A middle-crustal cross section from the Rincon Range, northern New Mexico; evidence for 1.68-Ga, pluton-influenced tectonism and 1.4-Ga regional metamorphism

Read, A.S., Karlstrom, K. E., Grambling, J.A., Bowring, S.A., Heizler, M.T., Daniel, C., 1999, A middle-crustal cross section from the Rincon Range, northern New Mexico; evidence for 1.68-Ga, pluton-influenced tectonism and 1.4-Ga regional metamorphism, in: Lithospheric structure and evolution of the Rocky Mountains, Part II, Karl E. Karlstrom guest editor, Rocky Mountain Geology, v. 34, n. 1, p. 67-91.

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 approximately 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 approximately 2 km. This change in dominant fabric orientation coincides with a regional change in metamorphic grade from near-granulite grade (approximately 650 degrees C, 4-6 kbar) in rocks with a subhorizontal fabric to amphibolite grade ( approximately 500 degrees C, 4-6 kbar) in rocks with a subvertical fabric. The shallowly dipping S₂ fabric and highest temperature assemblages are both centered around an approximately 1682-Ma granitic orthogneiss, the Guadalupita pluton, which engulfs the overturned lower limb of an approximately 15 km-scale, north-facing F₁fold. Porphyroblast-matrix microstructural studies suggest that S₁and S₂ formed during a progressive event that was synchronous with pluton emplacement and regional metamorphism at approximately 1682 Ma. Granite emplacement and its incorporation into the core of a fold-nappe at approximately 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 approximately 150 degrees C temperature gradient. However, metamorphic monazites aligned in S₂ yield U-Pb dates of approximately 1421 Ma, suggesting that monazite grew during renewed tectonism that reactivated the older subhorizontal fabric during approximately 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.