285-1 The importance of open-system magma chamber processes in forming a large and long-lived Half Dome-Cathedral Peak magma chamber in the Tuolumne intrusive complex, California

Abstract:

Cal Barnes’ pioneering research on crustal-scale magma systems emphasized the importance of open-system magma processes. Cal also showed that bridging the gap between macro-scale field geology and micro-scale geochemical analysis leads to critical insights about the complex growth and evolution of plutons. This contribution celebrating Cal’s remarkable career highlights both aspects in understanding the growth and evolution of the 95-85 Ma, 1,100 km² Tuolumne intrusive complex (TIC), Sierra Nevada, California.

The TIC is composed of four asymmetrically nested granodiorite units with contacts representing large gradational zones where the ~700 km² porphyritic Half Dome (pHD) and Cathedral Peak (CP) units are exposed in the inner TIC. Geologic mapping of this ~8 km wide pHD-CP zone indicates a consistent ring pattern of the following concentrically arranged gradational compositional variations remarkably identical in succession across the TIC: The pHD-typical <3 cm euhedral hornblende, biotite, and <1 cm titanite gradually decrease in abundance toward the CP and are absent in the CP. K-feldspar phenocrysts <3-4 cm in the pHD first drastically increase their size to <12-15 cm K-feldspar megacrysts, then gradually decrease to 2-4 cm phenocrysts in the CP and even smaller sizes toward leucogranites in the CP interior. The megacrystic zone is also typically marked by widespread megacryst clusters and schlieren-bound tubes and troughs. K-feldspars also gradually change shape from more tabular in pHD to more blocky where megacrystic and back to tabular in the CP with interruptions in this trend indicating magma erosion.

We propose that the pHD-CP transition tracks the solidification front of a fossilized magma body that underwent gradual in-mixing of ascending CP magma into pHD across the 700 km² TIC interior, keeping it above the solidus ~90-85 Ma (see Durning and Memeti, this meeting). Replenishing magma interrupted a gradually freezing and fractionating magma chamber until the system shut down while preserving older “bathtub rings” of crystallization. Regardless of the size/number of increments that built the inner TIC, the pHD-CP units crystallized as a single entity that was interconnected across the inner TIC. Additional observations support this model: (1) whole rock U-Pb zircon ages are consistent along strike around each transition unit and young inward; and (2) biotite cooling temperature in host rock were reset during pHD-CP time >5 km away from the CP.

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