The Colvin Creek Body, a Metavolcanic and Metasedimentary Mafic Inclusion in the Keweenawan Duluth Complex, Northeastern Minnesota

Abstract

The Northern Colvin Creek body (CCB) within the 1100 Ma Duluth Complex (Complex), northeastern Minnesota, is a very large, rotated inclusion of Keweenawan basalt and sedimentary rock metamorphosed to pyroxene hornfels facies by immersion in the Complex. The inclusion is about 2500 m in strike length with a stratigraphic thickness of 800 m. Volcanic and sedimentary features indicate a stratigraphic top to the northwest; strike is about N60°E and dips are 70° to 90° to the northwest. Previously the CCB has been interpreted as an oxidized, metamorphosed basalt of the North Shore Volcanic Group (Tyson 1976) and by Severson and Hauck (1990) as an intrusive unit of the Partridge River intrusion (PRI). The body consists of two metavolcanic units and an overlying metasedimentary unit. One of the metavolcanic units is cut by a sill. The stratigraphic package is bounded to the north by a weakly recrystallized olivine gabbro and to the south and east (?) by a poorly exposed, moderately metamorphosed, oxide-rich, fine-grained augite troctolite that shows local assimilation of the margins of the inclusion; both of these igneous units are parts of the Partridge River Gabbro Complex of the PRI. In the two volcanic units mineralogy consists of plagioclase-augite-olivine-orthopyroxene-magnetite-ilmenite. Concentrations of augite replace amygdules that are round, sheeted, and pipe-like; recrystallized piagioclase phenocrysts are locally present. Grain sizes in these units range from very fine- to locally medium-grained, averaging fine-grained. Thin, discontinuous, irregular augite veins are common. Metasedimentary rocks are anorthositic gabbro to gabbroic anorthosite in mineralogical and chemical composition and are of relatively constant proportions of plagioclase-diopside-orthopyroxene-magnetite-ilmenite, with lesser hematite, hercynite, geikielite and apatite. Sedimentary features include millimeter to centimeter scale density-graded modal layering and cross-beds. Texture is fine-grained with very uniform grain size throughout the unit. The contact between the volcanic and overlying sedimentary rocks is a thin (0-2 m) interval of ferrosalite pyroxene- and plagioclase-rich rock with locally abundant cordierite, garnet, biotite, and hercynite. The inclusion is associated with a magnetic high similar to many others in the Partridge River Gabbro Complex. It is uncertain if the rocks of the Colvin Creek body are the cause of this magnetic high, or simply overlie a buried anomaly. Geochemical work has confirmed the unit subdivisions established in the field. It has indicated essentially isochemical metamorphism and has given evidence that the metavolcanic units are probably equivalent to intermediate olivine tholeiites of the North Shore Volcanic Group. The metasedimentary rocks are more problematic; they are not analogous to any of the typical interflow sandstones of the North Shore Volcanic Group as described by Jirsa (1980, 1984). At about 350 m they are as thick as the total measured section of North Shore Volcanic Group interflow sedimentary rocks, show no rock fragments, no quartz, no conglomeratic horizons, and no intercalated volcanic rocks. These metasedimentary rocks, however, appear to match a Keweenawan sandstone exposed near Phantom Lake, north of Two Harbors. The similarities include: both rocks are strongly magnetic, bedded and cross-bedded; plagioclase-rich and quartz-poor; and of uniform fine grain-size. Neither of these units can be strictly correlated with any other in the Keweenawan system.