| 摘要: | 大南澳變質雜岩區之玉里帶為典型台灣地區出露高壓相變質礦物群之地質帶。然而在含有藍閃石等高壓相礦物的岩石卻與含有枕狀構造之變質基性岩相接觸卻是相當令人無法理解的事宜。今針對該岩塊以岩象學以及礦物化學的觀點進行研究以及解析該岩石所經歷之演化過程。 經由岩象學觀察，變質基性岩與石榴石-綠簾角閃岩兩種不同岩性之岩種具有不同的礦物組成。變質基性岩組成礦物群有：綠簾石、綠泥石、石英、鈉長石、陽起石、方解石、榍石、白雲母等；而石榴石-綠簾角閃岩的組成礦物群為：石榴子石、角閃石、綠簾石、石英、綠泥石、鈉長石、磷灰石等。而部分角閃石斑晶內具有藍閃石之包裹體，角閃石本身亦具有多期變質的特徵。 利用全岩成分解析兩種岩類的母岩來源，結果顯示變質基性岩來自於島弧系統的玄武岩，而石榴石-綠簾角閃岩來自於洋底沉積物與凝灰岩的混合體。在石榴石-綠簾角閃岩內發現有變質基性岩之岩塊，顯示在變質之前兩種岩性及以共同存在，分別為洋底熔岩與沉積物的關係，並且在隱沒及抬升的過程中受到相同的變質作用。 進一步利用掃描式電子顯微鏡搭配能量散射光譜儀對研究標本內之礦物進行定性與半定量之分析。角閃石主要分為鹼性角閃石與鈣角閃石兩大類，鹼性角閃石全部屬於藍閃石，鈣角閃石分為淺閃石、韭閃石、陽起石、普通角閃石等。進一步將共生礦物群套入前人研究中得到角閃石至少經歷了三期以上的變質相的改變。第一期為藍色片岩相，在此時期中發育藍閃石；之後變質相轉變為綠簾-角閃岩相，原先的藍閃石外緣開始增生鈣角閃石(普通角閃石、淺閃石、韭閃石)，變質基性岩最外層的玻璃質物質亦在這個時候再結晶而生成角閃石等其他礦物之斑晶。在最後抬升的過程回到綠色片岩相發育大量的綠色礦物，並且在這個時期受到大量的熱液作用，導致許多礦物的表面受到蝕變並且結晶許多含有鉻元素之晶體。 綜合礦物化學、岩象觀察、以及全岩分析的結果。該岩石為洋底沉積物與島弧系統之火山岩混合的岩石，並且在隱沒的過程中被拉至藍色片岩相的變質環境，而後離開隱沒帶被移置至增積岩體內部受到綠簾-角閃岩相之變質作用，在抬升的過程中回到綠色片岩相並且受到大規模的熱水換質作用最後抬升並出露於地表。 Yuli belt is a typical geologic province of high pressure minerals. Amphibolites with glaucophane inclusions contacting with metapillow is quite ynusual. In this study, we use petrography and mineral chemistry to construct these rocks and to define their evolutions. According to the petrographic observation, mineral constructions of metabasite are epidote, chlorite, quartz, albite, actinolite, muscovite, calcite, and sphene. Mineral compositions of Garnet-Epidote Amphibolite are garnet, amphibole, epidote, quartz, calcite, chlorite, apatite, and albite. Some amphibole exhibits poikiloblastic textures with glaucophane inclusions. And these amphiboles also show polymetamorphism. Bulk rock analysis is quite powerful to define the protolith of metabasite and garnet-epidote amphibolite. The results show that the metabasite came from island arc basalt; and the garnet-epidote amphibolite were mixed by volcanic ash, tuff and ocean floor sediment. Some metabasite were found in the garnet-epidote amphibolite. It implies that these two rocks were coexisting before metamorphism. In addition, these two rocks went through the same metamorphism. Using SEM and EDS to perform qualitative and semi-quantitative analyses of the mineral chemistry are very effective. The amphiboles were classified to Alkali and Calcic amphiboles. The alkali amphiboles in this study were glaucophane. And the calcic amphiboles were classified to edenite, pargasite, actinolite, and hornblende. The amphiboles went through three stages metamorphism. The first stage was blueschist facies, then epidote-amphibolite facies, the last stage was greenschist facies. The glaucophane were overgrown by calcic amphiboles, and the least rimed by actinolite. When the rocks were uplifted, the hydrothermal alteration then occurred. Many minerals were replaced by quartz, and many grain surfaces were altered. In area altered by hydrothermal water chrome element can be found. Combining mineral chemistry, petrography, and buck rock analysis, this rock includes metabasite, ocean floor sediment, and volcanic ash. When the protolith sub-ducted to blueschist facies, it grow glaucophane. Then it transferred to epidote-amphibolite facies and growth the calcic amphiboles. Finally, the rock was uplifted to greenschist facies to the surface. |
