A basanite–nephelinite glass suite from early submarine Kilauea defines a continuous compositional array marked by increasing
concentrations of incompatible components with decreasing SiO2, MgO, and Al2O3. Like peripheral and post-shield strongly alkalic Hawaiian localities (Clague et al. in J Volcanol Geotherm Res 151:279–307,
2006; Dixon et al. in J Pet 38:911–939, 1997), the early Kilauea basanite–nephelinite glasses are interpreted as olivine fractionation products from primary magnesian
alkalic liquids. For early Kilauea, these were saturated with a garnet–phlogopite–sulfide peridotite assemblage, with elevated
dissolved CO2 contents responsible for the liquids’ distinctly low-SiO2 concentrations. Reconstructed primitive liquids for early Kilauea and other Hawaiian strongly alkalic localities are similar
to experimental 3 GPa low-degree melts of moderately carbonated garnet lherzolite, and estimated parent magma temperatures
of 1,350–1,400°C (olivine–liquid geothermometry) match the ambient upper mantle geotherm shortly beneath the base of the lithosphere.
The ~3 GPa source regions were too hot for stable crystalline carbonate and may have consisted of ambient upper mantle peridotite
containing interstitial carbonate–silicate or carbonatitic liquid, possibly (Dixon et al. in Geochem Geophys Geosyst 9(9):Q09005,
2008), although not necessarily, from the Hawaiian mantle plume. Carbonate-enriched domains were particularly susceptible to further
melting upon modest decompression during upward lithospheric flexure beneath the advancing Hawaiian Arch, or by conductive
heating or upward drag by the Hawaiian mantle plume. The early Kilauea basanite–nephelinite suite has a HIMU-influenced isotopic
character unlike other Hawaiian magmas (Shimizu et al. in EOS Tran Amer Geophys Union 82(47): abstr V12B-0962, 2001; Shimizu et al. in Geochim Cosmochim Acta 66(15A):710, 2002) but consistent with oceanic carbonatite involvement (Hoernle et al. in Contrib Mineral Petrol 142:520–542, 2002). It may represent the melting products of a fertile domain in the ambient upper mantle impinged upon and perturbed by the
sustained plume source that feeds later shield-stage magmatism. 相似文献
Abstract A deep section of accretionary complex, the metamorphosed Susunai Complex, is observed on Sakhalin Is., Russia. High pressure part of pumpellyite-actinolite facies metavolcanics, metacherts and metapelites are well exposed and constitute a tectonic pile preserving primary structures related to underplating of the oceanic crust. Three stages of deformation, D1 through D3, suggest successive deformation during subduction, underplating and exhumation of the complex. Oceanic material in the complex is more abundant than other well documented ancient accretionary complexes (e.g. the Shimanto Belt in southwest Japan and the Ghost Rocks Formation in Alaska), which were shallowly underplated. At Susunai, deep down-stepping of a décollément has scraped off the upper part of the oceanic crust, primarily the pillowed basalt horizon. This down-stepping results from crustal weakening as overpressured water is released from the fractured oceanic crust during metamorphism. 相似文献
Abstract The geology and geochemistry of pyroclastic flows and fallout tephras formed during the Karasugasen dome eruption in the Daisen–Hiruzen Volcano Group in southwest Japan have been examined in detail. The Karasugasen lava dome erupted at about 26 ka. The eruption began with a vulcanian ash fall, and this was followed by at least eight block and ash flows and a pumice flow. The block and ash flows were produced by the successive collapses of a growing lava dome. This main eruption phase was followed by an eruption of vulcanian ash falls, and finally ended with a sub-Plinian pumice fall. This eruption sequence is typical of the Daisen Volcano during the last three eruption events, which occurred at 58, 26 and 17 ka. The magma produced during the Karasugasen eruption was a typical adakite, with extremely high Sr/Y ratios and low HREE/LREE ratios compared to normal arc lavas. The chemistry of the Karasugasen lavas is almost identical to other Daisen–Hiruzen lavas that were produced from eruptions over an interval of a million years. The continuous supply of a huge amount of adakitic magma (>100 km3) for such a long period suggests a massive homogeneous source material, such as molten Philippine Sea Plate slab. Slab melting is a plausible mechanism for the production of the adakitic lavas at Karasugasen, and hence the Daisen–Hiruzen Volcano Group. 相似文献
Summary Fe and Mn distribution in the pumpellyite group minerals (W8X4Y8Z12056-n(OH)n) has been studied by using57Fe Mössbauer spectroscopy. The studied Fe-pumpellyites, belonging to the pumpellyite-julgoldite series, were collected from two localities; metabasites in the Tokoro belt, Hokkaido, and gabbroic sills in the Shimane Peninsula, Japan. Okhotskite, an Mn3+-dominant pumpellyite group mineral, was separated from the ores of metamorphosed manganiferous iron ore deposits in the Tokoro belt.57Fe Mössbauer spectrum of Tokoro Fe-pumpellyite is composed of two Fe2+- and two Fe3+-doublets. On the basis of the single crystal structure refinements of Al-pumpellyites published so far, doublets were assigned to Fe
W2+
(IS= 1.01 andQS = 2.73 mm/s), Fe
X2+
(IS = 0.97 andQS = 3.18 mm/s), Fe
X3+
(IS = 0.29 andQS =1.37 mm/s) and Fe
Y3+
(IS = 0.36 andQS = 2.09 mm/s), whereIS is isomer shift relative to a metallic iron absorber andQS is quadrupole splitting. The Mössbauer spectrum of the Mitsu Fepumpellyite is composed of three doublets assigned to Fe
X2+
(IS= 1.14 andQS = 3.20 mm/s), Fe
X3+
(IS = 0.36 andQS =1.13 mm/s) and Fe
Y3+
(IS = 0.37 andQS= 1.93 mm/s). These assignments show strong preference of Fe3+ in the X-site. The Mössbauer spectrum of the okhotskite is composed of two doublets by Fe
X3+
(IS= 0.37 andQS = 1.13 mm/s) and Fe
Y3+
(IS = 0.42 andQS = 2.18 mm/s). The area ratio shows that Fe
X3+
:Fe
Y3+
ratio is 94:6. On the basis of chemical and Mössbauer analyses, Mn
X3+
:Mn
Y3+
ratio is given as 19:81, indicating stronger Y-site preference of Mn3+ than Fe3+, what is consistent with Jahn-Teller theory. Al, Mn3+ and Fe3+ prefer the Y-site in this order.
Eine57Fe Mössbauer-Studie von Mineralen der Pumpellyit-Okhotskit-Julgoldit-Serie
Zusammenfassung Die Fe- und Mn-Verteilung in Mineralen der Pumpellyit-Gruppe (W8X4Y8Z12O56-n(OH)n) wurde mittels Mössbauer-Spektroskopie studiert. Die untersuchten Fe-Pumpellyite der Pumpellyit-Julgoldit-Serie stammen von zwei verschiedenen japanischen Lokalitäten: von Metabasiten des Tokoro-Gürtels, Hokkaido, und von Gabbro-Sills der Shimane Halbinsel. Okhotskit, ein Mn3+-dominiertes Mineral der Pumpellyit-Gruppe, wurde aus Erzen einer Mn-führenden Eisenerzlagerstätte des Tokoro-Gürtels separiert. Das57Fe Mössbauer-Spektrum der Tokoro Fe-Pumpellyite zeigt zwei Fe2+- und zwei Fe3+-Doubletten. Auf Grund bisher publizierter verfeinerter Einkristall-Strukturuntersuchungen von Al-Pumpellyiten werden diese Doubletten folgendermaßen zugeordnet: Fe
W2+
(IS = 1.01 undQS = 2.73 mm/s), Fe
X2+
(IS = 0.97 undQS = 3.18 mm/s), Fe
X3+
(IS = 0.29 undQS =1.37 mm/s) und Fe
Y3+
(IS = 0.36 undQS = 2.09 mm/s).IS bezeichnet dabei die Isomer-Shift relativ zu einem metallischen Eisenabsorbenten,QS das Quadrupole-Splitting. Diese Zuordnungen belegen den bevorzugten Einbau von Fe3+ in die X-Position. Das Mössbauer-Spektrum von Okhotskit zeigt zwei Doubletten bei Fe
X3+
(IS = 0.37 undQS = 1.13 mm/s) und Fe
Y3+
(IS = 0.42 undQS = 2.18 mm/s). Das Flächenverhältnis zeigt, daß das Verhältnis Fe
X3+
:Fe
Y3+
94:6 ist. Auf Grund der chemischen und der Mössbauer-Analysen wird das Mn
X3+
:Mn
Y3+
Verhältnis mit 19:81 angegeben. Mn3+ zeigt somit eine stärkere Präferenz für die Y-Position als Fe3+, ein Resultat, das mit der Jahn-Teller-Theorie konsistent ist. Der bevorzugte Einbau in die Y-Position ist, in dieser Reihenfolge, Al>Mn3+>Fe3+.
Welded tuffs in the Bogopol and Sijanov groups were sampled at 27 sites from 12 caldera formations in the Sikhote Alin mountain range around Kavalerovo (44.3°N, 135.0°E) for chronological and paleomagnetic studies. KAr age dates show that the welded tuffs erupted between 66 Ma and 46 Ma. All sites yield reliable paleomagnetic directions, with unblocking temperatures higher than 560°C. The high-temperature component at 12 sites and the medium-temperature component at 3 sites in the Bogopol Group show reversed polarity (D = 193.7°, I = −57.6°,95 = 8.1°). The high-temperature component at 11 sites in the Sijanov Group showed both reversed and normal polarities and its mean direction reveals no detectable deflection from north (D = −2.9°, I = 59.6°,95 = 11.2°). The combined paleomagnetic direction of the two groups yields a paleomagnetic pole of 250.5°E, 84.1°N (A95 = 8.8°), which falls near Cretaceous paleomagnetic poles from Outer Mongolia, Inner Mongolia, the North China Block and the South China Block. The Sikhote Alin area appears not to have been subjected to detectable motion with respect to East Asia since about 50 Ma. This implies that the Sikhote Alin area behaved as an integral part of East Asia during the opening of the Japan Sea at about 15 Ma. However, significant separation between the paleomagnetic poles of East Asia and Europe during the Jurassic-Paleogene implies a major relative movement between these two blocks since the Paleogene. 相似文献
One of the keys to understanding the origin of Archaean greenstone belts lies in the geological relationships between mafic and ultramafic greenstones, felsic to intermediate volcanic rocks and terrigenous sediments. Traditional models for greenstone belt evolution have been based on in-situ stratigraphic relationships. Most of these models, for example an oceanic island-arc developed on oceanic basement, back-arc basins, and the recently popular plume model, predict concordant stratigraphic relationships among the various greenstone belt lithologies. However, rather than being depositional in nature, several authors have indicated that many of the relationships between the different lithologies in greenstone belts are in fact tectonic, suggesting an allochthonous origin for most greenstone sequences. All of these latter models make analogies to Phanerozoic tectonic processes involving accretion of oceanic materials with volcanism related to both plate subduction and rifting.
In this paper, we have evaluated the geological relationships between volcanic rocks and sediments in three regions in the Superior province, where the accretion of oceanic material can be documented, and direct comparisons are made to geological processes in Phanerozoic accretionary complexes. In the Malartic area in the southeastern Abitibi Subprovince, 3 to 4 km thick slices of komatiite and tholeiite, with intercalated terrigenous sediment, are tectonically imbricated and are overlain by calc-alkaline volcanics which postdate tectonic stacking. In both the Larder Lake region of the southwestern Abitibi belt and in the Beardmore-Geraldton belt, at the south-eastern limit of the Wabigoon belt, slices of iron-rich tholeiite and chemical sediments of an oceanic origin are tectonically imbricated with terrigenous sediment.
The Malartic-Val d'Or area is considered to be an example of accretion of an Archaean oceanic plateau, while the Larder Lake and the Beardmore-Geraldton regions are potentially typical of accretion of normal oceanic crust in an arc-environment. Phanerozoic accretion of oceanic crust is accompanied by a step-back in subduction, and in this paper we suggest that oceanic crust accretion may have been the principal mechanism by which the locus of subduction migrated towards the south of the Superior province. Asthenospheric upwelling associated with the isolated sinking plate may have been responsible for widespread late-magmatism. This scenario requires that magmas be erupted through previously accreted volcanic, plutonic and sedimentary material. Furthermore, later ridge subduction will result in transpressional tectonics and eruption of mafic sequences over mature and immature volcano-plutonic sequences. The combined result of the plate tectonic scenario envisaged would result in the well-described “cyclic stratigraphy” of many granite greenstone sequences. 相似文献
Abstract Fossil dolphins belonging to the extinct family Kentriodontidae are small to medium-sized toothed cetaceans, which probably include the ancestors of some living species. Kentriodontids are known from rocks of Late Oligocene to Late Miocene age in various parts of the world. Among kentriodontids, species in the subfamily Kentriodontinae (e.g. species of Kentriodon Kellogg, 1927) are the most ubiquitous and generalized; these are now known from latest Oligocene to earliest Miocene strata in New Zealand and Patagonia, and Middle Miocene deposits in Maryland, Virginia, California and Japan. The diversity, morphologies and distributions of Miocene species of Kentriodontinae seem to parallel those of the living species of mostly pelagic delphinids in the subfamily Delphininae, and the fossil group may have been an ecological or behavioral/functional counterpart of the latter. Kentriodontines are inferred to have been wide-ranging neritic to pelagic animals that ate small fish and other nectonic organisms; they were probably active echolocators, and might have formed large schools. They are relatively common as fossils and, therefore, are potentially useful for intercontinental correlations of marine deposits. 相似文献