Naxos: metamorphism, P-T-t evolution (A)

Transcription

1 Naxos: metamorphism, P-T-t evolution (A) 1. Regional geological setting The Cyclades are an island group in the Aegean Sea southeast of the Greek mainland and located centrally in the Attic Cycladic Metamorphic Belt. Fig. 1 shows the exact position of Naxos, the island this paper focuses on. The region is strongly formed by tertiary magmatic activity, caused by the subduction of the African plate underneath the Aegean microplate, a process which is still going on today (cf. Liati et al., 2009). The Cyclades including Naxos currently are in a back-arc-position (cf. Liati et al., 2009), the active trenches are located in a semicircle from the Ionian Sea in north-western Greece, via the south of the Peloponnesos and Crete to east of Rhodes (Wijbrans & Mc Dougall, 1988) and can be seen in Fig. 1. South-west of Naxos lies the Hellenic volcanic arc where subduction causes recent volcanism, especially on Mylos and Thera (cf. Wijbrans & Mc Dougall, 1988). The Cycladic Metamorphic Belt primarily consist of blueschist (glaucophane-schist) grade rocks, which are locally overprinted by a later metamorphic event which produced greenschist to upper amphibolite facies rocks. Widespread intrusion of granitoids followed the regional metamorphism (cf. Altherr & Siebel, 2002). On Naxos an Upper Unit, Fig. 1: The Aegean Sea, the red rectangle frames Naxos; consisting of unmetamorphosed edited figure from Innocenti et al. (2010) sediments and a Lower Unit consisting of a lower grade marble-schist sequence and a migmatite core that includes Mesozoic sediments, S-type granites and Variscan orthogneisses can be distinguished (cf. Gautier, Brun & Jolivet, 1993, after Gärtner et al., 2011). The island consisting basically of metamorphic rocks is also intruded by a granodiorite which is responsible for local contact metamorphism in a range of about 500 m from the contact (cf. Wijbrans & Mc Dougall, 1988).

2 2. Metamorphic events During its geological history Naxos experienced several metamorphic events. The first main metamorphic event, called M 1 occurred about Ma ago (cf. Gueydan et al., 2009) under eclogite to glaucophane-schist conditions (cf. Liati et al., 2009; Wijbrans & Mc Dougall, 1988). As the facies diagram (Fig.2) shows the glaucophane-, or blueschist facies indicates high-pressure low-temperature conditions which imply a fast subduction velocity and/or a high subduction dip angle, as the metamorphic parameters P and T are dependent on the burial rate (cf. Gueydan et al., 2009). Environments with blueschist conditions are possibly the deepest parts of accretionary wedges or continental collision zones with a sufficiently thickened crust (cf. Wijbrans & Mc Dougall, 1988). Another important factor is a high exhumation rate to preserve the high-pressure mineral assemblage, that means to prevent recrystallization under increasing temperature caused by heatproducing radioactive elements ( cf. Gueydan et al., 2009; Wijbrans & Mc Dougall, 1988). Taking the data from Feenstra (1985), Wijbrans & Mc Dougall (1988) stated that the early glaucophane schist metamorphism, M1, in south-east Naxos reached metamorphic temperatures of C and pressures of about 900 MPa (Wijbrans & Mc Dougall, 1988) while the rocks in the Attic Cycladic Belt in general reached pressures from MPa. The second metamorphic event, M 2 occurred later and caused local overprinting under greenschist to amphibolite conditions meaning that the temperature of the second event was higher. Wijbrans & Mc Dougall (1988) proposed that the peak of M2 metamorphism on Naxos occurred between 15,0 Ma and 19,8 Ma ago, based on their 40 Ar/ 39 Ar age spectrum analysis of hornblende samples from Naxos and that the event was caused by a shortlived transient heat source (Wijbrans & Mc Dougall, 1988). They depicted the temperatures from Jansen and Schuiling (1976) as ranging from 420 to 690 C, but estimated that the temperature could have been as much as 100 C lower, because of C02-rich metamorphic fluids that decreased the melting point of the minerals (cf. Wijbrans & Mc Dougall, 1988). Later studies indicated that these C0 2-rich fluids are not representative for the M2 metamorphic event (cf. Baker et al., 1989), so the conclusions relating to the temperature are invalid. Some smaller thermal events accompanied the M2 metamorphism, the intrusion of a granodiorite in the west of Naxos and a thermal anomaly which formed in northern Naxos and is possibly associated with a magma intrusion (cf. Wijbrans & Mc Dougall, 1988). Buick (1991) mentions the age of the granodiorite intrusion as Ma. Fig. 2: Metamorphic facies diagram (Press and Siever, 1994)

3 3. Petrology The metamorphic rocks on Naxos have many chemically different lithologies, such as mainly calcitic and in some parts dolomitic marbles, metapelites and amphiboles. M1 mineral assemblages in the mica schists on Naxos include phengite, glaucophane, paragonite, chlorite, garnet, chloritoid and albite. Glaucophane, epidote, chlorite, paragonite, phengite and actinolite can be found in metavolcanic rocks (cf. Wijbrans & Mc Dougall, 1988). In south-east Naxos the M1 mineral assemblage is preserved best. The M2 metamorphic event caused a Barrovian metamorphic overprint with varying intensity, which developed during non-coaxial mid-lower crustal extension (Buick & Holland, 1989). Wijbrans & Mc Dougall (1988) edited a geological map (Fig. 3) from Jansen (1973) to visualize the differences in the grade of M2 overprinting on Naxos in different zones. The dashed lines represent isogrades which are defined by the occurrence of minerals as proxy for the P-T conditions and which separate the mineral zones. The isogrades are defined as corundum-in isograd ( C), biotite-in isograd (ca. 500 C), chloritoid-out isograd ( C), sillimanite-in isograd (620 C), kyanite-out isograd (ca. 660 C) and melt phase in the metapelites ( C) and illustrate an increase of the temperature and therefore effect of the M2 overprinting from barely detectable in the south-east to complete recrystallization in and around the migmatite Fig. 3: Schematic geological map of Naxos including major lithologies (1=Marble, 2=mica schist, 3=migmatite, 4 =unmetamorphosed core. rocks, 5=granodiorite) and isograds (edited by Wijbrans et al. The map also shows 1988, after Jansen, 1973) the different lithologies on Naxos. Marbles occur widespread on the island, but generally in western Naxos while mica schists surround the migmatite core which is located in the northwest of Naxos. The west of the island is dominated by the granodiorite intrusion. Between the migmatite core and the intrusion some non-metamorphosed rocks can be found.

4 4. P-T-path The scattering of high-pressure metamorphic rocks from the Rhodope massif in the north to Crete and the Peloponnese in the south (Fig. 1) is due to a southward migration of the thrust front and subduction through time, and to the post-orogenic Aegean extension (Jolivet et al., 2003). Fig. 4 shows the evolution of the Aegean region in cross-section. The end of subduction of the Vardar Ocean plate led to the collision of the Apulian platform with Eurasia. Due to this collision the lower part of the Apulian crust has been subducted, while the upper crust formed an accretionary wedge and took part in orogenesis. Later, slab retreat with delamination of upper and lower crust led to extension and the formation of the backarc basin in which Naxos currently lies. According to (Jolivet et al., 2003) two types of subduction-behavior exist, one is the steady-state subduction with a constant thermal structure of the subduction complex leading to a single P/T gradient and similar P/T paths throughout the evolution [...] and one non-steadystate with changes in the geodynamic context such as slab retreat (Aegean) producing different P-T evolutions through time (Jolivet et al., 2003). Therefore slab retreat does not only explain the scattering of metamorphic rocks from Rhodopes to Crete, but also the variation of the P-T conditions over time which can be seen in Fig. 5. The diagram shows the maximum P -T values and exhumation P-T- paths of several islands and areas in and around the Aegean sea which are related to the mentioned subduction with the temperature ( C) on the x-axis and the pressure on the y-axis (kbar). Striking is a general trend of decreasing temperatures from north (Rhodopes) to south (Crete) and old to young. The thermal conditions changed during the extension-driven exhumation during the southward slab retreat (cf. Jolivet et al., 2003). While a Fig. 4: Evolution of the Aegean region cold regime was always associated with proximity of the slab [ ] [the] backarc domain was warmer and the P-T- paths followed a loop toward high temperatures (Jolivet et al., 2003) which is very striking in the graph that represents Naxos. The graph shows an increase of the temperature from ca. 550 C to almost 700 C followed by a decrease of the temperature to a level lower than 500 C during exhumation. Due to it's late exhumation during the Miocene the blueschists on Naxos underwent higher temperatures and show evidence of partial melting (cf. Jolivet et al., 2003). Therefore the M2 event which Wijbrans & Mc Dougall, (1988) proposed to be caused by a shortlived transient heat source is very likely to have it's origin in relatively late Miocene exhumation with a warm exhumation path in a backarc position.

5 In general P-T data and P/T ratios provide important information about subduction velocity and subduction dip angle. Gueydan et al. (2009) made an approach to use P/T ratios as a proxy to quantify dip and velocity of continental subduction and stated that the variations of the P/T ratio between the Cyclades and the Cretan blueschists stand in relation to variation of subduction velocity and dip angles. Fig. 5: Exhumation PT-paths; x-axis = T ( C), y-axis = P (kbar)

6 References Altherr, R. & Siebel, W., 2002, I-type plutonism in a continental back- arc setting: Miocene granitoids and monzonites from the central Aegean Sea, Greece, Contributions to Mineralogy and Petrology, v. 134, pp 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, v. 53, pp Buick I. S., 1991, The late Alpine evolution of an extensional shear zone, Naxos, Greece, Journal of the Geological Society, London, v. 148, pp Buick, I. S. & Holland, T. J. B., 1989, The P-T-t path associated with crustal extension, Naxos, Cyclades, Greece, Geological Society, London, Special Publications, v. 43, pp Gärtner, C., Bröcker, M., Strauss, H., Farber, K., 2011, Strontium-, carbon- and oxygen-isotope compositions of marbles from the Cycladic blueschist belt, Greece, Geological Magazine, v. 148, pp Gueydan, F., Le Garzic, E., Carry, N., 2009, P/T ratio in high-pressure rocks as a function of dip and velocity of continental subduction, Lithosphere, v. 1, pp Innocenti, F., Agostini, S., Doglioni, C., Manetti, P., Tonarini, S., 2010, Geodynamic evolution of the Aegean: constraints from the Plio- Pleistocene volcanism of the Volos-Evia area, Journal of the Geological Society, London, v. 167, pp Jolivet, L., Faccenna, C., Geoffé, B., Burov, E., Agard, P., 2003, Subduction Tectonics and Exhumation of high-pressure metamorphic rocks in the Mediterranean Orogens, American Journal of Science, v. 303, pp Liati, A., Skarpelis, N., Pe-Piper, G., 2009, Late Miocene magmatic activity in the Attic-Cycladic Belt of the Aegean (Lavrion, SE Attica, Greece): implications for the geodynamic evolution and timing of ore deposition, Geological Magazine, v. 146, pp Press, F. & Siever, R., 1994, Understanding Earth, W. H. Freeman and Company Wijbrans, J. R. & Mc Dougall, I., 1988, Metamorphic evolution of the Attic Cycladic Metamorphic Belt on Naxos (Cyclades, Greece) utilizing 40 Ar/ 39 Ar age spectrum measurements, Journal of Metamorphic Geology, v. 6, pp Secondary References: Feenstra, A., 1985, Metamorphism of bauxites on Naxos, Greece, Ph.D. Thesis, University of Utrecht, Geologica Ultraiectina, v. 39, pp Gautier, P., Brun, J. P., Jolivet, L., 1993, Structure and kinematics of Upper Cenozoic extensional detachment on Naxos and Paros (Cyclades Islands, Greece), Tectonics, v. 12, pp Jansen, J. B. H., 1973, Geological map of Naxos, I.G.M.E. Athens Jansen J. B. H., and Schuiling, R.D., 1976, Metamorphism on Naxos: petrology and geothermal gradients, American Journal of Science, v. 276, pp