anorthosite

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ANORTHOSITE

Introduction and Definition

Anorthosites are monomineralic ultrabasic igneous rocks which are principally composed of calcic

plagioclase. The name anorthosite was first introduced by Thomas.S.Hunt in 18! in describing the

plagioclase rich rock of the "abrador area#$anada. The name was deri%ed from the &erman description for the

triclinic feldspars'anorthosefeldspars. Anorthosite is a mantle deri%ed rock. (t is commonly associated with

other mafic and basic rock types mainly distributed within )recambrian shield areas and cratons.

Mineralogy

Anorthosite is principally composed of calcic plagioclase. *onomineralic anorthosites are composed

of high $a+ plagioclase ranging from labradorite to anorthite. )artially monomineralic types display a

compositional range from oligoclase to bytownite , $a+ abundance is greater than -/ or An-0.

Albite when present is generally secondary in origin metasomatic origin. The colour of the

plagioclase in the outcrop or hand specimen is %ariable and can be identified by its distinct play of colours.

)rimary accessory minerals include' oliine,high to moderate *g+0# pyro2ene' orthopyro!eneand

rare clinopyro2ene# ores' chro"iteand #e$Ti$% o!ides. Secondary accessory minerals include alteration and

desilication products of plagioclase' scapolite# alkali feldspar# 3uart4# rare garnet# corundum# calcite5 pyro2ene'

uralite# amphibole# epidote. *ica and 4ircon are generally rare or absent.

&etrography

Te2tural and structural types in anorthosite may be grouped as igneous or metamorphic in character.

Igneous Te!tures

The most common igneous te2ture recorded is the 6hypidio"orphic granular6 te2ture with euhedral

or anhedral intergranular accessories. 'u"ulate or layered te!tures is common in "ono"ineralic

anorthosite. &orphyriticte2tures with phenocrystal and ground mass plagioclase ,of similar or dissimilar0

composition also occur. 7are glo"eroporphyritic te2tures ha%e been obser%ed in some anorthosites.

Anorthosite phenocrystals may be lineated or twinned or 4oned. &round mass minerals are generally

subhedral.


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Igneous Structures

(ayering and segregation of plagioclase and chromite ore as distinct conformable bands are

obser%ed in some monomineralic anorthosites. Associated ultramafic rocks may also be layered with an

igneous stratigraphy. The band thickness can %ary from laminar , 1m0 to layered ,9 1m0 on a %ertical scale.

The disposition of the bands may be sub hori4ontal to %ertical. :anding may be continuous or discontinuous.

(ensoid shapesor stringersof anorthosite bounded abo%e and below by mafic bands ha%e been recorded in

some anorthosites.

Meta"orphic Te!tures

*etamorphic te2tures common reported include' gneissic) augen) protoclastic or "ylonitic. Augen

or gneissic te2tures suggest granulite to amphibolite grade regional metamorphism. *ylonitic or protoclastic

te2tures suggest shearing or faulting of the anorthosite. *neissic +andingcan de%elop by folding deformation

in layered anorthosite. *eta;igneous te2tures may be seen in some anorthosites which ha%e undergone low to

medium grade regional metamorphism. +riginal igneous features ,4oning# twinning0 may be obliterated or

partially altered.

Meta"orphic Structures

Augen structure and thick gneissic banding may be seen in metamorphosed anorthosites.

Mode of Occurrence

Anorthosite displays se%eral modes of occurrence' they include the following types'

(ayers

*onomineralic layers of anorthosite occur within a lopolith or basin shaped layered ultramafic

comple2# generally abo%e the gabbro or troctolite or chromite layers# e.g. :ush%eld (gneous $omple2 of South

Africa.

Discrete intrusions

Anorthosite can occur as discrete batholithic or stock like intrusi%es within cratonic granulitic rocks.

They occur as discrete residual hills or massifs# e.g. +ddanchatram and *anaparai anorthosite.


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Di,e - Sills

Anorthosite dikes and sills ha%e been obser%ed in ophiolite se3uences and oceanic gabbros of mid

oceanic ridge affinity. The dyke or sill may be differentiated %ertically to display distinct bands of basal mafic

and upper plagioclase minerals.

Mig"atite 'onfiguration

"eucosomes of some migmatite rocks may be mainly composed of calcic plagioclase to be

anorthositic in composition. These anorthosites are deri%ed from palingenetic metamorphic processes.

'lassification of Anorthosites

Anorthosites ha%e been classified using se%eral parameters' mineralogy# petrography# and mode of

occurrence.

I.*S Sche"e

This scheme is based on the modal abundance of plagioclase with respect to oli%ine and pyro2ene.

+nly the igneous %arieties are represented. The nomenclature is gi%en in the diagram is self e2planatory.

(e/is D Ash/al0s Sche"e


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Ashwal6s scheme is based on age#petrography#association#mode of occurrence# and geological

en%ironment of the anorthosites. He proposed the following categories'

Archean megacrystic anorthosite# )rotero4oic anorthosite# +ceanic anorthosite# +phiolitic anorthosite#


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%ertical (ayers' illow "ake

#olded and "eta"orphosed layers' Bisknaesset# +phiolitic and +cean floor anorthosite# +bducted oceanic

crust anorthosite.

+2 Massif or Adirondac, Type

These anorthosites occur as batholiths or stocks co%ering an area of about 1 km -. They are

associated with )recambrian orogens and their eroded e3ui%alents in cratons. High grade granulite and gneiss

are the common field associates. They are further subdi%ided into'

(a+radorite Type4

These anorthosites may be partly layered with An content ranging from C to D/ resembling gabbroic

or troctolitic anorthosite.

Andesine Type4

These are non layered massif types and display An content e3ui%alent to antiperthitic andesine.

Meta"orphosed Massif Type4

These types may contain garnet.

Be;Ti;> layers may occur in massif anorthosite.

c2 Meta"orphic Stratigraphic Type

These anorthosites are folded# deformed# metamorphosed# hydrothermally altered# and sometimes

sheared. They retain the layered conformable relations with the associated mafic and basic rocks. They occur

in Archean high grade gneiss se3uences. Schist and 3uart4ite may occur. They are subdi%ided on to the

following types'

Sitta"pundi Type4

These are highly folded# contorted# metamorphosed anorthosites with relict layering as e%idenced by

chromitite layers or boudins. The common structure is gneissic with plagioclase decomposition features. They

may contain remnant features resembling +phiolite.

Sa,eny Type4


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This type was proposed by :oulanger ,1D80 after sakenite rock of *adagascar. (t is composed of

calcic plagioclase ,9 8C/0# with minor spinel# sapphirine# and corundum. "ayering may be continuous or

discontinuous often associated with chromite bearing pyro2enite # amphibolite# marble# and 3uart4ite.

*eoche"istry

The silica content of anorthosite as a group %aries from E! to F/ indicating an @"T7A:AS($ silica

under saturated to silica saturated character. The $a+ ranges from C to -/ e2ceeding *g+GBe+. *g+

%aries from .1 to 1.1/. a-+ %aries from .DD to 11.1/. Al-+! is significant in the formation of

plagioclase. The %olume of the other o2ides are %ariable.

(n anorthosite containing oli%ine and orthopyro2ene# the *g+ / of the oli%ine and orthopyro2ene are

in direct proportion to the $a+/ of co;e2isting plagioclase. The $a+ of plagioclase may correlate to $a+ of

co;e2isting diopside if present. The $a+ I Al-+! ratio is %ariable suggesting differing protolith character and

compositions.

:a# Sr# 7b#


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*eneral

Anorthosites are deri%ed from mafic or basic magmas of mantle protoliths. Bractional crystalli4ation#

diapiric ,%ertical0 rise ,of plagioclase crystals along with melt material0# and lateral flow or inNection ,of

plagioclase melt mush by filter pressing processes0 may be in%ol%ed in the generation of anorthosites from

their parental melts.

*enetic Models for the different anorthosites

Archean Megacrystic AnorthositeO)hinney ,1D8-0#


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or be suspended in the magma mass and the minor mafic minerals accumulate at the base of the magma

chamber. The source magma melt may under go one or se%eral stages of differentiation to form the

anorthosite layers.

The following is en%isaged in multi;stage differentiation of anorthosite forming magma'

a. Bormation of ultramafic magma of oli%ine boninite compositional.

b. Separation of this magma into two fractions' one fraction is rich in *g+ and Be+# the other fraction is rich in

Al-+! # $a+ J a-+# by li3uid immiscibility.

c. >ertical layering of basal mafic fraction and upper felsic fraction.

d. $rystalli4ation of basal layer to form' dunite#peridotite#pyro2enite# and upper layer to form anorthosite.

Troctolite# gabbro or norite form in between these layers.

$hromite# magnetite# ilmenite rutile# )& magnetite are the associated

deposits. 7esidual Au may accumulate in the residual fluid to form an anorthosite hosted Au deposit.

Ophiolitic and Oceanic Anorthosites

The +phiolitic anorthosites are associated with ophiolite se3uences and their tectonic regimes. The

oceanic anorthosites are related to layered gabbro comple2es and *+7 magma chambers. Source melt is

tholeiitic basalt of *+7 or (sland Arc magmas ,basic to calc;alkaline0.

(unar Anorthosites

These anorthosites are either iron rich type ,ferroan0 or alkali rich type ,alkali rich0 based on Be+ and

$a+J a-+ respecti%ely. Anorthosite meteorites are called as &allasites2

Anorthosite Inclusions or 5enoliths

$ognate anorthosite is cogenetic with the enclosing older anorthosite. =enolithic anorthosite originate

from %aried sources in the upper mantle and lower crust.

Anorthosites of Ta"il Nadu

$onfirmed anorthosite occurrences in Tamil adu include the following' S(TTA*)@?(# KA?A>@7#

T+&A*A"A(# +??A$HAT7A*# *AA))A7A(# $H(&"


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I"portant References

(eelananda")'2 And M2 Narasi"ha Reddy5 ,1DD80. )recambrian anorthosites from peninsular (ndia

problems and perspecti%es. &eol.Sur%.(nd.Spl.)ub.o. EE# pp 1C- 1FD.

'hatter6ee)S2'2,1DE0. )etrography of the (gneous and metamorphic rocks of (ndia#*acmillan )ublication $o.

?elhi.

anorthosite

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