CA. 1.4 GA METAMORPHIC TEMPERATURES OF THE SOUTHWESTERN U.S. ESTIMATED FROM REGIONAL 40AR/39AR DATA

Proterozoic lithosphere of the SW U.S. was profoundly modified by a 1.4 Ga regional tectonic episode involving plutonism, metamorphism, and transpressive deformation. This tectonic episode altered the composition of the crust and established tectonic trends that influenced later magmatism, metallogenesis, and faulting. Sixty-eight new 40Ar/39Ar dates and nearly 700 compiled K/Ar and 40Ar/39Ar dates have been analyzed to constrain regional metamorphic temperatures during 1.4 Ga tectonism. We recognize four distinct thermal domains in the SW based on partial to complete resetting of argon systematics in hornblende, muscovite, and biotite: (1) Archean crust in the core of the Wyoming craton remained cooler than ~300°C (40Ar/39Ar biotite >1700 Ma); (2) rocks in ~200 km-wide zone near the southern margin of the craton reached ~ 300-350°C (40Ar/39Ar biotite reset to ~1400 Ma); (3) Proterozoic crust of Colorado and Arizona underwent amphibolite grade metamorphism at ~350-550°C (40Ar/39Ar muscovite ~1400 Ma, hornblende ~1700-1400 Ma), higher T and steep T-gradients prevailed in the aureoles of ~1.4 Ga plutons; (4) Proterozoic crust of northern New Mexico and southernmost Colorado (few ~1.4 Ga plutons) underwent uppermost amphibolite-grade metamorphism at ~500-700°C (1.4 Ga U-Pb monazite and zircon). Mid-crustal exposure levels (2-3 kbars) are prevalent throughout the region. The 1.4 Ga thermal domains correspond to lithospheric provinces established during 1.8-1.6 Ga assembly of the continental lithosphere of the SW. We interpret the regional T-field as a reflection of the differing responses of contrasting lithospheric columns to a widespread mantle thermal event. Thick Archean tectosphere resisted the thermal perturbation that preferentially affected young Proterozoic arc terranes. The S margin of the craton, attenuated during ca. 2.0 Ga rifting, suffered low-grade heating. The transition in S Colorado from mid- to upper amphibolite grade conditions could reflect different crustal exposure levels due to structural juxtaposition or upwarping along the Yavapai-Mazatzal province boundary.