Post-collisional two-stage magmatism in the East Sarmatian Orogen, East European Craton: evidence from the Olkhovsky ring complex

The Olkhovsky ring complex (ORC) is composed of a two-phase magmatic rock association with quartz dioritequartz monzodiorite-granodiorite suite in the outer ring (OR) and trondhjemite as the inner stock (IS). Zircon U–Pb dating reveals that the rocks of the OR and IS were emplaced at 2070 ± 9 Ma and 2044 ± 13 Ma, respectively. The OR rocks are intermediate and felsic, metaluminous, low to moderate alkali. Both rocks are enriched relative to primitive mantle in Rb, Ba, Sr, K, and depleted in Nb and Ti. The OR quartz diorite and IS trondhjemite are characterized by positive values εNd(T) = 2.2 and 1.6, respectively. The ORC belongs to intermediate I-type granite with nil or weak crustal contamination, whereas the IS trondhjemite shows adakite-like features that were contaminated with components from earlier magmatic pulses. The low degree of contamination of the hypothetical melt, its basaltic-andesite composition, the elevated potassium and magnesium contents, and trace element patterns of whole rock and zircon, suggest garnet-free lower crust or metasomatized mantle wedge as the magma source for the OR, in a post-collisional setting. The mechanism for the inner stock trondhjemite was probably similar, although the magma source was characterized by the presence of garnet with a moderate degree of melting.