Timescales of collisional metamorphism from Sm-Nd, Lu-Hf and U-Pb thermochronology: A case from the Proterozoic Putumayo Orogen of Amazonia

Abstract

The construction of mountain belts by continent-continent collision is a fundamental phase of the supercontinent cycle and an inevitable consequence of subduction-driven plate tectonics on Earth. Yet, quantitative estimation of the rate and duration of prograde orogenic metamorphism in high-grade collisional belts has proven elusive to constrain. Differences in the chemical partitioning and diffusive behavior of elements associated with the 147Sm-143Nd and 176Lu-176Hf isotopic systems in metamorphic garnet can provide valuable insights on the time scales over which this mineral grows and cools in regionally metamorphosed terranes. Here, we present new Sm-Nd and Lu-Hf isotopic results from multiple garnet and whole-rock aliquots from a granulite-grade metasedimentary rock from the Proterozoic Putumayo Orogen of South America. Using a combination of internal isochron dating by these two decay systems, combined with apatite U-Pb dating and pressure-temperature-time (P-T-t) constraints from mineral equilibria and chemical diffusion geospeedometry, we provide first-order insights into the time scales of a Proterozoic continent-continent collisional metamorphic event in Amazonia associated with its incorporation to the Supercontinent Rodinia. Our results indicate that these metasediments attained peak granulite-grade metamorphic conditions of nearly 0.62 GPa and 680 °C at approximately 1035 Ma, and started to cool slowly, presumably during exhumation, at a rate of ∼5 K/Myr. Simple diffusion scaling relationships and numerical modeling of Sm-Nd and Lu-Hf in garnet indicate that, while Sm and Nd were both fully re-equilibrated at peak temperatures and therefore date cooling, Lu and Hf underwent diffusive decoupling due to incomplete Hf re-equilibration. By modeling the impact of this differential re-equilibration in the development of the Sm-Nd and Lu-Hf isochrons, we constraint the duration of the prograde metamorphic path and argue that garnet growth in this sample took place over millions of years—approximately 25 Myr according to our preferred interpretation. The determined apatite U-Pb date vs. grain-size relationship cannot be reconciled with the thermal history described above when modeled using the existing experimental diffusion data, and instead suggest that much slower Pb diffusivities in this mineral compared to those predicted by the existing experimental data may have inhibited its resetting. We note that the inferred temperature-time history for the metamorphic loop of this segment of the Putumayo basement is comparable to that proposed for the metamorphism and exhumation of metasediments in the India-Asia collision zone, suggesting that similar tectonic processes and rates likely operated during the Mesoproterozoic Putumayo orogeny.