Rock Units of the Precaittbriail Basement in Colorado GEOLOGIC A I, Rock Units of the Precambrian Basement in Colorado By OGDEN TWETO GEOLOGY OF THE PRECAMBRIAN BASEMENT IN COLORADO U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1321-A Description and distribution of Proterozoic and Archean metamorphic and igneous rocks in outcrop and the buried basement in Colorado UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1987 DEPARTMENT OF THE INTERIOR DONALD PAUL HODEL, Secretary U.S. GEOLOGICAL SURVEY Dallas L. Peck, Director Library of Congress Cataloging in Publication Data Tweto, Ogden, 1912-1983 Rock units of the Precambrian basement in Colorado. (Geological Survey Professional Paper ; 1321-A) Bibliography: p. Supt. of Docs. No.: I 19.16:1321-A 1. Geology, Stratigraphic Pre-Cambrian. 2. Geology Colorado. I. Title. II. Series. QE653.T9 1986 551.7'1'09788 84-600001 For sale by the Books and Open-File Reports Section U.S. Geological Survey Federal Center Box 25425 Denver, CO 80225 CONTENTS Page Page Abstract ............................ Al Proterozoic intrusive rocks Continued Introduction .......................... 2 Routt Plutonic Suite Continued Definition of Precambrian basement .......... 3 Pitts Meadow Granodiorite ............. A29 Regional features ..................... 3 Tenmile and Bakers Bridge Granites . 29 History of investigations ................. 5 Granodiorite of the Grand Junction area . 30 Sources of information .................. 6 Plutons beneath eastern Plains ....... 30 Stratigraphic classification ................ 7 Berthoud Plutonic Suite ............. 31 Rock terminology ..................... 8 Silver Plume Granite ............ 31 Radiometric ages ..................... 9 Cripple Creek Granite ............ 33 Archean rocks ......................... 9 San Isabel Granite .............. 33 Proterozoic layered rocks ................... 10 Sherman Granite ............... 33 Gneiss complex ...................... 10 Rocks of the Mount Ethel pluton ..... 34 Biotitic gneisses ................... 11 St. Kevin Granite .............. 35 Felsic and hornblendic gneisses .......... 14 Syenitic rocks in Gunnison-San Juan region 36 Vallecito Conglomerate and Uncompahgre Formation . 18 Vernal Mesa Quartz Monzonite ....... 37 Vallecito Conglomerate ............... 18 Curecanti Quartz Monzonite ........ 37 Uncompahgre Formation .............. 19 Eolus Granite ................ 38 Las Animas Formation .................. 20 Trimble Granite ............... 38 Uinta Mountain Group .................. 21 Electra Lake Gabbro ............ 39 Proterozoic intrusive rocks .................. 22 Subsurface units ............... 39 Groupings of igneous rocks ............... 22 Mafic and intermediate dikes .......... 39 Routt Plutonic Suite ................... 23 Rocks of the Pikes Peak batholith ....... 41 Rocks of the Rawah batholith ........... 23 Pikes Peak Granite ............. 41 Rocks of the Boulder Creek batholith ....... 24 Associated subalkalic rocks ......... 43 Rocks of the Mount Evans batholith ....... 25 Windy Point Granite ............ 43 Gabbro of Elkhorn Mountain ............ 25 Redskin Granite ............... 44 Cross C^eek Quartz Monzonite ........... 26 Syenite and gabbro plutons ......... 44 Denny Creek Granodiorite ............. 26 Latest Proterozoic and Cambrian rocks .... 45 Kroenke Granodiorite ................ 27 Tectonic influences ................... 45 Granitic rocks in Taylor Park-Gunnison region . 27 References cited .................... 47 Powderhorn Granite and related rocks ...... 29 ILLUSTRATIONS Page PLATE 1. Geologic map of the Precambrian basement in Colorado. In pocket FIGURES 1-4. Maps showing: 1. Proterozoic and Archean age provinces in Rocky Mountain region .... A4 2. Major tectonic and geographic features in Colorado ............. 5 3. Principal Precambrian plutons and some major fracture zones in Colorado 13 4. Intrusive bodies and centers in exposed part of Pikes Peak batholith . 42 TABLE TABLE 1. Major Precambrian rock units in Colorado ............................................ A8 in GEOLOGY OF THE PRECAMBRIAN BASEMENT IN COLORADO ROCK UNITS OF THE PRECAMBRIAN BASEMENT IN COLORADO By OGDEN TWETO ABSTRACT The term Precambrian basement as used in this report applies to lutetium-hafnium ages in the range 1,740-1,800 m.y. obtained from all rocks of Precambrian age. Basement rocks are exposed in many igneous components of the gneisses suggest that the age of the pro­ places in mountain uplifts in central and western Colorado, but are tolith was little greater than the age of metamorphism and probably buried beneath younger rocks in more than 86 percent of the State. does not exceed 1,800 m.y. The exposed basement consists of variously metamorphosed layered According to recent studies, the Vallecito Conglomerate of the rocks and of intrusive igneous rocks, in about equal proportions. The Needle Mountains in southwestern Colorado is younger than and lies buried basement is of the same general character as the exposed unconformably upon the Irving Formation of the Proterozoic gneiss basement except for the presence in southeastern Colorado of a unit complex. This is a reversal of some former interpretations but is of moderately metamorphosed sedimentary and volcanic rocks (Las in agreement with the original classification. The Vallecito is a mas­ Animas Formation) not known elsewhere. sive unit of conglomerate and pebbly quartzite more than 1,500 m The Precambrian rocks are all of Proterozoic age except in a small (5,000 ft) thick. The Uncompahgre Formation nearby may be a lateral area near the northwest corner of the State, where Archean rocks equivalent of the Vallecito. The Uncompahgre consists of more than at the south margin of the Wyoming cratonic age province are pres­ 2,650 m (8,700 ft) of quartzite, slate, and phyllite. The Uncompahgre ent, mainly in the subsurface. As exposed at the Colorado-Utah bor­ and Vallecito are preserved in three areas in a northwest-trending der and westward, the Archean rocks consist of gneisses about 2,700 belt in the western San Juan Mountains region. This belt may ap­ m.y. (million years) in age and of the Red Creek Quartzite. The Red proximate the original depositional trough. Ages of the two forma­ Creek is not conclusively dated but is interpreted as-probably older tions are known only as being younger than a 1,695-m.y. granite than the 2,500-m.y. age boundary between the Archean and Protero­ and older than a 1,440-m.y. granite. zoic Eons. The Archean gneisses are mainly of granitic composition The Middle Proterozoic Las Animas Formation is recognized only and igneous origin. They are exposed only in small fault blocks sepa­ in the subsurface in southeastern Colorado. The formation consists rated from the Red Creek by zones of quartz mylonite. The Red predominantly of dark-gray to black slate, phyllite, graywacke, and Creek Quartzite consists of at least 4 km (13,000 ft) of metaquartzite, chert, but an upper part is dull red and contains minor volcanic and kyanite-bearing quartz-muscovite schist, and minor marble, all exten­ carbonate rocks. A thickness of more than 1,700 m (5,575 ft) has sively intruded by sills now metamorphosed to amphibolite. Faulting been penetrated in boreholes, without the base being reached. The that occurred before and at various times after deposition of the Red formation occupies a west-trending belt about 130 km (80 mi) long Creek indicates recurrent tectonism at the border of the Archean and as much as 50 km (31 mi) wide. It is interpreted as fill in a Wyoming province. rift incised in a terrane of mesozonal granite about 1,400 m.y. in The oldest and most widespread of the Proterozoic rocks in Col­ age. The Las Animas is tentatively correlated with rocks in the orado are gneisses in a complex that constitutes part of the floor Wichita Mountains province of southern Oklahoma that are thought on which younger Precambrian sedimentary units rest and the matrix to be younger than 1,400 m.y. and older than 1,200 m.y. into which igneous rocks were intruded. The gneisses were derived The Uinta Mountain Group constitutes the core of the eastern from a heterogeneous sedimentary and volcanic protolith. They are Uinta Mountains in northwestern Colorado and is present in the sub­ metamorphosed in the upper or the lower amphibolite facies in most surface eastward at least to Craig. The group, which is more than places in Colorado, but are of lower metamorphic rank in a few areas. 7 km (3.7 mi) thick, consists of quartzites accompanied by thick They are divided broadly into biotitic varieties, largely metasedimen- wedges of conglomerate and subordinate shale and siltstone. It lies tary, and felsic and hornblendic varieties, largely metavolcanic. These in part on Archean rocks at the southern border of the Wyoming rock units occur both in large discrete bodies and intimately inter- cratonic age province and evidently was deposited in a fault-bounded layered with one another. The principal species of biotitic gneiss is trough in that structural zone. The group ranges in age from about biotite-quartz-plagioclase gneiss, some of which contains sillimanite, 925 to 1,100 m.y. and is equivalent in part to the compositionally garnet, or cordierite. Metaquartzite and marble or calc-silicate rocks similar Belt Supergroup of Montana and Idaho and Grand Canyon occur locally in the biotitic gneisses. In the subsurface in eastern Supergroup of Arizona. Colorado, a metaquartzite-mica schist facies is predominant. The fel­ Igneous intrusion began during