LATERAL ZONATION, EVOLUTION, AND GEODYNAMIC INTERPRETATION OF MAGMATISM OF THE URALS - NEW PETROLOGICAL AND GEOCHEMICAL DATA

The principal characteristics of magmatism of the Urals are examined f or each stage of its evolution from oceanic to continental. Evidence o f the distinct geodynamic environments and corresponding magmatic seri es and their evolutionary successions in the modern structure of the M iddle and Southern Urals is contained in three large megablocks: a sut ure with mafic-ultramafic rocks and predominantly continental complexe s and two island-arc-continental blocks, which are composed of an isla nd-are, continental-margin, and continental zones. The megablocks comp ose a single geologic structure-the Ural orogen-with a continuous late ral zonation of magmatism. The lateral zonation of the mafic-ultramafi c complexes is expressed as the eastward depletion of the analogous ig neous rocks (olivine and clinopyroxene gabbroids) in incompatible elem ents. These compositional variations are of a gradual character in the rocks of the dunite-clinopyroxenite-gabbro series and seem to be expr essed as a series of stepwise changes in the ophiolites. The lateral z onation of the basaltoid and granitoid magmatism is characterized by t he opposite trends: the contents of incompatible elements increase eas tward, with this increase being more pronounced for elements that are predominantly contained in the sialic continental crust. Similar compo sitional trends were detected on the scale of large batholiths, distin ct zones, and the Urals as a whole and are accompanied by an eastward decrease in the age of magmatic series. Judging from the composition o f the anatectic granites and the orthogneisses hosting them, the conti nental crust composition of the northwestern and southeastern island-a rc-continental megablocks is different. The crust of the northwestern megablock corresponds perhaps most to the basement of the Russian Plat form, whereas that of the southeastern megablock is dominated by redep osited Paleozoic oceanic and island-are complexes.