RESEARCH· ·1970·

GE,OLOGICAL SURVEY RESEA·RCH 1970
WHOLE-ROCK Rb-Sr AGE OF THE PIKES PEAK BATHOLITH,
COLORADO
By C. E. HEDGE, Denver, Colo.
A.bstract.-Ten whole-rock samples from the Pikes Peak
batholith and 3 from the satellite Rosalie lobe yield a Rb-Sr
age for the time of batholith emplacement of 1,041 ± 13 million
years. All the major lithological variants were probably intruded
within less than 20 m.y. The Pikes Peak batholith represents.
the youngest major Precambrian granitic intrusion in the
Front Range.
The Pikes Peak batholith is the largest single mass
of granite in the Front Range of Colorado (fig. 1). It
is exposed as an approximately 1,200-square-mile body,
somewhat elongate in a north-south direction, in the
southern part of the Front Range. A significant satellite
mass, locally known as the Rosalie lobe (Hutchinson,
1960), occurs just north of the batholith, and several
small bodies of probable Pikes Peak Granite crop
out on the east slope of ~1ount Evans to the northwest
of the Rosalie lobe. Other granite bodies of similar
lithology are known in the region, but no other major
bodies of igneous rocks of an equivalent age are known
in the immediate area or elsewhere in Colorado.
The Pikes Peak Granite was first named by Cross
in 1894 and has since been the object of many studies.
The batholith is complex and contains mappable variants.
Gross and Heinrich ( 1965) discussed the characteristics
of some of the rocks of the southern part of
the batholith. In this area some of the variants have
been given local names. IIutchinson (1960) has mapped
and subdivided the northern part of the batholith, using
petrologic names for the respective subdivisions.
The samples in the present study include most of the
recognized variants and two late dike rocks which are
common throughout the batholith.
Several studies of mineral ages from the batholith
have been published. Most notable of these are by
Aldrich, Wetherill, Davis, and Tilton ( 1958), Hutchinson
(1960), and Giffin and l(ulp ( 1960). The published
aJges rure summarized in truble 1. The total spread in
TABLE I.-Published mineral ages from the Pikes Peak batholith
Sample locality Mineral Source
of data t
Manitou Springs area __ Biotite _____ _____ do _____ _
Zircon _____ _
Summit of Pikes Peak_ Biotite _____ _____ do _____ _
_____ do _____ _
Ute Pass _________________ do _____ _
Gold Camp Road __________ do ___ _, __
Pikes Peak Toll Road ______ do _____ _
Near Foxter _______________ do _____ _
Buffalo ___________________ do _____ _
Wellington Reservoir _____ do _____ _
area.
Mount Rosa _________ Riebeckite __ _
Redskin stock ________ Biotite _____ _
Microcline __ _
Redskin stock greisen _ Muscovite __ _
Boomer cupola ____________ do---_
Microcline __ _
Boomer cupola greisen_ Muscovite __ _
1 Sources of data:
1. Aldrich, Wetherill, Davis, and Tilton (1958).
2. Giffin and Kulp (1969).
3. Hutchinson (1960).
4. Gross and Heinrich (1966).
5. Hawley, Huffman, Hamilton, and Rader (1966).
Method
1 K-Ar
1 Rb-Sr
1 Pb2o7_pb206
1 K-Ar
1 Rb-Sr
2 K-Ar
2 K-Ar
2 K-Ar
2 K-Ar
3 K-Ar
3 K-Ar
3 K-Ar
4 K-Ar
5 K-Ar
5 Rb-Sr
5 K-Ar
5 K-Ar
5 Rb-Sr
5 K-Ar
Age
(m.y.)
980
1, 020
980
1,060
1,080
980
1, 030
1,020
1, 020
1, 080
1, 050
1, 050
1, 040
980
980
1, 020
990
1, 010
1, 000
mica ages, by both 1(-Ar and Rb-Sr methods, is from
980 to 1,080 million years. The mea;n mioa ttge vs 1,030
m.y., and, considering the normal analytical uncertainties,
the data are consistent with analytical scatter
around a single number. 1(-Ar analysis of a riebeckite
from a pegmatite -gave a similar result of 1,040 m.y.
(Gross and Heinrich, 1965) . One zircon from the Pikes
Peak batholith has been isotopically dated (Aldrich and
others, 1958), and, although discordant, the Pb 207 -Pb 200
age of 980 m.y. is compatible with the 1nica ages.
Hawley, Huffman, Hamilton, and Rader (19·66) have
reported I(-Ar mica ages for two s1nall satellite masses
on the western edge of the batholith. These ages and
two Rb-Sr ages of microclines from the same area are
also given in table 1.
(,.
B86
U.S. GEOL. SURVEY PROF. PAPER 700-B, PAGES B86-B89