Metamorphic evolution and geochemistry of kyanite eclogites in central Rhodope,northern Greece

In central Rhodope of northern Greece, kyanite eclogites were discovered in the area of Thermes. They are strongly overprinted and exhibit a multi-stage development of minerals and mineral assemblages formed during successive stages of the exhumation. The initial high-pressure assemblage was garnet+omphacite+kyanite+zoisite+phengite+rutile+quartz. Corundum, Fe-Mg-spinel, sapphirine and högbomite occur as products of a first, high-temperature overprint, still at high pressures, whereas various symplectites [corundum-plagioclase (pl), spinel-pl, sapphirine-pl, clinopyroxene-pl, biotite-pl, amphibole-pl] grew during subsequent stages of the exhumation. Diablastic amphibole+plagioclase formed as end-products of the amphibolitization. According to geochemical data, the protoliths of the kyanite eclogites were basalts to basaltic andesites with “volcanic arc” affinities. For the high-pressure stage of metamorphism, minimum PT conditions were around 19 kbar, 700°C, while for the initial stages of the overprint, high-pressure granulite-facies conditions prevailed (T>800°C, at P>15 kbar). The PT conditions of the amphibolite facies were 8–11 kbar, 580–690°C. The kyanite eclogites of Thermes record the highest temperatures of metamorphism within the whole of Rhodope.     

Identification of repeated Alpine (ultra) high-pressure metamorphic events by U–Pb SHRIMP geochronology and REE geochemistry of zircon: the Rhodope zone of Northern Greece

U–Pb sensitive high resolution ion microprobe (SHRIMP) zircon geochronology, combined with REE geochemistry, has been applied in order to gain insight into the complex polymetamorphic history of the (ultra) high pressure [(U)HP] zone of Rhodope. Dating included a paragneiss of Central Rhodope, for which (U)HP conditions have been suggested, an amphibolitized eclogite, as well as a leucosome from a migmatized orthogneiss at the immediate contact to the amphibolitized eclogite, West Rhodope. The youngest detrital zircon cores of the paragneiss yielded ca. 560 Ma. This date indicates a maximum age for sedimentation in this part of Central Rhodope. The concentration of detrital core ages of the paragneiss between 670–560 Ma and around 2 Ga is consistent with a Gondwana provenance of the eroded rocks in this area of Central Rhodope. Metamorphic zircon rims of the same paragneiss yielded a lower intercept ²⁰⁶Pb/²³⁸U age of 148.8±2.2 Ma. Variable post-148.8 Ma Pb-loss in the outermost zircon rims of the paragneiss, in combination with previous K–Ar and SHRIMP-data, suggest that this rock of Central Rhodope underwent an additional Upper Eocene (ca. 40 Ma) metamorphic/fluid event. In West Rhodope, the co-magmatic zircon cores of the amphibolitized eclogite yielded a lower intercept ²⁰⁶Pb/²³⁸U age of 245.6±3.9 Ma, which is interpreted as the time of crystallization of the gabbroic protolith. The metamorphic zircon rims of the same rock gave a lower intercept ²⁰⁶Pb/²³⁸U age of 51.0±1.0 Ma. REE data on the metamorphic rims of the zircons from both the paragneiss of Central Rhodope and the amphibolitized eclogite of West Rhodope show no Eu anomaly in the chondrite-normalized patterns, indicating that they formed at least under HP conditions. Flat or nearly flat HREE profiles of the same zircons are consistent with the growth of garnet at the time of zircon formation. Low Nb and Ta contents of the zircon rims in the amphibolitized eclogite indicate concurrent growth of rutile. Based on the REE characteristics, the 148.8±2.2 Ma age of the garnet–kyanite paragneiss, Central Rhodope and the 51.0±1.0 Ma age of the amphibolitized eclogite, West Rhodope are interpreted to reflect the time close to the (U)HP and HP metamorphic peaks, respectively, with a good approximation. The magmatic zircon cores of the leucosome in the migmatized orthogneiss, West Rhodope, gave a lower intercept ²⁰⁶Pb/²³⁸U age of 294.3±2.4 Ma for the crystallization of the granitoid protolith of the orthogneiss. Two oscillatory zircon rims around the Hercynian cores, yielded ages of 39.7±1.2 and 38.1±0.8 Ma (2σ errors), which are interpreted as the time of leucosome formation during migmatization. The zircons in the leucosome do not show the 51 Ma old HP metamorphism identified in the neighboring amphibolitized eclogite, possibly because the two rock types were brought together tectonically after 51 Ma. If one takes into account the two previously determined ages of ca. 73 Ma for (U)HP metamorphism in East Rhodope, as well as the ca. 42 Ma for HP metamorphism in Thermes area, Central Rhodope, four distinct events of (U)HP metamorphism throughout Alpine times can be distinguished: 149, 73, 51 and 42 Ma. Thus, it is envisaged that the Rhodope consists of different terranes, which resulted from multiple Alpine subductions and collisions of micro-continents, rather similar to the presently accepted picture in the Central and Western Alps. It is likely that these microcontinents were rifted off from thinned continental margins of Gondwana, between the African and the European plates before the onset of Alpine convergence.     

Corundum- and zoisite-bearing marbles in the Rhodope Zone,Xanthi Area (N. Greece): Estimation of the fluid phase pomposition

Summary In the area of Xanthi, N. Greece, regionally metamorphosed marbles which belong to the Upper Tectonic Unit of the Rhodope Zone occur as intercalations within gneisses and amphibolites. In one marble horizon abundant Al-rich minerals like corundum (ruby), spinel, Al-rich pargasite, zoisite, and anorthite were found. In some cases, spinel occurs as a rim around corundum and was formed as a reaction product of corundum + dolomite. Zoisite often coexists in equilibrium with anorthite and calcite. These relationships permit the calculation of the fluid phase composition which is found to be relatively poor in CO₂, thus indicating that abundant water was available-at least on a local scale-during the last, Barrow-type metamorphism. This water was probably supplied from the metapelites by dehydration reactions during the amphibolite facies overprint and invaded the rocks through fractures and shear zones and along the contacts between different rock types.

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Korund- und zoisit führende Marmore der Rhodope-Zone, Xanthi Gebiet (N-Griechenland): bestimmung der zusammensetzung der fluiden phase

Zusammenfassung Regionalmetamorphe Marmore, die zur oberen tektonischen Einheit der Rhodope-Zone gehören, kommen in der Xanthi-Region (N-Griechenland) als Einschaltungen in Gneisen und Amphiboliten vor. In einem dieser Marmorhorizonte konnten Al-reiche Minerale, wie Korund (Rubin), Spinell, Al-reicher Pargasit, Zoisit und Anorthit, gefunden werden. Spinell tritt bisweilen als Rand um den Korund auf, was auf eine Entstehung als Reaktionsprodukt von Korund + Dolomit hinweist. Zoisit koexistiert meist mit Anorthit und Calcit. Diese Paragenesen erlauben eine Abschätzung der Zusammensetzung der fluiden Phase: sie ist relativ arm an CO₂. Dies weist darauf hin, daß, zumindest lokal, während der Barrow-Metamorphose genügend Wasser verfügbar gewesen ist. Das Wasser stammt vermutlich aus Metapeliten, aus denen es durch Dehydrationsreaktionen während der amphibolitfaziellen Überpr:agung ausgepreßt wurde. Entlang von Bruch- und Scherzonen, bzw. an Kontakten unterschiedlicher Gesteinstypen, ist das Wasser in die betreffenden Gesteine eingewandert.

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With 3 Figures     

Jurassic ophiolites within the Valais domain of the Western and Central Alps: geochronological evidence for re-rifting of oceanic crust

Metabasic rocks from different parts of the Antrona ophiolites, Western Alps, as well as from the Misox zone, Central Alps, were dated using ion microprobe (SHRIMP) U-Pb analyses of zircon, in association with cathodoluminescence (CL) imaging. HP metamorphism must have affected at least the major part of the Antrona ophiolites, although HP relics are rarely preserved, probably due to the Lepontine metamorphic overprint. HP metamorphism has affected also the area of the Misox zone. The origin of the Antrona ophiolites is arguable. They were interpreted as part of both the Piemont–Ligurian (PL) and the Valais ocean, the two main oceans in the area of the Alps before Alpine convergence. SHRIMP-analyses of co-magmatic zircon domains from the Antrona ophiolites (Guggilihorn, Passo del Mottone and Quarata areas) yielded identical (within uncertainty) weighted mean ²⁰⁶ Pb/²³⁸U ages of 155.2±1.6 Ma, 158±17 Ma (or 163.1±2.4 Ma: one analysis; 1 error) and 155.6±2.1 Ma, respectively, interpreted as the time of crystallization of the magmatic protoliths. These Late Jurassic ages fit well to the time span considered for the formation of Piemont–Ligurian oceanic crust. The metagabbro of the Misox zone (Hinterrhein area), for which a Valaisan origin is generally accepted, gave also a Late Jurassic, PL protolith age of 161.0±3.9 Ma. The metamorphic zircon domains from the amphibolitized eclogite of Mottone yielded an age of 38.5±0.7 Ma, interpreted as the time of HP metamorphism. This age is in good agreement with the time of metamorphism reported from previous zircon SHRIMP-data for eclogites and amphibolites of other parts in the Valais domain. In order to bring in line the PL protolith ages with the Valaisan metamorphic ages, we suggest a scenario involving emplacement of part of the PL oceanic crust to the north of the newly formed Briançonnais peninsula, inside the Valais geotectonic domain. This paleotectonic configuration was probably established when younger Valaisan oceanic crust formed by spreading and re-rifting, partly within PL oceanic crust.     

Constraining the prograde and retrograde P-T-t path of Eocene HP rocks by SHRIMP dating of different zircon domains: inferred rates of heating, burial, cooling and exhumation for central Rhodope, northern Greece

Ion microprobe (SHRIMP) dating was carried out on different zircon domains from metamorphic rocks of the HP-HT terrane of central Rhodope, northern Greece, to constrain the timing of prograde and retrograde stages within a single tectono-metamorphic cycle. A well determined P-T-t(relative) path for the metamorphic rocks of this terrane was used as a petrological basis for the geochronological investigations. Ion microprobe work was assisted by cathodoluminescence (CL) images of the zircon crystals. The geochronological results revealed that Hercynian continental crust was subducted during the Eocene. Several stages of the Eocene tectono-metamorphic cycle – including both the prograde and retrograde parts of the P-T path above ca 300 °C, 0.3 GPa – were dated using zircons from the following rock types: (1) A deformed quartz vein probably formed at ca 300 °C, 0.3 GPa. Zircons in this vein precipitated from a hydrothermal fluid; they yielded an age of 45.3 ± 0.9 Ma which corresponds to the time of a low-T prograde stage of metamorphism. (2) In kyanite eclogites, zircons were entirely reset during eclogite-facies metamorphism. Resetting was very probably enhanced by the presence of fluids derived by H₂O liberating reactions close to the P-peak. They yielded an age of 42.2 ± 0.9 Ma. (3) Orthogneisses surrounding the kyanite eclogites contained zircons with magmatic oscillatory zoned cores, which yielded Hercynian ages of 294 ± 8 Ma (age of granitic protolith formation), whereas CL-bright, metamorphic rims yielded, like the eclogite zircons, ages of 42.0 ± 1.1 Ma. Therefore, both the eclogites and orthogneisses are interpreted to have approached maximum depth at around 42 Ma. (4) In a leucosome of a migmatized orthogneiss, oscillatory zoned zircons yielded an age of 40.0 ± 1.0 Ma. At this time the rocks reached maximum temperatures during early decompression. (5) A late pegmatite crosscutting the schistosity of amphibolites contained oscillatory zoned zircons that yielded a crystallization age of 36.1 ± 1.2 Ma. Thus, the whole tectono-metamorphic cycle above ca 300 °C, 0.3 GPa lasted from 45.3 ± 0.9 Ma to 36.1 ± 1.2 Ma, that is 9.2 Ma with an extreme error value of 2.1 Ma. Based on combined SHRIMP and petrological data, the average rates of heating and burial during subduction (above ca 300 °C, 0.3 GPa) are >94 °C/Ma and >15 mm/a, respectively. Rates of cooling and exhumation (also above 300 °C, 0.3 GPa) are calculated to be >128° C/Ma and >7.7 mm/a. The Eocene age of metamorphism in central Rhodope implies that the terrane of, at least, central Rhodope and the Cyclades very probably was part of the same continental crust. Received: 5 October 1998 / Accepted: 18 January 1999     

The youngest basic oceanic magmatism in the Alps (Late Cretaceous<Emphasis Type="Italic">;</Emphasis> Chiavenna unit,Central Alps): geochronological constraints and geodynamic significance

Cathodoluminescence-controlled radiometric dating (U–Pb SHRIMP) was carried out on zircon domains from metabasic rocks of the Chiavenna unit, a major mafic/ultramafic-bearing unit in the Central Alps. Co-magmatic zircon domains from amphibolites near Chiavenna and Prata areas yielded weighted mean ²⁰⁶Pb/²³⁸U ages at 93.0±2.0 and 93.9±1.8 Ma, respectively, interpreted as the age of crystallization of the magmatic protoliths. These ages fit well with the time of late spreading in the Valais Ocean, as suggested by previous paleogeographic reconstructions. Inherited zircon grains and/or core domains (Permo-Triassic, Carboniferous, Proterozoic) are abundant, indicating proximity of the Chiavenna unit to thinned continental crust. This is in line with the origin of this unit from subcontinental mantle sources, as suggested previously on petrological and structural grounds. Metamorphic zircon domains from one amphibolite near Chiavenna yielded a weighted mean ²⁰⁶Pb/²³⁸U age at 37.1±0.9 Ma, identical to the 38.5±0.9 Ma SHRIMP age of an amphibolitized eclogite of the Antrona ophiolites (Valais domain, Western Alps). Precise metamorphic ages were difficult to obtain from the composite (poly)metamorphic rim domains of the Prata amphibolite. This is attributed to the location of the Prata area close to the granulite-facies Gruf unit (metamorphosed at ca. 33 Ma) and to the 24–25 Ma old Novate granite, where metamorphic/fluid events probably caused multiple resetting to various degrees. The ca. 93 Ma old magmatism, identified for the first time in the Chiavenna unit, is the youngest basic oceanic magmatism reported in the Alps. The 37.1±0.9 Ma old metamorphism in the Chiavenna unit, attributed to the Valais domain, confirms the model suggesting stepwise younging of metamorphic ages from the south (Adriatic plate) to the north (European plate). It is older than metamorphism in the European margin (ca. 35–31 Ma) lying to the north of the Valais domain and younger than that in the Piemont–Ligurian Ocean (ca. 44–45 Ma) lying to the south of the Valais domain.Editorial responsibility: W. Schreyer