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Notes: Abstract The metamorphism on the island of Sifnos is characterized by the Eocene development of a coherent highpressure blueschist terrane and an early Miocene greenschist facies overprint. This study documents the metamorphic evolution of the blueschist assemblages, still preserved in the northern parts of the island, and their subsequent transformation into greenschists in the central and southern parts. The oxygen isotope geothermometry is based on calibrations for quartz, pyroxenes and magnetite (Matthews et al. 1983a) augmented by revised calibrations for the minerals muscovite (Δ Qz−Mu=1.55×106 T −2), epidote (δ Qz−Ep= 1.56+1.92Δ ps)106 T −2), and rutile (Δ Qz−Ru=4.54×106 T −2). Oxygen isotope analyses of minerals from the Blueschist unit of northern Sifnos give consistent fractionations which are independent of rock type. An average temperature of 455° C was obtained, although the scatter in temperatures deduced from the various geothermometers suggests that equilibration occurs under slightly changing physicochemical conditions. Analyses of minerals and whole rocks shows that pervasive equilibration in the presence of a common metamorphic fluid has not occurred. The minerals and whole rocks of the greenschists of central Sifnos are systematically enriched in 18O relative to the blueschist assemblages. Chemical data indicate that the greenschist overprint was accompanied by a metasomatic enrichment of Ca2+ and CO2. The petrologic, isotopic and chemical evidence favour a metamorphism governed by the infiltration of 18O-CO2 enriched aqueous solutions. It is reasonable to assume that this is connected with the Miocene magmatic activity observed throughout the Cyclades. The marbles separating the Blueschist from the Greenschist unit probably acted as barriers to fluid infiltration into the blueschists and were responsible for their preservation. The pressure of the blueschist metamorphism is estimated at 14±2 kbar, corresponding to a depth of ca. 50 km. The structural style and stratigraphy of Sifnos are suggestive of the subduction of a continental margin sequence. It is clear that the considerable tectonic depression may be associated with continental collision and underthrusting.

Notes: Abstract A new occurrence of deerite-bearing quartzites from blueschist metamorphic rocks of the Greek island of Sifnos is described, and analytical and X-ray data on deerite are presented.

Notes: Abstract The transformation from blueschist to greenschist facies forms a major part of the Alpine regional geodynamic evolution of the Cyclades. The transition in metabasic rocks on Sifnos involves the retrogression of eclogites, blueschists and actinolite-bearing rocks from high-pressure conditions which have been estimated at 460±30° C and 15±3 kb. Petrographically observed parageneses are interpreted by a sequence of hydration and carbonation reactions involving the breakdown of omphacite and glaucophane-bearing assemblages to albite+chlorite±actinolite±calcite assemblages. The retrograde processes are calculated to occur at pressures of 10 to 8 kb during the isothermal uplift of the Sifnos units. Oxygen isotope analyses of different rock types show that interlayered lithologies have experienced a large degree of isotopic equilibration during both metamorphic phases. However, chemically equivalent rocks show systematic increases in δ 18O from lowest values (∼8 to 11‰ in metabasics) in the unaltered blueschists in the upper stratigraphic levels to higher values (〉15‰ in metabasics) associated with greenschists in the deepest stratigraphic levels. Relict eclogites enclosed within greenschists have the lower δ 18O values typical of unaltered blueschist facies rocks. These isotopic gradients and the δ 13C and δ 18O compositions of carbonates demonstrate an infiltration mechanism involving the upward movement of 18O-enriched fluids whose compositions were buffered by exchange with marble units. Calculated minimum fluid/rock ratios for the blueschist-to-greenschist transition decrease from ∼0.4 in the deepest studied level (Central Sifnos) and ∼0.2 in the intermediate level (Kamares Bay samples) to an assumed value of zero in unaltered blueschists. These ratios may be lower if recycling of fluids occurred between schists and marbles. Infiltration of fluids became inhibited as the transformation advanced as a consequence of “sealing effect” of the hydration and carbonation reactions. Although infiltration most probably was a pre-requisite for the regional occurrence of the blueschist-to-greenschist transformation in the Cyclades, the evidence on Sifnos does not suggest the introduction of large quantities of fluid.

Notes: Abstract Melting relationships in the system K2O-CaO-Al2O3-SiO2-H2O have been reinvestigated using Schreinemakers analysis and hydrothermal experiments. The reaction sanidine+muscovite+zoisite+quartz+vapor =melt has been bracketed at 10, 15, and 20 kbars and 670–680, 680–690, and 690–700° C, respectively and it marks the lowest solidus temperatures in the system investigated. Below 10 kbars, experimental data on the beginning of melting in zoisite- or muscovite-bearing anorthite+sanidine assemblages have been obtained, which are not showing any differences and therefore point to melt compositions close to the feldspar-quartz join.

Notes: Abstract Measurements of 40Ar/39Ar age spectra on single white mica crystals of Tertiary age is a new means of obtaining detailed geochronological information of Alpine blueschists. Internal consistency of the data set, as well as excellent agreement with previous conventional K−Ar results, demonstrates that this new technique can be used with confidence to obtain information on the thermal evolution of young metamorphic belts. As mineral grains can be dated individually with this technique, problems related to multiple generations of mica occurring side in the rock can be addressed, potentially yielding much information which cannot be obtained easily by other dating techniques. Ages decrease from 41.7±0.3 Ma in the fresh blueschists and eclogites in northern Sifnos to ca 30 Ma in the more severely overprinted greenschists of central Sifnos. It is argued that this decreasing trend appears gradual, rather than stepwise. This gradual character of the age trend is taken to indicate that it may be caused by differential uplift and cooling, rather than by tectonic juxtaposition of different rock units. In addition, the rocks of central Sifnos contain mica with a plateau age of 18.9±0.3 Ma. This latter age is younger than previous estimates for the age of greenschist overprinting, and more in line with the age of high temperature metamorphism and granite emplacement elsewhere in the Cyclades.

Notes: Abstract Polymetamorphic rocks of Sifnos (Greece) have been investigated by Rb-Sr, K-Ar, and fission track methods. Critical mineral assemblages from the northern and southernmost parts of Sifnos include jadeite+quartz+3T phengite, and omphacite+garnet +3T phengite, whereas the central part is characterized by the assemblage albite+chlorite+epidote+2M 1 phengite. K-Ar and Rb-Sr dates on phengites (predominantly 3T) of the best preserved high P/itTmetamorphic rocks from northern Sifnos gave concordant ages around 42 m.y., indicating a Late Lutetian age for the high P/T metamorphism. Phengites (2M 1+3T) of less preserved high P/T assemblages yielded K-Ar dates between 48 and 41 m.y. but generally lower Rb-Sr dates. The higher K-Ar dates are interpreted as being elevated by excess argon. K-Ar and Rb-Sr ages on 2M 1 phengites from central Sifnos vary between 24 and 21 m.y. These ages date a second, greenschist-facies metamorphism which overprinted the earlier high-pressure metamorphic rocks.