香港国家地质公园粮船湾组火山岩岩石学研究

摘要

在香港国家地质公园西贡火山岩园区,核心地质景观是白垩纪粮船湾组(Kkh)火山岩优美的六方形石柱(柱状节理),它们的岩石类型长期存在熔岩和火山碎屑岩之争.笔者等经野外调查和薄片岩石学研究,确认粮船湾组火山岩实属一种特殊的熔岩——流纹质碎斑熔岩,以普遍的柱状节理、斑晶具有碎斑结构和珠边结构、基质发育霏细结构和流动构造为特征；它们不仅代表了香港地区中生代最晚期火山喷发的产物,而且构成了西贡破火山机构的中央侵出相岩穹.推断粮船湾组火山岩石柱是地球上已知面积最大的流纹质碎斑熔岩石柱群(～150 km2),目前所见的火山岩石柱仅是长期剥蚀后的残余部分.%Wonderful Hexagonal columns (columnar joints) of Cretaceous High Island Formation (Kkh) volcanic rocks make up the key landscape of the Sai Kung Volcanic Rock Region in southern Hong Kong National Geopark, but whether they belong to lava or pytoclastic rock has been thsputing. On the basis of field geological investigation and rock thin section observation, these volcanic rocks are determined to be a specific lava-rhyolidc porphyroclastic lava which is characterized by common columnar joints, porphyroclasdc & pearl -rim textures of phenocrysts, and felsitic texture & flow structure in groundmass. They are not only the latest Mesozoic products in Hong Kong but also constitute a large central extrusive fades dome within the Sai Kung Caldera. This porphyroclasdc lava dome is inferred to be the largest one (~150 km2) so far known in the world, and its residual parts after long-term erosion are current landscape which can be seen in the Sai Kung Volcanic Rock Region.