Zeolitic Diagenesis of Oligocene Pyroclastic Rocks of the Metaxades Area, Thrace, Greece
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Zeolitic diagenesis of Oligocene pyroclastic rocks of the Metaxades area, Thrace, Greece P. TSOLIS-KATAGAS AND C. KATAGAS Department of Geology, University of Patras, 261 10 Patras, Greece Abstract The unstable glassy component of the Oligocene tuffaceous sediments of the Metaxades area, Thrace, Greece, has undergone extensive zeolitic diagenetic alteration. The authigenic minerals occur as crypto- to micro-crystalline aggregates making up most of the matrix in the altered tufts and as precipitates in cavities produced by dissolved glass fragments. Glass-shard pseudomorphs are ubiqui- tous and most of them are partly filled by one or more of the minerals smectite, heulandite, mordenite and silica. The heulandite group zeolites are mostly high-silica calcium-rich heulandites showing intermediate thermal behaviour between most heulandites and clinoptilolites. Their Si/AI ratios are similar to clinoptilolite (4.74-5.19) but their divalent/monovalent cation ratios (1.5-3.24) are partly superposed onto the ratios of heulandite group 1 and 2 and differ considerably from the values of the relevant ratio in clinoptilolite. Rare K-rich clinoptilolite crystals have been identified in one sample only. Based on field observations, compositions and paragenetic relationships of coexisting authigenic minerals and the absence of critical phases such as laumontite, analcime or authigenic albite, it is suggested that the Metaxades pyroclastic rocks underwent burial diagenesis intermediate between Iijima's (1978) zones II and III commonly developed in silica saturated environments. K E VWO R D S: heulandite, clinoptilolite, mordenite, burial diagenesis, zeolite, Thrace, Greece. Introduction cal parameters. Our investigation concentrated ZEOLITE-BEARING mineral assemblages in pyro- mainly on the crystal chemistry of the heulandite- clastic deposits of marine Tertiary basins are wide- group zeolites and the diagenetic history of the spread in the area of Eastern Rhodope, Bulgaria pyroclastic deposit of Metaxades. (Aleksiev and Djourova, 1975); their formation has been attributed by the above authors to the Geological setting so-called 'geo-autoclave' type. Zeolite diagenesis Zeolitic alteration products of acid pyroclastics is also of particular interest in western Thrace, outcrop in the Metaxades area, at the southern Greece, because of the abundance of volcano- part of the Tertiary Orestias basin (Fig. 1). The genie sediments in the Tertiary geologic record latter, started during the lower Lutetian on the of the area (cf. Fytikas et al., 1984). Acid pyro- metamorphic formations of the Rhodope Massif elastic deposits occur in the Metaxades area, near and the Cireum Rhodope unit, with coarse con- the Greek-Bulgarian border; this deposit has glomerates grading upwards into a sequence of been quarried for local building purposes and only interbedded siltstone, sandstone and clay layers. very recently gained the attention of investigators The pyroclastics overlay a middle to Upper interested in zeolitization. Clinoptilolite, clay Eocene 40-50m thick limestone (Andronopou- minerals and cristobalite were recognized in the los, 1977). Marine microfossils found in a lime- Metaxades deposit by Marantos et al. (1988) and stone bed at the upper parts of the pyroclastics Tsirambides et al. (1988). However, heulandite suggest a lower Oligocene age (Marantos et al., and mordenite had not been previously recog- 1988). Radiometric dates show also that volcanic nized in this deposit. During the past few years activity in the Greek part of Thrace developed the minerals of the heulandite group have been from 33 to 23.6Ma (Fytikas et al., 1984). The the subject of numerous studies, which have thickness of the pyroclastics in the studied area improved our knowledge on their crystallochemi- is about 35m; the rocks at the base differ from Mineralogical Magazine, March 1990, Vol. 54, pp. 95-103 Copyright the Mineralogical Society 96 P. TSOLIS-KATAGASANDC. KATAGAS f15 o j~ o jt o o o o o o ~ ..... "o-";" ~ .o ~ .o -']'" "~ =."-- - _\ / ...... N ThessalonikJ~'~%~ ~'~" 1 f r--. --. -- (a) k.<, -\.~-" -'7 ---7----7 ----7 ---- p-~ Quaternary "~o 0~176176 ~ ~-~--~ Neogene (b) .f'~.J..o o o o o o o /" "/~".'~o o o o o - I..~--.'"---~-~'\0 0 0 0 C [-~--] Pyroclastics [ ~"~.r -- o .~.IB ,~ETAXADE,~ ~ andstone-marl- clay 1" r~176 o'c phase ~o-- o o-- o--St- ~ co-r o ,1"-~3~ Numulitic limestone ~---~ Basal clastic series ]-~ Circum Rhodope unit L 5km I FIG. l. (a) Location of Metaxades area. (b) Simplified geological map of the Metaxades area (after N. Solakious and P. Papadopoulos, 1988) showing the sampling locations for the stratigraphic sections A (samples 1-15) and B (samples 16-32). those stratigraphically above in both primary and section show less pronounced changes in grain size authigenic mineralogy and texture. The basal bed between the base and top layers than do the more is a coarse sandstone with poorly to moderately basal units, and often bedding may be defined sorted angular to subangular extra-formational only by changes in colour or mineralogical compo- fragments of feldspar, muscovite, biotite, horn- sition. In the uppermost part of the section coarse blende, epidote, garnet and metamorphic rocks sandstones with crystal and lithic fragments of cemented by calcite; it grades upwards inversely metamorphic origin, very similar to those encoun- into a conglomerate consisting of subangular to tered in the basal beds, mark the end of the vol- subrounded pebbles of metamorphic rocks canic episode. This calcite-cemented sandstone is cemented by vitric and crystal light green col- overlain locally by a zeolitized tuff bed (sample oured, ash size, welded pyroclastics. The maxi- M15), the presence of which suggests the begin- mum clast size in the conglomerate is c. 10mm. ning of a new ash fall episode (cf. Lilov et al., The whole sequence above the conglomerate con- 1987). It is unclear whether the last episode was sists mainly of zeolitized vitric and crystal tufts a major or minor one, as the original thickness mixed with epiclastic grains derived from pre- of the pyroclastics of this cycle is unknown. existing rocks and is characterized by multiple graded bedding. The tufts are white to pale grey, Methods of study yellow or green in colour, thinly to medium bed- ded and locally thinly to thickly cross-laminated. Sampling was carried out in two supplementary There exists an overall decrease in grain size stratigraphic sections, each approximately 17m upwards as can be shown mainly by the coarsest thick; their locations are shown in Fig. l(b). particles of the beds. Individual graded beds Powder pack mounts were used to produce ran- encountered in the middle and upper units of the dom orientation for bulk X-ray diffraction (XRD) ZEOLITIC DIAGENESIS 97 TABLE i. Representative bulk rock and minemalogical compositions of altemed pyroclastlc and psammitic rocks from the Het~u~ades deposit, Samples ape apranged fPom upper to lower stratigraphic homizons and mineral assemblages ape arranged in decreasing relative abundance. (C=Cristo - ballte; Cr H=Heulandlte; I-6=Illite-smectite; M=Mordenite; S-Smectite). MI5 M13 M7 M4 M16 M21 M29 M27 M28 M32 6i02 70.77 68.96 71.37 73.98 71.39 71.19 67.80 73.19 70.15 45.98 AI203 11.73 10.86 9.95 9.90 iO.10 9.71 11.19 9.95 10.69 9.99 Fc203 0.77 2.97 0.99 0.38 0.90 0.70 0.88 1.06 0.91 3.63 MgO 0.61 1.18 0.58 0.q0 0.71 0.51 o.q0 0.95 0.99 1.39 Ca0 2.37 6.g8 2.65 1.63 2.54 2.99 3.60 2.83 3.28 20.40 Na20 1.69 0.53 0.55 0.30 O.6q 1.06 1.06 1.20 1.08 1.59 K20 2.79 g.95 2.79 9.80 3.17 2.89 3.21 2.q6 2.88 1.46 TiO 2 0.10 0.32 0.10 0.08 0.13 0.11 0.11 O.lq 0.13 0.39 P208 0.02 0.05 0.01 0.Ol 0.02 0.02 0.02 0.02 0.02 0.08 Hn0 0.01 0.06 O.Ol 0.02 0.02 0.09 0.08 O.1O 0.07 0.28 H20- 3.55 1.00 4.15 2.69 3.38 3.g8 3.90 3.25 3.42 0.S0 b.O.I. 5.29 6.86 6.90 4.92 6.21 6.87 7.06 5.19 6.49 13.66 Total 99,56 99.90 99.55 99.10 99.16 99.52 99.39 99.33 99.56 99.25 H,C,M I-S,Cc H,M,C,S II,C,M,6 H,M,S II,M,C,S II,C,M II,M,C,S H,M,C,S Cc,I-S mineral analyses. For each sample three mounts Petrography and authigenic mineralogy were prepared and investigated after heating at The mineralogical composition of representa- 260, 400 and 550~ to determine whether inten- tive altered pyroclastics as determined by micro- sity reduction of the (020) and other X-ray reflec- scopic observation and XRD analysis is given in tions of the heulandite-group zeolites occurred. Table 1. It is evident from this table that the pyro- For clay mineral identification the <2Fm size clastic pile is sandwiched between two sandstone fractions were obtained and oriented mounts were beds characterized by similar authigenic and detri- prepared on glass slides. The mounts were run tal mineralogy. Authigenic minerals in these using a Philips diffractometer and Ni-filtered Cu- 30-40cm thick beds include abundant smectite, Kc~ radiation. Glycerol treatment and heating at a mixed-layer illite-smectite and calcite (sample 490~ for 2hr aided their identification. Relative M32 at the base and M13 at the top of the pile).