Mineral chemistry and its genetic significance of olivine in Cenozoic basalts from the South China Sea
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We dredged lots of Cenozoic basalts from areas covered from the northern sub-slope to the southern sub- slope of the South China Sea. Based on the study on mineral chemistry of clinopyroxenes in these Cenozoic hasalts, this paper indicates that pyroxenes are mostly enstatite and a few of augite, sahlite and Ca-rich pyroxene. Pyroxene microlite has higher content in, Ca, Ti and Fe than pyroxene phenocryst, it may reflect that the evolution trend of host magma of pyroxene is coincidence with that of alkali rock series. The depth of magma chambers which calculated from equilibrium temperatures and pressures between clinopyroxene and melt are as follows, that of magma of tephrite is about 49km, that of magma of trachybasalt is about 25km, and that of magma of basalt is about 15km. Correspondingly, Equilibrium temperatures( K) of three types rocks mentioned above gradually decrease from 1535 1498 to 1429 to 1369. By using discriminant plot which developed from pyroxene and alkali discriminant diagram of host rock, Cenozoic basalt from the South China Sea belongs to intraplate alkali basalt. The results suggest that alkali basalt series in the study area may be the products of continuous evolution of mantle plume, which result from some physical and chemistry process including partial melting and fractional crystallization of mantle plume during the course of its ascent to the surface.Keywords:
Pyroxene
Phenocryst
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Picrite in island arcs has been rarely reported. However, several thick layers of picrite, which were rarely found in the world, were recognized in the Middle Devonian strata on the southern margin of the Altay Mountain. This paper presents new data of electronic microprobe analyses on clinopyroxenes from the layers of picrite and associated basalt in this area. It is shown that the clinopyroxenes from the picrite are exclusively diopside, whereas those from the basalt are diopside and augite, suggesting a continuous variation in composition. The crystallization temperature, pressure and depth of clinopyroxene phenocrysts from the picrite and basalt were calculated on the basis of the clinopyroxene-melt equilibrium principle. The temperature of mineral crystallization in depth, which represents the condition of picritic magma chamber, is estimated to be in the range of 1 350-1 387℃, and the corresponding pressure and depth are 16. 6-21.3 kPa and 55-70 km, respectively. In contrast, the depth of mineral crystallization in the shallow part, which represents the condition of basaltic magma chamber, is in the range of 37-41 km, indicating that the basaltic magma might have been an evolved magma formed by fractionation crystallization of primary magma under lower pressure. Moreover, the mineral chemistry of the clinopyroxenes shows that the volcanic rocks in this area belong to subalkaline series and were probably formed in an island arc setting.
Diopside
Phenocryst
Magma chamber
Fractional crystallization (geology)
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Amphibole
Phlogopite
Alkali feldspar
Diopside
Pyroxene
Phenocryst
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Phenocryst
Phlogopite
Alkali basalt
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Phenocryst
Pyroxene
Fractional crystallization (geology)
Trace element
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Trachyte
Phenocryst
Silicic
Fractional crystallization (geology)
Pyroxene
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Tertiary volcanic rocks in Yangnam basin comprise a continuous series of calc-alkline rocks, ranging in composition from basalts to rhyolites, which may be related to the Pacific plate subduction beneath the Japanese islands. In this paper, geochemical study was focused on the intermediate to matic Tertiary volcanic rocks in the Yangnam basin. Intermediate to mafic volcanic rocks of the Yangnam basin are mainly composed of basalt, basaltic andesite and andesite. Whole rock chemical analyses of rocks from the study area indicate that the all the rocks have characteristics of medium-K calc-alkaline series in the subalkali field. Major and trace element variations of the rocks describe well-defined trends with increasing differentiation which are generally consistent with control by fractional crystallization of phenocryst phases. Basalts and andesites have similar Sr, Nd isotopic ratios which suggest that these rocks might be derived from the same magma source. Quantitative modelling of the compositional variations in basaltic rocks suggests that the fractional crystallization of pyroxene and Fe-Ti oxides being required to account for generation of the most evolved basalt. However, the chemical evolution of the andesitic rocks can be interpreted as fractional crystallization with contamination by crustal rocks rather than simple fractional crystallization. Trace element data suggest that the primary magma of the basaltic rocks may formed by the 15∼20% partial melting of garnet peridotite in the mantle wedge, and andesitic magma was generated by the contamination of mantle-derived parimary magma with crustal materials during emplacement.
Fractional crystallization (geology)
Basaltic andesite
Andesites
Phenocryst
Magma chamber
Trace element
Peridotite
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Late Cenozoic lava flows of southwest Utah comprise olivine basalt, with subordinate hypersthene-bearing basalt and andesite. Some basic flows containing normative nepheline may be classified chemically and mineralogically as alkali olivine basalt, others as tholeiitic on the basis of the presence of a reaction relation between olivine and Ca-poor pyroxene. Most of the basic lava flows, however, are transitional between typical alkaline and typical tholeiitic basalt and contain both olivine and hypersthene in the norm. The andesites are mineralogically similar to calc-alkaline andesites but are chemically richer in K 2 O. Most of the rocks are only mildly alkaline yet are lacking any clear trend of iron enrichment. Sr and Ba are the most abundant trace elements and are strongly enriched in andesite relative to basalt. Feldspar compositions vary from plagioclase of An 70 to potash feldspar of Or 70 , and olivine shows a considerable compositional range, except in basalt which contains groundmass hypersthene. Clinopyroxene is only weakly zoned and the behavior of its Al z is different according to whether the host rock is chemically alkaline or tholeiitic. Coexisting titano-magnetite and ilmenite in the groundmass of some of the basalts indicate consolidation temperatures in the range of 1040° to 800°C and oxygen fugacities ( f o 2 ) between 10 −10.3 and 10 −13.6 . The basaltic rocks probably originated by partial melting in the mantle, and their diversity is thought to reflect events which took place in the mantle rather than near the Earth9s surface. It is unlikely that the andesitic magmas were produced by crystal fractionation of basaltic magma, particularly if this involved removal of plagioclase. Sr 87 /Sr 86 ratios ranging from 0.7037 to 0.7054 place a limit on the possible amount of contamination of basalt by old, radiogenic crustal material. Thus the andesite may be the result of contamination of basaltic magma by young or nonradiogenic crustal material, or it may represent liquid produced by partial melting in the mantle. In either case, material rich in Sr and Ba seems to have been involved.
Margin (machine learning)
Basaltic andesite
Alkali basalt
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Petrogenesis
Flood basalt
Deccan Traps
Alkali basalt
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Al-Haruj basalts that represent the largest volcanic province in Libya consist of four lava flow phases of varying thicknesses, extensions, and dating. Their eruption is generally controlled by the larger Afro-Arabian rift system. The flow phases range from olivine rich and/or olivine dolerites to olivine and/or normal basalts that consist mainly of variable olivine, clinopyroxene, plagioclase, and glass. Olivine, plagioclase, and clinopyroxene form abundant porphyritic crystals. In olivine-rich basalt and olivine basalt, these minerals occur as glomerophyric or seriate clusters of an individual mineral or group of minerals. Groundmass textures are variably intergranular, intersertal, vitrophyric, and flow. The pyroclastic, clastogenic flows and/or ejecta of the volcanic cones show porphyritic, vitrophric, pilotaxitic, and vesicular textures. They are classified into tholeiite, alkaline, and olivine basalts. Three main groups are recorded. Basalts of phase 1 are generated from tholeiitic to alkaline magma, while those of phases 3 and 4 are derived from alkaline magma. It is proposed that the tholeiitic basalts represent prerift stage magma generated by higher degree of partial melting (2.0–3.5%) of garnet-peridotite asthenospheric mantle source, at shallow depth, whereas the dominant alkaline basalts may represent the rift stage magma formed by low degree of partial melting (0.7–1.5%) and high fractionation of the same source, at greater depth in an intra-continental plate with OIB affinity. The melt generation could be also attributed to lithosphere extension associated with passive rise of variable enriched mantle.
Porphyritic
Phenocryst
Peridotite
Pyroxene
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The volcanic cone of Hulanhoboke Volcano in Tongjing basin,Baikal rift region,consists of volcanic debris such as volcanic bombs,volcanic ashes and mafic lava.Olivine xenocrysts characterize with higher Mg~# relative to olivine phenocrysts in basalt.Some plagioclase phenoerysts have inverse zone and core-mantle-rim textures,and the mantle of plagioclase phenocrysts shows decompressing decomposition texture.One kind clinopyroxene rim shows decomposition texture.Based on chemical composition,this basalt belongs to shoshonite series;the light rare earth elements are strongly enriched and differentiate from the heavy rare earth elements obviously.Their geochemical characteristics of trace elements and isotopic compositions are similar to OIB.Petrologic and geochemical research indicates that the source of olivine basalt and the fraction of magma may relate to mantle plume.The magma eruption was rather fast,which led the formation of observed decomposition texture in plagioclase and clinopyroxene xenocrysts.The magma was rarely contaminated by continental crustal m,terial.The studied magma captured a little shallow wall-rock which occurs as rhyolitic inclusion.
Phenocryst
Magma chamber
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