SYNCONTRACTIONAL CRUSTAL ANATEXIS AND DEFORMATION DURING EMPLACEMENT OF SIMILAR-TO-1435 MA PLUTONS, WESTERN NEEDLE-MOUNTAINS, COLORADO

Mesoproterozoic deformation and biomodal magmatism in southwest Colora do offer important insight into the nature and tectonic regime of 1.4 Ga magmatism in the western United States. We document syncontractiona l emplacement of coeval gabbroic and granitic plutons in the middle cr ust at similar to 1435 Ma in the western Needle Mountains. This is inc ompatible with extension-dominated ''anorogenic'' models proposed for the generation of 1.4 Ga plutonic rocks in the Southwest but lends fur ther support for synkinematic intracontinental magmatism in response t o regional tectonic strains created by transpressive or compressive pl ate margin interactions between 1500-1300 Ma. Intrustion of the simila r to 1435 Ma Electra Lake Gabbro and Eolus Granite into Paleoproterozo ic rocks caused local crustal anatexis and extensive thermal metamorph ism in the Needle Mountains. Deformation of Paleo proterozoic country rocks at similar to 1435 Ma is widespread but is concentrated in pluto n aureoles where thermally softened crust accommodated the strain. We interpret the kinematic framework of structural elements in the deform ed pluton aureoles to indicate a regional synmagmatic strain field inv olving subhorizontal N-S contraction and E-W extension. This is reflec ted by brittle fracture arrays and conjugate ductile shear bands fille d with remelted trondhjemitic gneiss, transposed amphibolite layers an d similar to 1435 Ma granite dikes, local solid-state ductile deformat ion in similar to 1435 Ma intrusive rocks, broad-scale folding of migm atites, and trends of similar to 1435 Ma granitic and gabbroic dikes. Aureole metamorphism and kinematics of deformation recorded in the Nee dle Mountains at similar to 1435 Ma are similar to those that are docu mented in other 1.4 Ga plutonic complexes throughout Colorado, Arizona , and New Mexico. This regional record argues for a widespread episode of synorogenic magmatism in the southwestern United States at 1.4 Ga.