FORMATION OF THE NEOARCHEAN BAD VERMILION LAKE ANORTHOSITE COMPLEX AT A CONVERGENT PLATE MARGIN, SUPERIOR PROVINCE, WESTERN ONTARIO, CANADA

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. New whole-rock major and trace element data for the various units of the Complex, oxygen isotope data for the anorthosite are presented in this study to constrain its the petrogenetic and geodynamic origin. A ²⁰⁷Pb/²⁰⁶Pb age of 2716 ± 18 Ma，yield by zircon U-Pb dating of associated granitic rocks intruding the BVLA Complex, is used to constrain the minimum intrusion age of the complex.

Primary igneous textures are locally well preserved in the BVLA Complex despite greenschist facies metamorphism and deformation. 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 indicate that the BVLA Complex was originally from the Mantle and didn’t undergo significant crustal contamination during its emplacement. The existence of primary calcic igneous plagioclase, coherent Nb negative 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 processes of fractional crystallization and accumulation of olivine, orthopyroxene, clinopyroxene, plagioclase and possibly amphibole can explain the mineralogical composition of the Complex.