Initiation and evolution of intra-oceanic subduction in the Uralides: Geochemical and isotopic constraints from Devonian oceanic rocks of the Southern Urals, Russia

Abstract

The Southern Uralides are a collisional orogen generated in the Late Devonian– Early Carboniferous by the collision of the Magnitogorsk island arc (MA) generated in the Early to Middle Devonian by intra-oceanic convergence opposite to the continental margin, and the continental margin of the East European craton. A suture zone of the arc to the continental margin, the Main Uralian Fault (MUF), is marked by ophiolites and exhumed high-pressure–low-temperature metamorphic rocks of continental origin. The preorogenic events of the Southern Urals and their geodynamic setting are traced by means of fluid-immobile incompatible trace elements (rare earth elements and high field strength elements) and Sr–Nd–Pb isotope geochemistry of the MA suites, in particular the protoarc suite with boninites and probably ankaramites, and the mature arc comprised of island arc tholeiitic (IAT) suites, transitional IAT to calc-alkaline (CA), and CA suites. The MA volcanics result in genetically distinct magmatic source components. In particular, depleted normal-mid-oceanic ridge basalt-type mantle sources with various enrichments in a slabderived aqueous fluid component are evident. The enriched component is not involved in significant amounts, as testified by the rather radiogenic Nd isotopes and unradiogenic Pb isotopes. Further information on the pre-orogenic events is provided by the Mindyak Massif metagabbros derived from diverse gabbroic protoliths that were affected by oceanic rodingitization, and subsequently by a high-temperature (HT) metamorphism related to the development of a metamorphic sole. The HT metamorphism has the same age as the protoarc volcanism, and constrains the initiation of subduction at approximately 410 Ma. Consequently, the maximum timespan between initial intra-oceanic convergence and final collision is approximately 31 my, a duration consistent with that of present-day ongoing collisions in the western Pacific. The characteristics of early volcanism and the traces of a metamorphic sole provide useful criteria to attribute most MUF ophiolites to the Tethyan type with a complex pre-orogenic evolution.