GEOCHRONOLOGY AND CHEMICAL VARIATIONS OF VOLCANIC ROCKS ALONG THE ARC-CONTINENT COLLISION ZONE IN EASTERN TAIWAN

臺灣東部弧－陸碰撞帶火山岩之定年與地球化學變化研究

莊文星(WEN-SHING JUANG) ； 陳汝勤(JU-CHIN CHEN)

《Collection and Research》 2期 (1990/12) Pp. 89-118

https://doi.org/10.6693/CAR.1990.2.3

摘要

The volcanic arc of eastern Taiwan consists of the Coastal Range and two offshore islets, Lutao and Lanhsu, which have generally been considered as the northern extension of the Luzon arc. The seismic profiles off southeastern Taiwan show that the ridges and troughs are parallel to each other, and the area between the eastern continental shelf and Lutao-Lanhsu can be divided into five physiographic units. The southern extension of the Coastal Range and the Lutao-Lanhsu ridges may represent double volcanic arcs. The Coastal Range volcanic arc runs through the eastern margin of the North Luzon trough extending to the western Cordillera of the northern Luzon while the Lutao-Lanhsu arc links with Batan and Babuyan and the active volcanoes of central Luzon. The volcanic rocks, basalts and andesites, from the Chimei Igneous Complex of the Coastal Range and from Lutao, Lanhsu and Hsiaolanhsu have been dated by K-Ar method. The K-Ar data indicate that the volcanic activity occurred from Early Miocene to Early Pliocene in the Coastal Range and Lanhsu, and from Pliocene to Pleistocene in Lutao and Hsiaolanhsu. Recent foraminiferal study also shows that the later stage of volcanic activity in Lutao is approximately equivalent to N22 (Pleistocene). A significant increase in incompatible elements (such as K, La, Rb, Sr, P and Th) is found in the Pliocene to Recent andesite samples. The chemical variations in the volcanic rocks may be related to crustal thickening linked with the transition from subduction to collision regimes. The ^(87) Sr/^(86) Sr ratios in Lutao and Hsiaolanhsu volcanic rocks are unusually high when compared with the intra-oceanic arcs. The Sr isotopic compositions seem to be heterogeneous for the Coastal Range and Lanhsu volcanic rocks. There is no apparent correlation between the Sr isotopic ratios and the eruption ages in the volcanic rocks from eastern Taiwan. The younger series of volcanic rocks have unusual island-arc Nd and Sr ratios that fall along the extension of mantle array; they appear to be the result of sediment contamination or a unique mantle composition. Incorporation of sediments from eastern China and Taiwan into a MORB-type mantle can explain the Nd and Sr isotopic ratios of the volcanics of eastern Taiwan. Certain unusual petrographic features of Lutao andesites such as the existence of cordierite, the occurrence of olivine phenocrysts in relatively siliceous andesites and frequent occurrences of amphibole in basaltic andesites may indicate that contamination must be considered in discussing the origin of these rocks.

關鍵字

Geochronology ； Chemical Variation ； Volcanic Rocks ； eastern Taiwan

並列摘要

臺灣東部火山島弧包括海岸山脈及外海的綠島和蘭嶼， 它常被認為是呂宋火山島弧北延的部分。根據東部外海震測結果，配合陸上地形分布，發現從大陸棚至綠島－蘭嶼間可以分成海脊或海槽之五個大致互相乎行，南北走向的地形帶。其中海岸山脈南延海脊與綠島－蘭嶼海脊形成了成雙的火山島弧。海岸山脈火山島弧向南延伸至呂宋北部科地勒拉山脈西部；而綠島－蘭嶼海脊則連接巴丹－巴浦耶群島，再向南延伸至呂宋島中部的活火山帶。從鉀一氬定年所得的年齡看來，臺灣東部海岸山脈、綠島、蘭嶼和小蘭嶼的火山活動，彼此間似有重疊的關係。舊期系列之海岸山脈與蘭嶼之火山活動早從中新世早期開始至上新世早期結束；繼之而起的是新期系列，由上新世至更新世的綠島和小蘭嶼的火山活動。最近由綠島集塊岩中的沉積物或火山岩裂隙中充填的鈣質沉積物中所含的有孔蟲化石研究也發現到，晚期的火山活動時期相當於有孔蟲N 22 (更新世)的時代。如以火山岩化學組成與火山活動的時間作一比較，顯然地，自上新世至近代綠島和小蘭嶼之火山岩，其不共容元素含量有顯著的增加。此可能導源於地體構造由隱沒過渡到碰撞的領域，由於地殼的增厚，其母岩漿與地殼物質交換或混染的機會逐漸增加，使得島弧火山岩岩漿中不共容元素含量隨之增加。綠島和小蘭嶼安山岩的鍶同位素初始值遠比由兩塊海洋板塊所形成的島弧安山岩高出許多；如此高的比值，頗不尋常。它表示安山岩的形成，不是與海水大量交換，就是混有大陸地殼物質。海岸山脈和蘭嶼火山岩的鍶同位素組成不甚均勻。鍶同位素初始值與火山活動的年齡，彼此間沒有明顯的共變閱係。新期系列的火山岩鍶與釹同位素比之分布主要是沿著地函帶，這種現象可能是大陸地殼或沉積物混染的結果，也可能是一種特殊的富化的地函。而利用原始地函混染了臺灣與大陸東部沉積物的說法，也可得到很好的解釋。在綠島安山岩中含有堇青石巨晶、橄欖石與石英共存、角閃石存在於玄武岩質安山岩中以及斜長石與角閃石的逆向環帶、熔蝕反應圈和多相組成等現象，均意味著經由岩漿的混合而形成綠島的火山岩是不容置疑的。

並列關鍵字

定年學 ； 地球化學 ； 火山岩 ； 臺灣東部。