阿尔金山北缘花岗岩的形成时代及其构造环境探讨

采用同位素稀释热电离质谱(IDTIMS)U Pb法单颗粒锆石测年技术,测得阿尔金山北缘阔什布拉克斑状花岗岩的结晶年龄为443±5Ma。花岗岩结晶年龄的确定,为讨论该地区早古生代构造岩浆演化提供了依据,也为该地区地层划分提供了重要的参考数据。     

U-Pb geochronology of basement rocks in central Tibet and paleogeographic implications

The ages and paleogeographic affinities of basement rocks of Tibetan terranes are poorly known. New U-Pb zircon geochronologic data from orthogneisses of the Amdo basement better resolve Neoproterozoic and Cambro-Ordovician magmatism in central Tibet. The Amdo basement is exposed within the Bangong suture zone between the Lhasa and Qiangtang terranes and is composed of granitic orthogneisses with subordinate paragneisses and metasedimentary rocks. The intermediate-felsic orthogneisses show a bimodal distribution of Neoproterozoic (920-820 Ma) and Cambro-Ordovician (540-460 Ma) crystallization ages. These and other sparse basement ages from Tibetan terranes suggest the plateau is underlain by juvenile crust that is Neoproterozoic or younger; its young age and weaker rheology relative to cratonic blocks bounding the plateau margins likely facilitated the propagation of Indo-Asian deformation far into Asia. The Neoproterozoic ages post-date Rodinia assembly and magmatism of similar ages is documented in the Qaidaim-Kunlun terrane, South China block, the Aravalli-Delhi craton in NW India, the Eastern Ghats of India, and the Prince Charles mountains in Antarctica. The Amdo Neoproterozoic plutons cannot be unambiguously related to one of these regions, but we propose that the Yangtze block of the South China block is the most likely association, with the Amdo basement representing a terrane that possibly rifted from the active Yangtze margin in the middle Neoproterozoic. Cambro-Ordovician granitoids are ubiquitous throughout Gondwana as a product of active margin tectonics following Gondwana assembly and indicate that the Lhasa-Qiangtang terranes were involved in these tectono-magmatic events. U-Pb detrital zircon analysis of two quartzites from the Amdo basement suggest that the protoliths were Carboniferous-Permian continental margin strata widely deposited across the Lhasa and Qiangtang terranes. The detrital zircon age spectra of the upper Paleozoic Tibetan sandstones and other rocks deposited in East Gondwana during the late Neoproterozoic and Paleozoic are all quite similar, making it difficult to use the age spectra for paleogeographic determinations. There is a suggestion in the data that the Qiangtang terrane may have been located further west along Gondwana’s northern boundary than the Lhasa terrane, but more refined spatial and temporal data are needed to verify this configuration.     

State regulation of agro-industries in developing countries. Governance systems in the vegetable oil industry of Malaysia and Zimbabwe

Some observers of agro-industrial development in the Third World recognize the nation state as an important locus for mediation of conflicts between different social forces but fail to analyze the regulatory mechanisms and their effect on the dynamics of particular agro-industries. This paper proposes an analytical model to study agro-industrial regulation in a national framework. The purpose is to investigate the dynamics between changing forms of state regulation and particular industry governance system in the vegetable oil industry in Malaysia and Zimbabwe. State regulation and industry governance systems differ in a number of ways between the two countries. However, in each country there is a correspondence between the dominant form of state regulation and the industrial governance system. The study confronts some of the basic assumptions within the food regime approach and stress the need to incorporate the properties of the crops in the analysis of (agro-)industry governance systems.     

Sediment supply from landslide-dominated catchments: implications for basin-margin fans

The sediment flux from a mountainous catchment can be expressed as a function of a landslide rate constant κ which accounts for the vigour of hillslope erosion. Since the incising drainage network flushes all or a portion of the products of hillslope erosion to a range front where fan deposition takes place, a conservation of solid sediment volume allows the fan area and progradation distance to be calculated. These parameters are related primarily to the discharge of sediment from the catchment and to local tectonic subsidence.
A survey of modern alluvial fans in a wide range of climatic and tectonic settings shows that the effects of climate and bedrock lithology cannot be discriminated in the scatter of data of catchment area vs. fan area. However, by focusing on over 100 fans in the arid and semiarid zone of SW USA, the impact of tectonic subsidence rate is unambiguous. Although further quantitative data on local tectonic subsidence rates are urgently required, our preliminary analysis suggests considerable potential for reconstructing palaeocatchments where basin tectonic subsidence rates can be estimated. The progradation distances of fans from the northern and southern margins of the Middle Devonian Hornelen Basin of Norway, and the western and north-eastern margins of the Mio-Pliocene Ridge Basin, California, allow catchment sizes and denudation rates to be approximated. Although unique solution sets are not possible, an iteration of parameter values allows plausible parameter combinations to be calculated which shed light on the tectonic and sedimentary history of the proximal basin and upland source regions. Model results suggest significant asymmetry in basin subsidence rates, catchment slopes and transport mechanics between the two margins.     

On farsidedness of palaeomagnetic poles: Magnetic refraction,sediment compaction and dipole off-set

Summary Three possible ways to explain the Caenozoically observed farsidedness of paleomagnetic poles (apart from lithospheric plate movements) are discussed: magnetic refraction, sediment compaction and dipole off-set. The dipole off-set, being a possible geomagnetic field property, will be of opposite sign on opposite hemispheres, and hence will not tend to smoothe out by sectorial averaging. Sediment compaction shallows the inclination on both hemispheres, and hence will tend to smoothe out by sectorial averaging, provided that sediment properties, site latitude coverage and number of investigations are equal (fairly unlikely).Magnetic refraction causes systematic directional distortions of the remanent magnetization in rocks of moderate to high magnetic intensity (or apparent susceptibility k_app=k(1+Q)) such as in many volcanics, some metamorphics, as well as in baked clays and slags, etc. A detailed discussion of this effect is given: If the k_app of the material is above 0.001 emu/cm ³ (×4 SI), this effect is likely to cause a significant palaeomagnetic refraction error of the NRM (typically a TRM or a CRM) of the rock. An apparent susceptibility of this order of magnitude is quite common in volcanic rocks; e.g. for oceanic floor basalts the average of k_app is about 0.02 emu/cm ³ corresponding to systematic errors (flattening) of some 3° to 6° in the inclination of a horizontal flow, depending on the latitude.To improve paleomagnetic results in general, a simple refraction correction is therefore suggested to be applied in the case of common two-dimensional (i.e. flat, elongated) geological bodies such as dykes, sills, lava flows and baked clays. Numerical solutions are given for the horizontal case, while a graphical solution is given for the general two-dimensional case.Being of systematic types, the refraction error together with the sediment compaction effect may be responsible for a major part of the observed farsidedness of the Caenozoic palaeomagnetic pole positions, the apparent farsidedness not yet beeing masked by the scatter of pole positions produced by older individual lithospheric plate movements.Presented at 2nd conference on New Trends in Geomagnetism, Castle of Bechyn, Czechoslovakia, September 24–29, 1990.     

The Tucson revised index of asteroid data

Attention is called to the availability of the TRIAD computer file, a compilation of all reliable physical parameters for minor planets.     

Electromagnetic Subsurface Imaging. A Case for an Adaptive Born Approximation

The process of interpretation of electromagnetic data has many facets of which fast approximate interpretation techniques is an intriguing one. A new variant of the Born approximation – the Adaptive Born approximation – is presented and exemplified through 1D and 2D imaging techniques for transient electromagnetic data. The Adaptive Born approximation is generally applicable in approximate inversion schemes for inductive electromagnetic data as a one-pass imaging algorithm. Though it is as simple to use as the ordinary Born approximation, it offers a more accurate inverse mapping.In the first part of this paper an attempt will be made to give an overview of fundamental concepts in electromagnetic subsurface imaging relevant for approximate inverse mappings and to outline major trends in present day modeling and inversion of electromagnetic data. This is of course an impossible task – certainly for this author – and much important work will not be mentioned in the limited space of the following. My apologies to the people who are not mentioned and whose research is not given credit here though it should have been. Naturally, my choice of references reflects the schools and circles I have been subjected to, but I hope that the list of references to developments in electromagnetic methods will point to papers of importance and thereby to other references for the interested reader.