Geology Reference
In-Depth Information
4.5.7 Two Buttes, Colorado
The Two Buttes (Fig.
4.13
) are located 310 km from the present day rift axis and
lies in the Arkansas valley (37
W), 240 km east of the Canyon city.
They are intruded into Triassic shales and sandstones and Cretaceous limestones.
Bergman (1987) classi
°
30
′
N, 102
°
30
′
ed them as minettes. According to Gibson et al. (1993), the
K/Ar technique yielded ages ranging between 27 and 35 Ma. Chemical analyses of
some of the K-rich mafic magmas are shown in Table
4.11
.
Williams (1936) described potassium-rich volcanic rocks from Hopi Butte,
Arizona and Navajo Mountain, Utah. In Hopi Butte, the alkaline rocks occur as
pyroclastics and lava
flows of limburgites, monchiquites, and nepheline-bearing
trachytes. The eruption of lavas in this area took place sometimes between late
Pliocene and Pleistocene. Williams described the occurrence of olivine leucitite
from Todilto Park of this petrographic province.
4.5.8 Potassic Rocks from Leucite Hills, Wyoming
Leucite-bearing lavas and hypabyssal rocks were
rst reported by Cross (1897) and
later by Kemp and Knight (1903). These rocks were more recently studied by
Kuener (1980). They cover an area of 2,000
2,500 km
2
in the Sweetwater County,
located in the southwestern part of Wyoming (Fig.
4.14
). The locality lies 100 km
-
Steam Boat
C
E
LEUCITE HILL
WYOMING
C
D
B
Boars Tusk
N.Table
S.Table
North Pilot
Matthew
Rock Springs
Endlich
Hallock
Hague
Black Rock
Orenda
B
N
C
Hatcher
Cross
Osborn
Emmons
Superior
Badgers
B
Zirkel
Teeth
A
Pre Laramie
B Laramie
C Wasatch
D
D
A
Point
of Rock
B
C
Green River
E Bridger
Cones
05 0
Pilot
Rock Springs
Fig. 4.14 Sketch map describing different localities known for leucite-bearing lamproites from
Leucite Hills (after Barton and van Bergen 1981)
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