Abstract

Amphibolites from the Manzano Mountains, New Mexico, include two chemically and microstructurally distinct amphibole populations. Petrographic and electron-microprobe studies show that early actinolite is overgrown and crosscut by younger foliation-forming hornblende. An older foliation defined by actinolite porphyroclasts (S1) is discordant to the regionally extensive hornblende foliation (S2). Microstructural relationships both in amphiboles and muscovite-chlorite schists indicate that S1 and S2 record two distinct episodes of metamorphism and deformation, rather than a single progressive event. 40Ar/39Ar geochronologic analyses on hornblende, actinolite, muscovite, and biotite constrain timing of the youngest metamorphic/deformational event. Most amphiboles yield complex 40Ar/39Ar age spectra, but two hornblende concentrates give preferred ages of 1410 ± 12 Ma and 1399.1 ± 5.4 Ma, and one actionlite has a preferred age of 1391.6 ± 5.0 Ma, suggesting cooling below ∼450°C at this time. These ages are interpreted to record the timing of near-peak metamorphism. Muscovite sampled over a 1.5-km vertical section of muscovite-chlorite schists shows an age discordance, with the structurally highest sample being ∼50 m.y. older than the structurally lowest sample. This age discordance is interpreted to sugest cooling from ∼300°C at 0.5°C/m.y. following the peak of ca. 1400-Ma metamorphism. Biotites from similar structural levels yield variable preferred ages, which range from 1402.6 ± 5.1 to 1267.8 ± 5.7 Ma and corroborate the slow cooling suggested by the muscovite results.

Together, the thermochronologic, structural, and petrologic data support a model of regional deformation, metamorphism, and mineral growth at ca. 1450–1400 Ma. These data add to a growing body of evidence from the southwestern United States that ca. 1400-Ma plutonism was not anorogenic, but rather was contemporaneous both with metamorphism and deformation.

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