Date of Award
Master of Science
Geological and Atmospheric Sciences
Carl E. Jacobson
The southeast Castle Dome Mountains of southwest Arizona record the Mesozoic to Cenozoic geologic evolution of the Southwest. The age and tectonic setting of the informally named metasedimentary and metavolcanic rocks of Slumgullion have occasioned considerable debate, although some previous researchers have postulated a Jurassic age. Possible correlatives to the rocks of Slumgullion include the McCoy Mountains Formation of southeast California and southwest Arizona and the Winterhaven Formation of southeast California.
Within the rocks of Slumgullion detrital zircon ages were obtained from various metasedimentary units, including quartz arenites, arkosic sandstones, and a lithic arenite, in order to place constraints on depositional age and provenance. Primary igneous zircon ages were also determined for a metadacite from the base of the rocks of Slumgullion and from a monzogranite, which previous workers interpreted as having intruded the rocks of Slumgullion.
Ages from U-Pb zircon dating reveal the base of the Slumgullion may be Jurassic (youngest zircon ca. 160 Ma), but the top of the sequence is no older than latest Cretaceous (three 78-77 Ma zircons). The metadacite and monzogranite appear to be older than the metasediments and likely represent the depositional basement to the Slumgullion sedimentary basin. After deposition of the latest Cretaceous Slumgullion unit, the monzogranite (158 Ma average crystallization age) was faulted over the former.
Age peaks on probability density plots show that quartz arenites from the rocks of Slumgullion have similar source regions as the basal McCoy Mountains Formation and the Winterhaven Formation. This age signature is also similar to that of the Jurassic ergs from the Colorado Plateau. The quartz arenites within the rocks of Slumgullion thus indicate the derivation of sediment from relatively distant source regions. The less mature sandstones in the rocks of Slumgullion represent a different history as indicated by the fact that their sediment is locally derived.
The Orocopia Schist, which is widely viewed as a subduction complex, was also studied from the Castle Dome Mountains. U-Pb detrital zircon data obtained from two quartzofeldspathic samples of the schist are consistent with previous results implying a depositional age of Latest Cretaceous-earliest Paleogene. Results from 40Ar/39Ar thermochronology are also consistent with previous studies. Muscovite ages of ca. 42 Ma from both the Orocopia Schist and upper plate gneiss indicate that slip had occurred on the Chocolate Mountains fault system by this time. There is an ~20 m.y. difference in biotite 40Ar/39Ar ages between the Orocopia Schist and gneiss, but that can be explained by reheating due to Miocene volcanism.
Jonathan Hunter Reis
Reis, Jonathan Hunter, "Jurassic and Cretaceous tectonic evolution of the southeast Castle Dome Mountains, southwest Arizona" (2009). Graduate Theses and Dissertations. 12198.