Isotopic constraints on the genesis of the Rogaland anorthositic suite (southwest Norway)

Abstract

Isotopic data were obtained on the Sveconorwegian (900–1200 Ma) anorthosites and associated norites and charnockites of the Rogaland igneous complex (SW Norway). The isotopic data ( I Sr = 0.703−0.7077; ϵ Nd = +0.4 to +5.5; 238 U 204 Pb (μ 1) ⋍ 8.1−8.2 ; δ 18 O = +5.2 to +6.6‰ ) suggest an upper-mantle origin for the parental magma of the anorthosites and related norites and jotunites or an origin in the lower crust by melting of mantle-derived basic rocks shortly after their formation. In two well-documented cases (the Egersund-Ogna and the Hidra bodies) a progressive contamination by U-depleted but not Rb-depleted lower-crustal material is apparent during the differentiation process; the late-stage liquids have higher I Sr (up to 0.7086), lower (but still slightly positive) ϵ Nd, less radiogenic Pb isotopic composition. The intrusive acidic rocks, some of them of the charnockitic type, display variable isotopic data ( I Sr = 0.7055−0.709; ϵ Nd = +0.5 to −0.8 ; Pb isotopic compositions comparable to that of some of the surrounding gneisses), suggesting that there are, in general, no direct cogenetic relations between the anorthosites and the charnockites. Our data point to an origin of some of these acidic rocks by anatectic melting of the surrounding high-grade metamorphic gneisses.