The Tectonic Setting and Origin of Cretaceous Batholiths within the North American Cordillera: The Case for Slab Failure Magmatism and Its Significance for Crustal Growth
Robert S. Hildebrand, Joseph B. Whalen, 2017. "The Tectonic Setting and Origin of Cretaceous Batholiths within the North American Cordillera: The Case for Slab Failure Magmatism and Its Significance for Crustal Growth", The Tectonic Setting and Origin of Cretaceous Batholiths within the North American Cordillera: The Case for Slab Failure Magmatism and Its Significance for Crustal Growth, Robert S. Hildebrand, Joseph B. Whalen
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In the standard model, Cordilleran-type batholiths form beneath volcanic arcs in thickened crust, but our survey of modern and ancient continental arcs revealed most to be regions of normal to thinned crust, not zones of crustal thickening. This suggested to us that the standard batholithic paradigm is flawed.
In order to better understand the batholiths, we explored (1) the 100–84 Ma La Posta and Sierran Crest magmatic suites of the Peninsular Ranges and Sierran batholiths, which formed after the 100 Ma Oregonian event due to closure of the Bisbee-Arperos seaway; (2) plutons and batholiths emplaced into the metamorphic hinterland of the 124–115 Ma Sevier event, which occurred in the Great Basin sector of the United States but, due to younger meridional transport, are now exposed in the Omineca belt and Selwyn Basin of Canada; and (3) Late Cretaceous–early Cenozoic intrusive rocks, such as the Coast, Idaho, and Boulder batholiths, which intruded a metamorphic hinterland during and after the Laramide event. The dominance of syn-to postdeformational emplacement and the distinctive slab failure–type geochemistry indicate that most, but not all, Cretaceous plutons within Cordilleran batholiths formed during and after arc-continent collision as the result of slab failure.
We interpret whole-rock geochemistry, as well as radiogenic and stable isotopes, to indicate that slab failure magmas involve only minor amounts of crust and are derived mainly from plagioclase-absent melting of garnet-bearing rocks in the mantle. Some suites, such as the <100 Ma Oregonian Sierran and Peninsular Ranges batholiths, have evolved Nd and Sr isotopes compatible with old enriched subcontinental lithospheric mantle. The well-known 0.70687/86Sri isopleth appears to separate rocks of Oregonian slab failure from rocks of older arc magmatism and is probably unrelated to any obvious crustal break; instead, it reflects involvement of old subcontinental lithospheric mantle in the slab failure magmas. To expand our findings we examined the geochemistry of Cenozoic slab window and Precambrian tonalite-trondhjemite-granodiorite suites and found them to share many similarities with the Cretaceous slab failure rocks.
Because most Cretaceous plutons in the North American Cordillera appear to represent juvenile additions to the crust, we argue that substantial volumes of continental crust are formed by slab failure magmatism. Slab failure rocks, especially those emplaced within the epizone, are richly metalliferous and make excellent exploration targets.