Stages and conditions of metamorphism of mafic granulites in the Early Precambrian complex of the Angara–Kan terrane (southwestern Siberian Craton)

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).

Two episodes of metamorphism in the rocks of the eastern Kitoi block (Sharyzhalgai uplift of the Siberian platform) according to the garnet amphibolite data

“…Оценки РТпараметров метаморфизма (показаны эллипсами) и предполагаемые РТтренды двухэтапной эволюции метаморфизма (серые стрелки) для гранатовых амфиболитов восточной части Китойского блока. Линиями показаны экспериментальные данные об устойчивости минеральной ассоциации Cpx+Opx+Pl: WN91 - [Winther, Newton, 1991], PB95 -[PatinoDouce, Beard, 1995], BL91 - [Beard, Lofgren, 1991], CW67 - [Choudhuri, Winkler, 1967], S81 - [Spear, 1981]. Пунктирной линией (+Grt) показана линия появления граната в основных породах [Green, Ringwood, 1967].…”

“…Lines show experimental data on the Cpx+Opx+Pl assemblage stability field: WN91 - [Winther, Newton, 1991], PB95 - [Patino Douce, Beard, 1995], BL91 - [Beard, Lofgren, 1991], CW67 - [Choud huri, Winkler, 1967], S81 - [Spear, 1981]. The dotted line (+Grt) indicate the line of garnet appearance in mafic rocks [Green, Ringwood, 1967].…”

Orthopyroxene-sillimanite granulites of the Angara-Kan block (SW Siberian craton): Constraints on timing of UHT metamorphism

“…The metamorphic history of the Angara-Kan block includes two episodes of high-grade metamorphism (Turkina and Sukhorukov, 2015). The first one occurred at 1.85-1.89 Ga (Urmantseva et al, 2012;Turkina and Sukhorukov, 2015) and reached granulite facies.…”

“…The metamorphic history of the Angara-Kan block includes two episodes of high-grade metamorphism (Turkina and Sukhorukov, 2015). The first one occurred at 1.85-1.89 Ga (Urmantseva et al, 2012;Turkina and Sukhorukov, 2015) and reached granulite facies. The second one comprises both granulitic and ultra-high temperature (UHT) metamorphism reaching up to T = 1070-1100°C and P = 9 kbar (Sukhorukov et al, 2018a, b) and took place at 1.77-1.8…”

“…If the older ages (1.87-1.83 Ga) of this rock are interpreted to be Pb-loss results (Lines 271-272, which are pure assumptions), why cannot these ~1.8 Ga ages? VS: Based on early geochronological data the first stage of high-grade metamorphism occurred at 1.87-1.89 Ga (Urmantseva et al, 2012;Turkina and Sukhorukov, 2015) and reached granulite facies (see, lines 50-51). The second stage of granulite and ultra-high temperature (UHT) metamorphism reaching up to T = 1070-1100°C and P = 9 kbar (Sukhorukov et al, 2018 a, b) and took place at 1.78-1.8 Ga (Turkina and Sukhorukov, 2015;Sukhorukov et al, 2018 b).…”