1. Permské postorogenní žíly v moldanubiku jižních Čech: K-Ar geochronologie pro porfyry z okolí Vimperku a felsický mikrogranit od Milevska.
- Author
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ŽÁČEK, VLADIMÍR, BURIÁNEK, DAVID, ŠKODA, RADEK, and PÉCSKAY, ZOLTÁN
- Subjects
RARE earth metals ,METAMORPHIC rocks ,FLUORAPATITE ,SPHENE ,SCHEELITE ,DIKES (Geology) ,APATITE ,OLIVINE - Abstract
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 dyke swarm 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.%, Ab
99 An1 ) 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 (Kf70-90 Ab4-27 Cls0-3 ), 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 K2 O (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. Two samples were dated. The diorite porphyry from Vimperk dyke swarm gave an K-Ar age of 292.5 ± 9.1 Ma (1s), and the Milevsko microgranite 300.9 ± 9.1 Ma (1s). These ages are interpreted as minimal, but close to magmatic age (see Table 5). The existing data show a close geochemical relationship to mafic dyke swarms from other Moldanubian localities: from Ševětín (Vrána et al. 1993), Sušice (Vrána 2004), Kaplice (Košler et al. 2001, Vrána et al. 2005) and Štěpánovice and Hamr near Cítov (Vrána and Janoušek 2006). The contents of major elements (Fig. 4a-d) and rare earth patterns (Fig. 4e) also show a similar character as the above-mentioned dykes (Fig. 4e, cf. Vrána and Janoušek 2006). There is also significant petrographic similarity to these dykes, the major mineralogical composition, but especially the appearance of baddeleyite (Fig. 3b) and the presence of accessory galena, scheelite and other ore minerals (cf. Vrána and Janoušek 2006). The signs of Vimperk dykes, especially the occurrence of miarolitic cavities and micrographic intergrowths of quartz and K-feldspar in the matrix, indicate the rapid cooling of the magma, and the subvolcanic character of the crystallization below the surface. The temperature of ~ 752 °C, and pressure of 0.58 kbar were estimated using amphibole and plagioclase-amphibole geothermobarometers. The Vimperk swarm is situated between the Sušice dyke swarm to the west and the Ševětín dyke swarm to the east, and indicates the occurrence of similar dykes in large areas of the Moldanubicum. All these dykes are related to the late orogenic Variscan magmatism in the southern part of the Moldanubicum bound to the north-south structures connected with the Lower Permian extension tectonics of the Blanice Graben. The dykes penetrated the chilled Variscan crust shallowly below the surface in the lowest Permian and represent the last intrusive rocks. By analogy with other similar dykes of the same structural position (Vrána and Janoušek 2006), the origin of primary magma can be interpreted as a mixing of basic magma originating from the lithospheric mantle, which has been contaminated to varying degree by the acidic material of the continental crust during the uplift to the shallow levels of the Variscan crust. [ABSTRACT FROM AUTHOR]- Published
- 2020
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