PALEOPROTEROZOIC IRON-FORMATIONS OF THE LAKE SUPERIOR REGION IN TIME AND SPACE

The Paleoproterozoic banded iron-formations (BIFs) of the Lake Superior region are exposed in seven geographically separate ranges within a 100,000 km² region on the southern edge of the Superior craton. New observations from drill holes in Northern Michigan, drilled through Paleozoic strata east of the exposed ranges, show a facies change from the exposed BIFs in the west to fine-grained ferruginous clastic rocks in the east, indicating an eastern land mass of Archean rocks. A similar relationship was documented previously in Minnesota, near the western end of the Mesabi Range, and indicates a corresponding western landmass. Together, these observations suggest that the basin for chemical deposition Lake Superior BIFs may not have extended far beyond the preserved iron ranges. Geochronological data, reported sporadically over the past 20 years, indicate that deposition of BIFs in all the ranges is broadly, if not fully, synchronous, and persisted for 50-60 m.y., starting about 1910-1900 Ma and continuing until 1850 Ma. Constraints include an 1850 Ma minimum age provided by the Sudbury impact layer that commonly conformably lies on the uppermost BIF. Ages of interlayered volcanic and intrusive rocks, as well as detrital zircons are additional constraints on the age and correlation of BIFs across the region. Detrital zircon data show that clastic input before, during, and briefly after BIF deposition was entirely from Archean sources in the Superior Craton, or pre-BIF clastic rocks derived from it. The basin was open to the south to an ocean where initial accretion of an arc terrane had occurred by 1900 Ma and where volcanic complexes continued to form throughout BIF deposition. Although those terranes shed no clastic sediments to the north during BIF deposition, extensive massive-sulfide-related hydrothermal vents may have been a significant source of iron. Shortly after 1850 Ma, rapid uplift and erosion of the volcanic terranes, signaling the onset of the contractional phase of the Penokean orogeny, flooded the BIF depositional basin with clastic sediments, as indicated by dominant and widespread populations of 1850 Ma detrital zircons in post-BIF clastic sediments. This northward flood of clastic sediments ended the quiescent, sediment-starved condition needed for BIF deposition.