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

摘要: The Fountain Formation near Eldorado Springs, CO, USA, shows evidence of alteration by hydrothermal fluids that precipitated authigenic potassium feldspar (adularia) as rims on detrital feldspars and interstitial cement deposits. Detailed 40Ar/39Ar step-heating experiments samples from the reveal age spectra having a characteristic hump at laboratory temperatures between 900 1080°C. humps appear to be related presence adularia. Laser analyses small grains adularia indicate can physically dissected, unlike gradients found in igneous feldspars. Such behavior is consistent with mixing gas two or more generations K-feldspar, each unique diffusion properties. studied here pulses ∼150°C migrated through 135 94 Ma, prior main phase Laramide tectonic activity region. limited occurrence cements suggests reactivation underlying Precambrian Idaho Springs-Ralston Creek shear zone was significant enough heat mobilize large quantities meteoric fluids. In addition, these data also suggest Cretaceous movements along Transcontinental Arch, correlated stratigraphic events, began approximately 40 million years earlier than previously thought.