MAGNETIC FABRIC OF THE SONJU LAKE INTRUSION, NORTHEASTERN MINNESOTA: EVIDENCE FOR INTERNAL STRUCTURE AND EMPLACEMENT DYNAMICS

The Sonju Lake intrusion in northeastern Minnesota is a 1200 m thick layered intrusion of Keweenawan age (1096 Ma). The intrusion is related to the development of the Midcontinent Rift and is an example of closed-system differentiation of a tholeiitic mafic magma. Previous petrographic, geochemical, and field studies have led to a comprehensive knowledge of the cumulate series, igneous stratigraphy and differentiation trend, while very little is currently known about the emplacement dynamics of the intrusion. Anisotropy of magnetic susceptibility (AMS) was measured at 32 sites across the intrusion to provide information about primary igneous fabrics. Since large outcrops are scarce and vegetation obscures most field fabrics, the magnetic fabric data allow us to investigate the mechanisms of fabric development in the layered series as well as provide constraints on the location of the feeder system, magmatic flow patterns, and emplacement history of the intrusion.

Most magnetic foliations dip shallowly, < 30°, while dip direction varies from SW to SE. The mean magnetic foliation of 104, 18° S parallels the mean mineral foliation of 100°, 20° S. Strongly oblate magnetic shape fabrics indicate that the shallowly dipping planar fabrics were enhanced by compaction of the crystal mush. Well-aligned magnetic lineations consistently plunge shallowly toward the southwest. We attribute the lineation pattern to a dike-like feeder located to the southwest, beneath the Finland Granite.

Magnetic fabrics observed in the layered series of the Sonju Lake intrusion are consistent with sub-horizontal emplacement of the intrusion and show evidence that the cumulate layers were deposited in a dynamic environment. Emplacement of the Sonju Lake intrusion was contemporaneous with the tectonic development of the Midcontinent Rift system and therefore is likely to have been controlled by rift structures. The older Finland Granite may have acted as a density barrier to the upward movement of the Sonju Lake mafic magma, forcing magma flow to the northeast.