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

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

Secular Geochemistry of Central Puerto Rican Island Arc Lavas: Constraints on Mesozoic Tectonism in the Eastern Greater Antilles

Island arc volcanism in the Greater Antilles persisted for >70m.y. from Middle Cretaceous to Late Eocene time. During theinitial 50 m.y., lavas in central Puerto Rico shifted from predominantlyisland arc tholeiites (volcanic phase I, Aptian to Early Albian,120–105 Ma), to calc-alkaline basalts (phase II, LateAlbian, 105–97 Ma), and finally to high-K, incompatible-element-enrichedbasalts (phases III and IV, Cenomanian–Maastrichtian,97–70 Ma). Following an island-wide eruptive hiatus, geochemicaltrends were reversed in the Eocene with renewed eruption ofcalc-alkaline basalts (phase V, 60–45 Ma). Progressiveincreases in large-ion lithophile elements (LILE)/light rareearth elements (LREE), LILE/high field strength elements (HFSE),LREE/HFSE, and HFSE/heavy rare earth elements (HREE) characterizethe compositional evolution of the first four volcanic phases.The shift in trace element compositions is mirrored by increasingradiogenic content of the lavas. Pb     

The Late Cretaceous Impact of the Trindade Mantle Plume: Evidence from Large-volume, Mafic, Potassic Magmatism in SE Brazil

When the subcontinental lithospheric mantle undergoes heatingand/or extension, some of the earliest mafic melts to be generatedare those rich in volatUes and potassium. In some cases, e.g.when a plume impinges on thick cratonic lithosphere or whenthe amount of extension is very small, K-rich mafic igneousrocks may be the only surface expression of mantle melting.The Alto Paranaiba Igneous Province, in SE Brazil, is one ofthe world's most voluminous mafic potassic provinces (>15000km³),which until recently was relatively unknown. The magmas wereemplaced into a narrow Proterozoic mobile belt close to thesurface margin of the Sao Francisco craton, and it is one ofseveral Cretaceous alkaline igneous provinces that are locatedaround the margin of the Parana sedimentary basin in Braziland Paraguay.Detailed geochemical analyses of samples from throughoutthe Alto Paranaiba Igneous Province show that it is composedof a relatively diverse suite of ultrapotassic-potassic, ultramaficmqfic,silica-undersaturated lavas and hypabyssal intrusions, i.e.kimberlites, madupitic olivine lamproites and kamafugitic rocks.These all have very high concentrations of incompatible traceelements and are all strongly enriched in light rare earth relativeto heavy rare earth elements (e.g. La/Yb=50-230). Wide variationsin major element ratios, which are unrelated to the effectsof crystal fractionation in these magmas (e.g. CaO/Al₂O₃), suggestthat the mafic potassic rocks were derived from a heterogeneousmantle source. They show relatively restricted ranges of initial⁸⁷Sr/⁸⁶Sr (070436-070588) and Nd₂₅ values of -4 to -8, intermediatebetween Group I and II South African kimberlites. T_DM Nd isotopemodel ages of 900 Ma suggest that the magmas were derived bythe remobilization of subcontinental lithospheric mantle thathad been enriched by small-volume K-rich melt fractions sincethe Late Proterozoic.New K/Ar ages for mica separates show thatthe kimberlites, madupitic olivine lamproites and kamafugiticrocks were emplaced together with large carbonatite-bearingplutonic complexes at 85 Ma. Reconstructions of plate motionsshow that, at this time, the location of the Alto ParanaibaIgneous Province coincided with the postulated position of thepresent-day Trindade(or Martin Vaz) plume. We propose that thewidespread Late Cretaceous alkaline magmatism in SE Brazil mayhave been caused by impingement of this plume on the base ofthe subcontinental lithosphere. Heat penetrating the lithosphere,both by conduction and advection by asthenospheric-source decompressionmelts, may have caused melting of the readily fusible partsof the lithospheric mantle and the genesis of mafic potassicand (after fractionation) carbonatite magmas. The Proterozoicmobile belt (the Brasilia Belt) appears to have acted as a Hhinspofrelative to the adjacent Sao Francisco craton, allowing greaterupwelling and melting of the asthenosphere. Subsequently, asthe craton passed over the plume, volcanism was switched off'until the Early Tertiary when the plume reemerged from beneaththe westward drifting South America continent and was the magmasource for oceanic-islands and seamounts of the Trindade-Vitriachain. Corresponding author     

Early Cretaceous Basalt and Picrite Dykes of the Southern Etendeka Region, NW Namibia: Windows into the Role of the Tristan Mantle Plume in Parana-Etendeka Magmatism

Abundant dykes in the southern Etendeka region, NW Namibia,mostly contain 8–20% MgO. Almost all can be allocatedto previously described Early Cretaceous magma types. Horingbaai-typebasalts–picrites occur up to 120 km inland. Some havesuperficially mid-ocean ridge basalt (MORB)-like compositions:(La/Nb)_n     

Petrogenesis of Proterozoic Lamproites and Kimberlites from the Cuddapah Basin and Dharwar Craton, Southern India

Proterozoic mafic potassic and ultrapotassic igneous rocks emplacedin the Cuddapah Basin and Dharwar Craton of the southern Indianshield are among the earliest recorded on Earth. Lamproitesintrude the basin and its NE margin, whereas kimberlites intrudethe craton to the west of the basin. Kimberlites occur in twospatially separate groups: the non-diamondiferous Mahbubnagarcluster that was emplaced at 1400 Ma and is of a similar ageto the Cuddapah lamproites, and the predominantly diamondiferousAnantapur cluster, emplaced at     

Isotope Geochemistry of Tertiary Igneous Rocks from the Isle of Skye, N.W. Scotland

New Pb isotope analyses are reported for forty-seven basic,intermediate and acid Tertiary igneous rocks from the Isle ofSkye and adjacent regions of northwest Scotland. New Sr isotopeanalyses and two Rb/Sr isochron dates are also reported forthe acid igneous rocks. These results are combined with publishedSr and Nd isotope data for Tertiary igneous rocks from Skyeand adjacent regions in order to examine the genesis and evolutionof Tertiary magmas in the Isle of Skye. Pb isotope analyses form a linear array on a Pb/Pb isochrondiagram, with a slope yielding an apparent age of 2920 ±70m.y. (2), interpreted as a mixing line between ca. 2900 m.y.-oldcrustal Pb and 60 m.y.-old mantle-derived Pb. Combined isotopicdata support a model of selective crustal Pb, Sr and Nd contaminationof basic and intermediate mantle-derived magmas, without significantmodification of major-element composition. This contaminationprocess is probably due to the breakdown of LIL-element-enrichedaccessory mineral phases, and does not necessarily produce linearmixing lines on isotope ratio plots of different elements. SkyeRedhills epigranites contain a predominant fraction of acidmagma which differentiated from mantle-derived basic precursors,and a subsidiary fraction of upper crustal melt. The proportionof this melt is estimated to rise from less than 10 per centin the older granites to around 30 per cent in the younger. Isotopic and geochemical data are used to draw conclusions aboutthe differentiation histories of the four principal magma typesof Skye: the Fe-rich and Fe-poor members of the Skye Main LavaSeries, Preshal Mhor Basalts and Redhills epigranites.     

Early Proterozoic Melt Generation Processes beneath the Intra-cratonic Cuddapah Basin, Southern India

Early Proterozoic tholeiitic lavas and sills were emplaced duringthe initial phase of extension of the intra-cratonic CuddapahBasin, southern India. ⁴⁰Ar–³⁹Ar laser-fusion determinationson phlogopite mica, from the Tadpatri Fm mafic–ultramaficsill complex, constrain the age of the initial phase of extensionand volcanism in the basin at 1·9 Ga. Despite their EarlyProterozoic age, the igneous rocks are unmetamorphosed, undeformedand remarkably fresh. They exhibit a wide range in MgO contents(4–28 wt %) and have undergone varying degrees of accumulationor crystal fractionation. Variable La/Nb ratios (1·2–3·7)and     

Source of the Quaternary Alkalic Basalts, Picrites and Basanites of the Potrillo Volcanic Field, New Mexico, USA: Lithosphere or Convecting Mantle?

The <80 ka basalts–basanites of the Potrillo VolcanicField (PVF) form scattered scoria cones, lava flows and maarsadjacent to the New Mexico–Mexico border. MgO ranges upto 12·5%; lavas with MgO < 10·7% have fractionatedboth olivine and clinopyroxene. Cumulate fragments are commonin the lavas, as are subhedral megacrysts of aluminous clinopyroxene(with pleonaste inclusions) and kaersutitic amphibole. REE modellingindicates that these megacrysts could be in equilibrium withthe PVF melts at 1·6–1·7 GPa pressure. Thelavas fall into two geochemical groups: the Main Series (85%of lavas) have major- and trace-element abundances and ratiosclosely resembling those of worldwide ocean-island alkali basaltsand basanites (OIB); the Low-K Series (15%) differ principallyby having relatively low K₂O and Rb contents. Otherwise, theyare chemically indistinguishable from the Main Series lavas.Sr- and Nd-isotopic ratios in the two series are identical andvary by scarcely more than analytical error, averaging ⁸⁷Sr/⁸⁶Sr= 0·70308 (SD = 0·00004) and ¹⁴³Nd/¹⁴⁴Nd = 0·512952(SD=0·000025). Such compositions would be expected ifboth series originated from the same mantle source, with Low-Kmelts generated when amphibole remained in the residuum. ThreePVF lavas have very low Os contents (<14 ppt) and appearto have become contaminated by crustal Os. One Main Series picritehas 209 ppt Os and has a _Os value of +13·6, typical forOIB. This contrasts with published ¹⁸⁷Os/¹⁸⁸Os ratios for KilbourneHole peridotite mantle xenoliths, which give mostly negative_Os values and show that Proterozoic lithospheric mantle formsa thick Mechanical Boundary Layer (MBL) that extends to 70 kmdepth beneath the PVF area. The calculated mean primary magma,in equilibrium with Fo₈₉, has Na₂O and FeO contents that givea lherzolite decompression melting trajectory from 2·8GPa (95 km depth) to 2·2 GPa (70 km depth). Inverse modellingof REE abundances in Main Series Mg-rich lavas is successfulfor a model invoking decompression melting of convecting sub-lithosphericlherzolite mantle (Nd = 6·4; T_p 1400°C) between90 and 70 km. Nevertheless, such a one-stage model cannot accountfor the genesis of the Low-K Series because amphibole wouldnot be stable within convecting mantle at T_f 1400°C. Thesemagmas can only be accommodated by a three-stage model thatenvisages a Thermal Boundary Layer (TBL) freezing conductivelyonto the 70 km base of the Proterozoic MBL during the 20 Myrtectonomagmatic quiescence before PVF eruptions. As it grew,this was veined by hydrous small-fraction melts from below.The geologically recent arrival of hotter-than-ambient (T_p 1400°C) convecting mantle beneath the Potrillo area re-meltedthe TBL and caused the magmatism. KEY WORDS: western USA; picrites; Sr–Nd–Os isotopes; petrogenetic modelling; thermal boundary layer     

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

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