THE FORMATION OF CENOZOIC CONTINENTAL CRUST DURING AN IGNIMBRITE FLARE-UP EVENT: EVIDENCE FROM HAFNIUM AND OXYGEN ISOTOPES IN SIERRA MADRE OCCIDENTAL ZIRCONS

New hafnium and oxygen isotopic data from the Sierra Madre Occidental (SMO) silicic large igneous province, northwest Mexico, agree with models that infer the addition of substantial volumes (>50 %) of juvenile material from the mantle to the continental crust during the Eocene to earliest Miocene. Furthermore, they refute competing models that infer crustal recycling, driven by enhanced heat flow, as being the overwhelmingly dominant process. A sample suite covering the spatial and temporal extent of the SMO contains a zircon population with unexpectedly homogeneous hafnium and oxygen values interpreted as being moderately primitive and young (i.e. Cenozoic mantle depletion). Mixing models using local xenolith populations as end-members are best resolved by melting and assimilation of 10 % mafic orthogneiss and 10 % Proterozoic paragneiss.

We revise the current tectonomagmatic model of the SMO to account for our data and to agree better with recent advances in understanding of the immediate post-SMO geological evolution of northwestern Mexico. Rather than being a solely continental arc setting, we propose that significant horizontal stretching of the continental arc began in the later Eocene coinciding with alkaline magmatism and continued throughout the Oligocene. Crustal extension allowed for greater infiltration of mafic magma from the mantle and greater addition of advected heat into the base of the crustal, thereby weakening it further. The mid-Oligocene ignimbrite flare-up(s) were broadly contemporaneous with the passage of an opening slab window under northwestern Mexico that increased local geothermal gradient further. Subsequently rifting became the dominant tectonic process and bimodal volcanism was established in the Miocene. We infer that both increased rifting of the magmatic arc above a retreating slab and the subduction of an opening slab window were required to generate the uniquely voluminous SMO rhyolites, and these processes were neither contemporaneous nor cospatial anywhere else along the Cordilleran magmatic arc.