GEOCHRONOLOGY AND GEOCHEMISTRY OF BASALTS IN THE WESTERN FOOTHILLS, TAIWAN

臺灣西部麓山帶玄武岩之定年與地球化學研究

莊文星(WEN-SHING JUANG)

《Collection and Research》 7期 (1996/06) Pp. 45-98

https://doi.org/10.6693/CAR.1996.7.2

摘要

Contemporaneous volcanism took place sporadically in Taiwan during the deposition of the Miocene sediments in the western basin, but the volcanic rocks are distributed in different areas within strata of different geologic ages. In northern Taiwan, nearly all the Miocene units contain contemporaneous volcanic effusions locally, each varying widely in size and in areal distribution. The Miocene volcanism events in western Taiwan were previously divided into three volcanic stages: the Kungkuan, the Chienshih and the Chiaopanshan volcanic stages. The volcanic rocks of the Kungkuan and the Chiaopanshan stages occur mostly as small, disseminated tuff lenses, patches or lava flows in the clastic sediments. Furthermore, the Chienshih stage of volcanism seems inactive, and the outcrops are scarce. Based on K-Ar age dating, Miocene volcanism in the western Foothills of Taiwan can be divided into two episodes： Early Miocene (21.2 ± 0.4 to 16.3 ± 0.4 Ma) and Middle to Late Miocene (14.1 ± 10.4 to 7.1 ± 0.5 Ma) equivalent to the Kungkuan and Chaopanshan stages, respectively. The studied areas of Kung-kuan volcanism include the Nankang-Shcnkeng. Shantzechia(Sulin-Shamia), and Ching-shuikeng (Jungho-Tucheng) areas, the areas of Chiaopanshan volcanism include Chiaopanshan(Tachi-Fuhsing), Kuanhsi- Chutung and other small scattered volcanic bodies(Chienshan, Heng-chi, Taliaodi and Mucha) chic rocks of the western Foothills are composed mainly of alkali basalts The Miocene basaltic r and tholciites. The alkali basalt is characterized by its hgher K, Ti, P, Ba, Li, Nb, Rb, Sr, V, Y, Zr and LREE contents as compared with the tholeiite. However, the very highly incompatible element ratios of both alkali basalt and tholeiite are similar to E-type MORB, strongly reflecting their derivation from a common fertile mantle source. Based on Ti-Zr-Y, Hf-Th-Ta and Nb-Zr-Y tectonomagmatic discrimination diagrams and in compatible element spidergrams, all the basaltic rocks from the Miocene western Foothills are similar to those of typical intraplate continental rifting basalts The systematic compatible trace element variations suggest that the possibility of fractional crystallization can not be excluded in the evolutionary history of the magmas. The fractionations of olivine, pyroxene and plagioclase are predominant in Kungkuan basalts. The diversity of systematic incompatible trace element variation in Kungkuan and Chaopanshan basalts suggests that amphibole fractionation can not be excluded for the derivation of Chiopanshan volcanics. REE data and other evidence suggest that the alkali basalt may have been derived from relatively deeper mantle which had been metasomatized by LILE-cnriched fluid through partial melting. But the tholeiite may have originated from unmetasomatized mantle lherzolite at relatively shallow levels by 5-10％ equilibrium batch partial melting. The volcanic rocks of the Western Foothills of Taiwan and those of the Penghu Islands are similar in geochemistry and geochronology. They are closely related to Cenozoic rifting tectonism along the Asiatic continental margin caused by the 3rd heating and rifting episodic evolution of the South China Sea. Sr, Nd and Pb isotopic compositions of Miocene basalts in western Taiwan and the Penghu Islands show that there regional variations with the Chiaopanshan stage volcanics characterized by a anomaly. The variation trends of highly incompatible element ratios demonstrate that the enrichment processes that produced the Dupal anomaly in the basalts from Miocene strata are gradational. Addition of upper continental crustal materials into the asthenosphere is difficult to explain the trace element variations of these basalts. It is suggested that the Dupal mantle component was formed by mantle metasomatism.

關鍵字

Basalt ； Western Taiwan ； Geochemistry ； K-Ar dating ； Petrogenesis

並列摘要

臺灣西部麓山帶在中新世沈積作用進行的時候，火山作用不斷地在沈積盆地中活動。在臺灣北部，幾乎所有的中新世地層，都在不同地區曾發現玄武岩質凝灰岩、凝灰質角礫岩或火山熔岩。以往依照火山岩露頭所處周遭沈積岩作地層對比，在時代上劃分為公館、尖石和角板山三個火山活動期。其中以公館和角板山期較活躍，而尖石期火山岩地表甚少露出。鉀－氬法定年的結果顯示，西部麓山帶的火山活動大致上可分為中新世早期(21.2±0.4~16.3 ± 0.4 Ma) 與中新世中晚期(14.1±0.4~7.1±0.5 Ma)二大階段，亦即相當於公館期與角板山期的火山活動歷史。公館期火山活動定年之區城包括南港一深坑區、清水坑區(中和－土城區)、山子腳區(樹林一山佳區)。角板期火山岩包括桃園縣復興鄉角板山及石門水庫一帶、新竹縣關西一竹東及其他零星分布之火山岩體(如桃園縣鶯歌尖山、臺北縣三峽土城與高雄縣旗山木柵等)。中新世西部麓山帶的玄武岩岩性主要可分為矽質與鹼性玄武岩二大類，兩者雖具有截然不同的主要與微量元素成分，但其不共容親岩漿元素間的比值接近定值，推測可能代表源自相同的來源地函，且來源地函具有經稀土元素富化現象。由地體構造環繞區別性元素Ti、Nb、Ta或Zr-Ti-Y，Nb-Zr-Y、Th-Hf-Ta等區別圖與整體不共容元素分布圖特性判斷，中新世西部麓山帶火山岩皆落入板塊內大陸玄武岩的分布範圍，顯示火山活動時，地體構造環境屬大陸斷裂體系。不論公館其或角板山期矽質或鹼性玄武岩，其共容元素Co、Cr、Ni和Sc均隨MgO/ΣFeO之增大而增高。由此系統性變化，可以看出中新世麓山帶玄武岩都是歷經固液相平衡，結晶分化在岩漿的分化過程中可能曾經發生過。公館期鹼性玄武岩以橄欖石、斜輝石的結晶分化為主，尚伴隨有鈦鐵氧化物或斜長石等礦物的結晶分化。公館期與角板山期鹼性玄武岩其不共容元素與MgO/ΣFeO之變化趨勢相背離，可能意謂著角板山期鹼性玄武岩曾受到閃石類礦物結晶分化作用的影響。根據同位素與微量元素的研究，目前推測矽質與鹼性玄武岩可能分別代表二個獨立的岩漿型態。鹼性玄武岩是富化的上部地函物質，經由10~20%部分熔融而得，而矽質玄武岩則是由較淺處未受換質之地函兩輝橄欖岩紋5~10%平衡部分熔融作用而產生。由於原始岩漿來源之地函物質的富化程度、部分熔融與歷經橄欖石、斜輝石或角閃石的結晶分化的差異，乃衍生出中新世西部麓山帶玄武岩岩漿演化的歧異。西部麓山帶與澎湖玄武岩化學與火山年代相仿，這一連串的火山活動可能是由於中國南海的擴張所引起的第三期熱力與斷裂作用所造成中新世西部麓山帶與澎湖玄武岩釹、鍶和鉛同位素具有區域性的變化，而西部麓山帶角板山期火山岩杜巴異常顯著。根據不共容元素比值的變化趨勢顯示賦存於中新世地層之玄武岩的富化程度是漸變的。由地殼物質的加入難以闡釋角板山期火山岩的微量元素之變異，可能是由於地函的富化導致具有杜巴異常的地函源特性。

並列關鍵字

玄武岩 ； 臺灣西部 ； 地球化學 ； 鉀-氬法定年 ； 岩石成因