Late-collisional granites in the Variscan Erzgebirge, Germany

The late-collisional Erzebirge granites (similar to 325-318 Ma) were emplac ed at shallow crustal levels in the Variscan metamorphic basement shortly a fter large-scale extension caused by orlogenic collapse. These granites com prise mildly peraluminious transitional I-S-types and strongly peraluminous S-type rocks, which can be subdivided into three major groups: low-F bioti te granites; low-F two-mica granites; and high-F, high-P2O5 Li-mica granite s. The highest degree of differentiation is reached in the Li-mica granites , which exhibit strongly elevated concentrations of P, F, Li, Rb, Cs, Ta, S n, W and U; but very low Ti, Mg, Co, Ni, Sr, Ba, Y, Zr, Hf, Th and rare ear th elements. Crystal-melt fractionation is the dominant process controlling the bulk composition of all groups of granites. However, metasomatic proce sses involving late-stage residual melts and high-T orthomagmatic fluids be came 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 th at the three granite groups cannot be derived from a common precursor magma . Their discrete compositions are source related, and are attributed to mel ting of quartzo-feldspathic and pelitic crustal lithologies in different pr oportions. Granites are common in the central European Variscides, but the Erzgebirge is unusual for the predominance of evolved Li-mica granites asso ciated with economically important Sn, W and U deposits. The abundance of L i-mica granites is attributed to a combination of favourable factors: (1) l ow degrees of anatectic melting of crustal protoliths; (2) wide distributio n of fertile lithologies rich in large-ion lithophile elements and ore elem ents; (3) extended magmatic differentiation by crystal-melt fractionation a nd subsequent autometasomatism.