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 to 17 Ma pluton. Deep wallrock Ireteba granite monazite grains have patchy secondary growth zones of Searchlight age overprinting primary growth zones of Ireteba age (similar to 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. EMP analyses show that patchy secondary zones in Proterozoic gneiss monazite grains were contemporaneous with intrusion of the Ireteba granite, not the Searchlight pluton. 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 hydrothermal fluids associated with intrusion of the Searchlight pluton caused monazite in the Proterozoic gneiss or Ireteba granite wallrock to partially recrystallize. Ireteba samples with the most intensely altered monazite were at the greatest paleodepths at the time of Searchlight intrusion and are the most deformed, suggesting that strain caused Ireteba monazite grains to partially recrystallize. 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 may have destroyed monazite. Strong focusing of hydrothermal fluid and heat into the roof zone prevented the development of a well-defined contact metamorphic aureole in Ireteba granite and Proterozoic gneiss wallrocks.