ANCIENT PLAGIOGNEISSES OF THE ONOT BLOCK OF THE SHARYZHALGAI METAMORPHIC MASSIF: ISOTOPIC GEOCHRONOLOGY

Abstract

The oldest rocks found at Precambrian shields worldwide are so-called gray gneisses, which are high-grade metamorphic granitoids of predominantly tonalite composition. These rocks are often older than 3.6 Ga and mark the stage when the first sialic crust was produced. Such rocks with an age close to 3.6 Ga were found and dated in the former Soviet Union in the Ukrainian Shield and Omolon Massif in northeastern Siberia. The identification and reliable dating of each new area with these ancient rocks provides better insight into the timing of the origin of the Earth’s early crust and the scale and mechanisms of these processes.