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

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 Midcontineni 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 (Tech. Rep. NRRI/TR-91/13a, Natural Resources Research Institute, University of Minnesota, Duluth, 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. Sutfide mineralization, consisting of 1-5 vol. % of disseminated pyrrhotite, cubanite, chalcopyrite andpentlandite, occurs in Units I, II, III, and VI. Oxygen isotopic analyses indicate that Unit II has experienced extensive crustal contamination. 5O values of Unit II range from 69 to 7-l%o and are 0 enriched compared with values of 5-1-6-8%o found in other units. Silica contamination is indicated based not only on 5' 0 values, but also by the predominance of orthopyroxene in the unit. Possible high-' 0 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 offractionation of a high-Al, olivine tholeiite magma. Modeling of trace element concentrations and variations in mineral chemistry suggest that discontinuities within the major units developed by in situ boundaryAayer equilibrium crystallization of solidification zones ~ 20—50 m 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. 5 0 values of uncontaminated rock types are strongly correlative with modal mineralogy, and can also be modeled by boundary-layer fractionation. A parental magma 5 0 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.