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 (