On the Thermal Evolution of Archean Cratons : High-Grade Metamorphism, Granite Formation and Long-Term Cooling

English Abstract:

High-precision geochronological and thermodynamic studies provide records of unprecedented detail on the thermal evolution of continents following major episodes of continental assembly and orogenesis in the Archean. Similar characteristics have been observed in all the major cratons worldwide. Metamorphic events affect very large areas quasi-simultaneously, indicating that the driving mechanism operates on a large-scale, but peak temperatures vary between terranes. The timing of high-grade metamorphism lags magmatism and tectonic amalgamation by several tens of million years. Final stage metamorphic events are associated with granite formation and emplacement. These events are followed by slow and long lasting isobaric cooling that persists for up to 1 Gy at rates of less than 1K/My. The source of the heat that is required to account for these phenomena remains controversial. The most common explanations are the underplating of basaltic melts due to a mantle plume or the instability of thickened lithosphere, which is also expected to generate basaltic melts, but they are not consistent with the significant time-lag that separates voluminous mafic magmatism and high-grade metamorphism.

We show that all these features are consistent with the thermal evolution of a new continental assemblage that adjusts to heat released by radioactive decay in crustal rocks. Quantitative results depend on the total amount of radioactive elements in the newborn crust, which can be determined by heat flow studies. Using heat flow and heat production data from the Archean Superior Province of the Canadian Shield, we show that peak temperatures of ≈ 800-900°C were reached a few tens of millions years after the final amalgamation event that occurred at ≈2.7 Ga. Lateral variations of peak metamorphic temperatures are accounted for by lateral changes of crustal heat production. We also show that peak temperatures are sensitive to the width of the newly accreted terranes. It is only after the final docking event that saw the width of the accreted belts exceed ≈400 km that the necessary conditions for high-grade metamorphism were met. The slow and long-term cooling that followed peak metamorphism is a straightforward consequence of the decay of heat producing elements in the crust.