GEOLOGY AND GEOCHEMISTRY OF THE EOCENE ZEOLITE-BEARING VOLCANICLASTIC SEDIMENTS OF METAXADES, THRACE, GREECE

The Eocene zeolite-bearing volcaniclastic sediments (Ca-rich clinoptilolite 51 % on average) of Metaxades, Thrace, Greece, are studied in terms of geology and chemical composition. The alternating formations along a vertical profile in the Metaxades main quarry face are: Discrete horizons of zeolite-bearing volcaniclastic tuffs, zeolite-bearing volcaniclastic tuffs including pebbles, zeolite-bearing volcaniclastic tuffs including thin silica-rich layers, and a calc-clayey horizon. A positive correlation is observed between MgO and CaO in the volcaniclastic tuffs including silica-rich layers. Sr, Rb and Ba are the most abundant trace elements in alllayers; among them, Sr is found to correlate positively with zeolite. The enrichment of sorne trace elements is mainly attributed to mineral abundances, mineral chemistry or leaching processes. The zeolite-bearing volcaniclastic sediments of Metaxades represent an inhomogeneous sequence, which was deposited in a shallow marine environment under turbulent to quiet sedimentary conditions.

However, previous studies are based on a relatively restricted number of samples representative of certain only sites of the zeolite-bearing volcaniclastic formations; hence, sorne differences in results and interpretations were inevitable (e.g., Tsolis-Katagas and Katagas, 1990; Tsirambides et al., 1993; Savin et  al., 1993).Because of the great petrologic and commercial interest of these zeolite-bearing rocks, a detailed investigation has been undertaken.
This study concerns the results from a) a detailed field knowledge through a geological mapping (scale 1:5,000) constructed for the first time, and b) the petrographical observations, mineralogical abundances, mineral chemistry and chemical varia-  The observations in this study are based on the examination of a complete series of samples taken from a representative stratigraphic column (l T) of total height 13 m, from the eastem site of the main quarry face at Metaxades area (Fi.~s. 2 and 3).
Firstly, the samples were cut and large area pohshed surfaces of them were macroscopically examined.Thin and polished thin sections were prepared for petrographic observations in transmitted light and for microprobe analysis of the mineral constituents, respectively.
X-ray diffractograms were obtained on randomly oriented samples, usin~a Philips diffractometer, equipped with a PC-APD diffractlOn software.The operating conditions were: CuKa radiation, use of monochromator and scanning speed 10 per minute over the interval 2-60 0 of 28.
Electron microprobe analyses were performed by an ARL-SEM with Tracor Northem EDS, TN 5502, ZAF correction.Operating conditions were: accelerating voltage 15 kV, beam current 20 nA and counting time 15 seconds.The minerals used as probe standards were orthoclase, wollastonite, corundum, periclase, albite, chromite, almandine and pyrophanite.AIthough special care was taken for clinoptilolite analysis by enlarging the electron beam and reducing the beam current and counting time, the volatilization of the alkalies was not avoided (see Tsirambides et al., 1993).Table 1 presents representative analyses of the main mineral constituents.
Bulk rock chemical analyses both for major and trace e1ements were performed on the profile samples and the results are presented in Tab1e 2. The analytical equipment used was XRF PW 2400 with X-ray spectrometer.Before chemical determinations of trace elements, the powder used for the pelets was heated at 900 0 C for four hours.
The mineralogical composition of the profile samples was estimated by XRD, using standard mixtures of the minera1s (Table 3).Considering the chemical composition of the minerals and their proportions, the chemical composition of the rock samples (Table 2) is in good agreement with their mineralogical composition (Table 3).

Geology
tions (major and trace elements) along a vertical profile of the main quarry face in the Metaxades volcaniclastic sediments.
The Metaxades upper Eocene zeolite-bearing volcaniclastic sediments belong to the Orestias Tertiary molassic basin of the northeastern Thrace in Greece.This meta-Alpine basin has an elongated shape an extends into Bulgaria.It is dominated by sediments of Eocene to Pleistocene age, deposited uncoformably on the crystalline basement of the Rhodope massif (Fig. 1).
The sedimentary formations occurring in this basin, according to data from Andronopoulos (1977), Tsirambides el al. (1993) and Koutles (Ph.D. in progress) are from bottom to surface: l.Eocene formations consisting of a) breccias-conglomerates (l 0-15m thick) which lie unconformably upon the metamorphosed basement.Their composition is phyllitic, gneissic, amphibolitic, quartzitic or andesitic; they display a grading from coarser to finer fragments upwards, b) gray siltstones ("" 100 m thick) with psammitic and marly interlayers, c) mostly semi-loose sandstones (40-50 m thick) of varying grain-size, including very thin clayey interlayers, d) white to pale gray, yellow or green zeolite-bearing volcaniclastic tuffs (20-25 m of visible thickness) conformably deposited on the sandstones or siltstones; a thin layer of gray marl is descernible on the upper part of the tuffs, e) loose white-yellowish marly limestone (5-10m thick) and f) limestone (less than 30 m thick) rich in fossils.
2. Oligocene formations of relatively great thickness, unconformably deposited on the Eocene formations.They consist of gray clays, red-yellowish sandstones, white-yellow siltstones and interlayers of marly limestone.
3. Quaternary sediments of varying thickness, deposited over aH previous formations.
The complete sequence of the aboye formations is not always present in aH sites of the basin.One or more formations may be 10caHy absent due to tectonic activity, erosion or not deposition.In the broader area, two main visible faults and several fractures have been observed.

Petrography
The white to pale gray volcaniclastic sediments of Metaxades display earthy streak and conchoidal fracture.Detailed observations showed that the lower part of the section studied, presents a massive to poorly bedded structure and poorly sorted components of the unwe1ded and coarse grained deposits, lacking internal structures or laminations.On the other hand, cross-bedding predominates at the intermediate part of the section.The volcaniclastic sediments here are fine-grained and may be divided in planar-and trough cross-bedded.The laminated upper layers consist mainly of non-volcanogenic components, especially of silt and clay size clastic grains.Manganese dendrites are noticed through the whole section.
The chemicaJ composilion of each mineral phase displays sorne differences in the various lithological layers (Table 1).The calc-c1ayey horizon is extremely fine grained and consequently it was impossible to be microprobe analysed.
Zeolite, which reprcscnls the 51 % on average in Ihe volcaniclastic tuffs. is c1inoptilolite according to ils thermal stabilily (Boles andSurdam. 1979: Tsirambides el al.. 1993;Filippidis, 1993).It appears as liny crystals and withoul any zoning.The Si/Al ratio values of c1inoplilolite from aH samples are> 4.7 and in agreement with different authors c1assification (Mumpton. 1960;Mason and Sand. 1960;Alietli. 1972;Boles. 1972: Alielli el al., 1977).The Si. Al.Ca and Mg values are more or less similar with those presented by Tsirambides er al. (1993), while the values of Na and K are much lower, due to volalilization during Ihe microprobe analysis.Tsirambides el al. (1993) give average values of 0.95 and 1.08 per unit cell, for Na and K. respectively, for the Ca•rich Metaxades clinoplilolite.Arnong the zeolite-bearing volcaniclastic tuffs.the c1inoptilolite of the discrete ones is Ihe richest in calciurn.In vcry low concentrations.iron (FC20)) was detectcd in the cli-noptilolite from the discrete volcaniclastic tuffs including pebbles.
Sanidine displays no chemical differences in the three categories of volcaniclastic tuffs, while plagioclase is albite in the discrete volcaniclastic tuffs and sodic oligoclase in the other two categories, displaying considerable contents of potassium in aH samples (Table 1).
Biotite displays a relatively high Mg/(Mg+Fe 2 +) ratio.In the discrete volcaniclastic tuffs this ratio shows a large variation and in sorne cases biotite reaches the field of phlogopite; in this second case part of potassium is replaced by calcium as in margarite.
Amphibole is tschermakitic hornblende both in the discrete volcaniclastic tuffs and in those containing silica-rich layers, displaying smaH differences in calcium and sodium contents (Table 1).

Geochemistry
Table 2 shows the concentrations of major and trace elements along the studied 1T profile.Element abundances for aH varieties of volcaniclastic tuffs are mostly similar and comparable to values given by Alexiev and Djourova (1988) for the southeastern Bulgarian zeoliferous volcaniclastic rocks of north-eastern Rhodope.Among the oxide values, considerable differences are observed mainly for Si0 2 , Al 2 0 3 and CaO (Fig. 5) in the layer of the calc-clayey horizon (sample 1T23) and in the uppermost part of the profile (sample 1T5) which although belonging to the variety of volcaniclastic tuffs including pebbles, displays the highest proportion of calcite (Table 3).
Among the trace elements, Sr, Rb and Ba are the most abundant in nearly aH the samples of the sequence.The trace elements Zr, Y, Zn, Nb, Pb and Ga show sorne minor concentrations while the rest (Cu, Ni, Co, Sc, Cr, V) are near or below detection limit (Table 2).

Discussion and conclusions
Considerable work has been done and published on the formation and occurrence of zeolites by reaction of silisic volcanic glass with pore water.Much attention has been focused on the alteration of ash   (l T5) and in the calc-clayey horizon (l T23) (Table 3).Among major element oxides, a positive correlation was observed between MgO and CaO only in the volcaniclastic tuffs including silica-rich layers (Fig. 6) with the exception of sample 1T75 which contains the highest percentage of calcite (9 %).Furthermore, Sr was found to correlate positively with CaO in the discrete volcaniclastic tuffs and the volcaniclastic tuffs including thin silica-rich layers, with a small deviation of the sample 1T75 (Fig. 7).
A correlation between mineral abundances and element concentrations (Tables 2 and 3) showed that Sr correlates positively only with clinoptilolite (Fig. 8) while it does not show any affinity with calcite.Rb does not show any correlation either with sanidine or clays and clinoptilolite.No correlation was also observed between Ba and sanidine, plagioclase, clinoptilolite or clays.Consequently, the enrichment of Rb and Ba in certain layers could be attributed either to their leaching through the volcaniclastic sequence, or to their varying contents in a certain mineral phase present in the different layers.layers in various geologic environments (e. g., Boles and Coombs, 1975;Ogihara andlijima, 1989, 1990).The zeolite-bearing volcaniclastic sediments of Metaxades are considered to be the alteration products of volcanic material (rhyo-dacitic in composition) in a depositional environment of low salinity (Tsirambides et al., 1993).In their study, Tsirambides et al. (1993) concluded that the mineralogical differences they observed among samples taken from the Metaxades area, reflect differences especially in original rock composition and to a less extent in the fluid chemistry.Clinoptilolite was formed largely from the replacement of volcanic glass.Tiny clinoptilolite crystals are abundant as interstitial cement and polycrystalline pseudomorphs of glass shards.Savin et al. (1993) concluded from oxygen isotope data that the formation of the diagenetic phases of cristobalite and smectite took place under different temperature conditions, less than 60°C and less than 45°C, respectively.Since both these minerals are closely associated with clinoptilolite, we consider that probably this significant zeolite was formed under such low temperatures.
The massive and/or graded bedding, observed in the lower part of the vertical profile studied, could be attributed to high energy pyroclastic flows which transported on land and deposited in the shallow sea large quantities of clastic material with variable grain size.Probably, the trough cross-bedding, observed mainly in the intermediate part of the profile, is the result of ripIe marks transport downwards in the Metaxades basin by means of pyroclastic flows.The upper layers of the section, consisted mainly of non-volcanogenic components, were most probably created under quiet sedimentary conditions.
According to the chemical results obtained along the vertical profile, the variations observed for SiO z , Al z 0 3 and CaO (Table 2) are due mainly to the high contents of calcite (50 % and 24 %) in the top layer of the volcaniclastic tuffs with pebbles Among the trace elements with minor concentrations, sorne of them such as Y, Zn, Cu, Ni, Cr and V, show an enrichment in certain layers and especially in the calc-clayey horizon.This enrichment could possibly be assigned to different mineralogy of this horizon.In summary, according to the data of the present study, it results that the Ca-rich clinoptilolite-containing volcaniclastic tuffs of Metaxades area, which are considered to be the alteration products of volcanic material in a depositional environment of low salinity (Tsirambides et al., 1993), do not represent a homogeneous sequence but alternate with tuffs including pebbles from the surrounding rocks, tuffs including silica-rich layers, and a calcclayey horizon.This lithological inhomogeneity is reflected both in the mineralogical composition and well as in the chemistry of the minerals present in the different formations.Furthermore, varying concentrations of certain trace elements in each mineral phase in the different layers as well as leaching processes, seem to be responsible for the enrichment of these elements in certain layers.The zeoli- Samples and analytical methodsnes, sandstones and siltstones, b) Plio-Pleistocene formations and c) FIuvioterrestrial formations.
Fig. 6.-MgO vs CaO along the IT vertical profile of Metaxades main quarry face.

Table 1 .
-Representative chemical analyses of the main mineral constituents in the different layers of the 1T vertical profile