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10
MORB
5
Bulk earth
BPM carbonatite
0
OlB
Missouri Breaks
-5
diatremes
Zortman alnoite
Bearpaw minettes
Winnett alnoite
-10
Leucite Hil ls
Bearpaw
latites
Wyoming
-15
Crazy Mtns
-20
Smoky Butt e
Highwood Mtns
-25
.702
.704
.706
.708
.710
.712
87
/
86
Sr
Sr
Fig. 5.15 A plot of ε Nd versus 87 Sr/ 86 Sr data from rocks of Wyoming province and comparison
with mid-oceanic ridges basalts (MORB), ocean island basalt (OIB) and bulk earth adopted from
different sources. Solid dots High-K minettes; Open circles Crazy Mountains and Bearpaw
Mountain carbonatites (Dudas et al. 1987); Leucite Hills (Volmer et al. 1984), Highwood
Mountains (O ' Brain et al. 1991) and Smoky Butte (Fraser et al. 1985) (after McDonand et al.
1992)
from both continental extensional zones and subduction-related settings. Such high
ratios distinguish the minettes from the lamproites of Montana province. Usually,
lamproites have higher concentration of such elements as Nb, Ti, and Zr and lower
LILE/HFSE ratio. In a plot of MgO versus SiO 2 , Macdonald et al. noted that in case
of K-rich ma
c lavas of the Bearpaw Mountains, there is decrease in the values of
Cr and Ni and increase in Ba, Ce, Nb, Rb, Y, Zr contents with respect to silica. The
MgO content has a negative correlation with respect to SiO 2 . Likewise Al 2 O 3 ,FeO t ,
CaO, K 2 O and P 2 O 5 systematically increase with the decrease of MgO (Fig. 5.14 ).
The Sr and Nd isotopic data for rocks from Missouri Breaks, Winnett alnoite,
Leucite Hills, Crazy Mountains, Bearpaw Mountains, Highwood Mountains and
Smoky Butte are summarized in Figs. 5.15 and 5.16 . The minettes and lattites show
considerable variation in 87 Sr/ 86 Sr ratio (0.70716
-
0.70978) and (0.70841
-
0.71008),
ε Nd ranges between
respectively acdonald et al. observed that
9.27 and
14.53 in
the minettes, and between
16.73 in the latites. Differential elemental
ratios of the K-rich rocks (La/Nb, Sm/Nd) with respect
15.94 and
ε Nd are shown in
Fig. 5.16 . They observed that the 87 Sr/ 86 Sr ratios have similar span as those from
other alkaline centres in Montana, such as K-rich rocks of Leucite Hills, Crazy
Mountains and Highwood Mountains but
to
ε Nd tend to be more radiogenic.
 
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