The origin of bornhardts

Some bornhardts are of lithological origin, others are tectonic (horsts), but most are not susceptible of explanation in either of these terms. They are developed in granite or gneiss that apparently is mineralogically similar to that underlying the adjacent plains, and they are not obviously defined by fault dislocations. For these bornhardts two major hypotheses have been advanced. According to many workers bornhardts are the last residuals surviving after long distance scarp retreat. For others they are structural forms developed on massive compartments that stand in marked contrast with the well‐jointed rocks that have been weathered and worn down to form the plains.

Both of these hypotheses are theoretically feasible, but the field evidence (in particular the observed contrasts in fracture density between the granite of hill and plain; the evidence of subsurface initiation of both major and minor forms; the occurrence of bornhardts in narrow valleys within upland complexes and at all levels within the landscape, not just on divides; the fracture‐delineated outlines of the residuals; the association of bornhardts and multicyclic landscapes; the evidence of phased exposure; and the antiquity of the forms) are all consistent with the two‐stage concept. On the other hand, there is no evidence of long‐distance scarp retreat, and much of the field evidence is difficult to explain in such terms.     

False metamorphic events inferred from misinterpretation of microstructural evidence and P–T data

Geometrical relationships involving inclusions and partial inclusions in metamorphic microstructures can be inadequate for inferring an order of crystallization and hence a metamorphic reaction. Unique spatial and/or chemical relationships need to be defined for mineral inclusions, in the context of a reference paragenesis, commonly the matrix assemblage. Corona microstructures are reliable indicators of metamorphic reactions, but require considerable care when used to infer reactions or changes in P–T conditions, owing to kinetic problems, as well as to changes in the effective reaction volume during changes across relatively broad P–T stability fields of assemblages. Mineral equilibria models, most commonly implemented through P–T pseudosections, may allow the order in which different minerals become stable along a given P–T path to be inferred. However, the order in which two minerals become stable may be different from the order in which two grains of these minerals nucleate. Furthermore, such diagrams cannot make predictions about which minerals will form porphyroblasts and which minerals will form inclusions in porphyroblasts. An evaluation of three examples from the Australian Proterozoic shows that modelling, in combination with inclusion‐host relationships, is a powerful tool for understanding the metamorphic evolution of a rock, but involves considerable uncertainty.     

扩展式开发战略与秦巴山区的开发*

本文以秦巴山区为例,论述了这类大范围山区应以扩展式的开发战略为宜。并重点研究了秦巴山区内部发展轴线的建立及其发展壮大的基本途径。     

南华夏造山带构造演化的新证据

华南的大地构造有两种解释,一种可称为加里东造山模式,另一种可称为华南阿尔卑斯模式。前者认为华南是一个早古生代造山带,其后便成为准地台。中生代时受到活化导致盖层与基底的褶皱、断裂、岩浆作用和成矿作用。但这个模式不能解释许多地质事实。 本文继许靖华等(1987)的模式之后,给出了一些新证据,其中包括皖赣蛇绿混杂岩、蓝田、西山和乐平构造窗以及庐山飞来峰。这些证据为华南的三叠纪造山作用和构造演化模式提供了强有力的支持。     

Subaerial disconformities,microkarst and paleosols in Ordovician limestones at Bowan Park and Cliefden Caves,New South Wales,and their geoheritage significance

The Ordovician Daylesford Limestone at Bowan Park and the Fossil Hill Limestone at Cliefden Caves have diagenetic and pedogenic features of microkarst, paleosols and calcrete associated with subaerial disconformities in their stratigraphic sequences, all of which, as an ensemble, have global geoheritage significance. The original shelly limestones, lime mudstones, and coralline limestones have selectively dissolved to form vugular limestone whose cavities have filled with sparry calcite and/or crystal silt. The limestones also have been calcretised to develop massive and laminar calcrete and calcrete ooids. Below disconformity surfaces are bleached limestone, crystal-silt and spar-filled fossil moulds and enlarged moulds, micro-breccia-filled moulds and fissures filled with crystal silt, calcrete pellets and calcrete ooids. The disconformity surfaces are irregular or undulating interfaces between lithologies, fissures and fissure-fills, and calcrete. Above disconformities there are limestone lithoclasts, remanié fossils, calcreted limestone, veined limestone, calcrete ooids, laminated calcrete, lithoclast grainstone, or calcrete-ooid grainstone, and lithoclasts with fossils moulds filled with crystal silt and/or spar. The lithological, stratigraphic and possibly landscape differences, make the subaerial diagenesis/pedogenesis in the Daylesford Limestone subtly different to that of the Fossil Hill Limestone. Subaerial disconformities and associated diagenesis/pedogenesis, as recorded in these formations, are not widely reported globally nor well represented in Ordovician limestones. The microkarst features provide insights into the types of subaerial diagenesis/pedogenesis during the Ordovician and into climate, landscape setting, paleohydrology, and groundwater/rainwater alkalinity. Consequently, the story of the Ordovician microkarst, paleosols and calcrete ooids is unique and globally of geoheritage significance as examples of subaerial alteration in an ancient high-rainfall, tropical climate volcanic island environment in a tectonically active region.     

北京西山沿河城东岭台组火山岩成因及其地质意义

北京西山沿河城东岭台组火山岩由底部(第1岩性段)玄武粗安岩、下部(第2岩性段)酸性火山碎屑岩和上部(第3、4岩性段)粗面岩、流纹岩组成。根据地球化学特征,东岭台组下部第1岩性段玄武粗安岩属于碱性系列,具有Coombs成分变异趋势,上部的中、酸性岩属于高钾钙碱性;东岭台组火山岩整体具有从碱质富集的基性岩向硅饱和的中酸性岩变化的跨越式成分变异趋势。东岭台组中—基性岩是富集Ba、Sr、LREE和K的幔源原始玄武质岩浆在中等压力条件下分离结晶的产物,岩石在成岩过程中受到了下地壳物质的混染。东岭台组上部第3、4岩性段的粗面岩或英安岩依据地球化学特征分为3大类——富铝钾质粗面岩、富铝钠质粗面岩或英安岩与贫铝粗面岩或英安岩。富铝钾质粗面岩是玄武质岩浆与中地壳岩石发生熔融反应的产物;富铝钠质粗面岩或英安岩是内侵的基性岩含水熔融的产物;贫铝粗面岩或英安岩有可能由中—基性岩分异而来,成岩过程中受到围岩混染。东岭台组下部第2岩性段的酸性火山岩形成于下地壳低钾岩石在高氧逸度条件下的低程度部分熔融。东岭台组上部第3、4岩性段的酸性火山岩中的低硅端元是基性岩浆与中地壳岩石发生熔融反应的产物;高硅端元由低硅酸性火山岩分离结晶演化而来。燕山早白垩世早、中期大规模高钾钙碱性岩浆活动很可能是幔源岩浆与陆壳发生熔融反应的产物。东岭台组火山岩是早白垩世中期地壳被内侵玄武质岩浆加热,进而导致地温梯度增高的直观表征;地壳深部温度升高和部分熔融直接促使地壳发生侧向韧性流动,致使早白垩世时期的燕山地区由山地演变为高原。     

中国东部中新世风尘沉积的发现及其研究意义

黄土高原红粘土古气候与古环境研究取得巨大进展。已知最老的红粘土时代达到22 Ma，在亚洲晚新生代环境演变研究，特别是在指示亚洲内陆干旱荒漠化、高原隆升的环境效应等方面有重要意义。结合野外实地调查和室内粒度、磁化率、扫描电镜和地球化学实验分析结果，对南京附近灵岩山剖面的沉积特征与风尘沉积证据进行了探讨。在灵岩山剖面玄武岩下覆的松散沉积层中有厚度超过4 m的风尘沉积层，时代在12 Ma以前，为迄今中国东部发现的新生代最老的风尘沉积物，是中新世季风气候出现之后的产物，推测与秦安红土剖面记录的15～13 Ma快速强烈粉尘沉积阶段一致，并和南极冰盖15 Ma开始扩大造成的全球降温事件有关联。     

Palynoassemblages associated with a theropod dinosaur from the Snow Hill Island Formation (lower Maastrichtian) at the Naze,James Ross Island,Antarctica

The Cape Lamb Member of the Snow Hill Island Formation at The Naze on the northern margin of James Ross Island, east of the Antarctic Peninsula, yielded a theropod dinosaur recovered near the middle of a 90 m thick section that begins at sea level, ends below a basalt sill, and is composed of interbedded green–gray massive and laminated fine-grained sandstones and mudstones. Sixteen palynoassemblages were recovered from this section, which yielded moderately diverse assemblages with a total of 100 relatively well-preserved species. The principal terrestrial groups (32%) are represented by lycophytes (8 species), pteridophytes (15 species), gymnosperms (13 species), angiosperms (21 species) and freshwater chlorococcaleans (3 species). Marine palynomorphs (68%) belong to dinoflagellates (61 species), chlorococcaleans (6 species), and one acritarch. The vertical distribution of selected species allows the distinction of two informal assemblages, the lower Odontochitina porifera assemblage from the base to its disappearance in the lower part of the section, and the remaining section characterized by the Batiacasphaera grandis assemblage. The global stratigraphic ranges of selected palynomorphs suggest an early Maastrichtian age for this section and the entombed dinosaur that is also supported by the presence of the ammonoid Kitchinites darwinii. These assemblages share many species with latest Campanian–early Maastrichtian palynofloras from Vega and Humps Islands, New Zealand, and elsewhere in the Southern Ocean, establishing a good correlation among them. The dominance or frequent presence of dinoflagellates throughout the section supports the general interpretation of a shelf marine depocenter. The consistent presence of terrestrial palynomorphs suggests contributions from littoral/inland environments.     

Imposition of a Proterozoic salient on a Palaeozoic orogen at the eastern margin of Gondwana

Regional analysis of the distribution of metamorphic fabrics and shear zones in the Palaeo-Mesoproterozoic Curnamona Province has enabled the deconstruction of a Cambrian fold arc that defines part of the eastern margin of Gondwana. We suggest a tectonic model whereby the arc formed at ca. 500 Ma, during accretion of Phanerozoic terranes to the eastern margin of Precambrian Australia. The regional fold arc is interpreted to have formed along an irregular plate margin comprising a SE-convex rigid promontory of Precambrian basement during initial accretion of the Phanerozoic terranes during the Cambrian. An early phase of dextral oblique-slip along shear zones in the south and west of the province indicates an initial WNW transport direction. As the arc was folded, a slightly later phase of sinistral oblique-slip shearing was initiated along shear zones in the east, in response to deflection of the arc around the rigid promontory.     

Timing of mineralization and source of fluids in a slate-belt auriferous vein system, Hill End goldfield, NSW, Australia: Evidence from Ar/Ar dating and O- and H-isotopes

⁴⁰Ar/³⁹Ar dating of metamorphic biotite and alteration muscovite from the auriferous veins and host rocks at the Hill End goldfield, N.S.W., Australia, has distinguished four major geological events, including the timing of gold mineralization. The earliest hydrothermal event occurred during the Middle Devonian Tabberabberan Orogeny (370–380 Ma) and resulted in the formation of quartz veins barren of Au. A second and major episode of vein emplacement occurred in the Early Carboniferous during the principal phase of metamorphism and deformation at 359–363 Ma. This was followed by Au accumulation in two stages: (1) after the major phases of quartz deposition, and (2) during and after the development of conspicuous internal vein laminations (˜ 357 Ma and ˜ 343 Ma, respectively). Two sources of fluid are proposed for vein and ore formation. The first is a local metamorphic fluid characterized by δ¹⁸O_H2O values of 8.9 to 12.5 per mil and δD_H2O values of −87 to −90 per mil. The second is a mixed ore fluid with δ¹⁸O and δD values in the range of δ¹⁸O_H2O 8.4 to 11 per mil and δD_H2O of −49 to −36 per mil. Progressive entry of this second fluid, sourced from trough-fill or deeper crustal rocks, is linked closely to cycles of gold precipitation at Hill End.