Carbonatite Sources of the Il'meny-Vishnevogorsk Complex: Evidence from Sr and Nd Isotope Data on Carbonates

摘要

The alkaline-carbonatite complex of the Paleozoic structure of the Urals is located in the Precambrian Sysert-Il'menogorsk block, which includes two structural levels and experienced a long-term evolution. The core of the block (lower structural level) is composed of the diaphthoritic gneiss granulites and migmatites of the Selyanka Sequence with the U-Pb zircon age of 1820 ± 70 Ma. The migmatite sample is characterized by the following values: (~(87)Sr/~(86)Sr)_(1700) = 0.7067, (~(147)Nd/~(146)Nd)_(1700) = 0.5099, and ε_(Nd) = -10.2. The core formation is related to the metamorphic events of the Karelian folding. The framing (upper structural level) is made up of similar sequences: Il'menogorsk Sequence in the south; and Vishnevogorsk, Shumikhin, and Chernovsk sequences in the north. The sequences consist of amphibolites and less common plagiogneisses and quartzites. They have low initial Sr and Nd ratios. The Shumikhin Sequence is characterized by the following values: (~(87)Sr/~(86)Sr)_(576) = 0.702-0.703, (~(147)Nd/~(146)Nd)_(576) = 0.5121, and ε_(Nd) from + 3 to + 4. The U-Pb datings suggest that the aforementioned sequences are pre-Uralian paleoceanic complexes with a metamorphic age of 643 ± 46 Ma for the Il'menogorsk Sequence and 576 ± 65 Ma for the Shumikhin Sequence. They were formed during the breakup of Rodinia. Based on U-Pb and Rb-Sr data, the crystallization age of the alkaline rocks and carbonatites is 440 Ma. Later metamorphism and granite formation in the Sysert-Il'menogorsk block were related to the Hercynian orogeny (360-320 Ma) and subsequent postcollisional extension (260-240 Ma).