Mineralogical, chemical, and isotopic (O, C, S, and Sr) analyses were performed on minerals and bulk rocks from a forearc basement section to understand alteration processes and compare with mid-ocean ridges (MOR) and ophiolites. Ocean Drilling Program Hole 786B in the Izu-Bonin forearc penetrates 103 m of sediment and 725 m into volcanic flows, breccias, and basal dikes. The rocks comprise boninites and andesites to rhyolites. Most of the section was affected by low-temperature (<100 ¿C) seawater alteration, with temperatures increasing downward. The rocks are partly (5--25%) altered to smectite, Fe-oxyhydroxide, calcite, and phillipsite, and exhibit gains of K, Rb, and P, loss of Ca, variable changes in Si, Na, Mg, Fe, Sr, and Y, and elevated Δ18O and 87Sr/86Sr. Higher temperatures (~150 ¿C) in the basal dikes below 750 m led to more intense alteration and formation of chlorite-smectite, corrensite, albite, K-feldspar, and quartz (¿chlorite). A 5 m thick hydrothermally altered and pyritized zone at 815 m in the basal dikes reacted with mixtures of seawater and hydrothermal fluids to Mg-chlorite, albite, and pyrite, and gained Mg and S and lost Si and Ca. Focused flow of hydrothermal fluids produced sericitization halos (Na-K sericite, quartz, pyrophyllite, K-feldspar, and pyrite) along quartz veins at temperatures of 200¿--250 ¿C. High 87Sr/86Sr ratios of chloritized (~0.7055) and sericitized (~0.7065) rocks indicate involvement of seawater via mixing with hydrothermal fluids. Low Δ34S of sulfide (-2 to -5.5?) and sulfate (12.5?) are consistent with input of magmatic SO2 into hydrothermal fluids and disproportionation to sulfide and sulfate. Alteration processes were generally similar to those at MORs, but the arc section is more intensively altered, in part because of the presence of abundant glassy rocks and mafic phases. The increase in alteration grade below 750 m and the mineralization in the basal dikes are analogous to changes that occur near the base of the volcanic section in MOR and the Troodos ophiolite. ¿ 1998 American Geophysical Union