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Volcanism in a late Variscan intramontane trough: the petrology and geochemistry of the Carboniferous and Permian volcanic rocks of the Intra-Sudetic Basin, SW Poland

Awdankiewicz M.

Geologia Sudetica

1999

Vol. 32, nr 2

83-111

The Carbonifeorus-Permian volcanic rocks of the Intra-Sudetic Basin represent products of late- to post-collisional volcanism associated with extension within the eastern part of the Variscan belt of Europe. The volcanic succession is subdivided into the older, calc-alkaline suite (the early and late Carboniferous) and the younger, mildly alkaline suite (the late Carboniferous and early Permian). The rhyodacites with subordinate basaltic andesites and andesites of the older suite show convergent plate margin affinities. The rhyolitic tuffs, rhyolites with less widespread trachyandesites and basaltic trachyandesites of the younger suite are largely characterised by within-plate affinities, with some gradations towards convergent plate margin affinities. This geochemical variation compares well with that found in some Tertiary-Recent extensional settings adjacent to former active continental margins (e.g. the Basin and Range province of the SW USA). The parental magmas for each suite of the Intra-Sudetic Basin possibly originated from similar, garnet free mantle sources at relatively shallow depths (within the subcontinental mantle?), but at variable degrees of partial melting (lower for the mildly alkaline rocks). The convergent plate margin-like geochemical signatures of the volcanic rocks may either have been inherited from their mantle sources, or be related to the assimilation of crustal rocks by the ascending and fractionating primary magmas. The intermediate-acidic rocks within each suite mainly originated due to fractional crystallisation of variable mineral assemblages equivalent to the observed phenocrysts (mainly plagioclase and pyroxenes, with hornblende and biotite in the calc-alkaline suite, and K-feldspar in the mildly alkaline suite). The trace element patterns of the volcanic rocks were also strongly influenced by fractionation of accessory minerals, such as spinels, ilmenite, zircon, apatite and others. The petrographic evidence (e.g. quartz phenocrysts with reaction rims, complexly zoned or sieve-textured feldspar phenocrysts) suggests that assimilation and/or magma mixing processes might also have taken place during the evolution of the magmas.