Collaborative Research: Caught in the Act- The Petrology of Modern Lower-Crust Formation and Foundering in the North Andean Arc

Earth's continents are thought to have mainly formed in convergent tectonic boundaries, where hydrous fluids transported to depth by subducting plates enable melting of the upper mantle and produce magmas that ultimately contribute to the growth of the overlying crust. Nevertheless, the processes responsible for driving the composition of primitive magmas generated in arc regions, which are basaltic, towards the bulk composition that characterizes Earth's continents, which are andesitic, remain intensely debated.

A major difficulty in understanding these 'differentiation' processes, and how they may couple with other first-order features of convergent continental margins, is the fact that the deep roots of these magmatically-active regions are rarely exposed in the Earth's surface. However, volcanic eruptions are sometimes capable of transporting deep-sourced rock fragments - known as 'xenoliths' - to the Earth's surface, providing Earth scientists with unique glimpses of processes occurring deep within the continents that are otherwise difficult to study.

In the northern segment of the Andean orogen (southern Colombia), a rare eruption from a recent volcanic vent transported an extraordinary cargo of xenolith fragments sourced from the deep roots of the arc and mantle section beneath it. Therefore, this locality presents a unique opportunity to help unravel the physical and chemical processes that operate during construction of continental crust in magmatically-active regions like the Andes.

This award supports two early-career Assistant Professors and a woman who is a PhD student to conduct a detailed geochemical, petrologic and geochronologic study of these unique xenoliths, and the rare eruptions that brought them to the surface. In particular, this research focuses on the 'gravitationally unstable' root of the arc, a section of crustal (i.e., chemically differentiated) rocks with densities greater than the underlying mantle, represented by lithologies such as garnet gabbros, hornblendites, and garnet-bearing pyroxenites.

Mechanical removal of density-unstable roots form the base of arcs, a process known as lithospheric foundering (aka, delamination), has not only been identified as a crucial process to explain the chemistry of Earth's continents but is also thought capable of inducing rapid changes in surface elevation, such as those driving the fast Neogene uplift of the Central Andean Plateau. Because the processes taking place in the lower orogenic crust are critical for understanding the uplift history of mountains, the geochemistry of the continents, and the geophysical imaging of the deep lithosphere, results from this investigation will have important implications for our understanding of the geochemical, structural and tectonic history of Andean-type plate margins.

In addition to the scientific research products that will result from this investigation, the PIs will collaborate with the Rochester Museum and Science Center (RMSC) on activities that will: 1) explain the origin and physicochemical principles of arc volcanoes to the public of Upstate NY; and 2) continue to organize campus visits for high-school participants of RMSC's 'Next Level Science' summer camps, to provide local students with a direct opportunity to experience college and learn more about upper-level STEM education.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.