Geochronology and geochemistry of Eocene-aged volcanic rocks around the Bafra (Samsun, N Turkey) area: Constraints for the interaction of lithospheric mantle and crustal melts

International audience40Ar-39Ar age, whole-rock chemical and Sr-Nd isotope data are presented for the post-collisional, Eocene (51.3-44.1 Ma)-aged volcanic rocks from the Bafra (Samsun) area in the western part of the Eastern Pontides (N Turkey) aiming to unravel their sources and evolutionary history. The studied Eocene volcanic rocks can be divided into two groups: analcime-bearing (tephritic lava flows and dykes) and analcime-free (basaltic to trachytic lava flows and basaltic dykes). The analcime-bearing volcanic rocks have a fine-grained porphyritic texture with clinopyroxene phenocrysts, whereas analcime-free volcanic rocks show a variety of textures including hyalo-microlitic microgranular porphyritic, intersertal, trachytic, fluidal and glomeroporphyritic. The volcanic rocks also show evidence of mineral-melt disequilibrium textures such as sieved, rounded and corroded plagioclases, partially melted and dissolved clinopyroxenes and poikilitic texture. Petrochemically, the parental magmas of the volcanic rocks evolved from alkaline to calc-alkaline lava suites and include high-K and shoshonitic compositions. They display enrichments in light rare earth and large ion lithophile elements such as Sr, K and Rb, as well as depletions in high field strength elements such as Nb, Ta, Zr and Ti, resembling subduction-related magmas. The analcime-bearing and -free volcanic rocks share similar incompatible element ratios and chondrite-normalised rare rearth element patterns, indicating that they originated from similar sources. They also have relatively low to moderate initial 87Sr/86Sr (0.7042 to 0.7051), high positive εNd(t) values (+ 0.20 to + 3.32), and depleted mantle Nd model ages (TDM1 = 0.63-0.93 Ga, TDM2 = 0.58–0.84 Ga). The bulk-rock chemical and Sr-Nd isotope features as well as the high Rb/Y and Th/Zr, but low Nb/Zr and Nb/Y ratios, indicate that the volcanic rocks were derived from a lithospheric mantle source that had been metasomatised by slab-derived fluids. Trace element modelling suggests that the parental magma(s) of the volcanic rocks represent mixtures of melts derived by low-degree (~ 5–10 %) partial melting of spinel-lherzolite (40–85%) and garnet-lherzolite (15–60 %) mantle sources. Sr-Nd isotopic modelling also suggests that a 25–35% lower crustal component was added in the parental magmas; AFC modelling additionally indicates minor upper crustal contamination during the evolution of the volcanic rocks. In conclusion, integration of the geochemical, petrologic and isotopic data with regional geology suggests that the analcime-bearing and -free volcanic rocks evolved from parental magma(s) derived from melts of a subcontinental lithospheric mantle and lower crustal sources

Evolution of K-rich magmas derived from a net veined lithospheric mantle in an ongoing extensional setting: Geochronology and geochemistry of Eocene and Miocene volcanic rocks from Eastern Pontides (Turkey)

International audienceThe Eocene and Miocene volcanic rocks between the cities of Trabzon and Giresun in the Eastern Pontides (NE Turkey) erupted as mildly and moderately alkaline magmas ranging from silica-saturated to silica-undersaturated types. 40Ar-39Ar dating and petrochemical data reveal that the studied volcanic rocks are discriminated in two: Lutetian (Middle Eocene) mildly alkaline, (basaltic rocks: 45.31 ± 0.18 to 43.86 ± 0.19 Ma; trachytic rocks: 44.87 ± 0.22 to 41.32 ± 0.12 Ma), and Messinian (Late Miocene) moderately alkaline volcanic rocks (tephrytic rocks: 6.05 ± 0.06 and 5.65 ± 0.06 Ma). The trace and the rare earth element systematic, characterised by moderate light earth element (LREE)/heavy rare earth element (HREE) ratios in the Eocene basaltic and trachytic rocks, high LREE/HREE ratios in the Miocene tephrytic rocks, and different degrees of depletion in Nb, Ta, Ti coupled with high Th/Yb ratios, show that the parental magmas of the volcanic rocks were derived from mantle sources previously enriched by slab-derived fluids and subducted sediments. The Sr, Nd and Pb isotopic composition of the Eocene and Miocene volcanic rocks support the presence of subduction-modified subcontinental lithospheric mantle. During the magma ascent in the crust, parental magmas of both the Eocene and Miocene volcanic rocks were mostly affected by fractional crystallisation rather than assimilation coupled with fractional crystallisation and mixing. The silica-undersaturated character of the Miocene tephrytic rocks could be attributed to assimilation of carbonate rocks within shallow-level magma chambers. The parental magmas of the Eocene volcanic rocks resulted from a relatively high melting degree of a net veined mantle and surrounding peridotites in the spinel stability field due to an increase in temperature, resulting from asthenospheric upwelling related to the extension of lithosphere subsequent to delamination. The parental magmas for the Miocene volcanic rocks resulted from a relatively low melting degree of a net veined mantle domain previously modified by metasomatic melts derived from a garnet peridotite source after decompression due to extensional tectonics, combined with strike-slip movement at a regional scale related to ongoing delamination

Gölköy yöresi (Ordu, KD Türkiye) Eosen yaşlı I-tipi şoşonitik plütonların mineral kimyası, tüm-kayaç jeokimyası ve petrolojisi

Doğu Pontidler Orojenik Kuşağı’nda (KD Türkiye), farklı boyut ve bileşimlere sahip Eosen yaşlı ortaç-felsik plütonlar yaygın olarak yüzeyleme vermektedir. Bunlardan, Eriko Tepe ve Göl Tepe Plütonları yaklaşık KB-GD ve D-B uzanımlı olup, Üst Kretase ve /veya Eosen volkanik ve sedimanter kayaçlar içerisine yerleşmiştir. Petrografik olarak; incelenen plütonlar, ince-orta taneli monzonit, monzodiyorit ve yer yer kuvars monzonit bileşimlidir. İncelenen Plütonlara ait kayaçlar, plajiyoklas (An35-67), K-feldispat (Or61-96), kuvars, klinopiroksen (Wo28-49En35-51Fs10-25), biyotit (Mg#: 0.53-0.73) ± hornblend (Mg#: 0.65-0.82), Fe-Ti oksit içermekte olup, monzonitik, poikilitik, pertitik, nadiren antirapakivi ve grafik doku gösterirler. Mineral termobarometre hesaplamaları, plütonların orta-sığ kabuk derinliklerindeki P-T koşullarında kristallendiğini ifade etmektedir. Petrokimyasal olarak; incelenen monzonitik plütonlar çarpışma sonrası, I-tipi, metalümin (A/CNK=0.76-0.93) ve şoşonitik karakterlidirler. Tüm-kayaç ana oksit ve iz element değişimleri, monzonitik plütonların gelişiminde fraksiyonel kristallenmenin önemli rol oynadığına işaret etmektedir. İncelenen plütonların ilksel mantoya normalize iz element diyagramları, birbirine benzer büyük iyon yarıçaplı litofil element, Th, Ce zenginleşmesi ve negatif Nb ve Ti anomalileri göstermektedir. Ayrıca, kondrite normalize nadir toprak element dağılımları, orta derecede zenginleşmiş konkav şekilli (LaN/LuN=9.3-12.6) ve negatif Eu anomalisi (EuN/Eu*=0.69-0.84) göstermekte olup, plütonların gelişimlerinde plajiyoklas ve klinopiroksen ± hornblend ayrımlaşmasını yansıtmaktadır. Elde edilen tüm jeokimyasal verilere göre, incelenen monzonitik plütonların çarpışma sonrası ortamda oluştukları ve zenginleşmiş litosferik manto ergiyiklerinden türeyen ana magmalardan itibaren geliştikleri ileri sürülebilir

Stratigraphy, mineralogy and depositional environment of the evaporite unit in the Askale (Erzurum) sub-basin, Eastern Anatolia (Turkey)

The study area is situated in the Askale sub-basin where the Early-Middle Miocene aged Askale Formation was deposited in a shallow marine to lagoonal environment, and consists of interstratifications of clastic sediments, carbonates and evaporites. The successions of the Askale Formation can be divided into four main members interfingering with one another both vertically and laterally, and composed of the sandstone-mudstone-limestone member, the evaporite member, the gravelstone-sandstone-mudstone intercalations and the limestone member. The evaporite unit comprises of secondary gypsum lithofacies formed by hydration of precursor anhydrite, anhydrite, gypsum-bearing limestone and claystone in the form of wedges and lenses. Massive, nodular, nodular-banded, laminated and laminated-banded gpysum lithofacieses in addition to chicken-wire and rare entrolithic structures were described, indicating a sabhka or a shallow water depositional environment. Alabastrine and porphyblastic textures of gypsum were identified within the all lithofacieses with abundant amount of anhydrite relics. Additionally, saponite and illite/smectite, calcite and dolomite, celestite, epsomite were also observed. Successions of the Askale Formation were deposited in stable subtropical climatic conditions within rapidly subsiding sub-basin resulted in conversion of sub-basin to shallow platform and even in lagoon environment. (C) 2015 Elsevier Ltd. All rights reserved

Mineral chemistry and thermobarometry of Eocene monzogabbroic stocks from the Bafra (Samsun) area in Turkey: implications for disequilibrium crystallization and emplacement conditions

<div><p>Monzogabbro stocks including felsic enclaves (monzosyenite) around the Bafra (Samsun) area at the western edge of the Eastern Pontides cut Eocene-aged volcanic and sedimentary units. The monzogabbros contain plagioclase, alkali feldspar, clinopyroxene, olivine, hornblende, biotite, apatite, and iron-titanium oxides, whereas the felsic enclaves contain alkali feldspar, plagioclase, hornblende, biotite, clinopyroxene, and iron-titanium oxides. Mineral chemistry data suggest that magmas experienced hydrous and anhydrous crystallization in deep and shallow crustal magma chambers. Several thermobarometers were used to estimate temperatures of crystallization and emplacement for the mafic and felsic magmas. Clinopyroxene thermobarometry yielded 1100–1232 C and 5.9–8.1 kbar for monzogabbros, and 931–1109 C and 1.8–6.9 kbar for felsic enclaves. Hornblende thermobarometry and oxygen fugacity estimates reveal 739–971°C, 7.0–9.2 kbar and 10^−9.71 for monzogabbros and 681–928°C, 3.0–6.1 kbar and 10^−11.34 for felsic enclaves. Biotite thermobarometry shows elevated oxygen fugacity varying from 10^−18.9–10^−11.07 at 632–904°C and 1.29–1.89 kbar for monzogabbros, to 10^−15.99 –10^−11.82 at 719–873°C and 1.41–1.77 kbar for felsic enclaves. The estimated zircon and apatite saturation temperatures are 504–590°C and 693–730°C for monzogabbros and 765–775°C and 641–690°C for felsic enclaves, respectively. These data imply that several phases in the gabbroic and syenitic magmas did not necessarily crystallize simultaneously and further indicate that the mineral compositions may register intervals of disequilibrium crystallization. Besides, thermobarometry contrasts between monzogabbro and felsic enclave may be partly a consequence of extended interactions between the mafic and felsic magmas by mixing/mingling and diffusion. Additionally, the hot felsic magma was close to liquidus conditions (crystallinity < 30%) when injected into cooler mafic magma (crystallinity > 50%), and thus, the monzogabbro stocks reflect hybrid products from the mingling and incomplete mixing of these two magmas.</p></div

Geochemistry of Miocene evaporites from the Askale (Erzurum, Eastern Turkey) area: constraints for paleo-environment

The Askale sub basin hosts Early Miocene evaporites intercalated with clastic sediments and carbonates. Gypsum and anhydrite rich evaporite samples are characterized by high CaO and SO4 contents, and low Na2O, K2O, MgO, and B contents. The Sr contents are 228 = 13100 ppm in evaporite samples, 169 = 992 ppm in claystone, 181 = 60090 ppm in marl, and 15150 ppm in limestone. All the samples are also characterized by enrichment in light rare earth elements (REE) with La-N/Lu-N = 0.667 4.243 and have variable Ce-N / Ce* (0.823 - 1.353) ratios. Measured Eu-N/Eu* values of the samples display strong and variable negative and positive Eu anomalies. 834SCDT and 818O values of gypsum and anhydrite samples have wide ranges from 21.30 parts per thousand to 25.62 %o, and 11.5 parts per thousand to 19.1 parts per thousand, respectively. Most of these values are heavier than expected Miocene marine gypsum composition and may be resulted from reduction and oxidation reactions of sulfide species in brines. Sr-87/Sr-86 ratios range from 0.707475 (Delta(SW) = -169.8) to 0.708175 (Delta(SW) = -99.8), close to and / or slightly lower than an Early Miocene marine isotopic composition. Petrochemical and isotopic data indicate that the Askale basin evaporites developed in subtropical conditions via multiple marine transgressions onto a shallow platform or lagoonal environment