Synthesis of Petrographic, Geochemical, and Isotopic Data for the Boulder Batholith, Southwest Montana Professional Paper 1793 U.S. Department of the Interior U.S. Geological Survey Cover photograph: Planar jointed, spheroidal weathered Butte Granite at Homestake Pass, Montana. Synthesis of Petrographic, Geochemical, and Isotopic Data for the Boulder Batholith, Southwest Montana By Edward A. du Bray, John N. Aleinikoff, and Karen Lund Professional Paper 1793 U.S. Department of the Interior U.S. Geological Survey U.S. Department of the Interior KEN SALAZAR, Secretary U.S. Geological Survey Marcia K. McNutt, Director U.S. Geological Survey, Reston, Virginia: 2012 For more information on the USGS—the Federal source for science about the Earth, its natural and living resources, natural hazards, and the environment, visit http://www.usgs.gov or call 1–888–ASK–USGS. 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Suggested citation: du Bray, E.A., Aleinikoff, J.N., and Lund, Karen, 2012, Synthesis of petrographic, geochemical, and isotopic data for the Boulder batholith, southwest Montana: U.S. Geological Survey Professional Paper 1793, 39 p. iii Contents Abstract ...........................................................................................................................................................1 Introduction.....................................................................................................................................................1 Study Description...........................................................................................................................................4 Petrographic Data..........................................................................................................................................5 Major Oxide Data ...........................................................................................................................................9 Mafic Group ...........................................................................................................................................9 Intermediate Group...............................................................................................................................9 Felsic Group ...........................................................................................................................................9 Trace Element Data .....................................................................................................................................12 Mafic Group .........................................................................................................................................12 Intermediate Group.............................................................................................................................12 Felsic Group .........................................................................................................................................12 U-Pb Zircon Geochronology.......................................................................................................................17 Methods................................................................................................................................................17 Results ..................................................................................................................................................18 Radiogenic Isotope Data ............................................................................................................................20 Petrogenetic Implications of Geochemical and Isotopic Data ............................................................20 Conclusions...................................................................................................................................................35 Acknowledgments .......................................................................................................................................35 References Cited..........................................................................................................................................35 Figures 1. Index map showing location of Boulder batholith relative to major geologic features in the region ..................................................................................................2 2. Map showing the Boulder batholith, southwest Montana ....................................................3 3. Quartz-alkali feldspar-plagioclase ternary diagrams showing modal compositions of Boulder batholith plutons ...............................................................................7 4. Variation diagrams showing major oxide abundances in samples of the Boulder batholith, southwest Montana ............................................................................11 5. Total alkali-silica variation diagram showing compositions of the Boulder batholith, southwest Montana ..................................................................................13 6. FeO*/(FeO*+MgO) versus SiO2 variation diagram showing compositions the Boulder batholith, southwest Montana ............................................................................14 7. Na2O+K2O-CaO versus SiO2 variation diagram showing compositions of the Boulder batholith southwest Montana ........................................................................15 8. Variation diagram showing molar major oxide compositions of the Boulder batholith, southwest Montana as a function of alumina and alkali saturation ................16 9. Trace-element, tectonic-setting-discrimination variation diagram showing the composition of the Boulder batholith, southwest Montana .........................................18 10. Variation diagram showing relative proportions of rubidium, potassium, and strontium in samples of the Boulder batholith, and its peripheral plutons, southwest Montana ...................................................................................................................19 iv 11. Chondrite-normalized rare-earth element diagrams for Boulder batholith samples ........................................................................................................................................21 12. Primitive mantle-normalized trace-element diagrams for Boulder batholith samples ........................................................................................................................22 13. Photographs of representative zircons and Tera-Wasserburg plots of sensitive high-resolution ion microprobe (SHRIMP) U-Pb dates from each sample dated in this study .........................................................................................................23 14. Graph showing epsilon neodymium versus initial strontium isotopic data for Boulder batholith samples ..................................................................................................33 Tables 1. Summary of petrographic characteristics for plutons of the Boulder batholith, southwest Montana ....................................................................................................6 2. Average mineralogic and chemical compositions of Butte Granite north and south of latitude 46.2°N. Boulder batholith, southwest Montana ......................9 3. Average chemical compositions for plutons of the Boulder batholith, southwest Montana, arranged in order of increasing silica content ................................10 4. SHRIMP U-Th-Pb data for zircon from plutons of Boulder batholith, southwest Montana ...................................................................................................................27 5. Summary of age data for plutons of Boulder batholith, southwest Montana. .................33 6. Rb-Sr and Sm-Nd isotopic data for samples of the Boulder batholith, southwest Montana ...................................................................................................................34 Appendix 1. Petrographic and geochemical data for samples of the Boulder batholith, southwest Montana ............................................................................................... link Synthesis of Petrographic, Geochemical, and Isotopic Data for the Boulder Batholith, Southwest Montana By Edward A. du Bray, John N. Aleinikoff, and Karen Lund Abstract the plutons is also characterized by distinct trace element abun- dances although absolute trace element abundance variations are relatively minor. Limited Sr and Nd isotope data for whole- The Late Cretaceous Boulder batholith in southwest rock samples of the Boulder batholith are more radiogenic Montana consists of the Butte Granite and a group of associ- than those for plutonic rocks of western Idaho, eastern Oregon, ated smaller intrusions emplaced into Mesoproterozoic to the Salmon River suture, and most of the Big Belt Mountains. Mesozoic sedimentary rocks and into the Late Cretaceous Elk- Initial strontium