EARLY PROTEROZOIC POSTCOLLISIONAL GRANITOIDS IN SOUTHWESTERN SIBERIAN CRATON

The basement structures of the southwestern Siberian craton include the cis-Sayan and Angara-Kan marginal uplifts that are parts of the Yenisei Ridge. The postcollisional granitoids abundant within the cis-Sayan uplift were earlier assigned to the Sayan and Shumikha complexes but there was no consensus on their geodynamic environments. Geochemical studies of the Sayan granitoids at the junction between the Biryusa block and the Urik-Iya graben fill and the Shumikha granitoids at the junction of the Onot greenstone belt and the Sharyzhalgai block of mafic gneisses have confirmed the postcollisional nature of both complexes. The Shumikha granitoids share some features of A-type granites, including those typical of the Primorsky rapakivi-like complex in the western Baikal region. The Sayan and Shumikha granitoids were dated by the U-Pb zircon age method as 1858±20 and 1871±17 Ma, respectively. Interpreted in the context of 1.8–1.9 Ga granite magmatism on the craton periphery, the intrusion of the two complexes most likely postdated the collision responsible for the formation of an Early Proterozoic supercontinent between 2.0 and 1.9 Ga. Together with the Primorsky and Taraka complexes and the Akitkan volcanoplutonic belt, the Sayan and Shumikha intrusions record the onset of postcollisional extension which failed to cause continent break-up and oceanic crust production rather than the formation of the Proterozoic supercontinent.