Pre-Keweenawan anorthosite inclusions in the Keweenawan Beaver Bay and Duluth Complexes, northeastern Minnesota

Abstract

Large numbers of calcic anorthosite inclusions varying in size from centimetres to several hundred metres occur in the Beaver River gabbro unit of the Beaver Bay Complex exposed along the North Shore of Lake Superior. Lesser numbers of similar inclusions occur in the Duluth Complex. Both host rocks crystallized at about 1.13 b.y. B.P. and are of Keweenawan age. The inclusions have been divided into four groups, based on outcrop and petrographic data, as follows: (1) Tectonite inclusions that were sheared and recrystallized. The tectonites consist of plagioclase (An 76–78 ), minor to accessory olivine (Fo 78 ), and accessory magnetite-ilmenite. (2) Igneous inclusions that have igneous structures and textures. Plagioclase ranges in composition from An 54 to An 80 . Principal mafics are augite (Wo 40 En 45 Fs 15 ) and orthopyroxene (Wo 2 En 80 Fs 18 ), but more iron-rich pyroxenes (Wo 38 En 40 Fs 22 ), including pigeonite (Wo 10 En 55 Fs 35 ), occur as much smaller grains in most igneous inclusions and are comparable to pyroxenes in host rocks. Textural evidence and mineral chemistry suggest that the Fe-rich pyroxenes, as well as the more albitic plagioclase, represent contamination. (3) Intermediate inclusions whose textures are intermediate between the tectonites and igneous rocks. Some of these appear to have been incompletely recrystallized, and some were modified to more igneous-appearing rocks by reaction with host rocks. (4) Cataclastic and brecciated inclusions having brittle deformation that is not apparent in the host rocks. Representatives of each group were analyzed for rare-earth elements (REE) and Sm-Nd, Rb-Sr isotopes. The chemical data correlate with the textural groups. For example, tectonites have low REE abundances with Nd about one times chondrites, whereas those in the igneous-textured group have higher REE abundances with Nd up to as much as seven times chondrites. Isotopic data show that the isotopic systems of the inclusions were disturbed and that contamination was introduced. Tectonites have 87 Sr/ 86 Sr and 143 Nd/ 144 Nd ratios that are less than those of the Beaver River gabbro when recalculated to 1.13 b.y. Igneous-textured inclusions have 143 Nd/ 144 Nd ratios that are lower than tectonite values at 1.13 b.y., but 87 Sr/ 86 Sr ratios vary widely, with some being greater than ratios in the host Beaver River gabbro. None of the inclusions has an isotope ratio that would allow a co-magmatic relationship with either the Duluth Complex or the Beaver River gabbro. Sm-Nd data from an incompletely equilibrated inclusion suggest recrystallization between 1.14 b.y. B.P. and 1.9 b.y. B.P., perhaps as a result of the 1.8 b.y. B.P. Penokean event. Calculations of a crystallization age for the inclusions are speculative but suggest crystallization at or before 1.9 b.y. B.P.