SOURCES OF GRANITIC MAGMA AND THE EARTH'S CRUST FORMATION IN THE CENTRAL AND SOUTHERN URALS: SM-ND AND RB-SR ISOTOPIC EVIDENCE

Abstract

We present new analytical data on the isotopic composition of whole-rock samples taken from 26 Paleozoic granitoid plutons in the central and southern Urals; the age of these plutons is known with confidence. In addition, new isotopic data were obtained for some pre-Riphean metamorphic rocks. Three groups of granitoid rocks pertaining to different crustal sources are recognized from the evolution of Sm-Nd and Rb-Sr isotope systems: (1) Early and Middle Carboniferous, Devonian, and Silurian low-K granitoids genetically related to the recycling of Vendian-Cambrian TTG protolith, which was formed 690-490 Ma ago as a result of partial melting of Neoproterozoic metabasic rocks; (2) Carboniferous and Early Permian medium- and high-K granitic rocks derived from the same TTG protolith metasomatically modified by alkaline melts and solutions related to an enriched mantle reservoir of Riphean (?) age, and (3) Late Permian high-K granites; their source is inferred to be either Riphean silicic igneous rocks or local zones of metasomatic alteration superimposed upon the Vendian-Cambrian crustal protolith. Neither granites with unequivocal isotopic signatures of the pre-Riphean basement nor post-Cambrian sources of granitic magma have been recorded. The proposed model suggests the growth of the Earth's crust beneath the Uralian Foldbelt in the Neoproterozoic due to the underplating by mantle-derived basic rocks. The subsequent differentiation gave rise to the enrichment of the upper crust in granitic material leaving restites in the lower crust.