CONTACT METAMORPHISM-METASOMATISM ASSOCIATED WITH THE LATEST EOCENE NORTHERN ITALIAN MOUNTAIN GRANITE INTRUSION, GUNNISON COUNTY, COLORADO

Near vertical Pennsylvanian black shales and carbonates along the west margin of the intrusive were converted to phlogopite-diopside-andalusite hornfelses and scapolite-diopside skarns with minor analcime. Talc and pyrophyllite formed in distal lower grade hornfelses. Diopside and epidote crystals, vesuvianite, and scapolite-analcime skarns occur on Italian Mountain at the SE margin of the intrusion. Free-standing crystals are attached to fine grained, granular diopside skarns. Basal portions of the crystals contain white mica, chlorite, and epidote inclusions. Highly acicular actinolite occurs as felted mats and inclusions in some crystals. Coarse grossularite-diopside-plagioclase-calcite skarns and scapolite-diopside skarns formed along the NW margin of the intrusion. Veinlets with outer zones of pale pink Al-rich epidote, containing a few wt % Mn02, and interiors of Al-rich epidote (no Mn) and thomsonite cut the scapolite skarns. Fine grained forsterite-Ti phlogopite-calcite skarns, some with deep blue lazurite granules (lapis lazuli), and calcite marbles with diopside, Ti phlogopite, and pyrite occur in the Anderson Prospect. Limestone (Miss.) and sandstone (Dev.) comprise the wallrocks along the NE margin of the pluton. The sandstone matrix crystallized to white mica and minor andalusite. The limestones were converted to chlorite-tremolite marbles with a strong foliation indicating shearing and flowage during metamorphism. Al-in-hornblende pressures, heat flow considerations, mineral equilibria, and geologic evidence indicate pressures of < 0.5 Kb during metamorphism. Peak temperatures were 400° to 450° in wallrocks closest to the intrusion and may have exceeded 500° in wallrocks close to intrusive rocks on two sides. CO2 was easily lost to the surface. Fluids rich in water and NaCl facilitated dissolution, transport of dissolved constituents, and precipitation of skarn minerals at temperatures well below those associated with higher CO2 pressures. Dissolution of once nearby halite or saline surface waters or shallow groundwaters moving south from the Eagle Basin supplied the NaCl.