Petrology and Emplacement of Reversely Zoned Gabbro-Diorite Plutons in the Smartville Complex, Northern California

Gabbroic plutons are part of the intrusive substructure of theSmartville Complex, a late Jurassic, rifted, ensimatic arc locatedin the northern Sierra Nevada of California. The plutons rangefrom unzoned, equant bodies of olivine gabbro less than 1 kmin diameter to elongate intrusions up to 25 km in length thatare reversly zoned from olivine gabbro cores to quartz dioriterims. The felsic rocks dip inward beneath the mafic core, indicatingthat this zoning reversal continues to depth. The zoned plutonshave relatively shallow keels. We interpret the reversed zoningas an emplacement feature, analogous to the compositional zoningin a zoned tephra sheet. It formed as a result of tapping analready zoned, deeper level magma chamber. Whether the originalzoning of the magma was concentric or stratiform cannot be readilydeduced. During emplacement, considerable amounts of cumulaterocks were mobilized. The mineralogy and geochemistry of the reversely zoned plutonsindicate that they contain two suites of rocks: a cumulate suiterepresented by olivine gabbro and olivine clinopyroxenite anda differentiated suite of non-cumulate olivine gabbros, gabbronorites,and diorites that lie along a compositional continuum and approximateliquid compositions. Plagioclase and olivine compositions inthe Smartville Complex cumulate suite are identical to thosein modern arc cumulates and are characteristic of the arc cumulatesuite. The differentiated rocks form a compositionally continuousseries that is geochemically very similar to a differentiatedsuite of arc tholeiitic basalts and andesites. Fractionationmodeling indicates that removal of mineral phases found in thecumulate gabbros from the mafic members of the differentiatedsuite can produce the lithologic variation seen in the zonedplutons. Plutons such as those in the Smartville Complex indicatethat there is a genetic link between cumulate rocks and a basalt-andesitefractionation trend in arcs, supporting the hypothesis thatarc andesites form by crystal fractionation. The gabbroic plutonsand related Alaska-type ultramafic complexes contain ultramaficcumulates that can rectify the discrepancy between the cumulatemode predicted by fractionation models and the observed modeof gabbroic cumulates in arcs.     

Comment on ' ... Magmatic Conditions of the Fish Canyon Tuff, Central San Juan Volcanic Field, Colorado ' by Whitney & Stormer (1985)

Magmatic conditi ons inferred from the composition of the FishCanyon Tuff by Whitney &Stormer (J. Petrology, 26, 726–62(1985)) and Stormer & Whitney (Am. Miner. 70, 52–64(1985)) differ from studies of other ash-flow tuff and calderasystems, in that they consider the magma to have been unzonedand to have resided at {small tilde}9 kb pressure just priorto eruption. We find these conclusions unconvincing becauseof incomplete sampling of the Fish Canyon Tuff, alteration ofthe tuff after emplacement, and, in particular, serious limitationsof coupled Fe-Ti oxide and two-feldspar geothermobarometry.     

Sand transport model of barchan dune equilibrium

Erosion and deposition over a barchan dune near the Salton Sea, California, is modelled by book-keeping the quantity of sand in saltation following streamlines of transport. Field observations of near-surface wind velocity and direction plus supplemental measurements of the velocity distribution over a scale model of the dune are combined as input to Bagnold-type sand-transport formulae corrected for slope effects. A unidirectional wind is assumed. The resulting patterns of erosion and deposition compare closely with those observed in the field and those predicted by the assumption of equilibrium (downwind translation of the dune without change in size or geometry). Discrepancies between the simulated results and the observed or predicted erosional patterns appear to be largely due to natural fluctuation in the wind direction. Although the model includes a provision for a lag in response of the transport rate to downwind changes in applied shear stress, the best results are obtained when no delay is assumed. The shape of barchan dunes is a function of grain size, velocity, degree of saturation of the oncoming flow, and the variability in the direction of the oncoming wind. Smaller grain size or higher wind speed produce a steeper and more blunt stoss-side. Low saturation of the inter-dune sandflow produces open crescent-moon-shaped dunes, whereas high saturation produces a whaleback form with a small slip face. Dunes subject to winds of variable direction are blunter than those under unidirectional winds. The size of barchans could be proportional to natural atmospheric scales, to the age of the dune, or to the upwind roughness. The upwind roughness can be controlled by fixed elements or by the sand is saltation. In the latter case, dune scale may be proportional to wind velocity and inversely proportional to grain size. However, because the effective velocity for transport increases with grain size, dune scale may increase with grain size as observed by Wilson (1972).     

Interlobate ice-sheet dynamics during the Last Glacial Maximum at Whitburn Bay, County Durham, England

This research reconstructs ice-sheet processes operating during the Late Devensian in northeast England. The article assesses the lithostratigraphy of the Devensian glacial tills of Whitburn Bay, eastern County Durham, and presents the first detailed analysis of petrological, geochemical and biostratigraphical data to reconstruct lithostratigraphy, provenance and iceflow pathways. Two Devensian tractions tills (the Blackhall and Horden tills) are separated by a boulder pavement, pointing to a switch in ice-bed conditions and the production of a melt-out lag prior to deposition of the upper traction till, the Horden Till. The Blackhall Till contains Magnesian Limestone, Carboniferous Limestone, Whin Sill dolerite and Old Red Sandstone, suggesting a northwesterly source, probably from the Midland Valley and the Southern Uplands. The Horden Till contains erratics and heavy minerals derived from crystalline bedrock sources in the Cheviot Hills and northeast Scotland. Within the Horden Till are numerous sand, clay and gravel-filled canals incised downwards into the diamicton which are attributed to a low-energy, distributed, subglacial canal drainage system. Coupled with hydro-fractures and the boulder pavement, this suggests that a partially decoupled, fast-flowing ice stream deposited the Horden Till. The uphill, landward direction of ice movement indicates that the ice stream was confined in the North Sea Basin, possibly by the presence of Scandinavian Ice.     

Deciphering the depositional environment of the laminated Crato fossil beds (Early Cretaceous,Araripe Basin,North‐eastern Brazil)

The laminated limestones of the Early Cretaceous Crato Formation of the Araripe Basin (North‐eastern Brazil) are world‐famous for their exceptionally well‐preserved and taxonomically diverse fossil fauna and flora. Whereas the fossil biota has received considerable attention, only a few studies have focused on the sedimentary characteristics and palaeoenvironmental conditions which prevailed during formation of the Crato Fossil Lagerstätte. The Nova Olinda Member represents the lowermost and thickest unit (up to 10 m) of the Crato Formation and is characterized by a pronounced rhythmically bedded, pale to dark lamination. To obtain information on palaeoenvironmental conditions, sample slabs derived from three local stratigraphic sections within the Araripe Basin were studied using high‐resolution multiproxy techniques including detailed logging, petrography, μ‐XRF scanning and stable isotope geochemistry. Integration of lithological and petrographic evidence indicates that the bulk of the Nova Olinda limestone formed via authigenic precipitation of calcite from within the upper water column, most probably induced and/or mediated by phytoplankton and picoplankton activity. A significant contribution from a benthonic, carbonate‐secreting microbial mat community is not supported by these results. Deposition took place under anoxic and, at least during certain episodes, hypersaline bottom water conditions, as evidenced by the virtually undisturbed lamination pattern, the absence of a benthonic fauna and by the occurrence of halite pseudomorphs. Input of allochthonous, catchment‐derived siliciclastics to the basin during times of laminite formation was strongly reduced. The δ¹⁸O values of authigenic carbonate precipitates (between ?7·1 and ?5·1‰) point to a ¹⁸O‐poor meteoric water source and support a continental freshwater setting for the Nova Olinda Member. The δ¹³C values, which are comparatively rich in ¹³C (between ?0·1 and +1·9‰), are interpreted to reflect reduced throughflow of water in a restricted basin, promoting equilibration with atmospheric CO₂, probably in concert with stagnant conditions and low input of soil‐derived carbon. Integration of lithological and isotopic evidence indicates a shift from closed to semi‐closed conditions towards a more open lake system during the onset of laminite deposition in the Crato Formation.     

Dissolution of Corundum and Andalusite in H2O-Saturated Haplogranitic Melts at 800{degrees}C and 200 MPa: Constraints on Diffusivities and the Generation of Peraluminous Melts

The mechanisms and kinetics of equilibration between peraluminousminerals and granitic melt were investigated experimentallyby the dissolution of corundum and andalusite into H₂O-saturatedmetaluminous haplogranitic melt at 800°C and 200 MPa. Mineraland haplogranitic glass rods were juxtaposed inside platinumcapsules, and then subjected to experimental conditions fortimes ranging from 12 to 2900 h. Upon melting, the mineral –meltinterface retreats with the square root of time. The compositionof the melt at the interface changes with time, but its ASI[aluminum saturation index = molar Al₂O₃/(CaO + Na₂O + K₂O)]remains constant at     

Rapid Variscan exhumation and the role of magma in core complex formation: southern Brittany metamorphic belt, France

ABSTRACT The high-grade migmatitic core to the southern Brittany metamorphic belt has mineralogical and textural features that suggest high-temperature decompression. The chronology of this decompression and subsequent cooling history have been constrained with ⁴⁰Ar/³⁹ Ar ages determined for multigrain concentrates of hornblende and muscovite prepared from amphibolite and late-orogenic granite sheets within the migmatitic core, and from amphibolite of the structurally overlying unit. Three hornblende concentrates yield plateau isotope correlation ages of c. 303–298 Ma. Two muscovite concentrates record well-defined plateau ages of c. 306–305 Ma. These ages are geologically significant and date the last cooling through temperatures required for intracrystalline retention of radiogenic argon. The concordancy of the hornblende and muscovite ages suggest rapid post-metamorphic cooling. Extant geochronology and the new ⁴⁰Ar/³⁹Ar data suggest a minimum time-integrated average cooling rate between c. 725 °C and c. 125 °C of c. 14 ± 4°C Ma^-1, although below 600 °C the data permit an infinitely fast rate of cooling. Mineral assemblages and reaction textures in diatexite migmatites suggest c. 4 kbar decompression at 800–750 °C. This must have pre-dated the rapid cooling. Emplacement of two-mica granites into the metamorphic belt occurred between 345 and 300 Ma. The youngest plutons were emplaced synkinematically along shallow-dipping normal faults interpreted to be reactivated Eo-Variscan thrusts. A penetrative, west-plunging stretching lineation developed in these granites suggests that extension was orogen-parallel. Extension was probably related to regional uplift and gravitational collapse of thermally weakened crust during constrictional (escape) tectonics in this narrow part of the Variscan orogen. This followed slab breakoff during the terminal stages of convergence between Gondwana and Laurasia; detachment may have been consequent upon a change in kinematics leading to dextral displacement within the orogen. Dextral ductile strike-slip displacement was concentrated in granites emplaced synkinematically along the South Armorican Shear Zone. Rapid cooling is interpreted to have resulted from tectonic unroofing with emplacement of granite along decollement surfaces. The high-grade migmatitic core of the southern Brittany metamorphic belt represents a type of metamorphic core complex formed during orogen-parallel extensional unroofing and regional-scale ductile flow.