WAS THE LATE CRETACEOUS-PALEOCENE MAGMATIC FIRESTORM CAUSED BY SLAB FAILURE?

Magmatism within the American Cordillera is commonly interpreted to have resulted from long-lived eastward subduction of Pacific oceanic lithosphere. However, the development of a model in which the Americas collided with a composite arc-bearing ribbon continent above a westward-dipping subduction zone was recently confirmed by seismic tomography and invites actualistic models that involve both subduction-related and slab failure magmatism.

Shut-down of arc magmatism in the ribbon continent just prior to 80 Ma, coupled with Late Cretaceous-Paleocene deformation in the Cordillera extending from Alaska to Tierra del Fuego, are interpreted to represent terminal collision of the ribbon continent with the Americas. A period of intense magmatism, commonly mineralized, occurred during and after the deformation over most of its range. The magmatism is generally associated with linear belts of rapid exhumation, which contrasts with the typical low-standing nature of magmatic arcs. These include the Late Cretaceous-Paleocene Alaska Range-Talkeetna Mountains magmatic belt; the Late Cretaceous-Paleocene syn-to post-deformational Ruby Range suite, as well as the Carmacks, Donjek, Tlansanlin, and Mount Creedon volcanics of the Yukon; the 1500 km-long band of syn-deformational tonalite sills of the Coast plutonic complex of British Columbia; and rocks of the 80-64 Ma Boulder batholith and associated Elkhorn volcanics, the 80 Ma Two Medicine, and the 75 Ma Adel volcanics of the northern US. In the Great Basin area magmatism is present only in the 75-50 Ma Colorado Mineral belt. From the Sonoran desert region of Arizona southward into Mexico a linear band of 76-55 Ma magmatism and associated porphyry copper deposits occurs. In South America, 73-62 Ma centered complexes and belt-parallel dike swarms of the Coastal batholith followed terminal collision as did magmatism of the 75-60 Ma Neunauco belt of northern Argentina.

A slab failure origin of the magmatism explains its distribution over the length of the Americas, its relationships to the deformation and contemporaneous exhumation, its commonly metalliferous nature, and its general absence in the Great Basin sector due to previous slab failure there during the Sevier event. Many slab failure magmas with SiO2>60% have Sr/Y>20, Nb/Y>0.4, La/Yb>10, and Gd/Yb>2.0.