Syncontractional Crustal Anatexis and Deformation during Emplacement of ~1435 Ma Plutons, Western Needle Mountains, Colorado

Abstract

Mesoproterozoic deformation and biomodal magmatism in southwest Colorado offer important insight into the nature and tectonic regime of 1.4 Ga magmatism in the western United States. We document syncontractional emplacement of coeval gabbroic and granitic plutons in the middle crust at ~1435 Ma in the western Needle Mountains. This is incompatible with extension-dominated "anorogenic" models proposed for the generation of 1.4 Ga plutonic rocks in the Southwest but lends further support for synkinematic intracontinental magmatism in response to regional tectonic strains created by transpressive or compressive plate margin interactions between 1500-1300 Ma. Intrustion of the ~1435 Ma Electra Lake Gabbro and Eolus Granite into Paleoproterozoic rocks caused local crustal anatexis and extensive thermal metamorphism in the Needle Mountains. Deformation of Paleoproterozoic country rocks at ~1435 Ma is widespread but is concentrated in pluton aureoles where thermally softened crust accommodated the strain. We interpret the kinematic framework of structural elements in the deformed pluton aureoles to indicate a regional synmagmatic strain field involving subhorizontal N-S contraction and E-W extension. This is reflected by brittle fracture arrays and conjugate ductile shear bands filled with remelted trondhjemitic gneiss, transposed amphibolite layers and ~1435 Ma granite dikes, local solid-state ductile deformation in ~1435 Ma intrusive rocks, broad-scale folding of migmatites, and trends of ~1435 Ma granitic and gabbroic dikes. Aureole metamorphism and kinematics of deformation recorded in the Needle Mountains at ~1435 Ma are similar to those that are documented 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.