A Stable Isotope Study of Fluid-Rock Interactions in the Saddlebag Lake Roof Pendant, Sierra Nevada, California

Abstract:

The Saddlebag Lake Pendant (SLP) is a ~ 100 km² zone of Ordovician-Cretaceous metasedimentary and metavolcanic rocks just east of the 95-85 Ma Tuolumne Intrusive Complex (TIC) in the Sierra Nevada of central California. Western SLP and adjacent parts of TIC are affected by the Steelhead Lake  Shear Zone (SLSZ), with leucogranitic dikes, abundant qz-tm veins, ductile epidote-chlorite alteration, and massive qz veins. While TIC shows uniform stable isotope values, isotope studies of other Sierra Nevada pendants evidence diversity of fluid sources: Jurassic seawater, Cretaceous magmatic fluids, metamorphic fluids, and meteoric-hydrothermal fluids. We conducted a stable isotope study of 49 samples from units across the SLSZ, focusing on the shear zone. Unlike other pendants, both δ¹⁸ O and δD values from SLSZ showed great variability, and most samples were not in isotopic equilibrium. Overall, δ¹⁸ O mineral values ranged from –1.5‰(plag) to +15.8‰(bt); mineral δD values ranged from –140‰(tm) to –67‰(bt). TIC δ¹⁸ O was +7.8 to +10.0 (plag) and +4.8 to +9.2 (tm), normal magmatic values, and δD were –105 to –75. Paleozoic and Triassic metasedimentary units had most qz δ¹⁸ O from +11.3 to +15.8, so within metamorphic range, and δD from –100 to –72 (ep and tm). Jurassic metasedimentary units (Sawmill) and Triassic metavolcanics (Koip) had largest isotopic variability: δ¹⁸ O qz from +8.1 to +14.8, plag from –1.1 to +11.8, but ep and tm between +1.3 and +9.3 and δD between –108 and –81. All lower (submagmatic) isotopic values were from a wider, possibly transtensional, part of the SLSZ, transected by Sawmill Canyon. Although TIC and many of the Paleozoic units do not show isotopic evidence for alteration, the Koip and Sawmill units were likely infiltrated by later magmatic waters, and then subjected to very localized meteoric water infiltration in the area surrounding Sawmill Canyon.