N. Jb. Miner. Abh. 2005, Vol.181/1, p. 81–93, Stuttgart, Januar 2005

The base of the Cycladic blueschist unit on Island () re-visited: Field relationships, phengite chemistry and Rb–Sr geochronology

Michael Bröcker, Münster and Leander Franz, Freiberg

With 6 figures and 4 tables

Abstract: The archipelago in the is an important study area for subduction-related metamorphism and the exhumation of high-pressure/low-temperature rocks. Of special importance for interpretation of the general tectonic development in the central Aegean region are tectonic windows that expose the rock sequences below the Cycladic Blueschist Unit (CBU). Previous work suggested that the lowermost dolomite-phyllite-quartzite sequence on Tinos Island represents such a tectonic sub- unit with a metamorphic and deformational history that is different to the overlying blueschist- and greenschist-facies rocks. The tectonic contact was interpreted as a thrust fault. A re-evaluation of the arguments used to support this interpretation suggests that this conclusion is questionable. Previous studies inferred that the basal sequences only underwent greenschist-facies metamor- phism and were not affected by a high-pressure event. However, mineral assemblage and phengite composition in the basal phyl- lites cannot be distinguished from those of overlying rock sequences, which undoubtedly have experienced high-pressure met- amorphism and a pervasive greenschist-facies overprint. Rb–Sr geochronology of phyllites and quartzites (phengite – whole rock pairs), previously interpreted to belong to the lower plate, yielded dates that are indistinguishable from values obtained for strongly overprinted rocks collected at higher lithostratigraphic levels. It can also be shown that sedimentary structures are pre- served in many places within the CBU. The presumed absence of such features was originally interpreted as a major contrast to the fossil-bearing basal sequences, indicative for different deformational styles. We postulate that the para-autochthonous basal unit beneath the CBU is not exposed on Tinos Island. Field observations, petrological and geochronological data of the Panormos area are fully compatible with the interpretation that the dolomite-phyllite-quartzite succession is an integral part of the CBU, as originally suggested by Melidonis (1980).

Key words: Cycladic Blueschist Unit, Basal Unit, Rb–Sr geochronology, Tinos, Greece.

Introduction tion represent fundamental parameters for understanding of the exhumation history. Metamorphic sequences be- The general structural, geochronological and P–T evolu- neath the Cycladic Blueschist Unit (CBU) were reported tion of the Cyclades archipelago in the Aegean Sea from Tinos, Evvia, and the Fourni Islands (Fig. 1; (Fig. 1) is well documented, but many details of the tec- Avigad & Garfunkel 1989, Avigad et al. 1997, tonometamorphic history still are not fully understood. A Shaked et al. 2000, Ring et al. 2001). These occurren- much debated issue concerns the importance of syn-oro- ces are considered to represent para-authochthonous genic versus post-orogenic extension for unroofing of units, which are separated from the structurally higher eclogite- to epidote blueschist-facies rocks (e. g. Avigad CBU by thrust faults. Similar field relations are known & Garfunkel 1991, Gautier & Brun 1994 a, b, Avi- from other parts of the Hellenides, e. g. the Olympos- gad et al. 1997, Jolivet & Patriat 1999, Avigad et al. Ossa region on mainland Greece (Fig. 1; Schermer 2000, Gautier 2000, Trotet et al. 2001a, b, Parra et 1990, 1993). al. 2002). In this context, tectonic windows that expose In the central Cyclades, a presumed basal unit beneath the rock sequences below the Cycladic blueschists are of the high-pressure/low-temperature (HP/LT) sequences special importance. The metamorphic P–T conditions of only have been reported from the Panormos area in NW these basal unit(s) and the timing of tectonic juxtaposi- Tinos (Avigad & Garfunkel 1989; Fig. 2). Findings of

DOI: 10.1127/0077-7757/2005/0181-0003 0077-7757/05/0181-0081 $ 3.25  2005 E. Schweizerbart’sche Verlagsbuchhandlung, D-70176 Stuttgart 82 M. Bröcker and L. Franz

on Tinos. Field observations, phengite composition and Rb–Sr geochronology lead to the conclusion that a cor- relation of the lowermost Panormos section with well- documented basal units (e. g. Almyropotamos unit on Evvia) is questionable and as yet unconfirmed.

Geological background

The Attic-Cycladic Crystalline Belt (ACCB; Fig. 1) can be subdivided into two major lithotectonic units, both consisting of numerous subunits. The upper group of units was not affected by HP/LT metamorphism. Indi- vidual segments either experienced greenschist- to am- phibolite-facies metamorphism or were not metamor- phosed (e. g. Okrusch & Bröcker 1990, Dürr 1986, and references therein). The lower group of units (which includes the CBU) has experienced two stages of met- amorphism: a HP/LT event (Late Cretaceous-Eocene) Fig. 1. Simplified geographic map of the Aegean region indicating and a greenschist- to amphibolite-facies overprint (Oligo- key locations discussed in the text. ACCB = Attic-Cycladic Cry- cene-Miocene) (e. g. Okrusch & Bröcker 1990, Dürr stalline Belt. 1986). On Tinos (Figs. 1, 2), a representative crustal segment of the ACCB is exposed in at least three structural sub- undeformed fossils in dolomitic , the absence of units (cf. Melidonis 1980, Bröcker & Franz 2000). glaucophane and distinct deformational characteristics in The highest unit consists of amphibolite-facies rocks (= phyllitic rocks, as well as the presence of an apparent Akrotiri Unit) which yielded K–Ar ages of c. 67 Ma structural discordancy, were interpreted as evidence for (Patzak et al. 1994). The greenschist-facies Upper Unit significant metamorphic and microstructural differences is build up by a disrupted meta-ophiolite sequence (up to compared to the overlying CBU. The tectonic contact be- about 250 m in thickness), comprising serpentinites, tween the CBU and the underlying metamorphics was in- ophicalcites, meta-gabbros and phyllitic rocks (Katzir terpreted as a thrust fault (Avigad & Garfunkel 1989). et al. 1996). The phyllites yielded Rb–Sr phengite-who- In this paper, we critically re-evaluate the arguments, le-rock ages between c. 92–21 Ma (Bröcker & Franz which were used to suggest the existence of a basal unit 1998). The youngest age is believed to approximate the

Fig. 2. Simplified geological map of Tinos (after Meli- donis, 1980). The outcrop area of the Akrotiri Unit (Patzak et al. 1994) is restricted to a small occurrence indicated by a black circle. The base of the Cycladic blueschist unit 83 timing of tectonic juxtaposition. The Akrotiri Unit and Above the fault zone, calcite-rich marbles (c. 50 m thick) the Upper Unit show no indications for high-pressure are intercalated with thin bands of quartzites. These metamorphism and hence are considered to represent the rocks are considered to belong to the BGU. Below the upper group of units of the ACCB. The tectonic contact calcite marbles, a discontinuous horizon of phyllites and at the base of the Upper Unit was interpreted as a low- quartzites (< 2 m thick) was recognized. The phyllite– angle normal fault (Avigad & Garfunkel 1989, Gau- quartzite layer is considered to belong to a lower plate tier & Brun 1994a, b, Patriat & Jolivet 1998). which mainly consists of dolomites to dolomite-rich Most of the island belongs to the Blueschist-Green- marbles (> 100 m thick; Avigad & Garfunkel 1989, schist Unit (= BGU; in the literature also referred to as Matthews et al. 1999). Findings of undeformed fossils Lower Unit or Intermediate Unit; Fig. 2), which correla- in the basal dolomites, the absence of glaucophane and tes with the lower group of units of the ACCB. The BGU distinct deformational characteristics in phyllitic rocks, mainly consists of marbles, calcschists, siliciclastic met- and the inferred presence of a structural discordance, asediments, cherts as well as basic and acid metavolca- were interpreted as evidence for significant metamorphic nic rocks (Melidonis 1980, Bröcker 1990). Melidonis and microstructural differences compared to the overly- (1980) showed that this succession can be subdivided ing rocks (Avigad & Garfunkel 1989). These authors roughly by three mappable sequences (labelled suggested that the lowermost sequence represents a dis- m3, m2 and m1 from top to bottom). The BGU has ex- tinct tectonic unit, which was only affected by low-grade perienced eclogite- to epidote blueschist-facies metamor- greenschist-facies conditions. According to their inter- phism (T = 450–500˚C, P >12 kbar) in the Late Cretace- pretation, the fault zone is a synorogenic thrust, because ous to Eocene and a greenschist-facies overprint (T = high-pressure rocks were juxtaposed onto a lower grade 450–500 ˚C, P = 4–7 kbar) at the Oligocene/Miocene series (Avigad & Garfunkel 1989, 1991, Avigad et al. boundary (e. g. Bröcker et al. 1993, Bröcker & En- 1997). ders 1999). Multi-equilibrium P–T estimates, based on Matthews et al. (1999) showed that the fault zone the compositional variability of chlorite and phengite in and adjacent rock volumes are characterised by carbon metapelites indicated three metamorphic stages during and oxygen isotope depletions, which were explained by exhumation (Parra et al. 2002): decompression from focused fluid-infiltration of externally derived fluids 18–15 kbar at 500 ˚C to 9 kbar at 400 ˚C was followed at along the tectonic contact. Temperature estimates based c. 9 kbar by a thermal overprint (400 to 550 ˚C) and fur- on dolomite-calcite solvus thermometry indicated c. ther decompression from 9 kbar at 550–570 ˚C to 2 kbar 350–420˚C for upper and lower plate marbles at the time at 420˚C. of shearing (Matthews et al. 1999). Owing to an appar- In the eastern part of the island (Fig.2) the Upper Unit ent temperature increase in the dolomites towards the and the BGU were affected by contact metamorphism contact from c. 300 to 370 ˚C, these authors assumed that (Avigad & Garfunkel 1989, 1991, Stolz et al. 1997, the lower plate experienced heating during thrusting and Bröcker & Franz 1994, 2000), caused by Miocene gra- suggested that a temperature of c. 300 ˚C possibly re- nitoids which were dated at 17–14 Ma (Altherr et al. cords the original low-grade conditions. 1982, Bröcker & Franz 1998).

Field observations and sample description The dolomite-phyllite sequence in NW Tinos The Panormos exposure is divided into a northern and Previous work southern part by a roughly west-east-trending valley filled with alluvium (Fig. 2). In the northern segment, the basal The lowermost parts of the metamorphic succession on section consists of dolomite, which is overlain by a dis- Tinos are exposed in the NW part of the island around continuous horizon of graphite-rich phyllite and quartzite the village of Panormos (Fig. 2; Melidonis 1980, Avi- (up to 2 meter thick), or a discontinuous calcschist layer, gad & Garfunkel 1989, Matthews et al. 1999). In followed by a calcite marble (several meters in thickness) this area, the rock pile is dominated by calcite-rich marb- with quartzite intercalations (mostly < 5 cm; rarely up to les which are underlain by a dolomite sequence. Origi- 20 cm, Figs. 3 a, b). In the southern segment (Fig. 4), nally, this succession (= m1 marbles) was completely as- graphite-rich phyllite is rare and the dolomite is overlain signed to the BGU (Melidonis 1980). This interpreta- by a white quartzitic layer. On top of this horizon a cal- tion was questioned by Avigad & Garfunkel (1989) cite marble with thin quartzite layers occurs. Up section, who recognized a tectonic contact within the marbles. this marble is overlain by a calcschist, a semipelitic 84 M. Bröcker and L. Franz

Fig. 3. Field occurrence of calcite marbles from the Panormos area: A. and B. Intercalations of mm- to dm-thick quartzite layers; C. discordant quartzite vein with bleaching zone; D. bedding structure, caused by differences in grain-size; E. and F. pebbly mudstone fabric.

schist and a calcite marble. Thickness and abundance of calcite; dolomite is present in some samples, but is al- quartzite intercalations are much smaller than in the ways much less abundant. Among the accessories, phen- northern part. On both sides of the valley, quartz veins gite is most common, but albite, quartz, chlorite and and pods (up to several meter in thickness) are common. graphite may occur in the groundmass as additional con- In the calcite marbles, bleached alteration halos around stituents. Phengite often is strongly enriched on bedding veins of all sizes were recognized (Fig.3c). surfaces. No glaucophane, garnet or omphacite were rec- Calcite marbles from the BGU were collected from ognized, but relics of HP rocks locally are preserved di- the m1 sequence (Melidonis 1980) around Panormos rectly above the calcite marbles (Bieling & Bröcker, Bay and Vathy (Fig. 2). All samples were taken from out- unpubl. data). crops with quartzite intercalations. There is general con- The basal dolomites only are exposed around Panor- sensus that these marbles belong to the BGU and thus mos. These massive, fine-grained rocks show no bedding were affected by HP metamorphism and a greenschist- and contain fewer and smaller phengite grains than the facies overprint. The marbles are fine-to medium- calcite marbles. An exception is the highly strained sam- grained, well-bedded rocks with white to bluish-grey ple 1422, collected close to the phyllite horizon, with up colour on the outcrop- or hand-specimen scale. Normally to c. 3 vol.% of phengite. At some places, a breccia fab- graded bedding and changes in grain-size between indi- ric of unclear origin was recognized with weak contrast vidual layers (Fig. 3 d) locally were recognized. Wide- between clasts and matrix. Sporadically, Upper Triassic spread is a pebbly mudstone fabric with clasts and mat- fossils are preserved (Melidonis 1980). rix solely consisting of carbonates (Figs. 3 e, f). The The mineral assemblage of well-foliated phyllite and clasts range in size from a few millimetres to several the quartzite mainly consists of phengite, chlorite, albite, centimetres. The mineral assemblage mostly consists of quartz, epidote and titanite. The base of the Cycladic blueschist unit 85

Analytical methods

Phengite compositions were determined with a CAME- CA SX-50 and a JEOL JXA-8900 R electron microprobe at the Mineralogisches Institut, Universität Würzburg and at the Institut für Mineralogie, TU Freiberg. Operating conditions for silicates were 15 kV acceleration voltage, 10–20 nA beam current and counting time of 20–30 s. The beam diameter was set at 3–5 µm. For standardiza- tion, natural and synthetic minerals were used. The raw data were corrected with a ZAF procedure. Represent- ative phengite analyses are shown in Tables 1, 2 and 3. Isotope analyses were carried out at the Zentrallabora- torium für Geochronologie at the Institut für Minera- logie, Universität Münster. For sample preparation, whole rocks (< 1 kg) were crushed in a steel mortar or using a jaw-breaker and disc mill. Whole rock powders were prepared in a tungsten carbide mill. For mica sepa- ration, crushed material was reduced in size either by grinding for only a few seconds in a tungsten carbide mill or by use of a disk mill. Following sieving fines Fig. 4. Schematic columnar section (not to scale) of the southern were removed and mica was enriched by use of a Frantz Panormos area. magnetic separator and by adherence to a sheet of paper.

Table 1. Representative electron microprobe analyses of high-Si phengites from calcite marbles. Sample 2011 2011 2011 2012 2012 2012 1227 1227 1227 1224 1224 1224 Spot A5 A10 B4 B3 C4 D6 D6 G2 G5 A2 D1 F3

SiO2 54.66 54.60 53.97 54.23 53.97 54.96 54.38 55.63 55.82 55.74 55.80 55.45 TiO2 0.15 0.12 0.26 0.15 0.30 0.21 0.31 0.19 0.09 0.21 0.21 0.28 Al2O3 22.43 22.10 22.70 23.11 22.88 22.61 22.64 22.17 21.14 20.32 20.16 20.85 Cr2O3 0.15 0.02 0.06 0.08 0.09 0.09 0.07 0.06 0.18 0.11 0.06 0.12 MgO 6.25 6.51 6.33 6.32 6.21 6.35 6.56 6.90 7.23 7.36 7.47 7.42 CaO 0.24 0.11 0.14 0.07 0.08 0.10 0.22 0.24 0.21 0.22 0.17 0.13 MnO 0.03 0.03 0.01 0.02 0.02 0.00 0.02 0.00 0.03 0.01 0.00 0.00 FeO 0.01 0.07 0.02 0.00 0.07 0.02 0.06 0.02 0.01 0.02 0.00 0.00 BaO 0.04 0.00 0.00 0.00 0.00 0.00 0.11 0.00 0.03 0.26 0.26 0.25 Na2O 0.08 0.07 0.15 0.12 0.11 0.06 0.25 0.10 0.10 0.09 0.07 0.08 K2O 11.10 10.81 11.23 10.78 11.00 10.86 10.89 10.59 10.35 10.43 10.66 10.60 Total 95.14 94.44 94.87 94.88 94.73 95.26 95.51 95.90 95.19 94.77 94.86 95.18 Structural formula on the basis of 11 oxygen Si 3.620 3.630 3.590 3.590 3.585 3.625 3.590 3.640 3.675 3.695 3.700 3.665 Ti 0.005 0.005 0.015 0.010 0.015 0.010 0.015 0.010 0.005 0.010 0.010 0.015 Al 1.750 1.733 1.779 1.803 1.792 1.757 1.762 1.708 1.640 1.588 1.575 1.624 Cr 0.008 0.001 0.003 0.004 0.005 0.005 0.004 0.003 0.010 0.006 0.003 0.006 Mg 0.617 0.646 0.628 0.624 0.615 0.624 0.645 0.673 0.710 0.727 0.739 0.731 Ca 0.017 0.008 0.010 0.005 0.006 0.007 0.015 0.017 0.015 0.016 0.012 0.009 Mn 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 Fe 0.000 0.005 0.000 0.000 0.005 0.000 0.005 0.000 0.000 0.000 0.000 0.000 Ba 0.000 0.000 0.000 0.000 0.000 0.000 0.005 0.000 0.000 0.005 0.005 0.005 Na 0.010 0.010 0.020 0.015 0.015 0.005 0.030 0.010 0.015 0.010 0.010 0.010 K 0.935 0.915 0.950 0.910 0.935 0.915 0.915 0.885 0.870 0.880 0.900 0.895 Total 6.969 6.958 6.994 6.961 6.973 6.947 6.987 6.945 6.937 6.944 6.956 6.958 86 M. Bröcker and L. Franz

Table 2. Representative electron microprobe analyses of high-Si phengites from phyllites. Sample 1221 1221 1221 1222 1222 1222 1418 1418 1418 1421 1421 1421 Spot 26 10 11 13 1 7 1–2 1–7 4–5 2–1 1–5 2–5

SiO2 53.02 52.89 53.84 53.49 53.66 53.58 54.99 55.17 56.00 54.62 53.37 53.99 TiO2 0.11 0.09 0.12 0.09 0.11 0.10 0.10 0.06 0.08 0.08 0.10 0.11 Al2O3 23.70 24.03 23.74 23.93 23.13 24.10 22.81 22.80 22.44 22.47 25.01 23.78 Cr2O3 0.03 0.06 0.05 0.04 0.07 0.08 0.06 0.11 0.15 0.04 0.04 0.07 MgO 5.23 5.11 5.33 5.32 5.35 5.22 5.69 5.82 6.20 5.37 4.87 5.26 CaO 0.10 0.00 0.00 0.01 0.01 0.00 0.00 0.00 0.00 0.00 0.00 0.00 MnO 0.01 0.02 0.00 0.02 0.00 0.00 0.00 0.03 0.00 0.00 0.00 0.00 FeO 1.26 1.20 1.30 1.35 1.76 1.27 1.21 1.22 1.03 2.11 1.74 1.85 BaO 0.00 0.02 0.02 0.03 0.01 0.04 0.09 0.17 0.20 0.17 0.17 0.07 Na2O 0.07 0.07 0.07 0.08 0.07 0.16 0.07 0.07 0.03 0.06 0.15 0.07 K2O 10.99 11.04 11.09 10.83 11.04 10.94 10.09 10.44 9.70 10.46 10.16 10.33 Total 94.51 94.55 95.56 95.19 95.20 95.48 95.12 95.89 95.84 95.38 95.61 95.53 Structural formula on the basis of 11 oxygens Si 3.553 3.542 3.566 3.554 3.577 3.551 3.632 3.625 3.659 3.626 3.524 3.571 Ti 0.006 0.005 0.006 0.004 0.005 0.005 0.005 0.003 0.004 0.004 0.005 0.005 Al 1.872 1.897 1.853 1.874 1.817 1.882 1.775 1.766 1.728 1.758 1.947 1.853 Cr 0.001 0.003 0.003 0.002 0.004 0.004 0.003 0.006 0.008 0.002 0.002 0.003 Mg 0.522 0.510 0.526 0.527 0.532 0.516 0.561 0.570 0.604 0.531 0.480 0.518 Ca 0.007 0.000 0.000 0.000 0.001 0.000 0.000 0.000 0.000 0.000 0.000 0.000 Mn 0.000 0.001 0.000 0.001 0.000 0.000 0.000 0.002 0.000 0.000 0.000 0.000 Fe 0.071 0.067 0.072 0.075 0.098 0.070 0.067 0.067 0.056 0.117 0.096 0.102 Ba 0.000 0.001 0.001 0.001 0.000 0.001 0.002 0.004 0.005 0.004 0.004 0.002 Na 0.009 0.010 0.009 0.011 0.009 0.020 0.009 0.009 0.003 0.008 0.019 0.009 K 0.939 0.943 0.937 0.918 0.939 0.925 0.850 0.875 0.808 0.886 0.856 0.872 Total 6.979 6.979 6.973 6.968 6.981 6.974 6.904 6.928 6.875 6.937 6.933 6.936

Table 3. Representative electron microprobe analyses of high-Si phengites from dolomites. Sample 1408 1408 1408 2017 2017 2017 2031 2031 2031 2013 2013 2013 Spot B1 D1B A1B A4 B3 D1A C1 C3 C4 G3 H2 H3

SiO2 53.91 53.49 53.14 52.62 52.93 52.46 52.57 52.66 52.63 52.93 52.52 52.43 TiO2 0.07 0.09 0.02 0.04 0.11 0.05 0.18 0.12 0.10 0.02 0.08 0.05 Al2O3 25.55 25.56 25.24 26.98 26.93 27.14 25.83 26.26 26.11 25.27 26.20 25.92 Cr2O3 0.05 0.00 0.04 0.08 0.02 0.10 0.08 0.08 0.16 0.05 0.10 0.08 MgO 5.27 5.31 5.48 4.69 4.59 4.39 5.18 5.14 4.96 5.38 4.84 4.96 CaO 0.09 0.08 0.08 0.08 0.13 0.06 0.09 0.03 0.18 0.21 0.21 0.17 MnO 0.03 0.00 0.00 0.03 0.00 0.01 0.00 0.04 0.03 0.02 0.00 0.00 FeO 0.00 0.02 0.01 0.01 0.00 0.03 0.00 0.01 0.00 0.04 0.00 0.00 BaO 0.01 0.07 0.06 0.00 0.09 0.08 0.25 0.28 0.18 0.04 0.05 0.00 Na2O 0.05 0.15 0.09 0.26 0.24 0.20 0.56 0.44 0.46 0.09 0.16 0.12 K2O 11.07 11.05 10.90 10.32 10.70 10.77 9.67 9.79 10.04 10.88 10.91 11.07 Total 96.10 95.82 95.06 95.11 95.74 95.29 94.39 94.83 94.85 94.92 95.07 94.81 Structural formula on the basis of 11 oxygens Si 3.525 3.510 3.515 3.465 3.470 3.460 3.488 3.479 3.481 3.509 3.477 3.482 Ti 0.005 0.005 0.000 0.000 0.005 0.005 0.009 0.006 0.005 0.001 0.004 0.003 Al 1.969 1.978 1.967 2.094 2.082 2.109 2.020 2.044 2.035 1.974 2.044 2.029 Cr 0.002 0.000 0.002 0.004 0.001 0.005 0.004 0.004 0.008 0.003 0.005 0.004 Mg 0.513 0.520 0.541 0.461 0.448 0.431 0.512 0.506 0.489 0.532 0.478 0.491 Ca 0.007 0.006 0.005 0.005 0.009 0.004 0.006 0.002 0.013 0.015 0.015 0.012 Mn 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.002 0.001 0.001 0.000 0.000 Fe 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.002 0.000 0.000 Ba 0.000 0.000 0.000 0.000 0.000 0.000 0.006 0.007 0.005 0.001 0.001 0.000 Na 0.005 0.020 0.010 0.035 0.030 0.025 0.072 0.056 0.059 0.011 0.021 0.016 K 0.925 0.925 0.920 0.865 0.895 0.905 0.818 0.825 0.847 0.920 0.921 0.938 Total 6.951 6.967 6.965 6.933 6.945 6.947 6.936 6.931 6.945 6.968 6.966 6.975 The base of the Cycladic blueschist unit 87

Table 4. Rb–Sr isotope results for samples from the Panormos area. Sample Rock type Mineral Size Rb Sr 87Rb/86Sr* 87Sr/86Sr ± 2 σ Age in Ma (µm) (ppm) (ppm) ± 2 σ 3513 marble phengite 355–250 157 15.6 29.1 0.717662 0.000021 24.2 ± 0.2 calcite 355–250 0.165 223 0.00215 0.707646 0.000010 3514 marble phengite 355–250 174 8.76 57.7 0.728136 0.000030 25.0 ± 0.3 calcite 355–250 0.243 354 0.00199 0.707670 0.000010 3515 marble phengite 355–250 234 8.75 77.5 0.733414 0.000016 23.5 ± 0.2 calcite 355–250 0.331 332 0.00289 0.707612 0.000010 calcite 250–180 0.487 335 0.00420 0.707608 0.000010 3519 calcschist phengite 355–250 336 1.96 503 0.867485 0.000032 22.0 ± 0.2 calcite 355–250 1.87 177 0.0307 0.710228 0.000011 3520 marble phengite 250–180 258 5.40 139 0.752749 0.000017 22.9 ± 0.9 calcite 355–250 0.371 271 0.00396 0.707560 0.000010 calcite 250–180 0.194 277 0.00203 0.707587 0.000010 3522 calcschist phengite 355–250 331 2.18 445 0.847719 0.000025 21.9 ± 0.2 calcite 355–250 2.76 194 0.0411 0.709171 0.000010 3521 marble phengite 250–180 290 1.73 494 0.876071 0.000062 24.0 ± 0.2 calcite 355–250 0.193 286 0.00195 0.707670 0.000010 2016 calcschist phengite 355–250 228 5.87 113 0.743238 0.000015 21.5 ± 0.2 calcite 355–250 0.415 448 0.00269 0.708797 0.000014 1221 phyllite phengite 250–180 245 39.8 17.8 0.716073 0.000015 23.3 ± 0.4 phengite 180–125 285 24.7 33.4 0.721286 0.000015 whole rock 105 38.7 7.84 0.712802 0.000017 1222 quartzite phengite 180–125 331 75.7 12.7 0.713695 0.000014 23.8 ± 0.3 whole rock 18.2 150 0.352 0.709546 0.000015 18.1 150 0.350 0.709556 0.000013 1418 phyllite phengite 250–180 352 14.8 69.1 0.733286 0.000028 24.1 ± 0.2 whole rock 1.52 0.710175 0.000015 1421 phyllite phengite 250–180 343 15.8 62.8 0.731496 0.000032 24.1 ± 0.2 whole rock 79.0 159 1.44 0.710477 0.000011 3518 quartzite phengite 355–250 377 2.23 498.4 0.880240 0.000072 24.0 ± 0.2 calcite + quartz >355 1.40 14.4 0.281 0.710586 0.000011 * The 87Rb/86Sr ratios were assigned an uncertainty of 1% (2 σ).

After hand-picking, mica concentrates (optically pure for mass fractionation is based on a 86Sr/86Sr ratio of > 99 %) were washed in ethanol (p. a.) in an ultrasonic 0.1194. Rb ratios were corrected for mass fractionation bath and repeatedly rinsed in H2O (three times distilled). using a factor deduced from multiple measurements of Whole-rock powders (about 100 mg) and phengites (c. Rb standard NBS 607. Total procedural blanks were less 4–28 mg) were mixed with a 87Rb-84Sr spike in teflon than 0.1ng (mostly < 0.05 ng) for Rb and 0.24 ng (mostly screw-top vials and dissolved in a HF–HNO3 (5 :1) mix- < 0.1 ng) for Sr. Based on repeated measurements, the ture on a hot plate overnight. After drying, 6 N HCl was 87Rb/86Sr ratios were assigned an uncertainty of 1 % added to the residue. This mixture was homogenized on (2σ). Uncertainties of the 87Sr/86Sr ratios are reported at a hot plate overnight. After a second evaporation to dry- the 2 σm are reported at the 2σm level. In the course of ness, Rb and Sr were separated by standard ion-ex- this study, repeated runs of NBS standard 987 gave an change procedures (AG 50 W-X8 resin) on quartz glass average 87Sr/86Sr ratio of 0.710308 ± 0.000042 (2σ,n= columns using 2.5 N HCl as eluent. Calcite (c. 9–62 mg) 20). All ages and elemental concentrations were calcu- was dissolved in 2.5 N HCl. Rb was loaded with H2Oon lated using the IUGS recommended decay constants Ta filaments; Sr was loaded with TaF5 on W filaments. (Steiger & Jäger 1977) by means of the Isoplot pro- Mass-spectrometric analysis was carried out using a VG gram version 2.49 (Ludwig 1991). Rb–Sr isotope results Sector 54 multicollector mass spectrometer (Sr) and a are shown in Table 4. NBS-type Teledyne mass spectrometer (Rb). Correction 88 M. Bröcker and L. Franz

Phengite composition

In order to characterise white mica used for geochro- nology (homogeneous or mixed populations?) and to constrain pressure-dependent differences in Si-contents, 14 samples were studied with the electron microprobe (8 dolomites, 3 phyllites, 1 quartzite and 5 calcite marbles). In the calcite marbles, white mica is more frequent and of larger grain-size than in the dolomites. Phengite often is strongly enriched on bedding surfaces, but is also found in the groundmass. In dolomites, white mica oc- curs in two modes: (1) as isolated grains randomly distri- buted in the carbonate matrix and, more rarely, (2) in mi- croveins. No systematic differences in mica composi- tions were recognized in samples collected above and below the fault zone (Fig. 5; Tables 1, 2, 3); the range in Fig. 6. Schematic columnar section (not to scale) of the Panormos- Si-content is similar (calcite marble: 3.36–3.76; phylli- Vathy area summarizing geochronological results of the study area te/quartzite: 3.29–3.71; sheared dolomite: 3.23–3.64; (this study; Bröcker & Franz 1998). dolomite: 3.36–3.55). Most phengite plots along the Al- celadonite-muscovite join, illustrating the importance of the Tschermak’s substitution [Si, (Mg, Fe2+) = AlVI,AlIV] mite (sample 1422) show a higher degree of recrystalli- with maximum Si contents of > 3.5 p. f. u. Noteworthy zation, as indicated by increased modal proportions of are extremely high Si-contents in some calcite marbles phengite with Si values <3.3 p.f.u. In the dolomites, tex- (up to 3.75 p. f. u.), which cannot be explained by beam turally different micas cannot be distinguished by Si val- overlap on SiO2-rich phases, because the studied micas ues; their variations in Tschermak’s substitution show no occur as isolated grains in a carbonate matrix. Within the consistent pattern. Both, in calcite marble and dolomite, shear zone, both phyllites and a ductily deformed dolo- small amounts of paragonite may coexist with phengite.

Fig. 5. Si–Al diagram for phengitic white mica: a. calcite marbles (153 spot analyses from 6 samples); b. phyllites and quartzites (199 spot analyses from 4 samples); c. sheared dolomite (39 spot analyses from 1 sample); d. massive dolomites (164 spot analyses from 7 samples). The base of the Cycladic blueschist unit 89

Paragonite substitution (Na/(Na + K) in phengite is < 0.1. ries is problematic. In the following, this interpretation is In carbonate rocks phengite has very low FeO concentra- re-evaluated based on field observations, phengite chem- tions (< 0.1 wt.%), resulting in XMg values > 0.99. MgO- istry and geochronological data. concentration is variable and ranges between 2.5 and 8 wt.%, with average contents around 4.8 wt.% (dolo- mites) and 6.8 wt.% (calcite marbles). In phyllite and Indications for a different deformation history? quartzite, FeO-content of phengite is higher, varying mostly from 0.7 to 2.2 wt.%. Only in rare cases, values According to Avigad & Garfunkel (1989), the preser- up to 4 wt.% were recognized. MgO-concentration va- vation of undeformed fossils in the basal dolomites is a ries between 2.6 and 6.3 wt.% with average values be- major contrast between the lowermost carbonate rocks tween 4.3–5.1 wt.% (XMg = 0.84–0.89). and the calcite marbles of the BGU, suggesting different deformation on both sides of the tectonic con- tact. This argument was strengthened by noting that the Rb–Sr geochronology BGU marbles on Tinos generally are lacking sedimen- tary features. Our field observations are at variance to In order to characterise the age of metamorphism and/or this conclusion. In the Panormos area, the calcite marb- deformation in the basal dolomite-phyllite sequence, we les commonly show a pebbly mudstone fabric (Figs. 3 e, analysed five samples from the phyllite/quartzite hori- f). This structure is interpreted here as a breccia of debris zon, collected at three different outcrops. In addition, flow origin derived from reworked carbonate hard- ages were determined for five calcite marbles and three grounds. A similar origin was suggested for black calcschists. Attempts to separate enough phengite for spotted marbles, which occur at higher lithostratigraphic dating of dolomites were unsuccessful, due to a combi- levels (see plate 7, page 105 in Bröcker 1990). Further- nation of small grain-size, low modal abundance and more, in the upper section of this unit a meta-conglome- small samples. The isotopic data are summarized in Ta- rate or meta-debris flow is locally preserved with suban- ble 4. gular to rounded clasts (< 20 cm), composed of marbles Phyllites and quartzite (whole rock, phengite) yielded and minor metabasic rocks (Bröcker 1990, Bröcker et ages that are indistinguishable from results obtained for al. 2004). At many places, pebbles are totally sheared severely overprinted greenschist-facies rocks from the and flattened, clearly documenting heterogeneous strain BGU, collected at higher lithostratigraphic levels. The within the same horizon. The sporadic preservation of studied samples yield Rb–Sr ages of 23.3–24.1 Ma (Ta- sedimentary structures, as well as the occurrence of un- ble 4; weighted average: 24.0 ± 0.3 Ma; n = 5). Previous deformed fossils at the base of the metamorphic succes- geochronological work on a calcschist considered to be- sion, are compatible with a model that suggests non-per- long to the basal sequence provided a Rb–Sr age (phen- vasive strain distribution within a continuous metamor- gite, whole rock) of 21.7 ± 0.2 Ma (Bröcker & Franz phic sequence. 1998). An additional sample collected from this occur- Avigad & Garfunkel (1989) also argued that the rence is now dated at 21.5 ± 0.2 Ma. Calcite marbles syn-kinematic crystallization of the phyllites contrasts from the Panormos area (n = 5) provide ages between with static growth of greenschist-facies minerals in the 22.9 and 25.0 Ma (Table 4; weighted average: 24.0 ± 0.7 overlying sequence. However, the phyllites outline a rel- Ma). Two calcschists from the southern part of the Pa- atively narrow tectonic zone. It is unlikely that the origi- normos section, collected above the tectonic contact, nal deformational fabric still is preserved in this fault yield ages of 22.0 ± 0.2 Ma and 21.9 ± 0.2 Ma, respec- contact. In addition, there is considerable dispute con- tively. Calcite marbles from Vathy yielded a pooled age cerning the amount of deformation during greenschist- of 24.2 ± 0.8 Ma (n = 3; Bröcker et al. 2004). facies overprinting of the BGU. Avigad & Garfunkel (1989) and Bröcker (1990) concluded that evidence for extensive penetrative deformation during retrogression is Discussion largely absent in most parts of Tinos. This interpretation was questioned by other groups working on this island, There is general consensus that in NW-Tinos a tectonic who established arguments for ductile extensional defor- contact separates calcite-rich marbles from a lower dolo- mation during the overprint (e. g. Gautier & Brun mite sequence that includes a thin phyllite-quartzite hori- 1994a, b, Jolivet & Patriat 1999, Parra et al. 2002). zon. However, the conclusion that this fault zone separa- According to Avigad & Garfunkel (1989), field tes sequences with different tectonometamorphic histo- mapping suggests the presence of a structural discor- 90 M. Bröcker and L. Franz dancy in the Panormos area. However, as elsewhere ob- metamorphic history is not significantly different to the served on Tinos, thickness of individual lithological P–T path recognized elsewhere in the BGU. layers is highly variable along strike. Thus, thinning or For calcite marbles and dolomites, it is realistic to sug- wedging out of distinct horizons is not necessarily an in- gest that Si-content of phengite is strongly affected by dication for discordant field relationships. But even if a bulk-compositional constraints, because white mica is structural discordancy can unequivocally be documented, among the accessories the most common or single sili- such a feature still can be related to a fault within the cate phase, and thus most likely will accommodate most BGU. of the available Si (Bröcker et al. 2004). The trend to- wards higher Si-values in phengite of calcite marbles Differences in metamorphic grade? (Fig. 5), if compared to phengite in dolomite, is inter- preted as an artefact of such bulk-compositional differ- Avigad & Garfunkel (1989) interpreted the absence of ences. The calcite marbles typically are intercalated with glaucophane in the phyllite-quartzite horizon as indica- quartzite layers on a cm- to dm scale (Figs. 3 a, b), sug- tion that blueschist-facies conditions were not attained in gesting increased availability of Si. However, it is inter- the basal sequences. Judging from mineral assemblages esting to record that the dolomites also contain high Si- mainly consisting of white mica, chlorite, quartz and al- phengite (Table 3; Fig. 5). The conclusion that, for cal- bite, low-grade metamorphism in the greenschist-facies cite marble and dolomites, the Si-content of phengite is was assumed. However, the absence of glaucophane in of doubtful barometric significance is further corrobo- rocks of suitable bulk-rock composition is of doubtful rated by the fact that Rb–Sr phengite dating of calcite significance for establishing differences in metamorphic marbles yielded ages of c. 24 Ma (this study; Bröcker grade. Glaucophane is also not found in many clastic et al. 2004). Previous geochronology in the Cyclades has metasediments of the BGU, due to pervasive green- shown that phengite ages (Rb–Sr, K–Ar, 40Ar-39Ar) of schist-facies overprinting (e. g. Bröcker 1990, Parra et c. 25–18 Ma are related to greenschist-facies overprint- al. 2002). ing and that ages around 53–40 Ma are related to blue- The silica content in phengite is variable and experi- schist-facies metamorphism (e. g. Altherr et al. 1979, mental work indicates that with increasing pressure the 1982, Wijbrans & McDougall 1986, 1988, Wijbrans Si-content increases in rocks containing the limiting as- et al. 1990, Bröcker et al. 1993, Bröcker & Franz semblage (e. g. Velde 1967, Massonne & Schreyer 1998). The fact that high-Si phengite of the calcite marb- 1987, Massonne 1991). Although not fully correct for les does not record the age of the HP stage, but the time non-buffered mineral assemblages, high-Si content in of the subsequent overprint is related to recrystallization phengite (Si ≥ 3.5) is often considered as indication for at lower pressure, which did not cause a significant mod- crystallization under HP conditions, indicating at least a ification of Si-content (for a detailed discussion see minimum pressure. The studied clastic metasediments Bröcker et al. 2004). and marbles do not contain the limiting assemblage It is possible that the basal rocks were affected by (phengite, K-feldspar, biotite, quartz, H2O). Therefore, lower grade blueschist-facies conditions than the overly- Si-in-phengite is not a reliable indicator for absolute ing sequences. Field relations of this kind were reported metamorphic pressures. A detailed thermobarometric from Evvia (Fig. 1) where high-pressure lithologies of evaluation of the metamorphic conditions in the basal se- the Cycladic blueschist belt were thrusted onto the Al- quences is beyond the scope of this paper, but a qualita- myropotamos unit (= basal unit; Shaked et al. 2000, tive estimate of metamorphic pressure can be deduced Katzir et al. 2000). Findings of glaucophane clearly in- from phengite composition alone. In the context dis- dicate that blueschist-facies conditions were also attained cussed here the emphasis is on the question whether dif- in the structurally lower unit and the presence of two dis- ferences in Si-content can be recognized in samples col- tinct high-pressure units, which have experienced differ- lected above and below the Panormos shear zone. It is ent P–T conditions, was suggested (Shaked et al. 2000, worth to emphasize that clastic metasediments collected Katzir et al. 2000). The assumed pressure difference above and below the tectonic contact show no systematic (lower unit: c. 10 kbar; upper unit: c. 12 kbar; Shaked et difference in phengite compositions. The range of Si- al. 2000) is currently not well-documented. Available es- content in phengite of the basal phyllites/quartzites is timates are based on Si-content of phengite not coexist- similar to values reported from BGU rocks (e. g. ing with the limiting assemblage. Gautier (2000) sug- Bröcker et al. 1993, Bröcker & Franz 1998, Parra et gested that the apparent pressure difference of c. 2 kbar al. 2002). Although not providing unequivocal evidence, might be an artefact of sampling at widely separated lo- this observation is in accord with the conclusion that the cations, due to slightly dipping sequences. The base of the Cycladic blueschist unit 91

Timing of metamorphism and shear zone activity the tectonic contact. The geochronological data is in ac- cord with the interpretation that the main activity in the Phengite ages currently are not available for the dolo- Panormos shear zone broadly coincides with the green- mites. Future dating of these rocks will complement ex- schist-facies overprint affecting the BGU. Due to prob- isting datasets, but most likely will not provide the key lems related to incomplete resetting and mixing of differ- to unravel the structural position of the lowermost Panor- ent mica generations, the exact age of this retrogression mos section, for the following reasons. (1) Phengite ages is unknown and can not be detected by multigrain dating. of the basal units on Samos and Evvia are similar to those commonly found for overprinted rocks of the CBU (24–20 Ma; Ring et al. 2001, Ring & Reischmann Conclusions 2002). (2) Due to an unclear relationship between petro- The results of this study indicate that a correlation of the logical and geochronological information, it is contro- basal sequences on Tinos with para-autochthonous units versially discussed whether these dates indicate the time like the Almyropotamos and Olympos-Ossa units on Ev- of HP metamorphism or the age of greenschist-facies via and mainland Greece is problematic. We do not overprinting (Bröcker et al. 2004). We expect that the question the general importance of thrusting at the base Panormos dolomites will provide similar ages, which can of the CBU, however, Tinos is not a place to corroborate not be interpreted unambiguously. this concept. Field observations, phengite chemistry and The phyllite–quartzite layer outlines a shear zone be- geochronological data of the Panormos area are compati- tween different carbonate rocks. Direct dating of shear ble with the interpretation that the basal sequences are an zones is possible if the studied mineral phase formed or integral part of the BGU, as originally suggested by Me- recrystallized during deformation, at or below the clo- lidonis (1980). The Panormos fault is considered here sure temperature for the specific mineral and isotope sys- as a tectonic contact within the BGU, with unknown tem. For the studied samples, this prerequisite is ful- amount of displacement, related to post-orogenic exten- filled. Peak-temperatures during the HP stage and the sion (cf. Gautier & Brun 1994a, b). subsequent greenschist overprint are considered to be The metamorphic P–T path of rocks occurring above ≤ 500 ˚C (Avigad & Garfunkel 1989, Bröcker et al. the fault zone is well-constrained by chlorite-phengite 1993), close to or below the closure temperature for the local equilibria data (Vidal & Parra 2000, Parra et al. Rb–Sr system of phengite, which is commonly consid- 2002), but similar information is not at hand for the dol- ered to be around 500 ˚C. Thus, phengite ages of the Pa- omite-phyllite sequence below the tectonic contact. The normos area do not represent cooling ages, but provide status of these rocks within the structural framework of constraints for timing of mica formation/recrystalliza- the Aegean region can only unambiguously be unra- tion. Correct interpretation of these dates is severely velled, if quantitative P–T estimates become available hampered by the presence of mixed mica populations. for the lowermost Panormos sequence and/or if geochro- As indicated by variable Si-contents, the phengite ali- nological studies will present evidence that the age of quots used for geochronology are heterogeneous and HP metamorphism is different to the overlying rocks. consist of grains representing different generations. In the phyllites, the shearing did not cause a complete rec- rystallization of white mica. Multigrain dating of such Acknowledgements populations can not provide a precise age for deforma- tion, but only yields an upper time limit for shear zone Thanks are due to Heidi Baier for laboratory assistance. activity at c. 24 Ma. We thank Dov Avigad, Martin Engi and Teddy Parra Greenschist-facies rocks from the BGU, which show for critical comments on an earlier version of this manu- similar compositional complexities in their phengite pop- script. Reviews by Robert Schmid and Jay Barton are ulations as the newly studied samples, yield conformable greatly acknowledged. This study was funded by the ages. Bröcker & Franz (1998) reported phengite ages Deutsche Forschungsgemeinschaft (grant BR 1068/8-1). of 22.4–23.5 Ma for calcschists collected above the m1 calcite marbles in the Vathy area. 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Received: May 28, 2004; accepted: July 21, 2004 Responsible editor: R. Klemd, Würzburg

Author’s addresses: Michael Bröcker, Institut für Mineralogie, Zentrallaboratorium für Geochronologie, Universität Münster, Corrensstr. 24, D-48149 Münster, Germany. E-mail: [email protected] Leander Franz, Institut für Mineralogie, TU Bergakademie Freiberg, Brennhausgasse 14, D-09596 Freiberg, Germany. E-mail: [email protected]