東台灣海岸山脈石梯坪凝灰岩及利吉層之年代學與地球化學研究

海岸山脈地層層序中都鑾山層為島弧火山層序，其最上層為石梯坪凝灰岩，屬於島弧火山演化末期的岩相，此層出露為中性至酸性的中酸性凝灰岩，而利吉層為大陸邊緣沉積物與海洋地殼蛇綠岩系之殘塊所組成，並於弧陸碰撞過程中堆積在海岸山脈地層層序中。本研究於月眉火山嶺頂地區上層的中酸凝灰岩層，以及下層的火山角礫岩層當中採集數個安山岩岩塊；於石梯坪地區的中酸凝灰岩層中採集一個輝長岩質包體，以及夾雜於岩層當中的安山岩岩塊，使用全岩地球化學與鋯石鈾鉛定年法分析，以討論其位於海岸山脈岩漿活動中的角色；在利吉地區以及電光地區利吉層的溪床上採集數顆蛇綠岩岩塊以及一顆沈積岩，透過定年以及地球化學特徵之結果，以討論其形成年代以及物質可能的來源。本研究在嶺頂地區並未獲得來自呂宋島弧的岩漿鋯石年代可做後續討論，而位於石梯坪凝灰岩層中的玄武質安山岩（SiO2 ＝ 55.9 wt.%）以及石門火山角礫岩的安山岩（SiO2 ＝ 60.6 wt.%），兩個樣本皆為低鉀的鈣鹼序列，且呈現大離子半徑元素（如：銫、銣、鈾、釷、鉀、鋇、鍶等）富集以及高場力鍵結元素（如：鈮、鉭、鈦等）虧損，屬於島弧岩漿的訊號。石梯坪地區共存於中酸性凝灰岩當中的輝長岩包體與安山岩的年代皆為4百萬年，輝長岩質包體為低鉀的鈣鹼序列，二氧化矽含量為48.3 wt.%，四個安山岩岩塊為中鉀的鈣鹼序列，二氧化矽含量為53.0至58.0 wt.%，所有的微量元素皆呈現大離子半徑元素富集以及高場力元素虧損。利吉層當中的蛇綠岩套年代為18百萬年，蛇綠岩套二氧化矽含量36.4至63.6 wt.%，岩性從基性到酸性都有，且微量元素含量變異很大，包括有銪正異常（18CWC01-1B）與負異常（18CWC01-2B）、MORB類型（18LC01-1B）以及E-MORB類型（18LC03-1B、18NSC01-1B）。而砂岩的年代結果呈現多峰值頻譜，包含火山活動期間所形成的岩漿鋯石年代，以及與華夏陸塊沈積物相似的訊號。全岩釹同位素的結果，在嶺頂地區釹值為+9.5及+9.8、石梯坪地區的安山岩質釹值為+1.3至+2.2，而輝長岩包體釹值為+0.8，利吉層的蛇綠岩套釹值為+8.7至+11.1。綜合實驗結果，本研究在嶺頂地區的結果為與前人研究相同，都屬於島弧岩漿的產物，但無法在年代部分給予新的討論。石梯坪地區所發現之安山岩岩塊以及輝長岩包體，與前人研究中北呂宋島弧岩漿末期噴發之地球化學特性與年代相近，本輝長岩為同時期之岩漿侵入所形成，本研究並提出在奇美火山4百萬年以來的岩漿活動模式應為三個階段：第一階段經較高程度部分熔融產生的基性岩漿上升至淺層的儲存庫，並發生地殼混染作用；第二階段經較低程度部分熔融產生的岩漿再次注入先前形成的路徑，結晶分異後形成中性岩漿；第三階段因再次注入的岩漿引發原儲存庫中的安山岩質岩漿上湧噴發，並捕獲了已形成圍岩的輝長岩，噴發後形成中酸性凝灰岩當中共存著輝長岩包體以及安山岩質的角礫岩。本研究分析的利吉層樣本，其中包含了東台灣蛇綠岩套以及沈積岩，蛇綠岩套中的岩石種類，地球化學數據因不同岩石來源而有差異，其年代分析結果指出，東台灣蛇綠岩年代為18-16百萬年，相較前人年代偏老，而在沈積岩中碎屑鋯石年代頻譜結果，則顯示其具有北呂宋島弧與華夏陸塊主要岩漿活動年代峰值的訊號。

關鍵字

東台灣海岸山脈；石梯坪凝灰岩；基性包體；利吉混同層；地球化學；鋯石鈾鉛定年學

並列摘要

The Tuluanshan Formation in the Coastal Range represents the volcanic sequences of island arc. The uppermost layer is Shihtiping Tuff, which was formed during the final phase (subaerial) of the island arc sequences and composed intermediate to felsic ignimbrite. In addition, Lichi Formation consist of sedimentary rocks, volcanic rocks from the island arc, and ophiolite pieces from the oceanic crust. This study used whole rock geochemistry and zircon U-Pb ages to analyze the andesites collected from both upper ignimbrite layer and lower volcanic breccia in Lingding area (Yuemei volcano); and one gabbroic enclave and several andesites collected from ignimbrite layer in Shihtiping area (Chimei volcano). Meanwhile, this study also analyzed the whole rock geochemistry and zircon U-Pb ages from several igneous rocks from ophiolite. Moreover, one sandstone which were collected from riverbed in Lichi and Dianguang area to clarify the origin and the age of Lichi Mélange. According to the results of the zircon U-Pb dating, we did not get the magmatic zircons form Lingding area. Geochemical data shows that the samples are basaltic andesite (SiO2 = 55.9 wt.%) to andesite (SiO2 = 60.6 wt.%) and belong to low-K calc-alkaline series. Trace elements show enriched in large ion lithophile elements (LILE, e.g. Cs, Rb, U, Th, K, Ba, and Sr) and depleted in high field strength elements (HFSE, e.g. Nb, Ta, and Ti) that represents the signal of the island arc magmatism. Both of the gabbroic enclave and the andesites from the ignimbrite layer in the Shihtiping area yielded weighted mean ages of 4 Ma. The gabbroic enclave (SiO2 = 48.3 wt.%) belongs to low-K calc-alkaline series and the other four andesitic samples (SiO2 = 53.0 to 58.0 wt.%) belong to middle-K calc-alkaline series. Their trace elements show LILE enrichment and HFSE depletion. Both of the igneous rock in the ophiolite from Lichi Mélange in Lichi and Dianguang area yielded weighted mean ages of 18 Ma. These rocks show a large variation of SiO2 concentration from 36.4 to 63.6 wt.%. They also show different types of trace elements concentration including Eu enrichment (18CWC01-1B) and depletion (18CWC01-2B), MORB type (18LC01-1B) and E-MORB type (18LC03-1B, 18NSC01-1B). The whole rock Nd isotopes in andesites from ignimbrite and breccia layers from Lingding area show εNd value of +9.5 and +9.8. The andesites from Shihtiping area show εNd values range from +2.2 to +1.3 and the gabbroic enclave has more enriched εNd value of +0.8. The igneous rocks from ophiolites in Lichi Mélange have εNd values from +11.1 to +8.7. The geochemical data of Shihtiping Tuff and Shihmen Volcanic Breccias from Lingding area in this study show the same island arc signals with previous researches. In the Shihtiping area, we can identify three sub-stages in Shihtiping Tuff during 4 Ma: At sub-stage 1, mafic magma generated by higher degrees of partial melting rose to the shallow reservoir and contaminated with the crustal materials. At sub-stage 2, a magma generated by lower degrees of partial melting injected into the magma reservoir and fractionated to produce an intermediate magma. At the final sub-stage, the recharged magma rose and triggered the intermediate magma assimilated with the gabbroic wall rock which was formed by mafic magma from sub-stage 1. Eventually, the intermediate magma erupted to form ignimbrite and contained andesitic breccias and coexisted with the gabbroic enclave. Age results from Lichi Mélange can be found the major magmatism signals from both the Northern Luzon Arc and Cathaysia.