<p> The opening of the Rio Grande Rift of Southern Colorado resulted in a wide variety of igneous features including the Spanish Peaks dual stocks, their surrounding radial dyke system and several proximal enigmatic plugs.</p> <p> Aspects of the crystallization history of two magmas have been studied by using petrographic observations and Crystal Size Distribution (CSD). These were used to examine opaques and feldspars from a rhyolitic dyke in the Spanish Peaks swarm and augite, olivine, and opaques from a gabbroic dyke within Huerfano Butte.</p> <p> Augite and opaques from Huerfano Butte reveal a history of no crystal fractionation during emplacement, no size dependent growth, and constant, continuous nucleation and growth.</p> <p> Huerfano Butte olivine and Spanish Peaks opaques have been subjected to resorption and depletion of nucleii. The resorbed olivine material was probably redeposited as augite. This could explain why Huerfano Butte augite has a much higher nucleation density than the other minerals.</p> <p> The phenocrystic feldspars from Spanish Peaks also have a deficit of nucleii, but they lack resorption features. Xenocrystic skeletal feldspars throughout the dyke lack a fines population and show resorption. The absence of fines is probably due to sorting of grains by density during magma convection before eruption.</p> / Thesis / Master of Science (MSc)

<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)

No description available.

No description available.