USG S Eruptive History and Chemical Evolution of the Precaldera and Postcaldera Basalt-Dacite Sequences, Long Valley, California: Implications for Magma Sources, Current Seismic Unrest, and Future Volcanism Professional Paper 1692 U.S. Department of the Interior U.S. Geological Survey This page intentionally left blank Eruptive History and Chemical Evolution of the Precaldera and Postcaldera Basalt-Dacite Sequences, Long Valley, California: Implications for Magma Sources, Current Seismic Unrest, and Future Volcanism By Roy A. Bailey Professional Paper 1692 U.S. Department of the Interior U.S. Geological Survey ii U.S. Department of the Interior Gale A. Norton, Secretary U.S. Geological Survey Charles G. Groat, Director U.S. Geological Survey, Reston, Virginia: 2004 For sale by U.S. Geological Survey Information Services Box 25286, Denver Federal Center Denver, CO 80225 This report and any updates to it are available online at: http://pubs.usgs.gov//pp/p1692/ Additional USGS publications can be found at: http://geology.usgs.gov/products.html For more information about the USGS and its products: Telephone: 1–888–ASK–USGS (1–888–275–8747) World Wide Web: http://www.usgs.gov/ Any use of trade, product, or firm names in this publication is for descriptive purposes only and does not imply endorsement of the U.S. Government. Although this report is in the public domain, it contains copyrighted materials that are noted in the text. Permission to reproduce those items must be secured from the individual copyright owners. Cataloging-in-publication data are on file with the Library of Congress (URL http://www.loc.gov/). Produced in the Western Region, Menlo Park, California Manuscript approved for publication, March 1, 2004 Text edited by George A. Havach Layout and design by Sara Boore Front Cover Long Valley Caldera from above Mammoth Rock on the southwest rim to Glass Moun- tain on the northeast rim (30 km). The resurgent dome is the forested ridge in the middle distance. The seismically and geothermally active south moat is the lowland in foreground. The White Moun- tains are snowcapped on the right horizon. View northeastward. iii In Memoriam Roy Alden Bailey (1929–2003) Roy A. Bailey received his B.A. in geology from Brown University in 1951 and his M.Sc. from Cornell University in 1953. He was a Fulbright Scholar in New Zealand in 1956, where he met his wife Patrice and began his career-long professional attachments. Roy began his career with the U.S. Geological Survey (USGS) as camp cook and geologic field assistant in New Mexico during the summers of 1950–52, and became a full-time geologist with the USGS in 1953 in Reston, Va. After work- ing 16 years as a geologist, he returned to graduate school at the Johns Hopkins University from 1969 to 1971, completing his Ph.D. in 1978 under the USGS Employee Training Program. His research with the USGS focused primarily on the study of large silicic volcanic systems. His fruitful collaboration with Robert L. Smith and Clarence S. Ross led to the publication of classic papers on resurgent calderas and chemical zonation in magma reservoirs. Roy’s principal field efforts involved geologic map- ping of Valles Caldera (Jemez Mountains, N.Mex.) and the Long Valley volcanic field in eastern California. Through careful mapping of the volcanic rocks in eastern California, he was able to identify the source of the widespread Bishop Tuff as a large resurgent caldera at the base of the eastern Sierra Nevada escarpment, now known as Long Valley Caldera. During the 1960s, Roy was an instructor in the National Aeronautics & Space Administration (NASA)’s Apollo training program, tutoring astronauts in the recognition of lunar volcanic features by comparing them to similar features on the Earth. During 1980–83, he served as coordinator of the USGS Volcano Hazards Program, which was greatly increased in size and com- plexity after the 1980 eruption of Mount St. Helens, Wash. In this managerial role, he deftly guided the Survey’s expanded moni- toring and research activities in Hawaii, Alaska, and the conterminous United States. In 1983, he rotated back into a research position, moved to Menlo Park, Calif., in 1984, and resumed his studies of the Long Valley volcanic field, where escalating seismicity and magmatic unrest raised concerns about a possible eruption. He retired in 1995, after 42 years of service, and continued working essentially full time as a Pecora Fellow (1995–97) and as Scientist Emeritus with the USGS Volcano Hazards Team (1997–2003). Roy was a Fellow of both the Geological Society of America and the Mineralogical Society of America and held member- ship in the American Geophysical Union and the Commission on Explosive Volcanism of the International Association of Vol- canology and Chemistry of the Earth’s Interior. Throughout his long and distinguished career, Roy was recognized by his peers for his perceptive geologic observations and exceptional mapping skills. His carefully documented studies of the products and processes of large explosive eruptions contributed to fundamental understanding of the evolution of caldera-forming magmatic systems. —David P. Hill This page intentionally left blank Contents Abstract ...........................................................................................................................................................1 Introduction.....................................................................................................................................................2 Acknowledgments .........................................................................................................................................3 Geologic-Tectonic Setting ............................................................................................................................4 Long Valley Volcanism ..................................................................................................................................7 Introduction............................................................................................................................................7 Precaldera Sequence ..........................................................................................................................7 Postcaldera Sequence ......................................................................................................................10 Caldera Suite ..............................................................................................................................13 Basalt Enclaves .................................................................................................................14 Early Intracaldera Basalt-Trachyandesite Lavas ........................................................14 Late Intracaldera Basalt-Trachyandesite Lavas .........................................................14 Extracaldera Basalt-Trachyandesite Lavas .................................................................14 Dacite Lavas ......................................................................................................................15 Mono-Inyo Suite ........................................................................................................................15 Basalt lavas........................................................................................................................16 Dacite lavas .......................................................................................................................16 Petrography and Chemistry........................................................................................................................17 General Relations................................................................................................................................17 Petrography .........................................................................................................................................18 Precaldera Sequence ...............................................................................................................18 Basalt ................................................................................................................................18 Trachyandesite ..................................................................................................................18 Dacite ................................................................................................................................21 Postcaldera Sequence .............................................................................................................21 Quenched-Basalt Enclaves .............................................................................................21 Basalt ................................................................................................................................21 Trachyandesite ..................................................................................................................22 Dacite-Rhyolite ..................................................................................................................22 Summary.............................................................................................................................22 Major- and Trace-Element Chemistry..............................................................................................25 Previous Work ............................................................................................................................25