基隆火山群安山岩岩象學與石英來源之再探討

Abstract

基隆火山群位於北台灣火山活動帶的東北部，其火山體包含了基隆山、本山、武丹山、草山與雞母嶺。此外，在基隆市東北方外海的基隆嶼因形成年代、岩性特徵和地理位置皆與基隆火山群十分接近，經常被歸類於基隆火山群中一起討論。在北台灣火山活動帶中的火成岩中，僅在基隆火山群裡發現火成岩中含有肉眼可辨的石英顆粒，這是很特別的現象，而此現象與基隆火山群的岩石命名有緊密的關聯。從較早的文獻至今，學者們有著不同的發現與討論，經由岩象觀察、主要元素含量、氧同位素、實驗岩石學等方式，探討應稱呼其為石英安山岩、安山岩或含有石英的安山岩等稱呼上的分歧，而本研究在前人文獻的基礎之上，再以岩象學礦物計數、全岩地球化學分析結果，來加以探討這個議題，認為基隆火山群的火山岩應稱為安山岩或含有石英的安山岩。 關於基隆火山群火成岩特有的石英顆粒，本研究以岩象搭配石英顆粒電子影像，探討其石英顆粒的來源。在電子影像中，發現石英顆粒外表大多具有平滑表面或者貝狀斷口，岩象觀察中，部分石英顆粒為自形、單顆分布，可能代表石英為岩漿本身晶出；而電子影像中極少部分石英顆粒有小面積類似沉積岩的不平滑表面，岩象學觀察中，石英顆粒有渾圓的外表，可能代表石英是捕獲周圍沉積岩而來。另外，在岩象學觀察中發現石英顆粒具有灣狀構造，其造成原因有可能為岩漿捕獲周圍沉積岩而將石英熔蝕，也可能是石英顆粒於岩漿庫生成後，在岩漿上升過程中經過溫壓改變而有不穩定的狀態而造成。綜合各種特徵，本研究推論石英來自岩漿晶出或者由沉積岩捕獲而來，兩者皆有可能，甚至是兩種來源皆存在。 另外，因基隆嶼在前人文獻中，表現出部分資訊與基隆火山群相近的訊息，但也有部分文獻發現基隆嶼和大屯火山群的資訊較為相關，於是有了基隆嶼歸屬的議題討論，本研究以全岩主要元素與微量元素、全岩鍶釹同位素，以及前段敘述的石英顆粒特徵觀察等方法，並參考前人文獻，認為基隆嶼和基隆火山群可能來自不同岩漿庫。The Chilung Volcano Groups is located in the northeastern part of the Northern Taiwan Volcanic Zone, including volcanoes such as Chilungshan, Penshan, Wutanshan, Tsaoshan, and Chimuling. Additionally, Chilungyu, located off the northeastern coast of Chilung City is frequently discussed in connection with the Chilung Volcano Groups due to its similar eruption age, petrographic characteristics, and geographic location. Among the igneous rocks in the Northern Taiwan Volcanic Zone, a distinctive feature unique to the Chilung Volcano Group is the presence of visible quartz grains in its igneous rocks. Previous researchers have provided various rock classifications for volcanic rocks from the Chilung Volcano Group, including dacite, andesite, and quartz-andesite. Their studies contented petrographic observations, whole-rock geochemistry, oxygen isotopic analyses, and experimental petrological study. Based on previous studies, this study examines this issue with classification by whole-rock major elements and petrography, and concludes that the volcanic rocks of the Chilung Volcano Groups should be referred to as andesite or quartz-andesite.Regarding the unique quartz grains found in the volcanic rocks of the Chilung Volcano Groups, this study investigate their origin using petrographic analysis combined with electron images of quartz grains. In the electronic images, it was observed that most quartz grains have smooth surfaces or conchoidal fractures. Petrographic observations revealed that some quartz grains are euhedral and single-particle, possibly indicating their crystallization from the magma itself. On the other hand, a small portion of quartz grains in the electronic images exhibit slightly uneven surfaces resembling sedimentary rocks, and petrographic observations showed that these quartz grains have rounded appearances, suggesting they might have been captured from surrounding sedimentary rocks. Furthermore, in petrographic observations, it was discovered that the quartz grains exhibit embayment texture, which could be the result of the magma melting and eroding the surrounding sedimentary rocks, or it could be due to unstable conditions caused by temperature and pressure changes as the quartz grains formed in the magma chamber and experienced variations during the magma ascent process. Taking into consideration all these features, this study concludes that the quartz grains in the volcanic rocks could have originated from either crystallization within the magma or from being captured from sedimentary rocks.This study employs methods including whole-rock major and trace elements, whole-rock strontium-neodymium isotopes, and the observation of quartz grain characteristics. By referencing previous literature, it is suggested that Chilungyu and the Chilung Volcano Group likely originate from distinct magma reservoirs.