamica (98)00024-7. high-temperature metamorphic core complex: Naxos, Acta, v. 19, p. 301–321, https://doi.org/10.3166/ga.19 Axen, G.J., 2007, Research focus: Significance of large- Cyclades, Greece: Geological Society of America .301-321. displacement, low-angle normal faults: Geology, Bulletin, v. 129, p. 93–117, https://doi .org /10 .1130 Durr, S., Altherr, R., Keller, J., Okrusch, M., and Seidel, E., v. 35, p. 287–288, https://doi.org/10.1130/0091-7613 /B31502.1. 1978, The median Aegean crystalline belt: stratigra- (2007)35[287:RFSOLL]2.0.CO;2. Carmichael, D.M., 1969, On the Mechanism of Pro- phy, structure, metamorphism, magmatism, in Cloos, Baker, J., and Matthews, A., 1994, Textural and isotopic grade Metamorphic Reactions in Quartz-Bearing H., Roeder, D., and Schmidt, K., eds., Alps, Apennines, development of marble assemblages during the Bar- Pelitic Rocks: Contributions to Mineralogy and Hellenides: Schweizerbart, Stuttgart, p. 455–476. Petrology, v. 20, p. 244–267, https://doi.org/10.1007 rovian-style M2 metamorphic event, Naxos, Greece: England, P.C., 1978, Some thermal considerations of the Al- Contributions to Mineralogy and Petrology, v. 116, /BF00377479. pine metamorphism, past, present and future: Tectono- p. 130–144, https://doi.org/10.1007/BF00310695. Cobbold, P.R., and Quinquis, H., 1980, Development of physics, v. 46, p. 21–40, https://doi.org/10.1016/0040 Baker, J., Bickle, M.J., Buick, I.S., Holland, T.J.B., and Mat- sheath folds in shear regimes: Journal of Structural -1951(78)90103-8. thews, A., 1989, Isotopic and petrological evidence for Geology, v. 2, p. 119–126, https://doi .org /10 .1016 /0191 England, P.C., and Holland, T.J.B., 1979, Archimedes and the infiltration of water-rich fluids during the Miocene -8141(80)90041 -3 . the Tauern eclogites: The role of buoyancy in the pres- Coggon, R., and Holland, T.J.B., 2002, Mixing properties M2 metamorphism on Naxos, Greece: Geochimica et ervation of exotic eclogite blocks: Earth and Planetary Cosmochimica Acta, v. 53, p. 2037–2050, https://doi of phengitic micas and revised garnet-phengite thermo- Science Letters, v. 44, p. 287–294, https://doi.org/10 .org/10.1016/0016-7037(89)90323-2. barometers: Journal of Metamorphic Geology, v. 20, .1016/0012-821X(79)90177-8. Bhattacharya, L., Mohanty, L., Maji, A., Sen, S.K., and p. 683–696, https://doi.org/10.1046/j.1525-1314.2002 England, P.C., and Jackson, J., 1987, Migration of the Raith, M., 1992, Non-ideal mixing in the phlogopite- .00395.x. seismic-aseismic transition during uniform and non- annite binary: Constraints from experimental data on Collins, W.J., Van Kranendonk, M.J., and Teyssier, C., 1998, uniform extension of the continental lithosphere: Geol Mg-Fe partitioning and a reformulation of the biotite- Partial convective overturn of Archaean crust in the ogy, v. 15, p. 291–294, https://doi.org/10.1130/0091 garnet geothermometer: Contributions to Mineralogy east Pilbara Craton, Western Australia: Driving mecha- -7613(1987)15<291:MOTSTD>2.0.CO;2. and Petrology, v. 111, p. 87–93, https://doi.org/10.1007 nisms and tectonic implications: Journal of Structural England, P.C., and Richardson, S.W., 1977, The influence of /BF00296580. Geology, v. 20, p. 1405–1424. erosion upon the mineral facies of rocks from different Bickle, M.J., and Baker, J., 1990, Advective-diffusive trans- Coney, P.J., 1980, Cordilleran metamorphic core complexes: metamorphic environments: Journal of the Geological port of isotopic fronts: An example from Naxos, Greece: An overview, in Crittenden, M.D., Jr., Coney, P.J., Society [London], v. 134, p. 201–213, https://doi.org Earth and Planetary Science Letters, v. 97, p. 78–93, and Davis, G.H., eds., Cordilleran Metamorphic Core /10.1144/gsjgs.134.2.0201. https://doi.org/10.1016/0012-821X(90)90100-C. Complexes: Geological Society of America Memoir England, P.C., and Thompson, A.B., 1984, Pressure–tem- Bickle, M.J., Hawkesworth, C., England, P.C., and Athey, 153, p. 7–34, https://doi.org/10.1130/MEM153-p7. perature–time paths of regional metamorphism I. Heat Conrad, W.K., Nicholls, L.A., and Wall, V.J., 1988, Water- D., 1975, A preliminary thermal model for regional transfer during the evolution of regions of thickened saturated and -undersaturated melting of metalumi- metamorphism in the eastern Alps: Earth and Plan- continental crust: Journal of Petrology, v. 25, p. 894– nous and peraluminous crustal compositions at 10 kb: etary Science Letters, v. 26, p. 13–28, https://doi.org 928, https://doi.org/10.1093/petrology/25.4.894. Evidence for the origin of silicic magmas in the Taupo /10.1016/0012-821X(75)90173-9. Evans, B.W., and Guidotti, C.V., 1966, The sillimanite- volcanic zone, New Zealand, and other occurrences: Bolhar, R., Ring, U., and Ireland, T.R., 2017, Zircon in am- potash feldspar isograd in western Maine, U.S.A.: Journal of Petrology, v. 29, p. 765–803, https://doi.org phibolites from Naxos, Aegean Sea, Greece: origin, Contributions to Mineralogy and Petrology, v. 12, /10.1093/petrology/29.4.765. significance and tectonic setting: Journal of Metamor- p. 25–62, https://doi.org/10.1007/BF02651127. Corfield, R.I., and Searle, M.P., 2000, Crustal shortening esti phic Geology, v. 35, p. 413–434, https://doi.org/10 Ez, V., 2000, When shearing is a cause of folding: Earth- mates across the north Indian continental margin, Ladakh, .1111/jmg.12238. Science Reviews, v. 51, p. 155–172, https://doi.org/10 NW India, in Asif Khan, M., Treloar, P.J., Searle, M.P., Bröcker, M., and Franz, L., 1998, Rb-Sr isotope studies .1016/S0012-8252(00)00020-9. and Qasim Jan, M., eds., Tectonics of the Nanga Parbat on Tinos island (Cyclades, Greece): Additional time Feenstra, A., 1985, Metamorphism of bauxites on Naxos Syntaxis and the Western Himalaya: Geological Society constraints for metamorphism, extent of infiltration- of London Special Publication 170, p. 395–410, https:// [PhD Thesis], Utrecht, Netherlands, Rijks Universiteit. controlled overprinting and deformational activity: doi.org/10.1144/GSL.SP.2000.170.01.21. Fletcher, R.C., and Hallet, B., 1983, Unstable exten- Geological Magazine, v. 135, p. 369–382, https://doi Cossette, É., Audet, P., Schneider, D., and Grasemann, B., sion of the lithosphere: A mechanical model for .org/10.1017/S0016756898008681. 2016, Structure and anisotropy of the crust in the Cy- Basin-and-Range structure: Journal of Geophysical Brown, M., 2002, Retrograde processes in migmatites and clades, Greece, using receiver functions constrained by Research, v. 88, p. 7457–7466, https://doi.org/10.1029 granulites revisited: Journal of Metamorphic Geology, in situ rock textural data: Journal of Geophysical Re- /JB088iB09p07457. v. 20, p. 25–40, https://doi.org/10.1046/j.0263-4929 search–Solid Earth, v. 121, p. 2661–2678, https://doi García-Casco, A., Torres-Roldán, R.L., Millán, G., Monié, .2001.00362.x. .org/10.1002/2015JB012460. P., and Haissen, F., 2001, High-grade metamorphism Buick, I.S., 1991a, Mylonite fabric development on Naxos, Cottle, J.M., Jessup, M.J., Newell, D.J., Searle, M.P., Law, and hydrous melting of metapelites in the Pinos terrane Greece: Journal of Structural Geology, v. 13, p. 643– R.D., and Horstwood, M.S.A., 2007, Structural in- (W Cuba): Evidence for crustal thickening and exten- 655, https://doi.org/10.1016/0191-8141(91)90027-G. sights into the early stages of exhumation along an sion in the northern Caribbean collisional belt: Journal Buick, I.S., 1991b, The late Alpine evolution of an exten- orogen-scale detachment: The South Tibetan detach- of Metamorphic Geology, v. 19, p. 699–715, https://doi sional shear zone, Naxos, Greece: Journal of the Geo- ment system, Dzakaa Chu section, Eastern Himalaya: .org/10.1046/j.0263-4929.2001.00343.x. logical Society [London], v. 148, p. 93–103, https://doi Journal of Structural Geology, v. 29, p. 1781–1797, García-Casco, A., Haissen, F., Castro, A., El-Hmidi, H., .org/10.1144/gsjgs.148.1.0093. https://doi.org/10.1016/j.jsg.2007.08.007. Torres-Roldán, R.L., and Millán, G., 2003, Synthe- Buick, I.S., and Holland, T.J.B., 1989, The P-T-t path associ- Cruciani, G., Franceschelli, M., Elter, F.M., Puxeddu, M., sis of staurolite in melting experiments of a natural ated with crustal extension, in Daly, J.S., Cliff, R.A., and Utzeri, D., 2008, Petrogenesis of Al-silicate-bear- metapelite: Consequences for the phase relations in and Yardley, W.D., eds., Evolution of Metamorphic ing trondhjemitic migmatites from NE Sardinia, Italy: low-temperature pelitic migmatites: Journal of Pe- Belts: Geological Society of London Special Publica- Lithos, v. 102, p. 554–574, https://doi.org/10.1016/j trology, v. 44, p. 1727–1757, https://doi.org/10.1093 tion 43, p. 365–369. .lithos.2007.07.023. /petrology/egg056. Buick, I.S., and Holland, T.J.B., 1991, The nature and dis- Deer, W.A., Howie, R.A., and Zussman, J., 1992, An In- Gautier, P., and Brun, J.P., 1994a, Crustal-scale geometry tribution of fluids during amphibolite-facies meta- troduction to the Rock Forming Minerals (2nd ed.): and kinematics of late-orogenic extension in the central morphism, Naxos (Greece): Journal of Metamorphic Harlow, Essex, UK Longman Scientific & Technical, Aegean (Cyclades and Evvia island): Tectonophysics, Geology, v. 9, p. 301–314, https://doi.org/10.1111/j 528 p. v. 238, p. 399–424, https://doi.org/10.1016/0040-1951 .1525-1314.1991.tb00525.x. DeMets, C., Iaffaldano, G., and Merkouriev, S., 2015, High- (94)90066-3. Burg, J.P., Guiraud, M., Chen, G.M., and Li, G.C., 1984, resolution Neogene and Quaternary estimates of Nubia– Gautier, P., and Brun, J.P., 1994b, Ductile crust exhumation Himalayan metamorphism and deformations in the Eurasia–North America plate motion: Geophysical and extensional detachments in the central Aegean North Himalayan belt (southern Tibet, China): Earth Journal International, v. 203, p. 416–427, https://doi (Cyclades and Evvia islands): Geodinamica Acta, and Planetary Science Letters, v. 69, p. 391–400, .org/10.1093/gji/ggv277. v. 7, p. 57–85, https://doi.org/10.1080/09853111.1994 https://doi.org/10.1016/0012-821X(84)90197-3. Dixon, J.E., and Robertson, A.H.F., eds., 1984, The Geologi- .11105259. Caddick, M.J., Konopásek, J., and Thompson, A.B., 2010, cal Evolution of the Eastern Mediterranean: Geological Grasemann, B., Schneider, D.A., Stockli, D.F., and Iglseder, Preservation of garnet growth zoning and the dura- Society Special Publication, v. 17, p. 824. C., 2012, Miocene bivergent crustal extension in tion of prograde metamorphism: Journal of Petrology, Dragovic, B., Samanta, L.M., Baxter, E.F., and Selverstone, the Aegean: Evidence from the western Cyclades v. 51, p. 2327–2347, https://doi.org/10.1093/petrology J., 2012, Using garnet to constrain the duration and (Greece): Lithosphere, v. 4, p. 23–39, https://doi.org /egq059. rate of water-releasing metamorphic reactions during /10.1130/L164.1.

46 Geological Society of America Bulletin, v. 1XX, no. XX/XX

Green, E.C.R., White, R.W., Diener, J.F.A., Powell, R., Hollister, L.S., 1966, Garnet zoning: an interpretation based Katzir, Y., John, E., Valley, W., Matthews, A., and Spi- Holland, T.J.B., and Palin, R.M., 2016, Activity-com- on the Rayleigh fractionation model: Science, v. 154, cuzza, M.J., 2002, Tracking fluid flow during deep position relations for the calculation of partial melting p. 1647–1651. crustal anatexis: Metasomatism of peridotites (Naxos, equilibria in metabasic rocks: Journal of Metamorphic Holness, M., 2008, Decoding migmatite microstructures, in Greece): Contributions to Mineralogy and Petrology, Geology, v. 34, p. 845–869, https://doi.org/10.1111 Sawyer, E.W., and Brown, M., eds., Working with Mig- v. 142, p. 700–713, https://doi .org /10 .1007 /s00410 /jmg.12211. matites: Mineralogical Association of Canada Short -001-0319-4. Guidotti, C.V., 1984, Micas in metamorphic rocks, in Bailey, Course 38, p. 57–76. Katzir, Y., Garfunkel, Z., Avigad, D., and Matthews, A., S.W., ed., Micas: Mineralogical Society of America Huet, B., Labrousse, L., and Jolivet, L., 2009, Thrust or 2007, The geodynamic evolution of the Alpine orogen Reviews in Mineralogy 13, p. 357–468, https://doi.org detachment? Exhumation processes in the Aegean: in the Cyclades (Aegean Sea, Greece): Insights from /10.1515/9781501508820-014. Insight from a field study on Ios (Cyclades, Greece): diverse origins and modes of emplacement of ultra- Guilmette, C., Indares, A., and Hébert, R., 2011, High-pres- Tectonics, v. 28, TC3007, https://doi .org /10 .1029 mafic rocks, in Taymaz, T., Yilmaz, Y., and Dilek, Y., sure anatectic paragneisses from the Namche Barwa, /2008TC002397. eds., The Geodynamics of the Aegean and Anatolia: Eastern Himalayan syntaxis: Textural evidence for Indares, A., White, R.W., and Powell, R., 2008, Phase equi- Geological Society of London Special Publication 291, partial melting, phase equilibria modeling and tectonic libria modelling of kyanite-bearing anatectic para p. 17–40. implications: Lithos, v. 124, p. 66–81, https://doi.org gneiss from the central Grenville Province: Journal of Keay, S., Lister, G., and Buick, I., 2001, The timing of /10.1016/j.lithos.2010.09.003. Metamorphic Geology, v. 26, p. 815–836, https://doi partial melting, Barrovian metamorphism and granite Hacker, B.R., Gerya, T.V., and Gilotti, J.A., 2013, Forma- .org/10.1111/j.1525-1314.2008.00788.x. intrusion in the Naxos metamorphic core complex, Cy- tion and exhumation of ultrahigh-pressure terranes: Jansen, J.B.H., 1973, Geological Map of Greece, Island of clades, Aegean Sea, Greece: Tectonophysics, v. 342, Elements, v. 9, p. 289–293, https://doi.org/10.2113 Naxos: Athens, Greece, Institute for Geology and Min- p. 275–312, https://doi.org/10.1016/S0040-1951 /gselements.9.4.289. eral Resources. (01)00168-8. Halpin, J.A., Clarke, G.L., White, R.W., and Kelsey, D.E., Jansen, J.B.H., and Schuiling, R.D., 1976, Metamorphism Kissel, C., and Laj, C., 1988, The Tertiary geodynamical 2007, Contrasting P–T–t paths for Neoproterozoic on Naxos: Petrology and geothermal gradients: Ameri- evolution of the Aegean arc: A paleomagnetic recon- metamorphism in macrobertson and Kemp Lands, east can Journal of Science, v. 276, p. 1225–1253, https:// struction: Tectonophysics, v. 146, p. 183–201, https:// Antarctica: Journal of Metamorphic Geology, v. 25, doi.org/10.2475/ajs.276.10.1225. doi.org/10.1016/0040-1951(88)90090-X. p. 683–701, https://doi.org/10.1111/j.1525-1314.2007 Jessup, M.J., Law, R.D., Searle, M.P., and Hubbard, M.S., Kohn, M.J., and Spear, F.S., 2000, Retrograde net transfer .00723.x. 2006, Structural evolution and vorticity of flow during reaction insurance for pressure-temperature estimates: Heinrich, C. A., 1982, Kyanite-eclogite to amphibolite fades extrusion and exhumation of the Greater Himalayan Geology, v. 28, p. 1127–1130, https://doi.org/10.1130 evolution of hydrous mafic and pelitic rocks, Adula Slab, Mount Everest Massif, Tibet/Nepal: implications /0091-7613(2000)28<1127:RNTRIF>2.0.CO;2. nappe, Central Alps: Contributions to Mineralogy and for orogen-scale flow partitioning: Geological Soci- Koukouvelas, I.K., and Kokkalas, S., 2003, Emplacement of Petrology, v. 81, no. 1, p. 30–38. ety, London, Special Publications, v. 268, p.379–413, the Miocene west Naxos pluton (Aegean Sea, Greece): Henry, D.J., Guidotti, C.V., and Thomson, J.A., 2005, The https://doi.org/10.1144/GSL.SP.2006.268.01.18. A structural study: Geological Magazine, v. 140, p. 45– Ti-saturation surface for low-to-medium pressure John, B.E., and Howard, K.A., 1995, Rapid extension 61, https://doi.org/10.1017/S0016756802007094. metapelitic biotite: Implications for geothermometry recorded by cooling age pattern and brittle defor- Kruckenberg, S.C., Ferré, E.C., Teyssier, C., Vander and Ti-substitution mechanisms: The American Min- mation, Naxos, Greece: Journal of Geophysical Re- haeghe, O., Whitney, D.L., Seaton, N.C.A., and Skord, eralogist, v. 90, p. 316–328, https://doi.org/10.2138/am search, v. 100, p. 9969–9979, https://doi.org/10.1029 J.A., 2010, Viscoplastic flow in migmatites deduced /95JB00665. .2005.1498. from fabric anisotropy: An example from the Naxos Jolivet, L., 2001, A comparison of geodetic and finite strain Hinsken, T., Bröcker, M., Strauss, H., and Bulle, F., 2017, dome, Greece: Journal of Geophysical Research– pattern in the Aegean, geodynamic implications: Geochemical, isotopic and geochronological charac- Solid Earth, v. 115, B09401, https://doi.org/10.1029 Earth and Planetary Science Letters, v. 187, no. 1-2, terization of listvenite from the Upper Unit on Tinos, /2009JB007012. p. 95–104. Cyclades, Greece: Lithos, v. 282-282, p. 281–297, Kruckenberg, S.C., Vanderhaeghe, O., Ferré, E.C., Teyssier, Jolivet, L., and Brun, J.P., 2010, Cenozoic geodynamic evo- https://doi.org/10.1016/j.lithos.2017.02.019. C., and Whitney, D.L., 2011, Flow of partially molten lution of the Aegean region: International Journal of Hodges, K.V., Walker, J.D., and Wernicke, B.P., 1987, crust and the internal dynamics of a migmatite dome, Earth Sciences, v. 99, p. 109–138, https://doi.org/10 Footwall structural evolution of the Tucki Mountain Naxos, Greece: Tectonics, v. 30, TC3001, https://doi .1007/s00531-008-0366-4. detachment system, Death Valley region, southeast- .org/10.1029/2010TC002751. Jolivet, L., Faccenna, C., Goffé, B., Burov, E., and Agard, ern California: Geological Society of London, Special Kreulen, R., 1980, CO rich fluids during regional metamor- P., 2003, Subduction tectonics and exhumation of high- 2 Publications, v. 28, no. 1, p. 393–408, https://doi.org phism on Naxos (Greece); carbon isotopes and fluid pressure metamorphic rocks in the Mediterranean oro- /10.1144/GSL.SP.1987.028.01.24. gens: American Journal of Science, v. 303, p. 353–409, inclusions: American Journal of Science, v. 280, no. 8, Holdaway, M.J., 2000, Application of new experimental https://doi.org/10.2475/ajs.303.5.353. p. 745–771, https://doi.org/10.2475/ajs.280.8.745. and garnet Margules data to the garnet-biotite geother- Jolivet, L., Famin, V., Mehl, C., Parra, T., Aubourg, C., Lagos, M., Scherer, E. E., Tomaschek, F., Münker, C., mometer: The American Mineralogist, v. 85, p. 881– Hébert, R., and Philippot, P., 2004, Strain localization Keiter, M., Berndt, J., and Ballhaus, C., 2007, High 892, https://doi.org/10.2138/am-2000-0701. during crustal-scale boudinage to form extensional precision Lu–Hf geochronology of Eocene eclogite- Holland, T.J.B., 2009, AX: A program to calculate activi- metamorphic domes in the Aegean Sea, in Whitney, facies rocks from Syros, Cyclades, Greece: Chemical ties of mineral end-members from chemical analyses: D.L., Teyssier, C., and Siddoway, C.S., eds., Gneiss Geology, v. 243, p. 16–35, https://doi.org/10.1016/j http://www.esc.cam.ac.uk/research/research-groups Domes in Orogeny: Geological Society of America .chemgeo.2007.04.008. /holland/ax (accessed July 2016). Special Paper 380, p. 185–210. Lamont, T.N., 2018, Unravelling the structural, metamor- Holland, T.J.B., and Blundy, J., 1994, Non-ideal interactions Jolivet, L., Lecomte, E., Huet, B., Denèle, Y., Lacombe, O., phic, and strain history of the “Aegean Orogeny,” in calcic amphiboles and their bearing on amphibole- Labrousse, L., Le Pourhiet, L., and Mehl, C., 2010, The Southern Greece, with a combined structural, petro- plagioclase thermometry: Contributions to Mineralogy North Cycladic detachment system: Earth and Plane- logical and geochronological approach [D.Phil thesis]: and Petrology, v. 116, p. 433–447, https://doi.org/10 tary Science Letters, v. 289, p. 87–104, https://doi.org University of Oxford. .1007/BF00310910. /10.1016/j.epsl.2009.10.032. Laurent, V., Jolivet, L., Roche, R., Augier, R., Scaillet, R., Holland, T.J.B., and Powell, R., 1998, An internally-consis- Jolivet, L., Faccenna, C., Huet, B., Labrousse, L., Le and Cardello, G. L., 2016, Strain localization in a fos- tent thermodynamic dataset for phases of petrologi- Pourhiet, L., Lacombe, O., Lecomte, E., Burov, E., silized subduction channel: Insights from the Cycladic cal interest: Journal of Metamorphic Geology, v. 16, Denèle, Y., and Brun, J. P., 2013, Aegean tectonics: Blueschist Unit (Syros, Greece): Tectonophysics, p. 309–343, https://doi.org/10.1111/j.1525-1314.1998 Strain localisation, slab tearing and trench retreat: Tec- v. 672–673, p. 150–169, https://doi.org/10.1016/j.tecto .00140.x. tonophysics, v. 597–598, p. 1–33, https://doi.org/10 .2016.01.036. Holland, T.J.B., and Powell, R., 2003, Activity-composition .1016/j.tecto.2012.06.011. Law, R.D., Searle, M.P., and Godin, L., 2006, Channel flow, relations for phases in petrological calculations: An Jolivet, L., Menant, A., Sternai, P., Rabillard, A., Arbaret, A., ductile extrusion and exhumation in continental colli- asymmetric multicomponent formulation: Contribu- Augier, R., Laurent, V., Beaudoin, A., Grasemann, B., sion zones, in Law, R.D., Searle, M.P., and Godin, L., tions to Mineralogy and Petrology, v. 145, p. 492–501, and Huet, B., 2015, The geological signature of a slab eds., Channel Flow, Ductile Extrusion, Exhumation https://doi.org/10.1007/s00410-003-0464-z. tear below the Aegean: Tectonophysics, v. 659, p. 166– in Continental Collision Zones: Geological Society Holland, T.J.B., and Powell, R., 2011, An improved and ex- 182, https://doi.org/10.1016/j.tecto.2015.08.004. of London Special Publication 268, https://doi.org/10 tended internally consistent thermodynamic dataset for Katzir, Y., Matthews, A., Garfunkel, Z., Schliestedt, M., and .1144/GSL.SP.2006.268.01.28. phases of petrological interest, involving a new equa- Avigad, D., 1996, The tectono-metamorphic evolution Le Pichon, X., and Angelier, J., 1979, The Hellenic arc tion of state for solids: Journal of Metamorphic Geol- of a dismembered ophiolite (Tinos, Cyclades, Greece): and trench system: A key to the neotectonic evolu- ogy, v. 29, p. 333–383, https://doi.org/10.1111/j.1525 Geological Magazine, v. 133, p. 237–254, https://doi tion of the Eastern Mediterranean area: Tectonophys- -1314.2010.00923.x. .org/10.1017/S0016756800008992. ics, v. 60, p. 1–42, https://doi.org/10.1016/0040-1951 Holland, T.J.B., Baker, J.M., and Powell, R., 1998, Mixing Katzir, Y., Avigad, D., Matthews, A., Garfunkel, Z., and (79)90131-8. properties and activity-composition relationships of Evans, B. W., 1999, Origin and metamorphism of ultra Le Pichon, X., and Angelier, J., 1981, The Aegean Sea:

chlorites in the system MgO-FeO-Al2O3-SiO2-H2O: basic rocks associated with a subducted continental Philosophical Transactions of the Royal Society of European Journal of Mineralogy, v. 10, p. 395–406, margin, Naxos (Cyclades, Greece): Journal of Meta- London, v. 300, p. 357–372, https://doi.org/10.1098 https://doi.org/10.1127/ejm/10/3/0395. morphic Geology, v. 17, p. 301–318. /rsta.1981.0069.

Geological Society of America Bulletin, v. 1XX, no. XX/XX 47

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.

Le Pichon, X., Lallemant, S.J., Chamot-Rooke, N., Lemeur, Palin, R.M., Weller, O.M., Waters, D.J., and Dyck, B., 2016, Society London], v. 137, p. 261–260, https://doi.org/10 D., and Pascal, G., 2002, The Mediterranean Ridge Quantifying geological uncertainty in metamorphic .1144/gsjgs.137.3.0261. backstop and the Hellenic nappes: Marine Geology, phase equilibria modelling; a Monte Carlo assessment Ramsay, J.G., and Huber, M.I., 1987, The Techniques of v. 186, no. 1-2, p. 111–125, https://doi.org/10.1016 and implications for tectonic interpretations: Geosci- Modern Structural Geology, v. 2: Folds and Fractures: /S0025-3227(02)00175-5. ence Frontiers, v. 7, p. 591–607, https://doi.org/10 London, Pergamon Press, 392 p. Lee, J., Hacker, B., and Wang, Y., 2004, Evolution of North .1016/j.gsf.2015.08.005. Rey, P.F., Teyssier, C., Kruckenberg, S.C., and Whitney, Himalayan gneiss domes: Structural and metamor- Parra, T., Vidal, O., Jolivet, L., 2002, Relation between the D.L., 2011, Viscous collision in channel explains dou- phic studies in Mabja Dome, southern Tibet: Journal intensity of deformation and retrogression in blueschist ble domes in metamorphic core complexes: Geology, of Structural Geology, v. 26, p. 2297–2316, https://doi metapelites of Tinos Island (Greece) evidenced by v. 39, p. 387–390, https://doi.org/10.1130/G31587.1. .org/10.1016/j.jsg.2004.02.013. chlorite–mica local equilibria: Lithos, v. 63, p. 41–66, Rey, P.F., Mondy, L., Duclaux, G., Teyssier, C., Whitney, Lee, J., McClelland, W., Wang, Y., Blythe, A., and McWil- https://doi.org/10.1016/S0024-4937(02)00115-9. D.L., Bocher, M., and Prigent, C., 2017, The origin of liams, M., 2006, Oligocene–Miocene middle crustal Patiño Douce, A.E., and Harris, N., 1998, Experimental contractional structures in extensional gneiss domes: flow in southern Tibet: Geochronology of Mabja constraints on Himalayan anatexis: Journal of Petrol- Geology, v. 45, p. 263–266, https://doi .org /10 .1130 Dome, in Law, R.D., Searle, M.P., and Godin, L., eds., ogy, v. 39, p. 689–710, https://doi.org/10.1093/petroj /G38595.1. Channel Flow, Ductile Extrusion and Exhumation in /39.4.689. Ring, U., and Layer, P.W., 2003, High-pressure metamor- Continental Collision Zones: Geological Society of Pattison, D.R.M., and Begin, N.J., 1994, Hierarchy of clo- phism in the Aegean, Eastern Mediterranean: Under- London Special Publication 268, p. 445–469, https:// sure temperatures in granulites and the importance of plating and exhumation from the Late Cretaceous until doi.org/10.1144/GSL.SP.2006.268.01.21. an intergranular exchange medium (melt?) in control- the Miocene to Recent above the retreating Hellenic Lister, G.S., Banga, G., and Feenstra, A., 1984, Meta- ling maximum Fe-Mg exchange temperatures: Miner- subduction zone: Tectonics, v. 22, 1022, https://doi.org morphic core complexes of cordilleran type in the alogical Magazine, v. 58A, p. 694–695, https://doi.org /10.1029/2001TC001350. Cyclades, Aegean Sea, Greece: Geology, v. 12, p. 221– /10.1180/minmag.1994.58A.2.99. Ring, U., Thomson, S.N., and Bröcker, M., 2003, Fast 225, https://doi.org/10.1130/0091-7613(1984)12<221: Pattison, D.R.M., and Tracy, R.J., 1991, Phase equilibria and extension but little exhumation: The Vari detach- MCCOCT>2.0.CO;2. thermobarometry of metapelites: Mineralogy Society ment in the Cyclades, Greece: Geological Maga- Mahar, E.M., Baker, J.M., Powell, R., Holland, T.J.B., and of America Reviews in Mineralogy, v. 26, p. 105–206. zine, v. 140, p. 245–252, https://doi.org/10.1017 Howell, N., 1997, The effect of Mn on mineral stabil- Pattison, D.R.M., Chacko, T., Farquhar, J., and Mcfarlane, /S0016756803007799. ity in metapelites: Journal of Metamorphic Geology, C.R.M., 2003, Temperatures of granulite-facies meta- Ring, U., Glodny, J., Will, T., and Thomson, S., 2007a, v. 15, p. 223–238, https://doi.org/10.1111/j.1525-1314 morphism: Constraints from experimental phase equi- An Oligocene extrusion wedge of blueschists-facies .1997.00011.x. libria and thermobarometry corrected for retrograde nappes on Evia, Aegean Sea, Greece: Implications for Malandri, C., Soukis, K., Maffione, M., Özkaptan, M., Vas- exchange: Journal of Petrology, v. 44, p. 867–900, the early exhumation of high-pressure rocks: Journal of silakis, E., Lozios, S., and van Hinsbergen, D.J., 2016, https://doi.org/10.1093/petrology/44.5.867. the Geological Society [London], v. 164, p. 637–652, Vertical-axis rotations accommodated along the Mid- Peillod, A., Ring, U., Glodny, J., and Skelton, A., 2017, https://doi.org/10.1144/0016-76492006-041. Cycladic lineament on Paros Island in the extensional An Eocene/Oligocene blueschist/greenschist-facies Ring, U., Will, T., Glodny, J., Kumerics, C., Gessner, K., heart of the Aegean orocline (Greece): Lithosphere, P-T loop from the Cycladic Blueschist Unit on Naxos Thomson, S., Güngör, T., Monie, P., Okrusch, M., v. 9, p. 78–99, https://doi.org/10.1130/L575.1. Island, Greece: Deformation-related reequilibration vs and Drüppel, K., 2007b, Early exhumation of high- Martin, L., Duchêne, S., Deloule, E., and Vanderhaeghe, O., thermal relaxation: Journal of Metamorphic Geology, pressure rocks in extrusion wedges: Cycladic Blue- v. 35, p. 805–830, https://doi.org/10.1111/jmg.12256. schist Unit in the eastern Aegean, Greece, and Turkey: 2006, The isotopic composition of zircon and garnet: A Peterson, J.W., and Newton, R.C., 1989, Reversed experi- Tectonics, v. 26, TC2001, https://doi.org/10.1029 record of the metamorphic history of Naxos, Greece: ments on biotite-quartz-feldspar melting in the system /2005TC001872. Lithos, v. 87, p. 174–192, https://doi.org/10.1016/j KMASH: Implications for crustal anatexis: The Jour- Ring, U., Glodny, J., Will, T., and Thomson, S., 2010, The .lithos.2005.06.016. nal of Geology, v. 97, p. 465–485, https://doi.org/10 Hellenic subduction system: High-pressure metamor- McClusky, S., Balassanian, S., Barka, A., Demir, C., Ergin- .1086/629323. phism, exhumation, normal faulting, and large-scale tav, S., Georgiev, I., Gurkan, O., Hamburger, M., Hurst, Pëto, P., 1976, An Experimental Investigation of Melting extension: Annual Review of Earth and Planetary Sci- K., Kahle, H., and Kastens, K., 2000, Global position- Reactions Involving Muscovite and Paragonite in the ences, v. 38, p. 45–76, https://doi.org/10.1146/annurev ing system constraints on plate kinematics and dynam- Silica-Saturated Portion of the System K O–Na O– .earth.050708.170910. ics in the Eastern Mediterranean and Caucasus: Journal 2 2 Al O –SiO –H O to 15 kbar Total Pressure: UK Natu- Roche, V., Laurent, V., Cardello, G.L., Jolivet, L., and Scail- of Geophysical Research–Solid Earth, v. 105, no. B3, 2 3 2 2 ral Environment Research Council. let, S., 2016, Anatomy of the Cycladic Blueschist Unit p. 5695–5719, https://doi.org/10.1029/1999JB900351. Philippon, M., Brun, J.P., Gueydan, F., and Sokoutis, D., on Sifnos Island (Cyclades, Greece): Journal of Geo- Means, W.D., 1989, Stretching faults: Geology, v. 17, 2014, The interaction between Aegean back-arc ex- dynamics, v. 97, p. 62–87, https://doi.org/10.1016/j.jog p. 893–896, https://doi .org /10 .1130 /0091 -7613 tension and Anatolia escape since Middle Miocene: .2016.03.008. (1989)017<0893:SF>2.3.CO;2. Tectonophysics, v. 631, p. 176–188, https://doi.org/10 Rosenberg, C.L., and Handy, M.R., 2005, Experimental Menant, A., Jolivet, L., Augier, R., and Skarpelis, N., 2013, .1016/j.tecto.2014.04.039. deformation of partially melted granite revisited: impli- The North Cycladic Detachment System and associ- Platt, J.P., 1986, Dynamics of orogenic wedges and the cations for the continental crust: Journal of Metamor- ated mineralization, Mykonos, Greece: Insights on uplift of high-pressure metamorphic rocks: Geologi- phic Geology, v. 23, p. 19–28, https://doi.org/10.1111/j the evolution of the Aegean domain: Tectonics, v. 32, cal Society of America Bulletin, v. 97, p. 1037–1053, .1525-1314.2005.00555.x. p. 433–452, https://doi.org/10.1002/tect.20037. https://doi.org/10.1130/0016-7606(1986)97<1037: Sawyer, E.W., 1996, Melt segregation and magma flow in Miller, C.F., Stoddard, E.F., Bradfish, L.J., and Dollase, DOOWAT>2.0.CO;2. migmatites: Implications for the generation of granite W.A., 1981, Composition of plutonic muscovite; Platt, J.P., Behr, W.M., Johanesen, K., and Williams, J.R., magmas, in Brown, M., Candela, P.A., Peck, D.L., genetic implications: Canadian Mineralogist, v. 19, 2013, The Betic-Rif arc and its orogenic hinterland: Stephens, W.E., Walker, R.J., and Zen, E.A., eds., The p. 25–34. A review: Annual Review of Earth and Planetary Third Hutton Symposium on the Origin of Granites Mizera, M., and Behrmann, J.H., 2015, Strain and flow in Sciences, v. 41, p. 313–357, https://doi.org/10.1146 and Related Rocks: Geological Society of America the metamorphic core complex of Ios Island (Cyclades, /annurev-earth-050212-123951. Special Paper 315, p. 85–94, https://doi.org/10.1130/0 Greece): International Journal of Earth Sciences Platt, J.P., Behr, W., and Cooper, F.J., 2014, Metamorphic -8137-2315-9.85. [Geologische Rundschau], v. 105, no. 7 p. 2097–2110, core complexes: Windows into the mechanics and Sawyer, E.W., 2001, Melt segregation in the continental https://doi.org/10.1007/s00531-015-1259-y. rheology of the crust: Journal of the Geological Soci- crust: Distribution and movement of melt in anatec- Newton, R.C., 1989, Metamorphic fluids in the deep crust: ety [London], v. 172, p. 9–27, https://doi.org/10.1144 tic rocks: Journal of Metamorphic Geology, v. 19, Annual Review of Earth and Planetary Sciences, v. 17, /jgs2014-036. p. 291–309, https://doi.org/10.1046/j.0263-4929.2000 p. 385–410, https://doi.org/10.1146/annurev.ea.17 Powell, R., and Holland, T.J.B., 1988, An internally con- .00312.x. .050189.002125. sistent thermodynamic dataset with uncertainties and Schmid, S.M., Scharf, A., Handy, M.R., and Rosenberg, Nijman, W., Kloppenburg, A., and de Vries, S.T., 2017, correlations: 3. Application to geobarometry, worked C.L., 2013, The Tauern Window (eastern Alps, Aus- Archaean basin margin geology and crustal evolution: examples and a computer program: Journal of Meta- tria): A new tectonic map, with cross-sections and An East Pilbara traverse: Journal of the Geological So- morphic Geology, v. 6, p. 173–204, https://doi.org/10 a tectonometamorphic synthesis: Swiss Journal of ciety [London], v. 174, p. 1090–1112, https://doi.org .1111/j.1525-1314.1988.tb00415.x. Geosciences, v. 106, p. 1–32, https://doi.org/10.1007 /10.1144/jgs2016-127. Powell, R., and Holland, T. J., 2008, On hermobarometry: /s00015-013-0123-y. Palin, R.M., Searle, M.P., Waters, D.J., Parrish, R.R., Rob- Journal of Metamorphic Geology, v. 26, p. 155–179, Searle, M.P., 2010, Low-angle normal faults in the compres- erts, N.M.W., Horstwood, M.S.A., Yeh, M.-W., Chung, https://doi.org/10.1111/j.1525-1314.2007.00756.x. sional Himalayan orogen; Evidence from the Anna- S.L., and Anh, T.T., 2013, A geochronological and Powell, R., Holland, T.J.B., and Worley, B., 1998, Calculating purna-Dhaulagiri Himalaya, Nepal: Geosphere, v. 6, petrological study of anatectic paragneiss and associ- phase diagrams involving solid solutions via non-linear p. 296–315, https://doi.org/10.1130/GES00549.1. ated granite dykes from the Day Nui Con Voi metamor- equations, with examples using THERMOCALC: Jour- Searle, M.P., and Cox, J., 1999, Tectonic setting, origin and phic core complex, North Vietnam: Constraints on the nal of Metamorphic Geology, v. 16, p. 577–588, https:// obduction of the Oman ophiolite: Geological Society timing of metamorphism within the Red River shear doi.org /10 .1111 /j .1525 -1314 .1998 .00157 .x . of America Bulletin, v. 111, p. 104–122, https://doi zone: Journal of Metamorphic Geology, v. 31, p. 359– Ramberg, H., 1980, Diapirism and gravity collapse in the .org/10.1130/0016-7606(1999)111<0104:TSOAOO>2 387, https://doi.org/10.1111/jmg.12025. Scandinavian Caledonides: Journal of the Geological .3.CO;2.

48 Geological Society of America Bulletin, v. 1XX, no. XX/XX

Searle, M.P., and Cox, J., 2002, Subduction zone meta- Structural Geology, v. 24, p. 1861–1884, https://doi.org in the Danba structural culmination of eastern Tibet: morphism during formation and emplacement of the /10.1016/S0191-8141(02)00035-4. Journal of Metamorphic Geology, v. 31, p. 909–935, Semail ophiolite in the Oman Mountains: Geological St-Onge, M.R., 1984, The muscovite-melt bathograd and https://doi.org/10.1111/jmg.12050. Magazine, v. 139, no. 3, p. 241–255, https://doi.org/10 low-P isograd suites in north-central Wopmay orogen, Wernicke, B., 1981, Low-angle normal faults in the Basin .1017/S0016756802006532. Northwest Territories, Canada: Journal of Metamor- and Range Province: Nappe tectonics in an extending Searle, M.P., Warren, C.J., Waters, D.J., and Parrish, R.R., phic Geology, v. 2, p. 315–326, https://doi.org/10.1111 orogeny: Nature, v. 291, p. 645–648. 2004, Structural evolution, metamorphism and resto- /j.1525-1314.1984.tb00592.x. Wernicke, B., 1985, Uniform normal-sense simple shear of ration of the Arabian continental margin, Saih Hatat St-Onge, M.R., 1987, Zoned poikiloblastic garnets: PT paths the continental lithosphere: Canadian Journal of Earth region, Oman Mountains: Journal of Structural Geol- and syn-metamorphic uplift through 30 km of structural Sciences, v. 22, p. 108–125. ogy, v. 26, p. 451–473, https://doi.org/10.1016/j.jsg depth, Wopmay orogen, Canada: Journal of Petrology, White, R.W., and Powell, R., 2002, Melt loss and the preser- .2003.08.005. v. 28, p. 1–21, https://doi.org/10.1093/petrology/28.1.1. vation of granulite-facies mineral assemblages: Journal Searle, M.P., Law, R.D., and Jessup, M.J., 2006, Crustal Stouraiti, C., Pantziris, I., Vasilatos, C., Kanellopoulos, of Metamorphic Geology, v. 20, p. 621–632. structure, restoration and evolution of the Greater C.P., Mitropoulos, P., Pomonis, P., Moritz, R., and White, R.W., Powell, R., and Clarke, G.L., 2002, The in- Himalaya in Nepal–South Tibet: Implications for chan- Chiaradia, M., 2017, Ophiolitic remnants from terpretation of reaction textures in Fe-rich metapelitic nel flow and ductile extrusion of the middle crust, in the upper and intermediate structural unit of the granulites of the Musgrave block, central Australia: Law, R.D., Searle, M.P., and Godin, L., eds., Channel Attic-Cycladic Crystalline Belt (Aegean, Greece): Constraints from mineral equilibria calculations in the

Flow, Ductile Extrusion, Exhumation in Continental Fingerprinting geochemical affinities of magmatic system K2O-FeO-MgO-Al2O3-SiO2-H2O-TiO2-Fe2O3: Collision Zones: Geological Society of London Spe- precursors: Geosciences, v. 7, p. 14, https://doi.org/10 Journal of Metamorphic Geology, v. 20, p. 41–55, cial Publication 268, p. 355–378, https://doi.org/10 .3390/geosciences7010014. https://doi.org/10.1046/j.0263-4929.2001.00349.x. .1144/GSL.SP.2006.268.01.17 Teyssier, C., and Whitney D.L., 2002, Gneiss domes and White, R.W., Powell, R., and Halpin, J.A., 2004, Spatially Searle, M.P., Cottle, J.M., Streule, M.J., and Waters, D.J., orogeny, Geology, v. 30, p. 1139–1142, https://doi focused melt formation in aluminous metapelites from 2010, Crustal melt granites and migmatites along the .org/10.1130/0091-7613(2002)030<1139:GDAO>2.0 Broken Hill, Australia: Journal of Metamorphic Geol- Himalaya: Melt source, segregation, transport and .CO;2. ogy, v. 22, p. 825–845, https://doi.org/10.1111/j.1525 granite emplacement mechanisms: Earth and Envi- Thompson, A.B., 1982, Dehydration melting of pelitic -1314.2004.00553.x.

ronmental Science Transactions of the Royal Society rocks and the generation of H2O-undersaturated gra- White, R.W., Pomroy, N.E., and Powell, R., 2005, An in-situ of Edinburgh, v. 100, Special Issue 1–2, p. 219–233, nitic liquids: American Journal of Science, v. 282, metatexite-diatexite transition in upper amphibolite- https://doi.org/10.1017/S175569100901617X. p. 1567–1595. facies rocks from Broken Hill, Australia: Journal of Seward, D., Vanderhaeghe, O., Siebenaller, L., Thomson, Tippett, C.R., 1980, A Geological Cross-Section through the Metamorphic Geology, v. 23, p. 579–602, https://doi S., Hibsch, C., Zingg, A., Holzner, P., Ring, U., and Southern Margin of the Foxe Fold Belt, Baffin Island, .org/10.1111/j.1525-1314.2005.00597.x. Duchêne, S., 2009, Cenozoic tectonic evolution of Arctic Canada, and its Relevance to the Tectonic Evo- White, R.W., Powell, R., and Holland, T.J.B., 2007, Progress Naxos Island through a multi-faceted approach of fis- lution of the Northeastern Churchill Province [Ph.D. relating to calculation of partial melting equilibria for sion track analysis, in Ring, U., and Wernicke, B., eds., thesis]: Kingston, Ontario, Canada, Queen’s University. metapelites: Journal of Metamorphic Geology, v. 25, Extending a Continent: Architecture, Rheology, and Tomaschek, F., Kennedy, A.K., Villa, I.M., Lagos, M., and p. 511–527, https://doi.org/10.1111/j.1525-1314.2007 Heat Budget: Geological Society of London Special Ballhaus, C., 2003, Zircons from Syros, Cyclades, .00711.x. Publication 321, p. 179–196. Greece—Recrystallization and mobilization of zir- Whitney, D.L., and Evans, B.W., 2010, Abbreviations for Smye, A.J., Bickle, M.J., Holland, T.J.B., Parrish, R.R., and con during high-pressure metamorphism: Journal of names of rock-forming minerals: The American Min- Condon, D.J., 2011, Rapid formation and exhumation Petrology, v. 44, p. 1977–2002, https://doi.org/10.1093 eralogist, v. 95, p. 185–187, https://doi.org/10.2138/am of the youngest Alpine eclogites: A thermal conun- /petrology/egg067. .2010.3371. drum to Barrovian metamorphism: Earth and Planetary Tracy, R.J., Robinson, P., and Thompson, A.B., 1976, Gar- Whitney, D.L., and Irving, A.J., 1994, Origin of K-poor leuco Science Letters, v. 306, p. 193–204, https://doi.org/10 net composition and zoning in the determination of somes in a metasedimentary migmatite complex by .1016/j.epsl.2011.03.037. temperature and pressure of metamorphism, central ultrametamorphism, syn-metamorphic magmatism and Spear, F.S., 1993, Metamorphic Phase Equilibria and Pres- Massachusetts: The American Mineralogist, v. 6, subsolidus processes: Lithos, v. 32, p. 173–192, https:// sure-Temperature-Time Paths: Mineralogical Society p. 762–775. doi.org/10.1016/0024-4937(94)90038-8. of America Monograph 1, 799 p. Tyler, I.M., and Ashworth, J.R., 1982, Sillimanite potash Wiederkehr, M., Bousquet, R., Schmid, S.M., and Berger, Spear, F.S., and Selverstone, J., 1983, Quantitative P-T paths feldspar assemblages in graphitic pelites, Stronian area, A., 2008, From subduction to collision: thermal over- from zoned minerals: Theory and tectonic applications: Scotland: Contributions to Mineralogy and Petrology, print of HP/LT meta-sediments in the north-eastern Contributions to Mineralogy and Petrology, v. 83, v. 81, p. 18–29, https://doi.org/10.1007/BF00371155. Lepontine Dome (Swiss Alps) and consequences re- p. 348–357, https://doi.org/10.1007/BF00371203. Urai, J.L., Schuiling, R.D., and Jansen, J.B.H., 1990, Alpine garding the tectono-metamorphic evolution of the Al- Spear, F.S., Hickmott, D.D., and Selverstone, J., 1990, deformation on Naxos (Greece), in Knipe, R.J., and pine orogenic wedge: Swiss Journal of Geosciences, Metamorphic consequences of thrust emplacement, Rutter, E.H., eds., Deformation Mechanisms, Rheol- v. 101, https://doi.org/10.1007/s00015-008-1289-6. Fall Mountain, New Hampshire: Geological Society ogy and Tectonics: Geological Society of London Spe- Wijbrans, J.R., van Wees, J.D., Stephenson, R.A., and of America Bulletin, v. 102, p. 1344–1360, https://doi cial Publication 54, p. 509–522. Cloetingh, S.A.P.L., 1993, Pressure-temperature-time .org/10.1130/0016-7606(1990)102<1344:MCOTEF>2 Vanderhaeghe, O., 2004, Structural development of the evolution of the high-pressure, metamorphic complex of .3.CO;2. Naxos migmatite dome, in Whitney, D.L., Teyssier, Sifnos, Greece: Geology, v. 21, p. 443–446, https://doi Stevens, G., and Clemens, J.D., 1993, Fluid-absent melting C., and Siddoway, C.S., eds., Gneiss Domes in Orog- .org/10.1130/0091-7613(1993)021<0443:PTTEOT>2 and the roles of fluids in the lithosphere: a slanted sum- eny: Geological Society of America Special Paper 380, .3.CO;2. mary?, Chemical Geology, v. 108, p. 1–17, https://doi p. 211–227. Woodsworth, G.J., 1977, Homogenization of zoned garnets .org/10.1016/0009-2541(93)90314-9. Vanderhaeghe, O., and Teyssier, C., 2001, Partial melting from pelitic schists: Canadian Mineralogist, v. 15, Stipp, M., and Kunze, K., 2008, Dynamic recrystalliza- and flow of orogens: Tectonophysics, v. 342, p. 451– p. 230–242. tion near the brittle-plastic transition in naturally and 472, https://doi.org/10.1016/S0040-1951(01)00175-5. Wortel, M.J.R., and Spakman, W., 2000, Subduction and experimentally deformed quartz aggregates: Tecto- Virgo, S., von Hagke, C., and Urai, J.L., 2018, 2018, Multi slab detachment in the Mediterranean-Carpathian re- nophysics, v. 448, p. 77–97, https://doi.org/10.1016/j phase boudinage: A case study of amphibolites in gion: Science, v. 290, no. 5498, p. 1910–1917, https:// .tecto.2007.11.041. marble in the Naxos migmatite core: Solid Earth, v. 9, doi.org/10.1126/science.290.5498.1910. Stipp, M., Stünitz, H., Heilbronner, R., and Schmid, S.M., p. 91–113, https://doi.org/10.5194/se-9-91-2018. Yin, A., 1991, Mechanisms for the formation of domal and 2002a, Dynamic recrystallization of quartz: Correla- Von Hagke, C., Bamberg, B., Virgo, S., and Urai, J.L., 2018, basinal detachment faults: A three-dimensional analy- tion between natural and experimental conditions, in Outcrop-scale tomography: Insights into the 3D struc- sis: Journal of Geophysical Research, v. 96, p. 14,577– De Meer, S., Drury, M.R., De Bresser, J.H.P., and Pen- ture of multiphase boudins: Journal of Structural Geol- 14,594, https://doi.org/10.1029/91JB01113. nock, G.M., eds., Deformation Mechanisms, Rheology, ogy, v. 115, p. 311–317, https://doi.org/10.1016/j.jsg and Tectonics: Current Status and Future Perspectives: .2018.02.014. Science Editor: Aaron J. Cavosie Geological Society of London Special Publication 200, Weinberg, R., and Hasalová, P., 2015, Water-fluxed melting Associate Editor: Nicholas Erik Timms p. 171–190, https://doi.org/10.1144/GSL.SP.2001.200 of the continental crust: A review: Lithos, v. 212–215, .01.11. p. 158–188, https://doi .org /10 .1016 /j .lithos .2014 .08 Manuscript Received 27 November 2017 Stipp, M., Stünitz, H., Heilbronner, R., and Schmid, S.M., .021. Revised Manuscript Received 9 September 2018 2002b, The eastern Tonale fault zone: A ‘natural labo- Weller, O.M., St-Onge, M.R., Waters, D.J., Rayner, N., Manuscript Accepted 4 April 2018 ratory’ for crystal plastic deformation of quartz over Searle, M.P., Chung, S.-L., Palin, R.M., Lee, Y.-H., and a temperature range from 250 to 700 °C: Journal of Xu, X., 2013, Quantifying Barrovian metamorphism Printed in the USA

Geological Society of America Bulletin, v. 1XX, no. XX/XX 49

Downloaded from https://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/doi/10.1130/B31978.1/4719075/b31978.pdf by guest on 05 June 2019