A model for the emplacement of the eastern compartment of the bushveld complex

The stratigraphy and geological position of the eastern compartment of the Bushveld Complex are described. A mechanical model for the initiation and growth of the eastern compartment of the Bushveld intrusion has been developed using thin elastic plate theory, assuming linked conical magma chambers. It is shown that the contribution to the pressure at the base of a cell by the restitutional force exerted by the roof of Rooiberg felsites is 10⁴ times as great as that of the layers of host in the cone. Both are minimal compared to the lithostatic pressure exerted by the magma pile. Roof deformation is therefore seen to be a more important process than sagging of the floor during intrusion—a feature which probably occurred during cooling, solidification and isostatic readjustment of the area.A stratigraphie model is proposed in which the intrusion of basic rocks into the Transvaal sequence is discussed in the light of continuous basin subsidence. Early submarine sedimentation in an irregularly-floored basin some 620 km in diameter situated on the Archaean craton gave rise to a 7.7 km thick sedimentary pile, to which was added some 7 km of subaerial basalts and felsites. Depression of the floor of the basin into the regime of maximum horizontal compression induced favourable conditions for the intrusion of a total of 2.5 km of diabase sills which further assisted the subsidence. The 9 km thick Bushveld Complex was intruded into the basal sections at points along a 010° trend in a regime characterised by shear failure. Early magma influxes gave rise to a laminated marginal zone forming a shallow cone, with associated sill activity, whilst continued later influxes filled the conical cell, transgressed the floor and uparched the roof. Partial melting in the regions beneath the Complex, exacerbated by continued crustal depression, gave rise to the late Bushveld granites.     

Geochemical constraints on the origin of mafic and silicic magmas at Cordón El Guadal, Tatara-San Pedro Complex, central Chile

The aim of this study is to quantify the crustal differentiation processes and sources responsible for the origin of basaltic to dacitic volcanic rocks present on Cordón El Guadal in the Tatara-San Pedro Complex (TSPC). This suite is important for understanding the origin of evolved magmas in the southern Andes because it exhibits the widest compositional range of any unconformity-bound sequence of lavas in the TSPC. Major element, trace element, and Sr-isotopic data for the Guadal volcanic rocks provide evidence for complex crustal magmatic histories involving up to six differentiation mechanisms. The petrogenetic processes for andesitic and dacitic lavas containing undercooled inclusions of basaltic andesitic and andesitic magma include: (1) assimilation of garnet-bearing, possibly mafic lower continental crust by primary mantle-derived basaltic magmas; (2) fractionation of olivine + clinopyroxene + Ca-rich plagioclase + Fe-oxides in present non-modal proportions from basaltic magmas at ∼4–8 kbar to produce high-Al basalt and basaltic andesitic magmas; (3) vapor-undersaturated (i.e., P _H2O<P _TOTAL) partial melting of gabbroic crustal rocks at ∼3–7 kbar to produce dacitic magmas; (4) crystallization of plagioclase-rich phenocryst assemblages from dacitic magmas in shallow reservoirs; (5) intrusion of basaltic andesitic magmas into shallow reservoirs containing crystal-rich dacitic magmas and subsequent mixing to produce hybrid basaltic andesitic and andesitic magmas; and (6)␣formation and disaggregation of undercooled basaltic andesitic and andesitic inclusions during eruption from shallow chambers to form commingled, mafic inclusion-bearing andesitic and dacitic lavas flows. Collectively, the geochemical and petrographic features of the Guadal volcanic rocks are interpreted to reflect the development of shallow silicic reservoirs within a region characterized by high crustal temperatures due to focused basaltic activity and high magma supply rates. On the periphery of the silicic system where magma supply rates and crustal temperatures were lower, cooling and crystallization were more important than bulk crustal melting or assimilation. Received: 2 July 1997 / Accepted: 25 November 1997     

Singularity and Nonnormality in the Classification of Compositional Data

Geologists may want to classify compositional data and express the classification as a map. Regionalized classification is a tool that can be used for this purpose, but it incorporates discriminant analysis, which requires the computation and inversion of a covariance matrix. Covariance matrices of compositional data always will be singular (noninvertible) because of the unit-sum constraint. Fortunately, discriminant analyses can be calculated using a pseudo-inverse of the singular covariance matrix; this is done automatically by some statistical packages such as SAS. Granulometric data from the Darss Sill region of the Baltic Sea is used to explore how the pseudo-inversion procedure influences discriminant analysis results, comparing the algorithm used by SAS to the more conventional Moore–Penrose algorithm. Logratio transforms have been recommended to overcome problems associated with analysis of compositional data, including singularity. A regionalized classification of the Darss Sill data after logratio transformation is different only slightly from one based on raw granulometric data, suggesting that closure problems do not influence severely regionalized classification of compositional data.     

Determining effective wellbore permeability from a field pressure test: a numerical analysis of detection limits

We propose a simple pressure test that can be used in the field to determine the effective permeability of existing wellbores. Such tests are motivated by the need to understand and quantify leakage risks associated with geological storage of CO₂ in mature sedimentary basins. If CO₂ is injected into a deep geological formation, and the resulting CO₂ plume encounters a wellbore, leakage may occur through various pathways in the “disturbed zone” surrounding the well casing. The effective permeability of this composite zone, on the outside of the well casing, is an important parameter for models of leakage. However, the data that exist on this key parameter do not exist in the open literature, and therefore specific field tests need to be done in order to reduce the uncertainty inherent in the leakage estimates. The test designed and analyzed herein is designed to measure effective wellbore permeability within a low-permeability caprock, bounded above and below by permeable reservoirs, by pressurizing the reservoir below and measuring the response in the reservoir above. Alternatively, a modified test can be performed within the caprock without directly contacting the reservoirs above and below. We use numerical simulation to relate pressure response to effective well permeability and then evaluate the range of detection of the effective permeability based on instrument measurement error and limits on fracture pressure. These results can guide field experiments associated with site characterization and leakage analysis.     

Middle-Late Alpine thermotectonic evolution of the southern Rhodope Massif,Greece

Abstract

The transition from the Alpine tectonic assembly to the exhumation of the units in the Rhodope metamorphic province in northernmost Greece has been refined by ⁴⁰Ar/³⁹Ar laserprobe mica analyses. Preservation of pre-Alpine (~ 280 Ma and 145 Ma) muscovite cooling ages at the western margin of the Rhodope indicate that subsequent events failed to reset the argon system thermally in white mica in the outcropping basement of this region. The central and eastern Rhodope are characterized by white mica cooling ages of 40–35 Ma with ages gradually decreasing to ca. 15 Ma near the eastern margin of the Strymon Valley. The Eo-Oligocene ages reflect the regional exhumation of the metamorphosed units to shallow crustal levels, with corresponding temperatures below ca. 350 °C, by 40–35 Ma. The younger cooling ages are attributed to the initiation and subsequent operation of the Strymon-Thasos detachment system since ca. 30 Ma. This study provides a crucial contribution to future regional tectonic models for the Rhodope region as it recognizes an early stage of development of the Strymon-Thasos detachment system, and has constrained the regional exhumation of the Rhodope metamorphic province since 40 Ma indicating that the regionally observed amphibolite facies metamorphism had terminated by this time. © 2000 Editions scientifiques et médicales Elsevier SAS     

Chemical and structural relations of epitaxial xenotime and zircon substratum in sedimentary and hydrothermal environments: a TEM study

Xenotime overgrowths on detrital zircon in siliciclastic sediments have been reported in numerous studies. However, in natural samples, solid solution of zircon and xenotime is limited to near-end-member compositions. In order to characterize the interface region between both minerals and to draw inferences on the growth mechanisms of authigenic xenotime, we studied xenotime overgrowths on detrital zircon grains from two Phanerozoic sandstone samples with contrasting post-depositional histories. In one sample, the small (≤10 μm), pyramidal xenotime overgrowths are of diagenetic origin and grew without major discontinuity on the detrital zircon grain. The second sample shows up to >50-μm-wide, porous and inclusion-rich, hydrothermal xenotime overgrowths on detrital zircon, whereas the transition zone between both minerals is accompanied by large pore volume. Chemical compositions of the xenotime precipitates from the two samples differ particularly in Y, REE, Th and Sc concentrations, whereas high MREE availability in the diagenetic sample and the presence of Sc in the hydrothermal sample, respectively, appear to have promoted xenotime growth. Transmission electron microscopy on electron-transparent foils cut from the interface region shows that both the diagenetic xenotime and the hydrothermal xenotime are crystalline and grew in optical and crystallographic continuity to their detrital zircon substrata. Only a narrow transition zone (≤90 nm—diagenetic sample, 200–300 nm—hydrothermal sample) between zircon and xenotime is in part made up of nanometre-scale crystalline domains that are slightly distorted and may have formed from dissolution–re-precipitation processes at the zircon rim along with precipitation from the respective fluid.     

Evidence for fullerenes in solid bitumen from pillow lavas of Proterozoic age from Mítov (Bohemian Massif, Czech Republic)

Andesitic pillow lavas containing biogenic, solid bitumen (SB) are a constituent of a Neoproterozoic volcanosedimentary sequence (Teplá-Barrandian unit, Bohemian Massif) in the Mítov area of the Czech Republic. A black shale formation that is crosscut by these andesitic basalts is 565 Ma old. Carbon disulfide extracts of two powdered samples of SB contain 0.2 and 0.3 ppm of C₆₀, respectively, as determined by high-pressure liquid chromatography. The peak assignment based on retention time is fully supported by high-resolution electron ionization mass spectrometry (EI-MS). No C₇₀ was detected, nor was C₆₀ found in two other SB samples from this locality. Other investigated carbonaceous samples from Bohemia (coals and anthracites of Upper Paleozoic age and anthraxolite, graphitoids, and graphite of Upper Proterozoic age) did not contain fullerenes at concentrations above the detection limit of 0.01 ppm. The absence of C₆₀ in these samples was confirmed by EI-MS. The proposed mechanism of fullerene formation involves a primary algal phase, generation of a hydrocarbonaceous mixture in the course of thermal evolution of the sedimentary series, and their high-temperature transformation related to the extrusion of basalt. An important feature for fullerene conservation was the enclosure of fullerenes in SB with a structure similar to glasslike carbon, where the fullerene was protected against oxidation.     

Reduction of surface subsidence risk by fly ash exploitation as filling material in deep mining areas

The possibility of filling empty underground spaces, which exist owing to the extraction of mineral raw materials, with fly ash and cement fly ash mixes has been studied for the purpose of reducing the impact of deep mining on the surface. The method of physical modelling was used to study the behaviour of fly ash mixes deposited in extracted mine spaces. The models were constructed for two different geometries of underground extracted mine spaces: into cavities of the type of large slits created in the course of mining in steeply lode deposits without subsequently filling the empty space and in vertical shafts of rectangular or circular cross-section with horizontal side headings that have partly caved in due to mine shocks or other seismic events.