Mid-Paleozoic initiation of the northern Cordilleran marginal backarc basin: Geologic, geochemical, and neodymium isotope evidence from the oldest mafic magmatic rocks in the Yukon-Tanana terrane, Finlayson Lake district, southeast Yukon, Canada

The Fire Lake formation of the Yukon-Tanana terrane in the Finlayson Lake region, Yukon, Canada, consists primarily of Late Devonian (ca. 365-360 Ma) mafic metavolcanic rocks and smaller volumes of mafic and ultramafic subvolcanic metamorphosed intrusions. In this paper, field, geochemical, and Nd isotope attributes of these rocks are presented in an attempt to understand their tectonic setting, the magmatic processes involved in their formation, and their roles in Cordilleran crustal growth. The mafic rocks of the Fire Lake formation exhibit a wide diversity of geochemical signatures and are classified into seven chemically defined suites: (1) backarc-basin basalt, (2) enriched mid-oceanic-ridge basalt (E-MORB), (3) oceanic-island basalt (OIB), (4) Th-rich OIB, (5) boninite, (6) island-arc tholeiite, and (7) light rare earth element (LREE)-enriched island-are tholeiite. The diversity of geochemical signatures is interpreted to represent variable mixtures of asthenospheric (MORB-type) mantle, subarc mantle wedge, and lithospheric (OIB-type) mantle with or without elemental contributions from the subducted slab and/or continental crust. These suites of rocks are also associated with fine-grained basinal sedimentary facies, variations in metavolcanic and metasedimentary unit thickness, extensional synvolcanic faults, and apparent extensional-fault-controlled emplacement of mafic intrusive rocks and hydrothermal volcanic-hosted massive sulfide mineralization. The suites also exhibit a broad spatial distribution; those with arc signatures (Nb/Th-mn < 1; mn-normalized to primitive mantle values) are located primarily in the western parts of the formation, and suites with nonarc signatures (Nb/Th-mn < 1) are located primarily within the eastern parts of the formation. Collectively, these geologic and geochemical attributes are interpreted to stem from the transition from are magmatism to the initiation of an extensional backarc basinal environment associated with an east-dipping subduction zone. The initiation of backarc-basin magmatism recorded in the Fire Lake formation was part of a much larger Late Devonian backarc basinal system forming along the western edge of the margin of North America. The Fire Lake formation is interpreted to represent (1) the commencement of Yukon-Tanana arc rifting and separation from the North American cratonic margin, and (2) the initiation of a marginal (backarc) basin (now the Slide Mountain terrane) inboard of the Yukon-Tanana arc system. This tectonic evolution likely occurred either as a result of slab rollback toward the west within the convergent margin responsible for Yukon-Tanana arc activity or as a result of the propagation of the Slide Mountain backarc-basin spreading ridges into the Yukon-Tanana are system. This Yukon-Tanana arc rifting episode was also broadly coincident with rifting and hydrothermal activity within rocks of the North American craton. The geochemical and isotopic signatures of magmatic rocks in the Fire Lake formation have some features similar to intraoceanic arc rocks (e.g., boninites, island-arc tholeiites), and many have juvenile Nd isotope signatures (i.e., epsilon(Nd(t)) > 0; most have epsilon(Nd(t)) > +5), suggesting that the pericratonic terranes of the northern Cordillera have a significant juvenile component. If this is the case throughout the Yukon-Tanana terrane, then the pericratonic terranes may have contributed much more juvenile material to Cordilleran crustal growth in the Phanerozoic than has previously been considered.