MICROSCOPIC AND GEOCHEMICAL STUDIES OF ANCIENT, POTENTIALLY RIFT-RELATED PLUTONIC IGNEOUS ROCKS IN COLORADO
Nino, Bella
Nino, Bella
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2024-12-18
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Abstract
The regional geological framework of the area I am studying involves a possible major northwest-trending Cambrian to Ordovician rift zone with abundant igneous rocks in parts of Colorado. These igneous rocks may be related to large volumes of Cambrian igneous rocks located along the same trend in Southern Oklahoma and nearby parts of Texas within the large-scale Southern Oklahoma rift zone. My project focuses on plutonic igneous intrusions located in the Wet Mountains in the southern part of the Front Range in Colorado and the Powderhorn District farther west. This thesis reports the results of microscopic and geochemical studies carried out on three main intrusions in the Wet Mountains as well as important rock units within the Powderhorn District. Samples of gabbro and pyroxenite from the McClure Mountain Complex show evidence of the accumulation of crystals on the floor of the magma chamber. Trace element contents in younger nepheline-bearing syenites suggest these rocks were partly derived from more than one batch of magma injected into a single magma chamber. Nepheline syenites also present in the Powderhorn Complex have somewhat different geochemical contents from those in the McClure Mountain Complex indicating they are not directly related. A large body of carbonatite in the Powderhorn Complex formed by an intrusion of an entirely separate magma that was injected in pulses. The presence of nepheline syenites in the Colorado intrusions requires the production of silica-undersaturated magmas, which could not have been directly related to silica-oversaturated igneous rocks in the Southern Oklahoma rift zone. The Colorado intrusions may have formed in a continuation of the Southern Oklahoma rift zone but were produced by different igneous processes.