Tinos Island Research Articles

Evolution of hydrothermal regime along a crustal shear zone, Tinos Island, Greece

Structural cathodoluminescence and SIMS δ18O analyses of quartz‐calcite veins are combined to constrain the evolution in space and time of fluid infiltration in an exhuming detachment shear zone (Tinos Island, Cyclades, Greece). Careful description of vein arrays shows that the plumbing system evolved into an interconnected network just beneath the ductile‐to‐brittle transition. Microtextures of quartz and calcite infilling veins display deformed relics and newly precipitated grains. High‐resolution δ18O mapping in relics yields a steady quartz‐calcite fractionation of 2 ‰ at ∼400°C, whereas fractionation increases in neograins from 2 to 5 ‰ in 190 m toward the fault, or a ∼150°C temperature fall. Cooling is associated with a shift of fluid stable isotope signature from 15 ‰ to 0 ‰ beneath the detachment. Results are interpreted as advective removal of heat by massive infiltration of surface‐derived fluids over depths of 10–15 km. Fluid penetration was promoted by the coalescence of late ductile‐early brittle veins, as the exhuming footwall crossed the ductile‐brittle transition. Only small amounts of fluids penetrated the ductile crust beneath the transitional rheology.

P-T conditions of quartz-calcite boudins and vein formation within a low-angle detachment fault in Tinos Island (Aegean Sea): a fluid-inclusion study

Fluid inclusions were investigated in successive generations of quartz-calcite boudins and veins in the vicinity of, and within, the low-angle detachment fault (of Early Miocene age) exposed on Tinos Island (Cyclades, Greece). Abundant boudins, veins, tension gashes and mineral segregations were formed during various stages of ductile and brittle shear along the detachment indicating fluid flow at various crustal levels, assisted motion and slip along the detachment. Three generations of fluid inclusions were identified: (1) syn-deformation aqueous inclusions; (2) local- late-deformation H2O-CO2 inclusions; (3) post-slip deformation, partly decrepitated aqueous and H2O-CO2 inclusions. The conditions of inclusion trapping correspond to the greenschist facies, at temperatures between 300 and 450°C and pressures in the range of 1.5–4.0 kb. A gradient of pressure of about 0.5 kb from the fault zone to the rocks of upper and lower plates is suggested. The results indicate that the syn-deformation fluid was NaCl-KCl-sulfate-dominated with a salinity of about 5 wt% NaCl equiv. Downward infiltration of meteoric water is a suitable source for this fluid. During the late stage of fault activity, in the brittle field, fluid was of H2O-CO2 composition. Very rapid exhumation of the entire section (unrelated to motion on the investigated fault) is marked by numerous decrepitation clusters of the fluid inclusions.

High‐Si phengite records the time of greenschist facies overprinting: implications for models suggesting mega‐detachments in the Aegean Sea

AbstractIn the lower main unit of the Attic‐Cycladic crystalline belt (Greece), white mica geochronology (Rb–Sr, K–Ar, 40Ar–39Ar) has established the timing of at least two metamorphic events: well‐preserved high‐pressure/low‐temperature (HP/LT) rocks yielded Eocene ages (c. 53–40 Ma) and their greenschist facies counterparts provided Oligocene–Miocene dates (c. 25–18 Ma). Marbles from Tinos Island contain high‐Si phengite with Rb–Sr (phengite–calcite) and 40Ar–39Ar white mica ages between 41 and 24 Ma. All Ar age spectra are disturbed and 40Ar–39Ar total fusion ages generally are 3–6 Ma older than corresponding Rb–Sr ages. Due to the polymetamorphic history, we consider inheritance from the HP stage as the most likely cause for the complex Ar age spectra and the older 40Ar–39Ar dates. This concept also suggests that the Rb–Sr system is more sensitive to modification during overprinting than the Ar isotope system, because resetting of the Sr isotope system can be accomplished more quickly by Sr exchange with other Ca‐rich phases, whereas lack of pervasive deformation and/or restricted availability of synmetamorphic fluids has favoured partial inheritance of the Ar isotope system. On Tinos, the lowermost part of the metamorphic succession has experienced a pervasive greenschist facies overprint. Si‐rich phengite from marbles representing this lithostratigraphic level yielded Rb–Sr ages of c. 24 Ma. If the earlier metamorphic history is not taken into account, such data sets may lead to the erroneous conclusion of Miocene HP metamorphism. This study indicates that this phengite experienced pervasive rejuvenation of the Rb–Sr isotope system during overprinting, without significant changes in Si content, due to bulk‐compositional constraints. This leads to the conclusion that in the absence of critical mineral assemblages the Si value of phengite is not a reliable indicator for metamorphic pressures in impure marbles. Recent studies have reported large displacements (>100 km) for detachment faults in the Aegean Sea. A critical parameter for such models is the age of HP metamorphism as deduced from white mica dating in the basal units of the Cyclades. We question the underlying idea of Miocene HP metamorphism and suggest, instead, that this age constrains the timing of the greenschist facies overprint and that the existence of mega‐detachments in the study area requires further investigation.

Description and conditions of formation of new unnamed Ag-Cu and Ag-Cu-Au sulfotellurides in epithermal polymetallic Ag-Au-Te mineralization, Tinos Island, Hellas

Two new unnamed Ag-Cu ((Ag,Cu)12Te3S2) and Ag-A u-Cu ((Ag,Au, Cu)9Te2S3) or ((Ag,Cu)2|Au2Te§59) sulfotellurides were identified in low sulfidation epithermal Au-Ag-Te mineralization at Panormos Bay, Tinos Island, Greece. The sul­ fotellurides occur in stage V of nine paragenetic stages as minute, up to 13 pm in length, moderately reflective, white-gray isometric grains, with yellowish to brownish tint, depending on the gold and copper contents. The Ag-Cu sulfotelluride coexists with hessite and sylvanite in early stage V, whereas the Ag-Au-Cu sulfotelluride is in- tergrown with petzite and kostovite in late stage V. Both sulfotellurides also occur along the cleavages of hessite and petzite and replace stage II tetrahedrite. Based on silicate, sulfide, and telluride stabilities and fluid inclusion data, the con­ ditions of formation in stage V and the sulfotellurides formed between 230° to 260 °C at fS2 = 10~9 to 1095, f0 2 = 1035·86, fTe2 = 10-·92 to 10'938, pH = 5.74, aHTe (aq) = 2C)-io.88 and aH2Te(aqJ = 10 l20a, approximately. The gold and silver ion content in stage V solutions was 0.24 and 0.079 ppb, respectively.

Description and conditions of formation of new unnamed Ag–Cu and Ag–Cu–Au sulfotellurides in epithermal polymetallic Ag–Au–Te mineralization, Tinos Island, Hellas

Description and conditions of formation of new unnamed Ag–Cu and Ag–Cu–Au sulfotellurides in epithermal polymetallic Ag–Au–Te mineralization, Tinos Island, Hellas

Mobility of metamorphic fluids inferred from infrared microspectroscopy on natural fluid inclusions: the example of Tinos Island, Greece

OH structure of metamorphic fluids has been studied by high temperature infrared (IR) microspectroscopy on natural fluid inclusions contained in quartz veins, over the temperature range 25–370 °C. Blueschist-facies veins from Tinos island core complex (Cyclades, Greece) display H2O–NaCl–CaCl2–CO2 inclusions whereas greenschist-facies veins contain H2O–NaCl ± CO2 inclusions. From 25 to 370 °C, peak positions of OH stretching IR absorption bands increase quasi-linearly with slopes of 0.25 and 0.50 cm−1 °C−1 for inclusions trapped under blueschist and greenschist conditions, respectively. Extrapolation to 400 °C yield peak positions of 3,475 cm−1 for blueschist inclusions and 3,585 cm−1 for greenschist inclusions. Because the smaller wave number indicates the shorter hydrogen-bond distance between water molecules, fluids involved in the greenschist event have a ‘loose’ structure compared with blueschist fluids. We suggest that these properties might correspond to a low wetting angle of fluids. This would explain the high mobility of aqueous fluids suggested by structural observation and stable isotope analysis.

PETROGRAPHIC AND ISOTOPIC CHARACTERIZATION OF THE MARBLE OF THE ISLAND OF TINOS (GREECE)*

Most outcrops of good‐quality crystalline marble in Mediterranean countries were exploited in Greek and Roman times. Hence, a wide variety of marble is now found in archaeological excavations. The precise determination of the provenance of a marble object is of great archaeological importance, and is now mostly obtained by coupling the petrographic study of a thin section with the analysis of the C and O stable isotopes, or by stereomicroscopy and EPR. The existing databases are considerable but still insufficient, because our knowledge about and study of ancient Mediterranean quarries remain incomplete. The contribution of this research is to add new petrographic and isotopic data on small quarries on the Greek island of Tinos that were exploited in antiquity. This marble belongs to the same geological horizon; it is quite pure, sometimes dolomitic and characterized by a low‐T, high‐P metamorphism that produced a limited recrystallization (MGS varying from 0.64 to 2.50 mm) on marine limestone protoliths. The petrographic features are quite distinctive: the fabric is strongly lineated and often stressed; and the accessory minerals are ubiquitary quartz, ore minerals, graphite and muscovite. As far as the isotopic data are concerned, δ13CPDB varies from 1.1 to 2.7, and δ18OPDB from −1.7 to −11.4. Both the features and the data have been compared with those of similar marbles used in antiquity, showing that their combination mirrors the fingerprint of the Tinos marble.

Fast extension but little exhumation: the Vari detachment in the Cyclades, Greece

Markedly different cooling histories for the hanging- and footwall of the Vari detachment on Syros and Tinos islands, Greece, are revealed by zircon and apatite fission-track data. The Vari/Akrotiri unit in the hangingwall cooled slowly at rates of 5–15 °C Myr−1 since Late Cretaceous times. Samples from the Cycladic blueschist unit in the footwall of the detachment on Tinos Island have a mean zircon fission-track age of 10.0±1.0 Ma, which together with a published mean apatite fission-track age of 9.4±0.5 Ma indicates rapid cooling at rates of at least ∼60 °C Myr−1. We derive a minimum slip rate of ∼6.5 km Myr−1 and a displacement of <∼20 km and propose that the development of the detachment in the thermally softened magmatic arc aided fast displacement. Intra-arc extension accomplished the final ∼6–9 km of exhumation of the Cycladic blueschists from ∼60 km depth. The fast-slipping intra-arc detachments did not cause much exhumation, but were important for regional-scale extension and the formation of the Aegean Sea.

Blueschist preservation in a retrograded, high‐pressure, low‐temperature metamorphic terrane, Tinos, Greece: Implications for fluid flow paths in subduction zones

The preservation of high‐pressure, low‐temperature (HP‐LT) mineral assemblages adjacent to marble unit contacts on the Cycladic island of Tinos in Greece was investigated using a new type of digital outcrop mapping and numerical modeling of metamorphic fluid infiltration. Mineral assemblage distributions in a large blueschist outcrop, adjacent to the basal contact of a 150‐meter thick marble horizon, were mapped at centimeter‐scale resolution onto digital photographs using a belt‐worn computer and graphics editing software. Digital mapping reveals that while most HP‐LT rocks in the outcrop were pervasively retrograded to greenschist facies, the marble‐blueschist contact zone underwent an even more intense retrogression. Preservation of HP‐LT mineral assemblages was mainly restricted to a 10–15 meter zone (or enclave) adjacent to the intensely retrograded lithologic contact. The degree and distribution of the retrograde overprint suggests that pervasively infiltrating fluids were channelized into the marble‐blueschist contact and associated veins and flowed around the preserved HP‐LT enclave. Numerical modeling of Darcian flow, based on the field observations, suggests that near the marble horizon, deflections in fluid flow paths caused by flow channelization along the high‐permeability marble‐blueschist contact zone likely resulted in very large fluid fluxes along the lithologic contact and significantly smaller fluxes (as much as 8 times smaller than the input flux) within the narrow, low‐flux regions where HP‐LT minerals were preserved adjacent to the contact. Our results indicate that lithologic contacts are important conduits for metamorphic fluid flow in subduction zones. Channelization of retrograde fluids into these discrete flow conduits played a critical role in the preservation of HP‐LT assemblages.

Relation between the intensity of deformation and retrogression in blueschist metapelites of Tinos Island (Greece) evidenced by chlorite–mica local equilibria

The late greenschist deformation of the Cycladic Blueschists unit on Tinos Island evolves along a NE–SW gradient of non-coaxial strain across a major extensional shear zone. Deformation is more intense and the greenschist overprint is also more severe close to the shear zone that separates the Cycladic Blueschists from the upper ophiolitic unit in the NE of the island. The compositional variability of chlorite and K-white mica (KWM) in metapelites sampled along a NE–SW cross-section is characteristic of their P–T conditions of crystallization and is mainly characterized by variations of their Si, Al, Fe and Mg contents due to the Tschermak substitution (Si IV(Fe, Mg) VI=Al VIAl IV). Multi-equilibrium P–T estimates based on the assumption of local equilibrium indicate that each sample recorded a part of the P–T history, which depends on the distance to the contact to the main shear zone, i.e. the intensity and the pattern of deformation. The combination of P–T estimates obtained from all the samples collected along the transect provides a continuous P–T path which is composed of two parts. It provides insights on the syn- and post-orogenic exhumation for some of the Cycladic Blueschists. In particular, we could distinguish two stages in the exhumation process. A first stage in the blueschist facies occurred along a cold P–T path during the construction of the Hellenides. The second exhumation stage occurred along a warmer P–T path during the formation of the Aegean sea. Between the two stages of exhumation, there is a thermal reequilibration of the crust and a partial thermal overprint of the blueschist parageneses.

Post-plutonic unroofing and morphogenesis of the Attic–Cycladic complex (Aegea, Greece)

The Attic–Cycladic complex of Greece comprises an Eocene high-P unit with blueschist occurrences. Unroofing of this unit took place in Oligocene–Miocene times and was accompanied by a regional low-P medium-T overprint and Miocene granitic plutonism. Apatite fission-track ages of 14 “crystalline” samples from the islands of Tinos, Mikonos and Serifos range between 13.1 and 5.3 Ma, corresponding to the middle and late Miocene. The frequency distributions of confined track lengths are characterised by high arithmetic means of 14.2–15.1 μm and by standard deviations from 0.9 to 1.6 μm. Thermochronological modelling of the data indicates rapid cooling between 10 and 6 Ma and subsequent deceleration of the cooling rates. For a short time, the Miocene plutons of Tinos, Mikonos and Serifos experienced maximum cooling rates above 50 °C/Ma. These exceptionally high cooling rates cannot be explained by strong vertical uplift and fast regional erosion. Such a process is disproved by preserved remnants of a former peneplain, including inselbergs and kaolinized tropical subsoil. On Mikonos, fast post-plutonic cooling of the lower plate was apparently accompanied by simultaneous sedimentation on the upper plate. We propose a post-plutonic cooling model which assumes strong periplutonic heat flow into much cooler host rocks and fast extensional unroofing.

Oceanic and orogenic fluid–rock interaction in 18O/ 16O-enriched metagabbros of an ophiolite (Tinos, Cyclades)

Two spatial scales of fluid–rock interaction in an ophiolite suite are revealed by oxygen isotope and hydrogen isotope studies of metagabbros on the island of Tinos (Cyclades, Greece). Sequentially formed mineral generations in the metagabbros include relict igneous augite, hornblende of sub-seafloor hydrothermal origin, and actinolite and albite formed by regional greenschist-facies metamorphism during orogenesis. With the exception of augite (δ 18O=4.4–5.6‰), the metagabbros are characterized by unusually high δ 18O values: hornblende (5.8–7.4‰), actinolite (6.5–10.2‰), feldspar (14.6–14.9‰) and whole rocks (7.0–10.5‰). Hornblende δ D values range from −57 to −66‰. The high δ 18O values and the δ D range of the hornblendes are compatible with interaction of oceanic gabbro with seawater that had previously been enriched in 18O/ 16O (δ 18O=6.5–8‰) by isotopic exchange at moderate to high temperatures. The high degree of oceanic alteration in the layered gabbros, mass balance calculations of isotopic exchange, and field evidence for early oceanic thrusting suggest that seawater could have penetrated deeply into the ocean crust, becoming 18O/ 16O-enriched through isotopic exchange with gabbros at progressively increasing temperature. Upward, down-temperature flow of the high-δ 18O water would be very effective in elevating the δ 18O values of gabbros. The regional greenschist metamorphic overprint of the ophiolite, possibly the result of continued thrusting and piling up of nappes during obduction, is characterized by localized fluid–rock exchange. Actinolite in massive gabbroic layers has δ 18O values (6.5–7.2‰) close to those of the hornblende, whereas in deformed meter-sized gabbroic blocks the amphiboles have significantly higher values (8.4–10.2‰). Likewise, albite in the gabbroic blocks has high δ 18O values of ca. 15‰ that are ascribed to meter-scale exchange with 18O-rich fluids derived from dehydration reactions in low-temperature hydrothermally altered basaltic host rock enclosing the blocks. Deformation-enhanced permeability facilitated fluid infiltration in gabbroic blocks, whereas the relatively undeformed, and therefore less permeable, massive gabbros experienced minor interaction with fluids. The orogenic fluid–rock interaction thus represents local-scale redistribution of hydrous mineral components introduced during seafloor hydrothermal exchange.

Calculation of the physicochemical parameters ΣS, ΣC, aH2S, I and of the metal ions content of the hydrothermall fluid that has precipitated Au-Ag-Te ores in Panormos - Liardades Bay area, Tinos Island, Cyclades

The Guldberg - Waage chemical equilibration method was applied to calculate the ionic content of the hydrothermal fluid that has precipitated Ag-Au-Te ores with milky and clear quartz in a stockwork of veins at Panormos - Liardades Bay. The Au-Ag-Te mineralization is developed in nine (I to IX) successive stages with Base Metals Tellurides, Native precious metals and a supergene stage at the end. The metallic and non-metallic ions content of the hydrothermal fluid was calculated for temperatures between 300°C and 190°C. Reconstruction of the evolution of pHysicochemical conditions in the fluid within the same temperature interval yielded calculated values of ΣΟ= IO"1 to 10°, a=10'29 to 109 2 and 1= 0.4

Geomorphology of the island of Tinos

Morphology of Tinos Island is controlled mainly by lithology and tectonics. A significant factor is also the climate; the very high humidity and the strong winds of NNE directions. These special climate characteristics cause intense chemical alteration and cellular weathering at schists and granodiorites. The relief is fairly mountainous. Mt. Tsiknias in the south is the highest mountain (725m). In the central part of the island there are Mesovouni (647m) and Prasa (614m). The slopes of the valley-sides vary a lot depending mainly on the original dip. The relief is smooth while slopes of 15-45% comprise 73% of the island. Three morphological units with different relief type can be distinguished, that results from lithological differences and the type of erosion. The first unit covers the larger part of the island and consists of mica-schists with marble intercalation. The second unit is observed at the mountains Tsiknias, Vouno and Marias. It consists of green schist, clorittalkschists. It is characterized generally of smooth relief with acute notched peaks and Tafoni. The third unit comprises the area of the plutonic rocks. It is characterized by dispersed rounded bulks of granite and spheroidal weathering. Petrologie examination showed biotitic-monzogranit in the area Steni-Volax and biotitic-granodiorit in Livada. In the spherical bulks of Livada Tafoni are created. The spheroidal weathering is owed to chemical processes. The weathered mantle was identified sericitization. The island is crossed by a watershed, which is parallel to the long axis of the island trending NW-SE, and separates the island in two asymmetric parts. The watershed is coincided to the axis of the folds. Pianation surfaces are observed at 100m, 200m, 300- 350m, 410-430m and at 600m. The topographic slopes along the coasts of Tinos present a wide range of values. The slopes of the coasts of the 500m cross-sectional coastal zone, are higher of those of the 1000m one. Clear relation between the slopes of these zones and the corresponding lithologie formations is not observed. It is believed that the main factors which controls the slope morphology of the coastal zone is the tectonic and the weathering.

A catastrophic mass‐mortality episode of gorgonians and other organisms in the Ligurian Sea (North‐western Mediterranean), summer 1999

In the late summer of 1999, an extensive mortality of gorgonians and other epi‐benthic organisms was observed in the Ligurian Sea (Mediterranean Sea) from the Tuscan Archipelago to Marseille. Quantitative data from Tino Island and Portofino Promontory indicated that the proportion of affected gorgonians ranged from 60% to 100% in populations having a density of 9–27.8 colonies m−2, suggesting that millions of sea fans died along the coast of Liguria. This mass mortality episode coincided with a sudden increase of sea water temperature down to more than 50 m depth. Laboratory analyses showed that the colonies stressed by high temperature also underwent extensive attack by microrganisms (protozoans and fungi), which are interpreted as opportunistic pathogens.

The magnetic fabric of metasediments in a detachment shear zone: the example of Tinos Island (Greece)

We present a magnetic fabric study (12 sites) in strongly deformed metasediments (micaschists and calcschists) from a well-exposed detachment shear zone at Tinos Island (Cyclades, Greece). A gradient of deformation is observed from the southwest side to the northeast side of Tinos. The detachment shear zone is contemporaneous with a greenschist retrogression episode (ca. 25–20 Ma). High-pressure (HP) (45 Ma) metasediments are preserved in the southwest side of Tinos. We compared the anisotropy of magnetic susceptibility (AMS, 158 samples) to the anisotropy of isothermal remanent magnetization (AIRM, 40 samples). AMS is primarily dominated by phyllosilicates whilst AIRM reflects the contribution of mixture of ferromagnetic grains (magnetite, hematite, goethite and pyrrhotite). Interestingly, we observe the appearance of goethite (αFeO·OH) is observed when approaching the detachment area. The orientation of the AMS and AIRM axes shows a general consistency. On the other hand, AIRM is more prolate and anisotropic than AMS. We document a close agreement between the AMS magnetic foliation and the structural cleavage that has been measured from the core sample. The magnetic lineation is generally parallel to the stretching lineation observed on the field. However, composite magnetic lineations suggest well-preserved petrofabrics contemporaneous to the 45 Ma HP episode. An increase in the AMS foliation anisotropy parameter is observed in micaschists when deformation increases. Unfortunately, no quantitative estimate of finite deformation is possible using AMS due to the dominant role of phyllosilicates and the complex ferromagnetic mineralogy.

Fluid-rock interaction and thermal evolution during thrusting of an Alpine metamorphic complex (Tinos island, Greece)

This study examines the fluid-rock interaction and thermal evolution along a thrust that juxtaposes calcite-rich marbles of high P-T metamorphic unit of the Attic-Cycladic Massif (Greece) on top of a lower-grade dolomite marble unit. The Tertiary thrust represents a major phase of tectonic movement related to the decompression of the Alpine orogen in the Hellenides. The stable isotope signatures of the thrust plane and adjacent sections of the footwall and hanging wall rocks are characterized by significant carbon and oxygen isotope depletions. The depletion is most pronounced in calcite, but is almost entirely missing in coexisting dolomite. The isotopic patterns in the thrust zone can be explained by the infiltration of an externally derived water-rich H2O-CO2-CH4 fluid [X C (=X CO 2+X CH 4)<0.05] at water-rock ratios on the order of 0.1 to 0.5 by weight. The fluid-induced calcite recrystallization is viewed as an important rheological control during thrusting. The temperature evolution of the footwall, hanging wall and mylonitic tectonic contact was determined by calcite-dolomite solvus thermometry. Histograms of calcite-dolomite temperatures are interpreted as indicating a heating of the footwall dolomite marble during the thrusting of the hotter upper plate. Conversely, the hanging wall marble unit was cooled during the thrusting. The calcite-dolomite thermometry of the thrust plane gives temperatures intermediate between the initial temperatures of the lower and upper marble units, and this leads to the conclusion that conductive heat transfer rather than fluid infiltration controlled the thermal evolution during thrusting.

Rb–Sr isotope studies on Tinos Island (Cyclades, Greece): additional time constraints for metamorphism, extent of infiltration-controlled overprinting and deformational activity

This study presents new Rb–Sr age data concerning the metamorphic evolution of the Attic-Cycladic Crystalline Belt which represents a complex polymetamorphic terrane within the Alpidic orogenic belt of the Hellenides. Two major groups of tectonic units can be distinguished. Metamorphism in parts of the upper units is commonly considered as a Cretaceous event. In contrast, the group of lower units experienced Tertiary high-pressure metamorphism which was followed by a medium-pressure overprint. We focus on the island of Tinos where a representative spectrum of the rock units found in the Cyclades is exposed in three tectonic units: the Upper Unit, the Intermediate Unit and the Basal Unit. The complete range of tectono-metamorphic and magmatic events affecting the Attic-Cycladic Crystalline Belt is documented by numerous petrological and tectonic studies. Phyllites and phyllonites from the ophiolitic Upper Unit yielded Rb–Sr apparent ages (phengite–whole-rock) between c. 92 and 21 Ma. The older age differs from the Cretaceous dates reported for upper unit rocks elsewhere in the Cyclades. It is suggested that the sequence studied belongs to the Jurassic ophiolites of the Hellenides rather than to Cretaceous occurrences. The spread to younger ages is related to non-pervasive rejuvenation and resetting of the Rb–Sr system during tectonic juxtaposition of the Upper Unit over the Intermediate Unit. The youngest age obtained so far for a sample from the Upper Unit (21 Ma) is believed to approximate the timing of tectonic juxtaposition which probably occurred during a regional greenschist-facies episode producing a pervasive overprint in the structurally lower tectonic unit. The major phyllite/meta-gabbro/serpentinite sequence of the Upper Unit is interpreted as an emplacement-related ductile shear zone which experienced reworking under brittle conditions. In the Intermediate Unit, a gradient in Rb–Sr ages from top (c. 40 Ma) to the bottom (c. 22 Ma) was recognized, which is interpreted to represent greater effects of fluid infiltration and overprinting in the lower parts of this unit, possibly controlled by variable intensity of deformation which might be related to tectonic juxtaposition onto the Basal Unit. We suggest that synmetamorphic stacking of all three tectonic units took place during an Oligocene–Miocene greenschist event. Extensional deformation continued after tectonic stacking and after intrusion of the main granite, as is indicated by a Rb–Sr whole-rock isochron (15.1∓0.6 Ma) for a ductilely deformed garnet-bearing leucogranite from the marginal parts of the main undeformed pluton. Application of the Rb–Sr dating technique provided no unequivocal evidence that previously published Eocene K–Ar and 40Ar–39Ar dates for high-pressure phengites from the lower units are significantly contaminated with excess argon.

Aspects of the biology of the bryozoan Pentapora fascialis in the northwestern Mediterranean

Colonies of the bryozoan Pentapora fascialis (Pallas) were monitored photographically from January 1993 to March 1994, in two quadrats at depths of 11 and 22 m at Tino Island (Ligurian Sea, Italy). Colony size, shape, growth and mortality were analysed. Zoarial growth was measured as change in the two-dimensional area of substratum covered per unit time. Colonies ranged in size from less than 30 to 2760 cm2. Colony shape was normally depressed globular, with an elliptic or subcircular perimeter, but tended to elongate in crowded situations. Although extremely variable in small colonies, growth was inversely related to colony size and was maximum in spring. Highest net growth rate was 211% in 11 months. No recruitment was observed in the quadrats during the study period. Partial mortality in colonies was low and was mainly due to overgrowth by algae and to siltation. Physical disturbance by an exceptionally severe winter storm in December 1993 caused total mortality of almost all colonies in the shallow quadrat.

Block rotations and continental extension in the central Aegean Sea: palaeomagnetic and structural evidence from Tinos and Mykonos (Cyclades, Greece)

The Aegean Sea, floored by an attenuated continental crust, is a Mediterranean-type back-arc basin formed above the subducting African plate. We investigated post-12 Ma block rotation in relation to back-arc extension in the area of the western Cyclades (central Aegean Sea). On Tinos island, NW-trending dacitic dikes and a Miocene monzogranite penetrated an Alpine metamorphic series. KAr analyses of 6 dike samples yielded an average age of 11.5 ± 0.4 Ma. The palaeomagnetic analyses of the dikes and the monzogranite reveal 23° vertical-axis clockwise rotation. We assume that the dacitic dikes intruded vertical (as they are today) and that no tilt correction is required. This is supported by the fact that no inclination anomaly is revealed when the palaeomagnetic data is compared with a tilt-corrected pole position from Kimi (Evia). The absence of tilting implies that post-12 Ma crustal thinning in the area of Tinos island was minor. On Mykonos, a low-angle normal fault dipping 30° to the NE juxtaposes a series of Oligo-Miocene molasse sediments above a 10–12-Ma granite. Published palaeomagnetic data revealed a considerable amount of inclination flattening and 22° clockwise rotation of the footwall granite. We show that tilting predated vertical-axis rotation, and that the (now) low-angle normal fault originated steeper, dipping ca. 54°. Planar-rotational fault geometry indicates extension of ca. 60%. In spite of the significant variations in the magnitude of post-12 Ma upper crustal extension across the Tinos-Mykonos transect, crustal thickness below the entire area changes only slightly. This indicates that upper-crustal levels were decoupled from the lower crust. The widespread Middle Miocene granitic magmatism in the Cyclades suggests that flow in the lower crust may have had a dominant role in maintaining the smooth Moho. Assuming that the western Cyclades constitute a set of NW-striking crustal blocks, it is estimated that vertical-axis rotations increased the N-S dimension of the area by ca. 31% (e.g. 25 km). The exact timing of this clockwise rotation is not well constrained, but it postdated the formation of low-angle normal faults and tilting, and may have taken place in the Late Miocene.