New age and petrological constraints on Lower Silesian basaltoids, SW Poland.
Cenozoic volcanic rocks, generally called "basalts", are ubiquitous in the area of Lower Silesia. Most of the known occurrences are situated on the Fore-Sudetic Block, and in the western part of the Sudetes Mts. In the Fore-Sudetic Block, basaltoid exposures have been described from the Jawor, Legnica and Zlotoryja, as well as Niemcza and Kowalskie-Zelowice, Targowica, and Ziebice areas. As far as the first region is concerned, basaltoid rocks occur on either side of the morphotectonic boundary of the Middle Sudetes. Basaltoid occurrences in the second region are scattered over a larger area. In the western part of the Sudetes, numerous exposures are associated with the northern part of the NE-orientated Ohfe volcano-tectonic graben that cuts the Bohemian Massif. The other basaltoid occurrences are located in a depression occupied by the so-called "Opole" Cretaceous strata and in its eastern surroundings, whereas isolated exposures are known from the Karkonosze and Zlote Gory Mts. Basaltoid volcanic rocks have also been drilled by numerous boreholes and their vast subsurface spread has also been confirmed by geophysical soundings (Cwojdzinski and Jodlowski, 1982; Badura and Przybylski, 2000).
The aim of our paper is to present preliminary results of new K-Ar datings and petrological studies of basaltoid rocks exposed between Zlotoryja in the NW and Nowa Cerekiew in the SE of Lower Silesia.
2. GEOLOGICAL SETTING
The Lower Silesian Cenozoic volcanic rocks, mostly basaltoids and their pyroclastics, occur between the western frontier of the country up to the St. Anna Mt. in the east (Fig. 1). These volcanics constitute the eastern part of the Central European Volcanic Province (CEVP), nearly 700 km long, and situated in front of the European Alpides (Cwojdzinski and Jodlowski, 1982; Blusztajn and Hart, 1989); or, following Kopecky (1966, 1978) and Ulrych et al. (1999, 2002a,b), in the marginal part of a vast rift system that extends between the Rhine River valley through Germany, Czech Republic, up to Poland. A few occurrences of volcanic rocks in Lower Silesia can be distinguished. In the Czech Republic, the volcanic Ohfe rift does occur, and its north-eastern continuation in Poland and Germany is the Zittau-Bogatynia rift (Alibert et al., 1987; Ulrych et al., 1999), and--probably--basaltoid occurrences in the Western Sudetes and their foreland.
Volcanic lavas in Lower Silesia are typical of brittle, unfolded regions. Basaltoid rocks are represented here by predominant tephrites and basanites, rare foidites, and singular basalts and trachybasalts (Bolewski and Parachoniak, 1982; Kozlowska-Koch, 1987; Wierzcholowski, 1993; Lorenc et al., 2004). Apart from magmatic rocks, the Lower Silesian basaltoid formation includes as well pyroclastic rocks, such as: volcanic breccias, tuffs, and tuffites. The occurrences of volcanic rocks in SW Poland are associated with deep-seated faults, and form "spot-like" concentrations (Cwojdzinski and Jodlowski, 1982; Dyjor and Kosciowko, 1986). Cwojdzinski and Jodlowski (1982) distinguished three regions of basaltic rock concentrations: Zittau-Zgorzelec-Luban, Legnica-Jawor- Zlotoryja, and Strzelin-Ziebice. Solitary occurrences of basaltoids are also known from other areas of SW Poland, like: the Ladek Zdroj area, Glubczyce Plateau, or Karkonosze Mts. Basaltoid rocks are hosted in differentiated bedrock, including: Cretaceous strata near Opole (melabasanites and melanephelinites), Palaeogene and Neogene strata in the Luban-Bogatynia zone (basanites and foidites), metamorphic rocks of the Niemcza-Strzelin (foidites and basanites) and Ladek Zdroj areas (basanites), Hercynian granitoids and their metamorphic cover of the Jawor-Strzelin-Zlotoryja (basanites, foidites, alkali basalts) and Karkonosze-Izera areas (basanites; cf. Birken-majer et al., 2004a; Lorenc et al., 2004).
[FIGURE 1 OMITTED]
Altogether, 314 surface exposures of basalt-like rocks have been counted in Lower Silesia, including: 156 small veins (sills and dykes), 89 fragments of lava flows, 44 volcanic necks, 22 plugs and veins bearing fragments of lava flows, and 3 isolated tuff and conglomerate occurrences (Jerzmanski and Sliwa, 1979). The magnetic and gravimetric data show that the Cenozoic basaltoid occurrences are associated with the so-called "basalt anomalies" which may point to a relatively shallow depth of volcanic rocks, whose subsurface extent is much more greater than the surfacicial one (Cwojdzinski and Jodlowski, 1982; Badura and Przybylski, 2000). The basaltoid veins attain diametres ranging from a few metres to a few tens of metres. An example of the exposed basalt vein is the so-called "Perkun fan" structure (Kozlowski and Parachoniak, 1960). The diameter of volcanic plugs changes from a few tens to a few hundred metres. The accompanying tuffs and volcanic breccias have only been preserved fragmentarily, undergoing weathering, erosion, and denudation. Typical examples of volcanic necks, showing a few effusive phases, can be found near Zlotoryja, and also in other parts of Lower Silesia, like close to Gracze and Niemodlin. The basaltoid lava flows are particularly frequent in the SW part of Lower Silesia (Jerzmanski and Maciejewski, 1968). The thickness of lava flows changes from a few to a few tens of metres. Their basement is usually represented by older crystalline rocks, rarely sedimentary rocks of the Upper Cretaceous or the Palaeogene and Neogene.
The pyroclastic rocks have been much eroded; they are usually strongly weathered and turned into clayey beds showing traces of tuffs, lapillae, and volcanic bombs (Dyjor and Kosciowko, 1986). The primary, much more widespread, extent of pyroclastic rocks can be inferred basing on the Mokrzeszow graben borehole log, wherein tuffs and tuffites occurring at a depth of 418-660 m have not been drilled through (Grocholski, 1977). In the upper part of the tuffite series, a 6-m-thick basalt layer was drilled. The preservation of such a thick volcanogenic series has only been possible due to its location in a tectonic graben, showing a tendency to tectonic subsidence. Minor admixture of granitoid blocks and other crystalline rocks found in tuffites, as well as plant remains indicate that these sediments have largely been redeposited in an aquatic environment. The sporomorph studies conducted on clays overlying the tuffite series point to a Late Oligocene age of the latter (Jaworska, 1975). Cwojdzinski and Jodlowski (1982) infer that a similar tuff/tuffite series could also occur in a deeper part of the Paczkow Graben fill.
The results of geochemical studies indicate that the majority of Lower Silesian basaltoid rocks originated in the upper mantle at depths ranging between 75 and 90 km (Wierzcholowski, 1993), whereas more alkaline varieties were formed due to magma differentiation at depths of 30-45 km. Most of the Lower Silesian basaltoids are, hence, a result of rapid processes of magma upheaval that made its differentiation impossible. The basalts contain enclaves of rocks derived from the mantle, such as: peridotites (harzburgites, lherzolites), dunites, and clinopyroxenites (Cwojdzinski and Jodlowski, 1982; Bialowolska, 1993; Wierzcholowski, 1993). The mantle origin of Lower Silesian volcanic rocks is also testified to by proportions of Sr, Hf and Nd (Alibert et al., 1987; Blusztajn and Hart, 1989; Ladenberger et al., 2004).
We sampled 13 surficial exposures of basaltoids and cores of two boreholes drilled at Mokrzeszow (no. 14) and Jezow Sudecki (B-5; no. 15; Fig. 1; Table 1). Most of the studied localities cluster close to the Sudetic Marginal Fault. The surficial exposures represent both lava flows (6 localities) and plugs (6); and one sample (no. 11) comes from a loose block, probably derived from a plug. The Nowa Cerekiew localities (nos. 1 and 2) are situated in the southeasternmost portion of Lower Silesia, in the Glubczyce Plateau. The Fore-Sudetic Block is represented by samples collected in the Strzelin (no. 3 --Pogroda, no. 4--Debowiec) and Strzegom-Zlotoryja areas (no. 5--Chroslice, no. 6--Koscielna Gora, no. 7 -Winnik, and no. 11--Krajow). The last area, although situated already in the Sudetic Block (SW of the Sudetic Marginal Fault), was sampled at Grodziec (no. 8), Kozow (no. 9), Debina (no. 10), Gorzec (no. 12), and Muchowskie Wzgorza Hills (no. 13). The Jezow Sudecki (B-5) borehole (no. 15) is also situated in the Sudetic Block, on the NE margin of the Karkonosze Massif, in the Intra-Sudetic fault zone. The second borehole (Mokrzeszow), in turn, is located shortly NE of the Sudetic Marginal Fault, within a tectonic trough filled with Palaeogene and Neogene sediments.
4. METHODS AND DATING RESULTS
K-Ar datings were made by one of us (Z. Pecskay) on whole rock samples, using the methodology applied by the Institute of Nuclear Research, Hungarian Academy of Sciences, Debrecen, Hungary (cf. Balogh, 1985; Birkenmajer and Pecskay, 2002; Birkenmajer et al., 2002, 2004b).
Approximately, 0.05 g of finely ground sample was digested in acids and finally dissolved in 0.2M HCl. Potassium was determined by flame photometry with a Na buffer, and Li international standard. The inter-laboratory standards Asia 1/65, LP-6, HD-B1, and GL-O were used for checking the measurements. Argon was extracted from the samples by RF fusion in Mo crucibles, in a previously backed stainless steel vacuum system. The [sup.38]Ar spike was added from gas pipette system and the evolved gases were cleaned using Ti and SAES getters and liquid nitrogen traps, respectively. The purified Ar was then transported directly into the mass spectrometer, and Ar isotope ratio was measured in the static mode, using a 15 cm radius magnetic sector-type mass spectrometer, built in Debrecen. Atomic constants suggested by Steiger and Jager (1977) were used for calculating the ages. All analytical errors represent one standard deviation, i.e., 68% of analytical confidence level. Since we base our analytical errors on the long-time stability of instruments, and on the deviation of our results obtained on standard samples from the inter-laboratory mean, the analytical errors are likely to be overestimated.
Petrological and mineralogical studies were made by two of us (E. Koszowska, A. Wolska) at the Department of Mineralogy, Petrology, and Geochemistry, Institute of Geological Sciences, Jagiellonian University in Krakow. Microscopic study of thin sections was performed using AMPLIVAL petrographic microscope. The morphology of minerals and their chemical composition were examined using scanning electron microscopy (JEOL 5410), equipped with an energy dispersive spectrometer Voyager 3100 (NORAN). Geochemical analyses were performed at the Activation Laboratories, Ltd., Canada.
Samples no. 5-13 gave dates ranging between 21-38 Ma, whereas sample derived from Mokrzeszow borehole (no.14) was dated to 44 Ma. Another borehole sample (Jezow Sudecki B-5; no. 15) is of 59 Ma age. On the other hand, the supposedly "Quaternary" basalts from the Debowiec area (no. 4; Fore-Sudetic Block) fall into the interval of 29-30 Ma. The southeasternmost occurrences of the Lower Silesian basalts at Nowa Cerekiew (nos. 1,2) display two generations of effusive activity: the older lava flows (26 Ma) are cut here by plugs dated to 22 Ma (Tables 1,2).
5. PETROLOGICAL AND MINERALOGICAL ASPECTS
The basaltoid rocks studied have dark-grey, nearly black colour, and are massive and very fine-grained. The amygdaloidal structures are very rare (sample no. 15). Microscopic observations indicate that the structure of these rocks is fine-grained, porphyric, whilst the texture is chaotic. Only in sample no. 12, medium-grained structure, and intergranular and rare fluidal texture, marked by the arrangement of platy minerals (plagioclases), were observed. The enclaves of host rocks (xenoliths) are very rare (samples no. 1,3). The presence of fragments of the oldest basaltoid rocks is observed in sample no. 2.
The oldest, K-Ar age-dated, basaltoids are represented by samples no. 14 (ca. 44 Ma) and 15 (ca. 58 Ma). These rocks are strongly altered when compared to the other samples studied. In sample no. 14, bowlingite (a mixture of secondary minerals from the serpentinite, chlorite, and saponite group) occurs as pseudomorphs after olivine phenocrysts. Pyroxene phenocrysts are very rare in these samples. In sample no. 15, bowlingitic pseudomorphs after olivine phenocrysts, and pseudomorphs after pyroxenes filled by secondary minerals (carbonates, silica) and relic crystals of pyroxene were observed. Crystals of Ca-rich clinopyroxenes (bearing ca. 4 wt. % of Ti[O.sub.2]) and opaque minerals (mainly titanomagnetite) occur in the groundmass.
The K-Ar age of samples no. 2, 3, 4, 5, 7, 8, 12, 13 was found to be 26-33 Ma.
In samples no. 3, 5, 7, 8, 12, olivine and pyroxene phenocrysts do occur. Their size ranges from 0.8-1.7 mm to 0.4-0.6 mm, respectively. In samples no. 2, 4, and 13, olivine phenocrysts are predominant; pyroxene phenocrysts are very rare and smallest in size (0.1-0.4 mm). These rocks are alkali basalts and basanites (Fig. 2).
Olivine phenocrysts in both types of basaltoid rocks are represented mainly by Mg-rich (core [Fo.sub.88-80], rim [Fo.sub.78-71]) chrysolite. Nickel and chromium are commonly present in olivine phenocrysts. These phenocrysts are altered to a different degree. In samples no. 2, 3, and 13, bowlingite occurs both in central parts and micro-cracks within olivine phenocrysts. The latter minerals are surrounded by red iddingsite rims (samples no. 5,7,12), and their margins are corroded in all the studied samples.
Pyroxene phenocrysts vary in size from 0.2-0.4 mm, through 0.5-0.7 mm, to 0.8-1.2 mm). They are represented by Ca-rich clinopyroxenes ([En.sub.41-34] [Fs.sub.14-9] [Wo.sub.53-48]-salite; where: En--enstatite, Fs--ferrosilite, Wo--wollastonite) showing a distinct zonal structure and displaying Fe-richer rims and Fe-poorer cores. Very rare sieve texture (alkali glass in a fine, mesh-like arrangement) was observed in central parts of pyroxene phenocrysts (samples no. 2,5).
In the groundmass of the basaltoid rock samples, clinopyroxenes and opaque minerals (iron oxides, ilmenite) were mainly observed. The Ca-rich clinopyroxene crystals ([En.sub.37-29] [Fs.sub.17-12] [Wo.sub.53-48]-salite; according to Poldervaart and Hess' (1951) nomenclature) display variable content of Ti[O.sub.2] (2-3 wt. %--sample no. 7, and 4-5 wt. % samples no. 1, 3, 8). In the groundmass, there also occur very small plates of plagioclases: labradorite ([An.sub.68-56] [Ab.sub.41-32] [Or.sub.51]), rare bytownite ([An.sub.74-72][Ab.sub.28-26]), and andesine (samples no. 1,2,5,7). Samples no. 3 and 8 are characterised by the presence of nepheline, analcite, and sodalite. Small olivine crystals are observed in the groundmass. They are represented by hyalosiderite ([Fo.sub.66-57]), iddingsitized to a variable degree. Brown alkali glass occurs in the interstices among mineral crystals.
K-Ar ages of samples no. 1, 6, 9, and 10 were determined to 20-24 Ma. The rocks represent alkali basalts and basanites (Fig. 2).
In samples no. 9 and 10, olivine phenocrysts are common (being 0.3-0.6 mm, 0.7-1.2 mm, and 1.6-2,5 mm in size), whereas small (0.4-0.6 mm in size) pyroxene phenocrysts are very rare. In sample no. 6, only olivine phenocrysts do occur, whereas in sample no. 1 olivine phenocrysts are strongly altered into a mixture of secondary minerals of the bowlingite type.
Olivine phenocrysts of the basaltoid rocks studied are represented mainly by Mg-rich (core [Fo.sub.83-79], rim [Fo.sub.77-71]) chrysolite. These phenocrysts are commonly altered to a variable degree. In samples no. 9 and 10, bowlingite-like alteration processes in central parts, and cracks in olivine phenocrysts were observed. In sample no. 9 olivine phenocrysts are overgrown by red iddingsite rims. The margins of olivine phenocrysts are corroded in all the samples.
[TABLE 2 OMITTED]
[FIGURE 2 OMITTED]
Pyroxene phenocrysts are represented by clinopyroxenes and have salitic composition ([En.sub.38-27] [Fs.sub.17-10] [Wo.sub.53-52]), showing as well a distinct zonal structure and often displaying sector (hour-glass) structures.
In the groundmass of the basaltoids, clinopyroxenes and opaque minerals (iron oxides, ilmenite) were mainly observed. The Ca-rich clinopyroxene crystals ([En.sub.36-29] [Fs.sub.16-12] [Wo.sub.53-51]) contain variable amount of Ti[O.sub.2] (2-3 wt. %--sample no. 10, and 4-5 wt. %--samples no. 1, 6, 9). In the groundmass of samples no. 1, 9, and 10, there occur very small plates of plagioclases (bytownite--[An.sub.74-73] [Ab.sub.26-24] [Or.sub.3-2] and andesine--[An.sub.43-38] [Ab.sub.54-49] [Or.sub.9-7]). Light minerals are represented by nepheline and, probably, sodalite in sample no. 6. Small olivine crystals are present in the groundmass (samples no. 6 and 9). These are hyalosideritic ([Fo.sub.68-65]) in composition, and commonly iddingsitized. Brown alkali glass occurs in interstices among the groundmass minerals.
According to TAS discrimination diagram (LeMaitre et al., 1989; Fig. 2), the analysed samples represent both tephrites/basanites (nos. 3,8,6,9,10) and basalts (nos. 1,2,4,5,7,12,13,14). Sample no. 15 (B-5 borehole) is altered to such a degree that its classification is not possible.
6. AGE CONSTRAINTS
Samples no. 5, 6, 7, 8, 9, 10, 11, 12, and 13 are generally low-potassium basaltic rocks (<0.9%).This is a typical feature of all Palaeogene-Neogene alkaline basaltic rocks of the Opole region (Table 2). According to radiometric ages, two main volcanic phases can be distinguished: 21-24.5 Ma, and 31.3-33.7 Ma (Birkenmajer and Pecskay, 2002; Birkenmajer et al., 2002, 2004b). Only one older age was determined on sample no. 11 (38.27[+ or -]1.55 Ma), probably due to the presence of excess argon. Therefore, this radiometric age can only be concerned as an analytical age. Basing on geological inferences, sample no. 15 can be correlated with sample no. SK-10 coming from Sniezne Kotly, Karkonosze Mts. However, analytical data contradict geological observations. The K-Ar age (26.01[+ or -]1.27 Ma) obtained on SK-10 is much more reliable than the radiometric age obtained on sample no. 15 (58.7[+ or -]5.9 Ma) which can be related to strong alteration of the core sample. Highly consistent ages (30.33[+ or -]1.09 Ma and 29.09[+ or -]1.07 Ma, resp.) were obtained on samples nos. 3 and 4. Basing on analytical data, it can be ruled out completely that this volcanic activity took place in the Quaternary, as it has been supposed in some earlier papers (Wronski, 1970). Two samples (nos. 1, 2) were collected at Nowa Cerekiew quarry. Following the field observations, the radiometric age of the lava flow (26.41[+ or -]1.03 Ma) is older than that of the plug (22.31[+ or -]0.87 Ma). These apparent ages are similar to those obtained on basaltic rocks occurring in the Opole region (Birkenmajer and Pecskay, 2002).
The oldest K-Ar age is an Eocene one. However, this age estimation can be older than the real geological age. The dominant phase of volcanic activity appears to have taken place in Oligocene times. This also holds true for the Pogroda area, wherefrom much more younger ages have been expected. The second peak of volcanic activity took place in the Early Miocene. Nevertheless, more detailed analytical and geological work should be made for the sake of a correct explanation of the basalt eruptive sequences in the Lower Silesian volcanic field.
This study has been supported in part by the Polish Committee for Scientific Research (KBN) through a grant no. 8 T12B 025 20 (to J. Badura). We are deeply indebted to Prof. Dr. K. Birkenmajer for helpful and stimulating discussions on the age and origin of Lower Silesian basalts. Constructive comments offered by two anonymous Referees are highly appreciated.
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Janusz BADURA (1) *, Zoltan PECSKAY (2), Ewa KOSZOWSKA (3), Anna WOLSKA (3), Witold ZUCHIEWICZ (3) and Boguslaw PRZYBYLSKI (1)
(1) Polish Geological Institute, Lower Silesian Branch, al. Jaworowa 19, 50-122 Wroclaw, Poland
(2) Institute of Nuclear Research of the Hungarian Academy of Sciences, 4001 Debrecen, Bem ter 18/C, Hungary
(3) Institute of Geological Sciences, Jagiellonian University, Oleandry 2A, 30-063 Krakow, Poland
* Corresponding author's e-mail: Janusz.Badura@pgi.gov.pl
(Received January 2005, accepted April 2005)
Table 1 K-Ar datings of alkaline basaltic rocks of Lower Silesia Lab. Sample Locality Dated K (%) No. No. fraction 6312 BPZ Nowa Cerekiew w.r. 0.97 1 plug 6311 BPZ Nowa Cerkiew w.r. 0.77 2 lava flow 6310 BPZ 3 Pogroda plug w.r. 0.72 6314 BPZ Debowiec w.r. 0.69 4 lava flow 6315 BPZ Chroslice w.r. 0.61 5 lava flow (?) 6309 BPZ Koscielna Gora w.r. 0.66 6 plug (?) 6313 BPZ 7 Winnik lava flow w.r. 0.76 6343 BPZ Castle Grodziec w.r. 0.61 8 plug 6344 BPZ 9 Kozow plug w.r. 0.63 6345 BPZ Debina lava flow w.r. 0.79 10 6346 BPZ Krajow w.r. 0.41 11 block of plug (?) 6347 BPZ Gorzec lava flow w.r. 0.76 6348 12 BPZ Muchowskie w.r. 0.82 13 Wzgorza plug 6349 BPZ Mokrzeszow w.r. 1.27 14 borehole 6350 BPZ Jezow Sudecki w.r. 0.46 15 B-5 borehole Lab. [sup.40]Ar [sup.40] K-Ar age (Ma) No. rad (ccSTP/g) Ar rad (%) * [10.sup.-7] 6312 8.491 68.9 22.31 [+ or -] 0.87 6311 7.955 69.1 26.41 [+ or -] 1.03 6310 8.503 66.1 30.33 [+ or -] 1.09 6314 7.867 64.7 29.09 [+ or -] 1.07 6315 6.643 57.6 27.88 [+ or -] 1.13 6309 5.434 63.6 20.99 [+ or -] 0.83 6313 9.273 58.5 31.28 [+ or -] 1.26 6343 7.746 48.7 32.16 [+ or -] 37 6344 5.191 47.7 21.14 [+ or -] 0.91 6345 7.592 57.4 24.46 [+ or -] 0.99 6346 6.225 58.1 38.27 [+ or -] 1.55 6347 1.004 [10.sup.-6] 45.4 33.67 [+ or -] 1.48 6348 1.018 [10.sup.-6] 69.6 31.62 [+ or -] 1.23 6349 2.208 [10.sup.-6] 8.0 44.1 [+ or -] 7.7 6350 1.072 [10.sup.-6] 14.2 58.7 [+ or -] 5.9
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|Author:||Badura, Janusz; Pecskay, Zoltan; Koszowska, Ewa; Wolska, Anna; Zuchiewicz, Witold; Przybylski, Bogus|
|Publication:||Acta Geodynamica et Geromaterialia|
|Date:||Jul 1, 2005|
|Next Article:||Crust deformation monitoring in the Polish part of Snieznik Massif-continuing researches.|