Isotopic Ages of Rocks in the Northern Front Range, Colorado

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.

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 with a few modifications. 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.

[Summary provided by the USGS.]