Polymetamorphism in the Panamint Mountains, California: A 39AR‐40AR study

Abstract

Metamorphosed upper Precambrian rocks in the Panamint Mountains show textural evidence for polymetamorphism. Previous isotopic studies have established the age of metamorphism as middle to late Mesozoic. This study was undertaken to try to determine the ages of the two metamorphic events using the 39Ar‐40Ar stepwise degassing technique of the K‐Ar method on biotite, muscovite, and hornblende separates. The resulting spectra indicate that metamorphism occurred during the intervals 170–150 Ma ago and 80–70 Ma ago. Many analyses yield ambiguous results because of slow cooling following metamorphism, contamination of the separates by refractory phases containing extraneous argon, and in some cases, the superposition of the two metamorphic events. The ages of metamorphism correspond to periods of emplacement of granitic rocks in the vicinity of the Panamint Mountains, suggesting that the high heat flux responsible for metamorphism was associated with the emplacement of the Sierra Nevada batholith. The ages of metamorphism and granite emplacement are diachronous with the development of major thrust faults in the western Great Basin, which suggests that metamorphism might have been triggered by the formation of nappes rather than causing the formation of nappes.