Tracking the origin of post-Variscan granites through the geochemical diversity of mafic microgranular enclaves: Strzegom-Sobótka Massif, Poland

Abstract

The Strzegom-Sobótka Massif, a composite post-Variscan granitoid intrusion in SW Poland, contains abundant mafic microgranular enclaves that provide crucial insights into crust-mantle interactions during magma evolution. We present new geochemical and Sr-Nd isotopic data from enclaves in its eastern part. These are mostly tonalitic and show narrow isotopic ranges (initial ⁸⁷Sr/⁸⁶Sr: 0.7054–0.7060; εNd(300): -2.9 to -1.8) with relatively young depleted mantle model ages (TDM) (1.24–1.65 Ga). In contrast, western enclaves, previously described in the literature, range from monzodiorite to tonalite and show more variable, more crustal isotopic compositions (initial ⁸⁷Sr/⁸⁶Sr: 0.7079–0.7113; εNd(298): -4.5 to -2.5) and older TDM ages (1.10–2.40 Ga). The western enclaves record extensive chemical and isotopic equilibration with the host hornblende-biotite granite during emplacement, while the isotopic similarity between the eastern enclaves and the host biotite granodiorite reflects derivation from a common juvenile hybrid source. This source incorporated ancient crustal and juvenile mantle-derived components, whereas the western suite was derived from evolved continental crust with amphibolite- and greywacke-like affinities. These contrasting signatures indicate assembly of the pluton from isotopically heterogeneous source domains during post-collisional lithospheric thinning. Geochemical similarities to Permian intermediate volcanic rocks of the North Sudetic Basin further suggest a common melting history within metasomatized subcontinental lithosphere reactivated during post-orogenic extension.