Journal of Petrology

Mineralogic and Oxygen Isotopic Studies of Open-System Magmatic Processes in the South Kawishiwi Intrusion, Spruce Road Area, Duluth Complex, Minnesota.

Insung Lee and Edward M. Ripley

ABSTRACT

The South Kawishiwi intrusion, located along the western margin of the Duluth Complex, Minnesota, is one of several composite intrusions that are found in the Complex. The Duluth Complex is the principal exposed plutonic portion of the 1.1 Ga Midcontinent Rift system. In the Spruce Road area the South Kawishiwi intrusion is divided into seven distinct units that are part of the broader South Kawishiwi Troctolite Series defined by Severson (1994). Units may be characterized as follows: Unit I - basal accumulation of heterogeneous gabbro, troctolite, and norite; Unit II - norite with abundant inverted pigeonite; Unit III - troctolite and olivine gabbro with local oxide-rich layers; Unit IV - melatroctolite, troctolite, olivine gabbro; Unit V - increased plagioclase abundance in troctolites and leucocratic troctolites; Unit VI - strongly altered troctolite; Unit VII - similar to Unit V, troctolite and leucocratic troctolite. Country rocks in the Spruce Road area are granodiorite to quartz monzonite of the Archean Giants Range Batholith. Sulfide mineralization, consisting of 1 to 5 volume percent of disseminated pyrrhotite, cubanite, chalcopyrite, and pentlandite occurs in Units I, II, III, and VI.

Oxygen isotopic analyses indicate that Unit II has experienced extensive crustal contamination. d18O values of Unit II range from 6.9 to 7.1 %o and are 18O enriched compared to values of 5.1 to 6.8 found in other units. Silica contamination is indicated based not only on d18O values, but also by the predominance of orthopyroxene in the unit. Possible high-18O contaminant rocks include the Giants Range Batholith and pelitic rocks of the Lower Proterozoic Virginia Formation or Biwabik Iron Formation.

Mass balance computations suggest that units in the Spruce Road area may be related through varying degrees of fractionation of a high-Al, olivine tholeiite magma. Modelling of trace element concentrations and variations in mineral chemistry suggest that discontinuities within the major units developed by in situ boundary layer equilibrium crystallization of solidification zones ~20 to 50 meters in thickness, followed by recharge of fresh magma. Upward enrichment of incompatible elements, olivine Fa content, and plagioclase Ab content may be effectively explained by this process. d18O values of uncontaminated rock types are strongly correlative with modal mineralogy, and can also be modelled by boundary layer fractionation. A parental magma d18O value of ~6.3%o is calculated for Unit VII based on olivine and plagioclase values, and is similar to that of several other large, layered mafic intrusives.

Keywords:

Duluth Complex; South Kawishiwi Intrusion; high-Al olivine tholeiite; open system crystallization; oxygen isotopes

Return to header

Current Contents ................

J. Petrology Editor's Page