Geochemical features, age and geodynamic environment of formation of the Late Jurassic vehrlite-gabbro il'deus massif of the Bryanta block (south-eastern rim of the North-Asian craton)
In the geological structure of the Dzhugdzhur-Stanovoy and Selenga-Stanovoy superterranes in the south-eastern rim of the North-Asian craton, a significant role is played by ultrabasite-basite complexes. The largest massif of the vehrlite-gabbro formation type of the Bryanta block of the Dzhugdzhur-Stanovoy superterrane is the Ildeus massif. For plagiovehrlites of this massif the Late Jurassic age of 154.7+3.7 million years was stated by 40Ar/39Ar method on plagioclase. This allows distinguishing the corresponding stage of ultramafite-mafite magmatism within this structure. The formation age of this intrusive coincides with the age of the Vesyolkinsky and Petropavlovsky massifs (154-159 million years) of the Selenga-Stanovoy superterrane that may be the evidence of the common Late Jurassic stage of ultramafite-mafite magmatism along the south-eastern rim of the North-Asian craton. The main geochemical peculiarities of ultrabasite-basites of the Ildeus massifs are the predominance of LREE over HREE; the concentration of large-ion lithophiles of gabbros and their depletion in highly charged Nb, Ta, Hf. It suggests that their formation is associated with the cease of subduction with a subsequent rupture of the subducted plate. In this case the asthenosphere matter entered the formed breach and this resulted in the rise of magmas. Their geochemical characteristics show both the features of the subduction origin and the intraplate sources. The conformity of spectra of REE and minor elements distribution, as well as a regular increase of their contents with a decrease in their basicity, allows assuming that ultrabasites, pyroxenites and gabbros are the differetiates of the single magmatic melts. The geochemical peculiarities of basites are the evidence of their crystallization from magmas with the features of both the subduction and the intraplate origin. In combination with the general geodynamic reconstructions it may be assumed that their formation of the massif may be related to the formation of the asthenosphere window as a result of a rupture of the subducted plate.