Petrology and geochemistry of the mafic and ultramafic dikes and intrusions in Aitkin, Pine and Carlton counties, Minnesota

Abstract

This study investigated the 1.1 Ga Midcontinent Rift, MCR, reverse-polarity dikes and intrusions in Carlton, Aitkin, and Pine Counties in Minnesota for evaluation of genetic relationships within the study area and with MCR rocks throughout the Lake Superior region. The breadth of new work includes petrography, mineral analysis, whole-rock geochemistry, and geochronology. The intrusive rocks in the study area are divided into eight subgroups, broadly characterized by high-TiO2 diabase (TiO2 > 3.0 wt%), low-TiO2 diabase (TiO2 < 2.2 wt%), and moderate-TiO2 phenocryst-rich mafic and ultramafic dikes. Genetic relationships are developed based on geochemical characteristics of the subgroups, which allow recognizing the effects of fractional crystallization, crustal contamination, and variations in source rock composition, and trace element ratio models (e.g. Th/YbPM versus Nb/ThPM and Nb/YbPM versus Th/YbPM), which quantify mixing lines between possible depleted- or enriched-source regions and degrees of assimilation of crustal material. These geochemical characteristics identified the following relationships within the subgroups and with other MCR intrusive suites: (1) within the high-TiO2 group, the Esko intrusion and Esko dike have geochemical characteristics similar to the 1.14 Ga Abitibi dike swarm in Ontario and 1.12 Ga Cu-depleted dikes in Michigan, while the Carlton County dike swarm and Denham dike have geochemical characteristics analogous to the 1.115 Ga Logan sills in Ontario; (2) within the low-TiO2 group, the Cloquet dike has geochemical characteristics analogous to the 1.114-1.107 Ga Nipigon sills in Ontario, and the Normal-polarity dikes are likely extensions of the adjacent Duluth dike swarm and similar to the Pigeon River dikes in Ontario; and (3) the moderate-TiO2 phenocryst-rich dikes are separated into two subgroups, the AG dike, a hornblende augite-olivine-phyric melagabbro with mildly alkaline characteristics, and the Kettle River dikes, an olivine-augite-phyric melagabbro to feldspathic peridotite with geochemical characteristics similar to the Tamarack Intrusive Complex. The AG dike composition was not directly analogous to any known MCR composition. The major- and trace-element composition of the Kettle River dikes along with olivine and chromite mineral compositions closely match the estimated parental composition of the Tamarack Intrusive Complex and are interpreted to represent distal expressions of a similar magmatic system. Thermodynamic modeling of fractional crystallization was completed to define further relationships between the subgroups and identify the ideal conditions in which the high-TiO2 content may be achieved. Higher pressure, reduced, and dry melts were most conducive for maximizing TiO2 contents, and peak TiO2 content was achieved right before Ti-rich spinel began to form. The highest values of >4.0 wt% TiO2 observed in the Carlton County dike swarm and Esko dike needed higher TiO2 starting compositions in the parental melt than the MCR reference compositions used for modeling. Other processes that increase TiO2 content include magma mixing models in which the primary composition modeled was combined with evolved melts at the peak of TiO2 enrichment or with low-degrees of partial melts of a basaltic source at the base of the crust.