Komatiitic parental magmas of the Archean Ujaragssuit Nunât ultramafic body, SW Greenland, identified from spinel chemistry

Abstract

Archean and Proterozoic layered intrusions represent an important portion of the igneous rock archive and their parental magma composition may provide crucial insights into the Earth’s magmatic and geodynamic evolution. Both komatiitic and boninitic parental magmas have been suggested for several major Archean layered intrusions, which could imply different tectonic settings for their formation. We studied the ~ 3.2 Ga Ujaragssuit Nunât layered ultramafic body from southern West Greenland (Ujaragssuit ultramafic body), which contains some of Earth’s oldest chromitites. Spinel major and trace elements, and whole-rock platinum group element compositions in massive chromitites from the Ujaragssuit ultramafic body, largely preserve primary igneous compositions. In contrast, spinels from most silicate-dominated ultramafic rocks were altered by metamorphic and metasomatic events. We collated a large spinel dataset to investigate variations in their parental magma compositions and tectonic settings using multivariate statistical analysis. Both the massive chromitites from the Ujaragssuit ultramafic body and chromitites from other Archean and Proterozoic ultramafic layered intrusion show high Cr/(Cr + Al) and Ti/V ratios in spinel, and high whole-rock Ir and Ru contents, which are consistent with those of komatiitic spinel. The compositions of chromitites suggest that the parental magmas of the Ujaragssuit ultramafic body are komatiitic, implying that the formation of these layered intrusions was related to mantle plumes. Our recognition of a komatiitic ultramafic body in North Atlantic Craton, where no komatiite has previously been reported, suggests that komatiitic magmas were a common feature among cratons.