Sapphirine-Bearing Granulites of the Anabar Shield

Resumo

The paper presents detailed data on the mineralogy, chemical composition, geothermobarometry, and U–Pb zircon isotopic dating of peraluminous sapphirine-bearing crystalline schists, which occur in association with biotite–garnet–sillimanite gneisses, hypersthene–plagioclase–potassic feldspar and high-K hypersthene orthogneisses, and Ti-rich and subalkaline metabasites of the Kilegir Formation of the Daldyn Group at the Anabar Shield. In contrast to known granulites of the Daldyn and Upper Ananbar groups, the rocks of this association are enriched in K, Rb, Ba, Th, and LREE and have elevated (La/Yb)_n. Thermobarometric studies of these rocks yielded the P–T parameters of the origin and evolution of the sapphirine-bearing granulites in the Anabar Shield, with peak values of UHT metamorphism in the ranges of T = 920–1000°C at P = 9–11 kbar. Isotope-geochronological data indicate that the rocks underwent a polymetamorphic evolution. The detrital zircon cores yielded ages of 3.36, 2.75, 2.6, and 2.5 Ga. Later overprinted metamorphic transformations of the detrital zircon formed rims, which are dated at 2.4, 2.3, 2.2, and 1.8 Ga. The timing of formation of the aluminous metasedimentary and associated metamagmatic rocks is estimated in the range of 2.5 to 2.4 Ga. A potential eroded source of the detrital zircons could be hypersthene plagiogneisses and metabasites of the Daldyn Group (whose premetamorphic age was no younger than 3.3 Ga) and products of their metamorphism (dated at about 2.7 Ga), and perhaps, also Na–K granites with an age of about 2.6–2.5 Ga, which still have not been found in the area but known at other shields and are enriched in radioactive (K, Th, and U) and trace elements. An additional source of clastic material for the aluminous sediments could be the two-feldspar magmatic rocks of rhyolite composition found in association with the gneisses and having the same model age.