Two geochemically different types of Precambrian crust in the Anabar Shield, North Siberia

Abstract

In the 2.7 Ga old Anabar Granulitic Complex and the 1.96 Ga old Lamuyka Complex, the chemical compositions of the most abundant rocks have been investigated. In the Granulitic Complex, these are two-pyroxene schists and hypersthene-plagioclase gneisses (enderbitoids) of felsic and intermediate composition, whereas the Lamuyka Complex is dominated by hornblende migmatites and potassic granitoids. In relationship to the average upper crust, the Archaean Anabar Granulite Complex contains more Fet, Mg, Ni, Co, Cr, V, Sc and Cu, and less of the LIL elements K, Nb, Li, Zr and Ba. In contrast, the Proterozoic Lamuyka Complex closely resembles an average Phanerozoic upper continental crust. Geological and geophysical data indicate that the two geochemically different complexes form separate large crustal segments within the Shield. The crustal segments consisting of Archaean rocks occupy approximately 80% of the surface. They appear to be relics of “primitive” sialic crust of roughly andesitic composition, whereas terrains of Proterozoic crust associate with belts of faulting. They can be interpreted as representing Archaean crust changed by intrusions and the addition of Si, K, H2O and LIL elements. In this way, new crust of granodioritic composition was generated. Such crustal transformation processes appear to be characteristic of old granulitic domains. The Anabar Shield, which occupies an area of 50 000 km2, is a region where the transformation was stopped at an early stage, just before the onset of large-scale generation of granitic batholiths. It is therefore a good target for the study of the early sialic crust.