POSSIBLE CRUSTAL CONTAMINATION OF MIDCONTINENT RIFT IGNEOUS ROCKS - EXAMPLES FROM THE MINERAL LAKE INTRUSIONS, WISCONSIN

Interlayered mafic - felsic intrusions from the Mineral Lake intrusive complex in northwest Wisconsin reflect the typical bimodal basalt-rhyolite compositional pattern of the Midcontinent Rift flood basalt province in the Lake Superior region. The later felsic intrusions were emplaced between the mafic intrusions and overlying basalt flows, and postemplacement fractional crystallization produced gradational mineralogical and geochemical variations. Isotopic and trace-element data for the Mineral Lake intrusions are consistent with mantle sources for both mafic and felsic intrusions, with compositional differences explained by the extent of fractional crystallization and crustal contamination or mantle source characteristics. Epsilon(Nd)-epsilon(Sr) plots of analyzed Midcontinent Rift igneous rocks define three largely separate isotopic fields that suggest separate sources. However, the spread in isotopic data and a spider diagram plot of mafic samples from the epsilon(Nd) = epsilon(Sr) = 0 field suggest a crustal component and derivation from depleted rather than chondritic mantle. Evolved felsic rocks plotting in two negative epsilon(Nd) - positive epsilon(Sr) fields can be explained by derivation from separate enriched mantle sources or crustal contamination or both.