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題 名 | Origin of REE-Depleted Leucogranites in Chinmen Island, SE Fujian=金門島稀土元素貧乏型淡色花崗岩之成因研究 |
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作 者 | 林蔚; 陳正宏; 李寄嵎; | 書刊名 | 中國地質學會會刊 |
卷 期 | 40:3 1997.07[民86.07] |
頁 次 | 頁587-606 |
分類號 | 358.43 |
關鍵詞 | 稀土元素貧乏型淡色花崗岩; 燕山晚期岩漿活動; 金門島; REE-depleted leucogranite; Late yanshanian plutonism; Chinmen island; |
語 文 | 英文(English) |
中文摘要 | 本文報導金門地區兩個稀土元素貧乏型淡色花崗岩的最新研究成果,並從野外產 狀及岩石化學的觀點,討論其可能的成因及重要性。這兩個侵入體在岩性及產狀上均呈現極 大的差別,出露在太武山者岩性接近正常淡色花崗岩,乃侵入到大武山片麻岩中;在田浦出露 者為幾由長英質所組成的白色侵入體,夾在田浦淡色花崗岩與混合岩之接觸帶間。其地球化 學上均呈現極低的稀土元素含量,在稀土元素標準化分佈圖中呈現低平的分佈趨勢,並帶有 些微的銪元素異常。「太武山型」的成因,可能是因為岩漿演化後期,富稀土元素的附屬礦物 結晶分異,導致岩漿中稀土元素的虧損。至於「田浦型」岩體因與混合岩的淡色體相連,顯 示此岩漿尚未脫離其源區,應為混合岩受淡色花崗岩岩漿侵入作用而部分熔融所致,其稀土 元素貧乏現象乃淡色體原岩的特徵。此研究對於金門地區燕山晚期淡色花崗岩岩漿活動的了 解頗有助益,淡色花崗岩漿可能在上升管道中或侵入時受到此類成分不同程度混染作用的影 響,化學成分上顯現出二元混合的線性關係。 金門地區位於長樂南澳變質帶的南段,經由本研究結果可以對此變質帶特有之混合岩化 變質核心雜岩(Metamorphic Core Complex)與燕山晚期岩漿活動的關係,提供下列幾點制約條 件:(1)此類混合岩應非過去一般相信的深域高度變質岩,而是由呈現淺位侵入特徵的燕山晚 期淡色花崗岩漿,侵位於上部地殼中已停止活動的塑性剪切帶,產生岩漿接觸圈型態的混合岩 化作用;(2)由混合岩接觸圈部分熔融所形成的稀土元素貧乏型淡色花崗岩,對燕山晚期花崗 岩漿所受的混染作用扮演了端成分的角色;(3)因淺位侵入快速冷卻的因素,部分淡色花崗岩 顯現出岩漿演化後期,受富稀土元素之附屬礦物結晶分異作用影響,而產生正常淡色花崗岩- 稀土元素貧乏型淡色花崗岩-細品岩-偉晶岩之岩漿演化序列。 |
英文摘要 | Some leucogranites in Chinmen are chemically distinct from the majority of Albian leucogranites on account of their high degree of the rare earth elements (REE) depletion. These REE depleted leucogranites may play an important role in unraveling the evolution history of the leucogranitic magmatism in the Changle-Nanao Metamorphic Belt. Two such samples are studied. One occurs in the Taiwushan gneiss characterized by homogeneous, structureless features with no evidence of metamorphic crystallization. The other occurs along the margin of a migmatite zone and is in direct contact with the normal type of Tienpu leucogranite. Generally, these intrusives have high silica and low REE contents and show relatively flat chondrite-normalized patterns, consistent with the nature of typical REE-depleted leucogranites. However, from field observation and geochemistry, two different origins can be distinguished. The Taiwushan-type was probably generated by the fractionation of REE-rich accessory minerals that controlled the processes of REE depletion in the late stage of leucogranitic magmatism. Conversely, the Tienpu-type might be the product of the partial melting of the host migmatite. These results provide evidence against the conventional concept of there being a deep metamorphic core complex. It is suggested here that the emplacement of the Albian leucogranitic intrusions into preceding ductile shear zones of the metamorphic belt at higher crustal levels must have been the major contributor in the formation of migmatitic aureoles. |
本系統中英文摘要資訊取自各篇刊載內容。