The internal structure and petrology of the Aya syenite-granosyenite-granite series (Gorny Altai)

This paper presents new data on the geology. geochemistry, isotopic charact eristics, and crystallization conditions of the rocks of the Aya syenite-gr anosyenite-granite association in Corny Altar. The authors refine the geolo gical structure of the Aya massif and present the mineralogical and petrogr aphic characteristics of the main rock varieties, Analysis of the peculiari ties of the behavior of the Rb-Sr and Sm-Nd isotope systems and of the geoc hemical specific features of rocks suggests different genetic origin of bas ic-rock-syenitic and granite-leucogranitic rocks. The origin of syenitic magmas abnormally enriched in Sr (up to 8000 ppm) an d Ba (up to 3000 ppm) is still under discussion; the generation of granosye nite-granite melts occurred under melting of metasomatized crust substrata. The effect of crystallization differentiation manifests itself only in the bodies of younger leucogranites, in which residual low-temperature melts a bnormally enriched in water and fluorine and having drastically increased R b-87/Sr-86 ratios are observed. By isotopic-geochronological data, the stud ied magmatism is of Early Triassic age (244+/-4.5 Ma. Rb-Sr method). Judgin g from the low initial ratios of Sr Isotopes (0.7052-0.7068), the crust mat ter which underwent melting was rather primitive in composition.