INVESTIGATION OF A MARGINAL REVERSAL AT THE BASE OF THE MINERAL LAKE INTRUSION, NORTHERN WISCONSIN

The Mineral Lake Intrusion is a mafic intrusion associated with the 1.2 Ga. Keweenawan rift. The intrusion was uplifted along a listric reverse fault, which allowed for the exposure of a full vertical section. Previous work suggested the presence of a marginal reversal at the intrusion’s base, but this conclusion was based on very limited data (three samples). This study presents the results of eleven additional samples taken from a near linear traverse from the lowermost 400m of the intrusion. In addition to whole rock geochemistry, the samples were analyzed using an electron microprobe for the chemical composition of plagioclase (plag), olivine (ol), clinopyroxene (cpx) and orthopyroxene (opx). From the base to 120m into the intrusion, the data shows systematic variation in mineral and whole rock compositions, which are consistent with the presence of a marginal reversal. For instance, there is an increase in cpx magnesium number (Mg#) from 0.45 to 0.70 and the Ti per formula unit (pfu) increases from 0.006 to 0.018. Plagioclase anorthite (An) content increases from An₄₉ to An₅₉. Orthopyroxene Mg# increases from 0.34 to 0.64 and the Ti pfu increases from 0.0025 to 0.01. The ol forsterite (Fo) content increases from Fo₂₄ to Fo₅₈, however olivine was not observed in all the samples. The whole rock Mg# was 0.19 at the base and increased to 0.40 at 120m. Whereas the TiO₂ decreased from 4.80 to 0.79. These data are consistent with a marginal reversal, because they show trends that are opposite of those normally created by mafic magma differentiation. The mechanism(s) that produced the reversal is currently unknown, although the intrusion as a whole shows evidence of multiple injections suggesting that multiple injections might have played a role in the formation of the marginal reversal. Future work will involve meter scale sampling to identify finer scale trends that may better constrain the mechanism(s) that produced the marginal reversal at the base of the Mineral Lake Intrusion.