Abstract

Biotite 40αr/39αr cooling ages from medium-pressure (500–600 MPa) rocks in the Watersmeet district, northern Michigan, suggest significant cooling–uplift and concomitant deformation during gneiss dome formation at~1755 Ma, well after the close of the 1870–1830 Ma Penokean orogeny. However, an 1822 Ma hornblende plateau date indicates that the isograds surrounding the dome are Penokean in age. We attribute gneiss dome formation and doming of Penokean-aged isograds to an episode of orogenic collapse superimposed on an earlier history of crustal shortening. This contrasts with the compressional origin for gneiss domes preserved in the low-pressure (200–300 MPa) Republic district. The different origins may reflect the fact that collapse was localized along the overthickened region of the orogenic belt. In contrast to the Watersmeet area, hornblende and biotite 40Ar/39Ar ages obtained from the Republic area are 1720–1680 Ma. Given the relatively shallow depth of this region, it is unlikely that temperatures remained above 500 °C for over 100 Ma following collision. We interpret these ages to reflect a major thermal event that may have been responsible for formation of the Republic metamorphic node. This interpretation is supported by the recent identification of an ~1730 Ma pluton that is likely the cause of a large, near-surface, negative gravity anomaly coincident with the node, and by the fact that the metamorphic node crosscuts Penokean structures.

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