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Chemical structure in granitic magmas – a signal from the source?

Published online by Cambridge University Press:  01 March 2009

J. D. Clemens
Affiliation:
Department of Earth Sciences, University of Stellenbosch, Private Bag X1, 7602 Matieland, South Africa Email: [email protected]
P. A. Helps
Affiliation:
School of Geography, Geology and the Environment, CEESR, Kingston University, Penrhyn Rd, Kingston-upon-Thames, Surrey KT1 2EE, UK
G. Stevens
Affiliation:
Department of Earth Sciences, University of Stellenbosch, Private Bag X1, 7602 Matieland, South Africa Email: [email protected]

Abstract

Though typically exhibiting considerable scatter, geochemical variations in granitic plutons and silicic volcanic deposits are commonly modelled as products of differentiation of originally homogeneous magmas. However, many silicic igneous bodies, particularly those classified as S-types, are internally heterogeneous in their mineralogy, geochemistry and isotope ratios, on scales from hundreds of metres down to one metre or less. The preservation of these heterogeneities supports recent models for the construction of granitic magma bodies through incremental additions of numerous batches (pulses) of magma derived from contrasting sources. Such pulses result from the sequential nature of the melting reactions and the commonly layered structure of crustal magma sources. Internal differentiation of these batches occurs, but not generally on the scales of whole magma chambers. Rather than being created through differentiation or hybridisation processes, at or near emplacement levels, much of the variation within such bodies (e.g. trace-element or Mg# variation with SiO2 or isotope ratios) is a primary or near-source feature. At emplacement levels, the relatively high magma viscosities and slow diffusion rates of many chemical components in silicic melts probably inhibit processes that would lead to homogenisation. This permits at least partial preservation of the primary heterogeneities.

Type
Research Article
Copyright
Copyright © Royal Society of Edinburgh 2010

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