An estimate for the bulk composition of juvenile upper continental crust derived from glacial till in the North American Cordillera

Abstract

We use a compilation of bulk chemical analyses for 2771 rocks and 3487 till samples (silt + clay fraction) from British Columbia, Canada to construct an estimate of juvenile upper crust composition in the North American Cordillera for comparison with upper crust estimates for older continental crust. Principle component analyses demonstrate that glacial tills are a robust natural average of their surface rock provenance. The chemical weathering during the generation and deposition of tills is small, yet they preserve the chemical weathering history of their source regions. Our results show that the upper continental crust of the Cordillera is broadly similar to other estimates in terms of the abundance of Si, Al, Mg, Ca, Na, Zr, Y, Sr, Ba, Th and U. The till-based upper crustal estimate, however, shows consistent and marked enrichment in Fe, Ti, Mn, P and HREE and depletion in LREE and K. These attributes are not obviously attributed to any analytical or sampling bias; surface rock averages and modern river sediments from British Columbia show identical patterns of enrichment and depletion as the tills. We attribute the unique character of our Cordilleran juvenile upper crust estimate to a significant component of basaltic soils in till generation, as compared to upper crustal estimates from more mature denuded terrains. Nonetheless, our crustal estimate remains overall ‘andesitic’, similar to those from Precambrian shields, supporting the notion that reworking of old crust is volumetrically far more important than production of juvenile crust in the growth of the North American continent.