RESEARCH· ·1970·

..
A
B
FIGURE 3.-Pinto Diorite, a typical orthogneiss from the Precambrian
complex, Little Belt Mountains. A, hand specimen
showing characteristic mottled appearance. Dark areas are
clusters of hornblende grains; light areas are clusters of
andesine grains. B, actinolite grains rimmed by hornblende.
Photographs by Richard B. Taylor and Sandra Brennan, U.S.
Geological Survey.
port. Corrections were made for drift, latitude, elevation,
and terrain effects. Terrain corrections were computed
out to a distance of 167 kilometers. An elevation
factor of 0.06 mgal per ft ( milligal per foot), based
on an assumed density of 2.67 g/cm 3 , was used in the
WITKIND, KLEINKOPF, AND KEEFER
B65
calculations of elevation and terrain corrections. V ertical
and horizontal positions for the stations were determined
from bench-mark data and topographic controls
shown on the 7ljz-minute quadrangle maps of the
U.S. Geological Survey. The accuracy of the Bouguer
gravity data is estimated to be 1 mgal.
Complete Bouguer gravity data across the buried
pluton show a striking gmvity-minimum anomaly of
about 15 mgal. The anomaly 'appeal'S to define the later'al
extent of the pluton, but little can be said wbout the
depth configumtion without detailed model studies of
the gravity data.
The gravity-minimum anoma,ly suggests that the
pluton may be about 3 miles wide and extend 4 to 6
miles northeastward from the outcrop along Carpenter
Creek. It also may extend at least 1 mile southwest of
the outcrop. The minimum anomaly is distorted at the
northeastern end probably by high densities of the intensely
deformed and altered metamorphic rocks in the
San Miguel area.
Two additional lines of evidence substantiate the
definition of the buried pluton by gravity data. First,
the axis of the gravity-minimum anomaly coTrel.aJtes
with a zone of quartz porphyry dikes and plugs which
are probably apophyses of the concealed pluton (fig.
2) . Second, the association of a gravity-minimum
anomaly with the Snow Creek Porphyry is predictable
from rock density studies. Results from three determinations
of Snow Creek Porphyry and 10 determinations
of Precambrian metamorphic rocks showed a
lower density for the porphyry of about 0.2 g/cm 3 • The
lowest gravity values in the area are defined by the
-165-mgal contour closure whch is over the north part
of the porphyry outcrop along Carpenter Creek.
REFERENCES
Catanzaro, E. J., and Kulp, J. L., 1964, Discordant zircons from
the Little Belt (Montana), Beartooth (Montana), and
Santa Catalina (Arizona) Mountains: Geochim. et Cos·
mochim. Acta, v. 28, no. 4, p. 87-124.
Keefer, W. R., 1969, Preliminary geologic map of the Neihart
(7%') quadrangle, Cascade and Judith Basin Counties,
Montana: U.S. Geol. Survey open-file map.
Witkind, I. J., 1969, Preliminary geologic map of the Barker
(7 1/ 2') quadrangle, Judith Basin and Cascade Counties,
Montana: U.S. Geol. Survey open-file map.
Woollard, G. P., and Rose, J. C., 1963, International gravity
measurements: Wisconsin Univ. Geophys. and Polar Research
Center, 518 p.