PERALUMINOUS GRANITES OF THE CORDILLERAN INTERIOR, WESTERN USA: HYBRID MAGMAS FROM DEEP, ANCIENT CRUST

Abundant peraluminous granitoid plutons located inland from the Mesozoic coastal magmatic arc of the western USA are ostensibly among the world's best candidates for a wholly crustal anatectic origin. They are characterized by: mineral assemblages including muscovite + biotite and, in fractionated rocks, garnet; uniformly felsic compositions, with SiO₂ >70 wt%; initial Nd and Sr isotopic compositions clearly reflecting ancient crust (e_Nd from -10 to -22, most <-15; ⁸⁷Sr/⁸⁶Sr 0.710 to 0.720), moderate to very high d¹⁸O (+8 to +14‰); abundant inherited zircon cores (mostly 1.0-2.7 Ga, some Mesozoic); and low calculated zircon saturation temperatures, almost invariably below 800º C (abundant inherited cores suggest Zr concentrations are overestimates of melt Zr - the magmas may have been even cooler).

Nonetheless, many of these plutons - especially the younger ones (Late Cretaceous-early Paleogene) - are distinct from many peraluminous granites worldwide in having high Na₂O (~4 wt%) and Na₂O/ K₂O ratios (typically >1) and Sr (~400-800 ppm), low heavy REE (1-5 x chondrite), and absence of negative Eu anomalies. They commonly plot with adakites in Sr-Y-La-Yb diagrams. Furthermore, field evidence for interaction between the granites and mafic magmas (synplutonic dikes; pillows with chilled, crenulate margins) is fairly common. Despite their obviously crustal isotopic compositions, they are generally isotopically less evolved than the ancient regional crust. By far the largest of the high Sr/HREE complexes is the Bitterroot lobe of Idaho batholith; similar but smaller intrusions are exposed in northern Nevada, eastern California, and along the Colorado River (NV-CA-AZ).

These characteristics imply that these granites are primarily derived from ancient crust but probably contain a juvenile component, and that the crustal melting event was associated in time and space with mafic magmatism. The modest juvenile component may have been derived from both older Mesozoic intrusions and the new basaltic input. Crustal melting occurred at great depth, probably >35-40 km, with limited feldspar and abundant garnet in the residue. The low magma temperatures apparently preclude generation by conventional dehydration melting mechanisms. Magmatism may reflect the changing tectonic environment when subduction shallowed and coastal batholithic activity terminated.