The Eocene green tuff belt of the Alborz structural unit, the so-called Karaj Formation, is composed mainly of a pyroclastic succession with some carbonate rocks. Pyroclastics are genetically linked to calc-alkaline magmatism of the back arc extensional setting. The belt is a part of
the broad Arabian-Central Iran collision zone. The petrographic data has shown that glass shards of various shapes and sizes are the main component of tuffs. The geochemical characteristics indicate that these rocks are trachyandesites, rhyodacites and rhyolites. Based on the detailed microscopy,
XRD, and electron microprobe analysis (EMPA), the following main minerals have been identified: clinoptilolite, montmorillonite and crystobalite. The glass shards are filled with clinoptilolite as a result of their alteration. According to the Si/Al classification of the zeolite group derived
from the chemical data, clinoptilolite is the main component. The zeolitization developed in the glass-rich tuffaceous Karaj Formation as a stratabound form. This type of zeolite occurrence could be of marine to lacustrine origin.
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