Geochemistry of the Tussaap Ultramafic Complex, Southern West Greenland

The Tussaap ultramafic complex (TUC) is located within the ~3.65 to 3.81 Ga Itsaq Gneiss complex of southern West Greenland. The TUC is part of numerous ultramafic enclaves in the Itsaq Gneiss complex that are thought to predate the gneisses. Some of these enclaves have been interpreted as lithospheric mantle material while others are interpreted as layered cumulates. In this study, measurements of mineral chemistry and whole rock major-, trace-, and platinum-group-element abundances of the previously unstudied TUC were used to evaluate its origin as cumulate or residual mantle material. Rhenium-Os isotopic systematics were used to place chronological constraints on the enclave. The TUC comprises dunites, ol-clinopyroxenites, amphibolites, and hornblende gabbros. The mineral chemistry of the TUC largely reflects secondary processes, and, where possibly shielded from these processes, show no indication of partial melting. The whole-rock FeO contents of the dunites are high, up to 15.5 wt%, and with variable iridium group platinum group elements (IPGE: Os, Ir, and Ru) are not consistent with a depleted mantle origin for the rocks. Inter-element variations in major- and trace-element geochemistry of the TUC fit well with fractional crystallization models. In such models the dunites are explicable as olivine and chromite cumulates, the ol-clinopyroxenites as clinopyroxene, olivine, and chromite cumulates, the amphibolites as clinopyroxene and Cr-magnetite cumulates, and the hornblende gabbros as crystallized melts. Variable relative and absolute IPGE abundances and enrichments in IPGEs relative to palladium group platinum group elements (PPGE: Pd and Pt) in the cumulate lithologies are accounted for if the parental melts of the TUC were sulphide undersaturated and platinum-group-element contents in the cumulates were controlled by their compatibility in fractionating silicate, oxide, and platinum group minerals. In contrast to cumulate lithologies, one hornblende gabbro sample was PPGE enriched relative to IPGEs, consistent with these samples originating as crystallized melts. Major-element variations in ol-clinopyroxenites and trace-element systematics in all rocks of the TUC suggest their parental melts had arc-like affinities. However, Re-Os isotopic measurements revealed that these melts were moderately enriched in 187Os relative to estimates for primitive upper mantle at 3.8 Ga. This largely contrasts with other less evolved refractory peridotite enclaves in the area which have less radiogenic Os isotopic compositions. These differences are interpreted to reflect the proportionally higher degree of assimilation of basaltic crust experienced by the parental melts of the TUC. In addition, relative to Phanerozoic intra-plate magmas the Pd content was high, which was not considered to be a melt source characteristic, but likely due to crustal assimilation by the parental melts of the TUC. The extent to which crustal assimilation was responsible for generating arc-like signatures in major and trace elements is unclear. Thus, there is no strong evidence that parental melts of the TUC formed in a subduction-zone environment. Relative to primitive upper mantle at 3.8 Ga, the TUC is moderately enriched in 187Os, resulting in inconclusive age determinations. Although this radiogenic Os isotopic composition is interpreted to be the result crustal assimilation, resulting TMA and TRD ages are younger than ~2.85 Ga, which places some uncertainty on the inferred Eoarchean age of the TUC and, therefore, other enclaves in the Itsaq Gneiss complex.