Petrology and Structural Relations of the Brule Lake Intrusions, Cook County, Minnesota

Abstract

The Duluth Complex splits into two east-trending tongues at its eastern extent to form a north ern and a southern prong which expose a strip of older Keweenawan volcanic rocks between them. Brule Lake is located at the westernmost extent of the strip of volcanics just east of the point where the gabbro prongs coalesce. The volcanic rocks are intruded by a series of large sill-like bodies which terminate against rocks of the nearby Duluth Complex. Thus, these intrusions were formed between the time of the eruption of the local Keweenawan volcanic rocks and the subsequent gabbroic intrusions.

The intrusive rocks of the Brule Lake area, which occur both as dikes and sills, consists of porphyritic intrusions and subsidiary non-porphyritic bodies. The gabbro porphyries are from 100 to JOO meters thick and extend for 2 to 8 kilometers along strike. The non-porphyritic intrusions are much smaller than the porphyritic units and occur as dikes and sills in the volcanic rocks and in the older porphyry units.

The Brule Lake intrusions were derived from magmas of two different sources. Type A magmas, which formed mainly large, porphyritic gabbro units, are older than those smaller dikes and sills formed by the type B magma.

Major and trace element concentrations suggest that the type A magmas were developed by a two-stage process. The first stage was one of extensive differentiation of a tholeiitic magma involving principally the fractionation of calcic plagioclase and Mg-rich pyroxene in relatively shallow-crustal magma chambers. The melt was then tapped off, leaving behind a residuum of anorthositic or gabbroic composition. The liquid continued to crystallize plagioclase (now more sodic) in a second chamber, the feldspar this time floating in the liquid. This phenocryst-rich liquid portion was then tapped from the top of the magma chamber and injected along planes of weakness in the overlying volcanic pile.

The type B magmas did not undergo the same degree of differentiation as the type A. They were probably derived through a process that involved olivine fractionation to a greater degree than occurred in the type A line of descent. They were then intruded along the same planes exploited by the earlier liquids, incorporating very little of the country rock.

The intrusive rocks of the Brule Lake area are chemically similar to other intrusive rocks of northeastern Minnesota, particularly the Logan Intrusions. Although there are chemical differences between the two groups, their overall similarity leads to a conclusion that their origins were probably quite similar and are related to the same phase of Keweenawan igneous activity.