ABSTRACT

The two predominant basement units just north of Cooke City, Montana are a pink, granitic-to-adamellitic gneiss and a grey, tonahtic-to-gramtic gneiss. In the field, the pink gneiss consistently intrudes the grey. The pink granitic gneiss has previously been well dated by Rb-Sr whole-rock methods at 2745 + 37 Ma, but the age of the grey gneiss could not be similarly resolved. U-Pb data from small (0.2 to 0 5 mg) zircon separates from the grey gneiss are highly discordant, and yield imprecise older intercept ages that vary with locality from 2.66 +.34/-.30 b.y. to 3.11 + .24/-.22 b.y. The morphologies of the zircons vary also. Detrital zircons from metasedimentary enclaves within the grey gneiss show a range of ²⁰⁷Pb/²⁰⁶Pb ages up to 3.15 b.y. The data permit two alternative interpretations: either the grey gneiss is up to several hundred million years older than the pink, or the two are nearly contemporaneous, but the grey gneiss has assimilated a higher proportion of older metasedimentary material. In either case, no evidence of pre-3.15 by crust has been found in this area.

Oxygen-isotope data are also consistent with a significant crustal oxygen component in the grey gneiss, which gives §¹⁸0 values of +7.0 to + 10.0‰ relative to SMOW The metasedimentary enclaves have still heavier oxygen, with § 0 = +7.9 to + 10.7‰. An unexpected outcome of the oxygen analyses was the discovery that the pink gneisses, previously inferred to be of mantle origin in view of their initial ⁸⁷Sr/⁸⁶Sr - .7005, contain the heaviest oxygen of all §¹⁸0 = +8.1 to + 12.3‰). It may be that the pink gneiss has been derived, not from a mantle source, but from a high-grade lower-crustal source, depleted in Rb and thus having nonradiogenic Sr but possessing a characteristically heavy oxygen-isotope signature.

Comparison of the chronologic data with results reported elsewhere in the Beartooth range supports the suggestion that the time of stabilization of continental crust varies across the range, with an older terrane in the east that is absent to the west and southwest.