Calc-alkalic Trend Research Articles

The Indosinian Granitoids of the Songpan–Garze–West Kunlun Orogenic Belt, China: Distribution, Petrochemistry, and Tectonic Insights

During the Indosinian orogeny, the Songpan–Garze–West Kunlun orogenic belt experienced significant tectonic and magmatic activity, leading to the widespread emplacement of granitoid bodies. This study provides a detailed petrochemical and geochemical analysis of these granitoids, offering new insights into their tectonic settings and magmatic evolution. The granitoids of this belt are systematically categorized into arc calc-alkaline and arc tholeiitic granitoids (ACG and ATG), cordierite peraluminous and muscovite peraluminous granitoids (CPG and MPG), potassium calc-alkaline granitoids (KCG), and peralkaline granitoids (PAG) suites. ACG and ATG types dominate early magmatism (230–190 Ma), reflecting a convergent tectonic setting, while KCG and MPG types magmatism, respectively, emerged 10–20 Myr and 15–25 Myr later, during post-collisional extensional phases. Geochemical analyses show that ACG and ATG granitoids follow calcic and calc-alkalic trends, while KCG and MPG display alkalic characteristics. These findings align with the region’s tectonic transition from the closure of the Paleo–Tethys Ocean to Late Triassic transpressional deformation. This study enhances the understanding of granitoid petrogenesis and provides valuable implications for regional tectonic evolution and mineral exploration.

Geochronology and geochemistry of Devonian magmatism in the Frontal cordillera (Argentina): geodynamic implications for the pre-Andean SW Gondwana margin

ABSTRACT Whereas north of 33°S Devonian calcalkaline magmatism is notably absent, a Devonian arc is well developed to the south of 33°S. The Carrizalito, Pampa de los Avestruces, and Papagayos plutons are located in the Frontal Cordillera between 34° and 35°S. U-Pb zircon SHRIMP and LA-MC-ICP-MS data from the plutons yield Early Devonian ages of 389 ± 3 and 409 ± 3 for Carrizalito, 413 ± 2 Ma for Pampa de los Avestruces, and 346 ± 3, 393 ± 5 and 412 ± 3 Ma for Papagayos. The younger zircon age from the Papagayos pluton suggests that the Devo nian magmatism continued during the Early Carboniferous. Carrizalito and Pampa de los Avestruces plutons display metaluminous to slightly peraluminous granodiorite composition, with SiO2 ranging from 65.1 to 67.8 wt.% and from 67.9 to 69.7 wt.%, respectively, and define a magnesian calc-alkalic trend. The Papagayos pluton is metaluminous to slightly peraluminous and slightly more evolved in composition (felsic granodiorite to monzogranite), with a SiO2 content that varies between 68.4 to 70.5 wt.%, defining a magnesian with ferroan enrichment and calc-alkalic to alkali-calcic trend. These geochemical characteristics are similar to those reported for the Carboniferous arc granitoids in the Frontal Cordillera and Western Sierras Pampeanas at 27°–30°S and the archetypical calc-alkaline granitoids of the Peruvian Coastal batholith, indicating an Early Devonian magmatic arc. Rare earth element patterns ([La/Yb]N = 10–20) with mostly slight to moderate negative Eu anomalies (Eu/Eu* = 0.62–0.80) and geochemical ‘spider-diagrams’ are similar to arc granitoids. Combined whole-rock Nd and Sr isotope data from the largest Carrizalito and Pampa de los Avestruces plutons (εNdt = −4.3 and −4.6; 87Sr/86Srt = 0.70704 and 0.70762; TDM = ca. 1.5 Ga) together with in situ U–Pb and Hf isotope data from magmatic zircon (εHft ranging from −16.5 to −5.4; TDM = ca. 1.7 Ga), suggest a common older continental lithosphere source. The presented data indicate tectonic segmentation in the pre-Andean margin controlling the presence or absence of Devonian subduction-related magmatism. This segmentation is best explained by changes in the tectonic plate configuration occurred in a long-lived convergent margin.

Petrogenesis of I-type granitoids from the Melrose Stock, east-central Nevada

The Melrose Stock in the Dolly Varden Mountains of east-central Nevada is one of the many Mesozoic intrusion s in the Basin and Range Province. It consists of monzonites, quartz monzonites, granodiorites, and granites sharply intruding Mississippian to Triassic units. Phenocrysts of plagioclase (An38–An24) with oscillatory zoning and albitic rims, hornblende ± diopside, and biotite are common. Coexisting phases include orthoclase, quartz and accessory magnetite, apatite, titanite, ilmenite, and allanite. Mineral compositions suggest that the intrusion was emplaced at ∼720 ± 40°C and 1.8–2.3 kbar. All rocks are metaluminous to slightly peraluminous, defining a calcalkalic trend in which the monzonites and syenites are shoshonitic. Rare earth element patterns indicate that all studied rock types are comagmatic. Harker plots show curvilinear trends with some kinks consistent with fractionation, and mixing/assimilation. Major-element modelling and petrographic evidence suggest three stages of fractionation/mixing: Stage 1 marked by the fractionation of diopside and plagioclase; Stage 2 by fractionation of plagioclase, hornblende ± orthoclase ± biotite, accompanied by mixing through convection; and Stage 3 by fractionation of biotite, hornblende, plagioclase, and orthoclase. Mineralogic, petrographic, and major- and trace-element data demonstrate that all rocks are I-type granitoids, suggesting a significant mantle contribution. Spider diagrams show troughs for Ti, P, and Nb, indicating magma genesis in a subduction-zone setting. Discrimination diagrams classify all rocks as late orogenic. Magma was therefore generated from mantle metasomatized by subduction, differentiated to a monzonitic magma, and emplaced in the thinned continental crust during a period of extension late in the cycle of Elko orogeny.

Tholeiitic vs Calc-alkalic Differentiation and Evolution of Arc Crust: Constraints from Melting Experiments on a Basalt from the Izu-Bonin-Mariana Arc

The liquid line of descent (LLD) for a representative basalt from the Izu–Bonin–Mariana (IBM) arc was investigated at 0·3 GPa in the presence of 0·49–2·83 wt % H2O to constrain the differentiation of arc magmas and the evolution of the arc's crust. This is of interest as differentiated continental crust may form as the middle crust of the intra-oceanic IBM system. The tholeiitic differentiation trend, which is often documented in the volcanic sequences of intra-oceanic arcs, is best reproduced by the LLD of an H2O-poor (∼0·5 wt %) basalt, whereas the calc-alkalic trend in the IBM rocks, which are likely to form middle crust of intermediate composition, mimics the LLD of a hydrous basalt with 2·5–3·0 wt % H2O. Magmatic temperatures estimated for IBM calc-alkalic rocks, however, tend to be higher than those for the hydrous LLD. It may thus be suggested that an alternative mechanism, mixing of basaltic and felsic magmas, could play a major role in calc-alkalic middle crust formation in the IBM. Seismic velocities of the inferred middle crustal rocks, obtained based on both the experimental results and theoretical calculations, agree well with the observed seismic crustal structure.

Archaean high-K granitoids produced by remelting of earlier Tonalite–Trondhjemite–Granodiorite (TTG) in the Sangmelima region of the Ntem complex of the Congo craton, southern Cameroon

We present a geochemical and isotopic study that, consistent with observed field relations, suggest Sangmelima late Archaean high-K granite was derived by partial melting of older Archaean TTG. The TTG formations are sodic-trondhjemitic, showing calcic and calc-alkalic trends and are metaluminous to peraluminous. High-K granites in contrast show a potassic calc-alkaline affinity that spans the calcic, calc-alkalic, alkali-calcic and alkalic compositions. The two rock groups (TTG and high-K granites) on the other hand are both ferroan and magnesian. They have a similar degree of fractionation for LREE but a different one for HREE. Nd model ages and Sr/Y ratios define Mesoarchaean and slab-mantle derived magma compositions respectively, with Nb and Ti anomalies indicating a subduction setting for the TTG. Major and trace element in addition to Sr and Nd isotopic compositions support field observations that indicate the derivation of the high-K granitic group from the partial melting of the older TTG equivalent at depth. Geochemical characteristics of the high-K granitic group are therefore inherited features from the TTG protolith and cannot be used for determining their tectonic setting. The heat budget required for TTG partial melting is ascribed to the upwelling of the mantle marked by a doleritic event of identical age as the generated high-K granite melts. The cause of this upwelling is related to linear delamination along mega-shear zones in an intracontinental setting.

小笠原群島父島に産する低Ｃａボニナイト系列岩の２つの組成変化トレンド

Chichijima, located in Ogasawara (Bonin) arc, Japan has been known as the type locality of boninite. Meanwhile, extremely evolved volcanics (SiO2>70 wt%) also occur together with the high-magnesian andesites. The evolved rocks have been considered as the products of differentiation from the boninitic parental magma by fractional crystallization processes. The differentiation processes are considered to produce low-Ca boninite through bronzite-andesite, dacite, quartz-dacite and quartz-rhyolite. Some of the quartz-dacites, however, would not be necessarily regarded as differentiates from the dacite magma. Quartz-dacites are enriched in SiO2 for the fractional crystallization products from dacite. Some quartz-dacites show mineralogical evidences of magma mixing, while such features can not be observed in the dacites. Quartz phenocrysts in the silica rich quartz-rhyolite exhibit euhedral form, but quartz-dacites have corroded quartz phenocrysts. The compositions of mafic pyroxene phenocrysts in quartz-dacites are similar to those in bronzite-andesites. Groundmass pyroxenes in some quartz-dacites include orthopyroxene and augite. In contrast, pigeonite and augite are ubiquitous groundmass pyroxenes in the quartz-free dacites. These chemical and petrographical features imply that the quartz-dacites have calc-alkalic affinities, whereas the dacites have tholeiitic affinities. The tholeiitic trend of the dacites might be formed by fractional crystallization under low fO2 conditions, judging from the crystallization of Fe-rich pigeonite phenocrysts, and the quartz-rhyolite would be produced from the dacite magma by fractional crystallization in the tholeiite suite. Mixing calculations between bronzite-andesite and quartz-rhyolite reproduce the observed chemical variations of quartz-dacites. Therefore, calc-alkalic trend of the quartz-dacites might be formed by internal magma mixing between the bronzite-andesite magma and the quartz-rhyolite magma.

Magmatic Degassing of Volatiles and Ore Metals into a Hydrothermal System on the Modern Sea Floor of the Eastern Manus Back-Arc Basin, Western Pacific

Magmatic fluids were degassed before and during the eruption of vesicular volcanic rocks that host the actively forming massive sulfides at the PACMANUS hydrothermal field in the eastern Manus back-arc basin, western Pacific. The dredged samples of fresh lavas, ranging in composition from basalt to rhyolite, define a calc-alkalic trend that is thought to have resulted from fractionation in a common magma chamber. These rocks have variable vesicularity and vesicle size distributions that record the degassing history of the magma. The highly vesicular basalt and basaltic andesite are the least fractionated melt and experienced preeruptive, syneruptive, and posteruptive degassing. The weakly vesicular felsic rocks formed from an evolved magma that was largely degassed before its eruption. Vesicularity tends to decrease with Si, K, Ba, and Zr and to increase with Ca, Mg, Fe, and Sc in bulk samples, suggesting that the degassing of volatiles was linked to crystal fractionation of the magma. A volatile-rich magma is indicated by high concentrations of H 2 O (0.9–2.5%) and Cl (to 0.45%) in mafic melt inclusions in phenocrysts of the basaltic andesite. A fluid phase in the melt inclusions indicates that the magma was saturated with volatiles in the magma chamber. Volatiles exsolve as an immiscible fluid with increasing crystal fractionation, and the composition of the degassed magmatic fluid changes with the evolving magma. The fluid is CO 2 -dominated during the degassing of weakly fractionated mafic magma and becomes a mixture of CO 2 and H 2 O as H 2 O is increasingly exsolved from the highly fractionated felsic magma. The ore metals in the degassed fluid, as inferred from the compositions of the metallic precipitates found in the vesicles of melt inclusions and matrix glass, progressively change from Ni + Cu + Zn + Fe in basalt and basaltic andesite, to Cu + Zn + Fe in andesite, to Cu + Fe in dacite, to Fe in rhyodacite, and to Fe + Zn (+ Pb?) in rhyolite. This trend suggests that magmatic fluids, released from a fractionating magma, could be a source of metal for various types of ore deposits and mineral occurrences. Significant amounts of magmatic fluid can be degassed during the fractionation of a shallow magma chamber. At least 1.7 percent H 2 O is estimated to have exsolved from the magma at PACMANUS. If a fluid is concentrated in a shallow magma chamber and is discharged along a favorable structural zone that extends to the sea floor, it will contribute large quantities of volatiles and ore metals directly to a sea-floor hydrothermal system, a process demonstrated for the geothermal systems of subareal volcanoes. The focused discharge of a magmatic fluid as a result of preeruptive degassing, particularly in a fractionated felsic magma, could be responsible for the Fe, Cu, Zn, and Pb metals in the sulfide chimneys at PACMANUS. By analogy, a magmatic fluid can provide a major source of ore metals for large or super-large volcanogenic massive sulfide deposits in the geologic record.

Geochemistry and Genesis of the S-Type, Cordierite-Andalusite-Bearing Capillitas Batholith, Argentina

The Ordovician Famatinian-age magmatic cordierite-andalusite-bearing Capillitas batholith, in the Pampean Ranges of northwestern Argentina, encompasses a coarsely porphyritic to equigranular two-mica monzogranite with equigranular, fine-grained, late leucogranitic muscovite-rich facies. This batholith exhibits sharp, discordant contacts with low-pressure biotite-andalusite-cordierite schists of the La Cébila Formation, locally developing biotite-cordierite-muscovite-bearing contact hornfels aureoles. The two-mica monzogranite contains cordierite, andalusite, and sillimanite, although cordierite and andalusite are more abundant in the leucogranitic equigranular facies. These minerals are not homogeneously distributed and the three minerals are found together only locally. The presence of biotite-rich xenoliths with a high amount of anhydrous aluminum silicates (andalusite, sillimanite) and cordierite, exhibiting textures similar to those of the host monzogranites, suggest that, at least in part, they have been incorporated into the magma and reequilibrated. The pressure during emplacement was probably 4 kbar under near-solidus temperature, thus preserving the anhydrous aluminum-silicate stability under high H2O activity. The major-element data indicate a peraluminous calc-alkalic trend with compositional gaps. They attest to the existence of two distinct magma pulses. The chondrite-normalized REE patterns and multi-element spidergrams point to a probable origin by crustal (metasediment?) anatexis for both pulses. Peraluminous granitic magma cannot be a primary melt of metaluminous quartz-amphibolite, since there is a great geochemical homogeneity of all the granitoids. Igneous xenoliths are absent and the isotopic compositions of the granitoids correspond to those of metasedimentary sources. Both major and trace elements point to a collisional tectonic environment of an inner-continental magmatic arc.

Geochemistry of hybrid granitoid rocks and of their biotites from central northern Portugal and their petrogenesis

In the area of Franzilhal, S. Lourenço and Pombal, central northern Portugal, occur rocks ranging from hornblende-biotite tonalite to muscovite-biotite granite. The distribution of some major and trace elements in the rocks and in the biotite shows an apparent calc-alkalic trend of differentiation. The behaviour of K, Li, Rb, Cs and K/Rb, Cr.10 3/Fe 3+, Ni.10 3/Mg, Li.10 3/Mg, Cs.10 3/K ratios of the rocks and of F of biotite cannot be explained by mineral fractionation, but can be described in terms of hybridization. The tonalite, granodiorites and biotite-muscovite granite are hybrid rocks formed by assimilation of pelitic sediments and dolomitic marbles by the granite magma. The fO 2 values range from 10 −14.8 to 10 −16.3; fO 2 attains higher values during the crystallization of hybrid rocks than in the uncontaminated muscovite-biotite granite (GIII) magma. The fH 2O and PH 2O values generally increase and the fH 2O/fHF ratio decreases in the hybrid rocks from tonalite to biotite-muscovite-bearing granodiorite and is smaller than in the uncontaminated granite magma.

Some Chemical Characteristics and Variations of Gneisses, Intermediate Intrusive Rocks, and Anorthosite in the Lac Rouvray Map-area, Quebec

A sequence of high-grade gneisses acted as the host for the intrusions of anorthosite and intermediate rocks in the Lac Rouvray area. Based on structural data both these intrusive episodes are closely related spatially as well as temporally.Differentiation trends of the intermediate rocks show enrichment in alkalies relative to iron and magnesia. General agreement exists between the Lac Rouvray series and average calc-alkalic trends of differentiation, suggesting magma generation at considerable depth with emplacement of the intrusions into the crust at some horizontal distance from the most actively deforming part of the orogen.It is now generally accepted that anorthosites are crystal cumulates and therefore compositionally different from their parental magma. Anorthositic rocks have been included in some of the figures for completeness. Owing to their unique composition and general lack of variation, they occupy isolated regions in the projections.High contents of silica typify the gneisses. With increasing silica, variation trends of major oxides are in most respects similar to those of the intermediate suite. Structural and textural evidence, however, demonstrates these rocks to be unrelated.Soda contents for the gneisses and intermediate rocks are nearly constant over a considerable range in potash and lime. Linear decrease in iron and magnesia with increasing alkali content is reflected by all rocks. Anorthositic rocks are restricted to the high-alkali end of this trend, followed by gneisses at lower values of alkalies. The intermediate suite dominates the iron- and magnesia-rich portion of the trend.Potash values for gneisses and intermediate rocks are relatively high over a wide range of silica percentages. Normative feldspar compositions for these rocks, however, show that they are not unusually potassic. Comparison of their alkali-lime indexes shows a significantly higher alkalinity for the intermediate rocks. The index reflects mainly the variations in lime and potash, owing to the nearly constant soda contents.

The Troodos ophiolitic complex was probably formed in an island arc

The Troodos ophiolitic complex in Cyprus has been widely regarded as a fragment of oceanic crust that was created in a mid-oceanic ridge. However, about one-third of the analyzed rocks of the lower pillow lavas and sheeted complex in it follows a calc-alkalic trend. This strongly suggests that the massif was created as a basaltic volcano in an island arc with a relatively thin oceanic-type crust rather than in a mid-oceanic ridge. Other chemical features and the structure of the massif are also consistent with an island arc origin. Some other ophiolitic complexes also appear to have been formed in island arcs.