Stable Isotope Evidence for the Role of Diffusion, Infiltration, and Local Structure on Contact Metamorphism of Calc-Silicate Rocks at Noth Peak, Utah

The effects of infiltration, diffusion, and local structureon the contact metamorphism of the Cambrian Weeks Formation,a calcareous argillite located in western Utah, were examinedusing stable isotope systematics and mineral mass-balance considerations.The emplacement of the Jurassic Notch Peak granitic stock resultedin metamorphism characterized with increasing proximity to theintrusion by phlogopite, diopside, and wollastonite isograds.The isograds are generally concentric around the stock, withdiopside and wollastonite isograds {small tilde}600 and 400m from the contact, respectively, but are deflected near a pre-intrusionfault where the wollastonite isograd is 1. 5 km from the contact.Where the isograds are concentric, the wollastonite isogradmarks an isotopic front with whole-rock ¹⁸O values approachingthe 9.5% value of the stock. In contrast, the ¹⁸O values inthe unmetamorphosed to diopside-grade rocks range from 16.3to 20.2%. Near the fault the isotopic front extends throughthe diopside zone, suggesting that the fault was a major conduitfor magmatic water. Water—rock ratios for the diopside- and wollastonite-gradesamples determined from mineralogic mass balance are nearlyone order of magnitude larger than ‘one-box’ closed-systemratios determined from shifts in stable isotope ratios. Chromatographicmodels for isotopic exchange and propagation of isograds showthat one-dimensional infiltration of magmatic water throughpore spaces would lead to an isotopic front at 50 m from thestock and the wollastonite isograd would be only 8 km from thestock. These distances are significantly smaller than observed.It is suggested that most of the magmatic water flowed throughfractures or thin permeable layers, with the extent of isotopicexchange between the magmatic water in these conduits and theadjacent rocks being controlled by the extent of reaction progressin the rocks. Considerations of CO₂—H₂O interdiffusivitiesin fluids indicate that removal of CO₂ from the reaction frontstoward the intrusion or the fault was sufficiently rapid tocontrol the geometry of the isograds.