台灣東部海岸山脈熔積岩產狀與年代學之研究

熔積岩為熱的岩漿物質與未固結的沉積物交互作用所形成的產狀，為一個良好的定年材料。前人文獻指出在海岸山脈中的熔積岩只存在於陸上環境，且為火山碎屑流所形成，但經由重新檢視熔積岩之定義，只要火山產物能保存其熱能，在水下環境也能產生熔積岩，大部分的國外文獻所判視的熔積岩是於含水環境形成。因此本研究利用沉積物流體化、岩象學等其他熔積岩的證據，在海岸山脈中有七個區域發現熔積岩，分別為：嶺頂、砂脈橋、月洞遊憩區、膽曼海岸、泰源隧道、馬武窟溪及七里溪等剖面，對各區域熔積岩的組合、種類與形成機制進行相關的探討，並針對馬武窟溪與七里溪區域熔積岩中的火山碎屑進行鋯石鈾鉛定年，藉此找尋海岸山脈南段火山頂層的噴發年代。研究結果顯示熔積岩由白色或黑色火山碎屑與凝灰質或鈣質的沉積岩所組成，這些組合中具有凝灰質砂岩的熔積岩可以視為火山島弧已成長至水面上的指標。熔積岩的種類除了砂脈橋與泰源隧道區域為流體狀熔積岩，其餘皆為塊狀與流體狀熔積岩同時出現，流體狀熔積岩的出現表示熔積岩的生成環境為含水環境，由此可知海岸山脈熔積岩形成於任意含水環境。海岸山脈熔積岩形成機制，不只是前人所述火山碎屑流，岩漿淺層侵入、熔岩流所形成的熔積岩也都有出露。成廣澳火山與都蘭山火山的範圍內的熔積岩位在石梯坪凝灰岩層與八里灣層的交界處，熔積岩鋯石鈾鉛定年結果為：成廣澳火山頂層噴發年代為6.3± 0.4 Ma、都蘭山火山頂層噴發年代為6.3± 0.3 Ma及6.8± 0.3 Ma，表示說這兩座火山島約在6 Ma時就已經成長超出水面了。成廣澳火山的樣本中發現大量的繼承鋯石，年代統計為214± 3 Ma，呼應前人所提出的在東台灣火山噴發時有抓取到古老陸塊的鋯石。都蘭山火山頂層噴發年代相較於前人的年代結果年輕了約2 Ma，透過分別計算鋯石核心與邊緣年代，發現有數顆鋯石有顯著的年代變化，鋯石邊緣的年代落在5-7 Ma，其中年代5 Ma的鋯石不在少數，可能表示有更年輕的噴發年代。

關鍵字

海岸山脈；火山島弧；火山岩相；熔積岩；鋯石鈾鉛定年

並列摘要

Peperite is an occurrence formed by hot magmatic materials such as magma, lava flow, and pyroclastic flow interaction with unconsolidated sediments. Previous account peperite is a good material for dating age. According to previous studies have found the peperite in the Coastal Range, eastern Taiwan. Peperite in volcanic arc succession and only exists in the subaerial environment formed by pyroclastic flows. However, review the definition of peperite, it can also exist in the subaqueous environments as long as the heat can be preserved in the hot magmatic materials. Further most foreign studies identified that the peperite is form in subaqueous environment. This study used sedimentary fluidization and petrological evidence to recognize the peperite in the Coastal Range, eastern Taiwan. Peperite can be found in seven outcrops: Lingding, Sand Dike Bridge, Yuedong, Danman, Taiyuan Tunnel, Mawukuchi and Chilichi. Describe the combination and types of the peperite in each outcrop. and zircon U-Pb age results for selected outcrops were also discussed in this study. Peperite are composed of white and black volcanic clasts with tuffaceous sandstone and calcareous sedimentary rocks. The peperite with tuffaceous sandstone can be regarded as an indicator that the volcanic arc has grown to sea level. Except that the areas of Sand Dike Bridge and Taiyuan Tunnels are fluidal peperite, occurrences in the other outcrops show both blocky and fluidal peperite. These fluidal peperite must formed in subaqueous environment, thus, peperite in the Coastal Range were not only exist in the subaerial eruptive facies but also in some subaqueous environment. Based on the relations between peperite and the surrounding lithofacies, the juvenile clasts which were formed peperite can be identified as the shallow magma intrusion, lava flow and pyroclastic flow in Mawukuchi, Yuedong and Lingding, respectively. Peperite in Mawukuchi and Chilichi are exposed at the junction between Tuluanshan Formation (Shitiping tuff) and Paliwan Formation young zircon U-Pb dating result yield a mean age of 6.3 ± 0.4 Ma in the Mawukuchi area and is similar to the previous results. However, a large number of inherited zircons can be found in this sample with a Cathaysian-type ages as the signal shows in Chimei and Lanyu volcanos. This result prove that the old continental fragment must have overlain the Luzon subduction zone and that inherited zircons were picked up during the ascent of magma to the magma chamber. Two peperite samples got zircon U-Pb dating results of 6.3± 0.3 Ma and 6.8± 0.3 Ma in the Chilichi area. Both of these ages are younger than the previous work (8.5± 0.2 Ma). These igneous zircons with ages from 7 to 5 Ma in the rim, and the amount of 5 Ma zircon isn’t less than other. So this result indicate that the Tuluanshan volcano may still have volcanism during 5 to 6 Ma.