Neodymium isotope geochemistry of felsic volcanic and intrusive rocks from the Yukon–Tanana Terrane in the Finlayson Lake Region, Yukon, Canada

Abstract

Devonian–Mississippian felsic rocks from the Finlayson Lake region have variable geochemical and Nd isotopic characteristics that provide insights into the tectonic and metallogenic evolution of the Yukon–Tanana terrane (YTT), and the northern Cordillera. Late Devonian (~365–360 Ma) calc-alkaline and tholeiitic arc felsic rocks in the mafic-dominated Fire Lake unit yield εNd350 = –4.8 and +0.1, respectively, and have 1.49–1.94 Ga depleted mantle model ages (TDM). Devonian–Mississippian (~360–356 Ma) felsic volcanic (Kudz Ze Kayah unit, Wolverine succession) and intrusive rocks (Grass Lakes suite) associated with volcanogenic massive sulphide (VMS) deposits have εNd350 = –7.8 to –9.5 with TDM = 1.59–2.25 Ga. A granitoid sample from the Early Mississippian (~350–345 Ma) Simpson Range plutonic suite has εNd350 = –12.9 and TDM = 2.01 Ga, similar to previously reported values for this suite. The VMS-associated Grass Lakes suite of granitoids has higher high field strength element (HFSE) and rare-earth element (REE) contents, and higher Zr/Sc, Zr/TiO2, Nb/La, and Zr/La values relative to the Simpson Range plutonic suite; these geochemical features are similar to coeval VMS-associated felsic volcanic rocks in the Kudz Ze Kayah unit. The identification of similar HFSE–REE-enriched felsic volcanic and subvolcanic intrusive rocks may aid in delineating prospective regions for VMS mineralization in the YTT and other continental-margin arc to back-arc environments. The geochemical and Nd isotopic data for these YTT felsic rocks suggest that they reflect episodic mid-Paleozoic arc (Fire Lake unit; Simpson Range plutonic suite) and back-arc magmatism (Kudz Ze Kudz unit; Wolverine succession) built upon a transitional basement with variable, but significant, influence from evolved (Proterozoic) crustal materials.

Petrology and U-Pb Geochronology of Footwall Porphyritic Rhyolites from the Wolverine Volcanogenic Massive Sulfide Deposit, Yukon, Canada: Implications for the Genesis of Massive Sulfide Deposits in Continental Margin Environments

“…In this model the porphyries formed in response to crustal melting and crustmantle mixing caused by basaltic underplating during backarc extension (Piercey et al, , 2003. In both porphyry suites, crustal melting and/or crustal contamination was quite important as indicated by the flat upper continental crust-normalized REE patterns (Fig.…”

“…Furthermore, the inherited zircon (Fig. 16, Table 3) and existing Nd isotope data (Piercey et al, 2003) the role for significant ancient crust in the genesis of these porphyries. There are subtle but important differences between the FP and the QFP.…”

Geochemistry and tectonic significance of alkalic mafic magmatism in the Yukon-Tanana terrane, Finlayson Lake region, Yukon

“…2, 3). The lowermost part of the Grass Lakes succession consists of pre-365 Ma quartz-rich, noncarbonaceous metaclastic rocks of unit 1, which are overlain by the -365-360 Ma mafic dominated arc-and back-arc rocks of the Fire Lake unit (Mortensen 1992;Grant 1997;Piercey 1999, 2000;Piercey et al 1999Piercey et al , 2001a. The KZK unit stratigraphically overlies the Fire Lake unit and consists of felsic volcanic and variably carbonaceous sedimentary rocks (Murphy 1998) (Figs.…”

“…Numerous workers have used geological, geochemical, and isotopic techniques to illustrate that the Devonian-Mississippian evolution of the YTT was a product of west-dipping continental arc magmatism, possibly built on the distal edge of the North American margin (e.g., Mortensen 1992 and references therein;Creaser et al 1997Creaser et al , 1999Grant 1997;Piercey et al 1999Piercey et al , 2001aPiercey et al , 2001b. In the Finlayson Lake region, arc magmatism is recorded primarily by the rocks of the -365-360 Ma Fire Lake unit and Money Creek thrust sheet (e.g., Grant 1997;Piercey et al 2001a) which lie to the south and west of the area of alkalic magmatism.…”

“…In recent years new stratigraphic mapping, trace element geochemistry, and Nd-isotope geochemistry of mafic rocks in the YTT have led to significant new advances in understanding the tectonic setting and origin of YTT mafic rocks (e.g., Creaser et al 1997;Grant 1997;Piercey 1999, 2000;Murphy 2001;Piercey et al 1999Piercey et al , 2001aPiercey et al , 2001b. For example, Creaser et al (1997) and Grant (1997) illustrated that mafic rocks of the Anvil Assemblage had arc-like trace element geochemical and Nd isotopic signatures that were incompatible with them being correlative to basalts of the Slide Mountain terrane, which have mid-ocean-ridge basalts (MORB) like signatures (Nelson and Bradford 1993;Nelson 1993;Plint and Gordon 1997).…”