Coexisting Baddeleyite and Zircon in Early Eocene Andesites of the Sikhote Alin: U-Pb Geochronology, Trace-Element Features, and Petrological and Tectonic Implications

Relatively large crystals of baddeleyite (up to 100 μm) and zircon (up to 400 μm) were found in Cenozoic subalkaline andesites in the north of Sikhote-Alin. The reasons for such a rare association of Zr minerals for volcanics are discussed based on the petrological characteristics of andesites, U-Pb isotope dating and the results of studying the contents of trace elements in baddeleyite and zircon. Andesites contain rare clinopyroxene phenocrysts, the crystallization of which occurred in the temperature range of 1090–1150°C. The fine-grained matrix is composed of pyroxene, plagioclase, hornblende, ilmenite and acid domains, which consist of K-Na feldspars, quartz and acid glass and were formed at T = 926°C. Accessory baddeleyite and zircon are confined to acid domains. U-Pb age of baddeleyite 52.7 ± 1.1 Ma (MSWD = 2.6) can be used as a preliminary estimate of the crystallization age of andesite melts. Age values Pb*/U of zircon are in a wide range from 46 to 56 Ma. The youngest age values (46–49 Ma) probably reflect a partial disturbance of the isotope system caused by high contents of U (up to 1.3 wt.%) and Th (up to 3.8 wt.%). Baddeleyite has a decrease in the concentrations of Hf (from 7742 to 2869 μg/g), Y and heavy REE, which could be associated with its crystallization in competition with amphibole. Deep negative Eu anomalies in baddeleyite and zircon suggest their growth simultaneously with feldspars. High concentrations of heavy REE, U and Th in zircon indicate its crystallization from enriched residual melts. The zircon crystallization temperature estimated using the Ti-in-zircon geothermometer (from 800 to 990°C) is comparable with temperature estimates for acid domains. Thus, the baddeleyite and zircon compositions suggest their crystallization at late stages of melt evolution, which could have occurred in an intermediate magmatic chamber. The studied Early Eocene andesites in the magmatic history of Sikhote-Alin occupy the period between the preceding Paleocene-Early Eocene A-type rhyolites (61–53 Ma) and the subsequent Eocene–Miocene basalts (40–20 Ma). This period is associated with the extension of the lithosphere due to the rupture of the continuity of the submerged oceanic plate and the opening of the "mantle window", and the studied early Eocene andesites probably mark the peak of this tectonic event.