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1 Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131 U.S.A.
2 Department of Earth, Atmospheric, and Planetary Science, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 U.S.A.
3 New Mexico Bureau of Mines and Mineral Resources, New Mexico Tech, Socorro, New Mexico 87801 U.S.A.
In the Rincon Range, north of Mora, New Mexico, a relatively abrupt regional change in dominant fabric orientation occurs within Paleoproterozoic rocks which are nearly continuously exposed for 
70 km in adjacent Laramide uplifts of the southern Sangre de Cristo Mountains. Near the village of Guadalupita, these rocks display a smooth but abrupt south-to-north change from subhorizontal to subvertical dominant foliation (S2) over a distance of 2 km. This change in dominant fabric orientation coincides with a regional change in metamorphic grade from near-granulite grade (650°C, 4–6 kbar) in rocks with a subhorizontal fabric to amphibolite grade (500°C, 4–6 kbar) in rocks with a subvertical fabric. The shallowly dipping S2 fabric and highest temperature assemblages are both centered around an 1682-Ma granitic orthogneiss, the Guadalupita pluton, which engulfs the overturned lower limb of an 15 kmscale, north-facing F1 fold. Porphyroblast-matrix microstructural studies suggest that S1 and S2 formed during a progressive event that was synchronous with pluton emplacement and regional metamorphism at 1982 Ma. Granite emplacement and its incorporation into the core of a fold-nappe at 1.68 Ga appears to have facilitated subhorizontal S2 fabric development late during the progressive S1/S2 event and heat from the granite enhanced regional metamorphic conditions to create the 150° C temperature gradient. However, metamorphic monazites aligned in S2 yield U-Pb dates of 1421 Ma, suggesting that monazite grew during renewed tectonism that reactivated the older subhorizontal fabric during 1.42-Ga regional metamorphism. Present geometries therefore reflect a superposition of major tectonometamorphic events at 1.68 and 1.42 Ga. This study suggests that: (1) large temperature gradients around plutons can cause regionally heterogeneous middle-crustal pressure-temperature–time–deformation (P–T–t–D) paths; (2) plutons may both localize and be localized by subhorizontal shear zones; and (3) middle-crustal rheologies are strongly influenced by thermal weakening near plutons.
Key Words: Rincon Range • Mora • Guadalupita • rheology • granite • Paleoproterozoic • Mesoproterozoic • metamorphism • Mazatzal orogeny • middle crust
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