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Nd Isotope Systematics of Coarse- and Fine-Grained Sediments: Examples from the Middle Proterozoic Belt-Purcell Supergroup

C. D. Frost and Don Winston
The Journal of Geology
Vol. 95, No. 3 (May, 1987), pp. 309-327
Stable URL: http://www.jstor.org/stable/30061938
Page Count: 19
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Since scans are not currently available to screen readers, please contact JSTOR User Support for access. We'll provide a PDF copy for your screen reader.
Nd Isotope Systematics of Coarse- and Fine-Grained Sediments: Examples from the Middle Proterozoic Belt-Purcell Supergroup
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Abstract

Sm-Nd isotopic compositions are reported for 20 fine- and 5 coarse-grained samples of the Belt-Purcell Supergroup of western North America. $\varepsilon_{Nd}$ and Sm/Nd ratios for fine-grained Belt-Purcell sediments are remarkably uniform, indicating crustal residence ages ranging only from 1.6 to 2.1 Ga. $\varepsilon_{Nd}$ and Sm/Nd ratios for coarse-grained sediments, however, are much more varied and contain evidence of Sm-Nd fractionation during sedimentary recycling. Assuming that this REE fractionation occurred between the time source areas were eroded and when sediment was deposited, a new method of calculating crustal residence age intervals is derived for the coarse-grained samples, which serves to place constraints on the average time since source materials were extracted from the mantle. The Nd data from both sample groups provide an average age estimate of $2.0 \pm 0.3$ Ga for the Middle Proterozoic crust from which most of the Belt-Purcell Supergroup was derived. The isotopic data, together with published data for possible source areas, are used to evaluate the provenance of the sediment succession. The isotopic results preclude derivation of the Belt-Purcell material entirely from the Archean portion of the North American craton but are compatible with sedimentologic evidence for derivation of fine-grained Belt-Purcell sediments from a western continent. When augmented by analysis of coarse-grained elastics such as those studied here, the kind of information provided by fine-grained sediments may better define models of crustal evolution.

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