DEFINITION OF CARBONATITE

Rare igneous rocks that contain more than 50

percent carbonate. Some carbonatites are calcitic (sovite)

whereas magnesio-carbonatites (rauhaugites) have dolomite

as the primary carbonate mineral. Ferro-carbonatites have

calcium-magnesium and iron carbonate minerals. There are

more than 350 carbonatite intrusions and volcanoes recognized

on Earth, with many of these in the East African rift

system. Northern Tanzania hosts the world’s most dense carbonatite

province, where active eruptions from centers such

as Ol Doninyo Lengai produce short-lived bizarre flows with

twisted spires, swirls, and oozes made of the extremely fluid

carbonatite. The carbonatite lavas flow with a viscosity similar

to that of olive oil, the lowest viscosity known for any

magma on Earth. Other carbonatites are associated with kimberlites

and alkalic rocks and are all located on continental

crust. Most carbonatites are very small, being less than 1

square kilometer in area, with the largest known complex

being 7.5 square miles (20 km2). They range in age from Proterozoic

(2.0 billion years old) to the present with most being

younger than 150 million years old.

The chemistry of carbonatites shows that they formed by

magmatic processes in the mantle. They have high concentrations

of rare elements such as phosphorous and sodium and

compositionally grade into kimberlites. Intrusion of both

kimberlites and carbonatites causes intense alteration of surrounding

country rocks, characterized by the addition of

sodium and potassium.

Carbonatites are thought to form by small amounts of

partial melting in the upper mantle, although there has been a

large amount of controversy about the origin of this volumetrically

minor group of igneous rocks. Some models suggested

that carbonatites were formed by the remobilization of sedimentary

limestones by heat from nearby silicic intrusions, but

these ideas have been largely discredited. Carbonatite magmatism

tends to occur repeatedly, but not continuously, for

hundreds of millions or even billions of years in the same

location, suggesting that some unusual aspect of the mantle

in these areas leads to the formation of magmas with such

rare compositions. It has been noted that carbonatite magmatism

in the African plate seems to coincide with distant plate

collisions or periods of increased global magmatic activity,

although the cause of these apparent trends is unknown.

See also DIVERGENT OR EXTENSIONAL BOUNDARIES;

IGNEOUS ROCKS; RIFTS.

Title Post:
Rating: 100% based on 99998 ratings. 99 user reviews.
Author:

Terimakasih sudah berkunjung di blog SELAPUTS, Jika ada kritik dan saran silahkan tinggalkan komentar