Journal of


  (Formerly Journal of the Czech Geological Society)

Original Paper

Stanislav Vrána, Slabý, Jiří Bendl

The Kaplice dyke swarm of biotite granodiorite porphyry and its relationship to the Freistadt granodiorite, Moldanubian Batholith

Journal of the Czech Geological Society, volume 50 (2005), issue 1-2, 9 - 17


  Abstract References

A possible link of the Kaplice dyke swarm with Freistadt granodiorite was studied. The swarm of biotite granodiorite porphyry dykes is associated with the Kaplice fault zone - a prominent late Variscan discontinuity in the Moldanubian Zone of the Bohemian Massif, featuring sinistral displacement of ca. 17 km. The dykes have a strongly porphyritic structure in which plagioclase, biotite - quartz phenocrysts combined represent about 50 vol. % of the rock. The microgranular and spherulitic textures of groundmass in the dykes indicate probable devitrification of the originally present glass, resulting from quenching of the melt carrying phenocrysts. This points to an intrusion of the dykes into relatively cold country rocks and thus to rather shallow emplacement level. The composition of biotite granodiorite porphyry dykes is characterized by analyses of major and trace elements in fourteen samples. The rocks are weakly peraluminous with A/CNK = 1.04 to 1.22, have a relatively low to moderate mg number and low Cr and Ni contents. K2O/Na2O varies in the range 0.47-0.83. The spread of abundances for major elements in Harker's plot and chondrite-normalized REE patterns are similar to published data for the central and marginal facies of the Freistadt biotite granodiorite. The porphyries have rather unradiogenic initial (age-corrected for 305 Ma) 87Sr/86Sr values for seven samples in the range of 0.7043-0.7067. Such isotopic composition would be compatible with an origin from a reservoir with fairly low time-integrated Rb/Sr ratio, probably remelted lower crustal (?metabasic) rocks, with or without contribution from the mantle-derived melts. Given the similarities in petrology and whole-rock geochemistry, the published monazite age of ca. 300 Ma for Freistadt granodiorite is likely to be a best age estimate for the biotite granodiorite porphyry dykes. Two dykes of hornblende quartz diorite porphyries are metaluminous, with A/CNK = 0.90 and K2O/Na2O is 0.36 and 0.42. The new 40Ar-39Ar datum of 303±5 Ma for the hornblende in one sample of quartz diorite dyke suggests intrusion of the dyke as within the error co-eval with emplacement of the Freistadt granodiorite and the Kaplice swarm of biotite granodiorite porphyry dykes. It is suggested that the regional distribution of granodiorite porphyry dykes between the major area of the Freistadt pluton near Freistadt and the northernmost outlier near Trhové Sviny approximates the regional extent of the hidden parts of the Freistadt pluton.

Journal of Geosciences, Published by © Czech Geological Society, with support from the Czech Geological Survey.
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ISSN: 1802-6222

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