Stable isotopic studies of mafic sills and proterozoic metasedimentary rocks located beneath the Duluth Complex, Minnesota

Abstract

Thin (3–20 m), picritic to noritic sills are found in the Proterozoic metasedimentary footwall rocks of the Duluth Complex, Minnesota. The sills were emplaced prior to the major intrusions that comprise the Duluth Complex, and underwent contact metamorphism along with the country rocks. Oxygen, hydrogen, and sulfur isotopic compositions of the sills indicate a varied history of isotopic exchange between minerals, melts, and hydrothermal fluids in the high temperature environment below the major plutonic bodies of the Midcontinent Rift system. The pre-Duluth Complex sills exhibit a range in δ 18O values from 4.9‰ to 14.8‰, with values between 6‰ and 7‰ generally found in sill interiors. High δ 18O values near sill contacts with high 18O metasedimentary rocks of the pelitic Virginia Formation or Biwabik Iron Formation, coupled with a smooth sigmoidal isotopic profile centered at the contact, suggest that oxygen diffusion was an important exchange mechanism. The elevated δ 18O values near the center of the thickest sills are thought to reflect the emplacement of isotopically contaminated basaltic magma. Dehydration reactions in the pelitic rocks of the contact aureole liberated high 18O fluids that enhanced subsolidus diffusive exchange. Advective displacement of the diffusion profiles toward the sill interior is less than 40 cm, and suggests that layer parallel flow dominated in the dehydration of the contact aureole. Elevated, but uniform δ 18O values (9.7‰ to 10.5‰) in thinner sills suggests that oxygen diffusivity was increased relative to country rocks due to enhanced porosity, perhaps related to extensive development of microcracks. Although δD values of the pelitic country rocks record a history of dehydration, systematic variations of δD (−64‰ to −143‰) and H 2O (0.15 to 5.40 wt.%) content are not found in the sills. δ 18O values of coexisting plagioclase and pyroxene from the sills indicate a close approach to isotopic equilibrium, and are consistent with a diffusion-dominant exchange process at temperatures near 500°C. Results of diffusion modelling suggest a duration of isotopic exchange that may have extended from tens of thousands of years to 1.4 Ma, depending on local controls of porosity and permeability, as well as rates of fluid production in the contact aureole. Localized areas of 18O and D depletion in the sills (values as low as 4.9‰ and −143‰, respectively) denote exchange with meteoric water after interaction with the high 18O metamorphic fluids in the contact aureole. Although all of the elevated 18O samples in the contact environment may have suffered 18O depletion, most exchange with meteoric water appears to be spatially localized, and is thought to reflect highly channelized, fracture-controlled fluid flow. Sulfur isotopic values of the sills are variable (−2.7‰ to 11.2‰), and indicative of an evolution involving pre-emplacement contamination of basaltic magma, and sub-solidus exchange with an H 2S-bearing metamorphic fluid. Sulfur contents exceed 3.0 wt.% only within troctolitic to melatroctolitic sills, and δ 34S values of 7.8‰ to 8.3‰ are strongly suggestive of pre-emplacement contamination by sulfur derived from a Proterozoic sedimentary unit. High δ 18O and δ 34S rocks, particularly at sill margins, are consistent with either hydrothermal precipitation of fine-grained sulfide minerals, or isotopic exchange between magmatic sulfides and an H 2S-bearing metamorphic fluid.