Hybrids, magma mixing and enriched mantle melts in post-collisional Variscan granitoids: the Rastenberg Pluton, Austria

Abstract

Abstract The composite Rastenberg Pluton in the South Bohemian Massif preserves an example of generation of relatively homogeneous granitoid hybrids by mixing of mafic and felsic magmas. Metaluminous melagranites and quartz monzonites, the main lithologies of the pluton, are interpreted to be hybrids. In contrast, a slightly peraluminous biotite granodiorite is considered to be a lower-crustal melt. In addition, abundant mafic ultrapotassic enclaves, country-rock lamprophyres and quartz monzodiorite bodies represent distinct lithospheric mantle-derived magmas, which were only slightly modified by fractionation and/ or magma mixing. Almost continuous linear chemical and isotope correlations joining the enclaves, quartz monzonites, melagranites and granodiorites indicate the importance of a mixing process in the generation of these rocks. Incompatible elements decrease with increasing silica and moderate negative Eu anomalies disappear towards the granodioritic endmember. Pb, Sr and Nd isotopes show typical crustal values in all granitoids but are more radiogenic in the ultrapotassic endmember. In the Rastenberg Pluton, the interaction of mantle- and crustal-derived magmas has produced unequivocal petrographic, chemical and isotopic evidence for mixing and mingling. Because similar features are lacking in most other Variscan plutons, we suggest that mantle magmas were not substantially involved in their genesis. Nevertheless, small-volumes of hybrids, involving variably enriched mantle-derived melts, do crop out locally throughout the central Variscides. In view of the generally strongly enriched nature of these small volume hybrid magmas, we suggest that voluminous mantle melting and large-scale magmatic underplating are unlikely to have occurred.