CHARNOCKITE

 

Charnockites are very high-grade metamorphic rocks.  They straddle the boundary between igneous & metamorphic.  Many charnockites have been given metamorphic and igneous names.  I’m not a hard-rock geologist, but I get the impression that no one really fully understands charnockites.  The literature really doesn't provide a consistent definition of these rocks.  My understanding is that charnockites are dark-colored, very coarsely-crystalline, quartzo-feldspathic with pyroxene, and are very high-grade metamorphic rocks (or at the very least, intimately associated with very high-grade metamorphic terranes).  Some charnockites have obvious foliation, while the foliation is subtle in other samples.  Foliation is best seen in the black pyroxene component.  The feldspar component of some charnockites has a distinctive glimmers when tilted in the light (see the large, lower feldspar crystal in the “Verde Butterfly Granite” photo below).

 


 

Charnockite (“Ubatuba Granite”) from near the town of Ubatuba in southeastern Brazil.  This late Neoproterozoic to Cambrian (~650-500 million years) charnockite has large, dark-greenish feldspars, black pyroxene, and some quartz.

 


 

Charnockite (“Verde Butterfly Granite”) from southeastern Brazil.  This rock is a garnetiferous charnockite (it’s also been called a hypersthene monzogranite).  It formed during the late Neoproterozoic to Cambrian (~650-500 million years ago).  It's got feldspar (greenish), pyroxene (black), quartz, and garnet (very deep red).

 


 

Charnockite (“Seaweed Green Granite”) - this Precambrian-aged garnetiferous charnockite is from Orissa State in the Eastern Ghats Orogenic Belt of eastern India.  This rock is also known by the less slimy names “Pocono Green Granite” and “Pine Green Granite”.  The very dark red garnet component of this rock is mostly mixed with the black pyroxene patches.  Note that the black pyroxene areas are generally elongated, and aligned more or less parallel throughout the rock (foliated texture).

 


Home page