The Petrogenetic Significance of Interstratified High- and Low-Ti Basalts in Central Nicaragua

Basalts erupted from recent volcanoes in central Nicaragua canbe divided into distinct high-and low-Ti suites. Low-Ti basaltshave higher concentrations of LILE and LREE than high-Ti basalts.In addition, low-Ti basalts have obviously higher Ba/La, La/Sm,and ⁸⁷Sr/⁸⁶Sr, and lower Ti/Zr, than high-Ti basalts. In contrast,there are no mineralogical or petrographic differences betweenthe two suites. The differences between the high-and low-Ti basalts of centralNicaragua are inherited from their source regions. The primarymagmas of both are generated in the mantle wedge. However, low-Tiprimary magmas come from parts of the wedge which bear a strongsubduction zone signature, including that of subducted pelagicsediment. On the other hand, the primary magmas of the high-Tibasalts are generated in parts of the wedge relatively freeof subduction zone influence. Subducted pelagic sediment can therefore be a key source componentat active continental margins as well as at island arcs. Pelagicsediment could also be responsible for subtle high-field-strengthelement fractionations within subduction zone magmas. The mantlewedge beneath Nicaragua, which is variably modified by the subductingplate, is relatively enriched suboceanic mantle.     

Volcanism in the Vitim Volcanic Field, Siberia: Geochemical Evidence for a Mantle Plume Beneath the Baikal Rift Zone

The Baikal Rift is a zone of active lithospheric extension adjacentto the Siberian Craton. The 6–16 Myr old Vitim VolcanicField (VVF) lies approximately 200 km east of the rift axisand consists of 5000 km³ of melanephelinites, basanites, alkaliand tholeiitic basalts, and minor nephelinites. In the volcanicpile, 142 drill core samples were used to study temporal andspatial variations. Variations in major element abundances (e.g.MgO = 3·3–14·6 wt %) reflect polybaric fractionalcrystallization of olivine, clinopyroxene and plagioclase. ⁸⁷Sr/⁸⁶Sr_i(0·7039–0·7049), ¹⁴³Nd/¹⁴⁴Nd_i (0·5127–0·5129)and ¹⁷⁶Hf/¹⁷⁷Hf_i (0·2829–0·2830) ratiosare similar to those for ocean island basalts and suggest thatthe magmas have not assimilated significant amounts of continentalcrust. Variable degrees of partial melting appear to be responsiblefor differences in Na₂O, P₂O₅, K₂O and incompatible trace elementabundances in the most primitive (high-MgO) magmas. Fractionatedheavy rare earth element (HREE) ratios (e.g. [Gd/Lu]_n > 2·5)indicate that the parental magmas of the Vitim lavas were predominantlygenerated within the garnet stability field. Forward major elementand REE inversion models suggest that the tholeiitic and alkalibasalts were generated by decompression melting of a fertileperidotite source within the convecting mantle beneath Vitim.Ba/Sr ratios and negative K anomalies in normalized multi-elementplots suggest that phlogopite was a residual mantle phase duringthe genesis of the nephelinites and basanites. Relatively highlight REE (LREE) abundances in the silica-undersaturated meltsrequire a metasomatically enriched lithospheric mantle source.Results of forward major element modelling suggest that meltingof phlogopite-bearing pyroxenite veins could explain the majorelement composition of these melts. In support of this, pyroxenitexenoliths have been found in the VVF. High Cenozoic mantle potentialtemperatures (1450°C) predicted from geochemical modellingsuggest the presence of a mantle plume beneath the Baikal RiftZone. KEY WORDS: Baikal Rift; mafic magmatism; mantle plume; metasomatism; partial melting     

Petrology and chemistry of recent lavas in the northern Marianas: Implications for the origin of island arc basalts

Petrologic and chemical data are presented for samples from five volcanically active islands in the northern Marianas group, an intra-oceanic island arc. The data include microprobe analyses of phenocryst and xenolith assemblages, whole rock major and trace element chemistry including REE, and Sr isotope determinations (⁸⁷Sr/⁸⁶Sr=0.7034±0.0001). Quartz-normative basalt and basaltic andesite are the most abundant lava types. These are mineralogically and chemically similar to the mafic products of other intra-oceanic islands arcs. It is suggested, however, that they are not typical of the ‘island arc tholeiitic’ series, having Fe enrichment trends and K/Rb, for example, more typical of calc-alkaline suits. Major and trace element characteristics, and the presence of cumulate xenoliths, indicate that extensive near surface (< 3 Kb) fractionation has occurred. Thus, even least fractionated basalts have low abundances of Mg, Ni and Cr, and high abundances of K and other large cation, imcompatible elements, relative to ocean ridge tholeiites. However, abundances of REE and small cation lithophile elements, such as Ti, Zr, Nb, and Hf are lower than typical ocean ridge tholeiites. The REE data and Sr isotope compositions suggest a purely mantle origin for the Marianas island arc basalts, with negligible input from subducted crustal material. Thus, subduction of oceanic lithosphere may not be a sufficient condition for initiation of island arc magmatism. Intersection of the Benioff zone with an asthenosphere under appropriate conditions may be requisite. Element ratios and abundances, combined with isotopic data, suggest that the source for the Marianas island arc basalts is more chondritic in some respects, and less depleted in large cations than the shallow (?) mantle source for ocean ridge tholeiites.     

The Four Plutonic Belts of the Transhimalaya-Himalaya: a Chemical, Mineralogical, Isotopic, and Chronological Synthesis along a Tibet-Nepal Section

The geochemistry (major, trace element, O- and Sr-isotope ratios)and petrology of the Transhimalaya, North Himalaya, High Himalayaand ‘Lesser Himalaya’ plutonic belts are comparedbased on the analyses of up to 492 samples. The composite Transhimalaya batholith is subalkaline or monzoniticin character rather than calc-alkaline. Its genesis was probablyclosely related to subduction processes associated with strike-slipmovement. It was emplaced on both sides of the boundary betweenan earlier metavolcanic arc and a continental margin. Two principalperiods of magmatic activity occurred: Upper Cretaceous and,particularly in this region, Eocene at the time of the India-Eurasiacollision when sediments may have become involved in the subductionprocess. Magmatic differentiation, characterized by two superimposedstages of evolution, and hybridization processes, involvingboth basic and acidic magmas, can account for the genesis ofthe different plutonic units. Although a continental contributionis implied, the isotopic data (6.8 < ¹⁸O < 9?2; 0?704< ⁸⁷Sr/⁸⁶Sr, < 0?707) preclude a significant contributionfrom either old crust or surface derived sediments. The North, High and ‘Lesser’ Himalaya plutonic beltsare fundamentally different and correspond to aluminous associationsof two groups of ages (Lower Palaeozoic for the ‘LesserHimalaya’ and part of the North Himalaya; Upper Cenozoicfor the High Himalaya and part of the North Himalaya). Theyare all high-¹⁸O (9 < ¹⁸O < 14) granites and adamelliteswith high initial ⁸⁷Sr/⁸⁶Sr, ratios (0?709 to < 0?740). TheLower Palaeozoic group was generated within the Gondwana continentalcrust, independent of any true orogenesis, with a probable butlimited contribution from the mantle. High Himalaya and NorthHimalaya Cenozoic plutons are directly linked to the activityof the Main Central Thrust. They were derived by similar anatecticprocesses of the same continental source rocks. The small butdistinct chemical and mineralogical differences among the plutonsare related to the increase in the intensity of anatexis ongoing towards the north and the east.     

Manam Island, Papua New Guinea: Petrology and Geochemistry of a Low-TiO2 Basaltic Island-Arc Volcano

Manam volcano consists of relatively mafic and compositionallysimilar tholeiitic basalts and low-SiO₂ andesites that are characterizedby notably low (mainly 0?3–0?35 weight per cent) TiO₂contents. These rocks provide an ideal opportunity to investigateboth the extent of depletion in their peridotite magma-sourceregions (which are evidently similar in many respects to thehighly depleted sources of boninitic magmas), and the interplayof the high-level processes of magma mixing, crystal fractionation,and upper crustal contamination, in an island-are volcano. Manamrocks have pronounced enrichments in Rb, Ba, K, and Sr relativeto the light rare-earth elements and, especially, to the high-field-strengthelements (Sr/Ti values are exceptionally high). However, thereis no compelling evidence that these enrichments were causedby addition of a hydrous, slab-derived component to the peridotitesource region. Nd and Sr-isotope ratios plot within the oceanicmantle array; ²⁰⁷Pb/²⁰⁴Pb values are only slightly higher thanthose for oceanic rocks; and the absence of hydrous minerals,the early crystallization and modal preponderance of plagioclaseover pyroxene, high estimated quenching temperatures, and lowwater contents in the Manam rocks, are all evidence that themagmas crystallized under markedly water-undersaturated conditions.Unusually anorthite-rich plagioclase phenoerysts in the morediffrentiated rocks may correspond to crystallization underhigher water-vapour pressures, possibly caused by influxes ofgroundwater, or they may be accidental xenocrysts. Fractionationof olivine, clinopyroxene, and spinel (early chromite followedby magnetite) has dominated the evolution of the magma series.However, clear correlations between incompatible trace-elementratios, ⁸⁷Sr/⁸⁶Sr, and 100 Mg/(Mg ? Fe^2?) values are convincingevidence for an accompanying mixing process—either of(1) two basaltic magma types (one more fractionated and lowerin ⁸⁷Sr/⁸⁶Sr than the other), or (2) pristine magmas and contaminantfrom basaltic conduit and reservoir wall rocks. Wall-rock contaminationis the less likely process, and is the more difficult one toidentify, particularly if it accompanied magma mixing.     

Minor-element and Sr-isotope geochemistry of tertiary stocks,Colorado mineral belt

Rocks of the northeast portion of the Colorado mineral belt form two petrographically, chemically and geographically distinct rock suites: (1) a silica oversaturated granodiorite suite; and (2) a silica saturated, high alkali monzonite suite. Rocks of the granodiorite suite generally have Sr contents less than 1000 ppm, subparallel REE patterns and initial ⁸⁷Sr/ ⁸⁶Sr ratios greater than 0.707. Rocks of the monzonite suite are restricted to the northeast part of the mineral belt, where few rocks of the granodiorite suite occur, and generally have Sr contents greater than 1000 ppm, highly variable REE patterns and ⁸⁷Sr/⁸⁶Sr initial ratios less than 0.706.Despite forming simple, smooth trends on major element variation diagrams, trace element data for rocks of the granodiorite suite indicate that they were not derived from a single magma. These rocks were derived from magmas having similar REE patterns, but variable Rb and Sr contents, and Rb/Sr ratios. The preferred explanation for these rocks is that they were derived by partial melting of a mixed source, which yielded pyroxene granulite or pyroxenite residues.The monzonite suite is chemically and petrographically more complex than the granodiorite suite. It is subdivided here into alkalic and mafic monzonites, and quartz syenites, based on the textural relations of their ferromagnesian phases and quartz. The geochemistry of these three rock types require derivation from separate and chemically distinct magma types. The preferred explanation for the alkalic monzonites is derivation from a heterogeneous mafic source, leaving a residue dominated by garnet and clinopyroxene. Early crystallization of sphene from these magmas was responsible for the severe depletion of the REE observed in the residual magmas. The lower Sr content and higher Rb/Sr ratios of the mafic monzonites requires a plagioclase-bearing source.The Sr-isotope systematics of the majority of these rocks are interpreted to be largely primary, and not the result of crustal contamination. The positive correlation of Rb/Sr and ⁸⁷Sr/⁸⁶Sr ratios for the least fractionated samples indicate that the sources from which parent magmas of both the granodiorite and monzonite suites were derived are Precambrian in age.     

Geochemistry of the Gabbro--Diorite--Tonalite--Trondhjemite Suite of southwest Finland and its Implications for the Origin of Tonalitic and Trondhjemitic Magmas

Rocks of the Proterozoic gabbro—diorite—tonalite—trondhjemitesuite of southwest Finland were analyzed for major elements,REE, Rb, Sr, Ba, U, Th, and isotopic composition of Sr and O.Petrographic continuity from hornblendite through hornblendegabbro, hornblende—biotite diorite, hornblende—biotitetonalite and trondhjemite is reflected in regular variationof major and trace elements in samples ranging from 42 to 74per cent of SiO₂. The suite is calc alkaline—trondhjemiticand is distinguished from ‘normal’ calc-alkalinesuites by the increase in Na₂O and decrease in K₂O and REE concentrationsfrom intermediate to silicic rocks. Prior to solidification, the magmas may have undergone additionof water from metamorphic country rock producing a variationof whole-rock O₁₈ values of from 6.0 to 11.3 per mil. This processmay have been accompanied by introduction of Rb and Sr and removalof Ba from intermediate to trondhjemitic magmas. A linear whole-rockisochron age of 1.9 x 10⁹ years may only be approximate owingto the possibility of Rb and Sr exchange. REE concentrationsshow regular variation through the suite and were not noticeablyaffected by exchange processes. Samples ranging from 50 to 60per cent SiO₂ show chondrite-normalized light REE contents rangingfrom 40 to 160 whereas heavy REE contents are constant at 10,and there are no Eu anomalies. Samples ranging from 60 to 74per cent SiO₂ have REE patterns which decrease in both lightand heavy REE with increasing SiO₂ to values of 10 and 1 timeschondritic values respectively, and show increasingly positiveEu anomalies. Three possible models for magma genesis are consistent withthe petrographic, major and trace element variations, isotopicdata, and experimental petrologic studies: (1) fractional crystallizationof a gabbroic liquid involving hornblende, plagioclase, andbiotite as the major precipitating phases; (2) partial meltingof amphibolite leaving a hornblende-rich residue; (3) partialmelting of amphibolite or eclogite leaving an eclogitic residue.Model (1) is preferred because of the presence of hornblende-cumulaterocks, and because there is a continuum of compositions fromgabbro to trondhjemite.     

The Arc Lavas of the Shirahama Group, Japan: Sr and Nd Isotopic Data Indicate Mantle-Derived Bimodal Magmatism

New Sr and Nd isotopic data are presented and integrated withprevious data for the Shirahama Group Mio-Pliocene medium-Kvolcanic are suite of south-central Honshu, Japan. Main resultsare: (1) The Shirahama lavas range in ⁸⁷Sr/⁸⁶Sr from 0.70315to 0.70337 and in ¹⁴³Nd/¹⁴⁴Nd from 0.51298 to 0.51306; the Srand Nd isotopic data cluster tightly within the mantle array,and all lie within an overlapping field of mid-ocean ridge basaltand ocean island basalt; (2) small differences exist among theShirahama tholeiitic series, calc-alkaline series and mixedlavas. The present isotopic data are consistent with a previouslypublished model, which proposes that chemical variations inmagmas of coexisting tholeiitic and calc-alkaline series areproduced through crystal fractionation from mantle-derived magmasof basalt and magnesian andesite, respectively. Moreover, thetholeiitic series and the calc-alkaline series are isotopicallyidentical. Thus, both magma series can be derived from a sourcemantle with the same isotopic composition, supporting the hypothesisof simultaneous generation of basalt and magnesian andesitemagmas from a single diapir rising through the mantle wedgeabove the subduction zone. The differences of water contentand temperature within the diapir are again thought to havebeen produced through dehydration and heating of an isotopicallyhomogeneous hydrous diapir. The isotopic data show that thehigh-SiO₂ lavas have the same isotopic compositions as moremafic lavas. These data and liquid lines of descent of the Shirahamamagmas suggest that even rhyolites can be produced by differentiationfrom mantle-derived magmas without crustal contamination. Analysesfrom 38 other arc volcanoes have been compiled to investigatethe intravolcano variability of ⁸⁷Sr/⁸⁶Sr. Twelve of these displayno intravolcano strontium isotopic variability, as is the casewith the Shirahama Group, but others show greater variationof ⁸⁷Sr/⁸⁶Sr from individual volcanic centers, presumably reflectingcrustal contamination. Most of the latter volcanoes are underlainby thick continental crust. It is noteworthy, however, thatthe greater variations of ⁸⁷Sr/⁸⁶Sr correlate with SiO₂ content;andesites or dacites, not basalts, from the same volcano havethe lowest ⁸⁷Sr/⁸⁶Sr, and these rocks are calc-alkaline in termsof FeO^*/MgO and SiO₂ Theoretically, assimilation of continentalcrust by the isotopically uniform Shirahama magmas could producethese relationships. Given that mantle-derived basalt and magnesianandesite both encounter continental crust on their ascent tothe surface, the hotter basalt magma would assimilate more crustalwallrocks than the cooler andesite, resulting in the basaltbeing more radiogenic. Fractional crystallization, magma mixing,and/or assimilation-fractional crystallization of these magmasin crustal magma chambers could produce large compositionalvariations, but the derivatives of the hotter basaltic magmas(tholeiitic series in the broad sense) would display greatercontamination than those derived from the cooler andesitic magmas(calc-alkaline series). ^*Telephone: 81-858-43-1215. Fax: 81-858-43-2184. e-mail: tamura{at}misasa.okayam-u.ac.jp     

Na-rich Partial Melts from Newly Underplated Basaltic Crust: the Cordillera Blanca Batholith, Peru

The late Miocene Cordillera Blanca Batholith lies directly overthick (50 km) crust, inboard of the older Cretaceous CoastalBatholith. Its peraluminous ‘S’ type mineralogyand its position suggest recycling of continental crust, whichis commonly thought to be an increasingly important componentin magmas inboard of continental margins. However, the peraluminous,apparent ‘S’ type character of the batholith isan artefact of deformation and uplift along a major crustallineament. The batholith is a metaluminous ‘I’ typeand the dominant high-silica rocks (>70%) are Na rich withmany of the characteristics of subducted oceanic slab melts.However, the position of the batholith and age of the oceaniccrust at the trench during the Miocene preclude slab melting.Instead, partial melting of newly underplated Miocene crustis proposed. In this dynamic model newly underplated basalticmaterial is melted to produce high-Na, low HREE, high-Al ‘trondhjemitic’type melts with residues of garnet, clinopyroxene and amphibole.Such Na-rich magmas are characteristic of thick Andean crust;they are significantly different from typical cole-alkaline,tonalite-grano-diorite magmas, and their presence along thespine of the Andes provokes questions about models of trondhjemitegenesis by melting of subducted oceanic crust, as well as anygeneralized, circum-Pacific model involving consistent isotopicor chemical changes inboard from the trench. KEY WORDS: batholith; modified ‘I’ type granite; Na-rich magma; thick crust ^* Corresponding author.     

Archean High-Mg Granodiorite: A Derivative of Light Rare Earth Element-enriched Monzodiorite of Mantle Origin

The Roaring River Complex, Superior Province, Canada, containsrocks varying from diorite and monzodiorite to granodioritewhich are characterized by high mg-numbers (0.43–0.62),high abundances of Cr (150 ppm), Sr (500–2000 ppm), Ba(1000–2500ppm), and P2O5 (0.5 wt.%), low Rb/Sr ratios (001–0.02),and steeply fractionated, subparallel REE patterns (Ce_n =65–170,Yb_n = 3–6) without Eu anomalies. The continuous compositionalvariation of the rock suite provides a basis for testing thevarious processes thought to have been important in the extractionof granodiorite magmas from the mantle during the Archean. Weconsider (1) the relative roles of partial melting, crystallizationfractionation, and other processes; (2) the role of garnet orother phases in controlling the steep REE patterns of the rocks;and (3) the chemical and isotopic composition of the sourceregion. The subparallel and decreasing REE patterns with increasingsilica, and the ten-fold variation and high abundances of Crand Ni within the diorite-granodiorite series are not consistentwith different extents of melting of basic crust. The scatterin bivariate plots for closely spaced samples does not supportsimple two-component mixing or liquid immiscibility. The compositionalvariation can be explained by crystallization differentiation(from 0 to 90%) of monzodioritic magma through separation ofdioritic cumulates containing clinopyroxene, hornblende, biotite,plagioclase, K-feldspar, and accessories. The compatibilityof the REEs resulted principally from crystallization of spheneand apatite. The parental monzodioritic magmas with their high mg-numbers,Ni, and Cr contents were derived from peridotitic source rocks(mg-numbers>0.80) with low Rb/Sr ratios (<0.02) and light-REEenrichment relative to chondrites. The differences in the REEpatterns of monzodiorite samples do not support, nor rule out,garnet in the residue for melting. If the monzodioritic meltswere derivatives of other melts, the parent melts would havebeen similar to high-Mg monzodiorites (‘sanukitoids’)recognized as components of other diorite-granodiorite bodiesin the region. An Rb-Sr whole-rock isochron (n = 25) yields a minimum crystallizationage of 2623 Ma (?19) with initial ⁸⁷Sr/⁸⁶Sr = 070134 (?000004;MSWD=l.8). Sm-Nd isotope data for six rocks yield _Nd (2623)=+0.8 ?0.3. The isotope data indicate a source region with long-termRb/Sr of 0.02, similar to depleted mantle, and light-REE depletionrelative to chondrites. The peridotite source to the diorite-granodioriteseries became light-REE enriched before melting through theaddition of a light-REE component of a fluid or melt. In generating Archean granodiorite with suitably high mg-numbers,and Ni, Cr, Sr, Ba, P2O5, and light-REE contents, these dataindicate: (1) the importance of crystallization differentiationof high-Mg monzodioritic parent magmas, (2) that the steep REEpatterns may be a characteristic of the source rocks, and (3)light-REE-enriched, peridotitic sources were melting and contributingsiliceous material directly to the Archean crust.     

The Petrology and Geochemistry of Mafic Igneous Rocks in the Anorthosite-Bearing Adirondack Highlands, New York

Igneous rocks of broadly basaltic composition are widely distributedin the anorthosite-bearing Adirondack Highlands of New York.They constitute a mafic series of rocks that occurs on the marginsof the anorthosite series and up to 50 km away from the anorthosite.On an Sr reference diagram, the mafic series has an apparentinitial ⁸⁷Sr/⁸⁶Sr value of 0.7036, which is consistent with1100 Ma subcontinental mantle. The series was probably emplacedin the interval 1150–1100 Ma, during which time anorthositeand granitic magmas were also emplaced. The rocks were metamorphosedshortly thereafter ({small tilde} 1088 Ma) under upper-amphiboliteand granulite facies conditions. Individual bodies of maficrocks range in thickness from small enclaves and layers >>1 m thick to large lensoid masses several tens of meters thick.Despite deformation and metamorphic recrystallization, manyof the rocks retain chiHed margins, cross-cutting relationships,and relict igneous textures. Selected samples of the mafic series have been analyzed fortheir major and trace element compositions. Metamorphism didnot significantly alter the igneous geochemical relationships,and the rocks retain mantle-like values for Zr/Nb, K/Zr, K/Rb,Rb/Sr, and ⁸⁷Sr/⁸⁶Sr. The most primitive rocks of the seriesare silica-undersaturated gabbroic troctolites, and the moreevolved rocks are basaltic in composition. The mafic seriesas a whole has high abundances of A1₂O₃, FeO, the light rareearth elements (LREE), and other incompatible trace elements.Even the most geochemically primitive compositions have highFeO contents. The Fe enrichment and Si depletion that are shownby chemically evolved compositions are consistent with a Fennertrend of fractionation. Low levels of normative di indicatethat high Fe is not a result of the extensive fractionationof cpx. The geochemical trends that are defined by the traceelements, including the REE, suggest the basaltic rocks maybe differentiates of a parental magma of gabbroic troctolitecomposition. The main compositional trend of the mafic seriescan be simulated by 61% crystallization of olivine and plagioclaseof a gabbroic troctolite, followed by 13% crystallization ofolivine, plagioclase, clinopyroxene, and titanomagnetite atthe final stages. The modelled ratio of olivine to plagioclasecrystallization changes from 1–8: 1 to 0–64: 1.These non-cotectic ratios may reflect a delay in the crystallizationof plagioclase relative to olivine, possibly as a result oflow nucleation rates. At later stages of differentiation, plagioclasewas more important in the crystallization of the series. Delayedcrystallization of plagioclase may also have resulted in thehigh A1₂O₃ contents and enhancement of Eu and Sr relative toother trace elements at early stages of differentiation. The mafic series and the silica-saturated anorthosite seriestogether form an anorthosite-norite-troctolite (ANT) suite.More than one mantle composition may have been involved in generatingthe Adirondack mafic magmas. The rocks retain geochemical evidenceof a source that was depleted in basaltic components (cpx) butenriched in Fe, Ti, K, and the LREE. Previously documented evidenceof anticlockwise cooling paths (Bohlen, 1987) and of a regionalgravity high centered beneath the Adirondack region (Simmons,1964) suggests that of the continental crust by basaltic magmasmay have underplating of the continental crust by basaltic magmasmay have been an important feature of the tectonic evolutionofthe region. A model of mantle upwelling beneath thinning continentalcrust explains the geochemically hybrid nature of the maficseries magmas. It is also consistent with a tectonic settingof incipient or failed continental rifting, to which the generationof the anorthosites is commonly attributed.     

Contrasting Archean and Proterozoic lithospheric mantle: isotopic evidence from the Shonkin Sag sill (Montana)

²⁰⁷Pb/²⁰⁴Pb versus ²⁰⁶Pb/²⁰⁴Pb model ages using Shonkin Sag data and published analyses for magmas of the Cenozoic Wyoming-Montana alkaline province (WYMAP) provide evidence of an Archean age for the subcontinental lithospheric mantle (SLM) associated with the Wyoming craton. The SLM imprint on magmas is expressed as Ba, Ta, Nb and Ti "anomalies" which correlate with radiogenic isotopic data, and it resembles a subduction imprint on Cenozoic south-western USA basalts (SWUSAB). However the latter give Proterozoic Pb isotope model ages. Although the Archean and Proterozic model ages may represent mixing lines, the fact that they resemble the ages for continental crust cut by WYMAP and SWUSAB respectively indicates that the age of the underlying SLM helped control the "isochron" slopes and inferred "ages". Lower ¹⁴³Nd/¹⁴⁴Nd and ²⁰⁶Pb/²⁰⁴Pb but comparable ⁸⁷Sr/⁸⁶Sr for WYMAP suggest that SLM associated with Archean cratons has lower Sm/Nd, U/Pb and Rb/Sr ratios than SLM associated with SWUSAB Proterozic terranes, regardless of when the subduction imprint or imprints developed. WYMAP magmas have high Pb/Zr ratios indicating that Archean SLM, like Archean continental crust, is enriched in Pb compared to Proterozoic SLM. If the enrichment was Archean, it implies that higher Archean heat flow enhanced Pb transfer from the subducting slab to overlying lithospheric mantle and crust. A subducted sediment imprint on the SLM is also consistent with high i¹⁸O values for the Shonkin Sag. Low TiO₂ in WYMAP may reflect a residual mantle TiO₂ phase. If so, the Nb "missing" from crustal and oceanic mantle reservoirs may reside in rutile of Archean SLM. Isotopic similarities between WYMAP and EM1 oceanic island basalts may reflect the presence of delaminated, Archean SLM in the oceanic mantle, although low Pb/Zr ratios and a lack of Ti, Nb and Ta anomalies in oceanic island basalts deserve further investigation.     

Geochemistry and origin of volcanic rocks of the Andes (26°–28°S)

Mesozoic to Recent volcanic rocks from a transect of the Central Andes between latitudes 26 ° and 28 ° South in northern Chile and Argentina show chemical and temporal zonation with respect to the Peru-Chile trench. Jurassic to Eocene lavas occur closer to the trench and are comparable to calc-alkaline rocks of island arcs. Eastwards they are followed by Miocene to Quaternary sequences of typical continental margin calc-alkaline rocks which have higher contents of K, Rb, Sr, Ba, Zr, and REE and also higher K/Na and La/Yb ratios. The rocks occurring farthest from the trench have shoshonitic affinities. The distribution of major and trace elements is consistent with a model in which magmas were derived by anatexis of an upper mantle source already enriched in LILE and located above the descending oceanic slab. It is suggested that the chemical variations across the volcanic belt reflect systematic changes in the composition of the magmas due to a decreasing degree of partial melting with increasing depth, and probably also due to the heterogeneity of the source materials.     

The Lead, Neodymium and Strontium Isotopic Structure of Ocean Ridge Basalts

Pb-, Nd- and Sr-isotope compositions and U, Pb, Sm, Nd, Rb andSr concentrations are reported for samples of basaltic glassand altered substrates from spreading centres in the Atlantic,Indian and Pacific Oceans. Correlations are shown to exist between^{208, 207, 206}Pb/²⁰⁴Pb ratios, and ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Ndratios in basaltic glasses, but they are dominated by samplesfrom the Mid-Atlantic Ridge. Whereas basaltic glasses from EastPacific spreading centres exhibit smaller variability in Pb,Sr and Nd isotope compositions than Atlantic samples, seamountsamples from the E. Pacific have a similar range of Pb-isotopecompositions as Mid-Atlantic Ridge glasses. Contamination ofbasaltic magmas by altered oceanic crust or sediments is notconsidered to be of prime importance in determining the isotopicstructures of MORB glasses. It is proposed that the isotopicheterogeneity in the mantle beneath the Pacific and Atlanticis similar, but magma generation processes associated with fastspreading ridges of the East Pacific more effectively eradicateheterogeneities in the erupted basalts. Alteration of oceanic crust is further investigated with respectto the relative response of the U–Pb, Sm–Nd andRb–Sr systems, and the role of recycled oceanic crustin producing the mantle heterogeneities is evaluated.     

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.     

Post-collisional granitoids from the Dabie orogen: New evidence for partial melting of a thickened continental crust

The geological implications of granitoid magmas with high Sr/Y and La/Yb are debated because these signatures can be produced by multiple processes. This study presents comprehensive major and trace element compositions and zircon SHRIMP U-Pb age data of 81 early Cretaceous granitoids and 4 mafic enclaves from the Dabie orogen to investigate partial melting of the thickened lower continental crust (LCC). On the basis of Sr/Y ratios, granitoids can be grouped into two magma series: (i) high Sr/Y granitoids (HSG) and (ii) normal granitoids with low Sr/Y. Relative to normal granitoids, HSG display the following distinct chemical features: (1) at given SiO₂ and CaO contents, the HSG have significantly higher Sr than normal granitoids, defining two different trends in Sr versus SiO₂, CaO diagrams; (2) highly depleted heavy rare earth element (REE) relative to middle and light REE with (Dy/Yb)_N and (La/Yb)_N up to 3.2 and 151, respectively; (3) variable and higher Nb/Ta; and (4) positive correlations among Sr/Y, (Dy/Yb)_N, (La/Yb)_N, and Nb/Ta.High Sr/Y, (La/Yb)_N, (Dy/Yb)_N, and Sr/CaO of HSG do not correlate with major elements (e.g., SiO₂). Large variations in these ratios at a given SiO₂ content indicate that these features do not reflect magma mixing or fractionation. HSG have higher Sr at a given CaO content and larger variation of (Dy/Yb)_N than old crustal rocks (including exposed basement, global mafic LCC xenoliths, high Sr/Y TTG suites, and adakites in modern arcs). This precludes inheritance of the HSG chemical features from these source rocks. Instead, the chemical features of the HSG are best explained by partial melting of the thickened LCC with garnet-dominant, plagioclase-poor, and rutile-present residual lithologies. The coupled chemical features of the HSG are not observed in post-collisional granitoids younger than ca.130 Ma, indicating removal of the eclogitic source and/or residuum of HSG underneath the orogen. These characteristic chemical relationships in the Dabie HSG may be applied to distinguish partial melts of thickened LCC from high Sr/Y intermediate-felsic magmatic rocks which do not show clear indications for melting depth.     

Contrasted melting relations in a pyrolite upper mantle under mid-oceanic ridge,stable crust and island arc environments

The pyrolite model composition provides a satisfactory source composition for mantle-derived magmas insofar as major elements and “compatible” trace elements are concerned but there is evidence for mantle inhomogeneity in the abundances of “incompatible” minor and trace elements (e.g., K, Ti, P, Rb, Sr, light rare earth elements etc.). The composition of a magma, assuming a constant source composition, varies according to the pressure, temperature and water pressure or water content of the source region. The latter two variables essentially determine the degree of partial melting of the source region and in considering the chemical composition of the melt and nature of the residual phases, this parameter is of prime importance.For high degrees (> 20% approx.) of partial melting of a pyrolite source region, magmas are of tholeiitic character but are of increasingly undersaturated and alkaline type for lower degrees of partial melting and high pressures. For any chosen degree of melting and fixed water content of the source region, magmas are more olivine-rich at higher pressures. For any chosen pressure and chosen degree of partial melting, magmas are less olivine-rich at high water contents (and thus lower temperatures). Quartz tholeiite magmas may be derived by ~ 30% melting of pyrolite under water-saturated conditions at pressures up to between 17 kbar and 20 kbar. These generalizations may be applied to understand the characteristic magmatism of mid-oceanic ridges, island chains, oceanic islands and orogenic regions.     

Sr Isotope and Trace Element Studies of the Great Dyke and Bushveld Mafic Phase and their Relation to Early Proterozoic Magma Genesis in Southern Africa

New Rb-Sr and trace element data are reported for the GreatDyke and Bushveld Mafic Phase layered intrusions. It is arguedthat geochemical characteristics, such as ⁸⁷Sr/⁸⁶Sr ratios andR.E.E. distribution patterns have been little modified by crustalcontamination. Rb-Sr data for whole-rocks of the Great Dyke yield an age of2514±16 m.y. and an initial ⁸⁷Sr/⁸⁶Sr ratio of 0.70261±4.Mineral data are consistent with these results. The low errorson the results indicate no significant variation of ⁸⁷Sr/⁸⁶Srratios of successive magmatic influxes emplaced in differentmagma chambers. Earlier Great Dyke magmas were highly Mg-richand represent extensive partial melts of the source material.One such influx is shown to have a high Rb/Sr ratio (0.25) anda fractionated R.E.E. pattern (Ce_N/YB_N 12). These ratios areconsidered to approximate those of the source region. The Bushveld Mafic Phase has been dated accurately for the firsttime and has a Rb-Sr age of 2095±24 m.y. Initial ⁸⁷Sr/⁸⁶Srratios increase in a stepwise manner upwards in the intrusionfrom 0.70563±2 to 0.70769±6. Each increase isabrupt and occurs at a horizon also characterized by a suddenirregularity in cryptic variation. The Mafic Phase was emplacedas a succession of magmatic influxes each of which had higher⁸⁷Sr/⁸⁶Sr ratio than its predecessor. The first magma was both Mg-rich (MgO 21.5 per cent) and SiO₂-rich(50–55 per cent SiO₂) and was derived by extensive partialmelting of a shallow level upper mantle source. This sourcewas characterized by trace element abundance ratios (e.g. Rb/Sr 0.25; K/Rb 90; Ce_N/Yb_N 11), similar to those of kimberlitesand some potassic lavas and comparable with those deduced forthe Great Dyke source region. It is postulated that when the Rhodesian and Kaapvaal cratonsstabilized, underlying refractory mantle became fixed theretoto form a proto-lithosphere. Shortly afterwards, at about 2800m.y. ago, this proto-lithospheric mantle was enriched by passagethrough it of fluids with kimberlitic trace element chemistry.This sub-cratonic mantle thereafter evolved with a relativelyhigh Rb/Sr ratio. Magmas derived from it have anomalous chemicalcharacteristics with respect to those of ocean-floor basalts,reflecting major differences in the evolution of their respectivesource regions.     

Petrogenesis of Tertiary Continental Intra-plate Lavas from the Westerwald Region, Germany

Tertiary volcanic rocks from the Westerwald region range frombasanites and alkali basalts to trachytes, whereas lavas fromthe margin of the Vogelsberg volcanic field consist of morealkaline basanites and alkali basalts. Heavy rare earth elementfractionation indicates that the primitive Westerwald magmasprobably represent melts of garnet peridotite. The Vogelsbergmelts formed in the spinel–garnet peridotite transitionregion with residual amphibole for some magmas suggesting meltingof relatively cold mantle. Assimilation of lower-crustal rocksand fractional crystallization altered the composition of lavasfrom the Westerwald and Vogelsberg region significantly. Thecontaminating lower crust beneath the Rhenish Massif has a differentisotopic composition from the lower continental crust beneaththe Hessian Depression and Vogelsberg, implying a compositionalboundary between the two crustal domains. The mantle sourceof the lavas from the Rhenish Massif has higher ²⁰⁶Pb/²⁰⁴Pband ⁸⁷Sr/⁸⁶Sr than the mantle source beneath the Vogelsbergand Hessian Depression. The 30–20 Ma volcanism of theWesterwald apparently had the same mantle source as the QuaternaryEifel lavas, suggesting that the magmas probably formed in apulsing mantle plume with a maximum excess temperature of 100°Cbeneath the Rhenish Massif. The relatively shallow melting ofamphibole-bearing peridotite beneath the Vogelsberg and HessianDepression may indicate an origin from a metasomatized portionof the thermal boundary layer. KEY WORDS: continental rift volcanism; basanites; trachytes; assimilation; fractional crystallization; partial melting     

Ultramafic Xenoliths in Plio-Pleistocene Alkali Basalts from the Eastern Transylvanian Basin: Depleted Mantle Enriched by Vein Metasomatism

Ultramafic xenoliths from alkali basalts in the Perjani Mountainsin the Eastern Transylvanian Basin (ETB) of Romania are mainlyspinel Iherzolites, although spinel harzburgites, websterites,clinopyroxenites and amphibole pyroxenites are also present.Amphibole veins cut some spinel peridotite samples. All arederived from the shallow lithospheric upper mantle. In general,textural variations are restricted to protogranular and porphyroclastictypes, compared with the more varied textures found in mantlexenoliths from the alkali basalts of the neighbouring PannonianBasin. Also, ETB peridotites are richer in amphibole. Thus,the mantle beneath the edge of the ETB is less deformed butmore strongly metasomatized than the mantle closer to the centreof the Pannonian Basin.Mineralogical and bulk-rock geochemicalvariations resemble those of spinel Iherzolites from other sub-continentalshallow mantle xenolith suites. There is no apparent correlationbetween deformation and geochemistry, and much of the majorand trace element variation is due to variable extraction ofpicritic melts. The REE patterns of separated clinopyroxenesfrom the peridotite xenoliths are mostly LREE depleted, althoughclinopyroxenes from regions adjacent to amphibole veins haveexperienced an enrichment in La and Ce and a change in theirSr and Nd isotopic values towards those of the vein, while stillretaining an overall LREE depletion. Clinopyroxenes from thewebsterites and clinopyroxenites are more variable. Amphibolein the hydrous pyroxenites and amphibole veins is strongly LREEenriched and is considered to be metasomatic in origin. ⁸⁷Sr/⁸⁶Srand ¹⁴³Nd/^l44Nd isotopic ratios of the xenoliths vary between07018 and 07044, and 051355 and 0 51275, respectively. Thesevalue are more depleted than those obtained for xenoliths fromthe Pannonian Basin. The lower ^l43Nd/^l44Nd and higher ⁸⁷Sr/Sr⁸⁶values are found in anhydrous pyroxenites, metasomatic amphibolesin veins and amphibole pyroxenites, and in the only exampleof an equigranular spinel Iherzolite in the suite.The ETB xenolithswere brought to the surface in alkaline vokanism which post-dateda period of Miocene to Pliocene subduction-related cak-alkalinevolcanism. However, the effects of the passage of either slab-derivedfluids or cak-alkaline magmas through the ETB lithospheric mantlecannot be discerned in the chemistry of the xenoliths. The metasomaticamphibole has ⁸⁷Sr/Sr⁸⁶ and ¹⁴³Sr/Sr¹⁴⁴ ratios similar to thehost alkali basalts, but the least evoked cak-alkaline magmasalso have similar Sr and Nd isotope compositions. The REE patternsof the amphibole resembk those of amphiboles considered to havecrystallized from alkaline melts. No preferential enrichmentin elements typically associated with slab-derivedfluids (K,Rb and Sr) relative to elements typically depleted in cak-alkalinemagmas (Ti, 2jr and Nb) has been observed in the vein amphiboles,although some interstitial amphibole is depleted in all incompatibletrace elements, including LREE. Thus, despite its position closeto the calc-alkaline volcanic arc of the Eastern Carpathians,we cannot readily detect any interaction between the lithosphericupper mantle beneath the ETB and subduction-related magmas orfluids. Metasomatism in the lithospheric mantle is instead largelyrelated to the passage of a primitive alkaline magma similarto the host alkali basal ^*corresponding author