烈嶼東崗地區基性岩脈群之活動與地球化學研究

Abstract

金門及烈嶼島分布著許多位態以東北走向為主、傾角近乎垂直的未變形基性岩脈群侵入花崗岩中。前人研究認為這些基性岩脈是中生代晚期非造山張裂環境下最後一次的岩漿侵入事件。然而經由野外探勘觀察到這些基性岩脈有互相截切情形，走向及岩性也不盡相同，暗示其年代、母岩漿或岩漿演化可能有所差異。本研究以烈嶼東崗地區出露的基性岩脈群做為研究材料，藉由野外調查和岩象學等方法，以截切關係、位態、岩性、礦物組成和組織將這些基性岩脈進行分組，並以岩石地球化學之全岩主量和微量元素地球化學分析進一步探討其地質演化的意義。 野外觀察發現岩脈位態分為東北與近南北走向，其中兩條岩脈有與圍岩混雜的現象。岩象分析結果顯示多數岩脈樣本具斑狀組織，礦物組成顯示岩性為安山岩、黑安山岩或石英閃長岩，基質礦物及斑晶礦物多為斜長石及角閃石，部分樣本含少量黑雲母、石英等礦物，礦物顆粒整體而言較細，依照截切關係、位態及岩象學分析結果將之分為具相對年代關係的五大期，其中兩個大期可再細分，一共八個小組。 由全岩主量和微量元素的分析結果得知，大部分樣本落於玄武岩質安山岩的範圍中，屬於中鉀至高鉀鈣鹼性系列岩石，蛛網圖中有明顯的島弧岩漿特徵，且有相似的結晶分異趨勢，部分主量和微量元素呈現較凌亂的分布趨勢，暗示岩漿源區性質不同，源區可能較接近尖晶石二輝橄欖岩的部分熔融曲線，且部分熔融程度不同並有受到隱沒板塊帶來物質的影響。除了最老的分期之外，這些地球化學變化特徵與年代分期似乎沒有太大關聯。 本區基性岩脈與華南沿海一帶的I型花崗岩和基性岩脈比較，顯示出相似的島弧地球化學特性，A型花崗岩相對較無島弧特徵，但三者均屬張裂環境。綜合上述結果及前人研究，認為花崗岩岩漿源區主要與地殼熔融有關，基性岩脈源區與受隱沒改造的岩石圈地函熔融有關，且源區成分不均質和部分熔融程度也有所變化。

Many mainly NE in strike and near vertical in dip, intruded into granite, non metamorphic mafic dikes are distributed in Kinmen and Liehyu island. Previous studies suggested that these mafic dikes were the last magma intrusion event in the late Mesozoic anorogeny extension environment. However, field trip observation observed that these mafic dikes are cross cut each other, and have different orientations and lithology, suggesting that their age, parent magma is different or undergone different evolution. In this study, we use mafic dikes in Dongang, Liehyu as the study materials, and group these mafic dikes by cross cutting relationship, orientation, mineral composition and rock texture, and further discuss their geological significance by whole-rock major and trace element geochemical analysis data. These dikes orientation divide into NE or near NS trend, two of the dikes are mingling with the host rocks. Petrographic analysis shows that most dike samples are porphyritic, the lithology is andesite, mela-andesite or quartz-diorite. The samples are generally fine grained, groundmass and phenocryst are mainly compose by plagioclase and hornblende, some samples contain a small amount of biotite or quartz etc. According to cross cutting relationships, orientations and petrographic analysis, these mafic dikes are divided into five major stages with relative chronological relationships, two stage can be further subdivided, eight groups in total. The results of whole-rock major and trace element analyses show that most of the samples fall in the area of basaltic andesite, belong to the medium to high potassium calc-alkaline series. All samples reveals arc-like characteristic in spider diagram, and similar fractional crystallization trend. However, some of the major and trace elements show a more scattered distribution, suggesting that the magma source characteristic is different. Magma source closer to the partial melting curve of the spinel lherzolite, have varying partial melting degree, and also influenced by subduction materials. Except earliest stage dikes, these geochemistry variation do not appear to correlate with relative age staging. These mafic dikes show similar arc-like geochemical characteristics compared with the I type granite and mafic dikes along the coast of Southeast China, the A-type granite no apparent arc-like characteristics, but all are in extension environment. The magma source of granite is mainly related to crustal melting, and the magma source area of mafic dikes is related to partial melting of lithospheric mantle modify by subduction materials, the composition of the source magma area is not homogeneous and the degree of partial melting also have variation.