RESEARCH· ·1970·

EXPLANATION
Pikes Peak • batholith
0
Rosalie lobe
TABLE 3.-Analytical data on samples from localities in figure 1
and table 2
Sample No.
Rb
(ppm)
Sr1 (ppm) RbS7f8r86 2 SrB7/SrS6 3 Apparent age
(m.y.)
1215 ________ 215 95. 4 6. 327 0. 7979 1, 023
1217 ________ 258 51. 1 14. 94 . 9191 1, 013
1218 ________ 240 4. 91 179. 0 3. 364 1, 060
1219 ________ 289 45. 2 19. 03 . 9864 1, 048
122 L _______ 221 144 4. 501 . 7754 1, 081
1222 ________ 157 15. 8 29. 91 1. 1452 1, 045
1223A _______ 220 63.4 10. 17 . 8545 1, 033
1223B _______ 541 4. 78 621. 5 9. 885 1, 055
PP-B _______ 199 160 3. 61 . 7605 1, 053
PP-2 ________ 160 249 1. 81 . 7354 1, 108
rlg-L _______ 160 55.4 7. 904 . 8380 1,048
rlg-2_ ------- 161 58. 5 8. 063 . 8261 1,052
rlg-3 ________ 144 56.4 8. 483 . 8176 1, 053
1 Normal Sr.
2 Rb and Sr concentrations determined by isotope dilution.
Constants:
Rb87 >.{J = 1.39Xlo-u yr -1
RbS7 = 0.283 g/g Rb
a Fractionation effects corrected for by normalizing observed SrBGfSrss ratios to 0.1194
tion that all the samples are the same age and began
with the same Sr isotopic composition. All the points
fit the line within their respective analytical uncertainties
and are, therefore, consistent with the singleage
interpretation. This best age would be ·1,041 m.y.
with an apparent uncertainty of 13 m.y. If the time of
emplacement of these samples diverged by more than
about 20 m.y., some of the points would deviate from
the line.
DISCUSSION
Although the Pikes Peak batholith is generally believed
to represent the youngest major period of Precambrian
igneous activity in the Front Range, some
discrepancies, which need to be clarified, still exist. The
Pikes Peak batholith has often been correlated, on the
8
16
Rb87 /Sr 86
24 32
FIGURE 2.-Isochron plot for total-rock samples from the
Pikes Peak batholith and Rosalie lobe. The two most
radiogenic samples (1218 and 1223B) .are not plotted, ·but
they lie on the projection of this isochrO'Il .
basis of lithology, with the Sherman Granite of the
Colorado-'Vyoming border area. This correlation is
erroneous, as the age of the Sherman Granite is 1,410
m.y. (Peterman and others, 1968). The Sherman
Granite is intruded by the Log Cabin batholith, which
is composed of Silver Plume Granite. This relationship,
together with an interpretation of the contact
between the Rosalie lobe and other bodies of Silver
Plume-like granite (Boos and Aberdeen, 1940), has
led some to the belief that the Pikes Peak batholith is
older than the Silver Plume Granite. The various
bodies of Silver Plume Granite, in the Front Range,
are all 1,400-1,450 m.y. old (Peterman and others,
1968; Hedge, 1968). Included in these are the Cripple
Creek Granite and the Indian Creek Granite of Boos
and Aberdeen ( 1940), both of which have been interpreted
as being younger than the Pikes Peak. Enough
data are now available to say with certainty that the
emplacement of the Pikes Peak batholith, 1,040 m.y.
ago, was t!1e last 1najor Precambrian igneous event in
the Front Range.
Though attempts can be made to decipher the mnplacement
history of a complex batholith such as the
Pikes Peak batholith on the basis of radiometric dat-