Bulletin of Geosciences, Vol. 79, No. 4, 221–229, 2004 © Czech Geological Survey, ISSN 1214-1119
Late Variscan Sušice dyke swarm (Moldanubian Zone): quartz micromonzodiorite dykes and their pyroxene gabbro xenoliths Stanislav Vrána Czech Geological Survey, Klárov 3, CZ-118 21 Praha, E-mail:
[email protected]
Abs t ract . The N-S trending dyke swarm located east of Sušice in south-western Bohemia is of augite quartz micromonzodiorite composition. These fine-grained, equigranular, non-porphyritic rocks include rare amygdaloidal varieties. They are of subalkaline/tholeiitic, metaluminous (A/CNK = 87 86 0.8–0.9) composition, with a relatively low mg # and low Cr and Ni contents. Sr–Nd isotope analyses of three samples yielded initial Sr/ Sr = 0.708 and 87 86 DM ≈ 1.2–1.4 Ga. The dykes carry rare xenoliths of two-augite gabbro (with Fe-rich A1 augite and Mg-rich A2 augite, Sr/ Sr also ~ 0.708) and disT Nd persed small xenocrysts of A1 augite. The early Fe-rich A1 augite indirectly indicates derivation of the xenoliths from a fractionated gabbro intrusion below the present erosion level. Rare xenoliths of biotite paragneiss exhibit selective melting of feldspars and the devitrification of glass into fibrous and skeletal feldspar aggregates. The Sušice dykes are most likely of Permian age, as suggested by previous paleomagnetic study, as well as by the dating of primary hornblende (272 Ma) in dykes from Ševětín that resemble those from Sušice in terms of petrology, whole-rock chemistry, and Sr–Nd isotopic composition. The dyke swarm is spatially related to the circular Sušice structure expressed in the surface topography, but there is insufficient evidence for determining a causal link. Ke y w o rd s : pyroxene quartz micromonzodiorite, dyke swarm, gabbro xenoliths, southern Bohemia, Moldanubian Zone
Introduction Several dykes of augite quartz micromonzodiorite have been found in the Moldanubian Zone of south-western Bohemia between Sušice and Strakonice (referred to here as the Sušice dyke swarm). These non-porphyritic, partly amygdaloidal, fine-grained rocks, which could be alternatively classed as trachyandesite, are products of high-temperature magmatic crystallization at a relatively shallow intrusion level. Due to the multitude of dyke types in this part of the Moldanubian Zone (Holub et al. 1993), generally interpreted as being associated with the Variscan granitoid intrusions of the Central Bohemian Pluton (Žežulková 1982, 1989) and the Moldanubian Pluton, this distinct group of Sušice dykes has been largely overlooked so far. The Sušice dykes arosed attention due to their similarity to augite microgranodiorite dykes near Ševětín, NNE of České Budějovice (Vrána et al. 1993, Košler et al. 2001). Paleomagnetic studies at one locality of the Sušice dykes indicated a probable Early Permian crystallization age (Krs and Vrána 1993). Similarly, Ar-Ar dating of titanian pargasite in one Ševetín dyke yielded an Early Permian (Upper Autunian) age (272 ± 2 Ma: Košler et al. 2001). In view of the geological evidence that the Sušice dykes represent the youngest igneous rocks in the area, and their paleomagnetic and petrological similarity to the Ševětín dykes, the Sušice dyke swarm is probably related to Permian volcanic activity in the Moldanubian Zone. While Permian volcanism produced large volumes of basaltic, andesitic, and rhyolitic rocks in the Saxothuringian Zone of northern Bohemia and Saxony (Prouza 1994), evidence of this activity in the Moldanubian Zone of the Bohemian Massif has been recognized only recently (Košler et al.
2001). Current geophysical models of the Moldanubian crust (Tomek et al. 1997) do not consider the possible role of Permian igneous activity, which could have potentially affected the structure and composition of the middle and lower crust of this tectonic zone.
Geological setting The studied area is part of the Moldanubian Zone of SW Bohemia. It is comprised mainly of sillimanite-biotite paragneiss and migmatite, leucocratic biotite migmatite, and orthogneiss. Calcite marble and calc-silicate gneiss intercalations are widespread in the paragneiss-migmatite units north of Strakonice and northeast of Sušice (Fig. 1). In the north, the metasedimentary complex was intruded by granitoids of the Central Bohemian Pluton. The hornblende-biotite Červená granodiorite forms an arcuate body near Sušice, gently dipping to the northwest. Granitoids of the Moldanubian Pluton occur in the southwest, along the border with Germany (Fig. 1). Much of the area shows the NE-SW structural trend typical of the Bohemian branch of the Moldanubian Zone, though this domain adjoins the NW-SE trending Šumava (Bohemian Forest) structural domain 7 km SW of Sušice.
Analytical techniques Whole-rock chemical analyses were performed in the laboratories of the Czech Geological Survey in Prague. Minor and trace element abundances were determined by XRF (Cr, Ni, Zn, Rb, Sr, Y, Nb, Sn, Ba and U) and INAA by the company Geoindustria, in Černošice, Czech Republic. 221
Stanislav Vrána Table 1. Localities of analysed augite quartz micromonzodiorite samples Sample No.
Locality
SN 15 and SN 16
Žichovice, 1.3 km south of the village and 100 m east of the road, loose blocks
SN 18
Kadešice, blocks 100 m SW of the village
SN 24
Kadešice, blocks 200 m SW of the village
SN 28
Nezdice, outcrop at the eastern edge of elevation above the quarry; 0.5 km north from the centre of Nezdice
Mineral composition was analysed by an electron microprobe Cam-Scan 4-90DV, equipped with EDX at the Czech Geological Survey. An accelerating voltage of 15 kV, a beam current of 3 nA, a counting time of 80 s, and ZAF correction program were used. Natural minerals were used as standards. Three micromonzodiorite samples and one gabbro xenolith were analysed for Rb–Sr isotopic composition in the laboratories of the Czech Geological Survey. Three Sm-Nd isotopic analyses were performed in the Laboratory of the Mineralogical–Geological Museum in Oslo, Norway. The analytical methods were described in Vrána et al. (1993).
nes (Morimoto et al. 1988) does not provide a detailed designation for these individual generations. To assist our description, the augite types are designated as follows: A1 augite (Wo 26.7–40.0 mol.%, Fs 26.8–34.7); A2 augite (Wo 40.9–43.1, Fs 11.2–12.3); A3 augite (Wo 35.0–46.8, Fs 18.5–30.6). A1 and A2 augites occur in the rare gabbro xenoliths and as xenocrysts in the quartz micromonzodiorite. A3 augite is a major mineral in the quartz micromonzodiorite dykes.
Field relations and petrography
The micromonzodiorite dykes considered here are several metres wide. About 10% of the material shows amygdaloidal texture, with amygdales 2 to 7 mm in diameter, filled by calcite, minor quartz or chlorite, and rare K-feldspar (Fig. 2). Xenoliths of the paragneiss country rock and vein quartz occur in minor quantities. Rare xenoliths of two-pyroxene gabbro were sampled at a locality 1.3 km south of Žichovice. The location of the analysed samples is shown in Table 1. In the augite quartz micromonzodiorite, crystallization started with minute tabular ilmenite, calcic plagioclase (An Note on augite terminology 65–45), and A3 augite, with lath-shaped plagioclase and anhedral A3 augite intergrown in a subophitic structure The studied rocks contain three generations of augite of (Fig. 3). The anorthite content in zoned plagioclase declines distinct compositions. The simple nomenclature of pyroxeoutward, to values as low as An 10 (Fig. 4a). Wedge-shaped spaces among plagioclase crystals are partly filled by micrographic K-feldspar-quartz intergrowths. The feldspar and pyroxene compositions of one of the dykes are shown in Fig. 4, and representative mineral analyses are presented in Table 2. Opaque minerals have been identified by reflected light microscopy, microprobe analyses, and for a single dyke by thermal demagnetization of a whole-rock sample (Krs and Vrána 1993). Ilmenite and pyrrhotite are the main opaque phases, while pyrite, chalcopyrite, sphalerite, and galena occur only in trace amounts. Magnetite is absent. Minor chlorite, calcite, actinolite, muscovite, prehnite, and titanite probably crystallized during a late autohydrothermal stage. Several generations of calcite, including massive calcite veins and drusy calcite coatings in open fractures, are associated with a thick calcite marble intercalation near Nezdice, close to one of the Figure 1. Geological sketch of the area around Sušice. Based on 1 : 50,000 maps published by the Czech Sušice dykes (Žák et al. 1997). It is probable that some of the late Geological Survey. 222
Late Variscan Sušice dyke swarm (Moldanubian Zone): quartz micromonzodiorite dykes and their pyroxene gabbro xenoliths
calcite generations correlated with the thermal/fluid conditions during the intrusion of the Sušice dykes.
a
Chemical and isotopic composition The chemical composition of the augite quartz micromonzodiorite dykes is presented in Table 3. As demonstrated by the multicationic plot of Debon and Le Fort (1983), these dykes are exclusively quartz monzonitic in composition (Fig. 5). In the diagram of Le Bas et al. (1986), five samples fall into the subalkaline/tholeiitic field. Three samples plot very near the quadruple point of basaltic trachyandesite-trachyandesite-basaltic andesite-andesite fields, while two samples plot close to the alkali-rich part of the andesite field. The rocks are metaluminous with A/CNK = 0.8–0.9, and have relatively low mg # and low Cr and Ni contents, indicating a prevalence of crustal material over primary mantle-derived melt. The Sr-Nd isotopic composition of three samples with initial Sr values near 0.708 and εiNd = –2.8 to –5.1 (Table 4) indicates an important crustal component. The spread in εiNd values probably points to variation in the proportion of primary mantle melt and assimilated crustal material.
b
Gabbro xenoliths Rare small clasts of two-augite gabbro, up to 4 cm long, have been found in one of the Sušice dykes 1.3 km SE of Žichovice. The mineral composition of the gabbro xenoliths is given in Table 5. Planimetric analysis of one xenolith (Fig. 6a) gave the following results (2400 points, vol.%): 30.7 A1 augite, 13.2 A2 augite, 55.0 plagioclase An 68–77, 1.1 ilmenite. The rock also contains traces of K-feldspar and sodic oligoclase. The Fe-rich A1 augite (Wo 27 to 40) is unusual in having a Mg/Fe ratio near unity, and thus plotting in the pyroxene miscibility gap, indicating high cooling rates and crystallization in a dry system. Textural information shows that the crystallization of A1 augite was followed by that of a Mg-rich A2 augite, the latter of which forms relatively large oikocrysts (Fig. 6b). A1 augite is associated with bytownite (An 73). This is in contrast to layered intrusions, where the crystallization of Fe-rich augite usually occurs relatively late, i.e. in the ferrodiorite stage, and is accompanied by oligoclase-andesine (An
