Calc-alkaline Andesites Research Articles

Co-existing calcic amphiboles in calc-alkaline andesites: Possible evidence of a zoned magma chamber

Hornblende-biotite andesites erupted from Mount Price and Clinker Peak volcanoes, southwestern British Columbia, contain two texturally and compositionally distinct calcic amphiboles: pargasitic hornblende xenocrysts and magnesio-hornblende microphenocrysts. Disequilibrium relationships exhibited by these amphiboles and associated minerals suggest that the magnesio-hornblendes precipitated under chemical and thermal conditions that were intermediate between those under which pargasitic hornblende and biotite, respectively, crystallized. Experimental studies of crystallization in double-diffusive systems (Chen and Turner, 1980; Turner, 1980; McBirney, 1980) suggest that these varied magmatic environments can be explained as a consequence of progressive crystallization within a zoned magma chamber. Although gravitational settling may have played a role, the observed mineral assemblages probably developed by convective mixing of crystals precipitated at the cooling margins with those crystallized in the interior of the compositionally stratified magma column.

Fluorine geochemistry of quaternary volcanic rocks from Southwestern British Columbia: Some petrogenetic implications

Fluorine contents in 38 Quaternary volcanic rocks, representing calc-alkaline andesite eruptive groups from the Garibaldi Lake area, were determined by a selective ion-electrode method. A close relationship is evident between F abundance and the type of ferromagnesian phenocrysts present in the andesitic rocks. Hypersthene andesites have the lowest F contents (142–212 ppm), whereas hornblende-biotite andesites exhibit the highest F values (279–368 ppm); hornblende andesites have intermediate F contents (238–292 ppm). The hornblende-free Desolation Valley basaltic andesite has a lower F content than the hornblende-bearing Sphinx Moraine basaltic andesite (122 ppm versus 317–333 ppm).

Petrology and geochemistry of the island of Sarigan in the Mariana arc; calc-alkaline volcanism in an oceanic setting

Isotopic studies of rocks from oceanic island arcs such as the Marianas indicate that little, if any, recycling of continental material (e.g. oceanic sediments) occurs in these arcs. Because oceanic arcs are on the average more mafic than the dominantly andesitic continental arcs, an important question is whether the andesites of continental arcs are produced by a fundamentally different (more complex?) mechanism than the lavas of oceanic arcs. An excellent opportunity for study of this question is provided by the island of Sarigan, in the Mariana active arc, on which calc-alkaline andesites (including hornblende-bearing types) are exposed along with more mafic lavas.

Archaean greenstone belt from the Central African Republic (Equatorial Africa)

Abstract The Bandas belt, one of two prominent Archaean greenstone belts in the Central African Republic (Equatorial Africa), is ca. 250 km long. At the southernmost part of the belt, a metasedimentary—metavolcanic rock suite is preserved only in brachysynclines. The suite can be divided into two lithostratigraphic units. The lower unit is composed predominantly of volcanic rocks, while the upper one contains mainly metasedimentary rocks. The volcanic rocks, which are part of a sequence ca. 3600 m thick, can be sub-divided according to stratigraphic position, lithology and geochemistry into three groups. The lowermost group includes low-K tholeiitic basalts depleted in light REE. The second group consists of tholeiitic basalts with light REE-enriched patterns and the third, uppermost, group includes andesites, which are similar in several respects to Recent calc-alkaline andesites. The tholeiitic basalts of the first two groups are probably related to different upper mantle sources. The andesites of the third group were produced either by fractional crystallization from a basaltic magma enriched in light REE or equilibrium melting of eclogite or garnet amphibolite.

Petrogenesis and tectonic setting of late Precambrian ensimatic volcanic rocks, central eastern desert of Egypt

Abstract Early stages in the geologic evolution of the central eastern desert of Egypt (CED) reflect an intense episode of ensimatic volcanic activity similar to modern magmatism of the ocean floors and island arcs. This paper reports results from studies of the petrology and petrogenesis, and interprets the significance of these Late Precambrian volcanic rocks. A three-fold stratigraphy is preserved in the basement of the CED. A basal section of oceanic crust includes ultramafics, gabbros and pillowed basalts. These older metavolcanics (OMV) are conformably succeeded by dominantly volcanogenic metasediments, which are in turn succeeded by a dominantly andesitic, calc-alkaline sequence of younger metavolcanics (YMV). The OMV and YMV are largely restricted to the CED in Egypt, but analogous terranes are found in northern Arabia. (40–400 ppm) and Ni (30–260 ppm). They are poor in K 2 O (0.05–0.92%), Rb (0.3–5.0 ppm) and Ba (11–89 ppm). On Ti-Zr-Cr-V-Ni-P discriminant diagrams, the OMV plot in the field of modern abyssal tholeiites. High K/Rb (450–1800) and light REE depletions support this inference, although K/Ba (25–45) is lower than modern mid-ocean ridge basalts (MORB). The sum of OMV geochemical characteristics requires that these magmas were derived by the fractional fusion of the mantle. It is suggested that the OMV were generated by 20–25% fractional melting of previously depleted mantle at depths of less than 60 km. Relatively little fractionation accompanied ascent to the surface, where the OMV were erupted in a primitive crustal environment, either a small oceanic rift or a back-arc basin. Metamorphism of the YMV resulted in little elemental redistribution. These andesites have sub-alkaline clinopyroxenes and major-element geochemical characteristics indistinguishable from modern calc-alkaline andesites. YMV andesites in the central and western CED have K/Rb = 400–600, K/Ba = 20–40 and are light REE-enriched and heavy REE depleted. High concentrations of Cr (50–150 ppm) and Ni (20–100 ppm) and low initial 87 Sr/ 86 Sr ratios (0.7028–0.7030) indicate that these magmas were generated by melting in the mantle. Modelling studies and consideration of experimental data indicate that these andesites were formed by 2–10% fractional fusion of hydrous, undepleted, garnet therzolite at depths of 65 km or more in the mantle. The data show that an intense episode of instability, convection, and widespread melting occurred in the mantle beneath Afro-Arabia at the end of the Precambrian.

Geology and petrology of Quaternary volcanic rocks, Garibaldi Lake area, southwestern British Columbia: Summary

Recent geochemical studies of convergent margin volcanoes have been strongly influenced by the concept that calc-alkaline andesites are primary magmas formed by partial melting of wet mantle peridotite above subducted oceanic crust (Fountain, 1979; Rogers and Ragland, 1980; Suzuki and Shiraki, 1980). Partial melting of the peridotite is thought to occur through access of volatiles released by dehydration of the descending oceanic crust (McBirney, 1969; Best, 1975), or of volatile-rich silicate melts generated by partial fusion of the crustal portion of the subducted plate (Ringwood, 1975). Such an interpretation is critically dependent upon the assumption that the composition of melts erupted at the surface has been little modified by crystal fractionation. This paper examines the petrogenesis of Pleistocene-Holocene andesites from the Garibaldi Lake area of southwestern British Columbia on the light of observed mineralogical and chemical variations. These andesites provide evidence that crystal fractionation processes have significantly modified magma compositions. Evolution of British Columbia9s continental margin during the past 10 m.y. has been dominated by a complex pattern of plate interactions (Riddihough, 1977; Keen and Hyndman, 1979). The present plate-tectonic setting of southwestern British Columbia is depicted in Figure 1. The Queen Charlotte fault system, which parallels the edge of the continental shelf from southeastern Alaska to a poorly defined “ridge-trench-fault” triple junction just north of Vancouver Island, marks the locus of right-lateral transcurrent motion between the Pacific and North American plates. En echelon spreading axes (Juan de Fuca Ridge, Explorer Ridge, Dellwood and J. Tuzo Wilson Knolls) offset by short transform segments form the Juan de Fuca-Pacific and Explorer-Pacific plate boundaries. The Explorer plate, detached from the Juan de Fuca plate as the spreading ridge system rotated to become more nearly orthogonal to the Queen Charlotte fault, has been shown to be moving independently during the past 3 m.y. (Riddihough, 1977). Left-lateral motion between the Juan de Fuca and Explorer plates occurs across the Nootka tranform fault system, which extends north-easterly from the northern end of the Juan de Fuca Ridge to Nootka Island off the west coast of Vancouver Island (Keen and Hyndman, 1979). South of the Nootaka fault, the boundary between the Juan de Fuca and North American plates is a zone of convergence; seismic data show that the oceanic lithosphere dips at 5° toward the margin and reaches depths of 4 to 5 km before disappearing under Vancouver Island (Riddihough, 1979). The absence of a clearly defined eastward-dipping seismic zone and the lack of a major bathymetric trench at the foot of the continental slope have contributed to uncertainty as to whether subduction has taken place recently. Riddihough and Hyndman (1979), Ando and Balazs (1979), and Riddihough (1979), however, have reviewed relevant geological, geodetic, and geophysical data and concluded that the Juan de Fuca plate is now continuously underthrusting western North America. The plate motion is largely aseismic, but a few Benioff-type events at about 50 to 60 km beneath southern Georgia Strait (Keen and Hyndman, 1979) provide some weak constraints on the present position of the subducted plate. Whether the Explorer plate is now being subducted beneath the continent is less certain Riddihough (1977) suggested the possibility that the oceanic lithosphere east of Dellwood Knolls has become locked to the North American plate.

The mineral deposits and metallogenesis of the Fiji Platform

The Fiji platform has a thick, youthful crust of ensimatic origin. Stratigraphy within the Fiji islands indicates a history restricted to the Cenozoic, with volcanic rocks, principally submarine fragmental types with related sediments, dominating the succession. There are also carbonate sediments and plutonic rocks of gabbroic to tonalitic composition which were intruded during the Tholo orogeny, an important middle to late Miocene tectonic event.Mineral deposits in Fiji include various types of massive sulfide; manganese ores; disseminated porphyry-type Cu-Au deposits; epithermal-telethermal Au-Te-Ag deposits; veins, skarns, and limestone replacements related to plutonism; and residual deposits such as iron sands and bauxite.Preorogenic rocks are generally arc tholeiites, but following the Tholo orogeny there was great chemical diversity--possibly owing to anatexis and contamination--and eruption of arc tholeiite, calc-alkaline andesite, shoshonitic rocks, and oceanic-type basalts. Contamination of magmas by watery metamorphic fluids is thought to be the cause of relative enrichment of chalcophile elements in postorogenic rocks of late Miocene to early Pliocene age. Nearly all the major mineral deposits in Fiji are associated with these rocks.Volcanic and structural factors exert a local control on mineralization. On a regional scale plate collision in Outer Melanesia led to the Tholo orogeny and deep fracturing of the Fiji platform--factors of fundamental importance to Fiji mineralization; however, a simple correlation of island-arc mineral deposits and subduction is not apparent.

Geochemistry of the andesite flank lavas of three composite cones within the Atitlán Cauldron, Guatemala

Three composite cones have grown on the southern edge of the previously existing Atitlan Cauldron, along the active volcanic axis of Guatemala. Lavas exposed on the flanks of these cones are generally calc-alkaline andesites, but their chemical compositions vary widely. Atitlan, the largest and most southerly of the three cones, has recently erupted mainly pyroclastic basaltic andesites, while the flanks of San Pedro and Toliman are mantled by more silicic lava flows. On Toliman, 74 different lava units have been mapped, forming the basis for sequential sampling. Rocks of all three cones are consistently higher in K2O, Rb, Ba and REE than other Guatemalan andesites. Atitlan’s rocks and late lavas from Toliman have high Al2O3 content, compared to similar andesites from other nearby cones. All major and trace element data on the rocks are shown to be consistent with crystal fractionation involving phases observed in the rocks. If such models are correct, significant differences in the relative proportions of fractionation phases are necessary to explain the varied compositions, in particular higher Al2O3 rocks have fractionated less plagioclase. We speculate that inhibition of plagioclase fractionation could occur in chambers where PH2O is greater and when repose intervals are shorter. The distribution of volcanic vents throughout Guatemala which show this postulated «inhibition of plagioclase fractionation» is systematic with such vents lying just to the south of the main axis. The andesites of the three cones cannot be simply related to the late-Pleistocene rhyolites which are apparently associated with cauldron formation, because unlike the andesites, the rhyolites have markedly depleted heavy REE abundances. Recent dacitic lavas from vents south of San Pedro volcano and silicic pyroclastic rocks which mantle the slopes the San Pedro may reflect residual post-cauldron rhyolitic volcanism.

Geochemistry, strontium isotope data, and potassium-argon ages of the andesite-rhyolite association in the Padang area, West Sumatra

Abstract Quaternary volcanoes in the Padang area on the west coast of Sumatra have produced two-pyroxene, calc-alkaline andesite and volumetrically subordinate rhyolitic and andesitic ash-flow tuffs. A sequence of andesite (pre-caldera), rhyolitic tuff and andesitic tuff, in decreasing order of age, is related to Maninjau caldera. Andesite compositions range from 55.0 to 61.2% SiO 2 and from 1.13 to 2.05% K 2 O. Six K-Ar whole-rock age determinations on andesites show a range of 0.27 ± 0.12 to 0.83 ± 0.42 m.y.; a single determination on the rhyolitic ashflow tuff gave 0.28 ± 0.12 m.y. Eight 57 Sr/ 26 Sr ratios on andesites and rhyolite tuff west of the Semangko fault zone are in the range 0.7056 – 0.7066. These ratios are higher than those elsewhere in the Sunda arc but are comparable to the Taupo volcanic zone of New Zealand and calc-alkaline volcanics of continental margins. An 87 Sr/ 86 Sr ratio of 0.7048 on G. Sirabungan east of the Semangko fault is similar to an earlier determination on nearby G. Marapi (0.7047), and agrees with 87 Sr/ 86 Sr ratios in the rest of the Sunda arc. The reason for this distribution of 87 Sr/ 86 Sr ratios is unknown. The high 87 Sr/ 86 Sr ratios are tentatively regarded to reflect a crustal source for the andesites, while moderately fractionated REE patterns with pronounced negative Eu anomalies suggest a residue enriched in plagioclase with hornblende and/or pyroxenes. Generation of associated andesite and rhyolite could have been caused by hydrous fractional melting of andesite or volcanogenic sediments under adiabatic decompression.

山陰地域の鮮新世三朝層群中の火山岩のＳｒ同位体比と起源物質

87Sr/86Sr ratios of the volcanic rocks from the Pliocene Misasa Group and the andesite from the Quaternary Daisen volcano in San'in district were determined. The alkaline basalt and the calc-alkaline andesite from the Misasa Group have a restricted range of 87Sr/86Sr ratios (basalt 0.7051-0.7052; andesite 0.7052) and Rb/Sr ratios (basalt 0.03-0.04; andesite 0.04) with low Rb (basalt 19.4-34.5ppm; andesite 29.2ppm) and Sr (basalt 631-851ppm; andesite 729ppm) contents. These results may indicate that the calc-alkaline andesite may not have been derived from the alkaline basaltic magma by fractional crystallization or assimilation of older sialic materials. An analysis of the Daisen andesite yields a 87Sr/86Sr ratio of 0.7050, which is almost identical with that of the Misasa andesite. In 87Sr/86Sr ratios, the Misasa alkaline basalt, calc-alkaline andesite and the Daisen calc-alkaline andesite are very similar to the Oki-Dogo alkaline basalt (0.7048) which carries ultramafic inclusions and is possibly of the upper mantle origin (Kaneoka et al., 1978). This similarity in Sr isotopic ratios suggests that both the Misasa and Daisen volcanic rocks may have been derived from the upper mantle similar to the source material of the Oki-Dogo alkaline basalt.

Paleomagnetism of the Upper Cretaceous Mount Somers Volcanics, Canterbury, New Zealand

The Mount Somers Volcanics of mid Canterbury in eastern South Island are a high-K calcalkaline andesite–dacite–rhyolite suite erupted subaerially approximately 95 Ma ago during the long Late Cretaceous episode of normal geomagnetic field. Thermoremanent magnetic directions at 46 sites, corrected for post–eruption tectonic tilt, are stable and group well (a95= 3·8°, K = 31·7) with a very steep mean inclination (I = –85°, D = 354°). The resulting paleomagnetic South Pole position is at 52° S 174° E, indicating that Canterbury was within 10° of the South Pole 95 Ma ago, and that little finite rotation of Canterbury relative to the South Pole has occurred since. The Mount Somers Volcanics were erupted prior to the separation of eastern New Zealand from West Antarctica along the Pacific–Antarctic ridge. They provide a mid–Cretaceous pole position that can be compared with pole positions of similar age from Australia and with the polar–wander path previously reported for the Chatham Islands over the past 75 Ma. The Mount Somers pole fits neatly onto the New Zealand polar-wander path, which is consistent with the Australian polarwander path when convergent displacements across the Pacific-Indian plate boundary, and spreading of the Tasman Sea, are allowed for. The paleomagnetic data also support the proposition that regional folding and oroclinal bending of the pre-Cretaceous Torlesse basement rocks mainly predates the extrusion of the Mount Somers Volcanics and took place primarily during the Early Cretaceous Rangitata Orogeny and not during the Cenozoic as implied in recent plate-tectonic reconstructions.

Petrology of the Pliocene volcanic rocks in Sanin district with special reference to the genetic relationship between the alkaline basalt and the calc-alkaline andesite(Abstracts of Papers Submitted for the Meeting of the Society)

Petrology of the Pliocene volcanic rocks in Sanin district with special reference to the genetic relationship between the alkaline basalt and the calc-alkaline andesite(Abstracts of Papers Submitted for the Meeting of the Society)

143Nd/ 144Nd, 87Sr/ 86Sr, and incompatible element variations in calc-alkaline andesites and plateau lavas from South America

Trace element, Sr and Nd isotope data on Andean calc-alkaline andesites and plateau basalts from Patagonia are presented. Seven plateau lavas have 87Sr/ 86Sr ratios of 0.7033–0.7052 and 143Nd/ 144Nd of 0.51300–0.51262 and on a plot of 87Sr/ 86Sr versus 143Nd/ 144Nd they all lie within the region defined by Atlantic volcanic rocks. Six andesites from Ecuador have a narrow 143Nd/ 144Nd range of 0.51300–0.51285 while the Chilean calc-alkaline rocks have much lower ratios of 0.51257–0.51236. On the Nd versus Sr isotope diagram the Ecuador samples, and to a lesser extent the Chilean samples, are subparallel to the trend of the Patagonian and Atlantic lavas but displaced to more radiogenic compositions. Since both island-arc tholeiites and Andean andesites exhibit this shift to more radiogenic compositions, their Sr isotope geochemistry in particular may reflect similar processes along destructive plate margins in both continental and oceanic environments. The available data are briefly evaluated and a model is discussed whereby Rb, K, and Sr are preferentially released in H 2O-rich fluids from the downgoing slab as a result of dehydration. Depending on the amount and composition of the Sr both in the overriding lithosphere and released from the slab this will variously affect the geochemistry of the resultant volcanics. In general, however, they are enriched in alkalis relative to say the rare earth elements and, providing their 87Sr/ 86Sr ratios are <0.706–0.707, they reflect displacement to more radiogenic Sr compositions due to the input of seawater-affected Sr from the subducted lithosphere.

Strontium isotope evidence for petrogenesis of Central American andesites

THE active volcanic belts of Mexico and of Central America can be considered in two groups (Fig. 1). The Mexican volcanic belt (MVB) forms an irregular line trending away from the Middle America Trench, running from Ceboruco and Colima in the west to San Martin on the Caribbean coast1. In contrast to the MBV, the active Central American volcanic belt runs some 1,200 km from southern Mexico (Tacana) and Guatemala to Costa Rica, broadly parallel with the Middle America Trench1. With the exception of the alkaline volcano San Martin2, these are all calc-alkaline andesite volcanoes1. Between Mexico and El Salvador, the volcanic chain is built on a crust which includes Precambrian and Lower Palaeozoic crystalline rocks3,4. Southeast of El Salvador, in Nicaragua and Costa Rica the crust is composed mainly of Cainozoic volcanic and sedimentary rocks resting on Cretaceous ultramafic complexes3–6. This change in basement character does not seem to be reflected in the composition of volcanic rocks7. Here we report new Sr isotope data for Cainozoic volcanic rocks from Costa Rica, and by comparing these with earlier published data we conclude that all of these volcanic rocks have their origin in the mantle and that a sialic contribution to their origin is insignificant.

On the origin of andesite in the northern Mariana Island Arc: Implications from Agrigan

Magmatic evolution on the active volcano of Agrigan in the northern Mariana Island Arc is interpreted as resulting in the production of calc-alkaline andesites by the fractional crystallization of high-alumina basalt. Basaltic products predominate, but the ratio of andesites to basalts increases with time up to an event of voluminous andesitic pyroclastic ejection accompanied by caldera-collapse; post-collapse lavas are entirely basaltic. Moderate iron-enrichment is demonstrated for the volcanic suite, with indications of a progressive, pre-caldera decrease in iron-enrichment; post-caldera lavas display a return to moderate Fe-enrichment. Overall, the lavas are enriched in the LIL elements (K, Rb, Ba, Sr) and depleted in Ti, Mg, Cr, and Ni. From the oldest to the youngest pre-caldera volcanic sequence, the LIL elements increase 3-6X while Ca and Mg decrease by 50% or more. Approximately constant K/ Rb (430±60) and 87Sr/86Sr (0.7032–0.7034) indicate consanguinity of the basalts and the andesites. Cumulate plutonic xenoliths, common in the lavas, are composed of mineral phases also encountered as phenocrysts. The following order of crystallization is indicated: olivine; anorthite-bytownite; clinopyroxene; orthopyroxene and titanomagnetite. Co-existing xenolithic olivines (Fo74–83) and plagioclase (An88–96) are typical of calc-alkaline island-arc assemblages and contrast with assemblages in the tholeiites from the Mariana Trough to the west. The relatively fayalitic composition and low abundances of Ni in olivines and Cr in clinopyroxenes indicate equilibrium with an already-fractionated liquid. These data, along with structural evidence, high Ca in the olivines, and comparison of the observed assemblages with experimental studies, suggests that these xenoliths formed as crystal cumulates at the floor of a shallow (≦ 7 km) crustal magma chamber.

Andesites of the Tongariro volcanic centre, North Island, New Zealand

The Tongariro volcanic centre comprises four major andesite massifs, Kakaramea, Pihanga, Tongariro and Ruapehu, and four smaller cones and flows, Maungakatote, Pukeonake, Hauhungatahi and Ohakune. Older lavas from Kakaramea, Pihanga and Tongariro were erupted from a series of vents aligned NW-SE and more recent lavas from vents aligned NNE-SSW. The most voluminous lava types in the centre are labradorite and labradorite-pyroxene andesite containing phenocrysts of plagioclase (An 68—An 48), orthopyroxene (mainly bronzite) and clinopyroxene, in a fine to medium-grained groundmass. Smaller amounts of pyroxene andesite (from Pihanga), olivine andesite (from NNE-trending vents), and hornblende andesite also occur. Pigeonite surrounds orthopyroxene in some of the pyroxene andesites. Average major and trace element compositions of each petrographic type indicate that all lavas are “normal” calc-alkaline andesites and low-Si andesites. Low-Si andesites, represented mainly by pyroxene and olivine andesites, are typified by higher FeO, MnO, MgO, Sr, Cu, Ni, V, Sc and Cr and lower Al 2O 3, Na 2O, K 2O, Rb, Ba, Zr and Pb than “normal” andesites. Some low-Si andesites are regarded as phenocryst-rich cumulates. The location of the Tongariro andesites and alignments of eruptive vents suggest a source from a subduction zone underlying the area. However, the lavas differ chemically from island arc andesites such as those of Tonga by having higher Na 2O, K 2O, Rb, Ba and Zr. This suggests some crustal contamination, but the overall chemistry of the andesites is inconsistent with an origin by mixing, en route to the surface, of high-alumina basalt with either Mesozoic greywacke-argillite or with a partial melt of the greywacke-argillite equivalent in composition to Taupo rhyolite. A model is presented in which oceanic crust assimilates Mesozoic and Cenozoic sediments at the eastern side of the North Island, and is subducted to produce amphibolite. This amphibolite would subsequently break down, hydrating the overlying upper mantle and lower crust, and, in steeply dipping subduction zones such as that under Tongariro, would produce phlogopite eclogite below 90 km. The latter will partially melt at 150–200 km and the resultant magma will fractionate in the upper mantle or lower crust to produce andesite.

Strontium isotope evidence for petrogenesis of Mexican andesites

THE Mexican volcanic belt (MVB) consists of several inactive and active volcanoes1 (Fig. 1). The active volcanoes include Colima1,2 and Ceboruco3 near the Pacific coast, Popocatepetl and Iztaccihuatl in central Mexico, and San Martin4 close to the Caribbean coast. The MVB rests on continental crust approximately 40 km thick5,6, and is associated with subduction of the Cocos plate below central Mexico3,7. The link between plate subduction and the disposition of the MVB is not simple since the latter makes an angle of about 20° with the Middle America Trench3. This complexity might be related to the structure of the continental crust5, or to a combination of subduction and extensional processes4. The most prominent products of volcanism of the MVB are the large composite volcanoes built up from successive eruptions of intermediate lavas and pyroclastic rocks3,8. The lavas of the active volcanoes shown in Fig. 1 are dominantly calc-alkaline andesite and dacite3,8–10. In contrast, the easternmost active volcano, San Martin, is built from lavas of a picritic basalt–basanitoid–alkali basalt–hawaiite association4,11,12. This volcanic area occurs at a change in direction of the MVB and might therefore be linked with extension or fracturing associated with the destructive margin setting of the MVB (ref. 4). Here we report new Sr isotope data for lavas from Ceboruco, Colima, and San Martin. These data, and published Sr isotope data for Cainozoic volcanics from central Mexico13 constitute evidence for petrogenesis of Mexican volcanic rocks.

The Petragenesis of some Archaean Volcanic Rocks from Southern Africa

Major, rare earth and transition elements, have been determined on a selection of volcanic rocks from greenstone belts in Rhodesia (∼2.6 by) and South Africa (∼3.4 by). In Rhodesia two distinct series can be recognized: a komatite-tholeiite series which occurs early in the greenstone belt evolution and apparently grades into a second, calc-alkaline, series at higher structural levels. Peridotitic komatites reflect higher degrees of partial melting than any Phanerozoic rocks so far observed and are therefore used to place limits on the composition of their source. At lower MgO contents they merge into low K tholeites which have significantly higher CeN/YbN and Rb/Sr ratios (at any MgO content) than those observed in modern ocean floor and island are environments. The calc-alkaline series is characterized by andesites which exhibit a marked heavy REE depletion, but similar light REE and transition metal contents to the more evolved tholeitic rock types. The continuum of compositions from komatiites to tholeiites and calc-alkaline andesites suggests that the bulk of greenstone belt volcanics could have been derived by differential partial melting, and polybaric fractionation of an essentially homogeneous peridotite source. Late stage dacitic lavas and intrusions probably reflect melting of a more eclogitic source region. The distinctive trace element geochemistry of Archaean volcanics, particularly the high Ni and low Yb values of the calc-alkaline rocks precludes direct comparison with modern island are associations. Rather the large decrease in liquidus temperatures (500 °C) with increasing structural height within greenstone belts, coupled with the fact that most of the volcanics could have been derived from an essentially homogeneous source, may suggest that greenstone belts developed in a rifting environment. It appears unlikely that the tholeiite/calc-alkaline association observed in the Archaean may be taken as an indication of subduction at that time.

Geneses of Calcalkaline Andesites as Demonstrated by the Examples : at Oshima-0shima and Other Areas

Geneses of Calcalkaline Andesites as Demonstrated by the Examples : at Oshima-0shima and Other Areas

5.Chemical compositions of calc-alkaline andesites and its phenocrysts, in Bandai Volcano, Nontheastern Japan

5.Chemical compositions of calc-alkaline andesites and its phenocrysts, in Bandai Volcano, Nontheastern Japan