Intrusive magmatism during early evolutionary stages of the Ural epioceanic orogen: U-Pb geochronology (LA ICP MS,NORDSIM, and SHRIMP II), geochemistry,and evolutionary tendencies |
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| Authors: | G B Fershtater A A Krasnobaev F Bea P Montero N S Borodina |
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| Institution: | (1) Zavaritskii Institute of Geology and Geochemistry, Ural Division, Russian Academy of Sciences, Pochtovyi per. 7, Yekaterinburg, 620151, Russia;(2) University of Granada, Granada, Spain |
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| Abstract: | In recent years extensive data have been obtained on all geologically important intrusive complexes in the Central and Southern Urals by U-Pb zircon geochronologic high spatial resolution techniques (LA ICP MS, NORDSIM, and SHRIMP II). This made it possible to revise the current concepts for the magmatic activity of the Ural Paleozoic orogen.Intrusive magmatism that occurred early in the evolution of the Ural orogen was focused mostly in the Tagil megazone, was characterized by several common features, and took place nearly simultaneously within both of its zones: the Platinum Belt and the Tagil volcanic zone.The composition of the parental magmas of all complexes of this age corresponded to an ultramafic or mafic source; i.e., the magma was derived from a mantle source. The gabbroids most closely approximating the composition of the parental magmatic melts show geochemical features of suprasubduction melts, such as negative HFSE (Nb, Ti, and Zr) and positive Ba and Sr anomalies. The REE patterns of these rocks display variable La/Lu ratios, which are usually higher than 1. These geochemical features suggest that this magmatic source was a metasomatized mantle wedge, above which (at a depth of 40–25 km) a block of the pre-Ural basement occurred in Ordovician-Silurian time. The Tagil megazone started to develop on this block. By the Devonian, i.e., by the time when the Magnitogorsk zone began to evolve (～400 Ma) and continental-margin gabbro-tonalite-granodiorite magmatism was initiated (360 Ma), this basement had been destroyed by orogenesis. The major phases of Paleozoic magmatism in the Urals likely corresponded to global epochs of tectono-magmatic activity, because they correlate well with known data on the evolution of the 87Sr/86Sr ratio in Paleozoic seawater. |
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