Geology Reference
In-Depth Information
5.6 Minor and REE Geochemistry of K-Rich
Silica
cient Volcanic Rocks from Highwood
Mountains
De
-
'
O
Brien et al. (1991) noted an overall increase in the REE abundance from prim-
itive to more evolved K-rich rock types. This is in agreement with the subtraction of
ma
c phases having low D-values for REE. According to them due to extensive
apatite fractionation from the magma, overall abundance is low and the patterns are
concave upward, re
ecting higher apatite D-values for the middle REE.
On the basis of geochemical studies of K-rich magmatic rocks from Highwood
Mountains, Smoky Butte, Haystack Butte and Crazy Mountains, O`Brien et. al.
suggested that in the generation of K-rich ultrama
c Highwood Mountain rocks,
phlogopite in the minettes probably played an important role in the generation of
minettes. According to O
'
Brien et al. the high enrichment of Ba (2,000
500 ppm)
-
and Sr (850
1,200 ppm) are in agreement with the idea that mica and clinopyroxene
were present in the mantle source region. Presence of relatively high La/Yb ratio
might indicate that garnet probably was an important phase in the residuum. This
view is based on available crystal/liquid partition coef
-
cient of these elements.
They found that the rocks from Highwood and Bearpaw Mountains have high
87 Sr/ 86 Sr ratios related to the Nd
Sm trends (Fig. 4.12, Chap. 4 ) of the rocks of
Missouri Breaks, Leucite Hills and Smoky Butte. Figure 5.18 related to different Pb
isotopic ratios of that region refer to heterogeneity of the mantle (O
-
Brien et al.
1991). Figure 5.20 refers to the minor element and REE concentration of K-rich
volcanic Rocks Associated with Rio Grande Rift.
The concentration of incompatible trace elements at Navajo-Hopi volcanic
'
eld
located towards the west of the ridge and in the two Buttes are very similar (Gibson
et al. 1993; Fig. 5.19 ). Samples of these localities are enriched in incompatible
elements; for example, (Ba /Y) N ranges between 17.5 and 27.6 in Navajo volcanic
field and between 10.3 and 20.4 at Two Buttes. They showed that their chondrite-
normalized incompatible element patterns show the presence of a trough at Nb and
Ta and the (La/Th) N ratio is equal to 1.5 and in the cases of Two Buttes, it is equal
to 1.6 in the Navajo volcanic
field. They further showed that the intrusive rocks at
the Two Buttes are relatively less enriched in the trace element contents than those
from the Navajo volcanic
field, but geochemical difference between the two
localities occur in the concentration of LREE with respect to HREE. For example,
the (La/Yb) N ratio of the Two Buttes range between 12 and 25 in contrast to 45
86
-
in the Navajo volcanic
eld.
They noted that the Oligo
Miocene magmas at Spanish Peaks and Dulce are rich
in LILE and depleted Ti. The dykes here have K 2 O/Na 2 O ratio > 1, and are
enriched in LREE with respect to HREE [(La/Yb) N = 7.46] and depleted in HFSE
such as Nb and Ta relative to LREE [(La/Ta) N = 1.5]. The dyke from Spanish Peak
has higher K 2 O content (7.13 wt%) compared to other dykes in the rift shoulders,
but the La/Ta ratio (1.0) is similar to that of Ocean Island basalt. Gibson et al.
(1992) found that the 87 Sr/ 86 Sr ratios of the rocks of the Two Buttes range between
-
 
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