Bar development in a fluvial sandstone (Westphalian 'A'), Scotland

The fluvial sandstone beneath the Mill Coal in the Westphalian ‘A’ of Scotland erosively overlies a lake mudstone. Slightly erosive surfaces within the sandstone, traceable for over 200 m, are used to divide it into two types of major sedimentary units termed type A and type B. Type A sand units are approximately 200 m wide, up to 7 m thick, convex upward, and lenticular in all directions. The constituent cosets overlap to the ENE and dip mainly at 1–2° downcurrent (NNW), but locally at 10–15°. Where thickest, type A sand units display a vertical facies sequence commencing with trough cross-bedded and massive sandstone, overlain by a thick zone of ripple cross-lamination, a thin zone of trough cross-beds, and a variably eroded silt drape up to 0.4 m thick. Attenuated lateral margins are dominated by flat bedded sandstone with primary current lineation. Type A sand units are interpreted as deposits which were accreted on to a large fluvial bar during successive flood events. The bar is thought to have had a similar topographic significance to sand waves described from the Brahmaputra and slip face bounded bars observed in the South Saskatchewan river. Palaeocurrents measured from trough cross-bed sets 0.3–1.0 m thick within type B sand units indicate flow to the WSW, perpendicular to the palaeoflow direction measured from type A units. In sections perpendicular to the WSW flow direction type B units are lenticular, and in ENE-WSW trending sections they can be traced for over 80 m at a constant thickness. Type B sand units are interpreted as the product of low stage channels which flowed across bar fronts and tops. The sandstone described herein is interpreted as a braided-type river deposit but is atypical, because it is fine grained and has an internal structure dominated by ripple cross-lamination and upper phase plane beds. The palaeoriver is thought to have been of low sinuosity, 7–10 m deep, with a high suspended load and large rapidly fluctuating discharge. At low stage a braided-type flow pattern developed around submerged bars. The regime of the palaeoriver was probably controlled by the fine sediment grain size and humid tropical climate.     

A MARITIME RESPONSE TO THE CRISIS IN AREA STUDIES

ABSTRACT. The area-studies model of global scholarship, based on dividing the world into a set number of large, quasi-continental regions, is under assault from a variety of intellectual and institutional forces. New, less rigid models of global scholarship are increasingly being called for by both scholars and funding agencies. One useful alternative, currently being explored at Duke University, reframes area studies around ocean and sea basins. Putting maritime interactions at the center of vision brings to light a set of historical regions that have largely remained invisible on the conventional map of the world.     

Interaction of Continental Lithosphere and Asthenospheric Melts below the Geronimo Volcanic Field, Arizona, U.S.A

Spinel lherzolite and pyroxenite inclusions from the Geronimovolcanic field, Arizona (and Dish Hill, California) record,in their constituent minerals, a chronology of diverse mantledepletion and enrichment events. Certain portions of the lithosphericmantle have remained relatively isolated for considerable periodsof time(1–4 b.y.) while wall rock adjacent to conduitsof basanite has been recently (< 0-2 b.y.) modified. Evidenceexists for a widespread ancient (1–4 b.y.) partial meltingresidue, now recognizable as MORB-like mantle below the southwesternU.S.A. Trace element enrichment (0?9 b.y.) has increased thelight rare earth element (LREE) and Sr content of many refractoryperidotites without any mineralogical changes to the host rock.The fluids/melt responsible for this enrichment have a complexhistory involving heterogeneous mantle sources. In contrast,modal metasomatism of the mantle (< 0.2 b.y.) in aureolesaround evolved derivatives of basanite has petrographicallyand chemically transformed this ancient partial melting residue.The metasomatic fluids responsible for such metasomatism havean asthenospheric mantle source identical to the host magma.It is proposed that modal metasomatism occurs in contact metamorphicaureoles that surround apophyses of basanitic silicate meltin the lithospheric mantle. The gradient in CO₂/(CO₂ ? H₂O)ratio that must surround such veins in the upper mantle (<20 kb) may encourage the development of enrichment fronts. Immediatelyadjacent to the vein, a wet zone with a relatively low CO₂/(CO₂? H₂O) ratio would allow a precipitation of mica ? amphibole.Beyond this a dry zone with a higher CO₂/(CO₂ ? H₂O) ratio wouldhasten chemical but not petrographic transformation of the wallrock.     

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     

Fenitizing Processes Induced by Ferrocarbonatite Magmatism at Swartbooisdrif, NW Namibia

The southeastern margin of the anorthositic Kunene IntrusiveComplex, NW Namibia, has been subsequently invaded by Mesoproterozoicsyenite, nepheline syenite and ferrocarbonatite dykes alongNE- and SE-trending faults. The first generation of carbonatiteintrusions frequently contains fenitized anorthositic wall-rockfragments set in a ferrocarbonatite matrix; later, subordinateveins of massive ferrocarbonatite are almost xenolith-free andcut through the main carbonatite dykes. A mantle source forboth carbonatite generations is constrained by their respectiveoxygen and carbon isotope compositions of ankerite (¹⁸O_SMOW8·91–9·73; ¹³C_PDB –6·98 to–6·76). Na-rich fluids, released from the meltparental to the ferrocarbonatites, caused the fenitization ofboth the incorporated anorthosite xenoliths and the borderinganorthosite, syenite and nepheline syenite. This process ismainly characterized by the progressive transformation of Ca-richplagioclase, K-feldspar and nepheline into albite and/or sodalite.The changing mineral modes indicate that the fenitizing fluidswere sodium-rich and strongly Si-deficient solutions, whichalso contained significant amounts of Sr, Ba, Nb and the lightrare earth elements. On the basis of mineral equilibria studies,it is possible to reconstruct the temperature conditions forcarbonatite emplacement (c. 830 ± 200°C) and recrystallization(c. 480 ± 130°C), and for the metasomatic formationof sodalite (c. 700 ± 70°C). KEY WORDS: anorthosite; fenitization; ferrocarbonatite; sodalite; stable isotopes     

Lithological and biochemical properties in sediments of Lama Lake as indicators for the late Pleistocene and Holocene ecosystem development of the southern Taymyr Peninsula, Central Siberia

In order to evaluate the sensitivity of aquatic and terrestrial ecosystems to climatic changes, lithological (sediment structure and color, grain size, physical properties) and biochemical (TOC, TOC/TN, δ¹³C of TOC and carbonates) investigations were carried out on an 11.12 m-long sediment core from Lama Lake (Central Siberia, Putorana Plateau). According to the pollen data, the sequence represents the termination of the Pleistocene, and the entire Holocene. It is composed of highly terrigenous and stratified clays and silts. Sediment structure, grain-size distribution, carbonate contents and physical properties of the sediment indicate that glaciers were present in the catchment area of Lama Lake during the period Oldest Dryas to AllerØd. For the same time period, δ¹³C values of TOC give indications of a perennial ice cover. Since the AllerØd, organic matter accumulation has increased, caused by an increasing input of land vegetation and aquatic primary production as revealed by relations TOC contents, TOC/TN ratios and δ¹³C values of TOC. During the Holocene climatic optimum, in late Preboreal and Boreal times, biogenic primary production in Lama Lake reached its maximum and the vegetation in the catchment area changed from grassy tundra to a dense forest. From the Atlantic period to the present, small variations in δ¹³C values of TOC and TOC contents are probably related to the location of Lama Lake on the border between grass and wooded steppe, leading to responses of environmental conditions to even small changes in climate.