A Mafic Layered Intrusion of Keweenawan Age Near Finland, Lake County, Minnesota

Abstract

A gently dipping sequence of layered, differentiated, mafic rocks are exposed in the vicinity of Sonju Lake, Minnesota. These rocks, here named the Sonju Lake intrusion, are in fault contact along their eastern edge with diabasic olivine gabbro and troctolite. To the south, prismatic granodiorite appears to intrude the top of the layered units, and to the north, the base of the intrusion is bounded by diabasic troctolite. All of the rock units in the area are presumed to be Late Precambrian in age.

The Sonju Lake intrusion has well-developed igneous lamination and centimeter-scale, gravity stratified, and cryptic layering. The intrusion has a stratigraphic thickness of approximately 3300 feet (1000 meters). The rock units, from the base to the top, are picrite (105 feet), a transition zone (150 feet), troctolite (965 feet), two-pyroxene gabbro (780 feet), augite gabbro (680 feet), apatite-rich ferrodiorite (265 feet), and a granodiorite unit (350 feet). The crystallization sequence based on stratigraphy and textures is (chromian spinel)-olivine-plagioclese-augite-pigeonite(inverted)-ilmenite-magnetite-apatite-alkali feldspar-quartz.

Compositions of major minerals vary systematically with stratigraphic height. Electron microprobe and optical studies show that olivine varies from Fo71 in the picrite to Fo12 in the uppermost portion of the apatite- rich ferrodiorite, plagioclase from An83 in the picrite to An46 in the ferrodiorite, and augite from Wo43En46Fs11 in the two-pyroxene gabbro to Wo40En25Fs35 in the uppermost portion of the apatite-rich ferrodiorite.

A calculated bulk composition for the Sonju Lake magma is tholeiitic and is between the high-alumina olivine diabase magma of the Pigeon Point sill and the more alkali and titaniferrous tholeiitic magma of the Logan Sills, Canada.

In bulk composition, layering, crystallization sequence, and differentiation the Sonju Lake intrusion has many affinities with other Keweenawan intrusions as well as the Tertiary Skaergaerd intrusion of East Greenland. The data presented in this thesis place new chemical and physical restraints on models of Late Precambrian petrogenesis.