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JOURNAL OF PETROLOGY
VOLUME 41
NUMBER 12
PAGES 1743–1757
2000
Crystal Fractionation and the Evolution of
Intra-plate hy-normative Igneous Suites:
Insights from their Feldspars
H. NEKVASIL∗, A. SIMON AND D. H. LINDSLEY
DEPARTMENT OF GEOSCIENCES AND THE CENTER FOR HIGH PRESSURE RESEARCH, STATE UNIVERSITY OF
NEW YORK, STONY BROOK, NY 11794-2100, USA
RECEIVED JANUARY 14, 1999; REVISED TYPESCRIPT ACCEPTED APRIL 26, 2000
Feldspars and normative feldspar constituents of bulk magma show
trends supportive of fractional crystallization in the three main types
of hy-normative intraplate suites that contain qz-oversaturated
rocks: ocean island and continental alkalic suites, ocean island
tholeiitic suites and continental tholeiitic suites. These suites are
characterized by the presence of a single feldspar in each suite
member, a shift of this feldspar from plagioclase to alkali feldspar,
and K enrichment of alkali feldspar with decreasing temperature in
the trachytic members. The modal feldspars provide evidence for a
reaction relationship between feldspars and indicate a build-up
of magmatic volatile content towards saturation with progressive
fractionation of a parental magma having low initial volatile content.
The feldspar and normative feldspar evolutionary paths are unique
for each of the three suite types but similar for different suites within
the same type. This characteristic extends to the felsic members,
making it easy to distinguish between rhyolitic or granitic rocks
from the different suite types. The feldspars in natural volcanic
suites commonly show evidence for a polybaric history, particularly
in the least-evolved suite members. Late-stage feldspars of the
intermediate members and feldspars of the most evolved members
show paths indicative of significantly lower temperature and pressure
regimes.
Significant igneous activity, as manifested by intraplate
oceanic volcanic sequences, continental bimodal volcanism, alkalic volcanic sequences, massif anorthosite
complexes and ‘anorogenic’ granites, is associated with
hotspot and continental rift environments. A major characteristic of this magmatism is the production of suites
of compositionally diverse rocks that are temporally and
spatially associated. Much is still unknown about the
processes that have led to the diversity of magmas within
such suites or to the global diversity of intra-plate suite
types.
Many ocean island and continental hotspot (and continental rift) related suites are characterized by rocks with
high alkali/silica ratios that fall into the alkalic field of
Irvine & Baragar (1971; Fig. 1) regardless of their state
of silica saturation. Such alkalic suites range from those
that are strongly silica undersaturated, containing rocks
such as nephelinites (or leucitites in the more potassic
suites), to mildly silica-undersaturated suites [the Kennedy series of Miyashiro (1978)] containing alkali basalts,
silica-undersaturated trachytes and phonolites, and,
finally, to hy-normative suites [the Coombs trend of
Miyashiro (1978)] that include olivine basalts, trachytes
and rhyolites. Hy-normative alkalic suites that contain
silica-oversaturated rocks on ocean islands are widespread
and include those from Ascension Island (e.g. Harris,
1983), the Samoan Islands (MacDonald, 1968), Terceira,
Azores (e.g. White et al., 1979) and Clarion Island, Mexico
(Bryan, 1967).
Some hy-normative suites of the ocean islands that
contain silica-oversaturated members are subalkalic (Fig.
1) and show the classic tholeiitic Fe-enrichment trend
(e.g. the Thingmuli volcanic suite, Iceland, Carmichael,
1964; the Alcedo and Pinzon suites, Galapagos, Baitis &
Lindstrom, 1980; Geist et al., 1995; eastern Canary
∗Corresponding author. E-mail: [email protected]
 Oxford University Press 2000
KEY WORDS:
alkalic; intra-plate; feldspars; fractionation; suites
INTRODUCTION
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VOLUME 41
Fig. 1. Variation of total alkalis with silica in lavas of the hy-normative
alkalic suites of Ascension Island (Harris, 1983), Clarion Island, Mexico
(Bryan, 1967) and Terceira, Azores (Schmincke, 1973; Self & Gunn,
1976; White et al., 1979; Mungall & Martin, 1995), in lavas of the
tholeiitic ocean island suites of Thingmuli, Iceland (Carmichael, 1964)
and Volcan Alcedo and Pinzon, Galapagos (Baitis & Lindstrom, 1980;
Geist et al., 1995) and in lavas of the transitional Easter Island suite
(Haase et al., 1997). Average calc-alkaline rocks from Best (1995) are
shown for reference by the crosses. The continuous line represents the
alkalic–tholeiitic boundary of Irvine & Baragar (1971).
Islands, Schminke, 1973). Such suites contain rocks that
range from olivine tholeiites to ferrobasalts and sodic
rhyolites. At any selected silica content of an intermediate
or felsic member, these tholeiitic rocks differ from those
of the alkalic suites not only by their lower alkali contents,
but also by their higher MgO, CaO and total iron
contents (Fig. 2). Some suites, like that of Easter Island
(Haase et al., 1997; Fig. 2), exhibit compositional characteristics intermediate to the hy-normative alkalic and
tholeiitic trends.
Hy-normative alkalic rocks of the continental hotspot
regions (e.g. the Nandewar Volcano, N.S.W., Australia,
Abbott, 1969; Stolz, 1985) share many features with the
hy-normative ocean island alkalic suites that reach silicaoversaturated compositions (e.g. Ascension Island, Harris,
1983). Both display higher Na2O, lower CaO, higher
Al2O3 and higher total alkali contents (for a given silica
content) than lavas of the ocean island tholeiitic suites
(Figs 3 and 4). The Na2O/K2O ratios, however, can
vary significantly between different alkalic suites (e.g.
Nandewar and Ascension Island lavas).
A third distinctive type of hy-normative suite is restricted
to continental settings. It is characterized most commonly
by bimodal tholeiitic basalt–potassic rhyolite volcanism
[e.g. Yellowstone, Hildreth et al., 1981; Snake River Plain
(SRP), Stout & Nicholls, 1977; Leeman, 1982]. Only
rarely are lavas of intermediate composition associated
with these bimodal suites. The mafic rocks include highly
distinctive ferrobasalts with low silica contents (but which
are not necessarily ne-normative) and high Fe and alkali
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contents [e.g. Craters of the Moon suite (COM), Idaho,
Kuntz et al., 1992; Stout et al., 1994]. These ferrobasalts
are also associated with potassic trachytes and rhyolites.
Such units are classified as potassic by us not because of
exceptionally high K2O contents, but because of high
K2O/Na2O ratios. They are also termed ‘tholeiitic’ (despite the presence of many alkalic members within the
suites) because of the tholeiitic nature of the associated
least-evolved mafic units.
Figure 3a shows the variation of alkalis with silica in
Snake River Plain (SRP) basalts, rhyolites, and spatially
and temporally associated intermediate COM rocks.
These units form a trend with increasing mg-number
(Fig. 3b) that extends from the subalkalic field towards
lower silica contents and more alkalic compositions (Fig.
3a), and to higher-silica rocks that lack the strong sodium
enrichment typical of alkalic suites (Fig. 4a). This trend
begins at its low-silica end with a distinctive strong
decrease in SiO2 and concomitant increase in FeOT,
P2O5 and K2O abundance with decreasing mg-number.
Massif anorthosite or rapakivi-type granite suites and
large layered tholeiitic intrusions (e.g. Skaergaard) may
represent plutonic counterparts to the volcanic continental tholeiitic suites (Duchesne, 1990; Frost & Frost,
1997). Fine-grained dikes within anorthosite suites consist
of ‘high-Al’ gabbro, ferrodiorite ( jotunite) and monzonite. The associated syenites and potassic granites at
least in part reflect liquid compositions (e.g. in the Laramie Anorthosite Complex, Scoates et al., 1996). Although the associated fine-grained gabbros are commonly
subalkalic, the intermediate liquid compositions are typically alkalic (Figs 5 and 6). Figure 5a shows the similarities
between (1) high-Al gabbros of the Laramie Anorthosite
Complex (LAC), Wyoming and SRP basalts; (2) LAC
ferrodiorites and COM ferrobasalts; and (3) potassic
granites of the LAC and rhyolites of the Snake River
Plain. Syenites from the Sybille pluton (Figs 5 and 6)
contain up to 30 modal % of accumulated alkali feldspar
(Scoates et al., 1996; this accounts for their compositional
deviation from the COM trachytes towards higher K2O
and Al2O3 contents (Fig. 6b and f ).
The granites associated with massif anorthosites
strongly resemble ‘anorogenic’ granite batholiths such as
the Wolf River batholith, the main body of the Pikes
Peak Batholith (e.g. Emslie, 1978, 1991; Frost & Frost,
1997), and the wiborgite of the Fennoscandian regions
(e.g. Vorma, 1976). These similarities, shown particularly
well by the Na2O and CaO variations with silica (Fig.
7a and d), suggest common elements in the evolutionary
history of potassic granites and massif anorthosites. Likewise, similarities in the evolutionary history of late-stage
alkalic intrusions (e.g. of the Pikes Peak batholith, Barker
et al., 1975) and felsic rocks of the volcanic alkalic suites
(e.g. of the Nandewar suite, Fig. 7) are suggested by the
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Fig. 2. Variation of major oxides with silica in lavas of the hy-normative alkalic suites of Ascension Island, Clarion Island, Mexico, and Terceira,
Azores, in lavas of the tholeiitic ocean island suites of Thingmuli, Iceland, and Pinzon, Galapagos, and in the lavas of the transitional Easter
Island suite. Average calc-alkaline rocks from Best (1995) are shown for reference by the crosses. Symbols are described in the legend. Sources
of data are as in Fig. 1.
similarities in bulk compositions and mineral assemblages.
The role of crystal fractionation
Ocean islands
Crystal fractionation has been repeatedly called upon as
the mechanism for inducing compositional diversity in
ocean island lavas. This interpretation is aided by the
isotopic similarity among associated suite members (e.g.
Geist et al., 1995; Mungall & Martin, 1995; Kar et al.,
1998). Fractionation is supported by continuous major
and trace element trends within suites, continuous mineral compositional trends, preservation of specific trace
element characteristics of the basalts in the rhyolites
within a suite, and the results of a variety of petrogenetic
modeling efforts.
It has been proposed that fractionation of first plagioclase, titanaugite and olivine from a basalt, followed
by an olivine-free, magnetite-bearing assemblage and,
finally, a fayalite, ferroaugite, alkali feldspar and perhaps
kaersutite-bearing assemblage explains the salient geochemical features of the hy-normative alkalic suites from
the ocean islands (White et al., 1979; Harris, 1983; Haase
et al., 1997). Crystal fractionation is also considered the
dominant process producing the tholeiitic ocean island
associations that consist of olivine basalt, tholeiite, ferrobasalt, icelandite, oceanic dacite and oceanic rhyolite. In
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Fig. 3. (a) Variation of total alkalis with silica in lavas of the hynormative alkalic suites of the Nandewar Volcano, N.S.W., Australia
(Abbott, 1969; Stolz, 1985), Ascension Island (Harris, 1983) and Clarion
Island, Mexico (Bryan, 1967), in lavas of the tholeiitic ocean island
suites of Thingmuli, Iceland (Carmichael, 1964), and in the continental
tholeiitic suite of the Snake River Plain (SRP, Stout & Nicholls, 1977;
Leeman, 1982) and Craters of the Moon (COM), Idaho (Leeman et
al., 1976; Stout et al., 1994). The continuous line represents the
alkalic–tholeiitic boundary of Irvine & Baragar (1971). (b) Variation in
molar mg-number with silica content for the SRP basalts and COM
rocks. Sources of data are as in (a).
these rocks, however, olivine, rather than clinopyroxene,
plays a more dominant role during the early stages of
fractionation (e.g. Carmichael, 1964; Baitis & Lindstrom,
1980). Olivine followed by titanomagnetite crystallization
would enrich the liquid in silica until the olivine-free,
plagioclase-, diopsidic augite- and pigeonite-bearing assemblage of icelandite is attained. Fractionation of plagioclase would further enrich the liquid in alkalis and
silica to produce rhyolitic compositions (e.g. Carmichael,
1964; Baitis & Lindstrom, 1980).
Partial melting is a less commonly invoked process,
not only because of the inherent thermal problems of
melting basaltic crust, but also because it is inconsistent
with the observed trace element characteristics of many
suites (e.g. White et al., 1979; Geist et al., 1995; Mungall
& Martin, 1995).
Continental hotspots and rifts
There is much less of a consensus about relationships among suite members in continental settings,
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particularly for tholeiitic suites in which isotopic variability between members is characteristic (e.g. Laramie
Anorthosite Complex, Scoates et al., 1996; Craters of
the Moon, Idaho, Leeman et al., 1976) and may even
occur in units of the same lithology. Leeman et al.
(1976) and Stout et al. (1994) suggested a fractionation
relationship for the COM suite, but the relationship
of the COM rocks to the SRP basalts remains
unclear. The ubiquity of cumulate rocks in the plutonic
continental tholeiitic suites such as the massif anorthosite
suites is direct evidence for crystal accumulation, but
whether this process links associated rocks is still much
debated. Scoates et al. (1999) experimentally investigated
the relationship of associated rocks of the LAC at
9 kbar—a pressure appropriate for the aluminous pyroxene megacrysts common in least-evolved members
of continental tholeiitic suites. They determined that
fractionation of clinopyroxene + olivine + plagioclase
from an anhydrous high-Al gabbroic melt (compositionally similar to continental tholeiite, such as
from the SRP) could produce the silica depletion and
strong Fe enrichment before silica enrichment that
characterizes continental tholeiitic suites. If fractionation
of a high-Al gabbroic magma can generate the rocks
of continental tholeiitic suites, then it must occur
primarily at depths greater than that of emplacement
to account for the commonly observed isotopic heterogeneities. Such heterogeneities could perhaps arise by
interaction of discrete batches of fractionated magma
with isotopically different wallrock during ascent to the
final level of emplacement.
The relationship of rhyolites to basalts of continental
bimodal suites remains problematic. Nash et al. (1996)
pointed out the compositional similarities of the older
(>16 Ma) Yellowstone silicic tuffs to the interstitial glass
of the Columbia River Plateau basalts. This, along with
the unlikelihood of a wholly crustal source for many
potassic rhyolites (e.g. Hildreth et al., 1981), suggests at
least the possibility of crystal fractionation from basalt.
If the fractionation path yields rocks similar to those of
the SRP–COM suite, then the high Fe contents of the
intermediate magmas would make their eruption very
difficult (and such intermediate fractionation products
rare at the surface), thereby accounting for the commonly
bimodal nature of such suites.
Continental hy-normative alkalic suites containing qzoversaturated rocks (e.g. Tertiary and Quaternary volcanoes of the Massif Central, France, Downes, 1987; the
Miocene volcanic field of New South Wales, Australia,
Stolz, 1985; the Gardar Province, Southern Greenland,
Upton & Emeleus, 1987; silica-oversaturated volcanics
of the Kenya rift, MacDonald, 1987) retain many of the
characteristics of ocean island alkalic suites. As is the
case for ocean island alkalic suites, members of the
continental alkalic suites exhibit significantly less isotopic
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EVOLUTION OF INTRA-PLATE IGNEOUS SUITES
Fig. 4. Variation of major oxides with silica in SRP and COM lavas compared with those of the Nandewar, Thingmuli and Ascension Island
suites. Symbols are as described in legend. Sources of data are as in Fig. 3.
diversity than members of continental tholeiitic suites.
Crystal fractionation is commonly invoked to link associated mafic and intermediate rocks in continental
alkalic suites (e.g. Abbott, 1969; Dondolini & Nekvasil,
1999). The origin of the commonly associated peralkaline
rhyolites and granites, however, remains problematic.
They may arise from a combination of processes involving
crystal fractionation and compositional changes during
volatile exsolution (MacDonald, 1987; Upton & Emeleus,
1987).
Clearly, crystal fractionation is considered by many to
play a major role in the evolution of magmas from a
variety of hy-normative suites. As direct evidence for this
process is generally not available, indirect support for or
against it must come from a variety of directions. One
is the nature of mineralogical variations among associated
rocks, particularly of minerals that demonstrate strong
compositional sensitivity to intensive and extensive conditions and to evolutionary process. Feldspar is uniquely
suited for study of hy-normative suites in that it shows
strong compositional changes as the host changes from
mafic. Furthermore, its composition is highly sensitive to
the potassium content of the host magma, a compositional
variable that remains distinct for each of the suite types.
The discussion that follows focuses on the compositions
of feldspar from rocks of natural hy-normative suites
to evaluate the following: (1) whether the least-evolved
feldspars in rock units of hy-normative suites show any
trends that support a cogenetic origin of suite members;
(2) if any such trends are similar for suites of the same
type, and how different they are between the three
suite types; (3) whether feldspar compositions preserve
evidence for a multi-stage history (e.g. polybaric fractionation).
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Fig. 5. (a) Variation of total alkalis with silica in non-cumulate rocks
of the Laramie Anorthosite Complex, Wyoming (LAC), compared with
lavas from the SRP, COM and Nandewar. Sources of data: high-Al
gabbro, Mitchell et al. (1995); Maloin Ranch Pluton, Kolker & Lindsley
(1989); Sybille Pluton, Fuhrman et al. (1988), Scoates et al. (1996) and
J. S. Scoates (personal communication, 1999); ferrodiorite, Mitchell et
al. (1996); monzodiorite, Scoates et al. (1996); Sherman Granite, Zielinski
et al. (1981), Edwards (1993) and Frost et al. (1999). (b) Variation in
molar mg-number with SiO2 content. Arrows indicate the path of
decreasing mg-number. Data for the Nandewar, COM and SRP lavas
as in Figs 1 and 3.
THE EVOLUTION OF FELDSPAR
COMPOSITIONS IN MAGMAS OF
LOW BULK WATER CONTENT
Water-undersaturated conditions are prevalent in basalts
of all three types of hy-normative suites. Basalts from
ocean island and continental tholeiitic suites generally
contain less water (<1 wt %; e.g. Anderson & Brown,
1993; H. Nekvasil, unpublished data, 2000) than those
of the alkalic suites, which may reach >2 wt % in some
hawaiites (H. Nekvasil, unpublished data, 2000). Even
the higher water contents of the alkalic magmas are low
enough to ensure water-undersaturated conditions until
decompression to low pressures or until a significant
amount of fractionation has occurred at elevated pressure.
A large temperature interval of crystallization is expected if the water content starts low and builds up
towards saturation in the more evolved units. That the
more evolved rocks of hy-normative suites contain higher
volatile contents than their less evolved counterparts is
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DECEMBER 2000
readily seen in analyses of the rhyolites, comendites and
pantellerites of the alkalic suites (MacDonald, 1987), and
by the presence of hydrous minerals in the most evolved
units of the tholeiitic suites (e.g. Carmichael, 1964; Frost
et al., 1999). If fractional crystallization forms such suites,
then this increase in water content would arise through
fractionation under H2O-unbuffered conditions. During
H2O-unbuffered crystallization, the water content builds
up towards saturation as anhydrous minerals precipitate.
Under H2O-buffered conditions, changes in the activity
of water during crystallization are greatly restricted. This
is caused by precipitation of hydrous minerals or exsolution of a large amount of a fluid phase.
During H2O-unbuffered crystallization, feldspar compositions can change markedly because of the large
crystallization temperature interval. If the melts are still
hot when the feldspar reaches the compositional region
around the ‘nose’ of the ternary feldspar solvus, ternary
feldspar will form. Changes in ternary feldspar compositions are sensitive not only to small changes in
temperature and melt composition (Nekvasil, 1990), but
also to H2O-unbuffered or H2O-buffered conditions
(Nekvasil, 1994). If the melt is saturated with two ternary
feldspars, and if a peritectic rather than a coprecipitational (cotectic) relationship exists between the
coexisting feldspars, feldspar compositional evolution
takes on particularly distinctive characteristics (Nekvasil,
1992b). In the peritectic region of the two-feldspar multiple-saturation field of a multicomponent system, sequences of ternary feldspar compositions that are
generated by equilibrium crystallization are very different
from those generated by fractional crystallization
(Nekvasil, 1994).
Figure 8 shows calculated feldspar lines-of-descent for
a model trachyte (compositionally within the four-component system Ab–Or–An–H2O) of low bulk water content, during H2O-unbuffered and H2O-buffered
equilibrium and fractional crystallization [modified from
Nekvasil (1994)]. Both the equilibrium and fractional
crystallization cases assume equilibrium between coexisting phases. For equilibrium crystallization, the precipitating phases remain in contact with melt; for
fractional crystallization, minerals are removed from the
melt soon after they crystallize. The major differences in
the paths of feldspar evolution at low bulk H2O content
(Fig. 8) result from odd (peritectic) and even (eutectic)
regions along the plagioclase–alkali feldspar–L (PAL)
surface. At low water contents, saturation with alkali
feldspar can occur in either the cotectic or the peritectic
region of the PAL surface. In the former case, upon
reaching saturation in two feldspars, these will co-precipitate until the peritectic region is reached. In the latter
case (and also if the melt reaches the peritectic portion
of the PAL surface), alkali feldspar is produced only by
reaction between plagioclase and liquid. If plagioclase
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NEKVASIL et al.
EVOLUTION OF INTRA-PLATE IGNEOUS SUITES
Fig. 6. Variation of major oxides with silica for non-cumulate rocks of the LAC compared with lavas of the SRP and COM. Symbols as
described in legend. Sources of data as in Figs 3 and 5. Arrows indicate the path of decreasing mg-number.
crystals remain available for reaction during equilibrium
crystallization, the melt will stay on the PAL surface (Fig.
8a and b); once plagioclase is used up, only alkali feldspar
precipitates. During ideal fractional crystallization (i.e.
crystals are removed immediately upon precipitation),
the melt leaves the PAL surface as soon as the peritectic
region is reached and alkali feldspar crystallizes instead
of plagioclase (Fig. 8c and d).
During the final stages of H2O-unbuffered equilibrium
crystallization at pressures above >5 kbar in the simple
haplogranite system (Luth et al., 1964) and the low
temperatures of nearly H2O-saturated conditions, the
melt may reach the cotectic region of the albitic part of
the PAL surface (dark gray curves, Fig. 8a and c). If two
feldspars are still present (e.g. Fig. 8a), the reaction
between plagioclase and melt is replaced by coprecipitation of both feldspars. If only alkali feldspar is
present at this stage (e.g. Fig. 8c), its solidus intersects
the wide low-temperature portion of the ternary feldspar
solvus and two feldspars will be in the final assemblage.
These feldspars will have the compositions of those at
the ternary eutectic of the haplogranite system.
Alkali feldspar that crystallizes under H2O-unbuffered
conditions in melts with low H2O content undergoes a
period of Or enrichment because of the continual expansion of the solvus with decreasing temperature. This
effect is enhanced at higher pressures because of greater
water solubility and lower solidus temperature. The expansion of the solvus also affects the composition of
plagioclase that is in equilibrium with alkali feldspar
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Fig. 7. Variation of major oxides with silica for sodic and potassic rocks of the Pikes Peak (PP) Batholith and Wolf River (WR) Batholith
compared with those of the LAC and Nandewar. Sources of data: PP—Gross & Heinrich (1965), Barker et al. (1975), Wobus & Anderson (1978),
Simmons et al. (1986), D. Smith (personal communication, 1997); WR—Van Schmus et al. (1975) and Anderson & Cullers (1978); LAC and
Nandewar rocks as in Figs 3 and 5.
during H2O-unbuffered equilibrium crystallization (Fig.
8a). Its composition decreases in Or content with decreasing temperature as the melt crosses the PAL surface.
Although this model system is limited to the fivecomponent system Ab–Or–An–Qz–H2O, feldspars from
numerous rock suites follow one or more of these paths.
To a first approximation, additional components act
mainly as diluents that depress melting temperatures but
do not affect the main compositional trends of the model
system. Relating the model results to natural feldspars
requires a further consideration. Crystal fractionation
obviously removes crystals from contact with the melt
from which they crystallized. Under conditions of ideal
fractionation, either the liquid or crystals would be absent,
and the rocks would represent either accumulated phases
or removed liquids. However, it is likely that in nature
fractionation is stepwise and polybaric, with periods of
in situ crystallization punctuated by removal of batches
of liquid (with perhaps some entrained crystals). Therefore, the phenocrysts in any specific unit of a fractionated
suite may reflect the cooling history during the last in situ
crystallization stage of the fractionation process, rather
than represent relict grains entrained in the liquid during
a stage of liquid removal at higher pressure. In other
words, although a bulk magma (crystals + liquid) may
represent the composition of a liquid derived by crystal
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EVOLUTION OF INTRA-PLATE IGNEOUS SUITES
Fig. 8. Generalized evolutionary paths of liquid (L), plagioclase (P)
and alkali feldspar (A) of a model trachyte of low bulk water content
in the system Ab–Or–H2O undergoing H2O-unbuffered (a, c) or H2Obuffered (b, d) equilibrium or fractional crystallization. Black curves in
(a) and (c) show the feldspar paths anticipated for pressures below
>5 kbar; the dark gray curves are for >5 kbar or higher. Light gray
curve indicates the path of liquid evolution at the lower pressure.
Dashed lines connect coexisting feldspars. Diagrams are modified from
Nekvasil (1990, 1992b, 1994) and Brown (1993).
fractionation at a greater pressure, the minerals actually
seen in the respective lava or plutonic body would not
necessarily represent the fractionated mineral assemblage.
Only the compositionally least-evolved (i.e. highest-temperature) mineral phases in a lava could represent accidentally entrained phases that reflect the mineral phases
involved in the higher-pressure fractionation. This is of
particular relevance when using phenocryst assemblages
to model liquid lines of descent arising from polybaric
fractionation, and should be kept in mind during the
ensuing discussion.
FELDSPAR–LIQUID RELATIONS IN
OCEAN ISLAND SUITES
Tholeiitic suites
Normative feldspar constituents of lavas from the ocean
island tholeiitic suites of Pinzon, Galapagos (Baitis &
Lindstrom, 1980) and Thingmuli, Iceland (Carmichael,
1964) projected into the system Ab–Or–An define a tight
trend with falling temperature across the plagioclase
saturation surface toward the Ab–Or sideline (gray arrows, Fig. 9). Different ocean island tholeiitic suites deviate little from this path. Even lavas from the
‘transitional’ Easter Island suite show a trend indistinguishable from that of Thingmuli and Pinzon. This
suggests that compositional differences like those shown
in Figs 1 and 2 among ocean island tholeiitic suites
Fig. 9. Projected normative feldspar constituents of bulk lavas (not of
the glass coexisting with feldspar) from the tholeiitic suites of Thingmuli,
Iceland (Carmichael, 1964, 1967) and Pinzon, Galapagos (Baitis &
Lindstrom, 1980) and from the ‘transitional’ Easter Island lavas (Haase
et al., 1997). Gray arrows show the generalized liquid line-of-descent
of the bulk lavas. The most An-rich feldspar from each lava from
Thingmuli and Easter Island is shown by the black symbols. The black
arrows were constructed such that the back-tangent to the liquid path
of descent at any particular lava intersects the feldspar line-of-descent
at the composition of the highest-temperature feldspar present in the
lava. Solvus isotherms calculated using the thermodynamic model of
Elkins & Grove (1990), and the program SOLVCALC (Wen & Nekvasil,
1994) are shown for 1 kbar and 1000–800°C. A portion of the consolute
curve of the solvus at this pressure is shown by the bold gray curve.
The calculated 5 kbar plagioclase (P)–alkali feldspar (A)–liquid (L)
surface in the system Ab–Or–An–H2O from Nekvasil (1992a) is outlined
by the dry and H2O-saturated limiting curves (fine gray lines).
have little effect on the evolution of normative feldspar
compositions.
Compositions of the highest-temperature modal feldspar (as indicated by phenocrysts or phenocryst cores) in
each lava flow from Thingmuli (Carmichael, 1967) range
from bytownite to oligoclase (Fig. 9). The feldspar path
(as well as the liquid path, Fig. 9) is fully consistent with
crystal fractionation of a parental magma of low K2O
content. Such magmas should contain plagioclase as the
sole feldspar over much of the crystallization interval
(Fig. 9; Brown, 1993; Nekvasil, 1994). Alkali feldspar
would appear only at the end-stages of fractionation as
either the sole feldspar or with plagioclase (if the solvus
were intersected) depending upon the pressure. At low
pressures, saturation with alkali feldspar would not occur
until the temperature was below 800°C.
Hy-normative alkalic suites
The normative feldspar constituents of lavas from
Ascension Island (Harris, 1983) and Terceira, Azores
(Schmincke, 1973; Self & Gunn, 1976; White et al., 1979;
Mungall & Martin, 1995) form a trend that extends from
compositions more potassic than those of tholeiitic suites
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Fig. 10. Projected normative feldspar constituents of the alkalic lavas
and least-evolved feldspars of Ascension Island (Harris, 1983) and
normative feldspar constituents of lavas from Terceira, Azores
(Schmincke, 1973; Self & Gunn, 1976; White et al., 1979; Mungall &
Martin, 1995). The black arrows indicate the feldspar line-of-descent
based on the highest-temperature feldspar within each lava. The gray
arrows indicate the feldspar-line-of descent within an individual lava.
Feldspar–liquid relations of lavas from Thingmuli, Iceland (Carmichael,
1964) and Pinzon, Galapagos (Baitis & Lindstrom, 1980) are shown
for comparison. Solvus isotherms calculated using the thermodynamic
model of Elkins & Grove (1990), and the program SOLVCALC (Wen
& Nekvasil, 1994) are shown for 10 kbar and 1000–800°C. A portion
of the consolute curve of the solvus at this pressure is shown by the
bold gray curve.
(e.g. Pinzon) at high normative An content, to compositions that are virtually indistinguishable (Fig. 10)
from those of tholeiitic suites at low An contents. The
highest-temperature modal feldspar of each lava, when
taken over the entire suite, forms a trend (black arrows
in Fig. 10) different from that of the tholeiitic suites.
Each lava contains only a single feldspar; the feldspar
changes from plagioclase in the less evolved units to alkali
feldspar in the more evolved units. The details of feldspar
zoning at the trachytic stage—the highly informative part
of the crystallization history—are complex because of
the presence of exsolved ternary feldspars. However,
Harris (1986) described cognate quartz syenite blocks
that contain feldspars with andesine–oligoclase cores and
anorthoclase rims, as well as granitic blocks that contain
cryptoperthite more potassic than anorthoclase. These
features suggest reverse zoning of alkali feldspar during
the trachytic stage, a trend that is shown by the black
arrows in Fig. 10.
The presence of a single least-evolved feldspar that
changes from plagioclase to alkali feldspar (at the dotted
line in Fig. 10) supports fractional crystallization at low
bulk magmatic water contents (Fig. 8c) as a process
leading to diversity of Ascension Island magmas. If the
phases in the plutonic ejecta accumulated during stepwise
crystal fractionation, then the anorthoclase-rimmed oligoclase is direct evidence for a peritectic relationship
NUMBER 12
DECEMBER 2000
between the feldspars. The trend of the highest-temperature feldspars suggests that the temperature of intersection of the solvus, and thus the onset of this peritectic
reaction, may be as high as 850°C. The cryptoperthitic
alkali feldspar probably crystallized soon after this stage,
while temperatures were still high enough to form strongly
ternary feldspars. The higher potassium content of the
cryptoperthite relative to the anorthoclase implies H2Ounbuffered conditions (Fig. 8c), as is also suggested by
the volatile-rich nature of the late-stage rhyolites.
Although the projected trend of least-evolved feldspars
in the alkalic suites shows a fractionation topology (Fig.
10), this need not imply that these least-evolved feldspars
were actually involved in the supposed fractionation
events that led to the diversification of the magmas within
the suite. They might instead form during the earliest
stages of low-pressure differentiation of batches of magma
that have already been produced at higher pressures
by differentiation of what may even be a feldspar-free
assemblage. As low-pressure liquidus phases, the feldspars
would reflect the fractionation relationship of the bulk
magmas even if they played no role in the actual fractionation events. The experiments of Dondolini & Nekvasil (1999) support this possibility. They determined that
the best match between bulk lava compositions and
experimental fractionation-derived hy-normative alkalic
liquids occurs at >9 kbar under ‘no volatiles added’
conditions (>2 wt % bulk H2O content). At this pressure
kaersutite dominates the fractionating assemblage and
plagioclase does not appear until the very last (trachytic)
stages of crystallization. If feldspar were among the first
phases to crystallize at a lesser pressure from the melts
derived by stepwise fractionation at 9 kbar, the firstformed feldspar compositions, taken over all successive
batches of melt, would describe a path that merely
emulated fractionation, owing to the relationship of the
bulk lava compositions. Further cooling of each magma
at the lower pressure of feldspar saturation would result
in feldspar lines-of-descent for the magma that would
deviate from the main path of least-evolved feldspars
across the suite. Such deviations are readily seen in alkalic
suites and exemplified by feldspars from Ascension Island
lavas (Fig. 10), and suggest significantly lower temperatures (and hence, lower pressure) than the main
fractionation event. The low-temperature feldspar linesof-descent are similar to the lines-of-descent shown by
feldspars of the more evolved members of tholeiitic suites
(Fig. 9).
FELDSPAR–LIQUID RELATIONS IN
CONTINENTAL SUITES
Hy-normative alkalic suites
Hy-normative alkalic suites from continental settings vary
from low-K sodic ones (e.g. Aden and Little Aden, Cox
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NEKVASIL et al.
EVOLUTION OF INTRA-PLATE IGNEOUS SUITES
Fig. 11. Projected normative feldspar constituents of alkalic lavas
and compositions of highest-temperature crystalline feldspars from the
Nandewar lavas from Stolz (1985, 1986). Lavas from Ascension Island
(Harris, 1983) and Aden and Little Aden (Cox et al., 1970) are shown
for comparison. Black arrows indicate the main path of feldspar
evolution based on more complete data from Stolz (1986). The dotted
line indicates the compositions of plagioclase rimmed by alkali feldspar
in a mafic trachyte. Gray arrows indicate the possible line of descent
of the groundmass feldspars of the hawaiites and trachyandesites. Solvus
isotherms and consolute curve are as in Fig. 10. A portion of the
consolute curve of the solvus at this pressure is shown by the bold gray
curve. The calculated 5 kbar plagioclase (P)–alkali feldspar (A)–liquid
(L) surface in the system Ab–Or–An–H2O from Nekvasil (1992a) is
outlined by the dry and H2O-saturated limiting curves (gray lines).
Some feldspars from mafic trachytes are zoned from
Ab-rich cores to Or-rich rims. This trend is extended to
more potassic compositions by groundmass feldspar
(Stolz, 1986). Feldspars of peralkaline trachytes and comendites also show K enrichment, albeit at temperatures
close to 100°C lower, with compositions reaching Or50An2
before reversing. Late-stage feldspars of the alkali rhyolites
and comendites become more albitic and close to binary
(Or38An0).
Both feldspar paths of the Nandewar suite indicate the
presence of a single stable feldspar throughout much of
the crystallization history. This direct evidence for a
reaction relation between plagioclase and liquid, together
with reverse zoning of alkali feldspar, is suggestive of
H2O-unbuffered fractional crystallization at low initial
water contents (Fig. 8c) for each path. However, as
discussed for Ascension Island lavas, the first-formed
feldspar need not have been part of the fractionating
assemblage at depth. The low mg-number of 65 for
the hawaiite suggests that it is not a primitive melt.
Furthermore, at the pressure of best agreement between
experimentally produced liquids and bulk lavas of the
suite (9 kbar, Dondolini & Nekvasil, 1999, unpublished
data, 2000) plagioclase is not a liquid phase and does
not appear until trachytic melts are produced.
et al., 1970) that resemble the ocean island suites of
Ascension Island (Harris, 1983) and of Terceira, Azores
(Schmincke, 1973; Self & Gunn, 1976; White et al., 1979;
Mungall & Martin, 1995) to potassic ones such as the
Nandewar volcano (Stolz, 1985). Both are shown in Fig.
11. The more potassic Nandewar lavas contain more
potassic feldspars. Within the suite of hawaiites, trachyandesites, tristanites and mafic trachytes, least-evolved
feldspars show continua of compositional evolution
around the nose of the feldspar with little evidence for
stable coexistence of two feldspars at any stage of cooling
(Stolz, 1986). For the most part, the feldspar line-ofdescent for each lava is superimposed upon the trend of
least-evolved feldspars across the suite to mafic trachytes,
with differences only in the abundance of specific feldspars
and in the compositions of least-evolved feldspars. Exceptions to this occur for several trachyandesites in which
plagioclase phenocrysts show strong reverse zoning and
the groundmass phenocrysts display higher An contents
than the phenocryst cores (Stolz, 1986). These reversely
zoned plagioclase phenocrysts coexist with partially resorbed aluminous subcalcic augite and aluminous bronzite, both of which are absent from the groundmass.
These textural features and mineral assemblages record
a major decompression event (e.g. Fram & Longhi, 1992)
in some of the least-evolved lavas. This lower-pressure
trend continues in feldspars from units more felsic than
mafic trachyte (Fig. 11).
Least-evolved feldspars of the continental tholeiitic suite
represented by SRP basalts and rhyolites and COM lavas
(Stout et al., 1994) form a trend similar to that of the hynormative alkalic suites (Fig. 11), albeit with more extreme K enrichment of feldspar compositions. Only a
single feldspar is present in each lava. This feldspar
changes from potassic plagioclase in basalt, to highly
ternary feldspar in trachyte and, finally, to more albitic
low-Ca alkali feldspar in rhyolite. Normative feldspar
constituents of the lavas also follow a path similar to that
of alkalic suites, except at the lower-temperature stages
of evolution. The less evolved SRP and Yellowstone
rhyolites are clearly more potassic than those of alkalic
suites, and this is reflected in the higher temperature of
alkali feldspar saturation (close to 1000°C for the COM
suite, Fig. 12). Both the paths of least-evolved feldspars
and liquid are consistent with crystal fractionation at low
bulk water contents under H2O-unbuffered conditions.
Just as for the hy-normative alkalic suites, feldspar rims
and groundmass of SRP basalts show evidence for a
secondary path that trends to significantly lower Or
contents (Fig. 12). Between the cores and rims of feldspars
from basalt to tristanite lavas is commonly a region with
higher An and lower Or than the cores of the phenocrysts
(Stout et al., 1994). The lower Or content is also found
in phenocryst rims and groundmass feldspar (Fig. 12).
Continental tholeiitic suites
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VOLUME 41
Fig. 12. Feldspar–melt relations for the SRP basalts and rhyolites
(Stout & Nicholls, 1977; Leeman, 1982), COM lavas (Leeman et al.,
1976; Stout et al., 1994) and Yellowstone rhyolites (Hildreth et al., 1981).
The SRP–COM liquid line-of-descent is shown by the dark gray
arrows. The solid black arrows connect the cores of feldspar phenocrysts
within the lavas. The thin black arrows show the compositional evolution
of feldspars’ rims and groundmass feldspar in basalts through tristanites
(Stout et al., 1994). Shown for comparison are normative feldspar
contents of non-cumulate rocks of the LAC. Data from Scoates et al.
(1996) and Frost et al. (1999). Other features are as in Fig. 11.
This secondary path is consistent with a late-stage decompression event. The dominance of plagioclase and
olivine phenocrysts in the SRP and COM basalts, yet
the impossibility of obtaining the major element trends
observed in the lavas (Fig. 4) by fractionation of these
phases without also fractionating a significant amount
of clinopyroxene (e.g. Thompson, 1972), lends further
support for a secondary low-pressure crystallization event
that led to the formation of most of the observed
phenocrysts.
The normative feldspar compositions of the finegrained rocks of the Laramie Anorthosite Complex and
its associated granitic rocks form a trend that resembles
that of the SRP–COM (Fig. 12). Intermediate compositions of the former, however, show slightly higher
Or contents than do the COM trachytes. This may
reflect both higher-temperature and higher-pressure conditions for the LAC rocks, consistent with the plutonic
nature of the LAC rocks and extrusive nature of the
COM suite. This difference is less clear for the most
evolved rocks. However, the overall similarities suggest
that a similar process induced the compositional diversity
for both the plutonic and volcanic suites.
The intermediate and silicic rocks of the continental
tholeiitic suites are clearly more potassic than are those
of the alkalic suites. This difference discriminates between
silicic members of alkalic and continental tholeiitic suites
if both types are present or only a few lithologies are
exposed. The Pikes Peak batholith presents a good example of both cases. The main unit consists predominantly of granite, whereas the late-stage units are a
NUMBER 12
DECEMBER 2000
Fig. 13. Normative feldspar compositions of sodic and potassic rocks
of the Pikes Peak Batholith compared with those the LAC and Nandewar
rocks. The sodic and potassic liquid lines-of-descent are indicated by
the black and gray arrows, respectively. Sources of data: PP—Gross &
Heinrich (1965), Barker et al. (1975), Wobus & Anderson (1978),
Simmons et al. (1986), D. Smith (personal communication, 1997);
LAC—Fuhrman et al. (1988), Kolker & Lindsley (1989), Mitchell et al.
(1995), Scoates et al. (1996) and Frost et al. (1999).
variety of alkalic rock types, including granite (Barker et
al., 1975). The normative feldspar constituents of the
whole rocks clearly differentiate between members of the
two suite types (Fig. 13). The sodic members (Noblett et
al., 1993) lie along the alkalic trend exemplified by the
Nandewar lavas (Fig. 13), but the potassic units lie along
the LAC trend. Also shown in Fig. 13 are rocks from the
Honkajoki Rapakivi Granite Complex (HC) of southern
Finland (Rämö, 1986). The rocks with intermediate An
contents in the HC complex display higher normative
Or contents than do the main units of the LAC. Some
members of both the COM and the LAC units show
this degree of Or enrichment (Figs 11 and 13). The highK character of the mafic rocks from the HC is preserved
in the granites. These similarities in trends of normative
feldspar constituents between the rocks of anorthosite
and rapakivi-type granite suites suggest a common evolutionary history (e.g. Frost & Frost, 1997).
SUMMARY
Trends of both feldspar compositions and bulk magma
normative feldspar constituents in the three main types
of hy-normative magmatic suites reflect the uniqueness
of peritectic vs cotectic relations between feldspars and
liquids in magmas with low bulk water content. Because
these are unlikely to reflect random correlation, the
trends are consistent with (albeit not proof of ) polybaric
fractional crystallization as the primary process that induces diversity in basaltic through intermediate members
of hy-normative intraplate suites. Furthermore, the continuous nature of the trends between intermediate rocks
(e.g. trachytes) and felsic rocks in many suites suggests a
1754
NEKVASIL et al.
EVOLUTION OF INTRA-PLATE IGNEOUS SUITES
cogenetic link to the less evolved suite members. Feldspar–
liquid relations cannot preclude a combination of partial
melting and fractional crystallization of the resulting
liquid, but make it likely that the source of any partial
melts is related to the suite, not a random member of
the associated crust.
Evolutionary trends of feldspar compositions and
normative feldspar constituents of the bulk rocks are
unique for each suite type, yet strikingly similar for
different suites of the same type. This implies that feldspar
compositions and the normative feldspar constituents of
the whole rocks can be used to evaluate the suite type
of a partly exposed subgroup of magmatic rocks. Therefore, this tool can be used to assess the origin of anorogenic
granite bodies even if few intermediate or mafic units
are exposed.
Feldspars of many volcanic suites show strong evidence for a major decompression event. If crystal
fractionation induces the compositional evolution of
these hy-normative suites, this fractionation is likely to
be polybaric. Because the phase relations of hynormative suites appear strongly affected by pressure
(e.g. Scoates et al., 1999; A. Dondolini & H. Nekvasil,
unpublished data, 2000), a polybaric history implies
that the phases involved in the fractionation may not
be preserved in the lavas.
ACKNOWLEDGEMENTS
We would like to acknowledge the helpful reviews of
J. Longhi, S. A. Morse and R. Wiebe. We would also
like to thank the numerous workers whose major efforts
to characterize the chemistry of the myriad of suites
discussed in this paper made this manuscript possible.
Particular thanks go to Diane Smith and her colleagues
who shared invaluable unpublished information on the
Pikes Peak batholith, to Ben Edwards for data from
his M.S. thesis on the Sherman Granite, and to Carol
and Ron Frost for a pre-publication copy of their
1997 paper. This work was supported by National
Science Foundation grants EAR9614322 to H.N.,
EAR9304699 to D.H.L. and the Center for High
Pressure Research (EAR8920329). This paper is MPI
Manuscript 279.
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