Formation of the Neoarchean Bad Vermilion Lake Anorthosite Complex and spatially associated granitic rocks at a convergent plate margin, Superior Province, Western Ontario, Canada

Abstract

The Bad Vermilion Lake Anorthosite Complex (henceforth, the BVLA Complex) in western Ontario is one of the well-exposed, anorthosite-bearing, Archean layered intrusions in the Superior Province, Canada. This study presents new whole-rock major and trace element data for the various units of the Complex, oxygen isotope data for the anorthosite, and major and trace element data for the spatially associated granitic rocks intruding the BVLA Complex to constrain their petrogenetic and geodynamic origin. Zircons from granitic rocks have yielded a 207 Pb/ 206 Pb age of 2716 ± 18 Ma, constraining the minimum intrusion age of the Complex. Despite deformation and greenschist facies metamorphism, primary igneous textures are locally well preserved in the BVLA Complex. Its whole-rock major and trace elemental compositions and the oxygen isotopic systematics appear not to have been substantially modified by deformation and metamorphism. Mantle-like oxygen isotope signatures and major and trace element compositions are inconsistent with significant crustal contamination of the BVLA Complex during its emplacement. The existence of primary calcic igneous plagioclase, coherent negative Nb anomalies (Nb/Nb*=0.08-0.88), and geochemical similarities between gabbros from the BVLA Complex and gabbros from Cenozoic arcs collectively suggest an intra-oceanic subduction zone geodynamic setting for the Complex. Near-flat REE patterns in the various units of the BVLA Complex suggest that they were derived from melting of a shallow source beneath a subarc mantle wedge. Trends in immobile major (e.g., MgO) and trace (e.g., Zr) element data indicate that the mineralogical composition of the Complex can be explained by fractional crystallization and accumulation of olivine, orthopyroxene, clinopyroxene, plagioclase and 3 possibly amphibole. Compositionally, the bordering granitic rocks are A 2-type and strongly enriched in Th and REE (>100 times chondrite) and depleted of Ba, Sr, Eu and Ti. We suggest that they formed in a post-collisional, extensional, tectonic regime following emplacement of the BVLA Complex in an oceanic arc.

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

“…World map showing the distribution and age of Archean anorthosite occurrences (modified after Zhou et al, 2016) based on information from the references cited in Table S2. The numbers correspond to the occurrences listed in Table S2.…”

Petrogenetic and geodynamic origin of the Neoarchean Doré Lake Complex, Abitibi subprovince, Superior Province, Canada

“…Approximately 8 mg of sample powder were weighed into spring-loaded sample holders, evacuated overnight at ca. 150 • C, and then placed into nickel reaction vessels and heated in vacuo at 300 • C for further 3 hours to remove surface water (see Polat and Longstaffe 2014;Zhou et al 2016). The samples were then reacted overnight at ca.…”

“…Despite their small volumes, anorthosite-bearing layered intrusions are abundant in Archean cratons, particularly in western Greenland and the Superior Province of the Canadian Shield (Windley and Garde 2009;Ashwal 2010;Hoffmann et al 2012;Percival et al 2012;Polat et al 2015). Examples of well-preserved anorthosite-bearing layered intrusions in the Superior Province include the Doré Lake, Bell River, Mulcahy Lake, Shawmere, Bird River, Pipestone, Bad Vermilion Lake and Nelson River complexes (Bell 1978;Simmons and Hanson 1980;Corkery 1985;Ashwal et al 1983;Ashwal, 1993;Morrison et al, 1985;Edgar and Sweeny 1991;Chown et al 1992;Mortensen 1993;Maier et al 1996;Peck et al 1998Peck et al , 1999Hartlaub et al 2004;Gilbert 2007;Bédard et al 2009;Yang et al 2012;Yang and Gilbert 2014;Zhou et al 2016). In addition, anorthosites occur in Proterozoic orogenic belts, Phanerozoic ophiolites and modern oceanic crust (Ashwal 1993(Ashwal , 2010.…”

“…Similarly, they are comparable to the  18 O of the 2970 Ma Fiskenaesset and the ca. 2720 Ma Bad Vermilion Lake anorthosite complexes(Polat and Longstaffe 2014;Zhou et al 2016). The much wider range of  18 O in the altered plagioclase separates (+4.77 to +7.96‰) is consistent with alteration over a range of metamorphic conditions, with the lowest values…”