Abstract

Nineteen U-Pb zircon analyses from the Still-water complex basal zone and associated rocks are interpreted as meaning: (1) the Stillwater igneous complex was emplaced about 2,750 m.y. ago; (2) there is little recorded time difference (less than 60 m.y.) between intrusion of the Stillwater complex and emplacement of siliceous igneous rocks at the southeastern end of the complex; (3) the Stillwater complex intruded sediments or metasediments containing approximately 3,140 m.y. old (or older) zircons; and (4) the Stillwater complex contact metamorphism did not reset U-Pb clock in zircons from exposed hornfels, although hornfels zircons appear to have lost from about 10 to 40 percent of their lead due to intrusion of the complex. The discordant zircon data appear to be best explained with an episodic lead loss mechanism; lead having been lost about 2,750 and 100 m.y. ago. The possibilities, however, of uranium gain at these times, or some continuous diffusion lead loss, or both, may not be excluded.

Four apatite U-Pb analyses suggest apatite in the Stillwater complex vicinity lost lead or gained uranium about, 1,600 m.y. ago during an event that did not significantly influence the zircon ages.

U-Th-Pb data from Mouat quartz monzonite magnetite indicate either uranium and thorium are heterogeneously distributed in the magnetite and/or uranium and thorium are very susceptible to laboratory acid leaching.

Limited plagioclase data did not yield useful age information; rather they showed that the lead isotopic compositions of the Stillwater complex anorthosite have been disturbed since the complex was emplaced.

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