Abstract

• Tonnge basaltic field is situated 100 km north of Mandalay in Tigyaing Township, Sagaing Region of Myanmar. It is regarded as an intraplate Quaternary basaltic field and located to the north of wellknown Singu-Kabwet basaltic lava field of Central Myanmar. In this study, zeolites filling in the amygdales of the olivine basalts from Tonnge area have been reported and studied by field observation, transmitted light microscopy and X-ray diffraction analysis. The zeolite group minerals are identified as natrolite, phillipsite, mordenite, laumontite, analcime, and epistilbite. Non-zeolite mineral phases in the amygdaloidal infillings are recognized as smectite, quartz (as amorphous silica), and carbonates. Zeolites and associated minerals are interpreted to be developed possibly as a consequence of late-stage hydrothermal circulation. Moderately to weakly alkaline hydrothermal solutions are responsible for the formation of zeolites in the basalts. Due to the occurrence of different zeolite minerals, it can be deduced that a change in fluid composition occurred as temperature dropped during their formation.