Metalamprophyre dike deformation in the Homestake Reservoir Quadrangle, Leadville, Colorado

Abstract

Field relations and microstructural observations show that intrusion of mafic, ultrapotassic, lamprophyre dikes in the Northern Sawatch Range near Leadville, Colorado localized shear zone deformation ca. 1.4 Ga. This suggests that lithologic contrasts and advective heating associated with magmatism contributed to localizing intracontinental deformation associated with a widespread tectonothermal event that affected the southwestern U.S. during the Mesoproterozoic. Dikes are concentrated in a distinct structural domain characterized by east striking, subvertical foliations in a localized zone of Paleoproterozoic hornblende gneiss, calc-silicate gneiss, and biotite gneiss country rock. Observations show that preexisting fabrics in Proterozoic units preferentially localized north south extension, which accommodated dike emplacement. Following emplacement, dikes were metamorphosed and sheared along their margins within relatively unaffected country rock. The TitaniQ titanium and quartz thermometer yields deformation temperatures in dikes of at least 564 + or - 45 °C, hotter than previously calculated deformation temperatures of mylonite in the nearby Homestake Shear Zone (HSZ) and similar to deformation temperatures of the nearby Slide Lake Shear Zone (SLSZ). This implies that during deformation, the dikes were still hot from their recent intrusion and this may have localized early stages of progressive strain accumulation. Dike emplacement and subsequent deformation demonstrates that strain was more widely distributed than in previously mapped, strictly defined 1.4 Ga shear zones. This study identifies how the newly recognized element of strain partitioning in mafic dikes, in addition to coeval emplacement of St. Kevin batholith and deformation in the HSZ and SLSZ, suggests a multi-faceted, kinematically-linked system of deformation ca. 1.4 Ga, which illustrates the dynamic middle continental crustal effects of Proterozoic intracontinental deformation.