Evidence for Neoarchean hydrous arc magmatism, the anorthosite-bearing Mayville Intrusion, western Superior Province, Canada

Abstract

The Neoarchean (2742.8 ± 0.8 Ma) megacrystic anorthosite-bearing Mayville layered intrusion of the Bird River greenstone belt in the Superior Province of Canada is well preserved and well exposed. Given its good preservation, we studied the Mayville Intrusion through a combination of extensive field and petrographic observations, mineral chemistry, and whole-rock major and trace element and Sm-Nd and U-Th-Pb isotopic analyses to unravel its petrogenesis and geodynamic setting, and determine whether its amphibole is of igneous origin and indicative of hydrous magmatism. The Mayville Intrusion intrudes pillow basalts and gabbros of the Mayville Assemblage, which was erupted onto and emplaced into the Mesoarchean phase of the Maskwa Lake tonalite-trondhjemite-granodiorite (TTG) Batholith. Some of the anorthosites and leucogabbros of the Mayville Intrusion have undergone extensive shearing and hydrothermal alteration and had their Sm-Nd and U-Th-Pb isotope systems reset. Despite metamorphism and deformation affecting the Mayville Intrusion, it has largely retained its primary igneous minerals and textures, and trace element and Sm-Nd and Pb isotope compositions. Abundant interstitial magmatic amphibole oikocrysts in the anorthosites and leucogabbros exhibit whole-grain optical continuity and well-preserved igneous grain boundaries with magmatic plagioclase, indicating that the Mayville Intrusion crystallized from a hydrous magma.The Mayville Intrusion has variably positive εNd (2743 Ma) values (+0.7 to +3.9) that are indicative of its derivation from a depleted mantle source. The anorthosites and leucogabbros of the Mayville Intrusion crystallized from a hydrous Ca- and Al-rich tholeiitic parental magma that fractionated from a hydrous primitive arc tholeiitic parental magma. These lithologies were derived by partial melting of a depleted, shallow hydrous sub-arc mantle wedge source. The parental magmas to the gabbros, leucogabbros and anorthosites of the Anorthosite to Leucogabbro Zone subsequently experienced minor contamination by Mesoarchean TTGs (Maskwa Lake TTG Batholith). The field relationships and trace element geochemistry of the Mayville Intrusion and its proximity to the continental arc Maskwa Lake TTG Batholith indicate that it formed in a Chilean-style continental back-arc geodynamic setting. The findings of this study demonstrate that hydrous arc magmatism and modern-style plate tectonic processes have been operating since at least the Late Archean.