U. S. DEPARTMENT OF THE INTERIOR U.S. GEOLOGICAL SURVEY

GEOLOGIC MAP OF THE COLLEGIATE PEAKS WILDERNESS AREA AND THE GRIZZLY PEAK CALDERA, SAWATCH RANGE, CENTRAL COLORADO

By

C.J. Fridrich, Ed DeWitt, Bruce Bryant, Steve Richard, and R.P. Smith

Pamphlet to accompany MISCELLANEOUS INVESTIGATIONS SERIES MAPI-2565 1998 CONTENTS Explanatory notes 1 Description of Map Units 1 Acknowledgments 26 References Cited 26

FIGURES

3. Summary geochemical diagrams for rocks of the Grizzly Peak caldera. 2 4. Summary geochemical diagrams for Cretaceous and Tertiary intrusive rocks in the Aspen area. 6 5. Summary geochemical diagrams for the Italian Mountain intrusive complex. 7 6. Summary geochemical diagrams for altered intrusive rocks at Middle Mountain and Winfield Peak. 8 7. Summary geochemical diagrams for rocks of the Mount Champion area. 9 8. Summary geochemical diagrams for intrusive rocks of the Mount Princeton Pluton. 10 9. Summary geochemical diagrams for intrusive rocks of the Twin Lakes Pluton. 11 10. 40Ar- 39 Ar release spectra for hornblende from sample R8-28-84-4 of mafic border of Twin Lakes pluton. Width of bars on spectra is 2-sigma uncertainty. 13 11. Summary geochemical diagrams for Cretaceous plutons east of the Twin Lakes pluton. 15 12. Summary geochemical diagrams for 1.4-Ga plutons and plutons assumed to be 1.4 Gain the central Sawatch Range area. 17 13. Concordia diagram for zircon from sample IC-6 of the St. Kevin Granite. Size of symbol represents 2-sigma analytical uncertainty. 18 14. Summary geochemical diagrams for some 1.65-Ga and older plutons in the central Sawatch Range area. Plot includes the Kroenke Granodiorite, granite of Henry Mountain, and granite of Mount Yale. 21 15. Summary geochemical diagrams for some 1.70-Ga and older plutons in the central Sawatch Range area. Plot includes the Denny Creek Granodiorite and unnamed gabbro. 23 EXPLANATORY NOTES Tp Post-resurgent plutons (Oiigocene)-Numer­ ous dikes and small stocks ranging in compo­ sition from high-silica rhyolite to lati-andesite In order to facilitate use of this large map, two smaller (monzodiorite) concentrated within the Griz­ index maps have been prepared, a location map (fig. 1) that zly Peak caldera. Rhyolite porphyry dikes are has place names used in the description of map units, and a typically light greenish or bluish gray and simplified bedrock geologic map (fig. 2). The simplified strongly flow foliated. Rhyodacite dikes are geologic map consolidates the units within the description much less numerous that rhyolite dikes and of map units and deletes all surficial, glacial, and Miocene are medium gray or grayish green. Dacite and younger stratified deposits. The compositional names and lati-andesite dikes are dark olive green or of all igneous rock units are based on the R R nomencla­ 1 2 greenish black and commonly have boul­ ture of De La Roche and others (1980). For rocks in the der-size (to >5 m) inclusions of pink, Grizzly Peak caldera, additional modifiers used are: low-sil­ medium-grained, nonfoliated granite. Also ica for rhyolites with < 73% Si0 and high-silica for rhyo­ 2 includes the composite Sawmill stock (Cru­ lites with > 76.5% Si0 . Supporting geochemical data for 2 son, 1973), a 1.6-km-diameter, dark-green­ igneous units are shown in figures 3-15. Summary geochro­ ish-gray to greenish-gray, porphyritic to nologic data for samples from the map area are in table 1. equigranular tonalite near the ghost town of Ruby, in the center of the caldera, and a DESCRIPTION OF MAP UNITS smaller stock southeast of Ruby, referred to as the Red Mountain stock (Holtzclaw, 1973). Many of the post-resurgent dikes are radial to Qs Surficial deposits (Holocene)-Alluvium, the Sawmill stock. Rhyolite porphyries con­ alluvial-fan deposits, talus, landslide deposits, tain about 5% phenocrysts of quartz, two colluvium, solifluction deposits, swamp feldspars, and biotite; rhyodacite and dacite deposits, rock glaciers, and rock avalanches porphyries contain 10-40% phenocrysts of younger than Pinedale glacial event. Locally plagioclase and biotite, ±hornblende; contains small areas of ground moraine and lati-andesite porphyries contain about 10% lateral moraine. Surficial versus glacial phenocrysts of plagioclase and lesser amounts deposits not differentiated in Italian Mountain of biotite. The Sawmill stock consists of a area; all Quaternary deposits there designated small body of phenocryst-poor rhyolite por­ as surficial. Small areas of bedrock that have phyry emplaced into a larger intrusive body moved during landsliding or glacial deposits that ranges from margins of phenocryst-rich that have slumped are not shown as surficial porphyritic-aphanitic biotite-hornblende gra­ deposits; instead, fault symbols are shown at nodiorite porphyry to a core of nearly the head of such deposits equigranular, fine- to medium-grained biotite Qg Glacial deposits (Pleistocene)-Glacial till, tonalite. including ground, lateral, and terminal Compositionally, the lati-andesite dikes are morainal deposits, and sand and gravel in alkali-calcic, (fig. 3A), metaluminous (fig. deposits of Pinedale and older glacial events. 3B), have average alkali enrichment (fig. 3C), Locally contains small areas of colluvium and and average iron enrichment (fig. 3D). The till that has slumped most felsic dikes are also alkali-calcic (fig. Tdu Dry Union Formation (Pliocene and Miocene)- 3A), are mildly peraluminous to strongly per­ Light-brown, poorly consolidated sandy silt­ aluminous (fig. 3B), have an average to sodic stone and interbedded friable sandstone, con­ alkali enrichment (fig. 3C), and are Mg-rich glomerate, and volcanic ash. In map area, only (fig. 3D). Some late dikes are marginally present in the upper Arkansas River graben alkalic, because they have compositions of from east of Mount Elbert to south of Granite quartz latite (fig. 3A) and are very Mg rich Rock units of the Grizzly Peak caldera-Plu­ (fig. 3D). tons of the Grizzly Peak caldera consist of Oligocene age assignment for the Sawmill three age groups. Late-resurgent and post­ stock based on K-Ar biotite date of 32.9 ±1.1 resurgent plutons together form an east-west Ma (Fridrich and others, 1991) belt of dikes and stocks across the central Resurgent plutons part of the caldera. The oldest rocks are Tl Late-resurgent plutons (Oligocene)-Four resurgent plutons of the composite laccolith stocks, each less than 1 km in diameter, and sev­ in the north-central part of the caldera eral related dikes. Stocks are on Pine Creek, at

1 1.3 Strongly Peraluminous + Grizzly Peak caldera 1.2 Mildly Peraluminous t:, Lincoln Gulch stock (units Trl, Tr2, and Tr3) + late dikes (unit Tp) 1.1 \\ • fiamme (parts of units Tgl, Tgm, Tghl, Tgu, and Tgh2) z~ ~ '0.9 + + 0.8 +

0.7 so 60 (wt. Si02 percent)

ISOO

Kpi(Kp +Nap) i 1000 ' +• D

IN + ~ I (FeO + 0.89Fe:P3 )I(FeO + 0.89Fe:P3+ MgO) u r:t."' AlkGr A

~------L------~------L-----~ 1000 JSOO 2000 2500 3000

R1= 4000Si -ll,OOO(Na + K)- 2000(Fe + Tl)

Figure 3. Summary geochemical diagrams for rocks of the Grizzly Peak caldera. Data from C.J. Fridrich (1986; unpub. data, 1994). (A) R 1R2 major element classification diagram (De la Roche and others, 1980). Alk Rhy, alkali rhyolite; Rhy, rhyolite; R Dac, rhyodacite; Dac, dacite; And, Andesite; And-Bas, andesitic basalt; Lat And, lati-andesite; Q Lat, quartz latite; Q Tr, quartz Trachyte; Tr, trachyte; Lat, latite; T Bas, trachybasalt; T And, trachyandesite; Lat Bas, latibasalt. Volcanic rock names used because of fine-grained nature of most rocks. See figure 4 for the equivalent plutonic rock names. Fields of alkalinity modified slightly from those in DeWitt (1989) based on Ed DeWitt (unpub. data, 1994).

(B) Alumina saturation diagram (Si02 versus NCNK). A, molar Al20 3; C, molar CaO; N, molar Na20; K, molar K20. (C) Alkali classification diagram (K20/(K20+Na20) versus Si02). Field boundaries from Ed DeWitt (unpub data, 1994). (D) Iron enrichment classification diagram ((Fe0+0.89*Fe20 3)/(Fe0+0.89*Fe20 3+Mg0) versus Si02). Field boundaries slightly modified from those in DeWitt (1989) based on Ed DeWitt (unpub. data, 1994).

Red Mountain, west of Bowman Creek, and ±1 A Ma (original Kennecott data recalculated eastofTelluriumLake (Perkins, 1973). Stocks by Fridrich and others, 1991, using decay con­ are typically porphyritic-aphanitic and stants of Steiger and Jager, 1977). Although strongly altered. Stocks are spatially coinci­ these dates are older than the dates on the oldest dent with zones of strong hydrothermal alter­ resurgent plutons, we consider these plutons to ation and weak mineralization resembling that be younger. The original geochronologic data of porphyry molybdenum deposits. for the age calculations are lacking and we sus­ Composition inferred from relict pheno­ pect that the material dated may not have been crysts and phenocryst pseudomorphs to be free of impurities, especially clay minerals that generally granodioritic in composition. No could contain excess argon. The stocks are pre­ chemical analyses available for the stocks. sumed to have been emplaced late in the resur­ Oligocene age assignment based on K-Ar gence of the caldera because they coincide dates from one stock. The stock on Pine Creek with ring fracture zones that were reactivated has K-Arbiotitedates of35.7±1.2Maand 36.8 during caldera resurgence

2 Oldest resurgent plutons-The Lincoln abundant plagioclase, biotite, and quartz, Gulch stock, named by Candee ( 1971) and lesser magnetite and alkali feldspar, and described by Fridrich and Mahood (1984), is minor hornblende. Also includes a ring dike a composite laccolith consisting of three that is well exposed at East Red and along intrusive bodies, as described below. All southeastern margin of the caldera. three bodies are porphyritic-aphanitic All three plutons range in composition throughout (both phenocrysts and ground­ from granodiorite to tonalite, are alkali-calcic masses become coarser toward the cores) and (fig. 3A), mildly peraluminous (fig. 3B), have contain locally abundant inclusions (to 1 m no preferential alkali enrichment (fig. 3C), diameter) of Early Proterozoic metasedimen­ and are average to slightly Mg rich (fig. 3D). tary gneiss (unit Xgn) and lati-andesite por­ The slope of data points on the alumina satu­ phyry. Includes several related dikes found ration diagram is surprisingly low. Most both within and outside of the composite lac­ igneous rocks having less than 60% Si02 are colith normally metaluminous. The plutons bear a Tr3 Youngest pluton (Oligocene)-Concentri­ strong chemical similarity to the Italian cally zoned from leucocratic granodiorite mar­ Mountain intrusive complex (unit Tim) gins to a more mafic granodiorite core. Largest Grizzly Peak Thff body is exposed on high ridge, north of Grizzly A phenocryst-rich (20-40%) lithic lapilli Peak. Margins are medium pinkish tan to tan ash-flow tuff (Fridrich and others, 1991) that gray; core is medium greenish gray. Margin varies in composition from high- to low-silica

contains about 45% phenocrysts of plagio­ rhyolite (about 77-71% Si02). Two horizons clase, quartz, alkali feldspar, biotite, and less are noted that contain as much as 20% sub­

abundant magnetite. Core contains about 60% rhyolitic (66-57% Si02) fiamme (collapsed phenocrysts of abundant plagioclase and pumice lumps) in addition to the rhyolitic biotite, lesser amounts of quartz and alkali fiamme and ash found in the rest of the tuff. feldspar, and sparse magnetite Predominant lithic fragments are Early Prot­ Tr2 Intermediate-age pluton (Oligocene)­ erozoic metasedimentary gneiss (unit Xgn), Concentrically zoned from leucocratic grano­ St. Kevin Granite (unit Ys), Denny Creek diorite margins to a tonalite core. Exposed Granodiorite (unit Xd), and a high-silica rhy­ from mouth of Tabor Creek on the northwest, olite porphyry resembling the precaldera rhy­ nearly to Grizzly Peak on the southeast. Mar­ olite lava (unit Tf). Grizzly Peak Tuff is gins are dark grayish green to medium green­ subdivided everywhere into four composi­ ish gray; core is medium salt-and-pepper tional subunits; no unit-wide cooling breaks gray. Margins have about 35% phenocrysts are present between subunits. The distinction of abundant plagioclase, quartz, and biotite, between subunits is arbitrary and is based on and lesser alkali feldspar and magnetite. Core composition and on the positions of two very contains about 55% phenocrysts of abundant thick megabreccias in the caldera filL Local plagioclase and biotite, and lesser quartz, cooling breaks formed as chilled margins hornblende, magnetite, and alkali feldspar. around breccias intercalated in the tuff. Oligocene age assignment for all three Intracaldera tuff has biotite and hornblende bodies is based on K-Ar biotite date of 34.8 K-Ar dates of 33.4 ±1.0 Ma and 33.8 ±1.1 ±1.1 Ma (recalculated from data in Obradov- Ma, respectively (Fridrich and others, 1991) ich and others, 1969, by Fridrich and others, Tgh2 Upper heterogeneous subunit (Oiigocene)­ 1991) for unit Tr2 Heterogeneous tuff layer containing larger Tr1 Oldest pluton (Oligocene)-Concentri­ and more mafic fiamme than rest of Grizzly cally zoned from leucocratic granodiorite Peak Tuff. Layer caps the upper rhyolite sub­ margins to a tonalite core. Exposed from unit (Tgu). Exposed only on high ridge at mouth of Tabor Creek on the northwest, to head of Mountain Boy Gulch. Subunit has a near Grizzly Peak on the southeast. Mar­ thickness of 40 m, is medium rusty red (or gins are light bluish or yellowish gray; core black where vitrophyric) and contains fiamme is medium salt-and-pepper gray. Margins that have the identical range of compositions, contain about 30% phenocrysts of abundant but different colors, than those in the lower plagioclase, quartz, alkali feldspar, and heterogeneous tuff layer (unit Tghl). Matrix biotite, and lesser amounts of magnetite. of layer is a low-silica rhyolite, similar in Core contains about 60% phenocrysts of composition to the immediately underlying

3 tuff (unit Tgu). Fiamme in the layer range in subunit (Tgm). Exposed at Garfield Peak and color from white to very light bluish gray for on slope west of Ruby. Subunit is 2::50 m high- to low-silica rhyolite, medium rusty red thick, is medium to dark gray, has a for dacite, and dark rust for lati-andesite con­ medium-silica rhyolite matrix, and contains taining phenocrysts of biotite-hornblende­ fiamme ranging in color from bluish white for orthopyroxene or hornblende-orthopyroxene­ high-silica rhyolite, light gray for medium­ biotite and low-silica rhyolite, medium gray for dac­ Tgu Upper rhyolite subunit (Oligocene)-Rhyo- ite, and black for lati-andesite containing phe­ lite tuff containing numerous intercalated nocrysts of biotite-hornblende-orthopyroxene wedges, sheets, and tongues of megabreccia, and hornblende- orthopyroxene-biotite. All of as well as layers strewn with giant, internally the non-rhyolitic fiamme have about 40% shattered boulders. Preserved only in the phenocrysts northeastern part of the caldera, where it Tgm Middle rhyolite subunit (Oligocene)-Dark­ overlies a >300-m-thick layer of caldera-col- charcoal-gray, dark-reddish-brown, and lapse megabreccia (unit Tm). Typically medium-lavender-gray tuff. Welding varies medium brown, orange brown, or reddish but predominantly is dense. Groundmass brown; very lithic rich and contains lithics crystallinity increasingly granophyric (with predominantly of Early Proterozoic gneiss feldspars phenocrysts over 1 mm in long (unit Xgn), St. Kevin Granite (unit Ys), dimension) toward the center of the caldera. Denny Creek Granodiorite (unit Xd), and gra­ Extensively exposed on western side of nodiorite of Sayers (unit YXs). Thickness caldera. Interbedded with megabreccia on about about 400 m. Contains about 35% phe- eastern side of caldera. Where granophyri­ nocrysts of plagioclase, quartz, alkali feld- cally crystallized, subunit is dark and pyro­ spar, biotite, and magnetite, and trace clastic textures are difficult to discern. amounts of zircon, apatite, and monazite. Thickness ranges from about 250 to 600 m. Biotite and magnetite are about twice as Subunit typically has about 30% phenocrysts abundant as in the medium- silica rhyolite of plagioclase, quartz, alkali feldspar, biotite, (unit Tgm). and magnetite, in decreasing concentrations. Composition is low-silica, ranging from Biotite and magnetite are more than twice as

about 70 to 72.5% Si02. Low-silica rhy­ abundant as in the high-silica lower rhyolite olitic fiamme are alkali-calcic (fig. 3A), subunit, Tgl). Contains trace amounts of zir­ mildly to strongly peraluminous (fig. 3B), con, apatite, monazite, and allanite. have a slight sodic enrichment (fig. 3C), and Composition is medium-silica rhyolite,

no significant iron enrichment (fig. 3D). ranging from about 72.5 to 75% Si02• Other, more mafic fiamme have a range in Medium-silica rhyolite fiamme are similar in composition, from as mafic as calc-alkalic composition to low-silica rhyolite fiamme,

lati-andesite and calc-alkalic quartz latite to but some are metaluminous at Si02 concen­ calc-alkalic rhyodacite (fig. 3A). These trations of more than 72% (fig. 3B), indicat­ mafic fiamme are metaluminous (fig. 3B) ing that they are hydrothermally altered. and have little apparent alkali or iron enrich­ Some of these altered fiamme are also notably ment (figs. 3C and D). Quartz latite fiamme sodic (fig. 3C) are enriched in Th and are commonly sodic, Tgl Lower rhyolite subunit (Oligocene)-White, suggesting that they are not the product of gray, light-yellow, light- to medium-red, tan, alteration but instead may represent a dis- lavender, or black (where vitrophyric) rhyo­ tinctly more alkalic magma than the bulk of lite tuff. The predominant lithic is the pre­ the Grizzly Peak Tuff represents. caldera rhyolite lava (unit Tf). Exposed Sample of outflow tuff south of Twin extensively around the north, west, and south Lakes Reservoir and west of Granite that is margins of the caldera. At base, the tuff is a probably equivalent to the upper rhyolite nearly monolithologic tuff breccia. Welding subunit has a K-Ar biotite date of 33.3 ±1.0 varies from predominantly dense to poor. Ma Thickness ranges from about 0.5 to 1.0 km. Tgh1 Lower heterogeneous subunit (Oligocene)­ Contains about 20% phenocrysts of quartz, Heterogeneous tuff layer containing larger sanidine, and plagioclase in subequal and more mafic fiamme than rest of Grizzly amounts, minor biotite, and traces of mona­ Peak Tuff. Layer caps the middle rhyolite zite, allanite, zircon, and apatite.

4 Composition is high-silica rhyolite, rang­ head of Tellurium Creek. Crops out as a flow

ing from about 75 to 77% Si02• No chemical beneath the Grizzly Peak Tuff and as a small data are available for most of the high-silica stock of porphyry which underlies the flow. rhyolite Closely resembles the first-erupted high-sil­ Caldera-Collapse Breccia ica rhyolite of the overlying Grizzly Peak Extremely coarse rock-avalanche breccia Tuff (unit Tgl) in chemistry and phenocryst forming wedges, sheets, and tongues interca- assemblage lated within the intracaldera Grizzly Peak Tw Intrusive rocks of the White Rock pluton (Oli­ Tuff. Most clasts longer than 1 m are inter- gocene)-Light-gray, medium- to fine­ nally shattered. Composed dominantly of grained, equigranular, biotite-hornblende gra­ clasts of Early Proterozoic Denny Creek Gra­ nodiorite and associated rocks. Forms White nodiorite (unit Xd), St. Kevin Granite (unit Rock pluton or laccolithic intrusive complex, Ys), metasedimentary gneiss (unit Xms), and 15 by 15 km in extent, that intrudes Paleozoic granodiorite of Sayers (unit YXs) which and Mesozoic strata in the far western part of formed the wallrocks of the Grizzly Peak the map area (Bryant, 1979). Some small caldera. Less common constituents include dikes and sills associated with pluton not intrusive rocks of the Twin Lakes Pluton shown at scale of map. Main rock type is gra­ (units Tt, Ttl, and Ttm) and precaldera rhyo- nodiorite, which consists, in decreasing lite (unit Tf). Everywhere divided into the amounts, of oligoclase to andesine, quartz, following three subunits potassic feldspar, biotite, opaque minerals, Tm Megabreccia (Oligocene )-Caldera-collapse hornblende, and minor amounts of apatite, breccias containing little or no tuff matrix. zircon, sphene, and allanite. Typical clast size is tens of meters long; The main pluton ranges from tonalite to maximum clast size is at least 0.5 km long. granodiorite, straddles the border between Minor seams of tuff matrix are present between alkali-calcic and calc-alkalic (fig. 4A), ranges some giant clasts. Exposed extensively in from metaluminous to mildly peraluminous eastern and northeastern part of caldera . (fig. 4B), and shows no strong alkali enrich­ Tmt Clast-supported tuff megabreccia (Oli­ ment or iron enrichment trends (fig. 4C, D). gocene)-Caldera-collapse breccia, but typi­ Rocks as mafic as gabbro-diorite to diorite are cal clast size is smaller and has an average associated with the granodiorite, but consti­ length of < 10 m and a maximum length of tute a minor percentage of the pluton. Dikes about 50 m. Exposed between East Red and are predominately granodiorite in composi­ Grizzly Peak. Large masses of tuff matrix tion. No minor element geochemistry is surround the shattered giant clasts. Most tuff available. matrix is nonwelded or poorly welded, but Oligocene age assignment based on K-Ar large masses show increased welding biotite date of 34.8 ± 1.0 Ma from sample to inward. Composition of the tuff matrix is west of map area, as recalculated by Wallace similar to that of either underlying or overly­ (1993) from data in Obradovich and others ing tuff (1969) Ttm Matrix-supported tuff megabreccia (Oli- Tim Intrusive rocks of the Italian Mountain intru­ gocene )-Caldera-collapse breccia, similar to sive complex (Oligocene)-Medium- to tuff megabreccia (unit Tmt), but clasts are dark-gray, medium-grained, equigranular matrix supported, are typically well rounded, quartz diorite to light-gray, fine-grained, por­ and most range between 2 and 10 m in diame- phyritic dacite. Exposed in far southwestern ter. Clasts composed dominantly of Denny part of map area. Cunningham (1976) identi­ Creek Granodiorite (unit Xd), Twin Lakes fied three intrusive centers which are not sep­ Pluton (units Tt, Ttl, and Ttm), and St. Kevin arated on this map. Intrudes rocks as young Granite (unit Ys). Matrix is poorly welded, as the Pennsylvanian Belden Formation. lithic rich, low-silica rhyolite unit of the Griz- Quartz diorite consists, in decreasing zly Peak Tuff. Exposed in a single area in the amounts, of plagioclase, quartz, potassium northeast part of the caldera, at the head of feldspar, hornblende, biotite, epidote, opaque Negro Gulch minerals, and minor amounts of sphene, apa­ Tf Precaldera rhyolite lava (Oligocene)­ tite, and zircon. Rocks of intermediate com­ Grayish white or pink high-silica rhyolite position (described by Cunningham (1976) as lava and porphyry. Exposed southeast of the granodiorite and quartz monzonite) have less

5 1.3 Strongly Peraluminous 1.2 .- Mildly Peraluminous •• Intrusive rocks near Aspen \ I A altered Cretaceous(?) diorite (part of unit Kd) \\ • I 1.1 • t + Cretaceous aplite, quartz porphyry (unit Kr) I \ I I o White Rock pluton (unit Tw) ~ I I cfJO 0 z A - D 0 0 --- ~ '0 - - 0 -- 0.9 --- I -i Vetysodlc I 0 0 --/~'- B / 0 0.8 Field of unaltered Metaluminous igneous roeks ~/

0.7 '-----'--~'____Jc______,___ ~----'----____j 0+ / so 60 70 • /'(]6 0 o 0 i 0 Average Fe rich. Vety Fe rich ·' Mg rich I . i 60 Sodic c I I i 0 1500 fjN I Average Vetypotassic 0 I.••• 0 /oo + g I 0.2 0.4 0.6 0 < Kpi

R 1= 4000Si- ll,OOO(Na + K)- 2000(Fe + Ti)

Figure 4. Summary geochemical diagrams for Cretaceous and Tertiary intrusive rocks in the Aspen area. Data from Bruce Bryant (1979; unpub. data, 1994). (A) R1R2 major element classification diagram (De la Roche and others, 1980). Alk Gr, alkali granite; Gr, granite; Gd, granodiorite; Ton, tonalite; Di, diorite; Gb-Di, gabbro-diorite; Mz Gb, monzogabbro; Mz Di, monzodiorite; Q M, quartz monzonite; Q Sy, quartz syenite; Sye, syenite; Sy Di, syenodiorite; Sy Gb, syenogabbro. Fields of alkalinity modified slightly from those in DeWitt (1989) based on Ed DeWitt (unpub. data, 1994). (B) Alumina saturation diagram (Si02 versus A/CNK). A, molar Al20 3; C, molar CaO; N, molar Na20; K, molar K20. (C) Alkali classification diagram (K20/(K20+Na20) versus Si02). Field boundaries from Ed DeWitt (unpub data, 1994). (D) Iron enrichment classification diagram ((Fe0+0.89*Fe20 3)/(Fe0+0.89*Fe20 3+Mg0) versus Si02). Field boundaries slightly modified from those in DeWitt (1989) based on Ed DeWitt (unpub. data, 1994).

plagioclase and hornblende, more quartz, suite has no preferential alkali enrichment. potassium feldspar, and biotite, and lower The suite varies from very Mg rich to very Fe concentrations of opaque minerals. Late rich (fig. 5D) Some late dikes are very Mg dikes and vent-facies rocks are porphyritic rich. These wide ranges in Fe enrichment and have a fine-grained groundmass. and alkali enrichment distinguish these intru­ Includes large east-striking dike west of Ital­ sive rocks from other Oligocene rocks in the ian Mountain. area. In terms of major-element composition, the bulk of the Italian Mountain intrusive The suite ranges in composition from complex closely resembles the resurgent plu­ tonalite to granite (fig. SA) and is metalumi­ tons (Lincoln Gulch Stock) of the Grizzly nous (tonalite and vent-facies rocks) to Peak caldera. strongly peraluminous (fig. 5B). A large range in alkali enrichment is noted, from very Oligocene age assignment based on fis­ sadie tonalite and granodiorite to average sion-track zircon date of 33.9 ± 3.9 Ma from granodiorite (fig. 5C), although most of the porphyritic granodiorite, the youngest rock in

6 1.3 Strongly Peraluminous Italian Mountain intrusive complex (unit Tim) A quartz diorite 1.2 Mildly Peraluminous -- \ granodiorite 6. \ \ I + • I melagranodiorite 1.1 ••• •• r I • quartz monzonite \ I I ~ I 6. • vent facies z I 6. I 6. dacite porphyry ~ + • • 0.9 - - Verysodic I • f B I 0.8 ---~"'Field of unaltered Metaluminous igneous rocks

'------'--...:_____L.__ _i__ __ .._,______.______j ,:,t' 0.7 so 70 y () ... Mg rich J Average Very Fe rich i ~e rich/ ! I I ! + ' I I ' c I I / ./ ./ • I Potassic Very potassic /.• / 6.. , / • li I , A <)'v' Very Mg rich Kpi(K.p + Nap) ·:6./1/ I i 1000 + .,I / D !. / g"' / f riJ so + o.s 0.6 0.7 0.8 0.9 1.0 soo i (FeO + 0.89FeP3)1(FeO + 0.89Fe p J'" MgO) AlkGr A

c______L______L______l______j 1000 1500 2000 2500 3000

R1= 4000Si- ll,OOO(Na + K)- 2000(Fe + Ti)

Figure 5. Summary geochemical diagrams for the Italian Mountain intrusive complex. Data and rock names from Cun­ ningham (1976) (A) R1R2 major element classification diagram (De la Roche and others, 1980). Alk Gr, alkali granite; Gr, granite; Gd, granodiorite; Ton, tonalite; Di, diorite; Gb-Di, gabbro-diorite; Mz Gb, monzogabbro; Mz Di, monzodiorite; Q M, quartz monzonite; Q Sy, quartz syenite; Sye, syenite; Sy Di, syenodiorite; Sy Gb, syenogabbro. Fields of alkalinity modified slightly from those in DeWitt (1989) based on Ed DeWitt (unpub. data, 1994). (B) Alumina saturation diagram (Si02 versus A/CNK). A, molar Al20 3; C, molar CaO; N, molar Na20; K, molar K20. (C) Alkali classification diagram (K20/(K20+Na20) versus Si02). Field boundaries from Ed DeWitt (unpub data, 1994). (D) Iron enrichment classification diagram ((Fe0+0.89*Fe20 3)/(Fe0+0.89*Fe20 3+Mg0) versus Si02). Field boundaries slightly modified from those in DeWitt (1989) based on Ed DeWitt (unpub. data, 1994).

the intrusive sequence (recalculated from in eastern half of map area, from Mount Cunningham and Naeser, 1975, by Cunning­ Elbert to Mount Columbia. Also includes ham and others, 1995) stock east of Mount Champion (Van Loenen Twf Felsic plutons and dikes of Winfield Peak and and others, 1989). Rhyolite, rhyolite por­ Middle Mountain (Eocene)-Light-gray, phyry, and felsic dikes consist of tan, and white, very fine-grained, porphyritic fine-grained groundmass of quartz and potas­ rhyolite, rhyolite porphyry, and granite plugs sium feldspar and 5-15% phenocrysts of and dikes, and areas of hydrothermally euhedral quartz, perthitic sanidine, albite, and altered rocks, breccia, and minor pegmatite minor biotite. Granite consists of orthoclase, (Ranta, 1974; Gunow, 1983). Intrusive quartz, plagioclase, biotite, and minor rocks and breccia have molybdenum pros­ amounts of magnetite, apatite, zircon, musco­ pects and minor gold-silver prospects. Gran­ vite, fluorite, and molybdenite. ite and hypabyssal rocks restricted to area of Although most of the rocks at Winfield are Winfield Peak and Middle Mountain. Rhyo­ hydrothermally altered to varying degrees, lite and rhyolite porphyry dikes widespread rhyolite and granite appear to have been

7 alkali-calcic (fig. 6A), mildly peraluminous (< 50 ppm) and high concentrations of Nb (fig. 6B), sodic (fig. 6C), and have no major (>40 ppm). Distinguished from resurgent iron enrichment (fig. 6D). Similar rocks east intrusive dikes and stocks of the Grizzly Peak of Mount Champion have approximately the caldera and from Cretaceous rhyolite (unit Kr) same major element composition (fig. 7a), but by low concentrations of Zr, Sr, and high con­ are markedly peraluminous because of their centrations of Nb. Dikes for which chemical extensive conversion to clay minerals (such analyses indicate certainty in classification rocks plot off scale of fig. 7B), and are very are indicated on map with small box on dike potassic because of their secondary potassium symbol. Dikes that lack chemical samples feldspar (fig. 7C). but that are inferred to belong to this unit on Minor-element chemistry distinguishes the basis of petrography are indicated by no these felsic rocks from other suites of felsic box on the dike symbol. rocks related to the Grizzly Peak caldera or to Eocene age assignment based on K-Ar Cretaceous plutonism. Least-altered rhyolite biotite date of 38.8 ± 1.3 Ma from the Middle is characterized by low concentrations of Zr Mountain porphyry, one of the oldest rocks at

1.3,------==------, Strongly Peraluminous ' 0 Intrusive rocks near Winfield £ Intrusives at Middle Mountain (part of unit Twf) 1.2 Mildly Peraluminous [weak sericitic alteration] \ /------o Intrusives at Winfield Peak (part of unit Twf) 1.1 [weak to strong sericitic alteration to 0 potassic alteration]

0 0

Si02 (wt. percent) .·· Mg rich / Average ~e rich/ Very Fe rich c 0 £ £?~ / ' d • / / ISOO 6 / ,/' ,' Potasaic I I f I I I I I I I ' I i Kpi

1000 1500 2000 2500 3000

R 1=4000Si -ll,OOO(Na+ K)-lOOO(Fe+ Ti)

Figure 6. Summary geochemical diagrams for altered intrusive rock at Middle Mountain and Winfield Peak. Data from Gunow (1983) and Ranta (1974). (A) R1R2 major element classification diagram (De Ia Roche and others, 1980). Alk Gr, alkali granite; Gr, granite; Gd, granodiorite; Ton, tonalite; Di, diorite; Gb-Di, gabbro-diorite; Mz Gb, monzogabbro; Mz Di, monzodiorite; Q M, quartz monzonite; Q Sy, quartz syenite; Sye, syenite; Sy Di, syenodiorite; Sy Gb, syenogabbro. Fields of alkalinity modified slightly from those in DeWitt (1989) based on Ed DeWitt (unpub. data, 1994). (B) Alumina saturation diagram (Si02 versus A/CNK). A, molar Al20 3; C, molar CaO; N, molar Na20; K, molar K20. (C) Alkali classification diagram (K20/(K20+Na20) versus Si02). Field boundaries from Ed DeWitt (unpub data, 1994). (D) Iron enrichment classification diagram ((Fe0+0.89*Fe20 3)/(Fe0+0.89*Fe20 3+Mg0) versus Si02). Field boundaries slightly modified from those in DeWitt (1989) based on Ed DeWitt (unpub. data, 1994).

8 1.3 L Strongly Peraluminous Mount Champion area 2 A Cretaceous rhyolite dikes (unit Kr) L ~ Mll~y Pe"'lumlnous Cretaceous(?) rhyolite dikes (unit Kr) (some altered dikes plot off diagram B)) 1.: • Tertiary stocks and dikes (unit Twt) ~ f (some highly altered rocks plot off diagram B) ~ ~ 0.91 Verysodic I r~ 0.8 ~ Field of unaltered • ~ Metaluminous igneous rocks • • 60 (wt. SI02 percent) Mg:~/ A~e ~erichVeryFe"1 c •. > • / ' ·~~~/ 1500 ... I I Potassic Very potassic I I I I I '

Kfli(Kfl + Na/)) i 1000 + f + 0.5 0.6 0.7 0.8 0.9 500 i (FeO + 0.89Fe;P3)1(Fe0 + 0.89Fep3+ MgO) II r:t.N A ••

-'-----_j 1000 1500 2000 2500 3000

= R 1 4000Si-ll,OOO(Na + K)- 2000(Fe + Tl)

Figure 7. Summary geochemical diagrams for rocks of the Mount Champion area. Data from R.E. Van Loenen ( 1989; unpub. data, 1994). (A) R1R2 major element classification diagram (De la Roche and others, 1980). Alk Gr, alkali granite; Gr, granite; Gd, granodiorite; Ton, tonalite; Di, diorite; Gb-Di, gabbro-diorite; Mz Gb, monzogabbro; Mz Di, monzodiorite; Q M, quartz monzonite; Q Sy, quartz syenite; Sye, syenite; Sy Di, syenodiorite; Sy Gb, syenogabbro. Fields of alkalinity modified slightly from those in DeWitt (1989) based on Ed DeWitt (unpub. data, 1994). (B) Alumina saturation diagram (Si02 versus A/CNK). A, molar Al20 3; C, molar CaO; N, molar Na20; K, molar K20. (C) Alkali classification diagram (K20/(K20+Na20) versus Si02). Field boundaries from Ed DeWitt (unpub data, 1994). (D) Iron enrichment classification diagram ((Fe0+0.89*Fe20 3)/(Fe0+0.89*Fe20 3+Mg0) versus Si02). Field boundaries slightly modified from those in DeWitt (1989) based on Ed DeWitt (unpub. data, 1994).

Winfield, and K-Ar biotite date of 37.7 ± 1.2 Monzonite (Dings and Robinson, 1957), the Ma from the granite, one of the youngest latter of which is interpreted as border facies rocks at Winfield (dates recalculated from of the pluton. The border consists, in data in Ranta, 197 4) decreasing amounts, of plagioclase, potas­ Tmp Intrusive rocks of the Mount Princeton pluton sium feldspar, hornblende and biotite, (Eocene)-Light-tan to greenish-tan, quartz, opaque minerals, and minor amounts medium-grained, equigranular to slightly por­ of sphene, zircon, and allanite. Main body phyritic tonalite to granodiorite and aplite. (granodiorite) consists of characteristically Exposed southwest and east of Mount Yale. turbid, normal and oscillatory zoned oligo­ Forms large pluton south of map area (Dings clase, slightly turbid to clear microcline and and Robinson, 1957; Shannon, 1988; Toul­ perthite, quartz, actinolitic hornblende, min and Hammarstrom, 1990). Rock types biotite, magnetite, ilmenite, apatite, zircon, exposed in map area restricted to granodiorite and sphene. Aplite contains traces of musco­ and aplite. Includes Mount Princeton Quartz vite in association with biotite, but no horn­ Monzonite and Mount Pomeroy Quartz blende.

9 Most of the intrusive rocks range from of the map area. Fission-track dates from tonalite to granodiorite (fig. 8A). Tonalite sphene as old as 39.4 to 41.2 ± 3.2 Ma and (not exposed in map area), is alkali-calcic zircon as old as 36.6 ± 0.4 Ma suggest the (fig. 8A), metaluminous (fig. 8B), average to pluton is Eocene (Shannon, 1988). A minus- slightly potassic (fig. 8C), and average to Mg 400-mesh zircon fraction from a sample along rich (fig. 8D). Granodiorite is alkali-calcic to Chalk Creek, south of Mount Princeton, has a calc-alkalic (fig. 8A), metaluminous to mildly 208Pbf232Th date of 36.6 Ma. If an upper inter­ peraluminous (fig. 8B), and average to Mg cept date of 1700 Ma is assumed for the zir­ rich (fig. 8D). Features that distinguish the con, a lower intercept date of 36.6 ± 0.4 Ma is Mount Princeton pluton from other intrusive obtained (E. DeWitt and R.E. Zartman, rocks in the area are its highly alkali-calcic unpub. data, 1990). Two 40Ar- 39 Ar horn­ chemistry and Mg-rich character. blende dates of 36.3 ± 0.3 Ma and 37.1 ± 0.3 Eocene age assignment based on unpub­ Ma from the pluton also suggest a latest lished fission-track and U-Th-Pb dates and Eocene age (Toulmin and Hammarstrom, published 40Ar- 39Ar dates from samples south 1990)

1.3 ------~=------~ Strongly Peraluminous 1.2 Mildly Peraluminous Mount Princeton pluton (unit Tmp) + main body 1.1 • o mafic border

·o.9 I I Verysodlc I / I B I I 0.8 I Metaluminous ,/ ./. 1// 0.7 L------'-~---'-----'------'--,----'------' , I so 60 70 Si0 (wt. percent) 2 :- I '/ 0 / Mg rich / Average ~e rich/ Very Fe rich 60 I I · 0.· ' I I l c I I ~ I I I I i /. / / 1500 Very potassic ff i ~~ / so 0 // *#..,~//.· • VeryMgrich Kpi

IN +• (FeO + 0.89FeP3)1(Fe0 + 0.89Fe p t- MgO) QSy ~u ~r ~N J A

__j 1000 1500 2000 2500 3000

R = 4000Si- ll,OOO(Na + K)- lOOO(Fe + Ti) 1

Figure 8. Summary geochemical diagrams for intrusive rocks of the Mount Princeton Pluton. Data from J.R. Shannon (1988; unpub. data, 1994) and Ed DeWitt (unpub. data, 1994). (A) R1R2 major element classification diagram (De Ia Roche and others, 1980). Alk Gr, alkali granite; Gr, granite; Gd, granodiorite; Ton, tonalite; Di, diorite; Gb-Di, gabbro-diorite; Mz Gb, monzogabbro; Mz Di, monzodiorite; Q M, quartz monzonite; Q Sy, quartz syenite; Sye, syenite; Sy Di, syenodiorite; Sy Gb, syenogabbro. Fields of alkalinity modified slightly from those in DeWitt (1989) based on Ed DeWitt (unpub. data, 1994). (B) Alumina saturation diagram (Si02 versus A/CNK). A, molar AI20 3; C, molar CaO; N, molar Na20; K, molar K20. (C) Alkali classification diagram (K20/(K20+Na20) versus Si02). Field boundaries from Ed DeWitt (unpub data, 1994). (D) Iron enrichment classification diagram ((Fe0+0.89*Fe20 3)/(Fe0+0.89*Fe20 3+Mg0) versus Si02). Field boundaries slightly modified from those in DeWitt (1989) based on Ed DeWitt (unpub. data, 1994).

10 Intrusive rocks of the Twin Lakes pluton masses. Cuts all other parts of the pluton. Rocks described below form the Twin Consists of 70-85% groundmass of very fine Lakes pluton, a group of intrusive rocks that grained quartz, a mix of feldspars, sericite, forms a large pluton and associated dikes in and 15-30% phenocrysts, in decreasing the east-central part of the map area. The amounts, of turbid to clear oscillatory-zoned Twin Lakes Granodiorite of Brock and Barker and albite-twinned plagioclase, euhedral to forms part of the Twin Lakes pluton; much of strongly embayed quartz, potassium feldspar, it is equivalent to the main body, discussed biotite slightly altered to chlorite and opaque below minerals, skeletal hornblende mostly replaced Tti Intermediate-composition porphyry dikes by opaque minerals, chlorite, minor epidote, (Paleocene)-Gray, fine-grained, porphyritic sphene, and opaque minerals. hornblende-biotite tonalite dikes and irregu­ Dikes are predominantly tonalite, a compo­ larly shaped plugs. Exposed throughout the sition not found in many of the other units of Twin Lakes pluton as small dikes and concen­ the Twin Lakes pluton (fig. 9A). The tonalite trated in the Winfield Peak area as larger is alkali-calcic (fig. 9A), metaluminous to

1.3 ,------=-----, Twin Lakes pluton Strongly Peraluminous -' main body (unit Tt) 1.2 Mildly Peraluminous • mafic border (unit Ttm) o leucocratic core (unit Ttl) 1.1 'V aplite (part of unit Tt) + + intermediate-composition dikes (unit Tti) z~ felsic dikes (unit Ttt) ~ main phase in breccia sheets 0.9 • 0.8 •

0.7 so 70

I 60 l c 1 1SOO g"" Ill Vfr/ potassic so 0 Kpi(K.p + Nap) Vfr/ Mg rich i 1000 • .+ + .. D f • + i (FeO + 0.89Fe:P3)1(Fe0 + 0.89Fep 3+ MgO) I r:t."" AltGr A

_L___ _L ____~ 1000 ISOO 2000 2SOO 3000

R 1= 4000Si- ll,OOO(Na + K) -lOOO(Fe + Ti)

Figure 9. Summary geochemical diagrams for intrusive rocks of the Twin Lakes Pluton. Data from Wilshire (1969), Ed DeWitt (unpub. data, 1994), and Stein and Crock (1990). (A) R1R2 major element classification diagram (De la Roche and others, 1980). Alk Gr, alkali granite; Gr, granite; Gd, granodiorite; Ton, tonalite; Di, diorite; Gb-Di, gabbro-diorite; Mz Gb, monzogabbro; Mz Di, monzodiorite; Q M, quartz monzonite; Q Sy, quartz syenite; Sye, syenite; Sy Di, syenodiorite; Sy Gb, syenogabbro. Fields of alkalinity modified slightly from those in DeWitt (1989) based on Ed DeWitt (unpub. data, 1994). (B) Alumina saturation diagram (Si02 versus A/CNK). A, molar Al20 3; C, molar CaO; N, molar Na20; K, molar K20. (C) Alkali classification diagram (K20/(K20+Na20) versus Si02). Field boundaries from Ed DeWitt (unpub data, 1994). (D) Iron.enrichment classification diagram ((Fe0+0.89*Fe20 3)/(Fe0+0.89*Fe20 3+Mg0) versus Si02). Field boundaries slightly modified from those in DeWitt (1989) based on Ed DeWitt (unpub. data, 1994).

11 mildly peraluminous (fig. 9B), shows no par­ common along east-west-trending contacts ticular alkali enrichment, and is average to with Early Proterozoic rocks on either side of Mg rich (fig. 9D). The composition of these Lake Creek. Cuts and is gradational into dikes is believed to be close to the average mafic border (unit Ttm ), and it is gradational composition of the magma at depth that was into leucocratic core (unit Ttl). Aplites, emplaced and fractionally crystallized to form including aplite body at Monitor Rock the main body of the Twin Lakes pluton (unit (Wilshire, 1969), cut the main body and are Tt) gradational to it. Includes normal facies of Ttf Felsic porphyry dikes (Paleocene)-Light­ Brock and Barker (1972). Fine-grained, tan, medium-grained, strongly porphyritic equigranular granodiorite restricted to out­ biotite granite and rhyolite dikes. Largest crops at East Red, just inside the ring fault of dike exposed on east side of Twin Lakes plu­ the Grizzly Peak caldera; may be a ton, from east of Twin Peaks, southward to higher-level textural equivalent of the east of Prospector Gulch in Taylor Park. Dike medium- to coarse-grained granodiorite out­ strikes northerly, dips steeply, and is 17 km side the caldera. Fine-grained granodiorite long and 13-26 m wide. Other, smaller dikes has been down-dropped in the caldera. in east-central part of map area. Largest dike Coarse-grained granodiorite contains variable is quartz monzonite porphyry of Brock and concentrations of conspicuous Carlsbad­ Barker (1972); other bodies include monzo­ twinned perthite and orthoclase phenocrysts nite porphyry and some quartz latite por­ as large as 5 em by 15 em and slightly phyry, both of Brock and Barker (1972). embayed quartz phenocrysts as large as 1.2 Largest dike has distinctive texture and con­ em (Chapman, 1935; Wilshire, 1969; Brock tains fine-grained groundmass and and Barker, 1972). Consists, in decreasing coarse-grained phenocrysts of potassium feld­ order, of oscillatory-zoned and albite­ spar (1-3.5 em long), plagioclase, and lesser twinned oligoclase, perthite and orthoclase, quartz (0.4-0.8 mm diameter). Other dikes quartz, biotite, opaque minerals, hornblende, have fine-grained groundmass of quartz, pla­ sphene, and minor amounts of apatite, zircon, gioclase, and potassium feldspar, and pheno­ and trace allanite. crysts and clots of oscillatory-zoned and Main body is a granodiorite that is twinned oligoclase, embayed quartz, biotite, alkali-calcic (fig. 9A). Points that plot in and very minor hornblende. calc-alkalic field are lacking the normal con­ Granite and rhyolite are alkali-calcic to centration of coarse-grained potassium feld­ calc-alkalic (fig. 9A), are moderately to spar phenocrysts. The granodiorite is strongly peraluminous (fig. 9B), and, consid­ metaluminous to mildly peraluminous (fig. ering the limited number of samples, have no 9B), is average to sodic (fig. 9C), and average apparent alkali enrichment and do not display to Mg rich (fig. 9D). Distinctive chemical significant iron enrichment trends (figs. 9C features of the granodiorite are its high Sr

and D). Distinguished from dikes related to concentration (650-1050 ppm) and low P20 5 Grizzly Peak pluton (units Tp and Tl) by (0.18 weight percent average). Aplite bodies strongly zoned plagioclase phenocrysts and are granite that is alkali-calcic (fig. 9A), high Sr concentration (700-1 ,000 ppm). mildly peraluminous (fig. 9B), and that lacks Dikes for which chemical analyses indicate significant alkali enrichment or iron enrich­ certainty in classification are indicated on ment trends (figs. 9C and D). map with small box on dike symbol. Dikes Fission-track apatite dates of 16-30 Ma, that lack chemical samples but that are fission-track sphene and zircon dates of 39-44 inferred to belong to this unit on the basis of Ma, K-Ar biotite date of 42 Ma, and Rb-Sr petrography are indicated by no box on the biotite dates of 49-56 Ma (summarized by dike symbol. Wallace, 1993, 1995) from main body of gra­ Tt Main body (Paleocene)-Light-gray and tan, nodiorite and from aplite are uplift and cool­ medium- to coarse-grained, equigranular to ing dates of the Paleocene pluton strongly porphyritic biotite granodiorite and Ttl Leucocratic core (Paleocene)-Light-gray to associated compositionally layered rocks and creamy-white, medium-grained, equigranular aplite bodies. Major rock unit in east-central biotite granite. Distinctive leucocratic nature part of area, from south flank of Mount Elbert and lack of potassium-feldspar phenocrysts to Taylor Park. Layered granodiorite most separate core from other units of the Twin

12 Lakes pluton. Exposed on Jenkins Mountain 1.0 and in upper reaches of North Fork of Clear Creek. Grades into main body (unit Tt). ( Lacks conspicuous perthite and orthoclase 0.1 phenocrysts. Cut by minor aplite and inter­ ~ =- mediate porphyry dikes. - - Leucocratic core is an alkali-calcic granite 0.01 (fig. 9A) that is mildly peraluminous (fig. 9B), very sodic (fig. 9C), and shows no sig­ nificant iron enrichment (fig. 9D). Character­ 80 ized by very low Th ·concentrations (<1.2-3.0

ppm) and low P20 5 (<0.05 weight percent). Outcrop extent corresponds to Th low on ... 60 radiometric maps of the central Sawatch 1- Range (D.L. Campbell and J. Duval, written commun., 1988) 40 - RS-28-84-8 Ttm Mafic border (Paleocene)-Dark -greenish­ Hornblende gray, medium-grained, equigranular to Tg Date= 66.29 Ma slightly porphyritic hornblende gabbrodiorite, Tp Date= 63.82 +/- 1.44 Ma monzodiorite, and diorite. Exposed discon­ 20 - tinuously on the southwestern margin of the pluton near Grizzly Peak, near the southwest­ em corner of the pluton near Huron Peak, and 0 I I l I I I I I I 100 on the northwestern margin of the pluton, 0 within the Grizzly Peak caldera, near the head of South Fork of Lake Creek. Both grada­ Figure 10. 40Ar-39 Ar release spectra for hornblende tional into and intruded by main body. from sample R8-28-84-8 of mafic border of Twin Lakes plu­ Includes border facies of Brock and Barker ton. Width of bars on spectra is 2-sigma uncertainty. (1972). Consists, in decreasing amounts, of oscillatory-zoned and albite-twinned andesine to oligoclase, hornblende, potassium feldspar Belden Formation (Bryant, 1970). Possibly having minor perthitic texture, quartz, opaque equivalent to the Late Cretaceous Pando Por­ minerals, biotite, sphene, and minor amounts phyry north of Leadville (Pearson and others, of apatite and zircon. 1962). Exposed as dikes paralleling foliation Mafic border has an average composition in Early Proterozoic metasedimentary gneiss of diorite (fig. 9A), is alkali-calcic (fig. 9A), west and north of Grizzly Peak caldera, from metalurrlinous (fig. 9B), lacks any alkali New York Peak to Mount Champion (Luding­ enrichment (fig. 9C), and varies from Mg rich ton and Yeoman, 1980), and as similar dikes to Fe rich (fig. 9D). Characterized by high in area centered on Granite, in the Arkansas concentrations of Sr (800-1,200 ppm). Dis­ River graben. Quartz porphyry contains con­ tinguished from Cretaceous diorite (unit Kd) spicuous phenocrysts of quartz and muscovite by alkali-calcic nature. and is composed, in decreasing amounts, of Paleocene age assignment based on quartz, plagioclase, potassic feldspar, biotite, 40Ar-39Ar hornblende plateau date of 63.8 ± muscovite, opaque minerals, and minor 1.4 Ma from sample collected north of Lake amounts of epidote and apatite. Aplite and Pass (table 2, fig. 10; Shannon and others, altered aplite contain fewer phenocrysts and 1987) less biotite. Kr Quartz porphyry, granite, and aplite (Creta­ Near Aspen, unaltered sills and plugs range ceous)-Light-tan to cream or pink, fine­ from granodiorite to granite (fig. 4A), are grained, equigranular, leucocratic quartz por­ strongly peraluminous (fig. 4B), sodic (fig. phyry, muscovite granite, aplite, and hydro­ 4C), and Fe rich to very Fe rich (fig. 4D). thermally altered aplite. Exposed in far Hydrothermally altered rocks are calc-alkalic northwestern part of area, near Richmond (fig. 4A), as a result of feldspar destruction, Hill, where sills of all rock types cut Paleo­ and strongly peraluminous (fig. 4B). Dissem­ zoic rocks as young as the Pennsylvanian inated pyrite in the altered rocks explains the

13 very Pe-rich nature of the quartz porphyry calcic (fig. 4A), metaluminous (fig. 4B), and aplite. In the area from Mount Champion potassic (fig. 4C), and very Pe-rich (fig. 4D). to northwest of Independence Pass, unaltered North of Missouri Mountain, diorite forms dikes are granite that is alkali-calcic (fig. 7A), a 1.5 by 3 km, east-west-elongate stock cut by strongly peraluminous (fig. 7B), sodic (fig. the eastern border of the Twin Lakes pluton. 7C), and average to Mg rich (fig. 7D). The diorite, between Lake Fork and Pecks Altered dikes in this area are much more Peak, is the granodiorite unit of Brock and strongly peraluminous and, although not Barker (1972) and is medium grained, shown on figure 7B, they would plot above equigranular, and consists, in decreasing the field of unaltered rocks (fig. 7B). Altered amounts, of andesine, quartz, biotite, horn­ dikes are very potass,ic (fig. 7C) and have an blende, potassium feldspar, opaque minerals, average iron enrichment (fig. 7D). Minor ele­ and minor amounts of sphene and apatite. ment geochemistry (Mosier and others, 1980) The diorite is calc-alkalic (fig. 11A), metalu­ indicates that the Cretaceous dikes in this area minous (fig. 11B), and has no apparent alkali can be distinguished from dikes related to the or iron enrichment (fig. llC and D). The Grizzly Peak caldera by their high Sr concen­ diorite has high concentrations of Sr (750-800 trations and low Zr and Nb concentrations at ppm), which is typical for rocks of this unit.

comparable Si02 concentrations. Dikes for At and west of the Lienhart Mine, an irreg­ which chemical analyses indicate certainty in ular-shaped, east-west-elongate body intrudes classification are indicated on map with small the Kroenke Granodiorite. This body is com­ box on dike symbol. Dikes that lack chemical positionally zoned, from dark-greenish-black, samples but that are inferred to belong to this coarse-grained, equigranular gabbro to tan, unit on the basis of petrography are indicated medium-grained, equigranular granophyre. by no box on the dike symbol. Mafic rock types are most numerous at the Cretaceous age based on K-Ar whole-rock southwestern margin of the body, on the date of 65.4 ± 2.4 Ma (Van Loenen and oth­ south-facing slopes above Frenchman ers, 1989) for dike north of Independence Creek. Felsic rocks crudely overlie the mafic Pass and 65.3 ± 2.4 Ma (Hedlund, 1985) for rocks toward the top of the ridge, but the body dike northeast of Granite, both of which are lacks well developed igneous layering. Gab­ interpreted as minimum ages for emplace­ bro consists of predominantly actinolitic ment of the dikes hornblende and plagioclase, and lesser biotite, Kd Dioritic intrusive rocks (Cretaceous)-Dark­ apatite, potassium feldspar, quartz, and mag­ greenish-gray, medium-grained, equigranular netite. Tonalite has plagioclase, quartz, horn­ hornblende-biotite diorite to granodiorite blende, biotite, potassium feldspar, and lesser stocks and sills. Exposed south of Aspen in opaque minerals, apatite, sphene, and allanite. the Richmond Hill area as sills and discordant Granophyre contains more quartz and less bodies cutting Paleozoic strata as young as hornblende than tonalite. The body ranges the Pennsylvanian Belden Formation. Also from olivine gabbro to granodiorite (fig. 11A; exposed as stocks east of the Twin Lakes plu­ olivine gabbro plots above the diagram), and ton in three areas: north of Missouri Moun­ averages tonalite. Rocks in the range diorite tain; at and west of the Lienhart Mine; and to granodiorite are calc-alkalic (fig. 11A), along Morrison Creek west of the Arkansas metaluminous to mildly peraluminous (fig. River. liB), and have no alkali or iron enrichment Near Richmond Hill the diorite is the horn­ (figs. llC and D). Distinctive features of the blende quartz diorite of Bryant ( 1970, 1979), intrusive body at the Lienhart Mine include which is in contact with quartz porphyry (unit its zoned nature and the low concentration of Kr). Contact relations of the two igneous Rb (65 ppm average). rocks are poorly exposed (Bryant, 1979). The body along Morrison Creek, west of Diorite near Richmond Hill is an altered, por­ the Arkansas River, is poorly exposed phyritic rock containing phenocrysts of horn­ because of forest and glacial cover. The intru­ blende and plagioclase. Altered diorite sive rock is finer grained and more porphy­ consists, in decreasing order, of plagioclase, ritic than the other diorite bodies and contains epidote, chlorite, opaque minerals, quartz, more quartz-pyrite-chalcopyrite veinlets and carbonate minerals, muscovite, and minor altered zones. No chemistry is available for sphene and apatite. Altered diorite is alkali- this body.

14 l.3r------Strongly Peraluminous Cretaceous plutons 1.2 Mildly Peraluminous ----- • • pluton near Lienhart Mine (part of unit Kd) I .0. Lincoln Porphyry 1.1 ~l~ /:;. I + Pando Porphyry ... I / 6 /:;. / .. o pluton near Pecks Peak (part of unit Kd) _t::r ______0.9 • B 0.8 ,------~-"Field of unaltered Metaluminous Igneous rocks

60 (wt. SI02 percent) j Mg rich / Average ~e rich/ Very Fe rich . I I I I c I I / / I I 1500 I . i ~.::.l8. / f. / VeryMg rich ,/1. ,.-// Kpi i / + f • D + 1.0

(FeO + 0.89Fep3 )I(FeO + 0.89Fe:P3+ MgO)

AlkGr A

__j____ _l______L_ ____ j 1000 1500 2000 2500 3000

= K)- R1 4000Si- ll,OOO(Na + 2000(Fe + Ti)

Figure 11. Summary geochemical diagrams for Cretaceous plutons east of the Twin Lakes pluton. Data from Ed DeWitt ( unpub. data, 1994 ). Lincoln Porphyry included for purposes of camparison to the Twin Lakes pluton. (A) R 1R2 major element classification diagram (De Ia Roche and others, 1980). Alk Gr, alkali granite; Gr, granite; Gd, granodiorite; Ton, tonalite; Di, diorite; Gb-Di, gabbro-diorite; Mz Gb, monzogabbro; Mz Di, monzodiorite; Q M, quartz monzonite; Q Sy, quartz syenite; Sye, syenite; Sy Di, syenodiorite; Sy Gb, syenogabbro. Fields of alkalinity modified slightly from those in DeWitt (1989) based on Ed DeWitt (unpub. data, 1994). (B) Alumina saturation diagram (Si02 versus NCNK). A, molar Al20 3; C, molar CaO; N, molar Na20; K, molar K20. (C) Alkali classification diagram (K20/(K20+Na20) versus Si02). Field boundaries from Ed DeWitt (unpub data, 1994). (D) Iron enrichment classification diagram ((Fe0+0.89*Fe20 3)/(Fe0+0.89*Fe20 3+Mg0) versus Si02). Field boundaries slightly modified from those in DeWitt (1989) based on Ed DeWitt (unpub. data, 1994).

Cretaceous age assignment for all bodi~s Middle Jurassic)-Exposed at Hunters Hill of diorite is based on observation that the in far southwestern part of area (Zoerner, 63.8-Ma Twin Lakes pluton cuts the diorite 1974). Includes: north of Missouri Mountain. Also, Knopf Mancos Shale (Upper Cretaceous)-Dark­ (1926) suggested that dioritic rocks in the gray, silty shale and thin beds of siltstone and Aspen area were older than 67-74-Ma quartz fine-grained sandstone. About 55 m thick porphyry sills (summarized in Bryant, Dakota Sandstone (Lower Cretaceous)­ 1979). Similar dioritic rocks in the northern White to light-gray and yellowish-gray Sawatch Range have K-Ar dates as old as 70 quartzite, sandstone, conglomeratic sand­ Ma (Wallace and Blaskowski, 1989; Wallace, stone, and dark-gray carbonaceous shale. 1993; 1995). We infer, therefore, that the About 92 m thick diorite bodies in the map area are older than Morrison Formation (Upper Jurassic)-· 64 Ma, but no older than Cretaceous Greenish-red and greenish-gray shale, silt­ KJ me Mancos Shale through Entrada Sandstone, stone, mudstone, and sandstone. At least 115 undivided (Upper Cretaceous through m thick 15 Entrada Sandstone (Middle Jurassic)­ Leadville Limestone (Lower Mississip­ White to yellowish-gray and pale-red, cross­ pian)-Consists of two members. Thickness bedded sandstone and a few lenses of con­ 45-67 m glomerate near the base. About 55 m thick Castle Butte Cliff Member-Gray, massive P!Pmb Maroon Formation through Belden Formation, limestone undivided (Lower Permian through Penn­ Red Cliff Member-Gray to dark-gray, thin sylvanian)-Exposed in far western part of to thick-bedded dolomite and a few beds of map area, from Richmond Hill to south of limestone in lower part. Contact metamor­ Italian Mountain (Bryant, 1970). Includes: phosed to marble near Tertiary intrusive bod­ Maroon Formation (Lower Permian to Mid­ ies. Thickness 45-67 m dle Pennsylvanian)-Grayish-red to red­ Chaffee Group (Lower Mississippian? and dish-brown calcareous sandstone, mudstone, Upper Devonian)-Consists of three forma­ conglomerate, and limestone. Minor anhy­ tions. Thickness 30-60 m drite in lower part. Thickest unmetamor­ Gilman Sandstone (Lower Mississippian or phosed sections exposed on Timbered Hill Upper Devonian)-Dolomitic sandstone, and southwest of Hunters Hill, in far south­ sandy dolomite, and sedimentary breccia western part of map area. Exposed as large Dyer Dolomite {Upper Devonian)-Light­ screens within White Rock pluton, where to dark-gray, thin- to thick-bedded dolomite rocks are contact metamorphosed to green and and a few beds of gray limestone gray calc-silicate hornfels and white quartzite. Parting Formation (Upper Devonian)­ Approximate thickness in map area 920 m White to tan quartzite, gray to green shale, Gothic Formation (Middle Pennsylvanian)­ and dolomitic shale. Thickness 30-60 m Gray, tan, and brown calcareous sandstone, Fremont Limestone (Upper Ordovician)­ siltstone, shale, silty limestone, and lime­ Gray, massive dolomite. Thickness 10 m. stone. Sparse anhydrite and gypsum beds. Included with Manitou Dolomite in south­ Locally metamorphosed to hornfels, quartz­ western part of map area ite, and marble adjacent to Tertiary intrusive Harding Sandstone (Middle Ordovician)­ rocks. Apparent thickness 136-920 m. True Dolomitic sandstone containing fossil fish thickness difficult to estimate because of plates. Thickness 1-3 m. Included with Mani­ thrust faults, especially the Elk Range thrust, tou Dolomite in southwestern part of map area that are localized within the formation Manitou Dolomite (Lower Ordovician)­ Belden Formation (Middle and Lower Penn­ Gray dolomite containing white chert string­ sylvanian)-Black to dark-gray limestone, ers and nodules in upper part. Thickness dolomite, shale, and carbonaceous shale con­ 45-100 m taining a few sandstone beds and thin lenses of Peerless Formation (Upper Cambrian)­ chert pebble conglomerate near base. Pyritic in Brown-weathering, locally glauconitic dolo­ part. Crops out poorly. Best exposed south­ mitic sandstone, sandy dolomite, and west of Taylor Pass. Contact metamorphosed gray-green and grayish-red shale. A few beds to marble and hornfels near Tertiary intrusive of white quartzite locally near base. Thick­ rocks. Some of the marble superficially resem­ ness 30-45 m bles marble of the Leadville Limestone. Sawatch Quartzite (Upper Cambrian)­ Apparent thickness 227-575 m. True thickness White quartzite containing some beds of difficult to estimate because of thrust faults brown dolomitic sandstone and at the base a M£1s Leadville Limestone through Sawatch Quartz­ quartz-pebble conglomerate less than 1 m ite (Lower Mississippian through Upper thick. Thickness near Cottonwood Pass 35 m. Cambrian)-All formations in unit exposed Thickness near Italian Mountain 67 m extensively in far western part of area, from Vs St. Kevin Granite (Middle Proterozoic)­ Richmond Hill to Italian Mountain (Bryant, Light-tan to pinkish-tan, fine- to medium­ 1970; Cunningham, 1976). Sawatch Quartz­ grained, equigranular biotite-muscovite gran­ ite exposed southwest and southeast of Cot­ ite (Tweto and Pearson, 1964). Undeformed, tonwood Pass (Brock and Barker, 1972). but flow-foliated and compositionally layered Sawatch Quartzite and Manitou Dolomite in places. Consists, in decreasing percent­ exposed beneath Grizzly Peak caldera fill ages, of microcline, quartz, plagioclase, along Bowman Creek. Includes: biotite, and muscovite, and trace amounts of

16 apatite, zircon, fluorite, and sillimanite. Most Granodiorite by equigranular texture, greater St. Kevin in map area corresponds to abundance of feldspar, lack of mafic border, fine-grained facies of Tweto (1974). Pegma­ much lower magnetic susceptibility, subdued tite sills and dikes as thick as 10 m common, aeromagnetic response (Tweto and Case, but not shown on map. Many small bodies 1972; Godson and others, 1985), and some­ that intrude unit Xms not shown. Some small what higher scintillometer readings, indicat­ bodies of granite and pegmatite west and ing higher concentration of radioactive south of New York Peak, in the western part elements. of the area, could be 1700-Ma rocks rather The granite is alkali-calcic (fig. 12A), than St. Kevin. Intrudes schist and metasedi­ mildly to strongly peraluminous (fig. 12B), mentary gneiss (unit Xms), gneissic grano­ average to potassic (fig. 12C), and has no diorite (unit Xgd), leucocratic and normal apparent iron enrichment (fig. 12D). Ele­ granite of Henry Mountain (units Xhl, Xh), vated concentrations of Zr (300-600 ppm) and Kroenke Granodiorite (unit Xk), and cuts Th ( 40-60 ppm) are common. Muscovite-rich Grottos Granodiorite (unit Yg) at the head of granite and pegmatite are identical in chemis­ Lost Man Creek. Distinguished from Grottos try to the main body, but slightly more

1.3 Strongly Peraluminous Browns Pass Granite 1.2 + main body (part of unit YXbp) , aplite (part of unit YXbp) 1.1 Grottos pluton • main body (unit Yg) ~ • mafic border (unit Ygm) ~ St. Kevin Granite 0.9 o main body (part of unit Ys) o pegmatite, muscovite granite (part of unit Ys)

0.8 Granodiorite of Sayers 'f main body (unit YXs)

0.7 • main body in breccia sheets so 60 70

SI02 (wt. percent) Q' l 60 i ISOO sf1'1) so 0 0.2 0.4 KfJI(K.!J + N!ap)

1000 i+ I + soo i (FeO + 0.89Fe;A )/(FeO + 0.89FI!PJ + MgO) n rJf.M AlGI' A

1000 ISOO 2000 2SOO 3000

= R 1 4000Si • ll,OOO(Na + K) • ZOOO(Fe + TI)

Figure 12. Summary geochemical diagrams for 1.4-Ga plutons and plutons assumed to be 1.4 Gain the central Sawatch Range area. Data from Ed DeWitt (unpub. data, 1994), Barker and Brock (1965), and Tweto and Pearson (1964). (A) R 1R 2 major element classification diagram (De la Roche and others, 1980). Alk Gr, alkali granite; Gr, granite; Gd, granodiorite; Ton, tonalite; Di, diorite; Gb-Di, gabbro-diorite; Mz Gb, monzogabbro; Mz Di, monzodiorite; Q M, quartz monzonite; Q Sy, quartz syenite; Sye, syenite; Sy Di, syenodiorite; Sy Gb, syenogabbro. Fields of alkalinity modified slightly from those in DeWitt (1989) based on Ed DeWitt (unpub. data, 1994). (B) Alumina saturation diagram (Si02 versus A/CNK). A, molar Al20 3; C, molar CaO; N, molar Na20; K, molar K20. (C) Alkali classification diagram (K20/(K20+Na20) versus Si02). Field boundaries from Ed DeWitt (unpub data, 1994). (D) Iron enrichment classification diagram ((Fe0+0.89*Fe20 3)/(Fe0+0.89*Fe20 3+Mg0) versus Si02). Field boundaries slightly modified from those in DeWitt (1989) based on Ed DeWitt (unpub. data, 1994).

17 strongly peraluminous (fig. 12B). Elevated tos Granodiorite on Smuggler Mountain, but concentrations of Rb (340-380 ppm) are char­ not mapped in this reconnaissance. Much acteristic. granite is highly fractured and susceptible to In the northern Sawatch Range, the St. landslide failure, as exhibited by large rock Kevin has a seven-point whole-rock Rb-Sr avalanche that extends from Mount Shimer to isochron date of 1420 ± 100 Ma (recalculated below Weller Lake along the Roaring Fork by DeWitt using program of Ludwig, 1990, River. Consists, in decreasing amounts, of and data of Pearson and others, 1966). Large microcline, plagioclase, quartz, biotite, mus­ uncertainty in the Rb-Sr isochron date is covite, and trace amounts of apatite and mag­ probably due to inhomogeneity of initial netite. Grades into and intrudes main body Sr-isotope ratio, which averages 0.7034 ± (unit Yg) and intrudes metasedimentary 0.007. U-Pb zircon discordia upper intercept gneiss (unit Xms) date of 1395 ± 65 Ma (recalculated by DeWitt Yg Main body (Middle Proterozoic)-Medium­ using program of Ludwig, 1990, and data of tan to grayish-tan, medium- to coarse­ Doe ·and Pearson, 1969) is for two least dis­ grained, equigranular biotite tonalite to gran­ cordant zircon fractions and their acid leaches ite in large, irregularly zoned pluton in north­ or same two zircon fractions and a forced western part of map area. Contains lower discordia intercept of 70 Ma. conspicuous magnetite and sphene. Tectoni­ Two zircon fractions for an outcrop of St. cally undeformed, but slightly flow-foliated Kevin Granite that cuts metasedimentary in places. Named herein for exposures along gneiss (unit Xms) east of Italian Mountain, in the Roaring Fork River, at and below The the southwestern part of area, have a U-Pb Grottos, on the steep south-facing slopes of discordia upper intercept date of 1446 Ma Smuggler Mountain, and on the steep, north­ (fig. 13, table 3). No uncertainty is calculated east-facing slopes of New York Peak. Pluton for the regression, as we have only two dated extends north beyond map area into the zircon fractions. The date of 1446 Ma is Hunter and Fryingpan Creek drainages, as slightly older than the dates discussed above, indicated by surface geology and aeromag­ and may indicate that the St. Kevin Granite is netic signature (Campbell, 1981), and west, slightly older than previously thought. beneath Phanerozoic rocks, toward Castle Intrusive rocks of the Grottos pluton Creek. Prominent aeromagnetic high coin­ cides with central part of pluton (Campbell Y g I Leucocratic bodies (Middle Proterozoic)- and Wallace, 1986). Granodiorite consists, in Light-tan to creamy-tan, fine- to medium­ decreasing amounts, of plagioclase, quartz, grained, equigranular biotite-muscovite gran­ biotite, microcline, magnetite, sphene, and ite. In small outcrops north and south of The muscovite, and trace amounts of apatite, Grottos and southwest of Taggerts Lake. allanite, zircon, and epidote. Granite consists, Other small bodies probably present in Grot- in decreasing amounts, of quartz, microcline, plagioclase, biotite, muscovite, and magne­ OAOO ,------·- tite, and trace amounts of sphene, apatite, and St. Kevin Granite zircon. Intrudes schist and metasedimentary SampleiC-6 gneiss (unit Xms) and St. Kevin Granite (unit 0.300 Ys) in upper reaches of Lost Man Creek. Grades laterally into and also cuts discor­ r dantly its mafic border (unit Ygm). Grades 0.200 :c / (-200+270) laterally into and also is cut discordantly by tN '\ / its leucocratic part (unit Ygl). / 0.100 Composition of the body ranges from Yq,'~ (-50.+100) Upper intercept at 1446 Ma monzodiorite to granodiorite and averages ;//:. granodiorite (fig. 12A). All the compositions 0.000 0.00 1.00 3.00 4.00 are alkali-calcic (fig. 12A), are metaluminous to mildly peraluminous (fig. 12B), are aver­ age to potassic (fig. 12C), and mostly have no Figure 13. Concordia diagram for zircon from sample apparent iron enrichment (fig. 12D). Ele­

IC-6 of the St. Kevin Granite. Size of symbol represents vated concentrations of Ti02 (1-2 weight per­ 2-sigma analytical uncertainty. cent), P20 5 (0.5-0.9 weight percent), Sr

18 (300-500 ppm), and Zr ( 400-600 ppm) com­ rich. Characteristically, the granite has Sr and mon in granodiorite and tonalite contrast to Zr concentrations less than 100 ppm and U most 1400-Ma plutons in the southwestern concentrations of 4-14 ppm. United States. Exceptions are 1400-Ma mafic Middle Proterozoic age is based on Rb-Sr plutons such as the Mount Evans and Oak geochronology of Wetherill and Bickford Creek plutons in the Front Range of Colorado (1965) and U-Th-Pb geochronology by Ed (Aleinikoff and others, 1993; Noblett and oth­ DeWitt and R.E. Zartman (unpub. data, 1990). ers, 1987), which resemble the Grottos Gran­ Sample #C0-64 of Wetherill and Bickford odiorite. (1965), from along the Taylor River southwest Age assignment based on undeformed of the map area, has a whole-rock potassium nature, chemical similarity to known feldspar-plagioclase-muscovite isochron of 1400-Ma plutons in Colorado, and fact that 1380 ± 19 Ma (recalculated by DeWitt using body is cut by St. Kevin Granite program of Ludwig, 1990). One zircon frac­ Ygm Mafic border (Middle Proterozoic)-Dark­ tion from a sample along the Taylor River greenish-gray to dark-gray and tan, medium­ southwest of the map area has a 207Pbf206pb grained biotite monzodiorite and tonalite. date of 1406 Ma (Ed DeWitt and R.E. Zart­ Most abundant on southwestern margin of man, unpub. data, 1990) pluton near McArthur Mountain. Smaller YXs Granodiorite of Sayers (Middle or Early Prot- outcrops at Upper Grottos Campground near erozoic)-Medium- to dark-gray, medium- to mouth of Ptarmigan Creek and in north­ coarse-grained, equigranular to strongly por­ east-trending sill-like body between Midway phyritic biotite quartz syenite to granodior­ Pass and Scotts Lake. Very rich in biotite, ite. Mild to strong flow foliation strikes sphene, and magnetite. Monzodiorite and northeast and dips variably. Subsolidus fabric tonalite consist, in decreasing amounts, of present in southern outcrops of granodiorite. plagioclase, biotite, quartz, microcline, mag­ Composed, in decreasing amounts, of per­ netite, sphene, and epidote, and trace amounts thite, oligoclase to albite, quartz, biotite, chlo­ of apatite, allanite, and zircon. Notable for ritized biotite, opaque minerals, and minor, lack of hornblende. Intrudes schist and but conspicuous, apatite and zircon. Exposed metasedimentary gneiss (unit Xms). northwest of Winfield Peak and south of Say­ Monzodiorite and tonalite are alkali-calcic ers mineral monument. Intrudes Kroenke (fig. 12A), metaluminous (fig. 12B), potassic Granodiorite (unit Xk) and metasedimentary (fig. 12C), and are Fe rich to very Fe rich (fig. gneiss (unit Xms). Includes complex of lay­ 12D). Anomalously high concentration of Sr ered gneisses of Brock and Barker (1972).

(500-800 ppm), Ti02 (2-3 weight percent), Igneous rock has wide range in composi­ and P20 5 (1.5-2.0 weight percent) character­ tion, from quartz syenite to granodiorite. ize the mafic border Quartz syenite is alkalic (fig.12A), mildly Yt Granite of Taylor River (Middle Protero­ peraluminous (fig. 12B), very potassic (fig. zoic)-Light-tan, fine- to medium-grained, 12C), and very iron rich (fig. 12D). Grano­ equigranular to slightly porphyritic musco­ diorite is calc-alkalic (fig. 12A), mildly pera­ vite-biotite granite. Informally named by luminous (fig. 12B), and has no preferred DeWitt and others (1985) for exposures in the alkali or iron enrichment (figs. 12C and D). Fossil Ridge area, southwest of the map Characteristically, the granodiorite of Sayers

area. Crops out southwest of Dinner Station has high K20 (5-6.3% ), high Zr and Sr (500 Campground in Taylor Park and along Illinois to 800 ppm each), and high Ba (2800-4500 Creek. Intrudes metasedimentary gneiss (unit ppm). Xms), quartzite (unit Xq), and granite of Age assignment based on flow foliated to Henry Mountain (unit Xh). Undeformed and slightly tectonized texture, unusual alkalic unaltered except where cut by extensive composition, which is indicative of 1.4-Ga northwest-striking, high-angle shear zone in plutons in the southwestern United States (E. far southern part of area. Extends southwest DeWitt, unpub. data, 1993), and fact that the into Fossil Ridge area where it forms a pluton body intrudes the Kroenke Granodiorite 18 by 18 km in extent. Was included in parts YXbp Browns Pass Granite (Middle or Early Protero­ of units Yg and YXg ofTweto (1979). zoic)-Light-tan, medium-grained, equi­ Chemically the granite is alkali-calcic, granular, undeformed, leucocratic biotite­ strongly peraluminous, and is average to iron muscovite granite and aplite. Originally

19 named the Browns Pass Quartz Monzonite undeformed, but compositionally layered, (Barker and Brock, 1965), the body is leucocratic biotite granodiorite, granite, and redescribed herein as granite because of new biotite aplite. Minor subsolidus fabric locally chemical analyses, map relations, and addi­ overprints igneous compositional layers. tional petrographic studies. Exposed in south­ Major rock unit in eastern third of map area; ern part of area, from southwestern Taylor smaller outlier west of Mount Elbert. Equiv­ Park to Avalanche Gulch on east slope of alent to granodiorite of Fairview Peak in Fos­ Mount Yale. Small masses of Browns Pass sil Ridge area to the southwest (De Witt and Granite along Langhoff Gulch at eastern edge others, 1985). Not recognized in Sawatch of map and east of Twin Lakes Reservoir at Range north of map area (Tweto, 1974; Van northeastern edge of map may be part of sepa­ Loenen, 1985; Wallace and others, 1986). rate 1.4-Ga intrusive body along Big Union Intrudes metasedimentary gneiss (unit Xms), Creek to the east of the map. Intrudes Denny Creek Granodiorite (unit Xd), gneissic Kroenke Granodiorite (unit Xk), granite of granodiorite (unit Xgd), and granite of Mount Mount Yale (unit Xy), Denny Creek Grano­ Yale (unit Xy). Thin slivers of metasedimen­ diorite (unit Xd), metasedimentary gneiss tary gneiss (unit Xms) present along much (unit Xms), and quartzite (unit Xq). Consists of the contact between Kroenke and Denny of medium-grained, equigranular to porphy­ Creek, as if Kroenke preferentially intruded ritic biotite granite, fine-grained, equigranular the metasedimentary gneiss unit. Overall, biotite-muscovite granite, and fine-grained, leucocratic rock has compositional layers leucocratic aplitic masses and dikes. Granite defined by biotite-rich and biotite-poor mate­ consists, in decreasing order of microcline and rial. Complexly layered Kroenke Granodior­ perthite, quartz, characteristic turbid oligo­ ite resembles schlieren or migmatite in clase, biotite and chloritized biotite, opaque outcrop. Consists, in decreasing order, of minerals, muscovite, and minor apatite, allan­ plagioclase, quartz, potassic feldspar, biotite ite, and zircon. Aplite contains same miner­ and biotite altered to chlorite, opaque miner­ als, but lesser amounts of opaque minerals, als, hornblende, and minor amounts of biotite, allanite, and zircon. sphene, apatite, allanite rimmed by epidote, The granite is alkali-calcic (fig. 12A), and zircon. Aplitic varieties have less biotite, mildly to strongly peraluminous (fig. 12B), sphene, and opaque minerals than main gran­ and has no preferred alkali enrichment and no odiorite. iron enrichment (figs. 12C and D). Compared Body ranges from granodiorite to granite to most 1.4-Ga plutons in the southwestern (fig. 14A), is alkali-calcic to calc-alkalic (fig. United States and Colorado (Ed DeWitt, 14A), mildly to strongly peraluminous (fig. unpub. data, 1993), the Browns Pass has low 14B), sodic to very sodic (fig. 14C), and aver­ Rb concentrations (average of 200 ppm) for age to Mg rich (fig. 14D). Distinctive fea­ an evolved granite. Aplitic bodies and dikes tures of the Kroenke include its Na-rich and of the granite have a chemistry similar to the Mg-rich nature, and low concentrations of Zr

main granite (fig. 12). and P20 5 (175 ppm and 0.14 weight percent, Middle or Early Proterozoic age assign­ respectively). Barker and others (1976) clas­ ment based on fact that aplitic masses and sify the Kroenke as a trondhjemite because of dikes of Browns Pass cut the Kroenke Grano­ its sodic chemistry. diorite and that the chemistry of the granite is Early Proterozoic age assignment based on similar to dated 1.4-Ga plutons in central Col­ published Rb-Sr and unpublished U-Pb orado. Barker and Brock ( 1965) suggested sphene analyses. Barker and others (1974) that the Browns Pass was older than the obtained a 1670 ± 100 Ma date (Model 1 date Kroenke because brecciated rocks on the recalculated by De Witt using the program of western slope of Mount Yale that they Ludwig, 1990) from 6 whole-rock samples of inferred were older than the Browns Pass Kroenke in the Sawatch Range (also cited by were cut by the pluton. These brecciated Reed and others, 1987). A U-Pb sphene date rocks were not identified as Kroenke in their of 1645 ± 5 Ma was obtained from a sample mapping. of Kroenke just south of Rockdale, along Xk Kroenke Granodiorite (Early Proterozoic)­ Clear Creek (J. Aleinikoff and J. Reed, unpub. Light-gray, medium-grained, equigranular, data, 1991)

20 Henry Mountain pluton 1.3 .------:;:::-=------, • main body, outside Collegiate Peaks (unit Xh) Strongly Peraluminous • main body, inside Collegiate Peaks (unit Xh) 1.2 Mildly Peraluminous • leucocratic granite, inside Collegiate Peaks (unit Xhl) o Kroenke Granodiorite (unit Xk)

1.1 Granite of Mount Yale \ ,/--- main body (part of unit Xy) / aplite (part of unit Xy) I questionable B Verysodic

X c

ISOO Very potassic i Kpi(Kp +Nap) + I + soo i (FeO + 0.89FeP3)1(Fe0 + 0.89Fe p J+ Mg()) I r;t."' A

L----~---~---~----~ I 000 I SOO 2000 2SOO 3000

R =4000Si-ll,OOO(Na + K)- ZOOO(Fe + Ti) 1

Figure 14. Summary geochemical diagrams for some 1.65-Ga and older plutons in the central Sawatch Range area. Plot includes the Kroenke Granodiorite, granite of Henry Mountain, and granite of Mount Yale. Data from Ed De Witt (unpub. data, 1994), Barker and Brock (1965), and Barker and others (1976). (A) R1R2 major element classification diagram (De la Roche and others, 1980). Alk Gr, alkali granite; Gr, granite; Gd, granodiorite; Ton, tonalite; Di, diorite; Gb-Di, gabbro-diorite; Mz Gb, monzogabbro; Mz Di, monzodiorite; Q M, quartz monzonite; Q Sy, quartz syenite; Sye, syenite; Sy Di, syenodiorite; Sy Gb, syenogabbro. Fields of alkalinity modified slightly from those in DeWitt (1989) based on Ed DeWitt (unpub. data, 1994). (B) Alumina saturation diagram (Si02 versus NCNK). A, molar Al20 3; C, molar CaO; N, molar Na20; K, molar K20. (C) Alkali classification diagram (K20/(K20+Na20) versus Si02). Field boundaries from Ed DeWitt (unpub data, 1994). (D) Iron enrichment classification diagram ((Fe0+0.89*Fe20 3)/(Fe0+0.89*Fe20 3+Mg0) versus Si02). Field boundaries slightly modified from those in DeWitt (1989) based on Ed DeWitt (unpub. data, 1994).

Intrusive rocks of the Henry Mountain pluton Henry Mountain. Consists, in decreasing Xhi Leucocratic granite bodies (Early Protero­ amounts, of microcline, quartz, sodic oligo­ zoic)-Light-tan to creamy-white, medium­ clase, muscovite, biotite, minor opaque min­ grained, equigranular to slightly porphyritic, erals, and trace amounts of apatite and leucocratic granite to alaskite. Exposed to zircon. the east and west of Taylor Pass, at the The leucocratic granite to alaskite is on the northwest head of Taylor Park. Includes the border of alkali-calcic and calc-alkalic (fig. granite (unit p£g) of Bryant (1970). Gran­ 14A), is mildly to strongly peraluminous (fig. ite to alaskite cuts and is gradational into 14B), and has no apparent alkali or iron granite of Henry Mountain (unit Xh). Leu­ enrichment (figs. 14C and D). cocratic body contains distinctive quartz The leucocratic granite is assumed to be a phenocrysts but lacks the microcline phenoc­ late-crystallizing equivalent of the granite of rysts and abundant biotite of the granite of Henry Mountain

21 Xh Granite of Henry Mountain (Early Protero­ compositionally layered rock, having layers zoic)-Pinkish-tan and black, coarse­ of biotite-rich and biotite-poor granodiorite to grained, undeformed, strongly porphyritic granite. In many western exposures, layers biotite-muscovite granite. Informally named are buckled and folded, giving the rock the by De Witt and others ( 1985) for exposures in appearance of "tiger stripes." Intrudes Fossil Ridge area to southwest of map area. metasedimentary gneiss (unit Xms) and Includes the porphyritic quartz monzonite gneissic granodiorite (unit Xgd). Consists, in (unit p£pq) of Bryant (1970) in northern decreasing amounts, of oligoclase, quartz, Taylor Park. Exposed continuously along the perthite, biotite, opaque minerals, and minor southwestern side of Taylor Park, from north amounts of sphene, zircon, apatite, and allan­ of Taylor Pass to south of Dinner Station ite. Campground. Intrudes metasedimentary Chemically, leucocratic or "tiger stripe" gneiss (unit Xms) and gneissic granodiorite part ranges from granodiorite to granite, is (unit Xgd). In Hunter-Fryingpan Wilderness alkali-calcic to calc-alkalic (fig. 15A), to north of map area, Hell Gate Porphyry strongly peraluminous (fig. 15B), and has no (Tweto and Pearson, 1964) is the northern strong alkali or iron enrichment (figs. 15C, equivalent of the granite of Henry Mountain. D). Similar to the main Denny Creek Grano­ Granite contains conspicuous phenocrysts of diorite (unit Xd), the leucocratic body has ele­ perthitic microcline and quartz. Consists, in vated concentrations of Zr and Y decreasing amounts, of microcline, quartz, Xd Main body of Denny Creek Granodiorite albite to sodic oligoclase, biotite, muscovite, (Early Proterozoic)-Dark-grayish brown to magnetite, and ilmenite, and minor amounts greenish-black, coarse-grained, equigranular of apatite, zircon, allanite, and epidote. to strongly porphyritic, undeformed to The body ranges from granodiorite to gran­ strongly tectonically foliated biotite-rich gra­ ite, is alkali-calcic (fig. 14A), mildly to nodiorite. Exposed from Independence Pass strongly peraluminous (fig. 14B), is slightly to beyond southern border of map area. Orig­ potassic (fig. 14C), and has no iron enrich­ inally named the Denny Creek Granodiorite ment (fig. 14D). The granite is characterized Gneiss (Barker and Brock, 1965). Granodior­ by moderate to high Y (40-60 ppm) and low ite is undeformed east of a line from west of Ba (many analyses < 550 ppm) for a rock that Mount Elbert, through Vicksburg, west of the contains so much potassium feldspar. summits of Mount Oxford and Mount Har­ Early Proterozoic age assignment based on vard, and on to the mouth of Cottonwood published and unpublished U-Th-Pb geochro­ Creek. Denny Creek Granodiorite south of nology. Bickford and Boardman (1984) and Texas Creek to Cottonwood Pass is mostly Bickford and others (1989) report a U-Pb undeformed, but contains some zones of upper intercept date of 1701 ± 10 Ma for zir­ highly strained granodiorite. Much of the con from the granite at Indian Head, south of Denny Creek north of Texas Creek and south the Fossil Ridge area. This date is in agree­ of Hamilton is strongly tectonized. Exposed ment with a 207Pbf2°6Pb date of 1693 Ma from both at type area along Denny Creek, south­ one fraction of zircon collected from a sample west of Mount Yale, and along the cliffs on of the granite along the Taylor River, south­ either side of Clear Creek, below Vicksburg. west of the map area (Ed De Witt and R.E. Denny Creek intrudes metasedimentary Zartman, unpub. data, 1990) gneiss (unit Xms), quartzite (unit Xq), and Intrusive rocks of the Denny Creek Pluton gneissic granodiorite (unit Xgd). Granodior­ Xdl Leucocratic bodies (Early Proterozoic)­ ite consists, in decreasing amounts, of oligo­ Tawny-brown to brownish-gray, medium­ clase, quartz, perthite, biotite, ilmenite and grained, equigranular, compositionally lay­ magnetite, and minor amounts of sphene, apa­ ered and tectonically foliated biotite grano­ tite, allanite, zircon, and chlorite. Distin­ diorite. Exposed in central part of area, from guishing features include the abundance of southwestern margin of Grizzly Peak caldera biotite (as much as 20 volume percent), vari­ along Tellurium Creek, to south of the Three able but high percentage of sphene, and low Apostles, north of Texas Creek. Also concentration of hornblende. exposed along northwestern margin of Griz­ The body ranges from monzodiorite to gra­ zly Peak caldera. Grades into main Denny nodiorite, but averages granodiorite (fig. Creek Granodiorite (unit Xd). Typically is 15A). Most compositions are calc-alkalic, but

22 1.3,------=-:=------, Strongly Peraluminous Denny Creek pluton 1.2 + main body (unit Xd) Mildly Peraluminous _----· • • main body in brec~ia sheets '\ --+ -f "tiger stripe" part of leucocratic body (part of unit Xdl) 1.1 e .. \ .. /---.-~ :-.: + questionable Denny Creek / ~-J+ •• o Gabbro (unit Xgb) ,'I...... 0.9 I I •• Verysodic I I I I ,. • -----/--t~_ B I I I 0.8 0 / I _// Field of unaltered I / I Metalumin~/// igneous rocks I .I ' 0.7 '----lJ--'--"-----'----L----'-T---'-----' 70 • . so 60 ~ ·~/. ~+. .• ' / / Mg rich / Average rich.' Very Fe rich ~a ....., .. , + . I I I I c I I .... -: I I I I 1+ I 1500 •• I I Very potassic f. ' I /+ .,/. 0.4 ~ VfllY Mg rich ./. Kpl(Kp +Nap) /.~-·~ i 1000 ••• + •• • D f D. + 500 i (FeO + 0.89Fe:P3)1(Fe0 + 0.89Fe p J'" MgO) u ~N I AltGr A

1000 1500 2000 2500 3000

R = 400081 -ll,OOO(Na + K) -lOOO(Fe + Ti) 1

Figure 15. Summary geochemical diagrams for some 1.70-Ga and older plutons in the central Sawatch Range area. Plot includes the Denny Creek Granodiorite and unnamed gabbro. Data from Ed DeWitt (unpub. data, 1994), and Barker and Brock (1965). (A) R 1R 2 major element classification diagram (De Ia Roche and others, 1980). Alk Gr, alkali granite; Gr, granite; Gd, granodiorite; Ton, tonalite; Di, diorite; Gb-Di, gabbro-diorite; Mz Gb, monzogabbro; Mz Di, monzodiorite; Q M, quartz monzonite; Q Sy, quartz syenite; Sye, syenite; Sy Di, syenodiorite; Sy Gb, syenogabbro. Fields of alkalinity modified slightly from those in DeWitt (1989) based on Ed DeWitt (unpub. data, 1994). (B) Alumina saturation diagram (Si02 versus A/CNK). A, molar AI20 3; C, molar CaO; N, molar Na20; K, molar K20. (C) Alkali classification diagram (K20/(K20+Na20) versus Si02). Field boundaries from Ed DeWitt (unpub data, 1994). (D) Iron enrichment classification diagram ((Fe0+0.89*Fe20 3)/(Fe0+0.89*Fe20 3+Mg0) versus Si02). Field boundaries slightly modified from those in DeWitt (1989) based on Ed DeWitt (unpub. data, 1994).

some granodiorite is alkali-calcic, probably for a rock so low in Si02• Also, the granodior­ the result of early crystallization of abundant ite does not become more Fe rich as differen­ biotite (fig. 15A). The most mafic bodies are tiation index increases. metaluminous, but much of the granodiorite Age designation based on observation that ranges from moderately to strongly peralumi­ Denny Creek is cut by the Kroenke Granodior­ nous (fig. 15B); as a whole the Denny Creek is ite and that the Denny Creek has been region­ mildly peraluminous. No strong alkali enrich­ ally deformed, possibly twice, in its western ment patterns are evident (fig. 15C). Most of exposures. No geochronologic data are avail­ the compositions are average to very Fe rich able for the Denny Creek Granodiorite (fig. 15D). Characteristic features of the Xkd Kroenke Granodiorite and Denny Creek Grano­ Denny Creek are its high Zr concentration diorite, undivided (Early Proterozoic)­ (averaging 400 ppm), high Y concentration Mixture of Denny Creek Granodiorite, much

(50-70 ppm), low slope on the R1R2 diagram, of it strongly deformed, and Kroenke Grano­ and mildly to strongly peraluminous character diorite. Includes minor bodies of amphibolite

23 (unit Xam) and metasedimentary gneiss (unit Ridge area to the southwest range in composi­ Xms). Exposed at and near the Three Apostles tion from tonalite to granite, are ca~c-alkalic Xdi Diorite (Early Proterozoic)-Medium-gray­ to alkali-calcic, and moderately to strongly green, medium-grained, equigranular to peraluminous. slightly porphyritic, well flow-foliated Early Proterozoic age assignment based on plagioclase-hornblende diorite. Prominent fact that bodies are cut by the granite of dike exposed south of Alan Lake where it cuts Henry Mountain, the Denny Creek Grano­ metasedimentary gneiss (unit Xms) and is cut diorite, and the Kroenke Granodiorite by Kroenke Granodiorite (unit Xk) and by Xy . Granite of Mount Yale (Early Proterozoic)- simple pegmatite bodies believed to be related Light-cream to white, fine- to coarse- grained, to granodiorite of Sayers (unit YXs). Body of equigranular to porphyritic, massive to foli­ metasedimentary gneiss, Kroenke ated, leucocratic granite to alaskite. Exposed Granodiorite, and simple pegmatite bodies at on Mount Yale and possibly within gneissic Alan Lake too small to be shown on map. granodiorite (unit Xgd) to the east of Mount Tectonic fabric locally superimposed on flow Yale. Intrudes metasedimentary gneiss (unit foliation near contact with metasedimentary Xms) and gabbro (unit Xgb). Granite of gneiss. Contact between diorite and Mount Yale and gneissic granodiorite (unit granodiorite of Sayers (unit YXs) obscured by Xgd) on east slopes of Mount Yale are inter­ intermediate-composition porphyry dike of layered~ age relations of the two are equivo­ the Twin Lakes pluton that is not shown on the cal. Includes complex of brecciated gneisses map because of its small size. Consists, in of Brock and Barker (1972). Typically has decreasing order, of poorly twinned two parts, a coarse-grained, equigranular to plagioclase, phenocrysts of hornblende, porphyritic, perthite-rich biotite granite, and a perthite, quartz, biotite, epidote, opaque fine-grained, equigranular, leucocratic to alas­ minerals, and minor amounts of allanite, kitic aplite that may or may not be associated apatite, sphene, and zircon. No chemical or with pegmatite bodies. Biotite granite con­ geochronologic data are available to further sists, in decreasing order, of perthite and refine the age of the diorite microcline, quartz, biotite, plagioclase, mag­ Xgd Gneissic granodiorite (Early Proterozoic)­ netite, muscovite, apatite, and minor amounts Homogeneous to heterogeneous gneissic gra­ of allanite. Aplite consists of perthite and nodiorite to granite bodies, many having lay­ microcline, quartz, plagioclase, magnetite, ers of alternating biotite-rich and biotite-poor biotite, muscovite, and minor amounts of zir­ material. Superimposed subsolidus fabric con. common, but not widespread. Exposed along The main body ranges from granodiorite to Bowman Creek at northwest end of Taylor granite, is alkali-calcic (fig. 14A), mildly to Park, at the head of Sayers Gulch, at Grizzly strongly peraluminous (fig. 14B), and pre­ Peak south of Jenkins Mountain, on the south dominantly shows no strong alkali or iron slope of Mount Elbert, and along the heavily enrichment (figs. 14C and D). The aplite is an forested east-facing slopes of Mount Harvard, alkali granite (fig. 14A), strongly peralumi­ Mount Columbia, and Mount Yale. Bodies at nous (fig. 14B), and very Fe rich (fig. 14D). the first three localities are homogeneous, lay­ Characteristic features of the granite are its ered biotite granodiorite. Those at the other Rb-rich and Zr-rich nature (150-175 ppm and localities are more heterogeneous, range from 180-600 ppm, respectively), and low Sr con­ biotite· granodiorite to leucocratic granite, and centration (40-160 ppm). contain inclusions of metasedimentary gneiss. Early Proterozoic age assignment based on Outcrops east of Mount Yale contain great observation that granite of Mount Yale is variety of plutonic rock types and migmatite. intruded by the Denny Creek Granodiorite Intrudes metasedimentary gneiss (unit and Kroenke Granodiorite Xms). Consists, in decreasing order, of per­ Xg b Gabbro and metagabbro (Early Protero- thite, oligoclase, quartz, biotite, muscovite, zoic)-Dark-greenish black, medium- to opaque minerals, and minor amounts of zir­ coarse-grained, equigranular to slightly foli­ con, allanite, apatite, and sphene. ated hornblende gabbro. Mappable bodies There are no chemical data for bodies . most common in southeastern part of map within the map area. However, similar area, from north of Cottonwood Pass to east of gneissic granodiorite bodies in the Fossil Mount Yale. Smaller, unmapped bodies are

24 along Frenchman and Three Elk Creeks, east caldera. Interlayered amphibolite, actinolite of Mount Columbia. Intrudes metasedimen­ gneiss, and intermediate-composition tary gneiss (unit Xms), but is most numerous metavolcanic rocks are most common south as large inclusions in Denny Creek Granodior­ of Mount Elbert, in the northwest-trending ite (unit Xd) and granite of Mount Yale (unit belt at the Three Apostles, and in the Xy). Consists, in decreasing order, of north-trending belt east of Mount Yale. Typi­ coarse-grained aggregates of plagioclase and calrock types in northwest-trending belt at the hornblende in a finer grained matrix of quartz, Three Apostles include: amphibolite gneiss; hornblende, biotite, and minor amounts of biotite-amphibole gneiss; migmatite; quartz­ allanite, sphene, and opaque minerals. feldspar gneiss; quartz-feldspar-biotite gneiss Chemical data for this unit are minimal. and schist; fine-grained, equigranular granite The two samples indicate that these mafic gneiss; actinolite-rich amphibolite; and minor rocks range from gabbro to gabbro-diorite pegmatite and aplite bodies arranged parallel (fig. 15A; gabbro plots above the diagram), to compositional layers. Mica schist is most are alkali-calcic (fig. 15A), metaluminous common in the mass southwest of Dinner Sta­ (fig. 15B), potassic to very potassic (fig. 15C), tion Campground on the western edge of Tay­ and Mg rich to very Mg rich (fig. 15D). At lor Park. Mass on south flank of Mount Yale least two suites probably are present, one with contains gneissic granite and mixed gneiss moderate Sr concentrations (400-600 ppm) (unit Xgn). Protoliths for the metasedimen­ and one with very high Sr concentrations tary gneiss unit likely include graywacke, sili­ ( 1,500-1,800 ppm) ceous graywacke, siltstone, and minor Xam Amphibolite (Early Proterozoic)-Dark- amounts of intermediate-composition volca­ greenish-black, medium-grained, equigran­ nic rocks and basalt. Chemical data are not ular, well-foliated hornblende-plagioclase available for most of the rock types. Includes gneiss and amphibole schist. Exposed in small biotite-quartz-plagioclase gneiss of Brock and bodies west of Harrison Flat, east of Mount Barker (1972). Yale, and south of Cottonwood Pass. Many Metasedimentary gneiss along the upper amphibolite masses are too small to be shown reaches of Sayers Gulch in the Grizzly Peak at the scale of the map. Relict pillow textures caldera is extensively brecciated near expo­ preserved in outcrops south of Cottonwood sures of felsic porphyry dikes (unit Ttf) of the Pass and farther south in Tincup area (Brock Twin Lakes pluton. The breccia contains and Barker, 1972) suggest that amphibolite angular fragments of gneiss, but in several bodies are metamorphosed basalt flows. areas lacks a matrix, as variously rotated Includes hornblende-plagioclase gneiss and pieces of gneiss abut each other. Brecciation amphibolite of Brock and Barker (1972). increases in proximity to the felsic porphyry Consists, in decreasing order, of andesine, dikes of the Twin Lakes pluton. Gas stream­ hornblende, biotite, quartz, opaque minerals, ing and fluidization above the advancing and minor amounts of sphene and apatite magma are believed to have created the brec­ Xms Metasedimentary gneiss (Early Proterozoic)- cia that contains no matrix, but solution col­ Heterogeneous assemblage of quartzofelds­ lapse also could have contributed to the pathic gneiss, micaceous gneiss, amphibolite, texture of the breccia and minor granitic intrusive rocks. Unit wide­ Xq Metasedimentary gneiss and quartzite (Early spread in map area. Largest mass is northwest Proterozoic)-Light-gray to tan, fine-grained of Grizzly Peak caldera, but extends from Tay­ to pebbly, undeformed to strongly deformed lor Pass to Deer Mountain, and on to Mount quartzite, quartz-pebble conglomerate, and Elbert. Smaller masses in the southern part of mica schist. Quartzite and conglomerate grade the Grizzly Peak caldera near Lake Pass, south into graywacke and siliceous graywacke of of Twin Lakes Reservoir, east of Mount unit Xms. Exposed north of Illinois Creek and Columbia, east and west of Mount Yale, south to the west of Prospector Gulch in Taylor Park of Cottonwood Pass, and in northwest-trend­ and south of Cottonwood Pass. Conglomerate ing belts at the Three Apostles and on the west and quartzite beds best developed northeast of side of Taylor Park. Texas Lakes. Aluminous graywacke and Quartzofeldspathic gneiss contammg pelites most common south of Cottonwood biotite, muscovite, garnet, and sillimanite is Pass where andalusite, garnet, and sillimanite common north and west of the Grizzly Peak are noted (Brock and Barker, 1972). Includes

25 biotite-muscovite-quartz-plagioclase schist Barker, Fred, Peterman, Z.E., Henderson, W.T., and Hil­ and muscovite-quartz schist units of Brock dreth, R.A., 1974, Rubidium-strontium dating of the and Barker (1972). Unit separated from trondhjemite of Rio Brazos, New Mexico, and of the metasedimentary gneiss (unit Xms) because Kroenke Granodiorite, Colorado: U.S. Geological Sur­ quartzite, conglomerate: and schist more sim­ vey Journal of Research, v. 2, p. 705-709. ilar to metasedimentary rock units in Fossil Barker, Fred, Arth, J.G., Peterman, Z.E., and Friedman, Irv­ Ridge area to southwest (De Witt and others, ing, 1976, The 1.7-1.8-b.y.-old trondhjemites of south­ 1985) and to schist in the Tincup area to the western Colorado and northern New Mexico; south (Brock and Barker, 1972) Geochemistry and depths of genesis: Geological Soci­ Xgn Mixed gneiss, mica schist, and gneissic granite ety of America Bulletin, v. 87, p. 189-198. (Early Proterozoic)-Mixture of light-gray Bickford, M.E., and Boardman, S.J., 1984, A Proterozoic migmatitic gneiss, mica schist, and gneissic volcano-plutonic terrane, Gunnison and Salida areas, granite. Equivalent to brecciated gneiss unit Colorado: Journal of Geology, v. 92, p. 657-666. of Brock and Barker (1972). Exposed in far Bickford, M.E., Shuster, R.D., and Boardman, S.J., 1989, southeastern part of map area, south of Cot­ U-Pb geochronology of the Proterozoic volcano­ tonwood Creek. Not mapped in this study; plutonic terrane in the Gunnison and Salida areas, distribution on map from Brock and Barker Colorado, in Grambling, J.A., and Tewksbury, B.J., (1972) eds., Proterozoic geology of the southern Rocky Mountains: Geological Society of America Special Paper 235, p. 33-48. ACKNOWLEDGMENTS Brock, M.R., and Barker, Fred, 1972, Geologic map of the Mount Harvard Quadrangle, Chaffee and Gunnison Counties, Colorado: U.S. Geological Survey Geologic Numerous individuals and organizations assisted our Quadrangle Map GQ-952, scale 1:62,500. mapping in the Collegiate Peaks Wilderness Area and the Bryant, Bruce, 1970, Geologic map of the Hayden Peak surrounding parts of the central Sawatch Range. The U.S. quadrangle, Pitkin and Gunnison Counties, Colorado: Forest Service offices in Leadville, Aspen, Gunnison, and U.S. Geological Survey Geologic Quadrangle Map Salida provided assistance in scheduling helicopter flights GQ-863, scale 1:24,000. into the area for mapping and sample collection. We partic­ ---1971, Geologic map of the Aspen quadrangle, Pitkin ularly would like to thank Gene Eide, Don Rogers, Jamie County, Colorado: U.S. Geological Survey Geologic Lind, Barney Lyons, and James Paxon, Jr., for their assis­ Quadrangle Map GQ-933, scale 1:24,000. tance. Personnel from the U.S. Bureau of Mines shared their ---1979, Geology of the Aspen 15-minute quadrangle, knowledge of mining activity and mineral prospects in the . Pitkin and Gunnison Counties, Colorado: U.S. Geolog­ area. We thank Rick Tschauder, Steve Ludington, Alan Wal­ ical Survey Professional Paper 1073, 146 p. lace, Jim Shannon, and Rich Van Loenen for sharing their Bryant, Bruce, and Naeser, C.W., 1980, The significance of knowledge of ore deposits and regional geology of the area fission-track ages of apatite in relation to the tectonic with us. The manuscript was reviewed by Alan Wallace and history of the Front and Sawatch Ranges, Colorado: Karl Evans and benefited from their comments. Geological Society of America Bulletin, v. 91, p. 156-164. Bryant, Bruce, Naeser, C. W., and Stegen, R.G., 1990, Reconnaissance fission-track geochronology of the REFERENCES CITED Aspen mining district, central Colorado, in Beaty, D. W., Landis, G. P., and Thompson, T. B., eds., Carbon­ Aleinikoff, J.N., Reed, J.C., Jr., and DeWitt, Ed, 1993, The ate-hosted sulfide deposits of the Colorado Mineral Mount Evans batholith in the Colorado Front Range; Belt: Economic Geology Monograph 7, p. 301-307. revision of its age and reinterpretation of its structure: Campbell, D.L., 1981, Aeromagnetic and complete Bouger Geological Society of America Bulletin, v. 105, p. gravity anomaly maps of the Hunter-Fryingpan Wilder­ 791-806. ness Area, Pitkin County, Colorado: U.S. Geological Barker, Fred, and Brock, M.R., 1965, Denny Creek Grano­ Survey Miscellaneous Field Studies Map MF-1236-C, diorite Gneiss, Browns Pass Quartz Monzonite, and 2 sheets, scale 1:50,000. Kroenke Granodiorite, Mount Harvard Quadrangle, Campbell, D.L., and Wallace, A.R., 1986, Aeromagnetic Colorado, in Cohee, G.V., and West, W.S., eds., map of the Holy Cross Wilderness Area, Eagle, Lake, Changes in stratigraphic nomenclature by the U.S. and Pitkin Counties, Colorado: U.S. Geological Sur­ Geological Survey, 1964: U.S. Geological Survey Bul­ vey Miscellaneous Field Studies Map MF-1841-B, letin 1224-A, p. A23-A26. scale 1: 100,000.

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