Late-Collisional Granites in the Variscan Erzgebirge, Germany

Abstract

The late-collisional Erzgebirge granites (∼325–318 Ma) were emplaced at shallow crustal levels in the Variscan metamorphic basement shortly after large-scale extension caused by orogenic collapse. These granites comprise mildly peraluminous 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.