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Isotopic Compositional Changes Across Space, Time, and Bulk Rock Composition in the High Lava Plains and Northwestern Basin and Range, Oregon. GSA 2009 abs. #224-5. Mark T. Ford Oregon State University Fordm@geo.oregonstate.edu. Richard Carlson Dept. of Terrestrial Magnetism - PowerPoint PPT Presentation
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Isotopic Compositional Changes Across Space, Time, and Bulk Rock Composition in the High Lava Plains
and Northwestern Basin and Range, Oregon
Mark T. FordMark T. Ford Oregon State UniversityOregon State University
Fordm@geo.oregonstate.edu Fordm@geo.oregonstate.edu
Anita L. GrunderAnita L. Grunder Oregon State University Oregon State University
Grundera@geo.oregonstate.eduGrundera@geo.oregonstate.edu
Richard CarlsonRichard Carlson Dept. of Terrestrial Magnetism Dept. of Terrestrial Magnetism
Carlson@dtm.ciw.eduCarlson@dtm.ciw.edu
GSA 2009 abs. #224-5
0 50 100 Miles
0 80 160 km
NWBR
HLP
Overview: Volcanic episodes and estimated volumes Focus on the 12 Ma to Recent rhyolites
Time-transgressive nature
Bulk rock composition
Isotope composition
Implications of heat flux on petrogenesis in the HLP and NWBR
12 – 0 Ma HLP and NWBR volcanism
Volume estimate 2,000 km3 to 2,500 km3
Basalts < 20 Ma in gray
Rhyolites in purple
Ash flow tuffs in yellow
Age progression in rhyolites
One post-progression rhyolite: Iron Mt.
2.89 Ma
HLP Rhyolite
• Volume declines in time
• Heightened activity 7-7.5 Ma, just after basalt pulse at 7.5-8 Ma (Jordan et al., 2004)
NWBR rhyolites
• not younger than ~5 Ma
Black ages - measured
Colored ages – interpolated:
Num
ber
Comparison to suites: Cascades, SRP, Iceland
Tholeiitic vs. Calc-alkaline suites Clearly separated on FeO – SiO2 diagram, except at highest silica
Tholeiitic vs. Calc-alkaline suites Clearly separated on FeO – SiO2 diagram, except at highest silica
Can we use this to help separate NWBR and HLP samples?
FeO – SiO2 diagram from the study area
Nearly all NWBR are “Low FeO”, HLP is variable to high FeO
• High Fe/Si focused along a belt in the HLP• Variability in composition to the East in the HLP• All tuffs high Fe/Si, large-volume tuffs in East
Glass Buttes Juniper Ridge
Zero lineHigh Fe/Si
Low Fe/Si
Within suite variation relative to FeO vs. SiO2
Within suite Fe/Si enrichment
Juniper Ridge and Glass Buttes
30 km
Fe-Si zero line
0.7 Ma
1.2 MaSuite evolution
Hig
h Fe
/Si
Low
Fe/
Si
What might this be telling us about the role of crust in making the rhyolites – or about the thermal inputs into the system?
Lets examine isotopic systems to gain some insights…
143 N
d/14
4 Nd
87Sr/86Sr(i)
Nd- and Sr-isotopic variations of the rhyolites
– some with elevated Sr isotopic ratios
crustal addition
87Sr/86Sr(i)
143 N
d/14
4 Nd
Comparison to basalts
Some of elevated Sr ratios may be due to parental magmas with high ratios
crustal addition
87S
r/86S
r (i)
Longitude
Longitude vs. Sr isotopic ratios:
Will the real crustal signature please stand up
West East
OR
Cas
cade
s ra
nge
Crustal addition
“Basalt-like”
206Pb/204Pb
207 P
b/20
4 Pb Pelagic sediments
or continental crust
206Pb/204Pb vs. 207Pb/204Pb correlation diagram
206Pb/204Pb
207 P
b/20
4 Pb Pelagic sediments
or continental crust
206Pb/204Pb vs. 207Pb/204Pb correlation diagram
87S
r/86S
r (i)
18O
Magmatic18O vs. 87Sr/86Sr correlation diagram
Matrix of crustal influence
HLP NWBR1 or more crustal signatures (Sr, Pb, O isotopes) 16 5No crustal factors (potential fractionates) 8 + Iron Mt 6
high Fe/Si low Fe/Si1 or more crustal signatures 11 5No crustal factors (potential fractionates) 5 3 + Iron Mt
Within the HLP:
Conclusions: HLP and NWBR are a single bimodal province with time-transgressive rhyolitic volcanism from 12 Ma to Recent
NWBR rhyolites are dominantly low FeO/SiO2
HLP rhyolites have more chemical diversity, especially to the east with high FeO/SiO2 along the axis of the plain
Within suite temporal evolution to higher FeO/SiO2 and greater crustal contribution
High heat flux creates a feedback in the crust that yields both a more mafic crust and more crustal melt in the HLP, including voluminous ignimbrites
Acknowledgements: NSF funding; Ilya Bindeman: Oxygen isotopes; Jenda Johnson: animation
Time for a short movie?…
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