ON THE ORIGIN OF EARLY PROTEROZOIC GNEISS DOMES AND METAMORPHIC NODES, NORTHERN MICHIGAN

Biotite Ar-40/Ar-39 cooling ages from medium-pressure (500-600 MPa) ro cks in the Watersmeet district, northern Michigan, suggest significant cooling-uplift and concomitant deformation during gneiss dome formati on at similar to 1755 Ma, well after the close of the 1870-1830 Ma Pen okean orogeny. However, an 1822 Ma hornblende plateau date indicates t hat the isograds surrounding the dome are Penokean in age. We attribut e gneiss dome formation and doming of Penokean-aged isograds to an epi sode of orogenic collapse superimposed on an earlier history of crusta l shortening. This contrasts with the compressional origin for gneiss domes preserved in the low-pressure (200-300 MPa) Republic district. T he different origins may reflect the fact that collapse was localized along the overthickened region of the orogenic belt. In contrast to th e Watersmeet area, bornblende and biotite Ar-40/Ar-39 ages obtained fr om the Republic area are 1720-1680 Ma. Given the relatively shallow de pth of this region, it is unlikely that temperatures remained above 50 0 degrees C for over 100 Ma following collision. We interpret these ag es to reflect a major thermal event that may have been responsible for formation of the Republic metamorphic node. This interpretation is su pported by the recent identification of an similar to 1730 Ma pluton t hat is likely the cause of a large, near-surface, negative gravity ano maly coincident with the node, and by the fact that the metamorphic no de crosscuts Penokean structures.