Date of Award

7-7-2020

Publication Type

Doctoral Thesis

Degree Name

Ph.D.

Department

Earth and Environmental Sciences

Keywords

Anorthosites, Archean, Layered intrusions, Petrogenesis, Superior Province, Tectonic setting

Supervisor

Ali Polat

Rights

info:eu-repo/semantics/openAccess

Abstract

Field relationships, petrographic observations, mineral chemistry, and whole-rock major and trace element and Sm-Nd and Pb radiogenic isotope geochemistry of Neoarchean megacrystic anorthosite-bearing layered intrusions and spatially and temporally associated volcanic and sedimentary rocks in Archean greenstone belts in the western Superior Province of Ontario and Manitoba, Canada, were utilised to constrain their petrogenesis, and the tectonic settings in which they formed. Thoroughly determining the petrogenesis and tectonic settings of these Archean megacrystic anorthosite-bearing layered intrusions and spatially associated greenstone belt volcanics has very important implications for whether modern-style plate tectonics processes operated in the western Superior Province during the Neoarchean. The Neoarchean Haines Gabbroic Complex, Bird River Sill and Mayville Intrusion include megacrystic anorthosites and leucogabbros and gabbros, melagabbros, pyroxenites, peridotites and chromitites and intrude or are closely spatially associated with pillow basalts in the Neoarchean Shebandowan and Bird River greenstone belts of Ontario and Manitoba, respectively. These layered intrusions and pillow basalts are intruded by tonalite-trondhjemite-granodiorite (TTG) batholiths, represent oceanic crust, and formed in an oceanic setting proximal to pre-existing continental crust. The pillow basalts were erupted onto pre-existing Neoarchean-Mesoarchean TTG batholiths and the Bird River Sill and Mayville Intrusion were emplaced into the pre-existing Mesoarchean Maskwa Lake TTG Batholith I. Basaltic to rhyolitic volcanic rocks were erupted onto the Haines Gabbroic Complex and Bird River Sill and constitute part of volcanic and sedimentary formations that formed after the emplacement of these layered intrusions. The Haines Gabbroic Complex, Bird River Sill and Mayville Intrusion and their host greenstone belts have undergone greenschist- to amphibolite-facies metamorphism and variable alteration and deformation (shearing). The primary mineralogy of the Haines Gabbroic Complex has been largely altered to metamorphic plagioclase and actinolite, however, its cumulate textures are still preserved. The primary mineralogy and cumulate textures of the Bird River Sill and Mayville Intrusion are variably well-preserved and consist of calcic plagioclase (An63-99), clinopyroxene, orthopyroxene, olivine, chromite and magnesiohornblende. Oikocrystic magnesiohornblende envelops cumulus calcic plagioclase in the megacrystic anorthosites and leucogabbros. The Haines Gabbroic Complex, Bird River Sill and Mayville Intrusion are of tholeiitic affinity and crystallised from hydrous Ca- and Al-rich tholeiitic parental magmas. The hydrous nature of these parental magmas is demonstrated by the occurrence of magmatic amphibole (magnesiohornblende) and calcic plagioclase megacrysts in the megacrystic anorthosites and leucogabbros of the Bird River Sill and Mayville Intrusion. The hydrous Ca- and Al-rich tholeiitic parental magmas to these layered intrusions fractionated from hydrous boninitic-like or primitive arc tholeiitic parental magmas that were derived by variable degree partial melting of variably depleted mantle sources (εNd = +0.53 to +4.72). The parental magmas to the Bird River Sill and Mayville Intrusion underwent minor contamination by pre-existing Mesoarchean crust (Maskwa Lake TTG Batholith I). The Mayville Intrusion was cogenetic with the pillow basalt-bearing Mayville Assemblage into which it intrudes. The Haines Gabbroic Complex was not cogenetic with the spatially and temporally associated Greenwater and Burchell assemblages. The anorthosites, leucogabbros and peridotites of the Bird River Sill were not cogenetic with the Northern Lamprey Falls Formation into which it intrudes, whereas the gabbros of this layered intrusion were cogenetic with this formation. Geological relationships, lithological associations, mineral chemistry and whole-rock trace element geochemistry (e.g., negative Nb-Ti anomalies) indicate that the Haines Gabbroic Complex, Bird River Sill and Mayville Intrusion, and the pillow basalts that they intrude or are closely spatially associated with, formed in continental back-arc geodynamic settings akin to the modern Circum-Pacific subduction zones. The Neoarchean Peterson and Bird River formations in the Bird River greenstone belt and the spatially associated Mesoarchean Maskwa Lake TTG Batholith I formed in a continental arc setting. The Neoarchean Burchell Assemblage in the Shebandowan greenstone belt formed in a mature intra-oceanic continental arc setting. Modern-style plate tectonics processes operated in the western Superior Province during the Neoarchean and hydrous arc magmatism has occurred since this time.

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