The Bedrock Geology of a Portion of the Cramer 15' Quadrangle, Lake County, Minnesota

Abstract

The area studied consists of sections 20 - 29 and 32 - 36 of T60N, R6W of the Cramer, Minnesota 15' quadrangle. Units exposed include the anorthositic "series," troctolitic "series," and felsic "series" of the Duluth Complex, volcanic rocks of the North Shore Volcanic Group, melagabbroic-gabbroic intrusive rocks, diabasic rocks, and a hypabysal intrusive. Bedrock units are locally covered by a variety of glacial deposits related to the Rainy Glacial Lobe of the Wisconsin Ice Age. Troctolites and gabbros of the troctolitic series of the Duluth Complex display cummulate textures and regular cryptic (chemical) variation in the anorthite content of plagioclase (An78 to An58), the forsterite content of olivine (Fo70 to Fo50), and in the FeO/FeO+MgO ratio in augite (22 - 40). Orientations of igneous laminations as well as the areal distribution of rock types and cryptic variations strongly suggest the troctolitic series is a sill-like intrusion which differentiated in place as the result of crystal settling.