Low‐ and high‐temperature alterations of volcanic rocks in the northwestern Philippine Sea, and association with volcanic settings

Petrological and geochemical characteristics of low- and high-temperature alterations were investigated in volcanic rocks from the northern Philippine Sea. The assemblage of secondary minerals is divided into two groups based on microprobe and XRD analyses. The first group, comprising samples from the Kyushu-Palau Ridge and Urdaneta Plateau, includes calcite, phillipsite, clay minerals and Fe-oxyhydroxides, reflecting low-temperature alteration. The content of secondary deposition is high in the Kyushu-Palau Ridge, but low in the Urdaneta Plateau. The second group, consisting of samples from the Amami Plateau, includes secondary alkali feldspar, analcime, and chlorite, reflecting high-temperature hydrothermal alteration. As groundmass alteration is dominant in the studied volcanic rocks, the alteration index, defined as the ratio of secondary minerals in voids to the total material count, is useful for estimating alteration degree when precise identification of secondary materials is difficult in highly vesicular rocks. Alteration indexes correlate with bulk H2O+ and loss on ignition (LOI) contents, and bulk major and trace element compositions of the studied volcanic rocks change with alteration. The difference of alteration types in three regions may depend on the tectonic setting of volcanism. That is: (i) the Kyushu-Palau Ridge is covered with the last lava flow of rifting volcanism associated with spreading of the Shikoku Basin, and the volcanics reacted with circulating seawater under low temperatures and oxidative conditions; (ii) the Urdaneta Plateau was formed as a result of intra-plate volcanism, and the volcanics were under seafloor weathering without seawater circulation associated with volcanic activity; and (iii) the Amami Plateau was produced by Cretaceous arc volcanism, which suffered high-temperature alteration associated with intrusive activity of tonalite.