Low‐ and high‐temperature alterations of volcanic rocks in the northwestern <scp>P</scp>hilippine <scp>S</scp>ea, and association with volcanic settings

<jats:title>Abstract</jats:title><jats:p>Petrological and geochemical characteristics of low‐ and high‐temperature alterations were investigated in volcanic rocks from the northern <jats:styled-content style="fixed-case">P</jats:styled-content>hilippine <jats:styled-content style="fixed-case">S</jats:styled-content>ea. The assemblage of secondary minerals is divided into two groups based on microprobe and <jats:styled-content style="fixed-case">XRD</jats:styled-content> analyses. The first group, comprising samples from the <jats:styled-content style="fixed-case">K</jats:styled-content>yushu‐<jats:styled-content style="fixed-case">P</jats:styled-content>alau <jats:styled-content style="fixed-case">R</jats:styled-content>idge and <jats:styled-content style="fixed-case">U</jats:styled-content>rdaneta <jats:styled-content style="fixed-case">P</jats:styled-content>lateau, includes calcite, phillipsite, clay minerals and <jats:styled-content style="fixed-case">F</jats:styled-content>e‐oxyhydroxides, reflecting low‐temperature alteration. The content of secondary deposition is high in the <jats:styled-content style="fixed-case">K</jats:styled-content>yushu‐<jats:styled-content style="fixed-case">P</jats:styled-content>alau <jats:styled-content style="fixed-case">R</jats:styled-content>idge, but low in the <jats:styled-content style="fixed-case">U</jats:styled-content>rdaneta <jats:styled-content style="fixed-case">P</jats:styled-content>lateau. The second group, consisting of samples from the <jats:styled-content style="fixed-case">A</jats:styled-content>mami <jats:styled-content style="fixed-case">P</jats:styled-content>lateau, 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 <jats:roman>H<jats:sub>2</jats:sub>O</jats:roman><jats:sup>+</jats:sup> and loss on ignition (<jats:styled-content style="fixed-case">LOI</jats:styled-content>) 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 <jats:styled-content style="fixed-case">K</jats:styled-content>yushu‐<jats:styled-content style="fixed-case">P</jats:styled-content>alau <jats:styled-content style="fixed-case">R</jats:styled-content>idge is covered with the last lava flow of rifting volcanism associated with spreading of the <jats:styled-content style="fixed-case">S</jats:styled-content>hikoku Basin, and the volcanics reacted with circulating seawater under low temperatures and oxidative conditions; (ii) the <jats:styled-content style="fixed-case">U</jats:styled-content>rdaneta <jats:styled-content style="fixed-case">P</jats:styled-content>lateau 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 <jats:styled-content style="fixed-case">A</jats:styled-content>mami <jats:styled-content style="fixed-case">P</jats:styled-content>lateau was produced by <jats:styled-content style="fixed-case">C</jats:styled-content>retaceous arc volcanism, which suffered high‐temperature alteration associated with intrusive activity of tonalite.</jats:p>