Welding of pyroclastic rocks is generally thought to occur by mechanical expulsion of interstitial gas from a deposit as it compacts under its own weight. We propose here that volatile resorption and compression can also be important factors in welding. We describe densely welded rocks which cannot be explained by loading and re-evaluate the welding process taking into account the effects of volatile resorption into the glassy fragments. Intra-caldera Oligocene ignimbrites from the Rhodope Mountains, Bulgaria, and intrusive tuffs of the Loch Ba ring dyke, Mull in Scotland are intensely welded but some samples lack foliation (eutaxitic texture). Fiamme and glass shards show no preferred orientation in thin section or on outcrop scale. Fiamme are sometimes complexy deformed into U or S shapes. We propose that these textures are the consequence of volatile dissolution in the glass and gas compression. Scaling analysis indicates that there are two regimes for gas behaviour following emplacement of hot pyroclastic deposits which depend on the relative characteristic time-scales of compaction, gas escape and gas resorption: a gas escape regime and a gas retention regime. During explosive eruption, glassy pyroclasts are decompressed to less than one atmosphere pressure and are outgassed. During deposition and subsequent burial in a thick hot deposit, volatiles will be retained and soluble gases (H 2 O) can be resorbed back into the glass, defining the gas retention regime. Poorly soluble gases (air) are compressed to small fractions of their original volume, resulting in destruction of pore spaces and vesicles in pumice. In some circumstances, such as volcanic vents, the volume changes involve isotropic strain and implosion of the tuff. Resorption of water greatly speeds up welding during compactional loading by reducing the viscosity of the glass. Welding is inhibited not only by lower temperatures but also by entrainment of insoluble atmospheric gases. The two regimes also can explain the common presence of post-emplacement gas escape pipes in non-welded ignimbrites and their rarity in densely welded ignimbrite. Factors that are likely to promote the gas retention regime include fine grain size, low collapsing columns, a large thickness of tuff and incorporation of external water.

The potassic (K) to ultrapotassic (UK) volcanic rocks cropping out in the Vardar Zone of Macedonia and southern Serbia span in age from Late Miocene (6.57 Ма)

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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.