Marinace Green
This is a truly spectacular rock. It is commonly found as a piece of dimension stone, a rock used for structural stone and decoration while polished. In that case, it is known as “Marinace Granite”, although for geologists it is very far from granite. In “Dimension stone” terms, any igneous or metamorphic rock, regardless of composition, tends to be called granite – geologists would break those rocks into dozens of different types based on textures and compositions.
This rock started its existence 1.7 billion years ago in what is today the Bahia state of eastern Brazil. At the time, a deep rift basin formed as part of a continental landmass rifted apart. Sediments, including coarse-grained river gravels, flowed into this lowland basin. The gravels that flowed into this basin included chunks of granite and gneiss, igneous and metamorphic rock derived from the surrounding continent.
These sediments were then buried, lithified into rocks, and shoved even deeper. Once they were buried deep enough, the minerals that were stable in the rocks began to change. Minerals that were stable at shallow depths were no longer stable under these conditions. The rocks were buried to what geologists call “greenschist” facies conditions. This facies received its name because minerals such as chlorite and epidote are stable under those conditions and, interestingly, they are green.
The rock started as a conglomerate. It was buried, metamorphosed, and the cobbles began turning a brilliant green. You can still see many of the textures from the sedimentary protolith preserved in the metamorphic rock. Greenschist facies conditions aren’t extremely hot, so chemical reactions are slow; they can start but not fully finish before metamorphism completes. Despite the metamorphism, the sedimentary cobbles are still in tact and visible, and some of them even show the metamorphic foliations that were present before they were even eroded. In one clast you can even see how the metamorphism affected the rim but has yet to reach the core.
Truly spectacular rock. So far I haven’t been able to find from Google where the term”Marinace” comes from, the formation is named the “Ouricuri do Ouro Formation”.
-JBB
Image credit: James St. John https://flic.kr/p/V4BW15
References: http://bit.ly/2sh8eH0 http://bit.ly/2tGnNMS http://sp.lyellcollection.org/content/294/1/33.short
