摘要
菲尔德斯半岛的火山岩系是太平洋板块向南极板块分段俯冲后期的产物,主要由玄武岩、玄武安山岩和安山岩组成,并有少量英安山。其同位素年龄为64.6±1—43±2Ma。属古新世到始新世。本区火山作用可分为两个阶段,不同阶段形成的火山岩中,主要氧化物、稀土元素和微量元素含量呈现出的规律性变化,主要与该阶段的岩性组合有关。同位素地球化学研究表明,本区原始岩浆是由上地幔部分熔融产生的,在深部经分离结晶作用上升到高位(浅部)岩浆房。岩浆喷发之前,玄武安山质的初始岩浆在高位岩浆房中发生分异,形成了不同成分的派生岩浆分带。在半岛的不同地区,基底导浆断裂于不同的火山作用阶段先后发生周期性拉张,切过各派生岩浆带,致使不同成分的派生岩浆喷出地表,形成了相应成分的火山岩组合。
The volcanic rocks series in the Fildes Peninsula is the product of the later subduction of the Pacific plate beneath the Antarctic. The series consists mainly of basaltic, basalt-andesitic and andesitic rocks and a small amount of dacitic volcanic rocks. Its isotope age ranges from 64.6±1 to 43±2 Ma, belonging to the Palaeocene and Eocene. Volcanism in the area is divided into two phases. Basaltic pyroclastic rocks and lavas of the Jasper Hill Member (Eb) and Agate Beach Member (Era) were formed in the first phase of volcanism. Volcanism of the second phase formed pyroclastic and pyroclastic-sedimentary rocks of the Fossil Hill Member (Eh), basaltic, basalt-andesitic, andesitic and dacitic lavas and breccia lavas of the Block Hill member (Ey), and basaltic lavas of the Long Hill member (Ech). An intermission of eruption existed between the two phases so that volcanic rocks in the top part of the Agate Member were subjected to weathering.
The geological characteristics of volcanism in the area are related to the lithological assemblage of volcanic rocks of various phases. A common feature of the main oxides of volcanic and subvolcanic rocks of the two phases is the presence of a high Al_2O_3 content. The average- SiO_2 content is 53.63％, essentially coinciding with that in the parent magma in the high-level magma chamber. Volcanic rocks of the first phase exhibit features of the tholeiitic series. Volcanism of the second phase is indicative of the trend of calc-alkaline evolution. The chondrite-normalized REE patterns of various rock types in the area exhibit slight enrichmen in LREE, and are essentially parallel. The ΣREE, ELREE and ΣHREE increase gradually with an increase in acidity. LIL and transition metallic elements also show regular changes with lithological differences of volcanic rocks. The above-mentioned facts suggest that volcanic rocks in the area are all products of comagmatic differentiation.
The results of studies of Sr, Nd and Pb isotopes indicate that primitive magmas in the area Were formed by partial melting in the mantle. The water-rich fluids needed by partial melting might come from dehydration of subducting oceanic crustal materials. After the formation of primitive magmas, fractional crystallization of mafic minerals occurred in the deeplevel magma chamber. Before the magmatic eruption, basalt-andesitic parent magma in the high-level magma chamber was subjected to differentiation, forming magma zones of different compositions. In various phase, magma-conducting basement faults cut through different magma zones in different parts of the peninsula, resulting in eruption of magmas of different compositions on the surface and the formation of a relevant volcanic rocks assemblage.
出处
《地质学报》
EI
CAS
CSCD
北大核心
1991年第3期232-249,共18页
Acta Geologica Sinica
基金
国家南极委员会
国家自然科学基金4870113项目
地质行业基金会
中国地质科学院资助
