The petrogenesis of albitized Early-Permian trachyandesites from Świerki quarry (Lower Silesia, Poland) - constraints on spilitization supported by mineralogical and geochemical data

Abstract

Trachyandesites from Świerki quarry (Lower Silesia, Poland), related to the post-collisional extension-related magmatic activity developed within the Intra-Sudetic Basin during Early-Permian times, are well-known for the occurrence of mineral associations typical of spilites (low-grade metamorphic rocks affected by Na-metasomatism). This paper deals with the reconstruction of the evolution of these rocks and puts constraints on the spilitization mechanism and its role in redistributing main and trace elements based on cathodoluminescence microscopy and spectroscopy combined with electron microprobe analysis. Our study reveals that the trachyandesites were strongly affected by low-temperature albitization, which occurred during the post-magmatic stage of the rock formation. The replacement of primary green luminescent andesine-labradorite (An39-50Ab47-58Or2–3) by weakly-luminescent to dark-brown luminescent secondary albite (Ab∼99 mol.%) was related to the CO2-induced, crystallographically-controlled, interface-coupled dissolution-reprecipitation mechanism, triggered by hydraulic fracturing of fluids and/or contractional cooling forces. The replacive albite forms isolated and homogenous patches along polysynthetic twinning planes of andesine-labradorite and/or within intergranular fractures. It is closely associated with such minerals as calcite, chlorite and kaolinite, and exhibits enhanced microporosity. Moreover, the low-temperature alteration was accompanied by physicochemical fluctuations (alkaline-acidic) during rock formation as revealed by two generations of fluoroapatite: (1) violet-pink (REE3+-activated) overprinted by (2) yellowish (Mn2+-activated) luminescent domains. We also report that primary (green luminescent) plagioclases (andesine-labradorite) are rimmed by bright-blue luminescent Ab-rich (An2-7Ab34-61Or32–64) alkali-feldspars followed by dull-blue luminescent Or-rich alkali-feldspars (An2Ab38-44Or54–60). The bright-blue luminescent crystals contain higher amounts of BaO (∼0.59 wt.%) and TiO2 (∼0.08 wt.%) than the dull-blue luminescent domains (∼0.12 and 0.03 wt.%, respectively). Hence, the difference in CL intensity in blue wavelengths are ascribed to the presence of both Al-O−-Al centres, related to a coupled KSi-BaAl substitution, and Al-O-Ti bridges. Not only the geochemical affinities of the rocks are peculiar to those reported for within-plate basalts, but also reveal similarities with arc-related geotectonic environments. Trachyandesites sourced from enriched subcontinental lithospheric mantle source owing to the strong LREE and LILE enrichment relative to Nb and Ta, while the presence of positive ZrHf anomalies coupled with low Nb/U ratios points to the involvement of crustal material during magma evolution. The spilitization has predominately resulted in the increase of Cs and decrease of Sr concentration among strongly-altered samples, while its influence on REE distribution was insignificant.