- 期刊
花蓮瑞穗地區藍閃石片岩之特性與成因
The Characteristics and Petrogenesis of Glaucophane Schists in Juisui Area, Hualien
摘要
花蓮瑞穗地區產出極具經濟價值之石礦首推蛇紋石，此外亦伴隨產出少量之竹葉石（藍閃石片岩）。此二者皆係海洋地殼受變質作用而生成，後者尤為板塊構造演化產生之重要指標岩類，此其為本研究之旨趣。野外及岩象分析結果顯示，藍閃石片岩及伴生岩類之角閃石可分為三類，一為環帶角閃石變質斑晶，反映由角閃岩相進入藍閃石片岩相之高壓變質環境；二為鈣角閃石變質斑晶，反映降級變質作用的過程；三則為綠色片岩中普遍常見之鈣角閃石。本研究利用地質溫壓計推算主要岩相之生成環境，藍閃石片岩生成溫度約在470℃以上。由吉布士法推算其伴生岩類則為增溫增壓之過程。綜合各類分析結果，藍閃石片岩類的原岩初始為一受深埋之環境，隨增高之溫壓進入角閃岩相之變質階段，而後再受板塊推擠而使進入高壓的藍閃石片岩相環境。後續地殼抬升解壓過程，復返綠色片岩相之變質環境，使得岩體外圍受到降級變質作用，而僅在核心位置未及平衡，藍閃石片岩得以保存。
並列摘要
The most valuable ore deposits in Juisui area of Hualien is serpentinite; glaucophane schist (aka bamboo-leaf stone) is an accompanying minor deposit. Both were formed from metamorphosed oceanic crust, the glaucophane schist is especially important rock type indicating the plate tectonic evolution. The aim of this study is to investigate the characteristics and petrogenesis of glaucophane schists. The field investigation and petrographic observations reveal three kinds of amphiboles in glaucophane schists and associated rocks: (1) compositional zoning porphyroblast which reflects a metamorphic environment changed from amphibolite facies to glaucophane schist facies; (2) Ca-amphibole porphyroblast which implies a retrograde metamorphism; and (3) non-zoning porphyroblast in green schists. Results of geothermobarometer study show that under similar pressure of around 11KBar, the temperature of formation at garnet-glaucophane schists was over 470℃. The P-T path of epidote-amphibole-quartz schists calculated by Gibbs method shows at an increasing temperature and pressure. Results of all above indicate that the protolith of the glaucophane schists in Juisui area was initially in deep burial environments, then to amphibolite metamorphic facies, and eventually subjected to tectonic plate movements and transferred to high-pressure glaucophane facies. In the subsequent decompressional rifting process, the block of glaucophane schists were transferred to greenschist facies environments. The fringe of the block was affected by a retrograde metamorphic process, while the glaucophane schists remained intact in the center of the block.