Tectonometamorphic Evolution of Cycladic Subduction Zone Rocks: the Syros Blueschist-Eclogite Terrane Iii
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TECTONOMETAMORPHIC EVOLUTION OF CYCLADIC SUBDUCTION ZONE ROCKS: THE SYROS BLUESCHIST-ECLOGITE TERRANE III FACULTY John B. Brady, Smith College John T. Cheney, Director, Amherst College Tekla A. Harms, Amherst College John C. Schumacher, Bristol University, UK STUDENTS Scott J. Dougan, Amherst College Jessica M. Driscoll, Amherst College Njoki W. Gitahi, Amherst College Elizabeth A. Holley, Pomona College Stacey K. Kepler, Amherst College Laura Frye-Levine, Smith College Eli D. Lazarus, Williams College Gabriel J. Nelson Carleton College Emily C. Pope, Colorado College Eric Purcell, Beloit College VISITORS in Greece Cameron Davidson, Carleton College Emily Dubinsky, Amherst College Andreas Magganas, University of Athens Frank Spear, Rensselaer Polytechnic Institute EVOLUTION OF CYCLADIC SUBDUCTION ZONE ROCKS: SYROS BLUESCHIST-ECLOGITE TERRANE III JOHN T. CHENEY Amherst College JOHN B. BRADY Smith College TEKLA A. HARMS Amherst College JOHN C. SCHUMACHER Bristol University REGIONAL INTRODUCTION metabasalts with clinopyroxene (omphacite) + garnet + glaucophane + epidote, felsic The Cycladic islands, located in the Aegean metavolcanics with jadeite + quartz, Sea (Figure 1), contain high-pressure metapelites with muscovite + glaucophane + metamorphic rocks that are believed to be the garnet + epidote, marbles containing dolomite dismembered roots of the mountain belt + calcite ± quartz ± epidote ± phlogopite, formed during Eurasia-Africa subduction ultramafic rocks, and quartzites (Schliestedt, (Lister and Raouzaios, 1996), which began in 1986; Ridley, 1984b: Dixon and Ridley, the Eocene (Tomaschek et al., 2003) or late 1987). Maximum metamorphic condition for Mesozoic (Bröcker and Enders, 1999). The the high-P event of 460°C and 14 kilobars at Cyclades are part of the Attic-Cycladic Sifnos (Schliestedt, 1986) are similar to the complex, an island belt of crystalline 480°C and 16 kilobars proposed for Syros by culminations linking continental Greece with Trotet et al. (2001a). Turkey. The complex consists of two main tectonic units. The upper unit contains various A younger metamorphic event has been intercalated fragments of ophiolites, Permian related to widespread late Oligocene/early sedimentary rocks and high temperature Miocene extension throughout the Attic- metamorphic rocks. In contrast, the lower unit Cycladic belt (Wijbrans et al., 1993). This is polymetamorphic and consists of a series of second event overprints the earlier blueschists thrust sheets containing pre-Alpine basement, and eclogites, culminating at Naxos in a Mesozoic marble, metavolcanics and migmatite dome. On most islands and in metapelites. The polymetamorphic nature of particular on Syros and Sifnos, this second this lower unit is manifest by: 1) high- event is recorded in the rocks as a pressure, blueschist facies metamorphism, 2) pervasive–yet localized– greenschist facies normal regional metamorphism, and 3) contact overprinting (Trotet et al., 2001b). metamorphism associated with the intrusion of Delamination of supracrustal rocks proximal granitic rocks (Schliestedt et al., 1987). to active subduction provided the cooling mechanism to preserve the blueschists and The high-pressure rocks are best preserved on eclogites (Wijbrans et al., 1993) and may have the islands of Sifnos and Syros. Mineral been responsible for exhuming deeply buried assemblages vary with protolith and include (>50 km) rocks. 20° 25° 42° 42° Bulgaria Tiranë Istambul Italy Albania Thesoliniki 40° 40° Greece H H H H Turkey H H H H H H Agean H Athens 38° H Sea 38° H African H H Plate H Syros Motion 36° inferred 36° extension: Hellenic o km Tre Iráklion x ple nch HH H H Pliny/Strab 0 100 Mediterranean H H H OMC - M.Weinelt Sea Trench Com 34° 34° 20° 25° 24° 25° 26° 27° 39° 39° Euboia 38° 38° Andros Tinos Ikaria Kea Mykonos Cyclades Kithnos Syros Paros Naxos Serifos 37° 37° Sifnos Amorgos Ultramafic rocks Ios Granitic rocks Milos Blueschist-facies carbonates Cycladic blueschist belt km OMC - M.Weinelt after Blake et al. (1984) GMT Jan 11 16:32 0 50 100 36° 36° 24° 25° 26° 27° By J.C. Schumacher 1999 Figure 1: Tectonic map of the Agean Sea with the location of Syros and the Cycladic Islands (top) and the important rock types in the Attic-Cycladic complex (bottom). SYROS V gneiss of Vari C St. Michalis UM upper marble unit UM N G B breccia B Lawsonite Point Kastri G metagabbro serpentinite/knockers main ophiolitic unit C conglomerates Kastri marble Pirgos marble Kini meta-basites glaucophane- / mica schists Ermoupolis undivided V V Vari Finikas Katergaki Point alluvium 0 1 2 km nappe/fault South Point Figure 2: Generalized Geology of Syros modified from Höpfer and Schumacher (1997) conclusions of previous workers based on The rocks on these islands are isoclinaly field (Dixon, 1969), geochemical (Seck et al., folded and extensively sheared and flattened. 1996), and isotopic (Putlitz et al., 2000) data. Four generations of deformation have been This result raises the interesting question of recognized in terms of fabric and why a coarse-grained igneous protolith should porphyroblast relations in rocks from Sifnos lead to a coarse-grained metamorphic rock by Lister and Raouzaios (1996). Thus, rocks containing all new minerals. The massive from Syros and Sifnos have been intensely character of the original gabbros appears to deformed and significantly recrystallized so have had a strong influence on their that most of the original igneous and metamorphism (coarse texture, little sedimentary textures have been obliterated hydration) and deformation (little fabric, (Lister and Raouzaios, 1996; Dixon and coherent blocks) during subduction and Ridley, 1987). exhumation. GEOLOGY OF SYROS The occurrence of multiple generations of high-P minerals, hornblende as inclusions in The rocks of Syros can be broadly divided into glaucophane, intricate chemical zoning of two tectono-stratigraphic units as shown on high-P minerals, and partial to complete Figure 2 (Schumacher et al., 2000): (I) "euhedral" pseudomorphs after lawsonite that metasedimentary and metavolcanic rocks and contain inclusions of garnet all attest to the (II) remnants of oceanic crust. The lowermost complexity of the PTt path that was followed rocks of Unit I are metamorphosed felsic tuffs, by these rocks during subduction and mafic schists, marbles, and finely-laminated exhumation. One constraint on this path is manganese cherts. These rocks give way provided by the occurrence of calcite upwards to a section dominated by marbles. pseudomorphs after aragonite that are The two main lower marble horizons are widespread in the marbles of Syros. These typically dolomitic, in part, and are separated striking features consist of polycrystalline from each other by glaucophane-schists, bundles of calcite rods that are elongate and greenschists (retrograde), and minor quartzites oriented at a high angle to the matrix foliation. and manganese cherts. Brady et al. (2004) have used the chemical Unit II consists of several discrete, fault- composition, shape, and occurrence to bounded packages of blueschist/eclogite-facies postulate that the aragonite grew in a preferred mafic rocks that contain minor serpentinite. orientation during high-P metamorphism and The mafic rocks occur with a variety of that it was subsequently topotacticaly replaced textures and modes but most are either fine by calcite during exhumation. grained, glaucophane-rich blueschists or According to Dixon and Ridley (1987), there coarse-grained (>1cm), massive omphacite- or is but one penetrative fabric that affects nearly glaucophane-rich rocks. These rock types have all in the rocks of Syros. More recently, been interpreted as metabasalt and Rosenbaum et al. (2002) have identified at metagabbro, respectively (e.g. Dixon and least three phases of deformation associated Ridley, 1987). This hypothesis has been with the high-pressure metamorphism. The verified by 38 whole-rock XRF and INAA earliest fabric is preserved as inclusion trails in analyses for 18 fine-grained and 20 coarse- garnet grains and it is commonly at an angle to grained samples reported by Brady et al. the matrix schistosity. High-pressure minerals (2000) and Schumacher et al. (2001, 2004). define the matrix schistosity, which is parallel These results show that the protoliths of the to lithologic contacts. Some glaucophane, the coarse-grained mafic rocks are indeed gabbros lawsonite, and the pseudomorphs after that have been chemically differentiated by lawsonite appear to postdate the main fabric. fractional crystallization, whereas the There is some strain partitioning reflected by protoliths of the fine-grained mafic rocks are the massive cores of the metagabbros and largely undifferentiated ocean floor basalts. some of the breccias. This dominant This interpretation is consistent with the schistosity is the one identified by Dixon and Ridley (1987) and glaucophane shear bands that the agreement between the 55 Ma U-Pb locally cut it. Superimposed on these high-P ages and the ~ 55 Ma Ar-Ar ages of white deformation fabrics is a late locally-developed micas from the same samples supports the fabric associated with greenschist conclusion that the metamorphism is Eocene. overprinting. They do not, however, explain how zircon crystallized from basaltic MORB-like magma. Cheney et al. (2000) have reported 206Pb - One likely source of Zr in a basalt is that it 238U ages from zircon in two blueschists. Ion resides in augite. When augite breaks down microprobe spot ages were obtained from during hydration,