Hydrothermal Flow Systems in the Midcontinent Rift: Oxygen and Hydrogen Isotopic Studies of the North Shore Volcanic Group and Related Hypabyssal Sills, Minnesota

摘要

Rift-related lavas of the North Shore Volcanic Group (NSVG) are intruded by plutonic rocks of the Duluth Complex along the unconformity between the NSVG and the underlying Proterozoic metasedimentary rocks (Animikie Group) and Archean volcano-sedimentary and plutonic rocks. Heat associated with the emplacement of the mafic intrusions generated fluid flow in the overlying plateau lavas. #delta#~(18)O values for whole rocks from the NSVG and hypabyssal sills range from 5.5 to 17.7% and 5.3 to 11.5%, respectively, and most values are higher than those considered "normal" for basaltic rocks (5.4 to 6.0%). In general, there is a positive correlation between whole rock #delta#~(18)O and water content, which suggests that elevated #delta#~(18)O values are related primarily to secondary mineral growth and isotopic exchange during hydrothermal alteration and metamorphism. #delta#~(18)O_(H_2O) values computed from amygdule-filling minerals such as smectite, chlorite, and epidote found in low- to high-temperature metamorphic zones range from approx -1 to 6% with an average value of approx 3%. Smectite in the lower-grade zones gives computed #delta#D_(H_2O) values between -26 and -83%, whereas epidote in the higher-grade zones gives #delta#D_(H_2O) values of -15 to 6%. Fluid isotopic compositions computed from epidote and smectite values are suggestive of the involvement of at least two fluids during the early stages of amygdule filling. Fluid #delta#D and #delta#~(18)O values determined from epidote at the higher metamorphic grades indicater that seawater dominated the deeper portions of the system where greenschist facies assem-blages and elevated #delta#~(18)O values were produced in flow interiors, as well as margins. Smectite isotopic compositions suggest that meteoric water was predominant in the shallower portions of the system. The increase in #delta#~(18)O values of massive flow interiors with depth is interpreted as a result of rock interaction with a fluid of constant oxygen isotopic composition with increasing temperature. The stable isotopic data are supportive of previous suggestions that seawater was involved in the hydrothermal system associated with the Midcontinent Rift. Although the origin of the seawater remains problematic, it appears that marine incursions may have occurred during the late stages of Portage Lake volcanism, and periodically thereafter.