APPLICATION OF GEOELECTRIC METHODS USING BURIED ELECTRODES IN EXPLORATION AND MINING1

Various geoelectric methods which have been developed and applied in the last 10–20 years in ELGI are discussed. These methods which use buried electrodes are: hole-to-surface gradient mapping to detect bauxite deposits in sinkholes below a resistive screening layer; in-mine gradient profiling to map the basement topography below galleries; and the hole-to-surface version of geoelectric layer tracing to find outcrops of mineralized zones penetrated by drillings. Data processing procedures have been developed on the basis of common concepts and hypotheses to link theoretical models with geological structures. The objects investigated are determined as the difference between the theoretical models and geological structures. The predominant part of the real electric field measured above the geological structures is the theoretical field related to the theoretical model. The effects of the objects (the anomaly) are superposed on the theoretical field but their extent is small compared with the values of the latter. The theoretical field and the anomaly depend strongly on the separation from the sources. For this reason the anomalies are difficult to recognize. Therefore the ratio of the theoretical field to the measured one is computed, since σ_a, the apparent specific conductivity, is proportional to this ratio. It is demonstrated that since the changes in the σ_a curve depending on the location of the observation point are small, the anomalies can easily be recognized on the curve. The σ_a, curve computed in the above way reflects the objects better than the originally measured electric fields. Examples illustrate the solution of the above-mentioned geological problems by the practical application of adequate geoelectric methods using buried electrodes.     

Parameterization of the Melting Regime of the Shallow Upper Mantle and the Effects of Variable Lithospheric Stretching on Mantle Modal Stratification and Trace-Element Concentrations in Magmas

Parameterization of melting phenomena in the upper mantle hasprimarily focused on two basic themes, namely the physical andchemical processes that govern partial melting. Parameterizationof physical processes mainly refers to establishing relationshipsbetween parameters such as the temperature, pressure, matrixand melt flow geometry, lithospheric stretching, and volumeof magma. By contrast, parameterization of chemical processeslargely implies unravelling the relationships between type anddegree of melting, and source and melt composition. Few attemptshave been made, however, to interrelate the two processes. Thepresent work is an effort to provide a link between physicaland chemical parameters associated with mantle melting and toallow in-depth modelling of partial melting processes in upwellingasthenosphere in a rigorous yet simplified manner. Several correlationsamong the most important physical parameters (e.g., equilibrationand extrusion temperature and pressure of magma, melt fractionand thickness, stretching factor, etc.) are explored. On thisbasis, a model for the compositional stratification of the lithosphereis proposed, and its bearing on the nature of intra-oceanicarc magmatism is emphasized. Trends of melting residues in termsof modal olivine and clinopyroxene are calculated for a widerange of possible potential temperatures that may be appliedto xenolith or abyssal peridotite suites to constrain furthertheir original depth of upwelling. Dry solidus equations fordepleted peridotite compositions are also derived that may beused to infer the effects of volatiles on the melting of refractorysupra-subduction zone mantle. The sensitivity of certain elementsto temperature variations during melting in a column of ascendingmantle is highlighted using Ni as an example, and the dangersof using single-value distribution coefficients to predict concentrationsof transition metals in magmas are emphasized. MORB-normalizedmulti-element profiles calculated using a variety of sources,mantle potential temperatures, and stretching factors are presented,and the differences between instantaneous and pooled melts arediscussed. A technique to calculate mineral proportions duringtransformation of garnet lherzolite to spinel lherzolite, togetherwith estimates of the modal composition of fertile spinel andgarnet lherzolite are included. Selected trace-element abundancesin various sources [bulk silicate Earth, depleted MORB (mid-oceanridge basalt) mantle, N-MORB) and distribution coefficientsfor common rock-forming minerals are also tabulated.     

Tracing Lithosphere Evolution through the Analysis of Heterogeneous G9-G10 Garnets in Peridotite Xenoliths, II: REE Chemistry

Following previous publication of major–minor elementdata, this paper presents rare earth element (REE) data forheterogeneous (chemically zoned) garnets belonging to the peridotitesuite of mantle xenoliths from the Jagersfontein kimberlitepipe, South Africa. The rim compositions of the garnets in thehighest temperature–pressure (deepest) deformed peridotitesshow a typical megacryst-like pattern, of very low light REE(LREE) increasing through the middle REE (MREE) to a plateauof heavy REE (HREE) at c. 20 times chondrite; these compositionswould be in equilibrium with small-volume melts of the mid-oceanridge basalt (MORB) source (asthenosphere). With decreasingdepth the garnet rims show increasing LREE and decreasing HREE,eventually resulting in humped relative abundance patterns.A set of compositions is calculated for melts that would bein equilibrium with the garnet rims at different depths. Theseshow decreasing relative abundance of each REE from La to Lu,and the La/Lu ratio of the melts increases with decreasing depthof formation. Modelling of the effects of crystal fractionationshows that this process could largely generate the sequenceof garnet rim and melt compositions found with decreasing depth,including the humped REE patterns in high-level garnets. Consideringthe behaviour of major–minor elements as well as REE,a process of percolative fractional crystallization is advocatedin which megacryst source melts percolate upwards through peridotitesand undergo fractionation in conjunction with exchange withthe peridotite minerals. The initial megacryst melt probablyincludes melt of lithospheric origin as well as melt from theMORB source, and it is suggested that the process of percolativefractional crystallization may form a variety of metasomaticand kimberlitic melts from initial megacryst melts. Repeatedmetasomatism of the lower lithosphere by such differentiatingmelts is suggested by consideration of garnet core compositions.Such metasomatism would progressively convert harzburgites tolherzolites by increasing their CaO content, and this may accountfor the fact that the Cr-rich diamond–garnet harzburgiteparagenesis is commonly preserved only where it has been encapsulatedin diamonds. KEY WORDS: cratonic lithosphere; garnet zoning; mantle xenoliths; megacryst magma; metasomatic melt     

Geochemical Constraints on Leucogranite Magmatism in the Langtang Valley, Nepal Himalaya

A complex of crustally derived leucogranitic sills emplacedinto sillimanite-grade psammites in the upper Langtang Valleyof northern Nepal forms part of the Miocene High Himalayan graniteassociation. A series of post-tectonic, subvertical leucograniticdykes intrude the underlying migmatites, providing possiblefeeders to the main granite sills. The leucogranite is peraluminous and alkali-rich, and can besubdivided into a muscovite–biotite and a tourmaline–muscovitefacies. Phase relations suggest that the tourmaline leucogranitescrystallized from a water-undersaturated magma of minimum-meltcomposition at pressures around 3–4 kbar. Potential metasedimentaryprotoliths include a substantial anatectic migmatite complexand a lower-grade mica schist sequence. Isotopic constraintspreclude the migmatites as a source of the granitic melts, whereastrace-element modelling of LILEs (Rb, Sr, and Ba), togetherwith the Nd and Sr isotopic signatures of potential protoliths,strongly suggest that the tourmaline-bearing leucogranites havebeen generated by fluid-absent partial melting of the muscovite-richschists. However, REE and HFSE distributions cannot be reconciledwith equilibrium melting from such a source. Systematic covariationsbetween Rb, Sr, and Ba can be explained by variations in protolithmineralogy and P–T–a_H2O. Tourmaline leucogranites with high Rb/Sr ratios represent low-fraction-melts(F{small tilde} 12%) efficiently extracted from their protolithsunder conditions of low water activity, whereas the heterogeneoustwo-mica granites may result from melting under somewhat highera_H2O conditions. The segregation of low-degree melts from sourcewas probably by deformation-enhanced intergranular flow andmagma fracturing, with the mechanisms of migration and emplacementcontrolled by variations in the uppercrustal stress regime duringlate–orogenic extensional collapse of the thickened crust.     

STRUCTURAL GLACIOLOGY OF A TEMPERATE MARITIME GLACIER: LOWER FOX GLACIER,NEW ZEALAND

This paper describes the structural glaciology of the lower Fox Glacier, a 12.7 km‐long valley glacier draining the western side of the Southern Alps, New Zealand. Field data are combined with analysis of aerial photographs to present a structural interpretation of a 5 km‐long segment covering the lower trunk of the glacier, from the upper icefall down‐glacier to the terminus. The glacier typifies the structural patterns observed in many other alpine glaciers, including: primary stratification visible within crevasse walls in the lower icefall; foliation visible in crevasses below the lower icefall; a complex set of intersecting crevasse traces; splaying and chevron crevasses at the glacier margins; transverse crevasses forming due to longitudinal extension; longitudinal crevasses due to lateral extension near the snout; and, arcuate up‐glacier dipping structures between the foot of the lower icefall and the terminus. The latter are interpreted as crevasse traces that have been reactivated as thrust faults, accommodating longitudinal compression at the glacier snout. Weak band‐ogives are visible below the upper icefall, and these could be formed by multiple shearing zones uplifting basal ice to the glacier surface to produce the darker bands, rather than by discrete fault planes. Many structures such as crevasses traces do not show a clear relationship with measured surface strain‐rates, in which case they may be ‘close to crevassing’, or are undergoing passive transport down‐glacier.     

High-Pressure Granulites (Retrograded Eclogites) from the Hengshan Complex, North China Craton: Petrology and Tectonic Implications

Both high- and medium-pressure granulites have been found asenclaves and boudins in tonalitic–trondhjemitic–granodioriticgneisses in the Hengshan Complex. Petrological evidence fromthese rocks indicates four distinct metamorphic assemblages.The early prograde assemblage (M₁) is preserved only in thehigh-pressure granulites and represented by quartz and rutileinclusions within the cores of garnet porphyroblasts, and omphacitepseudomorphs that are indicated by clinopyroxene + sodic plagioclasesymplectic intergrowths. The peak assemblage (M₂) consists ofclinopyroxene + garnet + sodic plagioclase + quartz ±hornblende in the high-pressure granulites and orthopyroxene+ clinopyroxene + garnet + plagioclase + quartz in the medium-pressuregranulites. Peak metamorphism was followed by near-isothermaldecompression (M₃), which resulted in the development of orthopyroxene+ clinopyroxene + plagioclase symplectites and coronas surroundingembayed garnet grains, and decompression-cooling (M₄), representedby hornblende + plagioclase symplectites on garnet. The THERMOCALCprogram yielded peak (M₂) P–T conditions of 13·4–15·5kbar and 770–840°C for the high-pressure granulitesand 9–11 kbar and 820–870°C for the medium-pressuregranulites, based on the core compositions of garnet, matrixpyroxene and plagioclase. The P–T conditions of pyroxene+ plagioclase symplectite and corona (M₃) were estimated at     

Cambrian enigmas

Bizarre soft-bodied animals from the Cambrian, principally the Burgess Shale of British Columbia, are throwing new light on the major diversification of early metazoans. A distinctive range of new body-plans hint at explosive rates of evolution, but the underlying mechanisms are still a matter for conjecture. Whether these unusual fossils suffered extinction because of bad design or bad luck is uncertain, but some evidence suggests that chance factors played an important role.