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1.
Field, chronologic, chemical, and isotopic data for late Cenozoic basaltic rocks from the northwestern United States illustrate the relationship between crustal structure and tectonic forces in controlling the genesis and evolution of continental volcanism. In the northwestern U.S., the first major episode of basaltic volcanism was triggered by crustal rifting in a “back-arc” environment, east of the westward-migrating volcanic arc created by the subduction of the Juan-de-Fuca plate beneath the North American plate. Rifting and volcanism were concentrated by pre-existing zones of crustal weakness associated with boundaries between the old Archean core of the continent and newly accreted terranes. Basalts erupted during this time (Columbia River, Steens Mountain) show evidence of significant fractionation histories including contamination by crust of varying age depending on the crustal structure at the eruption site. Presumably this reflects ponding and stagnation of primary magmas in the crust or at the crust-mantle interface due to their encounter with thick crust, not yet extended and still containing its low-density, easily fusible component. Continued rifting of this crust, and modification of its composition through extraction of rhyolitic partial melts and deposition of the fractionation products from primary basaltic melts, coupled with a shift in stress orientation roughly 10.5 Ma ago, allowed relatively unfractionated and uncontaminated magmas to begin reaching the surface. In the western part of the region (Oregon Plateau), these magmas tapped a mantle source similar to that which produced most of the ocean island basalts of the northern hemisphere. To the east (Snake River Plain), however, the mantle sampled by basaltic volcanism has isotopic characteristics suggesting it has preserved a record of incompatible element enrichment processes associated with the formation of the overlying Archean crustal section some 2.6 Ga ago.  相似文献   

2.
The numerous Miocene-Recent alkaline volcanic outcrops in the Antarctic Peninsula form a substantial volcanic province, the least well-known part of a major belt of alkaline volcanism that extends between South America and New Zealand. The outcrops consists mainly of aa and pahoehoe lavas and hyaloclastites which locally contain accidental nodules of spinel lherzolite and other mantle-derived lithologies. The province is predominantly basaltic with two major differentiation lineages: (1) a sodic series of olivine and alkali basalt, hawaiite, mugearite, trachy-phonolite and trachyte; and (2) a relatively potassic, highly undersaturated series of basanite, tephrite and phono-tephrite. All the lavas show varying effects of fractionation by crystallization of olivine and clinopyroxene, joined by plagioclase in the hawaiites to trachytes. Fractional crystallization can probably explain most of the chemical variation observed within each outcrop, but variable partial melting is necessary to account for the differences in incompatible element enrichment between the two series, and between the individual outcrops. The degree of partial melting may not have exceeded 3%, as is the case for many other alkaline magmas.The volcanism is an intraplate phenomenon but there is no correlation in timing between the cessation of subduction and the inception of alkaline volcanism. The activity cannot be related to the passage of the coupled Pacific-Antarctic plate over a stationary mantle hot-spot. Although the precise causal relationship with tectonic setting is unknown, regional extension was a prerequisite for giving the magmas rapid access to the surface.  相似文献   

3.
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4.
The paper reviews the stratigraphy, style of activity and some aspects of the petrology of Tertiary to Recent sodic alkaline volcanic rocks in Kenya, eastern Uganda and northern Tanzania. Repeated extrusions of basaltic and nephelinitic volcanics occurred from Miocene times onwards, confirming indications from chemical data that magmas of these compositions were parental. At some central volcanoes, a basalt-trachyte-phonolite series evidently arose by fractional crystallization of basaltic magma, whereas various courses of crystallization from a nephelinitic parent led to the production of phonolites, tephrites and basanites as well as olivine-and melilite-bearing nephelinites and melanephelinites. Phonolitic and trachytic volcanics which dominate an area of repeated upwarping (the Kenya dome) probably originated by processes of partial melting rather than by differentiation of basaltic magma. The basalt-trachyte association which characterizes many central volcanoes north and south of the dome can perhaps best be explained by postulating independent sources for the basic and salic volcanics.  相似文献   

5.
Nisyros island is a calc-alkaline volcano, built up during the last 100 ka. The first cycle of its subaerial history includes the cone-building activity with three phases, each characterized by a similar sequence: (1) effusive and explosive activity fed by basaltic andesitic and andesitic magmas; and (2) effusive andextrusive activity fed by dacitic and rhyolitic magmas. The second eruptive cycle includes the caldera-forming explosive activity with two phases, each consisting of the sequence: (1) rhyolitic phreatomagmatic eruptions triggering a central caldera collapse; and (2) extrusion of dacitic-rhyolitic domes and lava flows. The rocks of this cycle are characteized by the presence of mafic enclaves with different petrographic and chemical features which testify to mixing-mingling processes between variously evolved magmas. Jumps in the degree of evolution are present in the stratigraphic series, accompanied by changes in the porphyritic index. This index ranges from 60% to about 5% and correlates with several teochemical parameters, including a negative correlation with Sr isotope ratios (0.703384–0.705120). The latter increase from basaltic andesites to intermediate rocks, but then slightly decrease in the most evolved volcanic rocks. The petrographic, geochemical and isotopic characteristics can be largely explained by processes occurring in a convecting, crystallizing and assimilating magma chamber, where crystal sorting, retention, resorption and accumulation take place. A group of crystal-rich basaltic andesites with high Sr and compatible element contents and low incompatible elements and Sr isotope ratios probably resulted from the accumulation of plagioclase and pyroxene in an andesitic liquid. Re-entrainment of plagioclase crystals in the crystallizing magma may have been responsible for the lower 87Sr/86Sr in the most evolved rocks. The gaps in the degree of evolution with time are interpreted as due to liquid segregation from a crystal mush once critical crystallinity was reached. At that stage convection halted, and a less dense, less porphyritic, more evolved magma separated from a denser crystal-rich magma portion. The differences in incompatible element enrichment of pre-and post-caldera dacites and the chemical variation in the post-caldera dome sequence are the result of hybridization of post-caldera dome magmas with more mafic magmas, as represented by the enclave compositions. The occurrence of the quenched, more mafic magmas in the two post-caldera units suggests that renewed intrusion of mafic magma took place after each collapse event.  相似文献   

6.
The Tertiary volcanism of Eastern Macedonia and Western Thrace (Greece) developed in association with the sedimentary basin which formed, from Eocene to Oligocene, along the southern margin of the Rhodope Massif.The volcanic products, ranging in composition from basaltic andesites to rhyolites, show an overall calc-alkaline orogenic affinity, while chemical characteristics identify different groups of rocks, probably reflecting minor differences among parent magmas. The observed evolution within any group of rocks is compatible with fractional crystallization processes acting on relatively shallow magma bodies.The Sr isotopic composition of rhyolitic member shows an initial87Sr/86Sr ratio comparable with that of basaltic andesites, reinforcing the hypothesis of a crystal/liquid line of descent.Geochemical and petrographic evidence, on the whole, suggests that the investigated orogenic association developed on an active continental margin characterized by a relatively thick crust, acting as a density filter for the basic magmas and facilitating their storage and fractionation within the crust itself. Minor contamination by interaction with host materials may also have occurred.Stratigraphic and K/Ar geochronological data indicate that the volcanic activity started in Upper Eocene and reached its maximum development in Upper Oligocene. From Lower Miocene, the volcanism shifted southward in the Central Aegean area and in part of Western Anatolia, coming to an end by Middle Miocene.The southward migration of the volcanic front has been interpreted as a consequence of the increase in the dipping of the Benioff zone, due to the decrease of penetrative strength after the main phase of continental collision.  相似文献   

7.
Forty-six new K-Ar age determinations are presented on whole rock samples and mineral separates from volcanic and subvolcanic rocks of Gran Canaria. The main subaerial shield building basaltic volcanism with estimated volume of about 1000 km3 was confined to the interval about 13.7 m.y. to 13.5 m.y. ago in the middle Miocene. Substantial volume (～100 km3) of silicic volcanics (trachyte and peralkaline rhyolite) were erupted with no detectable time break following the basaltic volcanism, essentially contemporaneous with formation of a large collapse caldera at 13.4±0.3 m.y. ago. Trachytic to phonolitic volcanism continued intermittently in the waning states of activity until about 9 m.y. ago. Following a long hiatus there was resurgence of volcanism with eruption of about 100 km3 of basanitic to hauyne phonolitic rocks of the Roque Nublo Group between about 4.4 m.y. and 3.4 m.y. ago in the Pliocene. After a hiatus of less than 1.0 m.y., olivine nephelinite magmas were erupted and this activity continued intermittently until relatively recent times, the younger eruptives being mainly basanitic in composition. The volume of volcanic products in this phase probably does not exceed 10 km3. Thus the volume of all the resurgent volcanism comprises less than 10 percent of the subaerially exposed part of Gran Canaria. The results show that the subaerial main shield building phase of volcanism in Gran Canaria, consisting of mildly alkali to transitional basalts, occurred over a time interval that was less than 0.5 m.y. Magmatic evolution on Gran Canaria appears to be similar to that found on other basaltic volcanoes in oceanic regions. Thus volcanoes in the Hawaiian, Marquesas and Society Islands all were built by basaltic lavas in similar short-lived episodes of volcanism. In some Hawaiian volcanoes, a resurgent phase of volcanism of strongly undersaturated basalts of small volume is recognized following a long hiatus, again similar to that found on Gran Canaria. The relatively large volume of silicic lavas erupted in Gran Canaria immediately following the main basaltic shield building phase is, however, not matched in the Pacific volcanoes mentioned.  相似文献   

8.
Calc-alkaline intermediate rocks are spatially and temporally associated with high-Mg andesites (HMAs, Mg#>60) in Middle Miocene Setouchi volcanic belt. The calc-alkaline rocks are characterized by higher Mg# (strongly calc-alkaline trend) than ordinary calc-alkaline rocks at equivalent silica contents. Phenocrysts in the intermediate rocks have petrographical features such as: (1) coexisting reversely and normally zoned orthopyroxene phenocrysts in single rock; (2) sieve type plagioclase in which cores are mantled by higher An%, melt inclusion-rich zone; and (3) reversely zoned amphibole phenocrysts with opacite cores. In addition, mingling textures and magmatic inclusions were observed in some rocks. These petrographic features and the mineral chemistry indicate that magma mixing was the most important process in producing the strongly calc-alkaline rocks. The core composition of normally zoned orthopyroxene phenocrysts and the mantle composition of reversely zoned orthopyroxene phenocrysts have relatively high Mg# (85–90) in maximum. Although basaltic and high-Mg andesitic magmas are candidate as possible mafic end-member magmas, basaltic magma is excluded in terms of phenocryst assemblage and bulk composition. HMA magmas are suitable mafic end-member magmas that precipitated high Mg# (90) orthopyroxene, whereas andesitic to dacitic magma are suitable felsic end-members. In contrast, it is difficult to produce the strongly calc-alkaline trend through fractional crystallization from a HMA magma, because it would require removal of plagioclase together with mafic minerals from the early stage of crystallization, whereas the precipitation of plagiolase is suppressed due to the high water content of HMA magmas. These results imply that Archean Mg#-rich TTGs (>45–55), which are an analog of the strongly calc-alkaline rocks in terms of chemistry and magma genesis, can be derived from magma mixing in which a HMA magma is the mafic end-member magma, rather than by fractional crystallization from a HMA magma.  相似文献   

9.
Since the onset of their eruptive activity within the Cañadas caldera, about 180 ka ago, Teide–Pico Viejo stratovolcanoes have mainly produced lava flow eruptions of basaltic to phonoltic magmas. The products from these eruptions partially fill the caldera, and the adjacent Icod and La Orotava valleys, to the north. Although less frequent, explosive eruptions have also occurred at these composite volcanoes. In order to assess the possible evolution Teide–Pico Viejo stratovolcanoes and their potential for future explosive activity, we have analysed their recent volcanic history, assuming that similar episodes have the highest probability of occurrence in the near future. Explosive activity during the last 35000 years has been associated with the eruption of both, mafic (basalts, tephro–phonolites) and felsic (phono–tephrites and phonolites) magmas and has included strombolian, violent strombolian and sub-plinian magmatic eruptions, as well as phreatomagmatic eruptions of mafic magmas. Explosive eruptions have occurred both from central and flank vents, ranging in size from 0.001 to 0.1 km3 for the mafic eruptions and from 0.01 to < 1 km3 for the phonolitic ones. Comparison of the Teide–Pico Viejo stratovolcanoes with the previous cycles of activity from the central complex reveals that all them follow a similar pattern in the petrological evolution but that there is a significant difference in the eruptive behaviour of these different periods of central volcanism on Tenerife. Pre-Teide central activity is mostly characterised by large-volume (1–> 20 km3, DRE) eruptions of phonolitic magmas while Teide–Pico Viejo is dominated by effusive eruptions. These differences can be explained in terms of the different degree of evolution of Teide–Pico Viejo compared to the preceding cycles and, consequently, in the different pre-eruptive conditions of the corresponding phonolitic magmas. A clear interaction between the basaltic and phonolitic systems is observed from the products of phonolitic eruptions, indicating that basaltic magmatism is the driving force of the phonolitic eruptive activity. The magmatic evolution of Teide–Pico Viejo stratovolcanoes will continue in the future with a probably tendency to produce a major volume of phonolitic magmas, with an increasing explosive potential. Therefore, the explosive potential of Teide–Pico Viejo cannot be neglected and should be considered in hazard assessment on Tenerife.  相似文献   

10.
Gabbroic and hornblendite xenoliths from La Palma, Tenerife and Lanzarote fall into three main groups based on petrography and chemistry. One group (comprising all xenoliths from Lanzarote and some from La Palma) consists of highly deformed orthopyroxene-bearing gabbroic rocks that show a strong affinity to N-MORB and oceanic gabbro cumulates in terms of mineral chemistry and REE relations. However, they show mild enrichment in the most incompatible elements (particularly Rb+Ba±K) relative to intermediate and heavy REE, and their Sr–Nd isotope ratios fall within or close to the N-MORB field. The second group (60% of the xenoliths from La Palma) are gabbroic cumulates with zoned clinopyroxenes (Ti–Al-poor cores, Ti–Al-rich rims) and reaction rims of hornblende, biotite and clinopyroxene on other phases. Their trace-element and Sr–Nd isotope relations are in general transitional between N-MORB cumulates and Canary Islands alkali basalts, but they show strong enrichment in Rb, Ba and K relative to other strongly incompatible elements. The third group (comprising some xenoliths from La Palma and all those from Tenerife) are undeformed gabbroic and hornblendite rocks in which hornblende and biotite appear to belong to the primary assemblage. These rocks show strong affinities to Canary Islands alkali basaltic magmas with respect to mineral, trace-element, and Sr–Nd isotope chemistry. The first two groups are interpreted as fragments of old oceanic crust which have been mildly to strongly metasomatized through reactions with Canary Islands alkaline magmas. The reaction process is a combination of enrichment in elements compatible with biotite (and hornblende), and simple mixing between N-MORB cumulates and trapped alkaline magmas. The third group represents intrusions/cumulates formed from mafic alkaline Canary Islands magmas. Modeling indicates that locally up to 50% new material has been added to the old oceanic crust through reactions with ocean island basalts. Reactions and formation of cumulates do not represent simple underplating at the mantle/crust boundary, but have taken place within the pre-existing oceanic crust, and are likely to have significantly thickened the old oceanic crust.  相似文献   

11.
The Lower Pliocene volcanic rocks occurring in the Gölcük area of SW Turkey exhibit alkaline major element trends with a general potassic character. The development of volcanism can be divided into 2 major stages such as trachytic ancient lavas/domes and tephriphonolitic, trachyandesitic to trachytic Gölcük eruptions (ignimbrites, lava/dome extrusions, phreatomagmatic deposits, and finally, young domes). Volcanic rocks consist primarily of plagioclase, clinopyroxene (which ranges in composition from diopside to augite and are commonly zoned), biotite, and phlogopite. Amphibole phenocrysts are restricted to the pyroclastic deposits. Pseudoleucites are also seen only in the lava/dome extrusions. Oxides and apatites are common accessory phenocryst phases. As would be expected from their potassic–alkaline nature, the volcanic rocks of the Gölcük area contain high amounts of LILE (Ba, Sr, Rb and K), LREE, and Zr. Concentrations of compatible elements such as Cr, Ni and V are very low, possibly indicating fractionation of olivine and clinopyroxene. Correlation of SiO2, Rb/Sr and MgO with 87Sr/86Sr (0.703506–0.704142) exhibit an increasing trend in the direction of crustal contamination. However, the isotopic compositions of Sr are not as high to indicate a high level of crustal contamination. Geochemical data are consistent with the derivation of Gölcük volcanic rocks from a metasomatized and/or enriched lithospheric mantle source during crustal extension in the area. This metasomatism was probably occurred by fluids released from the northward subduction between African and Eurasian plates during Tertiary, as the Gölcük volcanic rocks display features of island-arc magmas with having high Ba/Nb (>28) ratios, and Nb and Ti depletions. Lower Pliocene volcanism in the Gölcük was response to extensional tectonics.  相似文献   

12.
Extrusive carbonatites are described from the Miocene alkaline complex of the Kaiserstuhl, Rhinegraben, Western Germany. Agglutinated carbonatitic lapilli form pyroclastic rocks in which all components show forms acquired when a highly fluid melt was sprayed into the air by an explosive eruption: droplets, spherical and elliptical lapilli, rods, dumbbell and pear-shaped forms.Complete morphological analogies suggest a mechanism similar to the formation of “Pele's tears”, basaltic droplets formed by the eruption of the most fluid Hawaiian basaltic magmas. Evidence is provided by this example that CaCO3-carbonatitic magmas can exist in nature under surface conditions displaying extremely low viscosity.  相似文献   

13.
Based on detailed field, petrographic, chemical, and isotopic data, this paper shows that the youngest magmas of the active Nisyros volcano (South Aegean Arc, Greece) are an example of transition from rhyolitic to less evolved magmas by multiple refilling with mafic melts, triggering complex magma interaction processes. The final magmatic activity of Nisyros was characterized by sub-Plinian caldera-forming eruption (40?ka), emplacing the Upper Pumice (UP) rhyolitic deposits, followed by the extrusion of rhyodacitic post-caldera domes (about 31–10?ka). The latter are rich in magmatic enclaves with textural and compositional (basaltic–andesite to andesite) characteristics that reveal they are quenched portions of mafic magmas included in a cooler more evolved melt. Dome-lavas have different chemical, isotopic, and mineralogical characteristics from the enclaves. The latter have lower 87Sr/86Sr and higher 143Nd/144Nd values than dome-lavas. Silica contents and 87Sr/86Sr values decrease with time among dome-lavas and enclaves. Micro-scale mingling processes caused by enclave crumbling and by widespread mineral exchanges increase from the oldest to the youngest domes, together with enclave content. We demonstrate that the dome-lavas are multi-component magmas formed by progressive mingling/mixing processes between a rhyolitic component (post-UP) and the enclave-forming mafic magmas refilling the felsic reservoir (from 15?wt.% to 40?wt.% of mafic component with time). We recognize that only the more evolved enclave magmas contribute to this process, in which recycling of cumulate plagioclase crystals is also involved. The post-UP end-member derives by fractional crystallization from the magmas leftover after the previous UP eruptions. The enclave magma differentiation develops mainly by fractional crystallization associated with multiple mixing with mafic melts changing their composition with time. A time-related picture of the relationships between dome-lavas and relative enclaves is proposed, suggesting a delay between a mafic magma input and the relative dome outpouring. We also infer that the magma viscosity reduction by re-heating allows dome extrusion without explosive activity.  相似文献   

14.
Oligocene dome complexes of trachydacitic to rhyolitic composition are common in the southern portion of the Mesa Central physiographic province, which forms part of the southern Basin and Range extensional province as well as of the southern Sierra Madre Occidental volcanic province. Generally, dome complexes occur aligned with regional fault systems, mostly associated with the southern Basin and Range province, and thus suggesting that faults controlled the felsic magmas that formed these domes. Two distribution patterns are evident, one aligned NE–SW and another aligned NNE. The set of domes were emplaced at 33–28 Ma. Emplacement of domes occurred in three continuous phases starting with those of trachydacite affinity at 33–32 Ma, to trachydacite–rhyolitic at 32–31 Ma, and finally to those with rhyolitic composition at 31–28 Ma. Felsic magmas that originated the domes were apparently generated by partial melting at the base of the continental crust. Contrary to previous hypothesis, our evidence suggest that these magmas in these particular areas of the Mesa Central were not accumulated in large magma reservoirs emplaced at shallow levels in the crust, but crossed the continental crust directly. Since continental crust in this region is relatively thin (30–33 km), we propose that an intense extensional episode favored the direct ascension of these magmas through the brittle crust, with little interaction with the country rock during ascent to the surface, to end up forming aligned dome chains or complexes. Geochemical data favors this model, as the felsic rocks show no depletions in Nb and Th but instead relatively enrichment in these elements. REE show flat or concave up patterns, suggesting that the magmas involved enriched (fertile), metasomatized lithospheric fluids that generated partial melting at the base of the continental crust. Based upon these data, we infer an intra-plate tectonic setting for these rocks.  相似文献   

15.
Mount Drum is one of the youngest volcanoes in the subduction-related Wrangell volcanic field (80×200 km) of southcentral Alaska. It lies at the northwest end of a series of large, andesite-dominated shield volcanoes that show a northwesterly progression of age from 26 Ma near the Alaska-Yukon border to about 0.2 Ma at Mount Drum. The volcano was constructed between 750 and 250 ka during at least two cycles of cone building and ring-dome emplacement and was partially destroyed by violent explosive activity probably after 250 ka. Cone lavas range from basaltic andesite to dacite in composition; ring-domes are dacite to rhyolite. The last constructional activity occurred in the vicinity of Snider Peak, on the south flank of the volcano, where extensive dacite flows and a dacite dome erupted at about 250 ka. The climactic explosive eruption, that destroyed the top and a part of the south flank of the volcano, produced more than 7 km3 of proximal hot and cold avalanche deposits and distal mudflows. The Mount Drum rocks have medium-K, calc-alkaline affinities and are generally plagioclase phyric. Silica contents range from 55.8 to 74.0 wt%, with a compositional gap between 66.8 and 72.8 wt%. All the rocks are enriched in alkali elements and depleted in Ta relative to the LREE, typical of volcanic arc rocks, but have higher MgO contents at a given SiO2, than typical orogenic medium-K andesites. Strontium-isotope ratios vary from 0.70292 to 0.70353. The compositional range of Mount Drum lavas is best explained by a combination of diverse parental magmas, magma mixing, and fractionation. The small, but significant, range in 87Sr/86Sr ratios in the basaltic andesites and the wide range of incompatible-element ratios exhibited by the basaltic andesites and andesites suggests the presence of compositionally diverse parent magmas. The lavas show abundant petrographic evidence of magma mixing, such as bimodal phenocryst size, resorbed phenocrysts, reaction rims, and disequilibrium mineral assemblages. In addition, some dacites and andesites contain Mg and Ni-rich olivines and/or have high MgO, Cr, Ni, Co, and Sc contents that are not in equilibrium with the host rock and indicate mixing between basalt or cumulate material and more evolved magmas. Incompatible element variations suggest that fractionation is responsible for some of the compositional range between basaltic andesite and dacite, but the rhyolites have K, Ba, Th, and Rb contents that are too low for the magmas to be generated by fractionation of the intermediate rocks. Limited Sr-isotope data support the possibility that the rhyolites may be partial melts of underlying volcanic rocks. Received March 13, 1993/Accepted September 10, 1993  相似文献   

16.
Geology and volcanology of the Edd-Bahar Assoli area (Ethiopia)   总被引：1，自引：0，他引：1  
The paper presents geological and petrological data on one of the alkaline ranges developed along the borders of the Afar depression (Ethiopia). These alkaline ranges occur in a position transversal to the dominant NNW trend of the spreading zones of northern and central Afar which are characterized by magmas of tholeiitic affinity. The Edd-Bahar Assoli volcanic range consists of broad fields of basic lavas and numerous spatter cones outcropping in the area extending between 13°25′ and 13°75′ lat. N and 41°38′ and 42°15′ long. E. The mineralogical assemblage and the chemical data point to an alkaline nature for this range consisting mainly of alkali olivine basalts and basalts tending to hawaiites, the most evolved terms being largely subordinate. Petrologic differences between the Assab, Edd-Bahar Assoli and Erta Ale ranges are shown. The Edd-Bahar Assoli alkaline volcanism would be related to tectonic patterns trending both from NNW-SSE to N-S and from NE-SW to E-W. The supposed similarity with the transverse structure of the equatorial Atlantic ocean would thus not completely be ascertained in this zone. In Afar, the coexistence of an axial volcanism of tholeiitic affinity with an alkaline volcanism at the margin can better be explained by models based upon the upper mantle temperature distribution in a zone under oceanization.  相似文献   

17.
Chemical data are presented for the basic lavas of the two volcanic shields, Piton des Neiges and Piton de la Fournaise, which comprise Reunion Island. In addition, data for cumulate xenoliths have been used to predict mineral/melt distribution coefficient values for the Reunion magmas.The younger volcanic shield, Piton de la Fournaise, comprises two lava sequences, the >0.5−0.2-m.y. B.P. Primary Shield lavas, and the <0.2-m.y. B.P. Caldera Series lavas. Fractional crystallization models for these lavas indicate that olivine is the major fractionating phase during the evolution from the parental basalt composition to the average basaltic liquid. Only during the evolution of the older, Primary Shield lavas has the common fractionation of an ol + cpx + plag + mt assemblage resulted in the eruption of hawaiitic, ankaramitic and feldspar-phyric lavas. The restriction of the Caldera Series liquids predominantly to olivine fractionation and the extensive cotectic fractionation during the evolution of the Primary Shield sequences is interpreted in terms of the maturity of the volcanic center. The younger stages of evolution involve high magma input into a well-developed feeder and reservoir system, thus maintaining the liquids above a cotectic surface. Whereas, during the evolution of the Primary Shield lavas, lower magma input rates into a less well-developed feeder system increased the probability of the fractionating liquid attaining a cotectic surface. Fractional crystallization accounts for all the chemical variation observed for the Piton de la Fournaise basaltic magmas. The analytical data are closely comparable to the rare earth element (REE) and trace element fractionation curves predicted by least-squares calculations, this supports the use of such models in quantitative evaluation of fractional crystallization.A preliminary survey of Sr isotope values indicates that the oldest (>2 m.y. B.P.) lava sequences of Piton des Neiges may be derived from a source which was isotopically distinct from that of the <2 m.y. B.P. lavas of both volcanic shields. These latter sequences are remarkably consistent in both isotopic and trace element abundance implying a homogeneous source material and an invariable partial melting process. Partial melting calculations indicate that the basaltic lavas have been derived by 5–10% melting of a garnet-poor peridotite (cpx/gt 9). Systematic differences in the light- and heavy-REE patterns between similar basaltic provinces are interpreted to be a result of variation in the nature of the phases buffering the entry of light- and/or heavy-REE into the melt during partial fusion.  相似文献   

18.
The back-arc region of the Izu-Bonin arc has complex bathymetric and structural features, which, due to repeated back-arc rifting and resumption of arc volcanism, have prevented us from understanding the volcano-tectonic history of the arc after 15 Ma. The laser-heating 40Ar/39Ar dating technique combined with high density sampling of volcanic rocks from the back-arc region of this arc successfully revealed the detailed temporal variation of volcanism related to the back-arc rifting. Based on the new 40Ar/39Ar dating results: (1) Back-arc rifting initiated at around 2.8 Ma in the middle part of the Izu-Bonin arc (30°30′N–32°30′N). Volcanism at the earliest stage of rifting is characterized by the basaltic volcanism from north–south-trending fissures and/or lines of vents. (2) Following this earliest stage of volcanism, at ca. 2.5 Ma, compositionally bimodal volcanism occurred and formed small cones in the wide area. This volcanism and rifting continued until about 1 Ma in the region west of the currently active rift zone. (3) After 1 Ma, active volcanism ceased in the area west of the currently active rift zone, and volcanism and rifting were confined to the currently active rift zone. The volcano-tectonic history of the back-arc region of the Izu-Bonin arc is an example of the earliest stage of back-arc rifting in the oceanic island arc. Age data on volcanics clearly indicate that volcanism changed its mode of activity, composition and locus along with a progress of rifting.  相似文献   

19.
Magma mixing and magma plumbing systems in island arcs   总被引：3，自引：0，他引：3  
Petrographic features of mixed rocks in island arcs, especially those originating by the mixing of magmas with a large compositional and temperature difference, such as basalt and dacite, suggest that the whole mixing process from their first contact to the final cooling (= eruption) has occurred continuously and in a relatively short time period. This period is probably less than several months, considerably shorter than the whole volcanic history. There may also be a prolonged quiescent interval, lasting longer than several days, between the magmas' contact and the mechanical mixing. This interval will allow the basic magma to cool and produce a semi-solidified boundary which is later disrupted by flow movements to produce basic inclusions.Mixing of magmas of contrasting chemical composition need not be the inevitable consequence of the contact of the magmas. It is, however, made more probable by forced convection caused by motive force such as the injection of a basic magma into an acidic magma chamber. A short interval between their initial contact and the final eruption requires that the acid magma chamber has a small volume, of the same order or less than that the introduced basic magma.The volcanic activity of Myoko volcano, central Japan, of the last 100,000 years shows alternate eruptions of hybrid andesite by mixing of basaltic and dacitic magmas, and non-mixed basalt to basaltic andesite. There was a repose period of 20,000 to 30,000 years between eruptions. The acidic chamber, eventually producing the mixed andesite activity, is formed during the repose period by the « in situ » solidification of the original basic magma against its wall. The volume of the chamber is very small, probably about 10–2 km3. Basaltic magma with constant chemical composition is supplied to the shallow chamber from another deep seated basaltic chamber. The volume of the shallow magma chamber may be critical to the characteristics of volcanic activity and its products.  相似文献   

20.
The last eruptions of the monogenetic Bakony-Balaton Highland Volcanic Field (western Pannonian Basin, Hungary) produced unusually crystal- and xenolith-rich alkaline basalts which are unique among the alkaline basalts of the Carpathian–Pannonian Region. Similar alkaline basalts are only rarely known in other volcanic fields of the world. These special basaltic magmas fed the eruptions of two closely located volcanic centres: the Bondoró-hegy and the Füzes-tó scoria cone. Their uncommon enrichment in diverse crystals produced unique rock textures and modified original magma compositions (13.1–14.2 wt.% MgO, 459–657 ppm Cr, and 455–564 ppm Ni contents). Detailed mineral-scale textural and chemical analyses revealed that the Bondoró-hegy and Füzes-tó alkaline basaltic magmas have a complex ascent history, and that most of their minerals (～30 vol.% of the rocks) represent foreign crystals derived from different levels of the underlying lithosphere. The most abundant xenocrysts, olivine, orthopyroxene, clinopyroxene, and spinel, were incorporated from different regions and rock types of the subcontinental lithospheric mantle. Megacrysts of clinopyroxene and spinel could have originated from pegmatitic veins/sills which probably represent magmas crystallized near the crust–mantle boundary. Green clinopyroxene xenocrysts could have been derived from lower crustal mafic granulites. Minerals that crystallized in situ from the alkaline basaltic melts (olivine with Cr-spinel inclusions, clinopyroxene, plagioclase, and Fe–Ti oxides) are only represented by microphenocrysts and overgrowths on the foreign crystals. The vast amount of peridotitic (most common) and mafic granulitic materials indicates a highly effective interaction between the ascending magmas and wall rocks at lithospheric mantle and lower crustal levels. However, fragments from the middle and upper crust are absent from the studied basalts, suggesting a change in the style (and possibly rate) of magma ascent in the crust. These xenocryst- and xenolith-rich basalts yield divers tools for estimating magma ascent rate that is important for hazard forecasting in monogenetic volcanic fields. According to the estimated ascent rates, the Bondoró-hegy and Füzes-tó alkaline basaltic magmas could have reached the surface within hours to few days, similarly to the estimates for other eruptive centres in the Pannonian Basin which were fed by “normal” (crystal and xenoliths poor) alkaline basalts.  相似文献   

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