Hump-shaped Ar-40/Ar-39 age spectra in K-feldspar and evidence for Cretaceous authigenesis in the Fountain Formation near Eldorado Springs, Colorado

The Fountain Formation near Eldorado Springs, CO, USA, shows evidence of al teration by hydrothermal fluids that precipitated authigenic potassium feld spar (adularia) as rims on detrital feldspars and as interstitial cement de posits. Detailed Ar-40/Ar-39 step-heating experiments of samples from the F ountain Formation reveal age spectra having a characteristic hump at labora tory temperatures between 900 and 1080 degrees C. The humps appear to be re lated to the presence of adularia. Laser analyses of small grains of adular ia indicate that the hump can be physically dissected, unlike age gradients found in igneous feldspars. Such behavior is consistent with the mixing of gas from two or more generations of K-feldspar, each having unique diffusi on properties. The feldspars studied here indicate that two pulses of simil ar to 150 degrees C hydrothermal fluids migrated through the Fountain Forma tion at 135 and 94 Ma, prior to the main phase of Laramide tectonic activit y in the region. The limited occurrence of authigenic cements suggests that reactivation of the underlying Precambrian Idaho Springs-Ralston Creek she ar zone was significant enough to heat and mobilize large quantities of met eoric fluids. In addition, these data also suggest that Cretaceous movement s along the Transcontinental Arch, correlated with stratigraphic events, be gan approximately 40 million years earlier than previously thought, (C) 199 9 Elsevier Science B.V. All rights reserved.