西南极利文斯顿岛中部中生代－新生代火山岩岩石化学研究

PETROCHEMISTRY OF THE MESOZOIC CENOZOIC VOLCANIC ROCKS FROM THE CENTRAL PART OF LIVINGSTON ISLAND, WEST ANTARCTICA

出露在利文斯顿岛中部的白垩纪火山岩为玄武岩－玄武安山岩－安山岩－英安岩组合，基本属于钙碱性系列，但具有低钾拉斑玄武岩的某些特点。岩石富集Ｒｂ、Ｔｈ、Ｂａ，轻度亏损Ｚｒ、Ｔｉ、Ｈｆ、Ｎｂ，较强亏损Ｃｒ和Ｎｉ，而且随着岩浆从基性向酸性成分的演化，富集（或亏损）程度加大。岩石的轻稀土元素中度富集，岩石的（Ｃｅ／Ｙｂ）ＣＮ比值较低。白垩纪火山岩形成在一个造山环境，是南设得兰岩浆弧的一部分。然而，在利文斯顿岛不同地区出露的岩石其化学性质有所区别，因此认为它们可能源于不同的岩浆体系。在晚古生代至早中生代，利文斯顿岛中部处于一个独特的浊积扇的构造环境。主要产在利文斯顿岛中部和汉那角的鲍勒斯山组火山岩与其它地点的中生代火山岩相比，无论岩相学特征还是岩石化学性质均有所不同。因此认为它们的产生同时受到太平洋板块的俯冲作用和该地构造位置所控制。史莱夫角的橄榄玄武岩成分中碱高硅低，并且通常表现出高铝的特点，这些性质与百耳斯半岛的玄武岩极为相似。区域上火山活动不断发展，活动中心逐渐向北东迁移，导致粗玄岩岩颈和岩席生成，最后是英云闪长岩的侵入。更新世至现代的依诺特角组橄榄玄武岩以化学成分中的低硅高碱含量区别于中生代火山岩。岩石的高Ｍ值?

Cretaceous volcanic rocks outcropped in the central part of Livingston Island consist of a basalt basaltic andesite andesite dacite association principally belonging to calc alkaline series and characters of low potassium and low Mg.Rocks are enrich in Rb,Th,Ba,little depletion in Zr,Ti,Hf,Nb,and more strongly depleted in Cr and Ni.Such enrichment (or depletion )became stronger and stronger combining with the magma evolving from basic to more acid member.The rocks are moderate enrichment in light rare earth element and with low (Ce/Yb) CN ratios.These rocks were formed at a orogenic environment being a part of the South Shetland magmatic arc.However,there should be more than one magma system which caused the distinguishable features of the volcanic rocks from different locations in Livingston Island. The Central Livingston Island was at a special tectonic setting,fan depositional environment, during Late Palaeozoic Early Mesozoic time.The volcanic rocks of the Mount Bowles Formation mainly outcropped at the Central Livingston and the Hannah Point differ from the Mesozoic Cenozoic volcanic rocks in other locations both in petrography and petrochemistry,therefore,they have been considered as the products of a local magma system which was controlled by both the proto Pacific plate subduction and local tectonic setting.The olivine basalts from Cape Shirreff are with high alkali and low silica contents and usually have high aluminum content.These features are much similar to the basalts in Byers Peninsula where volcanic activities lasted from late Cretaceous to middle Cretaceous and produced a calc alkaline series.Regionally,the volcanic activities continued and progressively migrated northeastwards,resulted in the dolerite plugs and sills formed,tonalite intruded finally.Pleistocene to Recent olivine basalt of the Inott Point Formation is distinctive from the volcanic rocks of Mesozoic ages by high alkai and low silica content in chemical composition,of which the high M value and the abnormal high Fo olivine xenocryst should be the indicators of a primary magma directly from a deeper magma chamber.