Geochemistry, Nd, Pb and Sr isotope systematics, and U–Pb zircon ages of the Neoarchean Bad Vermilion Lake greenstone belt and spatially associated granitic rocks, western Superior Province, Canada

Abstract

The ca. 2720Ma Neoarchean Bad Vermilion Lake (BVL) greenstone belt, in the western Superior Province, Canada, is composed of a suite of tholeiitic to calc-alkaline basalts to rhyolites, volcaniclastic rocks, gabbros, and Timiskaming-type siliciclastic sedimentary rocks. The greenstone belt was intruded by Neoarchean granitic rocks, and underwent greenschist facies metamorphism and intense deformation, resulting in mobilization of many elements (e.g., Rb, Ba, Sr, K, U, Pb).The high-field strength element and rare earth element systematics of the volcanic and volcaniclastic rocks, and gabbros are consistent with subduction zone geochemical signatures, suggesting that the BVL greenstone belt formed in a magmatic arc setting. On the basis of lithological associations and trace element systematics, the BVL greenstone belt is defined as a fragment of a Neoarchean subduction-related ophiolite. Three rhyolite samples from the belt have yielded 2722±18Ma, 2706±13Ma and 2710±28Ma U–Pb zircon ages, representing the approximate age of the arc volcanism in the study area and development of a subduction zone between the western Wabigoon terrane to the north and the Wawa-Abitibi terrane to the south. The intrusion of the ca. 2664±15Ma late- to post-tectonic, potassic Ottertail Lake granite marks the end of tectonic accretion in the study area.Both the volcanic rocks and gabbros display large ranges of Nd (143Nd/144Nd=0.511600–0.512849; εNd (2720Ma)=+0.8 to+4.0), Pb (206Pb/204Pb=13.80–60.67) and Sr (87Sr/86Sr=0.701481–1.01154) isotopic compositions, suggesting that these isotope systems were variably affected by post-magmatic element mobility. Neither the Sm–Nd (2921±200Ma) nor Rb–Sr (2130±610Ma) system has yielded reliable regression (isochron) ages, reflecting the open-system behavior of these systems during metamorphism. Despite large uncertainties, Pb–Pb regression ages yielded by all rock types (2661±60Ma), and basalts and gabbros (2725±83Ma) agree with the zircon U–Pb ages of the rhyolites, suggesting that the U–Pb system was the most robust among all three systems.