THE MOUNT EVANS BATHOLITH IN THE COLORADO FRONT RANGE - REVISION OF ITS AGE AND REINTERPRETATION OF ITS STRUCTURE

The Mount Evans batholith, in the central Front Range of Colorado, is composed of a main phase of massive to conspicuously foliated monzogra nite and granodiorite and undeformed aplite and pegmatite. The Mount E vans batholith was previously considered to be part of the 1.7 Ga Rout t Plutonic Suite. New U-Pb zircon ages on four samples (granodiorite, monzogranite, and granite), however, indicate that the batholith was e mplaced at 1,442 +/- 2 Ma and belongs to the Berthoud Plutonic Suite. Most of the batholith has igneous textures and structures, except in t he vicinity of the Idaho Springs-Ralston shear zone where those featur es are tectonically recrystallized and foliated. Foliation elsewhere i n the batholith is a How structure. Zircons in two granodiorite sample s, collected near the shear zone (just south of the Colorado Mineral B elt), are reversely discordant by about 0.8%-2.1%, with a considerable spread in Pb-207/Pb-206 ages. Many of the zircons from these samples contain apatite, K-feldspar, and quartz inclusions that appear to repl ace zircon along cracks and imperfections from rim to core. We suggest that these inclusions formed during a Laramide ore-forming event and incorporated Early and Middle Proterozoic radiogenic lead scavenged fr om the country rock. The excess radiogenic lead caused the scatter and reverse discordance in the data.