Spatial migration of magmatic activity within the Caucasian segment of the Alpine belt in the early neogene under the conditions of geotectonic setting change: Isotope-geochronological data

Abstract

The Miocene epoch is the most important, possiblykey time range in the Cenozoic history of the Eurasian–Arabian collisional zone (EACZ), in which theNeotethys oceanic basin was finally closed, subduction under the folded structures of the Pontic Mountains and Lesser Caucasis was finished, and the “rigid”stage of convergence between the closed lithosphericplates and microplates and, consequently, uplift andthe formation of grandiose mountain chains and largeplateau of the Caucasus segment within the Alpine–Himalayan folded belt (AHB) started. Change in theregional geotectonic setting resulted in the evolutionof the character of volcanic activity in this part of theEarth from supersubduction magmatism of the continental margin to the typical continental intraplatemagmatism.Without detailed analysis of all (more than 10) published petrological models suggested in recent decadesfor explanation of the reasons for the origin of intraplate magmatism within Eastern Anatolia and theCaucasus, we should note that their totality providestwo basic assumptions.(1) The Late Cenozoic volcanic activity on the territory of the Caucasian segment results from activity ofthe asthenospheric and subasthenospheric mantle.Spreading of the Red Sea plume in the northern direction along the zone of the Levantian transform faultunder the EACZ resulted in the formation of a “hotfield” in the mantle, and the geotectonic setting ofconvergence of continental plates does not play a keyrole in the origin and evolution of young volcanismthere [1, 2, and others].(2) Continental collision is the main reason forintraplate volcanic activity on the territory of the considered region in the Neogene–Quaternary period.Interaction between the large plate and a number ofmicroplates and terranes resulted in steepening, breakoff, and melting of a slab of the oceanic part of theArabian plate (delamination of the lithosphere underthe region, decompression melting in the lower part ofthe crust, etc.), which was previously subducted underthe Pontic Mountains and Lesser Caucasus. In turn,this resulted in passive uplift of the asthenosphericmantle under the Caucasian segment of the AHB andthe origin of magmatism with geochemical characteristics of the intraplate type [3 and others].Undoubtedly, establishment of precise age limitsfor the period of change in the type of magmatic activity in the Caucasus and Eastern Anatolia occurringagainst the background of the gradual evolution of thegeodynamic regime and geotectonic setting in thisregion is one of the fundamental tasks during study ofyoung continental volcanism of the Earth. The previous longterm investigations demonstrated that,according to the geochemical and isotope–geochemical characteristics, Late Miocene magmatism in theGreater Caucasus (8.5–6.0 m. y. ago), Lesser Caucasus (9–5 m. y. ago), and Eastern Anatolia (11–5 m. y.ago) were related to the intraplate type [3–5 and others], whereas the Early Oligocene (35–28 m. y. ago)volcanic activity of the Lesser Caucasian–Pontiac arc(LCPA) was undoubtedly developed in the regime ofthe continental margin above the subduction zone(Fig. 1). In this relation it is necessary to mention thatmarine carbonate rocks of the Burdigal stage are abundant in the central part of Eastern Anatolia. This provides evidence for the existence of the Neotethis deepwater basin there until the middle of the Neogene(16–15 Ma) [7].The period from the end of the Oligocene until thesecond half of the Miocene, thus, covering the transformation of magmatism type in the Caucasus region,was usually considered previously as amagmatic or asa time period with a small scale of volcanic activity[6 and others]. Several years ago we obtained thefirst evidence for the fact that in the Middle Miocene