Paleoproterozoic OIB- and MORB-Type Rift Volcanics of the Kursk Block, Eastern Sarmatia: Petrology and Geodynamics

Abstract

The metabasites of the Tim Formation occur in the Paleoproterozoic Orel, Tim–Yastrebovka, and Avilsky synforms in the Archean Kursk block of Eastern Sarmatia. The volcanics are picrites, picrobasalts, basalts, and basaltic andesites metamorphosed to the greenschist and epidote-amphibolite facies. Their zircon (SIMS) age is 2099 ± 8 Ma. Based on their (Gd/Yb)n and Ti/Y ratios, the volcanics of the Tim Formation are subdivided into two types. The first one is OIB-type metabasites with (Gd/Yb)n > 2 and Ti/Y > 500. The parental melts of these rocks corresponded to ferropicrite and were derived by melting a garnet-bearing mantle source. The rocks of the second type are MORB-type volcanics with (Gd/Yb)n < 2 and Ti/Y < 500, which implies that the magma was derived from a garnet-free source at a relatively shallow depth. Their parental melts could be variously enriched MORB-type picrites. These magmas, which are contrastingly different in petrogenesis but had approximately equal age, were likely generated at various interaction stages between the asthenospheric mantle and eclogitized oceanic crust, subcontinental lithospheric mantle, and continental crust in the destruction environment of a subducted oceanic slab at the accretionary–collisional interaction between the Kursk domain and Volga–Don orogen.