The Alnö alkaline and carbonatitic complex, east central

28 Pages·2005·2.38 MB·English

The Alnö alkaline and carbonatitic complex, east central Sweden – a petrogenetic
study
Jaana Hode Vuorinen
Department of Geology and Geochemistry, Stockholm University, SE-106 91 Stockholm, Sweden
The doctoral thesis consists of a summary and the following papers:
Paper I
Hode Vuorinen J., Hålenius U., Whitehouse M.J., Mansfeld J. and Skelton A.D.L. (2005).
Compositional variations (major and trace elements) of clinopyroxene and Ti-andradite from
pyroxenite, ijolite and nepheline syenite, Alnö Island, Sweden. Lithos 81, 55-77. Reprinted with
permission.
Paper II
Hode Vuorinen J. and Hålenius U. (2005). Nb-, Zr- and LREE-rich titanite from the Alnö alkaline
complex: crystal chemistry and its importance as a petrogenetic indicator. Lithos, in press. Reprinted
with permission.
Paper III
Hode Vuorinen J. and Skelton A.D.L. (2004). Origin of silicate minerals in carbonatites from Alnö
Island, Sweden – Magmatic crystallisation or wall rock assimilation? Terra Nova 16, 210-215.
Reprinted with permission.
Paper IV
Skelton. A., Hode Vuorinen, J., Arghe F., and Fallick A. Chromatographic modeling of fluid-
rock interaction across a carbonatite-gneiss transition zone, Alnö, Sweden. Re-submitted to
Contributions to Mineralogy and Petrology.
Paper V
Hode Vuorinen J., Skelton, A., Andersen T., Taylor, P. and Claesson S. Geochemical (major and
trace elements, Nd-, Sr- and Pb-isotopes) characteristics of the Alnö alkaline complex, Sweden
– Petrogenetic implications. In manuscript.
Field work, sampling and petrographic studies in all papers were carried out by the author, except in Paper IV where
the petrographic work was made by Alasdair Skelton and Fredrik Arghe (Department of Geology and Geochemistry,
Stockholm University). The author performed the electron microprobe analyses in Papers I, II and III with assistance
from Hans Harrysson (Uppsala University) and Paula McDade and Peter G. Hill (University of Edinburgh, Scotland).
SIMS analyses of REE in minerals for Paper I were performed by the author with assistance from Martin Whitehouse
(SMNH), whereas Joakim Mansfeld (Department of Geology and Geochemistry, Stockholm University) is responsible
for the LA-ICP-MS analyses in Paper I. Stable isotope (C and O) analyses on calcite samples in Paper IV were done by
Klara Hajnal, Department of Geology and Geochemistry, Stockholm University. O-istope analyses on alkali feldspars
in Paper IV were performed by A. Fallick, SUERC, East Kilbride, Scotland. Sample selection and preparation for
analyses in Paper IV was done by the author. Whole rock major and trace element analyses for Paper V were carried
out by ACME Labs, Canada. Laboratory work and analyses of Sr and Nd whole rock isotope composition was done
by the author with assistance by Marina Fischerström and Hans Schöberg (SMNH). The author is responsible for all
interpretations in Papers I, II and III in collaboration with co-authors. In Paper V, the Pb-isotope data interpretations
are made by T. Andersen. Major and trace element modelling in Paper V was made in collaboration with A. Skelton.
All interpretations in Paper V are by the author and co-workers. In Paper IV, the author answers for the mineralogical
and petrological descriptions and has provided input to the discussion and interpretations.

Introduction and Nd-concentrations, they are believed to
In 1960-61 the dispute concerning the existence buffer effectively against crustal contamination
and definition of carbonatite magmas was settled processes and might thus preserve the isotope
as an eruption of sodium-carbonatite lava flows composition of their mantle source(s) (Bell
was observed at the volcano Oldoinyo Lengai & Blenkinsop, 1987). Since most carbonatite
in Tanzania. Years prior to this, the concept occurrences are limited to extensional
of carbonatite magmas had been proposed continental tectonic settings, e.g. East African
(Eckermann, 1948), but was not fully accepted Rift System, the Rhinegraben in Germany and
by the scientific community. At the same time the Gardar rift in south Greenland, they may
as the eruptions at Oldoinyo Lengai, the first also provide chemical information relating to
successful experimental studies were also continental break-up. In addition, the intrusion
carried out on carbonatitic melts, seemingly ages for carbonatites spread all the way back
confirming the possibility of their existence. to Achaean times (Bell & Tilton, 2002). This
The events that occurred in 1960 were first makes it possible not only to characterize the
described by Dawson (1962a and 1962b) after recent composition of the sub-continental
which Oldoinyo Lengai has taken on a special lithospheric (and astenospheric) mantle, but it
significance in volcanological and petrological also presents a unique opportunity to monitor
research in general, and more specifically in the evolution of the mantle all the way back to
the study of carbonatites. Consequently, many the Achaean. This is a time perspective that is
studies and models of carbonatite genesis unmatched by other, mantle derived rocks such
constructed from observations of the lavas as MORB (Mid-Ocean Ridge Basalts).
at Oldoinyo Lengai have been argued to be
generally applicable to carbonatite genesis. Petrological and mineralogical definitions
This is despite the fact that the composition of Alkaline silicate rocks have, despite being
the lavas erupted at Oldoinyo Lengai is unique very rare in the Earths crust, given rise to a
with respect to the >400 carbonatites recognized bewildering range of names in the igneous
worldwide. For example, the carbonatite lavas nomenclature, reflecting the geochemical and
of Oldoinyo Lengai are alkali-rich in contrast mineralogical complexity of this group of
to most other known carbonatites which are rocks. In older literature, rock names have often
either Ca- and/or Mg-rich. This discrepancy been given with reference to type locality as
remains an issue of controversy and the nature for example alnöite, alvikite, beforsite (Alnö
of carbonatite formation remains uncertain, and surroundings), fenite, melteigite, hollaite,
other than that they are mantle-derived and sövite (Fen, Norway), ijolite (Iivaara, Finland)
spatially associated with alkaline silicate rocks. and jacupirangite (Jacupiranga, Brazil). Many
Carbonatite petrologists remain undecided as of the older names are now obsolete, and a
to whether (1) carbonatites are derivative melts general consensus regarding the essential rock
formed by unmixing or fractionation from a names has been reached and is presented by Le
mantle-generated alkaline silicate magma or Maitre et al (2002).
(2) carbonatite are mantle-derived magmas Variable use of the term alkaline
unrelated to an associated silicate melt since the has also flourished in the literature since the
time of extraction from the mantle. beginning of the study of such rocks and is
The study presented in this thesis aims excellently summarized by Sörensen (1974).
to further our understanding of the genesis of The term alkaline should only be applied to
alkaline silicate rocks and carbonatites in general rocks with Na and K in excess of that needed
by studying a particular complex in detail to form alkali feldspars (Si-saturated to -
(i.e. the Alnö complex, east central Sweden). oversaturated systems) or feldspathoids (Si-
This is important to petrologists, because undersaturated systems). This leads to mafic
carbonatites provide a chemical “window” minerals, i.e. clinopyroxenes and amphiboles,
through which we can examine the composition often with appreciable Na- and/or K-contents.
of the mantle. For example, due to the unique Rocks with Na O+K O/Al O >1 should be
2 2 2 3
geochemical characteristics of carbonatites termed peralkaline, a term which should not
(and associated silicate rocks), e.g. high Sr- be confused with, or used synonymous with
3
agpaitic. The term agpaitic should only be the genesis of SiO -poor magmas. Their modal
2
applied to peralkaline nepheline syenites with mineralogies are highly variable with modal
rare complex Zr and Ti silicates such as melilite varying from 10 vol% (melilitolite) to
more than 65 vol% (ultramelilitolite). Some
eudialyte melilitolites are essentially oxide-melilite
Na Ca (Fe2+,Mn2+) Zr Si O (O,OH,H O) (O rocks whereas others contain up to 50 vol% of
15 6 3 3 25 73 2 3
H,Cl) primary carbonates (Dunworth & Bell, 1998).
2
For a comprehensive review of melilitolite
and mosandrite mineralogy and classification schemes, the
(Ca,Na) (Ca,Ce) (Ti,Nb,Al,Zr)(Si O )(O,F) ] reader is referred to the work of Dunworth &
3 4 2 7 4
Bell (1998).
Of the more common alkaline silicate rocks Carbonatites are defined as igneous
occurring world-wide, the ijolite series rocks carbonate rocks in which the modal percentage
and nepheline syenites are most frequently of carbonates is more than 50 % (Le Maitre,
encountered. The ijolite series (melteigite- 2002). If the dominating carbonate is calcite the
ijolite-urtite) is defined as being essentially rock is a calcite carbonatite (calciocarbonatite),
clinopyroxene-nepheline rocks. Following the which is called sövite if it is coarse grained and
classification of Le Maitre (2002) a melteigite alvikite (Eckermann, 1948) if fine grained.
contains less than 30% nepheline; an ijolite However, although both alvikite and sövite are
between 30 and 70 % nepheline and an urtite is calciocarbonatites, they are distinctly different
composed of more than 70% nepheline. Ijolite chemically with respect to trace elements (Le
and urtite may sometimes carry small amounts Bas, 1999). Mg-rich carbonatites are dolomite-
of alkali feldspars and as these increase in bearing and sometimes referred to as beforsite
quantity, the rocks pass into nepheline syenite although this term is becoming somewhat
(via malignite = mela-nepheline syentie). obsolete. The term dolomite cabonatite or
Olivine is sometimes found as an important magnesiocarbonatite should be applied to
constituent in melteigite (Dunworth and Bell, those rocks. Fe-rich carbonatites are a bit more
2001; Verhulst et al, 2000). Accessory phases difficult to recognize as Fe in carbonatites
in all rocks of the ijolite series and in nepheline does not only reside within carbonates but is
syenite include Ti-rich andradite, which locally also contained within silicates (which may be
may become a major constituent, titanite, derived from wall-rock or may have crystallized
apatite, perovskite, phlogopite/biotite, iron- in the carbonatite magma) or magnetite and/or
titanium oxides and Fe-sulfides. Interstitial hematite which is an important cumulus phase
calcite is usually present and may become a in some carbonatites. This has lead to some
major phase. According to recommendations ambiguities as to how to correctly classify
by the IUGS (Le Maitre, 2002), ijolite series this rather rare type of carbonatite. Gittins &
rocks in which calcite increase to more than Harmer (1997) recognized the difficulties and
10 vol% (but is less than 50 vol%) should be recommended that the term ferrocarbonatite
called calcite- or carbonatitic ijolites. Late- should be used only when the modal mineralogy
stage and secondary phases in ijolite series of the rocks is not known (i.e. the term should
rocks and nepheline syenite include cancrinite, be used solely in a chemical sense), whereas
natrolite and sodalite. Alnöite and kimberlitic terms as siderite carbonatite, magnetite-calcite
alnöite are potassic ultramafic dyke rocks with carbonatite etc. should be used when the modal
clinopyroxene (diopside-augite), olivine and mineralogy is known. Natrocarbonatite is a
phlogopite as chief phenocryst constituents. fine-grained carbonatite lava composed chiefly
Calcite and sometimes melilite are abundant of nyerereite [(Na,K) Ca(CO ) ] and gregoryite
2 3 2
in the groundmass along with the already [(Na,K) CO ]. This is an extremely rare type
2 3
mentioned minerals. Garnet and perovskite of carbonatite and its occurrence is at present
may also be found. Melilitolites are rare on a restricted to the volcano Oldoinyo Lengai in
worldwide scale but are important components Tanzania.
of many alkaline-carbonatite complexes and
play an important role in the understanding of
4
Carbonatite petrogenesis – a literature carbonatite genesis are discussed, two of which
review argue that carbonatite magmas are derived from
a carbonated silicate parent:
A majority of all known carbonatites worldwide
are associated with alkaline silicate and/or
(a) carbonatite magma is produced through
ultramafic rocks, i.e. phonolites, nephelinites,
immiscible separation from a carbonated silicate
melilitites and kimberlites, or their plutonic
magma which can be nephelinitic or phonolitic
equivalents, but carbonatites apparently
(with or without preceding crystal fractionation)
occurring without any associated silicate
(Kjarsgaard & Hamilton, 1988, 1989a;
rocks, are also known. The almost ubiquitous
Kjarsgaard & Peterson, 1991; Kjarsgaard et al,
association however, indicates that it is difficult
1995; Church & Jones, 1995 among others)
to separate the origin of carbonatites from the
origin of the associated silicate rocks. Still the
(b) carbonatite magma is primary and
relationships between the different rock types
derived through melting of a carbonated
are not completely understood and a number
peridotite (Wyllie & Huang, 1975, 1976a;
of theories exist regarding their origin. Bell
Bailey, 1993; Lee & Wyllie, 1997a; Wyllie &
et al (1998) note that the diverse associations
Lee, 1998; Harmer, 1999; Harmer & Gittins,
that exist (i.e. carbonatite–nephelinite/ijolite,
1998)
carbonatite–phonolite/nepheline syenite,
carbonatite–syenite, carbonatite–kimberlite,
(c) carbonatite magma is produced through
carbonatite–lamprophyre, carbonatite–
fractional crystallization of a carbonated silicate
melilitite/melilitolite, carbonatite–pyroxenite
magma (Cooper & Reid, 1998; Lee & Wyllie,
and dolomitic carbonatite–olivine-rich
1994; Otto & Wyllie, 1993)
ultrabasites), argues against any simple model of
magma generation. This raises the question as to
The three models are not thought to be mutually
whether the associated rocks are generated from
exclusive and the genesis of carbonatite may
a common parental magma, and if so by what
reflect the combined effects of one or more of
processes, or if they are derived independently
these processes.
of one another. Essentially, three models for
(a) (b) MgO + FeO*
Na O + K O
2 2
Na O + K O
Lc CO -saturated 2 2
2
NA
ol
wt%
Ls+Lc
dol
(3) AB (3)
2.5 GPa (1) NA
1.0 GPa
(4)
Ls
EN (2)
(1) EN 1.0 GPa SV cc
SiO + Al O AB CaO + MgO CaO
2 2 3
SiO + Al O
+ TiO + FeO* 2 2 3
2
+ TiO
2
Figure 1. Comparison of miscibility gaps (curves at 1.0 and 2.5 GPa), liquid compositions from carbonated lherzolite (dark grey
field), natural rock compositions (AB, EN, NA and SV) and possible magmatic processes (paths 1-4) in silicate-carbonate systems
at mantle to crustal conditions. The miscibility gaps are at 1200°C and show the effect of pressure (a) and composition (b). NA
= natro-carbonatite, SV = average sövite, AB = alkali basalts and EN = evolved nephelinites. Arrows are: (1) progressive melt-
ing path for carbonated peridotite, (2) cooling path of carbonated, undersaturated mantle silicate melt through the crystallization
of amphibole (pargasite), (3) fractional crystallization path of mantle carbonatite liquid to NA and (4) metasomatic trend for the
formation of weh-rlite and average sövite by successive reaction of lherzolite with more magnesian carbonatite liquid. From Lee
& Wyllie (1997a).
5
a) b)
NaO+KO NaO+KO
2 2 2 2
Projected from CO Projected from CO
2 2
Two-liquid field Wt% Wt%
Two-liquid field
ne ne
ne ne
phl mel cc phl cpx+ne+mel cc
cpx cc cc
SiO+AlO cpx mel CaO+MgO SiO+AlO cpx CaO+MgO
2 2 3 2 2 3
TiO FeO TiO FeO
2 2
Figure 2. Two hypothetical paths for late-stage evolution in ultramafic alkaline complexes. The star shows the position of evolved
melt and the arrows show it’s progressive evolution during fractionation. In (a) melilite is stable in the crystallizing sequence and
the liquid reaches the miscibility gap. In (b) melilite is not stable and the liquid evolves toward the calcite liquidus field (ne+cpx+phl
crystallization). From Veksler et al. (1998a).
Liquid immiscib ility has become the of progressive partial melting of carbonated
most favoured model to explain carbonatite peridotite, or of fractional crystallization
genesis (Simonetti & Bell, 1994; MacDonald et of carbonated silicate liquids intersects the
al, 1993; Morogan & Lindblom, 1995; Peterson, miscibility gap under mantle conditions (paths
1990; Ruberti et al, 2002; Beccaluva et al, 1992 1-4, Fig. 1a), and concluded that it is unlikely
among many others) and has been shown by that silicate-carbonate immiscibility occurs
experimental studies to be a possible process during magmatic processes in the mantle.
for carbonatite genesis (Lee & Wyllie, 1998a, However, Lee & Wyllie (1997a) note that as the
1998b, 1997b; Kjarsgaard & Hamilton, 1988; MgO-content of evolving nephelinites decrease
among others). Interestingly, studies of liquid en route to crustal depths, their compositions
immiscibility in nature have claimed carbonatite are driven close to the silicate volume of the
unmixing from a variety of silicate rocks (i.e. miscibility gap (AB-EN in Fig. 1b) indicating
pyroxenite [Morogan & Lindblom, 1995], that immiscible carbonate-rich liquids may be
peralkaline wollastonite nephelinite [Church generated at crustal pressures. Furthermore, it
& Jones, 1995; Peterson, 1990], melilitite has been shown that if the parental silicate liquid
[Stoppa & Cundari, 1995] and nepheline reaches the miscibility gap during fractionation,
syenite [Beccaluva et al, 1992]). Additionally, more alkali-rich carbonatite liquids can form
carbonatites of widely differing compositions if the parental melt has a higher Na O+K O/
2 2
(i.e. Na-rich to Mg-rich) have been suggested CaO-ratio. The experimental results (cf. Lee
to form through unmixing from a carbonated & Wyllie, 1998a; 1998b; 1997a among others)
parent. Liquid immiscibility has even been have also shown that natrocarbonatites cannot
claimed to occur under mantle conditions (e.g. be parental to calciocarbonatites as has been
Kogarko et al., 1995; Pyle & Haggerty, 1994 proposed (Le Bas, 1989). Finally, experimental
among others). Lee & Wyllie (1996) showed studies have also shown that the compositions
that pure calciocarbonatites (> 80 % CaCO ) of dolomite-carbonatites are far removed from
3
cannot form by immiscibility and must be the miscibility gap and that such magmas are
cumulates. Thus most naturally occurring probably not derived through unmixing, but
carbonatites plot in the so-called “forbidden must represent primary magmas (Lee et al.,
volume” in CaO+MgO-FeO* - Na O+K O 2000; Lee & Wyllie, 1998a; 1997a). The case
2 2
– SiO +Al O +TiO –space, and in their current for primary mantle carbonatite melts have
2 2 3 2
state (chemical composition) cannot represent mostly been argued by Harmer (1999), Harmer
the carbonatite fraction that was separated from a & Gittins (1998; 1997) and Bailey (1993).
carbonated silicate parent. Lee & Wyllie (1998a, The idea stems, in part, from the observation
1997a) further showed that no proposed paths that carbonatite sometimes occur without any
6
association to alkaline silicate rocks. There is of carbonatite magma through (extreme)
experimental evidence that partial melting of fractionation of a carbonated silicate parent
mantle peridotites containing CO could yield (normally “carbonated nephelinite”) has been
2
carbonatite liquid with a composition (major argued by some authors (Cooper & Reid,
elements) corresponding to dolomite carbonatite 1998; Andersen, 1988; Veksler et al, 1998a;
(Dalton & Presnall, 1995 and 1996). However, Church & Jones, 1995). Some of these studies
experimental studies have also shown that not all invoke unmixing of a silicate and an (alkali-)-
carbonatite compositions are likely to represent carbonate fraction at an evolved stage following
primary melts, and it has been argued that prolonged fractionation of carbonated silicate
primary carbonatite magmas from the mantel parent magma (i.e. Andersen, 1988; Church &
should be dolomitic (Wyllie & Lee, 1998), Jones, 1995), while other evidence point to a
Consequently, many proposed primary mantle- straightforward fractionation without late-stage
derived carbonatite are not actually primary (cf. unmixing (i.e. Cooper & Reid, 1998; Veksler et
Lee & Wyllie, 1998a). However, more Ca-rich al, 1998a; see Fig. 2). In the study by Veksler et
compositions can be produced if the primary al (1998a), it is apparent that the fractionation of
dolomite carbonatite is protected from mantle melilite plays a key role in the evolution of the
lherzolite by metasomatic wehrlite. Through the silicate parent magma towards the miscibility
reaction [opx+melt→cpx+ol+CO (free fluid)], gap (cf. Fig. 2) and Cooper & Reid (1998)
2
a dolomitic melt can react progressively and showed that nepheline sövites from Dicker
reach calciocarbonatite compositions (Dalton & Willem in Namibia have compositions which
Wood, 1993: Lee & Wyllie, 2000a). One further allow co-precipitation of silicates and calcite,
limitation to the occurrence of primary mantle- i.e. they plot at the silicate-carbonate liquidus
derived carbonatites is that they should occur in field boundary (cf. Fig. 3 after Lee & Wyllie,
isolation, much like kimberlites (Eggler, 1989; 1998a). The fractional crystallization model
Lee & Wyllie, 1997a, 1998a). appears in the literature to be least favoured,
Petrological evidence for the formation and compared to the liquid immiscibility and
Figure 3. Silicate-carbonatite phase
MgO + FeO* MgO + FeO*
1.0 GPa relations at 1.0 GPa showing the
three major liquidus volumes for
Projected from the miscibility gap, silicate liquidus
Na O+K O CO field and carbonate liquidus field, and
2 2 2
Na O the liquidus surfaces between them.
2
+K O Contours for 10 wt% MgO+FeO* in-
wt% 2
tervals are shown. Nepheline sövites
from Dicker Willem (Cooper & Reid,
1998) would plot at the silicate-car-
Lc
bonate liquidus field boundary indi-
Ls
cating the possibility of co-precipita-
CaO CaO tion of calcite and silicate minerals.
SiO + Al O
SiO + Al O + TiO 2 2 3
2 2 3 2 + TiO
2
MgO + FeO*
1.0 GPa
Projected from
CO
2
Na O+K O
2 2
wt%
Carbonate liquidus
Silicate liquidus
CaO
SiO + Al O + TiO
2 2 3 2
7
primary mantle carbonatite models it has not
been given much attention.
Each of the above models has advantages and
short-comings, which have lead some authors
to speculate (and even conclude) that there are
“carbonatites and carbonatites”. For example,
Harmer (1999) pointed out a number of serious
short-comings of the fractional crystallization
model, which would appear to make it an unlikely
process for carbonatite genesis other than in a
few cases. A rather serious short-coming is that
for carbonatites to form as residual liquids, Ca
(and Mg) would have to behave incompatibly in
the crystallizing sequence. This does not seem
entirely likely as many of the associated alkaline
silicate rocks (i.e. melilitolite, pyroxenite, Figure 5. Location of KCL (Kola Carbonatite Line) after Dun-
ijolite) are dominated by (Mg-,) Ca-rich mineral worth & Bell, 2001. Other data from Mahotkin et al. (2000),
Kramm & Kogarko (1994), Zaitsev & Bell (1995) and Beard
assemblages and these elements thus appear to
et al. (1996).
behave compatibly. However, CO is likely to
2
behave incompatibly and may therefore become a process by which an immiscibly separated
enriched in residual liquids as shown by the carbonatite magma could be selectively retained
occurrence of interstitial calcite in many of the at depth, while the silicate magma continues
associated silicate rocks. The advantage of this its course upwards through the crust. This is
model compared to both the liquid immiscibility crucial, especially considering that a carbonatite
and the primary mantle-derived carbonatite liquid, due to its low viscosity, its lower density
magma hypotheses is that it could be expected relative to the silicate fraction and high volatile
to agree with the intrusive sequences observed content, should be expected to percolate more
in carbonatite complexes, e.g. carbonatites are easily upwards through the mantle and crust (cf.
generally intruded after associated silicate rocks. Hunter and McKenzie, 1989; McKenzie, 1985).
The liquid immiscibility model, fails to delineate Similarly, considering the incompatibility of
CO in the mantle (cf. Olafsson & Eggler, 1983)
2
and presuming that the silicate and carbonatite
melts form in the same source area, advocates
of the primary mantle carbonatite hypothesis
need either to explain why silicate melts would
form prior to carbonatite melts in the mantle
source, or why carbonatite melts are selectively
ponded at some level in the crust, while being
”overtaken” by the silicate melt. In contrast,
arguing in favour of primary mantle-derivation
of carbonatite, Harmer (1999), suggests that
because carbonatite melts are extremely reactive,
they are not able to move until channelways
lined by unreactive products have been created
by preceeding alkaline silicate melts.
Figure 4. Location of EACL (East African Carbonatite Line).
Plotted is data from Oldoinyo Lengai (Ne=nephelinites, Plume-lithosphere interaction
Ph=phonolites, P.B.=plutonic blocks), Ugandan carbonatites
Sr, Nd and Pb isotopes are extensively used when
(U.C.; Bell & Belnkinsop, 1987; Nelson et al., 1988), Canary
Islands (C.I.; Hoernle & Tilton, 1991) and mafic granulites trying to detect the origin of and relationships
(M.G.; Cohen et al., 1984). HIMU, DMM and EMI are mantle between different rock series. Carbonatites
end-members (Hart, 1988) defining the mixing line. From Bell
have been proven to be of particular use also
& Simonetti, 1996.
8
end-member has been referred to as the East
African Carbonatite Line (EACL, Fig. 4) (Bell
& Blenkinsop, 1987). A similar mantle mixing
line has also been identified for carbonatites
and associated alkaline silicate rocks from
the Kola Peninsula (Fig. 5) and is called the
Kola Carbonatite Line (KCL; Kramm, 1993;
Dunworth & Bell, 2001). However, deviations
from this common feature also exist and
carbonatites from China for example appear
to be derived from a completely different and
enriched lithospheric mantle source (Fig. 6; cf.
Ying et al, 2004).
Figure 6. MORB and OIB field compared to EACL (Bell & The associated alkaline silicate
Blenkinsop, 1987), Jacupiranga carbonatites (Huang et al., rocks in the East African Rift System or
1995), China carbonatites (Ying et al., 2004) and Italian car-
elsewhere however, do not show data that are
bonatites (Stoppa & Wolley, 1997). From Ying et al. (2004).
so straightforward and a number of alternative
models have been suggested also for the
when trying to characterize the composition generation of these rocks, including small
of the upper mantle under continents as degrees of partial melting of metasomatized,
carbonatites, due to their high Nd- and Sr- heterogeneous mantle, crustal contamination
contents effectively buffer against any effects and rheomorphism of fenites at crustal levels
of crustal contamination (Bell & Blenkinsop, (Kramm, 1994). Bell & Simonetti (1996)
1987). Pb-isotope ratios on the other hand can be pointed out that of all silicate rocks associated
used to monitor crustal contamination (because with carbonatites in Africa, phonolites show
the amount of Pb in mantle derived melts the greatest scatter in the εNd–εSr diagram.
generally is very low: Andersen, 1987) and to This requires a genesis model involving a third
characterize the mantle source in cases where component such as lower crustal granulites,
crustal contamination is not an issue. One of in addition to HIMU and EM1. The preferred
the most fundamental questions of carbonatite model for the generation of the natrocarbonatites
research today is whether the parental melts of and associated silicate rocks of Oldoinyo
carbonatites are derived from the astenosphere Lengai is that both the are produced by mixing
or the lithosphere or if they represent mixtures of of two mantle components (HIMU and EM1),
these components. Many recent studies (Bell & with the involvement of a third component to
Simonetti, 1996; Bell & Tilton, 2001; Simonetti support the generation of the silicate rocks (Bell
et al, 1998; Veena at al, 1998 among others) & Simonetti, 1996). The HIMU component is
have shown that carbonatites are indeed mantle attributed to a plume or “thermal cell” interacting
derived and that a component of astenospheric with the base of the sub-continental lithosphere
material is present in many carbonatite and and causing metasomatism. This provides a
alkaline silicate rocks. Radiogenic isotope data mechanism for generating nephelinitic and
from young carbonatites have shown that they carbonatitic melts through low-degree partial
are also very similar to OIB (Ocean Island melting. This type of two-stage model for the
Basalts), leading to the hypothesis that young generation of alkaline and carbonatitic magmas
carbonatites from East Africa are mixtures has also been proposed for the Deccan alkaline
between two principal mantle components province (Simonetti et al, 1998) and for the
(Bell and Dawson, 1995), i.e. HIMU and EM1 Sung Valley carbonatite complex, India (Veena
(HIMU=high 238U/204Pb, EM1=Enriched Mantle et al, 1998). The HIMU components in these
1, high Rb/Sr and low U/Pb and Sm/Nd). These two cases are attributed to the Réunion and
mantle components were originally defined on Kerguelen-Heard plume respectively, and in
the basis of Nd, Sr and Pb isotope ratios in OIBs. the case of Sung Valley the model involves
The young carbonatites of East Africa show mixing with an EM2-type mantle (=Enriched
such a consistency in isotopic data that the line Mantle 2, higher Rb/Sr than EM1 plus high
they represent plotting between the two mantle
9
U/Pb and low Sm/Nd). Isotope data from to mineralogy and mineral chemistry, a detailed
intrusive alkaline rocks also record complex petrographic and major- and trace element
evolution histories (like the silicate rocks at study of clinopyroxene and Ti-andradite from
Oldoinyo Lengai) and these data must therefore pyroxenite, ijolite and nepheline syenite was
be interpreted cautiously. The fact that many undertaken at the beginning of the project
silicate rocks of a complex show significantly (Paper I). The overall aim of this study was to
more scatter with respect to isotope data than evaluate the genetic relationships of these rocks
the carbonatites also means that establishment by processes such as fractional crystallisation,
of genetic relationships between the two, such possible contamination and magma mixing.
as liquid immiscibility, may become ambiguous During this study, important accessory phases
and should be evaluated carefully. were also chemically and petrographically
characterized. In doing so, it was recognized
that accessory titanite sometimes contained
The Alnö Complex
substantial amounts of, from a petrogenetic
point of view, important trace element such
Specific aims of the project
as Nb, Zr and LREE. This led to a separate,
A detailed petrogenetic study of the Alnö
thorough investigation of titanite with the aim
complex on the central Swedish east coast was
of detecting any compositional differences of
initiated by docent Viorica Morogan, then at
titanite between different rocks (e.g. pyroxenite,
the Department of Geology and Geochemistry
ijolite and nepheline syenite) and to evaluate its
at Stockholm University, in 1999. The key
petrogenetic significance (Paper II).
questions addressed in, and the specific aims of,
One of the most fundamental issues
this thesis are:
for the understanding of carbonatite genesis is
first to constrain what constituents observed
a) petrogenesis of the alkaline silicate rocks at
in a carbonatite have actually crystallized
Alnö (Papers I, II and V)
within the carbonatite magma and which are
potentially derived from surrounding wall-
b) petrogenesis of carbonatites at Alnö (Papers
rock. This has important implications since
III and V)
assimilated phases may substantially alter the
chemical composition of carbonatite magma.
c) evaluation of element and fluid transport
It also has implications for the interpretation
within and around an alkaline-carbonatite
of liquid immiscibility relationships between
intrusion (Paper IV)
carbonatites and associated silicate rocks
based only on petrography and mineral
d) evaluation of mantle source characteristics and
chemistry (identical liquidus mineralogies in
parental magma(s) and numerical modelling of
the immiscibly separated liquids, cf. Treiman
crystal fractionation and crustal contamination
and Essene, 1985). For this reason, a detailed
in the genesis of the Alnö rocks (Paper V)
petrographic and mineral chemistry study was
undertaken on three selected carbonatite dykes
Understanding the genesis of carbonatite
from Alnö in order to evaluate the mineralogical
magmas requires also that we understand the
contributions from surrounding wall-rock and
cause for the enormous compositional variations
to discuss the implications of this (Paper III).
displayed by their associated silicate rocks.
The importance of fluid-rock
Failure to do so, may lead to oversimplifications
interactions around alkaline-carbonatite
and misinterpretation of the relationships.
complexes is evidenced in the extensive fenite
Several studies of carbonatite genesis have short-
aureoles surrounding almost all reported
comings, the most serious being overlooking the
occurrences of this association. Such extensive
associated silicate rocks. The carbonatites of a
interaction transports elements from the intruding
complex are often thoroughly investigated while
magmas into adjacent wall rock and processes
associated alkaline silicate rocks in some cases
involved have been thoroughly studied in many
are only given brief mention. In order to fully
places (Morogan 1994 and 1989; Morogan
characterize the voluminously most important
& Woolley, 1988; Morogan & Martin, 1985;
silicate rocks of the Alnö complex with respect
10