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For maps, address mail orders to Maps Only U.S. Geological Survey, Information Services Maps may be purchased over the counter at the following Box 25286, Federal Center U.S. Geological Survey offices: Denver, CO 80225 Residents of Alaska may order maps from • FAIRBANKS, Alaska-New Federal Bldg, 101 Twelfth Ave. U.S. Geological Survey, Earth Science Information Center • ROLLA, Missouri-1400 Independence Rd. 101 Twelfth Ave., Box 12 Fairbanks, AK 99701 • STENNIS SPACE CENTER, Mississippi-Bldg. 3101 Compositional Characteristics of Middle to Upper Tertiary Volcanic Rocks of the Bolivian Altiplano By Edward A. du Bray, Steve Ludington, William E. Brooks, Bruce M. Gamble, James C. Ratte, Donald H. Richter, and Eduardo Soria-Escalante U.S. GEOLOGICAL SURVEY BULLETIN 2119 Major oxide and trace element data for lavas and ash-flow tuffs of the Bolivian Altiplano UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1995 U.S. DEPARTMENT OF THE INTERIOR BRUCE BABBITT, Secretary U.S. GEOLOGICAL SURVEY Gordon P. Eaton, Director For sale by U.S. Geological Survey, Information Services Box 25286, Federal Center Denver, CO 80225 Any use of trade, product, or firm names in this publication is for descriptive purposes only and does not imply endorsement by the U.S. Government Library of Congress Cataloging-in-Publication Data Compositional characteristics of middle to upper Tertiary volcanic rocks of the Bolivian Altiplano I by Edward A. du Bray ... [et al.]. p. em. -(U.S. Geological Survey bulletin ; 2119) Includes bibliographical references. Supt. of Docs. no.: B 19.3:2119 1. Volcanic ash, tuff, etc.-Bolivia-Composition. 2. Geology, Stratigraphic­ Tertiary. I. du Bray, E. A. II. Series. QE75.B9 no. 2119 [QE461] 557.3 s-dc20 95-3059 [552'.2'0984] CIP CONTENTS Abstract. ......................................................................................................................... Introduction . .. ... .. ... ... .. .. .. ... .... .. .. .... ... .. .. .. ......... .......... .. ... .. ... .. ... .. ... ... .. ...... 1 Petrography..................................................................................................................... 4 Geochemistry.................................................................................................................. 4 Analytical Methods................................................................................................. 4 Unaltered Rocks ...... .. ...... .. ... .. .. .. .. ... .. ... ... .. ...... ... ... ... .... .. .. .. .. ... .... ........ 4 Altered Rocks......................................................................................................... 18 Discussion and Conclusions .. .. .... ... .......... .. ... .. ... .. .. .. ....... .. .. .. ... .......... ........ ........ ... 20 Unaltered Rocks..................................................................................................... 20 Altered Rocks . .... ... ... .. .. ..... .. .... .. .. ............. .. .. .... .. .. .... ... .... ....... .. ... .. .. ... .. .. ... 21 References Cited............................................................................................................. 21 Appendixes-Brief Descriptions of Samples................................................................. 23 FIGURES 1. Maps showing volcanic features and sample locations, Altiplano and Cordillera...................................................... 2 2. Total alkali-silica variation diagram showing analyses of volcanic rock samples..................................................... 5 3. Histogram of Si02 contents......................................................................................................................................... 16 4. Variation diagrams showing abundances of major oxides.......................................................................................... 17 5. Variation diagram showing rubidium and strontium abundances............................................................................... 18 6. Histogram of rubidium-strontium ratios...................................................................................................................... 18 7. Ternary variation diagram showing the relative proportions of rubidium, potassium, and strontium........................ 19 8. Trace element-tectonic setting discrimination variation diagrams............................................................................. 19 9. Ternary AFM variation diagram .. ......... ... ... ... .. ... .... ... .... .. .. ... ... .. ......... .. ...... .. ... .. .. .. ......... .. ... .. .. .. .. .. .. .. .. 20 TABLES 1. Chemical compositions of unaltered volcanic rock samples from the Bolivian Altiplano........................................ 6 2. Chemical compositions of altered volcanic rock samples from the Bolivian Altiplano............................................ 14 III Compositional Characteristics of Middle to Upper Tertiary Volcanic Rocks of the Bolivian Altiplano By Edward A. du Bray, Steve Ludington, William E. Brooks, Bruce M. Gamble, James C. Ratte, Donald H. Richter, and Eduardo Soria-Escalante ABSTRACT Bolivia during 1990-91. During fieldwork for the assessment, field parties visited many Cenozoic volcanic centers (fig. 1), Middle to upper Tertiary lava flows, hypabyssal flow particularly those with associated mineral deposits, and the domes, and ash-flow tuffs of the Bolivian Altiplano, includ­ volcanic rocks were sampled. Geochemical and petrographic ing those genetically associated with polymetallic vein data were obtained to aid in the characterization of these rocks deposits, are composed dominantly of metaluminous dacite. and in the interpretation of the geologic setting of the associ­ Major oxide and trace element compositions of these rocks, ated mineral deposits. Data and interpretations presented here which contain variable combinations and abundances of pla­ are byproducts of the mineral resource assessment. gioclase, biotite, hornblende, clinopyroxene, quartz, and Mesozoic and Cenozoic igneous rocks of the Andes in potassium feldspar, are similar to those of continental mar­ western South America have long been recognized as the gin, volcanic arc rocks throughout the world. Abundances of products of archetypal continental margin, subduc­ Al203, total iron, MgO, CaO, and Ti02 decrease with tion-related volcanic arc magmatism (Mitchell and Reading, increasing Si02, whereas K20 abundances increase with 1969; Dewey and Bird, 1970). Volcanoes in the western part increasing Si02. Compositional variation trends defined by of the South American arc have been more extensively stud­ these rocks primarily reflect crustal contamination of primi­ ied (see, for example, Harmon and Rapela, 1991) than those tive, asthenospheric, mantle-derived partial melts that have of the Altiplano, primarily because they are the locus of undergone subsequent evolution involving varying amounts ongoing igneous activity within the arc. of crystal fractionation. Alumina saturation indices of the A detailed review of central Andean geology is beyond Altiplano