Geologic Map of the Maysville Quadrangle, Chaffee County, Colorado

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western sub-domain boundary with the muscovite-sillimanite gneiss. The granite and pegmatite intrusions are fairly abundant in the muscovite-sillimanite gneiss, decrease in abundance in the amphibolite gneiss transition zone, and are rare to absent in the lineated amphibolite sub-domain. Figure 37. Outcrop of Proterozoic lineated amphibolite (Xal). Strong amphibole lineation plunges steeply to left, parallel to yellow pencil magnet. The characteristics of the lineated amphibolite suggest a fine-grained, nonlayered, massive protolith with mafic composition, possibly basalt flows. Associated sequences of well-layered gneisses may represent interlayered mafic, intermediate and felsic metavolcanic and metasedimentary rocks. Xag Amphibolite gneiss (Early Proterozoic) – The amphibolite gneiss is a significant lithology covering about 7 to 10 percent of the surface area of the Maysville quadrangle. It makes up about 20 to 25 percent of the crystalline bedrock area and about 25 to 30 percent of the Proterozoic terrane. Thus, it is one of the dominant lithologic units 148
of the Proterozoic metamorphic rock sequence. The amphibolite gneiss is commonly interlayered and transitional with hornblende intermediate gneiss (Xhig) and calc-silicate gneiss (Xcs) and is locally interlayered and transitional with biotite felsic gneiss (Xbfg) and muscovite felsic schist (Xmfs). In areas where the interlayered gneisses crop out, the orientation of gneissic layering is usually conformable, suggesting that the gneisses represent sequences of layered lithologies of contrasting composition. The orientation of the amphibolite gneiss layering is usually parallel or subparallel to the boundaries of the lithologic-structural domains. Amphibolite gneiss is generally poorly exposed and occurs as zones of fine rock float with limited, small outcrops. Amphibolite gneiss is dark gray to black and predominantly fine grained with well-developed, fine- to medium-scale (fraction of inches to feet) gneissic layering. The gneiss has simple mineralogy consisting predominantly of hornblende and plagioclase. Estimated modes show 60 to 80 percent amphibole, 20 to 40 percent plagioclase, 0 to 2 percent biotite, and trace to 3 percent sphene. Abundant epidote and minor quartz and magnetite are present in areas with calc-silicate overprint. Localized zones of amphibolite gneiss have minor biotite that usually occurs in localized weak to moderate foliated bands paralleling the gneissic layering. This biotite is interpreted to be related to a weak retrograde metamorphic overprint. Amphibolite gneiss has variably developed metamorphic foliation that parallels the gneissic layering. Most of the amphibolite gneiss is non-foliated and local areas have weak foliation produced by alignment of elongated amphibolites and minor retrograde biotite. Localized, discontinuous zones of mylonitic amphibolite have strong foliation. The amphibolite gneiss is present in a couple of small lithologic-structural domains in the Proterozoic terrane south of the South Arkansas River where it is the overwhelming lithology (less than ten percent mixed lithologies, generally Berthoud-type granite and pegmatite intrusions). Many areas mapped as predominantly amphibolite gneiss also include small irregular and discontinuous layers of massive amphibolite, mylonitic amphibolite, and calc-silicate-bearing amphibolite that are too small to map at 1:24,000 scale. Discontinuous layers of massive amphibolite (about 20 to 50 ft thick) are interlayered with amphibolite gneiss in the lithologic-structural domains in the southwest corner of the quadrangle. The massive amphibolites are generally more mafic rich with a 149
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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
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on U.S. Forest Service color aerial
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We use fractional map units on the
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Qpt Pinedale till, undivided (late
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weathered, but Mount Princeton quar
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stratified, matrix-supported, bould
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as rockfalls, rock avalanches, rock
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stream-channel deposits of all pere
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Figure 10. View west up South Arkan
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Figure 11. View west up South Arkan
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valley near the eastern map boundar
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lithologies are variable and depend
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Arkansas River in parts of Redman C
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angular to subangular. Deposits gen
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elations, age constraints from foss
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elated to the inferred Squaw Creek
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Figure 14. Crude bedding in coarse
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microcline, and muscovite. The latt
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Volcanic Ash Figure 17. View of eas
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Figure 18. Close-up view of intense
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The western zone of brecciated Pale
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Tr Rhyolite dikes (early to late Ol
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as the overall N47°E trend of the
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One age determination on the rhyoli
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northwest-trending fault. Thus, fie
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on the North Fork leucogranite intr
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A small 400 by 400 foot body of Nor
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N12°E, 62°NW (about 6,000 ft sout
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The age of the quartz latite porphy
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Tma Mount Aetna quartz monzonite po
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of the Maysville quadrangle and the
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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
Page 137 and 138: eplaced by muscovite. The western m
Page 139 and 140: sillimanite-quartz-muscovite that a
Page 141 and 142: sillimanite gneiss (Xmsg) and the m
Page 143 and 144: Figure 35. Outcrop of Proterozoic h
Page 145 and 146: samples) indicates they are compose
Page 147: Xal Lineated amphibolite (Early Pro
Page 151 and 152: that the present structural configu
Page 153 and 154: elated to rotation of domain blocks
Page 155 and 156: sometimes terminate at a cross stru
Page 157 and 158: limestone that is exposed in a wind
Page 159 and 160: quadrangle are intimately associate
Page 161 and 162: of the quadrangle. The north-northw
Page 163 and 164: quartz monzonite ring dikes (Tma).
Page 165 and 166: that truncates the Upper Arkansas g
Page 167 and 168: as at the mouth of Squaw Creek. How
Page 169 and 170: adjacent to the southwestern edge o
Page 171 and 172: graben and the Upper Arkansas Valle
Page 173 and 174: GEOLOGIC HAZARDS Potential geologic
Page 175 and 176: In addition, older landslides may h
Page 177 and 178: sparse vegetation. One such area is
Page 179 and 180: Table 7. Historical (pre-instrument
Page 181 and 182: 4262272N. ECONOMIC GEOLOGY Currentl
Page 183 and 184: A number of the larger pegmatite bo
Page 185 and 186: River placers near Buena Vista (Van
Page 187 and 188: The small mines and prospects along
Page 189 and 190: elatively recent (post-1970) explor
Page 191 and 192: contact of two Quaternary gravels (
Page 193 and 194: quadrangle. The first area is a clu
Page 195 and 196: y Sharp (1976) to be a fault slice
Page 197 and 198: 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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