TRIASSIC VOLCANISM IN THE EASTERN FORE-CAUCASUS: EVOLUTION AND GEODYNAMIC INTERPRETATION

Abstract

The aim of this paper is to describe the Triassic regional geodynamic framework of the Eastern Fore-Caucasus area from data on volcanics and related sedimentary deposits. These deposits, only known by drilling, represent the lowermost horizons of a slightly deformed Mesozoic and Cenozoic cover and rest unconformably on folded, mainly Devonian to Carboniferous, series. They are thought to be, for the most part, postcollisional.The sedimentary record of Early to Middle Triassic evolution allows visualisation of uplift events associated with continental basin formation, preceding and following the opening of a deep passively subsiding marine basin. Tectonic instability, indicated by tilted strata, prevailed throughout the whole period. The subsidence maximum, corresponding with a total immersion below sea level, occurred during the Olenekian (Early Triassic). During Middle Triassic times, the area was continuously uplifted and progressively emerged to become strictly continental with a fluvio-lacustrine sedimentation during the Late Triassic.Two periods of intense volcanic activity occurred during Early-Middle and Late Triassic times. The former comprises: (1) subaerial explosive eruptions of rhyolitic magma, the vents of which are presumably located about 50–100 km southwest of the study area; and (2) submarine effusive eruptions of basalts and rhyolites within an actively subsiding trough. The numerous unconformities within the volcano-sedimentary sequence testify to a tectonically active setting. During the Late Triassic, eruptions were subaerial, mainly explosive (nuées ardentes), and rhyo-dacitic. Basaltic and andesitic lava flows and domes were minor. In both eruptive periods, mafic magmas had a calc-alkaline affinity and their diversity would have been the result of partial melting of a heterogeneous metasomatized subcontinental mantle. Several compositional gaps within the acidic group reflect melting of distinct components of the continental crust. Fractional crystallisation or even partial melting trends can be deciphered within mafic and rhyolitic groups.The geodynamic background of Early-Middle Triassic volcanism was one of extension. The precise nature of the extension is unclear but could include active rifting, back-arc extension, or even transtension in a strike-slip fault zone. With their great contribution of felsic rocks (about 90%), the Late Triassic subaerial volcanics can trace an important crustal partial melting event. They could belong to an overheated zone of an Andean-type volcanic belt.