The geochemical composition of serpentinites in the Mesoarchaean Tartoq Group, SW Greenland: Harzburgitic cumulates or melt-modified mantle?

• Two groups of serpentinites are distinguished by their platinum-group element patterns.
• Pervasive serpentinisation has destroyed all primary minerals, including spinels.
• The bulk-rock compositions have similarities with Kohistan ultramafic cumulates.
• The combined geochemical data suggests a cumulate origin for these serpentinites.

Large slivers and enclaves (up to 100 × 2000 m) of serpentinites form tectonic imbricates within mafic supracrustal rocks of the Mesoarchaean Tartoq Group in SW Greenland. In this study we present new bulk-rock major, trace and platinum-group element data, and preliminary Re–Os isotope data for these serpentinites. Additionally, we present in situ major and trace element data for chromite and magnetite.Bulk-serpentinite major and trace element compositions allow us to rule out komatiitic, picritic, boninitic or residual mantle origins. Rather, the fractionated platinum-group patterns for the Tartoq Group serpentinites suggest a cumulate origin. The bulk-rock Re–Os isotope data shows Re-depletion ages (TRD) of ca. 2300, 2800 and 2950 Ma for the three analysed samples. These ages are younger than the 2986 ± 4 Ma minimum age of the Tartoq Group, and suggest disturbance of the Re–Os system during metamorphism or post-magmatic metasomatic events. Late resetting and metamorphic overprinting are in agreement with oxide mineral compositions, which are not primary as indicated by negligible Mg and Al contents and elevated Fe3 +# (> 55). Although the major and trace element data do not provide conclusive evidence for the petrogenesis of these Mesoarchaean serpentinites, we observe remarkable similarities with ultramafic rocks associated with the lower crustal cumulates. In particular, ultramafic cumulates of the Kohistan and Talkeetna island arc sections are compositionally comparable, if we allow for slight loss of MgO and addition of SiO2 during early alteration, serpentinisation and metamorphism. Based on these similarities, we propose an arc origin for the Tartoq Group serpentinites. Previous geochemical and structural studies of the enclosing Tartoq Group supracrustal rocks also point to formation in a subduction zone geodynamic setting. This suggests that this assemblage of imbricated mafic to ultramafic rocks and minor sediments represents a Mesoarchaean arc-related ophiolite.

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