LAUSR

Abstract The geodynamic significance of continental volcanoes located far from the plate boundaries remains highly controversial as exemplified by contrasting models that favor either a deep mantle plume rooted from the base of the mantle or, alternatively, the shallower subduction or lithospheric-related processes. The Changbaishan (also referred to as Paektusan or Baekdusan) volcanic field (CHVF), located in the interior of eastern Eurasian continent, provides a good opportunity to constrain the magma origin and geodynamic mechanism governing continental intraplate volcanism. Here, we review the volcanic geology, eruptive history, geochemical data on volcanic rocks and released gases and geophysical observations of the Changbaishan volcanoes with the aim to (a) reconstruct temporal and spatial evolution of eruptive activities, (b) identify source of the primary magmas, (c) delineate magma evolution in the crust, (d) highlight geodynamic significance of the CHVF volcanism, (e) characterize crustal magmatic structure, and (f) analyze recent dynamics with a focus on the 2002–2005 unrest episode at Tianchi caldera, the only active volcano in the area (last eruption in 1903 CE). The eruptive activities of the Changbaishan volcanoes can be divided into three main stages: (1) central vent and fissure eruptions of basaltic magmas started approximately in Pliocene and culminated in Early Pleistocene (ca. 5–1 Ma), forming a shield-like lava plateau; (2) multi-stage eruptions of voluminous silicic (and minor intermediate) magmas constructed cones of the polygenetic volcanoes (e.g., Tianchi, Wangtian'e and Namphothe) between Late Pliocene and Pleistocene (3.14–0.01 Ma); and (3) explosive silicic eruptions [e.g., the Millennium eruption (ME) in 946 CE] during Holocene dominated the Tianchi volcano and led to the formation of its summit caldera. Small-scale eruptions of basaltic magmas from monogenetic scoria cones (and minor fissures) were coeval with the Tianchi cone-construction stage (ca. 1–0.01 Ma). The elemental and Sr-Nd-Pb isotopic characteristics of the Changbaishan basalts indicate an enriched, heterogeneous mantle source with components from depleted mantle (DM), enriched mantle 1 (EM1) and subduction-related materials (e.g., recycled oceanic crust and sediments). The interaction between the DM-like peridotite and carbonatite melts released by subducted oceanic slab in the mantle transition zone (MTZ) led to the formation of carbonated peridotite characterized by low δ26Mg values. By contrast, origin of the EM1-like components remains highly debated. The alkaline basalts and intermediate to silicic volcanic rocks from the polygenetic volcanoes constitute an integrated spectrum of magma composition controlled by closed system fractionation according to their element co-variations and uniform Sr-Nd-Pb isotopic compositions. Subordinate mingling between trachyte and comendite has been reported only for the ME at Tianchi caldera. The occurrence of a big mantle wedge (BMW) with a continuous stagnant Pacific slab in the MTZ is responsible for origin of the Changbaishan volcanoes. On the basis of subduction dynamics of the Pacific plate, we present a Late Cenozoic geodynamic framework of NE Asia, which can account for formation of the present-day BMW system via: (a) shallow-angle subduction (55–25 Ma), (b) slab rollback and sinking into the MTZ together with trench retreat (25–15 Ma), and (c) slab bottoming, thickening and flattening in the MTZ (15–0 Ma). Constraints from reconstructed plate motion history, numerical simulation and present-day geophysical observation of the BMW lend support to our geodynamic model, which reconciles well with the Izanagi slab break-off, development of the Japan Sea and Late Cenozoic continental intraplate volcanism in NE China. In response to the Rayleigh-Taylor instability, a MTZ-derived plume incorporating fragments of carbonated peridotite, EM1-like components and the Pacific slab-derived materials ascended and experienced decompression partial melting at shallow depths to feed the Changbaishan volcanism. From the perspective of magma origin and geodynamic mechanism, the Changbaishan volcanoes can shed light on the potential relationships between origin of continental intraplate volcanism and deep subduction of oceanic lithosphere. The spatial distribution of the Changbaishan volcanoes shows that the magmas ascended along a NW-SE trending, strike-slip fault oriented perpendicularly to the major faults delimiting the Songliao Basin, NE China. This interpretation is consistent with the 2009–2013 epicenters of tectonic earthquakes, also suggesting a NW-SE trending, buried and seismically active deep fault in the crust. Geophysical and petrological constraints indicate the presence of magma reservoirs at crustal depth beneath the active Tianchi volcano, which are likely to have high thermal state and act as the source of heat and material for shallow hydrothermal system. In consideration of magma origin from the MTZ-derived plume, the volatile outgassing from the Tianchi volcano associated with deep subduction of the Pacific plate represents an important mechanism for liberating volatile elements (especially carbon) from Earth's interior to the exosphere. Tianchi caldera suffered an unrest episode between 2002 and 2005, as evidenced by increased shallow seismicity, surface uplift and changes in chemical and isotopic composition of the hydrothermal gases. Such volcanic unrest was triggered by pressurization of a 2–6 km depth magma reservoir, from which magmatic volatiles were released into shallow hydrothermal system. Tianchi caldera shows different types of hazards related to volcanic, tectonic, geomorphological and hydrological processes. Further monitoring and additional volcanological data, especially those on eruptive dynamics of the past eruptions, should be collected to better constrain the potential hazards of future eruptions and to improve early warning management.