Geology Reference
In-Depth Information
CANADA
11 2
O
111
O
11 0
O
109
O
108
O
Sweetgrass Hills
50-54 Ma
United States
Montana
United States
EXPLANATION
Exirusive
Inirusive
Havre
Alkalic Rocks
Bearpaw Mtns.
50-54 Ma
Dialtemes
intrusions
Algonic and
kimberlitic rocks
0
km
50
Little Rocky
Mtns.
60-67 Ma
48
O
Eagle
Buttes
50-53 Ma
Fort Benton
Missouri Breaks
Diatremes, 47-52 Ma
Highwood
Mtns.
50-53 Ma
Great Fa
Judith Mtns.
477-89 Ma
Moccasin
Mtns.
53-66 Ma
Little Selt
M
tns. 56 Ma
Adel Mtns.
60-75 Ma
47
O
Winnett
Sill, 50 Ma
Lewistown
Grassrange
Intrusions
Fig. 4.12 Volcanic
fields of Highwood Mountains and Bearpaw Mountains (after Macdonald
et al. 1992)
quartz-normative latites, which are either overlain or intruded by a variety of
potassic silica-undersaturated ma
c to felsic
flows, dikes, sills and stocks consti-
tuting a younger series (O
Brain et al. 1991). The Highwood Mountains igneous
rocks overlie and intrude a clast-supported water-laid conglomerate unit, which was
deposited on Cretaceous Eagle sandstone and Colorado Shale.
The young volcanic series is 700 m thick and is constituted mainly of ma
'
c
phonolite lava
flow and breccias with tuffs having nearly 10 % crystal. There are
two minette lava
flows. An individual
flow of ma
c phonolite is less than 10 m
thick.
O
7.5 km
2
) of shonkinite/
malignite, alkali syenite/monzonite pile. These rocks are hypabyssal equivalent of
ma
'
Brien et al. described intrusion of several stocks (1
-
c phonolites. The Baldy Stock alkali syenite/monzonite also outcrops in this
area. The K
Ar ages of these intrusives are 52.5
51.5
±
1.8 Ma for Highwood
-
-
Baldy syenite and 50.4
1.8 Ma for Shonkin Sag and Square Butte laccoliths,
which were intruded into the sedimentary rocks near the Highwood Mountains.
During the emplacement of the stock, there was doming of the sedimentary
sequence. High angle normal and reverse faults also were developed during the
process of doming. The stocks,
51.1
±
-
flows and the country rocks are cross cut by dikes
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