Shoshonitic magmatism in the Paleoproterozoic of the south-western Siberian Craton: an analogue of the modern post-collision setting

The Siberian Craton assembled in a Paleoproterozoic episode at about 1.88 Ga by the collision older blocks, followed at about 1.86 Ga by post-collisional felsic magma. We have found a set of extremely fresh mica-bearing lamprophyre-looking rocks within the Sharyzhalgay metamorphic complex of the south-western Siberian Craton. Zircon from these rocks yields a U-Pb TIMS age of 1864.7 ± 1.8 Ma, which coincides perfectly with the peak of the post-collisional granite ages and postdates by ~15 Ma the peak of ages obtained for metamorphism. The same ages were reported earlier for a mafic dyke with ocean island basalt (OIB) geochemical signatures and a Pt-bearing mafic-ultramafic intrusion found in the same region. Mineralogy, major and trace element geochemistry and Sr-Nd-Pb isotopes show that the studied rocks (1) have shoshonitic affinity, (2) are hybrid rocks with mineral assemblages which could not be in equilibrium, (3) where derived by recycling of an Archean crustal source and (4) resemble post-collision Tibetan shoshonitic series. The genesis of these rocks is considered to be due to melting of crustal lithologies and metasomatized lithospheric mantle within a subducted slab. Some of the resulting melts ascended through the lithospheric column and fractionated to low-Mg absarokites, whereas other melts were contaminated by orthopyroxenitic mantle material and attained unusual high- Mg mafic compositions. According to our model, the post-collisional magmatism (shoshonite-and OIB-type) occurred due to upwelling of hot asthenosphere through a slab window, when the active collision ceased as a result of the slab break off and loss of the slab pull force. Overall, our study shows that in the Paleoproterozoic shoshonitic melts were emplaced within a similar tectonic setting as seen today in modern orogenic systems.