Granulites of the Larba Block of the Dzhugdzhur–Stanovoi Superterrane: Reconstruction of the Formation Conditions

Abstract

Abstract —We discuss the rocks of the Larba granulite block in the Ilikan zone of the Dzhugdzhur–Stanovoi superterrane. The Larba block is dominated by basic schists and garnet–biotite–orthopyroxene and garnet–biotite–cordierite–sillimanite gneisses (metabasites and metapelites). Calculation of temperatures and pressures of mineral formation was carried out by multi-equilibrium geothermobarometry, which makes it possible to evaluate the degree of equilibrium of mineral compositions along with P–T parameters. The P–T estimates have shown metamorphism of aluminous gneisses under moderate-pressure granulite facies conditions (7–8 kbar, 800–850 °C). Orthopyroxene granulites formed under granulite-amphibolite transition facies conditions. The mineral compositions and parageneses in highly ferrous metabasites permitted estimation of the conditions of metamorphism, P = 4–5 kbar and T = 630–700 °C, and show no influence of earlier granulite facies metamorphism. The bimodal P–T distribution for most samples of aluminous gneisses most probably reflects progressive and near-peak conditions of granulite metamorphism. The time of enderbite magmatism is determined from the upper intercept of discordia with concordia at 2546 ± 52 Ma and should be verified. The age of metamorphic rims over enderbite zircon is 1882 ± 11 Ma. The model Nd age of enderbites, TNd(DM) = 2.57–2.58 Ga, is close to the age of the core of enderbite zircon and differs significantly from the model Nd age of the host metamorphic rocks (2.8–3.0 Ga). The Paleoproterozoic metamorphism of the Larba block rocks regionally coincides with the third stage of collisional granitoid magmatism of the southeastern Siberian craton and records the formation of this structure.