Electron-microprobe monazite dating of ca. 1.71-1.63 Ga and ca. 1.45-1.38 Ga deformation in the Homestake shear zone, Colorado: Origin and early evolution of a persistent intracontinental tectonic zone

The Homestake shear zone, one of the principal Precambrian structures withi n the Colorado mineral belt, has a history of tectonism that extends from t he Proterozoic to the Tertiary. New field mapping, microstructural analysis , and electron-microprobe U-Th-Pb monazite dates define the style, kinemati cs, and timing of multistage Proterozoic ductile deformation. Early subhori zontal shearing (D1) progressed to northwest-southeast crustal shortening ( D2) during a protracted ca. 1710-1630 Ma orogeny involving high-temperature metamorphism, partial melting, and emplacement of the ca. 1.7 Ga Cross Cre ek batholith. The subvertical, northeast trend of the shear zone was establ ished by D2 shortening. A major episode of simple-shear displacement occurr ed ca. 1.4 Ga during emplacement of the St. Kevin batholith. Complex kinema tics are recorded by 1380 Ma, southeast-side-down, dextral mylonite zones ( D3) and post-1380 Ma southeast-side-up, dextral ultramylonite-pseudotachlyt e zones (D4) that were both localized in D2 foliation domains. The successi on of ductile tectonites in the Homestake records > 300 m.y. of episodic de formation at progressively shallower crustal levels as Colorado evolved fro m an active accretionary margin (1.8-1.7 Ga), through crustal assembly (1.7 -1.6 Ga), to a tectonically active continental interior (post 1.45 Ga). The Homestake shear zone constitutes a classic example of ductile shear-zone r eactivation and the development of persistent tectonic zones in the contine ntal lithosphere.