Petrotectonics of chromite in Archean anorthosite-bearing layered intrusions: Implications for Archean tectonics

The origin of Archean layered anorthosite-gabbro-peridotite-chromitite intrusions and their relationship to the extant tectonic style on early Earth have been controversial for decades. The geochemistry and microtextures of chromite in Archean anorthosite-bearing layered intrusions have major implications for these issues, and the “anorthosite problem” posited by N.L. Bowen in 1917, because the chromite is highly refractory and resistant to chemical modification, and thus it preserves significant petrogenetic and tectonic information. This study presents a comparison of chromites from the Archean anorthosite-bearing layered intrusions of the Fiskenæsset complex (ca. 2.97 Ga) in southwest Greenland and the Mayville intrusion and Bird River sill (ca. 2.74 Ga) in the Superior Province of Canada. Amphibole occurs as secondary minerals and inclusions within the chromites, revealing a hydrous parental magma, which hints at a subduction zone setting. Moreover, the mineral chemistry of chromite from the non-anorthositic components of these layered complexes shows that the Fiskenæsset complex formed in an intra-oceanic arc setting, while the Mayville intrusion and Bird River sill formed in a continental back-arc setting. However, the chromites derived from the anorthosites from all the complexes define a hitherto unclassified category of chromite chemistry, for both oceanic-type and continental margin−type arcs. Our results suggest that the style of modern plate tectonics (intra-oceanic arcs and continental arcs) was present in the Archean and formed sufficiently thick continental crust to form continental arcs by the Mesoarchean−Neoarchean.