A back-arc origin for the Neoarchean megacrystic anorthosite-bearing Bird River Sill and the associated greenstone belt, Bird River subprovince, Western Superior Province, Manitoba, Canada

Comparisons Between Tethyan Anorthosite‐Bearing Ophiolites and Archean Anorthosite‐Bearing Layered Intrusions: Implications for Archean Geodynamic Processes

“…The low Zr/Y, La/Yb, and Th/Yb ratios exhibited by Archean anorthosite‐bearing layered intrusions corroborate their interpreted tholeiitic affinity (Table S2; Ross & Bédard, 2009). The high Ca and Al contents of the anorthosites and leucogabbros from these Archean anorthosite‐bearing layered intrusions, combined with their tholeiitic affinity and their having magmatic amphibole, strongly indicate that they crystallized from hydrous Ca‐ and Al‐rich tholeiitic magmas (Table S2; Polat, Frei, et al, 2018; Sotiriou, Polat, & Frei, 2019; Sotiriou, Polat, Frei, Yang, et al, 2019). The positive ε Nd values, negative Nb and Ti anomalies, low incompatible trace element abundances, and highly depleted N‐MORB‐normalized trace element patterns of Archean anorthosite‐bearing layered intrusions indicate that they were generated by high‐degree partial melting of very depleted subarc harzburgitic mantle wedge sources (Golowin et al, 2017; Pearce, 2008; Pearce & Peate, 1995; Polat et al, 2011: Polat, Longstaffe, et al, 2018; Saccani, 2015; Saunders et al, 1980; Sotiriou, Polat, Frei, Yang, et al, 2019; Sotiriou, Polat, & Frei, 2019; Sotiriou et al, 2020; Stern et al, 2003).…”

“…The high Ca and Al contents of the anorthosites and leucogabbros from these Archean anorthosite‐bearing layered intrusions, combined with their tholeiitic affinity and their having magmatic amphibole, strongly indicate that they crystallized from hydrous Ca‐ and Al‐rich tholeiitic magmas (Table S2; Polat, Frei, et al, 2018; Sotiriou, Polat, & Frei, 2019; Sotiriou, Polat, Frei, Yang, et al, 2019). The positive ε Nd values, negative Nb and Ti anomalies, low incompatible trace element abundances, and highly depleted N‐MORB‐normalized trace element patterns of Archean anorthosite‐bearing layered intrusions indicate that they were generated by high‐degree partial melting of very depleted subarc harzburgitic mantle wedge sources (Golowin et al, 2017; Pearce, 2008; Pearce & Peate, 1995; Polat et al, 2011: Polat, Longstaffe, et al, 2018; Saccani, 2015; Saunders et al, 1980; Sotiriou, Polat, Frei, Yang, et al, 2019; Sotiriou, Polat, & Frei, 2019; Sotiriou et al, 2020; Stern et al, 2003). Archean anorthosites and leucogabbros have high Ca and Al contents and highly calcic (up to An 100 ) plagioclase, indicating that their mantle source may have been refertilized by oceanic slab crust‐derived Al 2 O 3 ‐ and SiO 2 ‐rich adakitic or TTG melts prior to the generation of their hydrous boninitic and primitive arc tholeiitic parental magmas (Polat et al, 2011; Rollinson et al, 2010; Sotiriou, Polat, Frei, Yang, et al, 2019; Sotiriou, Polat, & Frei, 2019; Woelki et al, 2018; Wyman, 2019).…”

“…The Neoarchean Mayville Intrusion in the western Superior Province of Canada contains abundant well‐preserved magmatic amphibole that occurs interstitially to cumulus calcic plagioclase megacrysts (Sotiriou et al, 2020). Magmatic amphibole occurs interstitially to the megacrysts and as oikocrysts that envelop smaller plagioclase crystals in these Archean intrusions (Hoffmann et al, 2012; Mohan et al, 2013; Polat et al, 2011, 2012; Polat, Longstaffe, et al, 2018; Sotiriou, Polat, Frei, Yang, et al, 2019; Sotiriou et al, 2020). The magmatic amphibole in these Archean megacrystic anorthosite‐bearing layered intrusions most likely crystallized from an interstitial hydrous melt due to this melt reacting with cumulus plagioclase (Claeson & Meurer, 2004; Sotiriou et al, 2020).…”

“…The identification of magmatic amphibole in gabbroic cumulates has been interpreted to reflect their crystallization from hydrous magmas in a subduction zone geodynamic setting (Claeson & Meurer, 2004). Magmatic amphibole has been identified in anorthosites and leucogabbros in the Archean megacrystic anorthosite-bearing Fiskenaesset, Naajat Kuuat, Bird River, Mayville, Sittampundi, Konkanhundi, and São José do Jacuipe layered intrusions, which have all been interpreted to have formed in a subduction zone setting (Hoffmann et al, 2012;Mohan et al, 2013;Piaia et al, 2017;Polat et al, 2011Polat et al, , 2012Rollinson et al, 2010;Santosh & Li, 2018;Sotiriou, Polat, Frei, Yang, et al, 2019;Sotiriou et al, 2020). The Neoarchean Mayville Intrusion in the western Superior Province of Canada contains abundant well-preserved magmatic amphibole that occurs interstitially to cumulus calcic plagioclase megacrysts (Sotiriou et al, 2020).…”

“…Based on textural and chemical evidence, amphibole in some Archean anorthosite‐bearing layered intrusions has been interpreted to be of magmatic origin and be indicative of hydrous magmatism (Hoffmann et al, 2012; Mohan et al, 2013; Piaia et al, 2017; Polat et al, 2011; Rollinson et al, 2010; Santosh & Li, 2018; Sotiriou, Polat, Frei, Yang, & et al, 2019; Sotiriou et al, 2020). The textural evidence for magmatic amphibole includes serrated igneous boundaries with calcic plagioclase, whole‐grain optical continuity and extinction, its occurrence as interstitial oikocrysts, and its anhedral form (Hoffmann et al, 2012; Mohan et al, 2013; Piaia et al, 2017; Polat et al, 2011; Rollinson et al, 2010; Santosh & Li, 2018; Sotiriou, Polat, Frei, Yang, et al, 2019; Sotiriou et al, 2020). This textural evidence distinguishes magmatic amphibole from metamorphic amphibole, which does not have serrated grain boundaries, does not occur as interstitial oikocrysts, and is euhedral (Sotiriou et al, 2020).…”