The Sonju Lake Intrusion, located within the Beaver Bay Complex near Finland, MN, is the most completely differentiated intrusion related to the Midcontinent Rift System (Stevenson, 1974; Wieblen, 1982). The Sonju Lake intrusion exhibits a cumulate stratigraphy consistent with closed system differentiation of tholeiitic magma by fractional crystallization (Stevenson, 1974; Miller et al., 1993; Miller and Ripley, 1996). The Finland granite, composed of micrographic-textured ferromonzonite to leucogranite, forms the hanging-wall to the Sonju Lake intrusion and has geochemistry consistent with the Finland granite being a late stage felsic differentiate of the Sonju Lake intrusion (Miller and Green, 2002; Miller and Ripley, 1996). However, geophysical modeling of gravity and aeromagnetic data implies a volume of granophyre that approaches that of the Sonju Lake intrusion and therefore greatly exceeds the volume of felsic material that could be accounted for by differentiation of a mafic body of that size (Miller et al., 1990).
Miller and Ripley (1996) suggested that the Finland granite was emplaced first and acted as a density barrier to the upward movement of the mafic Sonju Lake magma. Since underplating of the mafic magma would be expected to cause melting in the lower portions of the Finland granite, the lithologically and chemically gradational contact between the two bodies might instead represent a mixing zone between the mafic magma of the upper Sonju and the partially melted base of the Finland granite. The major and minor element geochemistry of the Finland granite resembles an extreme differentiate of the Sonju Lake intrusion and precludes using lithogeochemical data to evaluate the extent of mixing between mafic and felsic magmas. The Finland granite and Sonju Lake intrusion have distinct and identifiable isotopic compositions of Nd. Samples from the leucogranite range from -3 to -3.6 εNd at 1096 Ma with an average of -3.4 εNd for five samples (Vervoort and Green, 1997, Vervoort et al., 2007, this study). Samples from the Sonju Lake intrusion, below the cumulus arrival of apatite, range from 1.47 to -1.62 εNd. The rocks above and including the uppermost cumulate layer of the Sonju Lake intrusion consistently exhibit decreasing values of εNd with increasing stratigraphic height closer to the Finland granite. If the consistent isotopic signature of the leucogranite (-3 to -3.6 εNd) is taken as the initial εNd of the Finland granite as a whole, it is clear that the underlying quartz ferromonzodiorite, with εNd compositions of -1.7 to -2, contains a more radiogenic Nd component attributable to the Sonju Lake intrusion. The quartz ferromonzodiorite may be a late-stage differentiate of the Sonju Lake intrusion that assimilated non-radiogenic Nd from the Finland granite. An assimilation-fractional crystallization model is presented to evaluate Nd isotopes in the upper portions of the Sonju Lake intrusion and overlying Finland granite. From this model it is estimated that the Sonju Lake intrusion assimilated an amount of Finland granite equal to approximately 1-1.5% of the total mass of the Sonju Lake intrusion. Anomalously negative εNd values for samples from the SLI-1 core that profiles the felsic-mafic contact west of the main exposure area are enigmatic.
University of Minnesota M.S. thesis. August 2011. Major: Geological sciences. Advisor: James Miller Jr. 1 computer file (PDF); vii, 90 pages, appendices A-B.
Dayton, Ryan N..
The geochemical evolution of the Sonju Lake intrusion: assimilation and fractional crystallization in a layered mafic intrusion near Finland, Mn..
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