The magmatic evolution of the Midcontinent rift: New geochronologic and geochemical evidence from felsic magmatism

The Midcontinent Rift System (MRS) is an example of a mafic-dominated continental rift where silicic magmatism is locally significant. The best-preserved example of this is on the NE limb of the rift along the North Shore of Lake Superior where rhyolites comprise a large percentage (up to 25%; Green, J.C., Fitz, T.J., III, 1993. Extensive felsic lavas and rheoignimbrites in the Keweenawan Midcontinent rift plateau volcanics, Minnesota: petrographic and field recognition. J. Volcanol. Geotherm. Res. 54, 177–196) of lava flows and where hypabyssal granophyric intrusive complexes are common. In this paper, we report U–Pb zircon ages, Nd isotopic compositions, and major and trace-element data for seven granophyric complexes of the MRS exposed in NE Minnesota.U–Pb zircon ages for the granophyres define two different age groups: an older group with ages from 1109 to 1106 Ma; and a younger group with ages from 1099 to 1095 Ma. These ages coincide with the “early” and “main” magmatic stages of Midcontinent rift evolution suggested by Miller and Vervoort [Miller, J.D., Jr., Vervoort, J.D., 1996. The latent magmatic stage of the Midcontinent rift: a period of magmatic underplating and melting of the lower crust. Inst. Lake Superior Geol., 42nd Ann. Mtg., Proceedings, vol. 42, pp. 33–35] and Miller and Severson [Miller, J.D., Jr., Severson, M.J., 2002. Geology of the Duluth Complex. In: Miller, J.D., Jr., Green, J.C., Severson, M.J., Chandler, V.W., Hauck, S.A., Peterson, D.M., and Wahl, T.E. (Eds.), Geology and Mineral Potential of the Duluth Complex and Related Rocks of Northeastern Minnesota. Minnesota Geological Survey Report of Investigations, vol. 58, pp. 106–143]. Although the two groups of granophyres have similar major and trace-element compositions, they have different Nd isotopic compositions. The older, or “early stage”, granophyres have more radiogenic Nd isotopic compositions (ɛNd(i) = −3.7 to −0.5) whereas the younger, or “main stage”, granophyres have more crustal, unradiogenic Nd isotopic compositions (ɛNd(i) = −7.6 to −3.1). The age correlative Nd isotopic signatures of the granophyres are broadly consistent with the ages and isotopic compositions of the rhyolites within the North Shore Volcanic Group (NSVG) and illustrate the episodic nature of the Midcontinent rift evolution.The early stage (1109–1106 Ma) of MRS magmatism is characterized by mafic mantle-derived magmas with minor amounts of silicic magmas. The evolved magmas were likely derived by partial melting of either earlier formed rift related rocks or older crust with near chondritic Nd isotopic composition. This was followed by a period of relative quiescence lasting about 5 million years from which no significant MRS magmatism has been preserved. The main stage of MRS magmatism resumes at ca. 1100 Ma with voluminous basaltic volcanism and mafic intrusions. The silicic magmas produced during this stage are more abundant and distinctly more crustal in character than during the early magmatic stage magmas. We suggest that these silicic magmas have been derived from partial melts of more evolved crustal sources, perhaps at higher levels in the crust.