Paleoproterozoic granitoids of the Don terrane, East-Sarmatian Orogen: age, magma source and tectonic implications

摘要

The Pavlovsk complex granitoid plutons and associated gabbro-diorites are confined to the East Sarmatian Orogen (ESO) that compose the western part of the junction between the Sarmatian and Volgo-Uralia segments of the East-European Craton. The plutons form the largest batholiths with an area of up to 2000 km(2) and a total length of more than 330 km. The conditions for their emplacement and crystallization, sources of melts and tectonic position remain debatable. Here we present new results on geochronological, geochemical and isotope-data on the Pavlovsk complex granitoids and associated gabbro-diorites and discuss these in conjunction with previously published data. The geochemical characteristics, such as elevated concentrations of Mg, K, Sr and Ba (usually > 500 ppm), ratios (La/Yb)(N) (25-73) and Sr/Y (23-223) in the absence of Eu anomalies of the Pavlovsk-type plutons are similar to high-Mg Archean granites. Zircon U-Pb data indicate the formation of the granitoids and associated gabbro-diorites during a short span at 2060-2075 Ma. The magma emplacement occurred in a transitional environment (from late-collisional to post-collisional) after the collision of two crustal segments (Sarmatia and Volgo-Uralia). We identify a significant ancient Archean continental crust in the magma source for the northern plutons and leucogranites including the presence of Archean zircon cores and Archean model T-Nd (DM) ages. All the plutons are dominated by a mixed magma source generated through the interaction of juvenile lower crustal and enriched mantle components. The Nd isotopic composition of the studied plutons is significantly more radiogenic as compared with the host Archean rocks (epsilon Nd (T) from + 0.3 to -4.2). Our geochemical data suggest that that the mantle source of the gabbro-diorites and granitoids were modified (metasomatized) by subduction-related processes. The contrasting geochemical and isotopic features of the granitoids in the ESO as compared to the Pavlovsk granitoids suggest different sources for the parent melts.