The source, magmatic contamination, and alteration of the Idaho batholith

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 delta O-18 (Zircon) throughout the batholith are relatively homogeneous at 6.9 +/- 0.9 parts per thousand (1 SD, n = 21 rocks), despite variations in age and chemistry. Samples from the Bitterroot lobe have an average delta O-18(Zircon) of 7.1 +/- 0.3 parts per thousand (n = 7 rocks), and samples from the Atlanta lobe have an average delta O-18(Zircon) of 7.0 +/- 1.0 parts per thousand (n = 9 rocks). Four epizonal Tertiary plutons have an average delta O-18(Zircon) of 7.2 +/- 0.2 parts per thousand (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 delta O-18(Zircon) of 4.0 +/- 0.1 parts per thousand (n = 1 rock). The zircon data show that the range of normal magmatic delta O-18 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 delta O-18. Average delta O-18(Quartz) for the entire batholith is 10.4 +/-1.1 parts per thousand (1 SD, n = 65 rocks) with a range of 7.7 to 12.4 parts per thousand excluding the Casto pluton. The spread of quartz values is 2.7 parts per thousand 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 delta O-18 during metamorphic events. Quartz data from the Atlanta lobe suggest closed system diffusional exchange during cooling. Igneous garnet delta O-18 values are higher than, and out of equilibrium with, those of coexisting zircon by 0.1-0.9 parts per thousand. Assuming an average delta O-18 for unaltered wall-rock metasediments, the incorporation of 10% metasediment into a magma after crystallization of zircon raises the delta O-18(Magma) by 0.9 parts per thousand, the largest measured Delta (Garnet-Zircon). These quartz and garnet delta O-18 from the Atlanta and Bitterroot lobes suggest magmatic contamination with a high-delta O-18 rock after the crystallization of zircon, and before that of garnet and quartz.