New K-Ar biotite and amphibole ages of granitoids of Sikhote-Alin (Russian Far East): Criteria of reliability and interpretation

Biotite and biotite-amphibole pairs from granitoids of the southern Sikhote-Alin-North Sakhalin orogenic belt are dated by the K-Ar method. The obtained ages are highly consistent with U-Pb zircon data (original and published data). In the zircon (U-Pb)-biotite (K-Ar)-amphibole (K-Ar) triad, the zircon and biotite define close, frequently identical, ages, whereas the amphiboles usually yield younger K-Ar ages, which is inconsistent with the idea of the relative stability of the K-Ar isotope system of amphibole to thermal impact. Our research confirms the existing opinion that the isotope system of biotite is more stable to hydrothermal-metasomatic alteration than that of amphibole. The younger age of amphibole in monzonitic associations is probably caused by the formation of amphibole after pyroxene at the late magmatic and early postmagmatic stages. The obtained results, with allowance for analysis of the reliability of the isotope K-Ar and U-Pb dates on different mineral phases, confirm the idea that the Albian stage played a special role in the geological evolution of the studied region and mark the peak of magmatic activity at 105-95 Ma. According to isotope dates, the Albian-Cenomanian time was a period of simultaneous formation of monzonites and granites in Sikhote-Alin, which is typical of the continent-ocean transform-plate-boundary setting. It is found that the distribution of granitoids of different age is determined by the boundaries of the terranes of orogenic belts.