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4) is in the
same order as mentioned above. Chemically the lamproitic rocks of Smoky Butte
(Table
4.10
) are similar, but mineralogically or texturally the rocks are different
because of different cooling history. According to Mitchell et al. (1987), the rocks
may be termed as hyalo-armalcolite phlogopite lamproite and sanidine-phlogopite
lamproite. These rocks were formed from rapidly quenched, high temperature,
uncontaminated lamproitic magma. These lamproitic rocks are highly enriched in
TiO
2
and thus, the phlogopites are rich in TiO
2
. Armacolites are also abundant. The
minettes are characterized by minerals, which are Al-de
The order of crystallization in the above-mentioned assemblages (1
-
cient and display very little
compositional variation. Sanidine and nepheline intergrowth occurs as pseudoleu-
cite. The lamproites are enriched in Ta, Hf, and light REE (La/Yb = 162
280) and/
or characterized by high MgO and Cr
2
O
3
contents. According to them the lam-
proites were derived from a metasomatized harzburgite source (2.5 Ga old) and they
were emplaced at their liquidus temperatures.
-
4.5.4 Potassic Rocks of Navajo-Hopi Province
Magmatism associated with Rio Grande rift (Fig.
4.13
) has been discussed in detail
by Gibson et al. (1993) and Ehrenberg (1982). The rift is Late Paleocene to Qua-
ternary in age and has a complex geological history. According to them a rift is a
part of a much wider series of events that took place in the western U.S.A. during
the Oligocene, when the area was subjected to extension and widespread mag-
matism in the Basin and Range province. Cordwell (1982) stated that topograph-
ically the Rio Grande rift widens from central Colorado into New Mexico and
Texas. He thought that the axis of the rift is delineated by a broad uplift and an axial
graben that is typically 40 km wide and 15 km deep. There are several localities
which are associated with potassic volcanism: Navajo Hoppi, Dulce dike swarms in
New Mexico and Colorado, Espanola Basin (New Mexico), Cieneguilla, Petaca, La
Madera, Ojo Caliente, El Rito Creek Vent, Cerro Negro, Spanish Peaks and Two
Buttes. Of these, Navajo Hopi, Dulce Dike swarms, Spanish Peak and Two Buttes
are known for K-rich ma
c rocks.
The Navajo volcanic
field (Mc Dowell et al. 1986) outcrops over an area of
100,000 km
2
in the western part of the U.S.A. on the Colorado Plateau. The
province (37
W) is located about 280 km from the present day axis of the
Rio Grande rift (Fig.
4.13
). According to Gibson et al., the Navajo volcanic
°
N, 110
°
field is
of Late Oligocene to Early Miocene in age (25 Ma, Roden et al. 1979; 26
24 Ma,
McDouwell et al.1986). In this locality minettes and trachybasalts crop out as lava
-
flows and dikes, sills and volcanic necks. These rocks include spinel- and garnet
lherzolite. Analyses of K-rich rocks from Navajo Hopi are given in Table
4.11
.
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