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

Thin (3-20 m), picritic to noritic sills are found in the Proterozoic metas edimentary footwall rocks of the Duluth Complex, Minnesota. The sills were emplaced prior to the major intrusions that comprise the Duluth Complex, an d underwent contact metamorphism along with the country rocks. Oxygen, hydr ogen, and sulfur isotopic compositions of the sills indicate a varied histo ry of isotopic exchange between minerals, melts, and hydrothermal fluids in the high temperature environment below the major plutonic bodies of the Mi dcontinent Rift system. The pre-Duluth Complex sills exhibit a range in del ta(18)O values from 4.9 parts per thousand to 14.8 parts per thousand, with values between 6 parts per thousand and 7 parts per thousand generally fou nd in sill interiors. High delta(18)O values near sill contacts with high O -18 metasedimentary rocks of the pelitic Virginia Formation or Biwabik iron Formation, coupled with a smooth sigmoidal isotopic profile centered at th e contact, suggest that oxygen diffusion was an important exchange mechanis m. The elevated delta(18)O values near the center of the thickest sills are thought to reflect the emplacement of isotopically contaminated basaltic m agma. Dehydration reactions in the pelitic rocks of the contact aureole lib erated high O-18 fluids that enhanced subsolidus diffusive exchange. Advect ive displacement of the diffusion profiles toward the sill interior is less than 40 cm, and suggests that layer pal allel flow dominated in the dehydr ation of the contact aureole. Elevated, but uniform delta(18)O values (9.7 parts per thousand to 10.5 parts per thousand) in thinner sills suggests th at oxygen diffusivity was increased relative to country rocks due to enhanc ed porosity, perhaps related to extensive development of microcracks. Altho ugh delta D values of the pelitic country rocks record a history of dehydra tion, systematic variations of delta D (- 64 parts per thousand to - 143 pa rts per thousand) and H2O (0.15 to 5.40 wt.%) content are not found in the sills. delta(18)O values of coexisting plagioclase and pyroxene from the si lls indicate a close approach to isotopic equilibrium. and are consistent w ith a diffusion-dominant exchange process at temperatures near 500 degrees C. Results of diffusion modelling suggest a duration of isotopic exchange t hat 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 p roduction in the contact aureole. Localized areas of O-18 and D depletion in the sills (values as low as 4.9 parts per thousand and -143 parts per thousand, respectively) denote exchan ge with meteoric water after interaction with the high O-18 metamorphic flu ids in the contact aureole. Although all of the elevated O-18 samples in th e contact environment may have suffered O-18 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 parts per thousand to 11.2 parts per thous and), and indicative of an evolution involving pre-emplacement contaminatio n of basaltic magma, and sub-solidus exchange with an H2S-bearing metamorph ic fluid. Sulfur contents exceed 3.0 wt.% only within troctolitic to melatr octolitic sills, and delta(34)S values of 7.8 parts per thousand to 8.3 par ts per thousand are strongly suggestive of pre-emplacement contamination by sulfur derived from a Proterozoic sedimentary unit. High delta(18)O and de lta(34)S rocks, particularly at sill margins, are consistent with either hy drothermal precipitation of fine-grained sulfide minerals, or isotopic exch ange between magmatic sulfides and an H2S-bearing metamorphic fluid. Copyri ght (C) 1999 Elsevier Science Ltd.