Just a few key features allow Earth to be easily recognized from its planetary neighbors: water, continental crust (made of granitic rocks) and plate tectonics. These features are likely interlinked, and appear to be essential for the evolution of a planet that can support human life. This study focuses on the origin and emplacement of granitic rocks, which form the stable continental crust, without which terrestrial life could not have evolved. The origin of granite has been the source of much debate for more than a century, as successive hypotheses survive, are modified, or abandoned in the face of new observations. They weigh in on the debate by examining two currently popular models for the evolution of the Sierra Nevada Batholith in California, which is home not only to the crown jewels of the National Park System (Yosemite, Sequoia and Kings Canyon), but which has long served geologists as a type example of how granitic crust is formed in the plate tectonic setting known as a 'subduction zone'. Subduction zones are regions of Earth's surface where one tectonic plate sinks beneath another, in the process generating both earthquakes and volcanoes. These tectonic regions are also referred to as 'arcs', for the arcuate patterns of volcanoes that such a setting develops, such as the Japanese, Aleutian or Marianas Islands. It is in these regions that granites are thought to form. When such arcs develop granitic rocks, which are buoyant and difficult to subduct beneath another tectonic plate, these eventually amalgamate to form the continents as seen today.
In this project, the team will investigate the two key means by which granitic rocks of the Sierra Nevada Batholith, California are thought to be created: crystallization differentiation, and partial melting of pre-existing lower and upper crust. Recent work has suggested that earlier-formed crust, which consists mostly of basalt and overlying sediments, is heated by the intrusion of more newly-formed basaltic melts, and partially melted to form granitic melts directly. These granitic melts then rise through existing crustal materials to accumulate at shallow depths. An alternative hypothesis is that newly formed basalts are transported to the upper crust directly, and there differentiate to form the granitic materials that now dominate the landscape of the Sierra Nevada. The contrasts between these two hypotheses are significant. In the first case, little new crust is being created during Late Mesozoic time?older crust is simply being recycled and remobilized. The age of the crust then is then older than the age of the youngest granites in this model. In contrast, when newly formed basaltic melts differentiate directly to form granite, then this represents a new addition of granite to the crust. There is no better place to test these models than at the Guadalupe Igneous Complex (GIC) of the western Sierra Nevada. This Jurassic-age pluton (~151 Ma) contains very rare exposures of the newly-formed basaltic melts that intruded the crust at the time that the GIC granites formed: our goal is to test whether such melts primarily caused heating and partial melting of pre-existing crustal materials, or whether they differentiated directly to form granite. Another advantage to our study of the GIC is that the rocks in this region provide an unusual glimpse into the crust into which the newly formed basalts were intruded. Unlike other granitic plutons of the Sierra Nevada, they can test whether pre-existing crustal materials provide an appropriate source material for the formation of granitic rocks within the GIC.
