Collaborative Research: Contribution of mafic magmatism to upper crustal batholiths: A case study of the Sierra Nevada batholith

The dense crust beneath Earth’s oceans is regularly driven beneath the continents in a tectonic process called subduction, which results in the formation of magmas. Such magmas ascend and create long chains of volcanoes like the Cascades of the northwest United States or the Andes in South America. Over time, magmatism at subduction zones has helped build Earth’s continents.

These magmatic processes concentrate silica to create thick and buoyant continents that stand higher than surrounding oceans and oceanic crust, which is a unique feature of our planet. This continental crust is an important source for resources essential to human existence, but the processes that concentrate silica in magmas are not fully understood.

This research will study magmatic processes in the Sierra Nevada mountain range of California, which is the ancient “plumbing system” from the insides of subduction zone volcanoes from hundreds of millions of years ago, now exposed at earth’s surface. This work will study the chemistry of mafic (more magnesium and iron-rich, lower silica) rocks that represent an important compositional ingredient to create the high-silica rocks that form the bulk of the continents.

Extensive existing work on the high-silica rocks at this location will provide context for new measurements of the mafic end-member composition to understand the magmatic processes that build continents. The research will support collaboration between Caltech and Pomona College, including the mentoring of a female graduate student (Caltech) and multiple undergraduate/post-baccalaureate students (Pomona), as well as early career support for a female faculty member (Caltech).

In addition, Earth Science classroom lessons and field trips for middle and high school students from the Big Pine Unified School District (BPUSD) in Owens Valley, located within study area will be developed and conducted. BPUSD serves a student population that is ~50% Native American and >40% Latinx, two under-represented groups in geosciences. The ultimate goal is to increase participation and interest of under-represented students in geosciences through place-based and culturally appropriate lessons that successfully aligned Indigenous ways of knowing and scientific practices with Western science models

The formation of high-silica arc batholiths is an enduring petrologic problem. During flux-melting of the mantle wedge at subduction zones primitive basalts are produced. Upon ascent into the crust, further differentiation of these basalts is required to form more silicic derivative melts.

Although field and experimental studies highlight the importance of lower crustal (>0.7 GPa) fractional crystallization of primitive basalts in generating high-silica melts, this process in detail cannot produce the composition of arc batholiths. In particular, deep crustal fractional crystallization generates peraluminous intermediate and silicic melts, compositions that are not widely observed in arc batholiths.

To reconcile these observations, this research will test the following hypothesis: Deep crustal differentiation produces high-Al, low-Mg basalts, as well as, evolved mildly peraluminous granitic melts. These melts represent endmembers that can mix to form the compositional diversity of granitoids observed in arc batholith. Testing this mixing-model hypothesis has been limited due to the relative lack of studies focusing on the mafic endmember.

Although volumetrically minor and relatively less-studied compared to high-silica granodiorites to granites that dominate batholiths, mafic plutons (non-primitive gabbros and diorites) are widely present in the upper crust of accreted arc sections. Through a collaboration between Caltech and Pomona College this research will investigate the bulk-rock and mineral major/trace element chemistry, geochronology, and oxygen & strontium isotopic compositions mafic plutonic bodies across a transect from a classic continental arc locality, the Sierra Nevada batholith.

This data will be placed in the context of both existing and new granitoid data, as well as, quantitative geochemical and rheologic models to understand whether these mafic plutonic bodies represent suitable mixing endmembers in the production of batholithic granitoids.

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.