Geochronology of intrusive and metamorphic rocks in the Pilot Range, Utah and Nevada, and comparison with regional patterns

Abstract

K-Ar and U-Pb isotopic studies demonstrate middle Mesozoic metamorphism and plutonism followed by Eocene plutonism in the Pilot Range, northeastern Great Basin. Combined with field relations, the age constraints indicate that local amphibolite-facies and widespread greenschist-facies metamorphism peaked between 165 and 150 Ma and that plutons were emplaced early in the deformational and metamorphic history, between 165 and 155 Ma. Cenozoic plutons and widespread dikes that were emplaced at about 40 Ma cut folds and faults, including a major detachment fault that placed unmetamorphosed on metamorphosed rocks. The Mesozoic and early Cenozoic history of metamorphism and plutonism in the Pilot Range is consistent with age data and field relations throughout the northeastern Great Basin that indicate Late Jurassic plutonism and metamorphism followed by Eocene and Oligocene plutonism and local metamorphism. In many areas, rocks that were metamorphosed during Late Jurassic time were cooled through Ar blocking temperatures for micas between 85 and 60 Ma. The widespread occurrence of these mica ages suggests a regional mechanism for cooling.