U-Pb Age of Detrital Zircon in the Cement of Early Precambrian Polymict Conglomerates of the Central Karelia Domain, Karelian Province, Fennoscandian Shield

摘要

U-Th-Pb LA-ICP-MS was used for dating detrital zircon extracted from the cement of Neoar-chean and Paleoproterozoic polymict conglomerates of the Karelian Province, Fennoscandian Shield. The chemical composition of terrigenous and volcanogenic rocks of the Gimolskaya Formation is presented for the first time. The data obtained showed that the detrital zircon from the cement of conglomerates of different ages yields similar results of about 2750 Ma. The clasts of the gneiss tonalite and plagioporphyry dominate in the Neoarchean conglomerates of the Sukkosero area. They are identical in chemical composition with the Neoarchean (2.78-2.73 Ga) TTG, granodiorite, and metadacite of the Central Karelian domain. These rocks are distinguished by an increased content of Ba (900-1200 ppm) and Sr (600-700 ppm), and the ratios Sr/Y = 34-90 and (La/Yb)_n = 17-30. The chemical composition of the cement (values of CIA < 55 and negative values of the function DF(x)) from -1.0 to -0.4 is similar to the composition of clasts, which means the cement is composed of finer clasts of the same rocks. The Paleoproterozoic conglomerates of the Vottomuks area contain zircon from felsic rocks of the Neoarchean basement. The chemical composition of the cement of the Paleoproterozoic conglomerates and the predominance of basaltic andesites of Sumian are evidence of the post-Sumian(<2.45 Ga) age of these rocks. The absence of the Paleoproterozoic zircon in cement could have resulted from its absence in the Paleoproterozoic source rocks. The age of Neoarchean conglomerates from the Gimolskaya Formation is less than 2.75 Ga.