燕辽地区中生代火山活动对古环境的影响

Effect of volatiles erupted from Mesozoic activities in Yanliao area of North China on paleo-environmental changes.

采用红外光谱和电子探针分析方法,开展对华北克拉通北缘燕辽地区中生代火山岩中斑晶矿物包裹体地球化学成分研究,并具体分析了燕辽地区中生代火山活动对古环境所造成的重大影响。研究表明,从中生代早期至晚期,燕辽地区火山活动喷出气体及其气溶胶对古环境造成的影响具有明显不同的演化特征。从早到晚表现为:(1)南大岭期玄武质火山喷出的剧毒气体富集 S 与 Cl,它们导致火山区附近地表温度下降和酸雨形成;(2)髫髻山期安山质—粗面安山质火山喷发形成的气体以高含量的 H_2O 和 CO_2为特征,其对周围环境造成的影响是引起地表温度上升;(3)张家口期粗面质和流纹质火山作用喷出的剧毒气体富集 Cl 和 F 两组分,它们主要是破坏大气臭氧层和形成酸雨;(4)辽西义县期和九佛堂期中酸性火山活动喷入大气圈剧毒火山气体的 S、Cl 和 F 含量均较高,火山喷出的大量气体及其气溶胶与火山灰一起导致辽西中生代大量带毛恐龙和孔子鸟等脊椎动物集群死亡,并形成化石层。本文从火山气体及其气溶胶对古环境影响的角度初步分析了辽西地区出露大量中生代脊椎动物化石的原因。

Yanliao Mesozoic （ 188 - 120Ma） volcanic rocks in North China Craton are composed of basalt-andesite-trachyte-rhyolite associations. Petrographically, Mesozoic volcanic rocks have porphyritic textures with phenocryst minerals of olivine, pyroxene, amphibole, biotite, plagioclase, alkali feldspar, quartz and Fe-Ti oxides setting in the groundmass of amphibole, biotite, plagioclase, alkali feldspar, quartz and glass. Maximum heights of the Mesozoic volcanic eruption columns in the Yanliao area are above 18 km based on the physically volcanologic modeling calculations, which have been considered to entering into the stratosphere. Electron microprobe and infrared ray spectrum analyses determined contents of major elements and volatiles （ including Cl, F, S, CO2 and H2O） of magma inclusions in the phenocryst minerals and groundmass glass of the Mesozoic volcanic rocks in Yanliao. The analysed results show that volatiles emitted are variable during the Mesozoic Yanliao volcanism from early to late. Volatiles released from Nandaling basaltic volcanic activities （-188Ma） in the early stage are enriched by S and Cl, possibly declining Earth＇s surface temperatures and forming acid rain. Volcanic degassing of Tiaojishan andesitic and trachyandesitic volcanism （174 - 161Ma） is characterized by enrichments of H20 and CO2, which has increased Earth＇s surface temperatures. Zhangjiakou trachytic and rhyolitic pyroclastic flows and lavas （148 - 141Ma） have erupted a large amount of C1 and F into the atmosphere, which resulted in acid rain and ozone depletion. Volatiles from the Yixian-Jiufotang intermediate and acidic volcanic rocks in the western Liaoning province （135 120Ma） have high concentrations of S, F and CI, leading to mass mortality events.