V13A-1136 INVITED
The Cycladic Blueschist Belt in the Central Aegean Sea: Resolving the Interplay between Alpine Orogeny and Back-arc Extension
The Aegean Sea, formed via extensional tectonics and floored by an attenuated continental crust, overprinted and dissected a once-continuous Alpine orogenic belt that stretched from mainland Greece to Anatolia. The Cycladic islands, in the central Aegean region, mainly comprise HP-LT metamorphic rocks (and their greenschist-facies derivatives) whose P-T conditions range at 12-15 kbars and 450-500 °C, straddling the blueschist-eclogite facies boundary. The protoliths are supracrustals metavolcanics and volcanoclastics alongside thick marble units that were deposited on the Pindos basin margin. Locally, such as on Syros and Sifnos, kilometer-thick, blueschist and eclogite-facies rocks are preserved intact allowing to explore the bottom of the orogenic edifice. 40Ar/39Ar ages of ~45Ma have been repeatedly obtained on Si rich phengites assessing the Eocene timing of the high-pressure metamorphism and crustal thickening. Upon decompression, the high- pressure rocks were overprinted in the greenschist-facies but locally as on Naxos migmatites were formed on the expense of eclogites at mid-crustal depth, at ~20 Ma. A series of granitoids penetrated the exhumed rock units during the Middle Miocene (until ~10Ma) in relation to whole-lithosphere back-arc extension.//The Cycladic blueschist belt, in the core of the extending Aegean region, is a suitable site to analyze the interplay between Mediterranean-type back-arc extension and the exhumation of the high-pressure metamorphic rocks. The Cycladic blueschist unit is sandwiched between lower pressure rocks: it is topped by greenschist- and amphibolite facies metamorphic rocks comprising metavolcanics interleaved with metamorphosed ultrabasic slices. The tectonic contact is a low-angle extensional detachment of significant lateral dimension and kinematic markers usually portray top-to-the-North sense of motion. Being stitched by mid-Miocene granitoids this is the oldest extensional discontinuity observed in the central Aegean. Where the original architecture of the Alpine orogenic belt was not severely obliterated, such as on Evia, a basal unit (Almyropotamos window) is exposed below the Cycladic blueschists unit. Within the basal unit, the presence of relict glaucophane and Si-rich phengite attest for a LT-HP metamorphism, but carbonates still preserve Lutetian nummullites indicating the basal unit metamorphism outlasted the Middle Eocene as well as cooling of the overlying Cycladic blueschists. The Cycladic blueschist unit is thus allochtonous on a regional scale: it was accreted into the orogenic wedge sometimes after the mid-Eocene. The time interval between the Eocene peak of eclogite metamorphism and the onset of back-arc extension in the Oligo-Miocene involved thrusting and contraction. In the central Aegean, the entire inventory of extensional structures operated subsequently to the emplacement of the Cycladic blueschist unit onto lower pressure sequences implying whole-lithosphere back-arc extension overprinted an Alpine orogen containing eclogites at relatively shallow structural levels. This resembles the mode of occurrence of eclogites in other mountain belts where back-arc extension played no role. Remarkably, despite significant crustal stretching only minor lateral metamorphic breaks can be identified in the Cyclades and the 12-15 kbar level of the former orogen are pervasively exposed over much of the archipelago.
V13A-1137 INVITED
Geodynamic evolution of an UHP Suture Zone in the Greek Rhodope
Detailed mapping of Alpine suture zone areas in the Greek Rhodope Metamorphic Complex and characterisation in terms of structures, petrology and geochronology reveal a tectonic wedge geometry with synmetamorphic SW shear at the base and NE shear on-top. These areas define a suture zone (Rhodope Suture Zone, RSZ) which developed between two continental plates of Variscan and Late Jurassic age (Turpaud & Reischmann 2005). It is subdivided into the Lower Rhodope Suture Zone (LRSZ) with material of mainly continental provenance and the Upper Rhodope Suture Zone (URSZ) with constituents of continental and oceanic origin. Both subunits contain microdiamond-bearing metapelites (Perraki et al. 2006) and record metamorphic stages starting from UHP in the Early/Middle Jurassic (≥ 180 Ma) to amphibolite facies conditions in the Late Cretaceous (≤ 79 Ma) (Bauer et al. 2007). The rocks of the LRSZ underwent uniaxial stretch during upward movements to lower crustal levels (≤ 12 kbar; 650°C) at ca. 144 Ma forming L\ggS tectonites (deformation stage D1). The rocks of the URSZ experienced delayed exhumation and static annealing near 16 kbar and ~800°C at ca. 160 Ma. For both subunits subsequent exhumation was controlled by NE-SW shear along the Rhodope Normal Fault (RNF), which kinematically divides the two subunits of the RSZ (deformation stage D2). The duration of activity along the RNF is constrained by the formation of large-scale folds which formed during shear-partitioning (deformation stage D3) at conditions of <8 kbar and 650°C. D3 probably emerged along a discrete steep fault (Avren Fault in Bulgaria), re-orients earlier high-grade structures and separates central and eastern parts of the Rhodope Metamorphic Complex. D3 characterizes the Mid- to Late Cretaceous amphibolite/greenschist facies thrust tectonics in the Aegean region. N-vergent thrusting of low-grade Mesozoic schists including Jurassic ophiolites east of the fault is coeval with SSW vergent nappe consolidation in the central and western region. The source rocks of the RSZ are proposed to originate from a Variscan microcontinent and an ocean basin to the north. This ocean is interpreted as the easternmost branch of the Triassic/Jurassic Neotethyan ocean (Meliata ocean – eo-Alpine phase), which underwent subduction and metamorphism together with the microcontinent since the Early Jurassic. REFERENCESBAUER, C., RUBATTO, D., KRENN, K., PROYER, A. & HOINKES, G. 2007. A zircon study from the Rhodope Metamorphic Complex, N-Greece: Time record of a multistage evolution. Lithos, in press. PERRAKI, M., PROYER, A., MPOSKOS, E., KAINDL, R. & HOINKES, G. 2006. Raman micro-spectroscopy on DIAMOND, graphite and other carbon polymorphs from the ultrahigh-pressure metamorphic Kimi Complex of the Rhodope Metamorphic Province, NE Greece. Earth and Planetary Science Letters 241, 672-685.TURPAUD, P. & REISCHMANN, T. 2005. Relationships between crustal blocks and UHP relicts, an example from Northern Greece. Geophysical Research Abstracts 0453, 7.
V13A-1138 INVITED
The Apuseni Mountains, Romania, a Variscan Collage of Ordovician Gondwanan Terranes
The basement of the Apuseni Mountains, Romania, consists of three pre-Variscan terranes, sutured during an Early Variscan amalgamation around 351 Ma (Balintoni et al., this volume). The northern Someş Terrane (ST) is predominantly gneissic, while the southern Baia de Arieş Terrane (BAT) is dominated by the presence of large carbonate lenses, although metagranites and other types of orthogneisses can be found. These two terranes are sutured through the Biharia terrane, probably an accreted island arc. LA-ICP-MS datings on zircons extracted from orthogneisses and metagranites were performed in order to constrain the age of ST and BAT. A number of previously CL-imaged crystals were ablated at the China's University of Geosciences, Wuhan. From ST we dated an orthogneiss occurring in structurally lowermost position, a metatuff situated in the upper strongly retrogressed part and a twenty detrital crystal population sampled from a metasandstone. The 206Pb/238U apparent ages were projected using the weighted average plots.A magmatic crystallization age of 472.8±5.0 Ma (Upper Early Ordovician) resulted for one of the orthogneiss samples, besides several older ages at 505.7, 566.3 and 708.2 Ma corresponding to inherited cores. Another sample from the same rock appeared strongly affected by lead loss during a later thermotectonic event, most of the apparent ages grouping around 352±14 Ma. This age is similar with the age of the suture between ST and BT (Balintoni et al., this volume). The main zircon population of one metatuff sample furnished an averaged age of 423±7.2 Ma, also found in two additional samples, but their significance is obscure for the moment. Two primary magmatic ages arise at 464.2 and 473.8 Ma, an older value of 758.7 Ma corresponding to an inherited core. Detrital zircon ages range between 534.8 and 2596.8 Ma. The younger value represents an upper age constraint for the protolith age of ST-rocks. From BAT we dated the Lupşa metaporphyroid and the Mihoeşti (MiG) and Muncel (MuG) metagranites. The 466±11 Ma age value recorded in the metaporphyroid was inferred to represent the primary crystallization age. Many inherited cores yielded ages scattered between 523±32 Ma and 1914 Ma, while no ages younger then 466±11 Ma were recorded. The MiG crystallized at 469.2±7.5 Ma, inherited ages of 501±22, 554.9 and 685.6 Ma being also recorded. A 468.4±4.8 Ma primary magmatic age was found for the MuG, this sample also containing abundant inherited ages dispersed between 507.3±5.5 Ma and 1018.2 Ma. A group of ages at 429±13 Ma, corresponds to the dominant population in the ST metatuff. The results indicate that the main episode of crustal construction for the ST and BAT was the Ordovician; both terranes possess an abundant Cadomian inheritance. Pre-Cadomian inheritances are rare, except for the detrital zircons. Only the zircons from the ST appear strongly affected by lead loss processes, possibly related with the profuse Variscan magmatism affecting the upper plate. The history of the ST and BAT is compatible with a primitive North-Gondwanan origin.
V13A-1139 INVITED
The Age of the Variscan Suture in the Apuseni Mountains, Romania, as revealed by LA-ICP- MS Zircon Dating
The structure of the Apuseni Mountains, Romania, consists of several Alpine nappe systems thrust over an autochthonous unit. The Bihor autochthonous unit and some components of the Biharia and Codru nappe systems are dominantly metamorphic basement unit. Three metamorphic complexes, called Someş, Biharia and Baia de Arieş, were separated in the metamorphic basement and ascribed to three different terranes amalgamated during Variscan Orogeny. During the collision, the Someş assumed the position of upper plate and the Baia de Arieş was included in the lower plate, squeezing the Biharia between them. The southern margin of the Someş terrane was invaded by the Arieş migmatites in its eastern part and several granite plutons in its western side. Migmatites and the Codru Moma granite have been sampled in order to date the collisional event. LA-ICP-MS dating of previously CL-imaged zircon crystals was conducted at the China's University of Geosciences, Wuhan. From the Codru Moma granite, eleven 206Pb/238U apparent ages ranging between 341.1±3.7 Ma and 360.9±3.88 Ma out of nineteen apparent ages ranging between 321.5±3.47 Ma and 371±3.98 Ma were considered for a weighted average projection, resulting a mean early Variscan age of 350.5±4.9 Ma (MSWD = 3.8). At this scale, the MSWD reflects the presence of non analytical scattering, being probably connected to a crystallization interval reasonably extending over 20 Ma or more. A single late Neoproterozoic age of 567±5.97 Ma was found in an inherited core. From the Arieş migmatites, we selected thirty U-Pb apparent ages between 342.5±3.67 Ma and 363.8±3.78 Ma (the same range as the Codru Moma granites), out of forty apparent ages between 323.5±5.74 Ma and 383±4.32 Ma. The mean age resolved by weighted average plot is of 351.8±2.6 Ma, (MSWD = 3.5). It clearly resulted that the Codru Moma granite and Aries migmatites were emplaced in the same time span, extending over a crystallization period around 20 Ma. We remark three inherited cores confined to the 460.8±4.86 Ma - 467.5±4.81 Ma interval (i. e. middle Ordovician). Major element geochemistry indicates for both Codru Moma granite and Arieş migmatites a metaluminous, magnesian, and calc-alkaline to calcic composition, similar for instance to Cordilleran batholiths, either on the outboard side (Arieş migmatites) or in the main range (Codru Moma granite). On this basis we conclude that they were generated in a subductional tectonic setting. The suture from the Apuseni Mountains between the Someş and Baia de Arieş terranes is the result of early Variscan subduction and amalgamation between the two terranes. Such geological processes are known in the Western Carpathians and Bohemia, indicating the Apuseni Mountains as the eastern prolongation of Variscan Western Europe.
V13A-1140
Setting and Paleozoic age of the HP-complexes in the South Carpathians
The metamorphic basement units of the South Carpathians (SC), being hardly overprinted by Alpine events, record a polymetamorphic evolution discernible by tectonic discontinuities, rock compositions indicative of different geotectonic settings and contrasting metamorphic assemblages. Very recent age determinations contribute to a better understanding of this evolution. Pre-Alpine high-pressure (HP) complexes appear in the structurally upper Alpine basement units of the SC: the Lotru Metamorphic Suite (LMS) of the Getic Nappe and the Fàgàraş (FM) and Leaota (LM) Massifs. In these units, HP-complexes are sandwiched in contrasting lithologies (gneisses, metapelites) forming structurally coherent metamorphic piles. This feature confirms large-scale imbrication along plate margins. However, the timing of the various HP-events is different compared to the dominant metamorphic overprint, the tectonic stacking of contrasting lithologic units, and igneous events. The deep-seated stacking (ca. 20 km) of the various units of the LMS postdates the HP-event and is coeval with the dominant metamorphic overprint and emplacement of concordant granitoids, grossly at the Ordovician- Silurian boundary (441.7±3.7 Ma, U-Pb zircon age). In the FM the gneiss unit on top of the eclogite-bearing unit records dominant ages similar to those of the LMS (442±2.4 Ma). However, this U-Pb age constrains only a maximum age for the tectonic stacking because the rocks are strongly overprinted by a medium-grade metamorphic event not recorded in zircon populations so far. Eclogites of the Leaota Massif appear as knockers in a semipelitic subduction-accretion complex overlaid by low grade thrust sheets which were paleontologically dated as Lower Paleozoic. U-Pb zircon dating yielded a metamorphic age of 537±13 Ma for the eclogites. The stacking process responsible for the enclosure of the eclogite-bearing unit postdated an intrusion age of a concordant metagranite below the subduction-accretion complex (479.1±6.5 Ma). In all three HP complexes magmatic ages reflect juvenile igneous suites (arc, back-arc basins) Late Precambrian to Early Paleozoic in age, as well as reworked crustal inheritance (by anatexis/crustal contamination) covering a wide spectrum of ages. Dominant West-African inherited age spectra and subordinate Avalonian ones attest complex (micro)plate-margin interactions of Gondwana-derived terrains during the Early Paleozoic.
V13A-1141
The Origin and Significance of Eclogite and Metagabbronorite Knockers from the Bughea Complex, South Carpathians
The metamorphic basement of the Leaota Massif, South Carpathians, consists of a flat-lying sequence of concordant units displaying internal lithologic and metamorphic contrasts. The Bughea Complex (BC), a mélange containing HP-rock lenses hosted in a semipelitic matrix, is one of these units situated between gneisses and units of pelitic/meta-igneous origin. Based on trace element patterns, three HP rock-types could be distinguished among the knockers: continental pelitic fragments which underwent VHP-metamorphism, N-MORB - derived eclogites, and E-MORB eclogites and metagabbronorites bearing a distinctive LILE enrichment and HFSE depletion. These features together with the decoupling of Nb and Ta as well as the incompatible behavior of Pb reveal a supra-subduction geotectonic context. The HP overprint is uneven. Prograde features are well documented by garnet zoning in eclogites and successive stages of textural-mineralogic overprint of igneous textures and assemblages (plagioclase, pyroxene, biotite). Calculated PT conditions are different among studied HP blocks ranging between 550-780°C, 1.7- 3.2 GPa. The water activity constantly increased during prograde eclogitization. The rocks from the matrix reached only 0.85-0.95 GPa and 555-585°C. The uneven preservation of mineral and textural relics in metagabbronorites as well as variable PT-conditions of the HP-overprint suggest that these rocks had experienced different burial-exhumation histories, returning to surface at different moments as the subduction progressed and being enclosed in a subduction-accretion complex. U-Pb zircon dating of eclogites constrained a Lower Cambrian age for the subduction event (537±13 Ma). The gneissic footwall complex displays a polymetamorphic history overprinted along localized concordant shear- zones by metamorphic assemblages similar to those in the BC matrix. This overprint is connected with the tectonic stacking over an active plate-margin, closely post-dating the emplacement of late collisional granites (479.1±6.5 Ma). The units on top of the BC document successively shallower evolution and emplacement dephts.
V13A-1142 INVITED
Polymetamorphic complexes in the eastern parts of the Balkan Peninsula: 600 Ma of geodynamic evolution
Polymetamorphic amphibolite-facies complexes are exposed in the eastern and central parts of the Balkan Peninsula in different Alpine tectonic zones and under different Cadomian to Alpine collisional and exhumation histories and regimes. All complexes consist mostly of biotite and two-mica gneisses and schists, and amphibolites. Strong Cadomian overprint led to intimate mixing (tectonometamorphic amalgamation) of crustal and mantle (and/or oceanic crust)-derived (serpentinized ultramafics, eclogites) products. The pre-Cadomian complex in Central Sredna-gora Mountains evolved through Cadomian collision with c. 617 Ma granites, Hercynian 340 to 250 Ma granitoids, Late Permian exhumation, and Triassic-Jurassic sedimentation followed by Mid-Cretaceous exhumation. P-T conditions never reached amphibolite facies in post-Cadomian times except for some shear zones. The pre-Cadomian amphibolite-facies complex in Sakar Mt. was intruded by c. 500 Ma old granites, deeply eroded in late Permian time, and covered with depositional contact by Triassic terrestrial and marine sediments. Both basement and Triassic cover suffered folding and amphibolite-facies metamorphism (c. 150 Ma BP) followed by exhumation. Included in the Srednogorie Late Cretaceous volcanic arc as crystalline cores, these complexes have been affected by latest Cretaceous exhumation. Amphibolite-facies polymetamorphic cores (Ograzhdenian complex) within the Serbo-Macedonian massif and other units in SW Bulgaria and the adjacent countries were subjected to intense Cadomian (560 – 520 Ma BP) synmetamorphic collision and granite activity. Some units suffered Cadomian collision under greenschist-facies with a Neoproterozoic to Cambrian diabase-phyllitoid complex or have been exhumed and directly covered by Cambrian (followed by Cambrian limestones) or Tremadocian marine sandstones. After Palaeozoic exhumation, their structure was sealed by Permian, Triassic and Jurassic terrestrial and marine sediments, and after intense Mid-Cretaceous folding and thrusting, exhumed again in Palaeogene times. In the Rhodope region, the Mesoproterozoic? to Neoproterozoic supracrustal Rhodopian complex is built up of kyanite-, garnet- and staurolite-bearing biotite and two-mica gneisses and schists, amphibolites, marbles, calcareous schists, quartzo-feldspathic gneisses (derived of possible arkosic or rhyolitic protoliths), and orthoamphibolite, metaperidotite and eclogite rootless bodies. Migmatites and anatexites crop out in the cores of Rhodopian domes. Mid-Cretaceous thrusting in greenschist-facies conditions is documented at the peripheries of the Rhodope massif, and probably contributed to considerable (up to 70 km; now about 50 km) crustal thickening in the interior. The cores underwent very late (Early to Late Palaeogene) exhumation. Eclogitic rootless bodies that witness HP to UHP metamorphic events crop out in all complexes mentioned. Eclogite formation is referred to different mechanisms (burial through subduction of oceanic crust; amalgamation of mantle and crustal slivers in depth; metamorphism of deep norite to troctolite intrusions; local increase of pressure and temperature along minor shear zones) and times (Cadomian, Hercynian, Cimmerian and/or Alpine).