Early Proterozoic Ilikta Formation: a Record of the Akitkan Orogen Evolution (Southern Siberian Craton)

Abstract

Abstract —New petrographic, geochemical, and geochronological (LA-ICP-MS) data have been obtained for the Early Proterozoic upper Ilikta Formation of the Sarma Group within the Baikal uplift, a basement inlier of the Siberian craton. The Ilikta Formation consists of polymictic and graywacke sandstones and shales of graywacke siltstone and silty-pelitic mudstone varieties, identified from major-element chemistry and petrography. The mineralogy and chemistry of the samples indicate their origin from a felsic source. The Ilikta Formation rocks eluded the Early Proterozoic regional metamorphism but underwent secondary alteration to different grades associated with early Paleozoic collisional events. The Ilikta Formation was deposited during the 1.91–1.95 Ga interval, as constrained by the ages of regional metamorphism (1.95–1.98 Ga) and granitic intrusion (1.86–1.91 Ga). These estimates are consistent with dating of detrital zircons from the Ilikta Formation sandstone which showed the youngest ages of 1955–1993 Ma. The detrital material for the Ilikta Formation deposition was apparently derived from the igneous and metamorphic rocks of the Early Proterozoic Akitkan orogen, and some amount of material additionally came from the adjacent Aldan superterrane located in the east. The reconstructed geodynamic environment of sedimentation, together with the petrography and chemistry of rocks, allow interpreting the Ilikta Formation of the Sarma Group as molasse accumulated in extension basins after the formation of the Akitkan orogen.