Estimates of the time-temperature history of geologic samples are often based on 40Ar/39Ar ages obtained through incremental-heating experiments on minerals with different Ar retentivities. Laser incre mental heating of single crystals of muscovite and biotite from an ∼1700 Ma monzogranite from a low-pressure- high-temperature metamorphic terrain in central Arizona suggested an essentially uniform distribution of radiogenic 40Ar in each crystal and yielded ages of 1412 ±5 Ma for the muscovite and 1410 ±10 Ma for the biotite. The similarity in these ages implies rapid (∼25 K/m.y.) cooling of the monzogranite after a reheating episode slightly before 1412 Ma. In contrast, detailed age mapping of single crystals from the same samples with the laser microprobe revealed large radiogenic 40Ar gradients indicative of very slow (<1 K/m.y.) cooling and a closure interval for Ar diffusion lasting >400 m.y. Evidence for such low cooling rates has important implications for the longevity of high thermal gradients in the Early-Middle Proterozoic lithosphere. In addition, inconsistency between the laser mapping and incremental-heating results for these micas suggests caution when interpreting incremental-heating data for slowly cooled samples.

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