MID-CRETACEOUS CLOSURE OF A MARGINAL SEAWAY BY WESTWARD SUBDUCTION AND MANTLE ORIGIN OF POST-COLLISIONAL SLAB FAILURE PLUTONIC SUITES IN THE NORTH AMERICAN CORDILLERA

A major collisional event occurred along the length of western North America from southern Mexico to Alaska during the mid-Cretaceous at ~100 Ma when a marginal basin – which had opened at about 135 Ma after the Late Jurassic-Neocomian Nevadan orogeny – closed above a west-dipping subduction zone.

In the best exposed area, located in western Mexico, a west-facing Albian carbonate platform, built atop the eastern margin of the Bisbee-Arperos seaway, was uplifted, exposed, drowned, buried by pelagic mud and Cenomanian flysch at 100 Ma, then overthrust from the west by rocks of the 128-105 Ma Santiago Peak-Alisitos arc and its basement.

Following collision, the tectonically thickened hinterland was intruded by 100-84 Ma tonalite-granodiorite plutonic complexes such as the La Posta and Sierran Crest magmatic suites – all with Sr/Y > 10, Sm/Yb > 2.5, Gd/Yb > 2, Nb/Y > 0.4, and La/Yb > 10, suggesting garnet-bearing, plagioclase-free sources. Their post-collisional nature and geochemistry are typical features of slab failure, not arc, plutons. Lackey showed that plutons of the post-100 Ma Sierran Crest magmatic suite had δ¹⁸O_zircon within, and close to, the range of mantle values, so were not derived from continental crust; yet they also have negative εNd₍₀₎ and ⁸⁷Sr/⁸⁶Sr_i > 0.706, which is surprisingly similar to Sierran mantle pyroxenite xenoliths and many basalts of the < 17 Ma Snake River Plain and the 44–7 Ka Big Pine volcanic field.

The most likely source for the slab failure magmas is the metamorphosed basaltic-gabbroic portion of the subducted oceanic plate. We infer that the evolved Sr and Nd isotopic ratios of the post-100 Ma plutonic rocks of the Sierra Nevada and Peninsular Ranges batholiths were derived from old, enriched SCLM by fractional melting. Other post-collisional suites, such as 100–85 Ma plutons within the Coast Range batholith of BC, have positive εNd and Sri < 0.704 similar to mantle plume and arc basalts, but preserve the distinctive high La/Yb, Sm/Yb and Gd/Yb and Sr/Y ratios of slab failure magmas, so apparently do not have old, enriched SCLM beneath them.

For slab failure magmatism, the SCLM typically belongs to the lower plate continental margin and not the arc, as the margin is pulled beneath the arc to isolate it from its formerly subjacent mantle. The 0.706 Sr_i isopleth reflects the differences in SCLM, not crust.