The Cocos and Carnegie Aseismic Ridges: a Trace Element Record of Long-term Plume-Spreading Center Interaction

The aseismic Cocos and Carnegie Ridges, two prominent bathymetricfeatures in the eastern Pacific, record 20 Myr of interactionbetween the Galápagos hotspot and the adjacent GalápagosSpreading Center. Trace element data determined by inductivelycoupled plasma-mass spectrometry in >90 dredged seamountlavas are used to estimate melt generation conditions and mantlesource compositions along the ridges. Lavas from seamount provinceson the Cocos Ridge are alkalic and more enriched in incompatibletrace elements than any in the Galápagos archipelagotoday. The seamount lavas are effectively modeled as small degreemelts of a Galápagos plume source. Their eruption immediatelyfollows the failure of a rift zone at each seamount province'slocation. Thus the anomalously young alkalic lavas of the CocosRidge, including Cocos Island, are probably caused by post-abandonmentvolcanism following either a ridge jump or rift failure, andnot the direct activity of the Galápagos plume. The seamountshave plume-like signatures because they tap underlying mantlepreviously infused with Galápagos plume material. Whereasplume heterogeneities appear to be long-lived, tectonic rearrangementsof the ridge plate boundary may be the dominant factor in controllingregional eruptive behavior and compositional variations. KEY WORDS: mantle plume; mid-ocean ridge; Galápagos; abandoned rift; partial melting of the mantle     

Wolf Volcano, Galapagos Archipelago: Melting and Magmatic Evolution at the Margins of a Mantle Plume

Wolf volcano, an active shield volcano on northern Isabela Islandin the Galápagos Archipelago, has undergone two majorstages of caldera collapse, with a phase of partial calderarefilling between. Wolf is a typical Galápagos shieldvolcano, with circumferential vents on the steep upper carapaceand radial vents distributed in diffuse rift zones on the shallower-slopinglower flanks. The radial fissures continue into the submarineenvironment, where they form more tightly focused rift zones.Wolf's magmas are strikingly monotonous: estimated eruptivetemperatures of the majority of lavas span a total of only 22°C.This homogeneity is attributed to buffering of magmas as theyascend through a thick column of olivine gabbroic mush thathas been deposited from a thin, shallow (<2 km deep) subcalderasill that is in a thermochemical steady state. Wolf's lavashave the most depleted isotopic compositions of any historicallyactive intraplate ocean island volcano on the planet and haveisotopic compositions (except for ³He/⁴He) indistinguishablefrom mid-ocean ridge basalt erupted from the GalápagosSpreading Center (GSC) 250–410 km away from the peak ofinfluence of the Galápagos plume. Wolf's lavas are enrichedin incompatible trace elements and have systematic major elementdifferences relative to GSC lavas, however. Wolf's magmas resultfrom lower extents of melting, deeper melt extraction, and agreater influence of garnet compared with GSC magmas, but Wolfand the GSC share the same sources. These melt generation conditionsare attributed to melting in a thermal and mechanical boundarylayer of depleted asthenosphere at the margins of the Galápagosplume. The lower degrees of melting and extraction from deeperlevels result from a thicker lithospheric cap at Wolf than existsat the GSC. KEY WORDS: caldera; Galápagos; mush; partial melting; plume     

New volcanological and volatile data provide strong support for the continuous existence of Galápagos Islands over the past 17 million years

The first systematic rock sampling of volcanoes along the Galápagos hotspot tracks (the aseismic Cocos, Carnegie, Malpelo and Coiba ridges and adjacent seamounts) in the area between the Galápagos Islands and Central and South America was carried out on R/V Sonne cruise 144-3. Guyot-shaped seamounts, paleo-beach or intertidal wave-cut platform deposits, the structure and texture of volcanic rocks, and low sulfur contents of fresh glasses dredged at these volcanoes imply that ocean islands existed continuously above the Galápagos hotspot for at least the past 17 million years. These new data significantly extend the time period over which the unique endemic Galápagos fauna could have evolved, providing a complete solution to the long-standing enigma of the evolution of Galápagos land and marine iguanas.     

The emergence of a Galápagos shield volcano, Roca Redonda

Roca Redonda volcano is a mostly submarine shield volcano that rises nearly 3 km from the adjacent seafloor. Over twenty lava flows and palagonite tuff are exposed in a 60 meter high oblong outcrop above sea level, and several other flows are exposed in the shallow water surrounding the islet. Thick, slightly alkaline picritic flows form the base of the section. Thinner picrites interbedded with sparsely porphyritic alkali-olivine basaltic pahoehoe toes characterize the upper section. The subaerial section probably records the filling of a palagonite tuff cone with younger lavas. Numerous fumaroles that may have a magmatic component are present in the shallow (<30 m) submarine zone and indicate that the volcano is probably still active. Three lava types are exposed: the basal picrites with 19% > MgO > 14%, high-Mg basalts with MgO of about 9%, and low-Mg basalts with MgO of about 6%. The Sr and Nd isotopic ratios of the three lava types are within analytical uncertainty. Olivine compositions indicate that the picrites are basaltic liquids that have accumulated olivine whose composition is in equilibrium with the host basaltic liquid. Apparently, basaltic magmas percolated through dunite and troctolite that had crystallized from slightly older Roca Redonda basaltic magma. Lavas from Roca Redonda have enriched trace element contents and isotopic ratios relative to nearby Wolf volcano, but they are quite similar to lavas from Cerro Azul and Ecuador volcanoes. The common characteristic of these volcanoes is that they lie on the periphery of the archipelago and are in a stage of subaerial growth. This suggests that Galápagos volcanoes may go through a juvenile alkaline stage before a mature tholeiitic stage, analogous to the Loihi stage of Hawaiian volcanism. A low ³He/⁴He ratio in olivine from one of the picrites indicates a small contribution by the Galápagos mantle plume. Received: 15 December 1997 / Accepted: 6 May 1998     

The Generation of Oceanic Rhyolites by Crystal Fractionation: the Basalt-Rhyolite Association at Volcn Alcedo, Galpagos Archipelago

Alcedo volcano is one of six shield volcanoes on Isabela Islandin the western Galpagos Islands. Although Alcedo is dominantiybasaltic, it is unusual in that it also has erupted 1 km³ ofrhyolite. The rhyolitic phase marked a 10-fold decrease in themass-eruption rate of the volcano, and the volcano has returnedto erupting basalt. The basalts are tholeiitic and range fromstrongly to sparsely porphyritic. Olivine and plagiodase arethe liquidus phases in the most primitive basalts. The MgO andNi concentrations in the most primitive basalts indicate thatthey have undergone substantial differentiation since extractionfrom the mantle. The rhyolites contain the assemblage oligoclase-augite-titanomagnetite-fayalite-apatiteand sparse xenoliths of quenched basalt and cumulate gabbros.Intermediate rocks are very rare, but some are apparently basaltrhyolitehybrids, and others resulted from differentiation of tholeiiticmagma. Several modeling approaches and Sr-, Nd-, and O-isotopicdata indicate that the rhyolites resulted from 90% fractionation(by weight) of plagiodase, augite, titanomagnetite, olivine,and apatite from the most primitive olivine tholeiite. The dataare inconsistent with the rhyolites originating by crustal anatexis.The extreme Daly gap may be caused by the large increase inviscosity as the basaltic magma differentiates to intermediateand siliceous compositions; highly evolved magmas are eruptibleonly after they become saturated with volatiles by second boiling.The close association of the hybrid intermediate magmas andmagmatic inclusions with the climactic plinian eruption indicatesmixing between mafic and silicic magmas immediately before eruption.Rhyolite production was favored by the decrease in supply ofbasaltic magma as Alcedo was carried away from the focus ofthe Galpagos hotspot. A three-stage model for the magmaticevolution of a Galpagos volcano is proposed. In the first stage,the supply of basaltic magma is large. Basaltic magma continuallyintrudes the subcaldera magma chamber, buffering the magmas'compositional and thermal evolution. As the volcano is carriedaway from the basaltic source, the magma chamber is allowedto cool and differentiate, as exemplified by Alcedo's rhyoliticphase. Finally, the volcano receives even smaller influx ofbasalt, so a large magma chamber cannot be sustained, and thevolcano shifts to isolated basaltic eruptions. KEY WORDS: Galpagos; oceanic rhyolites; fractional crystallization; Isabela Island ^*Corresponding author, e-mail: Geist{at}IDUI1.csrv.uidaho.edu. Telephone: 208-885-6491. Fax: 208-885-5724     

Dynamics of the Galapagos hotspot from helium isotope geochemistry

We have measured the isotopes of He, Sr, Nd and Pb in a number of lava flows from the Galapagos Archipelago; the main goal is to use magmatic helium as a tracer of plume influence in the western volcanoes. Because the Galapagos lava flows are so well preserved, it is also possible to measure surface exposure ages using in situ cosmic-ray-produced ³He. The exposure ages range from <0.1 to 580 Ka, are consistent with other chronological constraints, and provide the first direct dating of these lava flows. The new age data demonstrate the utility of the technique in this important age range and show that the western Galapagos volcanoes have been erupting distinct compositions simultaneously for the last ∼10 Ka. The magmatic ³He/⁴He ratios range from 6.9 to 27 times atmospheric (R_a), with the highest values found on the islands of Isabela (16.8 R_a for Vulcan Sierra Negra) and Fernandina (23 to 27 R_a). Values from Santa Cruz are close to typical mid-ocean ridge basalt values (MORB, of ∼9 R_a) and Pinta has a ³He/⁴He ratio lower than MORB (6.9 R_a). These data confirm that the plume is centered beneath Fernandina which is the most active volcano in the archipelago and is at the leading edge of plate motion. The data are consistent with previous isotopic studies, confirming extensive contributions from depleted asthenospheric or lithospheric mantle sources, especially to the central islands. The most striking aspect of the helium isotopic data is that the ³He/⁴He ratios decrease systematically in all directions from Fernandina. This spatial variability is assumed to reflect the contribution of the purest plume component to Fernandina magmatism, and shows that helium is a sensitive indicator of plume influence. The highest ³He/⁴He ratios are found in volcanoes with lowest Na₂O(8) and FeO(8), which may relate to source composition as well as degree and depth of melting. An excellent correlation is observed between ³He/⁴He and Nb/La, suggesting that the Galapagos plume source is characterized by high concentrations of Nb (and Ta). The major and trace element correlations demonstrate that helium is controlled by silicate melting and source variations rather than degassing and/or metasomatic processes. Although lavas with radiogenic isotopic compositions tend to have higher ³He/⁴He, the island-wide isotopic variability cannot be explained by simple two components mixing alone. The preferred model to explain the isotopic data includes a heterogeneous plume, centered at Fernandina, which undergoes polybaric melting, and spatial divergence and mixing with asthenospheric material at shallower depths. The unique regional pattern of the helium isotopic data suggests that helium is extracted more efficiently than other elements during the early stages of melting in the ascending plume.     

The Petrology and Geochemistry of Volcanic Rocks on Jeju Island: Plume Magmatism along the Asian Continental Margin

The incompatible element signatures of volcanic rocks formingJeju Island, located at the eastern margin of the Asian continent,are identical to those of typical intraplate magmas. The sourceof these volcanic rocks may be a mantle plume, located immediatelybehind the SW Japan arc. Jeju plume magmas can be divided intothree series, based on major and trace element abundances: high-aluminaalkalic, low-alumina alkalic, and sub-alkalic. Mass-balancecalculations indicate that the compositional variations withineach magma series are largely governed by fractional crystallizationof three chemically distinct parental magmas. The compositionsof primary magmas for these series, using inferred residualmantle olivine compositions, suggest that the low-alumina alkalicand sub-alkalic magmas are generated at the deepest and shallowestdepths by lowest and highest degrees of melting, respectively.These estimates, together with systematic differences in traceelement and isotopic compositions, indicate that the upper mantlebeneath Jeju Island is characterized by an increased degreeof metasomatism and a change in major metasomatic hydrous mineralsfrom amphibole to phlogopite with decreasing depth. The originalplume material, having rather depleted geochemical characteristics,entrained shallower metasomatized uppermost mantle material,and segregated least-enriched low-alumina alkalic, moderatelyenriched high-alumina alkalic, and highly enriched sub-alkalicmagmas, with decreasing depth. KEY WORDS: Jeju Island; magma genesis; mantle plume; subcontinental mantle     

Geochemical fingerprint of the primary magma composition in the marine tephras originated from the Baegdusan and Ulleung volcanoes

The intraplate Baegdusan (Changbai) and Ulleung volcanoes located on the border of China, North Korea, and East/Japan Sea, respectively, have been explained by appeals to both hotspots and asthenospheric mantle upwelling (wet plume) caused by the stagnant Pacific plate. To understand the origin of the Baegdusan and Ulleung volcanism, we performed geochemical analyses on the tephra deposits in the East/Japan Sea basins originating from the Baegdusan and Ulleung volcanoes. The volcanic glass in the tephra from the Baegdusan and Ulleung volcanoes ranged from alkaline trachyte to peralkaline rhyolite and from phonolite to trachyte, respectively. The tephra from the two intraplate volcanoes showed highly enriched incompatible elements, such as Tb, Nb, Hf, and Ta, distinct from those of the ordinary arc volcanoes of the Japanese islands. The straddle distribution of the Th/Yb and Ta/Yb ratios of the tephra deposits from the Baegdusan volcano may originate from the alkali basaltic magma resulting from mixing between the wet plume from the stagnant Pacific plate in the transition zone and the overlying shallow asthenospheric mantle. In contrast, the deposits from the Ulleung volcano show a minor contribution of the stagnant slab to the basaltic magma, implying either partial melting of a more enriched mantle, smaller degrees of partial melting of a garnet-bearing mantle source, or a combination of both processes as the magma genesis. Our study indicated that the Baegdusan and Ulleung volcanoes have different magma sources and evolutionary histories.     

Plume-Associated Ultramafic Magmas of Phanerozoic Age

A parameterization of experimental data in the 0·2–7·0GPa pressure range constrains both forward models of potentialprimary magma compositions that exit the melting regime in themantle and inverse models for computing the effects of olivinefractionation for any olivine-phyric lava suite. This is usedto infer the MgO contents of primary magmas from Gorgona, Hawaii,Baffin Island and West Greenland. They typically contain 18–20%MgO for wide variations in assumed peridotite source compositions,but MgO can drop to 14–17% for Fe-enriched sources, andincrease to 24–26% for fractional melts from Gorgona.Primary magmas with 18–20% MgO have potential temperaturesof 1520–1570°C. For Gorgona picrites with 24% MgO,the potential temperature and initial melting pressure wereabout 1700°C and 8·0 GPa, respectively; melting washot and deep, consistent with the plume model. There are importantrestrictions to magma mixing in mantle plumes. Primary magmasthat exit the melting regime are both well-mixed aggregate fractionalmelts and isolated fractional melts. The latter can originatefrom a hot plume axis and be in equilibrium with olivines havingmg-numbers of 93·0–93·6, but they have MgOcontents and thermal characteristics that are difficult to constrain. KEY WORDS: komatiite; picrite; basalt; MORB; olivine; mantle plumes; primary magmas; equilibrium melting; accumulated fractional melting     

Geochemical Evidence for Slab Melting in the Trans-Mexican Volcanic Belt

Geochemical studies of Plio-Quaternary volcanic rocks from theValle de Bravo–Zitácuaro volcanic field (VBZ) incentral Mexico indicate that slab melting plays a key role inthe petrogenesis of the Trans-Mexican Volcanic Belt. Rocks fromthe VBZ are typical arc-related high-Mg andesites, but two differentrock suites with distinct trace element patterns and isotopiccompositions erupted concurrently in the area, with a traceelement character that is also distinct from that of other Mexicanvolcanoes. The geochemical differences between the VBZ suitescannot be explained by simple crystal fractionation and/or crustalassimilation of a common primitive magma, but can be reconciledby the participation of different proportions of melts derivedfrom the subducted basalt and sediments interacting with themantle wedge. Sr/Y and Sr/Pb ratios of the VBZ rocks correlateinversely with Pb and Sr isotopic compositions, indicating thatthe Sr and Pb budgets are strongly controlled by melt additionsfrom the subducted slab. In contrast, an inverse correlationbetween Pb(Th)/Nd and ¹⁴³Nd/¹⁴⁴Nd ratios, which extend to lowerisotopic values than those for Pacific mid-ocean ridge basalts,indicates the participation of an enriched mantle wedge thatis similar to the source of Mexican intraplate basalts. In addition,a systematic decrease in middle and heavy rare earth concentrationsand Nb/Ta ratios with increasing SiO₂ contents in the VBZ rocksis best explained if these elements are mobilized to some extentin the subduction flux, and suggests that slab partial fusionoccurred under garnet amphibolite-facies conditions. KEY WORDS: arcs; mantle; Mexico; sediment melting; slab melting     

滇西澜沧老厂地区玄武岩岩石成因与构造意义

对滇西澜沧老厂地区玄武岩进行了系统的主微量元素和Nd-Pb同位素地球化学研究,结果表明该玄武岩为典型的洋岛玄武岩(OIB)。Nd-Pb同位素研究表明,玄武岩浆含富集地幔组分。老厂地区玄武岩浆活动可能与地幔热柱有关,玄武岩可能为地幔热柱(软流圈)熔融产生的岩浆与富集的岩石圈地幔岩浆混合的产物。     

Isotope Compositions of Submarine Hana Ridge Lavas, Haleakala Volcano, Hawaii: Implications for Source Compositions, Melting Process and the Structure of the Hawaiian Plume

We report Sr, Nd, and Pb isotope compositions for 17 bulk-rocksamples from the submarine Hana Ridge, Haleakala volcano, Hawaii,collected by three dives by ROV Kaiko during a joint Japan–USHawaiian cruise in 2001. The Sr, Nd, and Pb isotope ratios forthe submarine Hana Ridge lavas are similar to those of Kilauealavas. This contrasts with the isotope ratios from the subaerialHonomanu lavas of the Haleakala shield, which are similar toMauna Loa lavas or intermediate between the Kilauea and MaunaLoa fields. The observation that both the Kea and Loa componentscoexist in individual shields is inconsistent with the interpretationthat the location of volcanoes within the Hawaiian chain controlsthe geographical distribution of the Loa and Kea trend geochemicalcharacteristics. Isotopic and trace element ratios in Haleakalashield lavas suggest that a recycled oceanic crustal gabbroiccomponent is present in the mantle source. The geochemical characteristicsof the lavas combined with petrological modeling calculationsusing trace element inversion and pMELTS suggest that the meltingdepth progressively decreases in the mantle source during shieldgrowth, and that the proportion of the recycled oceanic gabbroiccomponent sampled by the melt is higher in the later stagesof Hawaiian shields as the volcanoes migrate away from the centralaxis of the plume. KEY WORDS: submarine Hana Ridge; isotope composition; melting depth; Hawaiian mantle plume     

Geochemistry, Petrogenesis and Geodynamic Relationships of Miocene Calc-alkaline Volcanic Rocks in the Western Carpathian Arc, Eastern Central Europe

We report major and trace element abundances and Sr, Nd andPb isotopic data for Miocene (16·5–11 Ma) calc-alkalinevolcanic rocks from the western segment of the Carpathian arc.This volcanic suite consists mostly of andesites and dacites;basalts and basaltic andesites as well as rhyolites are rareand occur only at a late stage. Amphibole fractionation bothat high and low pressure played a significant role in magmaticdifferentiation, accompanied by high-pressure garnet fractionationduring the early stages. Sr–Nd–Pb isotopic dataindicate a major role for crustal materials in the petrogenesisof the magmas. The parental mafic magmas could have been generatedfrom an enriched mid-ocean ridge basalt (E-MORB)-type mantlesource, previously metasomatized by fluids derived from subductedsediment. Initially, the mafic magmas ponded beneath the thickcontinental crust and initiated melting in the lower crust.Mixing of mafic magmas with silicic melts from metasedimentarylower crust resulted in relatively Al-rich hybrid dacitic magmas,from which almandine could crystallize at high pressure. Theamount of crustal involvement in the petrogenesis of the magmasdecreased with time as the continental crust thinned. A strikingchange of mantle source occurred at about 13 Ma. The basalticmagmas generated during the later stages of the calc-alkalinemagmatism were derived from a more enriched mantle source, akinto FOZO. An upwelling mantle plume is unlikely to be presentin this area; therefore this mantle component probably residesin the heterogeneous upper mantle. Following the calc-alkalinemagmatism, alkaline mafic magmas erupted that were also generatedfrom an enriched asthenospheric source. We propose that bothtypes of magmatism were related in some way to lithosphericextension of the Pannonian Basin and that subduction playedonly an indirect role in generation of the calc-alkaline magmatism.The calc-alkaline magmas were formed during the peak phase ofextension by melting of metasomatized, enriched lithosphericmantle and were contaminated by various crustal materials, whereasthe alkaline mafic magmas were generated during the post-extensionalstage by low-degree melting of the shallow asthenosphere. Thewestern Carpathian volcanic areas provide an example of long-lastingmagmatism in which magma compositions changed continuously inresponse to changing geodynamic setting. KEY WORDS: Carpathian–Pannonian region; calc-alkaline magmatism; Sr, Nd and Pb isotopes; subduction; lithospheric extension     

Alkaline Lavas in the Volcanic Front of the Western Mexican Volcanic Belt: Geology and Petrology of the Ayutla and Tapalpa Volcanic Fields

The Plio-Quaternary Ayutla and Tapalpa volcanic fields in thevolcanic front of the western Mexican Volcanic Belt (WMVB) containa wide variety of alkaline volcanic rocks, rather than onlycalc-alkaline rocks as found in many continental arcs. Thereare three principal rock series in this region: an intraplatealkaline series (alkali basalts and hawaiites), a potassic series(lamprophyres and trachylavas), and a calc-alkaline series.Phlogopite-clinopyroxenite and hornblende-gabbro cumulate xenolithsfrom an augite minette lava flow have orthocumulate textures.The phlogopite-clinopyroxenite xenoliths also contain apatiteand titanomagnetite and probably formed by accumulation of mineralsfractionated from an augite minette more primitive than thehost. The intraplate alkaline series is probably generated bydecompression melting of asthenospheric mantle as a result ofcorner flow in the mantle wedge beneath the arc. Alkaline magmasmay be common in the WMVB as a result of prior metasomatism(during Tertiary Sierra Madre Occidental magmatism) of the Mexicansub-arc mantle. Generation of the more evolved andesites anddacites of the calc-alkaline series is due to either combinedassimilation and fractional crystallization (AFC) or magma mixing.The preponderance of alkaline and hydrous lavas in this regiondemonstrates that these lava types are the norm, rather thanthe exception in western Mexico, and occur in regions that arenot necessarily associated with active rifting. KEY WORDS: arc basalt; subduction; alkali basalt; minette; hawaiite; metasomatism     

Plume-Lithosphere Interaction and the Origin of Continental Rift-related Alkaline Volcanism--the Chyulu Hills Volcanic Province, Southern Kenya

Geochemical data are presented for primitive alkaline lavasfrom the Chyulu Hills Volcanic Province of southern Kenya, situatedsome 100 km east of the Kenya Rift Valley. In addition to theirprimitive compositions, a striking and ubiquitous feature isa strong but variable depletion in K relative to other highlyincompatible elements when normalized to primitive mantle values.Semi-quantitative models are developed that best explain thepetrogenesis of these lavas in terms of partial melting of asource that contained residual amphibole (but not phlogopite).The presence of amphibole implies a source in the subcontinentallithosphere rather than the asthenosphere. It is suggested thatthe amphibole is of metasomatic origin and was precipitatedin the lithospheric mantle by infiltrating fluids and/or meltsderived from rising mantle plume material. A raised geothermas a consequence of the continued ascent of the plume materialled to dehydration melting of the metasomatized mantle and generationof the Chyulu Hills lavas. It is proposed that the Chyulu HillsVolcanic Province represents an analogue for the earliest stagesof continental rift initiation, during which interaction betweena plume and initially refractory lithosphere may lead to thegeneration of lithospheric melts. KEY WORDS: rift-related alkaline volcanism; residual amphibole; subcontinental lithosphere     

Quaternary volcanism near the Valley of Mexico: implications for subduction zone magmatism and the effects of crustal thickness variations on primitive magma compositions

The Valley of Mexico and surrounding regions of Mexico and Morelos states in central Mexico contain more than 250 Quaternary eruptive vents in addition to the large, composite volcanoes of Popocatépetl, Iztaccíhuatl, and Nevado de Toluca. The eruptive vents include cinder and lava cones, shield volcanoes, and isolated andesitic and dacitic lava flows, and are most numerous in the Sierra Chichináutzin that forms the southern terminus of the Valley of Mexico. The Chichináutzin volcanic field (CVF) is part of the E-W-trending Mexican Volcanic Belt (MVB), a subduction-related volcanic arc that extends across Mexico. The crustal thickness beneath the CVF (∼50 km) is the greatest of any region in the MVB and one of the greatest found in any arc worldwide. Lavas and scoriae erupted from vents in the CVF include alkaline basalts and calc-alkaline basaltic andesites, andesites, and dacites. Both alkaline and calc-alkaline groups contain primitive varieties that have whole rock Mg#, MgO, and Ni contents, and liquidus olivine compositions (≤Fo₉₀) that are close to those expected of partial melts from mantle peridotite. Primitive varieties also show a wide range of incompatible trace element abundances (e.g. Ba 210–1080 ppm; Ce 25–100 ppm; Zr 130–280 ppm). Data for primitive calc-alkaline rocks from both the CVF and other regions of the MVB to the west are consistent with magma generation in an underlying mantle wedge that is depleted in Ti, Zr, and Nb and enriched in large ion lithophile (K, Ba, Rb) and light rare earth (La, Ce) elements. Extents of partial melting estimated from Ti and Zr data are lower for primitive calc-alkaline magmas in the CVF than for those from the regions of the MVB to the west where the crust is thinner. The distinctive major element compositions (low CaO and Al₂O₃, high SiO₂) of the primitive calc-alkaline magmas in the CVF indicate a more refractory mantle source beneath this region of thick crust. In contrast, primitive alkaline magmas from the CVF and other regions of the MVB show compositional similarities to intraplate-type alkali basalts erupted behind the arc in the Mexican Basin and Range province. These similarities are consistent with the hypothesis that slab-induced convection in the mantle wedge beneath the MVB causes advection of asthenospheric mantle from behind the arc to the region of magma generation. Trace element systematics of primitive magmas in the MVB reveal substantial variability in both the extent of mantle wedge enrichment by subduction processes and in the composition of mantle heterogeneities that are related to previous extraction of alkaline to sub-alkaline basaltic melts. Received: 23 June 1998 / Accepted: 23 December 1998     

Flood Basalts, Basalt Floods or Topless Bushvelds? Lunar Petrogenesis Revisited

There is a conspicuous dichotomy in the conventional model oflunar petrogenesis between the total intra-crustal differentiationpostulated for the products of feldspathic volcanism in thelunar highlands and the near absence of differentiation postulatedfor the products of mare volcanism. Both the cumulate mantlemodel, and the selenotherm postulated to accompany genesis ofalleged ‘primary’ mare magmas by remelting of thosecumulates, imply supra-adiabatic thermal gradients in near-solidusmaterials throughout the lunar mantle 4·3–3·2Ga ago. This should have resulted in vigorous convective motion,which has not occurred. There is no positive europium anomalyin the average lunar highland crust. That crust cannot, therefore,have formed by plagioclase flotation from a lunar magma ocean,for which there is no other requirement. There is no negativeeuropium anomaly in the average mantle to be inherited by latermare basalts. Other rocky bodies of lunar size in the SolarSystem have accreted at rates that allowed incorporation ofplenty of volatiles and without forming global magma oceans.Partial melting in the presence of water, followed by near-surfacefractionation and volatile losses can explain the feldspathiccharacter, high incompatible element concentrations and lackof Eu anomaly in the lunar highlands. Volcanic eruption on theMoon must have been accompanied by selective volatilizationlosses of sodium, sulphur and other elements similar to theprocess seen on Io, which can account for the major differencesbetween terrestrial and lunar basalts. Siderophile element depletionin lunar lavas may reflect immiscible sulphide liquid and metalseparation, rather than global impoverishment in such elements,and large ore bodies may have formed close to the lunar surface.Mare basalt volcanism appears to have been a protracted, lowmagma productivity event with few similarities to terrestrialocean-floor, ocean-island, continental flood basalt or komatiitevolcanism. At low pressure the crystallization of plagioclasewell before pyroxene typifies those terrestrial mid-ocean ridgebasalt, ocean-island basalt and continental flood basalt magmas.A similar sequence is demanded of the postulated lunar primarymagmas. Mare basalt hand-specimen and pyroclastic glass beadcompositions do not, however, display the required crystallizationsequence and cannot represent the required primary melt compositions.The true erupted lava compositions which gave rise to the regolithcompositions across all the maria are much more feldspathicthan the majority of large hand specimens and, in common withbasalts on other planets, they are close to low-pressure plagioclase-saturatedcotectic residual liquids which have evolved by removal of gabbrosin crustal magma chambers, or perhaps in giant lava lakes akinto topless Bushveld complexes. Any further debate could be resolvedby a 100 m drill core in a few mare locations. Field provenanceof samples from Mars, a planet half covered by flood basaltsand products of central volcanoes, will be little better thanfor those from the Moon. It will be important to encourage multipleworking hypotheses, rather than to rush to a consensus. KEY WORDS: lunar; basalt; highland; magma ocean; europium     

地幔柱成矿系统:以峨眉山地幔柱为例

地幔柱沟通了地核、地幔、地壳各个圈层之间的物质与能量交换,提供了板内构造岩浆活动及成矿作用的一种重要的动力学机制。峨眉山地幔柱是晚古生代全球最显著的地幔柱活动之一,形成了多种有重大资源经济价值的矿床类型。以峨眉山地幔柱为例,对几种典型矿床类型的产出特征及成因进行了系统分析,阐述了地幔柱成矿系统中各种成矿作用与地幔柱构造岩浆活动的关系及成矿机理。(1)通过对部分典型岩浆硫化物矿床的地质地球化学特征和矿化特征分析,揭示了峨眉山大火成岩省不同矿化特征的岩浆硫化物矿床形成于统一的地幔柱岩浆活动体系,并与峨眉山玄武岩为同源演化关系,岩浆演化过程及硫化物熔离富集过程存在的差异造成了矿化类型的变异。(2)对攀西地区4个超大型钒钛磁铁矿矿床进行了详尽的地质地球化学分析,论述了成矿岩浆的性质、与峨眉山玄武岩的关系及成岩演化过程和成矿模式,表明成矿母岩浆来自于地幔柱,但经历了较大程度的地壳混染作用,提出岩浆的多次补给混合及结晶锋面上发生的双扩散造成的液态分层导致了韵律条带矿石的形成。(3)阐述了滇黔相邻地区玄武岩型自然铜和黑铜矿铜矿化现象,指出玄武岩岩浆气液阶段的自变质作用和玄武岩构造变质热液蚀变改造作用两种方式造成铜矿化富集,岩浆气液阶段的自变质作     

The Geochemistry of Lavas from the Gomores Archipelago, Western Indian Ocean: Petrogenesis and Mantle Source Region Characteristics

New mineral and bulk-rock analyses, as well as Nd, Sr and Pbisotope compositions are presented for lavas from Grande Comore,Moheli and Mayotte, thru of the four main islands of the ComoresArchipelago in the western Indian Ocean, and these data an usedto evaluate the petrogenesis, evolution and mantle source regioncharacteristics of Comorean lavas. The typically silica-undersaturated,alkaline lavas from all three islands can be grouped into twodistinct types: La Grille-type (LGT) lavas, which display strongrelative depletions in K, and Karthala-type (KT) lavas, whichdo not. With the exception of the lavas erupted by La Grillevolcano on Grande Comore, which exhibit the petrographic andgeochemical characteristics expected of primary mantle-derivedmagmas, all Comorean lavas analysed have experienced compositionalmodifications after they segregated from their source regions.Much of this variation can be explained quantitatively by fractionalcrystallization processes dominated by the fractionation ofolivineand clinopyroxene. Semi-quantitative modelling shows that theconsistent and fundamental difference in composition betweenK-depleted LGT lavas and normal KT lavas can be attributed topartial melting processes, provided amphibole is a residualmantle phase after extraction of LGT magmas at low degrees ofmelting. Low absolute abundances of the heavy rare earth elementsin LGT magmas are interpreted to reflect partial melting withinthe garnet stability field In contrast, KT magmas, which donot show relative K depletions, are considered to be the productsof somewhat larger degrees of partial melting of an amphibolefreesource at comparatively shallower depths. Whereas the Nd andSr isotopic compositions of Comorean lavas (which show a significantrange: ⁸⁷Sr/⁸⁶Sr = 0.70319–0.70393; ¹⁴³Nd/Nd = 0.51263–0.51288)bear evidence for a time-averaged depletion in incompatibleelements, the high incompatible element abundances of the lavasare interpreted to reflect the effects of a recent mantle enrichmentevent. At depths well within the garnet stability field thismantle enrichment is interpreted to have taken the form of modalmetasomatism with the introduction of amphibole (giving riseto the source of LGT magmas), whereas cryptic metasomatism tookplace at shallower levels (giving rise to the source of KT lavas).The Nd, Sr and Pb isotope signature of the majority of Comoreanlavas (both LGT and KT) is proposed to be the result of predominant4contributions from a somewhat heterogeneous source4 4 4 presentativeof the ambient sub-Comorean mantle, comprising a mixture betweena HIMU component and a component on the depleted portion ofthe mantle array (possibly the source of Indian Ocean MORB),with only limited contributions from an EM I plume component.The lavas erupted by Karthala volcano (the youngest Comoreanlavas), however, have significantly different isotopic compositionsfrom all other Comorean lavas (lower ¹⁴³Nd/¹⁴⁴Nd and higher⁸⁷Sr/⁸⁶Sr), suggesting increased contributions from the EM Icomponent. KEY WORDS: basalt petrogenesis; Comores; mantle geochemistry; ocean island basalts ^*Telephone: 27-21-6502921. Fax: 27-21-6503781 e-mail: alr{at}geology.uct.ac.za.     

Longitudinal Petrochemical Variation in the Mackenzie Dyke Swarm, Northwestern Canadian Shield

Previous magnetic fabric studies of the giant, radiating, 1.27Ga Mackenzie dyke swarm concluded that flow patterns withinthe dykes support the concept of a mantle plume that is centredbeneath the swarm focus and supplies magma to overlying floodbasalts and developing radial dykes. To examine petrochemicalimplications of the model, compositional variation within thebasalt sequence is compared with that of the dykes along a ‘streamline’ of the swarm between 400 km (just beneath the lavas)and 2100 km from its focus and in a parallel segment farthereast. Evolution of tholeiitic magmas of the main sampled streamis recorded in the upward change of composition in the lavasequence from mg-numbers of 70 to 35. Underlying (feeder) dykeshave a comparable range, but outward along the swarm the rangeof compositions narrows progressively towards its more evolvedend, and at 2100 km, dyke compositions match those in upperlevels of the lava sequence. REE and other trace element abundancesshow a similar contraction in range, and a shift towards moreevolved compositions, both upward in the lava sequence and outwardalong the swarm. Apart from complications owing to crustal contamination,fundamental attributes of the magma (e.g. Zr/Y) change littlewith stratigraphic level or distance from the focus. The moreeasterly stream differs in its high proportion of alkalic compositions,suggesting the existence of distinctive subswarms. Normativemineral variation plots are consistent with fractionation inhigh- (main stream) to low-level (eastern stream) crustal magmachambers. Mackenzie magmatism is compatible with the plume model.Domal uplift related to plume activity initiated central grabencollapse and outward-extending radial fractures, thus providingaccess to plume-derived magmas and loci for magma chamber anddyke swarm development. Multiple magma chambers, forming aroundthe apex, each fed relatively independent subswarms of dykes.Uplift, accompanying fractionation, provided increasing magmatichead by which fractionating magmas could be dispatched to successivelygreater distances. This, and crystal settling in transport,accounts for the increasingly evolved nature of dykes with distancefrom the source. KEY WORDS: mantle plume; flood basalts; dyke swarm; petrochemistry; Precambrian ^*Corresponding author.