西南极菲尔德斯半岛第三纪钙碱性火山岩的生成与演化

西南极菲尔德斯半岛第三纪火山岩的岩石学和岩石化学特征表明,它们基本属于钙碱性火山岩系列,是岛弧火山作用的产物。该岩石组合中,随岩石中SiO_2含量的增加,斜长石斑晶数量减少,微量元素Cr、V丰度降低,Sr、Ba丰度下降,这些揭示了岩浆中斜长石和单斜辉石的分离结晶作用。稀土元素的系统变化也证明了这一点。主元素和微量元素的定量计算所验证了岩浆的分离结晶作用演化过程。

THE FORMATION AND EVOLUTION OF TERTIARY CALC-ALKALINE VOLACNIC ROCKS IN FILDES PENINSULA, WEST ANTARCTICA

Tertiary volcanic rocks in Fildes Peninsula composed of basalt, basaltic andesite, andesite and dacite belong to a calc-alkaline series and are the typical products of island-arc volcanism, The rocks are chemically rich in Cao (> 7 %) and Al_2O_3 (usually over 17%) contents. The lower K_2O content and K_2O/Na_2O ratio, the higher total FeO content comparing with typical calc-alcaline basalt, however, indicate that they have some tholeiitic characteristics. Considering the whole rock association, the MgO, CaO and A_2O_3 contents decreasing and K_2O content and K_2O/Na_2O ratio obviously increasing with increasing SiO_2 concentraction suggest that this rock association can be put under a calc-alkaline rock series. The sharp depletion of transitional metal elements, Cr and Ni in basalts shows a fractional effect. The distribution patterns of large ion lithophile elements and rare earth elements also suggest that they are calc-alkaline basalts. The decreasing plagioclase phenocrysts and Cr,V concentration, the unchangible content of Ni,and the decreasing of Ba,Sr relative concentration from basaltic to dacitic rocks indicate that the evolution of this rock association has been mainly controlled by the fractional crystallization of plagioclase phenocrysts and clinopyroxenes and olivine was not an important fractional phase. The distribution patterns of REE also support above suggestion. The formation and evolution of the calc-alkaline magma is considered as follows: the mantle-derived Mg-rich magma by partial melting remained at a lower crust with fractionation of olivine, clinopyroxene and plagioclase; then, the fractional crystallization of plagioclase and clinopyroxene controlled a principal evolution from basaltic magma through basaltic andesite and andesite to dacitic rocks.