Geodynamic controls on Tertiary arc magmatism in Ecuador: Constraints from U-Pb zircon geochronology of Oligocene-Miocene intrusions and regional age distribution trends

We obtained U-Pb zircon ages of Late Tertiary intrusions in the northern Andes to provide robust time calibration points for the intrusive geochronologic framework of Ecuador which is mostly based on K-Ar data. Magmatism associated with intrusion emplacement ranges from 31 to 7 Ma, with ages mainly pooling in the Late Oligocene-Early Miocene. Where both K-Ar and U-Pb data exist for a given intrusive system, ages obtained by the different methods are usually concordant within 1-4 my. suggesting that K-Ar ages are accurate indicators for the timing of Tertiary arc magmatism on a regional multi-m.y. scale. Except for a single sample, the investigated intrusions completely lack externally inherited zircons, in agreement with dominantly zircon-poor, oceanic crustal basement domains. Spatio-temporal distribution trends of Tertiary arc magmatism inferred from screened U-Pb. K-Ar, and zircon fission track geochronologic data allow tracking of the progressive along-arc broadening of a flat slab segment below southernmost Ecuador in the Mid- to Late Miocene, and of moderate slab shallowing in northern-central Ecuador in the Late Miocene. These regional arc migration patterns correlate in time with the subduction of the buoyant Inca Plateau and the Carnegie Ridge seamount chain. The temporal distribution of Tertiary Ecuadorian arc magmatism indicates a Late Oligocene-Early Miocene arc magmatic flare-up event comprising widespread ignimbrite eruption and batholith construction. Initiation of the flare-up event coincides in time with accelerating, less oblique Farallon/Nazca-South America plate convergence, suggesting a positive feedback between convergence rates, asthenospheric melt production, mantle-crust melt flux, and upper crustal arc magmatic productivity in Ecuador. (C) 2010 Elsevier B.V. All rights reserved.