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).