Pyroclastic material of Eocene age (i.e., green tuffs of Karaj Formation) is one of the most abundant rock types in Alborz Mountain Range, a fold-thrusted structural unit extending in a sinuous form for almost 2000 km from Republic of Armenia in northwest to Afghanistan to the east. Despite vast distribution and volumetric importance of these pyroclastic rocks, their geochemistry has largely been neglected. To select suitable samples for geochemical investigation, a set of 88 of these rocks were examined under microscope. In order of abundances they are classified into; vitric tuffs, crystal rich tuffs, fluidal pumice tuffs, and lapilli tuffs. They were collected from a 14 km long, southwest-northeast trending cross section in central Alborz (north Tehran) in the course of studying their alteration to fuller’s earth.

Ten samples were selected for major element analysis, three crystal-rich tuffs and seven almost vitrophyric tuffs. They are of dacitic and rhyolitic compositions, respectively. On Harker variation diagrams the samples demonstrate a set of well-defined trends indicating differentiation by fractional crystallization. The minerals involved were feldspar +pyroxene +Fe-Ti oxides ±amphibole. Vitrophyric tuffs (with up to 83 wt% SiO2 content) are the extreme differentiation products of a dacitic magma, similar to the one that produced the crystal-rich tuffs. The latter looks to be the explusive eruption product of a partly crystallized dacitic magma. It is probably of no accident that the crystal-rich tuffs are very similar in composition to the dacitic lavas found in northwest Qazvin, some 150 km west of Tehran (Figure 1). The lavas are associated with a sequence of Eocene green tuffs which has not yet been analysed.