Isotopic and geochemical evidence for a recent transition in mantle chemistry beneath the western Canadian Cordillera

Abstract

[1] New petrologic, geochemical, and isotopic data are reported from a suite of mafic dike and lava flow samples collected from sites within the western Canadian Cordillera. Samples range in age from Eocene to Quaternary and document a significant transition in mantle chemistry that occurred sometime after 10 Ma. Eocene to late Miocene basalts emplaced as dikes within the Coast Mountains Batholith contain abundant hornblende, are enriched in large ion lithophile elements (LILE) (Ba, Rb, K), have negative high field strength element (HFSE) (Nb, Ta) anomalies, and were likely derived from lithospheric mantle ( 87 Sr/ 86 Sr = 0.70353-0.70486; e Nd = +2.5 to +5.7) that was stabilized after the cessation of arc magmatism in the area. By contrast, Quaternary lava flows have lower LILE concentrations, positive Nb-Ta anomalies, and were likely generated by upwelling asthenosphere ( 87 Sr/ 86 Sr = 0.70266-0.70386; e Nd = +7.4 to +8.8) or at least a mantle source with different chemical and isotopic characteristics. A regional comparison of mafic rocks from western Canada that are also Eocene to Quaternary in age indicates that the transition in mantle chemistry after 10 Ma was pervasive and widespread and was not limited to the present study area. This transition occurred $40 Ma after the cessation of Cordilleran arc magmatism in central British Columbia, suggesting that large-scale transitions in mantle chemistry beneath magmatic arcs may occur on the order of tens of millions of years after the final subduction of oceanic lithosphere, in this case as a result of lithospheric thinning by continental extension.

Foundering-driven lithospheric melting: The source of central Andean mafic lavas on the Puna Plateau (22°S–27°S)

“…However, if K and La/Ta values fi ngerprint extensive replacement of the lower lithosphere by upwelling asthenosphere, then there should be other systematic trends in the elemental and isotopic values in Puna primitive lavas. This has been observed in other orogens (e.g., Iberian Massif-Gutierrez-Alonso et al, 2011; Canadian cordillera- Manthei et al, 2010). Instead, Kay et al (1994) and subsequent studies of primitive Puna lavas (Kraemer et al, 1999;Drew et al, 2009;Risse et al, 2008Risse et al, , 2013 have Schnurr et al (2006), with sample locations and regional names from this study.…”

“…The mafi c volcanic record has been used to support other geological evidence for proposed lithosphere-removal events in a variety of tectonic settings, including the Colorado Plateau (Bird, 1979;Crow et al, 2011), Sierra Nevada (Farmer et al, 2002;Ducea, 2002), Iberian Massif (Gutierrez-Alonso et al, 2011), central Andes (Kay et al, 1994), Tibet (Chung et al, 2009), and Canadian cordillera (Manthei et al, 2010). Foundering is thought to generate mantle melts because foundering pieces of lithosphere make space for mantle asthenosphere to upwell, melt via decompression, and advect heat toward the base of the remaining lithosphere (Kay and Kay, 1993;Kay et al, 1994;Ducea and Saleeby, 1998).…”

Lithospheric mantle xenoliths sampled by melts from upwelling asthenosphere: The Quaternary Tasse alkaline basalts of southeastern British Columbia, Canada

“…An early framework for the Late Cenozoic volcanism in the Canadian Cordillera was provided by Bevier et al (1979) and Souther (1991) who divided the region into a series of belts and fields, each with a distinctive rationale such as subduction, extension or mantle plume activity. Since then, numerous studies of the volcanic belts have provided additional information on age and petrogenesis (e.g., Edwards and Russell, 2000;Preece and Hart, 2004;Abraham et al, 2005;Sluggett, 2008;Manthei et al, 2010;Mullen and Weis, 2013;Kuehn et al, 2015).…”

Mantle-drip magmatism beneath the Altiplano-Puna plateau, central Andes

“…Such syn-drip mantle melts should be predictably different from local magmatic arc products erupted prior to dripping, and thereby identifi able geochemically and/or isotopically as rocks generated by a source typical of the convective mantle. However, where regional geochemical shifts have been observed in areas suspected of delamination (Manthei et al, 2010;Gutierrez-Alonso et al, 2011;Putirka and Platt, 2012) those shifts to a more primitive asthenospheric source took place over time scales of tens of millions of years (15-30 m.y. ), which is interesting because the individual foundering events occur at ~1 m.y.…”