Petrology and Geochemistry of the Archean Rocks of the Jap Lake Area, Northeastern Minnesota
1987-05
Title
Petrology and Geochemistry of the Archean Rocks of the Jap Lake Area, Northeastern Minnesota
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1987-05
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Thesis or Dissertation
Abstract
The Jap Lake area, located at the eastern end of the Vermilion district in northeast Minnesota, contains a volcanic-sedimentary succession typical of many Archean greenstone-granite complexes of the Canadian shield, as well as lithologies representative of the lower to middle Proterozoic seen elsewhere in northern Minnesota. Structurally, the Archean sequence represents the southern limb of a large antiform. The bedding trends NW-SE and stratigraphic indicators give tops to the SW. The lowest stratigraphic unit, here named the Jap Lake greenstone unit, consists of massive and pillowed tholeiitic basalts locally intercalated with thin mafic tuffs, pillow breccias and hyaloclastites. Numerous mafic to ultramafic sills are interlayered and apparently penecontemporaneous with the basalts. The base of this unit has been intruded arid apparently removed by the 2. 7 Ga Saganaga Tonalite. Capping the thick greenstone unit locally are a laterally limited calc-alkaline dacitic lava flow and an associated fragmental unit. The entire volcanic sequence is overlain by a thick blanket of volcanogenic sediments and later capped unoonformably in the early Proterozoic (~2.0 Ga) by the Gunflint Iron Formation and Rove greywackes of the Animikie Group. Intruding the top of this sequence is the Keweenawan Duluth complex (~1.1 Ga). Chemical analyses of 17 samples from the Jap Lake Volcanic unit indicate that these rocks plot as tholeiitic to high-Mg tholeiitic basalts and diabases, basaltic komatiitic chilled margins, and as pyroxenitic and peridotitic ultramafic sills. These mafic-ultramafic rocks are characterized by moderate to high Mg0% (7-11% in the basalts to 20-32% in the peridotite sills) and low Ti0 2% (0.6-1.0% in the basalts to 0.3-0.4% in the peridotite sills) as well as low abundances of other incompatible elements (e.g. Zr, Y). The ultramafic sills are further characterized by low A12o3 (2-7%) and high Ni (700-1100 ppm) and Cr contents (2300-3500 ppm). Continuous chemical trends of major and trace elements from mafic to ultramafic rocks of the Jap Lake greenstone unit appear to indicate that such compositional variation is the result of crystal accumulation of early fractionating minerals (olivine, pyroxene) into a mafic magma. Computer-generated fractionation calculations based on major elements indicate that the peridotite sills could have been produced by the addition of olivine and minor pyroxene and Cr-spinel crystals (67-68% total cumulus) to a magma having a tholeiitic or a high-Mg tholeiitic composition. This suggests that all the rocks of the Jap Lake greenstone unit may have been formed from the same parental magma type. However, the tholeiitic basalts and diabases have REE patterns inherently different from those of the ultramafic sills and that cannot be explained by simple fractionation or accumulation alone. The mafic samples have REE patterns that are typified by slightly depleted LREE patterns, flat HREE patterns, and low to moderate total REE abundances (5-11X chondrites). The ultramafic samples, on the other hand, have entirely different REE abundances and patterns that are characterized by flat to slightly enriched LREE patterns, moderate HREE depletion, and low total REE abundances (2-5X chondrites). If olivine and pyroxene are the major fractionating phases then clearly these two rock types could not have been produced from the same parental magma. Based on field, petrographic and geochemical evidence, the following model is proposed for the origin of the Archean volcanicsedimentary pile. Tholeiitic basalts were erupted subaqueously at significant water depths and subsequently intruded by thin synvolcanic mafic sills at shallow levels in the volcanic pile, as evidenced by the common occurrence of quench textures on the sill margins. Penecontemporaneously, magma of a different origin (perhaps more Mg-rich) accumulated and began to fractionate in small near-surface magma chambers. Removal of residual liquid from these chambers left cumulus minerals and trapped some liquid to form the ultramafic sills. It is herein interpreted that the magmas which formed the Jap Lake greenstone unit were erupted and emplaced as ocean-floor basalts and diabases, most likely at an oceanic spreading center. The Elusion Lake dacites, positioned stratigraphically above the Jap Lake greenstone unit, have a calc-alkaline geochemical character that is entirely different from any of the earlier volcanism and represent a separate period of volcanic activity. These dacites were probably erupted in a volcanic (island) arc setting and represent the products of magma generated by melting of subducted oceanic crust. The Saganaga Tonalite batholith was also emplaced at this time and is interpreted to be the magma source for the dacitic lavas.
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A Thesis submitted to the faculty of the Graduate School of the University of Minnesota by Jeffrey David Vervoort in partial fulfillment of the requirements for the degree of Master of Science, May 1987. Plate 1 referenced in the thesis is also attached to this record.
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