Late-collisional granites in the Variscan Erzgebirge, Germany

The late-collisional Erzebirge granites (similar to 325-318 Ma) were emplaced at shallow crustal levels in the Variscan metamorphic basement shortly after large-scale extension caused by orlogenic collapse. These granites comprise mildly peraluminious transitional I-S-types and strongly peraluminous S-type rocks, which can be subdivided into three major groups: low-F biotite granites; low-F two-mica granites; and high-F, high-P2O5 Li-mica granites. The highest degree of differentiation is reached in the Li-mica granites, which exhibit strongly elevated concentrations of P, F, Li, Rb, Cs, Ta, Sn, W and U; but very low Ti, Mg, Co, Ni, Sr, Ba, Y, Zr, Hf, Th and rare earth elements. Crystal-melt fractionation is the dominant process controlling the bulk composition of all groups of granites. However, metasomatic processes involving late-stage residual melts and high-T orthomagmatic fluids became increasingly more important in highly evolved units and have modified the abundances of mobile elements (P, F, Li, Rb, Cs, Ba, Sr) in the Li-mica granites particularly. Isotopic and geochemical characteristics suggest that the three granite groups cannot be derived from a common precursor magma. Their discrete compositions are source related, and are attributed to melting of quartzo-feldspathic and pelitic crustal lithologies in different proportions. Granites are common in the central European Variscides, but the Erzgebirge is unusual for the predominance of evolved Li-mica granites associated with economically important Sn, W and U deposits. The abundance of Li-mica granites is attributed to a combination of favourable factors: (1) low degrees of anatectic melting of crustal protoliths; (2) wide distribution of fertile lithologies rich in large-ion lithophile elements and ore elements; (3) extended magmatic differentiation by crystal-melt fractionation and subsequent autometasomatism.