Tectonics and evolution of the Philippine volcanic arc

本研究旨在透过菲律宾与日本诸岛之比较，阐明岛弧之发展与历史。结果表明：(1)为了解弧交会处跨弧方向火山带发育演化条件，研究了九州中部裂谷相关岩浆活动，该地区构造背景与菲律宾吕宋岛西南部Macolod走廊相似。以河西火山区山川流纹岩为研究对象，进行了K-Ar和(U,Th)/He定年和岩石学研究，重建了其火山喷发历史和喷发方式。(2)采用无尖刺灵敏度法测定了菲律宾吕宋岛比科尔半岛37个样品的K-Ar年龄，以约束菲律宾海沟俯冲开始的时间。测得的K-Ar年龄范围为0-7 Ma，其中两个老异常值为27和43 Ma。结合先前报道的Leyte和棉兰老岛东部火山岩的K-Ar年龄，俯冲火山作用可能从北向南扩展：Bicol ~ 6.6 Ma， Leyte及其附近~ 3.5 Ma。(3) 2000年2 ~ 3月马荣火山喷发的熔岩以块状熔岩流的形式沉积在火山东南部，其弹道大小从炸弹大小到宝石大小不等。与更老的熔岩，特别是1968年至1993年的熔岩的化学数据相比，2000年的熔岩显示出较小的变化。2000年的岩浆有可能是1968年以来喷发的一批岩浆的一部分。包括2000年，这批玄武岩安山岩岩浆共喷发5次。(4)对夏威夷岛历史火山岩进行了年代学和岩石学研究，建立了年轻火山岩的K-Ar年代学。我们发现在热点火山中存在氩的质量分数，并且利用地质量样品可以充分减少外来的40Ar。

The purpose of this study is to elucidate the development of island arc and its history through comparison of Philippine and Japan islands. We found followings :(1) To understand the conditions of development and evolution of volcanic belt in the across-arc direction at the junction of arcs, we studied magmatism related to the rifting in central Kyushu, which has a tectonic setting similar to the Macolod Corridor, southwestern Luzon, Philippines. With a particular focus on the Yamakogawa Rhyolite of Hohi Volcanic Zone, we carried out K-Ar and (U,Th)/He dating and petrologic study and reconstructed its volcanic eruption history and mode.(2) K-Ar ages of 37 samples collected from the Bicol peninsula, the Luzon island, Philippines, were determined by the unspiked sensitivity method in order to constrain the timing of initiation of subduction along the Philippine Trench. The measured K-Ar ages range from 0-7 Ma with two old outliers of 27 and 43 Ma. Together with K-Ar ages previously reported on volcanics in Leyte and eastern Mindanao, subduction volcanism has likely propagated from north to south : 〜6.6 Ma in Bicol and 〜3.5 Ma in Leyte and its vicinity.(3) Lavas from the February to March 2000 eruption of Mayon Volcano were deposited as blocky lava flows on the southeast sector of the volcano, and ballistics that range from bomb to lapilli size. Compared with chemical data from older lavas particularly from 1968 to 1993, the 2000 lava shows minor variations. It is possible that the 2000 magma is part of a batch of magma that has erupted since 1968. Including 2000, there are then 5 eruptions for this batch of basaltic andesite magma.(4) We conducted geochronologic and petrologic study of historical lavas from the Island of Hawaii to establish K-Ar geochronology of young volcanic rocks. We found that mass fraction of argon occurs in hotspot volcanoes and that extraneous 40Ar can be sufficiently reduced by using groundmass samples.