REWORKED AND K-METASOMATIZED EJECTA FROM THE SUDBURY IMPACT OCCURS IN MULTIPLE BEDS IN THE GOGEBIC IRON RANGE, WISCONSIN, USA

The 1850 Ma Sudbury meteor impact spread debris across the Lake Superior region of the U.S. and Canada that has been documented in most of the iron ranges of the region 450 to 1000 km west of the impact site. Twenty four iron exploration drill holes on the Gogebic Iron Range, from 650 to 700 km from the impact point, provide a detailed view of the ejecta there. Unlike all other iron ranges, where a single layer of ejecta-bearing rocks lies mostly at the upper contact of the iron-formation, there are as many as eight ejecta-bearing beds within a 25 m section of otherwise fine-grained clastic rocks in individual drill holes on the Gogebic Range. Most beds are meter-scale or thinner turbidites consisting of both local sediments and ejecta. Ejecta includes devitrified glass spherules and shards, accretionary lapilli, and shocked quartz grains. In addition, there are several channelized debris flows as much as 20 m thick. These are coarse breccias of partly consolidated local sediments intermixed with ejecta. The ejecta-bearing beds are also unique in being enriched in potassium (up to 13 wt. % K₂O) and depleted in sodium (about 0.2 wt. % Na₂O). These seem unlikely to be primary compositions. The high K contents resulted from two independent processes: 1) the initial ejecta appears to have been potassic as shown by an abundance of primary potassium minerals, mostly microcline, which occurs as microphenocrysts and fragments, and as glass particles containing a mosaic of microcline exhibiting various chilled textures, 2)  the beds contain many ejecta fragments with relict primary textures that are now almost entirely microcline, suggesting K-metasomatism of the primary ejecta prior to reworking. The metasomatism may be analogous to formation of K-bentonites that form as terrestrial volcanic ash interacts with sea water in shallow alkaline marine conditions, where the formation of metasomatic K-feldspar is favored. The multiple K-rich ejecta-bearing beds in the Gogebic Range appear to have resulted from a unique combination of primary ejecta deposition in shallow shelf conditions where K-metasomatism took place, followed by repeated mobilization of that material in debris and turbidite flows bringing it to its present site of deposition.