Ion Probe U-Pb Dating of the Central Sakarya Basement: A peri-Gondwana Terrane Intruded by Late Lower Carboniferous Subduction/Collision-related Granitic Rocks

Ion probe dating is used to determine the relative ages of amphibolite-facies meta-clastic sedimentary rocks and crosscutting granitoid rocks within an important 'basement' outcrop in northwestern Turkey. U-Pb ages of 89 detrital zircon grains separated from sillimanite-garnet micaschist from the Central Sakarya basement terrane range from 551 Ma (Ediacaran) to 2738 Ma (Neoarchean). Eighty five percent of the ages are 90-110% concordant. Zircon populations cluster at similar to 550-750 Ma ( 28 grains), similar to 950-1050 Ma (27 grains) and similar to 2000 Ma (5 grains), with smaller groupings at similar to 800 Ma and similar to 1850 Ma. The first, prominent, population (late Neoproterozoic) reflects derivation from a source area related to a Cadomian-Avalonian magmatic arc, or the East African orogen. An alternative Baltica-related origin is unlikely because Baltica was magmatically inactive during much of this period. The early Neoproterozoic ages (0.9-1.0 Ga) deviate significantly from the known age spectra of Cadomian terranes and are instead consistent with derivation from northeast Africa. The detrital zircon age spectrum of the Sakarya basement is similar to that of Cambrian-Ordovician sandstones along the northern periphery of the Arabian-Nubian Shield (Elat sandstones). A sample of crosscutting pink alkali feldspar-rich granitoid yielded an age of 324.3 +/- 1.5 Ma, whilst a grey, well-foliated biotite granitoid was dated at 327.2 +/- 1.9 Ma. A granitoid body with biotite and amphibole yielded an age of 319.5 +/- 1.1 Ma. The granitoid magmatism could thus have persisted for similar to 8 Ma during late Early Carboniferous time, possibly related to subduction or collision of a Central Sakarya terrane with the Eurasian margin. The Central Sakarya terrane is likely to have rifted during the Early Palaeozoic; i.e. relatively early compared to other Eastern Mediterranean, inferred `Minoan terranes' and then accreted to the Eurasian margin, probably during Late Palaeozoic time. The differences in detrital zircon populations suggest that the Central Sakarya terrane was not part of the source area of Lower Carboniferous clastic sediments of the now-adjacent Istanbul terrane, consistent with these two tectonic units being far apart during Late Palaeozoic-Early Mesozoic time

Tectonic evolution of the intra-pontide suture zone in the Armutlu Peninsula, NW turkey

The Armutlu Peninsula and adjacent areas in NW Turkey play a critical role in tectonic reconstructions of the southern margin of Eurasia in NW Turkey. This region includes an inferred Intra-Pontide oceanic basin that rifted from Eurasia in Early Mesozoic time and closed by Late Cretaceous time. The Armutlu Peninsula is divisible into two metamorphic units. The first, the Armutlu Metamorphics, comprises a ?Precambrian high-grade metamorphic basement, unconformably overlain by a ?Palaeozoic low-grade, mixed siliciclastic/carbonate/volcanogenic succession, including bimodal volcanics of inferred extensional origin, with a possibly inherited subduction signature. The second unit, the low-grade Iznik Metamorphics, is interpreted as a Triassic rift infilled with terrigenous, calcareous and volcanogenic lithologies, including basalts of within-plate type. The Triassic rift was unconformably overlain by a subsiding Jurassic-Late Cretaceous (Cenomanian) passive margin including siliciclastic/carbonate turbidites, radiolarian cherts and manganese deposits. The margin later collapsed to form a flexural foredeep associated with the emplacement of ophiolitic rocks in Turonian time. Geochemical evidence from meta-basalt blocks within ophiolite-derived melange suggests a supra-subduction zone origin for the ophiolite. The above major tectonic units of the Armutlu Peninsula were sealed by a Maastrichtian unconformity. Comparative evidence comes from the separate Almacik Flake further cast

A LATE PALEOZOIC EARLY MESOZOIC MARGINAL BASIN ALONG THE ACTIVE SOUTHERN CONTINENTAL-MARGIN OF EURASIA - EVIDENCE FROM THE CENTRAL PONTIDES (TURKEY) AND ADJACENT REGIONS

Remnants of two 'Palaeotethyan' oceanic basins are exposed in the Central Pontides of northern Turkey, separated by a continental sliver and an oceanic arc. The southern basin corresponds to the main Tethys ('Palaeotethys'), which partially closed in Early Mesozoic time following northward subduction under the southern, active continental margin of Eurasia

LATE PALEOZOIC MARGINAL BASIN AND SUBDUCTION-ACCRETION - THE PALAEOTETHYAN KURE COMPLEX, CENTRAL PONTIDES, NORTHERN TURKEY

The Kure Complex, located in the Central Pontides of northern Turkey, is a c. 20 km thick pile of thrust-imbricated deep-sea sediments, intercalated with a dismembered ophiolite. The ophiolitic rocks include serpentinized harzburgite, massive gabbro (cumulate and isotropic), sheeted dykes and basic volcanic rocks. The extrusive rocks are mainly pillow lavas, sheet flows and lava breccias. Geochemical evidence of immobile major- and trace-elements and pyroxene chemistry indicate mid-ocean ridge (MOR) and volcanic arc basalt (VAB) compositions, and suggest that the Kure Ophiolite was generated above a subduction zone. Chrome spinel analysis also supports this interpretation. In some thrust sheets the extrusives are depositionally overlain by hemi-pelagic shales, passing up into terrigenous turbidites and shales, which are up to several hundred metres thick in intact successions. Cyprus-type massive sulphides, interpreted as precipitates from black smokers, are located along the lava-sediment contact. Individual, intact sediment units, up to several kilometres thick within the Kure Complex are separated by narrow (up to 10 m wide) zones of intense shearing, layer-parallel extension and melange formation. Asymmetrical folds, reverse faults, thrusts and duplex structures throughout the Kure Complex indicate mainly northwards emplacement

Geochemical evidence used to test alternative plate tectonic models for pre-Upper Jurassic (Palaeotethyan) units in the Central Pontides, N Turkey

Ease-west trending, inferred Palaeotethyan subduction-accretion complexes form much of the pre-Upper Jurassic basement of the Central Pontides. A number of specific tectonic units are present that include variably metamorphosed basic igneous rocks, as dismembered ophiolites. coherent stratigraphical successions, ordered thrust sheets, and blocks in melange. About 200 samples from a c. 200 km long N-S traverse were analysed by a high-precision X-ray fluorescence technique, supplemented by electron microprobe analysis. The basic igneous rocks have not experienced significant chemical mobility after formation and are thus useful for discrimination of tectonic settings of formation using well known binary and ternary plots and mid-ocean ridge basalt (MORB)-normalized 'spidergrams'. From N to S the main results are that basalts of the Kure Complex were erupted in a subduction-influenced tectonic setting, favouring interpretation as a dismembered ophiolite formed in a marginal basin along the S Eurasian margin. Thick, mainly volcanic, successions of the Cangaldag Complex show a subduction-influenced calc-alkaline composition, above a basement varying from MORB/island are tholeiite type to high-Mg (boninitic) type, with an inferred origin as an oceanic are above forearc basement. Meta-basites from the adjacent Bayam Melange indicate a supra-subduction zone origin, possibly as a dismembered forearc ophiolite (possibly related to the Cangaldag). A large overriding thrust sheet of mainly ultramafic rocks, the Elekdag, is interpreted as a supra-subduction zone ophiolite, based on depleted chrome spinel composition (analysed by microprobe). Eclogitic blocks in structurally underlying melange are of mid-ocean ridge (MOR) type. Further south, a northward-dipping, inferred subduction-accretion complex, the Domuzdag-Saraycikdag Complex, includes MOR-type meta-basites in the north, and more 'enriched' compositions in the south, of within-plate origin. Lastly, the geochemistry of meta-basites in the south, within the Kargi Complex. suggests a plume-MOR to within-plate type eruptive setting, without continental crustal influence

Formation of the Late Palaeozoic Konya Complex and comparable units in southern Turkey by subduction-accretion processes: Implications for the tectonic development of Tethys in the Eastern Mediterranean region

The southern margin of Eurasia, from the Balkan region eastwards, is widely envisaged as an active continental margin related to northward subduction, at least during Late Carboniferous-Early Cenozoic time. By contrast. the Late Palaeozoic setting of the southern (Gondwana) margin was previously interpreted as an intra-continental marginal basin related to southward subduction beneath the northern margin of Gondwana, or as part of a forearc complex (e.g. forearc basin) related to northward subduction beneath Eurasia. Palaeotethyan evolution is recorded in the Konya Complex (new name), an assemblage of Palaeozoic (Silurian-Carboniferous) meta-sedimentary and meta-igneous rocks that is exposed beneath metamorphosed Upper Permian-Mesozoic shelf-type sediments in central southern Turkey to the north of the Tauride Mountains. The Konya Complex is dominated by large thrust slices of mainly Devonian shallow-water platform carbonates (Bozdag unit). There is also a melange that is made up of lenticular sheets and blocks of mainly Lower Carboniferous shallow-water limestones, Silurian-Devonian black chert (lydite), pelagic limestones and rare blocks of mid-ocean ridge-type and within plate-type basaltic rocks. The blocks are set in a mainly terrigenous-derived siliciclastic matrix, locally including siliceous tuff. The matrix is interpreted as mainly deep-water turbidites and debris-flow deposits. An overlying, intact volcanic-sedimentary sequence includes chemically enriched extrusives (e.g. trachyandesites) that also exhibit a negative Nb anomaly, suggesting a subduction influence. Dykes crosscutting the carbonate platform units are relatively depleted and also show a subduction influence. Shallow-marine carbonates and terrigenous quartzose sediments of mainly Late Permian age are exposed above the Konya Complex in the west of the area. In contrast, Triassic non-marine, to shallow-marine, clastic sediments unconformably overlie the Konya Complex in the east of the area. Both the Permian and the Triassic sediments pass upwards without a break into a Middle Triassic-Upper Cretaceous platform carbonate succession, which was overthrust by accretionary melange and ophiolites during latest Cretaceous time. The Mesozoic carbonate platform and the underlying Konya Complex experienced polyphase deformation and partial high-temperature/low-pressure metamorphism related to Alpine (Late Cretaceous-Early Cenozoic) closure of Tethys. Comparison of Tauride and Pontide units suggests the deformed Konya Complex Upper Palaeozoic carbonate platform has Gondwanan affinities. We also compare the Complex with other Upper Palaeozoic units in southern Turkey, including the Tekedere unit (Lycian Nappes) and the Karaburun and Chios melanges. All of these units are interpreted to have formed by subduction/accretion processes. Alternative possible settings involve northward subduction, southward subduction, or terrane displacement (strike-slip). Northward subduction beneath Eurasia requires collisional assembly with Gondwana, possibly during the latest Triassic "Cimmerian orogeny" for which there is little evidence. A southward-dipping subduction zone would need to be located some distance outboard of the Gondwana margin as there is little evidence of Upper Palaeozoic arc magmatism in the Tauride platform. Emplacement as an exotic terrane, probably derived from further west in the Balkan region is also possible. (C) 2008 Elsevier B.V. All rights reserved

Role of tectonic-sedimentary melange and Permian-Triassic cover units, central southern Turkey in Tethyan continental margin evolution

Melanges play a key role in the interpretation of orogenic belts, including those that have experienced deformation and metamorphism during continental collision. This is exemplified by a Palaeozoic tectonic-sedimentary melange (part of the Konya complex) that is exposed beneath a regionally metamorphosed carbonate platform near the city of Konya in central Anatolia. The Konya complex as a whole comprises three units: a dismembered, latest Silurian-Early Carboniferous carbonate platform, a Carboniferous melange made up of sedimentary and igneous blocks in a sedimentary matrix (also known as the Hallo Group or Sizma Group), and an overlying Volcanic-sedimentary Unit (earliest Permian?). The Palaeozoic carbonates accumulated on a subsiding carbonate platform that bordered the northern margin of Gondwana, perhaps as an off-margin unit. The matrix of the melange was mainly deposited as turbidites, debris flows and background terrigenous muds. Petrographic evidence shows that the clastic sediments were mostly derived from granitic and psammitic/pelitic metamorphic rocks, typical of upper continental crust. Both extension- and contraction-related origins of the melange can be considered. However, we interpret the melange as a Carboniferous subduction complex that formed along the northern margin of Gondwana, related to partial closure of Palaeotethys. Blocks and slices of Upper Palaeozoic radiolarian chert, basic igneous rocks and shallow-water carbonates were accreted and locally reworked by gravity processes. Large (up to km-sized) blocks and slices of shallow-water limestone were emplaced in response to collision of the Palaeozoic Carbonate Platform with the subduction zone. The overlying Volcanic-sedimentary Unit (earliest Permian?) comprises alkaline lava flows, interbedded with volcaniclastic debris flows and turbidites, volcanogenic shales and tuff. The complex as a whole is overlain by shallow-water, mixed carbonate-siliciclastic sediments of mainly Late Permian age that accumulated on a regional-scale shelf adjacent to Gondwana. Successions pass transitionally into Lower Triassic rift-related shallow-water carbonates and terrigenous sandstones in the southwest of the area. In contrast, Triassic sediments in the southeast overlie the melange unconformably and pass upwards from non-marine clastic sediments into shallow-marine calcareous sediments of Mid-Triassic age, marking the base of a regional Mesozoic carbonate platform. During the latest Cretaceous-Early Cenozoic the entire assemblage subducted northwards and underwent high pressure/low temperature metamorphism and polyphase folding as a part of the regional Anatolide unit. (C) 2010 Published by Elsevier Ltd

Late Palaeozoic-Early Cenozoic tectonic development of the Eastern Pontides (Artvin area), Turkey: stages of closure of Tethys along the southern margin of Eurasia

Early Carboniferous-Eocene units exposed in the Arvin area document the development of the southerly, active continental margin of Eurasia. The oldest rocks exposed in the area are Early Carboniferous granites that regionally intrude schists and gneisses. The continental terrane rifted along the entire length of the Pontides (> 1000 km east-west) during the Early-Middle Jurassic. Subsidence of the rift basin in the Artvin area was accompanied by terrigenous debris flows, turbidites and deep-sea radiolarian muds, and was associated with local extrusion of chemically 'enriched' basalts. Swarms of subduction-influenced basic, intermediate, to locally silicic dykes, intruded high-grade metamorphic basement within the rift. A basement horst within the rift was covered by condensed pink ammonite-bearing pelagic facies. Large volumes of subduction-influenced basalts erupted during the later stages of extensional basin development (Mid-Jurassic), associated with volcaniclastic sedimentation. The Artvin Basin is interpreted as a supra-subduction rift associated with incipient arc magmatism. The basin was stratigraphically inverted in response to Late Middle Jurassic 'Neo-Cimmerian' deformation. It was then partially eroded and covered by Upper Jurassic continental, to shallow-marine sediments, together with localized eruption of 'enriched' (non-subduction-influenced) basalts. The margin collapsed during the Late Jurassic-Early Cretaceous, initiating deposition of pelagic carbonates and mixed terrigenous, biogenic and volcaniclastic gravity flows. Subduction during the Late Cretaceous then constructed the east Pontide magmatic arc and a thick volcaniclastic fore-arc apron to the south. Supra-subduction-type ophiolites and accretionary melange formed within Neotethys to the south during the Late Cretaceous and were emplaced regionally northwards onto the leading edge of the Pontide active continental margin during the latest Cretaceous. Continental collision during the Mid-Eocene telescoped the distal part of the active margin which was emplaced northwards onto the east Pontide continental basement. The geological evolution of Artvin area correlates with the Pontides further west and with the southern and northern Transcaucasus to the east. Our favoured tectonic model involves long-lived, episodic, northward subduction of Tethys. Finally, there is no evidence of 'Palaeotethyan' ophiolites in the eastern Pontides region