Crystalline Records of Mafic Arc Magmas Across the Sierra Nevada Batholith, California

Citation

Ryan-Davis, Juliet Rose (2025) Crystalline Records of Mafic Arc Magmas Across the Sierra Nevada Batholith, California. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/y34m-jj74. https://resolver.caltech.edu/CaltechTHESIS:06022025-035505104

Abstract

Our planet Earth is unique among the rocky planets of the solar system in having compositionally evolved continental crust—the reason for life as we know it. Processes that create continental crust occur at subduction zones where magmas differentiate, evolving from primary mantle-derived basalt to produce a diverse range of compositions that erupt at arc volcanoes and comprise the bulk of the continental crust. Many processes for differentiation have been proposed, yet, recognizing and evaluating the role of pre-existing crust in these processes remains challenging.

This thesis includes two studies of mafic intrusions in the Sierra Nevada batholith—the well-studied Mesozoic continental arc of the western North American Cordillera. The studies are grounded in field and petrographic observations. By targeting the intrusive record, they document the time-integrated effects of mafic magmatism over tens of millions of years within a volcanic arc.

Chapter 2 is a petrologic study of the most primitive intrusive rocks of the batholith, exposed in the Emigrant Gap complex. The complex ranges compositionally from ultramafic cumulates to intermediate granodiorite. The petrology, geochemistry and field relationships of these plutonic rocks reveal that—in addition to crystallization—open-system processes (melt-mush reactions and hybridization between the mafic products and a crustally contaminated felsic magma) produced the distinctive geochemical and petrologic trends of the complex.

Chapters 3 and 4 represent a broad study of mafic intrusions within a 250 km by 100 km swath of the central Sierra Nevada (37±0.5°N latitude). This regional perspective on mafic magmatism spans ~110 million years and crosses several tectonically-assembled belts, each with distinctive geology. The two chapters document systematic differences in field relationships and geochemistry of the mafic intrusions that vary by geologic belt, as the mafic magmas interacted with heterogeneous crust. Chapter 3 presents field relationships, ages, and major and trace element chemistry. Chapter 4 presents isotopic compositions (O, Hf, Sr, and Nd), revealing that contamination of mafic magmas by ~10–20% assimilation of the local crustal column is inevitable, and that it occurs early during differentiation.

Thesis Files