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 northern Sikhote-Alin. The reasons for such a rare association of Zr minerals in volcanics are discussed based on the petrological characteristics of the andesites, their U−Pb isotope dating, and contents of trace elements in the baddeleyite and zircon. The andesites contain rare clinopyroxene phenocrysts, which crystallized in the temperature range of 1090–1150°C. The fine-grained matrix is composed of pyroxene, plagioclase, hornblende, ilmenite, and felsic domains, which consist of K−Na feldspar, quartz and silicic glass and were formed at 926°C. The accessory baddeleyite and zircon are confined to the felsic domains. The obtained U−Pb age of the baddeleyite 52.7 ± 1.1 Ma (MSWD = 2.6) can be used as an estimation of the crystallization age of the andesite melts. The zircon 206Pb/238U age broadly varies within the range of 46 to 56 Ma. The youngest values (46–49 Ma) probably reflect the partial disturbance of the isotope system caused by the high contents of U (up to 1.3 wt %) and Th (up to 3.8 wt %). The baddeleyite shows a simultaneous decrease in concentrations of Hf (from 7742 to 2869 ppm), Y, and heavy REE, which may be explained by its competitive crystallization with amphibole. Well-pronounced negative Eu anomalies in the baddeleyite and zircon suggest their growth simultaneously with feldspars. High concentrations of HREE, U, and Th in the zircon indicate its crystallization from enriched residual melts. Zircon crystallization temperatures estimated using the Ti-in-zircon geothermometer (from 800 to 990°C) are comparable with temperature estimations for the felsic domains. The baddeleyite and zircon compositions imply that the minerals crystallized during the late stages of the melt evolution, perhaps in an intermediate magma chamber. In the magmatic history of the Sikhote-Alin, the Early Eocene andesites formed between the older Paleocene−Early Eocene A-type rhyolites (61–53 Ma) and the younger Eocene–Miocene basalts (40–20 Ma). This time span is thought to have been associated with lithospheric extension due to the break-up of the downgoing oceanic slab and the opening of a mantle window, with the Early Eocene andesites likely marking this tectonic event.