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Abstract The magnitudes of plastic strains of 104 metacherts were determined from the deformed shape of initially spherical radiolarians in the Sambagawa high- P type metamorphic belt of Western Shikoku, Japan. The strain magnitude increases with increasing metamorphic temperature from several per cent to 250%. The a2 /a3 ratio of strain ellipsoids in the higher metamorphic grades decreases with increasing metamorphic grade while the a1 /a2 ratio increases rapidly. The long axis of the strain ellipsoid for every grade is nearly parallel to the length of the metamorphic belt, suggesting that the flow direction of the synmetamorphic deformation was uniform along the belt. A map of strain zones within the Sambagawa high- P type metamorphic belt reveals that the metamorphic belt underwent a progressive bulk inhomogeneous shear deformation and that the high-grade zones represent a deep-seated boundary shear zone on the accretionary wedge between a subducting oceanic plate and the immobile rigid continental plate. 相似文献
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Miyake-jima volcano has erupted at least 13 times during theperiod 14691983. To understand the historic magmaticprocesses, we focus on the mineral assemblage and chemical compositionsof crystal-clots in single samples from each of the eruptions.Most of the historic lavas consist of nearly aphyric to weaklyporphyritic basalt to andesite, but there also exist megacryst-bearingrocks. The megacrysts are considered to be xenocrysts from adeep-seated plutonic body. Many samples of each eruption containtwo types of clots beside megacrysts, termed here B-type andA-type. The B-type clots are composed of olivine, clinopyroxeneand plagioclase, whereas the A-type clots additionally containmagnetite and orthopyroxene. Compositional relationships betweenthese mafic minerals suggest that the minerals in the same typeof clots are in equilibrium. Comparing the chemical compositionsof the minerals in the two types of clots in each sample, theyare derived from distinct magmas: the B-type clots from basalticmagma and the A-type clots from andesitic magma. During thehistoric activity, the magma plumbing system appears to haveincluded two magma storage systems: a deep-seated basaltic anda shallower andesitic one. In many cases, basaltic magma hasinjected into shallower andesitic magma to form mixed magma;however, andesitic magma has sometimes erupted alone withoutextensive injections of basaltic magma. Temporal variationsof mineral compositions in the clots and estimated whole-rockcompositions of the end-member magmas suggest that the basalticmagma has differentiated gradually since 1469, and that itsmagmatic temperature has fallen from 1220 to 1180°C. Conversely,the andesitic magma has changed in a complex fashion to becomemore mafic (the magmatic temperature rose from 1050 to 1100°C).As a result of this study, it is estimated that the basalticmagma after the 1983 eruption was the least mafic, and the andesiticmagma the most mafic, of the historic eruptions. KEY WORDS: andesite; basalt; crystal-clots; evolution of magma; Miyake-jima volcano; magma mixing 相似文献
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Juvenile ejecta from the September and October 1995 eruptionsof Ruapehu volcano, New Zealand, indicate that mixing occurredbetween relatively higher- and lower-temperature (high-T andlow-T) andesitic magmas. Compositional zonations in clinopyroxenephenocrysts provide direct evidence for a pre-eruption crystalmeltmush chamber containing low-T magma, and elucidate the processesof magma mixing and eruption, following the injection of high-Tmagma. Many phenocrysts with Fe-rich cores derived from low-Tmagma have extremely reverse zoned mantles around slightly resorbedcores. Mg-value [100Mg/(Mg + Fe)] increases from 6570to 相似文献
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Hydration reactions are direct evidence of fluid–rock interaction during regional metamorphism. In this study, hydration reactions to produce retrograde actinolite in mafic schists are investigated to evaluate the controlling factors on the reaction progress. Mafic schists in the Sanbagawa belt contain amphibole coexisting with epidote, chlorite, plagioclase and quartz. Amphibole typically shows two types of compositional zoning from core to rim: barroisite → hornblende → actinolite in the high‐grade zone, and winchite → actinolite in the low‐grade zone. Both types indicate that amphibole grew during the exhumation stage of the metamorphic belt. Microstructures of amphibole zoning and mass‐balance relations suggest that: (1) the actinolite‐forming reactions proceeded at the expense of the preexisting amphibole; and (2) the breakdown reaction of hornblende consumed more H2O fluid than that of winchite, when one mole of preexisting amphibole was reacted. Reaction progress is indicated by the volume fraction of actinolite to total amphibole, Yact, with the following details: (1) reaction proceeded homogeneously in each mafic layer; (2) the extent of the hornblende breakdown reaction is commonly low (Yact < 0.5), but it increases drastically in the high‐grade part of the garnet zone (Yact > 0.7); and (3) the extent of the winchite breakdown reaction is commonly high (Yact > 0.7). Many microcracks are observed within hornblende, and the extent of hornblende breakdown reaction is correlated with the size reduction of the hornblende core. Brittle fracturing of hornblende may have enhanced retrograde reaction progress by increasing of influx of H2O and the surface area of hornblende. In contrast to high‐grade rocks, the winchite breakdown reaction is well advanced in the low‐grade rocks, where reaction progress is not associated with brittle fracturing of winchite. The high extent of the reaction in the low‐grade rocks may be due to small size of winchite before the reaction. 相似文献
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Crustal thickening of the lower crust of the Kohistan arc (N. Pakistan) deduced from Al zoning in clinopyroxene and plagioclase 总被引:8,自引:0,他引:8
The lower-crustal rocks of the Kohistan complex (northern Pakistan) are mostly composed of metabasic rocks such as pyroxene granulites, garnet granulites and amphibolites. We have investigated P–T trajectories of the relic two-pyroxene granulites, which are the protolith of the amphibolites within the Kamila amphibolite belt. Aluminous pyroxene retains igneous textures such as exsolution lamellae developed in the core. The significant amount of Al in clinopyroxene is buffered by breakdown reactions of plagioclase accompanied by film-like quartz as a product at grain boundaries between plagioclase and clinopyroxene. Distinct Al zoning profiles are preserved in pyroxene with exsolution lamellae in the core and in plagioclase adjacent to clinopyroxene in pyroxene granulites. In the northern part of the Kamila amphibolite belt, Al in clinopyroxene increases towards the rim and abruptly decreases at the outer rim, and anorthite in plagioclase decreases towards the rim and abruptly increases near the grain boundary between plagioclase and clinopyroxene. In the southern part of the Kamila amphibolite belt, Al in clinopyroxene and anorthite in plagioclase simply increase towards the margins of the grains. The anorthite zoning in plagioclase is in agreement with the zoning profiles of Ca-Tschermaks and jadeite components inferred from variations of Al, Na, Ti and Fe3+ in clinopyroxene. Assuming that the growth surface between them was in equilibrium, geothermobarometry based on Al zoning in clinopyroxene coexisting with plagioclase indicates that metamorphic pressures significantly increased with increasing temperature under granulite facies metamorphism. The peak of granulite facies metamorphism occurred at conditions of about 800 °C and 800–1100 MPa. These prograde P–T paths represent a crustal thickening process of the Kohistan arc during the Early to Middle Cretaceous. The crustal thickening of the Kohistan arc was caused by accretion of basaltic magma at mid-crustal depths. 相似文献
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The two-dimensional distribution patterns of garnet grains incontact metamorphic rocks and the Sambagawa high P/T type metamorphicrocks were studied by means of Fry's map in order to investigatethe development of segregation during synmetamorphic shear flow.The distribution of garnet in the undeformed hornfelses is nearlyrandom, indicating that the nucleation sites are approximatelyrandom. The distribution patterns of garnet in the highly deformedSambagawa pelitic schists, however, can be divided into twotypes: one is strongly random, and another is strongly clustered.The random distribution occurs in the relatively lower grademetamorphic rocks, whereas the clustered one is found in thehigher grade zones. This segregation of garnet comes from thekinematic collision of garnet grains in shear flow and the apparentattractive force operating between garnet grains due to an overgrowthmechanism. 相似文献
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Chemical zoning of garnet is often used to deduce PTpaths of rocks by inverse calculation. To validate the derivedPT paths, it is desired to establish a method to predictthe chemical compositions of garnet theoretically. This studyproposes a new forward calculation of the formation of MgFeMngarnet from chlorite, which solves the non-linear simultaneousequations using nested iterative calculations. Growth of garnetconsuming chlorite and quartz was modelled in a MnOFeOMgOAl2O3SiO2H2Osystem, using the most recent thermodynamic data for the minerals.The prograde PT history of the Sambagawa metamorphicbelt, SW Japan, was modelled. To reproduce growth zoning, crystallizedgarnet was removed step by step from the system; perfect diffusionwas assumed for chlorite. The proposed model derived the evolutionof molar amounts and chemical compositions of MgFeMnchlorite and garnet. It successfully reproduced the shape ofthe observed chemical profile of garnet, although the temperaturecondition was higher than general observations. The Mn contentof the garnet core was generally high, and Mg/Fe ratio alwaysstarted rising rapidly after Mn was depleted. Thermodynamicproperties of minerals, initial chlorite composition, PTpath, H2O partial pressure, and Ca content in garnet were variedto test the behaviour of the system. The properties of Mn phasesinfluenced only the chemical composition of the garnet core.The temperature range in which garnet grew depended on the H2Opartial pressure or the Ca content in garnet. KEY WORDS: chemical equilibrium; chemical zoning; garnet; forward modelling; Sambagawa metamorphic belt 相似文献
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Tyatya Volcano, southwestern Kuril arc: Recent eruptive activity inferred from widespread tephra 总被引:2,自引:0,他引:2
MITSUHIRO NAKAGAWA YOSHIHIRO ISHIZUKA † TAKASHI KUDO MITSUHIRO YOSHIMOTO ‡ WATARU HIROSE YOSHIO ISHIZAKI NOBUO GOUCHI YOSHIO KATSUI ALEXANDER W. SOLOVYOW GENRIKH S. STEINBERG ARSLAN I. ABDURAKHMANOV 《Island Arc》2002,11(4):236-254
Abstract Tyatya Volcano, situated in Kunashir Island at the southwestern end of Kuril Islands, is a large composite stratovolcano and one of the most active volcanoes in the Kuril arc. The volcanic edifice can be divided into the old and the young ones, which are composed of rocks of distinct magma types, low‐ and medium‐K series, respectively. The young volcano has a summit caldera with a central cone. Recent eruptions have occurred at the central cone and at the flank vents of the young volcano. We found several distal ash layers at the volcano and identified their ages and sources, that is, tephras of ad 1856, ad 1739, ad 1694 and ca 1 Ka derived from three volcanoes of Hokkaido, Japan, and caad 969 from Baitoushan Volcano of China/North Korea. These could provide good time markers to reveal the eruptive history of the central cone, which had continued intermittently with Strombolian eruptions and lava flow effusions since before 1 Ka. Relatively explosive eruptions have occurred three times at the cone during the past 1000 years. We revealed that, topographically, the youngest lava flows from the cone are covered not by the tephra of ad 1739 but by that of ad 1856. This evidence, together with a report of dense smoke rising from the summit in ad 1812, suggests that the latest major eruption with lava effusion from the central cone occurred in this year. In 1973, after a long period of dormancy, short‐lived phreatomagmatic eruptions began to occur from fissure vents at the northern flank of the young volcano. This was followed by large eruptions of Strombolian to sub‐Plinian types occurring from several craters at the southern flank. The 1973 activity is evaluated as Volcanic Explosivity Index = 4 (approximately 0.2 km3), the largest eruption during the 20th century in the southwestern Kuril arc. The rocks of the central cone are strongly porphyritic basalt and basaltic andesite, whereas the 1973 scoria is aphyric basalt, suggesting that magma feeding systems are definitely different between the summit and flank eruptions. 相似文献
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M. TORIUMI 《Journal of Metamorphic Geology》1990,8(4):457-466
The dominant deformation mechanism during the Sambagawa metamorphism changes from brittle to ductile with increasing metamorphic temperature. The magnitude of plastic strains inferred from the shapes of deformed radiolaria in metachert increases sharply across the boundary between the epidote-pumpellyite-actinolite zone and the epidote-actinolite zone. The synmetamorphic crack density of metachert is an indicator of the contemporaneous brittle strain of rocks, and it decreases sharply as the grade reaches the epidote-actinolite zone. Hence, the ratio of the ductile strain to the brittle strain of metachert decreases rapidly across the transition to the epidote-actinolite zone of the Sambagawa metamorphic belt.
The sharp change of the ductile strain magnitude also takes place at the epidote-actinolite grade in the Shimanto metamorphic belt of Japan, an example of the intermediate pressure facies series of metamorphism. It is concluded that the transition from brittle to ductile deformation takes place at about 300-400°C. and is independent of pressure of metamorphism. 相似文献
The sharp change of the ductile strain magnitude also takes place at the epidote-actinolite grade in the Shimanto metamorphic belt of Japan, an example of the intermediate pressure facies series of metamorphism. It is concluded that the transition from brittle to ductile deformation takes place at about 300-400°C. and is independent of pressure of metamorphism. 相似文献
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