Structural studies in a Proterozoic gneiss complex and adjacent cover rocks, west Needle Mountains, Colorado

Abstract

Proterozoic rocks in the Needle Mountains include ca. 1750 Ma amphibolite-grade, metavolcanic and metaplutonic gneisses and ca. 1690 Ma granitoids that comprise the basement to the siliciclastic Uncompahgre Group. The mafic and felsic gneisses underwent synkinematic metamorphism and two phases of isoclinal folding and foliation development during D_B, prior to emplacement of the ca. 1690 Ma plutons. D_BC deformation caused folding of D_B fabrics in the gneisses, development of a subvertical, east-striking foliation in the granitoids, and generation of a macroscopic sigmoidal foliation pattern throughout the area prior to 1430 Ma. D_BC structures in the basement are correlated with macroscopic structures in the Uncompahgre Group, which was deformed into an east-trending cuspate synclinorium during this event. Gently plunging mineral lineations and asymmetric kinematic indicators in the basement record a component of dextral strike-slip shearing in domains of east-striking foliation and sinistral shearing in areas of northeast-striking foliation. A model for D_BC involving the development of conjugate strike-slip shear zones in response to north-northwest shortening is most consistent with the kinematic and fabric orientation data.

A zone of phyllite, derived largely from basement, occurs everywhere along the basement-cover contact. Kinematic indicators along and near the contact record upward movement of the cover relative to the basement on each side of the synclinorium and imply that the cover rocks are parautochthonous. Stratigraphic facing of the cover rocks away from the basement supports the interpretation of this contact as an unconformity at the base of the Uncompahgre Group. Alteration of the basement rocks along this contact involved hydration and the loss of CaO, MgO, SiO₂, and Na₂O. The phyllite zone is interpreted as a metamorphosed and deformed regolith that localized out-of-synform movement while the basement and its parautochthonous cover were folded together during D_BC.

Rocks in the Needle Mountains comprise part of the Colorado Province, one of several terranes that were possibly accreted to the Archean Wyoming Craton during the Proterozoic. Age constraints on the timing of deformation indicate that D_B and D_BC are representative of two regionally extensive deformational episodes. Pre-1700 Ma deformation is attributed to the assembly of volcanogenic terranes and their accretion to the Wyoming Craton along the Cheyenne Belt. Post-1700 Ma deformation resulted from regional north-northwest crustal shortening induced by tectonic interactions along the southern margin of the Colorado Province. These results support the hypothesis that terrane accretion was important in the Proterozoic crustal evolution of southwestern North America.