台灣北部竹子山火山亞群的火山地質與層序研究

Abstract

大屯火山群位於台灣的北方，比鄰大台北都會區，本區包括五個火山亞群(丁火朽山亞群、七星山亞群、磺嘴山亞群、大屯山亞群、竹子山亞群)，主要活動時間為0.8Ma~0.2Ma。過去研究對於此火山活動帶是否仍持續活動抱持保守態度，近年來Song et.,al (2000)與楊燦堯等人(1999)透過氦同位素的研究，指出現今台灣北部底下，相當可能具有岩漿庫的存在。若此結果屬實，台北都會區居民的生命財產立即蒙受鉅大的威脅，有鑒於此，火山地質調查工作即變為刻不容緩。 本研究範圍為位於大屯火山群北方的竹子火山亞群，過去研究對於此區域著墨較少，因受到東南方金山斷層活動以及台灣氣候環境帶來侵蝕作用的影響，竹子火山亞群所保存的火山特徵多已不完整，對於火山層序的重建帶來相當大的困擾。本區火山岩可分為兩輝角閃石安山岩以及輝石安山岩兩種主要岩性，安山岩均落於鈣鹼系列(Calc-alkaline)，經由TAS作圖得知本區火山岩都介於玄武岩質安山岩到安山岩質的範圍，而SiO2對K2O得知樣本多落於中鉀到高鉀的範圍，與前人報導的大屯火山群的地球化學特性一致。 本研究利用精確的2X2m LiDar地形資料，將竹子火山亞群依照熔岩流型態劃分為三個火山單元；A(金山、兩湖村)，B(竹子山、嵩山)，C(小觀音山)。並輔佐以熔岩流接觸行為、野外調查、地球化學特徵(氧化鉀相對含量變化)，對應此三火山單元生成先後，將竹子火山亞群分作三期噴發；早期噴發階段A(金山、兩湖村)，中期噴發階段B(竹子山、嵩山)，以及晚期噴發階段C(小觀音山)。

ABSTRACT The TVG located in Northest part of Taiwan, erupted in 2.5ma~0.2ma. There are five subgroups in TVG ; Chutzeshan Subgroup, Tatunshan Subgroup, Chishingshan Subgroup, Huangtsuishan subgroup and Tinghuohsiushan subgroup. Chutzeshan volcano subgroup (CVS) is an older part of the TVG, volcanic features are destroyed by weathering or faulting, to reconstruct eruption history with damage landform is difficult. According to LiDar DTM analysis, there are three major lava flows groups in CVS. Northen part lava flows group A (Jinshan – Lianghu ) are hornblende two-pyroxene andesite, to be provided with much damage landform and lower altitude. Middle part flows group B (Chutzeshan-Shunshan) are two-pyroxene andesite, it can be find a ring shape crater in Chutzeshan. Southen part lava flows group C has similarity lithology with middle (group B ), but by DTM analysis it can be defined a detachment crater (Siaokuanyinshan ).

Three groups A, B, C have different K2O content. Because of partial melting and fractional crystallization effect, Younger magma might have lower K2O content with older. In CVS, northen part lava flows group A have lower K2O content (<1.6), sou then part lava flows group C have higher K2O content (>2.2 ), middle part lava flows group B K2O content between group C and group A. Besides, by lava flows flow motion, DTM shows younger lava will blocked by older, combine this flow motion, each groups K2O content and andesites altitude, volcanic sequences of CVS can be defined to three stages : older stage (Group A), middle stage (Group B), younger stage (Group C).