Typical recognition features of the Changyu caldera in the eastern Zhejiang Province

Calderas are widely distributed in the huge Cretaceous silicic volcanic-plutonic complex belt along coastal area of Southeast China. Their genesis and related volcanic activity processes are important issues for understanding the tectono-magmatism and mineralization in SE China. Calderas are subcircular volcanic depressions. The formation of the caldera is associated with the collapse of the magma chamber roof following with large-volume pyroclastic-flows during explosive eruptions. The eroded caldera can be well identified by the juxtaposition of the intracaldera volcanic rocks at the same level as older surrounding rocks. The formation of caldera commonly undergoes multi-stage volcanic activity processes, including pre-caldera eruption, caldera-forming eruption, post-caldera eruption and caldera resurgence. Identifying the different eruption stages and magmatic activities are crucial for understanding the formation and evolution of calderas. The Changyu caldera from eastern Zhejiang, coastal SE China has typical recognition features, including: 1 The volcanic rocks are distributed as nearly circular geometry with a diameter of 12 km, and lake sedimentary rocks were locally developed within the caldera; 2 The rhyolitic lapilli welded tuff of the first unit juxtaposed against older crystal-rich volcanic rocks of Jiuliping Formation along the south margin of the caldera and they are bounded by normal fault zone. The Jiuliping Formation constitutes the wall and the possible basement of Changyu caldera; 3 The first, second and third tuff units of Changyu caldera are mainly distributed within the caldera, showing typical petrological characteristics of pyroclastic-flow facies. They are vertically distributed from base to top, suggesting that the eruption of the first unit led to the formation of the collapse caldera, while the second and the third volcanic units were formed by post-caldera eruptions, which further filled the caldera; 4 Rhyolite domes were developed in several places within the caldera, representing volcanic magma conduits of the caldera.