Upper Cretaceous trachyandesitic volcanic rocks with intermediate, sub-aluminous, potassic and alkaline compositions are intensively altered in the Hekimhan around. A volcanic material of 30 % was approximately altered as a result of seawater-rock interaction and thus neoformation minerals such as zeolite (analcime), carbonate (calcite, dolomite), silica (quartz) clay (smectite, illite, chlorite, kaolinite, mixed-layers C-S and I-S) and Fe-oxide / hydroxide (hematite, goethite) were occurred. The enrichments in Fe, Mg and Ca-oxides, and depletions in Na and K-oxides are happened in the rocks dependent upon the alteration. A little amount of depletion in Si and Al-oxide were also observed. Alteration processes in the volcanics comprise the four stages : (1) filling of vesicles with the neoformation minerals (particularly analcime, chlorite and C-S), (2) hydrolyses of glassy and micro-crystalline matrix, (3) replacement of felsic (plagioclase) and mafic (hornblend) minerals, (4) settling of Fe-minerals within the pore, surface and fissures of the rocks. The association of minerals requiring different physicochemical conditions in the same rock seem to be controlled by two major parameters. First is physicochemistry of sea + pore water (pH, salinity and/or alkalinity, composition), the second is textural feature (crystallinity degree, granularity, porosity etc.), mineralogical-chemical composition and alteration degree of rock (dissolution rate and quantity).