Flood Basalt from Mont Tourmente in the Central Kerguelen Archipelago: the Change from Transitional to Alkalic Basalt at ~25 Ma

The surface of the Cenozoic Kerguelen Archipelago, constructedon the Kerguelen Plateau in the southern Indian Ocean, is dominantlyflood basalt. With the objective of understanding the Cenozoichistory of the Kerguelen mantle plume, the age and geochemicalcharacteristics of this flood basalt province are being determinedby studying stratigraphic sections of basalt flows at severallocations. Sections from the NW, north–central, east andSE parts of the archipelago have been studied. Here we reportresults for a 597 m succession of lavas from Mont Tourmentefrom the Plateau Central, a region of the archipelago that hasnot been studied in detail. Mont Tourmente lavas range from     

The Depleted Mantle Component in Kerguelen Archipelago Basalts: Petrogenesis of Tholeiitic-Transitional Basalts From the Loranchet Peninsula

A geochemical study of 28 Ma tholeiitic to transitional basaltsfrom the Kerguelen Archipelago (Mont des Ruches and Mont Fontaine)indicates that three distinct magma types erupted within     

Experimental constraints on the storage conditions of phonolites from the Kerguelen Archipelago

Phase relations of a phonolite (K1) and a tephri-phonolite (K2) from the Upper Miocene lavas in the Southeast Province of the Kerguelen Archipelago have been investigated in the P/T range 100–500 MPa and 700–900 °C at two fO₂ conditions (~ NNO and ~ NNO+2.3) to clarify the differentiation and pre-eruptive conditions of these magmas. Crystallization experiments were performed in cold seal pressure vessels (CSPV) and internally heated pressure vessels (IHPV) at various X_H2O, under reducing (log fO₂ ~ NNO) and oxidizing conditions (log fO₂ ~ NNO+2.3). Under reducing conditions, the resulting phase assemblage for K1 was: titanomagnetite, nepheline, alkali feldspar, clinopyroxene and biotite; under oxidizing conditions, the assemblage was: magnetite, plagioclase, alkali feldspar, nepheline, titanite (minerals given in the order of appearance with decreasing T at 200 MPa for 4 wt% water in the melt). It is emphasized that an effect of fO₂ on the phase stability of feldspars and feldspathoides was observed. Comparison of the natural and experimental phase assemblages shows that the pre-eruptive conditions for K1 must have been in the log fO₂ range NNO+1–NNO+2, at pressures above 200–250 MPa. Assuming a temperature of 800 °C, the water content of the melt is constrained to be between 4 and 6 wt% H₂O. The pre-eruptive fO₂ conditions for the less evolved sample K2 are more oxidizing with log fO₂ close to NNO+2.3. The experimental results show that the enrichment of alkalis in residual melts during differentiation of tephri-phonolites is enhanced at high fO₂.Editorial responsibility: J. Hoefs     

Mineral Chemistry of Peridotites from Paleozoic, Mesozoic and Cenozoic Lithosphere: Constraints on Mantle Evolution beneath Eastern China

Major- and trace-element data on the constituent minerals ofgarnet peridotite xenoliths hosted in early Paleozoic (457–500Ma) kimberlites and Neogene (16–18 Ma) volcanic rockswithin the North China Craton are compared with those from thepre-pilot hole of the Chinese Continental Scientific DrillingProject (CCSD-PP1) in the tectonically exhumed Triassic (220Ma) Sulu ultrahigh-pressure (UHP) terrane along its southernmargin. P–T estimates for the Paleozoic and Neogene peridotitexenoliths reflect different model geotherms corresponding tosurface heat flows of 40 mW/m² (Paleozoic) and 80 mW/m² (Neogene).Garnet peridotite xenoliths or xenocrysts from the Paleozoickimberlites are strongly depleted, similar to peridotites fromother areas of cratonic mantle, with magnesium olivine (meanFo_92.7), Cr-rich garnet and clinopyroxene with high La/Yb. Garnet(and spinel) peridotite xenoliths hosted in Neogene basaltsare derived from fertile mantle; they have high Al₂O₃ and TiO₂contents, low-Mg-number olivine (mean Fo_89.5), low-Cr garnetand diopside with flat rare earth element (REE) patterns. Thedifferences between the Paleozoic and Neogene xenoliths suggestthat a buoyant refractory lithospheric keel present beneaththe eastern North China Craton in Paleozoic times was at leastpartly replaced by younger, hotter and more fertile lithosphericmantle during Mesozoic–Cenozoic times. Garnet peridotitesfrom the Sulu UHP terrane have less magnesian olivine (Fo_91.5),and lower-Cr garnet than the Paleozoic xenoliths. The diopsideshave low heavy REE (HREE) contents and sinusoidal to light REE(LREE)-enriched REE patterns. These features, and their highMg/Si and low CaO and Al₂O₃ contents, indicate that the CCSD-PP1peridotites represent a moderately refractory mantle protolith.Details of mineral chemistry indicate that this protolith experiencedcomplex metasomatism by asthenosphere-derived melts or fluidsin Mesoproterozoic, and subsolidus re-equilibration involvingfluids/melts derived from the subducted Yangtze continentalcrust during UHP metamorphism in the early Mesozoic. Tectonicextension of the subcontinental lithospheric mantle of the NorthChina Craton and exhumation of the Sulu UHP rocks in the earlyMesozoic induced upwelling of the asthenosphere. Peridotitessampled by the Neogene basalts represent newly formed lithospherederived by cooling of the upwelling asthenospheric mantle inJurassic–Cretaceous and Paleogene time. KEY WORDS: garnet peridotite xenoliths; North China Craton; lithospheric thinning; Sulu UHP terrane; UHP lithosphere evolution; mantle replacement     

A Bimodal Alkalic Shield Volcano on Skiff Bank: its Place in the Evolution of the Kerguelen Plateau

A bimodal volcanic sequence of 230 m thickness on Skiff Bank,a western salient of the northern Kerguelen Plateau, was drilledduring ODP Leg 183. The sequence comprises three main units:a mafic unit of trachybasalt flows sandwiched between two unitsof trachytic or rhyolitic flows and volcaniclastic rocks. Althoughinterpretation is complicated by moderate to strong alterationof the rocks, their original chemical character can be establishedusing the least mobile major and trace elements (Al, Th, highfield strength elements and rare earth elements). High concentrationsof alkalis and incompatible trace elements indicate that bothmafic and felsic rocks are alkalic. The felsic rocks may havebeen derived by partial melting of mafic rocks, followed byfractionation of feldspar, clinopyroxene, Fe–Ti oxidesand apatite. The mafic and felsic rocks have similar Nd andPb isotopic compositions; ²⁰⁶Pb/²⁰⁴Pb ratios are low (17·5–18·0)but, like the ¹⁴³Nd/¹⁴⁴Nd ratios (0·5125–0·5126),they are comparable with those of basalts from the central andsouthern Kerguelen Plateau (e.g. Sites 747, 749, 750). The Srisotopic system is perturbed by later alteration. There is nochemical or isotopic evidence for a continental crustal component.The bimodal alkalic character and the presence of quartz-phyricrhyolites is interpreted to indicate that the sequence formspart of a shield volcano built upon the volcanic plateau. Theage of 68 Ma, obtained on Site 1139 rocks by Duncan (A timeframe for construction of the Kerguelen Plateau and Broken Ridge,Journal of Petrology 43, 1109–1119, 2002), provides onlya minimum age for the underlying flood volcanic rocks. The highage indicates none the less that Skiff Bank is not the presentlocation of the Kerguelen plume. KEY WORDS: Ocean Drilling Program; Kerguelen Plateau; Skiff Bank     

东天山地区多头山铁铜矿床磁铁矿化学成分及其对成矿流体演化的指示

多头山矿床位于阿齐山-雅满苏成矿带西段,是东天山地区海相火山岩型铁铜矿床的代表,但目前缺乏对其矿石矿物的直接研究.磁铁矿是一种常见的矿石矿物,其化学成分可以用于指示成矿演化过程.在详细划分磁铁矿形成期次的基础上,对东天山地区的多头山矿床展开磁铁矿化学成分研究.结果表明按照磁铁矿的生成顺序和共生矿物组合的不同,多头山铁铜矿床中的磁铁矿从早期到晚期可以划分为M_1a、M_1b和M₂型.其中,M_1a型磁铁矿为粒状结构,与绿帘石-角闪石-黄铁矿共生;M_1b型磁铁矿也为粒状结构,与石英-绿帘石-角闪石-黄铁矿共生;M₂型磁铁矿则呈长条状产出,与角闪石共生.这3类磁铁矿都有较低含量的Ti（84×10-6~1 117×10-6）、Al（417×10-6~5 273×10-6）和高场强元素,属于热液型磁铁矿.与M₂型磁铁矿相比,前两类磁铁矿具有较高含量的Si、Ca、Al和Mn,可能受到微细包体的影响.从M_1a型到M₂型磁铁矿,Ti含量呈现逐渐降低的趋势,可能与结晶温度逐渐降低有关;V和Cr含量表现出先升高后降低的变化规律,暗示成矿流体的氧逸度先降低后升高.综合考虑区域地质特征及M₂型磁铁矿更加富Mg,表明有一定比例的海水参与到多头山矿床中磁铁矿形成的晚期阶段.      

Experimental constraints on the origin and evolution of mildly alkalic basalts from the Kerguelen Archipelago, Southeast Indian Ocean

This experimental study examines the role of clinopyroxene fractionation on major element trends and alkalinity variations in mildly alkalic basalts from the Kerguelen Archipelago, Southeast Indian Ocean. Equilibrium crystallization experiments were carried out on a natural basalt (MgO=5 wt.%, alkalinity index=0.10) over a range of pressures (0–1.43 GPa) and water contents (nominally dry to hydrous, 1.2 wt.% H₂O) under relatively oxidizing conditions (Δlog FMQ=+1 to +2) at 0 GPa and relatively reducing conditions (Δlog FMQ=0 to –2) at all higher pressures. The hydrous experiments at 0.93 GPa closely reproduce most of the compositional variations in the 24–25 Ma mildly alkalic lavas from the archipelago, which supports a major role for high-Al clinopyroxene fractionation (5–9 wt.% Al₂O₃) at pressures corresponding to the base of the Northern Kerguelen Plateau (15–20 km). However, clinopyroxene fractionation at depth fails to produce important changes in the alkalinity of the residual melts. The transition from tholeiitic to mildly alkalic basalts on the Kerguelen Archipelago thus reflects primarily changes in melting conditions (lower extents of partial melting at higher pressures), which is related to crustal and lithospheric thickening as distance from the Southeast Indian Ridge increased over time from 43 to 24 Ma.     

Mineral Chemistry and Crystallization Conditions of the Mboutou Layered Gabbro-Syenite-Granite Complex, North Cameroon

The Mboutou complex is one of a line of early Tertiary ringcomplexes which runs from Lake Chad to the Gulf of Guinea, noneof which has hitherto been described in detail. The main rocktypes are layered gabbros and gabbronorites, with minor bodiesof quartz-syenodiorite, quartz-syenite and hypersolvus granite.Feldspars form a continuum with exceptional compositional range,from An₈₅Ab₁₃Or₂ to around An₁Ab₄₆Or₅₅, and form an entirelyhypersolvus sequence with very strong zoning in the syenodiorites.Ca-rich clinopyroxenes (salite and calcic augite) and olivines(Fo_78–62) have restricted range. Orthopyroxene-bearingleucogabbros and syenodiorites contain minor orthopyroxene (En₆₂Fs₃₅Wo₃)and quartz; olivine and orthopyroxene never coexist. In moreevolved rocks amphibole (magnesio-hornblende to ferroedenite)and minor biotite, showing progressive Fe-enrichment, are theonly mafic silicates.Major-element rock chemistry, minor elementsin clinopyroxenes and biotite chemistry show that, notwithstandingits thoroughly anorogenic setting, Mboutou was, at the outset,only very mildly alkaline. Its more evolved members embarkedon a line of evolution with some calc-alkaline characteristics,probably because of ingress of water into residual batches ofmagma, a possibility supported by stable isotope data. Thischange in behaviour corresponded with the sudden appearanceof quartz and orthopyroxene, which was not in equilibrium withclinopyroxene on the two-pyroxene surface. Amphibole then becamethe main mafic silicate with further increase in . The more evolved rocks are relatively highly altered,but Fe-Ti oxide pairs suggest that was maintained near to and above the QFM buffer and the rangeof biotite compositions further suggests crystallization undera regime of decreasing . Biotites maintain alkaline characteristics throughout the sequence. Zoningpatterns in the ternary feldspars in the syenodiorites, andthe hypersolvus character of the final granite, limit maximumvalues of to < 1 kb, and suggest minimum temperatures for the end of crystallizationin the syenodiorites of{small tilde} 850 ?C.     

Mineral Chemistry and Pressure-Temperature Evolution of Two Contrasting High-pressure-Low-temperature Belts in the Chonos Archipelago, Southern Chile

The Chonos Metamorphic Complex forms part of a belt of low-grademetamorphic rocks in the Chilean Coastal Cordillera that areinterpreted as Palaeozoic–Mesozoic accretionary complexes.It comprises metapsammopelitic schists, metabasites and meta-ironstonesoccurring in two contrasting units. Special attention duringmicroprobe study of key samples was given to the chemical zonationof minerals. Subsequently, conventional geothermobarometry andthat using thermodynamic calculations were applied. The Easternbelt comprises rocks that are metamorphosed to pumpellyite–actinolitefacies conditions and show a low degree of deformation withwell-preserved sedimentary and igneous structures. Maximum P–Tconditions were around 5·5 kbar and 250–280°C.The rocks of the Western belt are characterized by a transitionbetween greenschist and albite–epidote–amphibolitefacies metamorphism and show a penetrative tectonic transpositionfoliation S₂ formed close to the pressure maximum. Maximum P–Tconditions vary around 8–10 kbar and 380–500°Coverstepping the stilpnomelane + phengite stability. High pressuresin this belt are confirmed by regionally distributed phengiteswith high Si contents up to 3·5 Si per formula unit.Regional distribution of maximum temperatures is reflected bythe composition of actinolitic hornblendes within the metabasites.In a garnet-bearing metabasite of the Western belt, oscillatorygrowth zoning of garnet was observed. The composition of correspondingmineral inclusions suggests that a prograde P–T path duringgarnet growth evolved from 7·5 kbar and 375°C toabout 9·4 kbar and 500°C. Late garnet grew synkinematicallywith penetrative deformation. The retrograde P–T pathin the rocks of the Western belt is constrained by the compositionof mainly late strain-free minerals and involves slight coolingduring decompression. Both belts are part of a subduction system.The apparent P–T gap between the belts is due to theirjuxtaposition during exhumation. The Eastern belt constitutesthe transition towards the backstop system of the accretionaryprism that is represented by the Western belt, whereas the absenceof very low grade rocks west of the Western belt is attributedto tectonic erosion, which was possibly caused by subductionof a ridge. KEY WORDS: Chonos Metamorphic Complex; accretionary complex; high-pressure–low-temperature metamorphism; oscillatory garnet zonation; phengite; P–T paths     

新疆玉海-三岔口铜矿黑云母矿物化学特征及成岩成矿意义

黑云母的化学成分对于揭示结晶条件、岩石成因、成矿演化以及含矿性评价等具有重要的指示意义.利用电子探针（EPMA）对新疆玉海和三岔口铜矿中的黑云母进行成分分析,结果表明与成矿相关岩体的黑云母具有富镁贫铁的特征,而无矿化岩体中的黑云母则呈现富铁贫镁的特征.分析结果显示矿化岩体中黑云母为再平衡镁质黑云母,而无矿化岩体中黑云母为原生铁质黑云母;它们的寄主岩石均属于I型花岗岩,形成于与俯冲相关的构造背景,但是与矿化相关岩体的岩浆源区为壳幔混合来源,而无矿岩体的源区为地壳来源,形成过程中有新生地壳组分的混染;两类黑云母的结晶温度为529~677 ℃,寄主岩体的固结压力为1.1~2.8 kbar,均形成于高氧逸度条件;此外,黑云母的Mg/Fe和Fe2+/（Fe2++Mg2+）还可以区分斑岩型铜矿的含矿性.      

湘东锡田燕山期A型花岗岩黑云母矿物化学特征及其成岩成矿意义

黑云母的化学组成特征对揭示花岗岩的源区特征、形成环境、后期热液作用以及成矿元素富集特征具有重要的指示意义.对与锡田钨锡多金属矿床成矿作用密切相关的锡田燕山早期花岗岩黑云母和长石成分进行了系统的电子探针分析.分析结果表明,黑云母具有富铁贫镁、高铝低钠的特征,其MgO和FeOT含量分别为0.12%~1.35%和15.47%~23.24%,类似于高铁黑云母;其含铁指数Fe/(Fe+Mg)较高,集中在0.87~0.99,属于铁叶云母;其长石主要以正长石和钠长石为主.这些特征暗示了寄主岩石源区以壳源为主.结合相关区域地质资料,表明锡田燕山早期花岗质岩浆具有较高的温度和较低的氧逸度.黑云母具有高的含铁指数、较高的结晶温度和低的氧逸度等特征均有利于锡成矿,可以作为勘探锡矿的标志之一.综合分析认为,在锡田花岗质岩浆演化过程中,岩浆结晶期后分异出的流体趋向于向富锡的方向演化,是锡田多金属矿床成矿流体的重要来源.      

河北阳原新生代玄武岩中橄榄岩捕虏体矿物化学:华北岩石圈地幔演化

橄榄岩及其中矿物的组成, 特别是单斜辉石的微量元素, 可以很好的揭示岩石圈地幔性质.在对河北阳原新生代玄武岩中橄榄岩捕虏体详细岩相学和矿物化学研究基础上, 重点分析了单斜辉石的激光原位微量元素, 讨论了新生代华北内部山带岩石圈地幔特征及其演化机制.阳原地区陆下岩石圈, 除个别样品的部分熔融程度为15%~20%外, 多数样品<5%, 显示该区的地幔演化是不均一的, 并表现为饱满与难熔-过渡型地幔的交叉并存.这种共存现象可以用软流圈物质对古老地幔进行不均匀侵蚀、改造和置换作用来解释.      

Fluid Evolution during HP and UHP Metamorphism in Dabie Shan, China: Constraints from Mineral Chemistry, Fluid Inclusions and Stable Isotopes

The Dabie Shan ultrahigh-pressure (UHP) metamorphic terraneis located in the eastern part of the east–west-strikingQinling–Dabie orogenic belt in China. A major mylonitizedcontact zone of 200–300 m width divides Dabie Shan intothe South Dabie Terrane (SDT) and the North Dabie Complex (NDC).Combined investigation of major and trace element geochemistry,fluid inclusions, and oxygen and hydrogen isotopes constrainsthe fluid history during the metamorphic evolution of the twometamorphic belts, which differ in their fluid and metamorphicevolution. Fluid inclusions in rocks from the SDT are mainlyaqueous with varying salinities, whereas those from the NDCare dominated by CO₂. Low     

鲁西费县中生代玄武岩及幔源捕掳晶的矿物化学：对岩石圈地幔性质的制约

鲁西费县中生代玄武岩形成于119Ma，为碱性玄武岩。该玄武岩中含有丰富的幔源橄榄石、斜方辉石和单斜辉石捕掳晶。橄榄石捕掳晶具有环状裂纹，其Mg^#值介于90．0～93．0之间，平均为91．5；斜方辉石和单斜辉石捕掳晶具有特征的反应边，斜方辉石的Mg^#值介于88．0～93．5之间，平均为90．4；单斜辉石的Mg^#值介于86．0～91．7之间，平均为88．4。捕掳晶的矿物化学类似于中国东部新生代玄武岩中地幔橄榄岩的矿物成分特征，这暗示捕掳晶应为寄主岩浆上升过程中捕获的地幔橄榄岩物质，并且反映了新增生的岩石圈地幔特征。费县玄武岩的岩石地球化学显示其具有地幔原生岩浆的特征，其高，Isr、低εNd(t)和亏损高场强元素等特征应与断离的俯冲板片(苏鲁造山带中的榴辉岩)与软流圈物质的混熔有关。     

Crystallization Conditions and Mineral Chemistry in the East of Tafresh,Central Iran,with Insights into Magmatic Processes

The Tafresh granitoids are located at the central part of the Urumieh-Dokhtar Magmatic Arc(UDMA) in Iran. These rocks, mainly consisting of diorite and granodiorite, were emplaced during the Early Miocene. They are composed of varying proportions of plagioclase + K-feldspar + hornblende ± quartz ± biotite. Discrimination diagrams and chemical indices of amphibole phases reveal a calc-alkaline affinity and fall clearly in the crust-mantle mixed source field. The estimated pressure, derived from Al in amphibole barometry, is approximately 3 Kb. The granitoids are I-type, metaluminous and belong to the calc-alkaline series. They are all enriched in light rare earth elements and large ion lithophile elements, depleted in high field strength elements and display geochemical features typical of subduction-related calc-alkaline arc magmas. Most crystal size distribution(CSD) line patterns from the granitoids show a non-straight trend which points to the effect of physical processes during petrogenesis.The presence of numerous mafic enclaves, sieve texture and oscillatory zoning along with the CSD results show that magma mixing in the magma chamber had an important role in the petrogenesis of Tafresh granitoids. Moreover, the CSD analysis suggests that the plagioclase crystals were crystallized in a time span of less than 1000 years, which is indicative of shallow depth magma crystallization.     

南岭中段黄沙铀矿区黑云母与绿泥石的矿物化学特征及其对成岩成矿的约束

黄沙铀矿区位于南岭成矿带中段龙源坝岩体中南部,区内成矿地质条件优越,铀资源找矿潜力大。本文在矿物原位分析技术(EPMA与LA-ICP-MS)的基础上,讨论了黑云母和绿泥石的矿物化学特征对成岩成矿作用过程的约束。结果显示,矿区印支期中粗粒黑云母花岗岩中黑云母为铁质黑云母,指示岩浆演化程度较高,氧逸度和结晶温度较高。黑云母是花岗质熔体中Rb、Ba、Nb、Ta、Sc、V、Co、Ni、Cr元素的主要载体,副矿物(独居石、锆石、磷钇矿等)是Th、Sr、Hf、Zr、Y的主要载体,U主要赋存于晶质铀矿和含铀副矿物中。绿泥石分黑云母假象绿泥石和脉状绿泥石两类,形成于中温、相对酸性、相对氧化性的环境下。黑云母假象绿泥石形成于成矿前期的面状蚀变,以相对富铁为特征,经历了溶解、沉淀过程,伴随晶质铀矿和含铀副矿物中铀的活化与再分配。脉状绿泥石形成于成矿期的热液蚀变,以相对富镁为特征,经历了溶解、迁移与沉淀过程,富U流体流经还原性介质时,流体中的U~(6+)被还原沉淀形成沥青铀矿。     

The Ballachulish Igneous Complex, Scotland: Petrography, Mineral Chemistry, and Order of Crystallization in the Monzodiorite-Quartz Diorite Suite and in the Granite

The igneous complex of Ballachulish is a composite calc-alkalinepluton of Caledonian age (412 ? 28 Ma), emplaced in Dalradianmetasediments at a pressure of 3 ? 0–5 kb (c. 10 km depth).The 4 by 7 km intrusion is composed of a zoned monzodiorite-quartzdiorite envelope with a distinct flowand deformation-foliation,surrounding a younger core of porphyritic granite. Two-pyroxene thermometry, Fe-Ti oxide thermobarometry, and stabilityrelationships of ternary feldspars, biotite, and amphibolesare used to calibrate the 3 kb isobaric crystallization sequencewith respect to the following parameters: the fractionationstage of the host rocks, the water content of the magmas, phasecompositions, and oxygen fugacity. Plagioclase, augite, andoxides generally yielded submagmatic temperatures due to theextensive recrystallization and re-equilibration of these phasesin the 900–l550?C subsolidus range. The ‘dry’monzodiorites apparently contained less than 1 wt. % initialmagmatic water, and remained H₂O-deficient and vapor-absentthroughout their entire crystallization range. In contrast,2.5–3 wt.% initial H₂O is estimated for the more fractionatedquartz diorites and the younger granites. The main crystallizationinterval for Opx–Cpx–Plg primocrysts in the dioritescovers c. 1100–950?C. Late-magmatic biotite and alkalifeldspar join the paragenetic sequence below 980?860?C, at fO₂near NNO. A solidus temperature of c. 900?C is inferred forthis ‘dry’ system, in which amphiboles are entirelysubsolidus. At the present level of emplacement, crystallizationintervals of {small tilde} 1050–690?C and{small tilde}900–680?C are suggested for the quartz diorites and thegranites, which probably terminated crystallization in the presenceof a hydrous fluid.     

Petrogenesis of Evolved Basalts and Rhyolites at Austurhorn, Southeastern Iceland: the Role of Fractional Crystallization

The Austurhorn intrusive complex in southeastern Iceland representsthe evolved hypabyssal remains of an eroded Tertiary (6–7Ma) central volcano. The complex consists of a layered gabbrointrusion, a composite granophyric stock, and abundant maficand felsic dikes. Mineralogical and geochemical trends amongcontemporaneous, compositionally diverse liquids from the complexprovide insight into the genesis of evolved basalts and rhyolitesin Iceland that are difficult to infer from studies of onlylavas. Mafic and felsic samples have comparable ranges in incompatibletrace element ratios (Ba/La and P/Ce) and Sr- and Pb-isotopes(O'Nions and Pankhurst, 1973; B. Hanan, pers. comm., 1988),suggesting derivation from a common parental composition. Majorand trace element variations throughout the Austurhorn suiteare consistent with fractional crystallization of the observedphenocrysts. Quartz-normative basalts were derived from parentalbasalt containing 7.8 wt.% MgO by extensive low-pressure crystallizationof olivine, augite, plagioclase, magnetite, and ilmenite. Thefractionating assemblage is consistent with the observed mineralogyof associated gabbro. Moreover, the cumulus mineralogy of thegabbro provides evidence for fractionation processes in a compositionalinterval not represented by dikes and sills (i.e., 54–63wt.% SiO₂).Diversity among the mafic dikes reflects severaladditional factors: (1) crystallization under conditions ofvariable oxygen fugacity; (2) separate mantle melting eventsthat produce different Ce/Yb values; (3) contamination of somemafic dikes at depth, presumably by interaction with felsicmagmas. Major and trace element trends among most felsic samples canbe modeled by fractionation of the observed mineral phases:plagioclase, K-feldspar, clinopyroxene, ilmenite, apatite, allanite,and zircon. Although crustal melting has been proposed for generatingIcelandic rhyolites, most Austurhorn felsic samples are unlikeliquids derived by melting of hydrated basalts. In particular,apatite and zircon have controlled the abundances of Zr, Hf,and the REE in the felsic rocks, but they are unlikely to beresidual phases during partial melting of basalt. One felsicdike, interpreted as a melt of an evolved source, shows petrographicevidence of in situ anatexis and also has anomalous trace elementabundances and unusually high ₂₀₆Pb/₂₀₄Pb.     

Geochemical Constraints on the Mantle Source of the Upper Permian Emeishan Continental Flood Basalts,Southwestern China

The widespread Emeishan igneous province in southwestern China comprises the Emeishan continental flood basalts (ECFB) and associated mafie-ultramafic intrusions. The ECFB have variable SiO₂, ranging from 43.6 to 52.1 wt%, Al₂O₃ from 5.0 to 12.6 wt%, and total alkali (K₂O + Na₂O) from 0.7 to 6.5 wt%. These oxides exhibit negative correlations with MgO (5.4 - 23.1 wt%), implying fractional crystallization of olivine and clinopyroxene, which occur as phenocrysts in the rocks. Linear correlations between Zr, Nb, and La suggest that crustal contamination is not important. The primitive-mantle-normalized trace-element patterns show that the ECFB are enriched in high-field-strength trace elements, large-ion-lithophile elements, and light-rare-earth elements, similar to ocean-island basalt. Incompatible element ratios of the ECFB, such as Zr/Nb (7-10), Th/La (0.1-0.15), and Rb/Nb (0.9-1.7), differ from those of primitive mantle, N-MORB, and continental crust, but are similar to ocean-island basalts from an enriched mantle source (EM-1). However, the ECFB have isotopic ratios (¹⁴³Nd/¹⁴⁴Nd = 0.51229 -0.51276 and ⁸⁷Sr/⁸⁶Sr = 0.70480-0.70647) that imply that the ECFB were derived from a homogeneous, primitive lower mantle carried upward by a mantle plume.

We propose that the original melts derived from the mantle plume were contaminated through interaction at shallower depth with an enriched lithospheric mantle. This model suggests that the lithospheric mantle beneath the ECFB was modified by subduction of an oceanic slab.