香港世界地质公园粮船湾组碎斑熔岩柱状节理构造特征

在香港世界地质公园西贡火山岩园区 ,白垩纪粮船湾组流纹质碎斑熔岩普遍发育完整的柱状节理 ,构成世界上面积最大的流纹质火山岩石柱群.文章在详细野外观察与测量的基础上 ,分析了粮船湾组石柱群原生柱状节理构造特征 ,初步探讨了其成因.结果表明 ,该区火山岩石柱截面形态以六边形为主 ,少量五边形及不规则状 ,直径以1~3m为主 ;完整的单个石柱高一般20~30m ;石柱整体向NW和SW陡倾.但是 ,粮船湾组火山岩非全部发育柱状节理 ,按石柱发育程度可分为极发育、一般发育(被破坏)和不发育三种类型.通过统计大量原生柱面的产状 ,发现各个方向的柱面均有发育 ,其无序性符合冷凝收缩的成因模式 ,认为后期次生节理构造及海蚀作用对原生柱状节理有一定程度的破坏.

Columnar joint structure of High Island Formation porphyroclastic lavas in the Hong Kong global geopark

The columnar joints of High Island Formation are the largest rhyolitic porphyroclastic lava columns in the Sai Kung volcanic rock regions ,Hong Kong global geopark .On the basis of field investiga-tion and measurement ,the authors pursue the structural features and origin of these primitive columnar joints .It shows that not all the porphyroclastic lavas of High Island Formation occurred columnar joints , the columnar joints can be divided into three types ,i .e .,well-developed ,sub-developed (destructed by secondary joints) and undeveloped .The sections of the volcanic columns are mainly hexagonal in shape , and partly pentagon and irregular .T heir diameters are 1~3 meters and the heights are generally 20~30 meters ,most of them dip steeply towards the NW-trending and SW-trending .Occurring measurements of primitive columnar joints show they have various directions in cylinder ,which conforms to the contraction-condensation model ,and secondary joints and marine abrasion destroyed morphologies of columnar joints .