Petrogenesis of Ultramafic Xenoliths from the 1800 Kaupulehu Flow, Hualalai Volcano, Hawaii

The 1800 Kaupulehu flow on Hualalai Volcano, Hawaii, containsabundant xcnoliths of dunitc, wehrlite, and olivine clinopyroxenitewith minor gabbro, troctolite, anorthosite, and wcbstcrite.The petrography and mineral compositions of 41 dunite, wehrlite,and olivine clinopyroxenite xenoliths have been studied, andclinopyroxene separates from eight of these have been analyzedfor Ba, K, Rb, Sr, rare earth elements, ⁸⁷Sr/⁸⁶Sr, and ¹⁴³Nd/¹⁴⁴Nd.Temperatures of equilibration obtained by olivine-spinel andpyroxene geothermometry range from 1000 to 1200 C. Mineralogicaldata combined with published fluid inclusion data indicate depthsof origin in the range of 8–30 km. The rarity of orthopyroxene, the presence of Fe-rich olivine(Fo⁸¹–⁸⁹) and clinopyroxene (Fs⁵–¹²), and the occurrenceof high TiO² in spinel (0.9–2.8 wt.%) and clinopyroxene(035–1.33 wt%) all indicate that the xenoliths are cumulates,not residues from partial fusion. The separated clinopyrox-eneshave ⁸⁷Sr/⁸⁶Sr (0-70348.0-70367) and ¹⁴³Nd/¹⁴⁴Nd (0.51293–0.51299)values that are different from Sr and Nd isotope ratios of Pacificabyssal basalts (>0.7032 and >0-5130, respectively). Also,clinopyroxenes and spinels in the xenoliths have generally higherTiO₂ contents (>O.35 and >0.91 wt.%, respectively) thantheir counterparts in abyssal cumulates (<0.40 and <0.70wt%,respectively). These differences indicate that the xenolithsare not a normal component of oceanic crust. Because the xenoliths and alkalic to transitional Hualalai lavashave similar values for Cr/(Cr + Al) and Cr/(Cr + Al + Fe³⁺)of spinels, ⁸⁷Sr/⁸⁶Sr of clinopyroxenes, and whole-rock ³He/⁴He,we conclude that the xenoliths are cumulates from such magmas.Multiple parental magmas for the xenoliths are indicated byslightly heterogeneous ⁸⁷Sr/⁸⁶Sr of clinopyroxene separates.Depths of formation of the xenoliths are estimated to be {smalltilde}8–30 km. Extensive crystallization of olivine in the absence of pyroxenesand plagioclase is a characteristic and prominent feature ofHawaiian tholeiitic magmatism. Dunite xenoliths crystallizedfrom alkalic magmas have previously been reported from MaunaKea Volcano (Atwill & Garcia, 1985) and Loihi Seamount (Clague,1988). Our finding of an alkalic signature for dunite xenolithsfrom a third Hawaiian volcano, Hualalai, shows that early olivinecrystallization should be considered a characteristic not justof Hawaiian tholeiitic magmatism but also of Hawaiian alkalicmagmatism.     

Volatiles in Basaltic Glasses from Loihi Seamount, Hawaii: Evidence for a Relatively Dry Plume Component

New H₂O, CO₂ and S concentration data for basaltic glasses fromLoihi seamount, Hawaii, allow us to model degassing, assimilation,and the distribution of major volatiles within and around theHawaiian plume. Degassing and assimilation have affected CO₂and Cl but not H₂O concentrations in most Loihi glasses. Waterconcentrations relative to similarly incompatible elements inHawaiian submarine magmas are depleted (Loihi), equivalent (Kilauea,North Arch, Kauai–Oahu), or enriched (South Arch). H₂O/Ceratios are uncorrelated with major element composition or extentor depth of melting, but are related to position relative tothe Hawaiian plume and mantle source region composition, consistentwith a zoned plume model. In front of the plume core, overlyingmantle is metasomatized by hydrous partial melts derived fromthe Hawaiian plume. Downstream from the plume core, lavas tapa depleted source region with H₂O/Ce similar to enriched Pacificmid-ocean ridge basalt. Within the plume core, mantle components,thought to represent subducted oceanic lithosphere, have waterenrichments equivalent to (KEA) or less than (KOO) that of Ce.Lower H₂O/Ce in the KOO component may reflect efficient dehydrationof the subducting oceanic crust and sediments during recyclinginto the deep mantle. KEY WORDS: basalt; Hawaii; mantle; plumes; volatiles     

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.     

Ultramafic inclusions in basaltic rocks from Hawaii

Ultramafic inclusions and the enclosing basaltic rocks were collected from a number of localities in the Hawaiian Islands; these and other specimens were studied by standard pétrographic techniques and with an electron microprobe. Emphasis was on determination of mineral assemblages, mineral compositions, and variations in composition. Sixty-eight inclusions and thirteen basaltic rocks are described, with partial chemical analyses (Ti, Al, Cr, Fe, Mn, Ni, Mg, Ca, Na, K) of olivines, orthopyroxenes, clinopyroxenes, and some feldspars and other minerals. Inclusions range from dunite to anorthosite, and basaltic hosts range from olivine nephelinite to olivine tholeiite. The inclusions are separable into three categories, which correlate with three groups basaltic hosts: Lherzolite inclusions are relatively poor in Fe, and the component minerals have limited ranges of composition. In Hawaii, lherzolite inclusions occur preferentially in extremely undersaturated hosts (olivine nephelinite, nepheline basanite, and ankaratrite). Other varieties of inclusions (dunite, wehrlite, feldspathic peridotite, pyroxenite) are relatively rich in Fe, and the component minerals have wider ranges of composition. These inclusions, together with gabbro, occur preferentially in hosts which are but moderately undersaturated (alkaline olivine basalt, hawaiite, and ankaramite). The sparse inclusions in nearly-saturated basalt (olivine tholeiite) are petrographically distinct from those in the other two categories. These correlations suggest that the inclusions and the enclosing basaltic rocks are genetically related. As the three suites of inclusions differ chemically, mineralogically, physically, and texturally, more than one origin is probable.     

Geochemical and Isotopic Evolution of Loihi Volcano, Hawaii

The Publishers wish to apologise for the incorrect representationof figure 10 which accompanied this paper. The correct figureis reproduced below.     

Geochemical and Isotopic Evolution of Loihi Volcano, Hawaii

A 680m thick section from the deeply dissected east flank ofLoihi Volcano was sampled using the Pisces V submersible toevaluate the volcano's geochemical evolution. Three types oflavas were recovered: tholeiitic, weakly alkalic and stronglyalkalic. The ratio of alkalic to tholeiitic lavas varies systematicallywith depth, from predominantly alkalic at the base of the sectionto tholeiitic at the top. Glasses from these rocks have similarratios of highly incompatible elements and Pb, Sr and Nd isotopes,but distinct ratios of highly to moderately incompatible elements.Partial melting modeling indicates that these tholeiitic andalkalic lavas could be derived by variable degrees of partialmelting of a slightly heterogeneous source. Many distinct parentalmagmas were generated for each rock type during the 100–150k.y. that the east flank section was formed. Crystal fractionationand olivine accumulation were the dominant processes controllingcompositional variation among lavas of the same rock type. Magmamixing features were observed in only a few of the lavas collected. Loihi typifies the preshield stage of Hawaiian volcanism whenthe volcano drifts closer to the focus of the hotspot. The compositionalvariation in Loihi's east flank section, which may represent40% of the volcano's extrusive history, is consistent with thepredicted increase in partial melting during this drift. Thetransition from dominantly alkalic to tholeiitic volcanism onLoihi was fitful but relatively rapid and is now nearly complete.This transition is the opposite of that which occurs duringthe post-shield stage of Hawaiian volcanism as the volcano migratesaway from the hotspot focus. Loihi's tholeiitic lavas overlap in ratios of incompatible traceelements and Pb, Sr and Nd isotopes with lavas from its moreactive neighbor, Kilauea. The small differences in major elementcontents between lavas from these adjacent volcanoes can beexplained by high-pressure orthopyroxene fractionation of Loihimagmas, which may be a consequence of a low magma-supply rate,or by slightly shallower depths of melt segregation for Kilaueamagmas. KEY WORDS: Loihi volcano; Hawaii; geochemistry; Sr-Nd-Pb isotopes     

Volatiles in pillow rim glasses from Loihi and Kilauea volcanoes,Hawaii

Volatiles and major elements in submarine glasses from Loihi seamount and Kilauea volcano. Hawaii were analyzed by high temperature mass spectrometry and the electron microprobe. Loihi glasses are subdivided into three groups: tholeiitic, transitional and alkali basalts. The glasses are evolved: Mg numbers range from 48–58. The alkalic lavas are the most evolved.Total volatiles range from 0.73 to 1.40 wt.%. H₂O shows a positive linear correlation with K₂O content [H₂O = 0.83 (± .09) K₂O + 0.08 (± .06)]. Concentrations of H₂O are higher in the alkalic lavas, but Cl and F abundances are highly variable. Variations in ratios of incompatible elements (K₂O, P₂O₅, H₂O) indicate that each group was derived from a distinct source. CO₂ contents range from 0.05 to 0.19 wt.% but show no systematic correlation with rock type or Mg #. A well-defined decrease in glass CO₂ content with increasing vesicularity is shown by the alkalic lavas. CO₂ may have been outgassed from the tholeiitic and transitional magmas prior to eruption during storage in a shallow magma chamber. Reduced carbon species (CO and CH₄) were found in small amounts in most of the alkalic samples. Although the redox histories of Hawaiian lavas are poorly known, these new data indicate the presence of a reduced source for Loihi magmas.The Kilauea tholeiitic glasses are evolved (Mg # 48.3 to 55) and have higher H₂O contents (av. 0.54 wt.%) than Loihi tholeiites (av. 0.42 wt.%) at the same Mg # (~55). Cl is distinctly lower in Kilauea glasses (0.01 wt.%) compared to Loihi glasses (0.09 wt.%). The data indicate significant source differences for the two volcanoes, consistent with results of other geochemical studies.Loihi tholeiites have distinctly higher ³He/⁴He ratios than Kilauea tholeiites and are the highest measured in submarine basalts (KURZ et al., 1983). These high ratios have been used to invoke a primitive source for Loihi basalts. The high Cl content of these basalts, the highest we have ever measured in submarine basalts, may be a fingerprint of this primitive source, as previously noted for Icelandic basalts (Schillinget al. 1980).     

Petrology of volcanic rocks from Kaula Island,Hawaii

A compositionally diverse suite of volcanic rocks, including tholeiites, phonolites, basanites and nephelinites, occurs as accidental blocks in the palagonitic tuff of Kaula Island. The Kaula phonolites are the only documented phonolites from the Hawaiian Ridge. Among the accidental blocks, only the phonolites and a plagioclase basanite were amenable to K-Ar age dating. They yielded ages of 4.0–4.2 Ma and 1.8±0.2 Ma, respectively. Crystal fractionation modeling of major and trace element data indicates that the phonolites could be derived from a plagioclase basanite by subtraction of 27% clinopyroxene, 21% plagioclase, 16% anorthoclase, 14% olivine, 4% titanomagnetite and 1% apatite, leaving a 16% derivative liquid. The nephelinites contain the same phenocryst, xenocryst and xenolith assemblages as the tuff. Thus, they are probably comagmatic. The strong chemical similarity of the Kaula nephelinites and basanites to those from the post-erosional stage Honolulu Group on Oahu, the presence of garnet-bearing pyroxenites in the Kaula nephelinites (which previously, had only been reported in the Honolulu volcanic rocks) and the similar age of the Kaula basanite to post-erosional lavas from nearby volcanoes are compelling evidence that the Kaula basanites and nephelinites were formed during a post-erosional stage of volcanism.Now at Occidental Petroleum, Bakersfield, CA, 93309, USA     

The Role of Tonalite and Diorite in Mauna Kea Volcano, Hawaii, Magmatism: Petrology of Summit-Region Leucocratic Xenoliths

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Petrology of Submarine Lavas from Kilauea's Puna Ridge, Hawaii

We have studied 30 quenched tholeiitic lava flows recoveredby 20 dredge hauls and one submersible dive along Puna Ridge,the submarine part of the East Rift Zone of Kilauea Volcano,Hawaii Glass grains from numerous additional flows were recoveredin turbidite sands cored in the Hawaiian Trough. These quenchedlavas document variable primary magma compositions; olivineand multiphase crystallization and fractionation; degassing;wall-rock stoping and assimilation; mixing in the crustal reservoirand the rift zone; entrainment of olivine xenocrysts from ahot, ductile, olivine cumulate body; and disruption of gabbrowallrocks in the rift zone. Glass grains in turbidite sands contain up to 15•0wt% MgO,in contrast to < 7•0wt% MgO for the sampled glass rindson lavas. The most forsteritic olivine phenocryst (F0₉₀₇) isin equilibrium with primary Kilauea liquid containing an average16•5 wt% MgO, but ranging from 13•4 to 18•4%.Lavas and glass grains have more restricted P₂O₅/K₂O and TiO₂/K₂Othan glass inclusions in olivine, because more diverse liquidstrapped as glass inclusions are mixed and homogenized beforeeruption. Variable trace element compositions in glass grainsand whole rocks indicate that the primary liquids form by partialmelting of mantle sources retaining clinopyroxene and garnet. Orthopyroxene xenocrysts formed at moderate pressures provideevidence for a sub-crustal staging zone. Chromite and olivinecrystallize in the crustal magma reservoir as the liquid coolsfrom an average 1346C to 1170C. Low viscosities of the primaryliquids (04 Pas) facilitate olivine settling, and the crystallizedolivine forms an olivine cumulate body at the base of the reservoir.Olivine is deformed as the hot ductile dunite body flows downand away from the summit. This flow drives instability of theHilina landslide on Kilauea. Dikes intrude the dunite, and magmaflowing through the dikes disaggregates and entrains olivinexenocrysts in Puna Ridge magmas. Primary liquids pond at or near the base of Kilauea's crustalreservoir because they are denser than more fractionated liquidsthat occupy the upper parts of the reservoir. The sulfur andwater contents of glass rinds indicate that fractionated liquidsnear the top of the reservoir degas at low pressure, a processthat increases their density and causes them to sink to levelswhere they mix with resident undegassed, near-primary liquid.The fractionated liquids near the top of the magma reservoiracquire excess Cl, owing to assimilation of hydrothermally alteredroofrocks. Magma flowing into the rift zone encounters and mixes with low-temperature,multiphase-fractionated melt. The mixed magmas typically containrare orthopyroxene, plagioclase as sodic as andesine, olivineas fayalitic as F0₇₅ and Fe-rich augite derived from the fractionatedmagma. Magma flowing through dikes also dislodged fragmentsof gabbroic wallrocks that occur as xenoliths. The interrelations in the Kilauean submarine lavas between hostglass and glass inclusion compositions, volatile contents andmineral chemistry reveal an extraordinarily complex sequenceof petrogenetic processes and events that are difficult or impossibleto determine in subaerial Kilauea lavas because of crystallization,reequilibration and degassing during or after their eruption. KEY WORDS: submarine lavas; petrology; Kilauea; Hawaii; magma mixing ^*Corresponding authorPresent address: Rosentiel School of Marine and Atmospheric Science, Division of Marine Geology and Geophysics, University of Miami, 4600 Rickenbacker Causeway, Miami, FL 33149-1098, USA     

Petrology and Thermal History of Highly Deformed Mantle Xenoliths from the Montferrier Basanites, Languedoc, Southern France: A Comparison with Ultramafic Complexes from the North Pyrenean Zone

The Oligocene basanites from Montferrier near Montpellier containvarious ultramafic xenoliths, predominantly spinel-lherzoliteslocally rich in amphibole, and a unique garnet-clinopyroxenite.The sulfide content of spinel-lherzolites is unusually highfor xenoliths and approaches that of MORB mantle sources. Thegarnet-clinopyroxenite, now consisting of recrystallized cpx+ garnet + opx + sp assemblage, results from subsolidus evolutionof a primary subcalcic clinopyroxene (plus minor spinel) compositionallysimilar to those experimentally obtained during high-pressurecrystallization of MORB; such an origin is also attested bythe Ni/Cu ratio of the coexisting sulfide component which isclose to 1. The spinel lherzolites display generally fine-grained porphyroclastictextures with varying proportions of porphyroclasts and neoblasts,and more rarely granuloblastic textures. Within each singlesample, the cores of the pyroxene and spinel porphyroclastshave nearly constant compositions. Significantly inhomogeneouschemical changes appear at the extreme edges of some large crystals,as well as in the fine-grained pyroxenes and spinel, whereasolivine composition is invariable. Such compositional variationsresult from the superimposed effects of two episodes of deformationand recrystallization. Geothermometric determinations show that all the spinel lherzolitesand the garnet-pyroxenite attained an equilibrium state around950?C. From a discussion on the origin of relict opx–cpx–spclusters and relatively high sulfide contents in peridotites,on the reconstructed primary paragenesis of the garnet-pyroxenite,and from a comparison with the North Pyrenean ultramafic associations,it is inferred that this equilibration occurred at the end ofan episode involving partial melting, crystallization, and subsolidusrecrystallization consequent on plastic deformation. Sulfideswere probably retained as sulfide melt early in this episodewhile metasomatism responsible for the crystallization of amphiboleoccurred late. This event is ascribed to a diapiric uplift upto a 45–50 km depth in relation with Oligocene riftingwhich started 35–40 m.y. ago in the Languedoc area. The compositional disequilibrium observed in a number of lherzolitesis related to a second episode of shearing deformation responsiblefor generating porphyroclastic textures, grading locally intomylonites. As a consequence of a concomitant cooling, Al andCr contents decrease in both pyroxenes, and Na content alsodecreases in clinopyroxene. The main chemical change of spinelconsists of a systematic substitution of MgAl₂O₄ by FeCr₂O₄,linked to the development of a porphyroclastic texture, i.e.to the degree of shearing deformations imposed upon the peridotites.The temperature decrease down to 750?C resulted from the ascentof previously equilibrated mantle blocks into colder parts nearthe Moho. This event occurred a few millions years before thecollection of xenoliths by the Montferrier basanites, probablyas the Cevennes fault zone below Oligocene grabens was reactivedat depth.     

Petrology and Geochemistry of Cretaceous Ultramafic Volcanics from Eastern Kamchatka

The origin, evolution and primary melt compositions of lateCretaceous high-K ultramafic volcanics and associated basaltsof Eastern Kamchatka are discussed on the basis of a study ofthe mineralogy and geochemistry of the rocks and magmatic inclusionsin phenocrysts. The exceptionally primitive composition of thephenocryst assemblage [olivine—Fo;_88–95, Cr-spinel—Cr/(Cr + Al) up to 85] provides direct evidence of the mantleorigin of primary melts, which were highly magnesian compositions(MgO 19–24 wt%). The rocks and meltsare characterizedby strong high field strength element (HFSE) depletion in comparisonwith rare earth elements, and high and variable levels of enrichmentin large ion lithophile elements (LILE), P, K and H₂O (0.6–12wt % in picritic to basaltic melts). _Nd values lie in a narrowrange (+107 to +91), typical of N-MORB (mid-ocean ridge basalt),but ⁸⁷Sr/⁸⁶Sr (0.70316–0.70358) is slightly displacedfrom the mantle array. High-K ultramafic melts from Kamchatkaare considered as a new magma type within the island-arc magmaticspectrum; basaltic members of the suite resemble arc shoshonites.The primary melts were produced under high-pressure (30–50kbar) and high-temperature(1500–1700C) conditions bypartial melting of a refractory peridotitic mantle. KEY WORDS: Kamchatka; Late Cretaceous magmatism; ultramafic volcanics; shoshonites ^*Corresponding author. Present address: Department of Geology, University of Tasmania, GPO Box 252C, Hobart, Tas., Australia     

Xenoliths in the Honolulu Volcanic Series, Hawaii

The Pleistocene to Holocene Honolulu Volcanic Series was eruptedfrom about 37 vents scattered over the older Koolau tholeiiteshield. The rocks of this series are compositionally zoned withrespect to the shield; near the Koolau caldera the predominantrocks are melilitenepheline basalts, but these give way outwardto nepheline basalts, and ultimately, at the apron of the shield,to alkalic olivine basalts. The xenoliths in these are likewisezoned: most of those in the caldera area consist of dunite,most of those at intermediate distances of lherzolite, and someof those in the apron of the shield consist of garnet pyroxeniteand peridotite. The zoning of the xenoliths, however, does notcoincide with that of the enclosing rocks. We believe that copiouseruption of Koolau tholeiite produced a lateral and verticalheterogeneity in the mantle beneath Oahu, and that the zoningin both Honolulu lavas and their xenoliths is caused by thatheterogeneity. The textures of the xenoliths indicate that thebasalts were mainly produced by fractional melting rather thanfractional crystallization. There is some evidence that thedunite xenoliths are mantle residua produced during the generationof the tholeiite, and that the Honolulu magmas were generatedat greater depths than the Koolau magmas, probably as a resultof elastic unloading.     

Petrology and Geochemistry of Mantle Peridotite Xenoliths from SE China

Geochemical data on Type I spinel peridotite and garnet peridotitexenoliths in Cenozoic basalts from SE China demonstrate thatthe lithospheric mantle under this region is heterogeneous.The depletion and enrichment shown by these peridotite xenolithsare not related to their locations as suggested earlier. Samplesfrom individual localities, at the continental margin or thecontinental interior, show large variational ranges from depletedharzburgite to fertile Iherzolite. The measured Nd and Sr isotopiccompositions of clinopyroxene separates range from _Nd 49 to160 and from ⁸⁷Sr/⁸⁶Sr 070256 to 070407, respectively. Thedepleted signatures of Sr and Nd isotopic compositions and major-elementcontents (low CaO and Al₂O₃ in most xenoliths require an olddepletion event, probably mid-Proterozftic, and the enrichmentof LREE in the depleted peridotites implies a young metasomaticevent shortly before Cenozoic magmatism. Major-element compositionsof the peridotite xenoliths are controlled largely by the degreeof partial melting, and the extra fertile peridotites (highCaO and Al₂O₃) are probably the products of interaction betweenperidotites and a basaltic component. The equilibrium P–Tconditions, determined from coexisting mineral phases, indicatethat these xenoliths equilibrated over a wide P–T range,from 770 to 1250 C and from 10 to 27 kbar. Calculated oxygenfugacities for most spinel peridotites range from near the FMQbuffer to 25 log units below. The late-stage metasomatism didnot change the redox state in the upper mantle. ^*Corraponding author     

苏鲁造山带中胡家林超镁铁质岩岩石地球化学特征

苏鲁造山带中胡家林超镁铁质岩地块主要由两部分组成:南部滑石山以蛇纹岩和蛇纹石化橄榄岩为主,夹薄层状石榴橄辉岩-(石榴)单斜辉石岩;北部胡家林主要由(石榴)单斜辉石岩组成,夹厚层状蛇纹岩。蛇纹岩-蛇纹石化橄榄岩低Al2O3、低Ca O和高Mg O,REE含量低,但LREE稍富集。石榴橄辉岩和(石榴)单斜辉石岩低Mg O和Ca O,高REE含量高,其稀土配分曲线均表现出单斜辉石单矿物的配分特征。这些超镁铁岩块中不同岩石的微量元素均具有Pb的正异常,弱的Nb、Ta的负异常,显示地壳流体交代信息。Nd、Sr、Pb同位素特征显示存在亏损地幔与地壳之间的混合作用。亲石元素含量最低的蛇纹石化石榴橄辉岩的Sr、Nd、Pb同位素组成受交代流体控制明显,而石榴橄辉岩和(石榴)单斜辉石岩不明显。胡家林和滑石山超镁铁岩中所含的交代地壳成分不同,胡家林样品受到含水熔体和富水流体的双重交代,滑石山样品主要受富水流体的交代。     

Petrology of Lower Crustal and Upper Mantle Xenoliths from the Cima Volcanic Field, California

Basaltic rocks of the Cima volcanic field in the southern Basinand Range province contain abundant gabbro, pyroxenite, andperidotite xenoliths. Composite xenoliths containing two ormore rock types show that upper-mantle spinel peridotite wasenriched by multiple dike intrusions in at least three episodes;the mantle was further enriched by intergranular and shear-zonemelt infiltration in at least two episodes. The oldest dikes,now metamorphosed, are Cr-diopside websterite. Dikes of intermediateage are most abundant at Cima and consist of igneous-texturedwebsterite and two-pyroxene gabbro and microgabbro of tholeiiticor calcalkalic parentage. The youngest dikes are igneous-texturedclinopyroxenite, gabbro, and olivine microgabbro of alkalicparentage. The dikes in peridotite are interpreted as partsof a system of conduits through which tholeiitic (or calcalkalic)and alkalic magmas fed lower-crustal intrusions, which are representedby abundant xenoliths of the same igneous rock types as observedin the dikes. Mineral assemblages of dikes in peridotite indicatethat an enriched uppermost mantle zone no thicker than 15 kmcould have been sampled. Because of their high densities, thegabbros and pyroxenites can occupy the zone immediately abovethe present Moho (modeled on seismic data as 10-13 km thick,with V_p 6.8 km/s) only if their seismic velocities are reducedby the joints, partial melts, and fluid inclusions that occurin them. Alternatively, these xenoliths may have been derivedentirely from beneath the Moho, in which case the Moho is notthe local crust-mantle boundary.     

The Petrology of Grospydite Xenoliths from the Zagadochnaya Kimberlite Pipe in Yakutia

The xenoliths of garnet–clinopyroxene–disthene rocks(grospydites and associated kyanite eclogites) from the Zagadochnayakimberlite pipe in Yakutia have been studied in detail. Contraryto previous data, the presence of a continuous range in thepyrope-grossular series of garnets is shown on the basis ofnumerous X-ray data and 17 chemical analyses of garnets. Thisconclusion is confirmed by the study of separate grains withkyanite intergrowths from the kimberlite heavy fraction, whichare present in the kimberlite as the result of destruction ofgrospydite xenoliths, and possible, of garnet-kyanite rocksalso. A close connection of the calcium content in the garnetwith the sodium content in the coexisting clinopyroxes is alsoshown. An increase in the chemical potential of sodium resultsin the stability of the pryoxene-kyanite assemblage insteadof a garnet of intermediate composition (50 percent of grossular).The interval of the miscibility gap between calcium-rich andcalcium-poor garnets is increased in this way. The data of chemicalanalyses of 14 pyroxenes from the xenoliths indicate that theydiffer in the high aluminium and sodium content from other pyroxenesof eclogite-facies rocks. Chromium-rich bands with a high chromiumcontent in coexisting garnet, pyroxene, and kyantic have beenoccasionally found in the xenoliths. A study has been made ofthe chrome-kyanite with 12.86 per Cr202. The presence of chromium-richminerals in the grospydite xenoliths confrms their connectionwith ultrabasic rocks.     

Petrogenesis of Dunite Xenoliths from Koolau Volcano, Oahu, Hawaii: Implications for Hawaiian Volcanism

Ultramafic xenoliths from Koolau Volcano on the island of Oahu,Hawaii, are divided into spinel lherzolite, pyroxenite, anddunite suites. On the basis of a study of the petrography andmineral compositions of 43 spinel lherzolites, 12 pyroxenites,and 20 dunites, the following characteristics of the dunitesin relation to the other nodule types and to Hawaiian lavasemerge. (1) The forstente content of olivines in the Koolaudunites (Fo_82.6-Fo_{89 7} ) overlap those of Hawaiian tholeiiticand alkalic lavas and are generally lower than those in abyssallherzolites and dunites and in Koolau spinel lherzolites. (2)Most of the dunites contain no orthopyroxene, all except twocontain chrome spinel, and a few contain interstitial plagioclaseand clinopyroxene. (3) Chrome spinels from the Koolau dunitesare distinctly higher in Cr/(Cr+Al), lower in Mg/(Mg+ Fe²⁺)and higher in TiO₂ than those from abyssal basalts and peridotites.Chrome spinels in the dunites correspond closely in compositionto chrome spinels in Hawaiian tholeiitic and alkalic lavas.(4) The abundance of dunite relative to other nodule types decreasesoutward from the central part of the volcano. The dunites areinterpreted not as residues of partial fusion of the mantlebut as crystal accumulations stored at shallow depths beneaththe central part of Koolau Volcano and derived from picriticmagmas parental to the shield-building tholeiitic lavas.     

Petrology and Mineral Chemistry of Plutonic Igneous Xenoliths from the Carbonatite Volcano, Oldoinyo Lengai, Tanzania

Magmatic plutonic rocks in the Oldoinyo Lengai pyroclasticsare jacupirangite, pyroxenite, ijolite, nepheline syenite andwollastonitite. Mainly cumulates, they are combinations of nepheline,clinopyroxene, Ti-andradite, spinel (sensu lato), apatite, perovskite,titanite, wollastonite, sulphides, mica, glass and K-rich feldspar,most of which are strongly zoned. Low analytical sums for glasses,vesiculation of intergranular glasses, and the generally explosivenature of the volcanicity point to significant concentrationof dissolved volatiles in the parent magma; the absence of hydrousphases suggests that the dominant volatile is CO₂. Cumulatetextures, widely variable modes, veining and variation in specimenconsolidation and metasomatism all indicate derivation froma structurally complicated and multiply injected sub-volcaniccomplex. Complex zoning of phases and mineral disequilibrium is attributedto convective percolation of fluids through permeable cumulates,possibly complicated by magma replenishment during crystallizationof individual magma batches. Olivine, mica and pyroxene megacrystsin some ijolites indicate polybaric crystallization; mixingof potassic and sodic magmas may be the cause of these megacryst-bearingijolites, but the main parent is highly evolved, of carbonatedijolitic (nephelinitic) composition and with Nd and Sr isotopecharacteristics slightly more depleted than Bulk Earth. KEY WORDS: xenoliths; ijolite; jacupirangite; nepheline syenite ^*Corresponding author. Telephone 031 650 4837. Fax: 031 668 3184. e-mail: jbdawson{at}glg.ed.ac.uk