The Rama Schist
This is another part of the complex of twisted, bent, metamorphosed, nearly 2 billion year old rocks that sit at the bottom of the Grand Canyon and make up some of the oldest, deepest rocks exposed anywhere in the western U.S.
The other parts of the schist we’ve seen, the Vishnu Schist and the Brahma Schist, look a lot like metamorphosed parts of the ocean, metamorphosed sedimentary rocks and metamorphosed basaltic rocks. Those sit in contact with granites that look like an island arc that slammed into North America about 1.7 billion years ago.
These rocks fit into that same story. The Rama schist is a third component in the mixture of schists; more felsic, quartz and plagioclase rich metamorphosed rocks.
These rocks have the chemistry of volcanic rocks erupted close to the Earth’s surface, as happens often in island arcs. The rocks may have been faulted or moved around prior to running into the growing continent, but they were pulled into the same mountain building and metamorphic event as the surrounding sediments and basalts.
These rocks were probably volcaniclastic before they were metamorphosed, produced by explosive volcanic eruptions. Some of the layering is even maintained in the second image. Based on those details, the protolith of these rocks (what they were before metamorphism) was probably a tuff.
In other words, it would be fully accurate to call this rock a tuff schist. You’re welcome.
-JBB
Image credit: Tisha Irwin (with permission, taken on sample on GC National Park Rim Trail) https://www.flickr.com/photos/tishairwin/14491015401 Visit her page: http://www.photonsandplutons.com/ Also used: Ilg et al., GSA Bulletin, 1996http://gsabulletin.gsapubs.org/content/108/9/1149.short Previous articles: https://www.facebook.com/photo.php… https://www.facebook.com/photo.php?fbid=718487278212319 https://www.facebook.com/TheEarthStory/posts/718917208169326