Geologic Map of the Maysville Quadrangle, Chaffee County, Colorado

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about N27°E and dips 74°NW. It cuts the Mount Princeton pluton (Tmpp) and comes within about 400 ft of the California leucogranite intrusion (Tcm). The second dike occurs as a composite dike along the central portion of the main northeast-trending rhyolite porphyry dike (Trp) that straddles Squaw Creek. For about 4,500 ft, this dike is composite, consisting of two phases present in the same dike structure. One phase is moderate-high phenocryst, rhyolite porphyry (Trp) with an aphanitic groundmass and the other phase is a moderate phenocryst, aplite to fine-grained granite. The dike is poorly exposed and is expressed by a mappable concentrated float zone. One float sample showed a sharp contact between the two phases, suggesting a composite dike rather than a zoned dike with the quenched rhyolite phase being the earlier intrusion. Shannon (unpublished data, 1981) conducted a modal analysis (2,000 points on slab stained for alkali feldspar) of the California fine-grained granite-rhyolite dike from the composite Trp-Tcf dike just within the boundaries of the Maysville quadrangle and about 3,800 ft west of Shavano Lake. The total mode shows about 24.5 percent quartz, 40.6 percent alkali feldspar, 34.3 percent plagioclase, 0.3 percent biotite-muscovite, and 0.3 percent magnetite. According to IUGS classification, California fine-grained graniterhyolite dikes are granite b. However, if plagioclase is albitic (An 5 or less) the rock would have a modified classification of alkali-feldspar granite (Ramsay and others, 1985). In general, the fine-grained granite dikes tend to have higher biotite-muscovite, garnet, and magnetite contents in comparison to the rhyolite porphyry dikes (Trp). Tcm California leucogranite (early Oligocene) – Most of this intrusion is in the Mount Antero quadrangle but a small sliver (about 400 feet long and less than 100 feet wide) is present on the north boundary of the Maysville quadrangle, about 2,500 feet west of Shavano Lake. The 2,000 by 900 foot rectangular intrusion of medium-grained biotitegarnet leucogranite (Tcm) occurs in the head of Squaw Creek. This body was considered to be a satellite body of the main California leucogranite intrusion in Browns Creek in the Mount Antero quadrangle (Shannon, 1988). The western contact is well exposed and is a north-northwest-trending fault contact between leucogranite and the Mount Princeton pluton (Tmpp). The eastern contact is not exposed but is also interpreted to be a north- 80
northwest-trending fault. Thus, field relations suggest that the leucogranite is exposed in a narrow horst block. The California leucogranite is a white, medium-grained, equigranular, biotitegarnet granite. Unpublished data from Shannon (1984) included a modal analysis (2010 points on slab stained for alkali feldspar) on a sample of the satellite California leucogranite intrusion from the Mount Antero quadrangle (about 1,400 feet north of the northern boundary of the Maysville quadrangle). The mode showed 27.0 percent quartz, 35.3 percent alkali feldspar, 35.2 percent plagioclase, 1.4 percent biotite-muscovite, 0.6 percent magnetite, and 0.4 percent garnet. Orange to reddish garnet is conspicuous in most samples of the California leucogranite and locally ranges up to about 2.0 percent of the rock. Mineral separates on samples of the California leucogranite intrusion from the Mount Antero quadrangle indicate the accessory mineral suite includes garnet, magnetite, ilmenite, monazite, fluorite, zircon, and rare tourmaline. The California leucogranite is classified as granite b (IUGS classification) or alkali-feldspar granite (Ramsay and others, 1985) depending on the composition of the plagioclase. Shannon (1988) compiled whole-rock chemical analyses of the main California intrusion in the Mount Antero quadrangle, including four samples of fine-grained granite border unit and dikes (Tcf) and two samples of medium-grained leucogranite (Tcm). The analyses show similar chemistry of alkali granite compositions (De la Roche and others, 1980) with very high SiO 2 (75.5 to 78.3 percent), K 2 O (4.1 to 5.5 percent), and Na 2 O (3.4 to 4.6 percent). Overall higher silica and rubidium contents in the California leucogranite compared with the Mount Antero leucogranite suggest a chemically, slightly moreevolved composition. The California leucogranite is younger than the Mount Princeton pluton (Tmpp, Tmpm and Tmpk phases), the Mount Aetna quartz monzonite ring dikes (Tma), and the Mount Antero leucogranite intrusion on the basis of cross-cutting relationships in the Mount Antero quadrangle (Shannon, 1988). The relative ages of the California and North Fork leucogranite intrusions could not be established by cross-cutting relations. Shannon (1988) suggested the similar mineralogy and chemistry of these intrusions support that they were emplaced close to the same time and just after the Mount Antero leucogranite. Two age determinations on the California leucogranite intrusion in the Mount 81
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OPEN-FILE REPORT 06-10 Geologic Map
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TABLE OF CONTENTS Introduction…
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mylonitic, augen textures………
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INTRODUCTION LOCATION AND ACCESS Th
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SAWATCH RANGE RIFT-SHOULDER UPLIFT
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of the South Arkansas River (subseq
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EXPLANATION Rio Grande Rift Neogene
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in descriptions of the geologic uni
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During the Early and Middle Paleozo
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Mountains, and White River uplifts
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field are the largest remnants of t
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Figure 5. Detailed geologic-structu
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with the Colorado mineral belt (Boo
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Shannon, 2005). Widmann and others
Page 29 and 30: the Upper Arkansas graben (fig. 5)
Page 31 and 32: 15-minute quadrangle, which is adja
Page 33 and 34: etween the northern and southern se
Page 35 and 36: on U.S. Forest Service color aerial
Page 37 and 38: We use fractional map units on the
Page 39 and 40: Qpt Pinedale till, undivided (late
Page 41 and 42: weathered, but Mount Princeton quar
Page 43 and 44: stratified, matrix-supported, bould
Page 45 and 46: as rockfalls, rock avalanches, rock
Page 47 and 48: stream-channel deposits of all pere
Page 49 and 50: Figure 10. View west up South Arkan
Page 51 and 52: Figure 11. View west up South Arkan
Page 53 and 54: valley near the eastern map boundar
Page 55 and 56: lithologies are variable and depend
Page 57 and 58: Arkansas River in parts of Redman C
Page 59 and 60: angular to subangular. Deposits gen
Page 61 and 62: elations, age constraints from foss
Page 63 and 64: elated to the inferred Squaw Creek
Page 65 and 66: Figure 14. Crude bedding in coarse
Page 67 and 68: microcline, and muscovite. The latt
Page 69 and 70: Volcanic Ash Figure 17. View of eas
Page 71 and 72: Figure 18. Close-up view of intense
Page 73 and 74: The western zone of brecciated Pale
Page 75 and 76: Tr Rhyolite dikes (early to late Ol
Page 77 and 78: as the overall N47°E trend of the
Page 79: One age determination on the rhyoli
Page 83 and 84: on the North Fork leucogranite intr
Page 85 and 86: A small 400 by 400 foot body of Nor
Page 87 and 88: N12°E, 62°NW (about 6,000 ft sout
Page 89 and 90: The age of the quartz latite porphy
Page 91 and 92: Tma Mount Aetna quartz monzonite po
Page 93 and 94: of the Maysville quadrangle and the
Page 95 and 96: Mount Aetna ring dikes (Tma), one i
Page 97 and 98: the margins of the pluton, includin
Page 99 and 100: Shannon (1988). The average of seve
Page 101 and 102: indicating a granite b composition
Page 103 and 104: analysis by Crawford (1913), two an
Page 105 and 106: locally tanish to pinkish gray. It
Page 107 and 108: quadrangle (tables 1 and 3) were fr
Page 109 and 110: slices of Paleozoic sedimentary roc
Page 111 and 112: south-southwest of Maysville is a 9
Page 113 and 114: There is one additional Paleozoic s
Page 115 and 116: near the southwest margin of the qu
Page 117 and 118: eastern contact of the diorite intr
Page 119 and 120: endoskarn zones. The two widely sep
Page 121 and 122: Xhig) along the southeastern contac
Page 123 and 124: Xgd Granodiorite (Early Proterozoic
Page 125 and 126: Hybrid granodiorite is locally deve
Page 127 and 128: Xcs Calc-silicate gneiss (Early Pro
Page 129 and 130: Many of the calc-silicate gneiss zo
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typically expressed by intermittent
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The field relations and characteris
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the north side of all three segment
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eplaced by muscovite. The western m
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sillimanite-quartz-muscovite that a
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sillimanite gneiss (Xmsg) and the m
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Figure 35. Outcrop of Proterozoic h
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samples) indicates they are compose
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Xal Lineated amphibolite (Early Pro
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of the Proterozoic metamorphic rock
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that the present structural configu
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elated to rotation of domain blocks
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sometimes terminate at a cross stru
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limestone that is exposed in a wind
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quadrangle are intimately associate
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of the quadrangle. The north-northw
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quartz monzonite ring dikes (Tma).
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that truncates the Upper Arkansas g
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as at the mouth of Squaw Creek. How
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adjacent to the southwestern edge o
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graben and the Upper Arkansas Valle
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GEOLOGIC HAZARDS Potential geologic
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In addition, older landslides may h
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sparse vegetation. One such area is
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Table 7. Historical (pre-instrument
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4262272N. ECONOMIC GEOLOGY Currentl
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A number of the larger pegmatite bo
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River placers near Buena Vista (Van
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The small mines and prospects along
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elatively recent (post-1970) explor
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contact of two Quaternary gravels (
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quadrangle. The first area is a clu
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y Sharp (1976) to be a fault slice
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quadrant of the Maysville quadrangl
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the large Paleozoic rock outlier) i
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WATER RESOURCES Water resources on
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water near Maysville is being produ
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to Rudy C. Epis: Colorado School of
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University, M.Sc. thesis, 50 p. Din
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Hutchinson, R.M., and Hedge, C.E.,
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Litsey, L.R., 1958, Stratigraphy an
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County, Colorado, in Berg, R.R., an
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Stark, J.T., and Barnes, F.F., 1935
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Tweto, O., 1987, Rock units of the
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quadrangle, Colorado: U.S. Geologic
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94A 391996 4263422 Prospect YXp;Xbf
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336A 396189 4274961 Prospect Xgdf B
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854 391849 4268852 42 Adit; Geochem
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Geologic Map of the Maysville Quadrangle, Chaffee County, Colorado

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