Age and Chemostratigraphy of the Finlayson Lake District, Yukon: Implications for Volcanogenic Massive Sulfide (VMS) Mineralization and Tectonics along the Western Laurentian Continental Margin

Abstract

The Yukon-Tanana terrane in the Finlayson Lake district, Yukon, represents one of the first arc–back-arc systems that formed adjacent to the Laurentian continental margin in the mid-Paleozoic. Back-arc rocks contain many large and high-grade volcanogenic massive sulfide (VMS) deposits. This study integrates U-Pb zircon geochronology, lithogeochemistry, and Hf-Nd isotopes to establish precise controls on tectonomagmatic activity adjacent to the western Laurentian margin in the Late Devonian to Early Mississippian. High-precision chemical abrasion- (CA-) ID-TIMS U-Pb zircon geochronology defines coeval arc (ca. 363.1 to 348 Ma) and back-arc (ca. 363.3 to 355.0 Ma) magmatism in the Finlayson Lake district that intruded continental crust of Laurentian affinity (e.g., Snowcap assemblage). Mafic and felsic rocks display geochemical and isotopic characteristics that are consistent with being formed from mixtures of depleted asthenosphere and enriched lithospheric mantle sources. These melts variably entrained Laurentian continental crust via high-temperature crustal melting due to basaltic underplating. The high-temperature back-arc felsic magmatism occurs at specific time periods coinciding with VMS deposits and supports previous genetic models for VMS mineralization that suggest elevated heat flow and hydrothermal circulation were due to regional-scale rift-related magmatism rather than from local subvolcanic intrusions. The short timescales and transient nature of tectonomagmatic events in the Finlayson Lake district suggest that rapid and complex subduction initiation of oceanic and continental crust fragments facilitated coeval compression, extension, and magmatism in the arc and back-arc regions. We thus reevaluate the presently accepted tectonostratigraphic framework of the Finlayson Lake district and suggest revised interpretations that shed light on VMS depositional environments and a possible broader association with the ca. 358Ma Antler Orogeny. Results of this study have implications for incipient tectonics, magmatism, and mineralization along the western Laurentian continental margin and other orogenic belts globally.