Replenishment, Crystal Accumulation and Floor Aggradation in the Megacrystic Kameruka Suite, Australia

Detailed field evidence indicates that the Kameruka Suite plutonsof the Bega Batholith, eastern Australia, grew by crystal accumulationon the floor of a magma chamber. Depositional features in theplutons, including mafic enclave channels, asymmetric enclavepillows and exotic rafts, load casts and flame structures, andgraded and trough cross-beds, indicate that the pluton builtprogressively upward. The general eastward dip of depositionalfeatures in the main pluton implies a lower western and uppereastern contact, consistent with a basal granite–migmatitecontact in the west and a sharp hornfelsic sidewall contactin the east. Mafic, felsic and composite dykes, most commonnear and below the basal western contact, are interpreted asconduits for magma chamber replenishment and imply open-systembehaviour during pluton construction. Textural relations arealso consistent with an open-system, cumulate origin. Typically,centimetre-scale grains of quartz, plagioclase and megacrysticalkali feldspar form a touching framework with interstices filledwith smaller biotite flakes and smaller intercumulus quartzand feldspar crystals. Alkali feldspar megacrysts vary fromeuhedral and unzoned, to mantled and partially replaced by plagioclase,to ovoid and completely pseudomorphed by quartz–albiteaggregates. The common occurrence of mantled and pseudomorphedalkali feldspar in mafic enclaves, and in hybrid tonalitic rocksforming the matrix to enclave swarms, suggests that replacementor resorption of granitic primocrysts was associated with maficreplenishments. The occurrence of all megacryst types at outcropscale implies extended alkali feldspar crystallization in differentparts of the chamber, thorough stirring during, or after, periodicreplenishment, and final settling in a cumulate mush. The bulkcomposition of the cumulate mush, represented by granodiorite,cannot represent the emplaced magma. Compositional variationcan be modelled by variable degrees of crystal accumulationfrom a parental, silica-rich melt represented by the silicicdykes. As dykes periodically fed the magma chamber, crystalsaccumulated on the floor, and more evolved melts probably eruptedfrom its roof. Thus, the average composition of the magma, andthe cumulus minerals, may have remained relatively constant,and the sublinear chemical trends that typify the Kameruka Suitesimply reflect differing proportions of melt and cumulate material.Sublinear chemical trends can also be explained by a restitemodel; however, the distinctive Ba, light rare earth elementand Zr spikes at high silica can be explained only by a cumulatemodel, which also explains why the low-silica granites of thesuite share the same chemical characteristics as the high-silicagranites. KEY WORDS: crystal accumulation; magma chamber; open system; granitoids; Kameruka; Australia     

Ultramafic Xenoliths from the Bearpaw Mountains, Montana, USA: Evidence for Multiple Metasomatic Events in the Lithospheric Mantle beneath the Wyoming Craton

Ultramafic xenoliths in Eocene minettes of the Bearpaw Mountainsvolcanic field (Montana, USA), derived from the lower lithosphereof the Wyoming craton, can be divided based on textural criteriainto tectonite and cumulate groups. The tectonites consist ofstrongly depleted spinel lherzolites, harzburgites and dunites.Although their mineralogical compositions are generally similarto those of spinel peridotites in off-craton settings, somecontain pyroxenes and spinels that have unusually low Al₂O₃contents more akin to those found in cratonic spinel peridotites.Furthermore, the tectonite peridotites have whole-rock majorelement compositions that tend to be significantly more depletedthan non-cratonic mantle spinel peridotites (high MgO, low CaO,Al₂O₃ and TiO₂) and resemble those of cratonic mantle. Thesecompositions could have been generated by up to 30% partialmelting of an undepleted mantle source. Petrographic evidencesuggests that the mantle beneath the Wyoming craton was re-enrichedin three ways: (1) by silicate melts that formed mica websteriteand clinopyroxenite veins; (2) by growth of phlogopite fromK-rich hydrous fluids; (3) by interaction with aqueous fluidsto form orthopyroxene porphyroblasts and orthopyroxenite veins.In contrast to their depleted major element compositions, thetectonite peridotites are mostly light rare earth element (LREE)-enrichedand show enrichment in fluid-mobile elements such as Cs, Rb,U and Pb on mantle-normalized diagrams. Lack of enrichment inhigh field strength elements (HFSE; e.g. Nb, Ta, Zr and Hf)suggests that the tectonite peridotites have been metasomatizedby a subduction-related fluid. Clinopyroxenes from the tectoniteperidotites have distinct U-shaped REE patterns with strongLREE enrichment. They have ¹⁴³Nd/¹⁴⁴Nd values that range from0·5121 (close to the host minette values) to 0·5107,similar to those of xenoliths from the nearby Highwood Mountains.Foliated mica websterites also have low ¹⁴³Nd/¹⁴⁴Nd values (0·5113)and extremely high ⁸⁷Sr/⁸⁶Sr ratios in their constituent phlogopite,indicating an ancient (probably mid-Proterozoic) enrichment.This enriched mantle lithosphere later contributed to the formationof the high-K Eocene host magmas. The cumulate group rangesfrom clinopyroxene-rich mica peridotites (including abundantmica wehrlites) to mica clinopyroxenites. Most contain >30%phlogopite. Their mineral compositions are similar to thoseof phenocrysts in the host minettes. Their whole-rock compositionsare generally poorer in MgO but richer in incompatible traceelements than those of the tectonite peridotites. Whole-rocktrace element patterns are enriched in large ion lithophileelements (LILE; Rb, Cs, U and Pb) and depleted in HFSE (Nb,Ta Zr and Hf) as in the host minettes, and their Sr–Ndisotopic compositions are also identical to those of the minettes.Their clinopyroxenes are LREE-enriched and formed in equilibriumwith a LREE-enriched melt closely resembling the minettes. Thecumulates therefore represent a much younger magmatic event,related to crystallization at mantle depths of minette magmasin Eocene times, that caused further metasomatic enrichmentof the lithosphere. KEY WORDS: ultramafic xenoliths; Montana; Wyoming craton; metasomatism; cumulates; minette     

Tidal influence in Cretaceous fluvial strata from Utah, USA: a key to sequence stratigraphic interpretation

Detailed models already exist that outline physical and temporal relationships in marine and marginal marine strata. Such models are still in their infancy in alluvial deposits. Recognition of tidal and estuarine influence in fluvial strata is critical to the development of high resolution sequence stratigraphic correlations between marine and non-marine strata. Strata that have previously been interpreted as low energy meandering river deposits contain sedimentary and biogenic structures that suggest a tidal influence. These structures include sigmoidal bedding, paired mud/silt drapes, wavy and lenticular bedding, shrinkage cracks, multiple reactivation surfaces, inclined heterolithic strata, complex compound cross-beds, bidirectional cross-beds, and trace fossils including Teredolites, Arenicolites and Skolithos. Although none of these structures is unique to tidal processes, the preponderance of data suggests that fluvial systems have been affected by tidal processes well inland of coeval shoreline deposits. These deposits rarely form a significant proportion of a depositional sequence; however, their occurrence allows time significant surfaces to be extended for tens or even hundreds of kilometres inland from coeval shoreline deposits. In Turonian through Campanian strata exposed in the Kaiparowits Plateau of southern Utah, tidally influenced facies are recognized within at least two distinct stratigraphic levels that were deposited during periods of relatively rapid base level rise. These strata form part of an alluvial transgressive systems tract. Landward of each of the marine transgressive maxima, tidal facies are present in fluvial channels that are completely encased in non-marine strata at distances up to 65 km inland from a coeval palaeoshoreline. Our work suggests that such deposits may have gone unrecognized in the past, but they form a significant component of alluvial strata in many depositional sequences. Although these tidally influenced fluvial deposits may be difficult to recognize, they are temporally equivalent to marine maximum flooding surfaces and provide a chronostratigraphic correlation between alluvial and nearshore marine deposits.     

Laos and the making of a 'relational' resource frontier

This paper seeks to reconsider the contemporary relevance of the resource frontier, drawing on examples of nature's commodification and enclosure under way in the peripheral Southeast Asian country of Laos. Frontiers are conceived as relational zones of economy, nature and society; spaces of capitalist transition, where new forms of social property relations and systems of legality are rapidly established in response to market imperatives. Customary property rights on the resource frontier can be seized by powerful actors in crucial political moments, preparing the territorial stage for more intensive phases of resource commodity production and accumulation. Relational frontier space is understood through the work of geographers such as Doreen Massey, who views the production of space as 'constituted though the practices of engagement and the power-geometries of relations'. In Laos, a twenty-first century resource frontier is being driven by new corporate investments in natural resources, and a supporting array of land reform programmes. The paper focuses on both the material and representational aspects of the production of the resource frontier, through policy and discourse analysis, and village level research in Laos' Khammouane province. By rethinking a dualist and hierarchical-scaled imaginary of frontier places, both rural people and local ecologies are shown to be key actors, in a complex, relational reproduction of frontier zones. An emerging Lao spatial and political assemblage – a form of 'frontier-neoliberalism'– is shown as producing dramatic changes in socio-natural landscapes, as well as new patterns of marginalisation and livelihood insecurity for a vulnerable rural population.     

Triassic carbonate submarine fans along the Arabian platform margin, Sumeini Group, Oman

ABSTRACT The Sumeini Group formed along the passive continental margin slope that bounded the northeastern edge of the Arabian carbonate platform. With the initial development of this passive continental margin in Oman during Early to Middle Triassic time (possibly Permian), small carbonate submarine fans of the C Member of the Maqam Formation developed along a distally steepened slope. The fan deposits occur as several discrete lenticular sequences of genetically related beds of coarsegrained redeposited carbonate (calciclastic) sediment within a thick interval of basinal lime mudstone and shale. Repeated pulses of calciclastic sediment were derived from ooid shoals on an adjacent carbonate platform and contain coarser intraclasts eroded from the surrounding slope deposits. Sediment gravity flows, primarily turbidites with lesser debris flows and grain flows, transported the coarse sediments to the relatively deep submarine fans. Channel erosion was a major source of intraformational calcirudite. Two small submarine fan systems were each recurrently supplied with calciclastic sediment derived from point sources, submarine canyons. The northern fan system retrogrades and dies out upsection. The southern fan system was apparently longer-lived; calciclastic sediments in it are more prevalent and occur throughout the section. The proximal portions of this fan system are dominated by channelized beds of calcirudite which represent inner- to mid-fan channel complexes. The distal portions include mostly lenticular, unchannelized beds of calcarenite, apparently mid- to outer-fan lobes. Carbonate submarine fans appear to be rare in the geological record in comparison with more laterally continuous slope aprons of coarse redeposited sediment. The carbonate submarine fans of the C Member apparently formed by the funnelling of coarse calciclastic sediment into small submarine canyons which may have developed due to rift and/or transform tectonics. The alternation of discrete sequences of calciclastic sediment with thick intervals of ‘background’ sediment resulted from either sea-level fluctuations or pulses of tectonic activity.     

Terrace scarp deflation as a renewable source for eolian sediments in an arctic periglacial setting

Glacial, glaciofluvial, and glaciolacustrine sediments deposited by the retreating Vibekes Glacier are being actively reworked into sand dunes and loess sheets on the tops of glaciofluvial terraces down-valley from Vibekes Glacier and Vibekes So. Active permafrost precludes trenching below 0.5 to 1.0 meters. However, sedimentary structures, deflationary and abrasion features in shallow and surface deposits are visible. Although an armored pavement inhibits sediment deflation on the horizontal terrace surfaces, a combination of fluvial erosion, mass-wasting, and eolian processes on terrace scarps provides a continuing source of sand- and silt-size materials for redeposition.