Tectonics and evolution of the Philippine volcanic arc

本文旨在通过对菲律宾群岛和日本群岛的比较，阐明岛弧的发展及其历史。我们发现：（1）为了了解弧结处跨弧方向火山带的发育和演化条件，我们研究了与菲律宾吕宋岛西南部马科洛德走廊构造背景相似的九州中部裂谷有关的岩浆活动。以峰日火山带的山子川流纹岩为研究对象，对其进行了K-Ar和（U，Th）/He定年及岩石学研究，重建了火山喷发历史和喷发模式。(2)为了确定沿着菲律宾海沟俯冲的起始时间，采用无加标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。测得的K-Ar年龄范围为0-7 Ma，有两个老的异常值，分别为27和43 Ma。结合之前报道的莱特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：比科尔为6.6 Ma，莱特及其附近为3.5 Ma。(3)2000年2月至3月马荣火山喷发的熔岩沉积在火山东南部的块状熔岩流中，弹道大小从炸弹到火山砾不等。与1968年至1993年的老熔岩的化学数据相比，2000年的熔岩显示出微小的变化。2000年的岩浆可能是自1968年以来喷发的一批岩浆的一部分。包括2000年在内，这批玄武安山岩岩浆共喷发了5次。(4)通过对夏威夷历史火山岩的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，氩的质量分数发生在热点火山和外来的40 Ar可以充分减少使用groundmass样品。

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.