Geologic Map of the Collegiate Peaks Wilderness Area and the Grizzly Peak Caldera, Sa Watch Range, Central Colorado

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Geologic Map of the Collegiate Peaks Wilderness Area and the Grizzly Peak Caldera, Sa Watch Range, Central Colorado 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- 39Ar 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.
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