The Brahma SchistThe Vishnu Schist unit that I’ve covered a couple times now is a group of metamorph
The Brahma SchistThe Vishnu Schist unit that I’ve covered a couple times now is a group of metamorphosed sedimentary rocks found deep in the Grand Canyon that were metamorphosed when an island arc ran into the North American continent 1.7 billion years ago.When these types of continental collisions occur, there tends to be a lot of different types of rock involved. Sedimentary rocks like those at the Earth’s surface are involved, magma bodies are involved, but often there are other rocks in the collision that tell the story of the plates involved. These photos show another of those units from deep in the Grand Canyon: the Brahma Schist.This schist is a rock type called an amphibolite. It is made up mostly of the minerals plagioclase and hornblende, with various amounts of biotite as well.That mineral assemblage is characteristic of what happens when basaltic rocks are metamorphosed. Basalts are very common throughout the solar system; they make up most of the Earth’s ocean crust and so they are often caught in collisions between continents and island arcs.In most places where the Brahma schist is exposed, the metamorphism has destroyed any textural remnant of the original rocks, but the 2nd photo here managed to catch a really cool one. These spheroid-objects are actually remnant pillow basalts produced when basaltic lavas erupted on the ocean floor. This schist spells out a big part of the process for how the basement in the grand Canyon formed; a continent and an island arc came together, uplifting mountains and metamorphosing rocks like these, pieces of the old ocean floor that got caught up in the process.-JBBImage credits: Tisha Irwin (with permission, taken on sample on GC National Park Rim Trail) https://www.flickr.com/photos/tishairwin/14307794598Visit her page: http://www.photonsandplutons.com/Also used: Ilg et al., GSA Bulletin, 1996http://gsabulletin.gsapubs.org/content/108/9/1149.shortPrevious articles:https://www.facebook.com/photo.php?fbid=71718732167564https://www.facebook.com/photo.php?fbid=717596974968016https://www.facebook.com/photo.php?fbid=718487278212319 -- source link
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