Stephanian-Early Permian basaltic trachyandesites from the Sławków and Nieporaz-Brodła Grabens near Kraków, Southern Poland

In the Kraków-Lubliniec section of the major Hamburg-Kraków-Dobrogea Fault Zone (HKDFZ), the Stephanian-Early Permian magmatic activity was simultaneous with subsidence/uplift of formed blocks. In the proximity of the uplifted areas the sedimentation started with fanglomerates, distally passing into playa siltstone and was accompanied by volcanism. In the Nieporaz-Brodła graben, basaltic trachyandesite a-a lava flows with autoclastic breccias reach ca. 150 m in thickness. The weathering recorded as adhesive rims around breccia clasts reflects palaeosoil formation during quiescence periods between the eruptions. The eruptions were effusive although scoriaceous lava indicates high volatile content of the magma. The water content before eruption had to be over 2 wt. percent. Only then the crystallisation model predicts accurately the type, composition and order of the crystallising phases. Thus, the basaltic trachyandesite magma originated from a source containing hydrous phase (i.e., amphibole or phlogopite). All the studied basaltic trachyandesites had fayalitic olivine on their liquidus reflecting the crystallisation stage in a magma chamber at crustal depths. During final decompression on the magma ascent due to water exsolution, the olivine was followed by plagioclase, spinel, augite, ilmenite, apatite, K-feldspar and residual high-K rhyolitic glass. The high potassium content of these volcanics was thus a primary feature reflecting the source geochemistry and differentiation trend, and its consequence was potassium metaso- matism. Due to water exsolution the glass was altered, however, homogenous interstitial glass survived in some rocks. The glass is strongly enriched in incompatible trace elements, showing that it is a highly fractionated residual melt fraction of the basaltic trachyandesite magma. Thus, the glass geochemistry reflects the trend of fractional crystallisation indicating that co-occurring K-rich felsic rocks were not derived from the same magma. All the basaltic trachyandesites studied have calc-alkaline to alkaline affinity. They contain fayalitic olivine and are low in MgO 0.5-0.80 and small Eu/Eu* > 0.9-1.0 anomalies suggesting fractionation of plagioclase. The Eu/Eu* anomaly is probably compensated by amphibole retaining in the source. Negative Nb, Ti anomalies suggest Fe-Ti oxide fractionation characteristic for calc-alkaline evolution trend. A significant enrichment in LREE relatively to HREE (La/Yb > 10) indicates subduction-related metasomatism. However, the described tectonic context is inconsistent with subduction related characteristics. Concluding, the observed geochemical characteristics indicate remarkable role of water in magma evolution. The volcanism was related to strike-slip dextral movement along the Kraków-Lubliniec section of HKDFZ, transformed into crustal extension and subsidence, the features typical for the formation of pull-apart basins, in the late stages of the Variscan orogen evolution

Alleged Carboniferous (Viséan) volcanism at the eastern margin of the Moravo-Silesian Basin, Kraków region, Southern Poland

The volcanic rocks, represented by clasts in the breccias occurring within the Viséan Culm sequence at Głuchówki, previously were considered to provide evidence of the earliest record of Variscan volcanism on the eastern margin of Moravo-Silesian Basin in the Kraków area. The breccias have been described as the deposits of submarine slides, accompanying the Viséan volcanic activity. This paper provides evidence of a different mode of origin. The present study, based on new field observations, combined with petrological and geochemical data, showed that volcanic rocks are major framework constituents of a breccia and are accompanied by clasts of spotty hornfels and Culm mudstone. Some clasts originated in erosion of the nearby Permian Zalas rhyodacite laccolith and the spotty hornfelses of the contact aureole, while other clasts came from the Culm mudstones. The breccia framework components are not rounded and poorly rounded, which reflects their short distance of transport. Therefore, an alternative scenario for the genesis of the breccia is proposed. The breccias represent material deposited in erosional pockets formed along the bedding of the Culm mudstones. The red iron oxide shells around clasts may indicate weathering in a warm continental climate, thus constraining the breccia formation to either the Permian or the Early or Late Triassic. Thus, the only well documented Viséan igneous rock in the study area remains the diabase, known from the Klucze borehole. Conversely, no volcanic rocks of this age have been documented in the area under consideration