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Calc-alkaline magma series
Calc-alkaline magma series
Calc-alkaline magma series
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TAS diagram showing chemical composition range of sub-alkaline volcanic rocks including calc-alkaline rocks (yellow area) and alkaline volcanic rocks (blue area)

The calc-alkaline magma series is one of two main subdivisions of the subalkaline magma series, the other subalkaline magma series being the tholeiitic series. A magma series is a series of compositions that describes the evolution of a mafic magma, which is high in magnesium and iron and produces basalt or gabbro, as it fractionally crystallizes to become a felsic magma, which is low in magnesium and iron and produces rhyolite or granite. Calc-alkaline rocks are rich in alkaline earths (magnesia and calcium oxide) and alkali metals and make up a major part of the crust of the continents.

The diverse rock types in the calc-alkaline series include volcanic types such as basalt, andesite, dacite, rhyolite, and also their coarser-grained intrusive equivalents (gabbro, diorite, granodiorite, and granite). They do not include silica-undersaturated, alkalic, or peralkaline rocks.

Geochemical characterization

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AFM diagram showing the difference between tholeiitic and calc-alkaline magma series
AFM diagram showing the relative proportions of the oxides of alkalis (A), iron (F), and magnesium (M), with arrows showing the compositional change path of the magmas in the tholeiitic and the calc-alkaline magma series (BT=tholeiitic basalt, FB=ferro-basalt, ABT=tholeiitic basaltic andesite, AT=tholeiitic andesite, D=dacite, R=rhyolite, B=basalt, AB=basaltic andesite, A=andesite; dashed line=boundary between tholeiitic and calc-alkaline compositions)

Rocks from the calc-alkaline magma series are distinguished from rocks from the tholeiitic magma series by the redox state of the magma they crystallized from. Tholeiitic magmas are reduced, and calc-alkaline magmas are oxidized, with higher oxygen fugacities. When mafic (basalt-producing) magmas crystallize, they preferentially crystallize the more magnesium-rich and iron-poor forms of the silicate minerals olivine and pyroxene, causing the iron content of tholeiitic magmas to increase as the melt is depleted of iron-poor crystals. (Magnesium-rich olivine solidifies at much higher temperatures than iron-rich olivine.) However, a calc-alkaline magma is oxidized enough to (simultaneously) precipitate significant amounts of the iron oxide magnetite, causing the iron content of the magma to remain more steady as it cools than with a tholeiitic magma.

The difference between these two magma series can be seen on an AFM diagram, a ternary diagram showing the relative proportions of the oxides of Na2O + K2O (A), FeO + Fe2O3 (F), and MgO (M). As magmas cool, they precipitate out significantly more iron and magnesium than alkali, causing the magmas to move towards the alkali corner. In tholeiitic magma, as it cools and preferentially produces magnesium-rich crystals, the magnesium content of the magma plummets, causing the magma to move away from the magnesium corner until it runs low on magnesium and begins to move towards the alkali corner as it loses iron and remaining magnesium. With the calc-alkaline series, however, the precipitation of magnetite causes the iron-magnesium ratio to remain relatively constant, so the magma moves in a straight line towards the alkali corner on the AFM diagram.

Calc-alkaline magmas are typically hydrous.

Geologic context

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Calc-alkaline rocks typically are found in the volcanic arcs above subduction zones, commonly in island arcs and particularly in continental arcs.

Petrologic origin

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Rocks in the series are thought to be genetically related by fractional crystallization and to be at least partly derived from magmas of basalt composition formed in the Earth's mantle. Trends in composition can be explained by a variety of processes. Many explanations focus on water content and oxidation states of the magmas.

Proposed mechanisms of formation begin with partial melting of subducted material and of mantle peridotite (olivine and pyroxene) altered by water and melts derived from subducted material. Mechanisms by which the calc-alkaline magmas then evolve may include fractional crystallization, assimilation of continental crust, and mixing with partial melts of continental crust.

See also

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References

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Further reading

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Revisions and contributorsEdit on WikipediaRead on Wikipedia

Calc-alkaline magma series

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from Grokipedia
The calc-alkaline series is a geochemical lineage of subalkaline igneous rocks, ranging from to rhyolite, distinguished by a differentiation trend in which the iron-to-magnesium ratio (FeO/MgO) remains relatively constant as silica content increases, in contrast to the iron-enrichment typical of tholeiitic series. These rocks are primarily generated in convergent tectonic settings, such as arcs and active continental margins, where of a hydrous, metasomatized mantle wedge occurs due to fluids derived from subducting oceanic slabs. The series is named for its calcic and alkaline affinities, with compositions often featuring high aluminum oxide (Al₂O₃ > 17 wt%) in basaltic members and variable contents that define low-K, medium-K, and high-K subtypes. Key geochemical hallmarks include enrichment in large-ion lithophile elements (LILE) like , , and , coupled with depletion in high-field-strength elements (HFSE) such as , , and , reflecting the influence of slab-derived fluids on the source . On the AFM (alkalis-FeO*-MgO) ternary diagram, calc-alkaline rocks plot along a trend parallel to the A-F join, avoiding significant iron enrichment due to early crystallization of Fe-Ti oxides like and hydrous minerals such as under water-saturated conditions. This differentiation is driven by fractional at moderate pressures (around 2 kbar), often involving magma mixing between basaltic and more evolved melts, which suppresses plagioclase fractionation and promotes andesitic compositions. In geological context, the calc-alkaline series dominates volcanic arcs worldwide, including the Cascades in , the , and the Japan arc, contributing significantly to growth through repeated subduction-related magmatism. Unlike the tholeiitic series prevalent in mid- ridges and ocean islands, which originates from mantle melting and shows FeO* enrichment on differentiation diagrams, the calc-alkaline trend is tied to oxidized, hydrous conditions that enhance the stability of and inhibit iron buildup. Experimental studies confirm that higher oxygen and in the source are critical for producing this series, with primary magmas often being high-alumina basalts (>8 wt% MgO) derived from fluxed by components.

Definition and Characteristics

Overview

The calc-alkaline series represents a major compositional trend within subalkaline igneous rocks (subalkaline rocks having relatively low contents), characterized by progressive enrichment in silica (SiO₂) with a relatively constant or decreasing iron-to-magnesium ratio (FeO*/MgO) as the differentiates, typically evolving from basaltic through andesitic to rhyolitic compositions. This series contrasts with the tholeiitic series, where iron enrichment dominates during early differentiation stages. The trend reflects specific magmatic processes that suppress iron accumulation, often involving or other hydrous minerals in the sequence. The term was first formalized by geologist Mason A. Peacock in 1931, who classified series based on the alkali-lime index—a diagnostic plot of CaO and Na₂O + K₂O versus SiO₂ content (the SiO₂ wt% where the sum of Na₂O + K₂O equals CaO, around 56-61 for calc-alkaline series)—to distinguish calc-alkalic suites from purely alkalic or calcic ones. Peacock's work introduced the calc-alkalic series as part of a classification of subalkaline magmas into categories including alkalic, alkali-calcic, calc-alkalic, and calcic. This provided a foundational framework later refined to distinguish the calc-alkaline from the tholeiitic series within subalkaline magmas. Calc-alkaline magmas predominantly yield intermediate to rock types, such as andesites and dacites, and are volumetrically significant in volcanic arcs and associated plutons. They are closely linked to convergent tectonic settings, especially zones, where hydrous flux from the subducting slab promotes in the overlying mantle wedge. The name "calc-alkaline" stems from the series' signature enrichment in calcium early in differentiation ("calc") followed by rising contents, though it differs fundamentally from the alkaline series by lacking extreme alkali saturation.

Key Properties

Calc-alkaline rocks are characterized by mineral assemblages dominated by , often calcic and oscillatory zoned, alongside mafic minerals such as (), , and pyroxenes including and . In more evolved compositions, such as dacites and rhyolites, and alkali may appear, while is typically rare or absent, reflecting the series' differentiation path that favors hydrous mafic phases over stability. These assemblages are indicative of under conditions where and are stable, often linked to subduction-related . Texturally, volcanic rocks in the calc-alkaline series, particularly andesites, commonly exhibit textures with phenocrysts of and set in a fine-grained groundmass, sometimes forming glomeroporphyritic clusters. In contrast, their plutonic equivalents like diorites and granodiorites display equigranular textures with interlocking crystals, facilitating identification in hand samples through the presence of visible or flakes. These features aid in distinguishing calc-alkaline rocks from tholeiitic series, where is less prevalent. Physically, calc-alkaline rocks are typically metaluminous, with aluminum saturation indices around 0.9-1.0, supporting the stability of and without excess alumina. The presence of indicates elevated water contents, generally 2-4 wt% H₂O, which enhances the stability of hydrous phases during evolution. , the archetypal calc-alkaline rock, have densities ranging from 2.4-2.8 g/cm³, reflecting their intermediate silica content and mineral composition suitable for forming stratovolcanoes in zones.

Geochemical Features

The major element compositions of calc-alkaline magmas exhibit systematic variations during differentiation, primarily illustrated through Harker diagrams, which plot oxide concentrations against SiO2 content. In these series, SiO2 typically increases from about 50 to 75 wt%, reflecting evolution from basaltic to rhyolitic compositions, while the FeO/MgO ratio decreases or remains subdued, preventing the pronounced iron enrichment characteristic of tholeiitic trends. A key diagnostic feature is the calc-alkaline index proposed by Peacock, defined as the SiO2 weight percent at which Na2O + K2O equals CaO; for calc-alkalic series, this intersection occurs around 58 wt% SiO2, distinguishing them from more calcic or alkalic suites. Notable trends include progressive enrichment in Al2O3, reaching 18-20 wt% in intermediate andesitic rocks due to accumulation, and a moderate increase in K2O that varies by subtype, from calcic (lower K2O) to calc-alkalic (higher K2O) compositions.

Trace Element Signatures

Calc-alkaline magmas are characterized by fractionated (REE) patterns, with significant enrichment in REE (LREE) relative to heavy REE (HREE), typically exhibiting (La/Yb)N ratios greater than 10. These patterns often show flat HREE profiles, attributed to the fractionation of , which preferentially incorporates HREE into its crystal structure during magma evolution. Such signatures are evident in various arc settings, where LREE enrichment reflects of a metasomatized mantle wedge. In terms of high field strength elements (HFSE) versus large-ion lithophile elements (LILE), calc-alkaline magmas display notable depletions in HFSE such as Nb and Ta, with abundances typically 10-20 times chondritic values but markedly lower relative to LILE when normalized. Conversely, LILE including Ba, Sr, and U are enriched, often by factors exceeding 100 times primitive mantle levels, due to input from subducted sediments and slab-derived fluids. This contrast highlights the role of subduction-related processes in modifying the mantle source composition. Primitive mantle-normalized spider diagrams for calc-alkaline rocks reveal distinctive features, including prominent negative troughs at Nb and Ta, reflecting their relative depletion, alongside a positive anomaly at Sr. These patterns also show enrichments in LILE such as Ba, , Pb, and Rb, creating a saw-tooth appearance that contrasts with smoother profiles in non-arc magmas. Specific trace element ratios further fingerprint calc-alkaline series, with Ba/La ratios commonly exceeding 20, as observed in high- variants from arc environments. Similarly, Th/Yb ratios greater than 1 are prevalent, signaling contributions from crustal contamination or slab-derived fluids that elevate Th relative to Yb. These ratios, combined with the overall anomalies, provide robust geochemical tracers for identifying calc-alkaline affinities in volcanic and plutonic suites.

Geological Settings

Tectonic Environments

Calc-alkaline magmas primarily form in convergent plate boundaries characterized by subduction zones, where the downgoing oceanic slab interacts with the overlying mantle wedge. These settings include both oceanic island arcs, such as the Aleutians and the Marianas, and continental arcs, like the Andes and the Cascades. In island arcs, magmas erupt through oceanic crust, while in continental arcs, they interact with thicker continental lithosphere, influencing magma compositions. The geometry of plays a key role in the distribution of in arc settings. generation for the calc-alkaline series typically occurs at depths of 100-150 km in wedge, directly above the Benioff zone where seismic activity delineates the subducting slab. Volcanic arcs are positioned approximately 100-200 km landward of the , aligning with the depth where slab-derived fluids induce . This depth range reflects the thermal structure of the environment, where the Benioff zone's inclination controls the locus of melting. Calc-alkaline magmas have been identified in subduction-related arcs since the , with examples from granite-greenstone terranes and orogenic belts. They are particularly prevalent in arcs, where cooler mantle temperatures and evolving conditions, including increased oxygenation, support the oxidized environments essential for their Fe-depletion trends. In the , hotter mantle conditions favored higher-degree melting, leading to variations such as tonalite-trondhjemite-granodiorite suites with calc-alkaline affinities, alongside more tholeiitic or komatiitic compositions.

Associated Rock Types

The calc-alkaline magma series is primarily represented by a volcanic suite spanning to and rhyolite compositions, with andesites dominating the intermediate range at 55-65 wt% SiO₂. These volcanic rocks form stratovolcanoes and associated edifices in subduction-related settings, featuring textures with phenocrysts of , , and . Plutonic equivalents of the calc-alkaline series include , , and , commonly assembled into large batholiths such as the in , where these metaluminous, I-type granitoids exhibit compositions from mafic to felsic . These intrusive rocks share geochemical affinities with their extrusive counterparts, reflecting slow cooling in the continental crust. Volcaniclastic deposits associated with calc-alkaline magmatism include lahars—debris flows incorporating volcanic material—and ignimbrites from explosive eruptions of dacitic to rhyolitic magmas, prevalent in arc environments where rapid preserves these proximal . Prominent global examples include the in the , where calc-alkaline volcanics such as andesites and dacites build stratovolcanoes like . In Japanese volcanic arcs, the series manifests in andesitic to dacitic lavas, with adakitic variants observed in fields like Okoppe in northern , characterized by high Sr/Y ratios.

Petrogenesis

Formation Processes

The formation of calc-alkaline magmas primarily occurs in subduction zone settings through hydrous flux melting in the mantle wedge. As the oceanic slab descends, it undergoes progressive dehydration, releasing water-rich fluids from hydrous minerals such as , , and lawsonite within the altered and overlying sediments. These fluids migrate upward into the overlying mantle wedge, where they lower the solidus temperature of to approximately 800–820°C at pressures of 2–3 GPa (corresponding to depths of 60–90 km), thereby inducing without requiring excessively high temperatures. The source materials for these magmas involve a combination of the mantle wedge and components derived from the subducting slab. The , typically lherzolitic in composition, undergoes flux-induced , while the slab contributes volatiles and soluble elements through the released fluids; in some cases, partial melts of the sediments or eclogitized basaltic crust may also contribute directly to the budget. This process enriches the initial melts with and incompatible elements, setting the stage for the characteristic compositions of the calc-alkaline series. Water plays a critical role in shaping the and chemistry of these magmas by altering phase stability during melting and early . Elevated H₂O contents suppress the formation of in the residue, favoring the of and other hydrous phases instead, which promotes the development of calcic trends (higher CaO relative to alkalis) and elevated aluminum contents in the resulting liquids. Additionally, increases the silica content of the melts by about 1 wt% for every 3 wt% H₂O added, contributing to the andesitic affinities observed in many calc-alkaline suites. The degree of in wedge typically ranges from 5% to 20%, producing primary basaltic s that are high in Al₂O₃ (often >16 wt%). These low to moderate extents, influenced by the of slab-derived fluids, result in relatively fertile, undepleted sources and help explain the metaluminous to slightly peraluminous nature of subsequent calc-alkaline rocks.

Magma Evolution Models

Calc-alkaline s evolve primarily through a combination of fractional , assimilation of crustal material, and mixing with other magma batches, processes that distinguish their differentiation paths from tholeiitic series by suppressing iron enrichment and enhancing silica content. These models explain the characteristic trends observed in major and trace elements, such as the decrease in FeO*/MgO ratios with increasing SiO₂, which arises from the specific assemblages involved in under hydrous conditions. Fractional crystallization plays a central role in calc-alkaline magma evolution, where the removal of and from hydrous basaltic parents drives the observed geochemical trends. fractionation, facilitated by elevated water contents (typically >2-4 wt%), delays saturation and promotes the early removal of iron-bearing phases, leading to a decrease in FeO* with increasing SiO₂, in contrast to the iron enrichment seen in tholeiitic systems. removal further contributes to this by extracting calcium and sodium, resulting in more sodic and potassic residual liquids that evolve toward andesitic to dacitic compositions. This process is particularly evident in settings, where stability is enhanced by pressures corresponding to 4-10 km depths. Assimilation-fractional (AFC) models account for the incorporation of crustal material into evolving magmas, which introduces large ion lithophile elements (LILE) such as , Rb, and Ba, contributing to the enriched signatures typical of calc-alkaline series. In DePaolo's seminal AFC framework, simultaneous crustal assimilation and crystal removal are modeled as coupled processes, where the of assimilated material to crystallized melt (r) controls the extent of ; low to moderate r values often fit observed LILE enrichments without excessive radiogenic shifts. This mechanism is especially relevant in thickened , where of metasedimentary or metaigneous wallrocks provides the LILE source, as demonstrated in studies of Andean and Cascade arc plutons. Magma mixing, or hybridization, further modifies calc-alkaline compositions by blending mantle-derived magmas with crustal melts in environments, producing intermediate hybrids with dispersed major and patterns. This occurs in upper crustal chambers where contrasts allow intrusion of basaltic replenishments into resident andesitic or rhyolitic magmas, leading to convective overturn and homogenization; includes disequilibrium textures like sieved and mingled enclaves in arc volcanics. In like the Sierra Nevada, such mixing enhances K₂O and LILE contents while buffering silica variability, contributing to the broad spectrum of erupted compositions. Quantitative aspects of these evolution models are often evaluated using the Rayleigh fractionation equation for s during fractional crystallization, which describes the concentration change in the residual as: CLC0=FD−1\frac{C_L}{C_0} = F^{D-1} where CLC_L is the concentration in the , C0C_0 is the concentration, FF is the fraction of melt remaining (0 < F ≤ 1), and DD is the bulk partition coefficient of the element between crystals and melt. For incompatible LILE (D ≈ 0.1-0.5) in -dominated assemblages, this equation predicts strong enrichments (up to 10-20 times values at F = 0.5), consistent with AFC-modified trends in calc-alkaline suites; compatible elements like Ni (D > 1 for /) show depletion, aiding discrimination from primitive sources. These models, when combined, provide a robust framework for simulating observed patterns in arc magmas.

Comparisons and Discrimination

Versus Tholeiitic Series

The calc-alkaline and represent two primary differentiation trends in subalkaline , distinguished primarily by their evolutionary paths in major element compositions. In the calc-alkaline series, the FeO/MgO ratio declines progressively with increasing silica content, reflecting early of Fe-Ti oxides like that deplete iron relative to magnesium. In contrast, the tholeiitic series exhibits an initial increase in the FeO/MgO ratio, reaching a peak around 55 wt% SiO₂ due to of and under relatively reduced conditions, before any later decline if differentiation continues. This iron-enrichment trend in tholeiitic magmas results from delayed oxide saturation, allowing ferrous iron to accumulate in the melt. Tectonic settings further differentiate the two series, with calc-alkaline magmas predominantly forming in mature continental or island where has led to thickened crust and prolonged residence. These environments facilitate hydrous, oxidized conditions that promote the characteristic trends. Tholeiitic magmas, however, are more common in early-stage , back-arc basins, or divergent settings like s, where magmas experience shorter crustal transit times and lower water contents. For instance, tholeiitic basalts dominate mid-ocean ridge volcanism, while calc-alkaline suites prevail in evolved such as the . Mineralogically, calc-alkaline rocks are often amphibole-rich, with and early Fe-Ti s stabilizing due to higher oxygen and , which suppress crystallization and maintain higher magnesium contents. Tholeiitic rocks, by comparison, feature early assemblages dominated by and , with clinopyroxene, reflecting , reduced conditions that favor these silicates before late-stage appearance. These differences influence the overall texture and stability of the magmas during ascent. Globally, calc-alkaline magmas dominate volcanic output in subduction-related arcs, underscoring their importance in formation. Tholeiitic magmas, while ubiquitous in production at ridges and in back-arc basins, represent a smaller fraction of arc , often confined to immature or intra-arc zones. This distribution highlights the role of crustal maturity in steering magmatic evolution toward one series over the other.

Diagnostic Diagrams

Diagnostic diagrams are essential graphical tools in for distinguishing calc-alkaline magma series from other volcanic rock series, such as tholeiitic, by plotting major or trace element compositions to reveal differentiation trends and tectonic affinities. These diagrams facilitate the of igneous rocks based on their chemical , particularly in subduction-related settings where calc-alkaline series predominate. Widely used since the mid-20th century, they provide a visual means to identify the suppression of iron enrichment characteristic of calc-alkaline differentiation, contrasting with the iron enrichment seen in tholeiitic series. The AFM diagram, a ternary plot of alkali (A = Na₂O + K₂O), iron (F = total Fe as FeO + Fe₂O₃), and magnesium (M = MgO) oxides normalized to 100% (all in weight percent), is a foundational tool for discriminating calc-alkaline from tholeiitic series. In this diagram, calc-alkaline rocks plot to the left of a curved boundary line separating the two fields, reflecting their lower iron content relative to magnesium during differentiation due to early removal of iron-bearing phases like magnetite. This boundary, empirically derived from global volcanic datasets, shows tholeiitic series trending toward the iron apex, while calc-alkaline series follow a more magnesian path. The diagram's effectiveness stems from its simplicity and applicability to subalkaline basalts to andesites, though it assumes anhydrous compositions and may require adjustments for altered rocks. Another key major element diagram is the Miyashiro plot, which uses the ratio FeO*/MgO (where FeO* is total iron as FeO) on the y-axis against SiO₂ (wt%) on the x-axis. Calc-alkaline series rocks plot below a dividing line that curves upward with increasing silica, indicating limited iron enrichment compared to tholeiitic series, which plot above and show pronounced FeO* maxima around 55-60 wt% SiO₂. Developed from analyses of and volcanics, this bivariate plot highlights the compositional gap between series, with the boundary empirically defined as a curved line separating the fields. For rocks, particularly in greenstone belts, the Jensen cation plot offers a specialized diagnostic tool, plotting (Fe + Ti + Al) versus Mg cations against (Fe + Ti) versus (Ca + Na + K) cations (all normalized to six oxygen equivalents). This ternary diagram distinguishes calc-alkaline from tholeiitic and komatiitic volcanics by positioning calc-alkaline fields toward higher alumina and lower iron relative to magnesium, aiding identification in metamorphosed terrains where oxide-based plots may falter due to alteration. Applied to ancient supracrustal sequences, it reveals calc-alkaline andesites and dacites plotting in dedicated fields, separate from high-Fe tholeiites, based on recalculated cation proportions that minimize volatile influences. Its utility lies in handling altered samples from greenstone belts, such as those in the Superior Province. Modern variants incorporate immobile trace elements to enhance discrimination, especially for altered or low-silica rocks, with diagrams like Nb/Y versus Zr/TiO₂ (both ratios) providing robust . In these plots, calc-alkaline arc basalts and andesites cluster in fields offset from basalts (MORB) and ocean island basalts (OIB), showing elevated Zr/TiO₂ but depleted Nb/Y due to subduction-related enrichment processes. Derived from multi-element analyses of global tectonic settings, these diagrams use ratios to avoid effects, with calc-alkaline fields typically at Nb/Y < 0.7 and Zr/TiO₂ > 20, contrasting tholeiitic fields that overlap more with MORB. They are preferred in tectonic interpretations for their resistance to alteration and integration of elements like Y, Nb, Zr, and Ti.

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