Rare earth element, 87Sr/86Sr,and 143Nd/144Nd compositions of Cenozoic orogenic dacites from Baja California,northwestern Mexico,and adjacent west Texas: evidence for the predominance of a subcrustal component

Dacitic lavas and ignimbrites were examined from seven localities that span the entire 700 km width of the mid- to late Cenozoic magmatic arc of northwestern Mexico and adjacent west Texas. These rocks have remarkably similar REE patterns that are parallel in the heavy REE and have modest negative Eu anomalies. Samples from three localities including Baja California, the Sierra Madre Occidental, and the Chihuahuan Basin and Range have initial ⁸⁷Sr/⁸⁶Sr between 0.7044 and 0.7050 and _Nd near 0.0±1.0. These dacites are isotopically similar to associated basalts, and they show no systematic isotopic variation that is correlated with age or composition of the basement. There is no evidence that magmas parental to these dacites interacted significantly with continental crust. Samples form three other localites in the Basin and Range vary in initial ⁸⁷Sr/⁸⁶Sr from 0.7051 to 0.7070 and _Nd from about -1 to –2. The composition of these rocks reflects contamination of the parental magmas by relatively small amounts of Precambrian crust. Collectively, the dacites of this study show much less isotopic variation than do Mesozoic granitoids (Farmer and DePaolo 1983) and late Cenozoic olivine tholeiites (Hart 1985) from similar transects of the western United States. The distinctive source region for the magmas parental to the Mexican dacites was relatively uniform isotopically, but it was enriched in LIL and HFS elements beneath the eastern Basin and Range.     

The Hemlo gold deposit,Ontario: A geochemical and isotopic study

The Hemlo deposit, near Marathon, Ontario, is one of the largest gold deposits in North America. It is stratiform within Archean metamorphosed volcano-sedimentary rocks. The main ore zone is composed of pyritic, sericitic schist, and massive barite. This is the first report of stratiform barite in the Archean of North America, but other occurrences have since been found west of Hemlo. The mineralization is substantially enriched in Au, Mo, Sb, Hg, Tl and V and lacks carbonate. Because of metamorphism and deformation of the body its genesis is uncertain.${}^{87}\text{Sr}{}^{86}\text{Sr}$ of .7017 for barite from the deposit is similar to that of the sedimentary barite west of Hemlo and to initial ratios of contemporaneous volcanic rocks. At the base of the main ore zone, barite with δ^34S of +8 to +12%. was deposited with ~0%. pyrite. Upward, both barite and pyrite get isotopically lighter, with minimum values for pyrite, to ?17.5%, in non-baritic schist forming the upper part of the ore zone. In drill section, Au grades correlate with the isotopic composition of pyrite. This, and the association of fractionated sulphide with sulphate, suggests that Au, pyrite and barite were deposited contemporaneously. The linked, asymmetric distributions of S minerals and isotopic distributions, which are continuous from section to section, and the isotopic similarity of the Hemlo and western barites are consistent with a syngenetic depositional model.Two sources for the S minerals are considered. In the first, exogenous sulphate from a restricted basin were partially reduced in a geothermal system to form ³⁴S-depleted sulphide. In the second, the sulphate and sulphide are of magmatic-hydrothermal origin. Sulphate and fractionated sulphide are uncommon in Archean rocks, but one or both occur with unusual frequency in major Archean gold deposits. Hydrothermal fluids of moderately high , containing sulphate and permitting isotopic fractionation between oxidized and reduced S species, may have favoured the dissolution, transport and precipitation of Au.     

Al,Si ordering in cordierite using “magic angle spinning” NMR

A method of determining the number of Al-O-Al bonds per unit cell from ²⁹Si nuclear magnetic resonance (NMR) data of synthetic cordierites with increasing Si, Al order is described. The number of Al-O-Al bonds is found to vary linearly with the logarithm of the annealing time. This may be correlated with previously published heat of solution data on similar samples (Carpenter et al. 1983) to determine the enthalpy change Δh, associated with a single Al?Si interchange in cordierite. Δh is found to be 8.1 kcal/mole. The NMR data show that the short range Al, Si order cannot be described in terms of twin domains of ordered orthorhombic cordierite. An ordering model derived from group theoretical constraints on possible Al, Si distributions within the hexagonal symmetry of the cordierite is found to provide a better fit to the NMR data.     

Slope process change and colluvium deposition in Swaziland: An SEM analysis

Electron microscope analysis of sixty samples taken from six colluvium sites in Swaziland has shown that the quartz grains exhibit marked edge abrasion in the uppermost beds of exposures. This edge abrasion is lacking in the lower beds in all sections examined and indicates slope process change during colluvium deposition. A model of slope evolution is provided, in which slope erosion progressively brings about more channelling and causes greater surface roughness because of the exhumation of more core stones and differentially weathered rock. This roughness causes increased edge abrasion.     

Petrography of low rank coals with reference to liquefaction potential

Ten sub-bituminous coals were analyzed petrographically and chemically as part of a program by the Alberta Research Council to investigate the compositional properties of Alberta coals and relationship to liquefaction potential. In addition, four lignites, two from Saskatchewan and one each from Texas and North Dakota, were studied for comparative purposes. Liquefaction experiments were carried out on three of the Alberta coals and on density fractions of the two U.S. lignites. Petrographically the ten sub-bituminous coals could be divided into two populations; one with high huminite and liptinite contents (>90%) and one with substantially lower contents (70–75%). The two U.S. lignites showed huminite plus liptinite contents of less than 70% while the Saskatchewan coals were somewhat higher. Reflectance measurements on the sub-bituminous coals showed a range of 0.33 to 0.51 with a rough correlation to the ASTM rank designations of these coals. Reflectivities determined on the lignites were generally lower. Float-sink fractionation of the two U.S. lignites and two sub-bituminous coals showed enrichment of huminite and liptinite in the lighter fractions and inertinite and mineral matter in the heavier splits. Examination of the relationship between liquefaction yields and petrographic composition showed a rough positive correlation with huminite-liptinite content. Examination of liquefaction residues suggested total conversion of the liptinite, extensive conversion of huminite and possible partial conversion and reactivity of the semifusinite maceral of the inertinite group.     

Interdecadal climate variability in the subpolar North Atlantic

The statistical relationships between various components of the subpolar North Atlantic air-sea-ice climate system are reexamined in order to investigate potential processes involved in interdecadal climate variability. It is found that sea surface temperature anomalies concentrated in the Labrador Sea region have a strong impact upon atmospheric sea level pressure anomalies over Greenland, which in turn influence the transport of freshwater and ice anomalies out of the Arctic Ocean, via Fram Strait. These freshwater and ice anomalies are advected around the subpolar gyre into the Labrador Sea affecting convection and the formation of Labrador Sea Water. This has an impact upon the transport of North Atlantic Current water into the subpolar gyre and thus, also upon sea surface temperatures in the region. An interdecadal negative feedback loop is therefore proposed as an internal source of climate variability within the subpolar North Atlantic. Through the lags associated with the correlations between different climatic components, observed horizontal advection time scales, and the use of Boolean delay equation models, the time scale for one cycle of this feedback loop is determined to have a period of about 21 years.     

Simulation of the effect of doubled atmospheric CO2 on the climate of northern and central California

A Local Climate Model (LCM) is described that can provide a high-resolution (10 km) simulation of climate resulting from a doubling of atmospheric CO₂ concentrations. A canonicalregression function is used to compute the monthly temperature (mean of daily-maximum-temperature) and precipitation for any point, given a set of predictor variables. Predictor variables represent the influence of terrain, sea-surface temperature (SST), windfields, CO₂ concentration, and solar radiation on climate. The canonical-regression function is calibrated and validated using empirical windfield, SST, and climate data from stations in the western U.S. To illustrate an application of the LCM, the climate of northern and central California is simulated for a doubled CO₂ (600 ppmv) and a control scenario (300 ppmv CO₂). Windfields and SSTs used to compute predictor variables are taken from general circulation model simulations for these two scenarios. LCM solutions indicate that doubling CO₂ will result in a 3 C° increase in January temperature, a 2 C° increase in July temperature, a 16 mm (37%) increase in January precipitation, and a 3 mm (46%) increase in July precipitation.     

River channel adjustments downstream from channelization works in England and Wales

Relatively little attention has been given to river channel adjustments that occur downstream from channelization works. This study is concerned with the nature of channel adjustments downstream from a total of 46 channelization works located in low and high energy environments in England and Wales. Channel changes are identified principally by the method of field survey and by reconstructing the original positions of eroded beds and banks. Use is also made of maps, aerial photographs, and engineering drawings of different dates and the technique of space-for-time substitution is applied. Enlargement of channel cross-sections through erosion had occurred downstream from a variety of types, sizes, and dates of channelization works. The maximum increase of channel size was 153 per cent. Out of a total of 14 sites with enlarged channel cross-sections, seven had undergone a change of width only, at a further three width increased rather than depth, and at the remaining four sites depth increases were dominant. These sites all have relatively high stream powers. Factors causing spatial variation of erosion included tree roots locally binding bank sediments and the occurrence of bends. Planform change had taken place at only one site. A further three high stream power sites had downstream reaches incised into bedrock and therefore did not exhibit adjustment. Channel enlargement is explained in terms of increased flood flows downstream from channelization works causing higher stream velocities, which in turn cause erosion, thereby increasing channel width and/or depth. Examination of flow records for 35 stations revealed flood events which would formerly have spread overbank but are now confined by the channelization works and are therefore likely to alter downstream flows. At sites with downstream change it is proposed that the energy of increased flows was sufficient to exceed a threshold required for erosion of perimeter sediments. By contrast the absence of change at a majority of sites in low energy lowland areas could be a reflection of both the incompetence of increased flows to erode and resistance provided by perimeter sediments. Sites with erosion features appear not to have yet attained new equilibrium conditions.     

Possible effects of pre-existing basement topography on thrust fault ramping

Finite-element models show that one way in which thrust ramps may arise is through the mechanical interaction between basement and overlying sediments. In the simplest case, shear coupling between a planar basement—sediment contact causes the differential stresses in the sediments to die out with depth and distance from the applied load. For such cases, curved thrust faults may result if the strength of the rock is exceeded. Basement topography may also affect the location and shape of ramps by acting as a stress concentrator, by producing a stress shadow and by changing principal stress orientations. Modeling suggests that whether or not these basement topographic features cause ramping will depend on the height and angularity of the feature as well as the rock types that overlie it.Under the assumption of linear elasticity and for given boundary conditions, the Poisson's ratio plays an important role in determining the orientation and magnitude of the principal stresses. Calculations using experimentally measured Poisson's ratios predict that the earliest maximum compressive stress directions should be nearly vertical in the more cratonward portions of thrust belts. However, the stress directions which are inferred to have occurred earliest in this part of thrust belts are nearly horizontal. This suggests that non-elastic or ductile processes have an effect on the propagation of thrust faults.