Positional accuracy and measurement error in digital databases of land use: an empirical study

This paper discusses the issues of positional accuracy and measurement error in the context of a large empirical study of landscape change in England and Wales. The epsilon band model of digitizing accuracy is used to make estimates of the levels of positional uncertainty and measurement error that is due to digitizing polygon outlines. The degree of error expected had the same polygons been captured in raster format is then determined. These results prompt a general discussion of the nature of error in spatial databases.     

Book Reviews

Regular grid sampling structures in the plane are a common spatial framework for many studies. Constructing grids with desirable properties such as equality of area and shape is more difficult on a sphere. We studied the distortion characteristics of recursive partitions of the surface of the globe starting with the octahedron and icosahedron polyhedral models. We used five different methods for mapping from the polyhedral model to the surface of the sphere: the Gnomonic projection, Fuller's Dymaxion projection, Snyder's equal area polyhedral projection, direct spherical subdivision, and a recursive polyhedral projection. We increased partition density using both a 4-fold and a 9-fold ratio at each level of recursive subdivision by subdividing to the 8th level with the 4-fold density ratio (65 536 cells per polyhedral face) and to the fifth level with the 9-fold density ratio (59 049 cells per polyhedral face). We measured the area and perimeter of each cell at each level of recursion for each method on each model using each density ratio. From these basic measurements we calculated the range and standard deviation of the area measurement, and the mean, range, and standard deviation of a compactness measurement defined as the ratio of (the ratio of the perimeter to the area of the cell) to (the ratio of the perimeter to the area of a spherical circle with the same area). We looked at these basic measurements and their statistics using graphs of variation with recursion level, sums of squares analyses of variation, histograms of the distributions, maps of the spatial variation, and correlograms. The Snyder projection performed best in area distortion and the Gnomonic projection performed best in compactness distortion. The Fuller projection and the Sphere method had moderate distortion in both area and compactness relative to the worst methods. There was little difference in distortion performance between partitions using the 4-fold density ratio and those using the 9-fold density ratio. Partitions based on the icosahedron performed better for all statistics than those based on the octahedron.     

云南哈播斑岩铜(-钼-金)矿床流体包裹体研究

哈播斑岩Cu-(Mo-Au)矿床产于哀牢山富碱斑岩带的南段,形成于青藏高原后碰撞阶段构造转换环境,属于陆-陆碰撞型斑岩矿床.根据脉体的交切关系,确定哈播矿床各种脉的演化序列为早期石英脉→石英-黄铜矿脉→石英辉钼矿脉.脉中流体包裹体的岩相学、显微测温和激光拉曼光谱分析等研究结果显示,各期脉中均有富气相包裹体、富液相包裹体和含子矿物多相包裹体,各种包裹体的气相均含有CO2、SO2、H2O等气体.各期脉中多种包裹体并存并具有相似的均一温度范围,富液相包裹体均一温度149～427℃,盐度ω(NaCleq)6.0％～15.0％;富气相包裹体均一温度205～405℃,盐度ω(NaCleq) 3.4％～19.0％;含子矿物多相包裹体均一温度305～516℃,盐度w(NaCleq) 33.5％～61.0％.哈播矿床的初始成矿流体由稳定共存、不混溶的低盐度流体和高盐度流体组成,高盐度流体是哈播矿床成矿元素迁移的主要载体.成矿流体在400℃左右发生“二次沸腾”、分相,温度下降和挥发分持续逃逸可能是Cu-Au成矿的诱因.Mo元素在成矿流体多次沸腾、分相过程中,持续优先分配进入高盐度流体中而逐步富集;温度下降,使含钼硫化物在流体中溶解度降低、沉淀,形成石英-辉钼矿±黄铜矿脉.     

长江中下游成矿带成矿规律和成矿模式

长江中下游成矿带作为中国东部重要的成矿带之一,成矿地质条件、成矿规律和成矿模式的相关研究已经十分深入,形成了一系列公认的理论研究成果,本文重点论述了成矿带成矿地质条件和成矿规律方面研究的新进展,并在此基础上构筑了成矿带的成矿模式。本文主要开展了以下总结工作:(1)将成矿带的构造要素突破以往按构造单元分解的做法,确立跨构造单元的"复合构造系统",结合成矿带成岩成矿作用特点,重新确定了长江中下游成矿带的范围,并对成矿亚带进行了重新划分;(2)收集汇总了长江中下游成矿带近年来国内外学者最新的成岩成矿年龄数据,总结了矿床时空分布规律;(3)从地质,地球物理和地球化学等方面,总结归纳了沿基底结合带复活的网状断裂系统,阐明了成矿带的控矿构造格架,分析了长江中下游成矿带的成因规律;(4)补充和完善了断隆区和断凹区的成矿模式,并初步构筑了长江中下游成矿带综合成矿模式。提出长江中下游成矿带内主体成岩成矿作用总体上分为走滑挤压阶段(146~135Ma)、走滑引张阶段(135~126Ma)和拉张伸展阶段(126~123Ma)等三个阶段,古太平洋板块作用导致的壳幔过程,引发含元古代弧岩浆岩源区活化和中生代的构造活化("双活化")是长江中下游成矿带形成的主要机制。     

High- and Low-Temperature I-type Granites

Abstract: I– and S-type granites differ in several distinctive ways, as a consequence of their derivation from contrasting source rocks. The more mafic granites, whose compositions are closest to those of the source rocks, are most readily classified as I– or S–type. As granites become more felsic, compositions of the two types converge towards those of lowest temperature silicate melts. While discrimination of the two is therefore more difficult for such felsic rocks, that in no way invalidates the twofold subdivision. If felsic granite melts undergo fractional crystallisation, the major element compositions are not affected to any significant extent, but the concentrations of trace elements can vary widely. For some trace elements, fractional crystallisation causes the trace element abundances to diverge, so the I– and S– type granites are again easily separated. Such fractionated S-type granites can be distinguished, for example, by high P and low Th and Ce, relative to their I-type analogues. Our observations in the Lachlan Fold Belt show that there is no genetic basis for subdividing peraluminous granites into more mafic and felsic varieties, as has been attempted elsewhere. The subdivision of felsic peraluminous granites into I– and S-types is more appropriate, and mafic peraluminous granites are always S–type. In a given area, associated mafic and felsic S-type granites are likely to be closely related in origin, with the former comprising both restite-rich magmas and cumulate rocks, and the felsic granites corresponding to melts that may have undergone fractional crystallisation after prior restite separation. We propose a subdivision of I-type granites into two groups, formed at high and low temperatures. The high-temperature I–type granites formed from a magma that was completely or largely molten, and in which crystals of zircon were not initially present because the melt was undersaturated in zircon. In comparison with low-temperature I–type granites, the compositions extend to lower SiO₂ contents and the abundances of Ba, Zr and the rare earth elements initially increase with increasing SiO₂ in the more mafic rocks. While the high-temperature I–type granite magmas were produced by the partial melting of mafic source rocks, their low-temperature analogues resulted from the partial melting of quartzofeldspathic rocks such as older tonalites. In that second case, the melt produced was felsic and the more mafic low-temperature I–type granites have that character because of the presence of entrained and magmatically equilibrated restite. High temperature granites are more prospective for mineralisation, both because of that higher temperature and because they have a greater capacity to undergo extended fractional crystallisation, with consequent concentration of incompatible components, including H₂O.     

Calculated phase equilibria involving chemical potentials to investigate the textural evolution of metamorphic rocks

Evolving pressure–temperature conditions during metamorphism drive changes in the stable mineral assemblage, mineral proportions and mineral compositions in rocks. These changes are achieved via the diffusion of components between minerals, fluid and melt, the driving force for diffusion being the gradients in chemical potential of the components developed spatially within the rock. This study utilises recent developments in the software thermocalc to investigate quantitatively chemical potential relationships in rocks, with the phases involved being (solid) solutions. Phase diagrams with chemical potentials as axes are used to understand better the spatial rearrangement of components during the metamorphic evolution of rocks and the metamorphic textures that result. In contrast to qualitative chemical potential diagrams, quantitative diagrams can be contoured for mineral composition, allowing consideration of chemical zoning in minerals. Furthermore, the amount of material required to diffuse to equalise chemical potentials can be calculated. We start by demonstrating the approach via an example of retrograde corona development in an ultra-high-temperature granulite. Whereas the use of chemical potentials to consider the retrograde development of corona textures is well known, they are also significant in considering the prograde history. The role of chemical potentials in prograde metamorphic textural evolution is highlighted in consideration of the consumption and growth of aluminosilicates during the kyanite-to-sillimanite reaction, and the growth of garnet porphyroblasts.     

On the importance of minding one’s Ps and Ts: metamorphic processes and quantitative petrology

This Special Issue comprises a selection of the papers given at a two‐day discussion meeting held at the University of Melbourne, Australia in June 2009 to celebrate Roger Powell’s 60th birthday. At this milestone, it is fitting to review Roger’s career to date. He has published ～200 scientific papers on topics that range from low‐ to high‐grade metamorphism, from low‐ to ultrahigh‐pressure (UHP) metamorphism, and from thermodynamics to kinetics. Most of Roger’s papers are multi‐authored and address important questions in the petrogenesis of metamorphic rocks. Roger is widely known for his work with Tim Holland to develop the most complete internally consistent dataset of thermodynamic properties of end members of phases necessary to undertake calculations on the conditions of formation and modification of metamorphic rocks. Additionally, Roger and Tim have developed activity–composition models for many of these phases, building on their important methodological developments in formulating such models. Roger is also responsible for the ongoing development of thermocalc , a thermodynamic calculation software package that may be used to undertake a wide range of phase diagram calculations, including P–T projections, P–T, P–X and T–X, compatibility diagrams and μ–μ diagrams. Together, Roger and Tim have changed the way we carry out quantitative phase equilibria studies. However, Roger’s contributions to metamorphic petrology go well beyond the development of phase equilibria methods and mineral thermodynamics. He has contributed significantly to our understanding of a range of metamorphic processes, and with an extensive array of co‐authors has shown how phase equilibria can be used to understand the evolution of metamorphic rocks in general terms as well as in specific terranes. The papers in this Special Issue cover the range from the stabilization of the continents to understanding the formation of orogenic gold deposits, from the stability of sapphirine–quartz‐bearing assemblages to the crystallization of melt in migmatites, from the effects of ferric iron and sulphur on the stability of metamorphic mineral assemblages in general to the effects of ferric iron and H₂O on the stability of eclogite in particular, and to the quantification of UHP metamorphism. It is our hope that in reading these contributions, you will be stimulated to seek a better understanding of metamorphic processes and to improve our quantification of the variables in metamorphism.     

Retrograde melt–residue interaction and the formation of near‐anhydrous leucosomes in migmatites

Considering physical segregation of melt from its residue, the chemical potentials of the components (oxides) are the same in both when segregation occurs. Then, as P–T conditions change, gradients in chemical potential are established between the melt‐rich domains and residue permitting diffusional interaction to occur. In particular, on cooling, the chemical potential of H₂O becomes higher in the melt segregation than in the residue, particularly when biotite becomes stable in the residue assemblage. Diffusion of water from the melt to the residue promotes crystallization of anhydrous products from the melt and hydrous products in the residue. This diffusive process, when coupled with melt loss from the rocks subsequent to some degree of crystallization, can result in a significant degree of anhydrous leucosome being preserved in a migmatite with only minor retrogression of the residue. If H₂O can diffuse between the melt segregation and all of the residue, then no apparent selvedge between the two will be observed. Alternatively, if H₂O can diffuse between the melt segregation and only part of the residue, then a distinct selvedge may be produced. Diffusion of H₂O into the residue may be in part responsible for the commonly anhydrous nature of leucosomes, especially in granulite facies migmatites. Diffusion of other relatively mobile species such as Na₂O and K₂O has a lesser effect on overall melt crystallization but can change the proportion of quartz, plagioclase and K‐feldspar in the resultant leucosome. The diffusion of H₂O out of the melt results in the enhanced crystallization of the melt in the segregation and increases the amount of resulting anhydrous leucosome relative to the amount produced if melt crystallized in chemical isolation from the residue. For high residue:melt ratios, the proportion of resulting near‐anhydrous leucosome can approach that of the proportion of melt present at the onset of cooling with only minor loss of melt from a given segregation required. Crystallization of melt segregations via the diffusion of H₂O out of them into the host may also play a major role in driving melt‐rich segregations across key rheological transitions that would allow the expulsion of remaining melt from the system.     

Contrasting P–T–t paths for Neoproterozoic metamorphism in MacRobertson and Kemp Lands, east Antarctica

Mineral equilibria modelling and electron microprobe chemical dating of monazite in granulite facies metapelitic assemblages from the MacRobertson Land coastline, Rayner Complex, east Antarctica, are consistent with an 'anticlockwise' Neoproterozoic P–T–t path. Metamorphism occurred at c. 990–970 Ma, achieving peak conditions of 850 °C and 5.6–6.2 kbar at Cape Bruce, and 900 °C and 5.4–6.2 kbar at the Forbes Glacier ∼50 km to the east. These peak metamorphic conditions preceded the emplacement of regionally extensive syntectonic charnockite. High temperature conditions are likely to have been sustained for 80 Myr by lithospheric thinning and repeated pluton emplacement; advection was accompanied by crustal thickening to maximum pressures of 6–7 kbar, followed by near-isobaric cooling. This P–T–t path is distinct from that of rocks in adjacent Kemp Land, ∼50 km to the west, where a 'clockwise' P–T–t path from higher- P conditions at c. 940 Ma may reflect the response of a cratonic margin displaced from the main magma flux. In this scenario, crustal shortening was initially accommodated in younger, fertile crust (MacRobertson Land) involving metasediments and felsic plutons with the transfer of strain to adjacent older crust (Kemp Land) subsequent to charnockite emplacement.