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Home , Elk Mountains (Colorado), Sawatch Range

JOHN D. OBRADOVICH } FELIX E. MUTSCHLER j- U.S. Geological Survey, , BRUCE BRYANT J

Potassium-Argon Ages Bearing on the Igneous and

Tectonic History of the and

Vicinity, Colorado: A Preliminary Report

Abstract: K-Ar ages for epizonal plutonic rocks together with field studies indicate that uplift of the began at least 72 m.y. ago. Vertical uplift of the Sawatch Range was fol- lowed or accompanied by gravity sliding of sedimentary rocks along the Elk Range thrust fault. The greatest volume of exposed intrusive rocks in the Elk Mountains, Ruby Range, and consists of granodioritic rocks of Oligocene age which are younger than the Elk Range thrust. Miocene and Pliocene(?) mafic dikes were emplaced after most of the structural features in the area had developed. The Treasure Mountain dome near Marble was formed by emplacement of a unique soda granite pluton about 12.5 m.y. ago. Discordant K-Ar and Rb-Sr ages for biotite from the Twin Lakes stock in the Sawatch Range suggest that this large pluton is of Late Cretaceous or early Tertiary age and lost argon during Oligocene plutonism, or is of Eocene age and represents a separate event not yet supported by other radiometric dates from the area.

CONTENTS Acknowledgments 1749 Figure Elk Mountains and vicinity 1749 1. Index map of Colorado 1751 Late Cretaceous and Paleocene(P) 1751 Plate Paleocene and early Eocene 1751 1. Sketch map showing igneous rocks and } Post-middle Eocene and pre-Oligocene . . . 1753 major structural features, Elk Moun- [• Oligocene 1753 tains and vicinity, Colorado . . . . J Miocene and Pliocene 1754 Table Central Sawatch Range 1754 1. K-Ar analytical data for micas, Elk Moun- References cited 1755 tains and vicinity, Colorado 1752

ACKNOWLEDGMENTS deformed by uplift, folding, and high-angle and thrust faulting. A variety of post-Paleozoic We thank Carl Hedge and Ogden Tweto for igneous rocks are also present, some of which their perceptive critical reviews of the manu- intrude others, thus establishing mutual age script and James Vigil for assistance in prepara- relations. The emplacement of some intrusive tion of some of the mica separates. bodies was controlled by faults and folds, and ELK MOUNTAINS AND VICINITY "1^65 ""' ™ ** ^ The Elk Mountains expose Precambrian Tertiary sedimentary rocks, including the crystalline rocks and Paleozoic and Mesozoic Ohio Creek Formation of Paleocene age and sedimentary rocks which have been complexly the Wasatch Formation of Paleocene and

Geological Society of America Bulletin, v. 80, p. 1749-1756, 1 fig., 1 pi., September 1969 1749

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Eocene age, are exposed to the west of the Elk the town of Marble (Vanderwilt, 1937) where Mountains in the Piceance basin, Ruby Range, it cuts dikes of groups 2 and 3. Intrusion of the and West Elk Mountains, where they are cut granite was responsible for the uplift of the by plutons correlative with some of the Elk dome, which has 5500 feet of structural Mountains intrusives. These Tertiary sedi- closure on the base of the Cretaceous Dakota mentary rocks are of interest because they offer Sandstone. significant clues to the igneous history of the In order to fix the ages of the various region. Both the Ohio Creek and Wasatch igneous events we have analyzed micas from Formations contain scattered pebbles of fine- igneous rocks of the Elk Mountains, Ruby grained intermediate igneous rock of volcanic Range, and Sawatch Range. The area we have aspect, and the Wasatch Formation includes studied bridges the geographic gap between the tuffaceous beds. The source for these igneous San Juan volcanic field (Fig. 1) where igneous materials is presumed to have lain to the east or activity extended from Late Cretaceous to northeast of the present Ohio Creek and Pliocene, but climaxed in Oligocene time Wasatch outcrops, inasmuch as clastic grain (Steven and others, 1967)—and the Leadville size increases eastward and east- and northeast- area and Colorado —where Late trending channels have been noted in the Cretaceous to Paleocene or Eocene, Oligocene, Wasatch. and Miocene igneous activity occurred (Hart, The post-Paleozoic intrusive plutons of the 1961, 1964; Pearson and others, 1962; Taylor Elk Mountains and vicinity can be divided and others, 1968). Our data, therefore, con- into four groups on the basis of field relations tribute to an understanding of the Cretaceous and petrographic similarities: and Tertiary igneous events along the Colorado (1) Felsic porphyries of the Aspen mining Mineral Belt (Tweto and Sims, 1963). district including the albite aplite porphyry, Analytical data and calculated ages are albite alaskite porphyry, and aplite of Knopf listed in Table 1, and sample locations are (1926) occur as sills and fault-controlled in- shown in Plate 1. Geologic ages are based on trusives on the west margin of the Sawatch the Geological Society of London Phanerozoic Range. Group 1 plutons are not exposed in time scale of 1964 (Harland and others, 1964), contact with intrusives of our other three except that the value of 12 m.y. (Holmes, groups, but Knopf (1926) presumed the felsic 1965) is retained arbitrarily for the Miocene- porphyries to be older than our group 2 Pliocene boundary in the western United granodiorites on structural grounds. Diorite States in preference to the value of about 7 porphyry, considered to be older than the m.y.1 given by Harland and others (1964). felsic porphyries by Knopf (1926), also occurs Obradovich is responsible for the K-Ar age in the Aspen district. Group 1 plutons were determinations. Mutschler and Bryant are emplaced before, during, and after high-angle responsible for the geologic interpretations. faulting along the west edge of the Sawatch The radiometric ages indicate that the Range. igneous rocks of groups 1, 2, and 4 are, re- (2) Stocks, laccoliths, sills, and dikes of spectively, (1) Late Cretaceous, (2) middle granodiorite and granodiorite porphyry in- Tertiary (Oligocene), and (4) late Tertiary trude rocks as young as the Wasatch Formation (late Miocene and early Pliocene). The K-Ar in the Ruby Range and West Elk Mountains data fit into and serve to place temporal limits and cut low-angle thrust faults in the Elk on the following igneous and structural history Mountains. High-angle faulting, younger than of the Elk Mountains region developed by the thrust faulting, occurred both during and Gaskill and Godwin (1966a, b), Gaskill and after emplacement of the group 2 plutons. others (1967), Godwin and Gaskill (1964), (3) Small gabbro porphyry and lamprophyre Mutschler, and Bryant. dikes and sills cut group 2 plutons in the west- central Elk Mountains and the northern Ruby 1 There are sufficient data available to argue that the Range. These mafic rocks appear to be younger Miocene-Pliocene boundary for the Pacific coast drawn than most of the major tectonic features in the on the basis of microfaunal and megafaunal stages and area and may be correlative with basalt flows mammalian provincial ages is older than the Miocene- in the surrounding region. Pliocene boundary as given by Harland and others (4) A unique soda granite is exposed in the (1964). See Durham (1954), Durham and others (1954), center of the Treasure Mountain dome near and Evernden and others (1961, 1964).

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o ''..'•' - FLAT z M,, ; - --. TOPS i"<, -, '- -:" y.;,'"'"K DENVEE oDotser o -^ O ^>7\= &, 'i. \! <^ ^ V- * = Xf. 'V\5 r0Leadville O Area of Plate I x^

0 25 50 75 100 MILES ' 1 [ 1 I __J I I Figure 1. Index map of Colorado.

Late Cretaceous and Paleocene (?) formed by westward gravity gliding of a sheet Uplift of the Sawatch Range during Late of upper Paleozoic and Mesozoic sedimentary Cretaceous and Paleocene (?) time was ac- rocks from the Sawatch Range (Bryant, 1966). companied by high-angle faulting along the The exact time of thrust faulting cannot be west margin of the Sawatch Range, intrusion fixed with confidence because the youngest of group 1 plutons, and probably by hydro- rocks involved are of Late Cretaceous age thermal silver-lead-zinc mineralization in the and the oldest post-thrusting intrusives are Aspen area. Micas from porphyries emplaced of late early Oligocene age. Thus the thrust before the end of this period of faulting have faulting could be as young as Eocene. ages of 67 to 72 m.y. (samples 1 and 2, Table 1). Volcanism may have occurred in the Similar ages have been obtained from por- Sawatch Range during Late Cretaceous to phyries of the Leadville district on the east Paleocene time since the Ohio Creek Forma- flank of the Sawatch Range (Pearson and tion contains pebbles of fine-grained igneous others, 1962). The uplift must be younger rock. than 73 to 75 m.y. because marine Cretaceous rocks in the Aspen area extend into the Paleocene and early Eocene Exiteloceras jenneyi Zone (fossils identified by By Paleocene and early Eocene time an W. A. Cobban). K-Ar ages of sanidine and extensive volcanic terrane existed in the source biotite from a bentonite in this zone on the area from which the Wasatch Formation was north flank of the have derived. Coarse conglomerates composed of been dated at 75.2 + 2.3 m.y. and 72.7 + 2.2 Precambrian basement, Paleozoic sedimentary, m.y., respectively (Dickinson and others, and volcanic rock fragments are most abundant 1968). in the lower part of the Wasatch Formation Probably at about the time of the high-angle (Gaskill and Godwin, 1963; Gaskill and others, faulting at Aspen, the Elk Range thrust fault 1967) suggesting that if the Sawatch Range

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TABLE 1. K-AR ANALYTICAL DATA FOR MICAS, ELK MOUNTAINS AND VICINITY, COLORADO Analysts: J. D. Obradovich, H. H. Mehnert, and Violet Merritt

Radiogenic Radio- Argon genic Calculated Sample Field K20* jO-io Argon Age No. No. Rock Type Group Mineral (percent) moles/gm (percent) (m.y.) 1 HNMP Quartz-muscovite Muscovite *9.37 10.17 89.0 72.2 ±2.2 porphyry i 2 1045 Aplite Biotite (a)t *7.71 8.11 87.1 70.0 ±2.3 (b) *8.04 8.14 87.0 67.4 ±2.2 3 TL Quartz monzonite ? Biotite 8.86 5.51 90.4 41.7 ±1.2 porphyry 4 386 Granodiorite Biotite 8.36 4.25 79.3 34.1 ± 1.4 5 WR-2 Granodiorite Biotite 7.98 4.03 89.1 33.9 ± 1.0 6 NY-5 Granodiorite porphyry 2 Biotite 7.29 3.68 83.2 33.9 ± 1.1 7 389 Granodiorite porphyry Biotite 8.48 3.93 84.5 31.2 ± 1.1 8 472 Granodiorite porphyry Biotite 7.14 3.09 77.1 29.1 ±1.0 9 390 Granodiorite J Biotite 7.85 3.39 84.0 29.0 ± 1.1 10 348 Soda granite porphyry \ . Biotite 8.74 1.62 60.6 12.5 ±0.6 11 391 Soda granite porphyry J Biotite 8.38 1.53 60.3 12.3 ±0.6

Decay constants K40: * All potassium determinations are in duplicate 10 1 Xe = 0.584 X 10~ yr- except those marked with an asterisk (*), which X0 = 4.72 X 10-10 yr-1 are single analyses. Atomic abundance K40/K 1.19 X 10~4 t Separate biotite concentrates from same speci- men.

Sample descriptions and locations: 1. Little Annie sill(?), Highland tunnel, North Midnight drift at about 8700 ft above sea level, 106°49'30", 39°07'57", Aspen quadrangle, Pitkin . White quartz-muscovite porphyry containing conspicuous pheno- crysts of muscovite to 3 mm in diameter and of plagioclase altered to albite and partly resorbed quartz to 1.5 mm in diameter in a groundmass of quartz and feldspar from 0.2 to 0.5 mm in grain size containing secondary sericite and calcite. Pyrite is disseminated. 2. Unnamed pluton, spot elevation 11,212 ft on east side of Richmond Hill, 106°48'02", 39°06'20", Hayden Peak quadrangle, Pitkin County. Gray fine-grained rock containing sparse phenocrysts of biotite and andesine to 1 mm in diameter in a groundmass of quartz, albite, potassic feldspar, and minor sericite with a grain size of 0.04 to 0.2 mm. 3. Twin Lakes stock, roadcut on Colorado Highway 82, at a point 600 ft west of Echo Canyon road, 106°28'25", 39°04'12", Mt. Elbert SW quadrangle, Lake County. Porphyritic quartz monzonite containing Carlsbad- twinned phenocrysts of potassic feldspar as much as 3 cm long, oligoclase 5 mm long, quartz 1 cm in diameter, and biotite to 2 mm in diameter in a groundmass from 0.3 to 0.6 mm in grain size composed of quartz, plagioclase, potassic feldspar, and minor hornblende. 4. Snowmass pluton, northeast end of Snowmass Lake, 107°02'26", 39°07'03", quadrangle, Pitkin County. Light-gray medium-grained rock with a hypidiomorphic granular texture and containing oligoclase-andesine, quartz, potassic feldspar, biotite, and minor hornblende. 5. Whiterock pluton from 11,280 ft altitude on Montezuma mine road, 106°50'16", 39°0'35", Hayden Peak quadrangle, Pitkin County. Light-gray rock with hypidiomorphic granular texture containing biotite, andesine, and quartz from 1 to 2 mm in grain size and interstitial potassic feldspar. Some pyrite on fractures and dis- seminated. 6. Lincoln Creek stock 200 ft from contact of Grizzly Mountain Rhyolite of Howell (1919) on New York Collection Canal, 106°38'29", 39°05'20", New York Peak quadrangle, Pitkin County. Greenish-gray rock containing phenocrysts of partly resorbed quartz to 5 mm in diameter, biotite to 3.5 mm, and andesine (mostly altered) to 5 mm in a fine groundmass of quartz, plagioclase, potassic feldspar, and secondary calcite, and chlorite from 0.04 to 0.1 mm in grain size. 7. Snowmass Creek sill, 300 ft north of Forest Service wilderness register, Snowmass Creek trail, about 107°01'08", 39°11'07", quadrangle, Pitkin County. Brownish-gray rock with a seriate porphyritic texture containing Carlsbad-twinned phenocrysts of potassic feldspar as much as 3 cm long and of quartz, plagioclase, potassic feldspar, and biotite as much as 1 cm in diameter in a very fine-grained groundmass.

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TABLE 1. (Continued)

8. laccolith, 100 ft south of top of gondola lift, Crested Butte ski area, 106°56'36", 38°53'20", Gothic quadrangle, Gunnison County. Light-olive-gray rock with seriate porphyritic texture containing potassic feld- spar phenocrysts as much as 8 cm long and of plagioclase, potassic feldspar, quartz, biotite, and hornblende from 0.5 to 7 mm in diameter in a very fine-grained groundmass, which is locally altered to epidote, sericite, and chlorite. 9. Paradise stock, southwest shore of small lake 550 ft west of Paradise mine, 107°03'28", 38°59'30", Oh-Be-Joyful quadrangle, Gunnison County. Light-gray medium-grained rock with hypidiomorphic granular texture con- taining oligoclase-andesine, potassic feldspar, quartz, biotite, and hornblende. 10. Crystal pluton, granite of Treasure Mountain, , 107°07'08", 39°03'16", Snowmass Mountain quadrangle, Gunnison County. Light-gray to pinkish-gray rock with a seriate porphyritic texture containing Carlsbad-twinned phenocrysts of perthitic potassic feldspar as much as 1.5 cm long and of albite, quartz, potassic feldspar, and biotite from 1 to 3 mm in diameter in a very fine-grained allotriomorphic granular groundmass of quartz, albite, and potassic feldspar. 11. Crystal pluton, granite of Treasure Mountain, south side of Crystal River, 107°07'17", 38°03'06", Snowmass Mountain quadrangle, Gunnison County. Rock similar to sample 10.

was the source area it reached its maximum Whiterock plutons. After these large gran- topographic relief during Paleocene and early odiorite plutons had essentially crystallized, Eocene time. The tuffaceous beds of the lower granodiorite porphyry dikes, sills, and lac- part of the Wasatch suggest that volcanic coliths were emplaced in the Elk Mountains activity occurred in the source area at this (sample 7, Table 1), the Ruby Range (sample time. 8, Table 1), and the West Elk Mountains. Post-middle Eocene and pre-Oligocene Small granodiorite stocks (sample 9, Table 1) cut the porphyry dikes and sills of the Ruby Along their northwestern edge the Elk Range and western Elk Mountains. Vein, Mountains are marked by high-angle faults limestone repkcement, and disseminated sul- and folds and by a west-facing monoclinal fide deposits containing lead, zinc, silver, iron, flexure that merges northwestward with the copper, nickel, gold, and molybdenum were monocline. North of the formed during this middle Tertiary plutonic , the Eocene is deformed along the Grand Hog- episode. At about this time, volcanism in the West back indicating that there the structural relief Elk Mountains produced an extensive pile of along the monocline is, at least in part, post- andesite and dacite agglomerates, tuffs, and middle Eocene. breccias, constituting the West Elk Breccia. In, and to the west of, the Elk Mountains, The stratigraphy of the extrusives and their structures believed to be related to this period relation to group 2 intrusives is not entirely of deformation are cut by undeformed group clear. We have not been able to find exposed 2 intrusives of Oligocene age. contacts between the breccia and the grano- Oligocene diorite porphyry laccoliths of the West Elk Oligocene granodioritic rocks of group 2 Mountains. A series of dacite porphyry dikes make up the greatest volume of exposed in- cut granodiorite porphyry plutons in the trusive rocks in the area. Emplacement of southern Ruby Range and also cut West Elk group 2 plutons was accompanied and fol- Breccia in the West Elk Mountains according lowed by renewed high-angle faulting (Gaskill to D. L. Gaskill (oral commun., 1967). We and others, 1967; Bryant, 1969, in press; have not been able to obtain unaltered biotite Mutschler, in press). suitable for K-Ar dating from the dacite Among group 2 plutons a consistent set of porphyry dikes, but on the basis of their crosscutting relationships occurs. First, large chemical and petrographic character and struc- bodies of granodiorite including the Mount tural setting, we tentatively assign them to Sopris, Snowmass (sample 4, Table 1), and group 2. A minimum age for the West Elk Whiterock (sample 5, Table 1) plutons were Breccia is given by a K-Ar date of 26.8 + 2.7 empkced in the Elk Mountains. The Elk m.y. for a welded tuff unconformably over- Range thrust fault formed one of the structural lying breccia (Steven and others, 1967, sample controls for emplacement of the Snowmass and 4, Table 2).

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Miocene and Pliocene Sawatch Range, we have analyzed biotites In the Treasure Mountain dome and from two plutons in the central Sawatch Range. adjacent parts of the Elk Mountains and Ruby Near Twin Lakes, on the east side of the range, Range small gabbro porphyry and lamprophyre a very coarse grained quartz monzonite dikes cut group 2 Oligocene granodiorite porphyry (Twin Lakes Quartz Monzonite porphyries, and some are in turn cut by the of Howell, 1919) intrudes Precambrian rocks. granite of Treasure Mountain. These mafic Near Independence Pass, on the crest of the dikes of group 3 are provisionally considered range, are felsic volcanic rocks that were termed to be Miocene to early Pliocene since they are Grizzly Peak Rhyolite by Howell (1919) and petrographically and chemically similar to Grizzly Mountain Rhyolite by Stark and radiometrically dated Miocene and Pliocene Barnes (1935). The felsic volcanics are younger basalts of the Primitive Area (E. E. than the Twin Lakes Porphyry (Howell, 1919). Larson, Univ. of Colorado, oral commun., The volcanics are, in turn, intruded by a 1968). Mafic igneous activity seems to have porphyry stock in the Lincoln Creek valley occurred intermittently over a time span ex- (Vanderwilt and Koschmann, 1932; Burbank tending from Miocene to the present. Basalt and Goddard, 1935; Stark and Barnes, 1935). flows occupy a variety of physiographic posi- We were unable to obtain a biotite separate tions in the region including various low-level from the sample of Grizzly Mountain Rhyolite surfaces in the valleys of the Colorado River we collected, but biotite from the porphyry and Roaring Fork. The youngest known flows, stock at Lincoln Creek gave a K-Ar age ~ 34 in the bottom of the Colorado River valley m. y. (sample 6, Table 1) which sets an upper near Dotsero, have been dated at about 4000 limit on the age of the volcanic rocks. Although years B.P. (Giegengack, 1962). previous workers have referred to the Lincoln Group 4 is represented by the soda granite Creek stock as a "rhyolite" or "granite" of Treasure Mountain (samples 10 and 11, porphyry, it is petrographically a granodiorite Table 1) which intruded Precambrian and at its chilled margin against the volcanic rocks; Paleozoic rocks southwest of Marble, Colorado, consequently, the age and composition of the in late Miocene or early Pliocene time, pro- stock suggest its correlation with the group 2 ducing a dome with a diameter of about 6 Oligocene granodiorites of the Elk Mountains. miles and causing extensive contact meta- Biotite from the Twin Lakes Porphyry gave morphism including formation of the famous a K-Ar age ~ 42 m. y. (sample 3, Table 1). from the Mississippian Leadville This result raises the possibility that the Twin Limestone. Small vein and manto deposits Lakes Porphyry represents an igneous event containing copper, lead, zinc, silver, and intermediate in age between the Late Creta- molybdenum were formed in the dome follow- ceous (group 1) and the Oligocene (group 2) ing emplacement of the granite. plutonic events in the Elk Mountains. It is also Highly altered rhyolite porphyries occur as possible that the Twin Lakes Porphyry is of dikes, sills, and breccia pipes in the western Late Cretaceous or early Tertiary age and that Elk Mountains, the Ruby Range, and Treasure the K-Ar age reflects argon loss due to thermal Mountain dome. In the Ruby Range these effects of the Oligocene plutonism. Some felsites cut the Oligocene granodiorite por- support for the latter interpretation is given phyries, but in the Elk Mountains felsites and by a Rb-Sr age on biotite from sample 3, granodiorite seem to have mutually cross- Table 1, of 49.7 + 4.5 m. y. (error at the 95-percent level) (Carl Hedge, oral commun., cutting relations. In the Treasure Mountain 11 1 area some rhyolite porphyry dikes are clearly 1968; \? = 1.39 X lO" yr^ ). A Rb-Sr age related to the granite of Treasure Mountain. on biotite from the Twin Lakes Porphyry of Age relations of the felsites are thus uncertain, 56 + 10 m. y. reported by Moorbath and and probably rocks of this type, of both others (1967) is in approximate agreement Oligocene and late Miocene or early Pliocene with this. However, until additional work, age, occur in the area. including dating of the batholith, is completed, we shall reserve CENTRAL SAWATCH RANGE judgment on the age of the Twin Lakes In an attempt to extend the igneous chronol- Porphyry. ogy developed for the Elk Mountains to the

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MANUSCRIPT RECEIVED BY THE SOCIETY FEBRUARY 11, 1969 PUBLICATION AUTHORIZED BY THE DIRECTOR, U.S. GEOLOGICAL SURVEY

* Publications used to compile map on Plate 1. All theses are available on microfilm or by interlibrary loan.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/80/9/1749/3417567/i0016-7606-80-9-1749.pdf by guest on 02 October 2021 (10) EXPLANATION Locality 1. Elk Range thrust fault 2. Grizzly Mountain Rhyolite of Stark and Barnes (1935) 3. Lincoln Creek stock Granite of Treasure 4. Mount Princeton batholith Mountain 5. Sopris pluton 6. Ruby Range 7. Snowmass pluton 8. Treasure Mountain dome 9. Twin Lakes stock Gabbro porphyry, 10. Whiterock pluton lamprophyre, and .o basalt dikes

10 MILES .2 J

r r

Volcanic rocks undivided West Elk Breccia Probably mostly of Age relative to gran- OUgoccne age •§= odiorite stocks of O Elk Mountains uncertain

Intrusive rocks undivided D O UJ O < u cc O

Precambrian crystalline rocks -y- Contact Anticline Syncline Overturned Monocline syncline

Fault Thrust fault Location of dated sample Number refers to sample number in table 1

SKETCH MAP SHOWING IGNEOUS ROCKS AND MAJOR STRUCTURAL FEATURES, ELK MOUNTAINS AND VICINITY, COLORADO Compiled from sources followed by an asterisk in References Cited and also from unpublished mapping bv F. E. Mutschler and Bruce Bryant. OBRADOV1CH AND OTHERS, PLATE 1 Geological Society of America Bulletin, v. 80, no. 9 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/80/9/1749/3417567/i0016-7606-80-9-1749.pdf by guest on 02 October 2021