Timescale and evolution of the intracontinental Tianchi volcanic shield and ignimbrite-forming eruption, Changbaishan, Northeast China

Around the Tianchi eruption cone, the Junjianshan Formation which builds the Tianchi basalt shield can be divided into three units. They are, from bottom to top, the Toudao, Baishan and Laofangzixiaoshan units, respectively. Individual basaltic flows of the Tianchi shield vary significantly in volume and in spatial distribution from a few to dozens of kilometers in length, hundreds to thousands of meters in width, and a few to dozens of meters in thickness. K–Ar age data indicate that the Tianchi shield was formed within a very short time period between 2.77 (± 0.08) and 0.31 (± 0.002) Ma. Our detailed mapping in combination with the K–Ar dating shows that the Toudao unit erupted at ∼ 2.77–1.99 Ma, the Baishan unit at ∼ 1.64–1.11 Ma, and the Laofangzixiaoshan at ∼ 0.31–0.58 Ma. The huge Millennium eruption from the Tianchi Volcano sustained a 35 km high column (HT) and lasted a few days, producing the comenditic ignimbrite around the volcano. Wiggle-matching measurements of a sample series of charcoal wood indicate the Millennium eruption might have happened around 1024 AD.Features of eruptions stemming from the same source surrounding the Tianchi Volcano, plus the volcanic evolution from effusive flows to explosive eruption, are interpreted to reflect crustal thickening within this intracontinental tectonic environment. Field observations and lava compositional variations suggest that crystal fractionation and magma mixing or mingling process must have played an important role in the development and evolution of the Tianchi Volcano.