MINERAL CHEMISTRY CONSTRAINTS ON THE ORIGIN OF THE MINERAL LAKE INTRUSION, NORTHERN WISCONSIN

The 1.2 Ga. Mineral Lake Intrusion (MLI) is a mafic intrusion located in northern Wisconsin that was emplacing during the Keweenawan rifting event. The emplacement level was shallow based on its location between Keweenawan volcanic rocks and the Paleoproterozoic basement rocks. Subsequent compression and uplift has exposed a 6000m cross-section through the whole intrusion. Thirty-three samples collected on a near linear traverse from the base to the top of the intrusion have been studied using the chemical compositions of clinopyroxenes (cpx), olivines (ol), and plagioclase (plag) as determined by electron microprobe analyses. These data reveal three distinct zones along the traverse. In the first zone, from the base upwards to 200m, the olivine compositions increase from Fo₅₁ to Fo₆₃ and the cpx Mg# increase from 0.62 to 0.74. The second zone extends from 200m to 3750m, where olivine compositions decrease from Fo₆₃ to Fo₅₁ and the cpx Mg# decrease from 0.74 to 0.58. The third zone starts at 3750m where olivine and cpx compositions dramatically increase to Fo₇₄ and Mg# 0.82, respectively. These geochemical changes correspond with distinctive fracturing within the samples. This third zone extends up to 5800m and is characterized by intermixed primitive (cpx Mg# 0.82, plag An₇₆) and evolved (cpx Mg# 0.45, plag An₄₂ ) units.

The increase of cpx Mg# and olivine forsterite contents within the first 200 meters is interpreted as a possible magmatic reversal. This reflects the initial magmatic injection from a deeper magma chamber or conduit. We propose that this deeper chamber was zoned with evolved magma surrounding a more primitive center. The evolved magma migrated first to formed the base and edges of the MLI whereas the more primitive liquid from the center of the deeper chamber followed the first pulse to form the second and third zones. We propose that the high MgO samples within the third zone reflects the late stage intrusion of primitive melts from the mantle into the upper portion of the MLI.