宋聖榮臺灣大學：地質科學研究所邱清龍Chiu, Ching-LungChing-LungChiu2007-11-262018-06-282007-11-262018-06-282007http://ntur.lib.ntu.edu.tw//handle/246246/54852龜山島位於沖繩海槽最西南端，是北台灣最年輕的火山，其最後一次噴發年代約為7 ka (Chen et al., 2001)。朱秋紅（2005）將MgO＞5 wt.%的龜山島火山岩視為高鎂安山岩。然而，典型的高鎂安山岩（Crawford et al.,1989; Tatsumi and Maruyama,1989; Tatsumi and Hanyu, 2003），如：玻安岩（Boninites）及瀨戶內海高鎂安山岩（Setouchi HMAs），兩者的氧化鎂、鎳和鉻含量均較龜山島高鎂安山岩高，且龜山島火山所處的地體環境是否能提供高鎂安山岩的形成，仍有待商榷，故將龜山島含鎂量較高的火山岩歸類為高鎂安山岩可能較不適當。因此，筆者根據氧化鎂的含量，將龜山島火山岩可劃分成兩群：I型安山岩（MgO＜4.5wt.%）及II型安山岩（MgO＞4.5wt.%）。I型安山岩分布於岩層的下半部，II型安山岩則分布在岩層的上半部，顯示龜山島的岩漿性質由早期的正常島弧岩漿轉變為後期具有較高MgO含量的岩漿。從微量元素分析結果顯示，除了II型安山岩具有較高之鎳（Ni）、鉻（Cr）含量外，其餘微量元素含量大致皆與I型安山岩相近。I型安山岩和II型安山岩的隕石標準化稀土元素分布圖（REE Pattern）和整體不相容元素分布圖（Spidergram）均具有極相似的型態，均呈現銪(Eu)元素負異常、富集的輕稀土元素(LREE) 和大離子岩石圈元素(LILE)，以及虧損的高場力元素(Ti, Nb, Ta)。另外，從龜山島鑽井岩芯觀察結果顯示，上半部岩層具有較多的上部地殼之捕獲岩，而下半部岩層則含有較少之捕獲岩，意味著II型安山岩可能受到相當程度的地殼混染作用，而I型安山岩則可能僅受輕微地殼混染作用的影響。本研究利用地殼混染（crustal contamination）及結晶分化（fractional crystallization）的觀念，配合龜山島地區的大地構造資料，建立龜山島火山岩的岩漿演化模式： 1. I型安山岩可能由玄武岩質岩漿經歷約45%的結晶分化作用而成； 2. II型安山岩則因受到南沖繩海槽擴張的影響，玄武岩質岩漿可快速上升，岩漿因而僅受輕微的結晶分化作用，並且在上升的過程捕獲上部地殼物質，受到約15∼25%的地殼混染作用，如此可形成II型安山岩。Kueishantao (KST) is a young volcanic island located at the southernmost part of the Okinawa Trough. It is mainly composed of andesitic lava flows and pyroclastic flows. Based on major-element geochemistry, KST andesites can be divided into two subgroups, in which Type I Andesites are defined as possessing lower MgO and SiO2, and Type II Andesites possessing higher MgO and SiO2. In general, both of them reveal similar trace-element compositions except that Type II Andesites exhibit high Ni and Cr contents. In primitive-mantle-normalized incompatible element diagram and chondrite-normalized REE pattern, Type I and II Andesites have highly similar patterns. They are characterized by enrichment of large ion lithophile elements (LILEs) and Th, U and Pb, and depletion of high field strength elements (HFSEs). Type I Andesites are in the lower part of core, and Type II Andesites are distributed in the topper part, indicating that KST magmatic evolution from low MgO magmas transferring into high MgO magmas. The amount of xenoliths in the drill cores increases from the bottom to the top, inferring that the degree of crustal contamination increases progressively. Base on the combination of results of chemical analyses, isotopic compositions and petrographic observation, we propose that magmatic evolution of KST is as follows. Firstly, Type I Andesites were produced by about 45% fractional crystallization of basaltic magmas, which were derived from melting of the mantle wedge induced by hydrous fluids released during dehydration reactions in the subducting Philippine oceanic lithosphere, prior to the opening of the southern Okinawa Trough. Secondly, as a result of the opening of the Okinawa Trough, quick ascending of magma may shorten its storage time in magma chamber and experienced less fractional crystallization. Finally, magma assimilated the continental crust materials (about 15-25%) during it ascended through the upper continental crust and then produced Type II Andesites.第一章 緒論...............................................1 1.1 區域地質背景.......................................1 1.2 前人研究...........................................5 1.3 研究動機...........................................6 1.4 研究目的...........................................7 第二章 研究方法...........................................8 2.1 標本採集..........................................8 2.2 野外工作..........................................8 2.3 顯微鏡岩象觀察....................................8 2.4 捕獲岩數量分析....................................8 2.5 全岩主要元素含量分析.............................16 2.6 全岩微量元素含量分析.............................16 第三章 分析結果..........................................18 3.1 野外觀察結果.....................................18 3.2 岩芯觀察與描述...................................19 3.3 岩象觀察.........................................19 3.4 主要元素分析結果.................................20 3.5 微量元素分析結果.................................29 第四章 討論..............................................35 4.1 龜山島高鎂安山岩？................................35 4.2 龜山島火山岩岩漿之可能成因探討....................38 4.3 地殼混染作用與計算..........................39 4.4 II型安山岩主要元素含量之回推......................41 4.5 結晶分化作用......................................46 4.6 結晶分化模式計算..................................47 4.7 龜山島火山岩岩漿演化與沖繩海槽之關係..............49 第五章 結論..............................................54 參考文獻.................................................56 圖版.....................................................61en-US龜山島岩漿演化沖繩海槽地殼混染結晶分化Kueishantaomagmatic evolutionOkinawa Troughcrustal contaminationfractional crystallizationthesis