INCREMENTAL PLUTONIC ASSEMBLY OVER TENS OF MILLIONS OF YEARS? U-PB GEOCHRONOLOGY AND GHOST STRATIGRAPHY IN PEGMATITIC GRANITE, RUBY MOUNTAINS, NEVADA

Zircon and monazite dating by iron microprobe imply that a batholithic volume (>600 km3) of gneissic pegmatitic leucogranite in the Ruby Mountains was assembled over long periods spanning the Late Cretaceous Sevier orogeny and Paleogene regional volcanism. Zircon U-Pb grain ages cluster between 92 to 68 Ma and 46 to 29 Ma. Pegmatitic granites form a mid-crustal injection complex of coalesced sheets, sills, dikes and irregular shapes that inflates a ghost Paleozoic stratigraphic framework and lesser nonpegmatitic Mesozoic granites. The pegmatitic granite bodies lack inherited Precambrian zircon, suggesting that any clastic grains from the proposed melt source of muscovite-rich pelite (Lee et al., 2003) may have been too fine to survive melting. This absence contrasts with inherited Precambrian zircon present in nonpegmatitic Jurassic, Cretaceous, and Paleogene granites, which were derived from different source rocks. Downward increasing proportions of pegmatitic granite from <30% at high structural levels to 95–100% in deepest canyon bottoms reflect stages of incremental assembly of its total volume. Structurally deep samples of the pegmatitic granite yielded only Paleogene U-Pb ages; individual shallower samples yielded dominantly Late Cretaceous and lesser Paleogene zircon. Zircon grain morphology, oscillatory zoning, and trace-element geochemistry suggest most zircons of both Cretaceous and Paleogene age are low-temperature magmatic crystals. Zircon crystallization temperatures <700°C estimated from Ti content are consistent with the highly evolved leucogranite bulk composition and with low calculated zircon-saturation temperatures for the granite. Differences in zircon Th content and Th/U between different age groups in individual samples suggest compositionally different melts in equilibrium with the zircon at different times of crystallization. These differences distinguish ~69 Ma zircon rims from older Cretaceous cores in one sample, Paleogene zircon from Cretaceous zircon in another sample, and younger Paleogene zircon from older Paleogene zircon in a third. The age patterns suggest that pegmatitic granite may have been partially melted and remobilized several times during long periods of repeated and sustained near-solidus temperatures in the middle crust.