only have been reported from the Panormos area in NW the ...€¦ · gad et al. 1997, Jolivet &...

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

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

Michael Bröcker, Münster and Leander Franz, Freiberg

With 6 figures and 4 tables

Abstract: The Cyclades archipelago in the Aegean Sea is an important study area for subduction-related metamorphism and theexhumation of high-pressure/low-temperature rocks. Of special importance for interpretation of the general tectonic developmentin 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. Thetectonic contact was interpreted as a thrust fault. A re-evaluation of the arguments used to support this interpretation suggests thatthis 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 rockpairs), previously interpreted to belong to the lower plate, yielded dates that are indistinguishable from values obtained forstrongly 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 tothe fossil-bearing basal sequences, indicative for different deformational styles. We postulate that the para-autochthonous basalunit beneath the CBU is not exposed on Tinos Island. Field observations, petrological and geochronological data of the Panormosarea are fully compatible with the interpretation that the dolomite-phyllite-quartzite succession is an integral part of the CBU, asoriginally suggested by Melidonis (1980).

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

Introduction

The general structural, geochronological and P–T evolu-tion of the Cyclades archipelago in the Aegean Sea(Fig. 1) is well documented, but many details of the tec-tonometamorphic history still are not fully understood. Amuch debated issue concerns the importance of syn-oro-genic versus post-orogenic extension for unroofing ofeclogite- to epidote blueschist-facies rocks (e. g. Avigad& Garfunkel 1991, Gautier & Brun 1994 a, b, Avi-gad et al. 1997, Jolivet & Patriat 1999, Avigad et al.2000, Gautier 2000, Trotet et al. 2001a, b, Parra etal. 2002). In this context, tectonic windows that exposethe rock sequences below the Cycladic blueschists are ofspecial importance. The metamorphic P–T conditions ofthese basal unit(s) and the timing of tectonic juxtaposi-

tion represent fundamental parameters for understandingof the exhumation history. Metamorphic sequences be-neath the Cycladic Blueschist Unit (CBU) were reportedfrom Tinos, Evvia, Samos and the Fourni Islands (Fig. 1;Avigad & Garfunkel 1989, Avigad et al. 1997,Shaked et al. 2000, Ring et al. 2001). These occurren-ces are considered to represent para-authochthonousunits, which are separated from the structurally higherCBU by thrust faults. Similar field relations are knownfrom other parts of the Hellenides, e. g. the Olympos-Ossa region on mainland Greece (Fig. 1; Schermer1990, 1993).

In the central Cyclades, a presumed basal unit beneaththe high-pressure/low-temperature (HP/LT) sequencesonly have been reported from the Panormos area in NWTinos (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

Fig. 1. Simplified geographic map of the Aegean region indicatingkey locations discussed in the text. ACCB = Attic-Cycladic Cry-stalline Belt.

undeformed fossils in dolomitic marbles, the absence ofglaucophane and distinct deformational characteristics inphyllitic rocks, as well as the presence of an apparentstructural discordancy, were interpreted as evidence forsignificant metamorphic and microstructural differencescompared to the overlying CBU. The tectonic contact be-tween the CBU and the underlying metamorphics was in-terpreted as a thrust fault (Avigad & Garfunkel 1989).

In this paper, we critically re-evaluate the arguments,which were used to suggest the existence of a basal unit

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

Geological background

The Attic-Cycladic Crystalline Belt (ACCB; Fig. 1) canbe subdivided into two major lithotectonic units, bothconsisting of numerous subunits. The upper group ofunits 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 (whichincludes the CBU) has experienced two stages of met-amorphism: a HP/LT event (Late Cretaceous-Eocene)and a greenschist- to amphibolite-facies overprint (Oligo-cene-Miocene) (e. g. Okrusch & Bröcker 1990, Dürr1986).

On Tinos (Figs. 1, 2), a representative crustal segmentof the ACCB is exposed in at least three structural sub-units (cf. Melidonis 1980, Bröcker & Franz 2000).The highest unit consists of amphibolite-facies rocks (=Akrotiri Unit) which yielded K–Ar ages of c. 67 Ma(Patzak et al. 1994). The greenschist-facies Upper Unitis build up by a disrupted meta-ophiolite sequence (up toabout 250 m in thickness), comprising serpentinites,ophicalcites, meta-gabbros and phyllitic rocks (Katziret al. 1996). The phyllites yielded Rb–Sr phengite-who-le-rock ages between c. 92–21 Ma (Bröcker & Franz1998). 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 occurrenceindicated by a black circle.

The base of the Cycladic blueschist unit 83

timing of tectonic juxtaposition. The Akrotiri Unit andthe Upper Unit show no indications for high-pressuremetamorphism and hence are considered to represent theupper group of units of the ACCB. The tectonic contactat the base of the Upper Unit was interpreted as a low-angle normal fault (Avigad & Garfunkel 1989, Gau-tier & Brun 1994 a, b, Patriat & Jolivet 1998).

Most of the island belongs to the Blueschist-Green-schist Unit (= BGU; in the literature also referred to asLower Unit or Intermediate Unit; Fig. 2), which correla-tes with the lower group of units of the ACCB. The BGUmainly consists of marbles, calcschists, siliciclastic met-asediments, cherts as well as basic and acid metavolca-nic rocks (Melidonis 1980, Bröcker 1990). Melidonis(1980) showed that this succession can be subdividedroughly by three mappable marble sequences (labelledm3, m2 and m1 from top to bottom). The BGU has ex-perienced eclogite- to epidote blueschist-facies metamor-phism (T = 450–500˚C, P >12 kbar) in the Late Cretace-ous to Eocene and a greenschist-facies overprint (T =450–500 ˚C, P = 4–7 kbar) at the Oligocene/Mioceneboundary (e. g. Bröcker et al. 1993, Bröcker & En-ders 1999). Multi-equilibrium P–T estimates, based onthe compositional variability of chlorite and phengite inmetapelites indicated three metamorphic stages duringexhumation (Parra et al. 2002): decompression from18–15 kbar at 500 ˚C to 9 kbar at 400 ˚C was followed atc. 9 kbar by a thermal overprint (400 to 550 ˚C) and fur-ther decompression from 9 kbar at 550–570 ˚C to 2 kbarat 420 ˚C.

In the eastern part of the island (Fig. 2) the Upper Unitand the BGU were affected by contact metamorphism(Avigad & Garfunkel 1989, 1991, Stolz et al. 1997,Bröcker & Franz 1994, 2000), caused by Miocene gra-nitoids which were dated at 17–14 Ma (Altherr et al.1982, Bröcker & Franz 1998).

The dolomite-phyllite sequence in NW Tinos

Previous work

The lowermost parts of the metamorphic succession onTinos are exposed in the NW part of the island aroundthe village of Panormos (Fig. 2; Melidonis 1980, Avi-gad & Garfunkel 1989, Matthews et al. 1999). Inthis area, the rock pile is dominated by calcite-rich marb-les which are underlain by a dolomite sequence. Origi-nally, this succession (= m1 marbles) was completely as-signed to the BGU (Melidonis 1980). This interpreta-tion was questioned by Avigad & Garfunkel (1989)who recognized a tectonic contact within the marbles.

Above the fault zone, calcite-rich marbles (c. 50 m thick)are intercalated with thin bands of quartzites. Theserocks are considered to belong to the BGU. Below thecalcite marbles, a discontinuous horizon of phyllites andquartzites (< 2 m thick) was recognized. The phyllite–quartzite layer is considered to belong to a lower platewhich mainly consists of dolomites to dolomite-richmarbles (> 100 m thick; Avigad & Garfunkel 1989,Matthews et al. 1999). Findings of undeformed fossilsin the basal dolomites, the absence of glaucophane anddistinct deformational characteristics in phyllitic rocks,and the inferred presence of a structural discordance,were interpreted as evidence for significant metamorphicand microstructural differences compared to the overly-ing rocks (Avigad & Garfunkel 1989). These authorssuggested that the lowermost sequence represents a dis-tinct tectonic unit, which was only affected by low-gradegreenschist-facies conditions. According to their inter-pretation, the fault zone is a synorogenic thrust, becausehigh-pressure rocks were juxtaposed onto a lower gradeseries (Avigad & Garfunkel 1989, 1991, Avigad et al.1997).

Matthews et al. (1999) showed that the fault zoneand adjacent rock volumes are characterised by carbonand oxygen isotope depletions, which were explained byfocused fluid-infiltration of externally derived fluidsalong the tectonic contact. Temperature estimates basedon dolomite-calcite solvus thermometry indicated c.350–420˚C for upper and lower plate marbles at the timeof shearing (Matthews et al. 1999). Owing to an appar-ent temperature increase in the dolomites towards thecontact from c. 300 to 370 ˚C, these authors assumed thatthe lower plate experienced heating during thrusting andsuggested that a temperature of c. 300 ˚C possibly re-cords the original low-grade conditions.

Field observations and sample description

The Panormos exposure is divided into a northern andsouthern part by a roughly west-east-trending valley filledwith alluvium (Fig. 2). In the northern segment, the basalsection consists of dolomite, which is overlain by a dis-continuous horizon of graphite-rich phyllite and quartzite(up to 2 meter thick), or a discontinuous calcschist layer,followed by a calcite marble (several meters in thickness)with quartzite intercalations (mostly < 5 cm; rarely up to20 cm, Figs. 3 a, b). In the southern segment (Fig. 4),graphite-rich phyllite is rare and the dolomite is overlainby a white quartzitic layer. On top of this horizon a cal-cite marble with thin quartzite layers occurs. Up section,this marble is overlain by a calcschist, a semipelitic


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

schist and a calcite marble. Thickness and abundance ofquartzite intercalations are much smaller than in thenorthern part. On both sides of the valley, quartz veinsand pods (up to several meter in thickness) are common.In the calcite marbles, bleached alteration halos aroundveins of all sizes were recognized (Fig.3 c).

Calcite marbles from the BGU were collected fromthe m1 sequence (Melidonis 1980) around PanormosBay and Vathy (Fig. 2). All samples were taken from out-crops with quartzite intercalations. There is general con-sensus that these marbles belong to the BGU and thuswere affected by HP metamorphism and a greenschist-facies overprint. The marbles are fine-to medium-grained, well-bedded rocks with white to bluish-greycolour on the outcrop- or hand-specimen scale. Normallygraded bedding and changes in grain-size between indi-vidual layers (Fig. 3 d) locally were recognized. Wide-spread is a pebbly mudstone fabric with clasts and mat-rix solely consisting of carbonates (Figs. 3 e, f). Theclasts range in size from a few millimetres to severalcentimetres. The mineral assemblage mostly consists of

calcite; dolomite is present in some samples, but is al-ways much less abundant. Among the accessories, phen-gite is most common, but albite, quartz, chlorite andgraphite may occur in the groundmass as additional con-stituents. Phengite often is strongly enriched on beddingsurfaces. No glaucophane, garnet or omphacite were rec-ognized, but relics of HP rocks locally are preserved di-rectly above the calcite marbles (Bieling & Bröcker,unpubl. data).

The basal dolomites only are exposed around Panor-mos. These massive, fine-grained rocks show no beddingand contain fewer and smaller phengite grains than thecalcite marbles. An exception is the highly strained sam-ple 1422, collected close to the phyllite horizon, with upto c. 3 vol.% of phengite. At some places, a breccia fab-ric of unclear origin was recognized with weak contrastbetween clasts and matrix. Sporadically, Upper Triassicfossils are preserved (Melidonis 1980).

The mineral assemblage of well-foliated phyllite andthe quartzite mainly consists of phengite, chlorite, albite,quartz, epidote and titanite.


Fig. 4. Schematic columnar section (not to scale) of the southernPanormos area.

Analytical methods

Phengite compositions were determined with a CAME-CA SX-50 and a JEOL JXA-8900 R electron microprobeat the Mineralogisches Institut, Universität Würzburg andat the Institut für Mineralogie, TU Freiberg. Operatingconditions 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 rawdata 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 orusing a jaw-breaker and disc mill. Whole rock powderswere prepared in a tungsten carbide mill. For mica sepa-ration, crushed material was reduced in size either bygrinding for only a few seconds in a tungsten carbidemill or by use of a disk mill. Following sieving fineswere removed and mica was enriched by use of a Frantzmagnetic 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 1224Spot 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 55.45TiO2 0.15 0.12 0.26 0.15 0.30 0.21 0.31 0.19 0.09 0.21 0.21 0.28Al2O3 22.43 22.10 22.70 23.11 22.88 22.61 22.64 22.17 21.14 20.32 20.16 20.85Cr2O3 0.15 0.02 0.06 0.08 0.09 0.09 0.07 0.06 0.18 0.11 0.06 0.12MgO 6.25 6.51 6.33 6.32 6.21 6.35 6.56 6.90 7.23 7.36 7.47 7.42CaO 0.24 0.11 0.14 0.07 0.08 0.10 0.22 0.24 0.21 0.22 0.17 0.13MnO 0.03 0.03 0.01 0.02 0.02 0.00 0.02 0.00 0.03 0.01 0.00 0.00FeO 0.01 0.07 0.02 0.00 0.07 0.02 0.06 0.02 0.01 0.02 0.00 0.00BaO 0.04 0.00 0.00 0.00 0.00 0.00 0.11 0.00 0.03 0.26 0.26 0.25Na2O 0.08 0.07 0.15 0.12 0.11 0.06 0.25 0.10 0.10 0.09 0.07 0.08K2O 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 3.665Ti 0.005 0.005 0.015 0.010 0.015 0.010 0.015 0.010 0.005 0.010 0.010 0.015Al 1.750 1.733 1.779 1.803 1.792 1.757 1.762 1.708 1.640 1.588 1.575 1.624Cr 0.008 0.001 0.003 0.004 0.005 0.005 0.004 0.003 0.010 0.006 0.003 0.006Mg 0.617 0.646 0.628 0.624 0.615 0.624 0.645 0.673 0.710 0.727 0.739 0.731Ca 0.017 0.008 0.010 0.005 0.006 0.007 0.015 0.017 0.015 0.016 0.012 0.009Mn 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000Fe 0.000 0.005 0.000 0.000 0.005 0.000 0.005 0.000 0.000 0.000 0.000 0.000Ba 0.000 0.000 0.000 0.000 0.000 0.000 0.005 0.000 0.000 0.005 0.005 0.005Na 0.010 0.010 0.020 0.015 0.015 0.005 0.030 0.010 0.015 0.010 0.010 0.010K 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


Table 2. Representative electron microprobe analyses of high-Si phengites from phyllites.

Sample 1221 1221 1221 1222 1222 1222 1418 1418 1418 1421 1421 1421Spot 26 10 11 13 1 7 1–2 1–7 4–5 2–1 1–5 2–5


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


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 2013Spot B1 D1B A1B A4 B3 D1A C1 C3 C4 G3 H2 H3


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


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


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 ± 0.2calcite 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 ± 0.3calcite 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 ± 0.2calcite 355–250 0.331 332 0.00289 0.707612 0.000010calcite 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 ± 0.2calcite 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 ± 0.9calcite 355–250 0.371 271 0.00396 0.707560 0.000010calcite 250–180 0.194 277 0.00203 0.707587 0.000010


3521 marble phengite 250–180 290 1.73 494 0.876071 0.000062 24.0 ± 0.2calcite 355–250 0.193 286 0.00195 0.707670 0.000010


1221 phyllite phengite 250–180 245 39.8 17.8 0.716073 0.000015 23.3 ± 0.4phengite 180–125 285 24.7 33.4 0.721286 0.000015whole 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 ± 0.3whole 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 ± 0.2whole rock 1.52 0.710175 0.000015

1421 phyllite phengite 250–180 343 15.8 62.8 0.731496 0.000032 24.1 ± 0.2whole 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 ± 0.2calcite + 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> 99 %) were washed in ethanol (p. a.) in an ultrasonicbath and repeatedly rinsed in H2O (three times distilled).

Whole-rock powders (about 100 mg) and phengites (c.4–28 mg) were mixed with a 87Rb-84Sr spike in teflonscrew-top vials and dissolved in a HF–HNO3 (5 :1) mix-ture on a hot plate overnight. After drying, 6 N HCl wasadded to the residue. This mixture was homogenized ona hot plate overnight. After a second evaporation to dry-ness, Rb and Sr were separated by standard ion-ex-change procedures (AG 50 W-X8 resin) on quartz glasscolumns using 2.5 N HCl as eluent. Calcite (c. 9–62 mg)was dissolved in 2.5 N HCl. Rb was loaded with H2O onTa filaments; Sr was loaded with TaF5 on W filaments.Mass-spectrometric analysis was carried out using a VGSector 54 multicollector mass spectrometer (Sr) and aNBS-type Teledyne mass spectrometer (Rb). Correction

for mass fractionation is based on a 86Sr/86Sr ratio of0.1194. Rb ratios were corrected for mass fractionationusing a factor deduced from multiple measurements ofRb standard NBS 607. Total procedural blanks were lessthan 0.1ng (mostly < 0.05 ng) for Rb and 0.24 ng (mostly< 0.1 ng) for Sr. Based on repeated measurements, the87Rb/86Sr ratios were assigned an uncertainty of 1 %(2σ). Uncertainties of the 87Sr/86Sr ratios are reported atthe 2 σm are reported at the 2σm level. In the course ofthis study, repeated runs of NBS standard 987 gave anaverage 87Sr/86Sr ratio of 0.710308 ± 0.000042 (2σ, n =20). All ages and elemental concentrations were calcu-lated using the IUGS recommended decay constants(Steiger & Jäger 1977) by means of the Isoplot pro-gram version 2.49 (Ludwig 1991). Rb–Sr isotope resultsare shown in Table 4.


Phengite composition

In order to characterise white mica used for geochro-nology (homogeneous or mixed populations?) and toconstrain pressure-dependent differences in Si-contents,14 samples were studied with the electron microprobe (8dolomites, 3 phyllites, 1 quartzite and 5 calcite marbles).In the calcite marbles, white mica is more frequent andof larger grain-size than in the dolomites. Phengite oftenis strongly enriched on bedding surfaces, but is alsofound 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 andbelow the fault zone (Fig. 5; Tables 1, 2, 3); the range inSi-content is similar (calcite marble: 3.36–3.76; phylli-te/quartzite: 3.29–3.71; sheared dolomite: 3.23–3.64;dolomite: 3.36–3.55). Most phengite plots along the Al-celadonite-muscovite join, illustrating the importance ofthe Tschermak’s substitution [Si, (Mg, Fe2+) = AlVI, AlIV]with maximum Si contents of > 3.5 p. f. u. Noteworthyare extremely high Si-contents in some calcite marbles(up to 3.75 p. f. u.), which cannot be explained by beamoverlap on SiO2-rich phases, because the studied micasoccur as isolated grains in a carbonate matrix. Within theshear zone, both phyllites and a ductily deformed dolo-

Fig. 6. Schematic columnar section (not to scale) of the Panormos-Vathy area summarizing geochronological results of the study area(this study; Bröcker & Franz 1998).

mite (sample 1422) show a higher degree of recrystalli-zation, as indicated by increased modal proportions ofphengite with Si values < 3.3 p. f. u. In the dolomites, tex-turally different micas cannot be distinguished by Si val-ues; their variations in Tschermak’s substitution show noconsistent pattern. Both, in calcite marble and dolomite,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 (199spot analyses from 4 samples); c. sheared dolomite (39 spot analyses from 1 sample); d. massive dolomites (164 spot analyses from 7samples).


Paragonite substitution (Na/(Na + K) in phengite is < 0.1.In carbonate rocks phengite has very low FeO concentra-tions (< 0.1 wt.%), resulting in XMg values > 0.99. MgO-concentration is variable and ranges between 2.5 and 8wt.%, with average contents around 4.8 wt.% (dolo-mites) and 6.8 wt.% (calcite marbles). In phyllite andquartzite, FeO-content of phengite is higher, varyingmostly from 0.7 to 2.2 wt.%. Only in rare cases, valuesup to 4 wt.% were recognized. MgO-concentration va-ries between 2.6 and 6.3 wt.% with average values be-tween 4.3–5.1 wt.% (XMg = 0.84–0.89).

Rb–Sr geochronology

In order to characterise the age of metamorphism and/ordeformation in the basal dolomite-phyllite sequence, weanalysed five samples from the phyllite/quartzite hori-zon, collected at three different outcrops. In addition,ages were determined for five calcite marbles and threecalcschists. Attempts to separate enough phengite fordating of dolomites were unsuccessful, due to a combi-nation of small grain-size, low modal abundance andsmall samples. The isotopic data are summarized in Ta-ble 4.

Phyllites and quartzite (whole rock, phengite) yieldedages that are indistinguishable from results obtained forseverely overprinted greenschist-facies rocks from theBGU, collected at higher lithostratigraphic levels. Thestudied samples yield Rb–Sr ages of 23.3–24.1 Ma (Ta-ble 4; weighted average: 24.0 ± 0.3 Ma; n = 5). Previousgeochronological work on a calcschist considered to be-long to the basal sequence provided a Rb–Sr age (phen-gite, whole rock) of 21.7 ± 0.2 Ma (Bröcker & Franz1998). An additional sample collected from this occur-rence is now dated at 21.5 ± 0.2 Ma. Calcite marblesfrom the Panormos area (n = 5) provide ages between22.9 and 25.0 Ma (Table 4; weighted average: 24.0 ± 0.7Ma). Two calcschists from the southern part of the Pa-normos section, collected above the tectonic contact,yield ages of 22.0 ± 0.2 Ma and 21.9 ± 0.2 Ma, respec-tively. Calcite marbles from Vathy yielded a pooled ageof 24.2 ± 0.8 Ma (n = 3; Bröcker et al. 2004).

Discussion

There is general consensus that in NW-Tinos a tectoniccontact separates calcite-rich marbles from a lower dolo-mite sequence that includes a thin phyllite-quartzite hori-zon. However, the conclusion that this fault zone separa-tes sequences with different tectonometamorphic histo-

ries is problematic. In the following, this interpretation isre-evaluated based on field observations, phengite chem-istry and geochronological data.

Indications for a different deformation history?

According to Avigad & Garfunkel (1989), the preser-vation of undeformed fossils in the basal dolomites is amajor contrast between the lowermost carbonate rocksand the calcite marbles of the BGU, suggesting differentdeformation histories on both sides of the tectonic con-tact. This argument was strengthened by noting that theBGU marbles on Tinos generally are lacking sedimen-tary features. Our field observations are at variance tothis conclusion. In the Panormos area, the calcite marb-les commonly show a pebbly mudstone fabric (Figs. 3 e,f). This structure is interpreted here as a breccia of debrisflow origin derived from reworked carbonate hard-grounds. A similar origin was suggested for blackspotted marbles, which occur at higher lithostratigraphiclevels (see plate 7, page 105 in Bröcker 1990). Further-more, in the upper section of this unit a meta-conglome-rate or meta-debris flow is locally preserved with suban-gular to rounded clasts (< 20 cm), composed of marblesand minor metabasic rocks (Bröcker 1990, Bröcker etal. 2004). At many places, pebbles are totally shearedand flattened, clearly documenting heterogeneous strainwithin the same horizon. The sporadic preservation ofsedimentary structures, as well as the occurrence of un-deformed fossils at the base of the metamorphic succes-sion, are compatible with a model that suggests non-per-vasive strain distribution within a continuous metamor-phic sequence.

Avigad & Garfunkel (1989) also argued that thesyn-kinematic crystallization of the phyllites contrastswith static growth of greenschist-facies minerals in theoverlying sequence. However, the phyllites outline a rel-atively narrow tectonic zone. It is unlikely that the origi-nal deformational fabric still is preserved in this faultcontact. In addition, there is considerable dispute con-cerning the amount of deformation during greenschist-facies overprinting of the BGU. Avigad & Garfunkel(1989) and Bröcker (1990) concluded that evidence forextensive penetrative deformation during retrogression islargely absent in most parts of Tinos. This interpretationwas questioned by other groups working on this island,who established arguments for ductile extensional defor-mation during the overprint (e. g. Gautier & Brun1994a, b, Jolivet & Patriat 1999, Parra et al. 2002).

According to Avigad & Garfunkel (1989), fieldmapping suggests the presence of a structural discor-


dancy in the Panormos area. However, as elsewhere ob-served on Tinos, thickness of individual lithologicallayers is highly variable along strike. Thus, thinning orwedging out of distinct horizons is not necessarily an in-dication for discordant field relationships. But even if astructural discordancy can unequivocally be documented,such a feature still can be related to a fault within theBGU.

Differences in metamorphic grade?

Avigad & Garfunkel (1989) interpreted the absence ofglaucophane in the phyllite-quartzite horizon as indica-tion that blueschist-facies conditions were not attained inthe basal sequences. Judging from mineral assemblagesmainly consisting of white mica, chlorite, quartz and al-bite, low-grade metamorphism in the greenschist-facieswas assumed. However, the absence of glaucophane inrocks of suitable bulk-rock composition is of doubtfulsignificance for establishing differences in metamorphicgrade. Glaucophane is also not found in many clasticmetasediments of the BGU, due to pervasive green-schist-facies overprinting (e. g. Bröcker 1990, Parra etal. 2002).

The silica content in phengite is variable and experi-mental work indicates that with increasing pressure theSi-content increases in rocks containing the limiting as-semblage (e. g. Velde 1967, Massonne & Schreyer1987, Massonne 1991). Although not fully correct fornon-buffered mineral assemblages, high-Si content inphengite (Si ≥ 3.5) is often considered as indication forcrystallization under HP conditions, indicating at least aminimum pressure. The studied clastic metasedimentsand marbles do not contain the limiting assemblage(phengite, K-feldspar, biotite, quartz, H2O). Therefore,Si-in-phengite is not a reliable indicator for absolutemetamorphic pressures. A detailed thermobarometricevaluation of the metamorphic conditions in the basal se-quences is beyond the scope of this paper, but a qualita-tive estimate of metamorphic pressure can be deducedfrom phengite composition alone. In the context dis-cussed here the emphasis is on the question whether dif-ferences in Si-content can be recognized in samples col-lected above and below the Panormos shear zone. It isworth to emphasize that clastic metasediments collectedabove and below the tectonic contact show no systematicdifference in phengite compositions. The range of Si-content in phengite of the basal phyllites/quartzites issimilar to values reported from BGU rocks (e. g.Bröcker et al. 1993, Bröcker & Franz 1998, Parra etal. 2002). Although not providing unequivocal evidence,this observation is in accord with the conclusion that the

metamorphic history is not significantly different to theP–T path recognized elsewhere in the BGU.

For calcite marbles and dolomites, it is realistic to sug-gest that Si-content of phengite is strongly affected bybulk-compositional constraints, because white mica isamong the accessories the most common or single sili-cate phase, and thus most likely will accommodate mostof the available Si (Bröcker et al. 2004). The trend to-wards higher Si-values in phengite of calcite marbles(Fig. 5), if compared to phengite in dolomite, is inter-preted as an artefact of such bulk-compositional differ-ences. The calcite marbles typically are intercalated withquartzite layers on a cm- to dm scale (Figs. 3 a, b), sug-gesting increased availability of Si. However, it is inter-esting to record that the dolomites also contain high Si-phengite (Table 3; Fig. 5). The conclusion that, for cal-cite marble and dolomites, the Si-content of phengite isof doubtful barometric significance is further corrobo-rated by the fact that Rb–Sr phengite dating of calcitemarbles yielded ages of c. 24 Ma (this study; Bröckeret al. 2004). Previous geochronology in the Cyclades hasshown that phengite ages (Rb–Sr, K–Ar, 40Ar-39Ar) ofc. 25–18 Ma are related to greenschist-facies overprint-ing and that ages around 53–40 Ma are related to blue-schist-facies metamorphism (e. g. Altherr et al. 1979,1982, Wijbrans & McDougall 1986, 1988, Wijbranset al. 1990, Bröcker et al. 1993, Bröcker & Franz1998). The fact that high-Si phengite of the calcite marb-les does not record the age of the HP stage, but the timeof the subsequent overprint is related to recrystallizationat lower pressure, which did not cause a significant mod-ification of Si-content (for a detailed discussion seeBröcker et al. 2004).

It is possible that the basal rocks were affected bylower grade blueschist-facies conditions than the overly-ing sequences. Field relations of this kind were reportedfrom Evvia (Fig. 1) where high-pressure lithologies ofthe Cycladic blueschist belt were thrusted onto the Al-myropotamos unit (= basal unit; Shaked et al. 2000,Katzir et al. 2000). Findings of glaucophane clearly in-dicate that blueschist-facies conditions were also attainedin the structurally lower unit and the presence of two dis-tinct high-pressure units, which have experienced differ-ent P–T conditions, was suggested (Shaked et al. 2000,Katzir et al. 2000). The assumed pressure difference(lower unit: c. 10 kbar; upper unit: c. 12 kbar; Shaked etal. 2000) is currently not well-documented. Available es-timates are based on Si-content of phengite not coexist-ing with the limiting assemblage. Gautier (2000) sug-gested that the apparent pressure difference of c. 2 kbarmight be an artefact of sampling at widely separated lo-cations, due to slightly dipping sequences.


Timing of metamorphism and shear zone activity

Phengite ages currently are not available for the dolo-mites. Future dating of these rocks will complement ex-isting datasets, but most likely will not provide the keyto unravel the structural position of the lowermost Panor-mos section, for the following reasons. (1) Phengite agesof the basal units on Samos and Evvia are similar tothose commonly found for overprinted rocks of the CBU(24–20 Ma; Ring et al. 2001, Ring & Reischmann2002). (2) Due to an unclear relationship between petro-logical and geochronological information, it is contro-versially discussed whether these dates indicate the timeof HP metamorphism or the age of greenschist-faciesoverprinting (Bröcker et al. 2004). We expect that thePanormos dolomites will provide similar ages, which cannot be interpreted unambiguously.

The phyllite–quartzite layer outlines a shear zone be-tween different carbonate rocks. Direct dating of shearzones is possible if the studied mineral phase formed orrecrystallized during deformation, at or below the clo-sure temperature for the specific mineral and isotope sys-tem. For the studied samples, this prerequisite is ful-filled. Peak-temperatures during the HP stage and thesubsequent greenschist overprint are considered to be≤ 500 ˚C (Avigad & Garfunkel 1989, Bröcker et al.1993), close to or below the closure temperature for theRb–Sr system of phengite, which is commonly consid-ered to be around 500 ˚C. Thus, phengite ages of the Pa-normos area do not represent cooling ages, but provideconstraints for timing of mica formation/recrystalliza-tion. Correct interpretation of these dates is severelyhampered by the presence of mixed mica populations.As indicated by variable Si-contents, the phengite ali-quots used for geochronology are heterogeneous andconsist of grains representing different generations. Inthe phyllites, the shearing did not cause a complete rec-rystallization of white mica. Multigrain dating of suchpopulations can not provide a precise age for deforma-tion, but only yields an upper time limit for shear zoneactivity at c. 24 Ma.

Greenschist-facies rocks from the BGU, which showsimilar compositional complexities in their phengite pop-ulations as the newly studied samples, yield conformableages. Bröcker & Franz (1998) reported phengite agesof 22.4–23.5 Ma for calcschists collected above the m1calcite marbles in the Vathy area. Meta-acidic rocks fromvarious locations within the BGU, which were datedwith both the Rb–Sr (whole rock, phengite, epidote) and40Ar-39Ar (phengite) methods, yielded almost concordantages clustering between 23–21 Ma (Bröcker & Franz1998). A distinct age difference is not observed across

the tectonic contact. The geochronological data is in ac-cord with the interpretation that the main activity in thePanormos shear zone broadly coincides with the green-schist-facies overprint affecting the BGU. Due to prob-lems related to incomplete resetting and mixing of differ-ent mica generations, the exact age of this retrogressionis unknown and can not be detected by multigrain dating.

Conclusions

The results of this study indicate that a correlation of thebasal sequences on Tinos with para-autochthonous unitslike the Almyropotamos and Olympos-Ossa units on Ev-via and mainland Greece is problematic. We do notquestion the general importance of thrusting at the baseof the CBU, however, Tinos is not a place to corroboratethis concept. Field observations, phengite chemistry andgeochronological data of the Panormos area are compati-ble with the interpretation that the basal sequences are anintegral part of the BGU, as originally suggested by Me-lidonis (1980). The Panormos fault is considered hereas a tectonic contact within the BGU, with unknownamount of displacement, related to post-orogenic exten-sion (cf. Gautier & Brun 1994 a, b).

The metamorphic P–T path of rocks occurring abovethe fault zone is well-constrained by chlorite-phengitelocal equilibria data (Vidal & Parra 2000, Parra et al.2002), but similar information is not at hand for the dol-omite-phyllite sequence below the tectonic contact. Thestatus of these rocks within the structural framework ofthe Aegean region can only unambiguously be unra-velled, if quantitative P–T estimates become availablefor the lowermost Panormos sequence and/or if geochro-nological studies will present evidence that the age ofHP metamorphism is different to the overlying rocks.

Acknowledgements

Thanks are due to Heidi Baier for laboratory assistance.We thank Dov Avigad, Martin Engi and Teddy Parrafor critical comments on an earlier version of this manu-script. Reviews by Robert Schmid and Jay Barton aregreatly acknowledged. This study was funded by theDeutsche Forschungsgemeinschaft (grant BR 1068/8-1).

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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-48149Münster, Germany. E-mail: [email protected]

Leander Franz, Institut für Mineralogie, TU Bergakademie Freiberg, Brennhausgasse 14, D-09596 Freiberg, Germany.E-mail: [email protected]
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