花蓮和平地區斷層岩石之微構造與磁學性質研究

Abstract

斷層假玄武玻璃是一種黑色玻璃質的岩石，它的形成是由於斷層極快速的滑動速率 (1 公尺/ 秒)局部熔融圍岩、冷卻所形成的。因此，和一般斷層岩石不同，斷層假玄武玻璃可以做為古地震曾經發生的重要且有力的指標之一,所以又被稱為”地震化石”。台灣地處地震頻繁的區域，假玄武玻璃的報導是出乎意料的稀少。前人研究所報導的花蓮和平溪假玄武玻璃，是目前台灣東部變質岩區唯一所發現的假玄武玻璃天然露頭。為了了解假玄武玻璃的熔融過程以及形成機制，我們透過化學分析、顯微構造研究以及磁學測量以研究和平溪的假玄武玻璃。 X光螢光分析化學成分結果指示與圍岩和超質碎岩相比，假玄武玻璃中的二氧化矽、氧化鈉相對減少，二氧化三鐵、氧化鉀相對增加，顯示假玄武玻璃是形成於不一致(部分)熔融。X光繞射分析礦物組成結果顯示，假玄武玻璃和圍岩的礦物成分大致相同，由石英、長石、鉀長石、綠泥石、黑雲母和白雲母所組成。在假玄武玻璃的樣本中存在非晶質的訊號，指示在假玄武玻璃中含有玻璃質物質，這也指示和平溪假玄武玻璃是源自於熔融的重要證據之一。原先圍岩中黑雲母的結晶訊號 (8.75度)，在假玄武玻璃當中，轉變為微緩隆起，指示黑雲母選擇性熔融形成玻璃質物質，黑雲母的熔融溫度(約750 攝氏) 因此可以用來作為假玄武玻璃熔融溫度的下限。利用掃描式電子顯微鏡，在假玄武玻璃當中找到微米等級的微晶體，支持和平溪假玄武玻璃為熔融起源。從化學分析的結果得知，假玄武玻璃的氧化鐵含量略高於圍岩，因此利用振動式測磁儀量測假玄武玻璃中所含有的磁性礦物的特性。磁滯曲線顯示和平溪假玄武玻璃以順磁性為主帶有非常微弱的飽和等溫殘磁。從磁滯參數(Ms)推算在假玄武玻璃中所含的磁鐵礦僅有百萬分之一的濃度。從穿隧式電子顯微鏡的吸收影像中，也有觀察到一些奈米尺度的黑點，可能是較重元素(鐵、鈦)的存在，且這些黑點侷限於假玄武玻璃熔融的部分。在掃描式電子顯微鏡背向散射影像也有發現一些小圓粒，利用能量散布儀分析為氧化鐵。 和平溪假玄武玻璃形成於不一致熔融，鐵的富集程度有限、氧化環境條件不足，使得和平溪的假玄武玻璃為順磁性。假玄武玻璃的形成機制主要牽涉超質碎化以及非常有限的局部摩擦熔融，超質碎岩和假玄武玻璃為同時期、不同機制所形成，兩者緊密共生，不易區分。在本研究中，透過化學、微構造分析方法，試圖解讀假玄武玻璃的熔融過程、熔融溫度和形成機制，並從而區分出兩種外型相似、經常共生的斷層岩石—假玄武玻璃和超質碎岩，對於假玄武玻璃的磁學性質也有初步的探究。

Unlike other fault rocks, fault pseudotachylytes form through frictional melting during an earthquake. Therefore, these rocks, regarded as earthquake fossils potentially hold valuable information on seismic deformation. Paradoxically, although Taiwan is a seismically active zone, reports of pseudotachylyte outcrops in Taiwan remain rare. Previous studies reported the Hoping River pseudotachylyte locality is by far the only natural pseudotachylyte outcrop in the metamorphic region of Taiwan. To understand the melting process and generation mechanism of pseudotachylyte, we apply geochemical, microstructural and magnetic approaches to investigate the pseudotachylyte veins in this study. X- ray fluorescence (XRF) geochemical analysis show that the pseudotachylyte melt, formed by incongruent melting, is depleted in SiO2 and N2O, and enriched in Fe2O3,and K2O compared with the result of ultracataclasite and host rock. X-ray diffraction (XRD) analysis suggest the composition of pseudotachylyte and the host rock are dominant by quartz, feldspar, biotite, muscovite, and chlorite. In 4-10 and 20- 30 2-theta degrees, glass humps are prominent in pseudotachylyte, which is an evidence of melt- origin of the pseudotachylyte. The existence of microcrystallite (tiny, new grown crystals) under scanning electron microscope (SEM) also supports its melt- origin. Since iron content of the pseudotachylyte is slightly higher (4 wt. %) than the ultracataclasite and granitic host rock, magnetic hysteresis measurements were performed under high field (up to 1 Tesla) using a vibrating sample magnetometer (VSM) to determine the nature of magnetic minerals. Magnetic hysteresis curves show the pseudotachylyte veins of the Hoping River are dominated by paramagnetic phases, with a very weak saturation isothermal remanent magnetization (SIRM). Magnetite concentration, inferred from these measurements, is on the order of a few ppm only. Submicron iron-oxide (possible) grains are discovered under transmission X-ray microscope (TXM), which may be formed by the breakdown of biotite during melting. The presence of iron oxide grains appears restricted to the melted part of the pseudotachylyte. The pseudotachylyte in the Hoping River are formed by incongruent melting. The enrichment of Fe did not strongly change the nature of magnetic properties, resulting in paramagnetic- dominant of the pseudotachylyte. The generation mechanisms of pseudotachylyte in the Hoping River involve ultracataclasis and extreme limited melting, leading to the coeval formation of ultracataclasite and pseudotachylyte.