Modelling the post-caldera plumbing system of Changbaishan volcano (China) from integrated geochemical, isotopic, geobarometric, and geophysical data

The Changbaishan Tianchi volcano is the largest active volcano of China and the 946 CE VEI 6 "Millennium eruption" (ME) is responsible for the formation of its summit caldera. After the ME, the volcanic activity is characterized by smaller eruptions most of which are recorded only in historical documents. Here we analyze the deposits of the ME and 1403 CE eruption outcropping in the Heishigou section, which is located east of the Chagbaishan Tianchi caldera. We present whole-rock, glass, mineral, isotopic, and geobarometric data to obtain information on the vertical extent of the caldera and post-caldera plumbing system of the volcano. Results show that the plumbing system of the 1403 CE eruption is located between 1.9 and 5.4 km depth, which is also the depth range of the shallower reservoir of the ME. Therefore, the magma reservoir of the ME was active also after the caldera collapse. The 1403 CE magma stored in the ME reservoir evolved by fractional crystallization of the residual ME trachytic magma and underwent the contamination of upper crust material. The 1403 CE event was triggered by the injection of less evolved, mafic magma from depth. The inferred depth of the shallower ME reservoir and that of the 1403 CE eruption is the same of the present-day main geophysical anomalies (low seismic velocity and low resistivity) recorded below the Changbaishan Tianchi caldera. In addition, the source of the deformations related to the uplift during unrest episodes between 2002 and 2020 suggests a refilling of the 1403 CE plumbing system. We conclude that the plumbing system of the 1403 CE, post-caldera activity at Changbaishan Tianchi volcano is the same of the ME eruption and it is still active. This system may be recharged from the arrival of less evolved magma from depth. The proposed conceptual model of the post-caldera volcanism at Changbaishan Tianchi represents a reference framework to interpret the signals of unrest and decipher the dynamics of the present-day plumbing system.