Sr‐Nd‐Hf‐Pb isotope and trace element evidence for the origin of alkalic basalts in the Garibaldi Belt, northern Cascade arc

Abstract

[1] In the Garibaldi Belt, the northern segment of the Cascade arc, basalts at Bridge River Cones, Salal Glacier, and Mt. Meager (BSM volcanic centers) are alkalic, atypical for an arc setting. Subduction signatures are negligible or absent from primitive alkalic basalts from Salal Glacier and Bridge River, while altered oceanic crust may have contributed a minimal amount of fluid at Mt. Meager. More evolved BSM basalts display trace element signatures considered typical of arc lavas, but this is a consequence of deep crustal assimilation rather than primary input from the subducted slab. Primary BSM basalts represent 3-8% melts that segregated from enriched garnet lherzolite at significantly higher temperatures and pressures (70-105 km) than calc-alkaline Cascade arc basalts. The BSM mantle source is significantly more incompatible element-enriched than the depleted mantle tapped by calc-alkaline Cascade arc basalts. The BSM basalts are also isotopically distinct from calc-alkaline Cascade arc basalts, more similar to MORB and intraplate basalts of the NE Pacific and NW North America. The relatively deep, hot, and geochemically distinct mantle source for BSM basalts is consistent with upwelling asthenosphere. The BSM volcanic centers are close to the projected trace of the Nootka fault, which forms the boundary between the subducting Juan de Fuca plate and the near-stagnant Explorer plate. A gap or attenuated zone between the plates may promote upwelling of enriched asthenosphere that undergoes low-degree decompression melting to generate alkalic basalts that are essentially free of slab input yet occur in an arc setting.

Evidence for trench-parallel mantle flow in the northern Cascade Arc from basalt geochemistry

“…For isotopic analysis, separate aliquots of sample powders were digested as outlined above after multi-step leaching in 6N HCl following Nobre Silva et al (2010). Pb, Sr, Nd, and Hf were separated from dissolved sample solutions using sequential ion exchange column chemistry.…”

Magnesium Isotopes as a Tracer of Crustal Materials in Volcanic Arc Magmas in the Northern Cascade Arc

“…Baker volcanic field, the Bridge River Cones, Glacier Peak, and the Chilliwack batholith (Tepper, 1996;Weis, 2013, 2015;Mullen and McCallum, 2014;Mullen et al, 2017). To ensure that no weathering has occurred, all samples were collected from the interior of lava flows, were confirmed to have low LOI contents (<1%), and were examined for signs of weathering in thin section (Mullen and Weis, 2013).…”

“…Baker, Glacier Peak, and the Chilliwack FIGURE 2 | (A) Ba and Nb contents of the North Cascades samples, as well as literature data from other volcanic arcs where available. The first labeled gray arrow approximates the effects of the addition of a subducted component (bulk sediment) prior to 10% partial melting of the depleted mantle, and the second approximates the effects of partial melting of the primitive mantle for the North Cascades based on Mullen and Weis (2013). Primitive mantle composition is from Sun and McDonough (1989).…”

“…(B) Sr isotope composition and Sr content of the North Cascades samples, as well as literature data from other volcanic arcs where available. The first labeled gray arrow approximates the effects of the addition of a subducted component (bulk sediment) to a primitive magma and the second approximates the effects of assimilation and fractional crystallization with a gabbro assimilant for the North Cascades based on Mullen and Weis (2013). The primitive magma composition is based on Mullen and Weis (2013).…”

“…Baker and the Bridge River Cones. The alkali basalts from the Bridge River Cones display trace element and Sr-Nd-Hf-Pb isotope compositions that indicate an enriched mantle source and the absence of a subducted component (Figure 2; Mullen and Weis, 2013). A slab gap between the stagnant Explorer and Juan de Fuca plates near the northern termination of the arc likely facilitated upwelling of enriched asthenosphere that produced these alkali basalts nearly free of slab input Weis, 2013, 2015;Mullen et al, 2017).…”