『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.
Keywords: Geochemistry; Till; Rock; Crust; Weathering; Cordillera』
1. Introduction
2. Geological background: the North American Cordillera
3. Geochemical data
4. Results
4.1. Principal components analysis
4.2. Major elements
4.3. Trace elements
5. Discussion
5.1. Do fine fractions of tills reflect bedrock compositions?
5.2. The surface environment recorded in glacial till geochemistry
5.3. Estimation of juvenile upper crustal composition
Acknowledgements
Appendix A. Supplementary data
References