THE BADGER COMPLEX: AGES, GEOCHEMISTRY, AND OXYGEN ISOTOPES OF TONALITES IN THE SEQUOIA REGION FOOTHILLS, SIERRA NEVADA, CA

The western Sierra Nevada batholith was constructed between 120 and 105 Ma during convergent margin magmatism that was typified by gabbroic ring-dike complexes, quartz diorites, and hornblende-biotite tonalites. Deep weathering of the tonalitic portions of the western Sierra obscures plutonic contacts and hinders confident estimates of magma production rates in the early Cretaceous Sierran arc. In this study, we use detailed mapping (including magnetic susceptibility), petrography, whole rock (XRF), U-Pb zircon geochronology (LA-ICP-MS), and oxygen isotopes (laser fluorination) to evaluate the origins of a large (>330 km²) tonalite complex west of Sequoia National Park. Key changes in magnetic susceptibility and in outcrops around Badger Creek suggest an internal contact with the Shadequarter tonalite and an adjacent tonalite mass to the north. U-Pb dating of zircon indicates that the tonalite complex is 102.8 to 107.6 Ma (n = 10 ages), with outcrops associated with the Shadequarter tonalite being 106.1 to 107.6 Ma, older than previously reported ages from Chen and Moore (1982). Over 50% of the outcrop area is a mass in the northern part of the complex that yields ages from 105.0 to 105.8 Ma; the youngest ages (102.8–103.0) are from the west-central area of the complex, suggesting that the locus of magmatism migrated westward during emplacement. In general, SiO₂ contents (60–68 wt.%) of different age plutons overlap as do the ranges of most major and trace elements, with the exception of Na₂O, which is higher in the 105 Ma tonalites. The measured δ¹⁸O values of zircon (~2mg concentrates) measured from the tonalites (6.8–7.3‰ n = 5) and calculated δ¹⁸O(WR) (8.0 to 8.8 ‰) are consistent with elevated supracrustal input to their sources compared to younger granodiorites in the central and eastern Sierra, a pattern that has been seen in dominantly tonalitic Fine Gold Intrusive suite (Lackey et al. 2012). Overall, the Badger complex shows relatively constant source chemistry over ca. 5 million years, and a significant, previously unrecognized episode of 105 Ma tonalitic magmatism in the Foothills region of the Sierra Nevada batholith.