classification of lamprophyres, lamproites

classification of lamprophyres, lamproites

t75
The CanndianM ineralogist
Vol, 34,pp. 175-186(1996)
KIMBERLITES'
LAMPROITES,
OF LAMPROPHYRES,
CLASSIFICATION
ROCKSX
AND LEUCITIC
MELILITIC,
AND THEKALSILITIG,
ALAN R. WOOLLEY
RoaLbndon SW SBD'U.K
Crornwell
NaturalHistoryMuseum,
Departuentof Mineralogy,
STEYEN C. BERGMAN
2j00 WestPIarc Parkway,Plano,Texas75075'U'S'A'
Research,
and
Production
Exploration
Technnlogy,
ARCOResources
ALAN D. EDGAR
Ontario,LondaUOntarioN6A587
Universityof Westem
Deparxwntof EarthSciences,
MICHAELJ. LEBAS
of leicester,IsicesterIEI 7RH'U'K'
of Geology,University
Department
ROGERH. MITCIIELL
Bay,OntarioP7B5EI
Thunder
InkehzadlJniversity,
of Geotogy,
Department
NICHOLAS M.S. ROCKT
Austalin"Australia
Australia.Nedlanh6009,Westem
of
Westem
University
Geology,
of
Department
BARBARA H. SCOTTSMITH
2M9
BrttishColumbin'WR
Boulevar4NorthVancoaver,
Petrology,2555Edgemount
Scox-Smith
ABSIRAC"I
rocls
The nomenclatureand classificationof lamprophyres,lamproites,kimberlites andthe kalsilitic, melilitic and leucitic
on the
IUGS-Subcommi-ssion
t{e,
by
groups
establghed
working
of
three
recommendations
The
detined.
are inaaequately
the aboveare
Systematicsof igneousRocksto resolveaspectsof tlle problem arepresentedfor discussion.New definitionsof
given, variouslyln mineralogicaland geochemicalterms,and a reviied sequencefor the systematicclassificationof the rocks
is provided which integrateswith the existingIUGS hierarchicalsystem.
Kewords: lamprophlre, lamproite,kimberlite, kalsilitic rocks, melilitic rocks, leucitic rocks, classification.
Sovnrtarnn
m€lilite,
l,a nomenclatureet la classificationdu clan dgs tamFrophyres,lamproites,kimberlites, et des rochesa kahilite,
de
recommandations
les
discussion
p9*
de
presentons
i"i
Nous
jamais
de
fagon
satisfaisante.
d6finies
6t6
leucite
n'ont
et
1*
Intemationale
I'Union
de
igrdes
roches
des
la
syst6matisation
dL
charg6e
par
trois groupesde travail 6tablis la sous-commission
min€ralogiques'
CesS"cientesG6ologiques.Nous pr6sentonsdesd6finitionJnouvellei desmembresde ce clan, en tennessoit
nouvellede classificationsyst6matique,int6gr6eau sch6mahidrarchiquedu systbmeexistant
soit gdochimiques,et une s6quende
de ITUGS'
(fraduit par la R6daction)
plassification'
Mots-cl6s:lamprophyre,lampro'ite,kimberlite, rochesi kalsilite, rochesi m6lilite, rochesl leucite,
* Recommendatrons
of the IntemationalUnion of GeologicalSciences(IUGS), Subcommissionon the Systematicsof Igneous
Rocks.
t Deceased.
176
TI{E CANADTAN MINERAIOGIST
IlqrnonucttoN
In the book "A Classificartonof lgneous Roclcsand
Glossary of Terms: Recommendations of the
International Union of Geological Siiences
Subcornmission
on the Systematicsof lgneous Rocks.
(LelVlaite et al. 1989),lamprophyres,lamproitesand
kimberlites were amalgamatedunder the hearting
"lamprophyric rocksoo.This was regarded as a
provisional expedientuntil, as noted in the heface of
the book, a more satisfactoryclassification could be
established.
In thebook"Lamproplryres",Rock(1991)
went even further and placed lamproites and
kimberlites in his "lamprophyre clan',. However, to
many petrologists, inclusion of lamproite and
kimberlite with the lamprophyresis either inappropriate or incorrect becausethe terrn tampropSe is
devalued,whereasthe classificationof lamproitesand
kimberlitesis not furthered.
During a meeting of the Subcommissionat the
Intemational Geological Congressin Washingtonin
1989, three working groups were establishedto
reconsiderthe classification of these rocfts, together
with certain feldspathoidatand leucitic, melilitii, and
kalsilitic rocks, which are also inadequatelydefinedin
Le Maitre et al. (1989).The working groupsincluded
members of the Subcommission, and several
specialists,particularly in the areasof kimberlitic and
lamproitic rocls, were invited to participate. This
contribution brings together the findings of the
working groups and the Subcommissionon tlese
topics, and is presentedas the best compromise
currently achievable. The recommendationsare
presentedfor discussionand shouldnot be regardedas
the definitive statementon the topic.
The objectiveof the Subcommissionis not to crearc
a highly detailed ttxonomy, either mineralogical or
chemical,but to provide a broadly basedclassification
to be _usedin a logical manner by any geologist.
The-classificationsuggestedfollows tle principtes
established
by Le Bas& Streckeisen
(1991).The classificationof theseexotic alkaline rocks is
not a trivial tash and there will probably never be, at
least in our lifetimes, a truly practical and workable
scheme.Advanceshave been made in the past two
decadesin kimberlite and lamproite nomenclature,
prinarily becauseof their economic importance.In
contrast, the vast array of lamprophyres has not
receivedthe sameamountof attention,and there will
be a slow natural evolution of our ideas regarding
their classification as a consequenceof subsequent
investigatious.
INTEGRAIoN wrrr{ TIIE
IUGS l:lrrnnncucAr, Sysru\4
Le Maite et al. (1989) and Le Bas & Streckeisen
(1991,Fig. 8). In the publishedhierarchicalsysrem,
lamprophyres, lamproites and kimberlites were
combined under "lamprophyric rockso', but if this
portmanteauterm is to be abandoned,then it must be
replaced by a logical hierarchy of classification for
thesethree groups of rocks that integratesthem with
the melilitic and leucitic rocks.
Lamprophyres do not lend themselvesto either
mineralogicalor chemicalclassificationon accountof
their very variable HrO and CO, contents,which are
primary in someinstancesandsecondaryin others,and
which are not taken into account in most proposed
classifications.Nevertheless,the members of the
working groupsand the Subcommissionagreedalmost
unanimouslythat the following proposalswere a step
in the direction of establishinga nomenclaturewith
minimum ambiguify and maximumusefulness.
It must be sfiessed,however,that the hierarchical
systempresentedhereis not definitive andhasminimal
genetic conlent. Any classification of a rock as a
lamprophyre,kimberliie or lamproiteusingthis system
shouldbe consideredprovisional until further investigations have been undertakenusing the specialized
literatureas a key to correctclassification.
h.oposEDSvsruraFoRTIrECtassmcetow
or Msr,trffIc, Kersrrnc, KrvrsFRlmc.Latwnomc
ANDLEUCmcRocrs aNo Lavpnopnynrs
Classificationof melilitic rocla
The melilite-bearingrock classificationis used for
rocks with >lUVo modal melilite. Triangular plots are
presented for plutonic (melilitolite) (Fig. 1) and
volcanic (melilitite) rocks @ig. 2), and Table I is
intendedfor melilitic rocks containingkalsilite.
If the mode cannotbe determined"then one should
apply the total alkalis versus silica CIAS) chemical
classification,as follows: (a) The rock should plot in
the foidite field. (b) If the rock does not contain
kalsilite but has larnite in the norm, then one should
apply Figure 3. (c) If nomrativelamite is greaterthan
l07o and KrO is less than NqO (wt7o), then it is a
melilitite or olivine melilitite. (d) If K2Ois greaterthan
NarO and KrO exceeds2 wt%o,then it is a potassic
melilitite orpotassicolivine melilitite. The latter canbe
termeda lcanngife,which mineralogicallyis a kalsilite
- leucite - olivine melilitite. (e) If normativelarnite is
lessthan lUVo,thenthe rock is a melilite nepheliniteor
a melilite leucitite according to the nature of the
dominantfeldspathoidmineral.
Classificationof lealsilitic rocks
The principal mineralsof the katsilitic roctrsinclude
Any classificationshouldbe capableof integration clinopyroxene,kalsilite, leucite, meliliteo olivine
and
with the IUGS hierarchical system described by phlogopite (Iable 2). These rocks cannot be
called
CI.-ASSIFICATIONOF LAMPROPTIYRES
Me l
177
TABIfi 1. SUGGESTEDNOMENCLAN'RE FOR
KAMAFUGITIC ROCKS
Rocknane
MineralAssemblage
Mafurite
Olivine-pyroxenekalsilitite
Kalsilite-leuote-olivinemelilitite
Kafilngite
Venarzite
Coppaeute
Kalsitt+,pblogopite-olivine-leucitenelilitite
Kalsilite-phlogopit€melilitite
pyroxene
olivine pyroxene
melilitolite melilitolite
(1974).From the point of view of the IUGS systemof
classification, the presenceof essential melilite or
leucite (or both) indicatesthat either the classification
Ol
Cpx that deals with melilitic or leucitic rocks should be
Plutonicrocks
applied.However,the presenceof kalsilite audleucite
Frc. 1. Classification
of the plutonic(melilitolite)melilitic is consideredpetrogenetically so distinctive and
rockswith modalmelilite>10VoO-eMaifreer al, 1989 important that the acceptedterm, kamafugite, should
be retained for this consanguineousseries of rocks.
Fig.B.3).
Table I indicatestheir nomenclatureas a function of
mineral assemblage.
Plutonic kalsilitic rocks of the Aldan and North
pyroxenitebecausethat lerm is reservedfor plutonic Baikal petrologicalprovincesof Russi4 which are not
rocls. The rock types maftrite and katungite,together kamafugitic, may be distinguished by the prefix
with the closely associatedleucitic rock ugandite o'kalsilite".Thus, synnyrite becomeskalsilite syenite,
(which is excludedfrom Table 1. asit doesnot contain andyakutitebecomeskalsilite-biotite pyroxenite.
kalsilite and is more logrcally classifiedas an olivine
leucitite), constitutethe kamafugitic seriesof Sahama Classffication of kimberlite s
peridotite
melilile-bearing
and pyroxenite
Kimberlites are currently divided into Group I and
Group tr (Smith et al. 1985, Skinner 1989). Group-I
Mel
kimberlites correspond to archetypal rocks from
Kimberley, SouthAfric4 which were formerly termed
"basaltic kimberlites" by Wagner (1914). Group-tr
kimberlites correspondto the micaceousor lamprophyric kimberlitesof Wagner(1914).
Petrologists actively studying kimberlites have
concluded that there are significant petrological
differencesbetweenthe two groups,althoughopinion
is divided as to the extent of the revisionsrequiredto
their nomenclature.Somewish to retain the sntus qw
(Ski:rner 1989),whereasothers(Mitchell & Bergman
1991, Mtchell 1994b) believe that the terminology
shouldbe completelyrevised (seebelow). Regardless,
the working group is unanimousin agreeingthat a
single definition cannotbe usedto describeboth rock
types.
Becauseof the mineralogicalcomplexity of the
melalite-bearing
ultramaflcvolcanics
rocks, a simple succinctdefinition cannotbe devised.
Following a conceptoriginally developedby Dawson
Ol
Cpx
(1980), the rocks may be recognizedns containing a
'
Volcanicrocks
of minerals.
characteristicassemblage
of Group-I
characterization
The
following
(meilitite)
Ftc. 2. Classification of the volcanic
melilitic
rocks with modal melilite >10Vo(Le Maitre er al. 1989, kimberlites is a.fterMitchell (1995), which is based
essentiallyon that of Mirchell (1986, 1994b),and
Fig.8.3).
178
T}IE CANADIAN MINERALOGIST
A
A
MELILITITE
X M E L I L I T EN E P H E L I N I T E
AA K 64
A
AL
A
A
20
ur 18
F
2rc
E
J14
ul
A
A
AA
?tz
=10
A
F'
(E
o
z8
oo
{io"oo
4 '+ ^"a
'ffi
:bf{
4
2
6
8'to12
r
xJ{rr
rf
-
lt
I
I
14 16 1820222426283.O
N O R M A T I V EN E P H E L I N E
Flc. 3. Plot of normativenephelineversusnormatle lamite for melilitic lavas showing
the proposedboundary(dashedline) betweenthe melilitites and melilite nephelinites.
The data plotted are taken from eastem,western atd southemAfric4 Europe, the
former USSR,Australia, andU.S.A., including Hawaii, andarevariously describedas
olivine melilitite, melilitite or melilite nephelinite (see Le Bas 1989, Table 2).
Also plotted are typical potassic melilitic volcanic rocks: A, melilite ankmatrite;
K, katungite;L, leucitemelilitite; M, potassicolivine melilitite; V, venanziteftalsilite
- phlogopite- olivine - leucite melilitite).
evolved from earlier "definitions" given by Clement
et al. (1984) andMtchell (1979).
Group-I kimberlites consist of volatile-rich (dominantly COr) potassic ultrabasic rocks commonly
exhibiting a distinctiveinequigranulartextureresul :ng
from the presenceof macrocrysts(a generalterm for
large crystals,fypically 0.5-10 mm in diameter)and,
TABII 2. IVID].IERAL
ASSEMBLAGESOF (ALSII,ITTC RocKs
Phl
Cpx
Lct
Kal
Mel
Ol
Maftrite-x-xx
Katungrtexxxxx
VenaDzitorrxxrr
Coppaelitexx-xxSymbolg:Phf phbgopite,Cpc olimpyroxrc, kt ltuire, Msl ndflita, OL olivine,
Gb: glas. x prcem, -: ab&d. Afrrr Mjtc,hel & Bsgoln (t991, Table2.3).
in some cases,megacrystsQargercrystals, t,?ically
1-20 cm) set in a fine-grainedmatrix. The assemblage
of macrocrystsand megacrysts,at leastsomeof which
are xenocrystic,includes anhedralcrystalsof olivine,
magnesianilmenite, plrope, diopside (in some cases
subcalcic),phlogopite,enstatiteand Ti-poor chromite.
Olivine macrocrystsare a characteristicand dominant
constituent in all but fractionated kimberlites. The
matrix contains a second generation of primary
euhedralto subhedralolivine, which occurs together
with one or more of the following primary minerals:
monticellite,phlogopite,perovskite,spinel (magnesian
ulviispinel - magnesiochromite- ulvdspinel magnetite solid solutions), apatite, carbonateand
serpentine.Many kimberlites contain a late-stage
poikilitic mica belonging to the barian phlogopite kinoshitalite series. Nickeliferous sffides and rutile
are common accessoryminerals.The replacementof
earlier-formed olivine, phlogopite, monticellite and
apatiteby deuteric serpentineand calcite is common.
Evolved members of the Group may be poor in,
CI/,SSIFICATION OF I.AMPROPHYRES
or devoidof, macrocrysts,andcomposedessentiallyof
second-generationolivine, calcite, serpentineand
magnetite, together with minor phlogopite, apatite
andperovskite.
It is evident that kimberlites are complex hybrid
rocks in which the problem of distinguishing the
primary constituentsfrom the entrained xenocrysts
precludessimple definition. The above characterization at0emptsto recognizethat the composition and
mineralogyof kimberlitesarenot entirely derivedfrom
a parentmagma,and the nongeneticterms macrocryst
and megacryst are used to describe minerals of
cryptogenic,i.e., unknown,origin. Macrocrystsinclude
forsteritic olivine, chromian pyrope, almandinepyrope, chromian diopside, magnesianilnenite and
phlogopitecrystals,that are now generallybelievedto
originate by the disaggregation of mantle-derived
lherzolite, harzburgite, eclogite and metasomatized
peridotite xenoliths.In most cases,diamond,which is
excludedfrom the above"definition", belongsto this
suiteof mineralsbut is muchlesscommon.Megacrysts
are dominatedby -agnesian ilnenite, titanian pyrope,
diopside, olivine and enstatile that have relatively
Cr-poor compositions(<2 vttvo CrrOr). The origin of
the megacrystsis still being debated(e.9., Mirchell
1986),and somepetrologistsbelievethat they may be
coguate.Both of thesesuitesof mineralsare included
in the characterization because of their common
presencein kimberlites.
It canbe debatedwhetherreferenceto thesecharacteristic constituents should be removed from the
oodefinition"of kimberlite. Strictly, minerals that are
known to be xenocrystsshould not be included in a
pehological definition, as they have not crystallized
from the parentalmagma.Smaller grains of both the
macrocryst-and megacryst-suiteminerals also occuro
but may be easily distinguishedon the basis of their
compositions.In this respect, it is important to
distinguisl' pseudoprimarygroundmassdiopsidefrom
macrocrystic or megacrysticclinopyroxene. Group-I
kimberlitesdo not usually containthe former exceptas
a productof crystallizationinducedby the assimilation
of siliceous xenoliths (Scott Smith et al. 1983). T'he
primary nature of groundmassserpophitic serpentine
wasoriginallyrecognizedbyMitchell & Putnis(1988).
Recentstudies(Smith et aI. 1985, Skinner 1989,
Mitchell l994b,lggs,Tainton & Browning1991)have
demonstrated
that GroupI andGrouptr kimberlitesare
mineralogicallydifferent andpetrogeneticallyseparate
rock-types. A definition of Group-tr kimberlites has
not yet been agreedupon, as they have been insufflciently studied.Mitchell (1.994b,1995)has suggesred
that theserocks arenot kimberlitic at all. andshouldbe
termed o'orangeite',in recognition of their distinct
characterand unique occturencein southernAfrica.
Wagner (1928) previously suggestedthat the rocks
which he initially termed micaceous kimberlite
(Wagner L91.4) be renamed "orangite" (src,). The
179
following characterization of the rocks currently
described as Group-II kimberlites or micaceous
kimberlitesfollows that of Mitchell (1995).
Group-tr kimberlites(or orangeites)belongto a clan
of ultrapotassic,peralkaline rocks rich in volatiles
(dominantly H2O), characterized by phlogopite
macrocrystsand microphenocrysts,together with
groundmassmicas that vary in composition from
phlogopite to'lefiafeniphlogopite". RoundedInacrocrysts of olivine and euhedral primary crystals of
olivine are common, but are not invariably major
constituents.Characteristicprimary phasesin the
groundmassinclude: diopside, commonly zoned to,
and mantled by, titanian aegirine; spinels ranging
in composition from Mg-bearing chromite to
Ti-bearing magnetite; Sr- and kEE-ich perovskite;
Sr-rich apatite; REE-ich phosphates(monazite,
daqingshanite);potassianbariantitanatesbelongingto
the hollandite $oup; potassium triskaidecatitanates
(K.{!g'O2); M-bearing rutile and Mn-bearing
ilmenite. Thesearesetin a mesostasisthat may contain
calcite, dolomite, ancylite and other rare-earth
carbonates,witherite, norsethite and serpentine.
Evolved membersof the group contain groundmass
sanidineand potassiumrichterite. Zirconium silicates
(wadeite, zircon, kimzeyitic garnet, Ca-Z-silicate)
may occur aslate-stagegroundmassminerals.Barite is
a commondeutericsecondarymineral.
Note that theserocks have a greatermineralogical
affinity to lamproites than to Group-I kimberlites.
However, there are significant differences in the
of minerals,as
compositionsand.overallassemblage
detailed above,that permit their discrimination from
lamproites(Mitchell L994b,1995).
Classffi cation of lamproites
The classification system of lamproites described
it
by Mitchell & Bergman(1991) is recommended;
involves mineralogical and geochemicalcriteria, as
follows:
Lamproites are characterizedby the presenceof
widely varying amounts(5-90 vol.Vo)of the following
primary phases: (1) titanian (2-10 wt%o TiO),
aluminum-poor (5-12 wtVo Al2O) phenocrystic
phlogopite,(2) titanian (5-10 wt%oTiO) groundmass
poikilitic "tetraferriphlogopite",(3) titanian (3-5 vtt%o
TiO) potassium (4-6 wt%o K2O) richterite, (4)
forsteritic olivine, (5) aluminum-poor(<l t*,t%oN2O3),
sodium-poor (<l wtvo Na2O) diopside, (6) nonstoichiometriciron-rich (14 wt%oFqOr) leucite, and
(7) iron-rich sanidine(typicafly L-5 vttvo Fe2Or).The
presenceofall the abovephasesis not requiredin order
to classi$ a rock as a lamproite.Any onemineralmay
be dominant,and this, togetherwith the two or tlree
other major mineralspresent,sufficesto determinethe
petrographicname.
Minor and common accessoryphases include
180
TIIE CANADIAN MINERALOGIST
priderite, wadeite, apatite, perovskite, magnesiochromite, titanian magnesiochromite,and magnesian
titaniferousmagnetite;lesscommonly,but characteristically, jeppeite, armalcolile, shcherbakovite,ilmenite
and enstatitealso arepresent.
The presenceof the following mineralsprecludesa
rock from being classified as a lamproite: primary
plagioclase,melilite, monticellite, kalsilite, nepheline,
Na-rich alkali feldspar, sodalite, nosean,hauyne,
melanite,schorlomiteor kimzsyite.
Lamproites conform to the following chemical
characteristics:molar K2O/1.{a2O
> 3, i.e., ulfiapotassic, (2) molar KrO/Al2O3 > 0.8 and commonly > L,
(3) molar (KrO + NqO)/Al2O3 typically > | i.e.,
peralkaline, (4) typically <1,0wtVo each of FeO and
CaO,TiO, l-:7 v,ttVo,
>2000andcommonly>5000ppm
Ba, >500 ppmzx, >1000ppm Sr and>200ppm La.
The subdivisionof the lamproitesshouldfollow the
scheme of Mitchell & Bergman (1991), in which
the historical terminology is discarded in favor
of compoundnames based on the predominanceof
phlogopite, richterite, olivine, diopside, sanidine and
leucite, as given in Table 3. It shouldbe notedthat the
term "madupitid' in Table 3 indicates that the rock
containspoikilitic groundmassphlogopite,as opposed
to phlogopitelamproite,in which phlogopiteoccursas
phenocrysts.
The complex compositional and mineralogical
criteria required to define lamproites result from the
diverseconditionsinvolved in their genesis,compared
with thoseof rocks that canbe readily classifiedusing
the IUGS system. The main petrogenetic factors
contributing to the complexity of composition and
mineralogy of lamproites are the variable nature of
their metasomatizedsource-regionsin the mantle,
depth and extentof partial melting, coupledwith their
conrmonextensivedifferentiation.
TABLE3. NOMENCT-ATURB
OFLAMPROITtsS
llislorical name
Classificationof volcanic leucitic roclcs
The leucitic rocks. after elimination of the
lamproites and kamafugites, should be named
according to the QAPF (volcanic) diagram with the
prefix '1eucite" or ooleucite-bearing"
as appropriate.
Rocks containing liftle or no feldspar, i.e., falling in
field 15 (foidite) of QAPF 0-e Maitre et al. 1989,
Fig. B.10), areleucitites,which are divisible into three
subfields:(a) QAPF subfield 154 phonolitic leucitite:
Foids 60-907oof light-coloredconstituents,and alkali
feldspar > plagioclase; (b) QAPF subfield l5b,
tephritic leucitite: Foids 60-907o of light-colored
constituents,and plagioclase > alkali feldspar; (c)
QAPFfield 15c,leucititesensustricto:Foids9G-1007o
of light-coloredconstituents,andleucitepracticallythe
solefeldspathoid.
The essentialmineralogyof the principal groupsof
leucitic rocls is given in Table 4. No unambiguous
chemical criteria have been found to distinguish this
group of rocks. In terms of the TAS plot, leucitites
extend significantly beyond the foidite field into
adjacentfields @g. 4). They are better distinguished
from lamproites by other compositionalparameters,
althoughevenheresomeoverlapoccurs.The chemical
characteristicsof the potassicrocks, and attemptsat
distinguishinglamproitesfrom certain leucitic rocks,
usinga varietyofcriteria, areexploredbyFoley et al.
(1987)andMitchell & Bergman(1991).
r op hyr es
Classifi cation of Lamp
Lamprophyres are mesocratic to melanocratic
igneous rocks, usually hypabyssal,with a pan:idiomorphic texture and abundantmafic phenocrystsof
dark mica or amphibole (or both) with or without
pyroxene,with or without olivine, setin a matrix of the
same minerals, and with feldspar (usually alkali
feldspar)restrictedto the groundmass.For the general
classffication of these rocks, see I* Mal:tre et aI.
(1989),andfor detaileddescriptions,
seeRock (1991).
Revisodname
Wyoniagite
diopside-leucite-pblogopitelsmproite
Orendite
diopside-sanidine,phlogopitelamproile
TABLE 4. MINERAIOGY OF TI{E PRINCIPAL ROUPS
OFLEUCIT1CROCKS
Madupite
diopsftle nafupitic lamproite
Cedricite
diopsftte.leucite lamproite
Masilite
leucite-richtedte lamproite
I*ucitite
Wolgidit€
diopside-lercite-richerie madupitic lamproite
Fihoyilo
lerrite-phlogopite lamproite
Verite
hyaloolivinediopsidepblogopite
Tephritic leucitite
Phonolitic leucitite
kucitct@hdte
L4cito basanite
Leucitephonolit€
Fortnite
laoproite
olivine.diopside-richterite madupitic laqtroite
hyalo-enstatite-phlogopie lamFroite
Cancalitc
ensutite-sanidin+'phlogopite laryroite
Jumi[ite
after Mitchell & Bergoan (1991)
cpx
x
r
x
x
r
x
I
r
x
x
x
>10%
x>x
x<r
xx-
<to%
>to%
x
$llnbols Cpc dinopyrorog I,a: lruite Pt plagioolase,Ss: midiao (lroducn ol
its emoluion), OL olivine. x prs€il, -: abs€d. A[ ths roc&snay @dab somo
n€pbslirc.
181
CT.ASSIFICATIONOF I.AMPROPFIYRSS
;e
Eto
q
Y8
o
AI
to
37
41
45
49
53 s7 61
SiOzwt%
65
69
73
FIG. 4. Percentagefrequencydisribution diagram for 112 samplesof leucitite plotted
in the TAS diagram,showingthat only about507oplot in the foidite fiel{ with major
concentrationsin the basanite-tephriteand phonotephritefields. Reproducedfrom
Fig. 23 oflr Bas et al. (1,992),Star indicatesthe peakofthe frequencydistribution.
The Subcommissionno longer endones the terms
'lamprophyric rocks", or'lamprophyre clan", used
as
by Le Maitre et al. (1989) and Rock (1991) to
encompasslamFrophyres,lamproitesand kimberlites,
becauselamproitesandkimberttes arebestconsidered
independentlyof lamprophyres.
5. If the rock is fine-grainedor glassyand has larnite
in the norm. the rock should be classified as a
MELILITICROCK.
6. Ifthe volcanic rock containsessentialleucite"with
or without phlogopite (biotite), or is from a minor
intrusion vdth mafic phenocrystsonly (generallymica
Rsusm SseuBrflAL Sysrsr,rron
or amphibole, or both), apply the following criteria
Crassn'yNc Iovnous Rocrs
sequentially:
(a) If the rock is free of leucite but rich in olivine
The revised hierarchy, which modifies that of the (typically 35-55 modal % including macrocrysts,
wall chart accompanyingLe Maitre et al. (1989) anid xenocrysts and phenocrysts), and one or morg
Figure 8 of Le Bas & Sneckeisen(L991), is given dominaal primary minerals in the groundmassare
below @igs.5, 6). Eachstatementis a preconditionfor monticellite, phlogopite, carbonate, serpentine or
the next in the sequence.
diopside, the rock should be classified as a
KIMBERLITE.
1. If the rock is fragmental, the rock should be @) If the rock containstitanian phlogopi0eas Al-poor
phenocrystsor groundmassgrains (or both), together
classifiedas aPYROCLASTIC ROCK.
with common Fe-rich leucite or forsteritic olivine
(or both), as well as one or more of titanian potassium
2. If the rock contains more than 50Vo pimary
carbonate. the rock should be classified as a richterite, Al-Na-poor diopside, Fe-rich sanidine,
CARBONATITB.
accessorywadeite and priderite in the groundinass,
thenthe rock shouldbe classifiedas a LAMPROITE.
3. If the rock contains >107a modal melilite and (c) Any rcrnnining leucitic rocks should be classified
M > 90Vo. the rock should be classified as a using the terms provided in the classification of
MELILITIC ROCK.
VOLCANIC LEUCMC ROCKS.
(d) Apart from certain pyroxene-and olivine-bearing
4. If the rock contains kalsilite. the rock should be rocks, e.g., ankaramiteand oceanite,which are not
classifiedas a KALSILITIC ROCK.
lamprophyres (for tlese rocks proceed to the
182
THE CANADIAN
Use pyroclasticrock
classification
Use charnockite
classification
Use ultramafic
classification
Is it
high-Mg?
Use TAS. If it falls in fields F or Ul.
usenonn ne r. norn ab classification
Ftc. 5. Flow chaxt (after Le Bas & Streckeisen1991) for
the classification of igneous rocks following the IUGS
schemeQ.e Maitre er al. 1,989),but modified for the
proposalspresentedin this report paper (dashedbox).
The 'horm ze versusnorm ab classification"is that siven
by Le Bas (1989).
QAPFITAS systems),any remainingrocks containing
phlogopite (biotite) or amphibole(or both) and those
ftom minor intrusionswith only mafic phenocrysts,go
to the LAMPROPIIYRE classification (Le Maitre
et al.1989).
7. If the rock is glassy or the mode is not otherwise
determinable,the rock may be lamproitic, if molar
K2O/NqO is greaterthan 3, molar K2O/N2O3greater
than0.8 andmolar (KrO + NqO)/AlrO, greaterthan I
(peralkaline); then go to the LAMPROITE classification.
8. The classificationofthe charnockitic,plutonic and
rcmaining volcanic rocks proceeds as described in
te Maihe et al. (1989).
MINERALOGIST
TnE PRoBLEMS oF CI-A.SSFICATIoN
Many schemesof classificationfor igneousrocks,
such as that based on the total alkali versr.s silica
method(TAS), have a major petrogeneticcomponent,
and ultimately all taxonony of igneous rocks v/ill
incorporategeneticfactors.For no otherigneousrocks
is the petrogeneticcomponentof classificationmore
important than for lamprophyres, lamproites and
kimberlites.Onecriticism of the IUGS classificationof
rocks, discussedby Le Bas & Streckeisen(1991),is
that it is deficient in a stronggeneticcomponent.This
is essentially an historical artifact of the pragmatic
approachoriginally adoptedby Streckeisento attain a
Becauseof the plethoraof classification
consensus.
systemsthat have been suggestedand applied in the
past, the Subcommissionfollowed a consensual
approach. The widespread adoption of the IUGS
systemwould seemto havejustified this approachfor
many igneousrocks. However, althoughmuch of the
IUGS system undoubtedlyhas some petrogenetic
significance, and is used in genetic discussion,for
examplethe TAS system,purely descriptivetermsmay
have to be applied where there is disagteementas to
interrelationshipsof rock suites. This would seem
to apply at present to the lamprophyres,for which
difficulties remain in erecting a petrogeneticclassification.
The main problemsof the classificationof lamprophyres, lamproites, kimberlites, feldspathoidal,
kalsilitic, melilitic, and leucitic rocks are briefly
outlinedbelow.
Inrnprophyres
The lamprophyresare a complex gtoup of rocks
thathavemineralogicalsimilaritiesto somekimberlites
and lamproites.Lamprophyresare difificult to classiS
unambiguouslyusing existing criteria. They are not
amenable to classification according to modal
proportions, such as the system QAPF, nor
compositional discrimination diagrams, such as
TAS (Le Mure et al. 1989). It seemsunlikely that
a simple taxonomic system viill be found unless
appropriategeneticcriteria are applied,that is, unless
the classification takes into account the genesis of
the rocks.
The term "lamprophyre" was introduced by von
Gumbel in 1874 for a group of dark rocls that form
minor intrusions,containphenocrystalbrown mica and
hornblende,but lack feldsparphenocrysts.Following
its introduction, the term was used by Rosenbusch
(1877) to encompassa wide variety of hypabyssal
rocks containing ferromagnesianphenocrysts,e.9.,
minette, kersantite, camptonite and vogesite.
Eventually, spessartite,monchiquite and alnoite also
were includedin the group.Thus, the group becamea
repositoryfor any mafic-phenocryst-richrock that was
CLASSIFICATIONOF LAMPROPIIYRES
melilite > 1070
183
use melilitic rocks classification
containskalsilite
leucite or
minor intrusion with
only mafic phenocrysts
K2O/Na2O> 3.0
molar K2O/AI2O3> 0.8
and peralkaline
difficult to categorize.Subsequently,Middlemost
(1986)andRock (1986,1991)extendedthe definition
further to include kimberlites, lamproites and even
rocks containing feldspar and leucite phenocrysts.
Reviews of the history of lamprophyrenomenclature
can be found in Rock (1991), Mitchel & Bergman
(1991)andMtcheU (1994a).
Rock (1991) used 'lamprophyre" synonymously
with "lamprophyte clan',, a term that included
lamprophyres,lamproites and kimberlites. To many
petrologists working on lamproites and kimberlites,
"lamprophyre" is an inappropriate general term.
Consequently,the working group and Subcommission
6vsrwfuelmiagly rejected the use of the term
lamprophyres(sensu
"lamprophyreclan" to encompass
Rosenbusch),kimberlitesandlamproites.
Mitchell (L994a) discussed at some length the
inadequaciesof the conceptof the 'lamprophyre clan"
and proposed adoption of the term "lamprophyre
facies" to conveythe conceptthat somemembersof a
petrologicalclan crystallizedunderdifferent conditions
than othermembersof the clan. Mitchell's approachto
the problemis determinedby a convictionthat systems
of classificationshouldbe geneticin character.
The working groups found it impossibleto devise
a definition of'lamprophyric rocks" that is not so
broad as to be almost petrologically meaningless.
For the lamprophyres(sezsa Rosenbuschor Rock),
the working group could not draft a satisfactory
definition, in part becauseRock (1991) included a
numberof rock types that differ mineralogicallyfrom
+lct+ol+
Fto. 6. Flow chart for the
melilitic. kalsilitic and
leucitic rocks and the
kimberlites, lamproites
andlamprophyres(dashed
box ofFig. 5). It is entered
after the "carbonates
>50Vo" box and exits to
the "charnockitic" box of
Fig. 5. The symbolsused
follow those of Kretz
(1983)wherepossible.
the generallyacceptedcharacteristicsof lamprophyres.
However, of greater significance is the realization
that members of the lamprophyre group have
distinctly different origtns, and thus it is unwise to
describeor group togetherrocks that are genetically
different.
Among the rocksincludedin the lnmprophyregroup
by Rock (1991) are alnoite and polzenite. These
containmore than L0 vol.Vomelilite, and thus arenow
consideredasvarietiesof melilitic rocks.$imililly, fts
abundantcarbonatein aillikite suggeststhat it may be
considereda variety of silicocarbonatite.
Minette is a biotite-rich lamprophyre. However,
certain mica-rich rocks that usually occur in minor
intrusions should not be called minette becausethe
mica is phlogopite (commonly titaniferous), and
the rock is alkaline. Such rocks might be befter
termed 'alkali minette" rather than "glimmerite" or
"phlogopitite", as it is the alkalinity rather than the
Fe/IVIgratio that is their characteristicfeature.
The widespreadusageof the term lamprophyrein
English, French and Germanpetrological literature is
in markedcontrastto its infrequencyin the extensive
Russianliterature. There the term is usually reserved
for alkaline rocks. Specific rock types, such as
kersantite" are considered as varieties of diorite.
Similarly, camptonite and alnoite are regarded as
variants of gabbro and melilitite, respectively
(Atdreevaet a\.1.985,Kononova1984).This approach
to lamprophyrenomenclature155imil6 to the facies
conceptproposedby Mitchell (L99ad.
184
TIIE CANADIAN MINERALOGIST
Lamproites
Lamproites have always been a difficult group
of rocks in terms of their identification and
nomenclafure.Recent interest in them has been
prompted by the discovery of economically viable
diamond-bearingvarieties, which has led to a
detailed re-examinationof the group and complete
revision of the nomenclature (Scott Smith &
Skinner 1983, Mitchell & Bergman 1991). These
revisions have resulted in the reclassificationas
lamproites of some rocks previously regarded as
kimberlites (e.g., Prairie Creek, Arkansas, and
Majhgawan,India).
It is only in the last few decadesthat lamproites
have been consideredto crysrallize from a distinct
type of magma. Formerly, the presenceof leucite,
the similarity of some olivine-bearing lamproite
to kimberlite, and the presence of some
"lamprophyrico'characteristics,e,g., abundantphenocrysts of pblogopite, led not only to a plethora of
names, but to an ill-defined place in petrological
taxonomy. The problem was further exacerbatedby
their geochemical characteristics,with the magrna
chemistry stabitzing a large number of unusual
minerals(K-Ba titanatesand silicates,K-Z silicates),
someof which can be usedas discriminantsfor these
rocks.For most examplesef lamproite,the presenceof
these minerals reflects derivation from a sourse
enriched in incompatible and large-ion-lithophile
elements; this enrichment distinguishes lamproitic
rocks from Group-I kimberlites 6d lam.Frophyres.
A full discussionof the nomenclatureof lamproites,
their relationshipsto other potassicand ultrapotassic
rocks, in both mineralogical and chemical terms,
together with a suggestedrevised nomenclature,is
found in Mitchell & Bergman (1991). The
Subcommissionessentiallyacceptedthe detailedwork
of Mitchell & Bergman(1991) and Scott Smith &
Skimer (1983), and neededonly to integrate these
rocks into the IUGS hierarchicalsvstem.
Melilitic roclcs
The main problemshere concernvolcanic melilitic
rocks. In Le Maitre et al. (1989), the coarser-grained
melilitolite is classified on the basis of modal
proportions,but a completelysatisfactoryclassification
for the finer-grained rocks was not attained. A
definition basedon rock chemistry is desirable,but
unfortunately these rocks cannot be distinguished
adequatelyftom other volcanic rocks in the TAS
system.However,the presenceof melilite in morethan
tracemodal amountsresultsin the formation of lamite
(or calcium orthosilicate)in the CIPW norm, and this
can be used as a potential discriminant. Although
lamite may appearin the norm of some melilite-free
nepheliniticrocks containingclinopyroxenerich in the
Tschermaks
component(H.S.Yoder,Jr.,pers.comm.),
we find that the lafier is typically expressedin the norm
as anorthite.
A furtherproblemremains,that thereis a continuous
seriesfrom melilitite, tlrough melilite nepheliniteto
nephelinite.Investigationof this problemindicatesthat
a reasonablyclear discriminantbetweenmelilitite and
melilite nepheliniteis normative larnite. In Figure 3,
samplesof melilitite and nephelinite are plotted in
terrnsof normativelamite versusnotmatle nepheline.
The bestdivision appearsto be at LlVolarrtrte.
When classilying the melilitic rocks, the following
should be taken into consideration:(a) The present
classification for melilitic rocks in I* Mao.neet al.
(1989,p. 12)is basedon thepresence
of modalmelilite
exceedingl0 vol.Voin either plutonic (melilitolite) or
volcanic (melilitite) occlurences,in combinationwith
M > 9AVo.O) In the IUGS scheme(see flow chart
accompanyingLe Maitre et al. 1989),their identification is made after excluding the lamprophyresbut
beforeenteringQAPF. It is now consideredpreferable
to identify melilitic rocks before lamproites,
kimberlitesandlamprophyres.(c) Evenin fine-gained
rocks, melilite canusually be idenffied in thin section
where it occurs in essential proportions, i.e.,
>10 voLvo.This assumes
therock is not altered;if it is,
Kimberlites
melilite is usually carbonated.(d) Some fine-grained
melilitic rocks are stronglypotassic,e.g.katungite,the
The Subcommissionconsidered it inappropriate potassic character usually being reflected in the
to re-investigatethe nomenclatureand definition of presenceof modal leucite or kalsilite (or both). (e)
kimberliles in detail, becausethis has been done Melilitite is characterizedby the presenceof melilite
extensively in the last few years by the many andperovskiteandcontainslessthan 38 wtVoSiO2and
specialistsof kimberlites (Skinner & Clement 1979, greaterthan 13 WVoCaO.
Dawson 1980,Clementet al. 1984,Mitchell 1986).
Nevertheless, a clear definition of kimberlite Kalsilitic rocl<s
should be formulated, particularly for purposes of
dist''rguishing these rocks from olivine lamproite,
Kalsittic rocls havenot previouslybeenconsidered
and for placing kimberlites in the hierarchical by the Subcommission.Thesefall into two groups:the
classification system.Currently, the classification of kamafugrticseriesof Sahama(1974),andthekalsilitekimberlite is undergoing revision, and the nomen- bearing syenitesand pyroxenites,e.9., synnyrile and
clatureadvancedby Mtchell (1994b)hasnot yet been yakutite, occurring in the Aldan and North Baikal
tully explored.
petrologicalprovincesofRussia(Kogarkoet al.1995,
CLASSIFICATION
185
OF T.AMPROPTIYRES
Kostyuk et al. 1990). Someof the kamatugitic rocks
contain leucite or melilite (or both) and might be
consideredfeldspathoidalor melilitic rocks.However,
the presenceof kalsilite is consideredso importantthat
it requiresassignmentoftheserocksto a specialgroup.
Foiditic and leucitic rocl<s
The problemsposedby theserocks arerestrictedto
the fine-grainedmembers.The coarser-grainedrocks
of equivalent composition containing nepheline or
leucite (or both) and, despiteheteromorphismin some
cases (Yoder 1986), can be satisfactorily classified
using the QAPF and other mineralogical systems
(Le Maitre et al. L989).
The boundariesbetweenthe feldspathoidalfield and
the basanite+ephrite,phonotephrite,tephdphonolite
andphonolitefields in the TAS systemarenot wholly
satisfactory, as they do not provide an acceptable
boundaryfor the nephelinitic and leucitic rocls. The
problem with regardto the leucitic rocks is illustrated
by Figure 4.
It is evident that leucitic rocks cannot be distinguishedchemicallyon the TAS diagram.However,as
leucite is, with few exceptions,a phenocrystphase,or
forms small but identifiable crystals,a modal system
shouldbe feasible.This approachwas not adoptedby
Le Maitre et al. (L989). The distinction between
nepheliniteandbasanitehasbeenconsideredby Le Bas
(1989).
AcIC{OWLEDGENmNIS
The authorsthank their colleaguesin the working
groups and the Subcommissionfor their constructive
approachto the controversialproblems encountered.
The members of the working groups and others
who contributed include D.S. Barker, J.L. Brandle,
A. Cundari,S.V. Efremovq J. Keller, S. Kravchenko,
E. Lazko, A.N. Mcl.aurin, H.O.A. Meyer, E.A.K.
Middlemost, T.F.D. Nielsen, A. Streckeisen,
E.J.Visentin,K.Yag1,H.S.Yoder,Jr. andB. Zanettin.
We acknowledgethe numeroususeful suggestionsof
two referees,particularly Frank Dudas, and of the
Editor for his major "tidying" operations on the
manuscript.
Sadly,Nick Rock died in Februaryof 1992,before
He is
he sawthe final dmft of theserecommendations.
included as a coauthorhere not only becauseof his
significant participation in the discussions of the
working groups, but also becauseof his immense
contributionto the investigationsof lamprophyresand
relatedrocks andthe enthusiasmwith which he studied
them. Although he may not have agreedwith all the
ideasand conclusionspresentedin this reporl he saw
most of its components,supporled them, and was
active in their discussionalmost to the final stage,
includingthe meetingin Brazil (1991),duringthe Fffi
International Kimberlite Conference, when agreement
was reached on some of the major problems.
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