On the origin of early Proterozoic gneiss domes and metamorphic nodes, northern Michigan

Biotite (super 40) Ar/ (super 39) Ar cooling ages from medium-P (500-600 MPa) rocks in the Waterman district suggest significant cooling-uplift and concomitant deformation during gneiss dome formation at approximately 1755 m.y., well after the close of the 1870-1830 m.y. Penokean orogeny, though an 1822 m.y. hornblende plateau date indicates that the isograds surrounding the dome are Penokean in age. The doming of the gneisses and of the isograds is attributed to an episode of orogenic collapse superimposed on an earlier history of crustal shortening. This is in contrast with the compressional origin for gneiss domes preserved in the low-P (200-300 MPa) Republic district. Hornblende and biotite (super 40) Ar/ (super 39) Ar ages obtained from the Republic area are 1720-1680 m.y. These ages are interpreted as reflecting a major thermal event that may have been responsible for the Republic metamorphic node.