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, w ith 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(Zir con) 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 th ousand (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 in dicate a similar magmatic source for all granitic rocks of the Idaho bathol ith 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 th an was previously inferred, despite some samples containing two micas and/o r garnet, or having a mildly peraluminous character. Quartz is more Variabl e than zircon in delta O-18. Average delta O-18(Quartz) for the entire bath olith is 10.4 +/-1.1 parts per thousand (1 SD, n = 65 rocks) with a range o f 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 th at calculated relative to zircon. Quartz data from the Bitterroot lobe and epizonal Tertiary plutons suggest the alteration of primary delta O-18 duri ng metamorphic events. Quartz data from the Atlanta lobe suggest closed sys tem diffusional exchange during cooling. Igneous garnet delta O-18 values a re 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 wa ll-rock metasediments, the incorporation of 10% metasediment into a magma a fter crystallization of zircon raises the delta O-18(Magma) by 0.9 parts pe r thousand, the largest measured Delta (Garnet-Zircon). These quartz and ga rnet delta O-18 from the Atlanta and Bitterroot lobes suggest magmatic cont amination with a high-delta O-18 rock after the crystallization of zircon, and before that of garnet and quartz.