PARENT MAGMA CHARACTERISTICS OF EARLY MIDCONTINENT RIFT ULTRAMAFIC INTRUSIONS

The Tamarack intrusion of Minnesota and the BIC intrusion of Upper Michigan are both ultramafic-mafic intrusions associated with the early magmatic stage of the 1.1 Ga Midcontinent Rift in the Lake Superior region. Both intrusions developed Cu-Ni-PGE-enriched sulfide mineralization due to contamination by sulfide-bearing Paleoproterozoic slates, into which they were emplaced. Both intrusions were emplaced in two major episodes, wherein the second emplacement event was followed by closed system fractional crystallization to produce well differentiated cumulate sequences. Despite similarities in mineralization, geologic setting, and emplacement/crystallization history, the two intrusions differ in several important aspects.

The most significant difference between the two intrusions is in their cumulate mineral paragenesis. The Tamarack intrusion follows a cumulate mineral progression of Ol -> Opx+Cpx+Ol -> Pl+Cpx+Opx -> Pl+Cpx+Opx+FeOx -> Pl+Cpx+Opx+FeOl. In contrast, the BIC intrusion follows a cumulate progression of Ol -> Cpx+Ol -> Cpx+Feox±Ol-> Pl+Cpx+Feox±Ap. In both cases, this phase layering is complimented by a smooth cryptic layering in mg# of olivine and pyroxene. The different cumulus parageneses between the two intrusions can be attributed to significant differences in their parent magma compositions. Both intrusions display chilled contacts with metasedimentary country rock that are characterized by fine-grained, olivine-phyric, prismatic feldspathic werhlite (BIC) or lherzolite (Tamarack). When the olivine phenocrysts are subtracted from the bulk compositions, the resultant “parent magma” compositions can account for the cumulate sequences observed in each intrusion.

The BIC parent magma is a tholeiitic magma with an mg# of about 68, is enriched in Ti and P, is low in Al, and has a high sloping REE pattern. It generally resembles the compositional characterisitics of the Disraeli and Hele intrusions in the Lake Nipigon area of Ontario. The Tamarack parent magma is also a tholeiitic magma with an mg# of about 71 and, compared to BIC, has high Si, Al, and Mg, low Ti and P, and a low sloping REE pattern. Its elevated Si would explain the stability of cumulus orthopyroxene and its higher Al concentration would explain the cumulus arrival of plagioclase soon after pyroxene saturation.