中国东南沿海晚白垩世长屿火山的活动过程与古环境意义SCIEI北大核心CSCD

火山是唯一能够直接反映地球深部存在岩浆的地球动力学现象,对于地球气候演变和宜居性具有重要影响。相比年轻火山,古老火山因其活动过程完整、且可能剥蚀出露多阶段火山喷发产物及岩浆通道、岩浆房等,从而为揭示火山岩浆系统演化和火山活动过程提供重要的研究窗口。本文选择中国东南沿海晚白垩世长屿破火山为研究对象,对其开展火山地质、岩石学、年代学及地球化学研究,以期揭示其火山活动历史及其对中国东南沿海白垩纪古环境的启示意义。长屿火山出露面积约100km^(2),呈近圆形分布,具有较厚的火山地层厚度(总厚度约570m),类似于破火山内的火山堆积特点。由早至晚三个喷发阶段形成的流纹质角砾凝灰岩都显示了典型的火山碎屑流相的特点,发育典型的条纹斑状结构,但表现出不同的晶屑、玻屑及岩屑含量以及熔结程度等岩相学特征,反映火山喷发从初始到高峰、再到减弱的过程,最后岩浆沿火山通道侵出形成流纹斑岩穹隆,标志着火山活动的结束。长屿火山的火山碎屑流式喷发伴随着快速的岩浆房塌陷,以及缺少普林尼式空落堆积,反映了火山活动发生在伸展的构造背景。系统的LA-ICP-MS锆石U-Pb年代学研究获得了不同阶段火山岩一致的形成年龄(97~96Ma),暗示它们具有较短的形成时限,是由同期火山岩浆活动先后喷发形成的。长屿火山岩高的SiO_(2)含量(67%~76%)以及分异的锆石微量元素地球化学特征,表明喷发岩浆来自晶粥提取的熔体,并有晶粥来源晶体的混入。此外,我们在长屿火山岩中发现了南洋杉型丝炭化木(贝壳杉型木属),暗示晚白垩世中国东南沿海地区可能为温暖湿润的亚热带山地气候环境,表明中国东南沿海白垩纪大规模火山活动,在古武夷山脉以东地区形成了地形高耸的海岸山脉。

Volcanological evolution and paleoenvironment of the Late Cretaceous Changyu volcano in the coastal SE China

Volcanoes are the only straightforward expression for the existence and activity of magma under Earth' s surface. Volcanic eruptions can significantly influence the evolution of the Earth' s environment and livability. Compared to younger volcanoes, ancient volcanoes have a more complete documentation of the eruptive history, and thus can expose multi-stage eruptive products, magma plumbing systems and even the magma reservoirs. As a result, they provide a research opportunity for exploring the volcanic magma system and volcanic activity processes. In this work, we conducted volcanological, petrological, geochronological and geochemical studies on the volcanic rocks from the typical Late Cretaceous Changyu caldera in coastal SE China in order to reveal its eruptive history and implications to the paleoenvironment of the Cretaceous SE China. The Changyu volcano has a circular shape with an area of 100km(2). The exposed volcanic rocks have a relatively large thickness of about 570m in total, indicating intra-caldera ignimbrite successions. All the rhyolitic lapilli tuffs of three eruption stages show lithofacies which are characterized by pyroclastic flow deposits. They have typical eutaxitic textures, with different crystal, glass shard, and lithic lapilli contents and welding intensity, which documents its eruption processes from the waning stage to the waxing stage. At the final stage, magma extruded along the magma conduit forming porphyritic rhyolite domes, marking the end of the eruption. The pyroclastic flow eruption of the Changyu volcano, accompanied by rapid caldera collapse, and the lack of a Plinian fall deposit, indicate that the volcanism occurred in an extensional tectonic regime. The systematic LA-ICP-MS zircon U-Pb dating results reveal that the crystallization ages of the Changyu volcanic rocks of the different eruption stages are practically identical within analytical errors (97 similar to 96Ma), indicating that they were produced successively in a short period by common volcanism. The Changyu volcanic rocks have high SiO2 contents (67% similar to 76%) and fractionated zircon trace element features, suggesting that the erupted magmas were formed by the extraction of melt from crystal mushes, but containing entrained crystals. Furthermore, we discovered carbonized wood fragments of the Agathoxylon, which indicate that the climate of the Cretaceous SE China was subtropical warm and humid mountain climate, and a coastal mountain range of SE China to the east of ancient Wuyi Mountain had occurred during the Cretaceous intensive volcanism.