Protoliths of Paleoproterozoic calciphyres from the Irkut block (Sharyzhalgai uplift of the Siberian craton): composition and origin
L.N. Urmantseva, O.M. Turkina, I.N. Kapitonov
Keywords: Calciphyres, geochemistry, isotopic composition, zircon, U–Pb age, Sharyzhalgai uplift, Siberian craton
Abstract
The paper presents data on high-grade silicate-carbonate rocks (calciphyres) from the Irkut block (Sharyzhalgai uplift, southwestern Siberian craton). Their origin and age were determined from the rock characteristics, U–Pb dating, REE content, and Hf isotope composition of zircon. The calciphyres occur both as independent section fragments and as interbeds within Paleoproterozoic garnet-bearing and alumina-rich (cordierite- and sillimanite-containing) gneisses. They were produced by metamorphism of terrigenous-carbonate sediments. The terrigenous component ranges in maturity from arenites and wackes to argillaceous rocks; this is consistent with the reconstruction of the sedimentary protoliths of paragneisses, which are predominant in the metasedimentary unit. The geochemical features of the calciphyres, their LREE enrichment relative to “pure” carbonate rocks, and a distinct Eu anomaly were inherited from the terrigenous component of calc-silicate sediments. The Nd model age (2.4–2.7 Ga) of the calciphyres and the value THf(DM-2st) = 2.5–3.0 Ga for zircon from these rocks indicate that carbonate accumulation was accompanied by the supply of terrigenous material, which formed during the erosion of Archean and Paleoproterozoic crust. Zircon from the calciphyres is similar to metamorphic zircon in REE patterns and Th/U ratios. It might have been of detrital origin and then recrystallized during high-temperature metamorphism. Terrigenous-silicate rocks were metamorphosed at ~1.87 Ga. This is close to the previous age estimates for the terrigenous-rock metamorphism (1.85–1.86 Ga) and the age of baddeleyite from apocarbonate metasomatic rocks (1.86 Ga).
|
