MULTIPLE EXTENSIONAL EVENTS IN THE SOUTH CENTRAL MOJAVE; EVIDENCE FROM DIKES IN THE COWHOLE MOUNTAINS

The Cowhole Mountains, located south of Baker, CA in the south central Mojave Desert, may be the easternmost exposures of the Independence Dike Swarm (IDS), which is an important structural and temporal feature of the Mesozoic continental margin. Other workers initially assigned a mid-Jurassic age (ca. 148 Ma) to the swarm, but recent work has suggested at least another extensional event during the Cretaceous (ca. 90-95 Ma). Multiple extensional events, with no apparent change in structural orientation or style over approximately 60 m.y., require a different tectonic interpretation for this region.

The majority of dikes in the Cowholes are near vertical and occur in approx. the same orientation as IDS dikes (ca. 300-320 degrees). Dikes sampled in the Cowholes are strongly bimodal in composition and aphanitic, with rare intermediate porphyritic dikes. Our goal is to decipher relative ages of dikes through the use of field relationships, geochemistry, and petrographic data, and to identify different types of magma and different styles of metamorphism. These relative ages could be used to differentiate between Jurassic and Cretaceous dikes. Although mafic and felsic dikes are in close proximity, there is no evidence of mingling or even crosscutting relationships that might suggest interactions or coeval implacement.

Lower greenschist facies metamorphism is ubiquitous, but primary igneous fabrics of the dikes are preserved, in addition to zoned plagioclase and igneous hornblende. Some plagioclase phenocrysts have been partially altered to sericite and epidote, and chlorite is common. In instances, clinopyroxenes have been replaced by clinoamphiboles, and euhedral sphenes are present. Complete major and trace element abundances have been obtained from mafic (SiO2 <53%, MgO=5-11.3%), intermediate (SiO2=54-65%), and rhyolitic (SiO2=70.3 - 79.3%) dikes. Element variation diagrams do not reveal unambiguous magma types or petrogenetic trends for the suite of dikes sampled. Mineral compositions have been obtained by SEM-EDS and microprobe, in order to identify primary igneous phases and constrain metamorphic reactions.