1. 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 Fordm@geo.oregonstate.edu Anita L. Grunder Oregon State University Grundera@geo.oregonstate.edu Richard Carlson Dept. of Terrestrial Magnetism Carlson@dtm.ciw.edu Carlson@dtm.ciw.edu GSA 2009 abs. #224-5

2. 050100 Miles 080160 km NWBR HLP

3. 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

4. 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

5. 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 :

6. Number Comparison to suites: Cascades, SRP, Iceland

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

8. 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?

9. FeO – SiO 2 diagram from the study area Nearly all NWBR are “Low FeO”, HLP is variable to high FeO

10. 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

11. Glass Buttes Juniper Ridge

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

13. 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

14. 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…

15. 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

16. 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

17. 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”

18. 206 Pb/ 204 Pb 207 Pb/ 204 Pb Pelagic sediments or continental crust 206 Pb/ 204 Pb vs. 207 Pb/ 204 Pb correlation diagram

19. 206 Pb/ 204 Pb 207 Pb/ 204 Pb Pelagic sediments or continental crust 206 Pb/ 204 Pb vs. 207 Pb/ 204 Pb correlation diagram

20. 87 Sr/ 86 Sr (i)  18 O Magmatic  18 O vs. 87 Sr/ 86 Sr correlation diagram

21. 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:

22. 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?…