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Migration of the Upper Cretaceous subduction-related volcanism towards the back-arc basin of the eastern Pontide magmatic arc (NE Turkey)

The Eastern Pontide active continental margin extends along the eastern part of the Black Sea with an E-W trend and comprises the parallel oriented northern, southern and axial zones. Each zone is fault bound and distinguished by distinctive magmatism, lithofacies and stratigraphy. In the northern zone, from the coast of the Black Sea to Torul, the cessation of Liassic bimodal volcanism and accumulation of anoxic carbonates of the Valanginian-Barremian was followed by bimodal volcanism which evolved from a tholeiitic (TH)-calc-alkaline (CA) to high K-CA composition. These volcanics erupted sporadically in asymmetric deep basins until the end of the Senonian. In the southern zone, around Torul and Gumushane, Liassic bimodal volcanism and the accumulation of the Malm-Cenomanian platform carbonate was succeeded by Campanian volcanism, i.e. much later than volcanism in the northern zone. A second epoch of CA-high K-CA andesitic volcanism occurred in the southern zone and is intercalated with a Campanian red pelagic limestone which is also widespread in the northern zone, and known as the marker lithologic unit in the eastern Pontides. South of the southern zone, in the Bayburt-Maden area, the pelagic limestone of the Early Cretaceous and overlying ophiolitic olistostromal melange (which contains Cenomanian MOR, WP and IA basaltic pillow lavas) are overlain by high-K calc-alkaline Campanian andesitic volcanic rocks, implying that the ophiolitic melange was formed in a back-arc environment. The major and trace element geochemistry and REE patterns of the Upper Cretaceous volcanic rocks suggest a variation from hydrous IA to anhydrous back-arc melting conditions on the subduction zone and the migration of arc magmatism laterally from the northern to the southern zones during the Senonian. This change to dehydration melting and the migration of the subduction-related Late Cretaceous volcanism towards the south in the eastern Pontide Magmatic arc, requires a south-dipping subduction polarity during the Senonian. Copyright # 1999 John Wiley & Sons, Ltd.

Insight into magma genesis at convergent plate margins – a case study from the eastern Pontides (NE Turkey)

Convergent plate margins are the most intense areas of granitoid magmatism on Earth. The Eastern Pontide Magmatic Belt in NE Turkey represents a paleo-arc with numerous quartz diorite to syenite intrusions, ranging in age from 142 to 56 Ma and being composed of K-feldspar, plagioclase, quartz, pyroxene, hornblende, biotite, and Fe-Ti oxides. The granitoids exhibit lowto high-K calc-alkaline, metaluminous to slightly peraluminous I-type features and contain abundant mafi c magmatic enclaves (MME). They are characteristically enriched in large ion lithophile elements (LILE) and light rare earth elements (LREE) relative to high fi eld strength elements (HFSE). Chondrite-normalized REE patterns are fractionated (LaN/LuN = 1.49–17.4) with pronounced negative Eu anomalies (Eu/Eu* = 0.46–1.77). Initial Sr/Sr values are between 0.7056 and 0.7079, and eNd(i) values between –5.3 and 1.6. Fractional crystallization, magma mixing/mingling and crustal contamination played an important role during magma evolution. All these characteristics, combined with the low values of K2O/Na2O, Mg-number, ASI and ratios of Al2O3/ (FeO+MgO+TiO2) and (Na2O+K2O)/(FeO+MgO+TiO2), suggest an origin by dehydration melting of mafi c (amphibolitic) or tonalitic lower crustal source rocks.

Sakarya Zonunun Güney Kesiminde Yüzeylenen Geç Kretase Alkalen Volkanitlerinin Jeokimyasal Özelliklerinin Karşılaştırılması

Sakarya Zonunun guney kesiminde Bayburt, Amasya-Gumushacikoy ve Ankara-Kalecik lokasyonlarinda Ust Kretase’nin son donemlerinde benzer stratigrafik seviyelerde yuzeylenen alkalen volkanitler makroskobik olarak iri analsimlesmis losit porfirleri ve iri piroksen porfirleri iceren iki farkli litoloji sunarlar. Bu kayaclar carpisma oncesi yasli okyanus kabugunun yitim acisinin diklesmesi sonucu, metasomatizmaya ugramis flogopitce zengin manto kayaclarinin dusuk dereceli kismi ergimesi sonucu olusan kaynak magmalardan itibaren olusmuslardir. Olusumlarini izleyen surecte alkalen magma, kabuk icerisindeki orta/sig derinliklerdeki magma odalarinda diferansiyasyona ugrayarak, iki farkli litolojide yeryuzune ulasmisdir.