TITANITE PETROCHRONOLOGY AND BORON-ISOTOPE GEOCHEMISTRY OF THE TUOLUMNE INTRUSIVE SUITE, CA: INSIGHTS INTO PLUTONIC GROWTH AND EVOLUTION

The Tuolumne Intrusive Suite (TIS), a concentrically zoned plutonic suite located in the central part of the Sierra Nevada Batholith (SNB), provides an opportunity to examine how protracted pluton assembly influences the textural and geochemical evolution of Cordilleran plutons. This study integrates titanite petrochronology, whole rock geochemistry, and boron isotopes to investigate the possible role of late-stage, low-temperature (autometamorphic) processes in the observed geochemistry of the TIS. The suite includes five textural units (younging inward: tonalite of Glen Aulin and granodiorite of Kuna Crest, equigranular and porphyritic Half Dome Granodiorite, Cathedral Peak Granodiorite, and Johnson Granite Porphyry). Inter- and intra-pluton age variation shows that these plutons become progressively younger inwards from the margins of the suite, consistent with incremental assembly from approximately 85 Ma to 95 Ma.

Titanite from a transect across the TIS analyzed via LASS-ICP-MS reveals spatial and temporal variations in U-Pb dates and trace-element chemistry that record a spectrum of processes. The decreasing difference between zircon and titanite U-Pb dates (Δtzrc-turn) toward younger units suggests a progressive shift from sub-solidus (650-700° C) re- or neo-crystallization in the outer units to super- or near-solidus conditions in the central units. Trace-element data show systematic chemical transitions, with older units exhibiting broad, multimodal distributions (e.g., ΣREE) and younger units displaying narrower, unimodal distributions, reflecting evolving conditions during pluton growth. Whole-rock boron isotopic compositions (δ¹¹B), analyzed via MC-ICP-MS, provide an additional perspective on fluid sources. Samples collected from hydrothermal features and wall rock possess the lightest δ¹¹B values. These data suggest that the TIS experienced episodic intrusion and thermal rejuvenation, with magmatic and autometamorphic processes stemming from a protracted pluton assembly. This study highlights the complex interactions of rock, melt, heat, and fluid in constructing Cordilleran plutons and offers a framework for understanding growth rates and compositional evolution.