WARD

NEDERLAND

GRAYS PEAK

ELDORADO SPRINGS

MOUNT RICHTHOFEN

BUCKHORN MOUNTAIN HORSETOOTH RESERVOIR

MASONVILLE

PANORAMA PEAK

DW-105

DW-107 DW-111

DW-094

DW-114DW-113

DW-106

DW-093DW-110

DW-112 DW-098

DW-109DW-099

DW-108

DW-097

DW-092

DW-119 DW-189

DW-115

EP-066

EP-067EP-015

DW-104

DW-101

DW-103

DW-102

DW-172DW-067

DW-089

DW-100

DW-116

DW-118 EP-004DW-090DW-096

DW-117

DW-095EP-005

EP-012EP-011

EP-009 EP-080

EP-055

EP-029EP-028

EP-082

EP-010

EP-081EP-079

DW-030

DW-085

DW-213

FC-039FC-036

DW-137-141,148-171, 226-228

DW-136

DW-135DW-126

DW-131

DW-129,130,132-134

DW-142, 147

DW-143

DW-125DW-121

DW-128

DW-127

DW-144DW-145

DW-146

DW-120DW-010 DW-004 DW-122 DW-124

DW-001

DW-003DW-009

DW-211

DW-203

DW-195

DW-205

DW-191

DW-194

DW-201

DW-192

DW-204

DW-015DW-183

DW-076

DW-006

DW-074DW-005

DW-008DW-002 DW-179DW-007

DW-178

FC-005

FC-012

FC-004FC-007

FC-006

FC-011FC-010

FC-013

FC-052FC-053

FC-032FC-017

FC-046

FC-054FC-057

FC-056

FC-037

FC-038

FC-043FC-040FC-041

FC-035FC-034

EP-054

EP-053EP-069

EP-068

EP-110

EP-107EP-108

EP-047

EP-098

EP-102

EP-099

EP-101

EP-100

EP-044

DW-071

DW-069

DW-070DW-068

DW-066

EP-042

DW-084

EP-060

EP-019

EP-092EP-095

EP-018

EP-089

EP-057

EP-058EP-059

EP-076EP-077

EP-078

EP-093

EP-056EP-074

EP-094

EP-017

EP-070

BA-028BA-027

BA-017BA-020

BA-019

BA-013

BA-016

BA-040

BA-048BA-049

BA-050

BA-029

BA-009

BA-030

BA-005

BA-006

BA-007

DW-206DW-034DW-020

DW-019DW-207DW-198

DW-196 DW-193 DW-031DW-202

DW-197DW-199

DW-200

DW-190

DW-029

DW-086

DW-036

DW-032

DW-042DW-021

DW-033DW-061

DW-079

DW-210

DW-225

DW-057DW-224DW-028

DW-082

DW-081

DW-209DW-087DW-088

DW-023 DW-022DW-083

DW-026

DW-188DW-182

DW-175 DW-072

DW-018

DW-075

DW-051DW-043DW-216

DW-045DW-046

DW-044DW-039

DW-052DW-062DW-035

DW-040 DW-218

DW-219

DW-078

DW-048DW-049 DW-223DW-217DW-050

DW-047DW-058

DW-027DW-024 DW-215

DW-025

DW-222DW-221DW-038

DW-041DW-037

DW-080

FC-019

FC-027

FC-020FC-021

FC-018FC-001

FC-024

FC-022

FC-025FC-002

FC-028

EP-065EP-064

EP-002

EP-023

EP-020

EP-028, 036, 037EP-038

EP-032

EP-039

EP-086

EP-087

EP-033

EP-034EP-035

EP-003

EP-001EP-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-084EP-007

EP-088

EP-040

EP-041

EP-085

EP-008

FC-023

FC-059

FC-029

BA-018

BA-022BA-023

BA-031BA-035BA-032

BA-024

BA-036BA-037

BA-021

BA-034

BA-033

BA-015

DW-053DW-054

DW-186

DW-181

DW-180DW-176

DW-055

DW-056

BA-026

BA-079BA-083

BA-084BA-082BA-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-015FC-016

FC-026

FC-030

FC-031

FC-033

FC-044

FC-045

FC-047FC-048

FC-049

FC-050FC-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-051EP-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-014DW-016

DW-017

DW-059

DW-060

DW-063

DW-064DW-065

DW-073

DW-077

DW-091

DW-174

DW-177

DW-184

DW-185

DW-208

DW-212

DW-214

DW-220

BA-001BA-002

BA-003BA-004

BA-008

BA-010

BA-011

BA-012

BA-014

BA-025

BA-038

BA-039

BA-041

BA-042

BA-043BA-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 2006-1051SHEET 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) agedata are shown on one map; Proterozoic data are on the other (sheet 1). A sample locationmap (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 (40K/K=1.19x10-4 mol/mol) and abundance constants generally used in the Western World to ones based on the currently accepted decay constant (40K/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 (87Rbλ=1.39x10-11/yr) to ones based on the currently accepted constant ( 87Rbλ=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 INTERIORU.S. GEOLOGICAL SURVEY

5 0 10 15 20 25 30 KILOMETERS

SCALE 1: 250 000105 0 5 15 20 MILES

e Bryant

ISOTOPIC AGES OF ROCKS IN THE NORTHERN FRONT RANGE,

Anna B. Wilson and Bruc

2006

COLORADO

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