Transcript COUNTRY ROCK MONAZITE RESPONSE TO INTRUSION OF THE SEARCHLIGHT PLUTON, SOUTHERN NEVADA John C.

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
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
Big Granite Mtn.
Ireteba Peaks
Copper 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:
Cu, Au, and Ag ore deposits in roof zone
– Ireteba
granite
– Proterozoi
c gneiss
• Lithologies
contain
monazite
and
sericitization
• Metamorphi
sm at 250400°C and
~0.15-0.4
GPa.
• Focus on
transects.
IR20
IR1
Comparison of country rock and
Searchlight granite intrusion
Pluton
δ18OSMOW (‰)
Age (Ma)
Ireteba (excluding
altered pendant
samples)
66±2 (Kapp et al., 8.2-8.8 (Townsend
2002)
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
IG-2
IG-3
IG-4
IG-5
10.0
-86
9.5
-88
9.0
-90
8.5
-92
8.0
-94
18O
SMOW
DSMOW
7.5
SL 7.0
-96
-98
6.5
-100
0.0
0.1
0.2
0.3
0.4
Distance from Searchlight contact (km)
0.5
D SMOW (‰)
18O
SMOW (‰)
IG-1
Ireteba Granite: Monazite Zoning &
Analysis Spots
IMP208Pb/232Th age
d18OSMOW
Ireteba granite monazite ages
16.5 Ma
0.16
IMP Ages: Townsend et al. (2000) all analyses
LA-ICP-MS Ages: This study
Searchlight Pluton
0.14
Relative probability
0.12
0.10
0.08
0.06
0.04
0.02
0.00
0
20
40
208
Pb/232Th monazite age (Ma)
60
80
Ireteba Monazite Ages
70
50
40
Average
208
Pb/
232
Th Age (Ma)
60
30
20
10
0.0
0.1
0.2
0.3
Distance from Searchlight Contact (Ma)
0.4
0.5
Ireteba Granite: Sample IR1
age
d18OSMOW
0.1
0.09
IR1 Primary n=9
0.08
Relative probability
208Pb/232Th
IR1 Secondary
0.07
n=11
0.06
0.05
0.04
0.03
0.02
0.01
0
5
7
δ18O SMOW
9
11
Ireteba Granite: Sample IR20
Relative probability
0.1
0.08
IR20 Primary
n=12
IR20 Secondary
n=3
0.06
0.04
0.02
0
5
6
7
8
δ18OSMOW
9
10
11
12
Gneiss Whole Rock Stable Isotopes
XG-10 XG-4
XG-5
XG-6
13
18
OSMOW(‰)
12
11
10
9
8
0.0
0.2
0.4
Distance from Searchlight contact (km)
0.6
Proterozoic Gneiss Monazite
LA-ICP-MS analysis pits and EMP analysis spots labeled
with ages in Ma with 1σ errors.
Gneiss EMP monazite ages
60
1412
50
Number
40
30
65
1312
20
1200
10
0
0
200
400
600
800
EMP Age (Ma)
1000
1200
1400
1600
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.