Geochronological limits of subalkaline magmatism in the Ket-Kap–Yuna igneous province, Aldan Shield

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Comparative U-Pb (zircon- and sphene-based, for the first time) and K-Ar (biotite-, amphibole, and whole rock-based) dating of monzonitoids and subalkaline granitoids of the Ket-Kap and Uchur volcanic-plutonic complexes (Ket-Kap-Yuna Igneous Province is one of the areas of tectonic-magmatic activation of the Aldan Shield), respectively, has been made. The new data, which are chiefly acquired by us for the first time, are close to each other and fall within the relatively narrow time interval of the mid-Early Cretaceous. The U-Pb sphene-based isochron datings indicate the formation time of the subalkali complexes at the Barremian-Aptian boundary. The values of SHRIMP age for zircons from the Uchur Complex subalkali diorites vary from 124.4 to 120.1 Ma; variations in U-Pb dates for monzonitoids of the Ket-Kap Complex fall in the narrower interval of 125.8–123.6 Ma. The extraordinary cases of highly polymodal distribution of SHRIMP-derived zircon-based dates are explained by the presence of detrital zircon crystals probably belonging to the protolith; this is interpreted by evidence for participation of the ancient continental crustal material in the formation of the source for the KKYuIP subalkali magmas. The range of the new K-Ar datings based on the whole rock composition of subalkali rocks and on the results of monomineral determinations is wider; however, it corresponds to the Early Cretaceous as well. This is significantly in contrast with the data given in the literature and geological survey reports. It was found for the first time that the duration of the subalkali magmatism manifestation in the KKYuIP is only several Ma and, according to the precise U-Pb dating, falls within the Aptian age of the Early Cretaceous epoch; this is sharply in contrast to the previously derived dates that referred to Early Jurassic-Cretaceous age of the magmatism. This time interval probably marks the main stage (relative quiet phase under strong compression) in the evolution of the setting of mutual sliding between the Siberian Craton and Amurian megablock, according to geodynamical reconstructions by A.I. Khanchuk and his colleagues.