Meet the Gneisses of the Grand CanyonI’ve been deliberately making a mistake in my last couple posts

Meet the Gneisses of the Grand CanyonI’ve been deliberately making a mistake in my last couple posts on the basement rocks in the Grand Canyon. The deepest portions of the canyon are replete with igneous rocks that I’ve been lumping together as the “Zoroaster Granite”. This is a mistake I made for simplicity; now I want to rectify it.Deep in the Grand Canyon there are a series of igneous rocks produced as the very crust in this area was being formed. Those rocks have a variety of compositions and they outcrop as a variety of independent plutons and intrusions. You can go out in the field and map the boundaries between them. They have different compositions. Each one tells a different story.The general term “Zoroaster Granite” isn’t enough here; the correct term is Zoroaster Igneous Complex. These rocks are coarse-grained igneous rocks like granites, but they range in composition from gabbros to granite and range in time over hundreds of millions of years.These images show two of these rocks. The darker rock is a sample of the Ruby Gabbro, a rock that has a basaltic composition and is probably actually older than the collision between the island arc and North America that brought these rocks together. On the other hand, we have a much younger piece of actual granite.The early bodies like the Ruby Gabbro are found as large plutons; bodies of magma of a single composition that are several kilometers wide, probably representing old magma chamber systems. These plutons probably formed the backbone of the island arc system that ran into North America to create all the other basement rocks we’re looking at.The more pink rock is a true granite. Granites like these are found throughout the Zoroaster igneous complex. They are up to 75 million years younger than the earlier plutons and in many cases intrude and cut right across them. These rocks were formed after the continental collision happened; as the previous rocks were shoved deeper in a gradually building mountain range, some of them melted, giving rise to granites that intrude throughout the complex.I’ve spent a lot of time in the metamorphic rocks at the base of the Grand Canyon so far. These rocks are extremely interesting and record key processes in the assembly of what is today the North American Continent. Because they’re buried in many places, it is only thanks to small windows like the Grand Canyon’s inner gorge that we are truly able to piece together the geologic history of this part of the continent. They track over 100 million years of geologic history themselves; they reflect more time than it has taken to build the entire Himalayan mountain range. But with that said, it’s time to go up!-JBBImage credit: Tisha Irwin (with permission, taken on sample on GC National Park Rim Trail): https://www.flickr.com/photos/tishairwin/14307789988Visit her page: http://www.photonsandplutons.com/Also used: https://www.flickr.com/photos/brewbooks/5206777343Previous articles:https://www.facebook.com/photo.php?fbid=71718732167564https://www.facebook.com/photo.php?fbid=717596974968016https://www.facebook.com/photo.php?fbid=718487278212319https://www.facebook.com/TheEarthStory/posts/718917208169326https://www.facebook.com/TheEarthStory/posts/719035941490786 -- source link

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