Eberhard Seidel

Geochronology of high-pressure rocks on Sifnos (Cyclades, Greece)

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+2M1 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 (2M1+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 2M1 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.

Thermobarometric data from a fossil zircon partial annealing zone in high pressure–low temperature rocks of eastern and central Crete, Greece

Abstract A fossil partial annealing zone of fission tracks in zircon is described from high pressure–low temperature (HP–LT) rocks of the Phyllite–Quartzite Unit (PQ) on the island of Crete, Greece. Correlation of regional trends in fission track age populations with independent thermobarometric and microstructural data, and with new experimental annealing results, allows a calibration of this low temperature thermochronological method to a degree hitherto not available from other field examples. The zircon fission track (FT) ages of samples from the PQ across Crete range from original detrital signature through reduced to completely reset. The annealing is the result of a single heating period related to the HP–LT metamorphism with near-peak temperatures lasting for only a few million years some time between 24±1 and 20±1 Ma. In eastern Crete, where rocks have experienced temperatures of 300±50 °C and pressures of 0.8±0.3 GPa, zircon FT ages range from 414±24 to 145±10 Ma. Ages above 300 Ma occur mostly near the east coast of the island in rocks which have not been heated to above ca. 280 °C and probably represent a pre-Variscan source. Track lengths are already indicative of a substantial annealing at this temperature. Most of the zircon FT ages from eastern Crete scatter within error around the stratigraphic age. Samples with apparent zircon FT ages significantly younger than the depositional age are only observed in areas where temperatures exceeded ca. 320 °C. Towards the west, a sudden decrease to very young ages ranging from 17±2 to 18±1 Ma reflects a complete resetting at ca. 350 °C. Short tracks, however, are still observed. Throughout the central and western part of the island, ages are consistently below 22 Ma. Thermobarometric data for this area indicate maximum temperatures of 400±50 °C and pressures of 1±0.3 GPa. Only samples from western Crete, which have been exposed to 400±50 °C, show exclusively long tracks. Consequently, the high temperature limit of the zircon partial annealing zone (ZPAZ) appears to be between 350 and 400 °C. A significant influence of elevated confining pressure on the stability of fission tracks in zircon is ruled out by the results of annealing experiments at 0.5 GPa and at different temperatures, which fit the curves previously obtained by other authors at ambient pressure.

K-Ar geochronology of different tectonic units at the northwestern margin of the Bohemian Massif

Abstract The polymetamorphic Moldanubian (MO) of the northeasthern margin of the Bohemian Massif has been thrust to the north onto the mainly Paleozoic sedimentary Saxothuringian of the Fichtelgebirge (FG). These two units have undergone polyphase deformation and the last regional event to affect both units was a low-pressure metamorphism in which temperatures decreased towards the north. In contrast, the nappe units of the Erbendorf-Vohenstrauss Zone (ZEV) and the Erbendorf Greenschist Zone (EGZ), which partly cover the border of the Moldanubian and the Saxothuringian, and the Munchberg nappe pile (MM), which lies on the Saxothuringian, were in parts subjected to a late medium-pressure metamorphic event. The ZEV, the EGZ, the MO and the FG are intruded by Late Carboniferous granites. Conventional K-Ar analyses, mainly of hornblendes and muscovites from the autochthonous FG and MO, the units beneath the nappes, have yielded exclusively Carboniferous dates. The oldest dates point to a regional cooling of the rocks which outcrop at the present-day surface at about 330-320 Ma, i.e., at the Early-Late Carboniferous boundary. The Late Carboniferous cooling history was largely governed by the thermal influence of the post-kinematic granites (320-295 Ma), especially in the FG and the northern MO. The high-grade metamorphic rocks in the western part of the ZEV and in the upper three nappes of the MM mostly yield dates around 380 Ma i.e., Early Devonian. The results show a relatively wide scatter. Moreover, biotites frequently appear to be older than the coexisting muscovites. Both observations indicate that the rocks underwent a later thermal influence. Whether some groups of older dates (e.g., 400 Ma) are due to excess argon or to inherited argon is still open to discussion. Slightly scattered muscovite dates around 366 Ma were obtained for the prasinite-phyllite series, one of the lower nappes of the MM. A single hornblende from the EGZ gave the same age. These two nappes have, therefore, probably been affected by a Late Devonian thermal and/or tectonic event. The muscovite dates obtained from the Paleozoic Bavarian lithofacies, the lowermost nappe of the MM ∗ , and the hornblende dates from the eastern part of the ZEV are indistinguishable from those of the autochthonous units FG and MO.

Eo-alpine metamorphism in the uppermost unit of the Cretan nappe system — Petrology and geochronology

The uppermost unit of the Cretan nappe system consists of ophiolites on the top, and an ophiolitic mélange at the base.Among the various constituents of the mélange, there are slices of low-P/high-T metamorphics. They form a variegated series consisting of tholeiitic ortho-amphibolites, para-amphibolites, andalusite and sillimanite-cordierite-garnet bearing mica schists, calcsilicate rocks, and marbles. The metamorphic sequence is locally intruded by early tectonic magmatites of gabbroic, dioritic and granitic composition. Critical mineral assemblages lead to a maximum temperature of about 700° C reached during metamorphism, at a total pressure of 4–5 kilobars. K — Ar dating on 6 hornblendes, 7 biotites and 1 muscovite yielded cooling ages of 75–66 m.y. and confirmed earlier results according to which the metamorphism and related magmatism took place in Late Cretaceous times.In order to evaluate the age relationships between the hightemperature metamorphics within the ophiolitic mélange and the ophiolites, hornblendes from ultramafic and mafic rocks of the ophiolite complex were dated by the K — Ar method. Hornblende from one schistose hornblendite forming a constituent of the ophiolites proper yielded 156 m.y. and thus provides a middle Jurassic minimum age for the formation of this piece of oceanic lithosphere. Four hornblendes of calc-alkaline gabbrodiorite dikes within the ophiolite complex gave distinctly lower K — Ar dates of about 140 m.y.. The dikes probably intruded after the detachment of the ophiolites in an island-arc or continental-margin environment.As a consequence, the high-temperature metamorphics and related intrusives in the ophiolitic mélange of Crete are genetically unrelated to the overlying ophiolites. The paleogeographic position of the crystalline terrane, slices of which are now incorporated into the ophiolitic mélange is still open to discussion.

Ophiolites on the Southern Aegean islands Crete, Karpathos and Rhodes: composition, geochronology and position within the ophiolite belts of the Eastern Mediterranean

Abstract Dismembered ophiolites on the Southern Aegean islands of Crete, Karpathos and Rhodes link the Jurassic ophiolites of the Hellenides (e.g., Pindos, Vourinos) and the Cretaceous ophiolites of the Taurides in southern Turkey (e.g., Antalya). The ophiolites of these islands do not form a continuous belt. There are significant differences in composition and age between the ophiolites of Crete in the west and those of Karpathos and Rhodes in the east. Crete: Peridotite relics in serpentinites are lherzolitic with high concentrations of Al2O3 and CaO (mean values of 2.7 and 2.0 wt.%, respectively). Spinels in these rocks are as rich in Al2O3 as those in fertile peridotites from orogenic lherzolite massifs. These rocks represent primitive, undepleted mantle material, indicating an origin at a slow-spreading ridge. The Cretan peridotites are intruded by gabbroic dikes ranging in composition from pyroxene gabbros to hornblende diorites and plagiogranites. The dominance of hornblende in these rocks and the geochemical signature indicate a subduction-related origin for these dikes. Hornblendites associated with the peridotites are regarded as metamorphic ferrogabbros, which were probably overprinted in high-temperature shear zones. K–Ar dating of hornblendites yielded ages around 160 Ma (Middle to Late Jurassic), indicating that these ophiolites are a part of the Jurassic ophiolite belt of the Balkan peninsula. Around 20 Ma younger K–Ar ages were derived for the gabbroic dikes within the peridotites. Karpathos and Rhodes: The peridotite relics in serpentinites from Karpathos and Rhodes are very low in Al2O3 and CaO and correspond to depleted harzburgites. The ultramafics are intruded by dolerite dikes which are very uniform in composition. The dikes show a trace element signature typical of island arc basalts. Both the depleted nature of the peridotites and the geochemistry of the dikes are typical of supra-subduction zone ophiolites. K–Ar dating of hornblendes from the dolerites yielded an early Late Cretaceous minimum age (around 90 Ma) for the ophiolites of the two islands. Both age and the remarkable similarity in composition and structure to ophiolite occurrences in southern Turkey demonstrate that the ophiolites of Karpathos and Rhodes belong to the Cretaceous ophiolite belt of the Eastern Mediterranean and the Middle East.

Uplift-related retrogression history of aragonite marbles in Western Crete (Greece)

In the low-grade, high-pressure (≈400°C, 10 kbar) metamorphic Phyllite-Quartzite Unit of Western Crete, widespread occurrences of aragonite marbles have been discovered recently. A sedimentary precursor is proved by relic structures (bedding, fossils). Partial or complete transformation of aragonite into calcite is ubiquitous. Compositional and microstructural features reflect the metamorphic history: (1) The high-pressure stage is documented by aragonite that is chemically characterized by incorporation of variable amounts of Sr and the lack of Mg. The most Sr-rich aragonite has about 9 wt% SrO (XSrarag=0.09). A compositional zoning observed in some aragonite crystals may be due to the prograde divariant calcite⇒aragonite transformation in the system CaCO3-SrCO3. Because the parent rocks probably were Sr-poor calcite limestones, one can speculate that strontium has been supplied from an external source under high-pressure conditions. (2) During uplift, calcite replacing aragonite did not equilibrate with unreplaced aragonite. Disequilibrium is indicated by highly variable compositions of calcite crystals that show topotactic relations to the host aragonite. The calcite compositions range from that of the host aragonite (Sr-rich and Mg-free) to Mg-bearing and Sr-poor. (3) Calcite that recrystallized during retrogression is generally Sr-poor (mean value ofXSr<0.005), Mg-bearing (XMg≈0.010), and chemically homogeneous. Because practically no Sr remains in the calcite, an interaction with a fluid phase is indicated. In fine-grained calcite marbles rich in solid organic matter, microstructural features indicative of former aragonite may be present. (4) The last stage of retrogression is documented by the appearance of radiating aragonite in veins and nodules. This aragonite, which shows neither deformation nor retrogression, was probably formed metastably in a near-surface environment.

Jurassic age of metamorphism at the base of the Brezovica peridotite (Yugoslavia)

Abstract Minerals from two amphibolites and two micaschists from the metamorphic sequence at the base of the Brezovica peridotite were dated by the K-Ar method. Two muscovites and one hornblende gave concordant ages of 161.5 ± 1.6 (2σ), 162.8 ± 1.9,and162.6 ± 3.4m.y. respectively while one hornblende gave a slightly discordant date of 169.0 ± 3.2m.y. The dates indicate cooling of the sequence during Middle to Upper Jurassic times. If one interprets the Brezovica sequence as a dynamo-thermal aureole, the cooling age is indistinguishable from the time of emplacement of the hot peridotite.

Chloritoid-bearing metapelites associated with glaucophane rocks in western Crete, Greece

In western Crete, Greece, a widespread occurrence of chloritoid-bearing metapelites with the main mineral assemblage chloritoid-phengitic white mica-Fe-rich chlorite-quartz was recorded to form the country rock of glaucophane-bearing metabasalts. Six bulk rock analyses of the metapelites conform to the compositional restrictions evaluated by Hoschek (1967) for the formation of chloritoid. Three microprobe analyses revealed chloritoid compositions low in Mg and Mn, and, consequently, high in Fe. The metamorphic grade documented in the metapelites is obviously related to a subsequent prograde metamorphism by which, in the adjacent meta-basalts, epidote is formed at the expense of lawsonite. No relict of a high-P, low-T assemblage, in part well preserved in the meta-basalts, was recognized in the chloritoid schists. The significance of the metamorphic history is briefly discussed.

THE KAKOPETROS AND RAVDOUCHA IRON-OXIDE DEPOSITS, WESTERN CRETE, GREECE: FLUID TRANSPORT AND MINERALIZATION WITHIN A DETACHMENT ZONE

Small iron deposits at Kakopetros and Ravdoucha in western Crete are hosted by an extensional detachment zone at the roof of the high-pressure–low-temperature metamorphic core complex known as the Phyllite-Quartzite unit. The iron oxides occur in a brecciated layer of phyllite, quartzite, and marble up to tens of meters thick. They fill fractures and vugs in the breccia and partly impregnate the marble. The iron oxides, which were formerly mined in open pits, are predominantly composed of goethite and subordinate oxyhydroxides of the manganomelane group. The field relationships and microstructures indicate that precipitation of the iron-oxide minerals was related to fluid flow focussed along the detachment fault. δ 18 O values of goethite indicate crystallization at low temperatures (31°–40°C) and at a shallow depth of about 1 km. Microscopic investigations show that the deposition of iron oxides was syntectonic and occurred during deformation in the uppermost crust. Similar iron oxides are reported from low-angle brittle detachment horizons in the Cordilleran metamorphic core complexes of North America and suggest that small iron- and manganese-oxide deposits of this type may be a characteristic feature of detachment zones.

Chloritoid-bearing metapelites associated with glaucophane rocks in western Crete, Greece: Additional comments

Problems related to the formation of chloritoid in metapelites, associated with lawsonite-glaucophane bearing metabasalts, in the quartzitephyllite series of western Crete (Greece) are discussed. It is supposed that chloritoid was formed, during prograde metamorphism, according to a gliding-equilibrium reaction of the type (Fe,Mg)-carpholite1+chlorite1 (Fe,Mg)-carpholite12+(Fe,Mg)-chloritoid12 +chlorite1→2+quartz+H2O ⇋ (Fe,Mg)-chloritoid2+chlorite2+quartz+H2O. This view is stipulated by the occurrence of ferrocarpholite-chloritoid schists in the southeastern part of central Crete. The assemblage chloritoid+ lawsonite recently recognized in western Crete provides evidence that the formation of chloritoid started well within the stability field of lawsonite.