Abstract The Changbaishan volcanic field located on the Gaima (Gaema, Gaiman) Plateau has witnessed plateau-forming eruptions along with the uplift of the Gaima Plateau. The Tianchi basaltic lava shield volcano was formed at the main peak of Changbaishan, with cone construction eruptions that formed a huge and steep trachytic composite cone on the gentle lava shield. At the peak of the Millennium Eruption (ME), the height of the eruption column reached 25 km and the bulk volume of tephra was about 120 km 3 . The ME eventually formed Tianchi caldera, after which several eruptions occurred, albeit of a much smaller scale. The magmas involved in the shield-forming eruptions are characterized by both alkalic series trachybasalt and basaltic trachyandesite and subalkalic tholeiite and basaltic andesite. In the cone-construction and ignimbrite-forming eruption stages, the magma is completely composed of alkalic series trachyte and comendite. The largest negative Eu anomalies observed in ME magmas indicate that plagioclase was strongly crystallized and differentiated.

Abstract The Cenozoic alkali basalts are widely exposed in the Jingpohu Volcanic Field, NE China. Previous volcanology and geochronology research has revealed that they were formed in three periods in the Miocene ( c. 29.23–13.59 Ma), Pleistocene ( c. 83.7 ka) and Holocene ( c. 5.5–5.2 ka BP). The Miocene and Pleistocene basalts consist of alkali olivine basalts, while the Holocene basalts are composed of alkali olivine basalts and leucite tephrites. Petrogenetic studies reveal that the primary magmas of the Miocene and Pleistocene alkali olivine basalts originated from partial melting of EM2-like garnet peridotites, and those of the Holocene alkali olivine basalts were derived from melting of EM1- and EM2-like garnet peridotites with higher garnet proportions. In contrast, the primary magmas of the Holocene leucite tephrites were derived from the melting of eclogites and peridotites. Combined with previous research, we suggest that melting of the mantle source region to generate Jingpohu alkali basalts was triggered by decarbonization and dehydration of the slabs stagnated in the mantle transition zone.