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青龍山榴輝岩中石榴子石變質斑晶之化學環帶現象及變質溫壓條件
The chemical zoning and metamorphic conditions for garnet porphyroblasts in Qinglongshan eclogites
摘要
榴輝岩之岩象特徵及礦物化學成份可記錄板塊隱沒及折返過程之變質歷史，以蘇魯超高壓變質帶中青龍山榴輝岩為例，其代表典型大陸岩石圈隱沒超過200公里之產物。青龍山榴輝岩合大量合水變質斑晶，部份標本含有石榴子石變質斑晶。岩象特徵顯示本研究樣品於峰期變質作用後，歷經變質班晶增生階段以及後成合晶發育階段。此區石英脈所合之高壓礦物組合與榴輝岩中變質斑晶相似，推測流體沿榴輝岩組成礦物之顆粒邊緣作用而形成合水變質斑晶，此外流體亦使石榴子石增生形成其複雜環帶現象之變質斑晶。由各變質階段之平衡礦物成份計算其變質條件，結果顯示青龍山榴輝岩峰期變質條件為749 -782°C、31.5-36.0 kb。而變質斑晶形成階段為岩體折返至角閃石一榴輝岩相(755-791°C、18.3-19.8 kb)，此變質壓力低於文獻中以納雲母形成條件為制約之結果。顯示榴輝岩體在初期折返階段持續與流體作用，可能依序形成不同之礦物相。
並列摘要
Petrographic characteristics and mineral chemistry of eclogites may record the metamorphic history during slab subduction and exhumation. The Qinglongshan eclogites in the Sulu ultrahigh-pressure (UHP) metamorphic belt, which was formed by continental subduction to depths greater than 200 km were investigated in this study. These eclogite samples are characterized by large amounts of porphyroblastic hydrous minerals, and some samples have garnet porphyroblasts. The petrographic evidence shows that the eclogites experienced multi- stage metamorphisms, which formed peak metamorphic assemblage, porph yroblasts, and symplect ites, respectively. In the studied area, quartz veins contain similar high-pressure mineral assemblage as the porphyroblastic phases in the eclogites. It was inferred that the formation of porphyroblasts may be related to the infiltrations of high-pressure fluids along mineral grain boundaries. Additionally, the fluid infiltrations could be responsible for the development of garnet porphyroblasts with complex chemical zoning. The metamorphic conditions were calcul ated from composition s of equilibrated minerals, indicating that the UHP peak metamorphism occurred under the condition of 749-782 ℃ and 31.5-36.0 kb. Subsequently, the eclogties retrograded to amphibole-eclogite facies (755-791 ℃ and 18.3-19.8 kb) to form the porphyroblastic phases during exhumation. The metamorphic pressure is significantly lower than the published result constrained by the format ion of paragonite prophyroblasts, implying that the eclogites were continuously interacted with fluids during early exhumation stage to form different porphyrobla stic minerals at varied P-T conditions.