1.COUNTRY ROCK MONAZITE RESPONSE TO INTRUSION OF THE SEARCHLIGHTPLUTON, SOUTHERN NEVADA
John C. Ayers, Scott Crombie, Calvin Miller, Yan Luo,
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. 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.
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.
4.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
7.Searchlight Pluton Panoramic
Big Granite Mtn.
Located in the Eldorado Mountains of Southern Nevada
Tilted to expose deeper portions of the pluton
9.Two lithologies in the wallrock of the SL pluton:
Lithologies contain monazite and sericitization
Metamorphism at 250-400°C and ~0.15-0.4 GPa.
Focus on transects.
Cu, Au, and Ag ore deposits in roof zone
10.Comparison of country rock and Searchlight granite intrusion
11.Ireteba Whole Rock Stable Isotopes
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?
13.Ireteba Granite: Monazite Zoning & Analysis Spots
14.Ireteba granite monazite ages
15.Ireteba Monazite Ages
16.Ireteba Granite: Sample IR1
17.Ireteba Granite: Sample IR20
18.Gneiss Whole Rock Stable Isotopes
19.Proterozoic Gneiss Monazite
LA-ICP-MS analysis pits and EMP analysis spots labeled with ages in Ma with 1σ errors.
20.Gneiss EMP monazite ages
21.Proterozoic Gneiss Monazite
22.Proterozoic Gneiss Monazite Ages
23.Gneiss wallrock zircon grains
24.Gneiss Wallrock Monazite and Zircon
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.