BLACK GRANITES

 

The “black granites” are popular and attractive decorative stones.  However, the term “black granite” makes a geologist cringe.  These rocks are anything but granite.  Granites are felsic intrusive igneous rocks, and as such are light-colored.  Dark-colored igneous rocks generally have a mafic chemistry, as do all the rocks shown below.  Such rocks are actually gabbros, norites, gabbronorites, and dolerites.  They are dominated by the minerals plagioclase feldspar and pyroxene.

 


 

Black Pearl Granite - a Proterozoic-aged gabbro from Guntur District, eastern Andhra Pradesh State, southeastern India.

 


 

Black Cambrian Granite - a Precambrian norite from Quebec, Canada.

Geologic unit & age:

Lac-St.-Jean Anorthosite Suite, late Mesoproterozoic, 1.14 to 1.16 b.y.

Locality: quarry at St. Nazaire, Chicoutimi County, southern Quebec, southeastern Canada.

 


 

Black Galaxy Granite - a Proterozoic-aged norite from India's Eastern Ghats Orogenic Belt.  This is probably the most popular and attractive black granite available in the commercial decorative stone trade.  It has blackish plagioclase feldspar and bronzite pyroxene crystals.  Bronzite is an orthopyroxene and it has a spectacular copper-colored play of color on cut & polished surfaces (even on broken surfaces).  The result is a rock surface reminiscent of sparkling stars in space, hence the commercial name “Black Galaxy”.  This rock is quarried in the Chimkurthi area, Prakasam District, Andhra Pradesh State, southern India.

 


 

Impala Black Granite - an attractive, 2 billion year old gabbronorite from South Africa.  It is composed principally of grayish plagioclase feldspar and black pyroxene.  It comes from the famous Bushveld Complex, a world-class example of an LLI (large layered igneous province).  LLIs often have economic concentrations of valuable metals, such as platinum and chromium.  North America has one economically significant LLI - the Stillwater Complex of Montana.

The Bushveld has a thick stratigraphy that is well documented.  The gabbronorite sample shown above comes from the upper Main Zone of the Rustenburg Layered Suite.  It dates to the mid-Paleoproterozoic (2054-2061 million years).  “Impala Black Granite” is quarried north of the town of Rustenburg in northeastern North West Province (= northwest of the city of Johannesburg), northeastern South Africa.

 


 

Indian Absolute Black Granite - the name “Absolute Black Granite” has been applied to relatively finely-crystalline mafic intrusive igneous rocks from many localities on different continents (e.g., Africa, India, China).  This sample is from a large Precambrian dike of dolerite (a.k.a. diabase; a.k.a. microgabbro) that cuts through igneous & metamorphic rocks of southern India's Dharwar Craton.

 


 

 

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