Quantitative elemental mapping of granulite-facies monazite: Textural insights and implications for petrochronology     

Owen M. Weller  Simon Jackson  William G. R. Miller  Marc R. St-Onge  Nicole Rayner 《Journal of Metamorphic Geology》2020,38(8):853-880

Texturally complex monazite grains contained in two granulite-facies pelitic migmatites from southern Baffin Island, Arctic Canada, were mapped by laser ablation-inductively coupled plasma-mass spectrometry (using spot sizes ≤5 µm) to quantitatively determine the spatial variation in trace element chemistry (with up to 1,883 analyses per grain). The maps highlight the chemical complexity of monazite grains that have experienced multiple episodes of growth, resorption and chemical modification by dissolution–precipitation during high-grade metamorphism. Following detailed chemical characterization of monazite compositional zones, a related U–Pb data set is re-interpreted, allowing petrologically significant ages to be extracted from a continuum of concordant data. Synthesis of these data with pseudosection modelling of prograde and peak conditions allows for the temporal evolution of monazite trace element chemistry to be placed in the context of the evolving P–T conditions and major phase assemblage. This approach enables a critical evaluation of three commonly used petrochronological indicators: linking Y to garnet abundance, the Eu anomaly to feldspar content and Th/U to anatectic processes. Europium anomalies and Th/U behave in a relatively systematic fashion, suggesting that they are reliable petrochronological witnesses. However, Y systematics are variable, both within domains interpreted to have grown in a single event, between grains interpreted to be part of the same age population, and between samples that experienced similar metamorphic conditions and mineral assemblages. These observations caution against generalized petrological interpretations on the basis of Y content, as it suggests Y concentrations in monazite are controlled by domainal equilibria. The results reveal a c. 45 Myr interval between prograde metamorphism and retrograde melt crystallization in the study area, emphasizing the long-lived nature of heat flow in high-grade metamorphic terranes. Such long timescales of metamorphism would be assisted by the growth, retention and dominance of high-Th suprasolidus monazite, as observed in this study, contributing to the radiogenic heating budget of mid- to lower-crustal environments. Careful characterization of monazite grains suggests that continuum-style U–Pb data sets can be decoded to provide insights into the duration of metamorphic processes.  相似文献   

蚀变岩帽的特征、成因以及在华南的分布探讨     

陈静  周涛发  张乐骏  孙艺  WHITE Noel C  李旋旋 《岩石学报》2020,36(11):3380-3396

Sillitoe（1995）蚀变岩帽（Lithocap）的定义为大范围富黄铁矿的硅化、高级泥化和泥化蚀变，在地质环境上位于古地表和浅成中-酸性岩浆侵入体之间。蚀变岩帽往往显示为突出的正地形，有助于寻找隐伏的斑岩矿化体。但蚀变岩帽在地表的范围往往多达几十个平方千米，又常常掩盖下覆斑岩矿床的蚀变矿化特征及其地球化学印记，因此大型的蚀变岩帽又给勘探工作带来一定的挑战。蚀变岩帽相关矿床的勘探需以地质填图为基础，结合近红外光谱分析（SWIR）进行蚀变填图，以及全岩地球化学以及矿物地球化学表现的元素或元素组合异常，来帮助定位热源或深部斑岩体。遥感和地球物理中的激电响应，也可以辅助定位岩体。华南地区的蚀变岩帽主要分布于长江中下游成矿带和东南沿海火山岩带。前人对安徽庐枞盆地中的矾山蚀变岩帽进行了系统研究，确定了矾山蚀变岩帽形成于白垩纪，与围岩砖桥组火山岩年龄一致。同位素和流体包裹体工作证明了形成矾山蚀变岩帽的流体主要为深部岩浆热液中的酸性气体与浅部大气降水的混合，在浅部高渗透率的火山岩及其岩性界面反应，广泛发育了一套硅化和高级泥化蚀变，指示与矾山相关可能存在斑岩和高硫型浅成低温热液铜金矿床。福建紫金山地区有中国最大的高硫型浅成低温热液矿床，主要赋存于紫金山蚀变岩帽中。紫金山蚀变岩帽的地质特征和蚀变分带已经研究的较为详细，但目前深部的侵入体还没有发现。浙江的蚀变岩帽是中国非金属矿产的重要来源，包括明矾石矿、地开石矿和红柱石矿等，这些蚀变岩帽与金属矿化的关系尚未有相关研究。根据目前的资料总结，有较多的蚀变岩帽分布在中国华南，这些蚀变岩帽特征典型，但目前的研究程度尚浅。现有的研究结果表明，华南的蚀变岩帽的成矿潜力巨大，可能存在一条巨型的斑岩-浅成低温矿床成矿带，具有广阔的找矿勘查前景，建议加强蚀变岩帽及相关矿床的找矿与研究工作。  相似文献   

Calcareous algae and cyanobacteria     

Michael C. Leaf  Jessica S. A. Gay  Matthew J. Newbould  Owen R. Hewitt  Steven L. Rogers 《Geology Today》2020,36(2):75-80

Algae is an informal term used to describe a broad group of simple organisms from the plant kingdom. The organisms included within this grouping are aquatic photosynthetic biota with an extensive range of life habits and forms. These organisms range from micron-sized unicellular forms to giant seaweeds and kelps, which can grow to several metres long. Both benthic and planktonic modes of life are known and display a wide variety of life cycles.  相似文献   

大数据驱动下的数字古地理重建：现状与展望     

张蕾  钟瀚霆  陈安清  赵应权  黄可可  李凤杰  黄虎  刘宇  曹海洋  祝圣贤  穆财能  侯明才  JAMES G. Ogg 《高校地质学报》2020,26(1):73

古地理学是一门强数据依赖性学科,古地理重建作为古地理学的核心任务之一,着眼于研究地质历史时期地球表面的地理、生物、气候面貌及其演化规律。随着大数据时代的来临,海量古地理数据的不断积累和计算机技术的高速发展使得标准化、智能化的数字古地理重建成为可能。文章通过介绍国内外与古地理相关的代表数据库及团队,总结其优缺点,提出大数据驱动下的数字古地理重建核心思路:(1)建立标准化的古地理学知识体系;(2)建立开放互动、动态更新的古地理数据库,并利用机器阅读技术等拓展数据来源;(3)建立标准化的古地理学数据质量控制体系;(4)利用机器学习技术建立各类型古地理重建模型,深度挖掘数据;(5)以可实时更新的智能数字地图集或多维动画形式输出成果。  相似文献   

Climatic signals in lacustrine deposits of the Upper Yacoraite Formation,Western Argentina: Evidence from clay minerals,analcime, dolomite and fibrous calcite     

João Paulo B. Gomes  Rodrigo B. Bunevich  Sandra N. Tonietto  Daisy B. Alves  Julice F. Santos  Fiona F. Whitaker 《Sedimentology》2020,67(5):2282-2309

Authigenic clays are an important control on reservoir quality in lacustrine carbonates but remain challenging to predict. Lacustrine depositional systems respond to climatic variations in rainfall, surface runoff and groundwater input, and evaporation, and result in rapid and frequent changes in lake volume; this is expressed through changing water depth and shoreline position. In the upper portion of the Early Palaeocene Yacoraite Formation of the Salta Basin in Argentina, extensive lacustrine deposits were deposited during the sag phase of rifting. Prior high-resolution stratigraphic studies have suggested that climatic factors control microbial carbonate sequences within a ‘balanced fill’ lake, with variation in the lake level having a major influence on facies association changes. This study characterizes the evolution of facies and mineralogy within the Yacoraite Formation, focusing on the distribution of clay minerals, making a link between the high, medium and low-frequency sequence stratigraphic cycles. The low-frequency transgressive hemicycle of the upper portion of the Yacoraite Formation is comprised of abundant siliciclastic facies, suggesting a wetter period. Microbialites occurring in this interval are coarse-grained and agglutinated. Detrital clay minerals such as illite and chlorite and associated siliciclastic sediments were input to the lake during high-frequency transgressive periods. During high-frequency regressive hemicycles, sedimentation was dominated by carbonate facies with Ca-rich dolomite and the authigenic clays are comprised of chlorite/smectite mixed-layers. By contrast, the low frequency regressive hemicycle records fine-grained agglutinated microbialite with horizons of fibrous calcite, more stoichiometric dolomite, barite and authigenic magnesian smectite. This indicates elevated ion concentrations in the lake under intense evaporation during an arid period. Understanding the conditions that are favourable for formation and preservation of authigenic clays within the lacustrine environment can improve understanding of reservoir quality in comparable economically important deposits.  相似文献   

Morphometry and Morphostructural Studies of the Parts of Gola River and Kalsa River Basins,Chanphi-Okhalkanda Region,Kumaun Lesser Himalaya,India     

Lal. Kumar  Joshi  G.  Agarwal  K. K. 《Geotectonics》2020,54(3):410-427

Geotectonics - Study of morphometric parameters such as stream length (Lu) and stream length ratios (Rl), bifurcation ratio (Rb), drainage density (D), form factor (Rf), circularity ratio (Rc),...  相似文献   

An extreme wave event in eastern Yucatán,Mexico: Evidence of a palaeotsunami event during the Mayan times     

Javier Lario  Chris Spencer  Teresa Bardají  Ángel Marchante  Victor H. Garduño-monroy  Jorge Macias  Sergio Ortega 《Sedimentology》2020,67(3):1481-1504

The Yucatán Peninsula, Mexico, has typically been considered a tectonically stable region with little significant seismic activity. The region though, is one that is regularly affected by hurricanes. A detailed survey of ca 100 km of the eastern Yucatán and Cozumel coast identified the presence of ridges containing individual boulders measuring >1 m in length. The boulder ridges reach 5 m in height and their origin is associated with extreme wave event activity. Previously modelled tsunami waves from known seismically active zones in the region (Muertos Thrust Belt and South Caribbean Deformed Belt) are not of sufficient scale in the area of the Yucatán Peninsula to have produced the boulder ridges recorded in this study. The occurrence of hurricanes in this region is more common, but two of the most destructive (Hurricane Gilbert 1988 and Hurricane Wilma 2005) produced coastal waves too small to have created the ridges recorded here. In this paper, a new tsunami model with a source area located on the Motagua/Swan Island Fault System has been generated that indicates a tsunami event may have caused the extreme wave events that resulted in the deposition of the boulder ridges.  相似文献   

Digital mapping of coastal boulders – high-resolution data acquisition to infer past and recent transport dynamics     

Fabian Boesl  Max Engel  Rodrigo C. Eco  JaN B. Galang  Lia A. Gonzalo  Francesca Llanes  Eva Quix  Helmut Brückner 《Sedimentology》2020,67(3):1393-1410

Coastal boulder fields provide clues to long-term frequency-magnitude patterns of coastal flooding events and have the potential to play an important role in coastal hazard assessment. Mapping boulders in the field is time and labour-intensive, and work on intertidal reef platforms, as in the present study, is physically challenging. By addressing coastal scientists who are not specialists in remote sensing, this contribution reports on the possibilities and limitations of digital applications in boulder mapping in Eastern Samar, Philippines, where recent supertyphoons Haiyan and Hagupit induced high waves, coastal flooding and boulder transport. It is demonstrated how satellite imagery of sub-metre resolution (from Pléiades and WorldView-3 imagery) enables efficient analysis of transport vectors and distances of larger boulders, reflecting variation in latitudes of both typhoon tracks and approaching angles of typhoon-generated waves. During the investigated events, boulders with a-axes of up to 8 m were clearly identified to have been shifted for up to 32 m, mostly along the seaward margin of the boulder field. It is, however, hard to keep track of smaller boulders, and the length of a-axes and b-axes including their orientation is often impossible to map with sufficient accuracy. Orthophotographs and digital surface models created through the application of an unmanned aerial vehicle and the ‘Structure from Motion’ technique provide ultra-high-resolution data, and have the potential to not only improve the results of satellite image analysis, but also those from field mapping and may significantly reduce overall time in the field. Orthophotographs permit unequivocal mapping of a-axes and b-axes including their orientation, while precise values for c-axes can be derived from the respective digital surface models. Volume of boulders is best inferred from boulder-specific Structure from Motion-based three-dimensional models. Battery power, flight speed and altitude determine the limits of the area covered, while patches shielded by the boulders are difficult to resolve. For some tasks, field mapping remains mandatory and cannot be replaced by currently available remote sensing tools: for example, sampling for rock type, density and age dating, recording of lithological separation of boulders from the underlying geological unit and of geomorphic features on a millimetre to decimetre-scale, or documentation of fine-grained sediment transport in between the boulders in supratidal settings. In terms of future events, the digital products presented here will provide a valuable reference to track boulder transport on a centimetre to decimetre-scale and to better understand the hydrodynamics of extreme-wave events on a fringing reef coastline.  相似文献   

Sediment transfer in an extremely low-gradient,low-relief and highly buffered system: Darwin Harbour catchment,northern Australia     

M. Nawaz  R. Wasson 《Australian Journal of Earth Sciences》2020,67(3):351-359

Abstract

Sediment yields from and sediment transfer within catchments of very low relief and gradient, which make up about 50% of Earth’s surface, are poorly documented and their internal sediment dynamics are poorly known. Sediment sources, their proportionate contributions to valley floors and sediment yield, and storage are estimated using fallout radionuclides ²¹⁰Pb_(ex) and ¹³⁷Cs in the catchments that drain into Darwin Harbour, northern Australia, an example of this understudied catchment type that appears to be globally at the extreme end of this category of catchments. Unchannelled grassy valley floors (dambos, or seasonal wetlands) trap ～90% of the sediment delivered from hillslopes by sheet and rill erosion. Further down valley, small channels transport ～10% of the sediment that escapes from the dambos, and the remaining sediment comes from erosion of the channels. In this case, the fractional sediment storage is very high as a result of the existence of dambos, a landform that depends for its existence on low gradients.  相似文献   

Zircon U–Pb geochronology,whole-rock geochemical,and Sr–Nd–Pb isotopic constraints on the timing and origin of Permian and Triassic mafic dykes from eastern North China Craton     

Liu  Shen  Feng  Caixia  Fan  Yan  Chen  Xiaoqing  Yang  Yuhong  Zhao  Huibo  Coulson  Ian M. 《中国地球化学学报》2020,39(6):862-886

Acta Geochimica - This work reports an important episode of extensional, mafic magmatism that impacted the North China Craton (NCC) during the Permo-Triassic and influenced the evolution of this...  相似文献