Petrogenetic investigation of Eoarchean to Neoarchean anorthosite-bearing layered intrusions

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Although they are volumetrically small, Archean anorthosite-bearing layered intrusions are widespread in most Archean cratons. These layered intrusions are typically associated with greenstone belts, and are intruded mainly by syn- to post-tectonic granitoids. Some of the intrusions occur in high-grade gneissic terrains. Despite their widespread occurrence, the petrogenesis of Archean anorthosites is not well understood. Several outstanding scientific questions remain to be resolved to constrain their petrogenetic origin and significance for the evolution of the early Earth. These questions include: (1) How did Archean anorthosites originate? (2) What is the petrogenetic significance of high-Ca (>An70) megacrystic plagioclase? (3) Why are megacrystic anorthosites restricted to the Archean rock record? (4) What type of crust (oceanic versus continental) did they intrude? (5) Do they display any systematic geochemical changes from Eoarchean to Neoarchean? (6) What was the geodynamic setting(s) that produced these rocks? To address these questions over the next five years, I plan to study Archean anorthosite-bearing layered intrusions in Western Australia and Canada. The intrusions to be studied include the 3.73 Ga Manfred intrusion in the Yilgarn Craton of Western Australia, the 3.47 Ga East Pilbara intrusion in the East Pilbara Craton of Western Australia, and the Neoarchean (ca. 2.75 Ga) Shawmere and Pipestone Lake intrusions in the Superior Province of Canada. I will systematically sample each lithological unit in these intrusions and analyze them for major and trace elements and isotopes (Sm-Nd, Th-U-Pb, Lu-Hf, O) to improve our understanding the petrogenetic origin of these intrusions. The petrographic and geochemical characteristics of each rock type in these layered intrusions are expected to reflect the petrological processes responsible for the compositional and isotopic variations in these rocks and their sources. These petrological processes can then be linked to the global-scale Archean geodynamic processes in the early Earth. The geochemistry of these intrusions will be used to constrain the effects of declining of mantle temperatures over one billion years (3.7-2.7 Ga) on the petrogenetic and geodynamic processes in the Archean. The results of this investigation will have the following significance: (1) enhance our knowledge on the petrogenetic origin of Archean anorthosite-bearing layered intrusions; (2) provide geochemical constraints on the relationship between the thermal evolution of the mantle and anorthosite formation in the early Earth; (3) allow us to test and refine the geodynamic models proposed for the origin of Archean cratons; and (4) benefit the Canadian resource industry by training geologists with the expertise needed to explore for Archean anorthosite-bearing intrusions that have a high potential for chromium, vanadium, copper, nickel, titanium, and platinum group element mineral deposits.