The Northern Snake Range (Nevada) represents a spectacular example of a
metamorphic core complex and exposes a complete section from the
mylonitic footwall into the hanging wall of a fossil detachment system.
Paired geochronological and stable isotopic data of mylonitic quartzite
within the detachment footwall reveal that ductile deformation and
infiltration of meteoric fluids occurred between 27 and 23 Ma.
Ar-40/Ar-39 ages display complex recrystallization-cooling relationships
but decrease systematically from 26.9 +/- 0.2 Ma at the top to 21.3 +/-
0.2 Ma at the bottom of footwall mylonite. Hydrogen isotope (delta D)
values in white mica are very low (-150 to -145 %) within the top 80-90
m of detachment footwall, in contrast to values obtained from the deeper
part of the section where values range from -77 to -64 %, suggesting
that time-integrated interaction between rock and meteoric fluid was
restricted to the uppermost part of the mylonitic footwall. Pervasive
mica-water hydrogen isotope exchange is difficult to reconcile with
models of Ar-40 loss during mylonitization solely by volume diffusion.
Rather, we interpret the Ar-40/Ar-39 ages of white mica with low-delta D
values to date syn-mylonitic hydrogen and argon isotope exchange, and we
conclude that the hydrothermal system of the Northern Snake Range was
active during late Oligocene (27-23 Ma) and has been exhumed by the
combined effects of ductile strain, extensional detachment faulting, and
erosion.