Geochemical and isotopic composition and inherited zircon ages as evidence for lower crustal origin of two Variscan S-type granites in the NW Bohemian massif

Abstract

Geochemical and Nd–Sr isotopic compositions and U–Pb zircon ages of two Variscan granites (Neunburg and Oberviechtach) from southern Oberpfalz, NW Bohemian massif, have been investigated in order to place constraints on their formation and on the crustal reworking. Both granites exhibit similar mineralogical, chemical and isotopic characteristics. They have peraluminous compositions (A/CNK ratios 1.2–1.3) and display high K2O/Na2O ratios of 2.2–2.3, consistent with typical S-type granites. In terms of trace elements, they show an enrichment of LREE and strong fractionation between LREE and HREE (LaN/YbN ratios 46 to 60). Compared with the primordial mantle, distinct negative anomalies of several trace elements (Ba, Sr, Nb and Ti) are also observed in both granites. They are further characterised by low initial eNd-values of −6.2 to −8.2 and high initial 87Sr/86Sr ratios of 0.7114 to 0.7147. Zircon U–Pb data indicate that the intrusion of both granites shortly post-dates the HT–LP metamorphism of the Moldanubian basement and crystallised at about 320 Ma. The samples studied contain zircons mostly having xenocrystic cores with diverse morphologies. These inherited zircons have Early Proterozoic to Early Palaeozoic ages. This points to melting of sources comprising substantial sedimentary rocks. The LaN/YbN and TbN/YbN ratios of both granites are the highest so far reported from granitoids within this region. Melting of lower crustal rocks leaving garnet as a restite phase in the source provides a viable mechanism to reproduce the REE characteristics.