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E-146, 701 C St. Maps For maps, address mail orders to U.S. Geological Survey, Map Distribution Maps Federal Center, Box 25286 Maps may ^ purchased over the counter at the U.S. Geologi- Denver, CO 80225 caj Survey offices where books are sold (all addresses in above list) and at the following Geological Survey offices: Residents of Alaska may order maps from ROLLA, Missouri-1400 Independence Rd. Alaska Distribution Section, U.S. Geological Survey, . DENVER Colorado-Map Distribution, Bldg. 810, Federal New Federal Building - Box 12 Center 101 Twelfth Ave., Fairbanks, AK 99701 . FAIRBANKS, Alaska-New Federal Bldg., 101 Twelfth Ave. Mafic and Ultramafic Xenoliths from Volcanic Rocks of the Western United States By H.G. WILSHIRE, C.E. MEYER, J.K. NAKATA, L.C. CALK, J.W. SHERVAIS, J.E. NIELSON, and E.G. SCHWARZMAN U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1443 UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1988 DEPARTMENT OF THE INTERIOR DONALD PAUL HODEL, Secretary U.S. GEOLOGICAL SURVEY Dallas L. Peck, Director Library of Congress Cataloging-in-Publication Data Mafic and ultramafic xenoliths from volcanic rocks of the western United States. (U.S. Geological Survey professional paper; 1443) Bibliography: p. Supt. of Docs, no.: I 19.16:1443 1. Rocks, Igneous West (U.S.) Inclusions. 2. Alkalic igneous rocks West (U.S.) 3. Rocks, Ultrabasic West (U.S.) I. Wilshire, Howard Gordon, 1926- . II. Series: Geological Survey professional paper; 1443. QE461.M234 1988 552'. 1 88-600047 For sale by the Books and Open-File Reports Section, U.S. Geological Survey, Federal Center, Box 25425, Denver, CO 80225 CONTENTS Page Abstract --------------------------- 1 Trace elements and isotopes in xenoliths and Introduction ------------------------- 2 host rocks Continued Acknowledgments --------------------- 2 Sr and Nd isotopes Continued Previous investigations ------------------- 2 Sr and Nd isotopes of megacrysts ---------- 28 Distribution and setting ------------------- 4 Problems in interpreting isotopic data -------- 28 Host rocks -------------------------- 8 Oxygen isotopes --------------------- 28 Xenoliths- -------------------------- 8 Origin and causes of variation among xenoliths ------- 28 Relations among xenolith types as indicated by composite Cognate versus accidental origin ------------- 28 xenoliths -------------------------- 12 Origin of lithologic variants within major ultramafic Proximity of mafic-ultramafic lithologies in the mantle groups ------------------------- 30 and sequence of formation --------------- 12 Salient features ------------------- 30 Systematic compositional variations in composite Physical conditions of emplacement and xenoliths ------------------------ 13 differentiation ------------------- 31 Trace elements and isotopes in xenoliths and host rocks - - - 17 Wallrock reactions and metasomatism -------- 32 Rare earth elements ------------------- 18 Origin of the main xenolith groups ------------ 35 Cr-diopside group ------------------ 18 Current concepts of mantle metasomatism: cause or effect of Composite Cr-diopside peridotite and Al-augite alkaline basaltic magmatism?- --------------- 37 pyroxenite --------------------- 20 References -------------------------- 40 Al-augite group ------------------- 20 Appendixes Mineral separates and megacrysts, all groups - - - - 22 I. Locality descriptions ----------------- 51 Problems in interpreting REE data --------- 24 II. Bulk chemical compositions and CIPW norms for host Sr and Nd isotopes- ------------------- 25 rocks ------------------------ 57 Sr isotopes of the Cr-diopside group --------- 25 III. Detailed descriptions of principal xenolith groups Sr isotopes of the Al-augite group ---------- 25 and composite xenoliths- -------------- 65 Sr isotopes of the feldspathic and bottle-green IV. Bulk chemical compositions and CIPW norms for pyroxene groups ------------------ 26 xenoliths ---------------------- 85 Sr and Nd isotopes of the Cr-diopside group ----- 26 V. Mineral analyses ------------------- 95 Sr and Nd isotopes of composite xenoliths of the VI. Systematic compositional trends of minerals in Cr-diopside and Al-augite groups ---------- 26 composite xenoliths ----------------- 131 Sr and Nd isotopes of the garnetiferous ultramafic VII. Trace element and isotope data for host rocks, group- ----------------------- 27 xenoliths, and megacrysts ------------- 167 ILLUSTRATIONS FIGURES 1-9. Maps of the Western United States showing: 1. Distribution of xenolith localities with respect to Cenozoic tectonic provinces -------------------- 3 2. Distribution of xenoliths with respect to heat-flow provinces, crustal thickness, and P-wave velocity -------- 9 3. Distribution of xenoliths in the Cr-diopside group ---------------------------------- 10 4. Distribution of xenoliths in the Al-augite group ----------------------------------- 10 5. Distribution of xenoliths with hydrous phases ------------------------------------ 11 6. Distribution of xenoliths in the feldspathic ultramafic group ---------------------------- 11 7. Distribution of xenoliths in the bottle-green pyroxene group ---------------------------- 12 8. Distribution of xenoliths in the garnetiferous ultramafic group --------------------------- 12 9. Distribution of xenoliths in the gabbroid and metagabbroid groups ------------------------- 13 in IV CONTENTS Page FIGURE 10. Alkali-silica variation of host rocks ------------------------------------------- 15 11. Chart of main xenolith groups --------------------------------------------- 16 12. Classification and nomenclature of ultramafic and gabbroic rocks --------------------------- 17 13, 14. Plots of modal compositions of xenoliths in the: 13. Cr-diopside group ------------------------------------------------- 20 14. Al-augite group -------------------------------------------------- 20 15. Photographs of Cr-diopside websterite dike in Cr-diopside Iherzolite, and Cr-diopside Iherzolite lenses in Cr-diopside websterite ------------------------------------------------------ 20 16. Schematic diagram showing sequence of emplacement of dike rocks and associated reaction zones in xenoliths - - - - 21 17-22. Photographs showing: 17. Crosscutting Cr-diopside websterite dikes in Cr-diopside Iherzolite xenolith, and crosscutting Cr-diopside websterite dikes in Cr-diopside Iherzolite, Balmuccia peridotite massif ---------------------- 22 18. Garnet pyroxenite crosscutting spinel pyroxenite layers in Cr-diopside Iherzolite, Lherz peridotite massif; Al-augite pyroxenite dikes isolating an inclusion