1.4 Ga Tectonism Recorded by Synchronous High Temperature Deformation, Granulite Facies Metamorphism and Magmatism, Wet Mountains, Colorado

Proterozoic rocks of the Wet Mountains represent part of a regional high temperature metamorphic culmination at 1.4-1.3 Ga, expressed in NE New Mexico and SE Colorado. Peak metamorphic temperatures exceeded 600° C, as recorded by varied parageneses including px-bearing mafic granulites and Kfs-Sil-Bt-Grt metapelites. ⁴⁰Ar-³⁹Ar cooling ages on amphiboles aligned with or overprinting foliations were entirely reset at 1.42 to 1.38 Ga. HT deformational fabrics attributed to ~1.4 Ga tectonism pervade the range, yet the gneisses host Mesoproterozoic plutonic rocks with A-type (anorogenic) geochemistry. The Wet Mts therefore present important relationships that bear on the current debate over a dynamic vs. anorogenic setting for 1.4 Ga plutonism. The dominant structural elements in the Wet Mts consist of migmatitic compositional layering of supracrustal protoliths and dynamic fabrics in orthogneiss developed during an early phase of metamorphism from 1.67-1.70 Ga. In discrete zones, non-coaxial fabrics developed through crystal-plastic flow of quartz and grain boundary recrystallization in feldspars, indicating simple shear within the amphibolite facies. The early NW- to W-striking foliation contain NE-trending mineral lineations associated with top-to-SW reverse kinematic indicators, reflecting convergent tectonics during Paleoproterozoic plutonism. Peripheral to Mesoproterozoic plutons, younger discrete zones of high temperature dynamic recrystallization strongly overprint older fabrics. Megacrystic granitoids were reduced to granular rock with a fine-grained, homogeneous texture. Relict elliptical shapes of Kfs megacrysts and streaked-out, fine-grained aggregates of biotite attest to high strain under upper amphibolite to granulite conditions. Granular textures in hand sample and ameboid serrate grain boundaries in thin section attest to processes of sub-grain rotation recrystallization and grain boundary migration, suggesting higher T or lower strain rates than recorded by earlier fabrics. A preliminary U-Pb zircon date from a deformed sill intruded into one such zone is ~1.48 Ga, providing a time constraint on HT deformation. Voluminous granite sills pervade wide border zones around the plutons, emplaced as sheets closely interpenetrating the gneissic layering. Commonly the sills cut gneissic fabrics at low angles and contain screens of host rock, but are well foliated and lineated concordant with fabrics in host rocks, attesting to syntectonic emplacement. Pegmatite and granite dikes emplaced at high angles to the foliation are variably buckled, confirming that magmatism and shortening perpendicular to layering were contemporaneous and time-progressive. Relationships in the Wet Mts reinforce the emerging pattern of important intracontinental tectonism at ~1.4 Ga. With widespread presence of melt under elevated regional temperatures, the bulk strength of the rocks was greatly diminished, leading to wholesale flow in the middle crust. These results point to dynamic processes affecting the middle crust at 1.4 Ga, in accord with dynamic models for 1.4 Ga plutonism.