Earth Surface Processes Team - Central Region

Isotopic Studies

A component of the U.S. Geological Survey's Central Colorado Assessment Project

Wayne Premo, Anna Wilson, and Bruce Bryant

U-Pb Zircon and Sphene Geochronology and Whole-Rock Nd Isotopic Results from the Boulder Creek Batholith

by Wayne Premo

This diagram is an example of a wider, ongoing study to understand the isotopic character and history of the Proterozoic basement of north-central Colorado. The Proterozoic Boulder Creek batholith shows rock types as indicated from modes (after Gable, 1980). Age results from U-Pb zircon and sphene analyses, combined with initial Nd isotopic data, help us to understand the age and origin of many of the basement terranes (Premo and Van Schmus, 1989; Premo and Fanning, 2000). (click on map for a larger view)

Boulder Creek Batholith diagram


Isotopic ages for Proterozoic rocks in the Estes Park and Denver West quadrangles

by Anna Wilson and Bruce Bryant

A map has also been compiled for all Phanerozoic rocks, which include mostly Late Cretaceous and Tertiary intrusives. Similar maps have been created for the Bailey and Fort Collins 1:100,000 quadrangles. All data, including references, given dates and uncertainties, and locations, are on a spreadsheet and will soon be available as a USGS publication. (click on map for a larger view)

geologic map of Denver West and Estes Park

Proterozoic Intrusive Rocks
(pale green shows extent of Proterozoic metamorphic rocks)

a pinkish box represents the Pikes Peak Granite on the map Yp--Pikes Peak Granite (approx. 1080 Ma)
a brown box represents the Middle Proterozoic granitic rocks on the map Yg--Middle Proterozoic granitic rocks (approx. 1400 Ma)
a green box represents the Early Proterozoic granitic rocks on the map Xg--Early Proterozoic granitic rocks (approx. 1700 Ma)

Proterozoic dikes

green lines represent dikes on the map Yam--Middle Proterozoic dikes (approx. 1400 and 1300 Ma)

Ages in Millions of years
(open symbols indicate vague or inexact locations)

40Ar/39Ar ages
dot indicating 40Ar/39Ar ages 540 to 1500 million years ago 540 to 1,500 (15)
dot indicating 40Ar/39Ar ages 1500 to 1900 million years ago 1,500 to 1,900 (4)
Rb/Sr ages
polygon indicating Rb/Sr ages 540 to 1500 million years ago 540 to 1,500 (12)
polygon indicating Rb/Sr ages 1500 to 1900 million years ago 1,500 to 1,900 (0)
K-Ar ages
star indicating K-Ar ages 540 to 1500 million years ago 540 to 1,500 (5)
star indicating K-Ar ages 1500 to 1900 million years ago 1,500 to 1,900 (0)
U/Pb ages
square indicating U/Pb ages 540 to 1500 million years ago 540 to 1,500 (4)
square indicating U/Pb ages 1500 to 1900 million years ago 1,500 to 1,900 (4)

gray lines represent 7 and one-half minute quad map boundary 7 1/2' quadrangle boundary
heavy black lines represent 30 by 60 minute quad map boundary 30' x 60' quadrangle boundary


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