香港安山-英安质古火山颈群

Andesitic-dacitic paleovolcanic neck cluster in Hong Kong

香港中侏罗世屯门组火山岩主要由安山质熔岩、凝灰岩和凝灰角砾岩夹少量凝灰质砂岩组成。其中凝灰角砾岩初期曾误认为是沉积砾岩。1990年香港地质调查组重新研究后,确认是火山成因的凝灰角砾岩。按岩相分析,屯门组火山岩可分为火山通道相安山-英安质熔岩和爆发角砾岩,还有爆发空落相凝灰角砾岩。火山通道相又可分为火山颈相和岩墙相,分布在屯门组东西两侧。在青山东麓,出露一列NNW向呈断续、线状分布的火山颈群。近年来新出版的《香港地质考察指引》和《香港工程地质实践》等地质著作仍将该区爆发角砾岩误认为是沉积成因的砾岩,在地质勘探和工程设计上造成混乱,导致不应有的经济和时间上的损失。本文综合最新研究成果并与世界各地同类火山岩的特征进行对比,确证火山通道相爆发角砾岩的存在,并发现呈线状分布的古火山颈群。

The middle Jurassic Tunmen Formation volcanic rocks in Hong Kong mainly consist of andesitic lava, tuff and tuff breccias intercalated with tuffaceous sands. The tuff breccia, which has long been erroneously assumed to be sedimentary conglomerate, was determined as volcanogenic tuff breccias by Geological Sur- vey Group of Hong Kong since 1990. Based on petrography, it is found that the Tunmen Formation vol- canic rocks may be divided into andesitie-dacitic lava and explosion breccia of volcanic conduit facies, and tuff breccias of explosion airfall facies. Volcanic conduit facies may be further divided into volcanic neck fa- cies and dike facies which are distributed in both east and west of Tunmen Formation volcanic rocks. The volcanic neck cluster with NNW-trending intermittent and linear distribution is outcropped on the east foothill of Qingshan. The explosion breccia was identified incorrectly as sedimentogenic conglomerate in some geological works published in resent years, which induced some confusions in the process of geologi- cal exploration and engineering design and some economic losses. By summarizing latest research results and comparing with similar volcanic rocks of other regions in the world, it is identified that volcanic con- duit facies explosion breccia is existed and paleovolcanic neck cluster is characterized by linear distribution in Kong Hong.