No abstract

Comparative U-Pb (zircon and sphene based, for the first time) and K-Ar (biotite , amphib ole, and whole rock based) dating of monzonitoids and subalkaline granitoids of the Ket Kap and Uchur vol canic plutonic complexes (Ket Kap-Yuna Igneous Province is one of the areas of tectonic magmatic acti vation 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 Bar remian-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; how ever, 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 Creta ceous epoch; this is sharply in contrast to the previously derived dates that referred to Early Jurassic-Creta ceous 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.

No abstract