Feasibility of AFC models for the petrogenesis of calc-alkaline magma series

 One of the most popular petrogenetic concepts is that of simultaneous assimilation and fractional crystallisation (AFC). The equations governing these processes are well known and widely applied. The power and attraction of the model are that, with the minimum of constraints, petrogenetic schemes can be erected for essentially any group of rocks. These models are usually based on the observed variations in a small number of trace elements and/or isotopic systems. In this paper we have generated well-constrained AFC models for the Variscan, calc-alkaline Quérigut complex in the French Pyrénées. Our models are based on geological constraints on the identities of the parent magma and candidate assimilants, and the Sm-Nd and Rb-Sr isotope systematics of the rocks. The modelling is successful in mimicking the isotope array for the actual rocks. However, the results are massively inconsistent with those produced using the major- and trace-element concentrations. Our major conclusion is that AFC modelling should be undertaken only with robust initial constraints on the compositions of the parent magma and the proposed assimilant, with due respect for crystal-liquid partition coefficients, in so far as they are known. The results should then be accepted only if the isotope-based model parameters are consistent with those derived from major- and trace-element variations. Received: 19 August 1994/Accepted: 23 March 1995     

Variation in the geochemistry of mantle sources for tholeiitic and calc-alkaline mafic magmas,Western Sunda volcanic arc,Indonesia

Quaternary lavas of the normal island-arc basalt—andesite—dacite association in the islands of Java and Bali range from those belonging to tholeiitic series over Benioff-zone depths of ～ 150 km to high-K calc-alkaline series over Benioff-zone depths of 250 km. More abundant and diverse calc-alkaline lavas are found over intermediate Benioff-zone depths. On average, basaltic lavas become slightly more alkaline (largely due to increased K contents) with increasing depth to the Benioff zone. Levels of incompatible minor and trace elements (K, Rb, Cs, Ba, Nb, U, Th, light REE) show a corresponding increase of almost an order of magnitude.Low average Mg-numbers (～ 0.52) and Ni and Cr abundances (15–25 and 35–60 ppm, respectively) of basaltic lavas suggest that few lavas representing primary mantle-derived magma compositions are present. Calculated primary basaltic magma compositions for most tholeiitic and calc-alkaline volcanic centres are olivine tholeiites with 15–30% ol. The single high-K calc-alkaline centre considered yielded transitional alkali olivine basalt—basanite primary magma compositions. These calculated magma compositions suggest that the percentage of mantle melting decreases with increasing depth to the Benioff zone (from >25 to <10%), while the corresponding depth of magma separation increases from ～ 30 to 60 km.Calculation of REE patterns for basaltic magmas on the basis of peridotitic mantle sources with spinel lherzolite, amphibole lherzolite or garnet lherzolite mineralogy, and model REE levels of twice chondritic abundances, indicates that change in the conditions of magma genesis alone cannot explain the observed change in light-REE abundances of basaltic lavas with increasing depth to the Benioff zone. Complementary calculations of the REE levels of mantle sources required to yield the average tholeiitic, calc-alkaline and high-K calc-alkaline basaltic magma indicate that light-REE abundances must increase from 2–3 to 7–8 times chondrites with increasing depth to the Benioff zone. The percentages of mantle melting favoured on REE evidence are lower than those indicated by major-element considerations.The observed variation in incompatible element geochemistry of mantle magma sources is thought to be related directly or indirectly to dehydration and partial-melting processes affecting subducted oceanic crust. The possible nature of this relationship is discussed.     

The genetic significance of almandine-pyrope phenocrysts in the calc-alkaline Borrowdale Volcanic Group,Northern England

A suite of garnet-bearing andesites and dacites from the Ordovician of N. W. England is described and major- and trace-element analyses of the garnet phenocrysts are presented. The garnets are of almandine-pyrope composition, with minor amounts of spessartine and grossular, and often show marked reversed zoning; the crystal becoming progressively enriched in pyrope towards the margin. Garnets from the dacites are consistently richer in almandine and spessartine than are those from the andesites.From a consideration of the chemistry of the garnet phenocrysts and host rocks, especially La and Y abundances, it is shown that garnet could not have been removed from the magma in quantities sufficient to affect the liquid composition. Consequently the magma must have evolved by some process other than crystal fractionation. It is proposed that the magma was generated by the partial melting of oceanic crust along an ancient Benioff zone, stored at depth (possibly at the crust/mantle interface) long enough for garnet to nucleate, and then transferred rapidly to the surface. Isobaric crystallisation of the garnet phenocrysts at depth could explain the reversed zoning observed.     

Sector-zoned augite megacrysts in Aleutian high alumina basalts: implications for the conditions of basalt crystallization and the generation of calc-alkaline series magmas

Several high alumina basalts from the Aleutian volcanic centers of Cold Bay and Kanaga Island contain large (up to 1.5 cm diameter) megacrysts of sector-zoned augite. The megacrysts are invariably euhedral with well developed {001}, {010} and {111} forms. All crystals display concentric bands that are rich in mineral and glass inclusions. The sector zonation typically occurs as well developed (010), (100), (111) and (110) sectors which grew at different rates. A comparison of the width of synchronous growth bands indicates that following relative growth rates: (111) ≫ (100) ∼ (110) > (010). Compositionally, SiO₂ and MgO abundances decrease, and TiO₂, Al₂O₃, FeO and Na₂O abundances increase in the different sectors in the order (111), (100) ∼ (110), (010). This order is identical to that deduced for the relative growth rates, implying that growth rate clearly had a role in the development of the sector zonation. Calculated pre-eruption H₂O contents of the basalts range from 1 to 3 wt% but actual (measured) post-eruption H₂O contents range from 0.01 to 0.3 wt%. Deteurium isotopic values are heavily depleted and range from −110 to −141‰ . Together these indicate significant vapor (H₂O) exsolution prior to eruption. Maximum H₂O abundances in primitive glass inclusions, thought to be most representative of the host liquid reservoir at the time of melt entrapment, systematically decrease from the core to the rim of one augite megacryst studied in detail. We conclude that the presence of sector-zoned augite is due to augite supersaturation and rapid crystallization brought about by magma decompression and volatile (H₂O) exsolution. The calculated pre-eruption H₂O contents of 1–3 wt% limit vapor exsolution and basalt crystallization to depths of less than 3 and more likely 1.5 km. Very rapid crystallization at very shallow depths makes it unlikely that the time scales between initial crystallization and final eruption are sufficient to permit appreciable amounts of fractional crystallization. Given that high alumina basalt fractionation is the dominant process for generating more evolved andesite, dacite and rhyolite magmas of the calc-alkaline suite, the inability of parental high alumina basalt to yield such derivative magmas in the low pressure environment places the likely site of fractionation in the high pressure environment, at or near the base of the crust. Received: 1 December 1997 / Accepted: 23 December 1998     

Paricutin re-examined: a classic example of crustal assimilation in calc-alkaline magma

Following its birth on the 20th of February 1943, the Mexican volcano Paricutin discharged a total of 1.38 km³ of basaltic andesite and andesite before the eruption came to an end in 1952. Until 1947, when 75% of the volume had been erupted, the lavas varied little in chemical or isotopic composition. All were basaltic andesites with 55 to 56% SiO₂, ¹⁸O of +6.9 to 7.0, and ⁸⁷Sr/⁸⁶Sr ratios close to 0.7038. Subsequent lavas were hypersthene andesites with silica contents reaching 60%, ¹⁸O values up to +7.6, and ⁸⁷Sr/⁸⁶Sr of 0.7040 to 0.7043. The later lavas were enriched in Ba, Rb, Li, and K₂O and depleted in MgO, Cu, Zn, Cr, Ni, Sr, and Co. The isotopic and other chemical changes, which appeared abruptly over a few months in 1947, are interpreted as the result of tapping a sharply zoned and density stratified magma chamber. Xenoliths of partially fused felsic basement rocks in the lavas have silica contents greater than 70%, ¹⁸O of +5.6 to 9.9 and ⁸⁷Sr/⁸⁶Sr between 0.7043 and 0.7101. In many respects they resemble samples of basement rocks collected from nearby outcrops. Three analysed samples of the latter have silica contents of 65 to 67%, ¹⁸O of +7.7 to 8.6, and ⁸⁷Sr/⁸⁶Sr between 0.7047 and 0.7056.These new data provide strong support for the original interpretations of Wilcox (1954), who explained the chemical variations by a combination of fractional crystallization and concurrent assimilation of up to 20 weight % continental crust. Except for a few trace elements, particularly Ba, Sr, and Zr, the chemical and isotopic compositions of the xenoliths and basement rocks that crop out nearby match the type of contaminant required to explain the late-stage lavas. Some of the discrepancies may be explained by postulating a contaminant that was older and richer in Ba, Sr, and Zr than those represented by the analysed xenoliths. Others can be attributed to chemical changes accompanying disequilibrium partial melting, contact metamorphism, and meteoric-hydrothermal alteration of the country rock. Many of the xenoliths show evidence of having been affected by such processes.The lavas were erupted from a zoned magma chamber that had differentiated by liquid fractionation prior to the eruption. The order of appearance of the lavas can be explained in terms of withdrawal of stratified liquids of differing densities and viscosities.     

Igneous banding, schlieren and mafic enclaves in calc-alkaline granites: The Budduso pluton (Sardinia)

This study deals with the origin of igneous layering in plutons, and, especially, the extent layering is related to mafic–silicic magma interactions. The Budduso pluton (Sardinia) shows three main scales of organization.(i) Large scale lithological variations correspond to three main magmatic units, with differentiation increasing from the Outer (hornblende-bearing biotite granodiorite/monzogranite) to the Middle (biotite monzogranite) and the Inner (leucomonzogranite) units. The striking homogeneity of ⁸⁷Sr/⁸⁶Sr initial ratios (0.7090 ± 4) and ε_Nd(t) values (− 5.6 ± 0.1) strongly suggests that magma isotopic equilibration was achieved prior to emplacement, whereas mixing/mingling structures observed within the pluton reflect second-stage processes involving broadly cogenetic components.(ii) Metre to decametre-scale igneous layering may be isomodal or modally-graded, locally with cross-layering. Biotite and plagioclase compositions are similar in both biotite-rich and quartzofeldspathic layers, as are the trace-element patterns which differ only by relative abundances. This precludes an origin by fractional crystallization. A penetrative submagmatic fabric superimposed on the layering and corresponding mainly to flattening can be ascribed to interference between pluton growth and regional deformation.(iii) Composite layering and schlieren are commonly associated to mafic microgranular enclaves, locally within synmagmatic shear zones or disrupted synplutonic dykes. In that case, there is a progressive shift in biotite X_Fe values from the core of enclave ( 0.65) to the host monzogranite ( 0.72): schlieren in the monzogranite show biotite X_Fe values similar to that of the host rock, whereas schlieren close to mafic enclaves show lower X_Fe values ( 0.69) towards those of enclave rims.These features can be ascribed to three main processes: (i) assembly of differentiated (± mixed/mingled) magmatic pulses; (ii) local hydrodynamic sorting related to density currents in a mush, and segregation of residual melt; (iii) mechanical disruption and chemical hybridization of mafic magmas during ascent or within the pluton related to magma dynamics. None of these processes affect the whole pluton but they are limited to specific magmatic units. Therefore, pluton growth by incremental assembly of magma batches is not incompatible with magma chamber processes.     

Isotopic studies of processes in mafic magma chambers: II. The Skaergaard Intrusion,East Greenland

Isotopic ratios of Nd and Sr have been measured in a suite of samples spanning most of the exposed stratigraphy of the Skaergaard intrusion in order to detect and quantify input (such as assimilated wallrock and fresh magma) into the magma chamber during crystallization. Unlike ¹⁸O and D, Nd and Sr isotope ratios do not appear to have been significantly affected by circulation of meteoric waters in the upper part of the intrusion. Variations in initial ⁸⁷Sr/⁸⁶Sr and _Nd suggest that the Skaergaard magma chamber was affected during its crystallization by a small amount (2%–4%) of assimilation of Precambrian gneiss wallrock (high ⁸⁷Sr/⁸⁶Sr, low _Nd) and possibly recharge of uncontaminated magma. Decreases in _Nd and increases in ⁸⁷Sr/⁸⁶Sr during the early stages (0%–30%) of crystallization give way to approximately unchanging isotopic ratios through crystallization of the latest-deposited cumulates. Modelling of assimilation-fractional crystallization-recharge processes using these data as constraints shows that the assimilation rate must have been decreasing throughout crystallization. In addition, the isotope data allow replenishment by an amount of uncontaminated magma equal to 20%–30% of the total intrusion mass, occurring either continuously or in pulses over the first 75% of crystallization. Comparison of the recharge models with published Mg/(Mg+Fe²⁺) data from Skaergaard cumulates shows that the modelled replenishment rates are not inconsistent with available major element data, although significant recharge during the final 25% of crystallization can be ruled out. The isotope data show that the Skaergaard magma could have incorporated only a small amount of the gneiss that it displaced from the floor of the chamber; assimilation appears to have taken place primarily across a partially molten zone that formed at the roof from the wallrock that was dislodged during emplacement. In the latest stages of crystallization (>75% crystallized), the Skaergaard magma may have become stratified into two separately-convecting layers, effectively insulating Layered Series cumulates from further contamination.     

俯冲带镁铁质堆晶岩的母岩浆恢复: 方法与实例

古老俯冲带/造山带结构的恢复是重建古板块构造的基石。然而,古俯冲带/造山带由于受到造山作用和随后强烈的构造改造、隆升与剥蚀作用,绝大多数弧岩浆岩尤其是玄武岩较为缺乏,通常仅保留了代表弧下地壳的超镁铁质-镁铁质堆晶岩。从严格意义上来说,镁铁质堆晶岩是造岩矿物、副矿物与粒间熔体的集合体,其全岩成分不能代表弧岩浆本身,因此运用其地球化学来讨论俯冲带演化存在诸多不确定性和多解性。在前人提出的平衡配分方法基础上,作者近年来建立了基于单斜辉石和角闪石的俯冲带镁铁质堆晶岩的母岩浆恢复方法,并根据样品在原始地幔标准化微量元素蛛网图中的异常变化情况,通过增减相关的副矿物成分予以校正。该方法广泛应用于中国东北、华南、中亚造山带和云开地区等古俯冲带/造山带镁铁质堆晶岩的母岩浆成分恢复。通过对比计算结果与全岩的实测成分,发现两种方法获得的Th/La、Ba/La和Ba/Nb比值差别不大,反映这些微量元素比值可以用于示踪古俯冲带地幔楔的改造过程。然而计算获得的Th/Yb比值则明显比镁铁质堆晶岩的全岩实测值偏高,因此该比值不适合用来示踪俯冲带中沉积物的贡献。

     

Experimental determination of the isotopic fractionation between gaseous CO2 and carbon dissolved in tholeiitic magma

Isotope fractionation of carbon between CO₂ and carbon dissolved in a tholeiitic magma measured in the range 1120–1280 ° C, 7.0–8.4 Kb varies from 4.6 to 4 in favor of CO₂. These results make possible to explain all deep seated ¹³C values from a restricted range of primary mantle ¹³C concentrations. They also suggest that carbon could be dissolved in basaltic magmas in a reduced form.     

The record of magma chamber processes in plagioclase phenocrysts at Thera Volcano,Aegean Volcanic Arc,Greece

Lavas and pyroclastic rocks throughout the volcanic stratigraphy of the Tertiary-Quaternary volcanic complex of Thera in the Aegean island arc display inhomogenous plagioclase populations and phenocryst resorption textures, interpreted as indicative of magma mixing. Plagioclase zoning characteristics studied by Nomarski and laser interferometry techniques establish three main categories of plagioclase: (i) inherited plagioclase (nucleated in endmember prior to initial mixing event) (ii) in situ plagioclase (nucleated in mixed or hybrid magma) and (iii) xenocrystic plagioclase. Nomarski contrast images and linearized compositional zoning profiles reveal striking differences between calcic and sodic plagioclases, depending on the composition of the lava in which they are hosted. These differences reflect the contrasting effects of changes in physical-chemical parameters in basic vis-a-vis more acidic melts during magma mixing and/or influx of new magma into the subvolcanic magma chamber, as well as the influence of magma chamber dynamics on plagioclase equilibration. Variations in bulk major and trace element abundances of Thera volcanic products reflect the dominant overprint of crystal fractionation, but decoupling between major and trace element fractionation models and variations in incompatible trace element distributions are all indicative of magma mixing processes, consistent with compositional and textural zoning in plagioclases.     

Fingerprinting feldspar phenocrysts using crystal isotopic composition stratigraphy: implications for crystal transfer and magma mingling in S-type granites

 Microsampling of cm-scale feldspar crystals within an S-type granite from the Lachlan Fold Belt of southeastern Australia has revealed complex internal Sr and Nd isotopic variations. The observed isotopic zonations are in part interpreted as recording feldspar crystallisation in a dynamically mixing magma system, the isotopic composition of which was varying in response to the influx of more mafic and isotopically more mantle-like magmas, the latter stages of which are now represented in modified form by microgranular enclaves. Similar core to rim isotopic variations in feldspar megacrysts from a microgranular enclave and the adjacent host granite strongly suggest megacrysts in the enclave were transferred from the granitic magma during crystallisation. Feldspar rims have higher ⁸⁷Sr/⁸⁶Sr_i and lower ɛ_Nd(i) than adjacent whole rock analyses, but match those of mineral separates from the surrounding enclave matrix. This suggests that the final stages of megacryst growth occurred in the presence of a component that had previously interacted with a high ⁸⁷Sr/⁸⁶Sr, low ɛ_Nd(i) component such as metasedimentary wall rocks. Isotopic heterogeneities are also presererved within different mineral phases in the enclave matrix, suggesting that differing phases grew at differing stages of equilibration between the enclave magma and its host granitic magma. Our results reveal major isotopic heterogeneities on a single crystal and also inter-mineral scale in a pluton which shows well constrained evidence for magma mingling. These results indicate the suitability of feldspars as recorders of isotopic change in magmatic systems, even those which have cooled slowly in the plutonic environment and suggest that much heterogeneity in plutonic systems may be overlooked on a whole rock scale. Received: 28 September 1998 / Accepted: 29 December 1999     

相图对广西栗木花岗岩斑晶及起源岩浆的热力学形成条件的分析

与压力、温度、源岩成分、部分熔融程度有关的岩浆起源量化分析是花岗岩研究的难点。以广西栗木第二阶段花岗岩为研究对象,在岩相学及晶体类型分析的基础上,选择熔体晶体群中的斑晶和小颗粒环带净边结构斜长石及自形石英等作为探讨花岗岩起源之初始岩浆的特殊晶体群,运用相图分析它们形成时的压力和温度,进而推导出初始岩浆形成时的压力约为0.43 GPa(相当于深约16km处的上地壳),开始熔融出岩浆的共结点温度约为720°,初始岩浆最终熔出温度略高于800°,源岩成分的基性程度Ab/An值大于7.8。研究还表明,选择的特殊晶体群没有经历岩浆上升途中的阶段性生长,并认为,对于钙长石含量低的An—Ab—Or—Q岩浆体系(An5%),斑晶条纹长石的成分能够反映花岗岩浆起源时的部分熔融程度。该研究克服了前人利用矿物化学成分计算只能得出岩浆侵位时结晶温度、压力而得不到部分熔融时的熔融温度和压力的缺点,可为花岗岩起源分析提供有益的参考。     

Trace element fractionation trends of tholeiitic magma at moderate pressure: Evidence from an Al-spinel ultramafic-mafic inclusion suite

Trace element data on an Al-spinel ultramafic-mafic inclusion suite in an analcimite support earlier proposals that the various inclusions are comagmatic and represent fragments of a layered tholeiitic ‘pluton’ which differentiated at pressures close to 8 kb. Ultramafic inclusions are dominantly pyroxenites, often websteritic, whereas the mafic inclusions are largely two pyroxene-plagioclase assemblages. Appropriate experimental data and abundances of Sc, Cr and V indicate that subcalcic clinopyroxene or relatively Ca-rich clinopyroxene was the major ferromagnesian phase fractionated, often accompanied by spinel in the early and middle stages of differentiation and, in the later stages, by titanomagnetite. Comparatively moderate decreases in Ni and Co suggest that olivine was a relatively unimportant fractionating phase. Clinopyroxene fractionation at moderate pressures should be assigned only a comparatively insignificant role in the production of evolved basaltic compositions.     

Trace element fractionation trends of tholeiitic magma at moderate pressure: Evidence from an Al-spinel ultramafic-mafic inclusion suite

Trace element data on an Al-spinel ultramafic-mafic inclusion suite in an analcimite support earlier proposals that the various inclusions are comagmatic and represent fragments of a layered tholeiitic ‘pluton’ which differentiated at pressures close to 8 kb. Ultramafic inclusions are dominantly pyroxenites, often websteritic, whereas the mafic inclusions are largely two pyroxene-plagioclase assemblages. Appropriate experimental data and abundances of Sc, Cr and V indicate that subcalcic clinopyroxene or relatively Ca-rich clinopyroxene was the major ferromagnesian phase fractionated, often accompanied by spinel in the early and middle stages of differentiation and, in the later stages, by titanomagnetite. Comparatively moderate decreases in Ni and Co suggest that olivine was a relatively unimportant fractionating phase. Clinopyroxene fractionation at moderate pressures should be assigned only a comparatively insignificant role in the production of evolved basaltic compositions.     

Mineralogical relations and magma mixing in calc-alkaline andesites from lake Atitlán,Guatemala

Summary Quaternary calc-alkaline andesites erupted form three neighboring volcanoes along the Guatemalan volcanic front have mineralogic compositions and textures which show varying degrees of disequilibrium. Basaltic andesites and andesites (SiO₂ % = 50–59), erupted from Atitlán volcano located nearer to the trench, have the lowest degree of disequilibrium. These lavas contain an anhydrous phenocryst assemblage of mildly bimodal plagioclase, olivine, augite opx, and magnetite. Orthopyroxene occurs at the expense of olivine with increasing whole rock SiO₂. Most pyroxene phenocrysts show a trend of Fe enrichment.Andesites from Tolimán (SiO₂% = 53–62) and San Pedro (Si0₂% = 54–67) volcanoes, located further away from the trench, show comparatively high and moderate degrees of disequilibrium, respectively. Tolimán andesites have bimodal plagioclase compositions and textures. Olivine persists with increasing whole rock Si0₂ and lacks clear modal relations with coexisting orthopyroxene and hornblende phenocrysts. When compared to Atitlán andesites, Toliman olivines are more forsteritic and pyroxenes contain higher proportions of Mg-rich rims, though normal zoned phenocrysts occur within the same rock. Tolimán andesite also have lower proportions of phenocrysts to microphenocrysts, more calcic plagioclase groundmass compositions, and higher modal phenocrystic magnetite. San Pedro andesites have disequilibrium assemblages similar to Tolimán andesites but are not as striking.Magma mixing is proposed as the dominant cause for observed disequilibrium. Disequilibrium features are preserved best in Tolimán and San Pedro andesites because inferred durations between mixing and eruption are shortest, and consequently, these mixed andesites more clearly record mafic and silicic endmember compositions. The mafic component is a relatively high temperature, high-Al basalt containing phenocrysts of Mg-rich olivine (Fo = 78–80), calcic plagioclase (An 70–80), augite and titanomagnetite. The silicic component contains quartz, sodic plagioclase (An 40–50), Fe-rich orthopyroxene and titanomagnetite. Short durations between mixing and eruption produce petrographic features which, in part, mimic the effects of increasing PH₂0 in a fractionating magma. Inferred mixing durations for Atitlánn andesites are longer and involve a less-silicic composition. The intervolcano disequilibrium relations suggest that as Si0₂ in a silicic endmember increases, the duration and efficiency of mixing decreases.

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Mineral-Reaktionen und Magma-Mixing in Kalk-Alkali-Andesiten vom Atitlan See, Guatemala

Zusammenfassung Quartäre Kalk-Alkali-Andesite von drei benachbarten Vulkanen aus dem Guatemala Vulkan-Gürtel zeigen anhand ihrer mineralogischen Zusammensetzung und ihrer Textur variierende Bedingungen des Ungleichgewichts. Basalt-Andesite und Andesite (SiO₂% = 50–59), die aus dem dem Trench am nächsten gelegenen Vulkan Atitlan eruptierten, lassen den niedrigsten Grad an Ungleichgewicht erkennen. Diese Laven führen eine Phenokristall-Assoziation bestehend aus leicht bimodalem Plagioklas, Olivin, Augit, Orthopyroxen und Magnetit. In Gesteinen mit steigendem GesamtgesteinsSiO₂, tritt Orthopyroxen auf Kosten des Olivins auf. Die meisten Pyroxene zeigen einen Trend zur Fe-Anreicherung. Andesite vom Toliman (Si0₂% = 53–62) und vom Vulkan San Pedro (Si0₂% = 54-67), die beide weiter vom Trench entfernt liegen, zeigen hohen bzw. mittleren Grad an Ungleichgewicht. Die Toliman Andesite sind durch bimodale Zusammensetzung der Plagioklase und Textur gekennzeichnet. Olivin bleibt auch bei steigendem SiO₂-Gehalt bestehen, und zeigt hinsichlich seiner Zusammensetzung keine Verbindung mit koexistierenden Phenokristallen von Orthopyroxen und Hornblende. Im Vergleich zu den Atitlan Andesiten, weisen Toliman Olivine höheren Forsteritgehalt auf, die Pyroxene zeigen häufiger Mg-reiche Ränder, obwohl normal zonierte Phenokristalle auch im selben Gestein auftreten. Die Toliman Andesite sind durch ein kleineres Verhältnis von Phenokristalle/Mikrophenokristalle, höheren Ca-Gehalt der Matrix-Plagioklase und höheren Gehalt an Magnetit-Phenokristallen gekennzeichnet. Die San Pedro Andesite zeigen Ungleichgewichts-Paragenesen vergleichbar mit denen der Toliman Andesite, jedoch nicht so auffallend. Es ist zu vermuten, da\ Magma-Mixing vorwiegend für die UngleichgewichtsParagenesen verantwortlich ist. Die Toliman- und San Pedro Andesite zeigen die best erhaltenen Anzeichen für Ungleichgewicht, da vermutlich der Zeitraum zwischen Magma-Mixing und Eruption am kürzesten gewesen ist, weshalb diese Andesite die gemischten mafischen bzw. salischen Endglieder am besten widerspiegeln. Das mafische Endglied ist ein Al-reicher Hoch-Temperatur-Basalt, der sich aus Phenokristallen von Mg-reichem Olivin (Fo = 78–80), Ca-reichem Plagioklas (An 70–80), Fe-reichem Orthopyroxen und Titanomagnetit zusammensetzt. Das salische Endglied besteht aus Quarz, Na-reichem Plagioklas (An 40–50), Fe-reichem Orthopyroxen und Titanomagnetit. Aufgrund des kurzen Zeitabstandes zwischen Magma-Mixing und Eruption entstehen petrographische Strukturen, die den Effekt von steigendem PH₂0 in einem fraktionierenden Magma widerspiegeln. Es wird vermutet, da\ bei den Atitlan-Andesiten mehr Zeit zwischen Mixing und Eruption vergangen ist; dies manifestiert sich in einem geringeren Anteil der salischen Komponente. Die Zusammenhänge der Ungleichgewichts-Bedingungen innerhalb eines Vulkans lassen vermuten, daß die Dauer und Intensität des Mixing mit zunehmender salischer Komponente abnimmt.

     

Neoproterozoic zirconium-depleted boninite and tholeiitic series rocks from Adola, southern Ethiopia

In Adola, southern Ethiopia, mafic and ultramafic igneous rocks occur in narrow, 4–10 km wide, north-south-trending belts bounded by high-grade gneisses and migmatites. The mafic/ultramafic rocks are complexly deformed and metamorphosed in greenschist to lower amphibolite facies and are thought to be tectonically dismembered parts of an ophiolite complex. Preliminary geochemical and geochronological data highlight that the high-grade rocks in southern Ethiopia and northern Kenya include a significant portion of juvenile rocks that were accreted at the same time as ophiolitic rocks at 885-765 Ma. This is also the time of widespread oceanic magmatism and closure in the Arabian-Nubian Shield to the north.The Adola mafic rocks were previously described as island arc tholeiites and mid-ocean ridge basalts (MORB). New chemical analyses on the Megado belt rocks reveal the presence of boninites and related dacites interspersed with tholeiitic rocks. The Adola boninites are similar to the Cambrian boninites in western Tasmania in having relatively low Zr/Sm (≤32). Boninites with similarly low ratios have not been reported from elsewhere.The Adola tholeiites have high Ti/Zr (150–300). Mixing between tholeiite and boninite magmas may have resulted in elevated Ti/Zr (80–126) in some Adola boninites. Otherwise, Ti/Zr in the latter is low (20–40). Low Ti/Zr is characteristic of Tertiary boninites in the west Pacific. The fact that both Ti/Eu and Zr/Sm increase from the Adola and Tasmania type to the Tertiary boninites at constant Ti/Zr suggests that Ti might be an element that is also metasomatically added to the source of boninites and raises doubts about the role of amphibole in boninite petrogenesis.     

Early tholeiitic and calc-alkaline arc magmatism of middle to Late Eocene Age in the southern Ogasawara (Bonin) forearc

Analysis of middle and upper Eocene rocks from the IBM forearc, including the Ogasawara and Mariana Islands, help illuminate early arc volcanism of the proto-IBM arc. Dredged volcanic rocks from the forearc are two-pyroxene basalt to andesite, and may be divided into two groups, tholeiite and calc-alkaline, on the basis of mineralogy, petrography, and bulk chemistry. Tholeiites are characterized by high HFSE contents, high crystallization temperatures, and low water contents. In contrast, the calc-alkaline rocks are characterized by low HFSE contents, low crystallization temperatures, and higher water contents. These characteristics indicate that magma genesis for the two series differed. The tholeiites resulted from high degrees of partial melting of slightly depleted mantle under anhydrous conditions, whereas the calc-alkaline rocks were generated by low degrees of melting of depleted mantle under hydrous conditions. We believe that differences in mantle depletion arose from compositional layering and fluid zonations caused by MORB volcanism and slab dehydration, respectively. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.