Zeolitic Alteration of Eocene Volcaniclastic Sediments at Metaxades, Thrace, Greece

Applied Clay Science, 7 (1993) 509-526 509 Elsevier Science Publishers B.V., Amsterdam

Zeolitic alteration of Eocene volcaniclastic sediments at Metaxades, Thrace, Greece

A. Tsirambides, A. Filippidis and A. Kassoli-Fournaraki Department of Mineralogy-Petrology-Economic Geology, Aristotle University of Thessaloniki, 540 06 Thessaloniki, Greece (Received May 12, 1992; revised and accepted November 27, 1992)

ABSTRACT

Tsirambides, A., Filippidis, A. and Kassoli-Fournaraki, A., 1993. Zeolitic alteration of Eocene volcaniclastic sediments at Metaxades, Thrace, Greece. Appl. Clay Sci., 7: 509-526.

The conditions of alteration of the upper Eocene zeolite-bearing volcaniclastic sediments at Metax- ades, Thrace, Greece, were examined using a variety of petrographic and mineralogical techniques. A fine silt- to clay-size matrix, containing abundant altered glass shards, predominates. The primary minerals include quartz, K-feldspar, plagioclase, micas (especially biotite) and some opaque minerals. The diagenetic phases, clinoptilolite, smectite and cristobalite, occur as microcrystalline aggregates within the matrix or as replacements of glass shards. The abundances of cristobalite and clinoptilolite are related chiefly to the original composition of the rocks. The abundance of cristobalite increases and that of clinoptilolite+ smectite decreases as the abundance of the matrix increases. The formation of clinoptiloliteand cristobalite took place either at near-surface temperatures or at slightly elevated temperatures (lower than 70°C). The smectite content of the altered volcaniclastics decreases with increasing depth in the 15 m thick exposed section. No other systematic variation in mineral assemblages with depth was observed. However, the ratio clinoptilolite/smectite increases systematically with depth suggesting to us that smectite may have formed subsequent to the formation of clinoptilolite and as an alteration product of clinoptilolite. The initial composition of the volcanic material and to a less extent the interstitial fluids, greatly affected the diagenetic mineral assemblages of the final alteration products.

INTRODUCTION

The variety of diagenetic minerals which can form in volcaniclastic sediments is large and reflects both the nature of the parent rock and the conditions under which the alteration occurs (Boles and Coombs, 1975; Mumpton, 1977; Boles and Surdam, 1979). Zeolites are among the most abundant and widespread diagenetic silicates in these rocks.

Correspondence to: A. Tsirambides, Department of Mineralogy-Petrology-EconomicGeology, Aristotle University of Thessaioniki, 540 06 Thessaloniki, Greece.

Zeolite-rich volcaniclastic rocks of Paleogene age, deposited in a marine environment, cover an area of several hundred square kilometers of Greece and Bulgaria in the eastern Balkan peninsula. These volcaniclastics consist of fine- to coarse-grained ash-falls and ash-flows, the latter of which contain a small but variable component of non-volcanogenic detritus of a wide range of grain sizes. The zeolitic alteration of these rocks has been studied by Aleksiev and Djourova ( 1975 ), Djourova and Aleksiev ( 1989 ), Tsirambides et al. ( 1989 ) and Tsolis-Katagas and Katagas ( 1990 ). There is little agreement among these authors about the temperature, fluid chemistry and other parameters which affected the alteration of the volcaniclastic rocks. Aleksiev and Djourova (1975) concluded that the zeolitic alteration of ash-flows in the Bulgarian territory occurred after deposition, as the result of infiltration of marine and meteoric water into hot ignimbrites. In contrast, Tsolis-Katagas and Katagas (1990) reported that the pyroclastic rocks in the Greek portion of the region were affected by burial diagenesis at temperatures intermediate between Iiji- ma's (1978) zones II and Ill. This study is an examination of the alteration products and processes of the zeolite-bearing volcaniclastic rocks of Metaxades, Thrace, Greece. Using a variety of petrographic and mineralogical techniques we have attempted to set limits on the conditions under which both zeolites and non-zeolitic diagenetic silicates were formed.

GEOLOGICAL SETTING

During early Tertiary time the huge meta-Alpine basin of Thrace (Greece), was formed and filled with molassic sediments that lie unconformably on the Alpine rocks of the greater region. The area of our study lies along the south- ern edge of an elongated sub-basin which extends into Bulgaria (Fig. 1 ). The broader area is part of the Rhodope geotectonic zone that straddles the Greek- Bulgarian border. The volcaniclastic sediments of Metaxades have an upper Eocene age. Fytikas et al. ( 1984 ) reported, in a study of the evolution of Ter- tiary to Quaternary volcanism in the Aegean region, that rhyo-dacitic volcanism occurred in the Rhodope area between 33 and 23.6 Ma. Aleksiev and Djourova (1975 ) interpreted the depositional environment as shallow marine. Solakius and Tsapralis ( 1987 ) come to the same conclusion on the basis of microfossils contained in the marl overlying the volcaniclastics and in marl pebbles included within the volcaniclastics. The structural features of the Metaxades deposits are similar to those listed by Fisher and Schmincke (1984) as characteristics of ash-falls and ash-flows. They exhibit massive-bedding in the upper and lower portions of the volcaniclastic sequence and thin to thick cross-bedding, as well as alignment-bed- ZEOLITICALTERATION OF EOCENE VOLCANICLASTICSEDIMENTS 511

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Fig. 1. Geologic map and geographic position of the Metaxades area, Thrace, Greece (from Bornovas and Rondogianni-Tsiambaou, 1983). ding, in the middle portion. No structures attributable to post-depositional processes have been reported in the literature or noted in this study. A representative stratigraphic column of the Metaxades area is shown in Fig. 2. During early Lutetian (early Eocene) time, a basal clastic series of breccias and conglomerates was deposited unconformably upon the meta- morphosed basement. This series is unconformably overlain by Priabonian (upper Eocene) volcaniclastic tufts which are the focus of this study. Arikas (1979) reported that these tufts are interbedded with thin layers of sand- stones, siltstones and claystones, but we did not observe any interbedded clastic zones in the outcrops we studied. The tufts are a product of calc-alkaline volcanism which began in late Eocene time and reached maximum intensity during the Oligocene, during which time a series of marls and claystones containing intercalated rhyolites and tufts, was deposited overlying the Eocene sediments. Only a thin (about 1 m) layer of these exists in our area. Finally, a transgressive sequence, about 2 m thick, of Miocene coastal sediments (mainly porous limestones, rich in marine microfossils, marls and sand- ,5 1 ~, A TSIRAMB1DES ErI ' A i

massive -i7" ~~¢'- - bedded

al ignment and l cross-bedded F2

Fa massive bedded

Fig. 2. Representative stratigraphic column of the Metaxades area, Thrace, Greece. ko= Miocene marly limestone; F~ = Oligoeene marl and clay with thin layers of sandstone and tuff; F2 = Upper Eocene volcanic tuff in layers alternating with thin sandstone, siltstone and claystone; F3 = Lower Eocene conglomerate and breccia; F4 = Metamorphic bedrock (amphibolites and gneisses ). Fig- ure also shows the detailed section of the stratigraphic interval sampled in this study. Con- structed using data from Arikas (1979), Solakius and Tsapralis (1987) and our own field observations. stones) was deposited on the Oligocene sediment (Solakius and Tsapralis, 1987). Tsolis-Katagas and Katagas (1990) estimated that the maximum depth to which the volcaniclastic sediments of the Metaxades area have been buried is 1500 m. The average geothermal gradient in Thrace at depths < 1400 m is 37 °C/km (Fytikas and Kolios, 1979). This would imply a maximum temperature of the volcaniclastic sediments of 70°C (assuming a surface temperature of 15 °C), in the absence of local heating by volcanic or hydrothermal activity.

MATERIALS AND METHODS

The geological observations in this study are based on the examination of three quarry faces and several outcrops. The samples analyzed were taken from vertical sections with thicknesses of 15 and 10 m in two of the quarry faces. None of the sections studied penetrated entirely through the volcaniclastic section, which was inferred by Tsolis-Katagas and Katagas (1990) from drilling data in nearby regions to have a maximum thickness of 35 m. ZEOLITIC ALTERATION OF EOCENE VOLCANICLASTIC SEDIMENTS 513

Thin sections were prepared for examination in transmitted light and pol- ished for electron probe microanalysis of clinoptilolite and feldspars. Abun- dance of vitrous matrix, glass shards and primary minerals was determined by point-counting of thin sections. At least 400 points were counted for each sample. Chemical analyses of representative rock samples were performed by AAS techniques. Samples were disaggregated and size-separated prior to X-ray diffraction (XRD) analysis. Disaggregation was done gently in order to retain, to the extent possible, the intrinsic grain sizes of the particles. Each sample was placed in H20 in a plastic bottle for two months, during which it was shaken twice a day. Three size fractions of each sample (75 to 20/tm, 20 to 2/~m and < 2/tm) were separated by wet sieving, gravity settling and centrifugation. Separated fractions were dried in an oven overnight at about 65°C. X-ray diffraction was done using a Philips diffractometer with Ni-filtered CuI~ ra- diation. Both randomly oriented samples and samples with preferred orien- tation were scanned over the interval 2 to 40 ° 20 at a scanning speed 1 ° per minute. Samples were re-analyzed after glycolation. Semi-quantitative estimates of the abundances of the minerals in each size fraction were made from the XRD data using the methods of Schultz (1964), Perry and Hower (1970) and Reynolds and Hower (1970). Clinoptilolite was distinguished from heulandite by comparing the intensity of (020) re- flection before and after heating at 450°C for 15 h (Boles and Surdam, 1979). Cell parameters of clinoptilolite were obtained using 21 reflections, NaC1 as an internal standard, and the computer program of Appleman and Evans (1973). Broken surfaces of samples were coated with gold and observed using a JEOL JSM-840 scanning electron microscope (SEM). Chemical analyses of clinoptilolite were made using a Cameca electron probe microanalyzer. To minimize volatilization of alkalis in the clinoptilolite, the electron beam spot size was enlarged and the counting time decreased.

RESULTS Petrography

The volcaniclastic sediments of Metaxades are predominantly white to pale gray with some yellow, brown, pink, or green coloration. They exhibit a dull or earthy luster and conchoidal fracture. To the unaided eye, they generally resemble well-sorted siltstones. Macroscopic description is given in Table 1. Microscopic examination of thin sections reveals a fine-grained relict vitro- clastic texture. A fine silt- to clay-size matrix, containing abundant altered glass shards, predominates. The matrix was probably derived from the alteration of very fine-grained ash. Abundances of matrix, shards and total pri- 514 A. TSIRAMBIDESEq Ai~

TABLE 1

Petrographic information of the samples studied

Sample ~ Petrographic description

E2 Fine-grained semi-friable tuff, yellow-white with earthy luster; massive bedding. E3 Fine-grained semi-friable tuff, yellow-white with earthy luster; massive bedding; manganese dendrites present. B3 Coarse-grained friable tuff, yellow-white with earthy luster; massive bedding. E7 Very fine-grained hard tuff, yellow-white with earthy luster; alignment bedding. E9 Fine-grained semi-friable tuff, yellow-white with earthy luster; alignment bedding; many manganese dendrites. C9 Medium-grained semi-fi'iable tuff, yellow-white with earthy luster: alignment bedding; many manganese dendrites. Bio Fine-grained semi-friable tuff, white with earthy luster; cross bedding. D~o Fine-grained semi-friable tuff, white with earthy luster; cross bedding. Medium-grained semi-friable tuff, yellow-white with earthy luster: many ellipsoidal fossiliferous marl pebbles; massive bedding. C12 Very fine-grained semi-friable tuff, yellow-white with earthy luster; cross bedding. C~3 Fine-grained semi-friable tuff, yellow-white with earthy luster; cross bedding; many manganese dendrites. BI4 Medium-grained semi-friable tuff, yellow-white with earthy luster; many ellipsoidal fossiliferous marl pebbles; massive bedding. ~Letters in samples refer to different collecting sites in the quarry faces; numbers denote depth below the surface (in meters). All samples are from horizon F2 (Fig. 2 ).

TABLE 2

Modal composition (%) of the samples analyzed

Sample Matrix Shards Pr,

E2 41 37 22 E 3 61 19 20 B 3 38 22 40 E7 71 15 14 E9 59 22 19 C9 49 19 32 B~o 61 20 19 Dto 66 19 15 All 63 9 28 C~2 67 20 13 Cl3 61 23 16 BI4 56 20 24 Prt = Total of primary minerals (quartz + feldspars + micas + opaques ). mary minerals are reported in Table 2. The primary volcanic minerals consti- tute less than 30 % of each sample studied. Angular to subangular quartz and feldspar crystals, which us~y exhibit undulatory extinction, and sometimes magmatic erosion, are the main sialic phases. Pl~oelase crystals are almost always sericitized and saussuritized. Very fine-grained aggregates of the saus- ZEOLITICALTERATION OF EOCENE VOLCANICLASTICSEDIMENTS 515

Fig. 3. Photomicrograph of thin section, showing clinoptilolite (Cpt) laths inside glass shard pseudomorph. Shard walls are lined with a thin layer of smectite (S). suritic minerals epidote and clinozoisite were detected in some samples. Tab- ular biotite, usually chloritized, minor amounts of muscovite, amphibole, some opaque phases and manganese dendrites are the predominant mafic or dark minerals. According to Filippidis (work under preparation ) the opaque phases contained in these rocks are moissanite (SiC) and ilmenite. Throughout the pyroclastic section the vitrous matrix and glass shards have undergone the most extensive diagenetic alteration. The diagenetic phases occur as microcrystalline aggregates inside the matrix or as precipitates in vein- lets or replacing glass shards. Clinoptilolite crystals are very abundant as interstitial cements or as polycrystalline pseudomorphic replacements of glass shards. They have lath- or tabular shapes, range from 2 to 80/tm in length (Figs. 3, 4) and exhibit extinction parallel to cleavage faces. The peripheral zone of the altered glass shards is often a rim of tiny ( < 2 #m) smectite crystals (Fig. 3). Smectite also forms rims around altered plagioclase crystals. Cristobalite occurs mainly as spherules or oval aggregates composed of small ( < 1/~m ) closely packed crystallites (Fig. 4 ). Further insight into the nature and abundance of alteration products comes from consideration of particle size distribution data and XRD analysis of separated size fractions. Particle size distributions of the samples disaggregated 516 A. TSIRAMBII)ES ET A:,

Fig. 4. Electron microscope photomicrograph showing plaly clinoptilolite (('pt) crystals and aggregates of closely packed cristobatile (('r) cryslalliles.

TABLE 3

Grain size distribution (in/~m ) of the samples analyzed

Sample 75-20 20-2 < 2

E~ 50 34 16 E~ 35 44 21 B~ 43 43 14 E7 50 50 0 E~ 48 40 12 (', 45 39 16 Bm 44 39 17 Djo 36 50 14 A~ 50 39 11 Ci2 37 45 18 CL3 48 39 13 Bl4 58 31 11 Average 45 41 14 ZEOLITIC ALTERATION OF EOCENE VOLCANICLASTIC SEDIMENTS 517 by soaking in H20 are given in Table 3. In most of the samples the percentages of the two coarsest fractions are similar. The clay-size ( < 2/tm) fraction ranges from 0 to 21% of the samples and averages about 14 %. X-ray mineralogy

The results of XRD analyses of the different size fractions are given in Ta- ble 4. Different minerals are concentrated in different grain sizes. Quartz, K- feldspar, plagioclase and the micas (the primary minerals) are concentrated in the coarser two fractions of almost all samples. Clinoptilolite was detected in all fractions of all samples. Although semi-quantitative estimates of the abundance of clinoptilolite cannot be made with great confidence from XRD data, this phase appears to be the most abundant phase of most fractions of most samples. Smectite is a common, abundant constituent of the < 2 pm fraction of most samples and is rare or absent in the coarser fractions. Cris- tobalite is more abundant in the 20-2/~m fraction of most samples and is present in much lower abundances or is absent in the finest fraction. The calcite detected in a few fractions of two samples occurs in veins or in small marl pebbles included within the pyroclastics. We have not detected discrete illite, mixed layer illite/smectite, heulandite or mordenite in these samples, although Tsolis-Katagas and Katagas (1990) have reported the presence of the latter two phases in these deposits. This will be discussed further below. Mineralogical compositions of the bulk samples were calculated from the abundances of the fractions (Table 3 ) and mineralogical compositions of the separated fractions (Table 4). These are tabulated in Table 5. Chemical analysis

The major exchangeable cation in the Metaxades zeolite is Ca, but consid- erable amounts of K, Na and Mg are also present (Table 6). The chemical parameter R = Si/(Si + Ti + A1 + Fe 3+ ), which represents the percentage of the tetrahedra occupied by Si, is 0.81. The ratio Si/A1=4.33 and (Ca+Mg)/ (Na+K) = 1.05. Cell parameters are given in Table 6. These values are similar to those reported for other clinoptilolites (e.g., Gottardi and Galli, 1985 ). Plagioclase composition ranges between oligoclase and andesine (Table 7). Considering the standard deviation of the analytical techniques, the chemical composition of the rock samples (Table 8 ) and their mineralogical composition (Table 5 ) are generally in agreement, except sample E 7 which displays a little higher than expected K20 content. Comparison with results of other studies

In some ways the mineralogical and petrographical results differ from those of previous studies. Tsolis-Katagas and Katagas (1990) reported the pres- 518 A. TSIRAMBIDES ET AL

FABLE 4

Mineralogical composition (%) of separated size fractions

Sample Size Q Ksp Pl M Cpt ('r ~ ( (/ira)

E2 75-20 4 5 82 9 20-2 3 4 75 t4 4 <2 17 83 E 3 75-20 8 6 6 8 13 8 16 35 20-2 6 15 67 12 2-0.2 6 94 <0.2 5 95 B 3 75-20 4 3 3 85 5 20-2 10 3 6 3 55 8 15 <2 50 50 E 7 75-20 8 9 13 29 41 20-2 10 10 11 25 44 <2* E~ 75-20 7 3 6 70 14 20-2 5 5 7 4 61 14 4 <2 59 41 C9 75-20 5 3 4 7 67 14 20-2 7 4 6 61 15 7 <2 43 57 Blo 75-20 8 1 l 5 53 23 20-2 6 5 9 6 44 30 <2 5 60 9 26 Djo 75-20 3 3 6 5 58 25 20-2 5 4 6 9 46 30 <2 74 6 20 Al~ 75-20 4 77 15 4 20-2 5 58 26 7 4 <2 48 5 47 CI2 75-20 8 5 6 53 28 20-2 7 5 7 9 41 31 <2 3 3 73 14 7 Cl3 75-20 3 10 3 4 50 21 20-2 3 3 4 4 6l 25 <2 94 6 Bj4 75-20 6 7 73 14 20-2 5 3 9 58 25 <2 73 7 20

Q = quartz; Ksp = K-feldspar; PI = plagioclase; M = mica; Cpt = clinoptilolite; Cr- cristobalite: S = smectite; C= calcite. *No sample of this fraction was obtained. ence of mordenite, which was not detected in our samples. More significantly, Tsolis-Katagas and Katagas (1990) reported the presence of heulandite, while our data indicate that the predominant zeolite phase is clinoptilolite (we found no heulandite). Our interpretation is based upon the results of extensive heat ZEOLITIC ALTERATION OF EOCENE VOLCANICLASTICSEDIMENTS 519

TABLE 5

Mineralogical composition (%) of bulk volcaniclastic sediments, calculated from data in Tables 3 and 4

Sample Q Ksp P1 M Cpt Cr S C

E2 3 4 69 9 15 Ea 3 tr tr 3 8 9 55 18 Ba 6 3 4 tr 67 6 13 E 7 9 9 12 27 43 E 9 5 3 6 tr 65 12 7 C9 5 tr 3 5 61 12 12 B~o 7 7 6 tr 51 23 4 Dlo 4 3 5 6 54 25 3 Alt 4 66 18 8 C,2 6 5 6 4 51 27 tr Cl3 3 6 3 3 64 21 B,4 5 tr 7 68 17 tr

Q=quartz; Ksp=K-feldspar; Pl=plagioclase; M=mica; Cpt=clinoptilolite; Cr=cristobalite; S = smectite; C = calcite; tr = traces ( < 3%).

TABLE 6

Microprobe analyses, structural formula and cell parameters of c~inoptilolite r from Metaxades

Oxide wt.% Structural formula Cell parameters

Element Atoms per Item Value 72 oxygen

SiO2 65.90 Si 29.33 a (A) 17.592 TiO2 0.01 Ti - b (A) 18.003 A1203 12.90 Al 6.77 c (A) 7.295 Fe2Oa 0.08 Fe 3+ 0.03 V (A) 3 2063.5 MnO 0.05 Mn 0.02 b 116 o MgO 0.40 Mg 0.27 CaO 3.90 Ca 1.86 SrO bdl Sr - BaO bdl Ba - Na20 1.10 Na 0.95 K20 1.90 K 1.08 H202 13.76 H20 20.4

Total 100.00 Z 36.13 X 4.18 R 0.81

'Average of three analyses; 2Estimated by difference; bdl= below detection limit; Z = Si + Ti + A1 + Fe a +; X = Mn+ Mg+Ca+Na+ K; R=Si/(Si+ Ti+AI+ Fe 3+ ). 520 A. TSIRAMBIDES 1,;'1 ~ l,

TABLE 7

Microprobe analyses of feldspars ~ of volcaniclastic sediments of Metaxades

Oxide 1 2 3

SiO2 58.26 63.35 66.79 TiO 2 i).(13 t).03 0.03 AI203 26.17 23.19 18.57 FeO 2 0. i 2 0,06 0.08 MnO bdl bdl 0, t i~ MgO bdl bdl bdl CaO 8.19 4.32 0, 13 SrO bdl bdl bdl BaO bdl bdl bdl Na20 6.72 8.60 3.9i K20 0.25 0.54 I 1.14 Total 99 74 100.09 100.75

Numbers of ions on the basis of8(O )

Si 2.61 2.80 3.01 AI 1.38 t.21 0.99 Ca 0.39 0.20 0.01 Na 0.58 0.74 0,34 K 0.01 0.03 0.64

Ab 59.2 76.3 34.3 An 39.8 20.6 1.0 Or 1.0 3.1 64.7

~Average of three analyses, 2FeO as total iron, bdl = below detection limit. treatments (Boles and Surdam, 1979) and is supported by careful electron probe microanalyses: the Si/A1 ratio is > 4 (4.33) and the ratio (Ca+ Mg)/ (Na+K) is 1.05. Alietti et al. (1977) defined a (Ca+Mg)/(Na+K) of 1 as the limit for distinguishing between clinoptilolite and heulandite. We made our analyses using an enlarged electron beam spot size and a short counting time in order to minimize the loss of Na and K. Koshiaris et al. (1987), using a method based on ion exchange capacity, reported that the clinoptilolite content of the Metaxades volcaniclastic sediments ranges from 40 to 60%. Marantos et al. (1989), using a comparable method, estimated the clinoptilotite content of the same sediments to be between 29 and 45%. Tsirambides et al. (1989), in a preliminary investigation, concentrated clinoptilolite in three samples with heavy liquids and reported clinoptilolite contents of between 66 and 72%, close to the values determined using XRD in this study. Another difference from the results of previous authors is that we have found clinoptilolite to be most abundant in the 75 to 20/~m fraction and not in the ZEOLITICALTERATION OF EOCENEVOLCANICLASTIC SEDIMENTS 521

TABLE 8

Chemical composition (in wt%) of bulk ' volcaniclastic sediments of Metaxades

Oxide E2 E7 C9 C12 C13 Bl4

SiO2 69.98 76.34 69.85 71.39 68.40 68.19 TiO2 0.07 0.06 0.05 0.05 0.11 0.09 A1203 11.61 10.90 12.03 10.92 12.18 11.18 Fe2032 0.59 0.20 0.79 0.25 0.81 0.84 MnO 0.01 0.01 0.04 0.01 0.02 0.04 MgO 0.67 0.25 0.57 0.47 0.57 0.68 CaO 3.19 0.68 3.35 2.71 3.37 4.45 Na20 1.01 0.51 1.51 0.65 I. 13 1.59 K20 2.68 6.67 3.07 3.12 2.82 2.43 P205 0.01 bdl bdl bdl 0.02 bdl L.O.I. 11.12 4.57 8.83 10.14 11.01 10.46

Total 100.94 100.19 100.09 99.71 100.44 99.95

1Average of three analyses, 2Fe203 as total iron, bdl = below detection limit.

< 2 am as reported by Petrov et al. (1984) for Bulgarian volcaniclastics. This indicates to us that our technique of disaggregating the samples by soaking them for a long period has aided the preservation of the intrinsic grain sizes of the particles.

DISCUSSION AND CONCLUSIONS

The altered volcaniclastics at Metaxades are not thick enough to be affected by a significant vertical temperature gradient within the altered zone. There- fore, variations in alteration products and processes are interpreted as con- sequence of differences in the original composition of the rocks or differences in the hydrologic regimes within the formation. The latter, if important, could have arisen either from intrinsic textural and mineralogical differences that might have affected porosity, permeability or fluid flow paths, or from differences in the degree to which surficially-derived fluids might have penetrated during alteration. Below, we present data to support the hypothesis that: ( 1 ) The differences in abundances of the alteration products cristobalite and di- noptilolite are related chiefly to differences in original compositions of the samples and that: (2) Smectite may have formed at the expense of clinoptilolite in a subsequent alteration process, perhaps related to the infiltration of surface waters. It is shown in Fig. 5 that the abundance of cristobalite (Table 5 ) is nega- tively correlated with the abundance of primary minerals (Table 2 ) and pos- itively correlated with the abundance of matrix as determined by optical mi- croscopy (Table 2). The two measures are not completely independent, 5')2 A. TSIRAMBIDESETA*,.

0 10

10 20 30 40 50 60 70 80 90 Matrix (%) Primaries (%)

Fig. 5. Correlation of cristobalite abundance with matrix and total primary minerals.

O i L i I i I ...... 0 2O 4O 6O 100 Matrix (%) Fig. 6. Correlation of clinoptilolite plus smectite abundance with matrix. because the matrix consists, in part, of the primary minerals. Clinoptilolite and smectite are observed to occur in association with glass shards and matrix. Surprisingly, we found no correlation between the abundance of glass shards and the abundance of clinoptilolite, smectite, or clinoptilolite + smectite. The abundance of clinoptilolite + smectite does decrease as the abundance of the matrix increases (Fig. 6), however, indicating that these phases are formed by alteration of the coarser components of the pyroclastics. The correlation between the abundance of clinoptilolite + smectite and the abundance of matrix was better than the corresponding correlation with clinoptilolite alone. This, in addition to the petrographic relationships between smectite and clinoptilolite, susgests a genetic relationship, to be discussed later, between those two phases. ZEOLITIC ALTERATION OF EOCENE VOLCANICLASTIC SEDIMENTS 523

It has been shown in many studies that the initial composition of volcanic material and the fluids in the diagenetic environment greatly affect the mineral assemblages and mineral composition of the final alteration products. Alteration of predominantly rhyo-dacitic volcanic detritus typically results in siliceous alkali-rich zeolites. Surdam and Parker (1972 ), Boles and Surdam ( 1979 ) and others have pointed out the importance of chemistry in control- ling the extent of alteration of volcanic glass to zeolites and zeolites to K- feldspar. Typically, glass becomes most intensely altered (i.e., to K-feldspar) by very saline and alkaline water and unaltered glass may be found in rocks exposed to the freshest water. As the cristobalite content of the rocks increases the amount of clinoptilolite+ smectite decreases (Fig. 7 ). We conclude that the formation ofclinoptil- olite and cristobalite took place either at near-surface temperatures or at slightly elevated temperatures (lower than 70 °C based upon estimated maximum depth of burial). We find no evidence for heating by hydrothermal fluids or shallow intrusives. The significant amounts of clinoptilolite found in Metaxades, suggest that the volcanic material was altered in a depositional environment of low salinity. Our conclusion is not always in agreement with those of previous workers. Aleksiev and Djourova (1975 ) concluded that the zeolitic alteration of the ash flows in the Bulgarian portion of the Rhodope massif occurred as the result of infiltration of marine and meteoric water into hot ignimbrites. Tso- lis-Katagas and Katagas (1990) suggested that the authigenic silicate minerals in Metaxades could have resulted from a burial diagenetic alteration at temperatures in the range 84-91 °C (boundary between Iijima's zones II and III). The only clay mineral recognized in this sequence is discrete smectite. Its

100

60 +

40 o

(..) '\ \\

I I L L I \ J I J I 20 40 60 80 100 Cr~obol~e (%) Fig. 7. Correlationof clinoptiloliteplus srnectiteabundance with cristobalite. 5')4 ~. TSIRAMBIDES E'F A~r

(l /[I [][>/ I0

15 I L I 10 20 30 40 50 Smec'flte Ck)

,% 10

15 0 10 20 30 40 Clkx:~otllo~e (%) / Smec'rlte (~)

Fig. 8. Correlation of depth with smectite (a) and clinoptilolite/smectite (b) abundance. formation may have been enhanced by a relatively low (Na + K)/H + activity ratio in the pore fluid. Discrete illite and mixed-layer illite/smectite, which often indicate formation in argillaceous sequences at elevated temperatures, are absent, despite the presence of potassium. The formation of smectite would have caused an increase in pH, concentration of silica and (Na + K)/H + ratio, offering thus a chemical environment favorable for the formation of clinoptilolite (Hay and Sheppard, 1977; Hay and Guldman, 1987). The formation of Ca-rich clinoptilolite may have caused an increase in the Na/K ratio in the solution, perhaps favorable for the formation of mordenite (Hawkins et al., 1978). The mineralogical differences we observed among the samples probably reflect differences especially in original rock composition and to less extent in the fluid chemistry. ZEOLITIC ALTERATION OF EOCENE VOLCANICLASTIC SEDIMENTS 525

The smectite content of the altered volcaniclastics decreases with increasing depth in the section, from approximately 15 % near the surface to 2 % at a depth of 14 m (Fig. 8a). We observed no other systematic variations in mineral assemblages with depth. However, the ratio clinoptilolite/smectite increases systematically with depth (Fig. 8b). This observation and the relationships described earlier suggest to us that smectite may have formed subsequent to the formation of clinoptilolite and as an alteration product of clinoptilolite. The decrease in smectite abundance with depth suggests that the intensity of formation of smectite from clinoptilolite may have depended upon the ability of (meteoric or saline) surface fluids to penetrate the pyroclastic section.

ACKNOWLEDGEMENTS

The first author is indebted to S. Savin for providing hospitality during his sabbatical at the Department of Geological Sciences, Case Western Reserve University, Cleveland, Ohio, where most of the analytical work was carried out. We express our gratitude to the Greek Ministry of Research and Tech- nology for partial financial support and to S. Savin who thoroughly com- mented on the manuscript.

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