Ion Microprobe Depth Profile Analysis of Micron-Scale Metamorphic Rims on Detrital Zircons from the Pelona-Orocopia-Rand Schist
Physical and Biological Sciences
Earth Science (EART 195 - Senior Thesis)
The subduction of oceanic plates beneath continents is one of the fundamental processes responsible for mantle convection and the creation of continental crust in magmatic arcs such as the ancient Sierra Nevada or the modern Cascades. Studies have shown that oceanic plates typically subduct at angles between 30° and 45°. However, many researchers now recognize that there is evidence of low angle (< 25°) subduction of the oceanic Farallon plate along the western margin of North America during the Late Cretaceous. Shallow or flat-slab subduction zones have been observed around the world and are known to play a key role in the way that continents deform, yet opportunities to study their inner workings are rare. The metamorphic Pelona, Orocopia and Rand Schists (PORS) of southeastern California and southwestern Arizona comprise a tectonically exhumed shallow subduction complex of Late Cretaceous to Paleogene age. Prior work has established that the original sediments from which the PORS were derived experienced a full cycle of deposition in the trench; subduction beneath the continent; accretion onto the overriding plate; and metamorphism synchronous with the low-angle subduction of the Farallon Plate. Exposures of metamorphic rocks that have undergone multiple subduction-related events provide us with insight into these obscure, large-scale processes that occur over million-year timescales. Thus, knowledge of the timescale of emplacement and metamorphism of the PORS is critical to understanding the tectonic evolution of western North America and, more generally, shallow subduction margins.
During subduction, thermal and fluid conditions were favorable for the growth of thin metamorphic rims on zircon mineral grains within the PORS. In this study, we employ ion microprobe depth profile techniques for U-Pb dating of micron-scale metamorphic rims on zircons in order to constrain the timing of metamorphism of the Orocopia Schist – the youngest member of the PORS – from the Chocolate Mountains Anticlinorium (CMA) of southeastern California and southwestern Arizona. Zircon U-Pb ages are ideal for geochronology because (1) zircon is highly resistant to chemical attack and (2) the dual decay of 238U and 235U recorded within the crystal has allowed us to better evaluate closed system behavior. Ion microprobe depth profiling provides an advantage over other methodologies due to the low sputtering rates – and thus high spatial resolution – achievable with this technique, thereby allowing us to resolve U-Pb ages from micron-scale rims.
We used the Sensitive High-Resolution Ion Microprobe with Reverse Geometry (SHRIMP-RG) at the Stanford-USGS Microanalytical Center to acquire 134 U-Pb dates from nine samples of Orocopia Schist and one sample of tectonically adjacent gneiss for comparison. SHRIMP-RG analyses of Orocopia Schist zircon from the CMA have revealed that (1) U-Pb ages from metamorphic rims range from 75 Ma to 43 Ma, providing new evidence that the Orocopia Schist underwent subduction-related metamorphism starting as early as 75 Ma and Paleogene exhumation-related metamorphism from early Paleocene up to middle Eocene time; (2) metamorphic rims exhibit a significant ~60 Ma age population, which is consistent with previous studies’ observations of this population; (3) metamorphic rims of the Orocopia Schist characteristically correspond to extremely low (< 0.1) Th/U ratios and low U content, which allows us to more easily identify these rims in the future; (4) metamorphic ages show evidence of a west-to-east younging trend, building on detrital zircon and 40Ar/39Ar evidence of this age trend from previous works.
