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Journal of Petrology Advance Access originally published online on September 15, 2009
Journal of Petrology 2009 50(10):1935-1954; doi:10.1093/petrology/egp060
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© The Author 2009. Published by Oxford University Press. All rights reserved. For Permissions, please e-mail: journals.permissions@oxfordjournals.org

Generating High Mg-numbers and Chemical Diversity in Tonalite–Trondhjemite–Granodiorite (TTG) Magmas during Melting and Melt Segregation in the Continental Crust

Amanda Getsinger1, Tracy Rushmer2,*, Matt D. Jackson3 and Don Baker4

1Department of Geology, University of Vermont, Burlington, VT 05405, USA
2Gemoc, Department of Earth and Planetary Sciences, Macquarie University, Sydney, NSW 2109, Australia
3Department of Earth Science and Engineering, Imperial College, London SW7 2AZ, UK
4Department of Earth and Planetary Sciences, Mcgill University, 3450 University Street, Montreal, Quebec, Canada H3A 2A7

RECEIVED JUNE 5, 2008; ACCEPTED AUGUST 12, 2009


  Abstract

Major, trace, and rare earth element compositions of both tonalite–trondhjemite–granodiorite (TTG) and modern adakite-like magmas are typically used in conjunction with batch melting experiments and models to infer source rock composition, depth of melting and tectonic setting. However, batch melting does not capture the impact of melt segregation processes on magma geochemistry. We have used melting experiments in conjunction with numerical modelling to investigate the impact of melt segregation on TTG arc crust formation. Our melt segregation equilibrium (MSE) experiments are designed to reproduce the local changes in bulk composition that are predicted by the numerical model to occur as buoyant melt migrates upwards along grain boundaries and accumulates to form a magma that leaves the source region. The MSE experimental results show distinct differences in the melt and solid phase compositions and solid phase stability when compared with direct partial melting experiments. They yield a significant reduction in hornblende and plagioclase modal proportions at lower temperatures and partial melt compositions that are lower in An and have higher Mg-numbers. These results suggest that dynamic melt segregation and equilibrium processes may have a significant impact on modes, melt compositions and geochemical indicators such as Mg-numbers. Mantle wedge interaction may not be necessary to generate varying Mg-numbers in TTG and adakite magmas. Moreover, the use of batch melting models or experiments to interpret these geochemical signatures may not be appropriate.

KEY WORDS: TTG; experimental petrology; melt segregation; Mg-numbers

*Corresponding author. E-mail: trushmer{at}els.mq.edu.au


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