Tectonics and evolution of the Philippine volcanic arc

本研究的目的是通过对菲律宾和日本岛屿的比较，阐明岛弧的发展及其历史。(1)研究了九州中部与菲律宾吕宋岛西南部马科洛德走廊构造环境相似的裂谷作用，以了解火山带在弧间交界处的发育演化条件。以和希火山带Yamakogawa流纹岩为研究对象，进行了K-Ar和(U，Th)/He定年研究，重建了其火山喷发历史和喷发模式。(2)对采自菲律宾吕宋岛比科尔半岛的37个样品进行了K-Ar年龄测定，以确定菲律宾海沟俯冲的开始时间。测得的K-Ar年龄范围为0~7 Ma，有27 Ma和43 Ma两个老的离群值。连同先前报道的莱特岛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传播：比科尔约6.6 Ma，莱特及其邻近地区约3.5 Ma。(3)2000年2月至3月马荣火山喷发的熔岩以块状熔岩流形式沉积在火山东南部，弹道学从炸弹到青金石大小不等。与来自更古老的熔岩，特别是从1968年到1993年的化学数据相比，2000年的熔岩显示出微小的变化。2000年的岩浆有可能是1968年以来喷发的一批岩浆中的一部分。包括2000年在内，这批玄武岩安山岩岩浆共喷发了5次。(4)对夏威夷岛历史熔岩进行了地质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，Ar的质量分数存在于热点火山中，而外来的40Ar可以通过使用基质样品来充分还原。

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.