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Mark T. Ford Oregon State University Fordm@geo.oregonstate

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

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Mark T. Ford Oregon State University Fordm@geo.oregonstate

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  1. 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 Carlson@dtm.ciw.edu Anita L. Grunder Oregon State University Grundera@geo.oregonstate.edu

  2. 0 50 100 Miles 0 80 160 km HLP NWBR

  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 km3 to 2,500 km3 Basalts < 20 Ma in gray Rhyolites in purple Ash flow tuffs in yellow

  5. 2.89 Ma One post-progression rhyolite: Iron Mt. Age progression in rhyolites 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. Comparison to suites: Cascades, SRP, Iceland Number

  7. Tholeiitic vs. Calc-alkaline suites Clearly separated on FeO – SiO2 diagram, except at highest silica

  8. Can we use this to help separate NWBR and HLP samples? Tholeiitic vs. Calc-alkaline suites Clearly separated on FeO – SiO2 diagram, except at highest silica

  9. FeO – SiO2 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. High Fe/Si Zero line Low Fe/Si Within suite variation relative to FeO vs. SiO2

  13. Suite evolution Within suite Fe/Si enrichment Juniper Ridge and Glass Buttes High Fe/Si Fe-Si zero line 1.2 Ma 0.7 Ma Low Fe/Si 30 km

  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. crustal addition Nd- and Sr-isotopic variations of the rhyolites – some with elevated Sr isotopic ratios 143Nd/144Nd 87Sr/86Sr(i)

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

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

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

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

  20. Magmatic d18O vs. 87Sr/86Sr correlation diagram 87Sr/86Sr(i) d18O

  21. Matrix of crustal influence Within the HLP:

  22. 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 Conclusions: Time for a short movie?… Acknowledgements: NSF funding; Ilya Bindeman: Oxygen isotopes; Jenda Johnson: animation

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