About Rudy Wahl Links Pictures Credit Email Home
Regional Structural Setting
Figure 13. Major carbonatite-alkalic intrusions and major regional faults. 1) Chipman Lake fenites and carbonatit dikes; 2) Killala Lake alkalic complex; 3) Prairie Lake cabonatite; 4) Port Coldwell alkalic complex; 5) Gold Range diatreme; 6) Slate Islands diatremes and carbonatite dike; 7) Neys diatreme; 8) McKellar Creek diatreme; 9) Dead Horse Creek diatreme: A) Michipicoten Island fault; B) Big Bay - Ashburton Bay fault and its extrapolated northern extension. Scale 1:1,584,000
Regional Structural Setting
The following discussion pertains to the Slate Islands, McKellar Creek, and Dead Horse Creek diatremes. The relationship of the Gold Range diatreme to regional geology is uncertain. The Neys diatreme is considered by the author to represent the release of high level accumulation of volatiles in a crystallizing alkalic magma, entirely different from the other structures, which represent mantle-derived volatiles. The volatiles of the Dead Horse structure were related to carbonatite intrusion, and in the case of the Slate Islands, perhaps, diabasic or gabbroic intrusion.
Figure 13 is a generalized sketch of major alkalic geologic features and structures associated with these features north of the Lake Superior basin. The Slate Islands occur at the junction of two regional faults (Figure 13) identified on the basis of geophysical data by Hinze et al. (1966). The Michipicoten Island fault passes south of the islands, and up along the southwest side of the island group, and is likely a curvilinear feature marginal to the Lake Superior basin. The Big Bay - Ashburton Bay fault crosses the Lake Superior basin on a northeast trend intersecting the Michipicoten Island fault immediately south of the Slate Islands. It continues northeastward, intersecting the shoreline in the area of Ashburton Bay along the west flank of the Port Coldwell complex. The Big Bay - Ashburton Bay fault is actually a zone of faulting, which, at its point of impinging on the mainland between the Slate Islands and Ashburton Bay, spans a width on the order of 30 km between Ashburton Bay Jackfish Bay north of Slate Islands. The north to east -of-north trending linears along the shoreline, that are commonly occupied by small steams, are a reflection of a series of subparallel shears and faults. On the basis of airphoto interpretation and topographic maps, the author has postulated the extension of the fault northward to the Killala Lake alkalic rock complex, and then beyond to the Chipman Lake fenites and carbonatite dikes (Sage 1978). Subsidiary faults possibly controlled the emplacement of the Prairie Lake carbonatite (Sage 1978). At Chipman Lake, fault has a left lateral offset of approximately 0.8 km (Sage et al. 1976). South of Lake Superior, this structure has been traced into Michigan and Wisconsin (Klasner and Turner 1973 et al. 1975). The Bib Bay - Ashburton structure is the site of considerable crustal thickening (Amith et al. 1966; O'Brien 1968), and divides the Lake Superior basin into sub-basins, indication that the structure was a topographic high in Late Precambrian time (White 1972). At the site of the Slate Islands, the trend of the Big Bay - Ashburton Bay fault swings east, and in the proximity of the Port Coldwell complex, it swings west. Perhaps these deviations in tend created points of dilatancy during movement, thereby controlling the site of the Slate Islands diatremes and Port Coldwell alkalic rock complex (Sage in preparation). North to northeast trending fractures, parallel and subsidiary to the main structure, likely controlled the emplacement of the McKellar Creek and Dead Horse Creek diatremes, and account for their long axis orientation. The Killala Lake alkalic rock complex occurs at the point of intersection of the extension of the Big Bay - Ashburtion Bay fault, and a southwest tending lineament, conformable to lithologic trends (Coates 1970), between the Killala Lake alkalic rock complex and Prairie Lake carbonatite. Most of the alkalic rock-carbonatite intrusive events are Late Precambrian in age, however, a middle Paleozoic age for the Slate Islands lamprohyre dike with carbonatite affinities suggests that alkalic magmatism, north of Lake Superior, along this trend, spanned at least 700 Ma.