MESOZOIC AND TERTIARY PLUTON SOURCES IN MOJAVE CONTINENTAL CRUST — ZIRCON U-PB AND LU-HF ISOTOPIC EVIDENCE

Lu-Hf isotopic systematics from U-Pb-dated zircon grains in 14 samples from ca.165-, 100-, 75-, and 20-Ma plutons show a range in ¹⁷⁶Hf/¹⁷⁷Hf ratios. The Mojave crust that hosts the plutons consists largely of Paleoproterozoic granitoids and gneisses and includes a Neoarchean to early Paleoproterozoic component expressed in various isotopes. The sampled Phanerozoic plutons, especially Cretaceous ones, commonly contain xenocrystic Proterozoic zircon. Hf isotopic ratios in U-Pb-dated magmatic and xenocrystic zircons provide insight on the magmatic sources of the Phanerozoic granitoids. Model Hf ages and εHf(t) allow subdivision of zircons into four groups. One group of Mesozoic and Tertiary magmatic zircons has Hf model ages ranging from ~1700 Ma to ~900 Ma, and a range of εHf(t) values consistent with magma mixing between a Paleoproterozoic crustal component and a smaller mantle-derived component. The magmatic zircons generally have concordant U-Pb igneous ages, have well developed crystal faces, have concentric zoning, and commonly have inherited cores. A second group consists of inherited zircons in Cretaceous and some Jurassic granitoids; Hf model ages of ~1800 to 1500 Ma overlap values from zircon in a representative Paleoproterozoic metagranite. Rounded to irregular grain shapes and poorly developed zoning are common. A third, more diverse group of zircons found mainly in Cretaceous granites differ in having significantly discordant U-Pb ages, lower ¹⁷⁶Hf/¹⁷⁷Hf ratios, and poorly constrained Hf model ages in the range of 3000 Ma to 2100 Ma. Their U-Pb age discordance, many giving Mesozoic ²⁰⁶Pb/²³⁸U ages, could reflect metasomatic and /or metamorphic resetting during Laramide orogeny and/or the sampled spots may record smearing across Mesozoic zircon rims and inherited older cores. The fourth group consists of two crystals, dated as Mesozoic by U-Pb, having extreme εHf(t) values near 50 and model ages near 4000 Ma. The high model Hf ages of groups 3 and 4 are consistent with derivation from partial melts of cryptic Archean-to-early Paleoproterozoic detrital components in the Mojave crust. Diverse Hf ratios for zircons imply that each of the Jurassic and Cretaceous granitoids includes melt contributions from multiple, diverse crustal components.