Volcanic belts as markers of the Mesozoic-Cenozoic active margin of Eurasia

Abstract

From the Triassic to the Present linear belts of calc-alkaline volcanism developed along the southern margin of Eurasia. In the Triassic the calc-alkaline belt can be traced in Turan, Fore-Caucasus, Crimea and Dobrogea. Its development is related to the closure of the Paleo-Tethys, when fragments of Gondwana (Iran, Afghanistan etc.) were approaching Eurasia. Since the Early Jurassic calc-alkaline volcanism has been related to subduction of the oceanic crust of the Mesozoic Tethys. Five stages of calc-alkaline volcanic activity can be distinguished: Early Jurassic, Middle Jurassic-Early Cretaceous, Late Cretaceous, Paleogene and Late Miocene-Quaternary. The configuration of the volcanic belts, the intensity of the volcanism and the chemistry of the erupted products change considerably from stage to stage. Mesozoic belts were most pronounced in the Pontian-Transcaucasian segment of the margin. Starting from the Late Cretaceous, and especially in the Cenozoic, the volcanic belts can be continuously traced from the Balkans to Afghanistan. Most of the belts were actually island arcs formed on the fragments of the Hercynian of Baikalian basement and in this respect similar to the island arc of Japan. Only the “incipient” island arcs of Senonian age, formed close to the passive margin of Gondwana, are the exceptions: they were built on oceanic crust and are geochemically close to the Mariana arc. At certain times (Early Jurassic, Eocene) parallel chains of island arcs existed. In general, the situation resembled the present-day convergent boundary in the Western Pacific. As closure of the Tethys proceeded, island arcs gave way to marginal continental volcanic belts with wide spread development of high-potassium, particularly shoshonitic, series. Peaks of volcanic activity in the Bajocian, Late Cretaceous, Eocene and Late Miocene-Present alternated with quiet periods and corresponded to the times when the convergence rate between Africa and Eurasia increased. The opening of the back-arc basins is reflected in outbursts of basaltic rift-type volcanism. These outbursts occurred in the Toarcian, Aalenian-Bathonian, Late Jurassic, Aptian-Turonian and Early-Middle Eocene.