Zeolites and zeolitization of acid pyroclastic rocks from paroxysmal Paleogene volcanism, Eastern Rhodopes, Bulgaria

An extensive bimodal volcanic suite developed in the Late Paleogene in the Eastern Rhoclopes Mountains. Southern Bulgaria. Most of the widespreacl and thick rhyolitic tuffs and ignimbrites, erupted during trvo rnajor Early' Oligocene acid pha- ses, were emplaced in a shallow rnarine environment and subsequently altered. K-rich Ca-clinoptilolite (in some places K-clinop- tilolite. mordenite or analcime). cla.v minerals (celadonite or/and smectite). adularia (or albite) and opal-CT replace the volcanic glass. The zeolitized pyroclastic rocks are strongl)' depleted in NInO. NalO. K1O, and enriched in CaO. Fe1O.l. TiOl and MgO. Regional zoning in distribution of the major glass-replacing minerals has been established: quartz + adularia (or albite) -F zeolites i1 the proxirnal to the r,e1t area: zeolites in the medial and clal' minerals in the distal area. Vertical zoning has been also recog- nized because only the basal pl,roclastic layer is tlansformed into adularia + quartz rvhereas clinoptilolite. accompanied by opal- CT ancl clay minerals. prevails in the lest of the section. The zeolitized -slass shards are characteristically zoned: (1) a thin rim (<5Um thick) of celadonire coats the sharcl surface retainins their ori-sinal shapel (2) a 30-50pm-thick zone. made of contiguous platy zeolite crystals. oriented perpenciicular to the nnr and (3) a central zone of large euhedral crystals. surrounding a central hollow area. Phenocrysts are not affected by the zeolitization and some relict -ulass is tbund in the top of the pyroclastic section. Accorcling to the semiqgantitative X-ray analysis and heat of immersion test of Cut-r.-rz et al. (1973) the clinoptilolite content in the zeolitized pyroclastic rocks varies fiom 38 to 72 rvt.%. The cation exchange capacitv values range betrveen 40 and 153 meq/ 100g and sh6w a si-enificant positive conelation with clinoptilolite content obtained by the heat of immersion test (r = 0.71). The lateral zoning ancl presence of t1'pical hvdrothermal minerals such as celadotite indicate that zeolitization of the thick Eastern Rhodopes pyr-oclastic series results fiom the operation of a lar-te low temperiiture h.vdrothermal systenl. The h1'drothermal solu- tions cgnsist of marine water heated b1' the anomalous -ueotherntal gradient of the active r.olcanic areas or/and b1' the hot pyro- clastic deposits.


Introduction
During the Paleogene, after the Mesozoic-Cenozoic sub duction leading to the closure of the Tethys Ocean. mi croplates of African origin collided with the southern edge of the Eurasian plate (Rrcou 1994). Significant col lision-related magmatism (YANEY & BAHNEVA 1980. YANEY et a!. 1998) occurred on the southern edge of the Eurasian plate and resulted in formation of many volca nic areas extending from the Alps to Northwestern Tur key. One of them is Eastern Rhodopes (Southern Bulga ria) where ca. 7000 km 2 are now covered by volcanic rocks. The Eastern Rhodopes volcanic association (37 to 25 Ma. Lrwv et a !. 1987) is bimodal. andesite-latitic (lo cally up to basaltic) and rhyolitic in composition. Inter mediate and acid varieties are present in almost equal volumes.
The acid volcanic glass. deposited in marine environ ment. was transformed into zeolites. mainly clinoptilo lite, less mordenite and analcime, accompanied by clay minerals. opal-CT and adularia. In the top of the pyro clastic sections in some places non-zeolitized glass is still preserved (ALEKSIEV et a !. 1997). After the disco very of the zeolitized tuffs (ALEKSIEV 1968) an impres-   . . . \ '-f:> n. . sive amount of work on the Eastern Rhodopes zeolitized pyroclastic rocks, dealing with zeolite mineralogy (KI ROV 1974, D.rouROVA 1976 and features of some zeolite deposits (ALEKSIEV & D.rouRov A 1982, D.rocRov A & ALEKSTEV 1990, ALEXIEV et al. 1997 and references therein, RA YNOV et a . IV AN OVA et al. 2001. has been published. ALEKSIEV & OJOUROV A (1975) proposed a geoautoclave hypothesis to explain zeolitisation of the thick pyroclastic series.
A large number of bore-holes were drilled between 1974 and 1982 in the six large economic deposits ( Fig. 1) of clinoptilolitized pyroclastics and in one of mordeni tized pyroclastics. Beli Plast. Gorna Krepost. Most and Golobradovo belong to the I st Early Oligocene phase of volcanism. Belia Bair, Jelezni Vrata and the uppermost parts of the Gorna Krepost -to the 2nd phase. The mor denite deposit Liaskovetz belongs to the 1st phase. Ac tually in exploitation are Beli Plast and Jelezni Vrata .
The quantity of the zeolitized material in these deposits varies between some tens to some hundreds of millions of tons. The results of the geological research are sum marized in RAYNOV et al. (1997 and references therein).
Despite the impressive literature concerning zeolites from Rhodopes some areas are poorly studied. especially to the northwest and to the south of the town Kardjali where the pyroclastics of the two major acid volcanic phases are widespread (Fig. I). The aims of our paper are to characterize the zeolitization in some typical geologi cal sections of the paroxysmal acid phases. their mor phology, chemistry and quantity of the zeolites. We re port the first data on the chemistry of phyllosilicates, ac companying the zeolites and studied the cation exchange capacity (CEC) of the zeolitized pyroclastics and its rela tion with zeolite contents. We present the lateral and vertical zonality in distribution of glass replacing min erals in the products of the climactic explosive phases and determine the relationship between volcanic envi ronment (proximaL medial or distal zones). type of py roclastic material and zeolitization. These results com bined with already published data allow a better under standing of the zeolitization process of thick acid pyroc lastic series in a marine environment.  2nd. 3rd and 4th Early Oligocene phases, respectively. In the Late Oligocene this volcanic activity was followed by dyke intrusion. Here. we report a brief description of the products of the acid phases because only the acid glass is zeolitized (Fig. 1). Bentonitized latitic beds sepa rate the pyroclastics of the acid phases.

The products of the acid phases of volcanism
The Priabonian acid phase includes the Lozen dome volcano, where no zeolitization had occuned (YANEY et al. 1975), and the initial eruptions in the Sheinovetz cal dera (IVANOV A et al. 2000) (Fig. 1).
The first two, most powerful and voluminous Early Oligocene phases (Fig. I) were the culmination of the Eastern Rhodopean explosive volcanism. Phase 1 was essentially explosive producing pumice and ash flows accompanied by tephra falls. The volcanic source was probably situated in the region of Kostino village. as in ferred from observed progressive decrease in pumice size and deposit thickness away from this area (YANEY 1995. YANEY & BARDINTZEFF 1996. The products of the flows, weakly to moderately welded ignimbrites and fall out tuffs. cover an area of ca. 65 x 15-30km. Their de position in a marine environment is clearly indicated by the presence of microfossils and underlying coral reefs and marine sediments. Large Priabonian volcanic edifi ces north and west of the source area restricted the mi gration of the pyroclastic flows. Only fall-out tuffs were deposited, also subaqueously, further north and west ward. To the south of Kostino. the pyroclastic flows reached coastal reefs in the area near the town of Kard jali ( Fig. 1 a). beyond which fall-out tuffs alternate with few 1-2 m-thick limestone beds. Only sediments are de posited in the distal zone -in the southernmost part of Djebel depression (BOYANOV & GoRANOV 2001). The 2nd phase (YANEY & B.,\RDINTZEFF 1996) is es sentially explosive in the northern part of the Eastern Rhodopes, where several large pumice-flow eruptions resulted in Borovitza caldera collapse ( Fig. 1 b). The cal dera fill consists of some km thick succession of strongly welded ignimbrites. accompanied by fall-out tuffs. The outflow units, occupying large areas to the south and east of the caldera, were deposited in a marine environment producing moderately to non-welded ignimbrites. They extend also into the southern part of the Eastern Rhoda pes where dome-volcano activity dominated. The total area covered by the pyroclastics of this phase is ca. 80 x 60 km. Only fall-out tuffs were deposited in the medial zone (Djebel depression and eastern part of the Eastern Rhodopes). in some places accompanied by biogenic li mestone. Limestone. marl. clay  Twenty three samples richest in zeolites from different geological sections have been selected for further inves tigations including microprobe analyses. XRD. TEM. and CEC measurements. • The zeolite composition has been obtained by electron microprobe analysis on the following instruments: Ca mebax electron microprobe equipped with an X-ray energy dispersive detector (EDX) (using 20 kV accelera tion voltage, 10 nA beam cunent and 7-10 IJm beam di ameter) at the University of Montpelier (�amples BU 3 to BU 14): Jeol 870 Superprobe (WDS, using 15 kV accele ration voltage. 10 nA beam current and 10 11m beam di ameter) at the University of Florence by F. 0LMI (sam ples from Gorna Krepost and Beli Plast deposits): Ca meca SX 100 electron mycroprobe (using 15 kV accele ration voltage. 20 nA beam current and 10 A beam diam eter) at the Institute of Petrology of Vienna University (samples 1806 to 1851 and BU 21 to BU23).
• The composition of rhyolitic clasts in the lst phase ig nimbrites has been analyzed by scanning of an area of 100 x 100 11m using the Jeol 735 Superprobe at Geologi cal Institute. Sofia (analyst Tz. IL YEY): the chemistry of glass shards in the 2nd phase ignimbrites at Gorna Kre post deposit -by the Jeol 870 Superprobe at University of Florence (analyst F. Ouvn).
• Zeolite accompanying phyllosilicates have been stud ied by TEM at CRMC2-CNRS. Luminy. Marseille (sam ples BU 4 to BU 15) and by microprobe at University of Florence (samples from Gorna Krepost deposit).
• The quantity of the mineral phases have been evaluated by semiquantitative X-ray diffraction on a D500 Sie mens diffractometer at Geological Institute. Sofia, ac cording to the method of PETER & KADIAN (1964) and CHANG (1974CHANG ( . 1975 ). The operating conditions were: monochromatised CuKu radiation. 40 kV. 30 rnA. contin uous scanning at a velocity of 1-2 '/min using an auto matically changeable diaphragm. The quantity of each  mineral has been obtained by measuring and calculation of its RIR (reference intensity ratio) where the intensity of corundum (113) peak at d = 2.085 A has been used for comparison. Especially for clinoptilolite the intensity of the peak at d = 3.96 A has been used.
• The heat of water absorption (heating to 350 'C ) ac cording to the method of CuLFAZ et al . (1973) has been measured at the University of Aix-Marseille-III for ob taining the zeolite quantity and distinguishing clinoptilo lite from heulandite.
• For CEC (cation exchange capacity ) measurements the ammonium acetate saturation method (MINATO 1997). modified by KnsoPOULOS (1999). has been followed using Metrohm 692 pH/Ion-Meter (at University of Aix Marseille lii) with an NH4 + -selective gas electrode and precision ± 5 meq/100 g. P. LEGGO (University of Cam bridge. UK) has duplicated the CEC measurements.

Studied volcanic sequences
First Early Oligocene acid phase    . CAs & WRIGHT 1995 : large thickness without stratification. texturally unmodi fied juvenile glassy shards. uniform composition. folded pumice fragments (excluding the cold air-fall deposits), and absence of columnar jointing. The folding of pumice fragments indicates moderate welding. Microfossils (fo raminifers, radiolarians). commonly found in the matrix. and coral fragments indicate emplacement in marine en vironment. Coarse (em-sized) pumice fragments appear only in the upper part of the unit (sample B U 9 from the Most deposit corresponds to this lew!). Decimetre-to -1 m-sized grey or beige ""balls" and single injection clastic dykes can be randomly seen. They consist of psammitic sediments with adularia and quartz in the mat ri x.

Second Early Oligocene acid phase
Ignimbrites and fall-out tuffs rich in zeolites have been sampled at four localities in the medial zone. One of the sampled areas is located 25 km SE of the Borovitza cal dera bounding faults, where Belia Bair and Jelezni Vrata zeolite deposits are situated. Five lithologic units ( d) The fourth pyroclastic flow unit (sample 1807) is sev eral meters thick and covered by white fall-out tuffs in terbedded with thin pyroclastic t1ow deposits. all petro graphically similar to those previously described. e) Epiclastic sediments, up to 150m in thickness, interca lated with reef bodies. They contain rounded em to dm sized latite fragments set in a pink to white matrix of zeolitised shards.   Average microprobe analy�e of gla�� �hard" in the Leolrtitcd ignimhnte" of the 2nd phac-c-Goma Krepo"t Jepu�it (number of anal) -.c.., 5 l.

Results
Petrographic and chemical composition of the zeolitized pyroclastics All studied samples are zeolite rich and their petrography and mineralogy are summarized in Table I.
The fall-out tuffs as well as the matrix of the ignimb rites are vitroclast rich and consist of arcuate. rod-like. crescent-shaped. triangular shards with concave sides (Fig. 4). The rocks contain also pyrogenic (mostly quartz. less sanidine, plagioclase and biotite, rarely amphibole. clinopyroxene and some accessories such as magnetite, apatite, zircon, titanite) and xenogenic minerals probably derived from the metamorphic basement (mosaic quartz, K-feldspar, muscovite). Pumice ( with tube-like and spherical vesicles), massive lithic clasts (spherulitic or felsitic rhyolite and perlite). some microfossils. frag ments of limestone, latitic and metamorphic rocks are also present. The ignimbrites are very rich in pumice clasts, varying in size. In the 1st phase ignimbrites they are strongly flattened (see above) while in the ignimbri tes of the 2nd phase the pumice clasts are not deformed.
The major and trace element distribution in the zeoli tized pyroclastic rocks is presented in Tables 2 and 3. The zeolitized ignimbrites of the 2nd phase (Belia Bair deposit) are richer in Si0 2 due to their high quartz and lithic clasts contents (Table 1 ). A comparison between two types of rhyolitic clasts (according to their K-'0 con tent) from the 1st phase, and between non-altered glass shards of the 2nd phase from Gorna Krepost deposit and zeolitized pyroclastics of the same phase. respectively. is shown in Fig. 5. The change in Si02 and Al20 3 contents is minor, but the zeolitized samples are strongly depleted in MnO. Na20 and K20 (i. e. these oxides are partly leached), and enriched in CaO, Fe-'03• Ti02 and MgO. In the altered pyroclastics CaO is incorporated in zeolites and the other three oxides in phy llosilicates.
The average LILE contents in the zeolitized pyroclas tics have a tendency to increase from the 1st to the 2nd phase (Rb from 214 to 222 ppm; Sr from 286 to 409 ppm: Ba from 88 to 170 ppm). HFS elements are immo bile during the zeolitization and the Nb/Zr ratio (Fig. 6) shows that the zeolitized rocks have their primary mag matic (collisional) signatures still preserved.

Glass-replacing authogenic minerals
All of the glassy volcaniclastic fragments (shards and pumice) are completely altered but their original shape is intraplate Zr ppm 1000 commonly well preserved (Fig. 4). The glassy shards in the matrix of the epiclastic layers are also altered. Only in few places in the topmost parts of the 2nd phase se quence up to 50 o/c of glass is unaltered (ALEKSIEV et al . 1997). The crystal clasts (sanidine. plagioclase, and bio tite) are not affected by the zeolitization.
The glass-replacing mineral association consists of zeolites, phyllosilicates. feldspars and opal-CT. The quantitative estimations (Fig. 7) (Table 5) has been fo und in the fine ash matrix of the uppermost py-     (Fig. 10). The X-ray data indicate that small amounts of mordenite are present in some samples (Fig. 7).
Phyllosilicates are present as green rims around the shards, essentially in the I st phase (Fig. 4) as well as iso lated brown or green-colored grains or clusters (average size of 40 j..lm) within the internal clinoptilolite zones.
Based on X-ray studies two minerals have been identi fied, a swelling one (smectite) and a mica-like mineral (celadonite). The microprobe analyses ( Feldspars. Near the presumable volcanic centre of the 1st phase (in the Shabandere section), adularia, accompa nied by small needles of mordenite ( Fig. 10 left), is the most abundant authigenic mineral in the pumice clasts in the topmost parts of the I st pyroclastic flow unit (Fig. 3 ). Clinoptilolite, associated with adularia, prevails in the matrix (Fig. lO right). Albite has been also (but rarely) identified in the largest pumice blocks. Adularia and quartz are the most abundant authigenic phases in the matrix of the ·'balls" and in the injection clastic dykes in the pyroclastic rocks of the I st phase as well as in the basal layers of the section in the whole proximal and me dial areas.

Discussion
Vertical and lateral distribution of the glass replacing minerals 1st and 2nd Early Oligocene acid volcanic phases. Both vertical and lateral zoning has been established summa rizing all available data on the spatial distribution of the secondary mineral associations (Fig. 12).
Ve rtical zoning. Vo lcanic glass from the basal layers of the proximal and medial pyroclastic deposits erupted from Kostino volcanic centre (1st phase) is always re placed by adularia + quartz ( + analcime in the area of Most deposit, DmuRov A 1976). All available data (pre-sent study. ALEKSIEV et al. 1997. RA YNOV et al. 1997 (1987) based on their studies in Moriantzi. Beli Plast and Kos tino. According to these authors Na decreases upward. K increases in the same direction. and Ca concentrates in the middle or upper parts of the section. An increase of K content in clinoptilolite towards the upper parts of the pyroclastic sequence is also detected in the Sheinovets caldera (IVANOVA 2002). We have also found zoning in the distribution of the extra framework cations in the Belia Bair deposit but in the opposite direction: Na increases and K decreases to the top of the section ( Fig. 9. Table 4 ).
Lateral zoning in the di stribution of glass replacing minerals has been also established in the Eastern Rhodo pes zeolitized pyroclastics. Close to Kostino -the vent area of the 1st phase (Shabandere section ) adularia. ac companied by clinoptilolite and/or mordenite, locally predominates (e. g. in the pumice-rich upper part of the

On the origin of the zeolitization in the Eastern Rhodopes
Several genetic models have been invoked to explain the widespread zeolitization of the Eastern Rhodopes pyro clastic rocks. KIROV (1974) argued that clinoptilolite was formed by reaction of rhyolitic glass with meteoric and groundwater according to the model of HAY (1966).
Later. a geoautoclave concept was introduced (ALEKSIEV & D.rouROVA 1975, ALEKSIEV et al. 1997. PoLGAR! et al. 1997) stating that the zeolitization occurs immediately after the submmine deposition of hot pyroclastic flows rapidly buried by merlying deposits. Autoclave condi tions are then created inside the tl ow units and at the ex isting pressure, temperature and fluid content the volca nic glass is transforming into zeolites. This hypothesis docs not explain the zeolitization of the fall-out tuffs as well as the matrix of the epiclastic layers. The high tem perature of the pyroclastic flows (600-700 'C CAs & WRIGHT 1995 ), the low amount of water carried by the flow and dissolved in the glass fragments with regard to the water needed for zeolite crystallization exclude this hypothesis (RAYNOV et al. 1997, HALL 1997. The last author argues also that the fluids in the pyroclastic flow are strongly acid while the zeolites can be formed under neutral to alkaline conditions only. According to GoT (1985) this hypothesis concerns only the submarine pyroclastic tl ow deposits that were cooled by mixing with meteoric and marine water. This specific homogenous hydrothermal environment results in zeoli tizaton but can not be called geoauthoclave (HALL 1997).

TARDI & GALLI
According to TsnsrsHVILI et al. (1992) and RA YNOV et al. ( 1997) the zeolitization of the Eastern Rhodopes pyroclastic sequences was caused by regional low-tem perature hydrothermal systems. SHEPPARD & H AY (2001) pointed out that the zeolites from Southeastern Europe are an example of formation in an open hydrologic sys tem but in marine environment whereas UTADA (2001) considered them as result of burial diagenesis. (1975) were the first to indi cate that all zeolitized Eastern Rhodopes pyroclastics are submarine deposits. This observation is strongly sup ported by our data: the 1st phase pyroclastics overlie Priabonian marine sediments and coral reefs; the top most part of the 2nd phase section includes (see Fig. 2) and is covered by biogenic limestone: in both phases microfossils are present in the pyroclastics (see Table 1) and ripple marks in some fall-out tuffs of the 2nd phase (Jelezni Vrata quarry, Chitlik outcrop). There are no in dications of subaeral deposition of zeolitized rock: nei ther accretionary lapillis nor zeolitized strongly welded ignimbrites (with fi amme) are known. On the slopes and the top of the latite Dambalak and Madj arovo volcanoes, south-east of Kardjali (Fig. 12), the acid pyroclastics with accretionary lapillis are probably deposited in subaerial environment, but they are not zeolitized. The some km thick strongly welded ignimbrites filling in the Borovitza caldera are not zeolitized as well (YANEV 1990). On the contrary, not all pyroclastics deposited in marine envi ronment are zeolitized, especially those building the vol-canic edifices (e. g. the lowest part of the Borovitza rhyo litic section, YANEV 1990. or the Priabonian Lozen vol cano. YA NEV et al. 1975 ) . HALL (1997) explains this by the acid environment existing in the volcanic edifices close to the vent areas.

ALEKSIEV & DJOUROVA
Most of the fe atures of zeolitized pyroclastics from Eastern Rhodopes are indicative of burial diagenesis (ac cording to the criteria of GoTTARDI 1989): low temper ature ( < 200 'C) authigenic minerals, presence of one or two zeolite minerals only over large areas. small crystal size, vertical zoning in the di stribution of the glass re placing minerals with thick (many tens of meters) min eral zones. According to JuniA (1988) the temperature of burial glass transformation into clinoptilolite and/or mor denite in the Neogene Green Tuff region of Japan is 41-55 'C. However, the lack or the small thickness of the glass-zeolite transition zone and the short time span of zeolitic alteration (see below) are different from the products of burial diagenesis (according to the critera of HAY & SHEPPARD 2001). Whereas the observed vertical zoning (adularia at the base of the 1st phase pyroclastics) could be explained by the depth depending burial dia genesis, the lateral zoning suggests a hydrothermal gene sis. The lateral zoning is evident as strong adularization in the proximal zones. Kostino area and Borovitza cal dera. formed at relatively high temperature and/or high activity of K: zeolitization in the medial zone in the con fines of the region of volcanic activity; bentonitization out of it. in the distal zone (Fig. 12). The wide distribu tion of celadonite within the zeolitized pyroclastics espe cially in the 1st phase may also indicate a hydrothermal origin. The celadonite is one of the most precisely docu mented cases of authigenic clay formation from the hy drothermal areas in mid-oceanic ridges (HILLIER 1995). Under hydrothermal conditions the temperature of zeo lite genesis is higher than during the burial diagenesis: the maximal temperature of clinoptilolite formation is 150-250 'C (BARTH-WrRscHING & HoLLER 1989); the temperature of mordenite formation in Wairakei geo thermal area is 150-230 'C.
The chemical difference between the unaltered volca nic glass and zeolitized pyroclastics in the Eastern Rho dopes (Fig. 5) argues for an open system, but the class ical hydrologic open system ( in the sense of HAY & SHEPPARD 2001) is developed for non marine pyroclas tics. altered by meteoric water. The low temperature of this type of alteration demands a very long time of zeoli tization of up to 10 Ma for thick pyroclastic series (TscHENICH 1992). The Paleogene Eastern Rhodopes py roclastics are deposited in a marine environment as their zeolitization has not taken very long time presuming that the zeolitized products of the two acid phases are sepa-rated by non zeolitized but bentonitized latitic tuffs : the total duration of all Early Oligocene phases is not more than 3-4 Ma (LILOV et al. 1987) and the zeolitization has not affected the top of the Lower Oligocene section nei ther the Upper Oligocene sedimentary cover.
The glass shards may have been replaced by dissolu tion-reprecipitation processes according to the models of PuTNIS (2002) with preservation of the shard morphol ogy and formation of the distinct zonality. Dissolution of the shard surface and precipitation of the celadoni te/smectite rims (Figs. 4 and 8), resulting in enrichment of Si and alkalis in the solution (HAY 1966), is the first stage of the glass replacement process. During the sec ond stage the glass from the shards interior continue to dissolve under the int1uence of the Si-and alkali enriched solution and high-Si zeolites crystallized, ac companied by different phyllosilicates and opal-CT. This process is similar to the replacement of perlite bodies by zeolites described by YA NEY et al . (1986 and NoH & BoLES (1989). In the pumice clasts deposited near to the vent, which are able to retain their initial high tem perature for longer time (KAMINSKI & JAUPART 1997), adularia (or albite) crystallized preferentially.
The zeolitizing hydrothermal solutions consist of ma rine water heated by the anomalous geothermal gradient of the active volcanic areas or/and by the hot pyroclastic rocks (LENZI & PASSAGLIA 1974. GoTTARDI & GALLI 1985. Even a difference in temperature between volca nic rocks and marine water as big as 30-40 "C can trig ger a hydrothermal circulation (HALL 1997). These solu tions percolate throughout the permeable pyroclastic ho rizons over large areas and zeolitize all glass containing rocks including epiclastics. The participation of the en dogenous solutions coming from the magma chambers in this process is notified by some authors (i. e. GoGISHVILI 1980. GOTTARDI 1989. TSCHENICH 1992, TSITSISHVILI et al. 1992 but at that stage of our work we can neither support nor decline this hypothesis.