U-Pb isotope and REE composition of zircon from the pyroxene crystalline schists of the Irkut terrane, Sharyzhalgai Uplift: Evidence for the Neoarchean magmatic and metamorphic events

Abstract

The Early Precambrian granulitegneissic com� plexes are typically characterized by a long evolution� ary history, which involves formation of their pro� toliths and repeated metamorphism related mainly with collisional events. Zircons from highgrade meta� morphic rocks usually consist of cores inherited from the protolith, and one or several mantles and rims that were formed during later metamorphism or partial melting. Highresolution methods of U-Pb dating make it possible to determine the age of cores and rims. However, knowledge of the traceelement com� position of zircon is required in order to interpret cor� rectly the obtained dates, because its zones that were grown or recrystallized at hightemperature metamor� phism significantly differ in terms of REE distribution from their magmatic analogues (1-3). This paper reports the results of study of zircons from highgrade basic rocks, i.e., pyroxene crystalline schists, which allowed us to determine the formation time of their magmatic protoliths and metamorphism, as well as to estimate the conditions of zircon formation. The available data indicate that hightemperature metamorphism and granite formation in the Irkut block occurred in several stages. It was reliably estab� lished that Paleoproterozoic metamorphism and related granite and charnockite formation spanned a time range of 1.88-1.85 Ga. These events were recorded in the formation of metamorphogenic zir� cons, their rims, and baddeleyite in the metamorphic rocks, as well as magmatic zircons in the granitoids (4-7). The lower age boundary of the Neoarchean stage of granulite metamorphism is constrained by the crystallization age of magmatic zircon from gabbro (2649 ± 6 Ma) metamorphosed under the granulite� facies metamorphism (7). Vein granites cutting across the crystallization schistosity in metagabbro have an age of 2562 ± 20 Ma. The Neoarchean granulite meta� morphism and collisional granite formation of close age (~2.6 Ga and 2.53 Ga) were also established in the adjacent Kitoi granulite gneiss block (6).