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Book Chapter

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

By
Donald J. DePaolo
Donald J. DePaolo
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Frank V. Perry
Frank V. Perry
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W. Scott Baldridge
W. Scott Baldridge
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Published:
January 01, 1992

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.

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Contents

GSA Special Papers

The Second Hutton Symposium on the Origin of Granites and Related Rocks

P. E. Brown
P. E. Brown
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B. W. Chappell
B. W. Chappell
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Geological Society of America
Volume
272
ISBN print:
9780813722726
Publication date:
January 01, 1992

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