Fractional crystallization, mafic replenishment and assimilation in crustal magma chambers: geochemical constraints from the Permian post-collisional intermediate-composition volcanic suite of the North-Sudetic Basin (SW Poland)
The Permian intermediate-composition lavas of the North-Sudetic Basin represent a high-K calc-alkaline suite emplaced in an extensional, intracontinental, post-collisional setting in the eastern part of the European Variscan belt. The lavas, in a total volume of over 100 km3, erupted from fissure vents or small shield-type volcanoes in several episodes separated by repose and sedimentation periods. An idealised eruptive episode comprised basaltic trachyandesites (plagioclase- phyric, clinopyroxene lavas) followed by predominant, main-series basaltic andesites (weakly porphyrytic, two-pyroxene microcrystalline lavas) and evolved basaltic andesites (weakly porphyrytic, two-pyroxene fine-grained lavas). This volcanic suite originated in magmatic systems where differentiation processes evolved with time from (I) fractional crystallization, producing the basaltic trachyandesites, through (II) fractional crystallization coupled with mafic replenishment, resulting in the main series basaltic andesite lavas, to (III) fractional crystallization, mafic replenishment and minor crustal contamination, producing the evolved basaltic andesites. The fractionating mineral assemblages changed during the successive stages and, apart from plagioclase, olivine, clinopyroxene and Fe-Ti oxides, included ortho- pyroxene (stage II and III) and apatite with zircon (stage III). The general trace element characteristics of the volcanic rocks (enrichment in Th, LILE, Nb and Zr, but with high Th/Nb and La/Nb ratios) are transitional between those of extension-related within-plate lavas and active continental margin lavas. These characteristics are inherited from enriched lithospheric mantle sources carrying a crustal signature related to subduction processes during the earlier stages of the Variscan orogeny.
