北臺灣大屯火山群七星山岩漿庫行為之研究

Abstract

七星山亞群為大屯火山群眾多的火山亞群之一，其主要山體包括七星山、白雲山、燒庚寮、竹篙山、紗帽山與七股山等。依據前人研究成果，七星山之岩漿活動可區分為三期，分別產出紫蘇輝石角閃石安山岩、含角閃石兩輝石安山岩及兩輝石角閃石安山岩等不同岩性。七星山東、西兩側各發育一條包含約四個噴發破裂口之裂隙帶。陽管處於2021年執行之地質鑽探計畫中，完成東側四口鑽井，並成功取火成岩之岩心資料。本研究旨在利用上述鑽井所獲岩心中之火成岩樣本，探討七星山下方潛在岩漿庫之性質與活動機制。岩心採樣位置集中於七星山東側蒸氣噴發破裂口帶，取樣自S1及S3-1-1兩口井。其中S1井取樣深度為4.3 - 4.6 m、9.4 - 9.8 m及11 - 11.4 m；S3-1-1井則取自5.7 - 6 m與7.4 - 8 m，共五段岩心區段。將岩心樣本製成岩石薄片進行岩象學觀察，確認其中所包含之礦物種類，將需要做礦物化學的礦物鎖定在斜長石、角閃石、輝石。製作岩石靶進行掃描式電子顯微鏡 (Scanning Electron Microscope，SEM)與電子能量散射分析(Energy-Dispersive X-ray spectroscopy，EDS)來觀察礦物外型及化學組成。將SEM-EDS所得到的結果，再利用電子微探儀(Electron Probe Microanalyzer，EPMA)對礦物進行化學成分的定量分析。礦物化學數據藉由前人文獻中的經驗公式，針對斜輝石進行地溫計與Fe-Mg擴散速率的計算、角閃石進行地壓計的計算、斜長石觀察其An值與Mg擴散速率之間的關係，推論此岩漿在地下所停留的時間及有岩漿補充的歷程。結合40Ar/39Ar定年分析結果，了解岩漿噴發的時間。並進行全岩主要元素、全岩微量元素、全岩鍶釹同位素地球化學分析，比對前人的地化特性。本研究樣本噴發年代太過年輕，因此無法得到40Ar/39Ar定年的結果。七星山全岩的地球化學成分均一，釹同位素值變化極小，顯示岩漿源區單一。藉由礦物化學分析結果得知，此岩心中的礦物生長環境在地下6-12公里之間的區域，晶體開始生長時的溫度在1120 - 1180 ℃間，透過擴散速率得知岩漿在噴發之前，最久於岩漿庫停留了約8000年的時間，而在岩漿噴發的前十年間，出現了密集的3次岩漿補充事件。The Chihsingshan sub-group is one of the sub-groups within the Tatun Volcanic Group. Its main volcanic edifices include Chihsingshan, Baiyunshan, Shaogengliao, Zhugaoshan, Shamaoshan, and Qigushan. According to previous research, the magmatic activity of Chihsingshan can be divided into three stages, producing various rock types such as pyroxene - hornblende andesite, hornblende-bearing two-pyroxene andesite, and two-pyroxene - hornblende andesite. On both the eastern and western flanks of Chihsingshan, fissure zones have developed, each containing approximately four eruptive vents. In 2021, a geological drilling project was conducted at Yangmingshan National Park Headquarters, during which four boreholes were completed on the eastern side, successfully retrieving igneous rock core samples.This study aims to investigate the characteristics and activity mechanisms of a potential magma reservoir beneath Chihsingshan by analyzing igneous rock samples obtained from the aforementioned boreholes. The core sampling sites are concentrated along the eastern fissure zone of Chihsingshan, associated with phreatic eruptions, specifically from boreholes S1 and S3-1-1. Samples from borehole S1 were collected at depths of 4.3 - 4.6 m, 9.4 - 9.8 m, and 11 - 11.4 m, while those from borehole S3-1-1 were taken from 5.7 - 6 m and 7.4 - 8 m, totaling five core intervals. The core samples were made into thin sections for petrographic observation to identify the phenocrysts, focusing mineral chemical analyses on plagioclase, hornblende, and pyroxene. Rock mounts were prepared for Scanning Electron Microscopy (SEM) and Energy-Dispersive X-ray Spectroscopy (EDS) to examine the mineral morphology and chemical composition. The SEM-EDS results were further analyzed using an Electron Probe Microanalyzer (EPMA) for quantitative chemical analysis of the minerals. Using empirical formula from previous studies, the mineral chemistry data were used to calculate pyroxene-based geothermometry and Fe-Mg diffusion rates, hornblende-based geobarometry, and to examine the relationship between the An content and Mg diffusion rate in plagioclase. These analyses help infer the residence time of magma in the subsurface and the history of magma recharge events. This research tried to date these samples using 40Ar/39Ar geochronology, however, it’s too young to get an 40Ar/39Ar age. Major and trace element analyses, along with Sr-Nd isotopic geochemistry of whole-rock samples, were conducted to compare with previously published geochemical characteristics. The whole-rock geochemistry of Chihsingshan is homogeneous, with minimal variation in Nd isotopic values, suggesting a single magma source. The results indicate that mineral crystallization occurred at depths between 6 and 12 km beneath the surface, with initial crystallization temperatures ranging from 1120°C to 1180°C. Diffusion rate calculations reveal that the magma remained in the reservoir for up to approximately 8000 years before eruption, with at least three intense magma recharge events occurring within the final ten years prior to the eruption.