WARD NEDERLAND GRAYS PEAK ELDORADO SPRINGS MOUNT RICHTHOFEN BUCKHORN MOUNTAIN HORSETOOTH RESERVOIR MASONVILLE PANORAMA PEAK DW-105 DW-107 DW-111 DW-094 DW-114 DW-113 DW-106 DW-093 DW-110 DW-112 DW-098 DW-109 DW-099 DW-108 DW-097 DW-092 DW-119 DW-189 DW-115 EP-066 EP-067 EP-015 DW-104 DW-101 DW-103 DW-102 DW-172 DW-067 DW-089 DW-100 DW-116 DW-118 EP-004 DW-090 DW-096 DW-117 DW-095 EP-005 EP-012 EP-011 EP-009 EP-080 EP-055 EP-029 EP-028 EP-082 EP-010 EP-081 EP-079 DW-030 DW-085 DW-213 FC-039 FC-036 DW-137-141,148-171, 226-228 DW-136 DW-135 DW-126 DW-131 DW-129,130,132-134 DW-142, 147 DW-143 DW-125 DW-121 DW-128 DW-127 DW-144 DW-145 DW-146 DW-120 DW-010 DW-004 DW-122 DW-124 DW-001 DW-003 DW-009 DW-211 DW-203 DW-195 DW-205 DW-191 DW-194 DW-201 DW-192 DW-204 DW-015 DW-183 DW-076 DW-006 DW-074 DW-005 DW-008 DW-002 DW-179 DW-007 DW-178 FC-005 FC-012 FC-004 FC-007 FC-006 FC-011 FC-010 FC-013 FC-052 FC-053 FC-032 FC-017 FC-046 FC-054 FC-057 FC-056 FC-037 FC-038 FC-043 FC-040 FC-041 FC-035 FC-034 EP-054 EP-053 EP-069 EP-068 EP-110 EP-107 EP-108 EP-047 EP-098 EP-102 EP-099 EP-101 EP-100 EP-044 DW-071 DW-069 DW-070 DW-068 DW-066 EP-042 DW-084 EP-060 EP-019 EP-092 EP-095 EP-018 EP-089 EP-057 EP-058 EP-059 EP-076 EP-077 EP-078 EP-093 EP-056 EP-074 EP-094 EP-017 EP-070 BA-028 BA-027 BA-017 BA-020 BA-019 BA-013 BA-016 BA-040 BA-048 BA-049 BA-050 BA-029 BA-009 BA-030 BA-005 BA-006 BA-007 DW-206 DW-034 DW-020 DW-019 DW-207 DW-198 DW-196 DW-193 DW-031 DW-202 DW-197 DW-199 DW-200 DW-190 DW-029 DW-086 DW-036 DW-032 DW-042 DW-021 DW-033 DW-061 DW-079 DW-210 DW-225 DW-057 DW-224 DW-028 DW-082 DW-081 DW-209 DW-087 DW-088 DW-023 DW-022 DW-083 DW-026 DW-188 DW-182 DW-175 DW-072 DW-018 DW-075 DW-051 DW-043 DW-216 DW-045 DW-046 DW-044 DW-039 DW-052 DW-062 DW-035 DW-040 DW-218 DW-219 DW-078 DW-048 DW-049 DW-223 DW-217 DW-050 DW-047 DW-058 DW-027 DW-024 DW-215 DW-025 DW-222 DW-221 DW-038 DW-041 DW-037 DW-080 FC-019 FC-027 FC-020 FC-021 FC-018 FC-001 FC-024 FC-022 FC-025 FC-002 FC-028 EP-065 EP-064 EP-002 EP-023 EP-020 EP-028, 036, 037 EP-038 EP-032 EP-039 EP-086 EP-087 EP-033 EP-034 EP-035 EP-003 EP-001 EP-030 EP-031 BA-068 BA-067 BA-072 FC-042 EP-104 EP-106 EP-096 EP-103 EP-043 EP-097 EP-105 EP-026 EP-083 EP-025 EP-084 EP-007 EP-088 EP-040 EP-041 EP-085 EP-008 FC-023 FC-059 FC-029 BA-018 BA-022 BA-023 BA-031 BA-035 BA-032 BA-024 BA-036 BA-037 BA-021 BA-034 BA-033 BA-015 DW-053 DW-054 DW-186 DW-181 DW-180 DW-176 DW-055 DW-056 BA-026 BA-079 BA-083 BA-084 BA-082 BA-081 BA-046 BA-078 BA-045 DW-012 DW-173 DW-187 CRAZY MOUNTAIN GLENDEVEY RAWAH LAKES CLARK PEAK BOWEN MOUNTAIN TRAIL MOUNTAIN GRANBY BOTTLE PASS BYERS PEAK LOVELAND PASS KEYSTONE BOREAS PASS COMO FAIRPLAY EAST GARO SAND CREEK PASS DEADMAN BOSTON PEAK CHAMBERS LAKE FALL RIVER PASS GRAND LAKE SHADOW MOUNTAIN STRAWBERRY LAKE FRASER BERTHOUD PASS MONTEZUMA JEFFERSON MILLIGAN LAKES ELKHORN HARTSEL EATON RESERVOIR SOUTH BALD MOUNTAIN KINIKINIK COMANCHE PEAK TRAIL RIDGE MCHENRYS PEAK ISOLATION PEAK MONARCH LAKE EAST PORTAL EMPIRE GEORGETOWN MOUNT EVANS MOUNT LOGAN OBSERVATORY ROCK EAGLE ROCK SULPHUR MOUNTAIN DIAMOND PEAK RED FEATHER LAKES RUSTIC PINGREE PARK ESTES PARK LONGS PEAK ALLENS PARK CENTRAL CITY IDAHO SPRINGS HARRIS PARK SHAWNEE TOPAZ MOUNTAIN FARNUM PEAK GLENTIVAR CHEROKEE PARK HAYSTACK GULCH BIG NARROWS CRYSTAL MOUNTAIN GLEN HAVEN RAYMOND GOLD HILL TUNGSTEN BLACK HAWK SQUAW PASS MERIDIAN HILL BAILEY WINDY PEAK MCCURDY MOUNTAIN TARRYALL VIRGINIA DALE LIVERMORE MOUNTAIN POUDRE PARK DRAKE PINEWOOD LAKE LYONS BOULDER RALSTON BUTTES EVERGREEN CONIFER PINE GREEN MOUNTAIN CHEESMAN LAKE HACKETT MOUNTAIN TABLE MOUNTAIN LIVERMORE LAPORTE CARTER LAKE RESERVOIR HYGIENE NIWOT LOUISVILLE GOLDEN MORRISON INDIAN HILLS PLATTE CANYON DECKERS WESTCREEK SIGNAL BUTTE ROUND BUTTE BUCKEYE WELLINGTON FORT COLLINS LOVELAND BERTHOUD LONGMONT ERIE LAFAYETTE ARVADA FORT LOGAN LITTLETON KASSLER DEVILS HEAD DAKAN MOUNTAIN MOUNT DECEPTION FC-003 FC-008 FC-009 FC-014 FC-015 FC-016 FC-026 FC-030 FC-031 FC-033 FC-044 FC-045 FC-047 FC-048 FC-049 FC-050 FC-051 FC-055 FC-058 FC-060 EP-006 EP-013 EP-014 EP-016 EP-021 EP-022 EP-024 EP-027 EP-045 EP-046 EP-048 EP-049 EP-050 EP-051 EP-052 EP-061 EP-062 EP-063 EP-071 EP-072 EP-073 EP-075 EP-090 EP-091 EP-109 DW-011 DW-013 DW-014 DW-016 DW-017 DW-059 DW-060 DW-063 DW-064 DW-065 DW-073 DW-077 DW-091 DW-174 DW-177 DW-184 DW-185 DW-208 DW-212 DW-214 DW-220 BA-001 BA-002 BA-003 BA-004 BA-008 BA-010 BA-011 BA-012 BA-014 BA-025 BA-038 BA-039 BA-041 BA-042 BA-043 BA-044 BA-047 BA-051 BA-052 BA-053 BA-054 BA-055 BA-056 BA-057 BA-058 BA-059 BA-060 BA-061 BA-062 BA-063 BA-064 BA-069 BA-070 BA-071 BA-073 BA-074 BA-075 BA-076 BA-077 BA-080 EP-112 FC-061 EP-111 BA-065 BA-066 DW-123 OPEN-FILE REPORT 2006-1051 SHEET 2 of 2 Version 1.0 105°00' SAMPLE LOCATIONS These maps, and the tables that accompany them, are a compilation of isotopic age determinations of rocks and minerals in four 1:100,000-scale quadrangles in the northern and central Front Range, Colorado. Phanerozoic (primarily Tertiary and Cretaceous) age data are shown on one map; Proterozoic data are on the other (sheet 1). A sample location map (sheet 2) is included for ease of matching specific localities and data in the tables to the maps. Several records in the tables were not included in the maps because either there were ambiguous dates or lack of location precluded accurate plotting. Introduction To illustrate the geological setting for the samples, the plutonic rocks are shown on the maps. The boundaries of the plutons are from the Geologic Map of Colorado (Tweto, 1979; digital version by Green, 1992) with a few modifications (Gable, 1984; A.B. Wilson, 2004, unpub. data). For ease of reference, we labeled each of the larger (and some of the smaller) plutons with a generally accepted name from the literature. As a convenience in using the data, we have informally named some plutons based on geographic features on or near those plutons. Those names are shown in parentheses. The maps show the type of age determination by symbol and age subdivisions by color. Seven different methods, each shown with a different symbol, were used to determine Phanerozoic ages: Ar/ Ar (circles), Rb/Sr (crosses), K-Ar (stars), U/Pb (squares), zircon fission track (diamonds), sphene fission track (inverted triangles), and apatite fission track (triangles). Four different methods were used to determine Proterozoic ages: Ar/ Ar (circles), Rb/Sr (crosses), K-Ar (stars), and U/Pb (squares). Many age determinations have been made on samples which were not located precisely on published maps, although some were located on small-scale geologic sketch maps. We located those samples as best we could on published 1:24,000-scale geologic maps and determined a latitude and longitude for that location. These uncertain or imprecise locations are indicated in the tables by a footnote and on the maps they are shown with open, or unfilled, symbols in the same shapes as explained previously. Many ages in these tables were determined before the use of the currently accepted values for the decay and abundance constants recommended by the IUGS Subcommission on Geochronology (Steiger and Jäger, 1977). We have corrected the K-Ar ages that used the older decay constant ( 40 K/K=1.19x10 -4 mol/mol) and abundance constants generally used in the Western World to ones based on the currently accepted decay constant ( 40 K/K=1.167x10 -4 mol/mol) by using the tables of Dalrymple (1979). We have corrected the Rb/Sr ages that used the older decay constant ( 87 Rbλ=1.39x10 -11 /yr) to ones based on the currently accepted constant ( 87 Rbλ=1.42x10 -11 /yr) by multiplying the age by 0.9788. On the two maps intrusions of similar ages are highlighted in color, and the dated samples of each age group are shown in the same color. On the map showing Proterozoic ages, samples are divided into mostly Early Proterozoic (1,900–1,500 Ma; green) and Middle and Late Proterozoic (1,500–540 Ma, but mostly Middle Proterozoic; purple). On the map showing Phanerozoic ages samples are divided into six groups: Paleozoic and Mesozoic (540–80 Ma; yellow); Late Cretaceous to early Eocene ("Laramide"; 80–50 Ma; blue), early and middle Eocene (50–40 Ma; pale blue); middle Eocene to early Oligocene (40–30 Ma; pink), early Oligocene to early Miocene (30–20 Ma; magenta); and early Miocene and younger (20–0 Ma, but mostly Miocene; red). Additional isotopic age data were intentionally not included in this compilation because the data are unreliable. This includes early attempts to date rocks based on chemical determinations of total uranium and lead in pitchblende in some samples from the Central City district. For the older isotopic age determinations on these uranium-bearing minerals see Marvin and others (1974). Phair (1979) discussed and corrected some of these data and concluded that there were two groups of ages at 58 Ma and 72.5 Ma. References Cited Dalrymple, G.B., 1979, Critical tables for conversion of K-Ar ages from old to new constants: Geology, v. 7, p. 558–560. Gable, D.J., 1984, Geologic setting and petrochemistry of the Late Cretaceous-Early Tertiary intrusives in the northern Front Range Mineral Belt: U.S. Geological Survey Professional Paper 1280, 33 p., scale 1:48,000. Green, G.N., 1992, The Digital Geologic Map of Colorado in ARC/INFO Format: U.S. Geological Survey Open-File Report OF-92-0507, scale 1:500,000. Marvin, R.F., Young, E.J., Mehnert, H.H., and Naeser, C.W., 1974, Summary of radiometric age determinations on Mesozoic and Cenozoic igneous rocks and uranium and base metal deposits in Colorado: Isochron/West, n. 11, 41 p. Phair, George, 1979, Interpretation of lead-uranium ages of pitchblende deposits in the Front Range, Colorado: Geological Society of America Bulletin, Part 1, v. 90, p. 858–870. Steiger, R.H., and Jäger, E., 1977, Subcommission on geochronology: Convention on use of decay constants in geo- and cosmochronology: Earth and Planetary Science Letters, v. 126, p. 359–362. Tweto, Ogden, 1979, Geologic map of Colorado: U.S. Geological Survey, scale 1:500,000. 40 39 40 39 106°00' 41°00' 40°30' 40°00' 39°30' 39°00' U.S. DEPARTMENT OF THE INTERIOR U.S. GEOLOGICAL SURVEY 5 0 10 15 20 25 30 KILOMETERS SCALE 1: 250 000 10 5 0 5 15 20 MILES e Bryant ISOTOPIC AGES OF ROCKS IN THE NORTHERN FRONT RANGE, Anna B. Wilson and Bruc 2006 COLORADO 5