| Summary |
Basalt flows and subalkaline to peralkaline ash-flow tuffs of mid-Tertiary age crop out in lower Canyon Santa Clara, 91 km north of Chihuahua City, Chihuahua, Mexico. The rocks are cut by two, east-west trending, normal faults (south blocks down). Between the faults, there is a north-south trending vent containing vertically foliated tuff. North-south trending Basin and Range-style normal faults cut the eastern portion of the area. A mildly alkaline (?) basalt, subalkaline Acantilado tuff, comenditic Cryptic tuff, and strongly comenditic Campana tuff compose a genetically related sequence. These rocks are stratigraphically equivalent to the Bellavista Volcanics (Mauger, 1979). At least AOO m of basalt, with a few interbedded rhyolite tuffs, are exposed. Near the top of the section there is a glassy andesite which resulted from mixing of basaltic and rhyolitic magmas. The Acantilado ash-flow rhyolite overlies the basalt. This unit forms prominent columnar-jointed cliffs averaging 25 m in height. Above the Acantilado tuff is a thick section of peralkaline Cryptic tuff. It contains a basal vitrophyre overlain by a red and white flowed zone. A highly porous zone containing vaporphase minerals overlies this, and continuing upward there is a thick section of micropoikiolitic rhyolite with Na-pyroxenes and Na-amphiboles in the groundmass. Breccias are seen at the top and bottom of each cooling unit. The uppermost unit is the Campana tuff. It forms massive, columnar-jointed cliffs which become 200 m thick atop Sierra La Campana, south of the field area. Typically the Campana is strongly eutaxatic and blue from groundmass riebeckite, but brown to pink varieties such as the Chocolate tuff, contain groundmass aegirine. Low initial Sr⁸⁷/Sr⁸⁶ ratios indicate that the rocks in this area were probably differentiated by fractional crystallization at shallow depths to produce rhyolite magmas that became progressively Na-enriched and Al-depleted. These liquids could have become peralkaline during later phases of differentiation. The magmas probably formed in a tensional back-arc environment that may have had a genetic relationship to early stages of the Rio Grande rift. |
| Local note | Joyner-"Presented to the faculty of the Department of Geology ... in partial fulfillment of the requirements for the degree Master of Science in Geology." |
| Local note | Joyner-"Presented to the faculty of the Department of Geology ... in partial fulfillment of the requirements for the degree Master of Science in Geology." |
| Local note | Joyner-"Presented to the faculty of the Department of Geology ... in partial fulfillment of the requirements for the degree Master of Science in Geology." |
| General note | Presented To the Faculty of the Department of Geology |
| General note | Advisor: Richard L. Mauger |
| Dissertation note | M.S. East Carolina University 1979 |
| Bibliography note | Includes bibliographical references (leaves 95-101). |
| Genre/form | Academic theses. |
| Genre/form | Academic theses. |
| Genre/form | Thèses et écrits académiques. |
