COUNTRY ROCK MONAZITE RESPONSE TO INTRUSION OF THE SEARCHLIGHT PLUTON, SOUTHERN NEVADA

We investigated how monazite grains in country rocks responded to the intrusion of the Miocene Searchlight pluton in southern Nevada. Country rock samples were collected from the roof zone and along transects on the flanks (wallrock) of the 16-17 Ma pluton. Deep wallrock Ireteba granite monazite grains have patchy secondary growth zones of Searchlight age overprinting primary growth zones of Ireteba age (~66 Ma). Shallow wallrock Proterozoic gneiss zircon grains define a discordia with an upper intercept age of 1.74 ± 0.02 Ga corresponding to crystallization of the protolith. Proterozoic gneiss monazite grains define a discordia with an upper intercept age of 1.64 ± 0.02 Ga and a poorly-defined lower intercept age of 75 ± 61 Ma that may correspond to the Ireteba intrusion. Patchy zones in Proterozoic gneiss monazite grains did not grow in response to intrusion of the Ireteba or Searchlight plutons because they yield only Proterozoic ages. Oxygen isotopes in Ireteba monazite, hydrogen and oxygen isotopes in whole rocks from the Ireteba transect, and oxygen isotopes in whole rocks from the Proterozoic gneiss transect show no systematic pattern related to the contact. No geochemical data support the hypothesis that Searchlight-derived magmatic fluids caused Ireteba monazite grains to partially recrystallize. Instead, they may have partially recrystallized in response to strain. In Proterozoic gneiss country rock monazite grains are present on the flanks but absent from the roof zone, suggesting that high fluid fluxes in the roof destroyed monazite. Strong focusing of Searchlight magmatic fluid and heat into the roof zone prevented the development of a well-defined contact metamorphic aureole in Ireteba granite and Proterozoic gneiss wallrocks.