Volcaniclastic rocks of basaltic to rhyolitic composition, interlayered with basaltic to icelanditic lava flows in a 1918-m drill hole in eastern Iceland are altered to zeolite- and pumpellytie-prehnite facies assemblages. Major alteration minerals are chlorites, ilite-chlorite mixed minerals, ilites, K-feldspar, laumontite, quartz, and sphene; others, include julgoldite and secondary allanite. Smectites as well as mordenite and heulandite are restricted to the upper 900 m. Epidote and minor pumpellyite, prehnite, wairakite, and anhydrite are restricted to the core below 900 m. The boundary between zeolite- and prehnite-pumpellyite facies is marked by the appearance of chlorite-epidote-quartz assemblages at around 900 m. The typical paragenesis prehnite-pumpellyite-epidote-chlorite occurs near the base of the drill hole at 1855 m. Textures indicate generally prograde metamorphic conditions. The broad compositional spectrum, ranging from basalt to rhyolite, abundance of vitric fragments, and high initial porosity and permeability of the volcaniclastic rocks were major factors governing alteration. The site of growth (crystal, shard, vesicle, pore, and vein) of a secondary phase strongly influences composition, habit, amount, paragenesis, and time of crystallization. For example, sphene and, to a lesser degree, epidote are less rich in Fe and better crystallized in plagioclase than in the adjacent matrix. The different assemblages developed within plagioclase crystals through the section most clearly reflect the metamorphic changes. The abundance of vitric rhyolitic clasts (shards and pumice) favored K, Al, Si-enriched fluids from which phases like illite and adularia crystallized. These phases were not found in interlayered basalt flows (Mehegan et al., this issue). Low Ca activity is also indicated by the occurrence of wairakite and pumpellyite rather than prehnite, which is common in the basalts. Secular changes during metamorphism in pore fluid composition are shown by zonations in mineral compositions. For example, the depletion of epidote and juldoldite-pumpellyite in Fe with time appears to reflect decreasing fo2. Bulk rock chemical changes are more intense in the volcaniclastic rocks compared to the basalt lavas. H2O+, Fe3+/Fe2+, and, in part, CO2 values are higher in the upper part of the hole, reflecting the higher porosity and glass content of the clastic rocks. They decrease markedly downhole, approaching values of the lava flows due to more prevasive metamorphic alteration and thus similar mineral assemblages and physical properties.