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

We report here the evidence of protracted metamorphic evolution of orthopyroxene-sillimanite granulite culminating in UHT metamorphism within the Paleoproterozoic granulite complex of the Angara-Kan block (SW Siberian craton). The Opx-Sil granulites were formed during UHT metamorphism (T = 1070–1100 °C and P = 9 kbar) at ca. 1.8 Ga which was followed by retrograde isobaric cooling down to the temperature around 900–950 °C. At ca. 1.74 Ga Opx-Sil granulites were trapped and partly reworked by a granitic melt with crystallization of perthitic feldspar at about 750 °C. The integration of all data on metamorphism and granitoid magmatism of the Angara-Kan block indicates two-staged metamorphic history. The first stage of the granulite facies metamorphism occurred in the time range of 1.89–1.85 Ga and was terminated by the emplacement of the 1.84 Ga post-collisional granites, the second stage of metamorphism occurred at UHT conditions and culminated in 1.78–1.8 Ga. The UHT metamorphism including the isobaric cooling retrograde stage and granite magmatism had a duration of 30 to 60 million years from 1.78–1.8 to 1.74–1.75 Ga.