Strontium, neodymium and lead isotopic compositions of deep crustal xenoliths from the Snake River Plain: evidence for Archean basement (original) (raw)
1985, Earth and Planetary Science Letters
Xenoliths of intermediate to felsic granulites found in evolved lavas from the Snake River Plain have been analyzed for Nd, Pb, and Sr isotopic compositions and related trace element contents. Overall, they exhibit wide ranges in present-day values of S7Sr/86Sr (0.70235-0.83011), 143Nd/144Nd (0.51023-0.51148) and 2°6Pb/~°4pb (13,43-24.65). The Rb-Sr, U-Pb, Th-Pb, and Sm-Nd decay schemes have been variably affected by granulite facies metamorphism and possibly by transport in the host magmas. However, Pb-Pb and Sm-Nd isochron systematics seemingly are preserved in many of the xenoliths and indicate essentially concordant metamorphic ages of about 2.8 Ga. Nd model ages are significantly older (ca. 3,1-3.4 Ga) for many of the xenoliths, Precambrian metasediments exposed along the southern margin of the Snake River Plain have Sm-Nd systematics similar to those of the xenoliths. These results suggest that at least two significant thermal events occurred during Archean evolution of the crust in this region: (a) early (ca. 3.1-3.4 Ga) additions of mantle-derived magmas to the crust, and (b) regional metamorphism at 2.8 Ga accompanied at least locally by magmatism. Concordance between the Sm-Nd and Pb-Pb ages suggests that these isotopic systems are little affected by high-grade metamorphism in this case. The distribution of xenoliths in Snake River Plain lavas supports the presence of Archean crust beneath much of southern Idaho, although such rocks rarely are exposed at the surface. Thus, Nd and Pb isotopic studies of crustal xenoliths can provide a useful means of determining the extent of crustal age provinces where surface exposures are lacking.
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