An extensive bimodal volcanic suite developed in the Late Paleogene in the Eastern Rhodopes Mountains, Southern Bulgaria. Most of the widespread and thick rhyolitic tuffs and ignimbrites. erupted during two major Early Oligocene acid pha ses, were emplaced in a shallow marine environment and subsequently altered. K-rich Ca-clinoptilolite (in some places K-clinop tilolite. mordenite or analcime). clay minerals (celadonite or/and smectite). adularia (or albite) and opal-CT replace the volcanic glass. The zeolitized pyroclastic rocks are strongly depleted in MnO. Na20. K20, and enriched in CaO. Fe201. Ti02 and MgO. Regional zoning in distribution of the major glass-replacing minerals has been established: quartz+ adularia (or albite)± zeolites in the proximal to the vent area: zeolites in the medial and clay minerals in the distal area. Vertical zoning has been also recog nized because on ly the basal pyroclastic layer is transformed into adularia+ quartz whereas clinoptilolite. accompanied by opal CT and clay mineral s. prevails in the rest of the section. The zeolitized glass shards are characteristically zoned: (I) a thin rim (< 5 �m thick) of celadonite coats the shard surface retaining their original shape: (2) a 30-50 , um-thick zone. made of contiguous platy zeolite crystals. oriented perpendicular to the rim and (3) a central zone of large euhedral crystals. surrounding a central hollow area. Phenocrysts are not affected by the zeolitization and some relict glass is found in the top of the pyroclastic section. According to the semiquantitative X-ray analysis and heat of immersion test of CcLFAZ et a!. (1973) the clinoptilolite content in the zeolitized pyroclastic rocks varies from 38 to 72 wt. 'lc. The cation exchange capacity values range between 40 and 153 meq/ 100 g and show a significant positive correlation with clinoptilolite content obtained by the heat of immersion test (r = 0.71). The lateral zoning and presence of typical hydrothermal minerals such as celadotite indicate that zeolitization of the thick Eastern Rhodopes pyroclastic series results from the operation of a large low temperature hydrothermal system. The hydrothermal solu tions consist of marine water heated by the anomalous geothermal gradient of the active volcanic areas or/and by the hot pyro clastic deposits.

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