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was probably a change in the east-dipping subduction 30 to 40 Ma ago to a strike-
slip margin near the coast of Paci
c because of the development of the San Andreas
transform system (Gibson et al. 1993). Lipman (1980) considered that the Rio
Grande depression was caused due to decreased rate of convergence and subduction
resulting in the back
floor of the asthenosphere and interaction with the overlying
lithosphere. Wernicke et al. (1987, cited in Gibson et al. 1993) however, presented a
different hypothesis, according to which extension related to thermal relaxation and
gravity-driven spreading of materials resulted in thickening of the crust during
Larmide compression. They thought that the Mid-Tertiary extension was con
ned
to the Rio Grande Rift. Due to thrusting during the Mesozoic period, there was
crustal shortening. Gans et al. (1989) however, believed that the spatial and tem-
poral distribution of Cenozoic magmatism has nothing to do with zones of
Mesozoic shortening or Cenozoic plate motions. According to them, the earlier
magmatism in the Basin and Range province, predates the beginning of crustal
extension and is related to upwelling of the asthenosphere but not due to stretching
of the lithosphere.
The region of potassic magmatism is far inland within the continental margin at a
distance of about 1,500 km and has been interpreted due to a shallow dipping
subduction zone beneath the area. Increase in the K
2
O content with respect to SiO
2
and the K
2
O/SiO
2
ratio of the Cenozoic volcanic
field is supposed to have good
correlation with the distance from the trench axis of the Pacific coast. This is related
to steepening dip of the slab 40 to 20 Ma ago after truncation and later detachment
of the slab by the San Andreas fault. Rowell and Edgar (1983) observed that except
for the Leucite Hills lamproites, there is in general NE
SW younging trend of the
K-rich Cenozoic magmatism, which coincides with the younging trend of the outer
limit of magmatism. On the basis of this observation, they suggested that potassic
magmatism in the western USA is related to subduction rather than intraplate
tectonic activity. Age determination of the potassic rocks, using K/Ar technique and
geochemical data of Cenozoic K-rich ma
-
c rock localities in an around Rio Grande
rift such as Spanish Peaks, Elkhead Mountains, Dulce dike swarms and Two Butte,
show that there is no simple correlation between age and spatial distribution of the
K-rich ma
c rocks in Colorado, New Mexico and Arizona. Helmstead and Doig
(1975) suggested that eclogite, xenoliths within Navajo-Hopi minettes were origi-
nally considered to be fragments of subducted Mesozoic oceanic crust, which was
perhaps genetically related to the host magma, but recent observation shows that
eclogites are of Precambrian age and may not be related to Cenozoic subduction-
related event. Roden et al. (1979) considered that they are unrelated to the genesis
of minettes. Variable temperature history of the garnets from the Thumb have been
considered to be due to local melt-mantle interaction, and the peridotite xenoliths
are part of the continental lithosphere and not associated with the Farallon plate.
According to Gibson et al. (1993) the Chondrite normalized plot of incompatible
element concentration in the K-rich ma
c magmas of the Rio Grande rift zone has
inter-elemental ratios similar to the plots of rocks produced due to subduction-
relatedvolcanism. The contents of LILE and LREE are more in the K-rich ma
c
rocks from the Rio Grande rift zone than in subduction-related volcanic rocks.
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