西南极纳尔逊岛Stansbury半岛火山岩地质和岩石学基本特征

THE VOLCANIC GEOLOGY AND THE PETROLOGICAL CHARACTERISTICS OF THE STANSBURY PENINSULA,NELSON ISLAND, WEST ANTARCTICA

西南极纳尔逊岛Stansbury半岛基本被玄武质、玄武安山质熔岩以及火山角砾岩和含植物化石的火山-碎屑沉积岩所覆盖。地层接触关系和火山岩岩石组合显示,该半岛的熔岩和火山角砾岩主要形成于第一阶段的火山活动,可与菲尔德斯半岛的碧玉山段以及玛瑙滩段地层对比;含化石的火山-碎屑沉积岩和角砾熔岩、火山角砾岩分别相当于长城站地区的化石山段和岩块山段的岩层,它们是晚期火山作用的产物。stansbury半岛和菲尔德斯半岛发育有基本相同的火山岩地层,并且都在化石山段地层中发现了植物化石,这表明两个半岛上的火山岩可能是早第三纪区域火山作用的共同产物。 半岛所出露的断裂大致平行菲尔德斯海峡并且控制了次火山岩的分布。 岩石学特征说明,Stansbury半岛的熔岩和次火山岩主要为玄武质和玄武安山质,岩石中的斑晶矿物包括偏基性的斜长石、单斜辉石。岩石化学成分中,随着SiO_2含量增高,Al_2O_3、CaO、MgO和铁含量下降,碱质则升高。和菲尔德斯半岛的火山岩一样,这个半岛的火山岩也基本沿着钙碱趋势演化。微量元素和稀土元素的丰度以及配分模式证明,这套岩石具有同源岩浆的亲缘关系并且可能经历了分离结晶作用过程。

The Stansbury Peninsula of the Nelson Island, a neighbor of Fildes Peninsula, on which the Chinese Great Wall Station is located, is covered with basaltic, basalt-andesi-tic lavas and breccias as well as fossiliferous volcanic-sedimentary clastic rocks. Based on the stratigraphical relationship and the rock's association the author considers that the stratigraphical division of volcanic rocks on Stansbury Penins ula is much similar to that in Fildes Peninsula. The volcanic rocks and volcanic-sedimentary clastic rocks are the products of two stages of volcanism and can be stratigraphically subdivided into three members. The breccias, interbedding basaltic, basaltandesitic lavas and breccias formed successively, which are comparable with that of Jasper Hill plus Agate Beach Members in Fildes Peninsula. Before the end of this stage some subvolcanic intrusives appeared. The flira-bear-ing volcanic sedimentary clastic rocks belonging to Fossil Hill Member as found on Fildes Peninsula represent a long time break of volcanic activities on the peninsula. A lots of agglomeratic lavas and breccia on the volcanic sediments near the east coast of peninsula are comparable with those of Block Hill Member on the Great Wall Station area and showes that the centre of volcanic activities of the second stage had migrated eastwards. The similar development of volcanic strata and the discovery of plant fossils provided an evidence for such a suggestion that the volcanic rocks on Stansbury Peninsula and on Fildes Peninsula resulted from the same regional volcanism in the Early Tertiary. The faults on the peninsula parallel to the Fildes Strait Fault are the main structure and control the distribution of subvolcanic intrusives in the Peninsula. The petrological characteristics of the volcanic rocks from the Stansbury Peninsula suggest that the lavas and subvolcanic intrusives are basicaly basaltic and basalt andesi-tic. The phenocrystals of rocks include plagioclase and clinopyroxene. The Al2O3, CaO,MgO and tatol iron contents of the chemical compositions of rocks gradualy decrease and the alkali increase with the decreasing SiO2 from basaltic through basalt an-desitic to andesitic rocks. The petrochemical evoluting trend of rocks is similar to that from the Fildes Peninsula, that is, belongs to the clac-alkaline series with some high-Als and low-K features. Rare Eaith Element and other Trace Element concentration and chondrite-normalized patterns show that the volcanic rock association is probably resulted from the same magma sourse and the similar fractional crystallization of plagio-clase and pyroxene as discussed on the volcanic rock from the Fildes Peninsula.