Petrogenesis of Cenozoic Basalts from Mongolia: Evidence for the Role of Asthenospheric versus Metasomatized Lithospheric Mantle Sources

Diffuse Cenozoic volcanism in Mongolia forms part of a widespreadtectono-magmatic province that extends from NE China to LakeBaikal, Siberia. Mafic lavas from the Gobi Altai, southern Mongolia(     

The Role of Continental Crust and Lithospheric Mantle in the Genesis of Cameroon Volcanic Line Lavas: Constraints from Isotopic Variations in Lavas and Megacrysts from the Biu and Jos Plateaux

We present a combined Sr, Nd, Pb and Os isotope study of lavasand associated genetically related megacrysts from the Biu andJos Plateaux, northern Cameroon Volcanic Line (CVL). Comparisonof lavas and megacrysts allows us to distinguish between twocontamination paths of the primary magmas. The first is characterizedby both increasing ²⁰⁶Pb/²⁰⁴Pb (19·82–20·33)and ⁸⁷Sr/⁸⁶Sr (0·70290–0·70310), and decreasing_Nd (7·0–6·0), and involves addition of anenriched sub-continental lithospheric mantle-derived melt. Thesecond contamination path is characterized by decreasing ²⁰⁶Pb/²⁰⁴Pb(19·82–19·03), but also increasing ⁸⁷Sr/⁸⁶Sr(0·70290–0·70359), increasing ¹⁸⁷Os/¹⁸⁸Os(0·130–0·245) and decreasing _Nd (7·0–4·6),and involves addition of up to 8% bulk continental crust. Isotopicsystematics of some lavas from the oceanic sector of the CVLalso imply the involvement of a continental crustal component.Assuming that the line as a whole shares a common source, wepropose that the continental signature seen in the oceanic sectorof the CVL is caused by shallow contamination, either by continent-derivedsediments or by rafted crustal blocks that became trapped inthe oceanic lithosphere during continental breakup in the Mesozoic. KEY WORDS: crustal contamination; CVL; megacrysts; ocean floor; osmium isotopes     

Lithospheric Mantle Evolution during Continental Break-Up: The West Iberia Non-Volcanic Passive Margin

Ultramafic (lherzolites, metasomatized peridotites, harzburgites,websterites and clinopyroxenites) and mafic igneous (basalts,dolerites, diorites and gabbros) rocks exposed at the sea-flooralong the West Iberia continental margin represent a rare opportunityto study the transition zone between continental and oceaniclithosphere. The igneous rocks are enriched in LREE, unlikeNorth Atlantic MORB. A correlation between their ¹⁴³Nd/¹⁴⁴Ndisotopic composition and Ce/Yb ratio suggests that they originatefrom mixing between partial melts of a depleted mantle sourcesimilar to DMM and of an enriched mantle source which may residewithin the continental lithosphere. Clinopyroxenes and amphibolesin the ultramafic rocks are LREE depleted and have flat HREEpatterns with concentrations higher than those of abyssal peridotites.Clinopyroxenes in the harzburgites are less LREE depleted buthave lower HREE concentrations. The clinopyroxenes in the GaliciaBank (GB) lherzolites have radiogenic Nd (¹⁴³Nd/¹⁴⁴Nd rangingfrom 0·512937 to 0·513402) and unradiogenic Sr(⁸⁷Sr/⁸⁶Sr ranging from 0·702100 to 0·702311)isotopic ratios similar to, or higher than, DMM (Depleted MORBMantle) whereas the clinopyroxenes in the Iberia Abyssal Plainwebsterites have low-Nd isotopic compositions (¹⁴³Nd/¹⁴⁴Nd rangingfrom 0·512283 to 0·512553) with high-Sr isotopicratios (⁸⁷Sr/⁸⁶Sr ranging from 0·704170 to 0·705919).Amphiboles in Galicia Bank lherzolites and diorites have Nd–Srisotopic compositions (¹⁴³Nd/¹⁴⁴Nd from 0·512804 to 0·512938and ⁸⁷Sr/⁸⁶Sr from 0·703243 to 0·703887) intermediatebetween those of the clinopyroxenes from the Galicia Bank andthe Iberia Abyssal Plain, but similar to the clinopyroxenesin the 5100 Hill harzburgite (¹⁴³Nd/¹⁴⁴Nd = 0·512865and ⁸⁷Sr/⁸⁶Sr = 0·703591) and to the igneous rocks (¹⁴³Nd/¹⁴⁴Ndranging from 0·512729 to 0·513121 and ⁸⁷Sr/⁸⁶Srranging from 0·702255 to 0·705109). The majorand trace element compositions of cpx in the Galicia Bank spinellherzolites provide evidence for large-scale refertilizationof the lithospheric upper mantle by MORB-like tholeiitic melts.The associated harzburgites did not undergo partial meltingduring the rifting stage, but, in earlier times, probably during,or even before, the Hercynian orogeny. Iberia Abyssal Plainwebsterites are interpreted as high-pressure cumulates formedin the mantle. Their high Sm/Nd ratios (from 0·43 to0·67) coupled with very low-Nd isotopic compositionsare best explained by a two-stage history: formation of thecumulates from the percolation of enriched melts long beforethe rifting, followed by low-degree partial melting of the pyroxenites,accounting for their LREE depletion. This last event probablyoccurs during the rifting episode, 122 Myr ago. The isotopicheterogeneities observed in the ultramafic rocks of the Iberiamargin were already present at the time of the rifting event.They reflect a long and complex history of depletion and enrichmentevents in an old part of the mantle, and provide strong argumentsfor a sub-continental origin of this part of the upper mantle. KEY WORDS: Iberia margin; mantle peridotites; igneous rocks; petrology; geochemistry     

扬子克拉通东北缘岩石圈地幔的Re-Os同位素地球化学:大陆岩石圈地幔的形成和演化的制约

苏皖地区新生代碱性玄武岩中有丰富的地幔橄榄岩捕虏体 ,测定了 2 0多个样品的Re Os元素丰度和锇同位素组成 ,结果 :Re =0 .0 2 7× 10 - 9～ 0 .375×10 - 9,Os=0 .112×10 - 9～ 3.35× 10 - 9,1 87Os 1 88Os=0 .117～ 0 .134。由代理等时线法1 87Os 1 88Os Yb获得该区岩石圈地幔早期熔体亏损事件的年龄为 1.7Ga(中元古代 )。苏皖地区岩石圈地幔的古老性表明它是元古代后岩石圈拆沉 减薄作用的残余地幔部分。岩石圈地幔经过了亏损 富集多阶段演化。     

Mantle Processes and Sources of Neogene Slab Window Magmas from Southern Patagonia, Argentina

Neogene plateau lavas in Patagonia, southern Argentina, eastof the volcanic gap between the Southern and Austral VolcanicZones at 46·5° and 49·5°S are linked withasthenospheric slab window processes associated with the collisionof a Chile Ridge segment with the Chile Trench at 12 Ma. Thestrong ocean-island basalt (OIB)-like geochemical signatures(La/Ta <20; Ba/La <20; ⁸⁷Sr/⁸⁶Sr = 0·7035–0·7046;¹⁴³Nd/¹⁴⁴Nd = 0·51290–0·51261; ²⁰⁶Pb/²⁰⁴Pb= 18·3–18·8; ²⁰⁷Pb/²⁰⁴Pb = 15·57–15·65;²⁰⁸Pb/²⁰⁴Pb = 38·4–38·7) of these Patagonianslab window lavas contrast with the mid-ocean ridge basalt (MORB)-like,depleted mantle signatures of slab window lavas elsewhere inthe Cordillera (e.g. Antarctic Peninsula; Baja California).The Patagonian lavas can be divided into a voluminous     

The Lithospheric Mantle beneath Continental Margins: Melting and Melt-Rock Reaction in Canadian Cordillera Xenoliths

Seven alkali basalt centers in the southern Canadian Cordilleracontain mantle xenolith suites that comprise spinel Cr-diopsideperidotites, spinel augite-bearing wehrlites and orthopyroxene-poorlherzolites, and minor pyroxenites. The Cr-diopside peridotitesappear to be residues of the extraction of Mg-rich basalts byup to 15% partial melting (median 5–10%) of a pyrolite-likesource in the spinel stability field. The xenoliths are similarto other mantle xenolith suites derived from beneath convergentcontinental margins, but are less depleted, less oxidized, andhave lower spinel mg-number than peridotites found in fore-arcsettings. Their dominant high field strength element depletedcharacter, however, is typical of arc lavas, and may suggestthat fluids or melts circulating through the Canadian Cordilleralithosphere were subduction related. Modeling using MELTS isconsistent with the augite-bearing xenoliths being formed byinteraction between crystallizing alkaline melts and peridotite.Assimilation–fractional crystallization modeling suggeststhat the trace element patterns of liquids in equilibrium withthe augite xenoliths may represent the initial melts that reactedwith the peridotite. Moreover, the compositions of these meltsare similar to those of some glasses observed in the mantlexenoliths. Melt–rock interaction may thus be a viablemechanism for the formation of Si- and alkali-rich glass inperidotites. KEY WORDS: Canadian Cordillera; mantle xenolith; peridotite; wehrlite; melt–rock reaction     

Mantle Dynamics: A Nd, Sr, Pb and O Isotopic Study of the Cameroon Line Volcanic Chain

The Cameroon line comprises a 1600-km long Y-shaped chain of< 30 m.y. old volcanoes and <70 m.y. old plutons extendinginto mainland Africa from the Atlantic island of Pagalu. Thedistribution of basaltic volcanic centres is ideal for comparingsub-continental and sub-oceanic sources for basalts and forstudying the influence of the lithosphere on magma generation.We report Nd, Sr, Pb and O isotopic data for more than thirty(principally basaltic) samples from all the main volcanic centrestogether with data for two granulite facies xenoliths. Thosebasalts which display no obvious evidence of crustal contaminationyield initial ⁸⁷Sr/⁸⁶Sr ratios ranging from 0.7029 to 0.7035,Nd between +2 and +7 and ²⁰⁶Pb/²⁰⁴Pb between 19?0 and 20?6.The Nd and Sr isotopic compositions define a field on the lefthand side of the ‘mantle array’ (that is with relativelyunradiogenic Sr) and include some data which show overlap withcompositions observed for St. Helena. ²⁰⁸Pb/²⁰⁴Pb ratios extendto 40?4—amongst the more radiogenic observed for alkalibasalts. The Nd and Sr isotopic data are similar in oceanicand continental sectors indicating that the magmas are derivedfrom generally similar mantle sources. Despite this overallsimple picture, the source of the Cameroon line volcanics hasin fact been variable in both time and space. Pb is less radiogenicand Sr is more radiogenic in transitional to hypersthene-normativecompositions. There is a progression to more radiogenic leadisotopic compositions with time for the Cameroon line as a wholethat is most strikingly displayed in the 30 m.y. eruptive historyof Principe. These space-time data are difficult to reconcilewith conventional plume models or with some dispersed ‘plumpudding’ models. The heterogeneities require isolationtimes considerably longer than the age of the south Atlanticsea floor (120 Ma). The eruptive lavas with the most radiogenicPb observed (accompanied by unradiogenic Nd) precisely straddlethe continental edge (i.e. occur in both oceanic and continentalsectors) with no dependency on Nd and Pb concentrations. A modelis proposed which links these observations with the destructionof lithosphere, and the impregnation of the uppermost mantleby the St. Helena hot spot during the formation of the SouthAtlantic ocean. This mantle was subsequently melted to formthe Cameroon line which appears to be derived from a risinghot zone initiated by the early plume activity. The magmaticproducts reflect the mantle mixing that took place during continentalbreakup, the consequent cooling and thickening of the lithosphereand the continued interaction between rising plume componentsand this lithosphere. The depth from which magmas are currentlybeing tapped at the continent/ocean boundary is estimated atless that 150 km.     

Petrogenesis and Implications of Calc-Alkaline Cryptic Hybrid Magmas from Washburn Volcano, Absaroka Volcanic Province, USA

The petrogenesis of calc-alkaline magmatism in the Eocene AbsarokaVolcanic Province (AVP) is investigated at Washburn volcano,a major eruptive center in the low-K western belt of the AVP.New ⁴⁰Ar/³⁹Ar age determinations indicate that magmatism atthe volcano commenced as early as 55 Ma and continued untilat least 52 Ma. Although mineral and whole-rock compositionaldata reflect near equilibrium crystallization of modal phenocrysts,petrogenetic modeling demonstrates that intermediate compositionmagmas are hybrids formed by mixing variably fractionated andcontaminated mantle-derived melts and heterogeneous siliciccrustal melts. Nd and Sr isotopic compositions along with traceelement data indicate that silicic melts in the Washburn systemare derived from deep-crustal rocks broadly similar in compositionto granulite-facies xenoliths in the Wyoming Province. Our preferredexplanation for these features is that mantle-derived basalticmagma intruded repeatedly in the deep continental crust leadingto fractional crystallization, silicic melt production, andhomogenization of magmas, followed by ascent to shallow reservoirsand crystallization of new plagioclase-rich mineral assemblagesin equilibrium with the intermediate hybrid liquids. The implicationsof this process are that (1) some calc-alkaline magmas may onlybe recognized as hybrids on purely chemical grounds, particularlyin systems where mixing precedes and is widely separated fromcrystallization in space and time, and (2) given the role ascribedto crustal processes at Washburn volcano, the variation betweenrocks that follow calc-alkaline trends in the western AVP andthose that follow shoshonitic trends in the east cannot simplyreflect higher pressures of fractionation to the east in Moho-levelmagma chambers in the absence of crustal interaction. KEY WORDS: petrogenesis; magma mixing; calc-alkaline; Absaroka Volcanic Province; ⁴⁰Ar/³⁹Ar dates     

Ultrapotassic Magmas along the Flanks of the Oligo-Miocene Rio Grande Rift, USA: Monitors of the Zone of Lithospheric Mantle Extension and Thinning Beneath a Continental Rift

Recent theoretical studies of rift tectonics have concludedthat their observed geophysical features, require that (1) extensionaffects a much wider zone of the underlying lithospheric mantlethan the crust; (2) early extension involves a comparativelywide zone that narrows with time. The Neogene evolution of thesegment of the Rio Grande rift between the Great Plains andColorado Plateau shows this theoretical pattern clearly. Thewidth of the crustal extension zone narrowed from {small tilde}170km in the Oligo-Miocene to {small tilde}50 km in the Pliocene.In contrast, both gravity and teleseismic studies indicate thatthe current width of the zone of thinned lithospheric mantle(ß = 2–3) beneath the rift is {small tilde}750km. To assess the contributions of lithosphere- and asthenosphere-derivedmelts to the magmatismassociated with the early phase of developmentof the Rio Grande rift, we have undertaken a 670-km geochemicaltraverse of Oligo-Miocene volcanism between latitudes 36 and38N. Our section is centered on the present-day axis of therift in the Espanola Basin. It extends from the Navajo volcanicfield, Arizona, to Two Buttes, SE Colorado, and intersects hypabyssalintrusions on the rift shoulders at Dulce, west of the rift,and Spanish Peaks to the east. We have sampled a diverse rangeof magma types that vary in composition from ultrapotassic toHy- and Ne-normative basalts. A geochemical profile along thistraverse shows a spatially symmetrical variation in elementand oxide ratios, such as Na₂O/K₂O and Ba/Nb, and also in Srand Nd isotope ratios. On the rift flanks and shoulders Oligo-Miocenevolcanism was dominated by K-rich mafic magmatism, whereas atthe rift axis tholeiitic and alkalic basalts with whole-rockcompositions similar to those of ocean-island basalts (OIB)were erupted. This symmetrical geochemical variation broadlyparallels the corresponding teleseismic lithosphere thicknessprofile and is a mirror image of the gravity profile. We interpret the OIB-type magmas at the rift axis as predominantlyasthenosphere-derived melts. These suggest that mantle upwelling,and melting by decompression, were occurring during the earlydevelopment of the Rio Grande rift The symmetrical variationof incompatible elements and isotope ratios in rocks about therift axis suggests that the sources of the K-rich mafic magmason the stable flanks and shoulders of the rift are not directlyrelated to the subduction of the Farallon plate: an asymmetricprocess. Instead, we propose that the K-rich mafic magmas onthe flanks and shoulders of the Rio Grande rift are derivedfrom the melting of a metasomatized layer in the lithosphericmantle during extension. *Present address: British Antarctic Survey, High Cross, Madingley Road, Cambridge CB3 0ET, UK     

Chemical and isotopic characteristics of fluids along the Cameroon Volcanic Line, Cameroon

Results of the chemical and isotopic analysis of the water and gases discharged from volcanic crater lakes and soda springs located along the Cameroon Volcanic Line were used to characterize and infer their genetic relationships. Variations in the solute compositions of the waters indicate the dominant influence of silicate hydrolysis. Na⁺ (40–95%) constitutes the major cation in the springs while Fe²⁺ + Mg²⁺ (70%) dominate in the CO₂-rich lakes. The principal anion is HCO₃ (>90%), except in the coastal springs where Cl-predominates. Lakes Nyos and Monoun have FeMgCaHCO₃⁻ type signatures; the soda springs are essentially NaHCO₃⁻ type, while all other lakes show similar ionic compositions to dilute surface waters. Dissolved gases show essentially CO₂ (>90%), with small amounts of Ar and N₂, while CH₄ constitutes the principal component in the non-gassy lakes. Active volcanic gases are generally absent, except in the Lobe spring with detectable H₂S. Stable isotope ratio evidence indicates that the bicarbonate waters are essentially of meteoric origin. CO₂ (δ¹³C = −2 to −8%₀ and He (³He/⁴He = 1 to 5.6R_a) infer a mantle contribution to the total CO₂. CH₄ has a biogenic source, while Ar and N₂ are essentially atmospheric in origin, but mixing is quite common.     

Mantle Sources and the Highly Variable Role of Continental Lithosphere in Basalt Petrogenesis of the Kerguelen Plateau and Broken Ridge LIP: Results from ODP Leg 183

Ocean Drilling Program (ODP) Leg 183 was designed to investigatethe origin and evolution of the large igneous province composedof the Kerguelen Plateau and Broken Ridge. Of the eight sitesdrilled, basalt was recovered from seven, five on the plateauand two on Broken Ridge. We present results from four of thesesites, 1136, 1138, 1141 and 1142. Although this large igneousprovince is interpreted as being derived from the Kerguelenmantle plume, the geochemical characteristics of basalt fromsome parts of the province indicate a role for continental lithosphere.The 118–119 Ma basalt flows recovered in the SouthernKerguelen Plateau (Site 1136) have a more subtle continentalsignature than shown by basalt at Leg 119 Site 738. A continentalsignature is absent in the 100–101 Ma tholeiitic basaltsat Site 1138 in the Central Kerguelen Plateau (CKP); their age-correctedNd–Sr–Pb isotopic values and incompatible elementratios are similar to those estimated for primitive mantle.These flows may represent a major mantle source in the Kerguelenstarting-plume head. The 20 basalt units identified are a productof magma chamber replenishment, fractional crystallization,and resorption of crystallizing phases. The topmost unit, Unit1, is a dacite that evolved from a basalt magma similar to thoserepresented by Units 3–22; unlike the basalts the dacitemagma was probably influenced by continental material. MiddleCretaceous (     

The Role of Lithospheric Mantle Heterogeneity in the Generation of Plio-Pleistocene Alkali Basaltic Suites from NW Harrat Ash Shaam (Israel)

Plio-Pleistocene volcanism in the Golan and Galilee (northeasternIsrael) shows systematic variability with time and location:alkali basalts were erupted in the south during the Early Pliocene,whereas enriched basanitic lavas erupted in the north duringthe Late Pliocene (Galilee) and Pleistocene (Golan). The basaltsshow positive correlations in plots of ratios of highly to moderatelyincompatible elements versus the concentration of the highlyincompatible element (e.g. Nb/Zr vs Nb, La/Sm vs La) and indiagrams of REE/HFSE (rare earth elements/high field strengthelements) vs REE concentration (e.g. La/Nb vs La). Some of thesecorrelations are not linear but upward convex. ⁸⁷Sr/⁸⁶Sr ratiosvary between 0·7031 and 0·7034 and correlate negativelywith incompatible element concentrations and positively withRb/Sr ratios. We interpret these observations as an indicationthat the main control on magma composition is binary mixingof melts derived from two end-member mantle source components.Based on the high Sr/Ba ratios and negative Rb anomalies inprimitive mantle normalized trace element diagrams and the moderateslopes of MREE–HREE (middle REE–heavy REE) in chondrite-normalizeddiagrams, we suggest that the source for the alkali basalticend-member was a garnet-bearing amphibole peridotite that hadexperienced partial dehydration. The very high incompatibleelement concentrations, low K content, very low Rb contentsand steep MREE–HREE patterns in the basanites are attributedto derivation from amphibole- and garnet-bearing pyroxeniteveins. It is suggested that the veins were produced via partialmelting of amphibole peridotites, followed by complete solidificationand dehydration that effectively removed Rb and K. The requirementfor the presence of amphibole limits both sources to lithosphericdepths. The spatial geochemical variability of the basalts indicatesthat the lithosphere beneath the region is heterogeneous, composedof vein-rich and vein-poor domains. The relatively uniform ¹⁴³Nd/¹⁴⁴Nd(Nd = 4·0–5·2) suggests that the two mantlesources were formed by dehydration and partial melting of anoriginally isotopically uniform reservoir, probably as a resultof a Paleozoic thermal event. KEY WORDS: basanites; lithospheric heterogeneity; magma mixing; amphibole peridotite; pyroxenites     

Development of a Continental Volcanic Field: Petrogenesis of Pre-caldera Intermediate and Silicic Rocks and Origin of the Bandelier Magmas, Jemez Mountains (New Mexico, USA)

The Miocene–Quaternary Jemez Mountains volcanic field(JMVF) is the site of the Valles caldera and associated BandelierTuff. Caldera formation was preceded by > 10 Myr of volcanismdominated by intermediate composition rocks (57–70% SiO₂)that contain components derived from the lithospheric mantleand Precambrian crust. Simple mixing between crust-dominatedsilicic melts and mantle-dominated mafic magmas, fractionalcrystallization, and assimilation accompanied by fractionalcrystallization are the principal mechanisms involved in theproduction of these intermediate lavas. A variety of isotopicallydistinct crustal sources were involved in magmatism between13 and 6 Ma, but only one type (or two very similar types) ofcrust between 6 and 2 Ma. This long history constitutes a recordof accommodation of mantle-derived magma in the crust by meltingof country rock. The post-2 Ma Bandelier Tuff and associatedrhyolites were, in contrast, generated by melting of hybridizedcrust in the form of buried, warm intrusive rocks associatedwith pre-6 Ma activity. Major shifts in the location, styleand geochemical character of magmatism in the JMVF occur withina few million years after volcanic maxima and may correspondto pooling of magma at a new location in the crust followingsolidification of earlier magma chambers that acted as trapsfor basaltic replenishment. KEY WORDS: crustal anatexis; fractional crystallization; Jemez Mountain Volcanic Field; Valles Caldera; radiogenic isotopes; trace elements     

Sources of Magmas and Conditions of Rock Formation in the Late Cenozoic Udokan Volcanic Plateau

Doklady Earth Sciences - The Udokan volcanic plateau differs from other volcanic regions of the Late Cenozoic volcanic province of East Asia in the high alkalinity of volcanic rocks, their...     

Parameterization of the Melting Regime of the Shallow Upper Mantle and the Effects of Variable Lithospheric Stretching on Mantle Modal Stratification and Trace-Element Concentrations in Magmas

Parameterization of melting phenomena in the upper mantle hasprimarily focused on two basic themes, namely the physical andchemical processes that govern partial melting. Parameterizationof physical processes mainly refers to establishing relationshipsbetween parameters such as the temperature, pressure, matrixand melt flow geometry, lithospheric stretching, and volumeof magma. By contrast, parameterization of chemical processeslargely implies unravelling the relationships between type anddegree of melting, and source and melt composition. Few attemptshave been made, however, to interrelate the two processes. Thepresent work is an effort to provide a link between physicaland chemical parameters associated with mantle melting and toallow in-depth modelling of partial melting processes in upwellingasthenosphere in a rigorous yet simplified manner. Several correlationsamong the most important physical parameters (e.g., equilibrationand extrusion temperature and pressure of magma, melt fractionand thickness, stretching factor, etc.) are explored. On thisbasis, a model for the compositional stratification of the lithosphereis proposed, and its bearing on the nature of intra-oceanicarc magmatism is emphasized. Trends of melting residues in termsof modal olivine and clinopyroxene are calculated for a widerange of possible potential temperatures that may be appliedto xenolith or abyssal peridotite suites to constrain furthertheir original depth of upwelling. Dry solidus equations fordepleted peridotite compositions are also derived that may beused to infer the effects of volatiles on the melting of refractorysupra-subduction zone mantle. The sensitivity of certain elementsto temperature variations during melting in a column of ascendingmantle is highlighted using Ni as an example, and the dangersof using single-value distribution coefficients to predict concentrationsof transition metals in magmas are emphasized. MORB-normalizedmulti-element profiles calculated using a variety of sources,mantle potential temperatures, and stretching factors are presented,and the differences between instantaneous and pooled melts arediscussed. A technique to calculate mineral proportions duringtransformation of garnet lherzolite to spinel lherzolite, togetherwith estimates of the modal composition of fertile spinel andgarnet lherzolite are included. Selected trace-element abundancesin various sources [bulk silicate Earth, depleted MORB (mid-oceanridge basalt) mantle, N-MORB) and distribution coefficientsfor common rock-forming minerals are also tabulated.     

The Petrogenesis of A-type Magmas from the Amram Massif, Southern Israel

The (     

华北克拉通中生代拆沉陆壳物质对岩石圈地幔的改造:来自橄榄岩捕虏体中角闪石的成分制约

对鲁西和太行山南段早白垩世高镁闪长岩中橄榄岩捕虏体中的角闪石进行了主量元素和痕量元素分析,并讨论了角闪石成分与改造岩石圈地幔熔体的性质与来源之间的关系。研究结果表明,橄榄岩类捕虏体中的角闪石主要为交代成因。在化学上,它们属于钙质角闪石。太行山南段符山橄榄岩捕虏体中的角闪石属于镁质普通角闪石和浅闪石或浅闪石质普通角闪石;鲁西铁铜沟橄榄岩捕虏体中的角闪石为韭闪石质普通角闪石、浅闪石质普通角闪石和镁绿钙闪石质普通角闪石。与板内橄榄岩捕虏体中的角闪石相比,鲁西和太行山南段早白垩世高镁闪长岩橄榄岩捕虏体中的角闪石具有相对高的Mg#(鲁西:86.0~90.8;太行山:90.7~94.2)和SiO2(鲁西:44.04%~46.98%;太行山:47.09%~49.79%)以及相对低的Na2O(1.92%~2.51%)、TiO2(0.01%~1.46%)、Nb(0.29×10-6~1.98×10-6)和Zr(1.61×10-6~5.34×10-6),这与来自俯冲带之上地幔楔橄榄岩捕虏体中的角闪石相类似。综合橄榄岩捕虏体的地球化学特征,可以判定华北克拉通早白垩世岩石圈地幔遭受了起源于拆沉陆壳物质熔融熔体的改造。     

Geochemistry and geochronology of mafic rocks from Bamenda Mountains (Cameroon): Source composition and crustal contamination along the Cameroon Volcanic Line

Mafic rocks from the Bamenda volcanic province along the Cameroon Volcanic Line have been dated from 17 to 0 Ma. Associated with some trachytes and rhyolites, this volcanism covers a period of more than 25 Ma. The studied rocks are basalts to mugearites. Most of them have been contaminated by continental crust during their transit to the surface. The oldest rocks are the most contaminated. One group of samples shows high Eu, Sr and Ba contents. This characteristic is not due to crustal contamination process, but has a mantle source origin. We argue that these characteristics have been acquired by mixing of melts formed by partial melting of mantle pyroxenites with melts formed in mantle peridotites. Such pyroxenites have been observed as mantle xenoliths in the Adamaoua province, and their chemical and isotopic compositions are consistent with such a model.     

古老大陆岩石圈地幔再循环与蛇绿岩中铬铁矿床成因

不同地区、不同时代蛇绿岩中不同类型铬铁矿岩的Re-Os同位素研究表明,在铬铁矿石或围岩中均存在极度亏损的具有大陆岩石圈地幔属性的物质。新疆达拉布特古生代蛇绿岩带中萨尔托海富Al铬铁矿岩的Os同位素组成为0.1109～0.1256,对应的模式年龄为3.5～0.6Ga;西藏班公湖—怒江中生代蛇绿岩带中东巧富Cr铬铁矿石及围岩Os同位素组成介于0.1175～0.1261,对应的模式年龄为1.5～0.1Ga;雅鲁藏布江中生代蛇绿岩带中罗布莎富Cr铬铁矿岩的Os同位素变化范围为0.1038～0.1266,对应的模式年龄为3.37～0.28Ga,而该带中不含矿的泽当二辉橄榄岩的Os同位素组成为0.1256～0.1261,没有古老大陆岩石圈地幔属性的物质存在,与新特提斯洋地幔Os组成较为接近。推测在蛇绿岩形成过程中,古老大陆岩石圈地幔参与循环有利于形成铬铁矿床,明确提出"熔体与古老大陆岩石圈地幔反应成矿"的假说,指出蛇绿岩带中存在的古老微陆块可能是找矿的指示标志。     

Interaction of Continental Lithosphere and Asthenospheric Melts below the Geronimo Volcanic Field, Arizona, U.S.A

Spinel lherzolite and pyroxenite inclusions from the Geronimovolcanic field, Arizona (and Dish Hill, California) record,in their constituent minerals, a chronology of diverse mantledepletion and enrichment events. Certain portions of the lithosphericmantle have remained relatively isolated for considerable periodsof time(1–4 b.y.) while wall rock adjacent to conduitsof basanite has been recently (< 0-2 b.y.) modified. Evidenceexists for a widespread ancient (1–4 b.y.) partial meltingresidue, now recognizable as MORB-like mantle below the southwesternU.S.A. Trace element enrichment (0?9 b.y.) has increased thelight rare earth element (LREE) and Sr content of many refractoryperidotites without any mineralogical changes to the host rock.The fluids/melt responsible for this enrichment have a complexhistory involving heterogeneous mantle sources. In contrast,modal metasomatism of the mantle (< 0.2 b.y.) in aureolesaround evolved derivatives of basanite has petrographicallyand chemically transformed this ancient partial melting residue.The metasomatic fluids responsible for such metasomatism havean asthenospheric mantle source identical to the host magma.It is proposed that modal metasomatism occurs in contact metamorphicaureoles that surround apophyses of basanitic silicate meltin the lithospheric mantle. The gradient in CO₂/(CO₂ ? H₂O)ratio that must surround such veins in the upper mantle (<20 kb) may encourage the development of enrichment fronts. Immediatelyadjacent to the vein, a wet zone with a relatively low CO₂/(CO₂? H₂O) ratio would allow a precipitation of mica ? amphibole.Beyond this a dry zone with a higher CO₂/(CO₂ ? H₂O) ratio wouldhasten chemical but not petrographic transformation of the wallrock.