Late Variscan Sušice dyke swarm (Moldanubian Zone): quartz micromonzodiorite dykes and their pyroxene gabbro xenoliths


Authors: Vrána S

Published in: Bulletin of Geosciences, volume 79, issue 4; pages: 221 - 229; Received 17 June 2004; Accepted in revised form 21 September 2004;

Keywords: pyroxene quartz micromonzodiorite, dyke swarm, gabbro xenoliths, southern Bohemia, Moldanubian Zone,

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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 = 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 87Sr/86Sr = 0.708 and TNdDM ≈ 1.2-1.4 Ga. The dykes carry rare xenoliths of two-augite gabbro (with Fe-rich A1 augite and Mg-rich A2 augite, 87Sr/86Sr also ~ 0.708) and dispersed 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.