Paper No. 9
Presentation Time: 10:30 AM
A NEW VIEW OF PETROGENESIS OF THE SONJU LAKE-FINLAND GRANITE SYSTEM WITH IMPLICATIONS FOR PGE CONCENTRATION MECHANISM
LUNDSTROM, Craig C., Dept of Geology, Univ of Illinois, 1301 W Green St, Urbana, IL 61801, SCHERER, Erik, Institut für Mineralogie, Universität Münster, Corrensstrasse 24, D-48149, Münster, Germany and IRELAND, Kayla, Dept of Geology, University of North Carolina, Chapel Hill, NC 27599, lundstro@uiuc.edu
The Sonju Lake Intrusion (SLI) is a 1200 m thick layered mafic intrusion that directly underlies the equal volume Finland Granite (FG). These intrusions, part of the Proterozoic Keweenaw rift, provide a puzzle to standard models of igneous petrogenesis: examination of mineral modes, mineral compositions and even incompatible trace element ratios show smooth gradation between the two bodies as if the two intrusions were genetically related. Yet, differences in Sr and Pb isotope ratios and simple volume relationships argue against any fractional crystallization-like process linking the intrusions. Lundstrom et al. (2011) provided an alternative model showing that SLI modes and compositional trends could be reproduced by a top down sill injection process of a uniform composition basalt with differentiation occurring by temperature gradient based diffusion-reaction above the sill. We build on this model to show that the observed PGE reef in the SLI could be explained by a similar process.
Illinois petrology classes have gathered major, trace and isotope data to test to the top down proposal. 32 samples including 10 from the bottom of the SLI (the SNA drill core) and 8 from the AC1 drill core through the Finland Granite were analyzed for δ56Fe with the prediction being that stratigraphically higher samples should have heavier δ56Fe. Results show that several Finland Granite samples have higher δ56Fe than the mean mafic earth while most of the SLI samples hover near this value. In addition, we have performed Rb-Sr and Lu-Hf mineral isochron dating 3 rocks spaced through the SLI. While in general both systems show considerable isochron scatter when all phases are included, it is notable that an isochron defined by 2 apatite fractions and WR for an apatite ferrogabbro in the upper SLI, previously dated by U-Pb badeleyite methods, provides low MSWD (0.15) yet differs from the U-Pb age by 8 Myrs. The scatter in isochron points for different phases is interpreted to reflect the differing extents of reaction between the interstitial fluid melt and the particular phase.
