nETPOmma, 2013, m0M 21, № 5, c. 554-568
P-T ESTIMATES AND TIMING OF THE SAPPHIRINE-BEARING METAMORPHIC OVERPRINT IN KYANITE ECLOGITES FROM CENTRAL RHODOPE, NORTHERN GREECE
© 2013 Evangelos Moulas", Dimitrios Kostopoulos4, James A.D. Connolly", and Jean-Pierre Burg"
aDepartment of Earth Sciences, ETHZentrum, 8092 Zurich, Switzerland, Sonneggstrasse 5, e-mail: evangelos.moulas@erdw.ethz.ch bDepartment of Mineralogy and Petrology, National and Kapodistrian University of Athens, Panepistimioupoli Zographou, 15784 Athens, Greece Received March 20, 2013
Abstract—Sapphirine-bearing symplectites that replace kyanite in eclogites from the Greek Rhodope Massif have previously been attributed to a high-pressure granulite-facies metamorphic event that overprinted the eclogitic peak metamorphic assemblage. The eclogitic mineralogy consisted of garnet, omphacitic pyroxene, rutile and kyanite and is largely replaced by low-pressure minerals. Omphacite was initially replaced by symplectites of diopside and plagioclase that were subsequently replaced by symplectites of amphibole and plagioclase. Garnet reacted during decompression to form a corona of plagioclase, amphibole and magnetite. Rutile was partly transformed to ilmenite and kyanite decomposed to produce a high-variance mineral assemblage of sym-plectitic spinel, sapphirine, plagioclase, and corundum. The presence of quartz and corundum in the kyanite eclogites is evidence for the absence of bulk equilibrium and obviates a conventional analysis of phase equilibria based on the bulk-rock composition. To circumvent this difficulty we systematically explore the pressure-temperature-composition (P-T-X) space of a thermodynamic model for the symplectites in order to establish the pressure-temperature (P-T) conditions at which the symplectites formed after kyanite. This analysis combined with conventional thermometry indicates that the symplectites formed at amphibolite-facies conditions. The resulting upper-pressure limit (~0.7 GPa) of the sapphirine-producing metamorphic overprint is roughly half the former estimate for the lower pressure limit of the symplectite forming metamorphic event. Temperature was constrained (T ~ 720°C) using garnet-amphibole mineral thermometry. The P-T conditions inferred here are consistent with thermobarometry from other lithologies in the Rhodope Massif, which show no evidence of granulite-facies metamorphism. Regional geological arguments and ion-probe (SHRIMP) zircon dating place the post-eclogite-facies metamorphic evolution in Eocene times.
DOI: 10.7868/S0869590313050038
INTRODUCTION
The Rhodope Massif is a metamorphic nappe pile that belongs to the Alpine-Himalayan suture and collision system (e.g. Ricou et al., 1998; Burg, 2012 and references therein). High-pressure rocks from the intermediate thrust sheets contain evidence of poly-metamorphic evolution at ultrahigh-pressure (UHP), high-pressure and amphibolite-facies conditions (Liati and Mposkos, 1990; Mposkos and Liati, 1993; Liati and Seidel, 1996; Mposkos and Kostopou-los, 2001). The pressure-temperature (P-T) evolution of these rocks is of geodynamic interest because it elucidates the processes of burial and uplift in convergent tectonic settings. Quartz-bearing kyanite eclogites from the intermediate thrust sheets were investigated in order to deduce their retrograde P-T path and constrain their geodynamic evolution. This study focuses on sapphirine-spinel-corundum-plagioclase-bearing symplectites after kyanite that formed during the post-eclogite phase of the metamorphic evolution.
The observation that the rock contains both corundum and quartz, but that these phases are always sep-
arated by plagioclase indicates that the rock mineralogy does not represent bulk equilibrium. The absence of bulk equilibrium precludes analysis of phase relations on the basis of the bulk-rock composition. As an alternative we assume that the symplectites after kyanite record mosaic equilibrium (Korzhinskii, 1959), in which case the local composition that dictated mineral chemistry is unknown and most likely varied during symplectite formation. The corundum-bearing sym-plectites replacing kyanite are a case in point. Because the symplectites contain corundum, it is evident that they are not in equilibrium with the quartz-bearing matrix and because the symplecitites contain sapphir-ine, plagioclase and spinel, it is evident that the reaction site around kyanite has gained MgO, FeO, Na2O and CaO from the surrounding matrix and therefore that the effective bulk composition of the kyanite domains varied with time. To establish the P-T conditions of the symplectite formation our approach here is to explore the range of conditions at which the addition of the aforementioned components would repro-
duce the observed symplectitic mineral assemblage after kyanite.
We begin by describing briefly the petrography and mineral chemistry of the high-pressure rocks (eclog-ites). Then, we discuss the petrological interpretation of the symplectites and present our thermodynamic analysis for the symplectites based on Gibbs free energy minimization. This analysis constrains the P-T conditions during retrogression and decompression of the high-pressure metamorphic rocks to T ~ 720°C and P < 0.7 GPa. To constrain the timing of this retrogression, we then present ion-probe (SHRIMP) zircon dating that places the post-eclogite-facies metamorphic evolution in Eocene times, coeval with the magmatic activity in the region. This dating is relevant because it constrains the regional geodynamic scenarios in which the metamorphic overprint has taken place. We conclude with a discussion of these scenarios in light of our P-T estimates, which indicate that the sapphirine-bearing symplectites formed at highgrade amphibolite-facies conditions.
GEOLOGICAL SETTING
The Rhodope Massif is a region belonging to the Alpine-Himalayan orogen (e.g. Burg, 2012 and references therein). It involves three main units (Fig. 1) namely, and according to their present-day structural position, the Upper, Intermediate and Lower Terranes (e.g. Burg et al., 1996). This terrane subdivision summarizes the results of previous works in the Bulgarian and Greek Rhodope (e.g. Burg et al., 1990; Papani-kolaou and Panagopoulos, 1981, respectively). These three units were intensely deformed in a non-coaxial deformation regime and are separated by ductile my-lonitic zones with regional top-to-SSW sense of shear (Burg et al., 1990, 1996). The studied kyanite eclogite crops out near Thermes (Fig. 1) as isolated boudins within amphibolite-facies mylonitic orthogneisses. The orthogneisses are strongly foliated and occasionally have a migmatitic texture. Ion microprobe (SHRIMP) zircon dating from the eclogites and from the leucosomes has provided two ages, 42.2 ± 0.9 and 40.0 ± 1.0 Ma (Liati and Gebauer, 1999); these latter authors interpreted these two dates as the age of the high-pressure (1.9 GPa) metamorphism for the eclog-ite and the age of migmatitisation of the orthogneisses, respectively. Ductile normal faulting related to regional extension was identified by structural studies (Burg et al., 1990; Koukouvelas and Doutsos, 1990; Kolo-cotroni and Dixon, 1991) while Jones et al. (1992) pointed out the probable relationship between the voluminous Tertiary magmatism and the extensional collapse of the Hellenic orogen. The southern Rhodope is now viewed as a metamorphic core complex (Dinter and Royden, 1993; Sokoutis et al., 1993; Brun and Sokoutis, 2007). Syn- to post-orogenic extension began in Paleocene-early Eocene in the northeastern Rhodope Massif (Bonev et al., 2006) whereas in the central Rhodope extension started in the Mid-
Eocene (Lips et al., 2000; Brun and Sokoutis 2007; Wüthrich, 2009) and structured the mid-Eocene (48— 43 Ma) to Oligocene marine basins (Krohe and Mpos-kos, 2002). Miocene extension, related to slab-retreat and formation of the Aegean Sea has produced fault-bounded grabens, predominantly on the southern side of the Rhodope Massif.
Kyanite-bearing eclogites belonging to the intermediate thrust units (eclogite-metabasic-gneiss sequence of Burg et al., 1996) were investigated to trace their metamorphic P-T path during post-high-pressure conditions. The thin sections studied here come from a 4 m long boudin near Thermes village (RH506; N 41°21'36.24''/E 24°57'56.88''). These eclogites were reported to record a complex polymetamorphic history including high-pressure metamorphic conditions (1.9 GPa and 700°C) followed by high-pressure granulite-facies (P > 1.5 GPa, T > 800°C) and amphib-olite-facies conditions (P = 0.8-1.1 GPa, T = 580-690°C; Liati and Seidel, 1996).
ANALYTICAL METHODS
Major-element mineral analyses and elemental maps were obtained at ETH-Zürich and the Institut für Geowissenschaften, Johannes Gutenberg-Universität, Mainz. At ETH-Zürich a Jeol JXA 8200 electron probe used while a Jeol JXA 8900RL was used at the Institut für Geowissenschaften, Johannes Gutenberg-Universität, Mainz. Both microprobes are equipped with 5 wavelength-dispersive spectrometers operating at an accelerating voltage of 15 kV and were operated with a 2 ^m beam diameter and 20 nA beam current. Natural and synthetic materials were used as standards and a CITZAF correction procedure was applied. The whole-rock composition of the studied samples was obtained using a Pananalytical Axios wavelength dispersive XRF spectrometer (WDXRF, 2.4 kV) at ETH-Zürich.
Zircon separation was done at the Max-Planck Institut für Chemie, Mainz, Germany. The samples were crushed with a hydraulic press and ground in a rotary mill. Zircons were extracted using a Wilfley table, Franz isodynamic magnetic separator, heavy-liquid (methylene iodide) separation and finally hand-picking under a binocular microscope. Selected zircon grains were mounted together with zircon standard grains 91500 and TEMORA 1 in epoxy resin, they were then sectioned and polished to approxima
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