Calc-Alkaline Magmatism

1
Calc-Alkaline Magmatism Francis, 2013
Pacaya
Agua
Acatenango
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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 Earths
history Arc 6 km3/yr MORB 20
km3/yr OIB 2 km3/yr
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/(Na2OK2O) becomes less than 1
(Peacock, 1931) between 56 61 wt. SiO2. This
and the other divisions of the classification
(calcic gt 61 and alkalic lt 56) are, however, no
longer used in their original sense.
3
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.
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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.
5
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
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Oceanic versus Continental Volcanic Arcs
Andes
Marianas Arc
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Marianas Arc
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Oceanic versus Continental Volcanic Arcs
The modal composition of calc-alkaline 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
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Gareloi
Mt. St. Helens
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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,
calc-alkaline, 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 K-felds, whereas phlogopite
phenocrysts become common in the more evolved
lavas.
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Rindjani, East Sunda Arc Indonesia
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Vanuatu Arc
Epi
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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.
17
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 calc-alkaline
andesites.
18
Ti is typically low (lt 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.
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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.
20
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
21
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.
22
Tholeiitic Index (THI) Fe4/Fe8
Fe4 FeO at 41 wt. MgO
Fe8 FeO at 81 wt. MgO
Zimmer et al, 2010
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THI, Water, and fO2 are typically well correlated
H2O wt. e(1.26-THI)/0.32
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Many of these differences can be understood in
terms of the effect of water pressure on phase
equilibria. In tholeiitic suites, dry
low-pressure 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 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. 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.
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The Effect of Water
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The presence of water dramatically lowers the
stability of plagioclase in basaltic magmas,
inhibiting its crystallization to temperatures
well below the liquidus.
The Effect of Water
Ironically, this leads to over saturation in
plagioclase at low pressures, and the development
of the strongly plagioclasephyric 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.
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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 (lt 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.
29
Melt Inclusion Data
Are andesite magmas mixtures of mantle-derived
basaltic magmas and rhyolitic partial melts of
the continental crust?
30
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.
31
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.
32
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).
33
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.
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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 SrNd correlation
diagram (Aleutians, Mariana), along with many OIB
suites, between the extreme isotopic compositions
of MORB and Bulk Earth. Some calc-alkaline
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.
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Evidence of subducted Amazon River sediment in
the mantle source of Martinique and St. Lucia,
compared to St. Kitts.
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Continental versus Oceanic Arcs
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Felsic Magmas More Contaminated
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M A S H
Mixing - Assimilation - Storage - Homogenization
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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-al
kaline 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
calc-alkaline 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.
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Calc-Alkaline Picrites 10-20 wt. MgO
41
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 Earths 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.
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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 sub-ducted.
43
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.
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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
calc-alkaline 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.
Archean melting
e-watering
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Mantle-Wedge Melting
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Now de-watering
Archean melting
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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).
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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.
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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
gt Th, U gt Sr, Pb, gt LREE gt HREE gt 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.
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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 lt Th, U lt
Sr, Pb, lt LREE lt HREE lt HFSE lherzolite residue
Kd LIL lt Th, U lt HFSE lt Sr, Pb, lt LREE lt
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.
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average continental crust
Oceanic versus Continental Volcanic Arcs
Problem Oceanic subduction-related volcanism
is dominantly basaltic, not andesitic. So where
is the continental crust made?
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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.
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Mantle Ocean Continent
crust crust
SiO2 45.2 49.4
60.3 TiO2 0.7
1.4 1.0 Al2O3 3.5
15.4 15.6 MgO
37.5 7.6 3.9 FeO
8.5 10.1 7.2 CaO
3.1 12.5 5.8 Na2O
0.6 2.6 3.2 K2O
0.1 0.3 2.5
Total 99.2 99.3 99.5
Cations normalized to 100 cations
Si 38.5
46.1 56.4 Ti
0.5 1.0 0.7 Al
3.6 16.9 17.2
Mg 47.6 10.6 5.4
Fe 6.0 7.9
5.6 Ca 2.8
12.5 5.8 Na 0.9
4.7 5.8 K
0.1 0.5 3.0 O
140.2 153.0
161.3 Mineralogy (oxygen units, XFe3
0.10) Quartz 0.0
0.0 13.0 Feldspar
13.2 57.3 64.3
Clinopyroxene 6.7 25.7
5.9 Orthopyroxene 18.3
4.1 14.7 Olivine
59.9 9.9 0.0
Oxides 1.8 3.0
2.0
Oceanic crust - MORB basalt
e1 / P
Continental crust - granite
e2
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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 Earths crust was
  associated with more
• catastrophic event(s), such as meteorite
  impact or mantle
• overturn?

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Modified after Hawkesworth and Kemp, 2006
Jack Hills zircon
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Growth of the Continental Crust - Continuous or
Episodic
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Continents have old Mantle Roots
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Boninites
Boninites are high-MgO andesites (SiO2 gt 53 wt.,
Mg no. gt 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 (lt 8.5 wt. CaO gt), 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.
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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 U-shaped 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.