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COUNTRY ROCK MONAZITE RESPONSE TO INTRUSION OF
THE SEARCHLIGHTPLUTON, SOUTHERN NEVADA
John C. Ayers, Scott Crombie, Calvin Miller, Yan Luo,
Miranda Loflin
Vanderbilt University
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
• Metamorphism at 250-400°C and ~0.15-0.4 GPa.
• Focus on transects.
IR1
IR20
Cu, Au, and Ag ore deposits in roof zone
Comparison of country rock and Searchlight granite intrusion
Pluton Age (Ma) δ18OSMOW (‰)
Ireteba (excluding altered pendant samples)
66±2 (Kapp et al., 2002)
8.2-8.8 (Townsend et al., 2000; Kapp et al., 2002)
Proterozoic gneiss ? ?
Searchlight 16.5 ± 1 Ma (Cates et al., 2003)
7.0, 7.1 (Bachl et al. 2001)
Ireteba Whole Rock Stable Isotopes
Distance from Searchlight contact (km)
0.0 0.1 0.2 0.3 0.4 0.5
18
OS
MO
W(‰
)
6.5
7.0
7.5
8.0
8.5
9.0
9.5
10.0
IG-1 IG-2 IG-3 IG-4 IG-5
DS
MO
W(‰
)
-100
-98
-96
-94
-92
-90
-88
-86
18OSMOWDSMOW
SL
Ireteba Granite: Monazite Zoning & Analysis Spots
d18OSMOWIMP208Pb/232Th age
Ireteba granite monazite ages
208Pb/232Th monazite age (Ma)
0 20 40 60 80
Relative probability
0.00
0.02
0.04
0.06
0.08
0.10
0.12
0.14
0.16
16.5 Ma
IMP Ages: Townsend et al. (2000) all analysesLA-ICP-MS Ages: This studySearchlight Pluton
Ireteba Monazite Ages
Distance from Searchlight Contact (Ma)
0.0 0.1 0.2 0.3 0.4 0.5
Average
208P
b/232
Th A
ge (Ma)
10
20
30
40
50
60
70
Ireteba Granite: Sample IR1
d18OSMOW208Pb/232Th age
5 6 7 8 9 10 110
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.08
0.09
0.1
IR1 Primary n=9
IR1 Secondary n=11
δ18O SMOW
Re
lati
ve
pro
ba
bil
ity
Ireteba Granite: Sample IR20
5 6 7 8 9 10 11 120
0.02
0.04
IR20 Primary n=12
IR20 Secondary n=3
δ18OSMOW
Re
lativ
e p
rob
ab
ility
Gneiss Whole Rock Stable Isotopes
Distance from Searchlight contact (km)
0.0 0.2 0.4 0.6
18O
SM
OW
(‰)
8
9
10
11
12
13
XG-6XG-5 XG-4XG-10
Proterozoic Gneiss Monazite
LA-ICP-MS analysis pits and EMP analysis spots labeled with ages in Ma with 1σ errors.
Gneiss EMP monazite ages
0 200 400 600 800 1000 1200 1400 16000
10
20
30
40
50
60
EMP Age (Ma)
Nu
mb
er
1412
1312
1200
65
Proterozoic Gneiss Monazite
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.