Geology Reference
In-Depth Information
4.5.3 Smoky Butte
Lamproitic rocks from Smoky Butte (47
W) were studied by Bergman
(1987) and Mitchell et al. (1987). The locality lies west of Jordan in the Gar
°
19
N, 107
°
3
eld
County, Montana. The butte is located at the centre of the series of the dikes and
plug-like bodies extending for a length of 3 km. The intrusive bodies lie along the
axis of a syncline. The rocks of Smoky Butte are K-rich and their chemistry is
similar to lamproitic rocks of Leucite Hills, Wyoming and West Kimberley prov-
ince of Australia. According to Mitchell et al. (1987), the K
Ar ages of the rocks of
Smoky Butte are 27 Ma old, and in effect, are older in age compared to those of
Leucite Hills, Wyoming. The dikes of northern section trend N32
-
°
E, but those at
the south trend N25
°
E. They are separated by a system of post-intrusion faults with
200
west.
The dikes are aphyric and melanocratic. They may also occur as light grey or
brown vesicular or galssy rocks with olivine and phlogopite phenocrysts. The dikes
have thickness ranging from a few cms up to 2 m at Shiprock. They may exhibit
300 m offset. The fault plane strikes east
-
-
flow differentiation features and have chilled margins.
In the northern section, highly vesicular lamproites and also autolithic and
heterolithic lamproite breccias are found. Similar lamproites are also found in
Instrument Butte in the southern section. Matison (cited by Mitchell et al. 1987)
described them as plugs, but they are cone-shaped buttes. According to Mitchell
et al. these buttes comprise complex breccias characterized by extrusive and hyp-
abyssal K-rich rocks. Ropy vesicular
ow tops can be observed at Top Contact
Butte and Radial Dike Butte. Mitchell et al. described quite a few
ows (about
20 cm thick), which are superimposed on one another at Top Contact Butte. They
reported the occurrence of vesicular pumice at Radial Dike Butte. Lamproitic
breccia containing sandstone xenolith and associated with ropy lava occurs at the
western part of Half Sediment Butte.
Bergman (1987) described the occurrence of extrusive K-rich rocks at Smoky
Butte. Mitchell et al. also found extrusive rocks occurring as bedded welded
autolithic lamproite lapilli and welded agglutinate tuffs in clay-rich matrices. These
extrusive rocks overlie or are marginal to the intrusives.
Complex breccias are absent in the southern section, where dikes occur as nearly
parallel intrusive bodies. The largest dike occurs at Smoky Butte, where the earlier
dikes are surrounded by latter batches of magma, which got devitri
ed to coarse-
grained rocks. Mitchell et al. described following four assemblages from the Smoky
Butte rocks:
(1) Olivine-phlogopite-armalcolite-diopside-glass,
(2) Olivine-phlogopite-armalcolite-leucite-glass,
(3) Phlogopite-armalcolite-diopside-leucite-sanidine
±
glass, and
(4) Phlogopite-armalcolite-diopside-sanidine-K
Ti-richterite with or without glass.
-
 
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