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?…