Origin and Evolution of the Late Middle Pleistocene Trachytes in the Changbaishan Tianchi Volcano (China/North Korea): Insights from Mineralogy, Geochronology and Geochemistry

Cenozoic trachytes are characteristic of some active volcanic fields in China. In particular, the origin and mechanisms of the evolution of trachytes from the Tianchi (Changbaishan) volcano (TV, China/North Korea) are poorly known. Here, we present new geochronological, geochemical and isotopic data on two trachytic suites outcropping on the northern and southern upper slopes of TV. Detailed zircon laser ablation-multicollector-inductively coupled plasma-mass spectrometry (LA-MC-ICP-MS) U-Pb dating, Rb-Sr isochron dating of plagioclase and hornblende, ⁴⁰Ar/³⁹Ar chronology with mineral chemistry, whole-rock element and Sr-Nd-Pb isotope data are used to explore their origins and evolutionary mechanisms during the late Middle Pleistocene. Our data indicate that the trachytes mainly consist of sanidine, orthoclase and plagioclase, with minor albite, quartz and hornblende. They formed at 0.353–0.346 Ma (lava flow from the northern slope) and 0.383–0.311 Ma (lava flow from the southern slope), respectively. The TV trachytes are characterized by high K₂O/Na₂O and AR values, with low A/CNK and Mg^# values. They are enriched in rare earth elements (REEs; except Eu), depleted in Sr and Ba, crystallizing at 742–858°C. The TV trachytes have high (⁸⁷Sr/⁸⁶Sr)_i values (0.70776–0.71195), positive ε_Nd(t) values (0.61–2.93) and radiogenic ²⁰⁶Pb/²⁰⁴Pb values (17.515–17.806). These values are similar to those of the Pleistocene and Holocene TV trachytes. Geochemical data indicate that they were formed by fractional crystallization from a basaltic melt and assimilated upper crust material. The trace element pattern of the studied trachytes is consistent with an evolution from basaltic melts representative of an enriched mantle source. The vent from which the trachytic magma was erupted probably collapsed inside the caldera during the TV ‘Millennium’ eruption (ME) in 946 AD. The contemporaneous emission of basaltic and trachytic magma during the Middle–Late Pleistocene suggests the coexistence of fissural basaltic volcanism and central-type trachytic volcanism, the latter of which was associated with a magma chamber in the upper crust during the TV cone-construction stage.