Oxygen Isotope Geothermometry of the Burial Metamorphic Rocks of the Precambrian Belt Supergroup, Glacier National Park, Montana

Abstract

Isotopic evidence supports the conclusions of other studies that the Precambrian Belt rocks have been subjected to high-grade diagenesis and low-grade metamorphism. Isotopic temperatures calculated from O 18 /O 16 ratios of coexisting quartz and illite range from 225°C to 310°C and are interpreted as being temperatures reached during metamorphism. Isotopic temperatures generally increase with maximum depth of burial. A plot of isotopic temperatures as a function of depth of burial can be extrapolated to 20°C (surface temperature) at approximately 5,500 m above our uppermost sample. This is consistent with the probable amount of overburden above this sample as inferred from stratigraphic evidence. The isotopic temperatures are also consistent with a model of equilibration during burial in a normal geothermal gradient. Feldspar underwent isotopic exchange during metamorphism but does not always appear to have attained isotopic equilibrium with quartz and illite. The isotopic data indicate some degree of disequilibrium between carbonate and quartz and suggest that the carbonate may have been more readily subject to retrograde exchange than was the silicate. The isotopic compositions of whole-rock samples vary with depth in the stratigraphic section, apparently reflecting post-depositional isotopic exchange.