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Crustal versus mantle sources of granitic magmas: a two-parameter model based on Nd isotopic studies

Published online by Cambridge University Press:  03 November 2011

Donald J. DePaolo ,
Frank V. Perry  and
W. Scott Baldridge
Donald J. DePaolo
Affiliation:
Donald J. DePaolo, Center for Isotope Geochemistry, Department of Geology and Geophysics ,University of California, Berkeley, California 94720,U.S.A.; Earth Sciences Division, Lawrence Berkeley Laboratory, Berkeley, California 94720, U.S.A
Frank V. Perry
Affiliation:
Frank V. Perry, Department of Geology, University of New Mexico, Albuquerque, NM 87131, U.S.A.
W. Scott Baldridge
Affiliation:
W. Scott Baldridge, Earth and Environmental Sciences Division, Los Alamos National Laboratory, Los Alamos, NM 87545, U.S.A.
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Abstract

Temporal and spatial variations in the Nd isotopic compositions of Tertiary caldera-forming rhyolite tuffs, and Cretaceous and Tertiary granites of the western U.S.A. are used as a basis for a model that accounts for the observed proportions of crustal versus mantle contributions to silicic magmas in terms of two parameters: the ambient crustal temperature and the rate of supply of basaltic magma from the mantle. The crustal contribution to silicic igneous rocks is measured in terms of the Neodymium Crustal Index (NCI). The relationships between crustal temperature, basalt supply and NCI are quantified using a model of a magma chamber undergoing continuous recharge, wall-rock assimilation and fractional crystallisation. From the model, a critical value of the ratio of basalt recharge-to-assimilation, (r/a)c, is deduced, which increases with decreasing crustal temperature. The r/a value must exceed (r/a)c to allow the volume of differentiated magma to increase, a prerequisite for developing large volumes of silicic magma. Strongly peraluminous (or S-type) magmas (NCI = 0·8–1), form under conditions of high crustal temperature and low basalt supply. Metaluminous or I-type granites form over a wide range of conditions (NCI = 0·1–1), generally where basalt supply is substantial. In individual long-lived volcanic centres, the large-volume high-silica ignimbrites are associated with the highest r/a and lowest NCI values, indicating that these magmas are typically differentiates of mantle-derived basaltic parents.

Keywords

Assimilationfractional crystallisationignimbritemagma chamberrhyolite
Type
Research Article
Information
Earth and Environmental Science Transactions of The Royal Society of Edinburgh , Volume 83 , Issue 1-2: Second Hutton Symposium: The Origin of Granites and Related Rocks , 1992 , pp. 439 - 446
Copyright
Copyright © Royal Society of Edinburgh 1992

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