Geology of the Mount Aetna Volcanic Center, Chaffee and Gunnison Counties, Colorado
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Geology of the Mount Aetna Volcanic Center, Chaffee and Gunnison Counties, Colorado By PRIESTLEY TOULMIN Ill and JANE M. HAMMARSTROM A description of a deeply eroded mid-Tertiary volcano on the southern fringes of the Mount Princeton batholith in the Sawatch Range U.S. GEOLOGICAL SURVEY BULLETIN 1864 DEPARTMENT OF THE INTERIOR MANUEL LUJAN, Jr., Secretary U.S. GEOLOGICAL SURVEY Dallas L. Peck, Director 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 UNITED STATES GOVERNMENT PRINTING OFFICE: 1990 For sale by the Books and Open-File Reports Section U.S. Geological Survey Federal Center, Box 25425 Denver, CO 80225 Library of Congress Cataloging in Publication Data Toulmin, Priestley. Geology of the Mount Aetna volcanic center, Chaffee and Gunnison counties, Colorado. · (U.S. Geological Survey bulletin ; 1864) Bibliography: p. Supt. of Docs. no.: I 19.3:1864 1. Volcanic ash, tuff, etc. -Colorado-Chaffee County. 2. Volcanic ash, tuff, etc.-Colorado-Gunnison County. 3. Volcanism-Colorado-Chaffee County. 4. Volcanism-Colorado-Gunnison County. 5. Geology Colorado-Chaffee County. 6. Geology-Colorado-Gunnison County. I. Hammarstrom, Jane Marie. II. Title. Ill. Series. QE75.B9 no. ;1864 557.3 s [552'.2'0978841] 88-600309 [QE461] CONTENTS Abstract 1 Introduction 1 Previous Work 3 Acknowledgments 4 Description of Rock Units 4 Precambrian Rocks 4 Gneissic Quartz Monzonite 4 Silver Plume(?) Granite 5 Paleozoic Sedimentary Rocks 5 Tertiary(?)-Quartz Diorite 6 Tertiary 6 Mount Pomeroy Quartz Monzonite 6 Petrography 6 Contact Relations 6 Alteration and Metamorphism 7 Origin 7 Mount Princeton Quartz Monzonite 7 Contact Relations, Metamorphism, and Alteration 7 Petrography 8 Calico Mountain Andesite 9 Lithology and Internal Structures 9 Contact Relations 10 Sewanee Peak Volcanics 11 Petrography 11 Contact Relations: Basal Rubble Zone 12 Breccias and Megabreccias 13 Mount Aetna Quartz Monzonite Porphyry 14 Form and Contact Relations 14 Petrography 16 Hoffman Park Granite 17 Mount Antero Granite 17 Minor Intrusive Bodies 18 Rock Chemistry 19 Mineral Chemistry 24 Hornblende 24 Biotite 28 Plagioclase 29 Potassium Feldspar 31 Sphene 31 Opaque Minerals 31 Apatite 32 Radiometric Dating 32 Development of the Complex 34 Regional Relations 37 References Cited 39 Appendix: Locations of Analyzed Samples 43 Contents Ill PLATE [In pocket] 1. Geologic map of the Mount Aetna volcanic center FIGURES 1. Map showing location of the Mount Aetna volcanic center 2 2-4. Photographs showing: 2. View of Mount Aetna from the southeast 3 3. Collapsed, stretched pumice fragments in welded tuff of the Calico Mountain Andesite, Billings Basin 9 4. Agglomerate horizon in the Calico Mountain Andesite 9 5. Map showing structure contours on the base of the Calico Mountain Andesite 10 6. Photomicrograph of shard structure in Sewanee Peak Volcanics 11 7-9. Photographs showing: 7. Basal contact of the Sewanee Peak Volcanics at Pome~oy Pass 12 8. Hematite-rimmed clast in the basal Sewanee Peak Volcanics 13 9. An exceptionally well exposed block surmounting an erosional pinnacle in the megabreccia near the major fork in Tomichi Creek 14 10. Map showing locations of analyzed samples 15 11. Harker variation diagrams for rocks of the Mount Aetna volcanic center and other rocks of the Mount .princeton igneous complex 27 12. AFM diagram for rocks of the Mount Aetna volcanic center and other rocks of the Mount Princeton igneous complex 28 13. Plot of normative quartz, plagioclase, and orthoclase diagram for rocks of the Mount Aetna volcanic center and other rocks of the Mount Princeton igneous complex 28 14. Plot of normative quartz, albite, and orthoclase diagram for rocks of the Mount Aetna volcanic center and other rocks of the Mount Princeton igneous complex 28 15. Plots of chondrite-normalized rare-earth element patterns for rocks of the Mount Aetna volcanic center and other rocks of the Mount Princeton igneous complex 29 16. Plots showing variation of Eu and the europium anomaly as functions of Si02 for rocks of the Mount Aetna volcanic center and other rocks of the Mount Princeton igneous complex 29 17. Diagram showing range of amphibole compositions from rocks of the Mount Aetna volcanic center and other rocks of the Mount Princeton igneous com plex plotted in terms of tetrahedral aluminum content and Mg/(Mg + Fe) 35 18. Ternary diagrams showing compositions of amphiboles from rocks of the Mount Aetna volcanic center and other rocks of the Mount Princeton igneous complex 36 19-21. Plots showing: 19. Biotite compositions in t~pns of end-member micas 36 20. Average biotite compositions in terms of fluorine content 37 21. Compositional zonation in plagioclase crystals from rocks of the Mount Princeton igneous complex 37 IV Contents TABLES 1. Major-element analyses of rocks of the Mount Princeton igneous complex 19 2. Minor-element analyses of rocks of the Mount Princeton igneous complex 24 3. Chemical analyses of minerals 30 4. Compositional ranges of rock-forming minerals in major rock units of the Mount Princeton igneous complex from electron-microprobe analyses 32 5. Published radiometric age determinations for rocks associated with the Mount Aetna volcanic center 38 6. Radio~etric age determinations made in connection with the present study 38 Contents V Geology of the Mount Aetna Volcanic Center, Chaffee and Gunnison Counties, Colorado By Priestley Toulmin Ill and Jane M. Hammarstrom Abstract minor intrusions for another 10 million years or so. These epochs, like those associated with other volcanic centers The Mount Aetna volcanic center is located in the along the west side of the northernmost part of the rift southwestern part of the Mount Princeton batholith, a zone, are distinctly older than the corresponding episodes quartz-monzonitic pluton of Oligocene age in the south in the San juan volcanic field proper to the south; this ern Sawatch Range of central Colorado. The batholith lies early magmatism may reflect crustal stresses premonitory just west of the Arkansas River graben, the northernmost to the opening of this part of the Arkansas River graben major element of the Rio Grande rift system. As presently Rio Grande rift zone. exposed, the volcanic center consists of a deeply eroded Chemically, the rocks of the complex are typical of complex of highly compacted, densely welded, propyliti calc-alkaline suites. In a regional context, the rocks of this cally altered intracaldera ash-flow tuffs, flows, and brec complex have compositions intermediate between mag cias that have been preserved within a trapdoorlike gra matic types recognized elsewhere (Front Range, San juan ben that subsided after the main stage of volcanism but volcanic field, Elk Mountains) and tend to blur distinctions before the end of intrusive magmatic activity. The earliest made in those areas. Major-element compositions are volcanism of which a record is preserved produced pre consistent with local differentiation of magmas from one dominantly andesitic flows, breccias, and a few ash flows; another, but contents of rare-earth elements suggest that these volcanic rocks were deposited on a rolling surface plagioclase fractionation was not a dominant factor in the that exposed Precambrian and Paleozoic rocks, as well as evolution of the volcanic rocks. a border facies of the Mount Princeton batholith. After an The compositions of the major rock-forming miner interval of erosion that exposed deeper levels of the als vary considerably, but their ranges in the volcanic batholith and developed a new surface of rugged high rocks and dominant quartz-monzonite units overlap one relief, great thicknesses of more silicic volcanics were another. Much of the variation reflects both magmatic and deposited, largely in the form of ash-flow tuffs. Penecon postmagmatic processes. The younger, more silicic units temporaneous collapse formed a caldera of unknown (granites and rhyolitic and aplitic dikes) are distinct in both dimensions, along the walls of which chaotic megabrec mineral chemistry and major- and trace-element rock cias were produced by collapse and mass sliding of the chemistry. slopes. The megabreccias are preserved mostly on the western and southwestern margins of the presently exposed structure. It is likely that ash flows from the INTRODUCTION Mount Aetna center extended as far as 75 kilometers east across the Arkansas River graben to form part of the This report presents some results of a study of an Thirtynine Mile volcanic field, the southern topographic boundary of South Park. assemblage of middle Tertiary igneous rocks in the southern The igneous activity associated with the Mount Sawatch Range of central Colorado (fig. 1). The study, Aetna volcanic center began more than 37 million years which began in the early 1960's as a part of a multidisci ago, perhaps about 40 million years. Comparison of radi plinary field and laboratory study of the chemical and ometric data from several different bodies and map units petrologic relations between "classic" base- and precious strongly implies that a complex history of intrusion, ero metal hydrothermal mineralization and the differentiation sion, volcanism, and tectonism took place here in an processes exhibited by a complex suite of calc-alkaline extremely short time. Some of the more silicic postvolcan intrusive rocks, led to the recognition of a deeply eroded, ic intrusives may have been emplaced as much as 4 highly altered eruptive center that may have been the source million to 5 million years afterwards, and minor magmatic of widespread volcanic deposits in the region. activity apparently continued to produce small dikes and The remnants of the volcanic center are preserved in a trapdoorlike subsided block, to which we shall refer as the Manuscript approved for publication, July II, 1988. "Mount Aetna subsided block." This block subsided along Introduction DENVERo 0 25 50 MILES N 0 25 50 KILOMETERS t • Leadville Upper Grizzly~ Arkansas Peak~ Valley Caldera 3 6 San Juan 380 0 Field San Luis () Valley Figure 1.