Zircon trace element, and O and Hf isotopic records of magma sources and pluton assembly in the Sierra Crest intrusions (Sierra Nevada batholith, USA)

Abstract:

The Sierra Crest Intrusions of the Sierra Nevada Batholith are the last major magmatic pulse associated with the Cretaceous flare-up. They are characterized by long assembly times (several 10⁶ years), and are normally zoned from marginal, horblende-biotite granodiorites to more felsic, K-feldspar megacrystic, biotite granodiorites.

Combined trace element and O and Hf isotopes on zircon are presented from the major Sierra Crest Intrusions. Zircon saturation temperatures (T_Zrc,sat) are similar and low (ca. 700°C) for most of the individual units, but Ti-in-zircon temperatures (T_Zrn,Ti) and trace element ratios contrast strongly between outer marginal units and inner megacrystic units (low T_Zrn,Ti ≈ T_Zrc,sat, high Yb/Gd, low Th/U, high and similar Hf, and high Eu/Eu*). Zircon O and Hf isotopes vary markedly across the suite (Δε_Hf = 15; Δδ¹⁸O = 2.5‰). Individual intrusive suites (gabbro to high-silica granite) record variable O-Hf variations; no correlation (John Muir), subtle binary or ternary arrays (e.g., Whitney, Sonora), or bimodal distribution of values (Tuolumne). In some cases single hand samples (small-volume mafic or felsic units), may record the entire variability within a suite. Inner megacrystic units generally have lower ε_Hf than outer marginal units. Whole rock geochemical data for the intrusive suites also show an increase in the "garnet signature" with time (higher Sr/Y and Dy/Yb).

The isotopic data are consistent with variable mantle sources and progressively cooler, more water-rich magmatism with a simultaneous shift to greater crustal involvement, and deepening of the magma sources. Magmatic underplating and intraplating of mafic arc magmas produced increasing crustal assimilation but under PT conditions that allowed production of more felsic, zircon-saturated, magmas. The isotopic variability requires that plutons are amalgams of many magmas mixed at varying scales before final solidification.