Field trip Naxos, Greece, course B, SS 2014: Prof. Dr. J. Urai Metamorphic fluids, Naxos, Greece Tilman Scheele Applied Geosciences EMR, RWTH Aachen Introduction Naxos is located in the central Aegean region in Greece, about 200 km southeast of Athens (Rye et al. 1976, Kreulen. 1980). It forms the largest island of the Attic-Cycladic crystalline belt with about 20 by 35 km (Jansen & Schuilin. 1976). The area marks a back-arc basin that formed due to the rollback between the subducting African plate and the Apulian-Anatolian microplate. It was exposed to major events of alpine orogeny, a compressional phase, dated to 50 Ma, followed by extension that formed this back-arc basin (Cao et al. 2013). Lithologically, the region can be divided into two major structural units, separated by low angle normal faults. The upper unit consist of ophiolites, heterogeneous sequence of unmetamorphosed sediments of Permian to Neogene age, a greenschist-facies with Cretaceous to Tertiary metamorphic ages, granitoids of Late Cretaceous and middle pressure / high temperature metamorphic rocks. This upper unit is mostly poorly exposed. The lower unit comprises a pre-alpine crystalline basement and metamorphosed volcano-sedimentary succession, it is sometimes named Cycladic blueschist unit (Gärtner et al. 2011). Regional geology of Naxos The Lithology of Naxos is mainly composed of a metamorphic complex with a migmatite dome core of northnortheast orientated elongation that is surrounded by a series of schist and marble units (Fig.1). The rocks of the metamorphic core complex show a trend of decreasing metamorphism, directed outwards from the core. Individual marble and schist units show a variation in thickness, from a centimetre scale up to hundred metres (Rye et al. 1976). In the western part of Naxos, a granodiorit borders the metamorphic core complex. The intrusive contact is covered partly by an overthrusted block that contains ultrabasic and basic rocks, as well as younger sedimentary rocks. In the eastern part of the island is a smaller exposure of this overthrust zone (Rye et al. 1976). Naxos has experienced at least two major regional tecto-metamorphic 1
events, caused by the Alpine orogeny, that can be distinguished. The first one, a phase of compression, was associated with large scale thrusting and a high pressure low temperature metamorphosis (M1), blueschist / eclogite facies and took place during early alpine subduction episode in Eocene, dated to 45 ±5 Ma (Cao et al. 2013). The Barrovian type second metamorphic event (M2) occurred during a time of continental extension with a Miocene peak high-temperature metamorphism, 20 to 15 Ma, and was associated with the development of localized thermal domes (Cao et al. 2013). The metamorphic peak conditions were estimated to occur at a pressure of 6-7 kbar with temperatures reaching from 450 C in the southeast, up to 670 C in the leucogneiss core (Cao et al. 2013). During this stage, tectonic events brought lower crustal rocks into contact with upper crustal units, along a major shallow dipping shear zone. The former mentioned igneous intrusive events are separated in time from the M2 metamorphic partial melting events (Cao et al. 2013). Figure 1: Geological map of Naxos with the different metamorphic schist and marble bands, the north-northeast elongated core and the igneous granodiorite in the west. The lower part shows a west-east tending profile. (Cao et al. 2013) 2
Metamorphism on Naxos Naxos shows metamorphic facies from glaucophane schist into migmatites of the amphibolite facies, but is unusual in showing a transition range of these metamorphic facies. Although it shows a broad range of metamorphism in its lithology, Naxos and none of the Cyclades show the complete, continuous range of metamorphism. Therefore two phases of metamorphism are commonly considered (Jansen & Schuiling. 1976). The metamorphic rocks on Naxos show an increasing grade of metamorphism with decreasing distance to the core of the metamorphic complex. Jansen & Schuiling (1976) described these rocks and divided them into six metamorphic zones, named after the most characteristic mineral assemblage in the particular zone. Starting from the outermost unit, that equals the rock with the lowest metamorphic grade, the first zone is the Diaspore Zone (I). It is located in southeastern Naxos and mainly consists of marble with lenses of metabauxite. The Chlorite-Sericite Zone (II) marks the second zone and is defined by the first appearance of corundum in metabauxite lenses, followed by Biotite-Chloritoid Zone (III) marked by occurrence of biotite. With the beginning of the disappearance of chloritoid in iron-rich politic schists, the Kyanite Zone (IV) begins, followed by the Kyanite-Sillimanite Transition Zone (VA) that shows the firs appearance of sillimanite. By disappearance of kyanite, the Sillimanite Zone (VB) begins. Finally, the Migmatite Zone (VI) forms the core of the metamorphic complex, marked by the beginning of evidences for anatexis (Jansen & Schuiling. 1976). Several studies on fluids during the metamorphism on Naxos exist. Isotopic studies, based on measurements of fluid inclusions in the metamorphic rocks, were carried out by Jansen & Schuiling (1976), Kreulen (1980) and Baker et al. (1989) to derive information about the fluid composition during metamorphism. The included fluids show a composition that is in general not related to the metamorphic grade of the rocks as for example the least metamorphosed schists contain abundant CO2-rich inclusions (Kreulen. 1980). Three possible sources for the CO2, present during the metamorphism on Naxos, have to be considered (Kreulen. 1980). The CO2 could be derived from decarbonation reactions in carbonate rocks like siliceous dolomites. Oxidation of organic matter in in rocks during metamorphism is seen as a possible source as well as an origin of the CO2 in deep-seated sources, represented by either juvenile CO2 or homogenized crustal carbon (Kreulen. 1980). Negative values for measured δ 13 C in a range of -1 to -5 suggest a greater part of the CO2 to be derived not by decarbonation, but a deeper seated source. Such values were mainly found in schists and pegmatites (Kreulen. 1980). In contrast, positive values of +1 to +5 of δ 13 C represent CO2 that formed by decarbonation and is found in siliceous dolomites (Kreulen. 1980). Kreulen (1980) interpreted the isotopic values that the CO2 has its origin in an external deep-seated source. Measured isotopic values for δ 13 C of -1 to -5 in CO2-rich fluid inclusions, containing 60-3
90mol% CO2 are predominant in rocks on Naxos and are considered to represent the common metamorphic fluids (Kreulen. 1980). The fluids probably have been produced by reactions in siliceous dolomites in higher grade rocks or even still deeper and migrated over large distances through the rocks. Values for deep seated CO2 would be expected about -4 to -8, which is 3 deeper than the actual δ 13 C values. A reason for this could be mixing of this deep seated CO2 rich fluids and CO2 derived by decarbonisation but with a higher amount of mantle CO2 (Kreulen. 1980). The CO2-H2O ratios indicate that quantities of fluids from an extraneous source were large and apparently much larger than those fluids that were produced by dehydration reactions in rock proper and indicate a high interaction ratio between fluids and rocks (Kreulen. 1980). Baker et al. (1989) came to a different interpretation of the isotopic values. The evidence for an interaction of fluid with calcite and dolomite horizons during the M2 metamorphic event indicates two major fluid events during this metamorphism. They measured δ 13 C and δ 18 O values in individual marbles bands in consideration of their location and nearness to contacts of pelites and vein networks that are associated with calc-silicates. The measured core values of 22 to 29 for δ 18 O and 1 to 3 for δ 13 C show two different dominant styles of alteration. The first alteration shows isotopic values to 15 to 17 for δ 18 O and -5 to 1 for δ 13 C, this drop was observed in marbles near to a contact of surrounding pelitic rocks. A second drop is in the isotopic composition of marbles along vein networks associated with development of calc-silicates with 14 to 16 in δ 18 O and -3 to -4 in δ 13 C (Baker et al. 1989). The presence of even small quantities of graphite in pelites, at temperatures below calcite-graphite equilibrium, makes the presence of mantle carbon during metamorphism unnecessary to explain the observed isotopic signatures (Baker et al. 1989). The isotopic data of fluid inclusions in Rocks on Naxos indicate a high amount of CO2 in the former metamorphic fluids. This amount is considered to originate either of a deep seated mantle source (Kreulen. 1980) or by dehydration of graphite bearing pelites (Baker et al. 1989). Also a certain amount of CO2 is derived by decarbonation reactions and mixed with a second source of CO2. References Baker, J., Bickle, M.J., Buick, I.S., Holland, T.J.B., Matthews, A., 1989. Isotopic and petrological evidence for the infiltration of water-rich fluids during the Miocene M2 metamorphism on Naxos, Greece, Geochimica et Cosmochimica Acta, Vol. 53, pp. 2037-2050 Cao, S., Neubauer, F., Bernroider, M., Liu, J., 2013. The lateral boundary of a metamorphic core complex: The Moutsounas shear zone on Naxos, Cyclades, Greece, Journal of Structural Geology, Elsevier, Vol. 54, pp. 103-128 Gärtner, C., Bröcker, M., Strauss, H., Farber, K., 2011. Strontium-, carbon- and 4
oxygen-isotope compositions of marbles from the Cycladic blueschist belt, Greece, Geol. Mag., Vol. 148 (4), pp. 511-528, Cambridge University Press Jansen, J.B.H., Schuiling, R.D., 1976. Metamorphism on Naxos: Pertology and geothermal gradients, American Journal of Science, Vol. 276, pp. 1225-1253 Kreulen, R., 1980. CO2-rich fluids during regional metamorphism on Naxos (Greece): Carbon isotopes and fluid inclusions, American Journal of Science, Vol. 280, pp. 745-771 Rye, R.O., Schuiling, D.R., Rye, D.M., Jansen, J.B.H., 1976. Carbon, hydrogen, and oxygen isotope studies of the regional metamorphic complex at Naxos, Greece, Geochimica et Cosmochimica Acta, Vol. 40, pp. 1031 to 1049, Pergamon Press, Great Britain 5