Late Cretaceous (85--80 Ma), syn-arc cleavage development in metamorphic rocks of the Ritter Range, central Sierra Nevada California

Extensive, high quality exposures make the Ritter Range an appropriate place to investigate the deformation field in wallrocks of the Late Cretaceous Sierran magmatic arc. Faulting, tilting, cleavage development and shearing above affected pre-batholithic strata of upper Paleozoic to mid-Cretaceous age. The authors present Ar/Ar data for metamorphic amphibole, white mica and biotite from upper Paleozoic and Early to Middle Jurassic rocks of the section. These data re interpreted in light of temperatures of metamorphism and microstructural analysis of each sample. Syn- to post-kinematic amphibole from a foliated and lineated mafic amphibolite yields an age of 85 Ma for amphibole-derived gas. Peak metamorphic temperature was under 500 C. at this locality, so negligible Ar loss in the amphibole is expected. The authors interpret 85 Ma as the age of amphibole growth and cleavage development in this sample. Syn- to post-kinematic white mica and four biotites from five samples yield ages clustering at about 80 Ma. Since the micas probably grew above their Ar closure temperatures, the ages of the post-kinematic micas are minima for their fabrics. These data indicate that the dominant cleavage in the upper Paleozoic to Middle Jurassic rocks of the Ritter Range formed penecontemporaneously with emplacement ofmore » the adjacent Mono Pass and Tuolumne intrusive series, and suggests that the heat of these granitoids was instrumental in catalyzing widespread deformation in their wallrocks.« less