INJECTION OF MAGMATIC FLUIDS FROM THE TUOLUMNE INTRUSIVE COMPLEX INTO THE EASTERN SIERRA CREST SHEAR ZONE: EVIDENCE FROM STABLE ISOTOPES (B, O, H) OF TOURMALINE

Boron, oxygen, and hydrogen isotope data from tourmaline related to the Eastern Sierra Crest Shear Zone (ESCSZ) document fluid migration that originated from the Tuolumne Intrusive Complex (TIC) into this Late Cretaceous dextral shear zone. Evidence for locally sourced metamorphic fluids become more apparent distal to the intrusion. Tourmaline’s resistance to isotopic exchange for B, O, and H makes it an excellent monitor of the early stages of the hydrothermal evolution of a system with a complex fluid history. Early stages of hydrothermal activity both preceded and overlapped the well-documented main-phase flux of meteoric-hydrothermal fluids during ductile shear zone deformation (Compton et al., 2017; Hartman et al., 2018). Tourmaline has three modes of occurrence: (1) pegmatitic dikes, (2) sheared veins, and (3) slickenside surfaces. Electron microprobe analysis of tourmaline indicate schorl compositions characteristic of granitoids and dravite-rich compositions characteristic of Ca-Mg-rich metamorphic lithologies (e.g., Jurassic Sawmill Group) present within veins and brittle slickenside fabrics. Tourmaline hosted by the Sawmill Group and TIC have δ¹⁸O values in the range of equilibrium with magmatic fluids (+6.9 to +8.2‰; n = 15). Alternatively, those from the Triassic Koip Group metavolcanics are higher (δ¹⁸O >8‰, n = 9). Coexisting plagioclase and epidote have δ¹⁸O values out of equilibrium with tourmaline, consistent with meteoric water infiltration into the ESCSZ both before and during dike intrusion. Most tourmaline δD values (–60 to –100‰; n = 23) are within the range of D/H equilibrium with magmatic fluids. Boron isotopic compositions decrease away from the TIC, consistent with a larger metamorphic contribution to the fluids. Undeformed tourmalines from TIC dikes have δ¹¹B values (+1.8 to +6.3‰) consistent with their magmatic arc origin. Lower δ¹¹B values (–2.8 to +0.3‰) from brecciated TIC dike tourmalines are interpreted to be the product of interaction with magmatic fluids that have experienced Rayleigh fractionation. Very low δ¹¹B values (<–5‰) from veins and slickensides hosted by the Koip Group indicate a local, metamorphic source for B. Intermediate δ¹¹B values from veins and slickensides hosted by the Sawmill Group suggest a mixed metamorphic/magmatic source of B.