Geologic Map of the Maysville Quadrangle, Chaffee County, Colorado

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front and Shavano fault zone. Tqlp Quartz latite porphyry hybrid dikes (late Eocene to early Oligocene) – At least two and possibly three quartz latite porphyry dikes are part of the zone of dikes crossing the southeast flank of Mount Shavano. The quartz latite porphyry dikes are medium to dark gray and very fine-grained (fig. 22). Minor outcrop/subcrop and narrow concentrated float zones indicate the dikes are about 1 to 6 ft thick. The hand-sample and petrographic character of the quartz latite porphyry suggests affinities with the Mount Aetna cauldron ring dikes (Tma). The rock also has a hybrid character imparted by a relatively mafic-rich appearing groundmass (about 0.2 mm grain size) and a variable population of larger crystals. The mafic groundmass predominantly consists of welldeveloped, flow-aligned, subhedral plagioclase laths with about 10 to 20 percent amphibole and biotite. The subhedral, lath character of the plagioclase in the groundmass is never observed in Mount Aetna ring dikes and is more characteristic of andesitic and basaltic composition rocks. The larger crystals include a population (4 to 8 percent) of subhedral amphibole crystals interpreted to be primary phenocrysts and a variable population (from less than 1 percent to more than 18 percent) of rounded and resorbedappearing crystals of alkali feldspar, quartz, plagioclase, and biotite. It is these larger crystals, especially the presence of alkali feldspars (locally with rapakivi overgrowths) to 0.80 inch size that impart a Mount Aetna ring-dike character (fig. 22). The larger, rounded crystals are clearly phenocrysts (or xenocrysts of phenocrysts) that have been partly, and variably resorbed in the dike. Most of the large plagioclase crystals have reaction coronas suggesting they are also xenocrysts, but some that lack reaction coronas may be primary phenocrysts. Three segments of one quartz latite porphyry hybrid dike indicate a nearly continuous dike for about 12,000 ft across the Maysville quadrangle. The dike is within 500 to 1,200 ft east of and subparallels the main northeast-trending rhyolite porphyry dike (Trp). Minor float suggest the dike may extend another 3,000 ft to the southwest to the North Fork valley. The dike is poorly exposed and occurs as narrow concentrated float zones with an overall trend of N36°E. Two outcrops of quartz latite porphyry hybrid dikes indicate N28°E 56°NW (about 3,000 ft east-southeast of Shavano Lake) and 86
N12°E, 62°NW (about 6,000 ft southeast of Mount Shavano) orientations. This is similar to the overall trend of N25°E 50°NW of the main northeast-trending rhyolite porphyry dike (Trp) and is also subparallel to the southeast margin of the Mount Aetna cauldron ring zone (Shannon, 1988). Three thin sections of quartz latite porphyry dikes (two from Maysville quadrangle and one from Mount Antero quadrangle) were studied. The quartz latite porphyry has accessory magnetite, sphene, apatite, and allanite, which is the typical suite of accessory minerals in the Mount Princeton pluton and the Mount Aetna intrusions. All three samples also show microscopic evidence of late magmatic, potassic alteration including secondary matted biotite replacing amphibole and local, patchy replacement of the groundmass by alkali feldspar. Figure 22. Billets and slabs of Tertiary quartz latite hybrid dikes (Tqlp) stained for alkali feldspar showing range of textures and xenocryst contents. 87
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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
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the Upper Arkansas graben (fig. 5)
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15-minute quadrangle, which is adja
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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 and 80: One age determination on the rhyoli
Page 81 and 82: northwest-trending fault. Thus, fie
Page 83 and 84: on the North Fork leucogranite intr
Page 85: A small 400 by 400 foot body of Nor
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
Page 131 and 132: typically expressed by intermittent
Page 133 and 134: The field relations and characteris
Page 135 and 136: 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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