Early-Devonian intrusive magmatism of the Urals as indicator critical stage Paleozoic history mobile belt

It is established that during all-Uralian stage of mantle-type magmatism of Early Devonian age (410-393 Ma) the mantle Ordovician-Silurian magmatism was changed by mantle-crustal and crustal types. At that time Tagil island arc system completed its evolution and Magnitogorsk island arc is formed. Early Devonian magmatic rocks in the Middle Urals are represented by gabbro-granite rock series in the Uralian Platinum Belt, the Tagil volcanic zone, Reft allochthon and paleocontinental zone of northwestern megablock, and in the Southern Urals were formed obduction-related ultramafic-mafic layered and ring intrusions, swarms of spessartite dikes in Kempirsai-Khabarny allochthon and in the Main Uralian Fault zone. Middle Uralian gabbros as a products of initial melt crystallization are represented by biotite-amphibole gabbro-norite with orthoclase. The main mechanism of the formation of these series is crystallization differentiation due to fractionation of “gabbro” cotectic represented by femic minerals and plagioclase. Gabbro-granite series of Kurmanka massif which begin the mantle-crustal magmatism at the active continental margin of the northwestern megablock has more comp­lex mechanism of formation. Along with the fractionation, the processes of partial melting of the hornblende gabbro which is product of water mafic magmatism played an important role in origin of the granitoid part of rock series. The main type of Early Devonian magmatism in the southern Urals is related to obduction. During an emplacement to the surface of blocks of oceanic lithosphere, which are represented by ophiolite complexes or serpentinite melange zone, mafic magmas form as a result of а break-off the upper mantle continuity. They compose layered dunite-clinopyroxene-gabbro intrusions, constantly accompanying harzburgite tectonites (serpentinite) separated from the mantle as well as swarms of spessartite dikes and rare bodies of the obduction-related granites formed due to partial melting of crustal material during the emplacement of hot mantle blocks. Igneous rocks have both potassium and sodium specialization. The rocks of the first type conclude the final stage of Ordovician-Silurian mantle magmatism, the second type begin Middle Devonian-Permian mantle-crustal magmatism. Evident differences in the geological position and composition of various Early Devonian massifs clearly indicate significant differences in the geodynamics of the Central and Southern Urals in the Early Devonian and confirm the former times ideas about sub-latitudinal blocking of the Urals