Tectonostratigraphy of the Neoproterozoic of the Northern Kiselikha Terrane (Yenisei Ridge), a Fragment of the Siberian Paleocontinent Active Margin

The Kiselikha Terrane is a continental block in the NW part of the Yenisei Ridge, which is composed of Precambrian rocks and separated from the main (cratonic) part of the ridge by the Isakovka ophiolite belt. The tectonic nature of this block is debatable owing to incomplete and contradictory data on the composition of rocks, their age, and geodynamic setting. The paper provides new data on the geology of the northern part of the terrane within a segment of coastal outcrops of the Yenisei River between the Porozhny and Osinovka granitic plutons. Three stratigraphic subdivisions are identified. (1) The Kiselikha Formation composes almost the entire studied area and includes foliated retrograde metamorphic rocks, mostly, of unclear protolith. The study of zircon grains, which were extracted from seven samples, revealed the identical “detrital” age distribution with the predominant Neoproterozoic ages in a range of 1000–800 Ma along with Archean and Paleoproterozoic clusters. The previously suggested products of synchronous rhyolite–dacitic volcanism are absent. (2) The Ust-Kutukas Formation composes a local area, where it includes (meta-)pillow basalts similar to a stratotype. (3) The Ust-Porozhnaya Unit (recognized in this work) was observed in two local outcrops. It is composed of metasedimentary rocks with erosion products of granitoids 790–700 Ma in age. Taking into account the previously published results, it is suggested that, in the first half of the Neoproterozoic, the Kiselikha Terrane was a part of the active margin of the Siberian Paleocontinent. It began separating in the middle of the Neoproterozoic due to back-arc continental rifting, which further led to the opening of the Isakovka oceanic basin. The synrift sedimentary material of the Kiselikha Formation was washed out from the pre-Neoproterozoic gneisses, intruded by the Early Neoproterozoic granites. Rifting was accompanied by intraplate basaltic magmatism, which was followed by eruption of pillow basalts of the Ust-Kutukas Formation. Serpentinite bodies, widespread in this part of the terrane, are not associated with crustal oceanic rocks and were probably exhumed during hyperextension of the continental crust. Subduction below the outer edge of the Kiselikha Block lasted also in the second half of the Neoproterozoic (790–620 Ma). At the beginning of the Vendian, the Isakovka oceanic basin was squeezed between the Siberian Craton and Kiselikha Terrane, which again became part of the parental continent. The rocks of the Kiselikha and Ust-Kutukas formations together with serpentinite bodies were thrust onto the terrane from the east.