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1.
The Angra dos Reis meteorite fell in 1869 and is a unique achondrite. It is an ultramafic igneous rock, pyroxenite, with 93% fassaite pyroxene which has 15.7% Ca-Tschermak's molecule, plus calcic olivine (Fo53.1; 1.3% CaO), green hercynitic spinel, whitlockite (merrillite), metallic Ni-Fe, troilite, as well as magnesian kirschsteinite (Ks62.3Mo37.7), within olivine grains, and celsian (Cs90.2An7.7Ab1.7Or0.4) which are phases reported in a meteorite for the first time, and plagioclase (An86.0), baddeleyite, titanian magnetite (TiO2, 21.9%), and terrestrial hydrous iron oxide which are phases reported for the first time in this meteorite. Petrofabric analysis shows that fassaite has a preferred orientation and lineation which is interpreted as being due to cumulus processes, possibly the effect of post-depositional magmatic current flow or laminar flow of a crystalline mush. The mineral chemistry indicates crystallization from a highly silica-undersaturated melt at low pressure. Since the meteorite formed as a cumulate, pyroxene crystals may have gravitationally settled from a melt which crystallized melilite first. Plagioclase would be unstable in such a highly undersaturated melt, and feldspathoids would be rare or absent due to the very low alkali contents of the melt. The presence of rare grains of plagioclase and celsian may be the result of late-stage crystallization of residual liquids in local segregations. Thus, the Eu anomaly in Angra dos Reis may be the result of pyroxene separation from a melt which crystallized melilite earlier, rather than plagioclase as previously suggested.  相似文献   

2.
The Moshirabad pluton is located southwest of the Sanandaj–Sirjan Metamorphic Belt, Qorveh, western Iran. The pluton is composed of diorite, monzodiorite, quartz diorite, quartz monzodiorite, tonalite, granodiorite, granite, aplite, and pegmatite. In this study 31 samples from various rocks were chosen for whole‐rock analyses and 15 samples from different lithologies were chosen for mineral chemical studies. The compositions of minerals are used to describe the nature of magma and estimate the pressure and temperature at which the Moshirabad pluton was emplaced. Feldspar compositions are near the binary systems in which plagioclase compositions range from An5 to An53 and alkali‐feldspar compositions range from Or91 to Or97. Mafic minerals in the plutonic rocks are biotite and hornblende. Based on the composition of biotites and whole‐rock chemistry, the Moshirabad pluton formed from a calc‐alkaline magma. Amphiboles are calcic amphiboles (magnesio‐hornblende or edenite). Temperatures of crystallization, calculated with the hornblende–plagioclase thermometer, range 550–750°C. These temperatures indicate that plutonic rocks have undergone some retrogressive changes in their mineral compositions. Aluminum‐in‐hornblende geobarometry indicates that the Moshirabad pluton was emplaced at pressures of 2.3–6.0 kbar, equal to depths of 7–20 km, but with consideration of regional geology, lower pressures than the above pressure range are more probable. Alteration of amphiboles can be the reason for some overestimation of pressures.  相似文献   

3.
Nine basaltic lava-flows, which vary in thickness between 60 feet and 300 feet, were established in the NW Rajmahals. The flows were, at places, laid down one above the other and, at others, were found to contain intervening intertrappean horizons. All the flows are essentially of basaltic composition and are made up of labradoritie plagioclase, pigeonitic and augitic pyroxene, opaque ore, primary glass and secondary minerals (palagonite, secondary silica, calcite and zeolite). The phenocrystic plagioclase ranges in composition between An72 and An62, while the constituents of the groundmass range between An50 and An17. The microphenocrysts of pyroxene are mainly augitic and occasionally pigeonitic while the constituents of the groundmass are essentially pigeonitic. The opaque minerals are magnetite and ilmenite. Petrographically, the lava-flows are more or less similar to one another. The first three flows are, however, more remarkably porphyritic and a little coarser in grain size than the six overlying flows. The eighth flow is devoid of palagonite. Calcite occurs only in certain portions of the second flow. There is a gradual increase in the percentage of primary glass from the first to the ninth flow with a corresponding fall in the total percentage of plagioclase and pyroxene. Statistical analysis of the grain size variation in the plagioclases was carried out and the results were found to be directly related to the prevailing rates of cooling in the different flows and also in the different horizons of the same flow. Modal analysis of the nine flows (in all, 98 samples) was carried out and this brought out some interesting results. Samples from three of the flows were analysed chemically and the corresponding norms were calculated. The order of crystallisation of the primary constituents was established from petrographic and petrological studies. The basaltic magma, which gave rise to the lava-flows of this region, does not appear to have undergone any significant differentiation during the course of its cooling and consolidation. The only discernible effect of crystallisati on differentiation was an enrichment of silica (and, perhaps, alkalis) in the residual liquids and no noteworthy enrichment of iron appears to have occurred at any stage.  相似文献   

4.
Tanna, one of the southernmost islands of the New Hebrides volcanic arc, is made of Late Pliocene to Recent island arc tholeiitic basalts and andesites, with SiO2 contents ranging from 45 to 57%. These lavas are highly porphyritic (30–50% in volume): phenocrysts of plagioclase are the most abundant, together with olivine and clinopyroxene. The groundmass contain plagioclase, augite, olivine, magnetite and glass; pigeonite, tridymite, sanidine and, rarely, biotite may also occur. The olivines and clinopyroxenes show an iron enrichment from the cores of phenocrysts to their rims and the groundmass crystals, but their compositional variations are not correlated with the Mg/Fe ratio of bulk host rocks, the most Fe-rich compositions being found in Mg-rich lavas. Plagioclase compositions range from An95 to An60 in the basalts and An60 to An50 in the andesites, but, within each group, they are not correlated with SiO2 or Na2O contents of host lavas. Consequently, the bulk major element compositions of Tanna volcanic rocks cannot be considered as primarily controlled by crystal separation from successive liquids. The oxyde-SiO2 variations diagrams, and the modal compositions and mineral chemistry show that crystal accumulation is the predominant mechanism accounting for bulk rock compositions. However, this does not exclude fractional crystallization: the variation of the calculated groundmass mineralogy strongly suggest the occurrence of crystal removal mainly clinopyroxene and magnetite.  相似文献   

5.
Island arc and continental margin (i.e. western Americas) lavas are divided (based on raw data from literature) into basalts (defined by absence of Ca-poor pyroxene, dominated by quartz-normative tholeiites); basaltic andesites and andesites (subdivided on basis of breaks in SiO2 histogram and taken as <56% and 56–63% SiO2; Ca-poor pyroxene present; amphibole and biotite absent); and hornblende (±biotite) lavas, which prove to be mainly relatively silicic andesites. Relative proportions of these types are (576 samples): 23% basalts, 29% basaltic andesites; 30% andesites; 18% hornblende andesites. The compilation emphasizes the dominance of calcic plagioclase (labradorite-anorthite) amongst the phenocryst phases. Pyroxenes are largely augite and hypersthene (En60–75); olivine (Fo65–85) is common through all compositions. There is an overall close similarity in chemistry and mineralogy between continental margin and island arc lavas, although small consistent differences are apparent (e.g. K2O, TiO2, P2O5).Modal data indicate that 70% of lavas are phenocryst-rich (20–60 vol.%), and that phenocryst contents show a bimodal distribution. Statistically and petrologically significant correlations are found between mineralogy and rock chemistry, most notably between total rock Al2O3 and modal phenocrystic plagioclase (found in all data groups, except hornblende andesites). This, and related data and correlations, indicate that the majority of orogenic magmas are modified by crystal fractionation (including crystal accumulation) processes dominated by plagioclase, and interpreted to occur under relatively low pressures. Dominance of plagioclase suggests phenocryst precipitation occurs typically in water-undersaturated magmas.  相似文献   

6.
Ibitira meteorite is interpreted as a strongly metamorphosed, unbrecciated, vesicular eucrite with a primary variolitic and secondary hornfelsic texture dominated by 60% pyroxene (bulk composition En37Fs48Wo15 exsolved into lamellae several micrometers wide of augite En32Fs27Wo41 and pigeonite En40Fs56Wo4) and 30% plagioclase An94 (mosaic extinction and variable structural state). Minor phases are 5% tridymite plates one-quarter occupied by plagioclase (An94) inclusions; several percent intergrowths of ilmenite and Ti-chromite with trace kamacite Fe99Co0.5Ni0.2 and narrow olivine (Fa83) rims; one grain of low-Ti-chromite enclosed in tridymite; trace troilite with kamacite Fe98Co1.0Ni0.9. Euhedral ilmenite, Ti-chromite, plagioclase and merrillite in vesicles indicate vapor deposition. These properties can be explained by a series of processes including at least the following: (1) igneous crystallization under pressure low enough to allow vesiculation, (2) prolonged metamorphism, perhaps associated with vapor deposition and reduction, to produce the coarse exsolution of the pyroxene and the coarse ilmenite-chromite intergrowths, (3) strong shock which affected the plagioclase and tridymite but not the pyroxene, (4) sufficient annealing to allow recrystallization of the plagioclase and tridymite, and partial conversion to the low structural state of the former.  相似文献   

7.
Chemical and petrographic analyses of 51 sequential lava flows from the central vent of Mayon volcano show cyclical variation. In the two most recent cycles, from 1800 to 1876 and from 1881 to the present, one to three basaltic flows are followed by six to ten andesitic flows. Modal and whole-rock chemical parameters show the most regular cyclical variation; calculated groundmass chemical parameters vary less regularly. There is also a long-term trend, over approximately 1700 years of exposed section, toward more basic compositions.The cyclical variation in modes and the chemical composition of the lavas apparently results from periodic influxes of basaltic magma from depth into a shallow magma system. Fractional crystallization of olivine, augite, hypersthene, calcic plagioclase, magnetite and pargasitic hornblende produces successively more andesitic lavas until the next influx of basaltic magma. Differentiation in a deep zone of magma generation is not excluded by the data, but is more likely responsible for the overall change toward more basic compositions than for the cyclical variation.Three points in a cycle — the beginning of basaltic lavas, the beginning of andesitic lavas and a leveling-off of SiO2, K2 O and K2O/Na2O values — correspond roughly to the beginning of frequent effusive eruptions (with or without an early Plinian eruption), frequent weak to moderately explosive (Strombolian) eruptions, and less frequent explosive (Vulcanian) eruptions, respectively. Recognition of the current stage in a cycle can give a qualitative indication of the nature of forthcoming eruptions. Changes in several specific parameters may precede basaltic lavas and allow early detection of basaltic influxes. These include minima in the glass inclusion/plagioclase phenocryst and phenocryst/groundmass ratios, vesicularity and groundmass TiO2, a decrease in hypersthene phenocrysts, and constant values for the whole-rock K2O/Na2O ratio. The Mayon area is densely populated, making prediction of eruption type important for safety and land-use planning.  相似文献   

8.
Many isolated grains of a reddish pleonaste-type spinel occur in fines and metabreccia samples, particularly 14 319. Electron microprobe analyses (104) of spinels and their associated phases include 58 of pleonaste which show Mg/(Mg + Fe) 0.44–0.62 and Cr/(Cr + Al) 0.017–0.134 (atomic), plus minor amounts of other ions, and differ greatly from almost all previously recorded lunar spinels; almost no spinels of intermediate composition were found. Two types of compositional zoning exist: a diffuse primary one with cores lower in Ti, and a narrow secondary one from reaction with matrix yielding rims higher in Cr, Ti, and Mn. At contacts with breccia matrix there is a narrow corona of almost pure plagioclase (An80-An94), free of opaque minerals and pyroxene. Two types of solid inclusions found in the pleonaste are calcic plagioclase, and tiny spherical masses of nickel-rich sulfide.  相似文献   

9.
Igneous material dredged from the Rio Grande rise, South Atlantic Ocean, includes basaltic rocks, some having mafic nodules and megacrysts, and volcanic breccias composed largely of basaltic fragments. These samples represent the only volcanic rocks recovered from this aseismic rise. Bulk compositions show alkalic basalt, trachybasalt, and trachyandesite; the rock types are similar to those of nearby Tristan da Cunha, Gough, and the Walvis ridge. Microprobe analyses show basaltic groundmass to have olivine, Fo85, pyroxene, Fs13Wo46, feldspar, An71, plus interstitial alkali feldspar. Mafic nodules and megacrysts have olivine, Fo86–90 and pyroxene Fs6–7.5Wo45–46; Al2O3 2.5–4 wt.%.The Rio Grande rise rocks have compositional characteristics of an alkalic basaltic suite, and not of mid-ocean ridge tholeiite. Based on mineral compositions, nodules and megacrysts in basalt are interpreted as cognate inclusions. Because oceanic alkalic basaltic rocks are almost invariably associated with islands and seamounts, the Rio Grande rise probably represents a series of alkalic-basalt islands that formed and eventually subsided during rifting of the South Atlantic; the dredged volcanic breccias are probably slump deposits from those volcanoes. This interpretation lends support to the Rio Grande rise having formed at a hot spot, but the possibility of alkalic rocks having formed along fracture zones should not be discounted.  相似文献   

10.
Gabbro xenoliths in a tholeiitic lava of Kahoolawe Island, Hawaii, a 1.3–1.4 Ma shield volcano, are 1–3 cm in size and comprised of plagioclase, clinopyroxene, and orthopyroxene. Gabbro textures — while intergranular and in part subophitic-are open due to 28–48 vol.% of vesicular basalt occupying xenolith space. Vesicles in and around the xenoliths are lined or filled with rhyolitic glass (segregation vesicles). The host is evolved tholeiite (MgO 6.1 wt%) with phenocrysts, microphenocrysts, and glomerocrysts of olivine, clinopyroxene, orthopyroxene, and plagioclase, and megacrysts (1 cm) of plagioclase. The Sr-isotope ratio of one xenolith is 0.70489; the host basalt ratio is 0.70460. Xenolith isotope composition, grain resorption, and clinopyroxene (Fs12.5–15Wo38–35.5), orthopyroxene (Fs19.5–24Wo4.1), and plagioclase (An68–65Or0.8–1.2) compositions suggest that these gabbros crystallized from Kahoolawe tholeiitic magma of essentially the same composition as the host basalt, but pre-dating the magma represented by the host. Based on the absence of intergranular Fe–Ti oxide phases from the pl+cpx+opx assemblages, and the open, vuggy textures, we envision crystallization on a reservoir roof at temperatures >1100°C. Entrainment of gabbro assemblages and plagioclase megacrysts from a roof mush/suspension zone occurred during convection associated with replenishment of the magma reservoir. These open-textured gabbro xenoliths are therefore not fragments of preexisting coarse-grained bodies such as sills or segregation veins. Rhyolitic glass in vesicles represents a gas-effervescence filtration process that forced fractionated residual liquids from the groundmass into voids associated with the xenoliths.Sirrine Environmental Consultants, Fremont, CA 94538  相似文献   

11.
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   

12.
The nature, origin, and tectonic significance of shoshonitic volcanism is currently the subject of widely differing views. In the type locality in the Absaroka Range, the rocks consist of a diverse group of lavas, all of Mid-Eocene age. High in the volcanic pile are subordinate volumes of absarokite and shoshonite flows, both of which contain calcic plagioclase and sanidine coexisting in the groundmass. Shoshonites contain plagioclase, olivine, and pyroxene phenocrysts; absarokites contain only olivine and pyroxene phenocrysts. A few absarokites contain modal leucite. A chemical study was made of six shoshonites and three absarokites from this area.Although resorption and zoning indicate disequilibrium in both types of rocks, there is no geochemical evidence of magmatic contamination. Calculated crystal/liquid distribution coefficients are in close agreement with previously determined values for basalts. Decrease in pressure during ascent is a likely cause for the observed disequilibrium.Mass balance calculations show that it would be possible to form a shoshonite by fractionation of olivine and pyroxene from an absarokitic magma. Trace element abundances are consistent with this relationship. Petrographic evidence suggests, however, that fractionation of plagioclase played a role in the formation of shoshonites.A high-pressure origin is consistent with rare earth compositions, and a high magmatic temperature is indicated by the composition of the plagioclase. A calculation of ln aKliquid2O for a reaction involving eclogite and a reaction involving a tested rock (Marsh and Carmichael, 1974) suggests a high-pressure origin for a latitude underlying the shoshonites and absarokites. This calculation is subject to large variances because of its sensitivity to estimated equilibrium temperatures.Comparison with lavas of similar composition indicates that shoshonites and absarokites of the Absaroka Range are influenced by their continental setting. There is little evidence for the generation of these magmas in a subduction environment.  相似文献   

13.
Physical and chemical analyses of distal tephra from the 1912 eruption of Novarupta, Alaska, show considerable variations in glass and mineral compositions. A combination of a 150°C range in temperature deduced from iron-titanium oxide geothermometry, and curved patterns in bivariant element plots of glass compositions indicate that a chamber of compositionally zoned magma existed prior to the eruption. Magma-mixing cannot explain these features. The magma chamber may have resembled the model recently proposed by McBirney (1980). A highly silicic, quartz-phyric magma with mean phenocryst compositions of An25 plagioclase, Fs42 orthopyroxene, at a temperature of 880°C and a water pressure of 1.4 kbar, was located above a more mafic, hotter magma, bearing phenocrysts of An45 plagioclase and Fs35, orthopyroxene.Our results on distal tephras compare favorably with those from a recently completed study at source by Hildreth (1983), suggesting that useful petrologic information about distant volcanoes can be obtained from both types of deposits. Compositionally heterogeneous abyssal tephra layers are common in the Gulf of Alaska. Eruptions from chambers of zoned magma may account for many of these layers.  相似文献   

14.
Tauhara dacites have petrographic, geochemical and isotopic characteristics which indicate an origin by magma mixing between andesite and rhyolite. Phenocrysts typically exhibit strong zoning near their rims, are resorbed or display fusion textures. Assemblages are not in equilibrium with host lavas and compositions are bimodal: plagioclase An23–43 and An66–91; orthopyroxene En44–51 and En69–79. Chemical and isotopic trends pass through the bulk compositions of high-alumina andresite and rhyolite which crop out in the vicinity of the dacite domes. Least squares mixing models indicate 40–75% of a rhyolite endmember mixed with andesite can generate the full range of dacite compositions. Subtle geochemical differences between domes suggest that magma mixing may have proceeded as three or more general episodes, each punctuated by several events. These episodes may have catalyzed some of the larger pyroclastic flow eruptions of Taupo Volcanic Zone in the past 50,000 years.  相似文献   

15.
A latite dome in northwest Arizona contains a rare occurrence of primary SO4-rich scapolite phenocrysts. The total phenocryst assemblage consists of plagioclase (An20?An33), hornblende, biotite, and scapolite (Me68). Microphenocrysts include allanite and oxidized low-Ti magnetite. Electron microprobe analyses show that the scapolite contains about 1.74 wt.% S, which indicates an atomic S/(S + C) of 0.58. Although scapolite occurs in xenoliths in volcanic rocks and diatremes, as well as a metamorphic mineral in granulites, its occurrence as a primary igneous mineral is extremely rare.Ca-rich scapolite has been crystallized experimentally by others from melts with a wide range of SiO2, CaO, and Na2O contents, at temperatures above 825°C and pressures ranging from 3 to 15 kbar. Comparison of scapolite from this latite with synthetic scapolite crystallized from nepheline syenite melt suggests that the Arizona phenocrysts crystallized under conditions of 850 to 900°C, 3–6 kbar total pressure, and unusually high ?CO2 and ?SO2. The rarity of scapolite as a phenocryst mineral suggests that high partial pressures of CO2 and SO2 are rare in the magmatic environment.  相似文献   

16.
The 29.5 Ma Wah Wah Springs Formation which erupted from the Indian Peak Caldera has an estimated volume of > 3900 km3 making it one of the largest ignimbrites on earth. The magma was calc-alkaline, dacitic (68 wt. % SiO2) and phenocryst-rich (38 vol.%). Phenocrysts include plagioclase (An 47), magnesio-hornblende, Mg-biotite, quartz, Fe-Ti oxides, diopsidic-augite, and rare Ca-poor pyroxene, in order of decreasing abundance. Apatite, zircon and pyrrhotite occurs as inclusions within phenocrysts. Atmospheric glass losses (1040 km3) account for bulk-rock compositions that have SiO2 contents ranging from 63 to 67 wt.%. Glass compositions are high-silica rhyolite.Phenocrysts equilibrated at temperatures ranging from about 790 to 850°C and oxygen fugacities approximately 2.6 log units above the QFM buffer. Confining pressure estimates using the aluminum-in-hornblende geobarometer calibrated for calc-alkaline volcanic rocks suggest a mean pressure of 230±50 MPa corresponding to 7.5±1.5 km depth. These estimates are consistent with caldera formation accompanying emplacement.Crystal compositions for phenocrysts and mineral inclusions within phenocrysts are remarkably homogeneous throughout the outflow tuff, although minor zoning does occur. Given the dacitic composition of the magma, the weakly zoned phenocryst population cannot be modeled to produce the observed high-silica glass (melt) indicating open-system behavior for the magma. The high-silica rhyolite glass is interpreted to be an artifact of efficient magma mixing accompanying addition of highly evolved magma, or melt to intermediate composition magma. Mixing was followed by magma hybridization. Additional support for this hybridization model includes: (1) physically and chemically distinct populations of augite; (2) minor but unbiquitous resorbed plagioclase, biotite and hornblende phenocrysts; and (3) reverse zoning in some of the plagioclase euhedra within pumice lapilli.  相似文献   

17.
The Chilas Complex is a major lower crustal component of the Cretaceous Kohistan island arc and one of the largest exposed slices of arc magma chamber in the world. Covering more than 8000 km2, it reaches a current tectonic width of around 40 km. It was emplaced at 85 Ma during rifting of the arc soon after the collision of the arc with the Karakoram plate. Over 85% of the Complex comprises homogeneous, olivine‐free gabbronorite and subordinate orthopyroxene–quartz diorite association (MGNA), which contains bodies of up to 30 km2 of ultramafic–mafic–anorthositic association (UMAA) rocks. Primary cumulate textures, igneous layering, and sedimentary structures are well preserved in layered parts of the UMAA in spite of pervasive granulite facies metamorphism. Mineral analyses show that the UMAA is characterized by more magnesian and more aluminous pyroxene and more calcic plagioclase than those in the MGNA. High modal abundances of orthopyroxene, magnetite and ilmenite (in MGNA), general Mg–Fe–Al spatial variations, and an MFA plot of whole‐rock analyses suggest a calc‐alkaline origin for the Complex. Projection of the pyroxene compositions on the Wo–En–Fs face is akin to those of pyroxenes from island arcs gabbros. The presence of highly calcic plagioclase and hornblende in UMAA is indicative of hydrous parental arc magma. The complex may be a product of two‐stage partial melting of a rising mantle diaper. The MGNA rocks represent the earlier phase melting, whereas the UMAA magma resulted from the melting of the same source depleted by the extraction of the earlier melt phase. Some of the massive peridotites in the UMAA may either be cumulates or represent metasomatized and remobilized upper mantle. The Chilas Complex shows similarities with many other (supra)subduction‐related mafic–ultramafic complexes worldwide.  相似文献   

18.
Single-crystal X-ray, optical, and microprobe study of pyroxenes in the Serra de Magéfeldspar cumulate eucrite indicate complex exsolution features from a slow cooling history. Two pyroxenes now exist: “low” orthohypersthene ( P21ca) as host ( ～82 vol.%) and augite ( C2/c) in four distinct habits. This pyroxene pair yields an apparent “equilibration” temperature of ～900°. These relations are typical for orthopyroxene of both the Stillwater and Kintoki-San types, indicating an original pigeonite pyroxene with a bulk composition En51Fs39Wo10. Variations in augite-hypersthene textural relationships suggest variable initial compositions from about Wo8 to Wo11. The bulk composition is intermediate to those of initial pigeonites in Moama and Moore County but the augite-hypersthene tie line is longer suggesting a slower cooling history. Our examinations of all three meteorites show that Serra de Magéaugite lamellae are as thick or thicker than those in the other meteorites, contrary to the measurement of Miyamoto and Takeda. The compositional data, textural relations, and existence of P21ca hypersthene suggest at least a comparable if not slower cooling history for Serra de Magé.  相似文献   

19.
Gabbroic rocks occur only in the west, and are the oldest intrusions in the Peninsular Ranges Cordilleran batholith. They comprise an olivine-pyroxene gabbronorite series and an amphibole gabbro series both of which contain abundant plagioclase and amphibole. They formed by crystal accumulation and in situ differentiation, in multiple intrusive complexes, and are not considered to be related by fractionation to the granitoid rocks of the batholith.Pure mineral separates of plagioclase, olivine, clinopyroxene, orthopyroxene, and amphibole were obtained by magnetic and heavy-liquid methods from a representative suite of gabbroic rocks. Their major- and trace-element contents were determined by X-ray fluorescence, and the data used to test hypotheses on the genesis and fractionation of the gabbros.The plagioclases range from An98 to An65 in composition, olivines, Fo79 to Fo70, occur in rocks where An>36. All clinopyroxenes are augite with Mg #'s varying from 81.1 to 64.7. Orthopyroxene occurs where An<92, and is generally inverted pigeonite or bronzite, and has Mg #'s ranging from 77.9 to 52.1. The amphiboles include tschermakite, tschermakitic hornblende, pargasite, pargasitic hornblende, ferroan pargasite, magnesio-hornblende, and magnesio-taramite, Mg #'s range from 80.4 to 62.5. Systematic chemical and mineralogical changes confirm that differentiation, controlled by mineral assemblages of plagioclase, olivine, spinel, and clinopyroxene initially, and orthopyroxene, amphibole, and magnetite later, took place between intrusive episodes and in situ.The highly clacic plagioclase coexisting with olivine and amphibole suggests that the gabbros were formed from hydrous mafic magmas. The modal mineralogy of the gabbros, and the chemistry of the minerals is very similar to that of the cumulate blocks of the Lesser Antillean volcanoes. These features confirm that the gabbros were derived from a hydrous mafic magma, with high Al2O3 and low TiO2 contents, typical of orogenic environments.Cumulate minerals from the gabbros show little or no zoning and are considered to have formed in equilibrium with the evolving melts. Selected trace-element contents and distribution coefficients are used to calculate the compositions of the melts. The calculations show that the melts in equilibrium with the olivine-pyroxene gabbronorite series contain approximately 100–200 ppm Ba, 200–400 ppm Sr, 30-10 ppm Ni, 20-10 ppm Co, and 300-100 ppm V. K/Rb ratios of the melts, derived from post-cumulus and prismatic amphiboles, are generally in the range 550-250. These values are typical of calc-alkalic basalts and andesites, and it is suggested that they may have erupted at the surface to form a coeval calc-alkalic volcanic sequence.  相似文献   

20.
Microprobe studies of unzoned plagioclases (An92–96) from crystal tuff of Mutnovskii Volcano and allivalite nodules of rocks from the Ksudach, Malyi Semyachik, and Golovnina Volcanoes revealed small inclusions of a dark-colored mineral that was later identified as spinel. Microprobe analyses showed that the grains are unzoned and spinel inclusions of different chemical compositions may occur in one plagioclase crystal. The spinel compositions form a clear extended single trend corresponding to the solvus zone of a solid solution that has not been described in the literature. The existence of this spinel trend in the solvus zone might have been due to early capturing of spinel grains by growing plagioclase crystals and their rapid cooling soon after eruption, resulting in hardening of the metastable solution. These spinels are supposed to form synchronously with plagioclase crystallization. The diversity of spinel compositions is explained by thermo diffusive equalizing of originally zonal spinel crystals after they were captured by plagioclase crystals or by their growth in crystallization haloes of anorthite.  相似文献   

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