Multistage Activity Research Articles

Foreland basins and gold-bearing conglomerates: a new model for the Jacobina Basin (Sa˜o Francisco province, Brazil)

From a lithological, structural and metamorphic viewpoint, the Jacobina Basin in the Sa˜o Francisco province (Brazil) is representative of the foreland basins that mark a characteristic evolutionary stage in the development of the Palaeoproterozoic orogenic belts around 2000 Ma ago. In addition, it contains gold-bearing conglomerates of economic interest that are presently mined. The general structure of the basin is one of stacked detrital and schist formations along sinistral strikeslip/overthrusting shear zones, with exposed deep-seated units in the east and shallower and younger rocks in the west. New 40Ar/ 39Ar geochronological data give minimum ages of 1940-1910 Ma for the main thrusting and wrenching events. Shearing was accompanied by metamorphic recrystallization that indicates a high geothermal gradient and a progressive loading of the underthrust units. Despite this final major tectonic imprint, at least five stages of basin evolution are indicated by traces of tectonic instability, such as debris-flow deposits, chaotic breccias and early dyke swarms. In this context the conglomerate layers found in the basin formations correspond to periods of erosion of a preceding stage in the basin development. Multistage hydrothermal activity is shown by widespread alteration within mineralized conglomerates (sulphide, fuchsite, tourmaline, chromite, rutile), leaching of ultrabasic rocks, and phlogopite and/or emerald mineralization in contact metamorphic aureoles around late- to post-tectonic granites. This tectonic evolution is coherent with the development of the Jacobina Basin in a foreland position and its progressive involvement in the Transamazonian collision belt. Resedimentation processes, preservation of earlier stages of hydrothermal alteration in pebbles, extensive hydrothermal activity and a high geothermal gradient favour a model of epigenetic mineralization for the gold-bearing conglomerates with enrichment during the progressive propagation of the basin. Comparison with similar mineralized conglomerates shows that this model can be applied to other Precambrian foreland basins.

Vein-type tungsten deposits of China and adjoining regions

Taking the South China Tungsten Province as representative, this paper deals systematically with the geological setting, geometry, paragenesis, wallrock alteration, and geochemical characteristics of Asian vein-type tungsten deposits on the basis of data acquired since the 1970's. The regional metallogenic conditions and the genesis of the deposits are discussed. The large-scale distribution of vein-type tungsten deposits in Asia is inferred to be controlled by tungsten-enriched formations, multi-period and multi-stage granitic activity, and favourable tectonic environment. The deposits are not only controlled by magmatic differentiation, but also by the overall evolution of the continental crust.

Evaluation of information systems: IS development in the Processing Company

Information systems (IS) are centrally important to the modern form of organization, but their impacts are not yet well understood. IS development is, in general, grounded in a quasi-scientific ‘formal-rationalism’, oriented chiefly towards technical aspects; yet the accompanying difficulties in implementation are due for the most part to ‘people problems’. In defining an IS as a computer system embedded in an organization, this paper argues that information systems are not purely technical systems, but also social systems. Consequently, work in this area necessitates an interactionist approach, taking the effects of information technology to be a product of neither the technical nor social aspects alone, but of their interaction. Evaluation is shaped, similarly, by the interaction of technique with social process: it is a multi-stage activity running through IS development. The history of IS development in the Processing Company illustrates these views and highlights the dysfunctionality of the formal-rational perspective.

Evaluation of Information Systems: Is Development in the Processing Company

Information systems (IS) are centrally important to the modern form of organization, but their impacts are not yet well understood. IS development is, in general, grounded in a quasi-scientific ‘formal-rationalism’, oriented chiefly towards technical aspects; yet the accompanying difficulties in implementation are due for the most part to ‘people problems’. In defining an IS as a computer system embedded in an organization, this paper argues that information systems are not purely technical systems, but also social systems. Consequently, work in this area necessitates an interactionist approach, taking the effects of information technology to be a product of neither the technical nor social aspects alone, but of their interaction. Evaluation is shaped, similarly, by the interaction of technique with social process: it is a multi-stage activity running through IS development. The history of IS development in the Processing Company illustrates these views and highlights the dysfunctionality of the formal-rational perspective.

Origin and evolution of the ureilite parent magmas: Multi-stage igneous activity on a large parent body

Analyses of olivine and pigeonite cores (interpreted as cumulate crystals) in eight low-shock ureilites show well-defined correlations between Fe/ X (where X is one of the minor elements Mn, Cr, Ca, Al, Ti, P, or Ni) and Fe/ Mg ratios. For the lithophile minor elements these trends are linear, with positive slope, and pass through or near the origin, indicating various degrees of FeO reduction of the parent magmas. The trends shown by P and Ni are consistent with this interpretation, and, additionally, require equilibrium crystallization of 20–27 mole % metal. Graphite was most likely the reducing agent, which implies that the ureilites equilibrated at different pressures, because the graphite fO 2 buffer is pressure dependent. There is no evidence for fractionation of olivine or pryroxene in any of these trends. The Ca, Ti and Al trends show significant scatter, indicating that there must have been multiple parent magmas, with very similar Mg/Mn and Mg Cr ratios, but different Mg Al , Mg Ca and Mg Ti ratios. CaO/Al 2O 3 ratios vary by a factor of ~3 among the eight magmas, and are superchondritic (~ 14–32, molar), similar to lunar mare basalts. We propose the following model for generation and crystallization of ureilite parent magmas (UPM). Chondritic material undergoes 10–25% partial melting, leaving a plagioclase-depleted residue. The magma or magmas form a differentiated crust on the ureilite parent body (UPB), with mafic cumulates and plagioclase cumulates spatially separated. Later remelting (<10%) of the mafic cumulates produced the UPM. Siderophile elements indicate that the source region of the UPM had not experienced segregation of metal such as would occur during core formation. In the source region, the pressure must have been at least 0.5–1 kb, because only at pressures this high does the graphite buffer allow iron oxide to be the stable phase relative to metallic iron. The graphite-bearing UPM were emplaced at various levels in the crust, where they were reduced to various extents, dependent on depth. Ureilites are early olivine-pigeonite cumulates from these magmas. Compositions of interstitial, low-Ca pyroxenes indicate that the trapped interstitial liquids from which they crystallized had lower Mn Mg , Cr Mg , Ca Mg , Al Mg and Na Mg ratios than the UPM, and indicate an abrupt change in liquid composition coincident with impact excavation of ureilites. This interstitial material may be derived by mixing a shock melt of nearby olivine-pigeonite cumulates with residual primary liquid. Our model predicts that the UPB had a differentiated crust, did not have a core, and was at least 235 km in radius. The lack of a ureilite-like body among spectrally analyzed asteroids suggests that either the spectral signature of the UPB is not that of ureilites themselves (perhaps of carbon-free feldspathic material instead), that the UPB is not intact, or that ureilites did not originate in the asteroid belt.

Aggregate Index Number Formula for the Productivity Planning Including Multi-stage Activity Units

Aggregate Index Number Formula for the Productivity Planning Including Multi-stage Activity Units