The epilogue of Palaeoproterozoic tectonics in Eastern Sarmatia (W. Russia): insights from ~2 Ga bimodal post-collisional magmatism of the Mikhailovsky far-field continental back-arc basin

In the Palaeoproterozoic Mikhailovsky far-field continental back-arc basin of Eastern Sarmatia, the most remote from the subduction trench, metasiliciclastic strata contain sheets of metabasalts, metadacites, and metarhyolites along with subvolcanic bodies of porphyric granite. The variably contaminated metabasalts (εNd_(T) = −4.8 to −7.1) have high concentrations of Sr, low Cr/Ni, (Gd/Yb)_n < 2, and Ti/Y < 500, suggesting a water-rich melting of a shallow source in the lithospheric mantle. Their differentiation was controlled by the subordinate role of plagioclase and a predominance of pyroxenes over olivine in the cumulus association. Melting of that older lithospheric mantle occurred in a flat-subducting oceanic slab. Spatial melting of the Archaean continental crust is caused by rising basic magmas accompanied by fluids under increasing heat flow. The 2.07–2.05 Ga old A2-type felsic volcanics and 2.02 Ga old porphyric granite were formed as a result of shallow, high-temperature crustal melting mainly of the Archaean TTG association under the influence of heat and fluids during intrusion of the basaltic magmas into the crust. This last pulse of the felsic magmatism after completion of the subduction is the terminal event of Palaeoproterozoic tectonics in Eastern Sarmatia.