Olivine gabbro (Pigeon Point Sill, Mesoproterozoic, ~1.1 Ga; Pigeon Point, Minnesota, USA) (40770721514)

Forfatter/Opretter:

James St. John

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Olivine gabbro (Pigeon Point Sill, Mesoproterozoic, ~1.1 Ga; Pigeon Point, Minnesota, USA)

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Olivine gabbro from the Precambrian of Minnesota, USA (public display, Geology Department, Wittenberg University, Springfield, Ohio, USA) Along the northern shore of western Lake Superior are numerous exposures of a lava flow-dominated succession called the North Shore Volcanic Group. This is equivalent to & the same age as the Portage Lake Volcanic Series of northern Michigan's Keweenaw Peninsula (<a href="https://www.flickr.com/photos/jsjgeology/albums/72157632266738191">www.flickr.com/photos/jsjgeology/albums/72157632266738191</a>). The North Shore and Portage Lake successions are ~1.1 billion years old and represent basalt lava flows, plus minor sedimentary rocks, that filled up an ancient rift valley. This old rift is the Lake Superior segment of the Mid-Continent Rift System, a tear in the ancient North American paleocontinent of Laurentia (see: <a href="https://minerals.usgs.gov/science/midcontinent-rift-minerals/images/MRS-map.png" rel="noreferrer nofollow">minerals.usgs.gov/science/midcontinent-rift-minerals/imag...</a>). Tectonic rifting started along this tear, exactly like the modern-day East African Rift Valley. Laurentia's Mid-Continent Rift System started and then stopped and was subsequently filled and buried. This ancient failed rift is now exposed on either side of Lake Superior in North America's Great Lakes. The sample shown here is from an intrusion emplaced at the same time as North Shore volcanics were being erupted. The rock is a phaneritic, mafic, intrusive igneous rock called gabbro. Gabbro has plagioclase feldspar and pyroxene. This particular specimen also has a fair amount of olivine, so it is an "olivine gabbro". Some might characterize it as a troctolite. Accompanying exhibit signage identifies it as an olivine diabase, but the crystals are too large for that - it appears to be a gabbro. The igneous body itself is a sill - a planar intrusion that parallels the country rocks. In contrast, an igneous dike is a planar intrusion that cuts across the country rocks (see: <a href="https://www.flickr.com/photos/jsjgeology/albums/72157632467411234">www.flickr.com/photos/jsjgeology/albums/72157632467411234</a>). Geologic unit & age: Pigeon Point Sill, late Mesoproterozoic, ~1.1 Ga Locality: unrecorded/undisclosed site at Pigeon Point, northern shore of Lake Superior, west of Isle Royale, adjacent to the Canadian-American border, far-northeastern Minnesota, USA

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CC BY 2.0

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Creative Commons Attribution 2.0