Coupled elemental and isotopic analyses of polygenetic zircons from granitic rocks by ion microprobe, with implications for melt evolution and the sources of granitic magmas

Sequential Pb isotopic and trace element analyses were applied to the geochemical characterization of polygenetic zircons in the San Bernardino and Granite Mountains intrusive suites, two contemporaneous but compositionally distinct suites emplaced in the early evolutionary stage of the western U.S. Cordillera magmatic arc. These coupled analyses allow characterization of both melt evolution and regional diversity in the principal rock types present in the crustal sources of these two suites. Zircon grains in both intrusive suites exhibit consistent compositional zonation — Hf solid solution in zircon generally increases with cooling and crystallization, and with other indices of melt evolution, including total REE abundances and especially the MREE as indicated by increasing Yb/Gd ratios. These temperature-dependent fractionations are superimposed on persistent parental melt compositional differences in Hf/Zr and Yb/Gd ratios. Premagmatic zircons of Paleoproterozoic to Mesoproterozoic age are common across multiple samples from each suite, and their age and compositional variations image the crustal sources of these magmas. The high K San Bernardino suite had a lithologically comparatively simple 1.7 Ga metaigneous source, whereas the Granite Mountains suite had a source distinguished by the presence of hotter, 1.7 Ga metaigneous rocks hybridized by later intrusion of 1.4 Ga rocks. The absence of evidence for Grenville-age or Paleozoic-age zircons severely limits input in these intrusive suites from either upper crustal sedimentary rocks at the level of emplacement or underthrust sediments derived from erosion of the continental margin.