Chlorine in tertiary basalts from the Hessian Depression in NW Germany

The chlorine concentration has been determined by a chemical method in 7 quartz tholeiites, 19 alkali olivine basalts, 9 basanitic alkali olivine basalts and 11 olivine nephelinites to be on average 80, 280, 720, and 400 ppm Cl respectively. If these basalts are products of decreasing degrees of partial melting of mantle rocks a regular increase of chlorine is to be expected in this sequence. The actual chlorine abundances are a function of partial losses of gases during rock consolidation and optimum stabilities of sodalite group minerals as major chlorine traps in alkalic basalts. The occurrence of sodalite and sodalite nosean solid solutions has been detected by microprobe in 7 out of 10 alkalic basalt species in grains smaller than 70 m. Quantitative analyses of 4 sodalite group minerals from the olivine nephelinites are listed. One contains the sodalite and the nosean molecule in a proportion one to one and must be formed above 1,050 °C according to the experimental results of Tomisaka and Eugster (1968) in the respective system. In the majority of the samples apatite contains less than 20% of the total chlorine of the basalts. Biotite as chlorine containing phase (about 900 ppm Cl) is rare. The proportion of chlorine which could be extracted from rock powders by boiling water is small. No general correlation between the element pairs Cl/S and Cl/K could be observed. Excluding tholeiites a tendency of a reversed correlation between chlorine and potential primary water (as indicated by the Fe₂O₃/FeO ratio) and between chlorine and silica can be derived.     

Gough Island: Evaluation of a fractional crystallization model

Gough Island is composed of an alkaline olivine basalt-trachyte series. A fractional crystallization model for the development of these rocks has been evaluated by correlating the geochemical trends of major and trace elements. Beginning with an alkali olivine basalt parent the major element abundances were used to determine the varying proportions of crystallizing minerals required to generate the various residual liquids. A least-squares computer model was used for this calculation. The modal proportions of cumulative minerals and trace element distribution coefficients were used to predict the trace element abundances in each rock type.Three significant trace element trends are observed in Gough Island rocks: (1) increasing rare earth (RE) abundance and relative light RE enrichment with increasing major element differentiation, (2) marked Eu, Sr, and Ba depletions in late stage trachytes, (3) Cr and M enrichment in picrite basalt.The trace element abundances predicted by the fractional crystallization model are in good agreement with these observed trends. A fractional crystallization process involving olivine, pyroxene, feldspar, and apatite accounts for all the significant major and trace element trends observed in Gough Island rocks.     

Tholeiitic and alkali basalts from the Mid-Atlantic Ridge at 43 ° N

Tholeiitic basalts dredged from the Mid-Atlantic Ridge (MAR) axis at 43 ° N are enriched in incompatible trace elements compared to the ‘ normal’ incompatible element depleted tholeiites found from 49 ° N to 59 ° N and south of 33 ° N on the MAR. The most primitive 43 ° N glasses have MgO/FeO*= 1.2 and coexist with olivine (Fo_90–91) and chrome-rich spinel. The tholeiitic basalts from the MAR 43 ° N are distinct from the strongly incompatible trace element depleted tholeiities found elsewhere in the Atlantic, and have trace element features typical of island tholeiities and MAR axis tholeiites from 45 ° N. Petrographic, major, and compatible trace element trends of the axial valley tholeiites at 43 ° N are consistent with shallow-level fractionation; in particular, evolution from primitive liquids with forsteritic olivine plus chrome spinel as liquidus phases to fractionated liquids with plagioclase plus clinopyroxene as major crystallizing phases. However, each dredge haul has distinctive incompatible trace element abundances. These trace element characteristics require a hetrogeneous mantle or complex processes such as open system fractional crystallization and magma mixing. Alkali basalts (～5% normative nepheline) were dredged from a prominent fracture zone at 43 ° N. Typical of alkali basalts they are strongly enriched (compared to tholeiites) in incompatible elements. Their highly fractionated rare-earth element (REE) abundances require residual garnet during partial melting. The 43 ° N tholeiites and alkali basalts could be derived from a garnet peridotite source with REE contents equal to 2 × chondrites by ～5% and 1% melting, respectively. Alternatively, they could be derived from a moderately light REE enriched source by ～25% and 9.5% melting, respectively.     

Die Verteilung der Lanthaniden in basaltischen Gesteinen

Several types of basaltic and related rocks from NW Germany have been analysed for 14 lanthanides and yttrium. Alkali olivine basalts (13 samples) are the most common products of the late Tertiary volcanism in Northern Hessia and Lower Saxony. One basalt intermediate in composition between alkali olivine basalts and tholeiitic basalts has been investigated (intermediate basalt) beside 3 samples of the tholeiitic type. Several rare effusive rock species occur in this area. The number of samples is indicated in brackets: nepheline basanite (1), olivine nephelinites (5), peridotite inclusions (2) from one of the above mentioned alkali olivine basalts. Trachytes (3) and phonolite (1) from the Westerwald area, also Tertiary in age. Three nepheline leucite tephrites from the Eifel area, Pleistocene in age, and pyroxenes from the Recent Stromboli (Italy) have been included in this investigation.The lanthanides and yttrium are analysed after chemical preconcentration controlled by the use of spikes. La, Ce, Pr, Nd, Sm, Gd, Dy, Er, Yb have been determined with good accuracy and precision by X-ray fluorescence, Eu, Tb, Ho, Tm and Lu by optical emission spectrography.Following earlier suggestions the distribution pattern of the lanthanides in basalts has been compared with that of chondrites. Tholeiitic basalts of the area under investigation show only a slight deviation from the relative distribution of the lanthanides in chondrites. The latter contain a twentieth of the absolute concentration in tholeiitic basalts. All other effusive rocks of this volcanic province have higher Y and La-Lu abundances and increasing ratios of La-Eu/Y, Gd-Lu (in brackets) in the following sequence: intermediate basalt (3.7); alkali olivine basalts (7.6); nepheline leucite tephrites (8.8); nepheline basanite (9.1); olivine nephelinites (10.2); phonolite (11.1); trachytes (11.6). The highest concentration of yttrium and of the lanthanides is observed in olivine nephelinites (up to 860 ppm Y, La-Lu).The observed increase in absolute concentration of the lanthanides and in relative accumulation of the light lanthanide elements from chondrites to tholeiitic basalts, to intermediate basalt, to alkali olivine basalts and to nepheline basanite makes a genetic interrelation in this sequence of rock types probable. Chondrites resemble garnet peridotites as potential main constituents of the upper earth's mantle. The pattern of the distribution of the lanthanides confirms a hypothesis that some tholeiitic basalts represent the most primitive of all basaltic magmas. Several models on the origin of both tholeiitic and alkali olivine basalts from potential source rocks or melts in the mantle have been checked with the data on the abundances of the lanthanides. There is still a lack of information on rare earths distribution in abundant rock forming minerals to completely exclude crystal fractionation under different pressures in the mantle as the origin of the different tholeiitic and alkali olivine basalt magmas. Alkali and gas accumulation (including the light lanthanides) in the upper parts of deep seated magma reservoirs should be considered as a potential source of the different alkali basalts. This is a process which has been observed by Richter and Moore (1966) in Hawaiian lava pools.The concentration of all and accumulation of the light lanthanides in the olivine nephelinites of our area is much too high to be explained by assuming an assimilation of sedimentary carbonate rocks in alkali olivine basalt melts.     

The Cenozoic Basaltic Rocks of Eastern China: Petrology and Chemical Composition

The Cenozoic volcanicity of eastern China is entirely basalticand occurred as relatively small eruptions widely dispersedin space and time, closely associated with graben basins andtheir regional bounding faults. Samples (157) from over 30 sitesin eastern China have been studied. They are predominantly alkalinebasalts, but vary in composition from olivine nephelinites andleucitites to quartz tholeiites. The majority are aphyric butsome contain olivine and clinopyroxene phenocrysts. Whole-rockanalyses (X-ray fluorescence) of all samples for the major and13 trace elements are used, as are the compositions of all themajor mineral phases determined by electron microprobe. It is argued that the most primitive basanites, alkali olivinebasalts, and olivine tholeiites represent primary or near-primarymagmas which were formed by different degrees of partial meltingof the upper mantle at different depths. The olivine tholeiitesrepresent larger degrees of partial melting (8–9%) ofa spinel peridotite at depths of <66 km. The alkalic basaltscarry xenoliths of spinel and garnet peridotite and appear tohave been derived by 1–7% partial melting of a garnetlherzolite (50% ol, 25% opx, 15% cpx, 10% garnet) at depths> 79 km. The olivine nephelinite may have formed by evensmaller degrees of partial melting. Most flows are not primary; the variations in their compositionsare consistent with fractional crystallization from the spectrumof primary parents created by varying degrees of partial meltingof a mineralogically heterogeneous source. The tholeiites havefractionated by the removal of clinopyroxene and some olivine;the alkali basalts by the removal of clinopyroxene with a smallerproportion of olivine. The incompatible behavior of Sr impliesthe absence of plagioclase from any of the fractionating assemblagesand, together with the high Al content of the pyroxene phenocrysts,suggests that much of the fractionation occurred at mantle depthsand pressures. The Cenozoic magmatism of eastern China is seen as a typicalexample of volcanism associated with continental extension.That is, small volumes of predominantly alkalic basalts andolivine tholeiites erupted over a prolonged period and associatedwith extensional basins and their bounding faults. As such,the province is distinct from continental flood basalt provinces.     

Basic lavas of the Archean La Grande Greenstone belt: Products of polybaric fractionation and crustal contamination

Despite the fact that some greenstone belts preserve the record of contemporaneous komatiitic and tholeiitic volcanism, a genetic link between the two is not widely accepted. The significance of a compositional gap seperating these magma types and differences in their respective degree of light rare earth element (LREE) enrichment, cited as evidence against a derivative relationship, are complicated by the possibility of crustal assimilation by magmas of komatiitic affinity. In the Archean La Grande Greenstone belt of northern Quebec a succession of metamorphosed tholeiitic basalts and younger, high-Mg, LREE-enriched andesites are preserved. The tholeiites are differentiated basaltic rocks whose chemical compositions appear to have been controlled by low pressure, gabbroic fractional crystallization and are similar to Type 1 MORB. Parental magmas were probably high-Mg liquids of compositions similar to komatiitic basalts which also occur in the greenstone belt. These high-Mg liquids are believed to be themselves the product of high pressure, OLIV+OPX fractional crystallization of more magnesian primary liquids of komatiitic composition. The higher La/Sm ratios of komatiitic basalts and tholeiites relative to komatiites in this belt, can be explained by small degrees of crustal assimilation. In the central part of the belt, late-stage, mafic igneous rocks have chemical compositions similar to Archean examples of contaminated volcanic rocks (e.g., Kambalda, Australia). These late-stage lavas consist of basalts and andesites with high-Mg, Ni and Cr abundances, LREE-enriched profiles and low Ti abundances. They are believed to be the products of crustal assimilation and crystallization of OPX-PLAG-CPX from high-Mg liquids of komatiitic affinity. The volcanic stratigraphy records the progressive effects of crustal contamination through time. A light sialic crust may have initially acted as a density barrier, preventing the eruption of primary high-Mg liquids and forcing fractionation at depth which produced more buoyant compositions. With subsequent thinning of the crust, the density barrier presumably failed, and primary liquids migrated directly toward the surface. Reaction of these liquids with tonalitic crust produced contaminated differentiates.     

Petrological and trace element geochemical features of the Okete Volcanics,western North Island,New Zealand

The Okete Volcanics form small volume monogenetic volcanoes situated around the flanks of larger tholeiitic cones of the Plio-Pleistocene Alexandra Volcanics, in the back-arc tectonic environment of western North Island, New Zealand. The lavas and tuffs of the Okete Volcanics have compositions which include basanites, alkali olivine basalts, olivine tholeiites, and hawaiites. Most rocks have Mg numbers >66, >250 p.p.m. Ni, >500 p.p.m. Cr, and often contain ultramafic xenoliths, which indicate that they are very close to being primary magmas. The Okete Volcanics show geochemical trends, from basanite to hawaiite, of progressive depletion of both compatible and incompatible trace elements, progressive increase in Al₂O₃, and heavy REE and Y enrichment with crossingover REE patterns in the hawaiites. These geochemical trends can be accounted for by varying degrees of partial melting of a light REE enriched garnet peridotite with subsequent modification of the melts near source or during ascent by fractional crystallization of olivine and minor clinopyroxene. Mass balance calculations cannot quantitatively constrain the degree of partial melting or fractional crystallization, but nevertheless indicate that the Okete alkali olivine basalts, olivine tholeiites, and hawaiites have been derived by successively larger degrees of partial melting relative to basanites, and have also been progressively more modified by fractional crystallization than have the basanites. Sources of the alkalic melts lay at depths corresponding to >20 kb, and most of the ultramafic xenoliths, apart from some which may be cognate cumulates, are unrelated to the magmas that brought them to the surface. Magmas have changed in composition with time from older smaller-volume volcanoes of basanite or alkali olivine basalt compositions, to younger and more voluminous volcanoes which contain hawaiites. The geochemical trends shown by the Okete Volcanics and their spatial association with voluminous tholeiitic volcanism, are features which are different from those observed elsewhere in the Pliocene to Recent basaltic fields of northern North Island, and may be related to their unique tectonic setting, situated in a distinct structural domain.     

Geochronology and geochemistry of late Cenozoic basalts from the Leiqiong area,southern China

The Leiqiong area, which includes the Leizhou Peninsula and the northern part of the Hainan Island, is the largest province of exposed basalts in southern China. Ar–Ar and K–Ar dating indicates that incipient volcanism in the Leiqiong area may have taken place in late Oligocene time and gradually increased in tempo toward the Miocene and Pliocene Epoch. Volcanic activities were most extensive during Pleistocene, and declined and ended in Holocene. Based on radiometric age dating and geographic distribution, Pliocene and Quaternary volcanism in Hainan Island can be grouped into two stages and six eruptive regions. The early volcanism is dominated by flood type fissure eruption of quartz tholeiites and olivine tholeiites whereas the later phase is dominated by central type eruption of alkali olivine basalts and olivine tholeiites. The systematic decrease of MgO, ΣFeO and TiO₂ with increasing SiO₂ content for basalts from Hainan Island indicates that fractional crystallization of olivine, clinopyroxene and Ti-bearing opaques may have occurred during magmatic evolution. From coexisting Fe–Ti oxide minerals, it is estimated that the equilibrium temperatures range from 895–986°C and oxygen fugacities range from 10^−13.4 to 10^−10.7 atmospheres in the basaltic magmas. The incompatible element ratios and the chondrite-normalized REE patterns of basalts from the Leiqiong area are generally similar to OIB. The Nb/U ratios (less than 37) in most of the tholeiitic rocks and the negative Nb anomaly observed in the spidergram of some basalts indicated that the influence of a paleo-subduction zone derived component can not be excluded in considering the genesis of the basalts from the Leiqiong area. The tholeiites in the Leiqiong area may have mixed with a more enriched lithospheric mantle component as well as undergone relatively larger percentages of partial melting than the alkali basalts.     

Fluorine in basalts from Iceland

The fluorine content of Icelandic tholeiitic and alkaline basalts matches values found in similar rocks from other areas. Covariation between fluorine and incompatible minor elements such as potassium or phosphorus is found in evolved tholeiites and alkali basalts. Lack of such covariation in primitive olivine tholeiites indicates that fluorine and other incompatible minor and trace elements are not controlled by minerals such as amphibole, mica or apatite in the mantle residue, and that the covariation between these elements in the evolved basalts cannot be inherited from the mantle. Model calculations on rocks from the Langjökull area show that olivine tholeiite suites are, if derived by simple fractional crystallization, enriched in incompatible elements much in excess of the increase due to crystal removal. These observations are taken to indicate that the well documented covariation between fluorine and other incompatible elements is not established until evolution of the basaltic magma has started in crustal holding chambers. The constancy of element ratios and enrichment in excess of what can be accounted for by crystal fractionation or incremental addition of new batches of primitive magmas does indicate (1) mineral control involving amphibole, mica or apatite and (2) addition of fluorine, potassium and phosphorous from an external source. It is argued that this source is the crustal envelope of the holding chamber.     

Petrology of the Western Reykjanes Peninsula, Iceland

The active tholeiitic volcanic zone of the Reykjanes Peninsulaconsists of five volcanic fissure swarms, the two westernmostof which are the subject of this petrological study. The recent(less than 12,000 years) extrusives of the swarms group morphologicallyand petrographically into small picrite basalt lava shields,large olivine tholeiite lava shields and tholeiite fissure lavas;formed in that chronological succession. The picrite basalts exhibit a primitive mineralogy with chromite,olivine (Fo 89) and plagioclase (An 90) as phenocrysts and mayrepresent a primary liquid from the mantle. Simultaneous crystallizationof olivine, plagioclase and augite to form glomerocrysts inthe fissure lavas indicate low pressure cotectic crystallizationconditions. Twenty-eight new major element chemical analyses of the lavasare presented. They are generally characterized by a low contentof alkalies and high CaO. The lavas constitute two main suites,a lava shield suite and a fissure lava suite. There is a positivecorrelation between the volume of individual lavas and the contentof incompatible elements of the lavas within each group. Likewisethere is an overall chemical trend through time demonstrated,for example, by a rise in K₂O from about 0.02 per cent to 0.24per cent during the last, approximately, 12,000 years. There is an apparent chemical zoning within each volcanic swarmsuch that the most evolved and youngest lavas are found in thecentral axial area of the swarm. This central area is also characterizedby graben subsidence, high magnetic anomalies and high temperaturethermal areas, all indicative of shallow magma reservoir(s).In spite of indications of fractional crystallization in theevolution of the olivine tholeiites and tholeiites, some otherprocesses must be sought to explain the volume chemistry relations.Cyclic volcanic activity is tentatively suggested to explainthe observed regular temporal variations within the swarm, eachcycle starting with the formation of picrite basalts.     

Quantitative Untersuchungen mit der Elektronen-Mikrosonde an Pyroxenen aus Basalten und Peridotit-Einschlüssen

The chemical composition of the pyroxenes and olivines of 12 basaltic rocks and 5 lherzolite nodules was determined quantitatively by electron micro-probe analysis. The composition of the pyroxenes depends on the type of basalt in which they occur. Tholeiitic basalts with normative quartz contain three pyroxenes: orthorombic pyroxenes, pigeonites and augites. All pyroxene phases are zoned and do not show any exsolution. Their Ti and Al contents (Ca-Tschermaks and Ti-augite molecules) are small. All pyroxene phases were formed under disequilibrium with each other and with the melt because of rapid quenching. The sequence of crystallization: orthopyroxene—pigeonite—augite could be established by their Cr content.The alkali olivine basalts undersatured in SiO₂ and the olivine nephelinites are characterized by Ti and Al-rich clinopyroxenes. The distribution of Ti and Al in the pyroxenes of the alkali olivine basalts shows a differentiation trend from the cores of the phenocrysts to their outer zones and to the crystals of the ground mass. Thereby the Ca-Tschermaks molecule is being replaced more and more by the Ti-augite molecule. The Ti content of the pyroxenes of the olivine nephelinites decreases in the last stage of differentiation because simultaneously increasing amounts of titaniferous magnetite crystallize.The pyroxenes of lherzolite peridotite nodules are characterized by high Al and low Ti contents which differ according to the type of basalt (alkali olivine basalt or olivine nephelinite) in which the nodules occur. The homogeneous distribution of the elements within the single grains indicates crystallization under equlibrium conditions. The conditions of their formation are comparable to those of Al-pyroxene peridotites in the upper mantle. The composition of pyroxenes of early accumulates of alkali basaltic melts differ from those of peridotite nodules. Therefore lherzolite nodules can be taken as residues of deeper peridotite masses.     

峨眉山玄武岩微量元素地球化学的初步研究

峨眉山玄武岩的微量元素丰度显示区域性差异,西岩区(盐源—丽江拗陷)玄武岩总体上比中、东岩区(康滇隆起和滇黔拗陷)玄武岩富相容元素而贫不相容元素,主要是因为它们的母岩浆经历的结晶分离的程度不同。比较演化的石英拉斑玄武岩、安山玄武岩往往富放射成因锶,反映其成因还涉及一定程度的地壳混染。相对主体拉斑玄武岩,东川碱性火山岩贫REE特别是LREE,推测源区发生过先期熔融事件。主要元素和REE的模拟计算表明,二滩粗面岩可以由玄武质母岩浆经分离结晶衍生,最可能的主要分离相是该区似层状辉长岩的矿物组合。     

Deep differentiation of alkali ultramafic magmas: Formation of carbonatite melts

The study of melt microinclusions in olivine megacrysts from meimechites and alkali picrites of the Maimecha–Kotui alkali ultramafic and carbonatite province (Polar Siberia) revealed that the melt compositions corrected for loss of olivine due to post-entrapment crystallization of olivine on inclusion walls (differentiates of primary meimechite magma) match well to the composition of nephelinites and olivine melilitites belonging to carbonatite magmatic series. Modeling of fractional crystallization of meimechite magmas results in the high-alkali melt compositions corresponding to the silicate–carbonate liquid immiscibility field. The appearance of volatile-rich melts at the base of magma-generating plume systems at early stages of partial melting can be explained by extraction of incompatible elements including volatiles, by near-solidus melts at low degrees of partial melting, and meimechites are an example of such magmas. Subsequent accumulation of CO₂ in the residual melt results in generation of carbonate magma.     

REE compositional characteristics of Hannuoba basalts

The Hannuoba basalts are a suite of associated rocks consisting of alkali basalts and tholeiites. The alkali basalts can be divided into the K-rich type and the Na-rich type, while the tholeiites are almost solely of the Na-rich type. These two types of basalt are characterized as being obviously rich in light rare earth elements, as is shown in the chondrite-normalized REE patterns exhibiting positive Eu anomalies. However, significant differences in (La / Yb)_N and Eu / Eu* ratios are noticed between these two types of basalt. They seem to have been derived from varying-degree partial melting of mantle rocks.     

Geochemistry and Petrogenesis of Three Series of Cenozoic Basalts from Southeastern China

Cenozoic basaltic volcanism in southeastern China was related to the lithospheric extension and asthenospheric upwelling at the eastern Eurasian continental margin. The cenozoic basaltic rocks from this region can be grouped into three different series: tholeiitic basalts, alkali basalts, and picritic-nephelinitic basalts. Each basalt series has distinctive geochemical features and is not derived from a common source rock by different degrees of partial melting or from a common parental magma by fractional crystallization. The mineralogy, petrography, and major and trace-element geochemistry of the tholeiites are similar to oceanic island basalts, implying that the mantle source for these Chinese continental tholeiites was similar to that of the oceanic island basalts—an asthenospheric mantle. The alkali basalts and picritic-nephelinitic basalts are enriched in incompatible trace elements, and their geochemical features can be interpreted as a result of partial melting of an enriched lithospheric mantle, or the mixing products of an asthenospheric magma with a component derived from an enriched lithospheric mantle through thermal erosion at the base of the lithosphere. But the lack of a transitional rock type and continuous variational trends among these basalts suggests that the mixing between asthenospheric magmas and lithospheric magmas probably was not significant in the petrogenesis of the basalts from SE China. Low-degree partial melting of enriched lithospheric mantle alone can account for the observed geochemical data from these basalts.     

The Petrology of the Tholeiites through Melilite Nephelinites on Gran Canaria, Canary Islands: Crystal Fractionation, Accumulation, and Depths of Melting

We report major and trace element X-ray fluorescence (XRF) datafor mafic volcanics covering the 15-Ma evolution of Gran Canaria,Canary Islands. The Miocene (12–15 Ma) and Pliocene-Quaternary(0–6 Ma) mafic volcanics on Gran Canaria include picrites,tholeiites, alkali basalts, basanites, nephelinites, and melilitenephelinites. Olivineclinopyroxene are the major fractionatingor accumulating phases in the basalts. Plagioclase, Fe–Tioxide, and apatite fractionation or accumulation may play aminor role in the derivation of the most evolved mafic volcanics.The crystallization of clinopyroxene after olivine and the absenceof phenocrystic plagioclase in the Miocene tholeiites and inthe Pliocene and Quaternary alkali basalts and basanites withMgO>6 suggests that fractionation occurred at moderate pressure,probably within the upper mantle. The presence of plagioclasephenocrysts and chemical evidence for plagioclase fractionationin the Miocene basalts with MgO<6 and in the Pliocene tholeiitesis consistent with cooling and fractionation at shallow depth,probably during storage in lower-crustal reservoirs. Magma generationat pressures in excess of 3•0–3•5 GPa is suggestedby (a) the inferred presence of residual garnet and phlogopiteand (b) comparison of FeO¹ cation mole percentages and the CIPWnormative compositions of the mafic volcanics with results fromhigh-pressure melting experiments. The Gran Canaria mafic magmaswere probably formed by decompression melting in an upwellingcolumn of asthenospheric material, which encountered a mechanicalboundary layer at {small tilde}100-km depth.     

Rare earth element concentrations in a suite of basanitoids and alkali olivine basalts from Grenada,Lesser Antilles

A suite of basanitoids and alkali olivine basalts from Grenada, Lesser Antilles were analyzed for rare earth elements. The REE concentrations of these rocks are characterized by a small variation in the heavy REE (7 to 9 times chondrite) and a large variation in the light REE (17 to 93 times chondrite). Among the possible mechanisms to account for the REE variations, fractional crystallization processes at low and high pressures, and partial melting processes (both batch melting and fractional melting) were examined, using the partition relationships of REE among silicate minerals and melts. It is suggested that the observed REE variations are best explained by variable degrees of batch partial melting, in which garnet is present as one of the solid phases through 2 to 17% melting of a garnet lherzolite parent rock.     

Petrologic variations in quaternary volcanic rocks,British Columbia,and the nature of the underlying upper mantle

Volcanic activity has produced Late Tertiary and Quaternary cinder cones and flows between the Snake River Plain, U.S.A. and the Yukon Territory, Canada. The rock types include basanites, alkali olivine basalts, high-iron basalts, hawaiites, ankaramites, nephelinites, and olivine tholeiites. The alkali olivine basalts, basanites and hawaiites sampled are chemically similar to rocks from the mid-Atlantic islands. Associated with the volcanic rocks are xenoliths of ultramafic rocks, gabbros, granites and granulites.Seismic data indicate that the Moho throughout the region dips eastward at a very shallow angle. The low velocity zone has been located beneath southern British Columbia and displays a topographic high trending northwest-southeast. The nephelinite was erupted from near the crest of this high with less undersaturated lavas erupted from along its flanks.The suite of ultramafic xenoliths spans a greater variety of rock types than can be generated by maximum amounts of partial melting of a uniform source material to produce the lavas in the region. Calculated residual olivine compositions in equilibrium with the magmas at low velocity zone depths and liquidii temperatures are more iron-rich than the typical lherzolite xenolith olivine. This suggests that the residua from the partial melting episodes which produced the volcanic rocks are different from the upper mantle lid above the low velocity zone as represented by the ultramafic xenoliths.     

Chemistry and distribution pattern of recent basaltic rocks in Iceland

A survey of Recent basaltic rocks in Iceland is presented. The basalts are classified into three groups: tholeiites, transitional alkali basalts and alkali olivine basalts. The basalts can be divided into petrological regions where the composition of lavas seem to have been fairly constant throughout postglacial and possibly late-Pleistocene time. The tholeiites delineate the crest region of the Mid-Atlantic Ridge as it transects Iceland, and the mildly alkali olivine basalts and the transitional alkali basalts characterize the flank volcanic zones. Tholeiitic and alkalic diffrentiated rocks appear to have a distribution in accordance with the basalt distribution pattern. There is some correlation between the chemistry of the zones and the crustal structure of Iceland. Areal discharge of volcanic rocks varies consistently between the petrological regions being highest in the tholeiite regions. The total output of volcanic rocks along the Mid-Atlantic Ridge in the Iceland area reaches maximum in middle Iceland.     

The origin of basaltic rocks in Niutoushan area— high pressure differentiation

The Niutoushan basaltic cone, consisting of subalkali (quartz-tholeiite and olivine-tholeiite) and alkali basalts, is Late Tertiary in age. Its major characteristics are generalized as follows:

1. Both early subalkali and late alkali bali basalts are formed under the same geological environment.
2. The continuity in chemical composition from subalkali to alkali and the low FeO/MgO in alkali basalts show that they are the products of cognate magmatic differentiation.
3. The change from low REE abundance and weak enrichment of LREE in subalkali to high REE abundance and strong enrichment of LREE in alkali basalts indicates obvious REE enrichment and fractionation during magmatic differentiation. Weak positive Eu anomalies in the REE patterns are indicative of their formation under low oxygen fugacity conditions.
4. According to the calculated values, 70–75% of the primary olivine tholeiitic magma had been separated as subalkaline basaltic magma, the rest residual magma became alkaline basaltic magma. This result is consistent to the field observation that the outcrop area of subalkali basalts is four times as much as that of alkali basalts.
5. The basaltic rocks of Niutoushan show an S-type distribution straddling the thermal barrier on Ol′-Ne′-Qu′ diagram and an evolution tendency for Ne to increase with increasing FeO/MgO. This is in agreement with the melting experimental data on olivine basalts at 10–20 kb.
6. Mantle-derived inclusions (spinel lherzolite) in this area occur in both alkali olivine basalts and olivine tholeiites. The latter is of extremely rare occurrence. The formation temperature and pressure of the inclusions in alkalibasalts and olivine tholeiites have been calculated. The results show that the alkaline basaltic magma was separated from the subalkaline basaltic magma at about 20 kb.

Basaltic rocks in Niutoushan were formed through the so-called “high pressure differentiation”, that is, at about 20 kb the crystallization of clinopyroxene and orthpyroxene resulted in the separation of subalkaline basaltic magma from the primary olivine tholeiitic magma, and then the residue gradually became alkaline olivine basaltic magma.