Ar-40/Ar-39 thermochronology of mesoproterozoic metamorphism in the Colorado front range

A low-pressure metamorphic episode in the Colorado Front Range has been ide ntified by the presence of staurolite, andalusite, cordierite, and garnet p orphyroblasts overprinting earlier assemblages. The overprinting assemblage s and reaction textures are most consistent with porphyroblast growth on a prograde metamorphic path with peak temperatures exceeding similar to 525 d egrees C. Twenty-eight Ar-40/Ar-39 dates on hornblende, muscovite, biotite, and microcline were used to infer the age and thermal conditions of metamo rphism. Muscovite and biotite Ar-40/Ar-39 ages fall mainly in the interval 1400-1340 Ma, consistent with cooling through the closure temperature inter val of micas (similar to 400 degrees-300 degrees C) after about 1400 Ma. In contrast, hornblende apparent ages (T-c similar to 500 degrees-550 degrees C) between 1600 and 1390 Ma reflect variable retention of radiogenic argon . Forward modeling of argon diffusion shows that the distribution of hornbl ende and mica ages is consistent with the partial resetting of argon system atics ca. 1400 Ma by a thermal pulse reaching maximum temperatures around 5 50 degrees C and decaying within <20 m.yr. These temperatures match the con ditions inferred from the overprinting assemblage; thus, muscovite and biot ite ages are interpreted to date the cooling phase of this metamorphic even t. This late metamorphism is broadly coeval with the intrusion of ca. 1400- Ma granitic plutons in the study area and throughout the southwestern Unite d States. However, thermal effects are observed far from pluton margins, su ggesting pervasive, regional crustal heating rather than restricted contact metamorphism. Our results suggest that ca. 1400-Ma metamorphism and pluton ism are manifestations of a regional thermal episode that both partially me lted the lower crust and pervasively metamorphosed. middle crustal rocks.