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incompatible elements as Rb, Sr, Zr, Ba with LREE and K. The compatible element
contents likewise increase with increasing MgO content. Beard and Glazner (1998)
observed that
87
Sr/
86
Sr ratios decrease and the
∈
Nd
values increase with
decreasing MgO. The Sr and Nd isotope compositions of these lavas are extreme,
compared to most other continental and oceanic rocks. The
87
Sr/
86
Sr ratios range
from 0.7121 to 0.7105 and
the
∈
Nd
values range from 16.9 to 15.4. The Pb isotopic
(
206
Pb/
204
Pb
207
Pb/
204
Pb
ratios
are
as
follows:
17.2,
15.5 and
*
*
208
Pb/
204
Pb
38.6).
Castor (2008) studied the Sulphide Queen carbonatites and associated alkaline
complexes, which contain the rare earth ore bodies at Mountain Pass in California.
The carbonatites are moderately dipping tabular intrusion associated with 1.40 Ga old
alkaline plutons of similar sizes and orientation. The Mountain Pass carbonatite and
alkaline rocks lie in a 130 km long narrow belt. Both the carbonatite and the asso-
ciated alkaline rocks are chemically and mineralogically unusual. The ma
*
c members
of the Mountain Pass alkaline rock suite belong to the ultrapotassic group, and are
most akin to the lamproitic subdivision. The association with signi
cant amounts of
carbonatite is also unique. Chemical similarities suggest that although the carbona-
tites, are slightly younger (1.38 Ga), they are related to the ultrapotassic rocks. It is not
clear if the carbonatite and associated alkaline rocks came from a common magmatic
parent, but they likely came from a common enriched source in the mantle. Analyses
of K-rich rocks from Deep Spring Valley are given in Table
4.13
.
4.5.11 Other Localities in U.S.A
Kalsilte-bearing ultrapotassic rocks have been described from a 350 km long belt,
which trends northwest-southeast, and stretches from the Darby Mountains of the
Eastern Seaward peninsula to Kobuk-Selawik low lands. The potassic rock com-
plex was emplaced during middle Cretaceous, 97
110 Ma ago. The rocks pre-
dominantly belong to peraluminous K-rich nepheline syenite series. The belt also
includes minor amount of ultrapotassic rocks similar in mineralogy and chemistry
to potassic rocks of Highwood Mountains of Montana. Some of the rocks are
characterized by high concentration of kalsilite.
-
4.6 Potassium-Rich Silica
-
Under Saturated Rocks
from Brazil
The Alto Paranaiba Igneous Province (Fig.
4.17
) is the most important potassium-rich
petrographic province that occurs around the margin of the Ordovician to Cretaceous
sedimentary Parana Basin in Brazil and Paraguay (Gibson et al. 1995). The Alto
Paranaiba Igneous Province comprises wide variety of igneous rocks in the form of
dikes, pipes, plugs, diatremes,
lava
flows, pyroclastic deposits and plutonic
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