Distinct Magma Batches Research Articles

Millennial timescale resolution of rhyolite magma recharge at Tarawera volcano: insights from quartz chemistry and melt inclusions

Most rhyolite eruption episodes of Tarawera volcano have emitted several physiochemically distinct magma batches (∼1–10 km3). These episodes were separated on a millennial timescale. The magma batches were relatively homogeneous in temperature and composition at pumice scale (>4 cm), but experienced isolated crystallisation histories. At the sub-cm scale, matrix glasses have trace element compositions (Sr, Ba, Rb) that vary by factors up to 2.5, indicating incomplete mixing of separate melts. Some quartz-hosted melt inclusions are depleted in compatible trace elements (Sr, Ti, Ba) compared to enclosing matrix glasses. This could reflect re-melting of felsic crystals deeper in the crystal pile. Individual quartz crystals display a variety of cathodoluminescence brightness and Ti zoning patterns including rapid changes in melt chemistry and/or temperature (∼50–100°C), and point to multi-cycle crystallisation histories. The Tarawera magma system consisted of a crystal-rich mass containing waxing and waning melt pockets that were periodically recharged by silicic melts driven by basaltic intrusion.

Magma chamber evolution prior to the Campanian Ignimbrite and Neapolitan Yellow Tuff eruptions (Campi Flegrei, Italy)

The Campi Flegrei (Campanian Region, Italy) experienced two cataclysmic caldera-forming eruptions which produced the Campanian Ignimbrite (39 ka, CI) and the Neapolitan Yellow Tuff (15 ka, NYT). We studied the minor eruptions before both these large events to understand magma chamber evolution leading towards such catastrophic eruptions. Major, trace element, and Sr and Nd isotope compositions of pre-Campanian Ignimbrite and pre-Neapolitan Yellow Tuff products define distinct geochemical groups, which are here interpreted as distinct magma batches. These batches do not show any transitional trend towards the CI and NYT eruptions. The CI and NYT systems are decoupled geochemically and isotopically. At least one of the pre-CI and one of the pre-NYT erupted magma batches qualifies as mixing endmembers for the large CI and NYT eruptions, and thus, must have been stored in reservoirs for some time to remain available for the CI and NYT eruptions. The least evolved, isotopically distinct magma compositions that are typical of the last phases of the NYT and CI eruptions did not occur before caldera-forming events. Based on the new data, we propose the following scenario: Multiple magma chambers with distinct compositions existed below the Campi Flegrei before the CI and NYT eruptions and remained generally separated for some time unless new magma was recharged. In each case, one of the residing magma reservoirs was recharged by a new large-volume magma input of intermediate composition from a deeper differentiating magma reservoir. This may have triggered the coalescence of the previously separated reservoirs into one large chamber which fed the cataclysmic caldera-forming eruption. Large magma chambers in the Campi Flegrei may therefore be ephemeral features, interrupted by periods of evolution in individual, separated magma reservoirs.

Tephra beds in deep-sea cores off northern New Zealand: implications for the history of Taupo Volcanic Zone, Mayor Island and White Island volcanoes

Twenty piston cores were collected from water depths of ∼ 650–3300 m from offshore Bay of Plenty, North Island, New Zealand. They contain tephra from 11 Okataina Volcanic Centre and 4 Taupo Volcanic Centre rhyolite eruptions during the last ∼ 50 kyr that produce a tephrostratigraphic framework across the tectonically and volcanically complex region of southern Havre Trough, Alderman Trough and the Bay of Plenty continental shelf. This allows correlation between offshore and onshore sequences up to 200 km from source volcanoes, covering much of Marine Isotope Stages 3 and 2. The framework temporally constrains poorly dated and newly recognised volcanic events. Macroscopic tephra layers from the peralkaline Mayor Island volcano are documented for the first time at pre-50, post-50, 40.5, 37.4, 22.2, and 14.2 ka, in addition to the well known 7 ka (Tuhua) event. These macroscopic layers represent some of the most explosive events from this volcano, and provide new marker horizons. They are dispersed up to 90 km north-east to east of the edifice. Minor tephra dispersal is also recorded at 17.8, 25.5, ∼ 35 and pre-50 ka. The tephra contain high SiO 2 (73–75.5 wt.%) glass and subordinate basaltic components, and each tephra represents a distinct magma batch that can be fingerprinted. The Mayor Island tephra form two temporal trends toward less evolved magma compositions, punctuated by a large caldera-forming event at 36 ka. The historically active, andesitic White Island volcano does not have widely dispersed tephra, and the oldest primary deposit found is ∼ 21 ka. Five pre-50 ka rhyolite eruptions from an unknown Taupo Volcanic Zone source provide evidence for explosive activity in a time interval poorly documented on-land. The cores demonstrate the patchy and uneven preservation of large magnitude tephra falls caused by local faulting, bioturbation and ponding in bathymetrically complex regions. Reworked tephra layers are common and often lack indicative lithological features. Such units could easily be misinterpreted as primary events without micro-beam geochemical analyses of glass shards.

Petrogenesis of dacite in an oceanic subduction environment: Raoul Island, Kermadec arc

Raoul Volcano in the northern Kermadec arc is typical of volcanoes in oceanic subduction systems in that it is composed mainly of low-K high-Al basalts and basaltic andesite. However, during the last 4 ka Raoul Volcano has produced mainly dacite magma in pyroclastic eruptions associated with caldera formation. The rocks produced in these episodes are almost aphyric containing only sparse crystals of plagioclase, clinopyroxene, orthopyroxene and magnetite. These apparent phenocrysts have chemical compositions that suggest that they did not crystallise from melts with the chemical composition of their host rocks. Rather they are xenocrysts and only their rims show evidence for crystallisation from their host melt. Chemical compositions of samples of the dacites show that each eruption has tapped a distinct magma batch. Compositional variations through the analysed suite cannot be accommodated in any reasonable model of fractional crystallisation from likely parental magma compositions. The hypothesis that best fits the petrology of Raoul Island dacites is one of crustal anatexis. This model requires heating of the lower crust by a magma flux to the point where dehydration melting associated with amphibole breakdown produces magma from a preconditioned source. It is suggested that Raoul is passing through an adolescent stage of development in which siliceous melts are part of an open system in which felsic and mafic magmas coexist.

Re-examination of crystal ages in recent Mount St. Helens lavas: implications for magma reservoir processes

U-series data for recent Mount St. Helens lavas suggest that crystallization preceded eruption by more than 0.5 ka but are complicated by possible evidence of crystal recycling and/or addition of radium to the liquid after crystallization. We report new ion and electron microprobe trace- and major-element data for plagioclase and pyroxene in these recent Mount St. Helens lavas and use these data to reassess 226Ra–230Th crystal ages by taking into account differences in the partitioning behavior of radium and barium and the effects of impurities in mineral separates. Revised 226Ra–230Th model crystallization ages are ∼2–4 ka for plagioclase (with the exception of the 1982 dacite) and ∼0.15–5.7 ka for pyroxene. In contrast to previous interpretations, no late-stage addition of Ra to the liquid after precipitation of the minerals is required. The variability of Ba concentrations measured in plagioclase is too large to be consistent with progressive crystallization from the same liquid or with diffusive re-equilibration of xenocrysts with a new host liquid. Ba heterogeneity limits the residence time of the crystals in a magma at high temperatures and also suggests that in most cases Ra–Th ages have not been significantly modified by Ra diffusion into or out of the crystals. High (226Ra)/Ba in plagioclase in the 1982 dacite relative to the host liquid likely reflects crystallization processes that precluded bulk crystal–liquid chemical equilibrium. One possibility is that of growth entrapment of surface enrichments during rapid crystallization, which could lead to less discrimination between Ra and Ba than predicted by calculated bulk partition coefficients. 226Ra–230Th crystal ages for the Castle Creek andesite and basalt that are younger than 230Th–238U ages of the same crystals could be explained by mixing of crystals into melts with different 230Th/232Th ratios, by combinations of older and younger crystal growth within the same magma, or, for the basalt, by diffusion of Ra into crystals at high magmatic temperatures. Average plagioclase ages in most of the samples of >2 ka imply that some significant mass fraction of the crystals in each flow was present simultaneously beneath the volcano. This observation could be consistent either with simultaneous storage of physically distinct magma batches or with incorporation of a population of similarly aged crystals into each successive magma batch.

Timing of magma extraction during the Campanian Ignimbrite eruption (Campi Flegrei Caldera)

A core drilled within the northern part of the city of Napoli has offered the unique opportunity to observe in one single sequence the superposition of the four pyroclastic flow units emplaced during the Campanian Ignimbrite (CI) eruption. Such a stratigraphic succession has never been encountered before in natural or in man made exposures. Therefore the CI sequence was reconstructed only on the basis of stratigraphic correlations and compositional data (in literature). The occurrence of four superposed CI flows, together with all the data available (in literature) allowed us to better constrain the chemical stratigraphy of the deposit and the compositional structure of the CI magma chamber. The CI magma chamber includes two cogenetic magma layers, separated by a compositional gap. The upper magma layer was contaminated by interaction with radiogenic fluids. The two magma layers were extruded either individually or simultaneously during the course of the eruption. In the latter case they produced a hybrid magma. But no evidence of input of new geochemically and isotopically distinct magma batches just prior or during the eruption has been found. Comparison with the exposed CI deposits has permitted reconstruction of variable eruption phases and related magma withdrawal and caldera collapse episodes. The eruption was likely to have began with phreatomagmatic explosions followed by the formation of a sustained plinian eruption column fed by the simultaneous extraction from both magma layers. Towards the end of this phase the upward migration of the fragmentation surface and the decrease in magma eruption rate and/or activation of fractures formed an unstable pulsating column that was fed only by the most-evolved magma layer. This plinian phase was followed by the collapse of the eruption column and the beginning of caldera formation. At this stage expanded pyroclastic flows fed by the upper magma layer in the chamber generated. During the following major caldera collapse episode, the maximum mass discharge rate was reached and both magma layers were tapped, generating expanded pyroclastic flows. Towards the end of the eruption, only the deeper and less differentiated magma layer was tapped producing more concentrated pyroclastic flows that traveled short distances.

Tephra sequences as indicators of magma evolution: 40Ar/ 39Ar ages and geochemistry of tephra sequences in the southwest Nevada volcanic field

Changes in rock chemistry with 40Ar/ 39Ar ages in tephra layers record the temporal and magmatic history of two volcanic systems in southwestern Nevada. Tephra layers from the Post-Grouse Canyon tephra sequence record three distinct groups. These groups are chemically distinct and have 40Ar/ 39Ar ages of 13.52±0.06, 13.31±0.18, and 12.95±0.10 Ma. The age groups correspond to three distinct chemical groups based on trace element distributions. These chemical groups cannot be related by any reasonable fractional crystallization or magma mixing model and are interpreted as distinct magma batches. The Pre-Rainier Mesa tephra sequence records two 40Ar/ 39Ar ages (12.79±0.10 and 11.84±0.18 Ma). The upper portion of this sequence is equivalent in age and chemistry to part of the overlying Rainier Mesa ash-flow sheet. The lower portion of the sequence is equivalent in age to the underlying Tiva Canyon ash-flow sheet but is chemically distinct from this sheet. The formation of this chemical group is consistent with mixing of low silica Tiva Canyon and high silica, low Th, Rainier Mesa magma. Post-Grouse Canyon magmas were most likely emplaced as a series of small, unrelated magma bodies, which allowed them to evolve independently. The mixed Pre-Rainier Mesa magma was produced by infilling of the Tiva Canyon magma chamber with Rainier Mesa-like magma after eruption of the Tiva Canyon ash-flow sheet at approximately 12.8 Ma. The upper portion of the Pre-Rainier Mesa tephra sequence represents eruption of Rainier Mesa magma less than 0.3 My. before that of the voluminous (1200 km 3) Rainier Mesa ash-flow sheet at approximately 11.71 Ma.

Weak chemical and mineralogical zonation in the Kaingaroa Ignimbrite, Taupo Volcanic Zone, New Zealand

The 0.23 Ma Kaingaroa Ignimbrite is a composite, multiple flow‐unit ignimbrite erupted from the Reporoa Caldera, Taupo Volcanic Zone, New Zealand. Reporoa Caldera evolved from a single vent during the initial stages of the Kaingaroa eruption, to a trapdoor caldera with the opening up of fractures along the eastern margin and asymmetric eruption of early phases of the ignimbrite. Finally, plate collapse occurred during the later stages of the eruption. Kaingaroa Ignimbrite pumice clasts range in composition from dacite to rhyolite. Five pumice types have been identified based largely on the geochemical variation of Rb and Sr and ferromagnesian mineral composition. Pumice types A, B, C, and D exhibit variations in mineralogy, trace element chemistry, and isotopic composition consistent with derivation from a weakly zoned magma chamber. A subordinate dacitic juvenile component (type E) appears to be unrelated to the Kaingaroa pumices by crystal fraction‐ation or assimilation, and is interpreted as a distinct magma batch that was incorporated into the eruption. The asymmetrical removal of magma from the chamber during caldera collapse is inferred to have resulted in a greater degree of drawdown along the eastern margin of the chamber, as suggested by the current spatial distribution of post‐caldera rhyolites.

Structure of volcanic glasses from the NMR-EPR perspective: a preliminary application to the Neapolitan Yellow Tuff

Glass samples from alkali-trachytic pumice of the Lower Member (LM, level LM1) and Upper Member (UM) of the Neapolitan Yellow Tuff at Campi Flegrei Caldera were studied by nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR) spectroscopy. The EPR data show that the relative occupancy of the different iron sites varies in samples from the Lower Member LM1 and Upper Member of the eruptive sequence. The LM1 glasses are characterized by a more homogeneous distribution of the cationic site population of iron compared with UM and by a different Fe3+/Al3+ substitution in the C2v symmetrical sites of the silicate framework. 29Si MAS NMR spectra on glasses indicate that LM1 glasses are more depolymerized than UM glasses, indicating the existence of structurally distinct magma batches. 27Al MAS NMR spectra indicate the occurrence in UM glasses of aluminum in both tetrahedral and octahedral coordination; the observed AlVI is believed to be a primary feature of the glass, since 27Al CP-MAS NMR experiments do not reveal detectable esa-hydrated Al. The Al in sixfold coordination could be ascribed either to the more abundant alkali–OH terminations in UM, in which alkalies would be extracted from their charge-balance role for Al, thus altering its fourfold structural position, or to permanent compaction during fast cooling and decompression of high-pressure melts. 23Na CP-MAS NMR experiments indicate the existence of Na–OH groups in both LM1 and UM glasses, relatively more abundant in the latter. Although preliminary, the results indicate the possible application of atomic-scale studies to variables that determine the regimes of explosive volcanism.

Isotope Geochemistry of the Oceanic Mantle Near the Bouvet Triple Junction

We have measured the isotopes of helium and lead in three suites of dredged basalts from the Southwest Indian Ridge, the America-Antarctic Ridge, and the Mid-Atlantic Ridge near the Bouvet triple junction. The magmatic 3He/ 4He ratios range from 6.5 to 14.2 times atmospheric (R a), which encompasses the entire range of existing MORB data, and differs significantly from the commonly accepted value for depleted MORB of roughly eight times atmospheric (Ra). Several samples from near the Bouvet triple junction have extremely low helium concentrations, low 3He/ 4He ratios, and display internal isotopic disequilibrium between glass and vesicles. These basalt glasses are, therefore, unique among the sample suite in having a significant contribution from post-eruptive accumulation of radiogenic 4He. Although these data do not reflect the mantle source, they can be used to calculate eruption ages for the basalts, which fall between 7 and 100 kyr, and demonstrates the potential utility of He-U ages for MORB. Lead isotopic compositions from the dredged basalt glasses range from depleted MORB to those of Bouvet Island itself ( 206Pb/ 204Pb of 18.2–19.6). The results are consistent with a mantle plume origin of Bouvet Island in that high 3He/ 4He and 206Pb/ 204Pb ratios are observed both on Bouvet island and the ridge segment immediately adjacent to it. The American-Antarctic Ridge has lower He, Sr and Pb isotope ratios than those of the Southwest Indian Ridge, which suggests a lesser influence of the plume. The dredge samples from near the triple junction have relatively low 3He/ 4He ratios (7.4 ± 0.2 R a), suggesting that the plume is presently centered beneath Bouvet Island rather than beneath the triple junction. The data from the ridge segment adjacent to Bouvet Island, on the Southwest Indian Ridge, define a trend toward high helium, strontium, and lead isotope ratios, which could be explained by mixing processes. A single dredge from one ridge segment at 7°E, between the Islas Orcadas and Shaka Fracture Zones, defines most of the helium isotopic variability found throughout the region and defines an isotopic composition unique in the region. The highest 3He/ 4He ratios were obtained from this ridge segment (∼14 R a) and are associated with relatively unradiogenic strontium and lead isotopic compositions. The remarkable isotopic variability over short distances along the Southwest Indian Ridge and the America-Antarctic Ridge may be related to their slow spreading rates and the lack of steady state magma chambers which could homogenize geochemically distinct magma batches. Alternatively, there is a different scale of heterogeneity in the mantle beneath this region.

Magma batches in the Timber Mountain magmatic system, Southwestern Nevada Volcanic Field, Nevada, USA

The common occurrence of compositionally and mineralogically zoned ash flow sheets, such as those of the Timber Mountain Group, provides evidence that the source magma bodies were chemically and thermally zoned. The Rainier Mesa and Ammonia Tanks tuffs of the Timber Mountain Group are both large volume (1200 and 900 km 3, respectively) chemically zoned (57–78 wt.% SiO 2) ash flow sheets. Evidence of distinct magma batches in the Timber Mountain system are based on: (1) major- and trace-element variations of whole pumice fragments; (2) major-element variations in phenocrysts; (3) major-element variations in glass matrix; and (4) emplacement temperatures calculated from Fe-Ti oxides and feldspars. There are three distinct groups of pumice fragments in the Rainier Mesa Tuff: a low-silica group and two high-silica groups (a low-Th and a high-Th group). These groups cannot be related by crystal fractionation. The low-silica portion of the Rainier Mesa Tuff is distinct from the low-silica portion of the overlying Ammonia Tanks Tuff, even though the age difference is less than 200,000 years. Three distinct groups occur in the Ammonia Tanks Tuff: a low-silica, intermediate-silica and a high-silica group. Part of the high-silica group may be due to mixing of the two high-silica Rainier Mesa groups. The intermediate-silica group may be due to mixing of the low- and high-silica Ammonia Tanks groups. Three distinct emplacement temperatures occur in the Rainier Mesa Tuff (869, 804, 723 °C) that correspond to the low-silica, high-Th and low-Th magma batches, respectively. These temperature differences could not have been maintained for any length of time in the magma chamber (cf. Turner, J.S., Campbell, I.H., 1986. Convection and mixing in magma chambers. Earth-Sci. Rev. 23, 255–352; Martin, D., Griffiths, R.W., Campbell, I.H., 1987. Compositional and thermal convection in magma chambers. Contrib. Mineral. Petrol. 96, 465–475) and therefore eruption must have occurred soon after emplacement of the magma batches into the chamber. Emplacement temperatures of the pumice fragments from the Ammonia Tanks Tuff show a continuous gradient of temperatures with composition. This continuous temperature gradient is consistent with the model of storage of magma batches in the Ammonia Tanks group that have undergone both thermal and chemical diffusion.

An outline geochemistry of rhyolite eruptives from Taupo volcanic centre, New Zealand

Taupo is one of the most productive and frequently active rhyolite complexes in the world. Its early history from 320 to ca. 65 ka is recorded by numerous lava domes with associated locally dispersed pyroclastic deposits, together with two poorly welded ignimbrites of greater extent, but is not well constrained by radiometric age data. From ca. 65 to ca. 28 ka, dome-building activity began to be accompanied by increasingly powerful explosive events, culminating in the major phreatomagmatic Oruanui eruption at 22.6 14C ka (26.5 ka calibrated age), during which the structure of the modern volcano was established. Twenty eight eruptions are recognised in the post-Oruanui sequence, with ages from 20.5 ka to 1740 calibrated years B.P., of which 27 were explosive (many with inferred or suspected accompanying effusive activity) and one purely effusive. Eruption products at Taupo are dominantly rhyolitic, and the ranges of 87Sr86Sr (0.70514 to 0.70725) and ϵNd (−3.1 to + 2.0) preclude a common origin for all the rhyolites. The majority of the rhyolites can be grouped into magma types on the basis of mineral assemblages, major-and trace-element compositions, and Sr and Nd isotopes. Domes erupted between 320 and ca. 65 ka are in total isotopically and geochemically diverse, but often occur in small spatially associated groups that share the same isotopic signature, suggesting a common source or magma chamber. Domes and widespread pyroclastics erupted from ca. 65 ka to ca. 28 ka record the growth of an isotopically homogeneous (87Sr86Sr = 0.70550–0.70559) magma chamber beneath the area now occupied by the northern part of Lake Taupo. This chamber produced the ca. 400 km3 (magma) Oruanui eruption at 26.5 ka. Development of this large chamber was accompanied by eruption of distinct magma types in other geographic areas of the volcano. All magma erupted in the post-Oruanui sequence is compositionally distinct from any observed Oruanui magma (e.g., 87Sr86Sr = 0.70597–0.70622). Compositional data allow eruptives from this sequence to be subdivided into four magma types, one dacitic, forming the first three post-Oruanui eruptions between 20.5 and 17 ka, and three subtly distinct rhyolite compositions. Detailed stratigraphic and chronological controls reveal that these three rhyolite types were erupted in three discrete periods from 11.8 to 9.95, 7.05 to 2.75 and 2.15 to 1.74 ka, and that compositional variations are stepwise not gradual. Ranges of chemical and isotopic compositions within the products of individual post-26.5 ka magma eruptions are trivial compared to variations between the three groups. Taupo volcanic centre is both highly productive (ca. 6.5 km3 ky−1) and frequently active, and in the latter characteristic represents an extreme style of behaviour among rhyolite volcanoes. This unusually high frequency of activity is paralleled by rapid, stepwise changes in the compositions of the rhyolites erupted, that often appear to reflect tapping of distinct batches of magma rather than evolution of a single gradually-evolving magma chamber. New rhyolite magma bodies of volumes ca. 35 km3 and ca. 400 km3 were apparently generated in only about 1.8 ky and 40 ky, respectively, prior to caldera-forming eruptions at 1.77 and 26.5 ka.

Petrology of basic and intermediate orogenic granitoids from the Sila Massif (Calabria, southern Italy)

AbstractHercynian gabbroic, dioritic and tonalitic rocks crop out in the neighbourhood of Rovale (Sila Grande, Calabria). They make up a crude rectangular outcrop with the western part consisting of gabbroic rocks and the eastern of dioritic and tonalitic rocks. They come into contact with medium to high grade metapelites on the western side and with heterogeneous granodiorites on the other sides. In the gabbroic body both opx ± ol bearing cumulates and amphibole differentiates occur and are characterized by the widespread presence of brown pargasite. Sporadic magmatic to subsolidus corona textures between olivine and plagioclase or orthopyroxene and plagioclase can be observed and their preservation clearly suggests a post‐tectonic emplacement for the gabbroic magma. Diorites and tonalites display hypidiomorphic textures free of olivine and orthopyroxene and bearing green Mg‐hornblende. The granitoids, on the basis of chemical data, display orogenic features of the continent‐continent collision type. The gabbroic rocks have high Al tholeiitic composition and fractionation of orthopyroxene and plagioclase played an important part in their evolution.The Rb/Sr isochron method did not give a precise emplacement age for the granitoids as a whole. Initial 87Sr/86Sr ratios (at 290 Ma) are higher in the gabbroic body (0.7091–0.7095) than in diorites and tonalites (0.7083–0.7092). Thus gabbroic rocks appear more displaced than diorites and tonalites towards crustal isotopic composition. The eNd data seem to confirm this feature, thus suggesting that the gabbroic rocks and diorites derived from distinct mantle magma batches. Interestingly, small isotropic gabbroic masses occur within the diorites and show general features that allow them to be considered as possibly parental with respect to the host diorites. The evolution to the dioritic composition might have occurred through fractionation and minor mixing with a more acidic component such as the northern granodiorites.Geochemical, Sr and Nd isotopic data indicate a scenario of a composite plutonic body formed by distinct magma batches of mixed crust and mantle origin.

Geochemistry of the Lyngdal hyperites (S.W. Norway): Comparison with the monzonorites associated with the Rogaland anorthosite complex

The Lyngdal hyperites (LYHY) crop out as two small, post-tectonic intrusions of gabbro-noritic to monzonoritic composition, at the extreme South of Norway. They display subophitic textures with plagioclase laths (An 40 to An 58), orthopyroxene (En 56 to En 62), clinopyroxene (Wo 47.5En 39Fs 13.5), ilmenite, magnetite and interstitial myrmekite fringes, K-feldspar, quartz, apatite and biotite. The SiO 2 content is very constant (50–53%), but K 2O ranges from 1% to 1.6% in differentiated facies. REE patterns are smooth [ ( La Yb ) n = 6 ]. The hyperites define an Rb Sr isochron of 910 ± 82 Ma (2σ), with an initial ratio of 0.7054. ϵNd is slightly positive (0 to +2) and average lead isotopic initial ratios are 17.45 [ ( 206Pb 204Pb ) i ] and 15.51 [ ( 207Pb 204Pb ) i ]. The LYHY represent magmatic liquids that have not fractionated large amounts of plagioclase. They are quite comparable to the rocks of the monzonoritic series that occur in the nearby Rogaland anorthosite complex. (1) The plagioclases have the same composition; plagioclase phenocrysts are locally present. (2) They display enrichments in TiO 2, P 2O 5 and K 2O. (3) They have comparable REE patterns. (4) The isotopic data show that the monzonorites and the LYHY derive from the same isotopic reservoir. The LYHY nevertheless differ from the monzonorites by their high-Al content (Al 2O 3 = 18.5%) and their lower FeO tot content ( FeO tot FeO tot + MgO = 0.62 ). The geochemical and isotopic diversity of the monzonoritic magmatism, including the LYHY, implies that each occurrence represents a distinct magma batch generated from a distinct source. The LYHY could tentatively be interpreted as a Fe-poor (Mg-rich) end-member of the monzonoritic magmatism occurring to the East of the anorthosite complex.

Petrogenesis of contrasting Hercynian granitoids from the Calabrian Arc, southern Italy

The granitoids of the southern Calabrian Arc have been investigated for whole-rock and phase chemistry, zircon typology, REE, Sr and Nd isotopes. Two distinct granitoid associations, which are related in time and partly in space, are present: a calc-alkaline one and a peraluminous one. The prevailing calc-alkaline association is compositionally expanded (SiO 2=48–70%) and biotite dominated, with tonalites and granodiorites as predominant rock types. The peraluminous association is compositionally restricted (SiO 2=67–76%) and contains two-mica ± Al-silicates. Distinct peraluminous typologies occur also as core facies within the calc-alkaline types. All granitoids are ilmenite-bearing. The Cittanova (CN), Villa S. Giovanni (VSG) and Capo Rasocolmo (CR) peraluminous granites display zircon typology, REE patterns, ϵ Sr t - (+51 to +113) and ϵ Nd t -values (−8.5 to −4.6) at 290 Ma, suggesting a dominantly quartzofeldspathic metasedimentary source. In more detail, the CN types and the CR-VSG types require heterogeneous and different sources. The calc-alkaline granitoids display very variable REE patterns (Ce N=25–227 and Yb N=3.5–18.5 in the tonalites-granodiorites) and variable age-corrected ϵ Nd t -values (−8.5 to −0.25), whereas Sr values vary little (+82 to +93). Thus, in terms of ϵ Nd t  ϵ Sr t covariation, the data points define a vertical array, which is inconsistent with a model involving crustal contamination by mantle derivatives. A more viable mechanism seems to be the melting of hydrous and heterogeneous mafic lower crust (and/or basic underplate), producing distinct magma batches evolving independently. Crustal contamination, mingling and fractionation processes may all have contributed to the observed geochemical variations within the granitoids. The peraluminous granodiorites occurring within the Serre and Capo Vaticano multipulse calc-alkaline plutons exhibit isotopic ratios ( ϵ Nd t = −6.11 to +0.33 and ϵ Sr t = +93 to +97) which are similar to those of the calc-alkaline host rocks, suggesting a possible genetic link. Geologic and geochemical data indicate a continental collision setting for this plutonic activity. The magmas were produced during late-thickening to exhumation phases following collision and moderate crustal overthickening, and were emplaced subsequently during a short time span. A mantle contribution should have favoured melting, which affected various sectors of the continental crust.

Axial and off‐axial heterogeneity of basaltic rocks from the East Pacific Rise at 12°35′N–12°51′N and 11°26′N–11°30′N

Surface ship operations and 40 submersible dives have provided a large amount of field observations and rock samples from two segments of the EPR at 12°35′N–12°51′N and at 11°26′N–11°30′N and from four adjacent seamounts centered at less than 18 km from the rise axis. The basaltic samples show a great variety of morphological features and a diversity in the proportions of early formed mineral phases and chemical composition. There is no correlation between lava flow morphology, geological settings, and compositions. Based on mineral and compatible element composition, these basalts were classified into three types. The least evolved olivine‐basalts and highly phyric plagioclase‐basalts (type A) have high Fo87–89, high Mg # (66–70), and high Ni (>100) contents. The most evolved plagioclase‐olivine basalts with or without pyroxene (type B and C) have relatively low Fo78–86, low Mg # (55–65), and low Ni (60–100 ppm) contents. Simple crystal fractionation (3–18%) accounts for some of the observed compositional range. However, based on their incompatible element contents, the above basalts are divided into three groups (the various types A to C are found in each group): depleted basalts with low K/Ti (0.04–0.15) ratios, low Zr (<100 ppm) and low Nb (<0.4 ppm) contents, undepleted basalts with the highest K/Ti (0.25–0.46), Zr (150–170 ppm), and Nb (8–16 ppm) contents and the transitional basalts having compositions between these two extremes. These three basaltic groups occur within limited portions of the axial graben and its limbs in areas of less than 10 km in length and less than 3–4 km in width near 12°43′N–12°51′N on the EPR. Similar volcanic diversity in a more limited area (<3 km2) is observed on off‐axial seamounts and other constructional features centered up to 6 and 18 km from the rise axis. Depleted and undepleted melts are believed to have been formed during sequential batch melting of a composite mantle. The transitional basalts probably result from a mixing of depleted and undepleted liquids which have undergone variable degrees of crystal fractionation. Two distinct mantle sources are distinguishable by their incompatible element contents and their susceptibility to melting; we suggest that a lherzolitic mantle, with minerals having a variable behavior under partial melting, constitutes this type of composite mantle. Indeed, clinopyroxene, enriched in incompatible elements, will be consumed at an earlier stage of melting than the olivine plus orthopyroxene. Eruption of compositionally distinct magma batches in close proximity on axial and off‐axial structures implies periods of quiescence between successive magmatic stages and cycles.

Monzonorites from Rogaland (Southwest Norway): a series of rocks coeval but not comagmatic with massif-type anorthosites

Duchesne, J.C., Wilmart, E., Demaiffe, D. and Hertogen, J., 1989. Monzonorites from Rogaland (Southwest Norway ): a series of rocks coeval but not comagmatic with massif-type anorthosites. Precambrian Res., 45:111-128. Monzonorites, members of the anorthositic suite of rocks, are usually considered as residual after the formation of massif-type anorthosites. In Rogaland (S.W. Norway) they occur as large dykes, intrusions and chilled margins to differentiated massifs, emplaced during and soon after the main anorthosite massifs, in granulite facies conditions. Ti, P and Fe are enriched in monzonorites and steadily decrease towards quartz mangerites, the FeO/FeO + MgO ratios varying slightly during alkali enrichment (Bowen trend). Trace element spidergrams show deep troughs in Rb, Th, Nb-Ta, Sr, Zr-Hf and Ti. The REE slightly decrease in the evolution with (La/Yb)S ratios about 9 and neutral to positive Eu anomalies. Several occurrences, however, show highly contrasted trace element features. Sr isotope ratios (/st) show a wide interval of variation (0.704-0.710) between the various dykes and intrusions without any correlation with the elements indicative of crustal contamination. In the Tellnes main dyke, evolution towards acidic rocks occurs without contamination and variation in Isr. Fractional crystallization with subtraction of apatite-bearing noritic cumulates can account for the major and trace element evolution from monzonorite to quartz mangerite, but is unable to explain the large differences between occurrences. It is concluded that monzonorites cannot be comagmatic (though coeval) with massif-type anorthosites. They result from the crystallization of distinct magma batches, possibly formed through partial melting of basic to intermediate rocks in the lower crust.

Axial volcanism on the East Pacific Rise, 10–12°N

Summary The East Pacific Rise axis, 10–12°N, consists of three discrete topographic or tectonic units each of which has erupted compositionally distinct magmas. The two major units have an axial bathymetric high, are bounded by transform or overlapping spreading centres and are shown to be discrete volcanoes. One of the units shows systematic variation in magma composition along the rift consistent with injection of lava down-rift and concomitant low-pressure fractional crystallization with distance from the topographic high. Compositional variation within and between magmatic units is largely controlled by fractional crystallization at low and intermediate pressures. The eastern and western limbs of a large overlapping spreading centre at about 11°45′N have compositionally distinct lavas, apparently from separate magma batches. Axial eruptions in the vicinity of a large transform fault (Clipperton Fracture Zone) show a large variation in composition and at least one independent magma injection. Highly fractionated lavas with compositions up to 59% silica are included in these eruptions. One of the axial tectonic units has, in addition to normal tholeiitic eruptions, some lavas highly enriched in incompatible elements such as potassium, titanium, strontium, barium, zirconium and lanthanum. These lavas are also isotopically enriched with respect to 87 Sr/ 86 Sr, suggesting that the mantle beneath some portions of the axis is compositionally heterogeneous. The presence of compositionally distinct magma batches, often in close proximity, suggests that magma chambers underlying the East Pacific Rise axis are small and probably intermittent.

Petrogenesis of a two-mica ignimbrite suite: the Macusani Volcanics, SE Peru

ABSTRACTThe Miocene-Pliocene Macusani ash-flow tuffs and glasses from SE Peru are a rare example of two-mica felsic peraluminous volcanic rocks. They outcrop in three separate tectonic basins of the Cordillera Oriental in the Central Andes. In the Macusani field, the rocks are characterised by andalusite and muscovite phenocrysts. Compositions are both very felsic and peraluminous, similar to two-mica granites. Field relations, age differences and isotopic heterogeneities suggest that several distinct magma batches were involved. Two separate magmatic stages are recognised: (1) partial melting and evolution at or near the source region, and (2) shallow-level crystallisation and eruption. Magma genesis involved partial melting of metapelitic materials, with internally controlled. High heat flux, rapid heating, elevated temperatures and F-rich compositions were essential for the production of these mobile, H2O-undersaturated magmas. Chemical variations between the erupted products can be attributed to different degrees either of partial melting in somewhat variable source materials and (or) of fractional crystallisation at shallow levels. We discuss some important differences between the magmatic evolution of the Macusani Volcanics and of Hercynian and Himalayan two-mica granites.

Poás volcano, Costa Rica: Geology of the summit region and spatial and temporal variations among the most recent lavas

The recent eruptive history of Poás volcano is described here on the basis of field mapping of the summit area. This period, which may represent 10 to 20,000 years and less than 10% of the history of the volcano, began with caldera formation. Subsequent events included: composite cone construction, faulting and flank subsidence, flank fissure eruption, and multiple crater collapses. Local and regional structures have controlled the locations of summit vents and flank cinder cones. Chemical compositions of lavas of known stratigraphic position demonstrate temporal magmatic variation at Poás. Three similar felsic to mafic magmatic sequences occurred at the summit. The first and second sequences were separated by flank and summit eruptions of a distinct magma batch, enriched in TiO 2 and P 2O 5. The three felsic to mafic sequences appear to be cyclical, and the repeated, similar variations are interpreted as progressive tapping of zoned magma bodies developed repeatedly from a common parent by similar crystal fractionation processes. The breaks between the three sequences correlate with shifts or major modifications of the eruptive center. The chemical composition of the most recent lava, erupted in 1954, indicates that the volcano is near the end of the present sequence. The lavas are calc-alkaline basalts and andesites that are similar to the mafic lavas that comprise the bulk of the Central American volcanic front. The samples can be divided into 3 spatial-temporal-geochemical groups. A summit group that represents the most recent activity; a TiO 2-rich group that erupted primarily on the south flank about 7500 yr. B.P.; and an Al 2O 3-rich group that erupted primarily from vents north of the presently active Main Crater. Geochemical variations and projections into pseudoternary CMAS diagrams suggest a moderate pressure for the summit group, that is, a magma chamber at intracrustal to subcrustal depths. The Al 2O 3-rich group define smaller primary phase volumes for olivine and plagioclase, which suggests a deeper and or more water-rich magma chamber. The TiO 2-rich group appears to be a batch of mixed magma with the summit group magma as one endmember and a less siliceous magma as the other.