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1 Isotopic and Geochemical Evidence for a Recent ISOTOPIC AND GEOCHEMICAL EVIDENCE FOR A RECENT TRANSITION IN MANTLE CHEMISTRY BENEATH THE WESTERN CANADIAN CORDILLERA by Christian D. Manthei A Prepublication Manuscript Submitted to the Faculty of the DEPARTMENT OF GEOSCIENCES In Partial Fulfillment of the Requirements for the Degree of MASTER OF SCIENCE In the Graduate College THE UNIVERSITY OF ARIZONA 2009 1 Abstract 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 87 86 mantle ( Sr/ Sr = 0.70353 – 0.70486; εNd = +2.5 - +5.7). By contrast, Quaternary lava flows have lower LILE concentrations, positive Nb-Ta anomalies, and were likely 87 86 generated by upwelling asthenosphere ( Sr/ Sr = 0.70266 – 0.70386; εNd = +7.4 - +8.8). A regional comparison of numerous 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 c.a. 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. Key Words: Coast Mountains, mantle chemistry, radiogenic isotopes, continental mafic magmatism, continental extension 2 1. INTRODUCTION Continental basaltic magmatism provides a unique opportunity to study the geochemical nature and protracted evolution of the subcontinental mantle [e.g., Allègre et al., 1981; Hawkesworth et al., 1993; Carlson et al., 2005]. While the preserved volcanic cover and long-term intrusive construction of plutons and batholiths document the magmatic history of arcs, post-subduction mafic magmatism is one of the few tools available to geochemists to study the mantle during and after the cessation of arc- magmatism. Major, trace element and isotope compositions of mafic magmas provide information on the affinity of melt sources (i.e. lithosphere vs. asthenosphere), metasomatic processes, and pathways of melt migration into the crust [e.g., Pearce and Cann, 1973; Winchester and Floyd, 1977; Hawkesworth et al., 1990; Schmidt and Poli, 2005]. Augmentation of the magma’s chemistry by crustal contamination during ascent and/or emplacement must be identified and ruled out, if the magmas are to be representative of a primary mantle source [Hawkesworth et al., 1990; McDonough and Sun, 1995]. Identifying potential contamination may be difficult in some cases, because the composition of assimilated mafic lower crust [Rudnick and Fountain, 1995] may resemble the lithospheric mantle [Farmer, 2005]. Continental basalts are typically generated during crustal extension (e.g., Basin and Range Province, East Africa), hot spot magmatism (e.g., Snake River Plain), and wet-melting associated with a magmatic arc (e.g., Cascadia, Andes). Basaltic rocks from these regimes can be used to resolve the chemical nature of the subcontinental mantle within a time-integrated framework [Farmer et al., 2002; Cousens and Bevier, 1995]. Evaluating the geochemical nature of the subcontinental mantle is important for our 3 understanding of the evolution of magmatic arcs and orogenic belts [e.g., Kay et al., 1994; Wernicke et al., 1996]. In this study, major, trace element and isotopic compositions of mafic dike and lava flows from western British Columbia (52.3° – 55.3°N; Fig. 1), accompanied by newly acquired 40Ar/39Ar geochronology, provide a time-integrated geochemical history of the mantle since the demise of the Cordilleran arc (~48 Ma). Data from this study are compared to regional basalts from British Columbia and the Yukon Territory, also ranging in age from Eocene to Quaternary, to demonstrate that two geochemically distinct mantle-sources produced spatially widespread basaltic magmatism in western Canada. A regional tectonic synthesis indicates that the onset of dextral transform motion along the Queen Charlotte fault system, and intermittent periods of highly oblique divergence and convergence between the North American and Pacific plates in the early Eocene [Engebretson et al., 1985; Stock and Molnar, 1988], drove continental extension in western Canada. This period of extension has been documented by the opening of large, normal fault-bounded basins [Rohr and Dietrich, 1992; Rohr and Currie, 1997; Dostal et al., 2001] and widespread mafic magmatism since the Eocene. 1.2 Cordilleran Tectonic Framework Canada’s western Cordillera is composed of several allochthonous terranes and discrete crustal fragments that were outboard of the paleo-Laurentian continent and accreted to the margin from Jurassic to Eocene time [Coney et al., 1980; Monger et al., 1982; Crawford et al., 1987, 1999; Colpron et al., 2007; Gehrels et al., 2009]. Figure 1 shows the location of the Stikine terrane, Coast Mountains Batholith, the metamorphic 4 pendants and terranes of the Central Gneiss Complex, the Gravina belt and the amalgamated Alexander-Wrangellia terrane, respectively. Readers are referred to Coney et al., [1980], Monger et al., [1982], Colpron et al., [2007], and Gehrels et al. [2009] for comprehensive reviews of the Cordilleran terranes and their interactions through time. 1.3 Jurassic – Eocene Magmatic Arc Cordilleran arc magmatism was active in western Canada from early Jurassic to early Tertiary time [Armstrong, 1988; van der Heyden, 1992], and occurred coevally with terrane accretion and greenschist to amphibolite facies metamorphism [Monger et al., 1982; Hollister, 1982; Gehrels and Saleeby, 1987; Crawford and Crawford, 1991; Stowell and Crawford, 2000]. The plutons of the Coast Mountains Batholith (CMB) were assembled over this ~120 Ma period, and presently represent the largest exposed batholithic complex in North America [Barker and Arth, 1984]. The batholith, occasionally referred to as the Coast Plutonic Complex, can be traced from the northwestern United States, through British Columbia, the Yukon Territory and into southeastern Alaska, comprising a total distance of ~1,700 km [Roddick and Hutchinson, 1974; Gehrels et al., 2009]. The igneous rocks of the CMB range in composition from gabbroic through leucogranite, although the majority of plutons are tonalite or granodiorite [Gehrels et al., 2009]. They are dominantly mantle-derived [Samson et al., 1989; Cui and Russell, 1995], but may contain ~10 – 50% recycled crustal material [Samson et al., 1991]. Periods of high magmatic flux (35 – 50 km3/my per km) at 160-140 Ma, 120-78 Ma and 55-48 Ma [Gehrels et al., 2009] correspond to periods of expedited batholith construction, and are 5 documented by numerous U-Pb crystallization ages of zircon from CMB plutons [Crawford et al., 1999; Gehrels et al., 2009]. The lack of plutonic zircon ages that post- date the early – middle Eocene (~48 Ma), is consistent with field, stratigraphic and geochronological data that indicate Cordilleran arc magmatism along the Canadian margin ceased during this time [Armstrong, 1988; van der Heyden, 1992]. In southern British Columbia, the Garibaldi and Pemberton belts (Fig. 1) form a discrete chain of magmatic edifices that are the northernmost extension of the active Cascade arc in the northwestern United States. The Garibaldi belt was active until very recently [c.a. 0.1 Ma; Armstrong, 1988; Coish et al., 1998], likely as a result of the ongoing subduction of the Juan de Fuca plate [Coish et al., 1998]. 1.4 Eocene – Quaternary Mafic Magmatism Numerous studies have focused on Eocene – Quaternary basalts (and similar mafic rocks) that are widely distributed throughout the western Canadian Cordillera. Basalts are preserved as large plateau-forming lava flows [e.g., 3,300 km3 Chilcotin Group Basalts; Bevier, 1983], small-volume cinder-cone and fissure eruptions [e.g., Carignan et al., 1994; Francis and Ludden, 1990], and dikes that intrude the igneous rocks of the CMB and metamorphic rocks of the Cordilleran terranes [Irving et al., 1992; Rohr and Currie, 1997; Crawford et al., 2005; Rusmore et al., 2005]. Increased mafic magmatism beginning at ~48 Ma corresponds with the initial stages of transform motion along the Queen Charlotte Fairweather fault system (Fig. 1), and intermittent periods of highly oblique divergence and convergence between the North American and Pacific plates [Engebretson et al., 1985; Stock and Molnar, 1988; Irving et al., 1992]. The 6 opening and subsidence of large extensional basins during the Miocene [Rohr and Dietrich, 1992; Rohr and Currie, 1997] occurred concurrently with shear-distribution in the North American plate [Rohr and Currie, 1997], increased mantle heat-flow [Edwards and Russell, 2000] and the emplacement of mafic dike swarms that trend perpendicularly to the direction of extension [Irving et al., 1992]. The delineation of separate volcanic fields in the western Cordillera has been approximated in Figure 1, after the work of several authors.
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