Abstract
Zircons from Cretaceous and Tertiary granitic rocks of the Idaho batholith preserve the best record of magmatic oxygen isotope ratios. The coupling of oxygen isotope data from refractory minerals, such as zircon and garnet, with that from less refractory minerals, such as quartz, reveals the complex evolution and alteration history of the Idaho batholith. Values of δ18O(Zircon) throughout the batholith are relatively homogeneous at 6.9±0.9‰ (1 SD, n=21 rocks), despite variations in age and chemistry. Samples from the Bitterroot lobe have an average δ18O(Zircon) of 7.1±0.3‰ (n=7 rocks), and samples from the Atlanta lobe have an average δ18O(Zircon) of 7.0±1.0‰ (n=9 rocks). Four epizonal Tertiary plutons have an average δ18O(Zircon) of 7.2±0.2‰ (n=4 rocks). These data indicate a similar magmatic source for all granitic rocks of the Idaho batholith except the Tertiary epizonal Casto pluton which has a δ18O(Zircon) of 4.0±0.1‰ (n=1 rock). The zircon data show that the range of normal magmatic δ18O is considerably more restricted than was previously inferred, despite some samples containing two micas and/or garnet, or having a mildly peraluminous character. Quartz is more variable than zircon in δ18O. Average δ18O(Quartz) for the entire batholith is 10.4±1.1‰ (1 SD, n=65 rocks) with a range of 7.7 to 12.4‰, excluding the Casto pluton. The spread of quartz values is 2.7‰ larger and typically higher than that calculated relative to zircon. Quartz data from the Bitterroot lobe and epizonal Tertiary plutons suggest the alteration of primary δ18O during metamorphic events. Quartz data from the Atlanta lobe suggest closed system diffusional exchange during cooling. Igneous garnet δ18O values are higher than, and out of equilibrium with, those of coexisting zircon by 0.1–0.9‰. Assuming an average δ18O for unaltered wall-rock metasediments, the incorporation of 10% metasediment into a magma after crystallization of zircon raises the δ18O(Magma) by 0.9‰, the largest measured Δ(Garnet-Zircon). These quartz and garnet δ18O from the Atlanta and Bitterroot lobes suggest magmatic contamination with a high-δ18O rock after the crystallization of zircon, and before that of garnet and quartz.
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