- UW Department of Geology and Geophysics
Field studies and U-Pb geochronology in the Sangre de Cristo Mountains, southern Colorado, provide new constraints on the Proterozoic tectonic evolution of southern Laurentia. Protoliths for basement gneisses and amphibolites were formed in an arc environment and underwent early penetrative deformation and metamorphism (D1 and M1) during formation of the Yavapai province. D1 deformation produced penetrative, subvertical, northwest-striking fabrics (S1) in rocks exposed throughout the range and is interpreted to have occurred during long-lived arc formation and accretion across northwest-striking tectonic boundaries. The ages of D1 and M1 are constrained by a suite of 1750–1730-Ma calc-alkaline intrusions in the southern part of the range and might have occurred as late as ca. 1710 Ma in the northern part of the range. The northeast-striking tectonic grain that was developed regionally during the culmination of the Yavapai orogeny is not recognized locally. Post-orogenic granitoid plutons were emplaced at 1695±2 Ma and 1682±3 Ma, broadly coeval with deposition of locally derived quartzite at the surface. Magmatism and sedimentation during this time are interpreted to represent contemporaneous responses to crustal extension during the ca. 60-m.y. inter-orogenic period between the Yavapai and Mazatzal orogenic events. D2 deformation is interpreted to represent the Mazatzal orogeny locally, and involved northwest-directed shortening and dextral shear localized along subvertical, northeast-trending high-strain zones. D2 was accompanied by amphibolite-facies metamorphism (M2) at 1637±6 Ma, and the quartzite is inferred to have been deformed during this time. Mesoproterozoic deformation (D3) produced a northeast-striking, subvertical tectonic foliation and localized shear zones between 1420 and 1412 Ma. D3 deformation was bracketed by the emplacement of two newly dated granitic intrusions at 1434±2 Ma and 1407±6 Ma. The map-scale geometry of these intrusions and coeval deformational fabrics suggest that ca. 1.4-Ga granites were emplaced into a broadly compressional stress field during subhorizontal northwest–southeast shortening. These new data and observations indicate that ca. 1.4-Ga granites are not anorogenic, consistent with tectonic models suggesting that widespread magmatism was broadly synchronous with intracontinental orogenesis at ca. 1.4 Ga.