中国东部碱性玄武质岩石中的辉石和角闪石巨晶

在中国东部,北起黑龙江、南到海南岛,广泛地分布着新生代碱性玄武质岩石,例如碱性橄榄玄武岩、碧玄岩、霞石玄武岩等。它们含有丰富的超镁铁岩包体和各种巨晶。已发现的有辉石、角闪石、石榴石、歪长石、钛铁矿和云母等巨晶。Harte(1978)把巨晶定义为大的单矿物单个晶体,它比超镁铁岩团块中的矿物粗得多(2—20cm)。然而有的地质学家把巨晶的粒度下限定在1cm(Eggler 等,1979)。笔者遵循这一定义和采纳后一界线,在此基础上着重对中国东部部分地区的辉石和角闪石巨晶进行研究,研究情况见表1。     

Origin of anorthoclase megacrysts in alkali basalts

Anorthoclase megacrysts commonly occur with low pressure cumulate nodules (olivine + clinopyroxene + kaersutite ± oligoclase) in alkali basalts and their differentiates. The absence of anorthoclase from the cumulate nodules indicates that anorthoclase remained suspended in the magma while the other minerals sank, forming the cumulates, assuming a congeneric origin for megacrysts and nodules. On this basis, density calculations indicate that anorthoclase crystallised from a magma of approximately trachyandesitic composition, while the anorthoclase megacrysts usually occur in magmas more basic than trachybasalt. Thus, the anorthoclase megacrysts and the associated cumulate nodules did not crystallise from the host magma, but were picked up from a high-level partly crystallised magma pool containing a more evolved alkali basaltic differentiate by a later surge of more basic liquid which then carried both anorthoclase megacrysts and fragmented cumulates to the surface.     

Model for upper mantle below Malaita,Solomon Islands,deduced from chemistry of lherzolite and megacryst minerals

Clinopyroxene, orthopyroxene, and garnet megacrysts show consistent increase of Na and Ti, and decrease of Cr, with increasing Fe/Mg. Three groups of clinopyroxenes occur with increasing Fe/Mg: subcalcic diopside, lamellar intergrowth with ilmenite, and augite. Chemical relationships indicate simultaneous crystallization of garnet, orthopyroxene and sub-calcic diopside megacrysts, and pyroxene thermometry-barometry indicates a trend from 29 kb?1,230 ° C to 25 kb?1,080 ° C as crystallization proceeded to higher Fe/Mg. Ilmenite-pyroxene thermometry suggests a mean of 965 ° C for crystallization of the intergrowths, but calibration depends on crystal-chemical assumptions. Lherzolite assemblages fall into three groups: two garnet-bearing types which equilibrated at 31 kb?1,150 ° C and 22 kb?900 ° C, and a type bearing Al-rich spinel which probably crystallized below 20 kb. The minerals from the lherzolites have lower Fe/Mg than the megacrysts. The simplest model involves: (i) metamorphic equilibration of lherzolitic rocks to the local geotherm, (ii) local melting of lherzolite at P > 30 kb, (iii) sequential crystallization of megacrysts as the magma rose intermittently, (iv) generation of alnöitic magma at P > 32 kb, and (v) eruption to surface with transport of megacrysts and lherzolitic xenoliths. Garnet, olivine, orthopyroxene and clinopyroxene in these Malaita xenoliths have lower Na, Ti, and P relative to their equivalents from southern African kimberlites. Only clinopyroxene contains K (up to 270 ppmw), and no Na was found in olivine.     

玄武岩中刚玉巨晶的包体研究：Ⅰ.更长石包体的发现及意义

本文对山东昌乐碱性玄武岩中刚玉巨晶的更长石包体及同一地区产出的歪长石巨晶进行了电子探针、红外吸收光谱测定,讨论了更长石包体与刚玉巨晶、歪长石巨晶间的成因关系,认为刚玉巨品、歪长石巨晶以及玄武岩中的其他巨晶构成了一个结晶序列,歪长石相对较晚结晶。     

汉诺坝玄武岩的主要特征及其形成的T、P、fo_2条件

汉诺坝玄武岩主要为由拉斑玄武岩系列与碱性玄武岩系列岩石组成的高原玄武岩。喷发时代为晚第三纪,喷发类型为裂隙式。一、玄武岩、巨晶、包体的组成常见的玄武岩种属为石英拉斑玄武岩、橄榄拉斑玄武岩、碱性橄榄玄武岩及碧玄岩,以碱性玄武岩系列较为少见。在碱性玄武岩系列中,常见巨晶及包体。巨晶有镁铝榴石、普通     

Petrology and Petrogenesis of Clinopyroxene-Rich Tholeiitic Lavas, Merelava Volcano, Vanuatu

The mineralogy, petrography and geochemistry of a suite of clinopyroxene-richolivine tholenite lavas from Merelava island, Vanuatu are described.Located at the southern end of the Northern Trough back-arcbasin, this suite displays all the characteristics of primitiveisland arc lavas: flat REE patterns, depleted HFSE, enrichmentin K-group elements relative to LREE, highly calcic plagioclase(to An_{9 3} and Cr-rich spinels (cr-number80) Analysis of groundmasscompositions demonstrates that the variation in MgO within thelava suite (from 13?7 to 4?3% MgO) represents only a small departurefrom a liquid line of descent. Some of the more primitive lavas contain low-Al₂O₃ clinopyroxenemegacrysts (mg-number = 100Mg/(Mg+Fe^{2 +} and ultramafic xenoliths,the latter ranging from fine-grained, tectonite wehrlites andchnopyroxene-bearing harzburgites, to coarse-grained cumulatewehrlites. The cumulate nodules, megacrysts and phenocrysts are shown tobe co-magmatic, and an empirical compositional relationshipis demonstrated for equilibrium olivine-clinopyroxene pairs,covering the observed fractionation range (mg-number_Cpx=0?6375mg-number_O1 + 35?3). On the basis that the most primitive olivine(mg-number _{91 7}) is close to the liquidus composition, thiscompositional relationship demonstrates that clinopyroxene (mg-number=94,and containing no Fe³⁺) was also a liquidus phase. Clinopyroxeneswith mg-number>94 are the product of local oxidation duringmixing of primitive, relatively reduced magmas, and more evolved,oxidized magmas. This mixing also gave rise to relatively narrow,reversely zoned, internal rims on many clinopyroxene and olivinephenocrysts, cumulus crystals, and clinopyroxene megacrysts. Fractionation modelling demonstrates that the most differentiatedsample with 19 wt.% Al₂O₃ can be derived from the most primitivesample with 10?3% Al₂O₃ by removal of 48% crystals of clinopyroxeneand olivine in the proportions 73:27 Plagioclase is a late crystallizingphase and has an insignificant role in the fractionation process. The parent melt composition (mg-number=77) is deduced from themost primitive olivine composition and the liquid line of descent,and is shown to contain equal amounts of MgO and CaO (137 wt.%),a high CaO/Al₂O₃ ratio of 1?3 and an unusually low Ni contentof 137 ppm. Data from published high pressure (8–20 kb)experiments on melting of peridotite and pyrolite do not providean explanati in for the large normative diopside component inthis parent melt (38 mol.%), and a hypothesis is proposed wherebyhigh degrees of melting of refractory Iherzolite or harzburgite+acomponent of lower crustal pyroxenite and/or wehrlite takesplace at the base of the crust (5–55 kb). At this depth,and initially under hydrous conditions, high degrees of meltingwould progressively eliminate orthopyroxene and then clinopyroxeneto produce a dunite residue. The liquid produced near the pointof clinopyroxene elimination would be compatible with the highCaO and Sc contents, and high Sc/Ni, Cr/Ni and D1/Hy ratiosof the lavas, and the refractory nature of the phenocrysts.     

Rubidium,strontium and the isotopic composition of strontium in ultramafic nodule minerals and host basalts

The concentrations of rubidium and strontium and the isotopic composition of strontium have been determined in minerals separated from ultramaflc nodules occurring in late Tertiary and Quaternary basalts of wide geographic distribution. Clinopyroxene, orthopyroxene and olivine from each of three Iherzolite nodules show a relatively wide range of ⁸⁷Sr/⁸⁶Sr disequilibrium and none of the minerals is in isotopic equilibrium with its host basalt. In two cases there is a correlation between ⁸⁷Sr/⁸⁶Sr and ⁸⁷Rb/⁸⁶Sr ratios of the nodule minerals, indicating apparent isochron relationships which may represent relict mantle events. Clinopyroxene and olivine from each of two wehrlite nodules are not in isotopic equilibrium, although the magnitude of the disequilibrium is smaller than that observed in the Iherzolite nodules. None of these ultramafic nodules can be a crystal cumulate from its host basalt, and it is doubtful that any type of genetic relationship exists. The strontium isotopic disequilibrium between nodule minerals seems to be a primary feature inherited from past mantle histories.     

Aspects of the mineralogy of alnöitic breccia,Malaita, Solomon Islands; comparison with continental kimberlites

Megacrysts of titanian pyrope, high-Ti bronzite, sub-calcic diopside to augite, and phlogopite in an alnöitic breccia, Malaita, Solomon Islands, have compositional features which distinguish them from silicates in kimberlites. Particularly important is the high content of Al₂O₃ in the Malaita silicates, which may reflect the higher Al₂O₃ content of alnöite than kimberlite. The phlogopite megacrysts fall into two populations, one richer in Mg and Cr, and lower in Ti, than the other. The pyroxenes may also fall into two groups characterized by high and low Mg, Cr and Ni. Two stages of crystallization may be responsible.     

Petrogenesis of Garnet Lherzolite and Megacrystalline Nodules from the Thumb, Navajo Volcanic Field

Garnet Iherzolite and megacrystalline nodules from The Thumbcomprise the deepest mantle sample recovered from the diatremesand intrusions of the Four Corners area. Discrepancies betweenvarious geothermometers applied to these nodules are believedto reflect zoning of Fe in garnets as well as problems in calibrationof the geothermometers. Because the pyroxenes are not zoned,the pyroxene solvus method probably provides more reliable temperatureestimates for this nodule suite than methods based on partitioningof Fe and Mg with garnet. The preferred T-P range of the nodules,950–1230 °C and 32–40 kb, is thought to reflecta localized perturbation of the ambient geotherm by a diapiricor igneous intrusion. Garnet Iherzolites with coarse texture show a trend from fertileto refractory compositions consistent with variable depletionby partial melting. These nodules are interpreted as previouslydepleted mantle rocks, most of which were relatively littleaffected by the thermal perturbation. Garnet Iherzolites withsheared texture are enriched in Fe and Ti relative to the coarseIherzolites and are suggested to have formed by deformationand metasomatism of the coarse lherzolites during intrusionof magma related to the thermal perturbation. The texturallydiverse ‘megacrystalline’ nodules are interpretedas precipitates from this liquid. They are similar to discretenodules from kimberlite pipes, but have unique characteristicsof their own. The above relationships are thought to recordthe process of deep-seated intrusion and partial crystallizationof evolved liquids, possibly related to the host lamprophyre,shortly before the time of eruption.     

苏鲁皖新生代玄武岩中巨晶组合的稀土元素特征

我国东部苏鲁皖地区新生代碱性玄武岩中,除了含有大量地幔橄榄岩类捕虏体以外,尚含有一定数量的石榴石、普通辉石和歪长石巨晶。这些巨晶是在地幔不同深度上从玄武岩中晶出的。巨晶组合的分离结晶作用对熔体稀土元素含量有很大影响。赋存巨晶的碱性玄武岩所具有的LREE富集、HREE亏损的稀土元素分配型式是由地幔橄榄岩类部分熔融程度、石榴石巨晶和普通辉石巨晶的早期高压熔离和玄武岩的结晶分异作用等综合因素造成的。     

Quaternary alkalic and sub-alkalic volcanism in South Auckland,New Zealand

A significant feature of Quaternary volcanism in New Zealand is the presence of several compact fields of monogenetic basaltic volcanism, in many respects comparable to back-arc volcanism of Japan and intra-plate volcanism in S.E. Australia. Volcanism in South Auckland between 1.56 m.y. (B.P.) and 0.51 m.y. has produced some 20 km³ of lava and scoria from at least 70 centres. Older lavas are alkalic (basanite and alkali-olivine basalt), and are associated with maars and small scoria cones and many contain mafic and ultramafic xenoliths and megacrysts. Younger subalkalic lavas (mainly transitional tholeiite) form large scoria cones with extensive lava aprons.Chemically and mineralogically the two types are distinct and represent separate parent magmas. Alkalic magma probably differentiated near source (<25 kb) by precipitation of kaersutite, olivine, and clinopyroxene. Subalkalic lavas were the product of more voluminous melting and observed differentiation trends may be due to Al-opx and subcalcic Al-cpx precipitation at 10 to 15 kb. Progressive melting of a mantle diapir rising through the low velocity zone can account for the temporal and petrologic relationships between the two magmas.     

Pargasitic-kaersutitic amphibole from a basanitic diatreme at the Rosenberg,North of Kassel (North Germany)

The tuff and basanite of Rosenberg 20 km NW of Kassel are thus far unknown occurrences of megacrysts and polycrystalline aggregates of pargasitic-kaersutitic amphibole and of xenoliths of amphibole-bearing peridotite and pyroxenite. The amphiboles precipitated successively with gradually increasing Fe/Mg from the same magma from which the basanite was ultimately derived. The amphibole fractionation commenced within the upper mantle but may have continued into higher levels within the ascending melt.Amphibole and phlogopite fractionation together caused an increased Na/K ratio in the Rosenberg basanite. The crystallization of amphibole and phlogopite is genetically interrelated with the violent eruptive nature of the Rosenberg volcano. The release of vapour during the final stage of amphibole crystallization is indicated by the existence of coarsely vesicular amphibole megacrysts.     

中国山东昌乐地区碱性玄武岩刚玉中记录的岩浆不混溶作用

Abundant melt-and fluid inclusions occur in corundum megacrysts of alkaline basalt from the Changle area,Shandong province,eastern China.One type of melt inclusions,i.e.muhiphase melt inclusions(glass bubbles daughter minerals)were identified,which occur along growth zones of host corundum megacrysts.Microthermometry and laser Raman microprobe analysis were performed on the melt inclusions.The bubbles within the melt inclusions are confirmed to be CO_2-rich phase and the daughter minerals are probably silicates,such as augite and okenite.The results of high temperature homogenization experiment strongly suggest that two immiscible melts,i.e.a H_2O-and CO_2-rich melt and an anhydrous and CO_2-poor melt were trapped by melt inclusions in corundum megacryst.     

The Mineral Chemistry of Ultramafic Xenoliths of Eastern China: Implications for Upper Mantle Composition and the Paleogeotherms

Ultramafic xenoliths of garnet lherzolite (?rare spinel), spinellherzolites, spinel harzburgites, clinopyroxenites, and clinopyroxenemegacrysts were collected from Cenozoic basalts in all partsof eastern China. From their modal composition and mineral chemistryall the xenoliths may be placed into three types representing:a fertile or more primitive mantle (garnet lherzolite and spinellherzolite), a refractory or more depleted mantle (spinel harzburgiteand dunite), and inclusions cognate with the host alkali basaltsat mantle pressures (pyroxenite and megacrysts). There are systematicdifferences between the mineral compositions of each type. Spinelshows a wide compositional range and the spinel cr-number [100Cr/(Cr + Al)] is a significant indicator of the xenolithtype. Spinel cr-number and Al₂O₃ of coexisting minerals (spinel,clinopyroxene, and orthopyroxene) are useful as refractory indicatorsfor spinel peridotite in that the cr-number increases and thepercentage of Al₂O₃ decreases with increasing degrees of melting.In garnet peridotite, however, the same functions vary withpressure, not degree of melting. According to P–T estimates,the various xenoliths were derived from a large range of depthsin the upper mantle: spinel peridotite from approximately 11to 22 kb (37–66 km), spinel/garnet lherzolite from 19to 24 kb (62–80 km), and garnet lherzolite from 24 to25 kb (79–83 km). We conclude that the uppermost mantlebeneath eastern China is heterogeneous, with a north-northeastzone of more depleted mantle lying beneath the continental marginand a more primitive mantle occurring towards the continentalinterior.     

Strontium isotope geochemistry of megacrysts and host basalts from southeastern Australia

Strontium isotopic data for megacrysts and lavas from six eruptive centers within the Newer Basalts province of southeastern Australia show that megacrysts of clinopyroxene are in isotopic equilibrium with associated basalts, but that megacrysts of kaersutite, ferrokaersutite, orthopyroxene and anorthoclase may exhibit slight disequilibrium with their host basalts. Furthermore, the anorthoclase megacrysts may be either more or less radiogenic than their hosts. The ⁸⁷Sr/⁸⁶Sr ratios for 14 basalts from throughout the province vary from 0.7035 to 0.7045 and it is proposed that anorthoclase, amphibole and orthopyroxene megacrysts which crystallized in isotopic equilibrium with one magma may have been caught up in a pulse of a later magma of a different isotopic composition. The variations in the ⁸⁷Sr/⁸⁶Sr ratios for the basalts are attributed to variations in the isotopic composition of their source regions. Such isotopic heterogeneity is supported by published data for ultramafic xenoliths which occur in the Newer Basalts lavas.     

Ultramafic Xenoliths from Bullenmerri and Gnotuk Maars, Victoria, Australia: Petrology of a Sub-Continental Crust-Mantle Transition

The basanite tuffs of Bullenmerri and Gnotuk maars, Victoria,enclose abundant xenoliths of spinel lherzolites, many of whichcontain amphibole ± apatite ± phlogopite. Thexenolith suite also includes cumulate wehrlites, spinel metapyroxenitesand garnet metapyroxenites. All xenolith types contain abundantlarge CO₂-rich fluid inclusions. Microstructural evidence forthe exsolution of spinel, orthopyroxene, garnet and rare plagioclasefrom complex clinopyroxenes suggests that all of the metapyroxeniteshave formed from clinopyroxene (± spinel ± orthopyroxene)cumulates by exsolution and recrystallization during coolingto the ambient geotherm. Pyroxene chemistry implies that a rangeof parental magma types was involved. Garnet pyroxenites showa series of reactions to successively finer-grained, lower-Pmineral assemblages, which imply a relatively slow initial upwardtransport of the xenoliths in the magma, prior to explosiveeruption. The same process has allowed crystallization of phenocrystsfrom small patches of interstitial melt within xenoliths oflherzolite, wehrlite and metapyroxenite. Critically selected P-T estimates for 16 garnet websteritesare consistent with published experimental studies of the spinel/garnetpyroxenite transition, and define a geotherm from 900 °C,11 kb to 1100 °C, 16 kb. Other published data extend thecurve down to c. 7 kb and up to 25 kb. This elevated geothermsuggests that the high regional heat flow is related to convectiveheat transfer by dike injection accompanying the vulcanism.T estimates for the lherzolites range from 850–1050 °C;comparison with the derived geotherm implies that the spinellherzolites are derived from depths of 30–55 km. Thiszone has low seismic velocities (V_p = 6.8–7.8 km/sec)and has thus previously been regarded as a thick, largely maficlower crust. The xenolith data show that this Mower crust' isdominantly ultramafic, with layers, dikes and some large bodiesof pyroxenites and mafic granulites. The anomalously low V_pmay be due to the high T, the high proportion of fluid-filledpore volume, and the magnesian composition of the lherzolites.The seismically defined Moho (V_p >8.0 km/sec) coincides withthe experimentally determined position of the spinel lherzolite-garnetlherzolite transition.     

The genesis of basaltic magmas

This paper reports the results of a detailed experimental investigation of fractionation of natural basaltic compositions under conditions of high pressure and high temperature. A single stage, piston-cylinder apparatus has been used in the pressure range up to 27 kb and at temperatures up to 1500° C to study the melting behaviour of several basaltic compositions. The compositions chosen are olivine-rich (20% or more normative olivine) and include olivine tholeiite (12% normative hypersthene), olivine basalt (1% normative hypersthene) alkali olivine basalt (2% normative nepheline) and picrite (3% normative hypersthene). The liquidus phases of the olivine tholeiite and olivine basalt are olivine at 1 Atmosphere, 4.5 kb and 9 kb, orthopyroxene at 13.5 and 18 kb, clinopyroxene at 22.5 kb and garnet at 27 kb. In the alkali olivine basalt composition, the liquidus phases are olivine at 1 Atmosphere and 9 kb, orthopyroxene with clinopyroxene at 13.5 kb, clinopyroxene at 18 kb and garnet at 27 kb. The sequence of appearance of phases below the liquidus has also been studied in detail. The electron probe micro-analyser has been used to make partial quantitative analyses of olivines, orthopyroxenes, clinopyroxenes and garnets which have crystallized at high pressure.These experimental and analytical results are used to determine the directions of fractionation of basaltic magmas during crystallization over a wide range of pressures. At pressures corresponding to depths of 35–70 km separation of aluminous enstatite from olivine tholeiite magma produces a direct fractionation trend from olivine tholeiites through olivine basalts to alkali olivine basalts. Co-precipitation of sub-calcic, aluminous clinopyroxene with the orthopyroxene in the more undersaturated compositions of this sequence produces derivative liquids of basanite type. Magmas of alkali olivine basalt and basanite type represent the lower temperature liquids derived by approximately 30% crystallization of olivine-rich tholeiite at 35–70 km depth. At depths of about 30 km, fractionation of olivine-rich tholeiite with separation of both olivine and low-alumina enstatite, joined at lower temperatures by sub-calcic clinopyroxene, leads to derivative liquids with relatively constant SiO₂ (48 to 50%) increasingly high Al₂O₃ (15–17%) contents and retaining olivine + hypersthene normative chemistry (5–15% normative olivine). These have the composition of typical high-alumina olivine tholeiites. The effects of low pressure fractionation may be superimposed on magma compositions derived from various depths within the mantle. These lead to divergence of the alkali olivine basalt and tholeiitic series but convergence of both the low-alumina and high-alumina tholeiites towards quartz tholeiite derivative liquids.The general problem of derivation of basaltic magmas from a mantle of peridotitic composition is discussed in some detail. Magmas are considered to be a consequence of partial melting but the composition of a magma is determined not by the depth of partial melting but by the depth at which magma segregation from residual crystals occurs. Magma generation from parental peridotite (pyrolite) at depths up to 100 km involves liquid-crystal equilibria between basaltic liquids and olivine + aluminous pyroxenes and does not involve garnet. At 35–70 km depth, basaltic liquids segregating from a pyrolite mantle will be of alkali olivine basalt type with about 20% partial melting but with increasing degrees of partial melting, liquids will change to olivine-rich tholeiite type with about 30% melting. If the depth of magma segregation is about 30 km, then magmas produced by 20–25% partial melting will be of high-alumina olivine tholeiite type, similar to the oceanic tholeiites occurring on the sea floor along the mid-oceanic ridges.Hypotheses of magma fractionation and generation by partial melting are considered in relation to the abundances and ratios of trace elements and in relation to isotopic abundance data on natural basalts. It is shown that there is a group of elements (including K, Ti, P, U, Th, Ba, Rb, Sr, Cs, Zr, Hf and the rare-earth elements) which show enrichment factors in alkali olivine basalts and in some tholeiites, which are inconsistent with simple crystal fractionation relationships between the magma types. This group of elements has been called incompatible elements referring to their inability to substitute to any appreciable extent in the major minerals of the upper mantle (olivine, aluminous pyroxenes). Because of the lack of temperature contrast between magma and wall-rock for a body of magma near to its depth of segregation in the mantle, cooling of the magma involves complementary processes of reaction with the wall-rook, including selective melting and extraction of the lowest melting fraction. The incompatible elements are probably highly concentrated in the lowest melting fraction of the pyrolite. The production of large overall enrichments in incompatible elements in a magma by reaction with and highly selective sampling of large volumes of mantle wall-rock during slow ascent of a magma is considered to be a normal, complementary process to crystal fractionation in the mantle. This process has been called wall-rock reaction. Magma generation in the mantle is rarely a simple, closed-system partial melting process and the isotopic abundances and incompatible element abundances of a basalt as observed at the earth's surface may be largely determined by the degree of reaction with the mantle or lower crustal wall-rocks and bear little relation to the abundances and ratios of the original parental mantle material (pyrolite).Occurrences of cognate xenoliths and xenocrysts in basalts are considered in relation to the experimental data on liquid-crystal equilibria at high pressure. It is inferred that the lherzolite nodules largely represent residual material after extraction of alkali olivine basalt from mantle pyrolite or pyrolite which has been selectively depleted in incompatible elements by wall-rock reaction processes. Lherzolite nodules included in tholeiitic magmas would melt to a relatively large extent and disintegrate, but would have a largely refractory character if included in alkali olivine basalt magma. Other examples of xenocrystal material in basalts are shown to be probable liquidus crystals or accumulates at high pressure from basaltic magma and provide a useful link between the experimental study and natural processes.     

An experimental study of spinel clinopyroxenite xenoliths from the Duncansby Ness vent,Caithness, Scotland

Xenoliths of coarse-grained spinel-clinopyroxenite up to 15 cm in size occur in tuff in an isolated Permian vent on the Caithness coast at Duncansby Ness. Highly altered fragments of chrome-spinel lherzolite and wehrlite are also found in the tuff and in a body of monchiquite within the vent. The spinel-clinopyroxenites consist of aluminous augite and aluminous pleonaste spinel (FeO/MgO = 0.9) and their texture suggests the spinel to have exsolved from the augite. Experiments on representative natural xenolith compositions at 18 kb (dry) indicate that all the spinel in the estimated average bulk composition (Sp_4.9Px_95.1) could have exsolved from an original homogeneous pyroxene. Initial fractionation of such a pyroxene from an alkali basaltic magma at P≥18 kb, 1450-1350 °C, would be followed by spinel exsolution at T< 1290 ° C. A similar origin by fractionation of a highly aluminous augite (± aluminous spinel) at high pressure, with subsequent spinel exsolution is proposed for spinel-clinopyroxenites from alkali basalts elsewhere in the world. The similarity of these xenoliths suggests that such a process may form an important stage in the evolution of some undersaturated basaltic rocks.     

The mineral chemistry and origin of xenoliths from the lavas of Anjouan,Comores Archipelago,Western Indian Ocean

Mineralogical data for xenoliths occurring as inclusions in the fissure erupted alkali basalts and the basanitic tuffs of Anjouan reveal three xenolith suites: 1) the lherzolites, 2) the dunites and wehrlites, 3) the gabbros and syenites. The dunite-wehrlite suite and the gabbro suite are shown to represent high-level cumulate sequences resulting from ankaramitic fractionation of the hy-normative shield-building lavas and cotecictic fractionation of the alkali basalt lavas respectively, whilst the syenitic xenoliths represent evolved high-level intrusions. Mineralogical and rare earth element (REE) data indicate that the most likely origin for the spinel lherzolite xenoliths is by extraction of a basaltic phase from spinel peridotite, leaving a light REE-poor spinel lherzolite residuum. REE models, constructed using model peridotite assemblages, imply that the hy-normative basalt lavas may be derived by partial melting of spinel peridotite at pressures of <20–25 kb leaving a residual lherzolite, and that the alkali basalt and basanite melts are formed by small degrees of melting of a garnet-peridotite source at pressures of >20–25 kb. The spinel lherzolite source for the hy-normative basalts has been accidentally sampled during explosive eruption of the alkali basalt and basanite magmas.     

Sector-zoned augite megacrysts in Aleutian high alumina basalts: implications for the conditions of basalt crystallization and the generation of calc-alkaline series magmas

Several high alumina basalts from the Aleutian volcanic centers of Cold Bay and Kanaga Island contain large (up to 1.5 cm diameter) megacrysts of sector-zoned augite. The megacrysts are invariably euhedral with well developed {001}, {010} and {111} forms. All crystals display concentric bands that are rich in mineral and glass inclusions. The sector zonation typically occurs as well developed (010), (100), (111) and (110) sectors which grew at different rates. A comparison of the width of synchronous growth bands indicates that following relative growth rates: (111) ≫ (100) ∼ (110) > (010). Compositionally, SiO₂ and MgO abundances decrease, and TiO₂, Al₂O₃, FeO and Na₂O abundances increase in the different sectors in the order (111), (100) ∼ (110), (010). This order is identical to that deduced for the relative growth rates, implying that growth rate clearly had a role in the development of the sector zonation. Calculated pre-eruption H₂O contents of the basalts range from 1 to 3 wt% but actual (measured) post-eruption H₂O contents range from 0.01 to 0.3 wt%. Deteurium isotopic values are heavily depleted and range from −110 to −141‰ . Together these indicate significant vapor (H₂O) exsolution prior to eruption. Maximum H₂O abundances in primitive glass inclusions, thought to be most representative of the host liquid reservoir at the time of melt entrapment, systematically decrease from the core to the rim of one augite megacryst studied in detail. We conclude that the presence of sector-zoned augite is due to augite supersaturation and rapid crystallization brought about by magma decompression and volatile (H₂O) exsolution. The calculated pre-eruption H₂O contents of 1–3 wt% limit vapor exsolution and basalt crystallization to depths of less than 3 and more likely 1.5 km. Very rapid crystallization at very shallow depths makes it unlikely that the time scales between initial crystallization and final eruption are sufficient to permit appreciable amounts of fractional crystallization. Given that high alumina basalt fractionation is the dominant process for generating more evolved andesite, dacite and rhyolite magmas of the calc-alkaline suite, the inability of parental high alumina basalt to yield such derivative magmas in the low pressure environment places the likely site of fractionation in the high pressure environment, at or near the base of the crust. Received: 1 December 1997 / Accepted: 23 December 1998