Condilsons of Metamorphism of the Colorado Yule Marble

摘要

The Yule Marble, which comprises many famous sculptures and monuments such as the Lincoln Memorial, formed from the metamorphism of the Mississippian Leadville Limestone in the contact aureole of the Treasure Mountain dome laccolith in Colorado. Metamorphic mineral assemblages in impure siliceous dolomitic strata reflect a drop in temperature with increasing distance from the contact at approximately O.lGPa of pressure, with some complications,. The samples recording the highest temperature mineral assemblages come from two different units within 30 m of the contact, and depending on the assumed fluid composition in equilibrium, reflect temperatures of -490-700°C, At the contact with the laccolith, the temperature was likely to have been close to the temperature of the laccolith, which was ~700°C Samples that were collected farther than 30 m from the contact are consistent with temperatures of ~35O-455°C, depending on the assumed equilibrium composition of the fluid. However, there are samples collected within 30 m of the contact whose mineral assemblages indicate peak temperatures of ~4.35±15°C, which is lower than the temperature predicted for this distance from the pluton, and inconsistent with the higher temperature assemblages found near to the contact The apparent lack of attainment of chemical equilibrium in these samples may have resulted from the short duration of the thermal pulse Consistent with this interpretation is a well-developed chilled zone along the laccolith's margin, indicating rapid cooling. From mineral assembles in two samples collected within 100 m of the Yule Marble quarry, we estimate the peak temperature of metamorphism at the quarry to be close to ~4.35±15°C.