西南极岩浆作用及构造演化

Magmatism and tectonic evolution of West Antarctica

西南极主要由哈格冰原岛峰群、南极半岛、瑟斯顿岛、玛丽·伯德地和埃尔斯沃思-惠特莫尔山脉五个各具特色的地壳块体组成。通过综述上述各块体主要的岩浆事件及其构造意义,旨在了解西南极的地质演化过程。西南极最古老的岩石为哈格冰原岛峰群地块的前寒武纪正片麻岩,时代为1238 Ma,记录了中元古代弧岩浆作用,其余四个地块记录了~500 Ma以来的地质演化过程。古生代时期,埃尔斯沃思-惠特莫尔山脉地块处于快速沉降的陆相断陷盆地环境,岩浆活动稀少,与罗斯造山运动形成的弧后伸展有关;玛丽·伯德地地块中—晚古生代发育一套与板块汇聚有关的岩浆作用,形成于活动大陆边缘环境;而南极半岛-瑟斯顿岛地块记录了石炭纪—二叠纪时期弧的发育。各地块的构造背景从侏罗纪开始明显分化,埃尔斯沃思-惠特莫尔山脉地块记录了侏罗纪板内岩浆作用,可能与大火成岩省有关;玛丽·伯德地地块发育的侏罗纪—早白垩世I型弧岩浆岩随时间转变为白垩纪中期的A型碱性岩浆岩,经历了由俯冲向裂解机制的转变;南极半岛-瑟斯顿岛地块侏罗纪—白垩纪为弧岩浆活动活跃期,同时也有大火成岩省火山活动的记录,是持续俯冲和裂解相互作用的产物。新生代岩浆作用以南极半岛地块为代表,弧岩浆作用持续到始新世,其时空分布特征与左行错断扩张脊的分段俯冲和碰撞有关。

West Antarctica is mainly composed of five distinct micro-continental blocks,namely Haag Nunataks,Antarctic Peninsula,Thurston Island,Marie Byrd Land and Ellsworth-Whitmore Mountains.In order to understand the geological evolution of West Antarctica,this paper presents a brief overview of the main magmatic events of the five blocks and their tectonic significance.The oldest rock is the Precambrian orthogneiss from Haag Nunataks with zircon U-Pb age of~1238 Ma,indicating the development of Mesoproterozoic arc magmatism in West Antarctica.The other four blocks preserve the geological records since~500 Ma.During the Paleozoic,the Ellsworth-Whitmore Mountains block was formed in a rapidly subsiding continental rift basin environment which was related to the back-arc extension caused by the Ross Orogeny,and the magmatic activity was rare.A set of convergencerelated magmatism occurred in the middle to late Paleozoic in Mary Byrd Land block,which was formed in an active continental margin environment.The Antarctic Peninsula-Thurston Island blocks record the development of the Carboniferous-Permian arc during this time.During the Mesozoic,the tectonic setting of these blocks began to differentiate since the Jurassic.The Ellsworth-Whitmore Mountains block records Jurassic intra-plate magmatism,which may be associated with large igneous province.In Marie Byrd Land,the lithology changed from I-type arc magmatic rocks to A-type alkaline magmatic rocks in the Jurassic-Early Cretaceous to the mid-Cretaceous period.This reflects a major change in tectonic setting from subduction to rifting during the mid-Cretaceous.The Jurassic-Cretaceous flare-up in arc magmatism record on the Antarctic Peninsula-Thurston Island blocks with a pulse of Jurassic large igneous provinces.These are the product of the interaction of continuous subduction and rifting.The Cenozoic magmatism was represented by the Antarctic Peninsula block with arc magmatism continuing until the Eocene.The temporal and spatial distribution of the arc magmatism was related to the subduction and collision of spreading ridge which was cut into several segments by sinistral transform faults.