Metamorphism of the Weeks Limestone, Notch Peak, Utah : the low grade reactions and fluid inclusions

Upper Cambrian argillaceous limestones of the Weeks Limestone were intruded and contact metamorphosed by the Notch Peak quartz monzonite stock during Jurassic times. The unmetamorphosed rocks contain the assemblage calcite + quartz + dolomite + muscovite + chlorite + plagioclase. The first signs of metamorphism occur at the phlogopite isograd where dolomite and muscovite react to form phlogopite and chlorite. At the muscovite-out isograd, muscovite and chlorite react to form phlogopite and calcic plagioclase. The diopside zone is entered when phlogopite breaks down to form diopside and K-feldspar. In areas where X_CO2 higher, tremolite and feldspar formed before diopside by the breakdown of chlorite and phlogopite. Diopside then formed when tremolite broke down.

In the higher-grade rocks, the assemblage wollastonite + vesuvianite + diopside + K-feldspar indicates extremely low values of X_CO2, despite the loss of 23% CO₂ by decarbonation reactions as determined by mass balance. Fluid inclusions were examined to determine whether fluid compositions were consistent with mineral equilibria. Three rock types were examined: 1) the North Peak granite, 2) skarns adjacent to the granite, and 3) high grade rocks from the Weeks Limestone. Inclusions are secondary and consist of an H2O-rich liquid and a vapor phase at room temperature. Fluid inclusion melting temperatures of -120°±120°C give salinity values ranging from 0 to 28 equivalent weight percent NaCl. Homogenization temperatures ranging from 110° to 210°C gives filling temperatures of 325°±125°C at 2 Kb. These filling temperatures indicate that fluid inclusions do not contain fluids from the presumed peak of metamorphism (400° to 500°C). Moderate salinities are consistent with the widespread occurrence of scapolite. The presence of salinity greater than that of the H₂O-NaCl eutectic and the absence of daughter crystals in inclusions imply that the fluids that passed through these rocks must have contained salts other than NaCl. Even though fluid inclusions contain late metamorphic fluids, their composition is consistent with fluid compositions indicated by mineral assemblages. The presence of H₂O-rich fluids does not appear to be a result of fluid-phase separation of a low density CO₂ phase, because evidence for "boiling" was not observed. Unless the CO₂-rich phase failed to be trapped, the H2O rich fluid composition only could have been attained by massive infiltration of a water rich-brine.