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1.
Are Arc Basalts Dry, Wet, or Both? Evidence from the Sumisu Caldera Volcano, Izu-Bonin Arc, Japan 总被引：2，自引：0，他引：2
TAMURA Y.; TANI K.; ISHIZUKA O.; CHANG Q.; SHUKUNO H.; FISKE R. S. 《Journal of Petrology》2005,46(9):1769-1803
Basalt–basaltic andesite (<55 wt % SiO2) and dacite–rhyolite(66–74 wt % SiO2) are the predominant eruptive productsin the Sumisu caldera volcano, Izu–Bonin arc, Japan. Themost magnesian basalt (8·5% MgO), as well as some ofthe other basalts, has a low Zr content (20–25 ppm), andcannot yield basalts with higher Zr contents (29–40 ppm)through fractionation and/or assimilation. The high- and low-Zrbasalts have different phenocryst assemblages, olivine, plagioclaseand pyroxene phenocryst chemistries, REE (rare earth element)patterns, and fluid-mobile element/immobile element ratios.Estimated primary olivine compositions are more magnesian (>Fo91)in the low-Zr basalts compared with those in high-Zr basalts(<Fo89). The low-Zr basalts contain up to 11 vol. % augite,but many high-Zr basalts are free of augite, which appears onlyin their more differentiated products. The low-Zr basalts areconsidered to be hydrous magmas in which olivine crystallizesfirst followed by augite and plagioclase, whereas the high-Zrbasalts are dry. The low-Zr basalts have higher U/Th ratiosthan the high-Zr basalts. We suggest that both dry and wet primarybasalts existed in the Sumisu magmatic system, each having differenttrace element concentrations, mineral assemblages and mineralchemistry. The lower contents of Zr and light REE and magnesianprimary olivines in the wet basalts could have resulted froma higher degree of partial melting (20%) of a hydrous sourcemantle compared with 10% melting of a dry source mantle. TheSr, Nd and Pb isotope compositions of the wet and dry basaltsare similar and are limited in range. These lines of evidenceindicate that a mantle diapir model might be applicable to satisfythe configuration of such a mantle source region beneath a singlevolcanic system such as Sumisu. KEY WORDS: degree of melting; hot fingers; isotopes; mantle diapir; mantle wedge 相似文献
2.
The Miocene Tanzawa plutonic complex, consisting mainly of tonalite intrusions, is exposed at the northern end of the Izu–Bonin – Mariana (IBM) arc system as a consequence of collision with the Honshu Arc. The Tanzawa plutonic rocks belong to the calc-alkaline series and exhibit a wide range of chemical variation, from 43 to 75 wt% SiO2 . They are characterized by relatively high Ba/Rb and Ce/Nb ratios, and low abundances of K2 O, LIL elements, and rare earth elements (REE). Their petrographic and geochemical features indicate derivation from an intermediate parental magma through crystal fractionation and accumulation processes, involving hornblende, plagioclase, and magnetite. The Tanzawa plutonic complex is interpreted to be the exposed middle crust of the IBM arc, which was uplifted during the collision. The mass balance calculations, combining data from melting experiments of hydrous basaltic compositions at lower-to-middle crustal levels, suggest that parental magma and ultramafic restite were generated by dehydration partial melting (∼ 45% melting) of amphibolite chemically similar to low-K tholeiitic basalt. Partial melting of hydrated mafic lower crust might play an important role in felsic middle-crust formation in the IBM arc. 相似文献
3.
A magnetic anomaly map of the northern part of the Philippine Sea plate shows two conspicuous north–south rows of long-wavelength anomalies over the Izu–Ogasawara (Bonin) arc, which are slightly oblique to the present volcanic front. These anomalies are enhanced on reduced-to-pole and upward-continued anomaly maps. The east row is associated with frontal arc highs (the Shinkurose Ridge), and the west row is accompanied by the Nishi-Shichito Ridge. Another belt of long-wavelength anomalies very similar to the former two occurs over the Kyushu–Palau Ridge. To explain the similarity of the magnetic anomalies, it is proposed that after the spreading of the Shikoku Basin separated the Izu–Ogasawara arc from the Kyushu–Palau Ridge, another rifting event occurred in the Miocene, which divided the Izu–Ogasawara arc into the Nishi-Shichito and Shinkurose ridges. The occurrence of Miocene rifting has also been suggested from the geology of the collision zone of the Izu–Ogasawara arc against the Southwest Japan arc: the Misaka terrain yields peculiar volcanic rocks suggesting back-arc rifting at ～ 15 Ma. The magnetic anomaly belts over the Izu–Ogasawara arc do not extend south beyond the Sofugan Tectonic Line, suggesting a difference in tectonic history between the northern and southern parts of the Izu–Ogasawara arc. It is estimated that the Miocene extension was directed northeast–southwest, utilizing normal faults originally formed during Oligocene rifting. The direction is close to the final stage of the Shikoku Basin spreading. On a gravity anomaly relief map, northeast–southwest lineaments can be recognized in the Shikoku Basin as well as over the Nishi-Shichito Ridge. We thus consider that lines of structural weakness connected transform faults of the Shikoku Basin spreading system and the transfer faults of the Miocene Izu–Ogasawara arc rifting. Volcanism on the Nishi-Shichito Ridge has continued along the lines of weakness, which could have caused the en echelon arrangement of the volcanoes. 相似文献
4.
The Izu–Ogasawara arc contains, from east to west, a volcanic front, a back-arc extensional zone (back-arc knolls zone), and a series of across-arc seamount chains that cross the extensional zone in an east-northeast and west-southwest direction and extend into the Shikoku Basin. K–Ar ages of dredged volcanic rocks from these across-arc seamount chains and extension-related edifices in the back-arc region of the Izu–Ogasawara arc were measured to constrain the volcanic and tectonic history of the arc since the termination of spreading in the Shikoku Basin. K–Ar ages range between 12.5 and 1 Ma. Andesitic to dacitic rocks of 12.5–2.9 Ma occur mainly on the western part of the chains. The western part of the chains are the locus of volcanism behind the front which erupted mainly calc-alkaline andesitic lavas. The youngest rocks (< 2.8 Ma), characterized by cpx-ol basalt, occur along the western margin of the back-arc knolls zone. Basaltic rocks of 12.5–2.9 Ma have relatively high concentrations of Na2 O (> 2.0 wt%), Zr (> 50 p.p.m.) and Y (> 20 p.p.m.) and low CaO (< 12 wt%). On the other hand, basalts of 2.8–1 Ma have lower Na2 O (< 1.8 wt%), Zr (< 50 p.p.m.) and Y (< 20 p.p.m.), but significantly higher CaO (> 12 wt%). The age inferred for the initiation of back-arc rifting (∼ 2.35–2.9 Ma: Taylor 1992 ) behind the current volcanic arc coincides with the time that basalt chemistry changed drastically (eruption of the low-Na2 O and high-CaO basalt). This implies that post-2.8 Ma volcanism in the back-arc knolls zone is associated with rifting. Similarly, the change in chemical composition might be explained by a different type of source mantle following rift initiation. Volcanism in the western seamounts ceased after the onset of rifting at ∼ 2.8 Ma. 相似文献
5.
Palaeo-Tokyo Bay is a relic of the Plio-Pleistocene Kazusa forearc basin in the Boso Peninsula of Japan. The sedimentary infill of palaeo-Tokyo Bay is characterized by shallow marine to paralic sediments of the middle to upper Pleistocene Shimosa Group. Sequence stratigraphical analysis has been used to describe spatial and temporal variations in the depositional systems of the lowest units of the Shimosa Group, deposited during the early stage of development of palaeo-Tokyo Bay. Three different type of depositional systems were recognized: sand ridge to shelf (SRS), shelf to delta (SDL) and shelf to non-deltaic nearshore (SNS) systems. They overlie early transgressive estuarine deposits infilling lowstand valleys incised in the south-eastern margin of palaeo-Tokyo Bay. These systems were developed during late transgressive through highstand stages of a relative sea level cycle, which may have been controlled by a glacio-eustatic sea level change at about 0·4 Ma. Spatial variation in depositional systems is largely identical to that in modern Tokyo Bay; environmental conditions similar to those prevailing at the present day probably characterized the early history of palaeo-Tokyo Bay. The timing of highstand systems tracts within a high frequency depositional sequence was analysed in terms of the effect of sedimentation rate, based on the mapping of a chronostratigraphical surface marked by the Hy4 volcanic ash layer. From spatial variations in sedimentation rate, it was possible to identify the diachronous evolution of highstand systems tracts from the SDL system, through the SNS system, to the SRS system. Time lag is indicated by major bounding surfaces, such as maximum flooding or downlap surfaces associated with a condensed section, which developed immediately above or below the Hy4 volcanic ash layer. The lag may be of the order of a few thousands to tens of thousands of years within a depositional sequence with a total of duration of about 100 000 years. 相似文献
6.
Osumilite-bearing Granulites in the Eastern Grenville Province, Eastern Labrador, Canada: Mineral Parageneses and Metamorphic Conditions 总被引：1，自引：1，他引：0
Osumilite-bearing metasedimentary gneiss occurs in the contactaureole of the Sand Hill Big Pond gabbronorite complex of Labradorianage in eastern Grenville Province, eastern Labrador, Canada.The osumilite (Os) occurs in association with cordierite (Cd),orthopyroxene (Opx), sillimanite (Sil), sapphirine (Sa), spinel(Sp), K-feldspar (Kf), plagioclase (PI), phlogopite (Ph), hematite(Hm),magnetite (Mt), corundum (Co), and quartz (Q) in zones adjacentto the gabbronorite intrusion. The osumilite-in isogard is delineatedat a distance of 1-3km from the intrusive contact. The osumiliteis characterized by enhanced Mg/(Mg + Fe) ratios (0.89-0.92)and relatively high K2O contents (4. 13-4.78 wt. % ) The compositionalvariation of the osumilite is best understood by the substitutions(Mg+Fe+Mn)+St=2A1 and (K+Na)+(Mg +Fe+Mn)=Al+vacancy. Symplectiticintergrowth of Opx-Cd-Kf-Q, which partly of completely replacedosumilite, is ubiquitously present. The stable osumilite-bearingassemblages (all with Hm, Mt, and P1) deduced from petrographicfeatures and from the phase relations in the KMAS system areOs-Sa-Cd, Os-Cd-Sil-Q, Os-Cd-Kf-Q, Os-Cd-Opx-Q, and Os-Opx-Kf-Q.The saphirine-bearing assemblages are restricted to silica-deficient(quartz-absent) zones of the gnesis, which include Sa-Os-Cd,Sa-Sil-Cd, Sa-Opx-Sp, Sa-Sp-Cd, and Sa-Sp-Co. Orthopyroxenecoronas mantling phologite reflect the breakdown reaction Ph+ Q= Opx+Kf+vapor under extremely low water activity in thevapour. Petrogenetic grids in MAS, KMAS, and KMAS-H2O-CO2appropriateto the mineral assemblages in the metasedimentary gneisses suggestthat the changes of the mineral assemblages in the area studiedreflect dehydration reactions Ph+Sil+Q=Cd+Kf+V and Ph+Q=Opx+Kf+V, and osumilite and sapphirine-forming reactions Opx+Cd+Kf+Q=Os,Cd+Kf+Q=Os+Sil+, Opx+Sil+Kf=Os+Sa, and Opx+Sil =Sa+ Cd. Relativelyhigh Mg/(Mg+Fe2+) (0. 64-0.88) in the whole-rock compositionand high oxygen fugacity (near hematite-magnetite buffer), togetherwith exceptionally high temperature ( 1000?C) and extremelylow water activity (0.2) at peak meta-morphic conditions mightstabilize the osumilite- and sapphirine-bearing assemblagesat middle or lower crustal levels. Relatively low water activityis probably caused by the relatively dry precursor, which hadbeen metamorphosed at upper amphibolite facies metamorphic conditionsbefore gabbronorite emplacement. 相似文献
7.
ABSTRACT Groundwater temperature at an arbitrary depth and at an arbitrary point is determined not only by heat transported by conduction but also by advection caused either by infiltration of rain, snowmelt or irrigated water, or by seepage from surface water bodies. Therefore, characteristic changes of groundwater temperature are observed in recharging and discharging areas within a groundwater flow system. The changes may be one-, two-, or three-dimensional, depending on individual situations. Since heat is a conservative quantity in the subsurface environment, groundwater temperature can be used as a tracer to reveal the regional structure of a groundwater flow system. A case study showing the importance of groundwater temperature in a regional groundwater survey is presented taking Nagaoka plain, Japan, as an example. The groundwater temperatures were measured in observation wells with diameters of 65 to 250 mm and depths of 20 m or more. Marked seasonal changes in temperature depth profiles showing advective effects in the horizontal direction from the Shinano River, and in the vertical direction from upper and lower aquifers, were observed. The temperature depth profiles were classified into six types. The distribution of these types does not contradict the regional structure of the groundwater flow system revealed by the potential distribution. As groundwater temperature is an easily measureable element in a hydrological survey, the method described in the present paper is appropriate for a field study in an uninstrumented groundwater basin. 相似文献
8.
MAKOTO TAKEUCHI 《Island Arc》2011,20(2):221-247
Detrital chloritoids were extracted from the Lower Jurassic sandstones in the Joetsu area of central Japan. The discovery of detrital chloritoids in the Joetsu area, in addition to two previous reports, confirms their limited occurrence in the Jurassic strata of the Japanese islands. This finding emphasizes the importance of the denudation of chloritoid‐yielding metamorphic belts in Jurassic provenance evolution, in addition to a change from an active volcanic arc to a dissected arc that has already been described. Possible sources for the detrital chloritoids from the Jurassic sandstones are the Permo–Triassic chloritoid‐yielding metamorphic rocks distributed in dispersed tectonic zones (Hida, Unazuki, Ryuhozan and Hitachi Metamorphic Rocks), which are in fault contact with Permian to Jurassic accretionary complexes in the Japanese islands. This is because all of these pre‐Jurassic chloritoid‐yielding metamorphic rocks have a Carboniferous–Permian depositional age and a Permo–Triassic metamorphic age, whereas a Permian–Triassic metamorphic age on the Hitachi Metamorphic Rocks remains unreported. In addition, most metamorphic chloritoids imply a former stable land surface that has evolved into an unstable orogenic area. Therefore, the chloritoid‐yielding metamorphic rocks might form a continuous metamorphic belt originating from a passive continental margin in East Asia. Evidence from paleontological and petrological studies indicates that the Permo–Triassic metamorphic belt relates to a collision between the Central Asian Orogenic Belt and the North China Craton. The evolution of the Permian–Jurassic provenance of Japanese detrital rocks indicates that the temporal changes in detritus should result from sequences of collision‐related uplifting processes. 相似文献
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