Early Proterozoic continental tholeiites from western Bergslagen, Central Sweden, II. Nd and Sr isotopic variations and implications from SmNd systematics for the Svecofennian sub-continental mantle

Abstract

A subset of nine least-altered, least-fractionated within-plate continental metatholeiites from western Bergslagen yields a SmNd isochron age of 1886 ± 48 Ma, with an ϵNd(T) of + 1.1. This age is in agreement with previous UPb zircon dating of a coeval felsic intrusion. Variation of ϵNd(T) in other subsets is shown to be caused by crustal contamination of the basic magma. To explain the remarkable isotopic homogeneity of the least-altered, least-fractionated metatholeiites and their combined LREE enrichment and isotopic depletion, three petrogenetic models are discussed. The first model involves incipient melting of a depleted mantle in the garnet stability field, the second model presumes complete homogenization of a depleted-mantle-derived melt contaminated by older continental crust, and the third model proposes melting of an enriched mantle source. In light of the depleted nature of the Baltic asthenosphere, absence of enriched (Archaean) crust in the Bergslagen area, negative HFSE (Zr, Hf, Ti) anomalies in the metatholeiites and enriched isotopic characteristics of lamprophyres occurring 50 km to the west, model three is preferred. In a tentative scenario, three time-scales are inferred from SmNd isotope systematics to generate the metatholeiites: (1) long-term (> 1 Ga) LREE depletion of the asthenosphere by extraction of continental crust; (2) shorter-term (pre-1.88 Ga BP) LREE-enrichment of this depleted mantle by supra-subduction zone modification of the mantle wedge during protocrust formation; and (3) short-term (around 1.88 Ga BP) generation of basic magma by melting of this enriched mantle as a result of continental rifting of the Early Proterozoic protocrust.