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Permian postorogenic dykes in Southern Bohemia: K-Ar dating of the Vimperk porphyries and the Milevsko felsic microgranite (Moldanubian Unit, Czech Republic)


Vladimír Žáček, David Buriánek, Radek Škoda, Zoltán Pésckay

Geoscience Research Reports 53, 2020, pages 137–148
Map sheets: Vimperk (22-34)

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Published online: 12 October 2020

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This study deals with petrography, geochemistry and K-Ar dating of the recently described Vimperk porphyry dyke swarm (diorite to granodiorite) and microgranite of the Milesko felsic dyke swarm described earlier in detail by Vrána (1999). The Vimperk porphyry swarm strikes N-S, and exceeds 15 km in the length. The dykes penetrate the metamorphic rocks of the Monotonous Group of Moldanubicum, small intrusions of biotite granite, and W-E trending dykes of syenite porphyry, as well (Žáček et al. 2009, Žáček et al. 2012b). The individual dykes are up to 4 km long and 2-15 m thick (Žáček et al 2012b, see Fig. 1). The Milevsko felsic dyke is more than 15 km long, and the thickness of the dykes (microgranite) is up to 10 m. The dykes with dominant N-S direction cut porphyritic amphibole-biotite melagranite (durbachite) of the Central Bohemian Plutonic Complex near its southern contact with the Moldanubian gneisses and migmatites (Vrána 1999). The studied sample Mi1 consists of a mosaic of quartz (30 vol.%), albite (25 vol.%, Ab99An1) and K-feldspar (15 vol.%), and accessory apatite, rutile and limonitized pyrite (Fig. 2e-f). Geochemically, the rock is highly evolved alkali feldspar granite. The Vimperk dykes are both significantly porphyritic and non-porphyritic with varying mineralogical composition (see Fig. 2a-d, Fig. 3a, Table 1, 2). The rocks are composed of 40-50 vol.% of normally zoned calcic plagioclase (An67-36), 5-20 vol.% of K-feldspar (Kf70-90Ab4-27Cls0-3), 5-20 vol.% of quartz, 5-35 vol.% of magnesiohornblende (Si = 7.20, Xmg = 0.71, but mostly altered to actinolite or chlorite), 5-15 vol.% of probable orthopyroxene (completely chloritized), up to 10 vol. % of brown Ti-rich biotite (3.4-4.5 wt.% TiO2, Xmg = 0.37-0.57), up to 5 vol.% of pinkish clinopyroxene and 1-5 vol.% of lath-shaped ilmenite. The accessories include fluorapatite, titanite, baddeleyite (Fig. 3b), and less abundant zircon, allanite, thorite, galena, scheelite, pyrrhotite and probable chevkinite. The secondary chlorite and actinolite can represent up to ca 35 vol.% of rock, minor are clinozoisite-epidote, secondary titanite II, albite, sagenite and calcite. Scarce miarolitic cavities are filled with quartz, chlorite, calcite and rare skeletal baddeleyite. The altered xenocrysts of plagioclase I (Fig. 2d) and rarely of K-feldspar (up to 5 vol. %) are also present along with mm-cm sized mafic inclusions (Fig. 2b).
Mean magnetic susceptibility of porphyries corresponds to 0.24 × 10-3 SI, and that of felsic microgranite is 0.02 × 10-3 SI. The field gammaspectrometry of porphyries yielded the average contents of K = 3.6 %, eU = 7.1 ppm and eTh = 19.7 ppm (Table 3). The potassium is bound mainly in K- feldspar, less in biotite. Uranium and thorium are preferably bound in baddeleyite (1.96 wt.% UO2 and 0.41 wt.% ThO2), and much less in zircon (0.17 wt.% UO2 and 0.14 wt.% ThO2), see Table 3.
Geochemically, the rocks of the Vimperk swarm (6 analysed samples, see Table 4), exhibit relatively varying chemical composition SiO2 (54.7-67.6 wt.%), MgO (1.0-5.0 wt.%), CaO (1.7-7.0 wt.%), and K2O (2.7-5.5 wt.%). The chemical composition ranges from basaltic andesite over trachyte to rhyolite (Fig. 4 a-f). These rocks are metaluminous to slightly peraluminous (A/CKN = 0.8-1.5), and have high-K calc-alkaline to shoshonitic character. The geochemical signatures are typical of intra-plate continental settings. The primitive mantle-normalized spider plots with Nb, Sr, Ti, and P troughs, and peaks in Th, U, and Pb, indicate an influence of crustal contamination of magma, which was generated through the partial melting of lithospheric mantle.