Abstract

We present results of study of mineral assemblages and PT-conditions of metamorphism of mafic garnet–two-pyroxene and two-pyroxene granulites in the Early Precambrian metamorphic complex of the Angara–Kan terrane as well as the U–Pb age and trace-element and Lu–Hf isotope compositions of zircon from these rocks and the zircon/garnet REE distribution coefficients. The temperatures of metamorphism of two-pyroxene granulites are estimated as 800–870 to ~900 °C. Mafic garnet–two-pyroxene granulites contain garnet coronas formed at 750–860 °C and 8–9.5 kbar. The formation of the garnet coronas proceeded probably at the retrograde stage during cooling and was controlled by the rock composition. The age (1.92–1.94 Ga) of zircon cores, which retain the REE pattern typical of magmatic zircon, can be taken as the minimum age of protolith for the mafic granulites. The metamorphic zircon generation in the mafic granulites is represented by multifaceted or soccerball crystals and rims depleted in Y, MREE, and HREE compared to the cores. The age of metamorphic zircon in the garnet–two-pyroxene (~1.77 Ga) and two-pyroxene granulites (~1.85 and 1.78 Ga) indicates two episodes of high-temperature metamorphism. The presence of one generation (1.77 Ga) of metamorphic zircon in the garnet–two-pyroxene granulites and, on the contrary, the predominance of 1.85 Ga zircon in the two-pyroxene granulites with single garnet grains suggest that the formation of the garnet coronas proceeded at the second stage of metamorphism. The agreement between the zircon/garnet HREE distribution coefficients and the experimentally determined values at 800 °C suggests the simultaneous formation of ~1.77 Ga metamorphic zircon and garnet. Zircon formation by dissolution/reprecipitation or recrystallization in a closed system without exchange with the rock matrix is confirmed by the close ranges of 176Hf/177Hf values for the core and rims. The positive εHf values (up to +6.2) for the zircon cores suggest that the protolith of mafic granulites are derived from depleted-mantle source. The first stage of metamorphism of the mafic granulites and paragneisses of the Kan complex (1.85–1.89 Ga) ended with the formation of collisional granitoids (1.84 Ga). The second stage (~1.77 Ga) corresponds to the intrusion of the second phase of subalkalic leucogranites of the Taraka pluton and charnockites (1.73–1.75 Ga).

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