北呂宋島弧的火山與岩漿演化

Evolution of Volcanoes and Magmas in the Northern Luzon Arc

賴昱銘(Yu-Ming Lai)

指導教授 ： 宋聖榮

國立臺灣大學/理學院/地質科學研究所/博士(2012年)

https://doi.org/10.6342/NTU.2012.02320

摘要

北呂宋島弧的火山岩，包括歷經弧陸碰撞後已被擠壓變形的台灣海岸山脈火山體，以及尚在台灣島與呂宋島之間的火山島嶼。本研究於海岸山脈進行地質調查、採樣、分析火山岩的地球化學資料以及氬氬年代，並配合前人研究，嘗試重建海岸山脈的火山成長過程以及岩漿演化的機制。
海岸山脈的火山岩分屬四座火山，其中最北段的月眉火山，其火山中心已經隨著島弧持續向北移動，而沒入歐亞大陸板塊的下方。其他三座火山分別為：奇美火山、成廣澳火山以及都蘭山火山。分佈於火山中心的岩相屬於近火山口相，而隨著與火山中心的距離增加，出露中間火山相以及遠離火山口相。火山體受到侵蝕而出露的部分，在奇美火山出露火山最底部的岩相，而往南受到的侵蝕作用逐漸變小，所出露的岩相也就為較靠火山頂部者，此與火山島抬升的先後時間有關。北呂宋島弧的火山演化可分成四個時期：第一時期始自南中國海板塊的隱沒作用產生岩漿，上湧於深海中噴發，隨著火山島逐漸成長而出露海水面，目前的巴丹及巴布亞島等正在活動的火山島，仍屬於此時期。第二時期為巴丹島以北至綠島等島嶼，火山島已隨著板塊移動並停止噴發。第三時期的火山島經弧陸碰撞而被抬升，並受到不同程度的侵蝕作用，海岸山脈的奇美、成廣澳及都蘭山等火山為此階段。第四時期中，火山島隨著板塊而隱沒，月眉火山的火山中心即因此而隱沒。
海岸山脈東河地區發現了一個在岩漿噴發前後都發生岩漿混雜作用的例子。較基性的岩漿在地底的岩漿庫停留，經過結晶分化與較酸性的岩漿分層，而來自下方新的岩漿進入岩漿庫造成岩漿混雜或混合作用，並記錄在礦物斑晶的化學不平衡中。岩漿庫的岩漿上湧噴發，較酸性的岩漿侵入堆積在火山邊坡或火山頂部尚未固結的火山凝灰岩堆積物而形成熔積岩，同時間較基性的岩漿噴發形成熔岩流與前者相遇，物理的混雜機制保留了兩種火山角礫岩以及介於其中的混和帶。
熔積岩在海岸山脈分佈很廣，由產狀可分為塊狀及流體狀兩種。熔積岩中火成作用的產物成分涵蓋了玄武岩質至石英安山岩質，而沉積作用產物則包括泥、砂及凝灰岩質沉積物。熔積岩存在於中酸凝灰岩層中的白色火山角礫岩或火山彈之中，代表著島弧火山成長至海水面附近所形成的岩相。依照火山島噴發成長的過程，火山的岩相組合為深海噴發相、淺海噴發相以及陸上噴發相，此岩相組合的規則可涵蓋全海岸山脈，包括綠島及蘭嶼火山島，表示北呂宋島弧的岩相層序可能皆為如此，至於能否適用於所有的島弧火山，需要更多的資訊。
海岸山脈的岩漿來自相同性質的源區，經過不同程度的結晶分化與少量的地殼混染後噴發形成。岩漿隨著時間具有富集K2O、輕稀土元素以及εNd值降低的趨勢。月眉火山不具結晶分化與地殼混染的現象；奇美火山具有結晶分化的現象，並在6.6百萬年後岩漿源區的成分改變，反應在微量元素與釹同位素值的變化上；成廣澳火山也受結晶分化的影響，並且在7.4百萬年前同樣源區成分改變；都蘭山火山結晶分化與地殼混染的情形並不明顯。海岸山脈的岩漿演化過程與北呂宋島弧西鏈火山島有相同的趨勢，表示其可能是西鏈火山島向北的延伸。北呂宋島弧火山岩的岩漿演化，可分為西鏈與東鏈火山島兩個階段來說明：西鏈火山島始自南中國海板塊隱沒時帶入岩漿源區的陸源沉積物，加上上部地函經部份熔融形成的岩漿相混合而成的岩漿。隨著時間隱沒沉積物的量逐漸增加，從老於15百萬年前開始逐漸增加至6到5百萬年前，最多可加入0.5 %的沉積物，而5百萬年後由於位於南中國海板塊上的隱沒洋脊進入隱沒帶，使得隱沒角度的改變，原本存留在隱沒帶中古老的遠洋沉積物質進入岩漿源區而改變岩漿成分。東鏈火山島在3百萬年後形成，由於古老的遠洋沉積物影響存在隱沒帶的不均勻性，導致岩漿成份的變化與年代無關。除源區混染之外，部分前人研究亦指出在局部地區存在著地殼混染的現象。

關鍵字

北呂宋島弧 ； 海岸山脈 ； 火山演化 ； 岩漿演化 ； 熔積岩

並列摘要

The Northern Luzon Arc includes the destroyed volcanoes in the Coastal Range (CR) of eastern Taiwan and volcanic islands between Taiwan and Luzon Island. According to the volcanic facies associations, the Sr-Nd isotopic geochemistry, and the geography of the region, four volcanoes were identified in the CR, namely Yuemei, Chimei, Chengkuangao and Tuluanshan. Moreover, the near-vent facies association showed different degrees of erosion in the volcanic edifices for Chimei, Chengkuangao and Tuluanshan. This implies that Yuemei’s main volcanic body may have been destroyed and submerged into the trench with the northward subduction of the Philippine Sea Plate. A hypothesis can be given for the evolution of volcanism and geomorphology during the arc’s growth and the arc-continental collision in the northern Luzon Arc. These volcanoes were built up from the seafloor to emerge as islands during the arc’s volcanism, and then they ceased. They were then uplifted and accreted after the collision. The oldest volcano, Yuemei, may have already been subducted into the trench.
Complex rocks, consisting of different lithologic breccias and sediments in the Tungho area, were formed by magmas and magma-sediment mingling. Based on field occurrences, petrography, and mineral and rock compositions, three components: mafic magma, felsic magma, and sediments can be identified. A mafic magma generated from depth and stayed to form a stratified magma chamber. After that, some mafic magma from the deep pushed the upper felsic magma rising and intruding, and partially mingled with magmas in the chamber. Then, both magmas exploded, the mafic lava cooled slowly and flowed along the slope of the seamount, at the same time, felsic magma intruded into wet tuffaceous sediments to explode and form peperites. Finally, a viscous mafic lava flow made contact with exploding felsic magma thereby allowing these two magmas to mingle in a limited zone again.
Ignimbrites are generally crop out on the top of volcanic sequences in the CR. White volcanic breccias or bombs and peperites could be found in this sequence. Peperites showed blocky and fluidal textures due to basaltic to dacitic juvenile volcaniclasts mingled with mud, sand or suffaceous sediments. Ignimbrites with white volcaniclasts and peperites were the distinctive lithofacies indicated to the erupted environment were reached to the sea level. Volcanic facies associations from deep marine, submarine to subaerial could be reconstructed. This rule may be applied to all of the volcanic islands in the Northern Luzon Arc.
Volcanic rocks in the CR show the increasing of K2O content and the LREE-enrichment and decreasing of εNd value from the older to younger volcanics. They formed from the same source and with different degrees of fractional crystallizations and crustal contaminations. Yuemei and Tuluanshan volcanoes showed no or less effect by these influence while Chimei and Chengkuangao volcanoes showed much fractional crystallizations and changed their source characteristics after 6.6 Ma and 7.4 Ma years ago, respectively. Magmatic processes in the CR were similar to the western volcanic arc chain based on their variations of geochemistry with ages. Magma evolution in the Northern Luzon Arc included two stages: the first one was started by the subduction of the South China Sea Plate beneath the Philippine Sea Plate and formed the western volcanic arc chain. Magmas were formed from two components mixing between depleted mantle and increasing accepted terrigeous sediments before 6 to 5 Ma years ago. After that, the subducted angles changed as a result of the ridge subduction. The old pelagic sediments which were stored due to the previous subduction might be entered to the magma source and caused the divergent from the mixing curve. The second stage was the eastern volcanic arc chain erupted after 3 Ma years ago. Magma components were mixing by depleted mantle and the old pelagic sediments. Furthermore, according to other studies, crustal contaminations were also happened in some particular areas.

並列關鍵字

The Northern Luzon Arc ； Coastal Range of eastern Taiwan ； volcanic evolution ； magmatic evolution ； peperite

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