DIVERSITY AND ORIGIN OF QUATERNARY BASALTIC MAGMA SERIES IN NORTHERN TAIWAN

臺灣北部第四紀玄武岩漿的起源與歧異

莊文星(WEN-SHING JUANG)

《Collection and Research》 4期 (1993/11) Pp. 125-166

https://doi.org/10.6693/CAR.1993.4.10

摘要

In northeastern Taiwan, the subduction of the Philippine Sea plate beneath the Ryukyu arc dips around 45 degrees northward to a depth of roughly 150 km. The volcano groups and volcanic islets in northern Taiwan are considered to be the western extension of the Ryukyu volcanic arc. The volcanic events of northern Taiwan are indicated by K-Ar age data to have occurred from Pliocene to Recent and are primarily of Quaternary age. The Quaternary basaltic activities are of special interest, which took place in Penchiahsu (high-MgO, low-alumina basalt to low-MgO, high-alumina basalt, 2.1-0.29 m. y.), Mienhuahsu (high-MgO, low-alumina basalt, 0.53-0.44 m. y.), Hunglushan (low-MgO, high-alumina basalt, 0.39-0.11 m. y.), Kuanyinshan (shoshonitic basalt, 0.53-0.2 m. y.) and Tsaolingshan (absarokite, 0.25-0.17 m. y.). The eruption ages of the volcanics of Penchiahsu, Mienhuahsu, Hunglushan, Kuanyinshan and Tsaolingshan as well as their geochemical characteristics, i.e. depletion of Nb, Ta and Ti all indicate that they are island arc affinities. The systematic chemical variation of the volcanic rocks in northern Taiwan suggests that the chemical differences may be related to the involvement of upper crustal material in the magma genesis. Either sediment contamination has possibly been involved during the formation of these rocks as suggested from ε_(Nd)-ε_(Sr) plot or source component mixing took place in the region of arc magma generation. The presence of meta-sandstones or quartz schists in the volcanics may generally reflect a certain degree of anatexis of sialic rocks. The higher Ni, Cr, and Mg values indicate that Tsaolingshan absarokite may represent a primary magma. Based on K, Rb, Cs and LREE data the primary magma may be derived from upper mantle peridotite which has been contaminated by the product of dehydration or partial melting of the subducted Philippine Sea plate. Obscure subduction zone beneath Kuanyinshan and Tsaolingshan and the plate boundary may offer a pathway by which the LILE-enriched fluid generated at deeper level can be supplied to the adjacent upper mantle peridotite and has caused mantle heterogeneity. This occurrence could therefore be responsible for creating a heterogeneous magma source. The existence of phlogopite lherzolite xenolith in the absarokite indicates that absarokite magmas were derived from the fertile source mantle material by smaller degrees of partial melting. A wide range of high-MgO, low-alurmina basalt, low-MgO, high-alumina basalt, shoshonite and absarokite in northern Taiwan have therefore been derived from a mantle source that has undergone variable degrees of partial melting and fluid contamination by the downgoing slab.

關鍵字

Geochronology ； Chemical Variation ； Basalt ； Petrogenesis ； Quaternary ； Northern Taiwan

並列摘要

臺灣北部火山區主要包括大屯火山群、觀音山、草嶺山、基隆火山群及外海的彭佳嶼、棉花嶼、花瓶嶼、基隆島和龜山島。根據地體構造與地球化學特性看來，臺灣北部陸上和外海島嶼的火山帶是琉球火山帶向西的延伸。這個火山帶由於菲律賓海板塊隱沒在琉球海溝西段所形成。由於菲律賓海板塊向北隱沒到歐亞大陸板塊之下，因而產生了臺灣北部的火山活動。鉀－氬法定年顯示火山活動發生於上新世至近代，以第四紀爲主。而第四紀的玄武岩漿活動，值得深入探討。其中彭佳嶼（高鎂低鋁玄武岩－低鎂高鋁玄武岩，2.1－0.29百萬年）、棉花嶼（高鎂低鋁玄武岩，0.53－0.44百萬年）、烘爐山（低鎂高鋁玄武岩，0.39－0.11百萬年）、觀音山（鹼長基玄武岩，0.53－0.2百萬年）和草嶺山（白榴基玄武岩，0.25－0.17百萬年）由彭佳嶼、棉花嶼、烘爐山、觀音山和草嶺山火山活動的時代，配合岩石具有島弧玄武岩之地球化學特徵，如鈦、鈮和鉭相對含量較低，顯示此等玄武岩類皆爲火山島弧的產物。就地球化學特性而言，臺灣北部第四紀火山岩之化學成分變化主要受制於地殼之混染與在班尼奧夫帶上之上部橄欖岩之部分熔融程度。由鈮－鍶同位素變化看來，北部火山岩之産生可能是由沉積物混入之地殼汚染或岩槳源富化混染之結果。火山岩中砂岩、石英片岩等之普偏存在似乎支持了矽鋁質混染之推斷。但鹼長基玄武岩質岩系鹼金屬含量高出一般島弧火山岩或沉積物甚多，難由沉積物混合或純粹由地函物質小比率部分熔融獲得合理解釋。目前的看法，乃是富化的地函岩漿源，經由不同比率的部分熔融而産生了北部的玄武岩漿，其中由白榴基玄武岩、鹼長基玄武岩、低鎂高鋁玄武岩至高鎂低鋁玄武岩部分熔融比率逐漸增高。草嶺山白榴基玄武岩鎳、鉻及鎂指示値稍高，顯示可能源生自一種原始的玄武岩漿。鉀、銣、銫和輕稀土元素的數據又顯示出此原始岩漿，可能導源於上部地函橄欖岩。此橄欖岩極可能已受到隱沒的菲律賓海板塊經脫水或部分熔融作用所産生物質的汚染。白榴基玄武岩含有金雲母二輝橄欖岩之地函超基性捕獲岩塊，也支持了富化地函物質歷經部分熔融産生白榴基玄武岩漿的說法。綜括而言，臺灣北部，性質上差異極大的玄武岩漿，包括高鎂低鋁玄武岩、低鎂高鋁玄武岩和鹼性玄武岩，乃導因於隱沒作用，經由不同程度的富化混染再歷經不同程度的部分熔融所衍生的結果。

並列關鍵字

定年學 ； 地球化學 ； 第四紀玄武岩 ； 臺灣北部 ； 岩漿分歧與演化