DISTRIBUTION OF PRE-1400 MA CRUST IN THE GRENVILLE PROVINCE - IMPLICATIONS FOR RIFTING IN LAURENTIA-BALTICA DURING GEON-14

Recent geochronological data show that geon 14 and older rocks are fou nd at least 700 km south of the Grenville front in the eastern Grenvil le province. In the southwestern Grenville province, comparable rocks extend only 200 km south; farther south, in the Central metasedimentar y belt and the Adirondack Mountains, the crust mostly postdates 1350 M a. We explain this contrast by postulating a major rifting event, comm encing at approximately 1430 Ma, during which large sections of older crust were detached from that part of southern Laurentia now occupied by the Central metasedimentary belt. Crust was also removed from the a rea between northwest Britain and Scandinavia. Failed arms extended (1 ) from Missouri to where the Belt Supergroup accumulated, (2) from the Central metasedimentary belt eastward into Labrador, and (3) between Baltica and east of Greenland. The geologic history following late geo n 14 rifting was dominated by a southward-subducting magmatic arc that eventually collided with southern Laurentia-Baltica, resulting in the Elzevirian orogeny at approximately 1200 Ma.