Duluth Complex and Associated Rocks of the Midcontinent Rift System – Minneapolis to Duluth, Minnesota, Volume T345

Published by the American Geophysical Union as part of the Field Trip Guidebooks Series, Volume 345.

The four and a half day itinerary of the Duluth Complex portion of Field Trip No. T-345 covers 960 km and includes 24 stops (Figs. 1 and 2, Table 1, pp. 42 and 43). As can be seen from the data in Table 2, the outcrop area of the 1,100 Ma Middle Proterozoic(Keweenawan) Duluth Complex (4,700 km2) is an order of magnitude greater than that of the 2,700 Ma Archean Stillwater Complex (194 km2). Beyond size and age, there are other contrasts between the Stillwater and the Duluth Complexes: The Stillwater Complex (Czamanske and Zientek, 1985) consists of a more or less continuous section of ultramafic to feldspathic units whereas the Duluth Complex (Fig. 3) consists of a series of separate anorthositic, troctolitic-gabbroic, and granitoid intrusions. The only ultramafic units in the Duluth Complex are limited sequences of feldspathic dunite found at the base of troctolitic intrusions (Stevenson, 1974) and small (