Abstract

The Coldwell Complex gives a U–Pb zircon age of 1188 ± 56 Ma, a U–Pb tritomite age of 1126 ± 6 Ma, and a Pb–Pb potash feldspar age of 1139 ± 55 Ma. These ages are all significantly older than the previously published Rb–Sr and K–Ar ages. We interpret these ages as follows: (1) emplacement of the complex at 1188 Ma as part of the initial stages of the Keweenawan rifting event at depth; (2) slow cooling leading to closure of the U and Pb systems in tritomite and potash feldspar between 1139 and 1126 Ma; and (3) continued slow cooling leading to closure of the Rb–Sr system in the complex at an average age of 1053 ± 10 Ma and of the K–Ar system at ~1035 Ma.

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