Models of basalt petrogenesis: Lower Keweenawan diabase dikes and middle Keweenawan Portage Lake Lavas,upper Michigan

Lower Keweenawan diabase dikes from Marquette-Baraga Counties, and middle Keweenawan Portage Lake Lavas from upper Michigan, can each be subdivided into two chemically distinct groups: a low TiO₂-P₂O₅ group characterized by higher A1₂O₃ content, higher Mg ratio, and lower total Fe and REE abundances than a high TiO₂-P₂O₅ group. Both groups, which are indistinguishable in the field, are enriched in the LREE relative to the HREE, and have similar normalized REE abundance patterns.The systematic variation of the REE's within the subgroups is consistent with a model of fractional crystallization of the observed phenocryst phases, olivine and plagioclase. The unique TiO₂/P₂O₅ content of the subgroups cannot be explained by magma mixing or fractionation from a single primary liquid. Thus, two mantle sources of differing depth and REE abundance are required to produce the TiO₂-P₂O₅ subgroups which are intercalated in the lava pile. Similar chemical trends within the dikes have been interpreted to mean that the same magmatic processes must have been operative during their emplacement in the early opening stages of the Keweenawan rift in Michigan.