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

摘要

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 #delta#~(18)O values from 4.9 per thousand to 14.8 per thousand, with values between 6 per thousand and 7 per thousand generally found in sill interiors. High #delta#~(18)O values near sill contacts with high ~(18)O 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 #delta#~(18)O 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 ~(18)O 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 #delta#~(18)O values (9.7 per thousand to 10.5 per thousand) 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 #delta#D values of the pelitic country rocks record a history of dehydration, systematic variations of #delta#D (-64 per thousand to -143 per thousand) and H_2O (0.15 to 5.40 wt.%) content are not found in the sills. #delta#~(18)O 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 deg 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 ~(18)O and D depletion in the sills (values as low as 4.9 per thousand and -143 per thousand, respectively) denote exchange with meteoric water after interaction with the high ~(18)O metamorphic fluids in the contact aureole. Although all of the elevated ~(18)O samples in the contact environment may have suffered ~(18)O 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 per thousand to 11.2 per thousand), 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 #delta#~(34)S values of 7.8 per thousand to 8.3 per thousand are strongly suggestive of pre-emplacement contamination by sulfur derived from a Proterozoic sedimentary unit. High #delta#~(18)O and #delta#~(34)S 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.