<p> The Rio Grande Rift is a major tectonomagmatic feature of the North
American craton. Physiographically, the present rift is recognized as a series of grabens
and half-grabens which extend for over 1000 km from south-central Colorado into
Chihuahua, Mexico. Rift structures, however, are recognized as far north as the
Colorado-Wyoming border. Basaltic magmatism within the rift began by 30 Ma in
southern New Mexico, and by 25 Ma in northern New Mexico and Colorado. Within-rift
magmatism is low in volume in comparison to rift-related activity on the rift shoulders and
flanks. </p> <p> Petrologic studies of Rio Grande Rift-related volcanics are numerous, yet
focus primarily on suites erupted in north-central New Mexico and Colorado. However,
recently published abstracts suggest areas of southern New Mexico are receiving much
needed attention. Initial attempts at characterizing the petrologic diversity and mantle
sources of rift-related volcanics have concentrated on major and trace element data, as
well as the application of Sr and Nd isotopes. Only minor attention has been given to Pb
isotope variations. </p> <p> This thesis contains the most recent compilation of Pb isotope data for
volcanic rocks erupted within the Rio Grande Rift region. The oldest rocks included in
this work are· monzonite stocks erupted c. 60 Ma within the Colorado Mineral Belt. Using
Pb isotope data in combination with trace element variations and Sr-Nd isotopes, a
model is developed which suggests the stocks were initially derived from mantle sources
with geochemical properties similar to those which produced rift-related basaltic
volcanics in northwest Colorado beginning at 25 Ma. </p> <p> Two chapters of the thesis are devoted to exploring the utility of Pb isotopes
as tracers of crustal influence in continental basaltic volcanism. One deals specifically
with documenting Pb isotope variations in the northwest Colorado region, whereas the
second focuses on variations in the Espanola Basin of north-central New Mexico. Major
results of the northwest Colorado study 1) suggest that the asthenosphere contributed to
early rift (25 Ma volcanism), 2) better characterize the geochemical signature of
lithospheric and asthenospheric sources during periods of active volcanism, and 3)
confirm earlier suspicions regarding the effects of crustal contamination in several rock
suites. Work on basaltic components of volcanism in the Espanola Basin indicate that
crustal contamination was also an important process in producing the observed Pb and
Sr isotopic variations in both early and later rift lavas. </p> <p> The remaining chapter of the thesis is a synthesis of all available rift data,
from northern Colorado to southern New Mexico. An analysis of changes in the Pb
isotopic corn position of the lithosphere with latitude is presented, as well as a cross-rift
transect of the central rift region. Further, a model which combines previously published
ideas on the tectonomagmatic development of the Rio Grande Rift and the Basin and
Range province is proposed. The most important results of the combined model are the
proposition that rifting began earlier than previously thought, and that the timing of
extension and magmatism in the Rio Grande Rift is very similar to that of the Basin and
Range province. Additional data from the rift, particularly the southern region, will help to
confirm or deny this model. </p> / Thesis / Doctor of Philosophy (PhD)
| Identifer | oai:union.ndltd.org:mcmaster.ca/oai:macsphere.mcmaster.ca:11375/19350 |
| Date | 01 1900 |
| Creators | Heikoop, Cherylyn |
| Contributors | Dickin, A. P., Geology |
| Source Sets | McMaster University |
| Language | English |
| Detected Language | English |
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