Download fenites associated with carbonatites

915
The Canadian Mineralogist
Vol. 46, pp. 915-932 (2008)
DOI : 10.3749/canmin.46.4.915
FENITES ASSOCIATED WITH CARBONATITES
Michael J. LE BAS§
9 Park Lands, Blandford Forum, Dorset DT11 7BA, UK
Abstract
A review of occurrences of potassic and sodic fenitization around carbonatites is presented, including several new examples
and oceanic ones. Taken in conjunction with the many examples quoted in the literature, conclusions are drawn that the K and
Na contents of carbonatitic melts that induce fenitization are primary constituent parts of both calciocarbonatitic and magnesiocarbonatitic melts, which have a source in the mantle, and that the K and Na are not derived from the crust during passage by
these melts. Potassic fenitization can lead to the production of pseudotrachytes with up to 16 wt% K2O; these are genetically
unrelated to common magmatic trachytes, which have only 5–10 wt% K2O.
Keywords: fenite, carbonatite, sodic metasomatism, potassic metasomatism, K-feldspar, pseudotrachyte.
Sommaire
Le but de ce travail est de passer en revue plusieurs exemples de fénitisation potassique et de fénitisation sodique développées autour des carbonatites, y inclus de nouveaux exemples, dont certains dans un contexte océanique. Ces exemples, ainsi que
de nombreux cas cités dans la littérature, mènent à la conclusion que les teneurs en K et Na des magmas carbonatitiques qui
provoquent cette fénitisation sont primaires, à la fois dans les magmas calciocarbonatitiques et magnésiocarbonatitiques, qui ont
leur origine dans le manteau. C’est donc dire que le K et le Na ne seraient pas dérivés de la croûte lors du passage de ces liquides
carbonatés. La fénitisation potassique peut mener à la production de pseudotrachytes ayant jusqu’à 16% de K2O (poids). Ces
roches n’ont aucune filiation génétique avec les trachytes magmatiques cummunes, qui ne contiennent que 5–10% de K2O.
(Traduit par la Rédaction)
Mots-clés: fénite, carbonatite, métasomatose sodique, métasomatose potassique, feldspath potassique, pseudotrachyte.
Introduction
Fenitization is defined as the process of alkali
metasomatism associated with igneous activity, usually
alkaline igneous activity. Carbonatites are not usually
recognized as alkaline, as most contain negligible
alkalis. However, it is argued here that many carbonatitic magmas were originally alkaline, and that the
alkalinity was lost during the process of fenitization.
Thus “alkaline igneous activity” may be understood to
be “alkali silicate and associated carbonatite igneous
activity”.
Although a side issue from the purpose of this
contribution, the use above of the word “associated”
requires explanation in light of the 2003 paper by
Gittins & Harmer. They asserted that usually “there is
no liquid line of descent relationship between silicate
magma and carbonatite magma”, and therefore the word
§
E-mail address: [email protected]
“accompanying” is more appropriate than “associated”.
However, the fact that ca. 80% of carbonatites occur
together with alkali silicate rocks in time and space
(Woolley 2003) is a strong argument that they are
associated in the genetic sense, and hence “associated”
is used here. The intimate proximity of silicate magma
and carbonatitic magma erupting up the same volcanic
stem is well illustrated by the three Tanzanian volcanic
edifices Mosonik, Oldoinyo Lengai and Kerimasi that
occur within 20 km of each other; each has coexisting
carbonatite and nephelinite.
In this contribution, I consider what the alkalis in
fenites tell us about “carbonatitic” processes, and build
on the many recent contributions by Andersen (1989),
Kresten (1991), Cooper & Reid (2000), Sindern &
Kramm (2000), Drüppel et al. (2005) and others. I
concentrate on those fenites specifically associated with
carbonatites. Therefore, fenites around ijolites, e.g.,
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at the type locality of ijolite at Iivaara, northeastern
Finland (Lehijarvi 1960), and those around syenites,
e.g., at the Mlanje Mountains in Malawi (Platt &
Woolley 1986), are not considered here.
Background Information
Problems arise in the cases where fenites occur
around both alkali silicate rocks and carbonatites,
and whether the fenites produced are the result of
the intrusion of the alkali silicate or the carbonatitic
magmas. Examples where they cannot be distinguished
are numerous and include some classic cases, e.g., at
Kovdor in the Kola Peninsula, northern Russia, and also
at Alnö, Sweden (von Eckermann 1948, Morogan &
Woolley 1988), Bingo, Democratic Republic of Congo
(Woolley et al. 1995), Chilwa Island, Malawi (Woolley
1969) and Fen, Norway (Brøgger 1921). As a result of
this uncertainty, these occurrences are excluded from
this analysis. Such exclusion greatly limits the number
of examples available for consideration, but sufficient
carbonatite-associated fenites remain to make this
study viable.
Fenites around carbonatites come in many varieties. Heinrich (1985) identified three principal types:
potassic, sodic–potassic and sodic. All are syenitic in
appearance, and some are all-too-easily mistaken for
igneous syenites unless close attention is paid to the
mineralogy, chemical composition and field relationships. As the purpose of this contribution is to examine
the role of the alkalis K and Na, only the more extreme
K and Na types that best illustrate their properties are
considered. They can be characterized as: 1) potassic,
and dominated by the presence of K-rich feldspar, and
2) sodic, and dominated by the presence of alkali feldspar with an alkali amphibole and sodic pyroxene. The
sodic–potassic fenites are intermediates between the
two extremes; therefore, data about them are not specific
to either potassic fenitization or to sodic fenitization.
The tables of analytical data given with this contribution are only representative data. The complete data
files of all tables may be obtained from the Depository
of Unpublished Data on the MAC website [document
Fenites cm46_915]. The majority of the samples are
stored in the Le Bas Collection at the Natural History
Museum, London.
The Term “Fenite”
The terms fenite and fenitization were coined by
Brøgger in 1921 for certain rocks of the intrusive
complex at Fen in southern Norway. He described fenite
as any rock, whether felsic or basic, produced by in situ
metasomatism of older country-rock in contact with the
igneous rocks of the Fen complex; however, he did not
apply the term to rocks produced by remobilization or
rheomorphism of the metasomatic rock, even if fenitic
in origin.
In this contribution, the term fenite is extended to
include rheomorphic and intrusive rocks because, as
will be shown, they are intrinsically part of the fenitization process, producing, for example, the intrusive rocks
described as “feldspathic breccia” and “rheomorphic
leucotrachytes” by Garson (1965), Sutherland (1965a),
Woolley (1969), Baldock (1973) and as “ochreous
breccias and tuffs” by Flegg et al. (1977) and described
below.
Heinrich (1966, Chapter 3) gave a valuable account
of occurrences of fenitization around carbonatites,
pointing out that intensive fenitization can ultimately
produce the singular composition, a “feldspar rock”, no
matter what the original chemical composition of the
protolith may have been. This process of “convergent”
fenitization is common for the potassic fenites, but
Vartiainen & Woolley (1976) have shown that it is less
common for sodic fenites.
Many of the examples used in this contribution
were unknown to Heinrich and result from more recent
decades of seeing and collecting fenites adjacent to
carbonatite complexes all over the world, both continental and oceanic.
Potassic Fenites
Field relationships show that potassic fenites are
developed mainly around sövitic carbonatites, some
fine-grained calcite carbonatites and, less commonly,
around other carbonatites of the same intrusive complex
(Dixey et al. 1955, Brown 1964, Garson 1965, von
Eckermann 1960, Woodward & Hölttä 2005). Furthermore, the evidence in the field shows that the formation of fenite is usually caused by the metasomatic
action of the earliest intrusive sövite, the later phases
of intrusion commonly appearing only to brecciate the
earlier-formed fenite without metasomatism (Garson
1965, McKie 1966, Le Bas 1977).
The feldspar rocks produced by fenitization of quite
different protoliths are commonly termed, if coarsegrained, “orthoclasite” being composed largely of Orca.
90 (Sutherland 1965a, Le Bas 1981). The term is not
always appropriate because the K-feldspar of the rock
may be microcline, as pointed out by Heinrich & Moore
(1970). The more general term “feldspathic fenite” is
more appropriate. However “orthoclasite” is used in
this contribution where that name is already employed
in published occurrences, so that there is no uncertainty
about the rock being considered. The term “syenitic
fenite” is avoided, despite the common syenitic appearance in the field, because of the possible subsequent
misinterpretation that it may be an igneous syenite
largely composed of alkali feldspar. Nor do these rocks
have evidence of a melt with solidus feldspar composi-
fenites associated with carbonatites
tions, lying as they do in the leucite solidus field of the
system quartz – nepheline – kalsilite.
Pseudotrachyte
In the fenites surrounding the carbonatites of the
Tundulu complex, Malawi, Garson (1962) traced in
detail, with supporting illustrations, the transition from
recrystallization of coarse-grained feldspathic rock to
feldspathic breccia to a mobilized product described as
trachyte that intruded the host rocks as small bodies and
dykes. Since they are not true trachytes, being neither
truly magmatic nor of the chemical composition of
common trachyte, the term “pseudotrachyte” is more
suitable, although the term “potassic trachyte” is used
in some cases to highlight the K-rich nature (Sutherland
1965b, Baldock 1973, Cooper & Reid 2000). These
rocks are petrographically composed of small fragments
of feldspar recrystallized to aligned laths of water-clear
K-rich feldspar identified as sanidine by Garson (1962),
in a microcrystalline matrix. X-ray powder diffraction
of similar rocks elsewhere has shown that the feldspar
is not sanidine but is structurally maximum microcline
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(Heinrich & Moore 1970, Heinrich 1985, Mian & Le
Bas 1988). Some pseudotrachytes in eastern Uganda
carry large fragments of feldspar crystals that can be
mistaken for phenocrysts, falsely giving the rock the
appearance of common trachyte (Sutherland 1965b).
Cooper & Reid (2000) described the process of
formation of potassic trachyte dykes as the melting of
a cupola of high-grade fenite in response to the advective flux of heat from rising carbonatitic magma or
fluid which, with continued fluid fluxing, produces a
melt with or without xenocrysts. This melt then could
be emplaced as dykes, sills and plugs into the fenitized
collar around the carbonatite intrusion.
This development of extreme K-feldspathization
associated with carbonatite complexes is not unusual
and, for example, is seen in dykes and plugs in the Toror
Hills (Sutherland 1965b) and at Bukusu (Baldock 1973),
both in eastern Uganda, and in the dykes at Seladinha in
San Vicente, Cape Verde Islands, described below. This
process may also have given rise to the potassic glassy
extrusive lapilli described by Rosatelli et al. (2003) at
the Rangwa caldera complex in western Kenya. Also,
well exposed at Landslide Gully on the western cliffs
of the Homa Mountain carbonatite complex in western
Kenya is a further example of a “pseudotrachyte” or
“ochreous breccia”, an intrusive sill-like sheet up to
4 m thick between carbonatite and shattered, fenitized
Archean Nyanzian country rocks (Le Bas 1977, Chapter
18). The sheet is a tuff composed of minute fragments
aligned in cross-cutting flow patterns. A CIPW norm of
the XRF-derived composition of the tuff (Table 1) gives
the calculated composition: orthoclase 44%, quartz
14%, calcite 25% and Fe–Ti oxides 17%.
Potassic fenites of Ruri Hills, western Kenya
The main host rocks to the Ruri carbonatites are
chlorite-grade Nyanzian (Archean) metabasalts, mostly
bluish green and fine grained, but with some blackish
variolitic-textured pillow lavas. Within a few meters
of the contact with the sövites of the South Ruri Hills,
the metabasalts change in color to dark green with
conspicuous whitish patches. Thin sections of this
rock show the matrix feldspar as indeterminate cloudy
plagioclase enclosing opaque material, formerly intergranular pyroxene. The white patches are prismatic
plates of euhedral clear K-feldspar, in some cases
rimmed by albite. Toward the contact, this clear feldspar
is seen in places to take on a trachytic texture parallel
to the contact, indicative of some remobilization, in a
matrix of fine-grained, indeterminate opaque material
and white mica. Xenoliths of this trachytic rock are
caught up in the adjacent sövite. Analysis of the grains
of clear feldspar indicated compositions varying from
Or89 to Or92. For several meters from the contact, the
feldspar-rich rock is commonly brecciated. Strongly
fenitized rocks identified near the carbonatite are in
some cases also seen away from the contact, but on
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the canadian mineralogist
inspection, are visibly adjacent to both sides of a
fracture or joint surface along which fenitizing fluids
evidently migrated.
The bulk composition of these rocks (Table 1)
shows progressive enrichment in K, corresponding
with the appearance and increase in K-feldspar and the
increases in Ba and Rb contents (Fig. 1). The fenites
analyzed and plotted in Figure 2 come from the twin
carbonatite complexes of South Ruri (RS) and North
Ruri (RN). Figure 2 shows that the path of fenitization
(a) is initially one of increasing Na, followed by an
increase in K (path b), producing the euhedral plates of
clear K-feldspar and ultimately the feldspar rock. The
path is one of falling temperature.
products after hornblende, all cut by quartz veins
(sample U 1071). Relict alkali feldspar from the host
rock is in some cases preserved in the cores of the
feldspars analyzed (Table 2). Some outcrops show a
much finer-grained rock of flinty and foliated appearance, and composed of fine-grained granular feldspars
with some quartz and chlorite, but with no apparent
signs of fenitization (U 1069). At other outcrops, thin
sections reveal fragments of the granular feldspar rock,
“Orthoclasites” of Wasaki Peninsula, western Kenya
Surrounding the large (2 3 2 km) Wasaki body of
sövite west of Homa Bay is a zone 200 to 300 m wide
of feldspar rock and feldspathic breccias in which
the dominant feldspar is Or85–95, with some albite
and aegirine-augite (Table 2). The Wasaki sövite is
locally brecciated, and was described by Le Bas (1977,
Fig. 10.1). It outcrops from Ugongo in the north to
Sokolo southward, where it extends underground
toward Kiumba and is penetrated by the Nyamaji
phonolitic vent that brings to the surface fragments of
granitic, sövitic and feldspar rocks, amongst others,
providing a better image of this large carbonatite intrusion. A selection of fenite compositions from least
fenitized to feldspar rock is presented in Table 3.
The basement rock is a hornblende-bearing porphyritic granodiorite described by McCall (1958). Even the
least fenitized sample collected is strongly fractured
and is, especially in the south, reduced to minute fragments of dusty albite-twinned plagioclase and quartz
set in a matrix of Fe-stained and chloritized secondary
Fig. 1. Concentrations of K2O versus Ba and Rb for K-fenites
developed next to sövites at Ruri, western Kenya.
Fig. 2. Normative feldspar plot for fenites from Ruri, western
Kenya (filled circles, labeled SR and NR for South and
North Ruri) and for fenites from Seladhina, San Vicente,
Cape Verde Island (filled squares). The Ruri feldspars initially show an albite-enrichment trend (arrow a), then an
orthoclase-enrichment trend (b) with increase in fenitization of the metabasalt protolith. The trend c for the fenitization of an essexite protolith (E) at Seladhina, also begins
with albite enrichment and then joins trend b of orthoclase
enrichment, culminating in a pure K-feldspar rock.
fenites associated with carbonatites
each rimmed by water-clear K-feldspar plates. Filling
the fractures is a mixture of Fe oxides, green aegirine–
augite prisms, Fe-stained feldspars and some carbonate.
Quartz veins are still evident. These rocks grade into
feldspathic breccias.
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At higher grades of fenitization, the plates of clear
feldspar are larger and more abundant, with fractures
containing a turbid and Fe-stained matrix in which
abundant small laths of clear K-feldspar occur (U965).
Other outcrops to the north and south of the Wasaki
sövite show K-feldspar rocks veined and penetrated by
needles of green sodic pyroxene and some analcime
(SK34). There is only a general increase in proportion
of the K-feldspar and size of crystals on approaching
the sövite; the actual distribution is variable, and probably related to the extent of brecciation accompanying
the fenitization. The coarser the grain size of the
feldspars, the closer the composition approaches pure
K-feldspar, and the greater the Ba content in most cases
(Table 2).
Figure 3 demonstrates the path of the fenitization,
with the early stage (a) of Si loss and K gain, then
increasing K (stage b) leading to the feldspar-rock
compositions near pure K-feldspar. It follows the same
path as that shown by Baldock (1973) for fenitization
around the Bukusu and other carbonatites in eastern
Uganda (Sutherland 1965b). In the area labeled Kfs
in Figure 3 are the mineral compositions of the K-rich
feldspar in the feldspar rocks, mostly Or87–97. Individual
grains show zoning, with K and Ba increasing toward
the rim.
Of the trace elements listed in Table 3, Ba shows
an appreciable increase with progressive fenitization
around the Wasaki sövite, Rb less so (Fig. 4). Total
Fe and Nb both show a poor correlation with progressive fenitization, as measured by increasing K2O. The
Nb content instead shows a positive correlation to
total Fe.
Fig. 3. Normative felsic components of the Wasaki whole-rock fenite compositions
plotted on the quartz – kalsilite – nepheline diagram (wt%) showing the progressive
increase of the orthoclase (Or) component as fenitization increases from G (the local
granodiorite) along trend arrows a and b. The points plotted in area labeled Kfs pertain
to electron-microprobe data on feldspar in the fenites and orthoclasites.
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the canadian mineralogist
Fenites on Brava, Cape Verde Islands
On the barren hillsides of Minhoto on the eastern
part of the island of Brava, coarse-grained sövite is
exposed from near the coast, across the Ribeira Grande
inland for a kilometer to Cachaço. The carbonatite
intrudes ijolite, and in the contact area there are “orthoclasites” (K-fenite in Fig. 5) that are usually brecciated.
The exposures in some cases are whitish, but more
commonly reddish and brecciated, the red color coming
from the Fe-rich matrix to the breccia. Right up to the
sövite, the ijolite in most places remains coarse grained
with unaltered nepheline, aegirine-augite, apatite and
titanite. In some cases, there is clear K-feldspar between
the nepheline crystals. In other cases, the nepheline is
replaced by cancrinite, and some prisms of pyroxene are
rimmed and replaced by biotite and brown to opaque
Fe-rich material.
Thin section petrography reveals that the “orthoclasite”, where not altered, is composed of fresh, Carlsbadtwinned feldspar crystals 5 mm long. In general, the
feldspar is fractured, in some cases comminuted. The
feldspar has not been analyzed, but optically, it has the
same clear, subhedral and Carlsbad-twinned habit of
K-rich feldspar seen in other “orthoclasites”.
The database (Table 4) is limited but reveals
increases in Ba and Rb similar to those noted above.
Strontium, Nb and the REE, normally in abundance in
carbonatites, show no increase with fenitization. Instead
here they decrease. The fenitizing fluids here, although
emanating from carbonatite, evidently did not transport
these elements.
Fenites on San Vicente, Cape Verde Islands
On the hillside north of the road at Seladinha da
Cal (GR 163663), midway between Mindelo and Ponta
do Calhau, is a brownish white northeasterly trending
dyke-like body 2 m wide that cuts extensively brecciated essexitic gabbro. In the middle of the body is a
Fig. 4. Concentration of K2O versus Ba and Rb for K-fenites
around the Wasaki sövite, western Kenya.
Fig. 5. Normative felsic components of potassic fenites plotted on the quartz – kalsilite – nepheline diagram (wt%)
showing the progressive increase of the orthoclase component as fenitization increases in the nepheline syenites
(ne sy) at Koga, northern Pakistan and in the ijolites (ij) at
Brava, Cape Verde Islands.
fenites associated with carbonatites
pale brown carbonatite dyke ca. 40 cm wide showing
dolomitization (MgO varies from 5 to 18 wt%). The
data in Table 5 are taken from samples across the dyke
body into the country rocks at Seladinha, with an additional set of samples prefixed HV and sample 82LV5
from similar occurrences 150 m to the northwest of
Seladinha, where the carbonatite dykes are better seen.
The whole area hereabouts exposes similar rocks.
The data plotted as filled squares on Figure 2 show
the same initial reduction in SiO2 (trend c) comparable
to the Ruri example, followed by a similarly marked
increase in K and ending with rocks of almost pure
K-feldspar composition. In thin section, the original
albite-twinned plagioclase of the essexite is seen to be
replaced progressively by almost clear K-feldspar, and
the original ophitic mafic minerals are altered to opaque
material with a matrix of carbonate. These country
rocks are considerably brecciated. Some fragments were
formerly ankaramite rather than essexite. In less brecciated areas nearby, ankaramite dykes are seen cutting
the essexitic gabbros.
Feldspar rock at Koga, Swat district,
northern Pakistan, 72°50’E, 36°20’N
Forming the top of the hill of Naranji Kandao is an
enormous body of feldspar rock capping a plug of white
carbonatite that intrudes and fenitizes nepheline syenite
and ijolite. The ca. 300 Ma Koga nepheline syenite is an
oval body of about 40 km2 and is the eastern member
of the Ambela granitic complex (Chaudhry et al. 1982,
Le Bas et al. 1987). Naranji Kandao hill dominates the
southern margin of the nepheline syenite. That syenite
is coarse grained with perthitic orthoclase and albite,
cloudy euhedral nepheline, minor sodic amphibole and
pyroxene, biotite, titanite and, in some cases, brown
garnet. Toward the carbonatite at Naranji Kandao,
the grey syenite becomes whiter, and the proportion
of K-feldspar increases and becomes notable as large
white crystals.
The carbonatite exposed in places on the Naranji
Kandao hillsides is a coarse-grained sövite with triplejunctioned calcite, minor prisms of apatite, and rounded
crystals of perthitic albite. Feldspar-rich syenite occurs
higher up the hillsides, where it is invaded by sövite and
K-feldspar rock. The hilltop is capped by almost pure
feldspar rock that includes occasional elongate patches
of the feldspathized nepheline syenite a meter across,
some cut by veins of feldspar rock and sövite. The
feldspar rock covers an area 1 km across surrounded by
a further 0.5 km collar of feldspar-rich rocks. It is very
coarse-grained and composed of cloudy, twinned plates
of microcline up to 2 cm across, commonly rimmed
by albite. Rare large prisms of green-brown aegirine
occur randomly. Jabeen & Mian (1992) provided
detailed descriptions of the rocks and compositions of
the feldspars (Or94–98 and Ab89–98). Figure 5 shows the
921
transition from nepheline syenite to K-feldspar rock
(analytical data in Table 6).
As in other K-dominant fenites, Ba increases as K2O
increases. The REE show no gain and instead decrease
because the Koga sövite, which contained ca. 400 ppm
La and ca. 800 ppm Ce, contributed nothing to the
nepheline syenite initially being fenitized. The sövite
compositions are given in Le Bas (1999).
922
the canadian mineralogist
Mian & Jabeen (1990) subsequently found evidence
for a second and minor carbonatite that produced
Na-fenitized rocks bearing magnesio-arfvedsonite.
K-fenites on Silai Patti, Northwest
Frontier Provinces, Pakistan
On the southern edge of the Himalayan Mountains
in Malakand Agency of northern Pakistan, near the
Afghanistan border, is a carbonatite sheet that occupies a southerly dipping thrust plane (the Main Mantle
Thrust) between amphibolites to the north and granite
gneisses and schists to the south. The sheet is 20 to
50 m thick and extends east–west for 12 km before
disappearing into inaccessible tribal areas. It was first
reported by Ashraf & Chaudhry in 1977, was studied
in detail by Mian (1987), and dated at 29 ± 5 Ma by Le
Bas et al. (1987). It comprises a white sheet of biotite–
apatite sövite and a later brown and more extensive
sheet of amphibole–apatite sövite. The sövites are
coarse grained, with triple-junctioned plates of calcite,
and include rounded prismatic crystals of apatite and
biotite flakes. Outcrops of mildly fenitized rocks away
from the carbonatites are few.
The biotite sövite, rarely more than 20 m thick,
is best developed in the east, where it fenitizes the
overlying granitic gneiss. The gneiss next to the sövite
contact is medium to coarse grained, well foliated
(marked by muscovite and Fe oxides), but weathered
and friable. The onset of fenitization is marked by the
disappearance of the muscovite and quartz, the foliation
being marked by alignment of feldspar, biotite and Fe
oxides. Toward the contact, the grain size increases,
with K-feldspar crystals (Or88–96), some grid-twinned,
reaching up to 2 3 3 cm, usually surrounded by clusters
of small granules of albite (Ab99), some albite-twinned,
Fig. 6. Whole-rock fenite compositions, each with two
feldspars, adjacent to carbonatites at Silai Patti, northern
Pakistan plotted on the quartz – kalsilite – nepheline diagram, showing the trends for the two successive stages of
fenitization of the Malakand granitic gneiss protolith, each
commencing with a phase of desilication.
giving an augen texture appearance. Red-brown biotite
flakes around the Fe oxides and the prisms of aegirine
are present in some cases (Table 7). The change in
whole-rock compositions (Table 8) is plotted in Figure
6, which shows the same patterns as those shown
by Woolley (1969) for the fenites of Alnö and other
carbonatitic complexes. The Na-fenites associated with
the amphibole sövite are discussed below.
Similar K- and Na-fenites occur adjacent to the
sheets of sövitic carbonatite at Loe Shilman on the
Pakistan–Afghanistan border northwest of Peshawar
(Mian & Le Bas 1986, 1987, 1988).
K-fenites at Bayan Obo, Inner Mongolia, China
Above the massive sheet of H8 dolomite carbonatite
emplaced in Middle Proterozoic Bayan Obo sediments,
large bodies of pure feldspar rock are developed in the
overlying H9 slates. Despite the many contributions
describing the Bayan Obo rocks, these feldspar-rich
rocks have not received much recognition; they are
briefly mentioned by Le Bas et al. (1992) and were
declared by Yuan et al. (2004) not to be slate but metasomatized trachytic volcaniclastic rock or volcanic tuff.
The feldspar rocks are best displayed in the quarry faces
above the enormous Main and East pits mined for REE,
Fe and Nb, and in the hanging wall at several places
along the length of the sheet (>10 km). Figure 7 shows
the two-stage fenitization seen above (Figs. 2, 3, 6); the
preliminary decrease in Si is followed by an increase
in K. Woolley (1969) and Vartiainen & Woolley (1976)
also demonstrated similar paths of fenitization for the
Chilwa, Alnö and other carbonatite complexes.
The feldspar-rich rocks are in sharp contact with
the mineralized H8 dolomite carbonatite body and
extend into the Proterozoic slates for 2–4 meters. They
are very fine grained, grey to white, and penetrated
by pure white veins 10 to 50 mm wide. Preliminary
Fig. 7. Fenitized H9 slate compositions at Bayan Obo, Inner
Mongolia, plotted on the quartz – kalsilite – nepheline
diagram, showing the progressive increase of the orthoclase component as fenitization increases, beginning with
desilication and ending with pure K-feldspar rock.
fenites associated with carbonatites
field interpretation of these flinty rocks was that they
are albitites cut by calcite veins, but XRF analysis
reveals that they are K-feldspar rocks cut by albite
923
veins (Table 9), with some later indistinct carbonate
veins. In thin section, the feldspar rocks are seen to
be microcrystalline, and electron-microprobe analyses
show that the bulk of the microcrystalline material has
the composition Or95–98 with some Ab99 and occasional
prisms of magnesio-arfvedsonite, especially toward
the outer, darker (graphitic) and less fenitized slates.
Similar rocks have also been identified in dykes nearby
as potassic trachytic tuffs with K2O > 15 wt% (Yuan et
al. 2000), and described as the product of metasomatic
fluids emanating from the H8 dolomite carbonatite
(Yuan et al. 2004).
Feldspathization of carbonatite tuff at Kruidfontein,
South Africa
The caldera-filling carbonatitic tuffs of the Proterozoic carbonatite complex at Kruidfontein, in South
Africa, provide evidence for both K and Na fenitization
of the tuff. As well as clasts of fresh barian K-feldspar
(Or85–93) in the sövitic tuffs and clasts of K-feldspar
(Or96–100) in the ankeritic tuffs presumably derived from
earlier fenites, pervasive and microcrystalline K-feldspar (Or97–100) occupies the matrix of the tuffs (Clarke
et al. 1991). In addition, veins of both K-feldspar (Or98)
and albite (Ab99) cut the mildly ankeritized tuffs.
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the canadian mineralogist
Sodic Fenites
(Including some Sodic–Potassic Types)
Sodic fenites are characterized by Na-rich amphibole
and an alkali feldspar or albite, with K-feldspar and
Na-rich pyroxene playing a lesser role. An extreme
example is the occurrence of albitite around the
Kirumba carbonatite in Kivu Province, Democratic
Republic of Congo (Denaeyer 1966).
Among the Tertiary carbonatite complexes of East
Africa, evidence of sodic fenitization is rare. In the
volcanic vent of Nyamaji (Le Bas 1977), fragments of
albitite occur that might result from sodium fenitization. On Homa Mountain, the central updomed mass of
Nyanzian metabasalts, andesites and rhyolites is mildly
fenitized to Na-amphibole-bearing fenites of uncertain
origin, but they are likely to be related to the emplacement of the swarms of carbonatite cone-sheets centered
under the mountain. The fenites are characterized by
widespread shattering and are cut by veinlets of minute
sheaves of aegirine and magnesio-arfvedsonite together
with crystals of albite projecting from the walls. An
apatite-group mineral and calcite also occur, in some
cases in conspicuous amounts. Cutting these veins
and along their edges are numerous veinlets composed
of fine-grained K-feldspar, which in places appear to
develop into patches composed of K-feldspar, aegirine
and Na-amphibole. These fenites were described and
analyzed by Sutherland (1969) and Le Bas (1977).
The Na-fenites at Silai Patti, North West Frontier
Province, Pakistan
Near the contact between granitic gneiss countryrock and the brown amphibole sövite sheet at Silai
Patti, veins of sövite cutting the K-fenitized gneiss are
bordered by aegirine prisms, indicating the passage
of later sodic fluids. The gneiss near the sövite sheet
is also recrystallized to a syenitic composition. The
early stages of fenitization are seen unfortunately only
in deeply weathered exposures in the field, where
Na-pyroxene and some Na-amphibole progressively
replace the quartz, and albite becomes more prominent.
Nearer to the sövite contact, the fenites are feldsparrich, with aligned prisms and aggregates of dark green
aegirine-augite, in some cases rimmed by blue to grey
pleochroic magnesio-arfvedsonite (Table 10). The albite
and K-feldspar occur in the proportions of 2:1 in the
coarser-grained fenites, with albite increasing in the
finer-grained fenites to 3:1. These variable proportions
show no relationship to distance from the contact, but
the field outcrops do suggest a relationship of distance
to former joints in the gneiss, along which fenitizing
fluids may be presumed to have traveled.
Table 11 provides representative analytical data for
the local granite fenitized to increasing degrees. Plots of
these data on a Qtz–Ks–Ne normative diagram (Fig. 6)
shows the contrasting paths between the K and Na
fenitization processes.
The Na-fenites at Bayan Obo, Inner Mongolia, China
As well as the K-fenites at Bayan Obo described
above, Na-fenites occur adjacent to sövitic carbonatite
dykes, and also at the contact with the massive H8 dolomite carbonatite intrusion (Le Bas et al. 1992, 2007),
and bordering the north–south-striking Wu sövitic
carbonatite dyke a kilometer to the north of the H8
carbonatite described by Yang et al. (2000, 2003).
At the western contact of the Wu dyke, the adjacent
H1 arkosic quartzite is strongly fenitized to a brilliant
blue rock for the first 15 to 40 cm. The bedding is
fenites associated with carbonatites
almost perpendicular to the contact, which is strongly
dislocated by shear. The bulk of the blue rock is a
meshwork of amphibole fibers varying in composition
from ferrowinchite to magnesioriebeckite to glaucophane (Table 12) that surround patches (the remains
of pebbles in the quartzite) of granular, fresh, twinned
albite crystals and occasional Fe oxide phases, with
some cloudy areas of microcline. Further from the
contact and marked by a sharp change in color from
blue to white, evidently indicating the limit achieved
by the metasomatic front, the white fenitized quartzite
is penetrated by an anastomosing network of dark
amphibole-bearing veinlets. The veining appears to
mark the path of migrating carbonate-free fenitizing
fluids along annealed fractures and joints. Former quartz
pebbles are recognizable, but are now represented by
granular aggregates of albite, small and fractured grains
of quartz, and some microcline, all with strained extinction and sutured margins. Between the aggregates are
veinlets of pale mica flakes (Table 13), with blue-grey
pleochroic amphibole prisms varying in composition
from magnesioriebeckite to magnesio-arfvedsonite
(Table 12) and some apatite prisms. Further from the
carbonatite dyke, the proportion of amphibole diminishes and disappears some 10 meters from the contact,
where low-Al phlogopite is present. At 34 m from the
contact, no mafic minerals are present, only fresh albite
(Ab95–97) and cloudy microcline (Or97–99) grains around
areas of strained quartz (Table 14).
Results of XRF and ICP–MS analyses of these
fenitized quartzites show (Table 15) an increase in
Na2O (sample 90/48 is recalculated to 3.15 wt% Na2O
without calcite and barite) and loss of SiO2 toward the
carbonatite. There are also increases in Sr, Ba, Sc, V,
Zn, Nb and REE (Figs. 8a, b). The strong enrichment
of REE in the fenite alongside the fractured and sheared
western margin of the carbonatite dyke is unique among
all the other carbonatites dykes nearby and is interpreted
925
as a later REE-rich hydrothermal event, most probably
related to the formation of the massive REE–Nb–Fe
orebodies for which Bayan Obo is famous. This leaves
Sr, Ba and the alkalis as the main elements of the
fenitizing process. Full analytical data for the minerals
in H1 fenites 90/48, 90/47, 90/49, 90/50 and 90/51 and
H3 fenites 90/40, 90/55 and 90/56 are given in Tables
15a–h, obtainable from the Depository of Unpublished
Data on the MAC website [document Bayan Obo
CM46_915].
On its eastern contact, the dyke cuts bedded H3
shale of the Bayan Obo sedimentary sequence with
a sharp and undeformed contact perpendicular to the
bedding. Instead of the blue fenitic zone to the west,
the fenite to the east is striped dark and light according
to the psammitic content of the beds. Next to the
contact, sample 90/40 is unusually potassic (K2O =
11.94 wt%, Table 15), with the dark bands composed
mainly of low-Al phlogopite flakes with minor prisms
of sodic amphibole, aegirine and granules of feldspar
(Table 15f). The lighter bands differ only in having a
higher proportion of feldspar. Ten centimeters from the
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the canadian mineralogist
contact (90/55), the dark bands are composed dominantly of wispy sodic amphibole (magnesioriebeckite to
magnesio-arfvedsonite), euhedra of titanite, rare brown
pleochroic aegirine crystals and phlogopite flakes, and
much feldspar as clear grains of albite Ab98–99 and
K-feldspar Or94–95 (Table 15g). A meter from the contact
(90/56), the bands of bedding are still evident, and the
mineralogy is little different apart from more biotite and
Fe oxides (Table 15h). Geochemical changes across this
fenite zone are not as marked as those in the western
contact zone: Na, Nb and REE show some increase and
SiO2 decreases (Table 15). The narrow potassic zone
next to the contact is chemically no different from the
potassic fenites described earlier. Its position here is
unusual, but it may mark a later phase of K-fenitization
following Na-fenitization, similar to events described
by Garson (1965) and Woolley (1969) at Kangankunde,
Malawi, and by Platt & Woolley (1990) for the carbonatites and fenites of Chipman Lake, Ontario.
Discussion
Heinrich (1985) rightly pointed out that there are
“infinite variations on a fenite theme”. Therefore,
fruitful discussion on the process of fenitization associated with carbonatites is focused on those occurrences
where the metasomatic fluids or emanations that induce
the fenitic effect can be traced to a carbonatite source.
This limits, as mentioned earlier, the examples available for study because it rules out the fenitization at
the numerous centers where carbonatites coexist with
alkali silicate rocks and at which the source cannot
be distinguished, e.g., at Alnö, Chilwa, Fen, Kola and
Meimecha–Kotui in Siberia. In these and many other
localities, it is not entirely clear whether the fenitization,
usually sodic, is related to the intrusion of carbonatitic
magma or of alkali silicate magma (generally syenitic
or ijolitic). In some cases, too, there is uncertainty
concerning the genesis of the syenite associated with
a carbonatite, for example at Xiluvo in Mozambique,
where a syenite, with K-rich feldspar (Or96–97) as the
most abundant phase together with low-Al phlogopite
or biotite, is reported (Melluso et al. 2004), supposedly
as an igneous rock but seemingly comparable with the
fenites considered above.
The examples of fenitization considered above
and many more in the literature (e.g., McKie 1966,
Woolley 1969, 1987) have led to the general observation, noted by several authors including Garson (1965),
Fig. 8. Plot against distance in H1 quartzite from the Wu carbonatite dyke north of Bayan Obo, Inner Mongolia, China showing:
(a) increase of Ba and Rb as the contact is approached; (b) increase of Sc, Zn, V and Nb toward the contact.
fenites associated with carbonatites
Baldock (1973) and Heinrich (1985), that feldspathic
fenitization tends to occur at the upper and subvolcanic
levels of carbonatite intrusion. These levels are also
usually the sites of the younger carbonatite occurrences, where relatively little erosion has taken place.
In contrast, sodic fenites are more commonly found
around the older complexes such as Oldoinyo Dili,
Tanzania (McKie 1966), where the deeper levels have
become exposed as the result of prolonged erosion. At
these deeper levels, a higher-temperature part of the
thermal gradient (> ca. 600°C) would have prevailed,
favoring the formation of sodic fenites, as deduced by
Rubie & Gunter (1983). This temperature relationship
accords with the earlier preferential loss of Na over
K during the magmatic evolution of a carbonatite,
as noted by Woolley (1982). One exception to the
depth–time relationship is seen at the >2000 Ma old
Kruidfontein carbonatite in South Africa (see above),
where K-rich fenites in the caldera-filling sediments
somehow escaped much erosion. Another is seen with
the trachytic fenites around the “Paleozoic Limestone”
[interpreted by Woolley (1987) as carbonatite] at Brent
Crater in Ontario (Currie 1971).
It is also a fact that extensive and intensive brecciation commonly occurs among the K-feldspar fenites
and, in cases of extreme brecciation, results in the
emplacement of pseudotrachyte tuffs or, if higher fluid
pressures exist, of intrusive sheets of the comminuted
material explosively reaching the surface to become
extrusive tuffs, as experimentally demonstrated by
Lorenz et al. (1991). Brecciation does not characteristically occur among the sodic fenites.
It would seem evident from this that a “head” of
volatiles or fluids must migrate to the topmost levels
of a chamber of carbonatitic magma. Most likely, the
fluid is H2O in part expelled from crystallizing carbonatitic magma in which up to 5% H2O at 1 kbar can be
dissolved, as recently confirmed by Keppler (2003).
The H2O may be an intrinsic part of the carbonatitic
magma or, more likely, may have been derived from
the country rocks traversed by the carbonatitic magma.
Explosive release of this fluid within the subvolcanic
complex could produce dense clouds of dust leading to
the formation of the tuffs described above or, if droplets
formed, to lapilli tuffs, as observed in the caldera of
the Kruidfontein carbonatite complex in the Transvaal
and in the Tertiary Rangwa carbonatite caldera of
western Kenya. The potassic glasses found at Rangwa,
recently described by Rosatelli et al. (2003), and the
newly discovered trachytic glasses intermingled with
dolomite carbonatite globules in the Massif Central
of France (Chazot et al. 2003), may have a similar
explosive origin.
It has also been observed that where there is more
than one phase of intrusion of a sövite, the earlier sövite
produces the feldspathic fenites, but the later intrusions
of sövite may not, only cutting the earlier sövite and
fenite, and apparently without reaction. Garson (1965)
927
commented on this effect in describing the occurrence
of fresh xenoliths of feldspar-rich fenites in some
sövites evidently produced by earlier sövites.
One feature common to many fenites is the absence
or very limited presence of calcite or other carbonate
mineral. Morogan (1994) did propose that for fenitization associated with carbonatite, the fluid phase would,
as could be expected, be dominated by CO2 over H2O,
and with abundant calcite formed. Were that the case,
one might expect to see in thin section in between the
fenitic minerals, calcite penetrating from the adjacent
carbonatite that induced the fenitization. But this is
not commonly the case, and it would seem as if there
is some impermeable membrane not permitting the
passage of CO2-bearing fluids from the crystallizing
carbonatitic magma. The answer to the question of the
nature of the fluid that carries the fenitizing agents may
lie in mineralogy.
The presence of amphiboles, micas and apatite,
particularly in the sodic fenites, suggests that the
fluid included hydroxyl and fluorine ions, a conclusion drawn by Gittins et al. (1990) as a result of
experimental work. However, they cautioned that the
role of H2O in the evolution of a carbonatitic magma
was probably minor rather than major. Jago & Gittins
(1991) provided further evidence for significant F in
carbonatitic magma. In a later discussion with Treiman
& Essene (1992), Gittins et al. (1992) reiterated more
strongly the evidence for a dominance of F over H2O
in carbonatitic magma. Treiman & Essene (1992) did
not dispute that a significant proportion of F may be
present in carbonatite-related fluids, but maintained that
in the case of the Husereau dyke at Oka, Canada, it “was
fluxed by H2O rather than by HF or F2”. Fluorine clearly
is a dominant factor, but whether it is in the vapor phase
or in the melt remains open to argument.
The near-absence of H2O (and F) in the K-feldspar
fenites indicates that these fenite-producing fluids
are different, notwithstanding the fact that explosive
action, presumably fluid-driven, is associated with
many K-feldspar fenites. Phlogopite reaction rims are,
however, commonly seen between sövite and its hosts,
e.g., on the west coast of Fuerteventura, Canary Islands,
and may indicate (OH), K and F activity at relatively
higher temperatures (Bailey 1966). This may also
have been the case at the phlogopite-rich K-rich fenite
margin at the eastern margin of the Wu dyke described
above. However, variability in the ratio F/(OH) is to
be expected, as demonstrated by Williams-Jones &
Palmer (2002) for the potassic fenites around the Amba
Dongar carbonatite in India, where F clearly is a major
component of the fenitizing fluids.
The significant presence of F in carbonatitic magma
is borne out by fluid-inclusion studies recently summarized by Veksler & Lentz (2006) and by Rankin (2005).
The trapped primary daughter minerals include halides
and the trapped fluids are halogen-rich, indicating that
F was present at an early stage.
928
the canadian mineralogist
It is significant that calciocarbonatites and magnesiocarbonatites each can induce both potassic and
sodic fenitization at their margins, and that the potassic
fenites produced by calciocarbonatites and the potassic
fenites produced by magnesiocarbonatites cannot be
distinguished unless calcite or dolomite is present.
Similarly, sodic fenites produced by calciocarbonatites
and by magnesiocarbonatites cannot be distinguished.
Fenites around calciocarbonatites have been discussed
above; examples around magnesiocarbonatites are less
common but nevertheless well reported in the literature,
for example at Kangankunde, Malawi, where zones
of potassic feldspathization form collars around the
magnesiocarbonatite vents (Garson 1965). At Newania
in Rajasthan, India, eckermannite-rich fenites surround
a three-km-long dolomite carbonatite sheet (Viladkar
1980) and are also associated with the Swartbooisdrif
ferrocarbonatite in Namibia (Drüppel et al. 2005).
Veksler & Lentz (2006) also reviewed the significance of melt inclusions in carbonatites, distinguishing
between inclusions of primary melts and inclusions
of late-stage fluids. Here it is the melt inclusions in
early liquidus minerals in carbonatitic magma, such as
apatite, that provide the telling evidence on the primary
composition of those magmas. After reviewing the data
derived from carbonatites in East and South Africa,
Europe, North America, and in Russia from East to
West, they concluded that in general the inclusions are
alkali-rich. Costanzo et al. (2006) and Rankin (2005)
came to broadly similar conclusions, based for the
most part on other carbonatite complexes, that REE as
well as Sr, Na and K are significant minor components.
Rankin in particular stated that evidence from inclusions trapped in carbonatitic minerals showed that Na
and K are, besides Ca and Fe, the main components
of ortho­magmatic carbonatitic fluids, commonly with
percentage levels of Sr, Ba and REE. Mitchell (2005)
also arrived at the conclusion in his wide-ranging study
that carbonatite-forming magma must contain Na and K.
Despite the evident appreciable contents of Na and K
in primitive carbonatitic magma, the Na and K invariably does not remain in the crystallized product of the
magma, but is apparently lost via late-stage fluids to
induce fenitization. Fluid-inclusion evidence coming
from Xie et al. (2006) in a new study of a Chinese
carbonatite lends further support to the hypothesis
that these fluids have high contents of K, Na, Ba, Sr
and REE and that “the melt–fluid inclusions should
represent the original carbonatite fluid” (Xie, pers.
commun., 2007).
As observed long ago in 1937 by Dixey et al. (1955)
and commented on by Kresten (1991), the element most
commonly added during fenitization, besides Na and K,
is Ba, much of it going into barian K-feldspar (see Table
14), particularly near the contact. Iron also is usually
introduced, appearing as finely disseminated oxides,
as are Sc, V, Zn, Sr and Rb, but not the REE in most
instances. Drüppel et al. (2005) listed Na, Sr, Ba, Nb
and LREE as the elements released by the carbonatite
into the fenitizing fluids at Swartbooisdrif in Namibia.
Strontium and Ba are common trace elements in
carbonatites, either in discrete mineral phases or in
solid solution, variously in the carbonates, phosphates
and phlogopite. There are noteworthy contents of Ba in
the core of phlogopite that are possibly mantle-sourced
(Gaspar & Wyllie 1982, McCormick & Le Bas 1996).
The common presence of biotite or phlogopite is
an indicator of K in the primary carbonatite, but the
usual almost total absence of Na from carbonatites is
problematic. The eruption of Na–K carbonate lavas
at Oldoinyo Lengai in northern Tanzania proves that
Na–K-rich carbonatitic magmas can exist, but such
compositions are not found in any of the other >500
carbonatite complexes in the world, and this has
relegated Oldoinyo Lengai to its status as an exceptional occurrence and an unlikely parental composition.
However, the work of Wallace & Green (1988) shows
that sodic dolomitic carbonatitic melts can be generated
from an amphibole-bearing mantle, which under suitable geothermal conditions could rise through the crust
and be a potential source for Na-metasomatism around
a dolomitic carbonatite intrusion. The recent observations by Bailey et al. (2006) give further evidence for a
mantle source of dolomite carbonatite melts.
The presence of the rare minerals burbankite
and carbocernaite in some carbonatites is a clear
indication that alkali-bearing carbonatitic magmas
can exist, as at Oldoinyo Lengai. Burbankite,
(Na,Ca)3(Sr,Ca,Ce)3(CO3)5, is reported by Kapustin
(1980) in several carbonatites, both as primary crystals
and at a late stage. Primary burbankite (also sylvite)
is recorded by Costanzo et al. (2006) trapped in fluid
inclusions in apatite of the Jacupiranga carbonatite.
Platt & Woolley (1990) reported burbankite in the dolomitic carbonatite at Chipman Lake, Bühn et al. (1999)
similarly in the carbonatite at Kalkfield in Namibia,
and Sindern et al. (2004) also in the calcite carbonatite
at Khibina, Kola Peninsula, Russia. But burbankite is
unstable; upon its breakdown, the alkalis are likely to be
lost, dissolved in fluids during the later-stage processes
of carbonatite crystallization (Zaitsev et al. 2002). The
fact that burbankite is unstable and that it includes many
of the elements involved in fenitization may be taken
as a pointer to the possible source of the components
of fenitization.
Carbocernaite, (Ca,Na)(Sr,Ce,Ba)(CO3)2 is even
more rare; it is reported by Kapustin (1980) in a few
carbonatites, by Drüppel et al. (2005) in the Swart­
booisdrif ferrocarbonatite in Namibia, and by Wall et
al. (1993) in a carbonatite dyke in Rajasthan, India. It
is a stable mineral, in some cases primary, but is also
formed by the breakdown of burbankite.
On the basis of isotope evidence, Zaitsev et al.
(2002) deduced that the burbankite-bearing carbonatites
at Vuoriyarvi, and probably at Khibina, in the Kola
Peninsula of Russia, have a mantle source, and that
fenites associated with carbonatites
these Kola carbonatites lack crustal contamination. For
other carbonatitic magmas passing through continental
crust, contamination with K and Na from the crust
remains a possibility. However, the mineralogical and
geochemical evidence presented above shows that the
Na and K fenitization associated with the carbonatites
in the Cape Verde and Canary Islands is no different
from fenitization associated with carbonatites in continental areas. But the Cape Verde Islands and arguably
the Canary Islands are on oceanic crust, and therefore
the carbonatitic magmas of the islands during passage
through oceanic crust would fail to receive significant K
and Na contamination and more likely receive none.
Therefore, the source of the alkalis must be in the
mantle. In the case of Na, the source most likely is
amphibole-bearing mantle as Wallace & Green (1988)
have shown possible. A phlogopite-bearing mantle
might similarly be the source for the K, and the K:Na
ratio in the mantle-derived carbonatitic magma would
then depend on the sources sampled during its passage
through the mantle.
Experimental evidence on the process of metasomatism, in particular fenitization, is required, but the
uncertain rates of metasomatic chemical reactions and
intra-lattice diffusion processes make the process difficult to be reproduced in the laboratory. More detailed
petrography and element-distribution maps are needed
to gain greater insight into the process of fenitization.
Summary
Significant features, some new, about alkalis and
carbonatites are:
1. Primary mantle-derived carbonatite melts carry
appreciable Na and K in widely varying proportions
that can be subsequently lost to fenitization.
2. Calciocarbonatites and magnesiocarbonatites of
any age can both produce potassic fenites that are indistinguishable. Likewise, both carbonatites can produce
sodic fenites that are indistinguishable.
3. Potassic fenites are characterized by a high
proportion of K-rich orthoclase or microcline, in the
composition range Orca. 90, or in some cases by concentrations of low-Al phlogopite or biotite.
4. Sodic fenitization is characterized by an abundance of sodic amphiboles and some pyroxenes, accompanied by K- and Na-feldspars.
5. The fenitizing fluids carrying the Na and K are
halide-rich, principally F; CO2 is commonly absent.
The H2O content varies with locality and may depend
on the country rocks.
6. Potassic fenites are formed mostly in the upper
levels of an intrusive body of calciocarbonatite and
magnesiocarbonatite, whereas sodic fenites more
commonly are formed earlier and at deeper levels and
at relatively higher temperature.
7. The first marked effect of fenitization is loss of
silica, followed by increase in the alkalis.
929
8. The ultimate product of potassic fenitization
can be pseudotrachyte with 15% K 2 O, which by
remobilization (manner remains uncertain) may be
intruded as dykes or, at high fluid pressures, can grade
into feldspathic breccias, ochreous breccias and tuffs
emplaced as near-surface sheets or, in extreme cases,
as extrusive tuffs.
9. Barium is characteristically enriched in potassic
and sodic fenites, in some cases also Fe, Sr, Sc, V, Zn
and Rb.
Acknowledgements
I have many thanks to make for the enjoyable years
spent cogitating on carbonatites, primarily to Basil King,
Tom Deans and Diana Sutherland, who introduced me
to them in East Africa and provided subsequent encouragement, and to John Gittins for illuminating reflections beginning in our research student days. I am also
indebted my many research students, including John
Dixon, Ihsanullah Mian, Neil Hodgson, Yang Xueming
and Anatoly Zaitsev, for permitting extensive use of
data from their theses and for collaborative work, and
to the other many colleagues, especially Alan Woolley,
with whom I most profitably shared discussions on and
visits to carbonatite complexes worldwide. This contributed also greatly benefitted from profound, critical and
helpful comments from editors Robert F. Martin and
David R. Lentz, and from reviewers Don Hogarth and
Yang Xueming on many major and minor points.
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Received November 27, 2006, revised manuscript accepted
October 1, 2007.