Petrogenesis of late cenozoic collision volcanism in western Anatolia, Turkey

Western Anatolia exhibits a record of almost all stages of a collision event and its related magmatic processes. Following an Eocene continent-arc collision, Western Anatolia region experienced a complete cycle of thickening and orogenic collapse. The early stage of collision- related volcanism, which was most evident during the Early Miocene (<21 Ma), produced a considerable volume of lavas and pyroclastic deposits covering a broad compositional range from basaltic andesites to rhyolites. The volcanic activity continued into the Middle Miocene with a gradual change in eruptive style and rock compositions. The Middle Miocene activity, formed in relation to localised extensional basins and was dominated by lava flows and dykes of basalts to andesites composition. Both the Early-Middle Miocene rocks have calc-alkaline and shoshonitic character. The late stage volcanism, from 11.0 to 8.3 Ma, was marked by alkali basalts and basanites erupted along the localised extensional zones. The Early-Middle Miocene volcanic rocks exhibit enrichment in LILE and LREE relative to the HFSE (characterised by negative Nb and Ta anomalies) and are characterised by high (^87)Sr/(^86)Sr and low (^143)Nd/(^144)Nd (-ɛno) ratios. These characteristics indicate a mantle lithospheric source region carrying a subduction component inherited from a pre-collision subduction event. Perturbation of this subduction-metasomatised lithosphere by delamination of the thermal boundary layer is the likely mechanism for the initiation of the post-collision magmatism. Trace elements systematics suggest that the Early-Middle Miocene series underwent a hydrous crystallisation (dominated by pargasitic amphibole) in deep crustal magma chambers. Subsequent crystallisation in shallower magma chambers follows two different trends: (1) anhydrous (pyroxene + plagioclase-dominated; and (2) hydrous (edenitic amphibole + plagioclase + pyroxene dominated).Trace element and isotope modelling shows that the Early-Middle Miocene rocks have been affected by assimilation combined with fractional crystallisation processes, and that the effects of assimilation decrease gradually from the Early Miocene into the Middle Miocene. This indicates a progressive crustal thinning related to the extensional tectonics that prevailed from the latest Early Miocene onwards. In contrast to the Early-Middle Miocene rocks, the Late Miocene alkaline rocks are characterised by low (^87)Sr/(^86)Sr and high (^143)Nd/(^144)Nd (+ɛnd) ratios and have OIB-type like trace element patterns characterised by enrichment in LILE, HFSE and L-MREE, and a slight depletion in HREE, relative to the N-MORB compositions. REE inversion modelling indicates that these rocks formed by partial melting (with degrees of ~2 to -10%) of a spinel + garnet Iherzolite source. Trace element and isotopic systematics are consistent with decompression melting of an enriched mantle asthenospheric source

Lithostratigraphy and petrology of Lachman Crags and Cape Lachman lava-fed deltas, Ulu Peninsula, James Ross Island, north-eastern Antarctic Peninsula: Preliminary results

This paper presents the preliminary results regarding the lithostratigraphy, petrography and petrology of James Ross Island Volcanic Group dominating the Lachman Crags and Cape Lachman lava-fed deltas in the Ulu Peninsula, James Ross Island north-eastern Antarctic Peninsula. Studied lava-fed deltas were produced via Late Miocene to Pleistocene sub-marine and sub-glacial volcanism and made up four main lithofacies: a- bottomset pillow lavas, peperites and associated volcanoclastic/siliciclastic deposits; b- foreset-bedded hyaloclastite breccias; c- intrusions (feeder dykes, sills, and plugs) and d- topset subaerial lavas. Collectively these lithofacies record the transition from an effusive subaqueous to an effusive subaerial eruption environment. All lava samples and dykes from bottomset, foreset and topset lava-fed delta associations are olivine-phyric alkali basalts and are mineralogically and geochemically homogeneous. These eruptive products display significant enrichments in alkali contents and have ocean island basalt (OIB)-type, intra-plate geochemical signatures characterized by enrichments in all highly to moderately incompatible trace elements relative to basaltic rocks from ocean ridge settings. Volcanic products from a number of different eruptive periods display limited variations in major and trace element relative abundances, indicating derivation from a relatively homogeneous mantle source. The results of quantitative modelling of geochemical data is consistent with the view that the primary melts from which these mafic alkaline rocks were originated are the products of relatively small degrees (~3-7%) of partial melting of a volatile-bearing, metasomatized mantle source. The magmatism is likely the result of extension-driven mantle upwelling

Illite occurrences related to volcanic-hosted hydrothermal mineralization in the biga peninsula, NW Turkey: Implications for the age and origin of fluids

A different approach to investigate the origin of fluids, temperature conditions, age of hydrothermal activity of mineralization in the Biga Peninsula, (Koru, Tesbihdere and Kumarlar) employed mineralogical (illite Kübler index, b cell dimension, polytype) and geochemical (major, trace/REE, O-H stable isotope and Rb/Sr dating) methods. The Kübler Index (KI) values of illites indicate different temperature conditions, such as low temperature (high-grade diagenesis) for Koru deposit, and high temperature (anchizone) for the Tesbihdere and Kumarlar deposits. The textural, mineralogical and geochemical data from illites show that these have potential for estimating the age of hydrothermal activity and fluid characteristics. Both mineralogical (high grade diagenetic to anchizonal KI, 1 M polytype, low d060 values) and geochemical (similar major and trace element composition to host-rocks, low octahedral Mg + Fe contents, oxygen and hydrogen isotope composition) data are compatible with commonly known hydrothermal illites. Stable isotope data of illites are well matched to similar data from fluid inclusions, which indicate mainly magmatic fluids. The Rb/Sr age (22.4 ± 2.3 Ma: latest Oligocene and lowest Miocene) of the illites coincides with plutonic intrusions that are the main instigators of hydrothermal activities related to the extensional tectonic regime in the Biga Peninsula. The mineralogical and geochemical data of illites have some important advantages with respect to the use of fluid inclusions in determining δD of hydrothermal fluids thereby leading to better understanding ore-forming hydrothermal condition

High-K volcanism in the Afyon region, western Turkey: from Si-oversaturated to Si-undersaturated volcanism

Volcanic rocks of the Afyon province (eastern part of western Anatolia) make up a multistage potassic and ultrapotassic alkaline series dated from 14 to 12 Ma. The early-stage Si-oversaturated volcanic rocks around the Afyon city and further southward are trachyandesitic volcanic activity (14.23 ± 0.09 Ma). Late-stage Si-undersaturated volcanism in the southernmost part of the Afyon volcanic province took place in three episodes inferred from their stratigraphic relationships and ages. Melilite– leucitites (11.50 ± 0.03 Ma), spotted rachyandesites, tephryphonolites and lamproites (11.91 ± 0.13 Ma) formed in the first episode; trachyandesites in the second episode and finally phonotephrites, phonolite, basaltic trachyandesites and nosean-bearing trachyandesites during the last episode. The parameter Q [normative q-(ne + lc + kls + ol)] of western Anatolia volcanism clearly decreased southward with time becoming zero in the time interval 10–15 Ma. The magmatism experienced a sudden change in the extent of Si saturation after 14 Ma, during late-stage volcanic activity of Afyon volcanic province at around 12 Ma, though there was some coexistence of Si-oversaturated and Si-undersaturated magmas during the whole life of Afyon volcanic province

Constraints on the composition and thermal structure of the upper mantle beneath NW Turkey: Evidence from mantle xenoliths and alkali primary melts

Ultramafic xenoliths entrained in the Late Miocene alkali basalts and basanites from NW Turkey include refractory spinel-harzburgites and dunites accompanied by subordinate amount of spinel-lherzolites. Whole-rock major element and mineral chemical characteristics indicate that the xenoliths are mostly the solid residues of varying degrees of partial melting (similar to 3 to similar to 15%), but some have mineral chemical signatures reflecting processes of melt/rock interaction. The mineral compositions of the undisturbed xenoliths have been used to estimate temperatures and, where possible, pressures of equilibration, and to construct a geothermal framework to interpret the thermal structure of the upper mantle beneath the region at the time of basaltic melt extraction. Most of the peridotite xenoliths have chemically homogeneous mineral phases reflecting ambient equilibrium pressure-temperature conditions. The combination of results from Ca-in-orthopyroxene geothermometer and Ca-in-olivine geobarometer yields pressure-temperature estimates for spinel-peridotites that fall in experimentally determined spinel-lherzolite field in CaO-Fe-M-O-Al2O3-SiO2-Cr2O3 (CFMASCr) system. These pressure-temperature data sets have been used to construct the Late Cenozoic geotherm of NW Turkey, which defines a depth-temperature array from about 1.4 GPa at 810 degrees C to 2.1 GPa at 1180 degrees C. Extrapolation of this array to the adiabatic upwelling curve of normal temperature asthenospheric mantle indicates an apparent lithospheric thickness of similar to 80 km. The geothermal gradient of NW Turkey is only slightly higher than that of the conventional conductive models (similar to 90 MW m(-2)) and may be interpreted as being a consequence of the thermal perturbation by the heat input into the relatively thin mantle lithosphere via the passage and emplacement of magma(s). In contrast, the calculations, based on the melt products of experimental phase equilibria, show that the compositions of the host magmas precipitating the most magnesian olivine (with 88.5 to 91.4% forsterite; Mg2SiO4) phenocrysts of alkaline melts reveal significantly higher anhydrous liquidus temperatures that range from about 1290 to 1410 degrees C. The results are inconsistent with melt generation by either deep-seated thermal anomalies associated with mantle plumes (T-p > 1450 degrees C) or melting of thermally perturbed (wet) mantle lithosphere, but are consistent with the adiabatic upwelling of normal temperature (T-p similar to 1300 degrees C) mantle asthenosphere probably along the lithospheric scale major shear zones

Geochemical characteristics of mafic lavas from the Neotethyan ophiolites in western Turkey: implications for heterogeneous source contribution during variable stages of ocean crust generation

The Late Triassic to Late Cretaceous age mafic lavas from the Neotethyan suture zone ophiolites in western Turkey exhibit a wide diversity of geochemical signatures, indicating derivation from extremely heterogeneous mantle sources. The rocks as a whole can be divided into three broad subdivisions based on their bulk-rock geochemical characteristics: (1) mid-ocean ridge basalts (MORB) that range in composition from light rare earth element (LREE)-depleted varieties (N-MORB; (La/Sm)(N) 1); (2) the ocean island basalt (OIB)-type alkaline volcanic rocks with significant enrichment in LILE, HFSE and L-MREE, and a slight depletion in HREE, relative to normal mid-ocean ridge basalts (N-MORB); and (3) the supra-subduction zone (SSZ)-type tholeiites originated from arc mantle sources that are characterized by selective enrichments in fluid-soluble large ion lithophile elements (LILE) and LREE relative to the high field strength elements (HFSE). The formation of MORB tholeiites with variable enrichments and depletions in incompatible trace elements is probably related to the processes of crust generation along an oceanic spreading system, and the observed MORB-OIB associations can be modelled by heterogeneous source contribution and mixing of melts from chemically discrete sources from sub-lithospheric reservoirs. Evaluation of trace element systematics shows that the inferred heterogeneities within the mantle source regions are likely to have originated from continuous processes of formation and destruction of enriched mantle domains by long-term plate recycling, convective mixing and melt extraction. The origin of SSZ-type tholeiites with back-arc basin affinities, on the other hand, can be attributed to the later intra-oceanic subduction and plate convergence which led to the generation of supra-subduction -type oceanic crust as a consequence of imparting a certain extent of subduction component into the mantle melting region. Mixing of melts from a multiply depleted mantle source, which subsequently received variable re-enrichment with a subduction component, is suggested to explain the generation of supra-subduction-type oceanic crust. The geodynamic setting in which much of the SSZ-type ophiolitic extrusive rocks from western Turkey were generated can be described as an arc-basin system that is characterized by an oceanic lithosphere generation most probably associated with melting of mantle material along a supra-subduction-type spreading centre

Investigation of helium isotope variations of alkaline volcanic rocks In Osmaniye Region Turkey

The noble gases, in particular helium isotopes, provide valuable information on the mantle source of recentbasaltic volcanism, especially when linked to trace elements and radiogenic isotopes. Although the alkalinevolcanics in the Kütahya region have been studied in detail by several works by means of trace element variations,radiogenic dating and isotope systematics, noble gas isotope compositions that could significantly contribute todynamics of volcanism have not been investigated yet. In this study which aims to fill the deficiency of such data,helium isotope compositions of olivine-basalts in Kütahya region are measured.In this study we report helium isotope compositions of olivine from basalts. In olivine-basalts in alkalinevolcanic rocks from the Kütahya region, western Turkey, are investigated. 3He/4He range from 3.0 to 5.19 Ra.These low helium isotope values are most probably indicative of a lithospheric mantle source. Kütahya alkalinevolcanics are enriched in incompatible elements and show prominent negative Ti, Nd and Ta anomaly. Primitivemantle normalized trace element concentration patterns clearly display continental crustal contamination.Chemical compositions and helium isotope systematics imply contribution of a lithospheric mantle source andcontamination of continental crust in the genesis of the Kütahya alkaline lava