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     

ISOTOPE STUDIES OF PIPEFLOW AT PLYNLIMON,WALES, UK

Residence times and flow paths of pipe and stream flow were studied during low flow in the Nant Gerig and Gwy experimental catchments at Plynlimon in mid-Wales, UK, using a two-month time series of natural deuterium and electrical conductivity data from perennial and ephemeral pipe flow, stream flow, groundwater and rainfall. Low flow in both the perennial pipe and the stream was maintained by ‘old’ groundwater discharge. This groundwater was at least 40 days old. Flow in the ephemeral pipe was dominated by old groundwater and was only slightly affected by direct inputs of new water. Although direct rainfall inputs contributed minimally to runoff in the perennial pipe and the stream, rainfall influenced the isotopic and chemical character of the groundwater. Rainfall also affected the water-table elevation, which determined the flashiness of the perennial pipe flow and whether the ephemeral pipe flowed. The isotope and electrical conductivity data suggest that storm runoff in both the main pipe and the stream is overwhelmingly old water. A sensitivity analysis suggests that the old water is supplied both from near-stream groundwater and upslope groundwater delivered by the ephemeral pipes.     

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.