MINERALOGICAL CONSTRAINTS ON THE MAGMATIC EVOLUTION OF THE MINERAL LAKE INTRUSION, NORTHERN WISCONSIN

The 1.2 Ga Mineral Lake Intrusion (MLI) is an ~6 km thick mafic intrusion within the Mellen Igneous Complex located just west of Mellen, Wi. It is capped by a concordant ferrodiorite and is cut by the Mellen Granite and a felsic granophyre. Olivine and plagioclase are cumulus phases throughout the MLI, whereas clinopyroxene is intercumulus. Mineral modes and mineral geochemistry suggest the presence of magma mixing zones in the middle and near the top of the intrusion. Plagioclase modes within the intrusion range from ~60% at the base to ~90% near the middle. Olivine is present at the base with modes ranging from 6 – 30% but gradually decreases until it is absent near the middle of the intrusion. Olivine does reappear higher in the intrusion indicating possible mixing or the presence of additional intrusive bodies. Clinopyroxene shows similar trends in modal composition but it is never fully absent. Overall, clinopyroxene, as either fresh crystals or crystals altered to hornblende, is more abundant in the upper half of the intrusion (modes of 25 to 40%) relative to the lower half (modes of 5-30%). An abrupt increase in modal clinopyroxene occurs at the top of the middle mixing zone. Samples taken from this ~500m thick mixing zone show the reappearance of olivine, an increase in clinopyroxene, decrease in plagioclase and distinctive microfractures. These microfractures are unique to this horizon which is not laterally continuous across the intrusion. Microprobe data show nearly constant anorthite content at ~An₆₀ throughout the intrusion with the notable exceptions of the middle mixing zone which contains An₇₁ plagioclase. Additionally, forsterite contents in olivine show only minor fluctuations with an average of Fo₅₃, with the exception of higher values in the middle mixing zone (Fo₇₉) and in zones near the top of the intrusion (Fo₇₆ and Fo₆₆). The upper portion of the intrusion also contains a ferrodiorite with An₄₅ plagioclase and Fo₁₆ olivine. Overall these mineral chemistry and modal data suggest that the lower portion of the MLI experienced nearly continuous injections of new magma which buffered plagioclase and olivine compositions, whereas the middle to upper portions of the intrusion are more heterogeneous, which is consistent with sporadic late stage magma mixing and intrusion events.