The Opening Time Of The Black Sea And The Evolution Of The Eastern Pontides Arc During Jurassic

The Black Sea is the largest marginal sea preserved in Earth.However,the time of opening and tectonic development of the Black Sea remain unresolved issues.Two main hypothesis have been proposed,including(1)the Black Sea is a narrow back-arc basin to the Pontides arc;(2)the Black Sea is a relict of the Paleotethys.The Eastern Pontides arc bounding the Black Sea southern shore systemically preserve the Mesozoic-Cenozoic magmatic,sedimentary,and structural records,which can provide important constraints on the above issues.This thesis discusses the generation and evolution of the Black Sea will be accroding to the studies on the Middle Jurassic-Lower Ecoene detrital zircons and the Jurassic igneous rocks in the Eastern Pontides.Determing the relationhip between the Black Sea and the Plaeotethys,namely resolving the time of opening of the Black Sea,depends on the geological records that indicate an exchange of detritus between its northern and southern margins or not during the Mesozoic-early Cenozoic.This study presents new detrital modal analysis and detrital zircon U-Pb age analysis,the detrital compositions of the Middle Jurassic-Lower Eocene sedimentary rocks in the Samsun,Gumushane,and Yusufeli basins,from the Eastern Pontides,Turkey.The sedimentary rocks in the Eastern Pontides were locally sourced from the recycle Gondwana affinity crustal basement and associated Mesozoic-Cenozoic igneous rocks.By comparing the detrital ages of the sedimentary samples from the Eastern Pontides and peripheral terranes,this study identifies distinct detrital zircon age spectra and chemical compositions between the northern and southern Black Sea terranes.These differences enable us to propose that there should have no exchange of detritus across the Black Sea and to infer a paleo-width of the Black Sea during the Mesozoic-early Cenozoic.Our data,in combination with regional structural,paleomagnetic,and seismic data from the circum-Black Sea region,and analogy with modern turbidite systems,suggest that the Eastern Black Sea was～500-1000 km wide in the Middle Jurassic-Early Eocene.We present new field and petrographic observations combined with in-situ zircon U-Pb and Hf isotopic data,as well as whole-rock and mineral geochemistry from the Yusufeli and Camlikaya batholiths.The Yusufeli batholith is composed of(cumulate)gabbro,(cumulate)hornblende gabbro,diorite,quartz diorite,and tonalite intrusions and(quartz)dioritic and rhyolitic dykes.The Camlikaya batholith consists of quartz diorite,tonalite,and granodiorite intrusions and quartz dioritic dykes.A total of 21 U-Pb dating ages indicate the intrusive rocks from the Yusufeli and Camlikaya batholiths were emplaced at 179-170 Ma and 151-147 Ma,respectively.The Al-in-hornblende geobarometor reveals that the magmas of two batholiths crystallized at upper crustal levels,～140-260 MPa(～4-9 km).The petro█arth element modeling collectively suggest that the Yusufeli batholith was dominated by plagioclase and clinopyroxene fractionation with earlier plagioclase crystallization than amphibole,whereas the Camlikaya batholith was dominated by the fractionation of amphibole accompanied by co-crystallization of plagioclase.Their distinct liquid lines of descent(LLD)and thermodynamic modeling results derived using the Rhyolite-MELTS software all suggest that the Yusufeli batholith is derived from damp(1-2 wt.%H2O)parental magma formed dominantly by decompression melting of mantle wedge in a back-arc setting.In contrast,the parental magma of the Camlikaya batholith is significantly wet(>2 wt.%H2O)and was formed through flux melting of main arc mantle wedge.Combined with the back-arc basin-related Jurassic sedimentary and structural records previously determined in the Southern Zone of the Eastern Pontides,the spatial distribution of Jurassic intrusive rocks can be explained by the southward subduction of the Paleotethys oceanic lithosphere in the Early to Late Jurassic.