COUNTRY ROCK MONAZITE RESPONSE TO INTRUSION OF THE SEARCHLIGHT PLUTON, SOUTHERN NEVADA John C. Ayers, Scott Crombie, Calvin Miller, Yan Luo, Miranda Loflin Vanderbilt University

Abstract 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 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. 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.

Objectives To study the response of wallrock monazite to contact metamorphism & magmatic fluid infiltration. To identify the geologic process associated with monazite ages measured in-situ.

Why study contact metamorphic aureoles Have better geologic control than regional metamorphism (small scale, simple geometry) Protolith compositions generally available Transects allow evaluation of effects of continuous changes in metamorphic grade Fluid fluxes and peak temperatures vary systematically in relation to the contact

Searchlight Pluton Panoramic Copper Mtn.Ireteba PeaksBig Granite Mtn.

Searchlight/ Ireteba plutons Located in the Eldorado Mountains of Southern Nevada Tilted to expose deeper portions of the pluton

Two lithologies in the wallrock of the SL pluton: –Ireteba granite –Proterozoi c gneiss Lithologies contain monazite and sericitization Metamorphi sm at °C and ~ GPa. Focus on transects. IR1 IR20 Cu, Au, and Ag ore deposits in roof zone

Comparison of country rock and Searchlight granite intrusion PlutonAge (Ma)δ 18 O SMOW (‰) Ireteba (excluding altered pendant samples) 66±2 (Kapp et al., 2002) (Townsend et al., 2000; Kapp et al., 2002) Proterozoic gneiss?? Searchlight16.5 ± 1 Ma (Cates et al., 2003) 7.0, 7.1 (Bachl et al. 2001)

Ireteba Whole Rock Stable Isotopes SL

Searchlight/Ireteba plutons Ireteba: Zircon and monazite ages indicate a crystallization age of 64 ± 2 Ma Searchlight pluton crystallized at 16.5 Ma during Miocene extension Rocks close to the Searchlight contact do not show evidence of hydrothermal alteration, but included monazites exhibit signs of recrystallization – caused by fluids?

Ireteba Granite: Monazite Zoning & Analysis Spots  18 O SMOW IMP 208 Pb/ 232 Th age

Ireteba granite monazite ages

Ireteba Monazite Ages

Ireteba Granite: Sample IR1  18 O SMOW 208 Pb/ 232 Th age

Ireteba Granite: Sample IR20

Gneiss Whole Rock Stable Isotopes

Proterozoic Gneiss Monazite LA-ICP-MS analysis pits and EMP analysis spots labeled with ages in Ma with 1σ errors.

Gneiss EMP monazite ages

Proterozoic Gneiss Monazite

Proterozoic Gneiss Monazite Ages

Gneiss wallrock zircon grains

Gneiss Wallrock Monazite and Zircon

Conclusions Monazite in deep wallrocks (Ireteba granite) partially recrystallized/reset and developed patchy zoning in response to Searchlight intrusion at 16.5 Ma. No good evidence that fluids were responsible for monazite recrystallization – perhaps it was strain? Monazite in shallow wallrocks (Proterozoic gneiss) had preexisting patchy zoning and lost some Pb in response to Ireteba intrusion at 65 Ma, but were unaffected by Searchlight intrusion. Monazite is absent from Proterozoic Gneiss in roof zone samples, suggesting that high fluid fluxes that formed hydrothermal ore deposits destroyed monazite. Focusing of fluids in roof zone prevented development of contact metamorphic aureole and monazite recrystallization on pluton flanks.