Geochemical trends of gabbro and granites in the Urals: Evidence of the evolutionary history of the mobile belt

Abstract

Principal geochemical features of gabbro and granites in the Urals, the two dominant rock groups of cotectic composition, most adequately reflect the dependences of the composition of the rocks on geological environment, which varied during the evolution of the orogen. The main geochemical trend of the gabbroids is proved to have an age coordinate and was controlled by the progressively more intense metasomatic modification of the mantle wedge as the major source of mafic magmas. The trend reflects the process that can be provisionally defined as “continentalization”. The gabbroids older than 380 Ma tend to plot within the composition field of the oceanic crust (roughly corresponding to N-MORB), whereas younger gabbroids (whose dominant mafic mineral is hornblende) plot within the field of continental mafic rocks. The behavior of such lithophile elements as K and Rb, whose concentrations increase in the course of rock evolution, unambiguously testifies that the trend corresponds to the evolution of the rocks from mafic to acid with time. During the “continentalization” of the mobile belt, the fertility of the magmatic source of the mafic rocks (mantle wedge) increased because of the progressively enhanced metasomatism of the wedge by fluid. Much of this fluid was generated in the subduction zone via the dehydration of hydroxyl-bearing minerals. In the Late Devonian, the dominant source of the fluid was amphibole in amphibolites (metamorphosed basites) and serpentine, and later the relative role of sediments involved in the subduction zone increased. The age trend is almost not reflected in the geochemistry of the granites, and their major geochemical trend can be defined as spatial. The obvious geochemical specifics of the Permian collisional granites in the northwestern and southeastern megablocks were controlled by compositional variations in the source material. Along with the newly-formed crust, the magmatic sources of granites in the northwestern megablock contained much rocks of the Mesoproterozoic granite–gneiss basement, whereas the sources of granites in the southeastern megablock was dominated by orthogneisses, which are metamorphic rocks of Paleozoic gabbro–tonalite–granodiorite–granite series.