Fission-track evidence of the Cenozoic thermal history of the Idaho batholith

Abstract

Apatite and zircon fission-track data are combined with published KAr biotite dates from the Idaho batholith to outline the Cenozoic thermal history of central Idaho. Apatite apparent ages reveal that parts of the batholith have been at temperatures below 100 ° C for over 50 Ma, suggesting that much of the batholith has remained at shallow crustal levels throughout Cenozoic time. Eocene plutonism and attendant hydrothermal circulation resulted in thermal resetting of apatite and zircon dates from near the plutons. Concordant zircon and biotite dates indicate that hydrothermal systems associated with Eocene plutonism operated at shallow crustal levels, so that cooling occurred rapidly as geothermal activity waned. The calculated depth to the 105 ° C apatite annealing isotherm suggests that as much as 3 km of erosion has occurred in deep canyons during the last 10 Ma, presumably associated with final erosion of the batholith following the establishment of drainage at low elevation along the Snake River Plain.