High-Pressure and -Temperature Subophiolitic Kyanite--Garnet Amphibolites Generated during Initiation of Mid-Tertiary Subduction, Palawan, Philippines

Metamorphic rocks exposed near the base of a pre-middle Eoceneophiolite in central Palawan, Philippines, preserve a recordof the P-T-t conditions under which the ophiolite was detachedand emplaced onto the rifted southeast margin of China. Garnetamphibolites in contact with mantle harzburgite preserve peakmetamorphic temperatures of 700–760°C and minimumpressures of 9 kbar, consistent with the presence of kyanite.Garnet grains which preserve strong prograde zoning (Pyp₁₁ incores to Pyp₄₃ in rims) contain inclusion assemblages that constrainpressures above 5–6 kbar at 4O0–500°C. Thisrepresents an early prograde clockwise path consistent withunderthrusting. Two amphibolite samples yield hornblende ³⁹Ar/⁴⁰Ar–³⁶Ar/Ar⁴⁰Arisochron ages of 34•0±0•6 Ma (2) (early Oligocene),which are indistinguishable from a 34•3±0•2Ma isochron age of muscovite from a kyanitechlorite-muscoviteschist that is interlayered with the amphibolites. The highpressures (equivalent to a depth of >30 km in oceanic lithosphere)and temperatures, and the age difference between ophiolite crystallizationand metamorphism of the sole, are incompatible with the rockshaving formed in a mature subduction zone or oceanic spreadingcenter. Instead, the timing, P-T conditions, and regional geologysuggest metamorphism in a nascent subduction zone. Rapid coolingand exhumation of the metamorphic rocks followed peak metamorphicconditions in the earliest Oligocene, perhaps by the rapid thickeningof the accretionary complex that is preserved beneath the ophiolite.The amphibolites formed during the inception of southward subductionwithin proto-South China Sea oceanic lithosphere south of theEurasian margin. This led to the formation of an arc-trenchsystem that accommodated extension along southeast China andthe opening of the South China Sea. KEY WORDS: ophiolite; thermobarometry; garnet amphibolite; subduction     

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.     

The Skaergaard Layered Series, Part VII: Sr and Nd Isotopes

The initial isotopic ratios of strontium and neodymium in theSkaergaard Layered Series vary both vertically and laterally,on every scale from the intrusion as a whole down to coexistingminerals in a single rock. The magma that filled the Skaergaardchamber was contaminated to various degrees with the metamorphicrocks through which it rose and was never completely homogenizedafter being intruded. The contamination was most pronouncedin contact zones and aureoles around rare xenoliths. The greaterconcentrations of lithophile trace elements in the Upper BorderSeries was previously attributed to assimilation of buoyantfragments of gneiss that collected under the roof, but mostof the rocks of the Upper Border Series are isotopically indistinguishablefrom those of the Layered Series. It is doubtful, therefore,that this part of the intrusion assimilated much more of themetamorphic basement than did the rest of the magma. Similarly,the marked increase in the concentrations of excluded elementsin the upper part of the Layered Series is not matched by achange in the isotopic character of the rocks and cannot beattributed to a later influx of new magma. Analyses of mineralsseparated from rocks with exceptionally mafic or felsic modalcompositions revealed marked inhomogeneities in the isotopiccompositions of their constituent minerals. For example, coexistingplagioclase and pyroxene from closely associated anorthositesand pyroxenites have very different initial isotopic ratiosof both strontium and neodymium. The same is true of mafic andfelsic layers in modally graded gabbros. These differences areunrelated to the low-temperature alteration shown by oxygenisotopes. They must have been introduced when the original gabbrowas largely crystallized and underwent local metasomatic replacementby nearly mono-mineralic mafic and felsic assemblages. KEY WORDS: Nd isotopes; Skaergaard; Sr isotopes     

Elemental redistribution in Tieschitz and the origin of white matrix

Abstract— Two types of mesostasis coexist within some porphyritic chondrules in Tieschitz. One type is smooth. The other, confined to chondrule margins, is blocky on a 5–10 μm scale. Mesostases in one porphyritic olivine-pyroxene (POP) chondrule and one porphyritic olivine (PO) chondrule were analysed by scanning electron microscopy (SEM) and energy-dispersive x-ray spectrometry (EDS), as was white matrix nearby. Mesostases in the PO chondrule and in four others were analysed by ion probe. Pyroxene phenocrysts or dendrites extend across contacts between smooth and blocky mesostasis with no compositional change. Relative to smooth mesostasis, blocky mesostasis is enriched in Al, alkalis, Ba, F, and Cl but depleted in Si, Fe, and Ca. White matrix fills channels between the chondrules. It is physically and chemically similar to blocky mesostasis, but three ion probe analyses indicate that, unlike the mesostases, it is poor in Sc and has variable and fractionated rare earth elements (REEs). Smooth mesostasis is interpreted as solidified primary chondrule liquid; whereas blocky mesostasis is its alteration product or, less likely, a precipitate replacing smooth mesostasis leached out by aqueous fluid. White matrix may have formed by secondary alteration or replacement of mesostases that had been expelled from chondrules during accretion, or as a precipitate filling interchondrule voids. Iron may have been lost from the bulk meteorite, but most other elements merely underwent internal redistribution. Disturbed isotopic systems indicate that aqueous fluid may have been active on the Tieschitz parent body only 2 Ga ago. If correct, this would be the first evidence that an ordinary chondrite parent body underwent internal reprocessing significantly later than 4.5 Ga ago.     

URBAN CARTOGRAPHY AND THE MAPPING OF CHICAGO*

ABSTRACT. City maps are among the oldest and most popular forms of cartographic representation, yet urban cartography has received only limited scholarly attention. A review of the early history of city mapping suggests a progression of representational forms from profile to perspective to plan, a progression that parallels urban growth. The history of urban mapping reflects a desire to comprehend the geography of the city for reasons of curiosity and practical necessity, but the map also may become an icon that engenders a sense of pride and belonging, if not allegiance and control. An examination of Chicago maps indicates that, in addition to evolution in representational styles, the subjects of maps change to serve demands of the moment. City maps not only constitute a rich historical archive but also are dynamic agents in urban development. The urban landscape is shaped by cartographic vision.     

Chemical Diversity of the Ueno Basalts, Central Japan: Identification of Mantle and Crustal Contributions to Arc Basalts

The Ueno Basalts of central Japan comprise a monogenetic volcaniccone complex that was active between 2·76 and 1·34Ma. Basalts were erupted at more than 14 centers scattered overa region 40 km in diameter. Alkali basalt was erupted first,followed by sub-alkaline basalt. Quasi-concentric expansionof eruption centers coinciding with uplift and with decreasingalkalinity of the lavas suggests that Ueno magmatism originatedfrom a mantle diapir as it mushroomed at the base of the lithosphere.Depleted asthenospheric mantle (alkali basalt), enriched lithosphericmantle (sub-alkaline basalt), and crustal components are identifiedas chemical end-members in the petrogenesis of the Ueno Basalts.Incompatible trace element abundances indicate that the Uenoalkali basalts are typical within-plate basalts, whereas thesub-alkaline basalts show strong affinities with normal arclavas. Sr–Nd–Pb isotopic compositions indicate thatthe mantle source of the alkali basalts was more depleted thanthat of the sub-alkaline basalts. About 7% melting of asthenosphericmantle in the garnet-lherzolite stability field produced theprimitive alkali basalts and 12% melting of spinel lherzolitewithin the subcontinental lithosphere produced the primitivesub-alkaline basalts. Isotopic compositions and fluid mobile/immobileelement ratios broadly covary with SiO₂ contents in the sub-alkalinesuite, and increasing silica content is associated with strongerEMII (Enriched Mantle II) isotope affinities and fluid mobileelement abundances. A progressive AFC (assimilation–fractionalcrystallization) model assuming assimilation of a low-K silicicmelt reproduces the chemical variations observed in the sub-alkalinesuite. Melting of a flattening mantle diapir at the base ofthe lithosphere is the dominant cause of Ueno magmatism, accompaniedby the assimilation of older arc crust. KEY WORDS: arc basalt; crustal assimilation; mantle heterogeneity; Ueno Basalts     

Wave-induced liquefaction: a modern example from the Bay of Fundy

Soft-sediment deformation features occur commonly on parts of intertidal sand bodies in Cobequid Bay, Bay of Fundy. These features are small- to intermediate-sized, slump-like bodies, 1-3 m² in area and located on the crest and upper stoss side of ebb megaripples. External modification of these slumps indicates that they formed before complete emergence. The deformed cross-bedding within these bodies extends to a depth of 0.15-0.35 m and shows that deformation occurred during slumping and flowage of liquefied sand down the megaripple stoss side. Field evidence and calculations strongly indicate that this liquefaction results from the impact of 0.1-0.3 m high waves breaking against the megaripple lee faces. Neither rapid drawdown of the water level nor earthquake shocks are reasonable alternative explanations. Indigenous wave activity provides an attractive substitute to tectonism as an explanation of soft-sediment deformation in ancient shallow-water sediments. Slow wave-induced compaction may also account for the relative scarcity of deformation structures in shallow marine sandstones.     

Potassic Volcanic Rocks in NE China: Geochemical Constraints on Mantle Source and Magma Genesis

Potassic volcanic rocks from the Wudalianchi, Erkeshan and Keluo(WEK) fields in NE China are located between the Mesozoic SongliaoBasin and the Palaeozoic Xing'am Mountains fold belt. Theserocks erupted during three main eruptive episodes-Miocene (9•6–7•0Ma), Pleistocene (0•56–0•13 Ma) and Recent (AD1719–1721)-and are subdivided into three types-olivineleucitite, leucite basanite and trachybasalt—on the basisof modal composition. In comparison with Cenozoic alkaline basaltsfrom East China that are similar to oceanic island basalts (OIBs),WEK volcanic rocks are lower in Al₂O₃, CaO, Fe₂O3 and Sc, buthigher in K₂O (3•5–7•1 wt %), K₂O/Na₂O (>1)and incompatible elements. High ⁸⁷Sr/⁸⁶Sr (0•7050–0•7056),low ¹⁴³Nd/¹⁴⁴Nd (0•51238–0•51250) and ²⁰⁶Pb/²⁰⁴Pb(17•06–16•61) ratios also distinguish them fromoceanic and Chinese basalts. Trace element and isotope dataindicate that a post-Archaean subcontinental lithospheric mantlesource similar to the postulated EM1 component (enriched mantlewith low ^l43Nd/¹⁴⁴Nd and moderate high ⁸⁷Sr/⁸⁶Sr) must haveplayed a significant role in magma generation. The source rockis considered to be refractory phlogopite-bearing garnet peridotiteheterogeneously enriched in both large ion lithophile elementsand light rare earth elements by ancient metasomatism duringProterozoic times. This source may have mixed recently withOIB-like melts, but has not been modified by subduction of theKula-Pacific plate. Primitive WEK potassic magma was generatedby a low degree of partial melting, initiated by an extensionalphase beginning in the late Tertiary, at pressures of 20–45kbar and in the presence of mixed volatile components of H₂O,CO₂ and halogens. KEY WORDS: potassic volcanic rocks; NE China; geochemistry; montle sourc ^*Corresponding author. Present address: Centre for Petrology and Lithoipheric Studies, School of Earth Sciences, Macquarie University, NSW 2109, Australia     

Experimental verification of aeolian saltation and lee side deposition models

We report results of experiments intended to test the validity of a model for aeolian saltation and the resulting pattern of deposition on the lee side of aeolian dunes. In steady sea-breeze conditions on a 3-m-tall dune at Point Año Nuevo, California, we measured simultaneously the near-brink wind speed and the deposition on both horizontal and lee face collector platforms. We then used the details of the deposition patterns to constrain approximate values of parameters in a numerical model of the deposition rate that incorporates the essence of the saltation process. Best fits to the data constrain a parameter that controls the probability distribution of liftoff speeds. In addition, the total vertical number flux of grains is constrained to roughly 10⁷?10⁸ grains m^?2 s^?1 at shear velocities of 0.33–0.40 m s^?1. The lee side deposition pattern, which shows the expected maximum in deposition rate at a distance of several decimetres from the brink, is also well fit by the model. In addition, simultaneous collection of horizontal and lee deposition patterns, along with the numerical simulation of these patterns, strongly implies that the windfield in the lee of this particular dune is best described as a non-recirculating wake. Grainflows on the lee face are caused by failure of grainfall depositional bumps. Our results suggest that the principal effect of increased wind speed is to increase the frequency of grainflows. rather than to increase their size, implying that very large, thick grainflows require a different mechanism.