Chapter 21: Neodymium, strontium, and trace-element evidence of crustal anatexis and magma mixing in the Idaho batholith

Variations in initial ¹⁴³Nd/¹⁴⁴ Nd in Late Cretaceous plutonic rocks along the South Fork of the Clearwater River (SFCR) supplement results of Sr and O studies, which demonstrate large-scale mixing in magmas forming the western margin of the Idaho batholith. These marginal or border phases of the batholith span the terrane boundary between Proterozoic crust of North America and late Paleozoic-Mesozoic intraoceanic arc terranes (WSD terranes), delineated by the Western Idaho suture zone (or WISZ). ^ɛNd^(t) values in Early Cretaceous and older, pre-accretionary plutons of the WSD range from +3 to +7.6, and average +5.7. Proterozoic orthogneisses and metasedimentary rocks range from -7.4 to -13.7 and -10.45 to -15.7, respectively. ^ɛNd^(t) in Late Cretaceous plutons of the SFCR decreases abruptly from west to east near the WISZ, varying inversely with ɛSr^(t). Although Sr isotopic evidence (Fleck and Criss, 1985) is consistent with a binary mixing model, Sm-Nd results modify those conclusions, suggesting that SFCR plutons may be divided into three groups. Group 1 plutons occur in a narrow zone (<4 km width) along the suture zone (WISZ). These bodies probably represent at least three-component mixtures of very high-Sr, arc-type magmas, one or more Proterozoic crustal components that may include lower crust, and a high-Nb, high-Zr component. Group 2 plutons are characterized by high ɛSr^(t).and nearly constant, low ɛNd^(t). These bodies are thought to represent mixtures of deep-seated partial melts of two different Proterozoic lithospheric types, possibly representing upper and lower crust. Plutons belonging to Group 3 have ɛNd^(t).values <-14 and probably incorporated substantial amounts of Proterozoic metasedimentary rocks, but mixing components are poorly defined.

Trace-element variations in SFCR rocks also reflect the arc terrane-continental crustal boundary as Nb, Zr, and Nd increase dramatically, whereas Sr, Rb/Nb, and Sm/Nd exhibit coincident decreases east of the WISZ. Modeling of these variations with the isotopic variations in Nd and Sr supports mixing, but precludes contamination-bulk-assimilation models. Correlated ɛNd, ɛSr, and δ¹⁸O within the SFCR favors mixing of crustal and subcrustal magmas rather than derivation of the melts entirely from subcontinental lithosphere.