Last stage of Variscan granitoid magmatism in the Strzelin Massif (SW Poland): petrology and age of the biotite-muscovite granites

Abstract

New petrographic and geochemical data show some differences between Variscan Bt-Ms granites occurring either as small plutons or dykes in the Strzelin Massif (SW Poland). The granites of the Gromnik and Górka Sobocka plutons are rich in micas and crystallized from "wet" magmas; the granites in the dykes and in the Gębczyce pluton are mica-poorer and cordierite-bearing rocks, derived from “dryer” magmas. The lower initial eNd values in the Bt-Ms granites of the dykes, compared with those in the plutons, reflect a more "crustal" signature of the former, possibly due to local crustal assimilation, via AFC, shortly before emplacement. Much more radiogenic initial 87Sr/86Sr ratios in the dykes, up to 0.726, further suggest the involvement of extraneous, hydrous crustal fluids enriched in 87Sr during the evolution of late-stage magma derivatives. The new U-Pb SHRIMP zircon age of 296 ± 6 Ma for the Gębczyce Bt-Ms granite shows that this body belongs to the third stage of magmatism in the Strzelin Massif. The U-Pb SHRIMP zircon data for the Bt-Ms granite dykes provide ages similar to those of their host rocks: c. 295 Ma for the Gęsiniec tonalite and the enclosed Bt-Ms granite, and c. 285 Ma for the Strzelin biotite granite and its Bt-Ms granite dykes. These new data from peraluminous rock-types complement our previous studies focused on the tonalites, granodiorites and biotite granites, and shed light on the late-stage igneous evolution of the Strzelin Massif.