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1.
Since the ultra-high pressure metamorphic rocks with coesite and diamond indicators,which are formed under the condition of deep-subduction of continental crust,have been recognized[1―4],exsolution structures of the ultra-high pressure minerals such as pyroxene,olivine,garnet and sphene in the various kinds of rocks have attracted the great interests and attention of many researchers[5―18].Some exsolution structures,such as clinopyroxene+rutile+apatite[13]or quartz+rutile+apatite[15]exsolutio…  相似文献   

2.
A single garnet clinopyroxenite xenolith found at the Dish Hill basanite cone near Ludlow, California, has well developed unmixing and reaction textures like those found in garnet pyroxenite inclusions in Hawaiian, African and Australian basalts and like those of pyroxenites in some European alpine peridotites. Reconstructed pyroxene compositions suggest that before unmixing the rock consisted of clinopyroxene and about 10% garnet plus spinel, but all of the garnet may have been dissolved in clinopyroxene. Most or all of the garnet formed by exsolution from clinopyroxene and by reaction between clinopyroxene and spinel in an open system. Following exsolution, the rock was deformed and partly recrystallized in the solid state. Similarity of compositions of exsolved and recrystallized minerals suggests recrystallization at P-T conditions similar to those of exsolution.The rock is not the chemical equivalent of the host basanite and cannot represent magma of basanitic composition crystallized in the mantle. Its history of deformation and recrystallization, like that of accompanying spinel lherzolite inclusions, supports the idea that the garnet clinopyroxenite is an accidental inclusion derived from the upper mantle.  相似文献   

3.
Abundances of transition elements (Ti, V, Cr, Mn, Fe2+, Co, Ni, Cu and Zn) in coexisting clinopyroxene and garnet are used to estimate clinopyroxene/garnet partition coefficients for these elements. The analyzed samples include eclogites, granulites and peridotites. The partition coefficients are sensitive to the major element composition of the mineral phases, although for individual transition elements they correlate with different chemical parameters. The partition coefficients of Ti correlate with the (FeO/MgO)garnet/(FeO/MgO)clinopyroxene ratio thus suggesting that the partitioning of Ti is a sensitive indicator of the physical (temperature-pressure) conditions of equilibration.  相似文献   

4.
Abstract Garnet clinopyroxenite containing porphyroclastic clinopyroxene with garnet lamellae from the Su-Lu ultrahigh-pressure (UHP) province, eastern China, records a three-stage evolutionary history. Stage A: precursor aluminous clinopyroxene associated with Mg-rich spinel was stable. Stage B: Mg-rich spinel and aluminous clinopyroxene recrystallized to form porphyroblasts of garnet and diopside with garnet lamellae, respectively. Thereafter, these porphyroblasts were granulated and recrystallized to form equigranular neoblasts in the matrix, being driven by subsolidus deformation. Stage C: the assemblage developed retrograde hornblende, epidote, spinel, chlorite, calcite, and dolomite. The estimated composition of the precursor aluminous clinopyroxene indicates that equilibrium conditions of the stage A were 900-1000°C, and 1.0-1.5 GPa. The neoblast garnet-clinopyroxene pairs, not conjectural, give 740 ± 50°C and higher than 2.5 GPa for the climax conditions of stage B. This implies that the Hujialin garnet clinopyroxenite was transported from a low dP/dT setting to a high dP/dT setting, probably related to subduction during stage B. The Hujialin garnet clinopyroxenite as well as adjacent UHP eclogite which records 700°C and 3.0 GPa as the maximum physical conditions, experienced amphibolite facies retrogression, suggesting that they shared a common P-T history during exhumation.  相似文献   

5.
Late Cenozoic alkali basalts in the Ganseong area of South Korea contain abundant ultramafic xenoliths and clinopyroxene megacrysts. Anhydrous clinopyroxene‐rich wehrlite–clinopyroxenites make up the majority of the xenolith population and range from wehrlite through olivine clinopyroxenite to clinopyroxenite. This study investigates the petrogenesis of wehrlite–clinopyroxenite xenoliths and clinopyroxene megacrysts on the basis of petrography and mineral and whole‐rock chemistry. Observations such as an absence of carbonate or apatite, high Ti/Eu ratio, and clinopyroxene‐dominated mineralogy lead us to rule out peridotite–melt reactions as the origin of the Ganseong wehrlites– olivine clinopyroxenites. The whole‐rock compositions (e.g. high abundance of CaO at a given MgO content and low abundance of incompatible elements, such as U, K, P, and Ti compared with mafic melts) indicate that the pyroxenites do not represent crystallized magma itself, but are rather cumulates with a small amount of residual liquid. Anhydrous and orthopyroxene‐free mineral assemblages, crystallization sequence of olivine→clinopyroxene→plagioclase, and mineral chemistries (e.g. low Cr# and high TiO2 abundances in spinels and high TiO2 and Na2O abundances in clinopyroxenes at a given Mg#) suggest that relatively anhydrous intraplate alkaline basalt is the most likely candidate for the parent magma. Texture and compositions of the clinopyroxene megacrysts preclude a cognate origin via high‐pressure crystallization of the host magma. The clinopyroxene megacrysts occupy the Fe‐rich end of the compositional trends defined by wehrlite–pyroxenite clinopyroxenes. Progressive decreases in Mg# and an absence of significant compositional gaps between pyroxenite xenoliths and clinopyroxene megacrysts indicate fractionation and differentiation of a similar parental magma. We suggest that the clinopyroxene megacrysts represent fragments of pegmatitic clinopyroxenites crystallized from more advanced fractionation stages of the evolution of a series of magmatic liquids formed Ganseong wehrlite–clinopyroxenites.  相似文献   

6.
Phase equilibria in a natural garnet lherzolite nodule (PHN 1611) from Lesotho kimberlite and its chemical analogue have been studied in the pressure range 45–205 kbar and in the temperature range 1050–1200°C. Partition of elements, particularly Mg2+Fe2+, among coexisting minerals at varying pressures has also been examined. High-pressure transformations of olivine(α) to spinel(γ) through modified spinel(β) were confirmed in the garnet lherzolite. The transformation behavior is quite consistent with the information previously accumulated for the simple system Mg2SiO4Fe2SiO4. At pressures of 50–150 kbar, a continuous increase in the solid solubility of the pyroxene component in garnet was demonstrated in the lherzolite system by means of microprobe analyses. At 45–75 kbar and 1200°C, the Fe2+/(Mg + Fe2+) value becomes greater in the ascending order orthopyroxene, Ca-rich clinopyroxene, olivine and garnet. At 144–146 kbar and 1200°C, garnet exhibits the highest Fe2+/(Mg + Fe2+) value; modified spinel(β) and Ca-poor clinopyroxene follow it. When the modified spinel(β)-spinel(γ) transformation occurred, a higher concentration of Fe2+ was found in spinel(γ) rather than in garnet. As a result of the change in the Mg2+Fe2+ partition relation among coexisting minerals, an increase of about 1% in the Fe2SiO4 component in (Mg,Fe)2SiO4 modified spinel and spinel was observed compared with olivine.These experimental results strongly suggest that the olivine(α)-modified spinel(β) transformation is responsible for the seismic discontinuity at depths of 380–410 km in the mantle. They also support the idea that the minor seismic discontinuity around 520 km is due to the superposition effect of two types of phase transformation, i.e. the modified spinel(β)-spinel(γ) transformation and the pyroxene-garnet transformation. Mineral assemblages in the upper mantle and the upper half of the transition zone are given as a function of depth for the following regions: 100–150, 150–380, 380–410, 410–500, 500–600 and 600–650 km.  相似文献   

7.
Aluminum incorporation into the high pressure polymorph of TiO2 with the structure of α-PbO2 has been studied from 10 to 20 GPa and 1300 °C by XRD, high-resolution 27Al MAS-NMR and TEM. Al-doped α-PbO2 type TiO2 can be recovered at atmospheric pressure. Al2O3 solubility in α-PbO2 type TiO2 increases with increasing the synthesis pressure. The α-PbO2 type TiO2 polymorph is able to incorporate up to 35 wt.% Al2O3 at 13.6 GPa and 1300 °C, being the substitution of Ti4+ by Al3+ on normal octahedral sites the mechanism of solubility. The transition to the higher pressure TiO2 polymorph with the ZrO2 baddeleyite structure, Akaogiite, has not been observed in the quenched samples at room pressure. The microstructure of the recovered sample synthesized at 16 GPa and 1300 °C points to the existence of a non-quenchable aluminum titanium oxide phase at these conditions.  相似文献   

8.
With the discovery and further studies of high- pressure (HP) to ultra-high-pressure (UHP) eclogites and UHP garnet lherzolite in the Altun Mountains[19], it becomes interesting if the country rocks of these HP-UHP metamorphic rocks also underwent HP-UHP metamorphism, which will be crucial for understand-ing the relationship of HP-UHP metamorphic rocks and their country rocks, the mechanism of their forma-tion and uplifting and the existence of continental deep subduction in the are…  相似文献   

9.
Phase equilibrium experiments were performed on typical ‘oceanic’ and ‘cratonic’ peridotite compositions and a Ca, Al-rich orthopyroxene composition, to test the proposal that garnet lherzolites exsolved from high-temperature harzburgites, and to further our understanding of the origin of ancient cratonic lithospheres. ‘Oceanic’ peridotites crystallize a garnet harzburgite assemblage at pressures above 5 GPa in the temperature range 1450–1600°C, but at 5 GPa and temperatures less than 1450°C, crystallize clinopyroxene to become true lherzolites. ‘Cratonic’ peridotites crystallize a garnet harzburgite assemblage at pressures above 5 GPa in the temperature range 1300–1600°C. Garnet-free harzburgite crystallizes from both ‘cratonic’ and ‘oceanic’ peridotite at temperatures above 1450°C and pressures below 4.5–5 GPa. Phase relations for the high Ca, Al-rich orthopyroxene composition essentially mirror those for ‘oceanic’ peridotite.The complete solution of garnet and clinopyroxene into orthopyroxene observed in all three starting compositions at temperatures near or above the mantle solidus at pressures less than 6 GPa supports the hypothesis that garnet lherzolite could have exsolved from harzburgite. The inferred cooling path for the original high-temperature harzburgite protoliths of garnet lherzolites differs depending on bulk composition. The precursor harzburgite protoliths of garnet lherzolites and harzburgites with ‘cratonic’ bulk compositions apparently experienced simple isobaric cooling from formation temperatures near the peridotite solidus to those at which most of these peridotites were sampled in the mantle (< 1200°C). The cooling histories for harzburgite protoliths of sheared garnet lherzolites with ‘oceanic’ compositional affinity are speculated to have involved convective circulation of mantle material to depths deeper than those at which it was originally formed.Phase equilibria and compositional relationships for orthopyroxenes produced in phase equilibrium experiments on peridotite and komatiite are consistent with an origin for ‘cratonic’ peridotite as a residue of Archean komatiite extraction, which has since cooled and exsolved clinopyroxene and garnet to become the common low-temperature, coarse-grained peridotite thought to comprise the bulk of the mantle lithosphere beneath the Archean Kaapvaal craton.  相似文献   

10.
Megacrysts in the Cenozoic basalt of the Tuoyun Basin,Southwest Tianshan   总被引：2，自引：1，他引：2  
Abundant megacrysts of clinopyroxene, amphibole, anorthoclase, and phlogopite are found together with deep-seated xenoliths in the Cenozoic basalt of the Tuoyun Basin, Southwest Tianshan. The megacrysts are mainly in the cone sheet formed at the early stage of the volcanic activity. Clinopyrox-ene megacrysts are located in the lower part of the profile, with amphibole and phlogopite megacrysts in the middle part and anorthoclase megacrysts in the upper part. The crystal integrity, absence of de-formation fabric and their relation to the host basalt suggest that they were crystallized from the host magma and quickly transported to the surface. The mineralogical studies imply that the clinopyroxene megacrysts are of Al-augite with higher Al2O3 (>9%). Amphibole megacrysts are kaersutite rich in TiO2 (>4.5%). Sulfide inclusions such as pyrrhotite occur in some clinopyroxene and amphibole megacrysts. Thermodynamic calculations reveal that pyroxene megacrysts formed under the temperature of 1185.85―1199.85℃ and the pressure between 1.53 and 1.64 GPa comparable to the crust-mantle boundary and amphibole megacrysts crystallized under the pressure of around 0.85 GPa, temperature about 1000℃ comparable to the depth of 30 km. Anorthoclase megacrysts crystallized under the pressure between 0.8―1 GPa,temperature about 900℃.The absence of Ti-rich inclusions such as rutile can be considered as an evidence of quick magma ascending. The P-T conditions estimated via py-roxene megacrysts and phenocrysts compose a P-T path with a steep slope. It can be considered as another evidence of quick magma ascending. However, the estimated temperatures for amphibole megacrysts are markedly lower than those for pyroxene megacrysts given the same pressure. It probably shows that the amphiboles have crystallized at the vanguard of magma and under the vola-tile-rich condition. Thus, we can conclude that the Cenozoic basalts are produced in an extensional tectonic setting and the processes governing crystallization and ascending of the megacrysts are very complex.  相似文献   

11.
The stability field of knorringite (Mg3Cr2Si3O12) is studied experimentally. Knorringite is stable at pressures above 10.5 GPa at 1200°C and 11.8 GPa at 1400°C. Below these pressures, knorringite decomposes to enstatite + eskolaite. A phase diagram of the pyrope-knorringite system is described based on the available experimental data. The solubility of the knorringite molecule in pyrope is essentially dependent only on pressure, and the Cr/Cr+Al value of garnet is considered to be an indicator of the minimum pressure of equilibration. Consideration of the genesis of Cr-rich pyrope and other peridotitic inclusions in diamonds indicates that the fractionation process should have taken place, at least at depths to ca. 240 km, to give rise to the Cr-rich complement of Cr-poor upper mantle materials such as undepleted lherzolite. The knorringite-rich peridotitic suite in diamond will be identified with this complement, which may be the material constituting the deep upper mantle.  相似文献   

12.
Wei  Lin  Masaki  Enami 《Island Arc》2006,15(4):483-502
Abstract Jadeite‐bearing eclogites and associated blueschists locally crop out in a greenschist facies area at Kuldkourla, near the Akeyazhi River in the western Chinese Tianshan region, northwestern China. Garnet in these metamorphic rocks shows prograde zoning with increasing Mg and decreasing Mn from the crystal center towards the rim, and is divided into Ca‐poor/Fe‐rich core and Ca‐rich/Fe‐poor mantle parts. The garnet cores include the assemblages of (i) jadeite/omphacite (Xjd = 0.34–0.96) + barroisite/taramite; and (ii) omphacite + barroisite/pargasite, with paragonite, epidote, rutile and quartz as major phases with rare albite. The garnet mantles rarely contain inclusions of omphacite, glaucophane, epidote, rutile and quartz. Major matrix phases of the pre‐exhumation stage are omphacite, glaucophane, paragonite, rutile and quartz. These mineral parageneses give pressure (P)‐temperature (T) conditions of 0.9 GPa/390°C?1.4 GPa/560°C for the stage of the garnet core formation, 1.8 GPa/520°C for the stage of the garnet mantle formation, and 2.2 GPa/495°C‐2.4 GPa/535°C for the peak eclogite facies assemblage in the matrix. The estimated P‐T conditions and continuous changes of mineral parageneses imply a counterclockwise P‐T path which is a combination of (i) an early prograde stage of high‐pressure/low‐temperature (HP/LT) blueschist facies and/or LP/LT eclogite facies; (ii) a later prograde stage involving compression with minimal heating; and (iii) a climax‐of‐subduction stage characterized by a slight decrease of temperature with increasing pressure. The negative dP/dT of the latest subduction stage is possibly a record of the following events after a continuous subduction and ridge approach: (i) material migration within the upper part of the subducting slab, which has an inverse thermal gradient caused by ductile flow and/or slab break during subduction; and/or (ii) temporary cooling of the wedge mantle–slab interface by continuous subduction of a relatively cold slab following subduction of a hotter ridge.  相似文献   

13.
The TiO2 contents of rutile-saturated melts ranging from basalt to rhyodacite have been investigated at P = 8–30 kbar and T = 1000–1300°C under hydrous, CO2-saturated, and volatile-absent conditions. Dissolved TiO2 is positively correlated with T and not strongly dependent on Ptotal. For fixed P and T, TiO2 content decreases markedly as the melts become more felsic. The distribution of TiO2 between rutile and liquid, expressed as a wt.% concentration ratio, D (rut/liq), is given by: In D = −3.16 + (9373T) + 0.026P − 0.152FM where T is in Kelvins, P in kbar and FM is a melt composition parameter, FM = [Na+K+ 2(Ca+Fe+Mg)]/Al· 1/Si in which the chemical symbols represent cation fractions. The first term expresses the competition of aluminate and titanate anions for charge-compensating cations, and the second term expresses the inverse dependence of dissolved TiO2 on SiO2 content. There is no apparent dependence of rutile solubility on water content.For ranges of probable solidus conditions, rutile saturation in basaltic, andesitic, and dacitic liquids requires 7–9, 5–7, and 1–3 wt.% TiO2, respectively. These concentrations are well in excess of those found in the respective rock types, so depletion in Nb, Ta, and Ti and reduced Nb/U and Nb/Th ratios in volcanic rocks erupted at convergent plate margins cannot be attributed to residual rutile in their source regions. Thus, Nb, Ta and Ti depletion must be an inherent property of the source region.We suggest that the island-arc source region has been depleted in Nb and Ta by a previous episode of melt extraction (MORB), zoning refining, or equilibration with a percolating melt or fluid. Such a process markedly depletes the LILE and HFSE element concentrations in the residuum, but ratios such as Nb/U, Nb/Th and U/Th remain relatively constant due to similar solid-melt partition coefficients. The depletion of Nb relative to Th in the source regions of island-arc magmas occurs during hybridization of the source by rutile-saturated (Nb/Ta-depleted) melts or aqueous fluids. If the hybridizing agent is a melt, a relatively felsic composition, produced under low T (900°) hydrous conditions, is required.  相似文献   

14.
The mineralogy adopted by a depleted harzburgite composition has been studied over the pressure interval 5–26 GPa at temperatures of 1300–1400°C. The pyroxene-garnet component of the harzburgite composition (harzburgite minus 82 wt.% olivine) transforms to majorite garnet by 18–19 GPa, and further disproportionates to the assemblage of garnet + stishovite + Mg2SiO4 spinel above 20 GPa. At still higher pressures, first ilmenite (22–24 GPa) and then perovskite MgSiO3 (24–26 GPa) are found to coexist with garnet. Garnet disappears at 26 GPa and almost complete transition to perovskite is achieved at this pressure. The mineral proportions and density profiles in the subducting oceanic lithosphere, modelled by a combination of 80% harzburgite + 20% primitive MORB compositions are calculated as a function of depth under conditions isothermal with surrounding pyrolite mantle, and also for a temperature distribution in which the slab is substantially cooler than surrounding mantle to below 700 km. Under isothermal conditions, the slab has a density similar to surrounding mantle to a depth of 600 km. However, between 600 and 700 km, the slab is up to 0.08 g/cm3 denser than surrounding mantle. This is caused primarily by the higher alumina content in pyrolite as compared to harzburgite, which causes the transition to perovskite in pyrolite to occur at substantially higher pressures than in harzburgite. The presence of alumina also smears out the garnet-perovskite transition in pyrolite over a depth interval of 50 km, whereas this transformation is much sharper in the harzburgite composition. Calculations based on the observed phase equilibria also show that a subducted cool slab remains much denser (by 0.1–0.3 g/cm3) than surrounding mantle to a depth of 700 km but possesses a density similar to surrounding mantle below this depth. These results have important implications for the dynamical behaviour of slabs possessing different thermal regimes when they encounter the 670 km discontinuity and also for the nature of this discontinuity.  相似文献   

15.
The extensive gneisses in the high‐pressure and ultrahigh‐pressure metamorphic terrane in the Dabie‐Sulu orogen usually show no evidence of eclogite‐facies metamorphism. The garnet‐mica‐plagioclase gneisses from the Qiliping region in the western Dabie Orogen, comprise garnet, phengite, biotite, plagioclase, quartz, rutile, ilmenite, chlorite, epidote, and hornblende. The garnet porphyroblasts, with inclusions of quartz, epidote, and rutile, exhibit slight compositional zonations, from core to mantle with an increase in pyrope and a decrease in spessartine, and from mantle to rim with a decrease in pyrope and grossular and an increase in spessartine. The high‐Si phengite indicates that the gneisses may be subjected to a high‐pressure metamorphism. By the P–T pseudosections calculated in a system NCKMnFMASHTO (Na2O‐CaO‐K2O‐MnO‐FeO‐MgO‐Al2O3‐SiO2‐H2O‐TiO2‐O) for two representative samples, the metamorphic P–T path, reconstructed by the compositionally zoned garnet, shows that the prograde metamorphism is characterized by a temperature increase with a slight pressure increase from the conditions of 17.6 ± 1.5 kbar at 496 ± 15°C to the peak‐pressure ones of 21.8 ± 1.5–22.7 ± 1.5 kbar at 555 ± 15–561 ± 15°C; the early retrograde stage is dominated by decompression with a temperature increase to the maximum of 608 ± 15–611 ± 18°C at 10.3 ± 1.5–11.0 ± 1.5 kbar; and the late retrograde one is predominated by pressure and temperature decreases. The mineral assemblages in the prograde metamorphism are predicted to contain garnet, glaucophane, jadeite, lawsonite, phengite, quartz, rutile, and/or chlorite, which is different from those observed at present. Such high‐pressure metamorphism can partly be reconstructed by the P–T pseudosection in combination with the high‐Si phengite and garnet compositions in the core and mantle. This provides an important constraint on the subduction and exhumation of the terrane during the continent–continent collision between the Yangtze and Sino‐Korean cratons.  相似文献   

16.
Lower crustal high grade metamorphic rocks have been successively found at Pamirs nearby the western Himalayan syntaxis, Namjagbarwa and Dinggye nearby the eastern Himalayan syntaxis and the central segment of the Himalayan Orogenic Belt, respec-tively[1―4]. In particular, some researchers deduced that there were probably eclogites at some locations[5]. Moreover, some geochronological data of these lower crustal granulites also have been accumulated. For example, the high-pressure granulit…  相似文献   

17.
M. Ebanu  A. Nagasaki 《Island Arc》1999,8(4):459-474
Kyanite-bearing ultrahigh-pressure (UHP) eclogites occur as blocks in orthogneisses at Yangzhuang, in the Junan area of the southwestern Sulu province, eastern China. Eclogites have variable bulk rock compositions, with Al2O3 = 16–27 wt%, FeO* + MgO = 6–22 wt% and CaO = 9–13 wt%. Major minerals are garnet, omphacite, phengitic white mica, zoisite, kyanite, rutile and an SiO2 phase. Fe-rich staurolite (Mg ? Mg# = 0.24 ± 0.01) and paragonite–margarite aggregates are rarely included in the cores of prograde zoned garnet. Metamorphic conditions ranged from 520 to 650°C and <1.4 GPa at an early prograde stage, and mostly reached 660–830°C and 2.7–3.5 GPa at the peak UHP stage. The estimated dP/dT of the prograde P–T path is less than 0.25 GPa/100°C at earlier stages and increases to 0.7–1.4 GPa/100°C just before the UHP stage. The kink of the prograde P–T path closely resembles the steady-state P–T paths proposed, assuming a two-parameter brittle-plastic shear stress model. The estimated P–T path adequately explains the absence of prograde lawsonite and sodic amphibole and the common occurrence of coexisting zoisite, kyanite and sodic-calcic amphibole in the UHP eclogites throughout the Sulu province. Simple clockwise prograde P–T paths for Sulu UHP eclogites proposed in earlier studies should be carefully re-examined.  相似文献   

18.
Modern adakite, Archean tonalite-trondhjemite- granodiorite (TTG) and adakitic rocks derived from lower continental crust are high Na and Al felsic rocks and are characterized by strong heavy REE and Y de- pletion and high Sr/Y and La/Yb ratios, which sug…  相似文献   

19.
Garnet clinopyroxenites occur within foliated dunite in the Higashi-akaishi peridotite mass, located within the subduction-type high-pressure/low-temperature Sanbagawa metamorphic belt. The garnet clinopyroxenites contain 3–80% garnet, and garnet and clinopyroxene are homogeneously distributed. Garnet crystals contain extensive, regular dislocation arrays and dislocation networks, suggesting that dislocation creep was the dominant deformation mechanism. Analyses of crystallographic orientation maps indicate similar grain sizes and aspect ratios for garnet and clinopyroxene, regardless of modal composition, indicating that these minerals deformed with similar degree of plasticity. Moreover, indexes of crystallographic fabric intensity (i.e., J-index and M-index) for both garnet and clinopyroxene tend to increase with increasing modal composition of garnet. Fourier-transform infrared spectroscopy analysis revealed that water content in garnet is ～60 ppm, whereas that in clinopyroxene is ～70 ppm. Olivine crystal-preferred orientations in the Higashi-akaishi peridotite mass, characterized by [0 0 1] (0 1 0), are thought to have developed during deformation under wet conditions. Consequently, we argue that the presence of water could act to enhance garnet plasticity during deformation. The results reveal contrasting influences of water on the deformation of garnet and diopside: under wet conditions compared with dry, the strain rate increases by two orders of magnitude for garnet but by an order of magnitude for diopside. Given the influence of water on the creep strength of garnet, garnet within the Higashi-akaishi mass may have become significantly as weak as clinopyroxene during deformation.  相似文献   

20.
The extinct Pleistocene volcano Muriah, situated behind the main Pleistocene—Recent Sunda magmatic arc in north-central Java, has erupted at least two contrasted groups of lavas. One group forms a well-defined compositional series (Anhydrous Series) from leucite basanite to tephritic phonolite, with olivine and tschermakitic clinopyroxene the main phenocrysts. The other group, the “Hydrous Series”, includes compositionally variable tephrites and high-K andesites with common plagioclase, biotite and amphibole. Lavas of the Anhydrous Series are much richer in LIL trace elements than the most potassic lavas of neighbouring active volcanoes, but relative HFS element enrichment is less pronounced. REE patterns have almost constant slopes from La (250–600 times chondrites) to Yb (5–10 times chondrites), while those of lavas of active centres are less light-enriched, and show flattening in the heavy REE. Anhydrous Series initial 87Sr/86Sr ratios (0.7043–0.7046) are lower than those of active centres (0.7047–0.7053). Hydrous Series lavas are intermediate in all these geochemical characteristics.The most mafic A-series leucite basanite, with Mg/(Mg + Fe2+) 0.69, 140 ppm Ni and 620 ppm Cr was probably derived from the primary magma for the series by fractionation of only 5 wt.% olivine. Its REE pattern suggests derivation from a garnet-bearing source. Experiments on this basanite, with up to 10% olivine and 20% orthopyroxene added, and in the presence of H2O and H2O/CO2 mixtures, have shown that for all but very high magma water contents, the olivine and garnet liquidus fields are widely separated by fields of phlogopite and clinopyroxene. There is no liquidus field of orthopyroxene. Hence, if magma production involved an equilibrium melting process alone, the most probable sources are of garnet-bearing phlogopite clinopyroxenite type. Alternatively, this magma may represent the end-product of interaction between a low-K basanite magma from a garnet lherzolite source in the asthenosphere and a phlogopite-bearing lherzolite zone in the lower lithosphere. Its production was probably related to crustal doming and extension superimposed on the dominant subduction regime. Hydrous Series magmas may have resulted from mixing between Anhydrous Series magmas and high-K calc-alkaline basaltic to andesitic magmas more directly related to subduction processes.  相似文献   

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