Coupled garnet Lu–Hf and monazite U–Pb geochronology constrain early convergent margin dynamics in the Ross orogen,Antarctica          下载免费PDF全文

G. Hagen‐Peter  J. M. Cottle  M. Smit  A. F. Cooper 《Journal of Metamorphic Geology》2016,34(4):293-319

The Ross orogen of Antarctica is an extensive (>3000 km‐long) belt of deformed and metamorphosed sedimentary rocks and granitoid batholiths, which formed during convergence and subduction of palaeo‐Pacific lithosphere beneath East Gondwana in the Neoproterozoic–early Palaeozoic. Despite its prominent role in Gondwanan convergent tectonics, and a well‐established magmatic record, relatively little is known about the metamorphic rocks in the Ross orogen. A combination of garnet Lu–Hf and monazite U–Pb (measured by laser‐ablation split‐stream ICP‐MS) geochronology reveals a protracted metamorphic history of metapelites and garnet amphibolites from a major segment of the orogen. Additionally, direct dating of a common rock‐forming mineral (garnet) and accessory mineral (monazite) allows us to test assumptions that are commonly used when linking accessory mineral geochronology to rock‐forming mineral reactions. Petrography, mineral zoning, thermobarometry and pseudosection modelling reveal a Barrovian‐style prograde path, reaching temperatures of ~610–680 °C. Despite near‐complete diffusional resetting of garnet major element zoning, the garnet retains strong rare earth element zoning and preserves Lu–Hf dates that range from c. 616–572 Ma. Conversely, monazite in the rocks was extensively recrystallized, with concordant dates that span from c. 610–500 Ma, and retain only vestigial cores. Monazite cores yield dates that overlap with the garnet Lu–Hf dates and typically have low‐Y and heavy rare earth element (HREE) concentrations, corroborating interpretations of low‐Y and low‐HREE monazite domains as records of synchronous garnet growth. However, ratios of REE concentrations in garnet and monazite do not consistently match previously reported partition coefficients for the REE between these two minerals. High‐Y monazite inclusions within pristine, crack‐free garnet yield U–Pb dates significantly younger than the Lu–Hf dates for the same samples, indicating recrystallization of monazite within garnet. The recrystallization of high‐Y and high‐HREE monazite domains over >50 Ma likely records either punctuated thermal pulses or prolonged residence at relatively high temperatures (up to ~610–680 °C) driving monazite recrystallization. One c. 616 Ma garnet Lu–Hf date and several c. 610–600 Ma monazite U–Pb dates are tentatively interpreted as records of the onset of tectonism metamorphism in the Ross orogeny, with a more robust constraint from the other Lu–Hf dates (c. 588–572 Ma) and numerous c. 590–570 Ma monazite U–Pb dates. The data are consistent with a tectonic model that involves shortening and thickening prior to widespread magmatism in the vicinity of the study area. The early tectonic history of the Ross orogen, recorded in metamorphic rocks, was broadly synchronous with Gondwana‐wide collisional Pan‐African orogenies.  相似文献   

用活断层资料分析中国大陆及其邻区晚更新世以来的构造应变特征   总被引：2，自引：0，他引：2  

荆燕  任金卫  王辉 《地震研究》2005,28(4):353-358

根据活断层资料,利用贝塞尔双三次样条模型对中国大陆及其邻区晚更新世以来的平均水平应变率场进行了计算。结果显示,中国大陆的构造膨胀率、构造剪切应变率、构造矩率和旋转角速度的分布都与地震的分布相似,呈条带状,并与活动地块的边界和主要断裂位置一致。另外,中国大陆的地壳变形具有西强东弱的整体特征。  相似文献   

Evolution of the Lithospheric Mantle beneath the Marsabit Volcanic Field (Northern Kenya): Constraints from Textural, P-T and Geochemical Studies on Xenoliths   总被引：1，自引：0，他引：1  

KAESER  BENJAMIN; KALT  ANGELIKA; PETTKE  THOMAS 《Journal of Petrology》2006,47(11):2149-2184

Xenoliths hosted by Quaternary basanites and alkali basaltsfrom Marsabit (northern Kenya) represent fragments of Proterozoiclithospheric mantle thinned and chemically modified during riftingin the Mesozoic (Anza Graben) and in the Tertiary–Quaternary(Kenya rift). Four types of peridotite xenoliths were investigatedto constrain the thermal and chemical evolution of the lithosphericmantle. Group I, III and IV peridotites provide evidence ofa cold, highly deformed and heterogeneous upper mantle. Textures,thermobarometry and trace element characteristics of mineralsindicate that low temperatures in the spinel stability field(750–800°C at <1·5 GPa) were attained bydecompression and cooling from initially high pressures andtemperatures in the garnet stability field (970–1080°Cat 2·3–2·9 GPa). Cooling, decompressionand penetrative deformation are consistent with lithosphericthinning, probably related to the development of the Mesozoicto Paleogene Anza Graben. Re-equilibrated and recrystallizedperidotite xenoliths (Group II) record heating (from 800°Cto 1100°C). Mineral trace element signatures indicate enrichmentby mafic silicate melts, parental to the Quaternary host basanitesand alkali basalts. Relationships between mineral textures,P–T conditions of equilibration, and geochemistry canbe explained by metasomatism and heating of the lithosphererelated to the formation of the Kenya rift, above a zone ofhot upwelling mantle. KEY WORDS: East African Rift System; Anza Graben; in situ LA-ICPMS; peridotite xenoliths; thermobarometry  相似文献   

Pasture, profit, and power: An environmental history of cattle ranching in Colombia, 1850-1950     

Shawn Van Ausdal 《Geoforum》2009,40(5):707-719

This article examines the expansion of cattle ranching from 1850 to 1950 into lowland forests of Colombia. Although most attention to ranching as a source of deforestation in Latin America has focused on the second half of the 20th century, in Colombia it has a much longer history. The article also examines the role of introduced African grasses in the process of pasture development. While it discusses their ability to suppress secondary-forest regeneration, it underscores their contribution to productivity gains to help explain their wide diffusion. Lastly, the article suggests that paying greater attention to the costs and labor of pasture formation can serve as a springboard to reexamine a number of common stereotypes about ranching: that the logic of livestock was not about producing beef; that cattle were primarily a means to control territory; and that ranching ‘profits’ stemmed from extra-economic sources. Although there is some truth to these explanations, the dominance of cattle throughout the countryside cannot be explained without taking the economic and productive logic of ranching into account. This, in turn, should also help push us to better understand the nature of landed power and the dynamics of agrarian change in Colombia.  相似文献   

Tectonic setting of the Balaram-Kui-Surpagla-Kengora granulites of the South Delhi Terrane of the Aravalli Mobile Belt,NW India and its implication on correlation with the East African Orogen in the Gondwana assembly     

Yengkhom Kesorjit Singh  Bert De Waele  Subrata Karmakar  Sunayana Sarkar  Tapas Kumar Biswal 《Precambrian Research》2010

Granulites are developed in various tectonic settings and during different geological periods, and have been used for continental correlation within supercontinent models. In this context the Balaram-Kui-Surpagla-Kengora granulites of the South Delhi Terrane of the Aravalli Mobile Belt of northwestern India are significant. The granulites occur as shear zone bounded lensoidal bodies within low-grade rocks of the South Delhi Terrane and comprise pelitic and calcareous granulites, a gabbro-norite-basic granulite suite and multiple phases of granites of the Ambaji suite. The granulites have undergone three major phases of folding and shearing. The F₁ and F₂ folds are coaxial along NE-SW axis, and F₃ folds are developed across the former along NW-SE axis. Thus, various types of interference patterns are produced. The granulite facies metamorphism is marked by a spinel–cordierite–garnet–sillimanite–quartz assemblage with melt phase and is synkinematic to the F₁ phase of folding. The peak thermobarometric condition is set at ≥850 °C and 5.5–6.8 kb. The granulites have been exhumed through thrusting along multiple ductile shear zones during syn- to post-F₂ folding. Late-stage shearing has produced cataclasites and pseudotachylites. Sensitive High Resolution Ion MicroProbe (SHRIMP) U–Pb dating of zircon from pelitic granulites and synkinematically emplaced granites indicate that: (1) the sedimentary succession of the South Delhi Terrane was deposited between 1240 and 860 Ma with detritus derived from magmatic sources with ages between 1620 and 1240 Ma; (2) folding and granulite metamorphism have taken place between ca. 860 and 800 Ma, and exhumation at around ca. 800–760 Ma; and (3) the last phase of granitic activity occurred at ca. 759 Ma. This shows, for the first time, that the granulites of the South Delhi Terrane are much younger than those of the Sandmata Granulite Complex of the northern part of the Aravalli Mobile Belt, the Saussar granulites of the Central India Mobile Belt and the Eastern Ghats Mobile Belt. Instead, they show similarities to the Neoproterozoic granulites of the Circum Indian Orogens that include the East African Orogen (East Africa and Madagascar), the Southern Granulite Terrane of India and much of Sri Lanka. We suggest that the South Delhi Basin probably marks a trace of the proto-Mozambique Ocean in NW India within Gondwana, that closed when the Marwar Craton, arc fragments (Bemarivo Belt in Madagascar and the Seychelles) and components of the Arabian-Nubian Shield collided with the Aravalli-Bundelkhand Protocontinent at ca. 850–750 Ma.  相似文献   

Modern and early Holocene mollusc fauna of the Ounianga lakes (northern Chad): implications for the palaeohydrology of the central Sahara     

Bert Van Bocxlaer  Dirk Verschuren  Georg Schettler  Stefan Kröpelin 《第四纪科学杂志》2011,26(4):433-447

The fresh and saline lakes of Ounianga Kebir and Serir in northeastern Chad are among the very few permanent aquatic ecosystems currently existing in the hyper‐arid core of the Sahara desert. The confirmed modern fauna of aquatic molluscs at Ounianga comprises three widespread species (Melanoides tuberculata, Biomphalaria pfeifferi and Lymnaea natalensis), of which only the first appears to maintain a thriving population. We recovered seven more species of gastropods, among which one is new to science (Gabbiella ouniangaensis), and one bivalve species from early Holocene outcrops of diatomaceous limestone at Ounianga Serir. All species except one are known to be commonly spread between lake and river systems by birds, and in its entirety this species‐rich fossil mollusc fauna of Ounianga Serir resembles that of other Saharan lake sites known to have been hydrologically isolated during the early Holocene. The impoverished modern mollusc fauna, which lacks even the widespread and opportunistic species Bulinus truncatus, may have become established relatively recently through new colonisation events, following eradication of the early Holocene mollusc fauna during a later episode of high salinity or desiccation. Copyright © 2011 John Wiley & Sons, Ltd.  相似文献   

Soil phytoliths from miombo woodlands in Mozambique     

Julio Mercader  Tim Bennett  Steven Simpson 《Quaternary Research》2011,75(1):138-150

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中非石油合作的战略意义与重点区域选择     

周晓波  吴家新  赵媛 《世界地理研究》2011,20(1):63-69,43

非洲是我国重要的战略合作伙伴,石油资源丰富,是世界八大产油区之一。近十年来非洲石油的产量及出口量迅速增长,在国际能源格局中地位越来越重要。随着我国经济社会的快速发展,我国石油进口规模不断扩大,对外依赖程度日益加深,石油安全问题也日益突出,使得中非石油合作在我国能源战略中的地位不断上升。通过对非洲主要石油生产大国资源与生产情况的比较,提出中非石油合作的重点区域是陆上的北非和海上的西非几内亚湾,并就进一步加强中非石油合作提出了对策建议。  相似文献   

非洲旅游产品体系规划研究     

骆高远 《世界地理研究》2011,(3):129-134

旅游产品是指旅游经营者为满足旅游者在旅游活动中的各种需求而向旅游市场提供的各种物质产品、精神产品和旅游服务的组合。旅游产品体系则是指旅游产品各种表现形式的集合,其中包括单项旅游产品,旅游线路产品和区域旅游产品。这些产品表现形式相互关联,是旅游资源开发、旅游业经营和管理的重点,是呈现给旅游者的核心价值所在。根据非洲的实际情况,针对国际旅游市场的需求特征,将非洲的旅游产品分为狂野非洲、风情非洲与和谐非洲三个层次,以明确的主题来组合产品、塑造形象、打造品牌,以嬴得客源市场,提升非洲整体的旅游竞争力。  相似文献   

居民情感视角下的城市移民品牌化路径研究     

罗志伟  林银斌  王敏 《世界地理研究》2022,31(6):1215-1226

随着情感成为城市品牌的重要资产,非营销人员介入的意义创造过程日益受到重视,从居民的角度解读情感的空间性为城市品牌研究提供了一个新的视角。运用案例研究和半结构式访谈的方法,以广州非裔聚居现象及其意义生产为例,探讨居民情感的发生如何受到媒体和生活经历的影响,审视居民情感对城市移民品牌塑造的作用,初步剖析居民情感驱动的城市移民品牌化的运作机制。研究表明,尽管地方政府并没有利用非裔聚居现象及其文化景观构建品牌叙事,但居民基于人地情感关系,感知、想象和认同地方的意义,进而无形地塑造着广州开放、包容的城市品牌。然而,居民情感也并非连续、统一的整体,城市内部的有机发展使居民与地方产生差异化的情感互动。在媒体表征与个体生活经历的影响下,居民对非裔群体的积极和消极情感分别对城市移民品牌化起到建设性和破坏性的作用。研究呼吁关注居民自下而上的情感表达对城市品牌构建的意义,以期为城市形象管理提供现实参考。  相似文献