RESEARCH· ·1970·

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GE·OLOGICAL SURVEY RESEARCH 1970
GEOLOGIC AND GRAVITY EVIDENCE FOR A BURIED PLUTON,
LITTLE BELT MOUNTAINS, CENTRAL MONTANA
By I. j. WITKIND, M·. D. KLEINKOPF, and W. R. KEEFER,
Denver, Colo.
. .J,
...
Abstra.ot.-Grnvity contours delineate a buried pluton that
has domed Precambrian metamorphic crystalline rocks exposed
in the core of the J .. ittle Belt Mountains, central Montana. 'l;he
pluton, probnbly of Tertiary age, appears to be a northeastwardtrendi.ng
ovoid body 3 miles wide and possibly as much as 7
miles long.
The Little Belt l\1ountains are one of several laccolithic
mountain groups that rise above the plains of
central l\1ontana (fig. 1). The core of the range is composed
of Precambrian crystalline metamorphic rocks
that have been intruded and uplifted by a compound
rhyolitic pluton of Tertiary age. Part of the pluton is
exposed, but most of it is still concealed beneath the
Precambrian rocks. Deposits of silver, lead, and zinc
.have been mined from the exposed part, and similar
deposits may be within the buried part.
The gross shape and size of the pluton have been
suggested by detailed geologic mapping and gravity
r----
1 -----------------------,
I
. I
~ Bearpaw Mts , .. Little Ro~
\ HlghwoodM~!:Mt/- ~
\ Area of r*J'ort·.,. 1 .. r;p='·· Judith Mts I
l!"'IOUIU·~
' Little Belt Mts ' )~~"'
-~ HELENAO ,m. Big Snowy Mts N Castle Mts@ t ,' G>'crazy Mts I
'-"' \ M 0 N T.A N A. :
\,. ...----------------___J
\ I
~ ...... _/\.J
0 100 200 MILES
APPROXIMATE
1.-Index mnp of Montana, showing the major Iaccolithic
and volcanic mountain groups (stippled areas) in
central Montana.
studies. l\1aps resulting f:rom the geologic investigations
have been placed on open file by the U.S. Geological
Survey ('Vitkind, 1969 ; l(eefer, 1969).
The Precambrian complex crops out in about 50
square miles in the southern third of the Barker 71j2-
minute quadrangle and the no~thern third ·of the Neihart
71h-minute quadrangle, Cascade and Judith Basin
Com1ties, cen1Jra:l Monttana (fig. 2). It consists of both
paragneisses and orthogneisses which have undergone
two episodes of metamorphism (Catanzaro and l(ulp,
1964). The complex is overlain by Paleozoic rocks
along its northern margin and by rocks of the Precambrian
Belt Supergroup to the south.
The pluton, composed of several quartz rhyolite porphyries
grouped as the Snow Creek Porphyry, was
intruded into the Precambrian basement during the
early Tertiary as indicated by preliminary l(-Ar data
(R. F. l\1arvin, written commun., 1969). Although the
pluton is exposed only in an area of about 11j2 square
miles (fig. 2), several lines of geologic ev1dence suggest
that it underlies a much larger part of the Precambrian
crystalline terrane : ( 1) sharp doming of the
Precambrian basement as reflected by moderate to
steep dips in the flanking sedimentary rocks ; ( 2) geographic
distribution, in the Precambrian complex, of
small quartz rhyolite porphyry dikes that are mineralogically
and chemically similar to the Snow Creek
Porphyry ; ( 3) prograde contact metamorphism in
some Precambrian metamorphic rocks ( microcline incompletely
converted to orthoclase, actinolite grains
with hornblende rimts, fig. 3)., and ( 4) local ·rureas where
the pluton can be seen to intrude and pass beneath the
metamorphic rocks.
Gravity data were collected at 146 stations across
the buried pluton (fig. 2). The data were referenced
to tlie absolute value determined by 'Voollard and Rose
( 1963) for station 'VA 124, Great Falls, l\1ont., air-
U.S. GEOL. SURVEY PROF. PAPER 70G-B, PAGES B63-B65
B63