Jim Mize
Slab break-off can induce magmatism derived from upwelling of hot asthenosphere (Hildebrand and Bowring 1999). Hildebrand (2009) uses this mechanism to explain both the magmatism in the Canadian Rockies from 75 Ma to 60 Ma, and as further evidence for the existence of a west-dipping slab off the coast of North America in the Mesozoic. Hildebrand (2009) hypothesizes that the rocks of the Coast Mountain Batholith in western Canada are the magmatic body related to slab break-off magmatism in the Cordillera.
Conventional notions of the west coast of Canada involve one or two east dipping-subducting slab(s) and the associated magmatic arc(s) (Armstrong 1988; Heyden 1992, source of figure 1). 
The observation that led to this model is the recognition of the Coast Belt as the deeply eroded root of a Cretaceous and Tertiary magmatic arc (Monger et al., 1972). Heyden (1992) is in favor of the two subducting slabs, while Armstrong advocates for a single, narrow magmatic belt from one subduction zone. In both these models, the Coast Mountain Batholith is created from subduction-related magmatism, which results in linear belts of magmas that cover a large range of compositions.
Hildebrand (2009) proposes that the Coast Mountain Batholith is instead the product of slab break-off magmatism. This magmatism is caused when hot asthenosphere wells up through the gap created in a torn slab. This type of magmatism would also produce linear belts of magmatism that cover a large range of compositions (Hildebrand and Bowring 1999).
Since both mechanisms for creating the Coast Mountain Batholith result in similar rocks, chemical analysis of the rocks probably won’t be helpful in evaluating which source produced them. However, the different sources for the magmas (asthenosphere vs lithosphere) might cause differences in the isotopic ratios of strontium and neodymium in the magmas. Subduction-related magmas should have an 87Sr/86Sr ratio greater than 0.7045 and εNd < 0, while slab break-off magmas should have an 87Sr/86Sr ratio less than 0.7045 and εNd > 0.
The few (n = 12) data points available from the Western North American Volcanic and Intrusive Rock Database (NAVDAT) vaguely favor the slab break-off hypothesis, with 87Sr/86Sr ratios equal to or slightly less than 0.7045 and εNd >/= 0. Similarly, Armstrong (1988) found that the 87Sr/86Sr ratio is less than 0.704 in the southern part of the Coast Mountains. However, the northern part of the coast mountains does not conform to this, with 87Sr/86Sr ratios ranging from 0.704 to 0.706. Samson and others (1991) found that the 87Sr/86Sr ratios ranged from 0.7049 to 0.7061, and that εNd was < 0.
Hildebrand (2009) addresses this issue by acknowledging that the break-off magmas could have assimilated some of the lithospheric mantle as they intruded. The lithospheric mantle is an extremely diverse source region (Menzies et al., 1987; Foley, 1992), so the magma assimilating something with a high Sr ratio and a low εNd value is completely reasonable. Hildebrand also asserts that rising magmas can assimilate up to 75% crustal derived material. The paper cited for this, Housh and McMahon (2000), does not propose a mechanism for how this much material could be assimilated. Housh and McMahon found that the Sr ratios and εNd values that they have found in their rocks are a product of mixing, and their isotopic values are consistent with assimilation of 10% to 75% crustal material, depending on the composition of the assimilant.
In summary, the proposal put forth by Hildebrand (2009) that the rocks of the Coast Mountain Batholith are a product of slab break-off magmatism is dubious, but plausible. There aren’t any clear indications that these rocks came from slab break-off magmas, because the magmas may have assimilated an indeterminate amount of the rocks that they intruded. Conversely, there isn’t any evidence that automatically rules out the plausibility of Hildebrand’s hypothesis. The rocks observed in the Coast Mountain Batholith are consistent with both subduction magma and slab break-off magma, so the hypothesis can be neither confirmed nor denied with only this evidence. The best path to verifying this hypothesis is to determine the validity of Hildebrand’s west-dipping subduction in the Mesozoic.
Works Cited:
Armstrong, R.L., 1988, Mesozoic and early Cenozoic magmatic evolution of the Canadian Cordillera, in Clark, S.P., Jr., Burchfield el, B.C., and Suppe, J., eds., Processes in Continental Lithospheric Deformation: Geological Society of America Special Paper 218, p. 55–91.
Foley, S., 1992, Vein-plus-wall-rock melting mechanisms in the lithosphere and the origin of potassic alkaline magmas: Lithos, v. 28, p. 435–453, doi: 10.1016/0024-4937(92)90018-T.
Hildebrand, R.S., and Bowring, S.A., 1999, Crustal recycling by slab failure: Geology, v. 27, p. 11–14, doi: 10.1130/0091-7613(1999)027<0011:CRBSF>2.3.CO;2.
van der Heyden, P., 1992, A Middle Jurassic to Early Tertiary Andean-Sierran arc model for the coast belt of British Columbia: Tectonics, v. 11, p. 82–97, doi: 10.1029/91TC02183.
Housh, T.B., and McMahon, T.P., 2000, Ancient isotopic characteristics of Neogene potassic magmatism in western New Guinea (Irian Jaya, Indonesia): Lithos, v. 50, p. 217–239, doi: 10.1016/S0024-4937(99)00043-2.
Menzies, M., Rogers, N., Tindle, A., and Hawkesworth, C.J., 1987, Metasomatism and enrichment processes in lithospheric peridotites, an effect of asthenosphere-lithosphere interaction, in Menzies, M.A., and Hawkesworth, C.J., eds., Mantle Metasomatism: London, Academic Press, p. 313–361.
Monger, J. W. H., J. G. Souther, and H. Gabrielae, Evolution of the Canadian Cordillera: A plate tectonic model, Am. J. Sci., 272, 577-602, 1972.
Samson, S.D., Patchett, P.J., McClelland, W.C., and Gehrels, G.E., 1991, Isotopic constraints on the petrogenesis of the west side of the northern Coast Mountains batholith, Alaskan and Canadian Cordillera: Canadian Journal of Earth Sciences, v. 28, p. 939–946.