Isotopic Compositional Changes Across Space, Time, and Bulk Rock Composition in the High Lava Plains and Northwestern Basin and Range, Oregon Mark T. Ford Oregon State University Anita L. Grunder Oregon State University Richard Carlson Dept. of Terrestrial Magnetism GSA 2009 abs. #224-5

Miles 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 km 3 to 2,500 km 3 Basalts < 20 Ma in gray Rhyolites in purple Ash flow tuffs in yellow

Age progression in rhyolites One post-progression rhyolite: Iron Mt Ma HLP Rhyolite Volume declines in time Heightened activity Ma, just after basalt pulse at Ma ( Jordan et al., 2004 ) NWBR rhyolites not younger than ~5 Ma Black ages - measured Colored ages – interpolated :

Number Comparison to suites: Cascades, SRP, Iceland

Tholeiitic vs. Calc-alkaline suites Clearly separated on FeO – SiO 2 diagram, except at highest silica

Tholeiitic vs. Calc-alkaline suites Clearly separated on FeO – SiO 2 diagram, except at highest silica Can we use this to help separate NWBR and HLP samples?

FeO – SiO 2 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 line High Fe/Si Low Fe/Si Within suite variation relative to FeO vs. SiO 2

Within suite Fe/Si enrichment Juniper Ridge and Glass Buttes 30 km Fe-Si zero line 0.7 Ma 1.2 Ma Suite evolution High 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 Nd/ 144 Nd 87 Sr/ 86 Sr (i) Nd- and Sr-isotopic variations of the rhyolites – some with elevated Sr isotopic ratios crustal addition

87 Sr/ 86 Sr (i) 143 Nd/ 144 Nd Comparison to basalts Some of elevated Sr ratios may be due to parental magmas with high ratios crustal addition

87 Sr/ 86 Sr (i) Longitude Longitude vs. Sr isotopic ratios: Will the real crustal signature please stand up WestEast OR Cascades range Crustal addition “Basalt-like”

206 Pb/ 204 Pb 207 Pb/ 204 Pb Pelagic sediments or continental crust 206 Pb/ 204 Pb vs. 207 Pb/ 204 Pb correlation diagram

206 Pb/ 204 Pb 207 Pb/ 204 Pb Pelagic sediments or continental crust 206 Pb/ 204 Pb vs. 207 Pb/ 204 Pb correlation diagram

87 Sr/ 86 Sr (i)  18 O Magmatic  18 O vs. 87 Sr/ 86 Sr correlation diagram

Matrix of crustal influence HLPNWBR 1 or more crustal signatures (Sr, Pb, O isotopes)165 No crustal factors (potential fractionates)8 + Iron Mt6 high Fe/Silow Fe/Si 1 or more crustal signatures115 No crustal factors (potential fractionates)53 + 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/SiO 2  HLP rhyolites have more chemical diversity, especially to the east with high FeO/SiO 2 along the axis of the plain  Within suite temporal evolution to higher FeO/SiO 2 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?…