Duluth Complex - Formation

Formation

Some 1,100 million years ago (mya) the North American craton began to split apart in the Midcontinent Rift. Over a period of some 15–22 million years, magma rose through the earth’s crust, separating the older formations and creating new rock in the area of the rift. The rock sequences thereby created are known as the Keweenawan Supergroup. Rocks of this group north of Lake Superior are the layers of the North Shore Volcanic Group and the adjoining formations of the Duluth and Beaver Bay Complexes.

The North Shore Volcanics originated c. 1109-1096 mya from hundreds of individual lava flows, forming six distinct tilted and partially stacked plateaus which total more than 8,000 meters in thickness. These tilt toward the syncline under Lake Superior, as shown in the adjacent picture of the Sawtooth Mountains, the slopes of which mirror those of the shoreline rocks. While principally basaltic, these flows also include rhyolites and other types. As part of the Middle Proterozoic Keweenawan sequence, these volcanic layers are part of one of the oldest, largest, and best-preserved plateau lava provinces in the world.

These volcanics created the "roof rocks" into which were emplaced the mafic formations of the Duluth Complex. Primarily formed after 1102 mya, the oldest formations are near Duluth, and the youngest to the northeast near Tofte. Insulated by the overlying roof rock, upwelling magma cooled slowly, and the mafic rock into which it cooled therefore is coarse-grained. These intrusions formed a sill some 16 km thick, primarily of gabbro, but with significant amounts of anorthosite and other related granitic rocks. The Duluth Complex is one of the largest intrusions of gabbro on earth, and one of the largest layered mafic intrusions known. It covers an area of 4715 km2. The lower portion of the intrusion along the northwestern margin consists of ultramafic cumulates with associated segregations of nickel, copper and platinum group elements. The upper differentiated portions include ilmenite bearing labradorite anorthosites.

Along its northern margin, the Duluth Complex adjoins older structures, the Archaen Ely Greenstones (once believed to be the oldest exposed rock on earth), and the ore-bearing Mesabi and Gunflint iron ranges deposited as part of the Animikie Group from the Penokean orogeny, a mountain-building event from Paleoproterozoic times. Those two Middle Precambrian ranges are thought to have once been joined, but intruding magma of the Duluth Complex baked and engulfed the center of the mountain chain, separating it into the two ranges present today.

To the east, the complex abuts and intrudes into the Rove Formation, an older structure composed of sedimentary rocks. Gabbro and diabase structures of the Duluth Complex trend generally from southwest to northeast, and the differential erosion has left a series of ridges comprising these harder mafic rocks rising from the softer sedimentary rocks of the Rove Formation. Elongated lakes lie in many of these depressions.

To the south near Lake Superior, rock strata of the Duluth and Beaver Bay complexes are interspersed with and underlie the extrusive rock of the North Shore Volcanic Group. The Beaver Bay Complex occupies the center of the North Shore Volcanics, and is slightly younger in age than the other mafic rocks of the Duluth Complex, dating from c. 1096 mya. The volcanics and more recent sedimentary rocks were once thought to be underlain by the Duluth Complex all the way across Lake Superior to Wisconsin, where gabbro formations also exist. The Duluth Complex was considered to be a giant lopolith, a lens-shaped structure depressed in the center, connecting gabbro exposures on opposite sides of the lake, but now is recognized to extend only a few kilometers south of Superior's North Shore.