Esenli et al.indd 1. INTRODUCTION The pyroclastic rocks have extensive zeolite reserves in nature. This is due to their high volcanic glass con- tent and porosity. Composition of volcanic material as well as the ease of circulation of solutions influences dissolution of volcanic glass and the formation of zeo- lites. The crystallization sequence of minerals and reac- tion products during rock–solution interaction depends on the chemical composition of the starting material, contact of solution with volcanic glass and minerals of the pyroclastic rocks, rock–solution interaction time, temperature, pressure, pH, geological environment and hydrological systems (BOLES, 1988; BARTH- WIRSHING & HOLLER, 1989). Analcime, mordenite and clinoptilolite are usually regarded as diagenetic Zeolitization of Tuffaceous Rocks in the Keşan Region, Thrace, Turkey Fahri ESENLİ, Bektaş UZ, Fikret SUNER, Vildan ESENLİ, Ö. Işık ECE and Işık KUMBASAR products of glassy volcanic material. They can form in closed or open hydrological systems, in deep-sea sedi- ments, in hydrothermal alteration zones and, as a result of low-grade burial metamorphism (IIJIMA & UTADA, 1966; HAY, 1981; GOTTARDI & GALLI, 1985). The distribution of analcime-bearing and analcime-absent zones differs horizontally or vertically depending upon chemical zoning in these environments. Inner or deeper zones are characterized by the existence of analcimes (HAY, 1981). Analcime is low silica zeolite, so that generally the Si/Al ratio is 2, while this ratio ranges from 2 to 3 for sedimentary analcimes (GOTTARDI & GALLI, 1985). Na is the only extra-framework cation in analcimes. Mordenite and heulandite–clinoptilolite are high silica zeolites. The Si/Al ratio of mordenites ranges from 4.2 to 5.9, and their cation contents per unit cell are 1.6–2.5 for Ca, 2.0–5.0 for Na and 0.1–0.8 for K (PASSAGLIA, 1975). However, in some examples K is reported as the dominant cation (COOMBS et al., 1997, and references therein). Heulandites have been distinguished from iso- structural clinoptilolites on the basis of their Si/Al ratios – heulandites having Si/Al<4 and clinoptilolites having Si/Al>4. The clinoptilolite structure is more stable than that of heulandite and calcic clinoptilolite (MASON & SAND, 1960; ALIETTI, 1972; BOLES, 1972; ALIET- TI et al., 1977). The cation contents in heulandites and clinoptilolites are highly variable and therefore Ca-, Na, K- and Sr-heulandites and Ca-, Na- and K-clinoptilo- lites are commonly known (COOMBS et al., 1997). The significance of zeolitization in tuffaceous rocks in SW Keşan (Thrace, northwest Turkey – Fig. 1), the occurrence, stratigraphic position and mineralogical characteristics of zeolite-bearing levels are discussed in this study. Some zeolite occurrences in the Upper Eocene–Oligocene pyroclastics from Thrace (in the vicinity of Gelibolu and Uzunköprü areas and the south- east of Keşan) have been previously reported (ESENLİ et al., 1997a; İÇÖZ & TÜRKMENOĞLU, 1997). These occurrences contain the heulandite–clinoptilolite type of zeolites in the Gelibolu and Uzunköprü areas and anal- cime and clinoptilolite in SE Keşan. In this study, the zeolites, including analcime, mordenite and heuland- ite–clinoptilolite, were described stratigraphically as alternate layers in the SW Keşan. The objective of this paper is to characterize the zeolitization and to explain the formation of analcime- and mordenite (+heulandite– clinoptilolite)-rich levels. Geologia Croatica 58/2 151–161 5 Figs. 4 Tabs. ZAGREB 2005 Key words: Analcime, Clinoptilolite, Heulandite, Mordenite, Tuffaceous rocks, Zeolitization, Thrace, Turkey. Istanbul Technical University, Geology Department, Maslak, 34469 Istanbul, Turkey; e-mail: esenlif@itu.edu.tr Abstract A 33 metre thick pyroclastic-rich zone of the Mezardere formation of Oligocene age is exposed in the Keşan region of Thrace, Turkey. In this zone, vitreous tuffs of dacitic composition have altered pri- marily to zeolites, including mordenite, heulandite–clinoptilolite and analcime. Silicification and alteration to clay minerals are common. Zeolite minerals have developed from volcanic glass, whereas some mordenites have formed from dissolution of heulandite-group zeo- lites. Although authigenic mineral paragenesis does not vary laterally, there is a marked vertical variation, particularly in zeolites. Morden- ite (+heulandite–clinoptilolite) and analcime do not coexist and have formed in different stratigraphic levels. This suggests that their chemi- cal environment is controlled by different hydrologic systems. Whole rock composition shows the relationship between chemis- try and secondary mineralogy. For example, whole rock trace element geochemistry indicates the natural selectivities of zeolites. There is also stratigraphic control on the chemistry and texture of mordenites. Specifically, (Na+K)/(Ca+Mg) ratios of mordenites decrease from the lower to the upper levels. Mordenites of the lower level show a fibrous habit while the upper level mordenites are needle-like in shape. The average Si/Al ratio in mordenites is 3.90, in heulandite- group minerals 3.95, and in analcimes 2.34. 152 Geologia Croatica 58/2 2. GEOLOGICAL SETTING 2.1. Thrace Basin The Thrace basin is surrounded by the Stradja massif in the north, the Rhodope massif in the northwest and the Menderes metamorphic massif in the south. It is on the Intra-Pontide suture zone, which was developed by the northward subduction of the northern branch of the Neo Tethys Ocean during the Late Cretaceous–Early Ter- tiary period (ŞENGÖR & YILMAZ, 1981; YILMAZ et al., 1997) and continental collision of the Stradja and Sakarya zones (OKAY & TÜYSÜZ, 1999). The Thrace basin started to open at the end of the Middle Eocene, and sedimentation started as a transgressive process; however, environments of deposition significantly changed at the end of the Early Miocene and were lat- er completed as regressive sedimentation (TURGUT et al., 1983, 1991). Thus, in the stratigraphic succes- sion, fine-grained sediments of flysch facies proceed to shale, marl, limestone, coal and sandstone indicat- ing lacustrine, fluvial and shallow marine depositional environments. During the Late Oligocene–Early Mio- cene, the basin was wholly elevated and previously deposited sedimentary rocks were subjected to erosion. The Middle Miocene tectonic activity caused folding and faulting along the edges of the basin. During the Late Miocene, thick successions of conglomerate, sand- stone, claystone and coal series were overlain on the pre-Miocene rocks across regional disconformities. The thickness of sedimentary rocks reaches up to 8 km in the centre of the Thrace basin (TURGUT et al., 1983, 1991). Volcanic eruptions were synchronous with the maxi- mum of the transgressive episode of the Thrace basin. Voluminous volcanic products with dacitic and andes- itic composition form tuffaceous interbeds within sedimentary rocks or the tuffaceous matrix of clastic rocks. Petrographic and geochemical classification of the Thrace volcanic rocks presented by YILMAZ & POLAT (1998) and GENÇ (1998) are as follows: calc alkaline–intermediate volcanic products were extruded during the Late Eocene–Early Miocene period. This Fig. 1 The location, geological map and stratigraphic section of the study area (SW Keşan). 153Esenli et al.: Zeolitization of Tuffaceous Rocks in the Keşan Region, Thrace, Turkey first phase is assumed to be a product of Tibetian-type volcanism developed under the N–S compressional regime of northwestern Anatolia. The second volcanic phase started during the Late Miocene and is represent- ed by alkaline basaltic lavas extruded sporadically in different parts of the Thrace region. 2.2. Keşan Region The Keşan Formation (Upper Eocene) is the lowest part of the stratigraphic succession in the study area (SW Keşan, Fig. 1). It consists of turbiditic sandstones and minor shales and marls that are conformably overlain by the Lower–Middle Oligocene Mezardere Forma- tion, described by TURGUT et al. (1983). The latter is unconformably overlain by the Miocene–Pliocene Ergene Formation. TURGUT et al. (1983) infers that the Keşan Formation was formed in a marine environment and the Mezardere Formation indicates lacustrine and shallow depositional environments. TERNEK (1949) and KOOP et al. (1969) described the volcanic activ- ity (andesitic and dacitic lavas and tuffs) in the Keşan region throughout the Oligocene epoch. KOOP et al. (1969) reported an approximately 20 m thick pyroclas- tic bed named the “Keşan tuffs” between the Keşan and Mezardere Formations. Towards the Upper Oligocene, andesitic, dacitic and trachytic lavas became dominant. ERCAN et al. (1998) reported that andesitic lava from the Keşan area yields a K/Ar age of 26.2±0.5 my (Late Oligocene, Chattian). In the study area (SW Keşan, Fig. 1), tuffaceous rocks repeatedly occur both in the upper part of the Keşan Formation and throughout the Mezardere Forma- tion. They are characterized by dacitic–andesitic tuffs interbedded with marls and sandstones. Zeolite-bear- ing tuffaceous rocks are constrained to the lower part of the Mezardere Formation, and are exposed between the villages of Karahisar and Kızkapanı in an area of about 1.5 km2 (Fig. 1). The thickness of this zone is 33 m in a quarry in the Kızkapanı (see Fig. 5) and 20 m in the Karahisar areas. Trachytic lavas that appear as hills in the study area are probably younger than the pyroclas- tics. 3. METHODS Nineteen samples have been studied stratigraphically in detail: fourteen zeolite-bearing pyroclastic, one non- zeolitic pyroclastic and four clastic rock samples. The samples were studied petrographically and mineralogi- cally by polarizing microscope and X-ray diffractom- eter. A Philips diffractometer with Cu (Kα) radiation was used for X-ray powder diffraction (XRD) analyses and powder samples were scanned at 1°2θ per min- ute. Major element compositions of rocks have been determined by the X-ray fluorescence (XRF) method. Powdered rock samples have been fused using LiBO2 and the resulting glass disks have been analyzed in the research laboratory of Turkish Glass Inc. Rigaku; Rix– 2000 model equipment was used for XRF analyses. Trace elements were analyzed by using Spectro Ciros Vision ICP–ES for Ba and Sc (0.200 g pulp sample by LiBO2 fusion) and Mo, Cu, Pb, Zn, Ni, As (0.50 g sample leached with 3 ml of 2–2–2 HCl–HNO3–H2O at 95°C for one hour, diluted to 10 ml) and by Perkin Elmer Elan 6100 ICP–MS for the other elements in the ACME Analytical Laboratories, Vancouver, Canada. Morphological characteristics and chemical composi- tions of zeolites (mordenite, analcime and heuland- ite–clinoptilolite) were investigated on a JSM–840 type scanning electron microscope (SEM) and a Tracor Northern 5400 energy dispersive X-ray spectrometer (EDX). Structural formulas of zeolite minerals were calculated on the basis of 72 oxygen for clinoptilolites and 96 oxygen for mordenites and analcimes using unnormalized oxide values. The quality of the analyses was calculated by evaluating the balance error given by GOTTARDI & GALLI (1985). The balance error of the mordenite and heulandite–clinoptilolite samples ranges 9–29% (however, Ba, Sr and Li were not analyzed). 4. RESULTS AND DISCUSSION 4.1. Whole Rock The zeolite minerals are closely related to vitric tuffs of dacitic–andesitic composition. There is no clear dif- ference in physical appearance between mordenite (+heulandite–clinoptilolite)-rich and non-zeolitic expo- sures in the field. They are massive and beige–pale green in colour. The analcime-rich outcrops, however, have a dark gray colour and are brittle which distin- guishes them from the mordenite (+heulandite–clino- ptilolite)-rich and non-zeolitic outcrops of tuffaceous rocks. Phenocrysts of tuffs consist of plagioclase (oligo- clase–andesine), biotite, amphibole (hornblende), quartz and opâque minerals. The phenocryst content ranges from 2 to 60 vol. %. Volcanic glassy material (glass shards and pumice fragments) has authigenetically transformed to zeolites (analcime, mordenite and heu- landite–clinoptilolite), smectite (dioctahedral montmo- rillonite), silica minerals (opal-CT and quartz) and cal- cite. Mordenite, heulandite–clinoptilolite, smectite and opal-CT could not be identified optically. Analcimes are mostly anhedral. Calcites occur as pore-filling crystals as well as the alteration products of plagioclases. Quartz is present as both phenocrysts and as a secondary min- eral, and the majority of it is of authigenic origin. The mineralogical compositions of the samples determined by XRD analysis are listed in Table 1. Samples from 1 to 19 are arranged stratigraphically from the lower to the upper zone (see Fig. 5). Samples numbered as 3, 7, 12 and 15 are of marl and sandy calcareous composi- tion and the rest is of tuffaceous material. Sample 8 is the only pyroclastic rock which does not contain any 154 Geologia Croatica 58/2 zeolite mineral. Mordenite is identified in twelve sam- ples and analcime is found only in samples 9 and 14. These minerals are generally the major components of rock samples according to the XRD patterns (Fig. 2). Analcime- and mordenite-bearing levels alternate in the stratigraphic sequence. Lesser amounts of heuland- ite–clinoptilolite are associated with mordenite in five samples (5, 6, 10, 11 and 19). Smectite is identified in ten samples, and it is the main mineral in sample 16. Opal-CT in three samples is from the middle level of the zeolite-bearing zone. The major element compositions of nine rock sam- ples and trace element analyses of eleven samples are given in Tables 2 and 3, respectively. Plotted on the Zr/TiO2–Nb/Y diagram of WINCHESTER & FLOYD (1977), most of the samples fall in the dacite–rhyoda- cite field (Fig. 3). Only sample 5 is trachyandesite and sample 9 is andesite. There is a relationship between the whole rock chemistry and secondary mineralogy of the samples. For example, the SiO2 content of analcime- rich samples is low, but high in mordenite-rich samples. CaO, except sample 8, is between 3.23 and 4.17 wt. %. The high CaO of sample 8 (9.17%) is probably due to the presence of calcite in this sample. Fe2O3 contents of samples 8, 9, 11, 14 and 16 are higher than those of other samples. According to the XRD results, sample 8 contains a trace amount of illite. On the other hand, thin-section observations revealed that illite–celadon- ite minerals seen in the matrix, in voids and in pheno- Sample Mineralogy 1 Mordenite+Quartz 2 Mordenite+Quartz+Feldspar 3 Calcite+Quartz+Illite 4 Mordenite+Smectite+Quartz 5 Mordenite+Heulandite–Clinoptilolite+Smectite+Quartz 6 Mordenite+Heulandite–Clinoptilolite+Smectite 7 Calcite+Quartz+Illite+Kaolinite+Feldspar 8 Quartz+Feldspar+Calcite+Illite 9 Analcime+Quartz+Feldspar 10 Quartz+Mordenite+Heulandite–Clinoptilolite+Opal-CT+Feldspar+Smectite 11 Mordenite+Quartz+Heulandite–Clinoptilolite+Opal-CT+Calcite 12 Quartz+Calcite+Feldspar 13 Mordenite+Smectite+Opal-CT+Feldspar+Calcite+Quartz 14 Analcime+Quartz+Smectite+Feldspar 15 Quartz+Calcite+Kaolinite+Illite+Feldspar 16 Smectite+Mordenite+Feldspar 17 Mordenite+Quartz+Feldspar+Smectite 18 Mordenite+Quartz+Smectite 19 Mordenite+Quartz+Feldspar+Heulandite–Clinoptilolite+Smectite Table 1 The mineralogical composition of pyroclastic and clastic rocks of SW Keşan determined by XRD. Sample 1 5 8 9 11 14 16 18 19 SiO2 65.91 66.17 56.60 61.07 65.49 61.47 63.22 66.43 67.36 Al2O3 14.01 13.49 14.00 16.01 13.50 15.45 14.18 13.29 13.93 Fe2O3 1.61 1.31 5.57 2.74 3.47 2.84 3.01 1.82 1.76 MnO 0.16 0.21 0.28 0.11 0.05 0.16 0.17 0.17 0.10 TiO2 0.28 0.34 0.85 0.64 0.35 0.42 0.60 0.38 0.33 MgO 1.31 1.71 2.35 2.37 1.63 2.20 2.85 1.90 1.98 CaO 3.51 3.54 9.17 3.53 3.30 3.28 3.12 4.17 3.23 K2O 1.66 2.65 2.98 1.95 2.92 2.10 2.83 2.87 2.74 Na2O 2.88 1.50 2.59 3.90 0.71 4.15 1.31 1.33 1.18 P2O5 0.11 0.07 0.35 0.14 0.08 0.13 0.13 0.12 0.09 LOI 8.56 8.01 5.26 7.54 8.50 7.80 8.58 7.52 7.30 Table 2 Whole rock chemistry (wt. %) of the samples. 155Esenli et al.: Zeolitization of Tuffaceous Rocks in the Keşan Region, Thrace, Turkey crysts of samples 8, 9, 10, 11 and 14, are products of alteration. These clay-group minerals were not detect- ed in XRD patterns except in sample 8. Therefore the presence of illite–celadonite minerals might explain the high Fe2O3 in samples 8, 9, 11, 14 and 16. In addi- tion, mordenites in these samples also contribute some amounts of iron. Indeed, single crystal analyses (Table 4) show that mordenites have a high content of Fe. K2O and Na2O range between 1.66–2.98% and 0.71–4.15%, respectively. Na2O of the non-zeolitic sample 8 is high- er than that of mordenite-rich samples and reaches its highest values in analcime-rich samples. The results of the trace element chemistry can be summarized as follows: (a) Co, V, Zn, Ni, Y, Sm, Eu, Gd, Tb, Dy, Tb, Ho, Er are higher in sample 8 containing no authigenic zeolite mineral; (b) Samples 9 and 14 are richer in anal- cime and Cs, Mo, Rb, U, Zr, Cu, Pb are higher in these samples, and (c) Ba, Sr, Th, La, Ce and As of morde- nite-bearing samples 1, 5, 10, 11, 13, 16, 18 and 19 (Table 3), are higher than the average values of all other samples studied. It is also determined that samples con- taining mordenite+heulandite–clinoptilolite have higher average values for Ba and Sr than those of the morde- nite-rich group. On the other hand, considering the stratigraphic positions of mordenite-bearing samples, the upper levels are richer in Ba, Rb, Sr, Ti, U, W and As, while the lower levels are richer in Mo, Cu, Pb, Zn, and Y–Lu series. 4.2. Zeolites Mordenites of the tuffaceous rocks (SW Keşan) show fibrous and needle-like habits (Figs. 4a and b). The mordenite needles are about 3–20 μm in length and 0.3–1 μm in width, whereas the widths of fibres of thread-like mordenites are generally less than 0.2 μm. Mordenites have formed as individual crystal and also as radial from a nucleus (Fig. 4c). They have mostly developed from volcanic glassy material. The fibres are growing from the top of the vitrified volcanic glass, which displays a parallel fractured alteration pattern as shown in Fig. 4d. However, some mordenites of sample 19 have been observed as tiny crystals on heulandite- group crystals (Fig. 4e). In this case, heulandites–clino- ptilolites transformed to mordenite crystals which were in the form of dissolved grains and do not exhibit their Fig. 2 The XRD patterns of analcime- and mordenite-bearing samples (a: sample 14, and b: sample 17). 156 Geologia Croatica 58/2 monoclinic idiomorphic habits. These heulandite-group minerals were probably transformed to mordenites after dissolution. PE-PIPER (2000) reported transformations between heulandite-group and mordenite-group miner- als, which are depending on the changes in temperature. KITSOPULOS (1997) also reported the mordenites from the Polyegos Island, Greece, which are draped over and formed from the crystals of heulandites. The average structural formulae of the Keşan mor- denites was calculated on the basis of 96 oxygen to [Ca1.7 Mg1.3 Na2.0 K1.6] [(Fe, Al)9.6 Si37.4 O96 ] · 28H2O. They show the Si/Al ratio ranging from 3.60 to 4.36 with an average value of 3.9 and the average value of the (Na+K)/(Ca+Mg) ratio is 1.2 ranging from 0.90 to 1.57 (Table 4). The obtained Si/Al ratio is lower than those corresponding to hydrothermal mordenites. PAS- SAGLIA (1975) showed that there are no large varia- tions in the chemical composition of mordenites with probably sedimentary and hydrothermal origin, how- ever, according to GODOVIKOV (1985) hydrothermal mordenites have higher silica and water contents than sedimentary mordenites. Chemical compositions of the Keşan mordenites exhibit variations across the strati- graphic sequence (see Table 4 and Fig. 5). The ratio of alkali/earth alkali cations decreases toward the upper levels. This difference can be clearly seen in the com- Sample 1 5 8 9 10 11 13 14 16 17 18 As 4 17 3 4 4 5 4 3 4 5 4 Ba 658 1585 333 623 1020 873 698 210 636 571 869 Ce 57.9 55.4 50.7 56.8 52.1 64.0 62.0 55.1 59.8 63.5 58.3 Co 6.3 1.8 12.6 7.7 5.3 3.6 3.9 4.0 6.9 6.8 7.7 Cs 4.3 2.9 2.9 11.3 2.5 2.6 3.1 12.2 4.4 3.0 5.3 Cu 9 4 6 11 10 6 6 7 7 6 8 Dy 2.33 1.46 3.72 2.94 2.73 2.47 2.56 2.50 2.47 2.25 2.29 Er 1.49 1.11 2.23 1.85 1.45 1.70 1.69 1.67 1.65 1.22 1.55 Eu 0.62 0.44 1.25 0.89 0.67 0.68 0.76 0.71 0.7 0.67 0.59 Ga 14.7 13.4 15.0 15.3 11.9 14.5 15.4 13.9 15.8 13.7 14.7 Gd 2.58 1.93 4.37 3.57 2.42 2.61 2.64 3.15 2.82 2.33 2.74 Hf 4.3 4.2 4.6 5.1 3.6 4.7 5.0 5.0 4.7 4.1 4.3 Ho 0.43 1.31 0.78 0.62 0.48 0.51 0.52 0.50 0.51 0.41 0.48 La 32.1 35.6 24.6 31.6 30.0 36.0 35.3 30.0 33.2 34.1 30.1 Lu 0.22 0.21 0.34 0.35 0.27 0.32 0.34 0.30 0.29 0.28 0.26 Nb 7.6 8.2 6.2 8.7 7.0 9.1 8.6 10.5 9.1 7.6 7.7 Nd 18.9 17.1 24.2 21.1 18.4 22.4 21.4 21.2 22.7 20.6 20.1 Ni 7 4 18 13 6 2 3 17 13 7 14 Mo 0.3 0.4 0.3 1.1 0.6 0.2 0.3 0.2 0.2 0.4 0.2 Pb 33 39 1 32 28 35 35 23 39 36 33 Pr 5.83 5.60 6.16 6.20 5.49 6.61 6.4 5.76 6.32 6.09 5.60 Rb 89.8 77.5 69.2 91.0 85.5 65.4 72.8 100.7 75.0 78.0 77.3 Sm 3.4 2.6 5.2 4.0 3.0 3.8 3.6 3.3 3.9 3.2 3.5 Sr 1112 1882 433 731 1679 1585 1060 201 819 920 1232 Ta 0.7 0.7 0.3 0.6 0.6 0.7 0.6 0.7 0.7 0.7 0.7 Tb 0.42 0.30 0.73 0.57 0.42 0.44 0.5 0.43 0.48 0.38 0.43 Th 18.7 1.8 6.9 17.7 16.3 23.6 19.8 16.6 19.7 17.7 18.0 Tl 0.8 0.5 0.5 0.6 0.3 0.4 0.3 0.3 0.2 0.5 0.5 Tm 0.25 0.17 0.31 0.32 0.25 0.29 0.32 0.25 0.27 0.21 0.27 U 4.4 13.3 2.1 6.7 5.5 7.1 7.2 5.5 5.3 4.4 6.2 V 42 35 113 62 50 29 43 42 48 33 53 W 3 4 1 2 1 1 1 1 1 3 2 Y 14.6 9.8 21.7 20.1 14.8 17.2 17.9 17.2 16.5 13.0 15.1 Yb 1.91 1.23 2.24 2.06 1.62 2.02 2.21 1.88 1.76 1.53 1.70 Zn 36 23 50 47 35 30 29 37 43 31 36 Zr 141 157 156 162 128 153 172 167 178 143 138 Table 3 Whole rock trace element composition (ppm) of the samples. 157Esenli et al.: Zeolitization of Tuffaceous Rocks in the Keşan Region, Thrace, Turkey parison of mordenites from the lower and upper levels. Cation contents change from [Ca1.0 Mg1.5 Na1.2 K2.7] for the lower level mordenites (sample 1) to [Ca2.0 Mg2.1 Na3.3 K0.5] for the upper level mordenites (sample 19). Mordenites of the lower level are poor in Na and Ca and rich in K, whereas mordenites of the upper levels are poor in K and rich in Na and Ca. Morphological proper- ties of these mordenites from the lower level, even the middle levels (samples 6 and 11), show a fibrous habit and mordenites from the upper level are needle-like in shape (Figs. 4a and b). Consequently, fibrous morde- nites are rich in K and needle-like mordenites are rich in Ca and Na in the Keşan region. STOCIA et al. (1992) showed that structural changes of mordenite-group minerals are due to chemical and thermal differentia- tion. HAWKINS (1981) also concluded that mordenites had a more tabular than fibrous morphology when they tended to be rich in Ca. Likewise, Ca-rich mordenites in the Şile region (NW İstanbul, Turkey) displayed pris- matic crystals with c-elongation similar to those of epis- tilbites (ESENLİ et al., 1997b). In spite of the fact that typically tabular mordenite have not been found in the Keşan samples, the relationship between the chemistry and morphology of fibrous and needle-like mordenites from the SW Keşan supports these findings. Further- more, RUDOLF & GARCES (1994) reported the rela- tionships between Si/Al ratios, c-dimensions and X-ray patterns of mordenites. But, the X-ray patterns and Si/ Al ratios of the Keşan mordenites collected from differ- ent stratigraphic levels do not show any significant dif- ference. XRD patterns of mordenite are slightly broaded (Fig. 2b). The d-spacing values (Ǻ) of principal reflec- tions of all studied mordenites are 3.49, 3.22, 9.15, 4.00, 3.40, 6.65 and 4.54. Heulandite-group minerals were found to have deve- loped directly from volcanic glass. Although, the exis- tence of heulandite–clinoptilolite minerals in the Keşan Fig. 3 Classification of samples based on Zr/TiO2 vs. Nb/Y diagram of WINCHESTER & FLOYD (1977). Sample 1 6 6 6 9 11 11 14 19 19 (Mineral) (M) (M) (M) (H–C) (A) (M) (H–C) (A) (M) (H–C) Si 37.45 37.68 37.64 28.07 33.87 36.83 29.80 34.92 37.38 27.87 Al 9.57 9.93 9.60 7.65 14.94 10.24 6.32 14.50 8.57 8.07 Fe 1.25 0.74 1.19 0.37 0.06 0.61 0.23 0.32 1.31 0.14 Ti 0.22 0.16 0.09 0.09 – 0.09 – – 0.22 0.14 Mg 1.50 0.90 0.82 0.90 0.38 1.19 0.77 – 2.13 1.22 Ca 0.97 1.74 1.85 2.13 0.22 1.83 1.09 0.16 2.03 1.91 K 2.69 1.37 2.32 0.60 0.06 0.98 0.50 0.06 0.50 0.37 Na 1.19 1.99 1.25 0.70 10.27 2.13 0.86 10.34 3.25 0.69 Si/Al 3.91 3.79 3.92 3.67 2.27 3.60 4.72 2.41 4.36 3.45 Na+K/Mg+Ca 1.57 1.27 1.34 0.42 1.03 0.73 0.90 0.34 Error % 22.6 23.4 21.1 8.9 30.1 18.5 28.9 38.2 18.1 12.1 Table 4 The unit-cell compositions of heulandite–clinoptilolite (H–C), mordenite (M) and analcime (A). 158 Geologia Croatica 58/2 samples were determined by XRD studies, their typi- cal crystal habit (monoclinic plate) was not observed in SEM studies. These grains are generally anhedral in shape and about 5 μm long (Fig. 4e). The average structural formulae of the Keşan heulandite-group zeo- lites was calculated on the basis of 72 oxygen to [Ca1.7 Mg1.0 Na0.8 K0.5] [Al7.4 Si28.6 O72] ·24H2O. This formula is close to the clinoptilolite-Ca referred by COOMBS et al. (1997). Si/Al ratios of the Keşan heulandite-group zeolites range from 3.45 to 4.72 and their average value is 3.95 (Table 4). According to this ratio, some of the analyzed heulandite-group zeolites are clinoptilolite while the others can be classified as heulandite. Their (Na+K)/(Ca+Mg) ratios have an average value of 0.50 and range from 0.34 to 0.73. Analcimes were observed by both optical micros- copy and SEM as idiomorphous and hypidiomorphous crystals, mainly bedded in volcanic glass (Fig. 4f). These minerals were generally below 0.05 mm in size. No transformation from any phenocrysts and zeolite mineral to analcime was observed; it occurred directly from volcanic glassy materials. The XRD patterns of a b c d e f Fig. 4 Photomicrographs of zeolite minerals: (a) fibrous mordenites in the sample 1; (b) needle-like mordenites in the sample 19; (c) needle- like mordenites with radial habit in sample 19; (d) fibrous mordenites which are growing from volcanic glass in the sample 1; (e) morden- ites over the anhedral heulandite-group minerals in sample 6; (f) analcime in volcanic glassy material in sample 14. 159Esenli et al.: Zeolitization of Tuffaceous Rocks in the Keşan Region, Thrace, Turkey analcime are very sharp (Fig. 2a) and spacing values (Ǻ) of the most important lines are 3.43, 5.65, 2.93 and 2.50 in order of their relative intensities. The chemis- try of analcime (Table 4) was slightly consistent with the typical unit-cell formula reported in the literature (MUMPTON, 1981; GOTTARDI & GALLI, 1985). The Keşan analcimes had a high content of Si and less- er content of Al and Na. The average value of the Si/Al ratio of two analcime grains was 2.34. 4.3. Occurrence The relative abundance and stratigraphic distributions of authigenic minerals of the pyroclastic-rich zone of the Mezardere formation are shown in Fig. 5. Morde- nite and heulandite–clinoptilolite are the most and the least abundant zeolite minerals in this zone, respec- tively (Table 1 and Fig. 5). These two minerals exist together at a level where analcime is absent. There are two separate analcime-rich levels in the middle parts of the zone (Fig. 5). Smectite is associated with all zeolite minerals and its abundance increases upward. Opal-CT is observed in minor amounts in association with mor- denite and heulandite-group zeolites in the middle lev- els. No petrographic evidence is found to suggest the time relationships for the occurrences of opal, smectite and zeolites. Authigenic quartz commonly occurs with analcimes. Zeolitic tuffaceous rocks in the Keşan region are interbedded with Lower Oligocene sediments of lacus- trine and shallow marine environments. Zeolitization is limited only to pyroclastics and not found in epi- clastics. The recurrent alternate horizons of mordenite (+heulandite–clinoptilolite) and analcime in about a 30 m thick zone could not be explained by variation of marine water chemistry. It is difficult to explain how major changes of shallow marine water composition can take place within short time intervals. Therefore, the zeolitization in the Keşan seems not to be developed in a marine environment. Moreover, the formation of analcime and mordenite in different levels could not be related to the chemistry of the host rock. Chemical com- position and petrographic characteristics of rocks where the zeolitization took place show no distinct variation with stratigraphy. Zeolitization probably took place in a lacustrine environment, being exposed to an open hydrological system. No lateral zonation of authigenic minerals was observed throughout the area (between the Kızkapanı and Karahisar localities). This implies that zeolitization took place at constant conditions laterally. But, as seen in Fig. 5, vertical mineralogical zonation was clearly observed in the field. The vertical variation of petro- graphic and mineralogical composition of the tuffa- ceous rocks (Fig. 5) can be examined in a quarry in the Kızkapanı area. But, the exposures of tuffaceous rocks are limited in the Karahisar area. In this area, morde- nite-rich levels correspond stratigraphically to the mor- denite-rich upper levels of the Kızkapanı section (Fig. 5). However, the lower levels of the Karahisar section have not been observed in the field, and thus no infor- mation was available. Taking only the upper levels of pyroclastics in two areas into consideration, there is no evidence showing the lateral variation in zeolitization. During zeolitization, the fluid chemistry probably changed over short time intervals. Hydrological and Fig. 5 Stratigraphically, rela- tive abundances of authi- genic minerals in the study area (SW Keşan). 160 Geologia Croatica 58/2 mineralogical boundaries have also changed as they were controlled by climatic parameters. Conditions in open hydrological systems have probably changed to those of a closed lacustrine environment causing alkali enrichment in the water. Two analcime-rich levels must have developed due to these changes. Such environ- mental changes took place during the Oligocene–Mio- cene in the Thrace region, especially in the south Thrace (TURGUT et al., 1983). In Oligocene time, the chemistry of pore water and ground water was suitable for the development of mor- denites (stratigraphically lower level in Fig. 5) in the Keşan region (actually SW Keşan). Towards the end of this period, first smectite and then heulandite–clino- ptilolite became associated with mordenite. Later, the following products formed sequentially: (a) analcime, (b) mordenite (mordenite+heulandite–clinoptilolite+ smectite+opal-CT), (c) second analcime, and finally (d) a longer mordenite formation period. Still later, smectite and heulandite–clinoptilolite association formed with mordenite. Zeolitization in the Keşan region have been controlled by the variation of water chemistry through time. 5. CONCLUSIONS In SE Keşan the stratigraphically lower zone of the tuffaceous rocks of Oligocene age have undergone zeo- litic alteration. This process has created a vertical zone of different zeolite minerals. Parent material, mainly volcanic glass, has been transformed to zeolite miner- als by the action of fluids probably in an open hydro- logical regime. However, there must be several inter- vals affected by various parameters during this episode. Thus, analcime- and mordenite(+heulandite–clinoptilo- lite)-rich levels formed repeatedly in the study area. The stratigraphic sequence of zeolite mineral paragenesis of at 30 metre thick tuffaceous zone can be summarized as: (mordenite) → (mordenite+heulandite–clinoptilo- lite) → (analcime) → (mordenite+heulandite–clinopti- lolite) → (mordenite) → (analcime) → (mordenite) → (mordenite+heulandite–clinoptilolite). This mineralogi- cal sequence reflects the changes in hydrological sys- tems and chemistry of waters during the zeolitization processes and emphasized the variation in geological environments. In the Early Oligocene changes in the cli- mate and depth of the lake water must have taken place in the SW Thrace. 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