Paleoproterozoic Shoshonite Mafic Associations of the Irkut Block (Sharyzhalgai Uplift, Southwest Siberian Craton): U–Pb Age and Conditions of Zircon Crystallization

Abstract

Abstract —The paper presents data on the composition and age of mafic rocks of the shoshonitic series in the Irkut block of the Sharyzhalgai uplift (southwest of the Siberian Сraton). According to the U–Pb dating of magmatic zircon, the formation of monzodiorites of the Poludennyi massif and gabbro-dolerites in the endo- and exocontact zones of the Toisuk pluton occurred at 1.87 and 1.86–1.85 Ga, respectively. The intrusion of mafic magmas and their underplating into the basement of the crust under postcollisional extension resulted in the near-coeval mafic and granitoid magmatism in the Irkut block between 1.87 and 1.84 Ga. The Paleoproterozoic mafic associations belong to the shoshonitic series and are characterized by enrichment in incompatible elements, including Zr, and low negative εNd(T) values. These geochemical and isotopic characteristics point to the derivation of magma from a long-lived enriched-mantle source, such as the subcontinental lithospheric mantle. The crystallization of zircon from the last portions of the evolved mafic melt is evidenced by low zirconium saturation temperatures (710–965 °C) and zircon enrichment in U and Th with increasing Th/U, reflecting the growth of concentrations of highly incompatible elements in the residual melt.