Calc-Alkaline MagmatismFrancis, 2013

PacayaAgua

Acatenango

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: 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/(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.

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

AndesMarianas Arc

MarianasArc

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.

averagecontinental

crust

Mt. St. Helens

Gareloi

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.

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.

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.

Tholeiitic volcanic suites are characterized by decreasing Al with slowly increasing Si in the range from basalt to andesite.

Calc-alkaline volcanic suites are characterized by increasing Al with increasing Si in the range from basalt to andesite.

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]

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.

The Effect of Water

The Effect of Water

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.

The presence of water dramatically lowers the stability of plagioclase in basaltic magmas, inhibiting its crystallization to temperatures well below the liquidus.

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 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.

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

M A S HMixing - 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 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.

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 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.

Adakites

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 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.

e-watering Archeanmelting

Mantle-Wedge Melting:

Nowde-watering

Archeanmelting

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?

averagecontinental

crust

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.

Vertical Accretion

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

Mantle Ocean Continent crust crust

Oceanic crust - MORB basalt e1 / P

Continental crust - granite e2

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 Hillszircon

Growth of the Continental Crust - Continuous or Episodic

Continents have old Mantle Roots

BoninitesBoninites 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 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.