Arc-continent collision in the Southern Urals: Petrogenetic aspects of the forearc-arc complex

Abstract

In the Southern Urals a well-preserved, rapidly created, arc-continent collisional orogen developed during the Devonian to Early Carboniferous. The Magnitogorsk arc includes a forearc represented by lavas, dikes and shallow intrusive rocks (boninites, arc tholeiites and calc-alkaline extrusive rocks), and a fully developed intraoceanic arc that developed during Early Devonian time (Emsian to Eifelian). The suture zone between the arc and the continent, the Main Uralian fault, is a melange zone containing ophiolite fragments, volcanic rocks derived from the arc, and sediments from the forearc basin. A petrogenetic characterization of the igneous suites from the Magnitogorsk zone and the Main Uralian fault has been based on rare earth element distributions, abundance of immobile, incompatible elements (Nb, Ta and Th), and Sr-Nd isotope systematics. The Magnitogorsk magmatic suites were generated from depleted mantle sources variably enriched with fluids or melts derived from a subducting slab. A Nb enrichment and high Th/Yb ratios with moderate high-field strength element depletion may reflect addition of an enriched ocean-island basalt-type component to the mantle wedge. The boninitic magmatism was related to an exceptionally high thermal anomaly responsible for high degree of melting and high rate of magma production, similarly to the Eocene magmatic event occurred in the Izu-Bonin-Mariana system in western Pacific Ocean. Arc-related igneous rocks similar to those of the Magnitogorsk zone occur in the Main Uralian fault melange. Here, ocean-island basalt-type intrusive rocks and late dikes indicate distinct magmatic sources with a typical enriched signature, suggesting a deeper origin with respect to most act lavas, unrelated to subduction. This enriched component was also involved in arc magma generation. It is inferred that the Devonian igneous rocks from the Main Uralian fault were intruded into already obducted lherzolitic ophiolites, and that the arc has been built on, or near, an older lithosphere including oceanic crust and undepleted mantle.