Turkish Journal of Earth Sciences




This study explains the relations between the depositional environment of a zeolitic tuff unit and its diagenesis. It gives new ideas of the juvenile phreatomagmatic origin of the zeolitic unit with its bentonitic cap rock, and finds that the diagenetic alteration of the volcanic ash deposition in a hot hydrologic system is penecontemporaneous and not epigenetic. A massive, fine-grained zeolitic unit has a sharp contact with the underlying shallow subaqueous rhyolitic dome intrusions and their surrounding volcanoclastic ejecta. Juvenile emanations from the basal subaqueous intrusions activated thorough zeolitic diagenesis within the overlying rhyolitic tuff deposits extending as far as the periphery of the underlying intrusions. The bentonitic cap rock suggests that the diagenesis diminished vertically with the weakened phreatomagmatic activity up to the overlying limestone. The lack of sedimentary evaporite minerals and scarcity of boron-bearing authigenic K-feldspar indicate a nonsaline-alkaline depositional and diagenetic environment during the zeolitic transformation. Geochemical data from the zeolitic tuff samples indicated that the main diagenetic factors were hydrolysis of the glassy tuff in an open hydrologic system, under high heat flow rates and one of several scales of ion transfer. Zeolitisation developed with a significant loss of alkaline elements and iron oxide, which were compensated for by an important gain in the alkaline earth elements and absorption of strontium. The rhyolitic glass was altered by hydrolysis to form smectite and clinoptilolite, resulting in the release of excess silica that was not removed from the system but was changed in crystal form to opal-CT.

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