A study of the temporal evolution of the El Capitan Granite using U-Pb zircon geochronology (2011)
Undergraduate: Miquela Ingalls
Faculty Advisor: Drew Coleman
Department: Geology
The El Capitan Granite sits in the Yosemite Valley Intrusive Complex, which formed by arc magmatism in the Cretaceous period during subduction of an oceanic plate westward beneath the North American plate. It is believed that this west to east arc migration can be traced using geochronologic data of plutons across the Sierra Nevada Batholith. Studies using older techniques have supported this magma migration as well as a coeval mixing relationship between the El Capitan Granite and the mafic bodies that are interspersed. In this study, we use ages determined by U-Pb zircon geochronology to address the following objectives: 1) calculate the filling rate of the El Capitan Granite across the exposed volume of the pluton; 2) determine whether the intrusion followed the proposed west to east arc migration; and 3) elucidate whether the mafic rocks are temporally related to the El Capitan Granite. A 206Pb/238U age of 105.45±0.25 Ma was determined for westernmost point (Cookie Slides), and an age of 105.51±0.58 Ma for the summit in the east. These statistically indistinguishable ages along with textural and compositional homogeneity suggest that the El Capitan Granite had a faster intrusion rate than neighboring intrusive suites. Given a concordia age of 103±0.15 Ma for a 2 km2 mafic complex (the diorite of the Rock Slides) sitting between the two granitic samples, it can be said that the intermingled mafic bodies intruded 2 My after the crystallization of the El Capitan Granite and were not the product of magma mixing.