Discordant isotopic ages and excess argon in biotites |
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| Authors: | Bruno J Giletti |
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| Institution: | Department of Geological Sciences, Brown University, Providence, Rhode Island 02912,USA |
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| Abstract: | Major isotopic age discordances are found at the eastern margin of a terrane in SW Montana which underwent metamorphism 1.6 AE ago (1AE= 109years). In Portal Creek, a one-mile portion of a traverse across this margin yields: (1) discordant whole rock Rb-Sr ages which approximate 2.8 AE; (2) seven biotite Rb-Sr ages concordant at 1.68 AE; (3) fifteen biotite K-Ar ages which are highly discordant and range from 1.63 to 3.25 AE; (4) ten generally concordant hornblende K-Ar ages at 1.9 AE. It is concluded that there was a major resetting event for the hornblendes at 1.9 AE ago. This was followed by a milder (?) event at 1.6 AE when the biotites became open systems while the hornblende did not. During this time, the biotites lost all their radiogenic87Sr, but actually gained radiogenic40Ar in amounts up to 16.0 × 10−4 scc Ar/g biotite. There is clear evidence of the incorporation of87Sr into minor amphibolitic layers which occur occasionally in these generally quartzo-feldspathic rocks. |
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