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1.
St. Paul Island is the youngest volcanic center in the Bearing Sea basalt province. We have undertaken a field, petrographic, and geochemical study of select St. Paul volcanic rocks in order to better understand their differentiation; specifically, to test the hypothesis that magmas erupted from individual Bering Sea basaltic volcanoes are not related by shallow-level processes such as crystal fractionation. Petrographically, all of the St. Paul volcanic rocks are olivine-, plagioclase-, and clinopyroxene-phyric. Textural features and modal contents of olivine phenocrysts, however, vary widely throughout the spectrum of basalt compositions. Although differing in size and abundance, olivine phenocrysts in all rock compositions are euhedral and commonly skeletal, suggesting rapid growth during ascent or eruptive quenching. None, however, display reaction textures with surrounding groundmass liquid. Compositionally, the St. Paul volcanic rocks are basalts and tephritic basalts and all have high contents of normative nepheline (8% to 16%). Concentrations of many major and incompatible trace elements display no clear correlations with bulk-rock SiO2 and MgO contents or modal abundances of phenocrysts, suggesting that much of the compositional diversity of these magmas reflects variable mantle sources and degrees of partial melting. Similarly, chondrite-normalized REE patterns show variable degrees of light REE enrichment (Lan=70–90) that do not correlate with bulk-rock mg-numbers. In contrast, concentrations of compatible trace elements (Ni, Cr, and Co) are positively correlated with MgO contents and modal percentages of olivine phenocrysts. Maximum forsterite contents of olivine phenocryst cores in most St. Paul rocks decrease with decreasing bulk-rock mg-number and are similar to the calculated equilibrium range. This is evidence that the high mg-numbers are magmatic and do not result from olivine accumulation. Instead, major and compatible trace element mass balance calculations support derivation of the low mg-number lavas from the high mg-number lavas mainly by olivine fractionation, which, in turn, implies that St. Paul magmas may have temporarily resided in crustal magma chambers prior to eruption. 相似文献
2.
《Chemical Geology》2003,193(3-4):181-193
Xenolith samples of marine terrigenous sediments and altered Jurassic MORB from Gran Canaria (Canary Islands) represent samples of sub-island oceanic crust. These samples are postulated to define end-members for crustal contamination of basaltic and felsic ocean island magmas. The meta-igneous rocks show great heterogeneity in oxygen isotope compositions (δ18O 3.3–8.6‰), broadly correlating with their stratigraphic position in the oceanic crust. Gabbros interpreted as fragments of oceanic crust layer 3 have δ18O values of 3.3–5.1‰, which is lower than MORB (5.7–6.0‰). Layer 2 lavas and dykes show a broader range of δ18O of 4.1–8.6‰. Therefore, high-temperature metamorphism seems to have been the dominant process in layer 3, while both high- and low-temperature alteration have variably affected layer 2 rocks. Siliciclastic sediments have high δ18O values (14.1–16.4‰), indicating diagenesis and low-temperature interaction with seawater. The oxygen isotope stratigraphy of the crust beneath Gran Canaria is typical for old oceanic crust and resembles that in ophiolites. The lithologic boundary between older oceanic crust and the igneous core complex at 8–10 km depth—as postulated from geophysical data—probably coincides with a main magma stagnation level. There, the Miocene shield phase magmas interacted with preexisting oceanic crust. We suggest that the range in δ18O values (5.2–6.8‰) [Chem. Geol. 135 (1997) 233] found for shield basalts on Gran Canaria, and those in some Miocene felsic units (6.0–8.5‰), are best explained by assimilation of various amounts and combinations of oceanic and island crustal rocks and do not necessarily reflect mantle source characteristics. 相似文献
3.
Early–Middle Jurassic igneous rocks (190–170 Ma) are distributed in an E–W-trending band within the Nanling Tectonic Belt, and have a wide range of compositions but are only present in limited volumes. This scenario contrasts with the uniform but voluminous Middle–Late Jurassic igneous rocks (165–150 Ma) in this area. The Early–Middle Jurassic rocks include oceanic-island basalt (OIB)-type alkali basalts, tholeiitic basalts and gabbros, bimodal volcanic rocks, syenites, A-type granites, and high-K calc–alkaline granodiorites. Geochemical and isotopic data indicate that alkaline and tholeiitic basalts and syenites were derived from melting of the asthenospheric mantle, with asthenosphere-derived magmas mixing with variable amounts of magmas derived from melting of metasomatized lithospheric mantle. In comparison, A-type granites in the study area were probably generated by shallow dehydration-related melting of hornblende-bearing continental crustal rocks that were heated by contemporaneous intrusion of mantle-derived basaltic magmas, and high-K calc-alkaline granodiorites resulted from the interaction between melts from upwelling asthenospheric mantle and the lower crust. The Early–Middle Jurassic magmatic event is spatially variable in terms of lithology, geochemistry, and isotopic systematics. This indicates that the deep mantle sources of the magmas that formed these igneous rocks were significantly heterogeneous, and magmatism had a gradual decrease in the involvement of the asthenospheric mantle from west to east. These variations in composition and sourcing of magmas, in addition to the spatial distribution and the thermal structure of the crust–mantle boundary during this magmatic event, indicates that these igneous rocks formed during a period of rifting after the Indosinian Orogeny rather than during subduction of the paleo-Pacific oceanic crust. 相似文献
4.
《International Geology Review》2012,54(14):1684-1708
Volcanic rocks that make up Faial Island, Central Azores, consist of four volcano-stratigraphic units, with ages between 730 ka and the present. Lavas range from alkali basalts to trachyandesites and belong to the alkaline-sodic series. The oldest unit is the Ribeirinha Volcanic Complex, generally characterized by low MgO contents. The Cedros Volcanic Complex is composed of basalts to benmoreites with low MgO contents. The Almoxarife Formation represents fissure flows, containing MgO contents similar to to slightly higher than those of the underlying Cedros Volcanic Complex. The youngest unit, the Capelo Formation, consists of mafic rocks with MgO values higher than those of the other units. Bulk-rock major and trace element trends suggest that differentiation of the three earliest units were dominated by fractional crystallization of plagioclase ± clinopyroxene ± olivine ± titanomagnetite. Capelo bulk-rock compositions are the most primitive, and are related to a period when volcanic activity was fed by deep magmatic chambers, and melts ascended more rapidly. Comparison among geochemical patterns of the trace elements suggests a strong similarity between the lavas from Faial and Pico islands. Corvo Island volcanism contrasts with the geochemistry of Faial and Pico lavas, reflecting its strong K and Rb depletion, and Th, U, Ta, Nb, La, and Ce enrichment. Absence of the Daly gap in the Faial volcanics is attributed to early crystallization of Ti-Fe oxides. The probable source of the Faial magma coincides with the MORB-FOZO array, which implies the presence of ancient recycled oceanic crust in the mantle source. Ratios of incompatible trace elements suggest the similarity of Corvo volcanic rocks with magmas derived from HIMU sources, whereas the Faial and Pico volcanic rocks could have been produced from sources very close to EMII-type OIB. 相似文献
5.
Strontium isotope data for Quaternary volcanic rocks from six separate areas in Papua New Guinea suggest a subdivision into two broad groups. One group consists of island volcanoes that have lower Sr37/Sr38 ratios (0.7034–0.7043) and show isotopic homogeneity within each area sampled. Except for the St. Andrew Strait samples, it is considered that these rocks were produced from relatively homogeneous source regions in the upper mantle. Volcanoes of the second group (on the Papua New Guinea mainland) have generally a higher and wider range of Sr37/Sr36 ratios, compared to those of the first group. It is thought that the magmas of these mainland volcanoes were affected by different degrees of sialic crustal contamination, or were derived from heterogeneous sources in the upper mantle. 相似文献
6.
Some workers have held that mid-ocean ridge basalts are fractionated from high pressure (15–30 kbar) picritic primary magmas
whereas others have favored primary magmas generated at about 10 kbar with compositions close to those of mid-ocean ridge
basalts. Of critical significance are presumed differences in composition between experimentally determined primary magmas
and the least fractionated mid-ocean ridge basalts. To evaluate the significance of these differences, all based on electron
microprobe analyses, we consider three sources of uncertainty: (1) analytical uncertainties for a single microprobe laboratory,
(2) systematic interlaboratory analytical differences, and (3) real variations in the possible compositions of primary magmas
that can be produced from a peridotite source at a given pressure. The first source of error is surprisingly large and can
account for a substantial part of the total variation of normative quartz (hypersthene calculated as equivalent olivine and
quartz) in FAMOUS basalts. The second is not as serious but remains undetermined for many laboratories. The third is potentially
the largest but is not yet fully documented. The least fractionated FA-MOUS basalts have high mg numbers (70–73) compatible with derivation from the mantle by direct partial melting with little or no subsequent fractional
crystallization. Because of the wide range of normative quartz content in these basalts, it appears necessary to consider
them as representatives of multiple parental magmas. When all the sources of uncertainty are taken into account, we conclude
that the experimental data by various investigators are all fairly consistent and favor derivation of the least fractionated
mid-ocean ridge basalts by at most only a small amount of fractional crystallization from primary magmas having a wide range
of normative quartz content and generated over a range of pressures from about 7–11 kbar.
Contribution No. 420, Department of Geosciences, The University of Texas at Dallas 相似文献
7.
《Russian Geology and Geophysics》2014,55(8):929-940
Petrological and geochemical studies performed with invoking data on the compositions of clinopyroxenes have clarified the conditions of formation of Vendian-Cambrian basaltic complexes in the Dzhida zone of the Paleoasian Ocean (northern Mongolia and southwestern Transbaikalia). The research was based on a comparative analysis with reference igneous basaltic associations. Of special importance are our microprobe data on trace and rare-earth elements in clinopyroxenes from igneous rocks of different present-day geodynamic settings, namely, N-MORB (Mid-Atlantic Ridge, Central Atlantic), OIB (Bouvet Island, South Atlantic), WPB (within-plate tholeiitic plateau basalts of the Siberian Platform), and boninites of ensimatic arcs (Izu-Bonin island arc, Pacific). The studies have shown that the paleo-oceanic structures in the district of the Urgol guyot formed during geodynamic processes under the impact of mantle plumes on oceanic spreading crust, which resulted in oceanic basaltic plateaus and within-plate oceanic islands. All these structures were later superposed by typical island-arc structure-lithologic associations. Formation of basalt complexes in the Dzhidot guyot district proceeded with a stronger effect of enriched plume melts of within-plate oceanic islands as compared with the Urgol guyot. This is evidenced from petrochemical and geochemical data showing the development of OIB-type magmatic systems on the oceanic basement. Data on clinopyroxenes confirm the participation of mantle plume in this process, which led to the evolution of magmas from typical oceanic basalts (MORB) to plateau basalts and OIB. 相似文献
8.
The rocks of Macquarie Island are part of the mid-Tertiary oceanic lithosphere from a major ocean basin. They were probably created at the Indian—Australian—Pacific spreading ridge.The basalts and dolerites are usually porphyritic, carry plagioclase (An87-80) as a dominant phenocryst phase with less abundant olivine (Fo89-85), chrome spinel and rare clinopyroxene (Ca45Mg50Fe5|Ca38Mg50Fe12) phenocrysts. Normatively the rocks range from ne- to Q-bearing, with most falling near the critical plane of normative silica undersaturation. Dykes tend to be more Fe-rich than lavas, and to include the more di-poor rocks. The rocks also range compositionally from typical ocean floor basalts through to varieties relatively enriched in some incompatible trace elements, particularly Nb (20–60 ppm), that otherwise retain ocean-floor basalt phenocryst assemblages, major-element compositions and Ti, Ni, Cr and Zr contents. This enrichment, also characteristic of ocean-floor basalts from the “abnormal” ridge segments near 45° N and 36° N (FAMOUS area) on the Mid-Atlantic Ridge, causes the rocks to plot away from the ocean-floor basalt fields on popular trace-element diagrams intended to identify tectonic affinities of basalts.The upper parts of the Macquarie Island oceanic lithosphere section can be thought of as a vertical slice through a magma column, differentiating at shallow levels. The layered and massive gabbros that underlie the basalts and dolerites are composed essentially of olivine, plagioclase and clinopyroxene. Olivine and plagioclase are cumulate phases in the layered rocks, clinopyroxene is postcumulus. Mineral compositions of the gabbros, particularly those of the layered rocks, are closely resembled by phenocryst compositions in the basalts and dolerites. Plagiogranites and trondheimites are unknown from the island, and norites very rare. Thus, Macquarie Island basalts, dolerites and gabbros form a distinctive igneous association that ought to make Macquarie Island-type ophiolite complexes from major ocean basins an easily recognized ophiolite type in continental orogenic terranes, even when dismembered. 相似文献
9.
Thor H. Hansteen Andreas Klügel Hans-Ulrich Schmincke 《Contributions to Mineralogy and Petrology》1998,132(1):48-64
Gabbroic and ultramafic xenoliths and olivine and clinopyroxene phenocrysts in basaltic rocks from Gran Canaria, La Palma,
El Hierro, Lanzarote and La Gomera (Canary Islands) contain abundant CO2-dominated fluid inclusions. Inclusion densities are strikingly similar on a regional scale. Histogram maxima correspond to
one or more of the following pressures: (1) minimum 0.55 to 1.0 GPa (within the upper mantle); (2) between 0.2 and 0.4 GPa
(the Moho or the lower crust); (3) at about 0.1 GPa (upper crust). Fluid inclusions in several rocks show a bimodal density
distribution, the lower-density maximum comprising both texturally early and late inclusions. This is taken as evidence for
an incomplete resetting of inclusion densities, and simultaneous formation of young inclusions, at well-defined magma stagnation
levels. For Gran Canaria, pressure estimates for early inclusions in harzburgite and dunite xenoliths and olivine phenocrysts
in the host basanites overlap at 0.9 to 1.0 GPa, indicating that such magma reservoir depths coincide with levels of xenolith
entrainment into the magmas. Magma chamber pressures within the mantle, inferred to represent levels of mantle xenolith entrainment,
are 0.65–0.95 GPa for El Hierro, 0.60–0.68 GPa for La Palma, and 0.55–0.75 GPa for Lanzarote. The highest-density fluid inclusions
in many Canary Island mantle xenoliths have probably survived in-situ near-isobaric heating at the depth of xenolith entrainment.
Inclusion data from all islands indicate ponding of basaltic magmas at Moho or lower crustal depths, and possibly at an additional
higher level, strongly suggestive of two main crustal accumulation levels beneath each island. We emphasize that repeated
magmatic underplating of primitive magmas, and therefore intrusive accretion, are important growth mechanisms for the Canary
Islands, and by analogy, for other ocean islands. Comparable fluid inclusion data from primitive rocks in other tectonic settings,
including Iceland, Etna and continental rift systems (Hungary, South Norway), indicate that magma accumulation close to Moho
depths shortly before eruption is not, however, restricted to oceanic intraplate volcanoes. Lower crustal ponding and crystallization
prior to eruption may be the rule rather than the exception, independent of the tectonic setting.
Received: 30 May 1997 / Accepted: 6 February 1998 相似文献
10.
Karlis Muehlenbachs Alfred T. Anderson Gudmundur E. Sigvaldason 《Geochimica et cosmochimica acta》1974,38(4):577-588
The volcanic rocks of Iceland are anomalous in their oxygen isotope content. Recent tholeiitic and transitional alkali basalts from Iceland range in (δO18 from 1·8 to 5δ7%. Most of the tholeiitic basalts and their phenocrysts are at least 1% lower in δO18 than unaltered basalts from other oceanic islands or oceanic ridges. The Icelandic basalts that resemble ridge basalts in δO18 also resemble them in major element chemistry. δO18 values of alkali olivine basalts are closest to those of other oceanic islands. Secondary alteration processes have lowered as well as raised the δO18 values of some Icelandic rocks, but such surface mechanisms cannot account for the distribution of oxygen isotopes in the Recent basalts of Iceland. Three mechanisms that could give rise to the low-O18 magmas are (1) exchange of oxygen between magma and low-O18 hydrothermally altered rock, (2) exchange with low-O18 meteoric water, or (3) an exceptional mantle under Iceland. None of the above models can satisfactorily account for all the observations. 相似文献
11.
Mafic and intermediate granulite xenoliths, collected from Cenozoic alkali basalts, provide samples of the lower crust in western Saudi Arabia. The xenoliths are metaigneous two-pyroxene and garnet granulites. Mineral and whole rock compositions are inconsistent with origin from Red Sea rift-related basalts, and are compatible with origin from island arc calc-alkaline and low-potassium tholeiitic basalts. Most of the samples are either cumulates from mafic magmas or are restites remaining after partial melting of intermediate rocks and extraction of a felsic liquid. Initial87Sr/86Sr ratios are less than 0.7032, except for two samples at 0.7049. The Sm-Nd data yield TDM model ages of 0.64 to 1.02 Ga, similar to typical Arabian-Nubian Shield upper continental crust. The isotopic data indicate that the granulites formed from mantle-derived magmas with little or no contamination by older continent crust. Calculated temperatures and pressures of last reequilibration of the xenoliths show that they are derived from the lower crust. Calculated depths of origin and calculated seismic velocities for the xenoliths are in excellent agreement with the crustal structure model of Gettings et al. (1986) based on geophysical data from western Saudi Arabia. Estimation of mean lower crustal composition, using the granulite xenoliths and the Gettings et al. (1986) crustal model, suggests a remarkably homogeneous mafic lower crust, and an andesite or basaltic andesite bulk composition for Pan-African juvenile continental crust. 相似文献
12.
The influence of amphibole fractionation on the evolution of calc-alkaline andesite and dacite tephra from the central Aleutians,Alaska 总被引：3，自引：1，他引：2
Jay D. Romick Suzanne Mahlburg Kay Robert W. Kay 《Contributions to Mineralogy and Petrology》1992,112(1):101-118
Petrologic studies of tephra from Kanaga, Adak, and Great Sitkin Islands indicate that amphibole fractionation and magma mixing are important processes controlling the composition of calc-alkaline andesite and dacite magmas in the central Aleutians. Amphibole is ubiquitous in tephra from Kanaga and Adak Islands, whereas it is present only in a basaltic-andesite pumice from Great Sitkin. Dacitic tephra from Great Sitkin do not contain amphibole. Hornblende dacite tephra contain HB+PLAG+OX±OPX±CPX phenocrysts with simple zoning patterns, suggesting that the dacites evolved in isolated magma chambers. Andesitic tephra from Adak contain two pyroxene and hornbelende populations, and reversely zoned plagioclase, indicating a more complex history involving mixing and fractional crystallization. Mass balance calculations suggest that the andesitic tephra may represent the complements of amphibole-bearing cumulate xenoliths, both formed during the evolution of high-Al basalts. The presence of amphibole in andesitic and dacitic tephra implies that Aleutian cale-alkaline magmas evolve in the mid to lower crust under hydrous (>4 wt.% H2O) and oxidizing (Ni–NiO) conditions. Amphibole-bearing andesites and pyroxene-bearing dacites from Great Sitkin indicates fractionation at several levels within the arc crust. Despite its absence in many calc-alkaline andesite and dacite lavas, open system behavior involving amphibole fractionation can explain the trace element characteristies of lavas found on Adak Island. Neither open nor closed system fractionation involving a pyroxene-bearing assemblage is capable of explaining the trace element concentrations or ratios found in the Adak suite. We envision a scenario where amphibole was initially a liquidus phase in many calc-alkaline magmas, but was later replaced by pyroxenes as the magmas rose to shallow levels within the crust. The mineral assemblage in these evolved lavas reflects shallow level equilibration of the magma, whereas the trace element chemistry provides evidence for a earlier, amphibole-bearing, mineral assemblage. 相似文献
13.
Maryannick Richard Herv Bellon RenC. Maury Eric Barrier Juang Wen-Shing 《Tectonophysics》1986,125(1-3)
K-Ar radiometric datings are presented for volcanic rocks from the Coastal Range of Taiwan and from Lanhsu and Lütao islands. The samples involved are basalts, andesites and dacites which show the main petrological and geochemical characteristics of island arc magmas. The K-Ar data show that volcanic activity occurred from Early Miocene to Early Pliocene times in the Coastal Range and in Lanhsu Island, and during Pliocene times (ca. 1.3–4.3 Ma) in Lütao Island. The geological significance of Early and Middle Miocene ages is discussed with respect to hydrothermal/metasomatic alterations which have affected most of the samples. A significant increase in incompatible elements (e.g., K and Sr) is shown to have occurred during Pliocene to Recent times, and is exemplified by the compositions of the Lanhsu, and Lütao volcanic rocks. The origin of these chemical variations is related to the magmatic effects of crustal thickening linked to the transition from subduction to collision regimes. 相似文献
14.
Tian-Yu Zhang Cai Li Jian-Jun Fan Ming Wang Ya-Dong Wu 《International Geology Review》2017,59(14):1786-1803
New whole-rock major and trace elements data, zircon laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) U–Pb ages, and zircon Hf isotope compositions were analysed for Early Cretaceous volcanic rocks, also called Meiriqieco Formation (MF) in the Duobuzha area of the Southern Qiangtang–Baoshan Block (SQBB), northern Tibet. Our aim is to clarify their petrogenesis and tectonic setting, and constrain the evolution process on the northern margin of Bangong–Nujiang suture zone (BNSZ) during Early Cretaceous time. The MF volcanic rocks are mainly composed of andesites with subordinate basalts and rhyolites with high-K calc-alkaline affinity. Zircon LA-ICP-MS U–Pb dating for two andesite and one rhyolite samples give uniform ages within error of ca.113, 114, and 118 Ma, respectively, indicating they were erupted on the Early Cretaceous. The MF andesites have variable zircon εHf(t) values (+0.5 to +10.5), which is different from those of MF rhyolites (+7.9 to +10.7). All the MF rocks are enriched in large ion lithophile elements, and depleted in high field strength elements, yielding the affinity of arc rocks. The MF basalts were most likely derived from the mantle wedge that was metasomatized by fluids released from subducting slab with the involvement of subducted sediments. The MF rhyolites were generated by partial melting of the juvenile mafic lower crust. The MF andesites are interpreted to have formed by mixing of the magmas that parental of the MF basalts and the MF rhyolites. In addition, a couple of distinctly magmatic sources are identified in the SQBB, and this may be related to mantle components injected into the continental crust. Combined with published geological data in the BNSZ and SQBB, we consider that the MF volcanic rocks are formed in a continental arc setting, suggesting that BNO were subducting during the Early Cretaceous time in the Duobuzha area. 相似文献
15.
Gordon G. Goles 《Lithos》1975,8(1):47-58
Five Ne-normative basalts are unusually rich in Cr, to a lesser degree in Co, and have very high La/Lu ratios and high contents of several excluded trace elements resembling kimberlites in these and other aspects. They seem to represent mantle-derived magmas, only slightly modified by shallow level crystal/liquid fractionation. Cr was apparently excluded from the crystalline phases in equilibrium with these magmas in the upper mantle.The Chyulu rocks differ in several respects from other Gregory Rift basalts, but closely resemble those from Jan Mayen Island near the Mid-Atlantic ridge. The generation of these unusual basalts is therefore not related simply to mantle composition or tectonic setting, but may reflect unusually high-T (1450°C), low-P (15–29 kbars) environments in the zones of last effective equilibration of these magmas with mantle rocks. 相似文献
16.
The opening of the North Atlantic Ocean began in the Late Paleocene and was accompanied by the eruption of submarine and subaerial basalts, which built up submarine plateau and ridges, islands, and volcanoes. The volcanic rocks are dominated by low-K tholeiitic basalts, which associate with almost coeval alkaline rocks (subalkali and alkali basalts and their derivatives, basanites, nephelinites, and others). The oldest alkaline volcanics (58–56 Ma) were formed during the opening of the oceanic rift at its shoulders, in northeastern Greenland and the western Norwegian shelf. It was recently found that 55–53 Ma-old alkali-ultramafic rocks are much more widespread at the eastern coast of Greenland than it was previously thought. The younger occurrences of alkali volcanism with pulses at 30, 10, 5 Ma, and up to the present day were formed on the young oceanic plate and newly formed islands and seamounts. To compare the oceanic and continental volcanism of this region, oceanic volcanics dredged during Cruise 10 of the R/V Akademik Kurchatov were reanalyzed using modern analytical methods (XRF and ICP-MS). This study showed that the oceanic and continental alkaline rocks are significantly different in petrochemical and geochemical characteristics, which is caused by differences in magma generation depths and compositions of the mantle source material. The primary continental alkaline magmas were initially more enriched in incompatible trace elements than oceanic ones. During the shallow-level differentiation of oceanic magmas, trace elements and alkalis could be accumulated in residual melts, but these processes occurred on a minor scale and depended on tectonic conditions. 相似文献
17.
Petrogenesis of a basalt-comendite-pantellerite rock suite: the Boseti Volcanic Complex (Main Ethiopian Rift) 总被引：2，自引：0，他引：2
Fiorenzo Ronga Michele Lustrino Andrea Marzoli Leone Melluso 《Mineralogy and Petrology》2010,98(1-4):227-243
Petrological and geochemical data for basic (alkali basalts and hawaiites) and silicic peralkaline rocks, plus rare intermediate products (mugearites and benmoreites) from the Pleistocene Boseti volcanic complex (Main Ethiopian Rift, East Africa) are reported in this work. The basalts are slightly alkaline or transitional, have peaks at Ba and Nb in the mantle-normalized diagrams and relatively low 87Sr/86Sr (0.7039–0.7044). The silicic rocks (pantellerites and comendites) are rich in sanidine and anorthoclase, with mafic phases being represented by fayalite-rich olivine, opaque oxides, aenigmatite and slightly Na-rich ferroaugite (ferrohedenbergite). These rocks were generated after prolonged fractional crystallization process (up to 90–95 %) starting from basaltic parent magmas at shallow depths and fO2 conditions near the QFM buffer. The apparent Daly Gap between mafic and evolved Boseti rocks is explained with a model involving the silicic products filling upper crustal magma chambers and erupted preferentially with respect to basic and intermediate products. Evolved liquids could have been the only magmas which filled the uppermost magma reservoirs in the crust, thus giving time to evolve towards Rb-, Zr- and Nb-rich peralkaline rhyolites in broadly closed systems. 相似文献
18.
A. Yu. Martynov 《Petrology》2013,21(5):471-488
Geochemical data obtained on volcanic rocks produced during the early evolutionary stages in Kunashir Island provide insight into certain important aspect of the evolution of the subduction system. The mafic lavas of all age intervals exhibit clearly pronounced across-arc geochemical zoning, which implies that these rocks were produced in the environment of a subducted oceanic slab. The high Ba/Th and U/Th ratios of basalts from the frontal zones suggest that an important role in magma generation was played by a low-temperature aqueous fluid. The arc lavas of the Early Miocene, Pliocene, and Pliocene-Pleistocene episodes in the evolution of the island arc system provide evidence of the melting of subducted sediments, which testifies, when considered together with the calculated P-T conditions under which the high-Mg basalts were derived, that backarc tectono-magmatic processes affected subduction-related magmatic generation. Active mantle diapirism and volcanic activity in the opening Kurile Basin resulted in the heating of the suprasubduction mantle in the rear zone, the involvement of the upper sedimentary layer of the oceanic slab in the process of melting, and the eventual generation of basaltic magmas with unusual geochemical characteristics. 相似文献
19.
Janet Hergt 《Contributions to Mineralogy and Petrology》2000,139(2):240-244
18 O values, the magmas retain initial 87Sr/86Sr Compositions of greater than 0.709 (e.g. Hoefs et al. 1980). Consistent with this is the lack of a radiogenic Os isotope
signature in these rocks (e.g. Molzahn et al. 1996) as would be expected with assimilation of continental crust. Comments
here are not concerned with the AFC calculations employed to describe the variations within the suite of samples from the Prince Albert mountains studied by Antonini et al., but rather, the extrapolation of these
to more primitive compositions.
Antonini et al. have revived the possibility that the unusual chemical features of the least evolved low Ti continental flood
basalts of the Ferrar Province reflect a crustal-level interplay between mantle-derived magmas and the continental crust.
It is argued here that they have not demonstrated their claim.
Received: 7 October 1999 / Accepted: 25 January 2000 相似文献
20.
Phase relations of a high-Mg basalt from the Aleutian Island Arc: Implications for primary island arc basalts and high-Al basalts 总被引：1，自引：1，他引：1
Many volcanic centers in the Aleutian Islands have erupted lavas that range in composition from high-Mg basalt (MgO>9 wt%) to more fractionated and voluminous high-Al basalts and basaltic andesites. The petrogenetic relationships between these rock types and the composition of primary magmas has been vigorously debated. The phase relations of a typical high-Mg basalt from the Makushin volcanic field on Unalaska Island provide important constraints on petrogenetic models. Results of one-atmosphere and moderate-to high-pressure (5–20 kb) anhydrous experiments are similar to results obtained from primitive MORB. At low pressures olivine is the liquidus phase joined by plagioclase and clinopyroxene at progressively lower temperatures. Clinopyroxene is the second phase to crystallize at pressures greater than 5 kb and replaces olivine on the liquidus at approximately 10 kb. Above 10 kb the liquidus pyroxene is aluminous augite and orthopyroxene is the second phase to crystallize. Glasses in equilibrium with olivine and clinopyroxene at intermediate-pressure (5 to 10 kb) are similar in composition to high-Al basalt. Plagioclase is not involved and most likely does not become a liquidus phase until the liquid has evolved significantly. Although our studies do not confirm the primary nature of high-Mg basalts they do support a model in which high-Al basalts are generated by moderate amounts of crystal fractionation from more primitive (high Mg/Mg+Fe, lower Al2O3) basaltic magmas near the arc crust-mantle boundary.Abbreviations
Ol
olivine
-
Cpx
Clinopyroxene
-
Pl
plagioclase
-
L
liquid
-
Sp
spinel
-
Pig
pigeonite
-
Opx
Orthopyroxene 相似文献