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Presents a study that examined neodymium isotope geochemistry of felsic volcanic and intrusive rocks from the Yukon-Tanana terrane (YTT) in the Finlayson Lake region, Yukon, Canada. Geological setting of the YTT in the Finlayson Lake region; Geochemical attributes of the intrusive rocks; Systematics of the neodymium isotopes.

Calc-alkaline batholiths of the Archaean Minto block, northeastern Superior Province, Canada, have pyroxene- and hornblende-bearing mineral assemblages inferred to have crystallized from hot, water-undersaturated magmas at 2·729–2·724 Ga. A regional amphibolite- to granulite-facies tectonothermal event at 2·70 Ga resulted in mild to negligible metamorphic effects on the dominantly granodioritic units. Geochemical, textural and thermobarometric studies define the crystallization history in compositions ranging from cumulate pyroxenite through quartz diorite, granodiorite, granite, and syn-magmatic gabbroic dykes. Early magmatic assemblages include orthopyroxene, clinopyroxene, plagioclase, biotite, Fe–Ti oxides and ternary feldspar, indicating crystallization from magmas containing <2 wt % H2O at 1100–900°C. Water enrichment in the residual melt induced hornblende crystallization at 5 ± 1 kbar, 800–600°C. Characterized by a continuum of large ion lithophile element (LILE)-enriched, high field strength element (HFSE)-depleted compositions, the I-type suite resembles modern continental arc batholiths in composition and size but not primary mineralogy. Magmatic arcs produced between 2·75 and 1·85 Ga commonly have charnockitic components, possibly because slab-derived fluids interacted with mantle wedges at ambient temperatures higher by ∼100°C than at present, producing large volumes of water-deficient magma. [ABSTRACT FROM PUBLISHER]

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) fight rare earth element (LREE)—enriched island-arc 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/Thmn. < 1; mn—normalized to primitive mantle values) are located primarily in the western parts of the formation, and suites with “nonarc” signatures (Sb/Thmn ≥ 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 arc 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 arc 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 intraoceanie arc rocks (e.g., boninites, island-arc tholeiites), and many have juvenile Nd isotope signatures (i.e., εNd(t) > 0; most have εNd(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. [ABSTRACT FROM AUTHOR]