Rb-Sr dating of Precambrian W (lower Precambrian) gneisses and granitic rocks that form the cores of antiforms or domes mantled by metamorphosed Precambrian X (middle Precambrian) bedded rocks in the Marenisco-Watersmeet area, northern Michigan, shows that the rock systems have been disturbed to different degrees by complex tectonic and thermal events subsequent to primary crystallization. The disturbance is indicated by a lowering of the whole-rock Rb-Sr ages of many samples of gneiss from 2,600 m.y. or older to about 1,800 m.y. and by the scattering of data points, although some of the points can be fit to secondary isochrons; probably the major mechanism was redistribution of previously accumulated radiogenic Sr, which is indicated by the high initial 87Sr/86Sr of the resultant isochrons. The disturbance is interpreted as having resulted from internal cataclasis and recrystallization accompanied by metamorphic segregation and (or) partial melting during reactivation of the gneisses about 1,800 m.y. ago. Variations in the degree of reconstitution and mobility of individual bodies of basement rocks are indicated by differences in structural reconstitution of the basement rocks and in the metamorphic grade of the Precambrian X bedded rocks.
