1. Pre-Miocene magmatism in the northern Rockies
Reed S. Lewis
Idaho Geological Survey
Arthur A. Bookstrom
U.S. Geological Survey

2. 1380 Ma
A-type
granites
From: Zientek et al, U.S.G.S. OFR

3. 1380 Ma granitic augen gneiss, Dworshak Reservoir

4. Ross and Villeneuve, 2003

5. From: Zientek et al, U.S.G.S. OFR 2005-1235
Brushy gulch-
Green Mtn.
fault ?
From: Zientek et al, U.S.G.S. OFR

6. Neoproterozoic
syenite and diorite

8. Taylor
Ranch
mylonite
zone

9. “early phase”
Idaho batholith

10. “main phase”
Idaho batholith

11. Sodic and potassic rock suites Na-rich (high Sr/Y) K-rich (low Sr/Y)

12. Biotite granodiorite (Na-rich)

13. Hornblende-biotite granodiorite
Hornblende-biotite granodiorite (K-rich)

14. sodic suite younger? (~85-65 Ma) deeper?
“outboard” (or not yet exhumed in east?)
higher Na, Al, Sr, Ba, and LREE
Sr/Y > 40
initial 87Sr/86Sr ~
relatively depleted and deeper(?) source

15. potassic suite older? (~100-85 Ma) “inboard” (or eroded away?)
shallower?
higher K, Rb, U, Mg
Sr/Y < 40
initial 87Sr/86Sr ~ 0.707
relatively enriched, more mafic, and shallower(?) source

17. Eocene?
Cretaceous
Eocene?

19. Sr concentrations
in granodiorites
high Sr
low Sr
high Sr

20. questions
What controlled the NW belts of Mesoproterozoic and Neoproterozoic intrusions? Rifts? Strike-slip faults?
What were the magma sources? Can we deduce something about the different pC terranes?
What can the K plutons tell us about “inboard” granite genesis? (any significance to chemical suites?)
What can the T plutons tell us about extensional processes?

21. biggest need: geochron
We badly need more SHRIMP/LA-ICPMS zircon dates (everywhere, but especially in the KT rocks)– problems with inheritance in past efforts
Ar-Ar dates in selected areas (some areas covered well)