南极南设得兰群岛中-新生代火山作用与地质环境

MESOZOIC-CENOZOIC VOLCANISM AND GEOLOGICAL ENVIRONMENT IN THE SOUTH SHETLAND ISLANDS, ANTARCTICA

本文对采自南设得兰群岛的乔治王岛、纳尔逊岛、利文斯顿岛、欺骗岛和南极半岛的火山岩和火山灰进行了岩石化学和Sr、Nd同位素分析 ,Ar Ar和K Ar年龄测定。研究表明 ,南设得兰群岛火山岩属于玄武岩 安山岩 英安岩岩石组合 ,它们的Sr、Nd同位素比值非常接近 ,87Sr/86Sr比值普遍比较低 ,为 0 .70 32 97- 0 .70 35 0 7;1 4 3Nd/1 4 4 Nd比值普遍比较高 ,为 0 .5 1 2 835 - 0 .5 1 3779,二者呈负相关 ,εNd 87Sr/86Sr相关图显示岩浆来源于亏损地幔。火山岩形成时代集中于96Ma、91Ma、78Ma、60Ma和 35Ma等时段。 35Ma之后 ,火山活动长时间消沉 ,直到晚中新世和第四纪才时有发生 ;这一漫长的历史时期 ,正是南极大陆冰盖逐渐形成之时。是气候变冷抑制了火山活动 ?还是火山活动的减弱和停止导致气候变冷 ?尚待进一步研究 ;但冷期火山活动减弱或停止 ,暖期火山活动活跃和增强 ,这一对应关系是存在的。火山作用的时空分布及岩石地球化学特征暗示 ,南设得兰群岛处于一个岛弧的地质环境 ,早期群岛基本上与南极半岛连在一起 ,在德雷克板块的俯冲下 ,群岛逐渐与半岛分离 ,形成布兰斯菲尔德海峡 (裂谷 ) 。

Volcanic rocks in the South Shetland Islands belong to the basalt andesite decite association and have lower 87 Sr/ 86 Sr ratios, 0.703297-0.703507, and higher 143 Nd/ 144 Nd ratios, 0.512835 -0.513779, both being negative correlative. Also, in the εNd 87 Sr/ 86 Sr diagram their samples fall in the depleted mantle domain. All of these indicate that these volcanic rocks came from a depleted mantle. Their formation ages are mainly 96Ma, 91Ma, 78Ma, 60Ma and 35Ma. The time space distribution and petrochemical characteristics of volcanic rocks suggest that the South Shetland Islands have been in a geological environment of island arc. At the early stage the South Shetland Islands were connected with the Antarctic Peninsula. Owing to the subduction of the Drake plate, the islands were separated from the peninsula. Then with back arc spreading and rifting, the islands were further far away from the peninsula, bringing about new volcanism. There was a long quiet period of volcanism from 35Ma to 8-6 Ma. It is in this long period that continental ice sheets were gradually formed in Antarctica. Although relationships between volcanic activity and climatic change need further study, such a relationship has existed indeed: volcanic activity weakens or stops in a cold epoch but revives and increases in a warm epoch.