浙东长屿破火山的典型识别特征

破火山在中国东南沿海白垩纪巨型硅质火山-侵入杂岩带中大量分布,其成因及相关的火山活动过程是理解中国东南部构造-岩浆与成矿作用的重要科学问题。破火山是近圆形的火山洼地,其形成与大量岩浆喷出的爆炸式火山碎屑流喷发伴随的岩浆房顶板塌陷有关。对于经历剥蚀的古老破火山,其内部岩石相对围岩来说更年轻,与围岩并列接触并有截然的界线,是识别破火山的重要依据。破火山一般经历多阶段的火山活动过程,包括塌陷前喷发、破火山形成喷发、破火山形成后喷发和破火山复活等,识别这些不同的火山喷发和岩浆活动阶段是理解破火山形成与演化的关键问题。浙东晚白垩世长屿火山具有典型的破火山识别特征:①火山岩呈直径约12 km的圆形分布,局部发育破火山内湖相沉积;②破火山南部第一阶段流纹质角砾熔结凝灰岩与较老的九里坪组富晶体火山岩在空间上并列,并以正断层带为界,九里坪组火山岩构成了长屿破火山的围岩和可能的基底;③长屿3个阶段的火山岩均主要分布在破火山内,具有典型的火山碎屑流相的岩相学特征,在垂向上依次叠置,反映了第一阶段火山活动导致了塌陷破火山的形成,第二阶段和第三阶段为破火山形成后喷发,喷发产物进一步填充破火山;④破火山内多处发育的流纹岩穹隆代表了破火山内部的多个火山岩浆通道。

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.