Evolution of the Kap Edvard Holm Complex: a Mafic Intrusion at a Rifted Continental Margin

The Kap Edvard Holm Layered Series forms part of the East GreenlandTertiary Province, and was emplaced at shallow crustal level(at depths corresponding to a pressure of 1–2 kbar) duringcontinental break-up. It consists of two suites: a gabbro suitecomprising olivine and oxide gabbros, leucocratic olivine gabbrosand anorthosites, and a suite of wehrlites that formed fromthe intrusion of the gabbros during their solidification bya hydrous, high-MgO magma. Ion microprobe analyses of clinopyroxenereveal chemical contrasts between the parental melt of the wehrlitesuite and that of the gabbro suite. Thin sills (1–2 mthick) of the wehrlite suite, however, have clinopyroxene compositionssimilar to the gabbro suite, and were formed by interactionwith interstitial melts from the host layered gabbros. All evolvedmembers of the gabbro suite have elevated Nd, Zr and Sr concentrationsand Nd/Yb ratios, relative to the melt parental to the gabbrosuite. These characteristics are attributed to establishmentof a magma chamber at depths corresponding to a pressure of10 kbar, where melts evolved before injection into the low-pressuremagma chamber. Anorthosites of the gabbro suite are believedto have crystallized from such injections. The melts becamesupersaturated in plagioclase by the pressure release that followedtransportation to the low-pressure magma chamber after initialfractionation at 10 kbar. The most evolved gabbros formed bysubsequent fractionation within the low-pressure magma chamber.Our results indicate that high-pressure fractionation may beimportant in generating some of the lithological variationsin layered intrusions. KEY WORDS: fractionation; ion microprobe; layered intrusions; rift processes; trace elements ^*Corresponding author.     

Reaction Between Ultramafic Rock and Fractionating Basaltic Magma II. Experimental Investigation of Reaction Between Olivine Tholeiite and Harzburgite at 1150-1050?C and 5 kb

We present results of experiments on mixtures of olivine tholeiiteand mantle harzburgite, at 5 kb and 1050–1150?C, underconditions of controlled hydrogen fugacity. The basalt end-memberwas Kilauea 1921 olivine tholeiite+3 wt.% H₂O, and the harzburgiteend-member was a mixture of olivine and orthopyroxene mineralseparates made from a mantle-derived lherzolite xenolith. Theexperiments on mixtures of basalt and harzburgite difl not reachequilibrium in runs ranging from 12 to 200 h duration. Relativelylarge concentration gradients persisted in both liquid and solidphases in mixed samples, whereas ‘control’ samplescontaining only basalt were reasonably homogeneous and wereprobably close to equilibrium. Compositions of solid phases produced, measured by electronmicroprobe, show a regular increase in Mg/(Mg+Fe) with increasingproportion of harzburgite at constant temperature, but olivineand clinopyroxene in mixed samples were not in Fe-Mg exchangeequilibrium. Modes measured for each sample show that the fractionof liquid relative to the amount of basalt in the sample wasconstant at constant temperature, and independent of bulk composition:reaction between 1921 basalt and harzburgite does not changethe mass of liquid in the system. Average experimental liquidcompositions for each sample were obtained by mass balance.Using Kds defined by the ‘control’ sample for eachtemperature, and mass balance constraints, phase assemblages(solid- and liquid-phase compositions and proportions) werecalculated for all mixtures. Whether samples included harzburgite or not, all average experimentalliquid compositions, and all predicted liquid compositions,for samples run at 1050?C, are high-alumina basalts by the definitionof Kuno (1960). By the criteria of Irvine & Baragar (1971),all but two average experimental liquid compositions in basalt-harzburgitemixtures, and all predicted liquid compositions in basalt-harzburgitemixtures, are calc-alkaline basalts and basaltic andesites,whereas liquids in samples containing only basalt are tholeiiticbasalts. Combined crystallization and reaction with harzburgitein the upper mantle will produce calc-alkaline derivative liquidsfrom an olivine tholeiite liquid under conditions of temperature,pressure, water and oxygen fugacity, and initial bulk compositionwhich would produce a tholeiitic liquid line of descent by crystallizationin a closed system. ^*Present address: Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543Present address: Grant Institute of Geology, University of Edinburgh, West Mains Road, Edinburgh EH9 3JW, UK     

The thermal structure of continental crust in active orogens: insight from Miocene eclogite and granulite xenoliths of the Pamir Mountains

Rare ultrahigh‐temperature–(near)ultrahigh‐pressure (UHT–near‐UHP) crustal xenoliths erupted at 11 Ma in the Pamir Mountains, southeastern Tajikistan, preserve a compositional and thermal record at mantle depths of crustal material subducted beneath the largest collisional orogen on Earth. A combination of oxygen‐isotope thermometry, major‐element thermobarometry and pseudosection analysis reveals that, prior to eruption, the xenoliths partially equilibrated at conditions ranging from 815 °C at 19 kbar to 1100 °C at 27 kbar for eclogites and granulites, and 884 °C at 20 kbar to 1012 °C at 33 kbar for garnet–phlogopite websterites. To reach these conditions, the eclogites and granulites must have undergone mica‐dehydration melting. The extraction depths exceed the present‐day Pamir Moho at ～65 km depth and suggest an average thermal gradient of ～12–13 °C km^?1. The relatively cold geotherm implies the introduction of these rocks to mantle depths by subduction or gravitational foundering (transient crustal drip). The xenoliths provide a window into a part of the orogenic history in which crustal material reached UHT–(U)HP conditions, partially melted, and then decompressed, without being overprinted by the later post‐thermal relaxation history.     

A Detailed Geochemical Study of Island Arc Crust: the Talkeetna Arc Section, South-Central Alaska

The Early to Middle Jurassic Talkeetna Arc section exposed inthe Chugach Mountains of south–central Alaska is 5–18km wide and extends for over 150 km. This accreted island arcincludes exposures of upper mantle to volcanic upper crust.The section comprises six lithological units, in order of decreasingdepth: (1) residual upper mantle harzburgite (with lesser proportionsof dunite); (2) pyroxenite; (3) basal gabbronorite; (4) lowercrustal gabbronorite; (5) mid-crustal plutonic rocks; (6) volcanicrocks. The pyroxenites overlie residual mantle peridotite, withsome interfingering of the two along the contact. The basalgabbronorite overlies pyroxenite, again with some interfingeringof the two units along their contact. Lower crustal gabbronorite(10 km thick) includes abundant rocks with well-developed modallayering. The mid-crustal plutonic rocks include a heterogeneousassemblage of gabbroic rocks, dioritic to tonalitic rocks (30–40%area), and concentrations of mafic dikes and chilled mafic inclusions.The volcanic rocks (7 km thick) range from basalt to rhyolite.Many of the evolved volcanic compositions are a result of fractionalcrystallization processes whose cumulate products are directlyobservable in the lower crustal gabbronorites. For example,Ti and Eu enrichments in lower crustal gabbronorites are mirroredby Ti and Eu depletions in evolved volcanic rocks. In addition,calculated parental liquids from ion microprobe analyses ofclinopyroxene in lower crustal gabbronorites indicate that theclinopyroxenes crystallized in equilibrium with liquids whosecompositions were the same as those of the volcanic rocks. Thecompositional variation of the main series of volcanic and chilledmafic rocks can be modeled through fractionation of observedphase compositions and phase proportions in lower crustal gabbronorite(i.e. cumulates). Primary, mantle-derived melts in the TalkeetnaArc underwent fractionation of pyroxenite at the base of thecrust. Our calculations suggest that more than 25 wt % of theprimary melts crystallized as pyroxenites at the base of thecrust. The discrepancy between the observed proportion of pyroxenites(less than 5% of the arc section) and the proportion requiredby crystal fractionation modeling (more than 25%) may be bestunderstood as the result of gravitational instability, withdense ultramafic cumulates, probably together with dense garnetgranulites, foundering into the underlying mantle during thetime when the Talkeetna Arc was magmatically active, or in theinitial phases of slow cooling (and sub-solidus garnet growth)immediately after the cessation of arc activity. KEY WORDS: island arc crust; layered gabbro; Alaska geology; island arc magmatism; lower crust     

Reaction Between Ultramafic Rock and Fractionating Basaltic Magma I. Phase Relations, the Origin of Calc-alkaline Magma Series, and the Formation of Discordant Dunite

This paper presents results of modelling reaction between peridotiteand fractionating tholeiitic basalt in simple and complex silicatesystems. Reactions are outlined in appropriate binary and ternarysilicate systems. In these simple systems, the result of reactionsbetween ‘basalt’ and ‘peridotite’ maybe treated as a combination of Fe-Mg exchange and mass transferreactions at constant Fe/Mg. Fe-Mg exchange in ternary and higher-ordersystems is nearly isenthalpic, and involves a slight decreasein magma mass at constant temperature. Mass transfer reactions,typically involving dissolution of orthopyroxene and consequentcrystallization of olivine, are also nearly isenthalpic in ternaryand higher-order silicate systems, and produce a slight increasein the magma mass at constant temperature. The combined reactionsare essentially isenthalpic and produce a slight increase inmagma mass under conditions of constant temperature or constantenthalpy. Initial liquids saturated in plagioclase+olivine will becomesaturated only in olivine as a result of near-constant-temperaturereaction with peridotite, and crystal products of such reactionswill be dunite. Liquids saturated in clinopyroxene+olivine willremain on the cpx-ol cotectic during reaction with peridotite,but will crystallize much more olivine than clinopyroxene asa result of reaction, i.e., crystal products will be clinopyroxene-bearingdunite and wehrlite rather than olivine clinopyroxenite, whichwould be produced by cotectic crystallization. The Mg/Fe ratioof crystal products is ‘buffered’ by reaction withmagnesian peridotite, and dunites so produced will have high,nearly constant Mg/Fe. Production of voluminous magnesian dunitein this manner does not require crystal fractionation of a highlymagnesian olivine tholeiite or picrite liquid. Combined reaction with ultramafic wall rock and crystal fractionationdue to falling temperature produces a calc-alkaline liquid lineof descent from tholeiitic parental liquids under conditionsof temperature, pressure, and initial liquid composition whichwould produce tholeiitic derivative liquids in a closed system.Specifically, closed-system differentiates show iron enrichmentat near-constant silica concentration with decreasing temperature,whereas the same initial liquid reacting with peridotite producessilica-enriched derivatives at virtually constant Mg/Fe. Reaction between fractionating basalt and mafic to ultramaficrock is likely to be important in subduction-related magmaticarcs, where tholeiitic primary liquids pass slowly upward throughhigh-temperature wall rock in the lower crust and upper mantle.Although other explanations can account for chemical variationin individual calc-alkaline series, none can account as wellfor the characteristics shared by all calc-alkaline series.This process, if it is volumetrically important on Earth, hasimportant implications for (Phanerozoic) crustal evolution:sub-arc mantle should be enriched in iron, and depleted in silicaand alumina, relative to sub-oceanic mantle, acting as a sourcefor sialic crust It is probable that inter-occanic magmaticarcs have basement similar to alpine peridotite, in which sub-oceanicmantle has been modified by interaction with slowly ascendingbasaltic liquids at nearly constant temperature. Discordantdunite bodies in alpine pendotite may record extraction of sialiccrust from the Earth's upper mantle. ^*Present address: Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543