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of lamproitic melt was generated within the deep lithosphere possibly due to back-
arc asthenospheric melting as a result of some process linked to extension and
eruption 27 Ma ago in Smoky Butte.
14.5.2 Generation of Potassic Rocks Associated with Rio
Grande Rift
Gibson et al. (1993) discussed about ultrapotassic magmatism associated with Rio
Grande Rift. According to them the rift development (Fig.
14.12
) was connected
with complex geological events in the western USA during Oligocene. During this
period there was extension and widespread magmatism in the Basin and Range
Province, as discussed in the preceding section.
Cenozoic magmatism was associated with plate motion. It is believed that there
was convergence between the North American and Farallon plate with the sub-
duction of the lithosphere. There was rapid fragmentation of the slab and during its
assimilation by the asthenosphere a slab `gap
'
was produced around 35 Ma ago
beneath northern Mexico and New Mexico. This was associated with a E
W-
spreading zone of calc-alkalic magmatism (Coney and Raynolds 1971), which is
extended up to the western margin of the Great Plains, and covers volcanic
-
fields of
Mogollon
fields. The Rio Grande
Rift is located close to the eastern edge of the Tertiary magmatic arc and the
development of the rift was initiated immediately after this volcanic episode. There
Datil (39
24 Ma), San Juan and later volcanic
-
-
UTAH/ARIZONA
COLORADO/NEW MEXICO
KANSAS
TWO
ESPANOLA
SPANISH
PEAK
S
BUTTES
NAVAJO
DULCE
BASIN
X
-100
mgal
RIO GRANDE
-200
-30
0
50
LITHOSPHERE
km
100
ASTHENOSPHERE
150
200
111
110
109
108
107
106
105
104
103
102
Degrees West
PRES
ENT
RIFT
OLIGO-MIO EXTN.
Fig. 14.12 Gravity versus distance profile along west
east direction for the Reo Grande Rift
region. The axis of the rift is delineated by a broad uplift on both sides of an axial graben, which is
540 km wide and 15 km deep (after Gibson et al. 1993)
-
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