A horst block was drilled in the center of the Mariana forearc near 20 ¿N during leg 125 of the Ocean Drilling Program. At this site 781, the drill penetrated a Pleistocene vesicular, porphyritic basalt at 72 m below the seafloor, and the top of the basalt corresponds to a high-amplitude reflection on seismic reflection profiles across the site. The thickness of the basalt unit can only be estimated to be between 13 and 25 m because of poor core recovery (28% to 55%). The presence of an upper glassy chilled zone and a lower, fine-grained margin suggest that the basalt unit is either a single lava flow or a near-surface sill. The basalt is an island-arc tholeiite (IAT) enriched in large-ion-lithophile elements relative to high-field-strength elements, similar to the submarine lavas of the southern Mariana Arc seamounts. The basalt layer, the youngest in situ igneous layer reported from the Izu-Bonin and Mariana forearcs, is enigmatic because of its location more than 100 km from the active volcanic arc. The sediment layers above and below the basalt unit are late Pliocene (about 2.5 Ma) and normally magnetized. The basalt has schlieren-like structures, reverse magnetization, and a K-Ar age of 1.68¿0.37 Ma. Thus, the basalt layer is probably a sill fed by magma intruded along a fault zone bounding the horst and graben in the forearc. The geochemistry of the basalt is consistent with a magma source similar to that of rocks from the magmatic axis (or volcanic front) of the island arc, and derived from a mantle source above the subducting Pacific plate. ¿ American Geophysical Union 1992