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Quaternary and Pliocene terrigenous sediments recovered during Ocean Drilling Program Leg 141 on the Chile Trench continental slope are represented by hemipelagic mud and distal and proximal turbidites with minor gravity-flow deposits. The coarse fraction is composed mainly of quartz and feldspars, with hornblende, epidote, and clinopyroxene dominating the heavy fraction. Volcanic glass, mica, orthopyroxene, tremolite-actinolite, olivine and "sialic" accessories (garnet, zircon, apatite, tourmaline, and sphene) occur in minor amounts. Clay minerals consist mainly of hydromica (illite) and chlorite, and smectite is a common minor component. The mineral assemblage found in the sediment sequences is derived from the adjacent Andean orogenic source area. Only the basal beds of the sedimentary blanket over the Taitao Ridge contains volcanic minerals possibly derived from local submarine outcrops of mafic igneous rock. Both the clay mineralogy and coarse-fraction mineralogy of the sediments are generally uniform, irrespective of the location of the holes on the accretionary wedge, of water depth, of depth below seafloor, of lithologic units, of tectonic deformation, or of lithification, and they do not change markedly with grain size or depositional style. The accretionary wedge terrigenous sediments are affected by hydrothermal alteration over the subducted Chile spreading ridge (Site 863). The hydrothermal effects are best demonstrated by extensive secondary smectite formation in the sediments. Subduction of the spreading ridge produces a "hydrothermal trace" in the accretionary wedge.

Volcanic rocks recovered during Legs 143 and 144 on the Allison, Resolution, Lo-En, Wodejebato, and MIT guyots belong to intraplate alkaline basalts. The basalts are strongly altered by hydrothermal processes, and some are affected by subaerial weathering. The low-temperature hydrothermal alteration, which is best represented in inner parts of sills on the Allison Guyot, led mainly to the formation of smectite, swelling chlorite, mixed-layer smectite-chlorite, and carbonates. Kaolinite, smectite, and hematite dominate among the subaerial weathering products of basalts on the Resolution Guyot and occur in the outer zones of sills on the Allison Guyot. The basalts from the Lo-En, Wodejebato, and MIT guyots have undergone low-temperature smectitization. Formation of mixed-layer smectite-chlorite and seΦentine is a result of high magnesium content in the primary basalt rather than of higher temperature.

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SUMMARY In the course of deep weathering of a biotite gneiss, biotite is transformed into mixed‐layer biotite‐vermiculite hydrobiotite. Later biotite‐vermiculite is transformed into biotite‐chlorite under the action of hydrothermal solutions at the stage of epigenesis.