Evidence for multi-stage metasomatism and metamorphism of the Mesoarchean Ulamertoq ultramafic body in southern West Greenland

Abstract:

Archean crust is widely exposed in southern West Greenland. Ultramafic rocks occur as lens-shaped bodies within TTG orthogneisses. We studied the petrology and geochemistry of the Ulamertoq ultramafic body located within the Mesoarchaean Akia Terrane to assess the metasomatism, metamorphism, and its origin. The Ulamertoq ultramafic body experienced multi-stage metasomatism. Phlogopite/biotite, amphibole-rich layers and associated orthopyroxenite are observed near the boundaries with intrusive granitic sheets and the host orthogneiss. Mg# of minerals and the abundance of HREE and HFSE (Ti, Zr) imply that the ultramafic rocks suffered local metasomatism by fluids/melts related to granitic sheets and the hosting orthogneiss. However, hydrous minerals, as well as orthopyroxene are not restricted to the boundaries with the granitoid rocks, are but distributed heterogeneously throughout the ultramafic body. Orthopyroxene in the extensively metasomatized peridotites occurs as fine and/or poikilitic grains with low Cr₂O₃, Al₂O₃, and CaO contents, which suggest a secondary origin, rather than a residual origin after partial melting. Trace elements marked by negative Sr and Eu anomalies in orthopyroxene and amphibole of the extensively metasomatized peridotites indicate that metasomatic agent for the extensive metasomatism of the main body is different from that of the local metasomatism. The geochemical variations of the whole-rock compositions can be explained by varying degrees of extensive metasomatism, which resulted in addition of orthopyroxene and amphibole to dunitic compositions, rather than representing a partial melting trend. Titanian clinohumite-bearing dunite was previously reported for these rocks and suggests initial hydration followed by dehydration during granulite facies metamorphism (Nishio et al., 2019). Forsterite contents of the less metasomatized dunites are similar to Mg# of the whole-rock chemistry, but lower than that of the Eoarchean Isua metadunite (Guotana, 2020, Ph.D. Thesis), which imply that the olivine of dunites possibly preserves the primary igneous composition. Forsterite and NiO contents in olivine are similar to those in olivine of komatiite. Therefore, the origin of the Mesoarchean Ulamertoq ultramafic body is cumulate from komatiitic magma.