花蓮瑞穗地區斑點片岩鈉長石變質斑晶之成因與變質溫度壓力演化; Original and metamorphic P-T evolution of porphyroblastic albites of spotted schists in Juisui, Hualien

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题名:	花蓮瑞穗地區斑點片岩鈉長石變質斑晶之成因與變質溫度壓力演化;Original and metamorphic P-T evolution of porphyroblastic albites of spotted schists in Juisui, Hualien
作者:	江曙帆;Shu-Fan Chiang
贡献者:	地球物理研究所
关键词:	溫壓途徑;大南澳片岩;鈉長石變質斑晶;斑點片岩;porphyroblastic albite;spotted schists;P-T path
日期:	2003-07-10
上传时间:	2009-09-22 09:53:49 (UTC+8)
出版者:	國立中央大學圖書館
摘要:	花蓮瑞穗地區的斑點片岩，是一種具有變質斑晶的泥質黑色片岩，只出現在玉里帶中。由於前人工作多集中在地層描述、地質構造，以及變形組構的研究，而有關瑞穗斑點片岩的岩石學研究卻相當有限，所以本研究針對斑點片岩做岩石學以及變質溫度壓力的研究，進而增進本區多次變質歷史資料的完整性。斑點片岩之共生礦物群為石榴子石+白雲母+斜長石+綠泥石+石英±榍石±電氣石±方解石±金紅石。石榴子石的成份主要介於鐵鋁石榴子石與錳鋁石榴子石之間，具有典型的鐘形生長環帶，其由核心到邊緣的錳含量有逐漸減少，而鐵含量逐漸增加的趨勢，此反映了一個前進變質作用。此外，石榴子石最邊緣偶有逆轉環帶出現，即原本鐘型遞減之錳含量邊緣出現一錳含量增加之小峰，而鐵含量也由原來逐漸增加轉變成局部減少的趨勢。故推論前進變質作用之後有降級變質作用疊加，使得石榴子石邊緣瓦解形成綠泥石。地質溫壓計計算結果顯示，斑點片岩之石榴子石由核心至邊緣記錄著升溫升壓的變質過程，變質溫度由486℃至546℃，壓力由7.4kbar至9kbar。吉布士法計算斑點片岩之石榴子石的溫壓途徑亦為升溫升壓的變質過程。綜合岩象觀察、礦物化學、地質溫壓計計算結果與鈉長石穩定生長範圍之熱力學模擬結果，吾人認為斑點片岩在一開始可能先深埋，前進變質到綠簾-角閃岩相變質環境，形成石榴子石和斜長石 (An>10)。後來在抬升解壓的過程中，斑點片岩回到綠色片岩相的變質環境，使得石榴子石邊緣受降級變質作用，反應生成綠泥石，流體作用使得白雲母崩解，導致鈉長石變質斑晶生成。 The spotted schists of Juisui area in Hualien are a kind of porphyroblast-bearing black schists, which only occur in the Yuli belt. The general geology of Juisui area is well known. However, the study on metamorphic petrology is very limited in spotted schists. This study on metamorphic petrology in spotted schists is expected to provide more information for understanding the multiple metamorphic histories of the Yuli belt. The petrology show mineral assemblages are garnet+ muscovite+ plagioclase+ chlorite+ qtz± sphene± tourmaline± calcite± rutile. The garnets are major almandine and spessartine and display well bell- shaped chemical zonation. Analytical results show that the growth zoning with ratio of Mn/Fe decreasing from core towards rim. That maybe occurred by prograde metamorphism. However, there are some reverse zoning with ratio of Mn/Fe increasing near the rim of garnet. Moreover, the results of petrological observation show the rim of garnets replace by chlorite. The replacement maybe occurred by retrograde metamorphism after the prograde metamorphism. The P, T estimates of the spotted schists from geothermobarometry for the garnets with growth zoning yield 486℃, 7.4kbar for the core and 546℃, 9kbar for the rim. The P-T path derives by Gibbs method is both increasing in temperature and pressure from core to rim. Study of petrology, mineral chemistry and geothermobarometry suggest that the P-T paths demonstrate systematically increases in pressure and temperature from core to rim. These probably reflect the deeply burial process and the metamorphic grade could be up to epitdote-amphibolite facies. The garnets should grow in epidote-amphibolite facies. Subsequently, some rim of garnets replace by chlorite in retrograde metamorphism. At the same time, the albite porphyroblasts grown in decompression process.
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