ACCESSORY MINERAL CONTROLS ON METAMORPHIC HEAT BUDGETS, ANATECTIC TIMESCALES, AND CRUSTAL DIFFERENTIATION

Radioactive heat production and transfer plays an integral role in the evolution and differentiation of Earth’s crust, with decay of K, U, and Th (i.e., heat-producing elements) generating over 99% of radiogenic heat. These elements are expected to be broadly incompatible during anatexis of crustal rocks. Consequently, most interpretive cross-sections of continental crust include a higher concentration of the heat-producing elements in the upper and middle crust and a complementary depletion in the lower crust. However, this assumption relies mainly on observations from xenoliths which are inherently exotic with little tectonic context. Further, the effects of heat-producing element redistribution during anatexis on metamorphic heat budgets and timescales remains abstract.

To explore this issue, we use the Kapuskasing Uplift (Canada) as a natural laboratory as it is a tilted Archean crustal section which was frozen during crustal differentiation and is comprised of upper-crustal greenstones, middle-crustal tonalite-trondhjemite-granodiorite (TTG) gneisses, and lower-crustal high-grade metabasites. We model anatexis of natural metabasite compositions ranging from fertile greenstones to residual amphibolites/mafic granulites melting, which have been shown to be suitable source rocks for TTG magma. Model results indicate that most of the heat-producing element budget is hosted in apatite, hornblende, K-feldspar, epidote, and melt. Apatite is particularly important in retaining heat-producing elements in the residual lower-crust as it is stable at suprasolidus conditions. However, the partitioning of heat-producing elements into low-density melt decreases the heat production of the system and increases heating times as metamorphism progresses. Our modelling suggests that 1) the mantle is an equal to or greater source of heat than heat-producing elements during the anatexis of metabasites 2) the lower crust does not become depleted in heat-producing elements during anatexis, and 3) anatexis of metabasites is expected to take >30 Ma to reach 850°C. These results indicate that the “depleted” lower crust is not always so and encourages a change in our current approach to estimating crustal heat-production in the deep crust.