Factors controlling the distribution of zeolites in the andesitic rocks ofthe Central Coastal Range of eastern Taiwan

This study is a survey of zeolites in the andesitic rocks of the Central Co astal Range (CCR) of eastern Taiwan, Methods of the study include field obs ervations and laboratory techniques such as X-ray diffraction, petrography, and scanning electron microscopy, It is found that zeolites are abundant only in the andesitic rocks along tw o streamlets, i.e., Tuwei-Hsi (TH) and Yungshui-Hsi (YH), and are rare to a bsent in others, Zeolites generally occur in hyaloclastites but rarely in l ava flows in the TH section. On the other hand, breccias are usually richer in zeolites than lava flows, peperites, and tuffs in the YH section. Natro lite is the unique zeolite in TH hyaloclastites, It usually associates with apophyllite and calcite. Phillipsite is the only zeolite in YH breccias. I t mostly occurs alone and occasionally associates with calcite, The duration of exposure to seawater is considered as the main factor influ encing the amount of zeolites in submarine erupted andesitic rocks of the C CR, The longer the andesitic rocks are exposed to seawater, the larger the amount of zeolites would be in them. On the other hand, the species of zeol ite and related authigenic minerals in the andesitic rocks of the CCR are p resumably controlled by the kind of their phenocrysts which are the essenti al minerals suffered from alteration.