Expansiveness of selected basaltoid weathering products in Lower Silesia, Poland

Krystyna Choma-Moryl

Instytut Nauk Geologicznych, Uniwersytet Wrocławski, pl. Maksa Borna 9; 50-204 Wrocław, Poland, e-mail: kmor@ing.uni.wroc.pl

Key words:  expansive soils, swelling strain, clay minerals, basaltoid weathering products

Abstract
The weathering products of Tertiary basaltic rocks in Lower Silesia comprise clays that contain various amounts of smectite-group minerals, illite, kaolinite, quartz and coarser fragments of non-weathered rock. Deposits of weathering products whose volume is of economic importance occur in Krzeniów near Złotoryja, Męcinka near Jawor and Dunino near Krotoszyce. The expansiveness of these weathering products is evaluated using laboratory analyses of swelling and shrinkage and the empirical nomograms of van der Merwe and Seed. The measured results of swelling of products dominated by smectite-group minerals, and of halloysite weathering products, indicate that the swelling depends on mineral composition. The smectite clays (Krzeniów and Jawor-Męcinka) display swelling strain ep in the range 20.3-31.8% whereas, for the halloysite products (Dunino), the range is 10.0-18.3%. The lowest linear shrinkage characterised the samples from Jawor-Męcinka and the highest, the weathering products with 79-81% clay fraction from Dunino. Most of the basaltic weathering products fall in the van der Merwe nomogram fields of low and medium potential expansiveness. High and very high potential expansiveness is shown by the Dunino weathering products. On the Seed nomogram, the weathering products plot within the field of low expansiveness except for some high and very high expansiveness values from the uppermost halloysite horizons at Dunino.

Geologia Sudetica, 39: 11-97.

Late Palaeozoic lamprophyres and associated mafic subvolcanic rocks of the Sudetes (SW Poland): petrology, geochemistry and petrogenesis

Marek Awdankiewicz

Wrocław University, Institute of Geological Sciences, Department of Mineralogy and Petrology, ul. Cybulskiego 30, 50–205 Wrocław, Poland; e-mail: marek.awdankiewicz@ing.uni.wroc.pl

Key words:  lamprophyres, Sudetes, mineral chemistry, trace elements, Sr-Nd isotopes, petrogenesis

Abstract
Lamprophyric magmatism in the Sudetes, in the eastern part of the European Variscides, occurred during a period of post-collisional extension in the Carboniferous. The lamprophyres (minettes, vogesites, spessartites, kersantites) and associated mafic rocks (monzonites, micromonzodiorites) were emplaced as dyke swarms and as scattered veins that cut the crystalline basement and, locally, the overlying molasse deposits. The dyke swarms are adjacent to major regional dislocations, represent distinctive magmatic centres that are related to separate magmatic systems and each are characterized by specific parental melts that have undergone individualised shallow-level differentiation processes. The two largest dyke swarms are associated with the Karkonosze and Kłodzko-Złoty Stok granitoid massifs: these show the widest geochemical and petrographic variation, due to more advanced differentiation in long-lived magmatic systems. In contrast, a small dyke swarm emplaced in the SW part of the Orlica-Śnieżnik Dome, unrelated to granites, is strongly dominated by minettes only. Geochemical characteristics of the mafic rocks studied herein vary from (ultra)potassic in the minettes to calc-alkaline in the micromonzodiorites and from primitive (Mg# = 80-60 in many lamprophyres) to evolved (Mg# down to 30 in some micromonzodiorites). Some richterite minettes show Nb-enriched trace element patterns, but negative Nb anomalies are more typical. Richterite minettes posesess εNd₃₀₀ and ⁸⁷Sr/⁸⁶Sr₃₀₀ values that range from +1.9 to -8.3, and from 0.7037 to 0.715, respectively. The other rocks in this study show negatively correlated Nd and Sr isotopic ratios, between these extremes. The geochemical data suggest three types of mantle source for the lamprophyres and associated mafic rocks: (1) An asthenospheric, depleted and later re-enriched source; (2) A lithospheric source contaminated by subducted crustal rocks; (3) A lithospheric source metasomatized by subduction-related fluids. The richterite minette magmas originated from low degrees of partial melting, under high H₂O/CO₂ conditions, of garnet-phlogopite-peridotites. The Nb-enriched and Nb-depleted minettes are derived from sources (1) and (2), respectively. Kersantite magmas originated from source (3). The factors of source mixing, variable depths and degrees of melting, and aggregation of melts all influenced the compositions of other primitive minette and vogesite magmas. The other rocks studied (spessartites, monzonites, micromonzodiorites) are variably differentiated. Zoning and other disequilibrium textures in phlogopite, biotite, amphibole and clinopyroxene phenocrysts, together with the presence of xenocrysts, xenoliths and enclaves (cognate, restitic, migmatitic) constrain several processes that were involved in the shallow-level evolution of magmas: mixing, fractional crystallization, assimilation of crustal rocks. However, post-magmatic replacement of the igneous phases by albite, chlorite, epidotes, actinolite, blue amphiboles, titanite, carbonates, prehnite, pumpellyite and grossularite-andradite partly obscures the magmatic assemblages and textures. There are four more general results of this study. First, there is evidence for a strong heterogeneity of the upper mantle and of the presence of subduction-modified mantle beneath the Sudetes during the Late Palaeozoic. Second, the lamprophyre magmas originated and evolved in spatially and petrologically distinct, vertically extensive magmatic systems that spanned the asthenospheric and lithospheric mantle and the lower/middle crust. Third, a broad spectrum of source-related and shallow-level magmatic processes gave rise to the emplacement of primitive, mantle-derived magmas and of variably evolved magmas. Fourth, close links existed between late Variscan tectonics, the location of lamprophyric magmatism, and the shallow-level emplacement processes of mafic dykes.