The Qixiangzhan Lava Flow at the Tianchi Volcano: Eruptive Dynamics, Emplacement Mechanism and Implications for the Formation of Long-lived Magmatic Systems Prior to Caldera-forming Eruptions

The 7 ka old Qixiangzhan lava flow (QXZ, Tianchi volcano) represents the last eruptive event before the 946 CE, caldera-forming ‘Millennium’ eruption (ME). Petrographic, whole rock, mineral composition, Sr-Nd isotopic data on QXZ show that: (a) the lava consists of two components, constituted by comenditic obsidian fragments immersed in a continuous, aphanitic component; (b) both components have the same geochemical and isotopic variations of the ME magma. The QXZ and ME comendites result from fractional crystallization and crustal assimilation processes. The temperature of the QXZ magma was about 790°C and the depth of the magma reservoir around 7 km, the same values as estimated for ME. QXZ had a viscosity of 10^5.5–10⁹ Pa s and a velocity of 3–10 km/yr. The emplacement time was 0.5–1.6 yr and the flow rate 0.48–1.50 m³/s. These values lie within the range estimated for other rhyolitic flows worldwide. The QXZ lava originated through a mixed explosive–effusive activity with the obsidian resulting from the ascent of undercooling, degassing and the fragmentation of magma along the conduit walls, whereas the aphanitic component testifies to the less undercooled and segregated flow at the center of the conduit. The QXZ lava demonstrates the extensive history of the ME magma chamber.