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Calc-Alkaline Magmatism
Francis, 2013
Agua
Acatenango
Pacaya
Calc-Alkaline Arc Magmatism
Subduction-related calc-alkaline magmatism is the second most important form of
volcanism on the Earth, and has apparently played a crucial role in the development of
continental crust over the Earth’s history:
Arc:
MORB:
OIB:
The term “calc-alkaline” is a corruption
of the term calc-alkalic originally
coined for a suite of co-magmatic
volcanic rocks in which the wt.% ratio
of CaO/(Na2O+K2O) becomes less than
1 (Peacock, 1931) between 56 & 61
wt.% SiO2. This and the other divisions
of the classification (calcic > 61 and
alkalic < 56) are, however, no longer
used in their original sense.
6 km3/yr
20 km3/yr
2 km3/yr
Calc-Alkaline Arc Magmatism
The term calc-alkaline has persisted for volcanic suites characteristically occurring in volcanic arcs
associated with zones of subduction. This ingrained assumption is, however, dangerous because
contamination combined with crystal fractionation can produce a spectrum of lava compositions that
exhibits all the traits of calc-alkaline volcanic suites, in the absence of subduction.
The term “calc-alkaline” is most commonly used in opposition to the term tholeiitic, these two terms
referring to two different types of liquid lines of descent in suites of comagmatic lavas. Although the
former are typical of compressional arc environments and the latter of rifting or hotspot environments,
these tectonic associations are not exclusive. For example, tholeiitic fractionation trends are commonly
observed in the early development of immature oceanic volcanic arcs as well as volcanoes associated
with zones of rifting along the arc, and both tholeiitic and alkaline volcanoes can be found in arcs,
commonly spatially associated with fracture zones in the subducting plate. It is important to remember
that the terms calc-alkaline and tholeiitic refer to the liquid line of decent of volcanic suites, rather than
to individual samples.
Typically subduction-related, with volcanic arcs
occurring above the 100-200 km contour on the
top of the Benioff zone down going slab. These
depths are well within the garnet stability field for
both
basaltic
(eclogite)
and
peridotitic
compositions.
The dominant mafic lavas of calc-alkaline suites are in the range between basalt and andesite,
typically more evolved than the tholeiitic basalts that dominate MORB and hotspots. This in part
reflects the effect of water content on crystal fractionation. Furthermore, unlike the voluminous
basaltic andesites of some flood basalt provinces that are commonly aphyric, calc-alkaline basalts to
andesites are characteristically strongly plagioclase-phyric. The more viscous nature of the magmas
result in the construction of central volcanoes with relative steep slopes compared to the shields
which characterize hot-spot volcanoes.
Cotopaxi
Mayon
Oceanic versus Continental Volcanic Arcs
Marianas Arc
Andes
Marianas
Arc
Oceanic versus Continental
Volcanic Arcs
The modal composition of calcalkaline
volcanic
suites
on
continents is shifted to higher Si
contents (andesites dominate) than
those of oceanic suites (basalts
dominate), and the mafic to
intermediate lavas that build the
strato-volcanoes of the continental
arcs are typically accompanied by
the eruption of voluminous felsic
ignimbrite sheets, along with the
intrusion of coeval granitoids whose
“dacitic” compositions commonly
occupy a population minimum in the
volcanic suite, between rhyolite and
evolved andesite.
average
continental
crust
Gareloi
Mt. St. Helens
There is a tendency for total alkalis, and K in particular to increase with distance and time behind individual
subduction zones. Calc-alkaline suites have been traditionally subdivided into low-K calc-alkaline, calcalkaline, and high-K calc-alkaline suites, as well as the extreme end member, shoshonite suites on the basis of
K2O versus SiO2 content.
Low-K calc-alkaline suites commonly exhibit transitional tholeiitic affinities, while the high-K calc-alkaline
suites have transitional alkaline affinities. The paths of crystal fractionation in this diagram are, however,
sensitive to pressure (and H2O pressure), and individual volcanic suites commonly (Sloko) transcend the field
boundaries.
In contrast to most calc-alkaline volcanic
suites, shoshonitic suites are characterized
by the early appearance of cpx phenocrysts
after olivine (ankaramitic primitive
magmas), the relatively late appearance of
plagioclase as a phenocryst, along with an
absence of true rhyolites. In primitive
lavas, the potassium is in groundmass Kfelds, whereas phlogopite phenocrysts
become common in the more evolved lavas.
Rindjani, East Sunda Arc
Indonesia
Vanuatu Arc
Epi
Tholeiitic versus Calc-Alkaline
Fractionation Trends
Calc-alkaline volcanic suites are
characterized by decreasing Fe and
with decreasing Mg, in the range
from basalt to andesite
Tholeiitic volcanic suites are
characterized by increasing Fe
and decreasing Al with decreasing
Mg in the range from basalt to
andesite, while Si rises quite
slowly with fractionation.
Calc-alkaline volcanic suites
are
characterized
by
increasing Al with increasing
Si in the range from basalt to
andesite.
Tholeiitic volcanic suites are
characterized by decreasing Al
with slowly increasing Si in
the range from basalt to
andesite.
These differences are greatest at the boundary between basalt and andesite
(SiO2 = 55 wt.%), where tholeiitic andesites commonly have Al2O3 content
less than 15%, in contrast to the higher Al2O3 contents (15-20+%) of calcalkaline andesites.
Ti is typically low (< 1.2 wt.%
TiO2) in calc-alkaline suites and
remains relatively constant and
then decreases slowly with
increasing Si.
In tholeiitic suites, Ti first
increases by a factor of 2 or more
at almost constant Si and then
decreases rapidly with increasing
Si in the range 50 to 55 wt.%
SiO2.
The Effect of Water
Early plagioclase fractionation drives tholeiitic basalts to very Fe-rich basaltic compositions to the point
at which an Fe-Ti oxide begins to crystallize Mg.
The presence of water inhibits the crystallization of feldspar in calc-alkaline magmas resulting in no Fe
build-up and leading to residual liquids that are poor in Fe.
The Effect of Water
Early plagioclase fractionation drives
tholeiitic
basalts
to
Al-poor
compositions and reduces the increase
in Si with decreasing Mg in residual
liquids.
The presence of water inhibits
plagioclase
crystallization
to
temperatures well below the liquidus.
The absence of plagioclase in the early
fractionating
mineral
assemblage
results in a continued increase in Al
with more rapidly increasing Si with
decreasing Mg into the andesite range
The Effect of Water
The appearance of an Fe-Ti oxide
on the liquidus of Fe-rich
tholeiitic basalts is clearly visible
in the tholeiitic suite. Note the
nearly 3 fold increase in Ti over a
very limited increase in Si.
Calc-alkaline suites are more
oxidized, Fe-Ti oxides crystallize
relatively early keeping Ti low
with increasing Si in the residual
liquids.
Tholeiitic Index (THI) = Fe4/Fe8
Fe4 = FeO at 4±1 wt.% MgO
Fe8 = FeO at 8±1 wt.% MgO
Zimmer et al, 2010
THI, Water, and fO2 are
typically well correlated
H2O wt.% = e[(1.26-THI)/0.32]
The Effect of Water
Many of these differences can be understood in terms of the effect
of water pressure on phase equilibria. In tholeiitic suites, dry lowpressure conditions favour the early appearance and fractionation of
plagioclase, which induces Fe-enrichment and Al depletion in the
derived residual liquids.
In calc-alkaline suites, however,
fractionation at elevated water pressures suppresses the
crystallization of plagioclase, as a result there is an absence of Fe
enrichment and Al depletion during fractionation from basalt to
andesite.
The presence of water dramatically lowers the stability of
plagioclase in basaltic magmas, inhibiting its crystallization to
temperatures well below the liquidus. The absence of plagioclase
in the early fractionating mineral assemblage prevents the build
up of Fe and leads intermediate residual liquids that are rich in Al
and Si.
The Effect of Water
The Effect of Water
The presence of water dramatically lowers the stability
of plagioclase in basaltic magmas, inhibiting its
crystallization to temperatures well below the liquidus.
Ironically, this leads to over
saturation in plagioclase at low
pressures, and the development of the
strongly plagioclase–phyric character
typical of basalts and andesites in
most calc-alkaline suites.
Upon
rising to the surface, calc-alkaline
magmas lose their dissolved water
and become supersaturated in
plagioclase
because
of
their
composition with respect to the
position of the one atmosphere cpx –
plag cotectic.
Further, when
plagioclase does come on the
liquidus at high water pressures, the
modal proportion of plagioclase in
the cumulate assemblage is much
higher than that at low pressures.
Summary of the differences
between Calc-alkaline and
Tholeiitic fractionation trends
Calc-alkaline volcanic suites are characterized by
decreasing Fe and increasing Al with decreasing Mg, in
the range from basalt to andesite, while Si rises relatively
rapidly with fractionation. Ti is typically low (< 1.2 wt.%
TiO2) and remains relatively constant and then decreases
slowly with increasing Si. Calc-alkaline volcanic suites
are commonly dominated by lavas of intermediate
composition, like andesite. The magmas of calc-alkaline
suites are also characterized by relatively high oxidation
states compared to the tholeiitic basalts of MORB or
OIB suites.
Tholeiitic volcanic suites are characterized by increasing
Fe and decreasing Al with decreasing Mg, in the range
from basalt to andesite, while Si rises quite slowly with
fractionation. These differences are greatest at the
boundary between basalt and andesite (SiO2 ~ 55 wt.%),
where tholeiitic andesites typically have Al2O3 content
less than 15%, in contrast to the higher Al2O3 contents of
calc-alkaline andesites. Ti first increases by a factor of 2
or more and then decreases rapidly with increasing Si in
the range 50 to 55 wt.% SiO2. Tholeiitic volcanic suites
are typically bimodal, with large volumes of basalt,
smaller volumes of rhyolite, and a relative paucity of
lavas with intermediate andesitic compositions.
Melt Inclusion Data
Are andesite magmas mixtures of
mantle-derived basaltic magmas
and rhyolitic partial melts of the
continental crust?
Trace element characteristics of calc-alkaline magmas:
Arc lavas are enriched in incompatible trace elements such as LIL elements and LREE compared to MORB, but like
flood tholeiites have marked negative anomalies in HFSE, such as Nb, Ta, etc.
High LIL/HFSE and LIL/LREE ratios (Ba/Nb, Th/Nb and Ba/La, Th/La) are the most characteristic trace element
features of calc-alkaline volcanic suites. Calc-alkaline lavas also commonly exhibit relative positive anomalies in Sr,
Pb, and Eu.
In contrast to their fractioned LREE, the HREE, Y, and Sc are significantly less fractionated, resulting in relatively
flat HREE profiles in comparison to the primitive lavas of both OIB and flood basalt suites, which have fractionated
HREE.
Although there is a tendency for continental calc-alkaline suites to have higher LIL and LREE contents, and smaller
HFSE anomalies, it does not appear possible to reliably distinguish between oceanic arcs and continental arcs in
terms of trace elements. There do, however, appear to be good correlations between continental sediment input at
subduction zone and trace element chemistry of associated calc-alkaline volcanic arcs (lessor Antilles, Sunda-Banda
Arc).
Surprisingly (?), the most primitive lavas in
many calc-alkaline suites (high-Al basalts)
are difficult to distinguish from MORB,
even in terms of LREE, and in a number of
cases primitive island arc basalts (IAB)
have lower REE and HFSE contents than
MORB. The only consistent difference is
the enrichment in LIL elements and Th in
primitive subduction-related basalts.
Trace element profiles of
calc-alkaline arcs
compared to E-MORB
Note the enrichment in LIL trace
elements (Cs, Rb, Ba, Rb, K), Pb,
and Sr; but depletions in HFSE
trace elements (Nb, Ta, Zr, Ti).
Calc-alkaline basalts have high concentrations of boron (10-50 ppm)
compared to MORB and OIB basalts (1-3 ppm), apparently reflecting
the subduction of boron-rich ocean sediments (50-150 ppm) and altered
oceanic basalt (to 300 ppm).
Significantly, however, the flux of
incompatible element enriched sediment into subduction zones does not
appear to be enough to explain their output in arc volcanism, suggesting
that there is not wholesale incorporation of ocean sediments into the
magmatic system, but rather preferential uptake from the slab and
sediments combined with selective scavaging from the mantle wedge.
There also appears to be systematic increase in the degree of
enrichment in LREE and LIL with the increasing K content of the
volcanic suite, with shoshonite suites exhibiting the most enriched
incompatible trace element profiles.
Sr-Nd isotopic Systematics
There does not appear to be a unique
isotopic signature for calc-alkaline
volcanism.
Many calc-alkaline
suites, both continental and oceanic,
fall in the upper left hand quadrant
of the Sr–Nd correlation diagram
(Aleutians, Mariana), along with
many OIB suites, between the
extreme isotopic compositions of
MORB and Bulk Earth. Some calcalkaline suites appear to be shifted
to higher 87Sr/86Sr ratios, with little
change in 143Nd/144Nd ratio with
respect to the “mantle array”.
Others fall in the lower right hand
quadrant with both elevated 87Sr/86Sr
and low 143Nd/144Nd ratios (Banda
Arc, lessor Antilles, northern
Chilean and Peruvian Andes),
attributed to the involvement of
subducted continental sediments
and/or crust in their petrogenesis.
Evidence of subducted Amazon River
sediment in the mantle source of Martinique
and St. Lucia, compared to St. Kitts.
Continental versus Oceanic Arcs
Felsic Magmas
More Contaminated
MASH
Mixing - Assimilation - Storage - Homogenization
Pb – Pb Systematics
Calc-alkaline suites that fall in the lower
right hand quadrant of the Sr-Nd diagram
also exhibit high 207Pb/204Pb and 208Pb/204Pb
ratios that trend towards old continental
signatures.
Calc-alkaline lavas have relatively high
values of Be10 (essentially below detection
limit in MORB and OIB), a short lived
isotope (t1/2 = 1.5 Ma) produced by cosmic
rays interaction with the atmosphere and
concentrated in deep sea ocean clays. This
is taken as strong evidence for the
subduction and incorporation of sediments
into arc magmas.
The amount of slab component in calcalkaline lavas estimated from isotopic
considerations is typically significantly less
than that estimated using incompatible trace
elements.
This may suggest that the
character of calc-alkaline lavas is in large
part determined by reaction and exchange
with the mantle wedge.
Calc-Alkaline Picrites 10-20 wt.% MgO
Primary magmas:
Slab-Melting Proponents:
In many Arc suites, the most primitive magmas are high-Al
basalts that are very similar to MORB in composition, except
in terms of LIL elements. The majority of these high-Al
basalts, however, have relatively low Mg nos. that could not
coexist with the Earth’s mantle, and in some cases they contain
less Ni and Cr than their associated andesites. These problems
have lead proponents to conclude that high Al-basaltic parental
magmas are generated by large degree melting of the eclogite
in the down going slab, rather than the mantle wedge.
Such models have trouble explaining the lack of fractionation
of HREE in calc-alkaline suites, but may be viable for the
origin of adakites, and Archean tonalites, both of which are
characterized by fractionated and depleted HREE. Slab
melting models also do not have convincing explanations for
the Sr and Nb anomalies that typify calc-alkaline lavas.
Feldspar should not be a phase in the residue, and most of the
possible Nb-bearing accessory phases would not be saturated
in basaltics melt (zircon, rutile, apatite, ilmenite, etc) at the PT
conditions of their formation.
Adakites
Adakites are dacites that,
unlike most calc-alkaline
dacites, exhibit fractionated
HREE, Y, and Sc abundances,
suggesting an important role
for
garnet.
This
distinctiveness has been taken
by many to indicate that
adakites are direct melts of the
subducting
slab
(leaving
residual eclogite with garnet).
In the modern era, adakites
erupt where young relatively
warm oceanic crust is subducted.
Higher pressure favours the presence of garnet in the
melting residue of basalt (garnet-amphibolite), which
preferentially holds back elements like Yb and Y, with
respect to Sm and Zr in the partial melt.
Primary magmas:
Mantle-Wedge Melting Proponents:
A compilation of high-MgO arc lava compositions indicates that
primitive Arc magmas range from MORB-like high-Al basalts
(Aleutians, Cascades) to more magnesian low-Al ankaramites
(South Pacific arcs), which fall in the cpx-out field with OIB and
flood high magnesian lavas in a plot of Al versus Si. Although
the occurrence of low-Al ankaramites is relatively rare in calcalkaline volcanic suites, they do have compositions that can
coexist with the mantle, and some have suggested that low Ni
and Cr contents of high-Al arc basalts reflects the fact that they
represent derived liquids that have fractionated from ankaramitic
parental magmas. Ankaramitic primary magmas are thought to
be generated by wet melting of the peridotite mantle wedge
above the slab, induced by dewatering of the down going slab.
The positive Sr anomalies, negative Nb anomalies, and elevated
87Sr/86Sr ratios at constant Nd isotopic ratios are all thought to
reflect the relative solubilities of these elements in hydrothermal
solutions, decreasing in the order Rb, Sr, Nd, Sm, and Nb. A
variant of this model, calls upon volatile-rich felsic melts,
produced by partial melting of the down going slab, to induce
melting in the overlying mantle wedge. Unlike basaltic melts,
felsic melts may be saturated in a phase such as rutile, which
could have retained HFSE elements in the melting residue of the
slab.
e-watering
Archean
melting
Mantle-Wedge Melting:
Now
de-watering
Archean
melting
Origin of Negative High Field-Strength Element Anomalies
in Calc-Alkaline Lavas 1:
Crustal contamination:
The trace element pattern of calc-alkaline lavas is essentially that of the continental crust, and the
lower parts of continental flood basalt succession commonly exhibit similar trace element patterns
due to the assimilation of crust. However, the fact that the lavas of oceanic arcs commonly exhibit
even more negative Nb anomalies than the lavas of continental arcs would seem to rule out crustal
contamination as the origin of low Nb signatures (chicken versus the egg problem).
Origin of Negative High Field-Strength Element Anomalies
in Calc-Alkaline Lavas 2:
Residual Rutile:
Many attribute HFSE anomalies to the presence of residual rutile in the eclogite of the subducting
slab. Rutile is a relatively common accessory phase in eclogite xenoliths brought up by kimberlites
and typically contains Nb contents exceeding 1000 ppm (up to a few wt.% Nb2O5). A major
difficulty for residual phase models is that virtually all arc magmas, with the possible exception of
the most felsic, have Ti contents that are too low for rutile saturation. Furthermore, most thermal
models indicate that present day subducted slabs dehydrate rather than melt.
Origin of Negative High Field-Strength Element Anomalies
in Calc-Alkaline Lavas 3:
Insolubility in Aqueous Slab Fluids:
The enrichment in incompatible trace elements in calc-alkaline lavas with respect to
MORB is essentially proportional their solubility in aqueous fluids, not their magmatic
partition coefficients:
LIL > Th, U > Sr, Pb, > LREE > HREE > HFSE
According to this model, HFSE are left behind in the subducted slab because they are
insoluble in the released aqueous fluids. The HFSE abundances of calc-alkaline lavas
reflects their levels in the mantle wedge before metasomatism by slab-derived fluids.
Origin of Negative High Field-Strength Element
Anomalies in Calc-Alkaline Lavas 4:
Reactive Flow:
Others hold that the characteristic trace element pattern of calc-alkaline lavas reflects the
chromatographic buffering of melts perculating through depleted mantle harzburgite in
the spinel stability field. Unlike LIL, LREE, Th, and U that are largely held in
clinopyroxene, HFSE are significantly partitioned into orthopyroxene (1/3) as well
clinopyroxene (2/3). This results in an incompatibility sequence that is the same as the
aqueous solubility sequence. As a result, the HFSE will be decoupled from the other
incompatible elements, held back in the mantle column.
harzburgite residue: Kd: LIL < Th, U < Sr, Pb, < LREE < HREE < HFSE
lherzolite residue:
Kd: LIL < Th, U < HFSE < Sr, Pb, < LREE < HREE
Regardless of the mechanism, it appears certain that the slab subducted into the mantle
will have elevated HFSE / incompatible element ratios with respect to MORB, and
especially with respect to the continental crust, representing the accumulation of
complementary calc-alkaline lavas. This high HFSE subducted material in turn over time
provides a convenient source for high-Nb OIB magmas.
Oceanic versus Continental
Volcanic Arcs
Problem:
Oceanic subduction-related volcanism is
dominantly basaltic, not andesitic.
So where is the continental crust made?
average
continental
crust
Vertical Accretion
Others suggest that the continental
crust may have formed by magmatic
under plating, in which basaltic
magmas emplaced as sills along the
base of the crust fractionate to
granodioritic tops and underlying
ultramafic cumulates.
The ultramafic cumulates are
preferentially lost over time by
delamination into the mantle, while
the buoyant dioritic tops rise and
interact with partially-melts of
overlying crust.
Mantle Ocean Continent
crust crust
SiO2
TiO2
Al2O3
MgO
FeO
CaO
Na2O
K2O
Total
45.2
0.7
3.5
37.5
8.5
3.1
0.6
0.1
99.2
49.4
1.4
15.4
7.6
10.1
12.5
2.6
0.3
99.3
60.3
1.0
15.6
3.9
7.2
5.8
3.2
2.5
99.5
Cations normalized to 100 cations
Si
Ti
Al
Mg
Fe
Ca
Na
K
O
38.5
0.5
3.6
47.6
6.0
2.8
0.9
0.1
140.2
46.1
1.0
16.9
10.6
7.9
12.5
4.7
0.5
153.0
56.4
0.7
17.2
5.4
5.6
5.8
5.8
3.0
161.3
Mineralogy (oxygen units, XFe3+ = 0.10)
Quartz
Feldspar
Clinopyroxene
Orthopyroxene
Olivine
Oxides
0.0
13.2
6.7
18.3
59.9
1.8
0.0
57.3
25.7
4.1
9.9
3.0
13.0
64.3
5.9
14.7
0.0
2.0
Oceanic crust - MORB basalt
e1 / P
Continental crust -
e2
granite
Other Origin(s) for the Continental Crust
•
Preferential recycling of Mg, either by hydrothermal circulation in
MORB crust and/or subaerial weathering, back into the mantle at
subduction zones.
•
Does the episodic nature of zircon dates indicate that the origin
and or growth of the Earth’s crust was associated with more
catastrophic event(s), such as meteorite impact or mantle
overturn?
Modified after Hawkesworth and Kemp, 2006
Jack Hills
zircon
Growth of the Continental Crust - Continuous or Episodic
Continents have old Mantle Roots
Boninites
Boninites are high-MgO andesites (SiO2 > 53 wt.%, Mg no. > 0.6), commonly found in the gap
between the trench and calc-alkaline volcanic arc of subduction zones. Other names for boninites have
included: high-Mg andesite, marianite, and sanukatoid. Magmas with boninitic affinities are also
common, however, in some ophiolite complexes (Troodos, Thetford), as the apparent parental magmas
of large Precambrian layered intrusions (Bushveld & Stillwater), and within the volcanic successions
Archean greenstone belts, where low-Ca boninites grade to komatiites in composition. Boninites can be
subdivided into high and low-Ca types (< 8.5 wt.% CaO >), with primitive high-Ca types having
abundant olivine phenocrysts followed by cpx and/or opx, while the low-Ca types typically have early
clino or orthoenstatite phenocrysts in addition to, or rather than, olivine. Unlike most calc-alkaline
lavas, plagioclase is never a phenocrysts phase, and only becomes well developed in the groundmass of
relatively evolved lavas.
Boninites are characterized by very low overall REE contents, characterized by increasing
depletion with respect to MORB, from the HREE to MREE. The LIL trace elements (and
sometimes LREE) are, however, relatively enriched, giving the overall characteristic Ushaped pattern. Such low U-shaped patterns suggest a very depleted source that has
experienced a latter enrichment in LIL and sometimes LREE. Such U-shaped trace element
patterns are reminiscent of those exhibited by mildly metasomatized lherzolite lithospheric
xenoliths.
Primitive boninite magmas are
unusually Si-rich, and are still
technically high-Mg andesites
rather than basalts.
This
characteristic, along with their
extremely low concentrations
of REE, have lead to a
consensus
that
boninite
magmas represent second-stage
melting of metasomatically
enriched mantle lithosphere at
elevated water contents.