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Precambrian rocks of the Sandia Mountains have been dated previously by the U, Th-Pb methods at about 1.46 Ga. Rb-Sr geochronology yields a similar age, 1.44 Ga, although local open-system conditions in the granite are indicated by data that fall off the whole-rock isochron. It is not known if this scatter reflects multi-domains in the granite or whole-rock-scale incipient alteration. Potassium-argon dates for micas yield 1.3 to 1.4 Ga, and Rb-Sr biotite dates yield 1.35 Ga; this age is commonly noted in north-central New Mexico. We interpret this date as either a thermal event affecting the granite, or possibly extended cooling at temperatures high enough to reset mineral Rb-Sr systematics. The Rb-Sr whole-rock age of the Juan Tabo metamorphic rocks at the north end of the Sandia Mountains is 1.61 Ga. Pegmatites and aplites of the same area yield a date of 1.41 Ga, but with a high initial 87Sr/86Sr ratio of 0.712 to 0.714. Granulitic xenoliths from the granite yield a date of 1.48 Ga, and may be older crust sampled by the granite.

Our geochronologic data, and the geochemical data of Majumdar (1985), do not support the hypothesis of Condie and Budding (1979) that there are two major plutons in the Sandia granite. We propose instead one major pluton with some slight and systematic chemical variation within it.

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