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1.
The purpose of this paper is to examine the transfer of the green economy from a global discursive level to institutionalization at the national level in Tanzania. While there is a growing amount of research discussing technological aspects of the green economy, less attention has been paid to policy implications and governance aspects, especially in developing countries. There is an increasing emphasis on technological and market-based solutions to environmental challenges globally and in the developed part of the world. However, in developing countries, ‘green growth’ often implies transformed control over natural resources – under schemes that are often driven from abroad. Over the last five to ten years, investments aimed at increasing productivity in the rural agricultural sector in developing countries have become a focus area of the green economy, but various concepts of green have become confused. Such (mis-) interpretation of the green economy has consequences for implementation and outcomes of various ‘green’ projects. Drawing on governmentality as well as the concept of institutional bricolage, I examine how the green economy discourse and policy at the global level have been re-shaped and re-interpreted to fit the existing agri-business initiative of the Southern Agricultural Growth Corridor of Tanzania (SAGCOT), which has been championed as a model for green economy implementation in Africa. I discuss how the green discourse has been ‘grabbed’ as an opportunity to ‘greenwash’ SAGCOT in its establishment and institutionalization.  相似文献   

2.
Information on the ocean/atmosphere state over the period spanning the Last Glacial Maximum – from the Late Pleistocene to the Holocene – provides crucial constraints on the relationship between orbital forcing and global climate change. The Pacific Ocean is particularly important in this respect because of its dominant role in exporting heat and moisture from the tropics to higher latitudes. Through targeting groundwaters in the Mojave Desert, California, we show that noble gas derived temperatures in California averaged 4.2 ± 1.1 °C cooler in the Late Pleistocene (from ～43 to ～12 ka) compared to the Holocene (from ～10 to ～5 ka). Furthermore, the older groundwaters contain higher concentrations of excess air (entrained air bubbles) and have elevated oxygen-18/oxygen-16 ratios (δ18O) – indicators of vigorous aquifer recharge, and greater rainfall amounts and/or more intense precipitation events, respectively. Together, these paleoclimate indicators reveal that cooler and wetter conditions prevailed in the Mojave Desert from ～43 to ～12 ka. We suggest that during the Late Pleistocene, the Pacific ocean/atmosphere state was similar to present-day El Nino-like patterns, and was characterized by prolonged periods of weak trade winds, weak upwelling along the eastern Pacific margin, and increased precipitation in the southwestern U.S.  相似文献   

3.
Most sulfide-rich magmatic Ni-Cu-(PGE) deposits form in dynamic magmatic systems by partial melting S-bearing wall rocks with variable degrees of assimilation of miscible silicate and volatile components, and generation of barren to weakly-mineralized immiscible Fe sulfide xenomelts into which Ni-Cu-Co-PGE partition from the magma. Some exceptionally-thick magmatic Cr deposits may form by partial melting oxide-bearing wall rocks with variable degrees of assimilation of the miscible silicate and volatile components, and generation of barren Fe ± Ti oxide xenocrysts into which Cr-Mg-V ± Ti partition from the magma. The products of these processes are variably preserved as skarns, residues, xenoliths, xenocrysts, xenomelts, and xenovolatiles, which play important to critical roles in ore genesis, transport, localization, and/or modification. Incorporation of barren xenoliths/autoliths may induce small amounts of sulfide/chromite to segregate, but incorporation of sulfide xenomelts or oxide xenocrysts with dynamic upgrading of metal tenors (PGE > Cu > Ni > Co and Cr > V > Ti, respectively) is required to make significant ore deposits. Silicate xenomelts are only rarely preserved, but will be variably depleted in chalcophile and ferrous metals. Less dense felsic xenoliths may aid upward sulfide transport by increasing the effective viscosity and decreasing the bulk density of the magma. Denser mafic or metamorphosed xenoliths may also increase the effective viscosity of the magma, but may aid downward sulfide transport by increasing the bulk density of the magma. Sulfide wets olivine, so olivine xenocrysts may act as filter beds to collect advected finely dispersed sulfide droplets, but other silicates and xenoliths may not be wetted by sulfides. Xenovolatiles may retard settling of – or in some cases float – dense sulfide droplets. Reactions of sulfide melts with felsic country rocks may generate Fe-rich skarns that may allow sulfide melts to fractionate to more extreme Cu-Ni-rich compositions. Xenoliths, xenocrysts, xenomelts, and xenovolatiles are more likely to be preserved in cooler basaltic magmas than in hotter komatiitic magmas, and are more likely to be preserved in less dynamic (less turbulent) systems/domain/phases than in more dynamic (more turbulent) systems/domains/phases. Massive to semi-massive Ni-Cu-PGE and Cr mineralization and xenoliths are often localized within footwall embayments, dilations/jogs in dikes, throats of magma conduits, and the horizontal segments of dike-chonolith and dike-sill complexes, which represent fluid dynamic traps for both ascending and descending sulfides/oxides. If skarns, residues, xenoliths, xenocrysts, xenomelts, and/or xenovolatiles are present, they provide important constraints on ore genesis and they are valuable exploration indicators, but they must be included in elemental and isotopic mass balance calculations.  相似文献   

4.
Cell phones present new forms of sociality and new possibilities of encounter for young people across the globe. Nowhere is this more evident than in sub-Saharan Africa where the scale of usage, even among the very poor, is remarkable. In this paper we reflect on the inter-generational encounters which are embedded in young people’s cell phone interactions, and consider the wider societal implications, not least the potential for associated shifts in the generational balance of power. An intriguing feature of this changing generational nexus is that while many young people’s phone-based interactions, from their mid-teens onwards, are shifting away from the older generation towards friendship networks in their own age cohort, at the same time they are repositioning themselves – or becoming repositioned – as family information hubs, as a consequence of their phone expertise. The paper draws on mixed-methods research with young people aged c. 9–25 years and in-depth interviews with older age-groups in 24 sites (ranging from high density poor urban to remote rural) across Ghana, Malawi and South Africa.  相似文献   

5.
《Comptes Rendus Geoscience》2019,351(2-3):260-268
Nano-polycrystalline diamond cubes with an edge length of 6 mm have been used as anvils for Kawai-type multianvil apparatus. The maximum pressure of ∼88 GPa was confirmed based on in situ X-ray diffraction measurements using nano-polycrystalline diamond anvils with a truncation of 1.0 mm, which is more than 50% higher than the pressure (∼56 GPa) achieved at the same press load using sintered-diamond anvils. The X-ray transparency of the nano-polycrystalline diamond anvil was found to be about 10–100 times as high as that of sintered-diamond anvils for a wide range of photon energies (∼30–130 keV), leading to high-quality X-ray imaging and diffraction profiles of the sample under pressure, even through the anvils. It is expected that further optimization of the pressure medium and gasket would lead to pressures far higher than 100 GPa without sacrificing the advantages of the multianvil apparatus over diamond anvil cells.  相似文献   

6.
The giant Jianchaling gold deposit is located in the Shaanxi Province, China. The mineralization is hosted by WNW-trending faults in the Mianxian-Lueyang-Yangpingguan (MLY) area. The mineralization can be divided into three stages based on mineralogical assemblages and crosscutting relationships of mineralized quartz veins. These stages, from early to late, are characterized by the mineral assemblage of: (1) quartz – coarse-grained pyrite – pyrrhotite – pentlandite – dolomite; (2) quartz – pyrite – gold – sphalerite – galena – carbonate – arsenopyrite – fuchsite; and (3) dolomite – calcite – quartz – fine-grained pyrite – realgar – orpiment.Three types of fluid inclusions have been recognized in this study based on petrographic and microthermometric measurements, including pure CO2 and/or CH4 (PC-type), NaCl-H2O (W-type), and NaCl-CO2-H2O (C-type) fluid inclusions. These fluid inclusion types are present in quartz from the Stage 1 and 2 assemblages, whereas the Stage 3 quartz only contains W-type fluid inclusions. The Stage 2 assemblage is associated with the mineralization at the Jianchaling deposit. Fluid inclusions of Stage 1 quartz homogenize mainly between 250° and 360 °C, with salinities up to 15.6 wt.% NaCl equiv., whereas the Stage 3 dolomite with homogenization temperatures of 160° – 220 °C and salinities of 1.1–7.4 wt.% NaCl equiv. This indicates that the ore fluid system evolved from CO2-rich, probably metamorphic hydrothermal to CO2-poor, meteoric fluid. All three types of fluid inclusions can be observed in the Stage 2 quartz, suggesting that this heterogeneous association was trapped from a boiling fluid system. These inclusions homogenized at temperatures of 200°–250 °C and salinities of 1.2–12.4 wt.% NaCl equiv. The estimated trapping pressures of the fluid inclusions are between 117 and 354 MPa in Stage 1, suggesting an alternating lithostatic–hydrostatic fluid system, which was controlled by a fault-valve at the depth of ~ 12 km.Two fuchsite samples collected from the Stage 2 polymetallic-quartz veins yielded well-defined 40Ar/39Ar isotopic plateau ages of 197 ± 2 and 194 ± 2 Ma, and 39Ar/36Ar-40Ar/36Ar normal isochrones of 198 ± 2 and 199 ± 2 Ma. This indicates that the mineralization at Jianchaling is Early Jurassic (ca. 198 Ma) in age. We propose that Jianchaling is an orogenic gold deposit, and formed during continental collision related to the northward subduction of the Mian-Lue oceanic plate during the Early Jurassic. We also conclude that the beginning of the continental collision between the Yangtze and the North China Cratons took place around 200 Ma.  相似文献   

7.
This study investigates the concentration and spatial distribution of Cu, Zn, Hg and Pb in the surface (0–2 cm) soils of a regional city in Australia. Surface soils were collected from road sides and analysed for their total Cu, Zn, Hg and Pb concentrations in the <180 μm and <2 mm grain size fractions. The average metal concentration of surface soils, relative to local background soils at 40–50 cm depth, are twice as enriched in Hg, more than three times enriched in Cu and Zn, and nearly six times as enriched in Pb. Median surface soil metal concentration values were Cu – 39 mg/kg (682 mg/kg max), Zn – 120 mg/kg (4950 mg/kg max), Hg – 44 μg/kg (14,900 μg/kg max) and Pb – 46 mg/kg (3490 mg/kg max). Five sites exceeded the Australian NEPC (1999) 300 mg/kg guideline for Pb in residential soils. Strong positive correlations between Cu, Zn and Pb, coupled with the spatial distribution of elevated soil concentrations towards the city centre and main roads suggest traffic and older housing as major sources of contamination. No spatial relationships were identified between elevated metal loadings and locations of past or present industries.  相似文献   

8.
Feminist political ecology (fpe) is at a crossroads. Over the last 2 years, feminist political ecologists have begun to reflect on and debate the strengths of this subfield. In this article, we contribute by pointing to the limited theorization of race in this body of work. We argue that fpe must theorize a more complex and messier, notion of ‘gender’, one that accounts for race, racialization and racism more explicitly. Building on the work of feminist geography and critical race scholarship, we argue for a postcolonial intersectional analysis in fpe – putting this theory to work in an analysis of race, gender and whiteness in Honduras. With this intervention we demonstrate how theorizing race and gender as mutually constituted richly complicates our understanding of the politics of natural resource access and control in the Global South.  相似文献   

9.
Detrital zircons from the Ordovician and Devonian sedimentary cover of the Siberian Craton were analyzed for U/Pb geochronology to understand their sediment provenances. Five main age-peaks were identified in the zircon U/Pb age-spectra: (1) Neoarchaean – early Palaeoproterozoic (2.7–2.4 Ga); (2) late Palaeoproterozoic (2.0–1.65 Ga); (3) minor early Neoproterozoic (1.0–0.75 Ga); (4) Ediacaran (0.65–0.60 Ga) and (5) Cambrian – Early Ordovician (0.54–0.47 Ga), reflecting the main magmatic events in the sediment source regions. The oldest zircons (groups 1 and 2) are derived from the Siberian Craton which amalgamated during the Neoarchean – Palaeoproterozoic. The Neoproterozoic zircons (groups 3 and 4) likely sourced from southwestern basement uplifts and Neoproterozoic belts of the Siberian margin such as the Yenisey Ridge and Baikal-Muya region. The provenance of the youngest zircons (group 5) can be traced to the Altai–Sayan fold-belt, where peri-Gondwanan microcontinents and island-arcs accreted to Siberia during late Neoproterozoic – early Palaeozoic progressive consumption of the Palaeo-Asian Ocean.  相似文献   

10.
Polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) are persistent organic pollutants (POPs) that are formed and released unintentionally from anthropogenic sources. The high persistence of PCDD/Fs results in the concentrations of these contaminants in environment decreasing only very slowly. Two transport pathways, air and water, carry PCDD/Fs into all regions of the world. Recently, more frequent extreme weather events, such as storms and floods, have been projected to occur as a result of global warming. Extreme weather events have a documented impact on the remobilization and subsequent bioavailability of POPs. In this study, three specific episodes, namely winter monsoon, southeast biomass burning and tropical cyclone (typhoon) events, which influence the environmental fate and transport of PCDD/Fs in Taiwan, were evaluated based on a climate change scenario. During the winter (northeast) monsoon period, the temperature and relative humidity observed in northern Taiwan decreases sharply. During this time, the quantity of PCDD/Fs adsorbed onto suspended particles, as observed at background sites, was found to increase from 300 ± 127 to 630 ± 115 pg I-TEQ g-TSP−1, which is even higher than that measured in Taipei City (438 ± 80 pg I-TEQ g-TSP−1). Hence, the winter monsoon not only brings cold air but also transports air pollutants and dust over long distances from mainland China to Taiwan. During the 2010 Southeast Asia biomass burning events (2010/3/22–3/28), the level of atmospheric PCDD/Fs were measured in central Taiwan (Mt. Lulin) and in the source region of northern Thailand (Chiang Mai); this revealed that the variations in atmospheric PCDD/F concentrations at these two sites followed a similar pattern. On 25 March 2010, the atmospheric PCDD/F concentration increased dramatically from 1.43 to 6.09 fg I-TEQ m−3 at Mt. Lulin and from 7.64 to 12.1 fg I-TEQ m−3 in northern Thailand. However, the atmospheric PCDD/F concentration decreased dramatically 1 day after the biomass burning event. Based on the measurements from a dated sediment core collected at a reservoir in northern Taiwan, the sharp increases in input fluxes of PCDD/Fs and mineral-derived elements levels in 1990 (20 ng I-TEQ m−2 year−1), 2001 (17 ng I-TEQ m−2 year−1), 2004 (16 ng I-TEQ m−2 year−1) and 2005 (15 ng I-TEQ m−2 year−1) seem to be a result of a deep turbid layer formed upstream due to landslides and/or mud flows during the typhoon season. This finding demonstrates the effect of typhoon events on the long-term remobilization of PCDD/Fs as well as supporting the hypothesis that such events would have the potential to remobilize pollutants that have been deposited previously.  相似文献   

11.
Aquatic humic substances (AHS) have been intensively studied because they have a number of significant biogeochemical functions in water chemistry. In order to evaluate such functions, which are regulated by their quantity and quality, a quantitative understanding of AHS is necessary. This study shows the seasonal and vertical changes in the AHS concentration in Lake Biwa, the largest monomictic lake in Japan, over 4 years. The concentration of dissolved organic matter carbon (DOM-C) and AHS carbon (AHS-C) ranged from 0.85–1.6 mg C/l and 0.32–0.71 mg C/l, respectively. The percentages of AHS-C in the DOM-C ranged from 32% to 65%. At all depths, the DOM-C decreased and the AHS-C slightly increased or remained at nearly the same level, suggesting that the quality of the water may have changed mainly due to changes in the composition of the organic matter. Although the AHS-C to DOM-C ratio fluctuated and had no seasonal tendency, the average AHS-C ratio increased during the 4 year monitoring period. Because AHS can influence the entire food web starting with phytoplankton, unraveling the mechanism by which they accumulate remains an important goal.  相似文献   

12.
Southern Peru contains important epithermal Au–Ag (± base metals) deposits, such as Canahuire, Tucari, Santa Rosa, Caylloma, Shila and Paula. The Chapi Chiara gold prospect is located in this region and is part of a paleo-stratovolcano of the Upper Miocene–Pliocene. The hydrothermal alteration of the prospect was characterized based on spectroradiometric data, geochemistry and petrography. The mineralogical data, interpreted based on reflectance spectroscopy, were spatialized using the sequential indicator simulation technique for producing probabilistic maps of alteration. The inner part of the paleo-stratovolcano (SW sector) is marked by three main cores of advanced argillic alteration (AAA) (quartz–alunite supergroup minerals–kaolinite–dickite ± topaz ± pyrophyllite ± diaspore) associated with topographic highs. The AAA1 core is surrounded by argillic alteration (quartz–illite–paragonitic illite–smectite ± pyrite) and propylitic alteration (quartz–plagioclase–chlorite–calcite–epidote–smectite ± kaolinite ± pyrite ± chalcopyrite ± magnetite). The central sector of the prospect, situated in the NE flank of the paleo-stratovolcano, is characterized by hydrothermal breccias structured towards N65E. The main mineral phases comprise quartz and abundant pyrite, sometimes with traces of As. Anomalous geochemical values of Ag, As, Bi, Hg, Se, Sb and Te coincide with high gold contents in this sector of the prospect. Jarosite and goethite are evidence of a subsequent supergene event. Based on the mineralogical characterization, we conclude the existence of a high sulfidation epithermal system in Chapi Chiara. Hypogene minerals of higher temperature in the SW sector of the prospect, such as diaspore, pyrophyllite and topaz in the AAA zone, and epidote in the propylitic alteration zone, can reveal that the system is currently in a relatively deep erosion level, suggesting its proximity in relation to the interface between a deep epithermal system and a mesothermal system.  相似文献   

13.
Aisling Gallagher 《Geoforum》2012,43(3):464-471
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14.
The Córrego do Sítio lineament is defined as a 16-km long, NE-SW-trending ductile shear zones, which controls fourteen gold deposits, including the Carvoaria Velha deposit. The dominant lithotypes of this deposit are metagrewackes with subordinate carbonaceous phyllites lenses of the Archean Rio das Velhas greenstone belt, which host gold mineralization. Metamafic dikes and sills are parallel and crosscut the host metasedimentary sequence. All these rocks exhibit alteration to quartz, carbonate and sericite, besides sulfides and sulfosalts. The main gold mineralization styles at the Carvoaria Velha deposit, and at Córrego do Sítio as a whole, are quartz-carbonate-sulfide ± sulfosalt veins of varied distribution. The veins are classified as: V1 veins – quartz-ankerite-pyrite-berthierite-gold – parallel to the main regional foliation Sn,; V2 veins – quartz-ankerite-pyrite – developed at extensional crenulation cleavage Sn + 1, and rarely gold mineralized; V3 veins – quartz-ankerite – filling Sn + 3 fractures, usually free of sulfides and sulfosalts; V4 veins – quartz-calcite – of restricted occurrence in metamafic dikes and sills. The latter crosscut the metasedimentary sequence, are extensional and display no preferential orientation. The most common ore minerals in all vein types are arsenopyrite, pyrite, berthierite, and pyrrhotite. Microprobe analyses reveal the presence of metallic phases rich in Sb, Pb and Co, such as stibnite, ullmanite, tetrahedrite, galena, cobaltite, which commonly overgrow the sulfides. Fluid inclusion studies trapped in quartz from V1, V2 and V4 veins have identified a H2O-CO2 ± CH4-NaCl mineralizing fluid that may contain KCl, MgCl2 e FeCl2. The presence of CH4 in fluid inclusions of the V1 and V2 veins reflect interaction of the fluid with the Córrego do Sítio and Santa Quitéria carbon-bearing metapelitic host units.Based on the microthermometric data, the hydrothermal fluid is interpreted to have evolved in at least two stages: i) an early fluid stage, trapped in smoky quartz, of moderate salinity (~ 8.5 eq. wt% NaCl), and minimal trapping temperature of 330 ± 17 °C; and ii) a late-stage fluid trapped in recrystallized quartz with lower salinity (~ 4.6 eq. wt% NaCl), and a minimal trapping temperature of 365 ± 33 °C. Isotopic values of δ18Ofluid (+ 7.9 and + 13.0‰), Δ33S (+ 0.3 to + 3.5‰) and δ34S (− 2.9 to + 6.1‰) suggest that indeed the hydrothermal fluid responsible for the gold mineralization at the Carvoaria Velha deposit must have had a metamorphic origin, and interacted with metasedimentary sequences during its ascending path. The addition of CH4 during fluid-rock interaction may have caused some decrease in ƒO2 of the fluid which, as a consequence, destabilized gold-bearing sulfur complexes, liberating S− 2 for the formation of Fe sulfides and sulfosalts (arsenopyrite, pyrite, pyrrothite and berthierite, etc.), and outcome gold deposition.  相似文献   

15.
《Applied Geochemistry》2006,21(2):305-317
There is significant random and systematic variability in As concentrations in numerous public water system wells in Minnesota. Arsenic concentrations fluctuate above and below the USA’s As drinking water Maximum Contaminant Level (MCL) of 10 μg/L. The average As concentration is commonly within one standard deviation of the MCL. Results of intensive sampling conducted over the course of approximately 1 year at 3 public water system wells is consistent with the analysis of historic As measurements. In some cases, significant As concentration variability was noted over a short period of time. In these wells, the As concentration was less than 10 μg/L shortly after pumping started, but the As concentration increased over time to a level exceeding 10 μg/L. In these wells, the As concentration decreased to below 10 μg/L again when pumping was briefly restarted after being stopped for 4 h. The As concentration variability is likely due to As adsorption reactions between Fe oxides in the well borehole and in the aquifer near the well borehole during periods when the pump is not in operation. When it is crucial to accurately determine true average As concentrations – for example, at one of the many wells that fluctuate above and below the regulatory As limit of 10 μg/L – it is worthwhile collecting samples frequently during pumping to more accurately determine the average As concentration. Determining a reliable average depends on the standard deviation (SD) of the measurements, with more measurements required if the SD is larger.  相似文献   

16.
The word “mazuku” in Swahili means “evil wind”. It corresponds to lowland (depressions) where carbon dioxide is released and, being heavier than air, accumulates at high – often lethal – concentrations (10 vol.% of CO2 in atmosphere can be considered as the deadly threshold, even for a short time exposure). Mazuku are abundant in Goma and surrounding areas and particularly in the area south of the large volcanic edifices of Nyiragongo and Nyamulagira volcanoes located in the most eastern part of DR Congo, W branch of the East African Rift System (EARS). Our extensive field surveys have indicated that mazuku are concentrated within to and around the densely populated city of Goma close to the N shores of Lake Kivu, mainly near fault or fissure networks. At a more local scale, depressions allowing CO2-rich gas accumulation are created by lava flow superposition, lava tunnels or cavity collapses, or directly associated with open fractures. People are killed by mazuku every year. Given political and social unrest coupled with the current important demographic and urban growths around Goma, the risks associated to mazuku are increasing accordingly. Mazuku are currently the most important natural risk in terms of human loss for the area and there is an urgent need for further research, more systematic mapping and monitoring of mazuku and for appropriate risk management to be implemented. This paper summarizes the current scientific knowledge on mazuku as well as new advances and a preliminary risk assessment performed recently in the frame of the GORISK project.  相似文献   

17.
New U–Pb SHRIMP zircon ages combined with geochemical and isotope investigation in the Sierra de Maz and Sierra de Pie de Palo and a xenolith of the Precordillera basement (Ullún), provides insight into the identification of major Grenville-age tectonomagmatic events and their timing in the Western Sierras Pampeanas. The study reveals two contrasting scenarios that evolved separately during the 300 Ma long history: Sierra de Maz, which was always part of a continental crust, and the juvenile oceanic arc and back-arc sector of Sierra de Pie de Palo and Ullún. The oldest rocks are the Andino-type granitic orthogneisses of Sierra de Maz (1330–1260 Ma) and associated subalkaline basic rocks, that were part of an active continental margin developed in a Paleoproterozoic crust. Amphibolite facies metamorphism affected the orthogneisses at ca. 1175 Ma, while granulite facies was attained in neighbouring meta-sediments and basic granulites. Interruption of continental-edge magmatism and high-grade metamorphism is interpreted as related to an arc–continental collision dated by zircon overgrowths at 1170–1230 Ma. The next event consisted of massif-type anorthosites and related meta-jotunites, meta-mangerites (1092 ± 6 Ma) and meta-granites (1086 ± 10 Ma) that define an AMCG complex in Sierra de Maz. The emplacement of these mantle-derived magmas during an extensional episode produced a widespread thermal overprint at ca. 1095 Ma in neighbouring country rocks. In constrast, juvenile oceanic arc and back-arc complexes dominated the Sierra de Pie de Palo–Ullún sector, that was fully developed ca. 1200 Ma (1196 ± 8 Ma metagabbro). A new episode of oceanic arc magmatism at ～1165 Ma was roughly coeval with the amphibolite high-grade metamorphism of Sierra de Maz, indicating that these two sectors underwent independent geodynamic scenarios at this age. Two more episodes of arc subduction are registered in the Pie de Palo–Ullún sector: (i) 1110 ± 10 Ma orthogneisses and basic amphibolites with geochemical fingerprints of emplacement in a more mature crust, and (ii) a 1027 ± 17 Ma TTG juvenile suite, which is the youngest Grenville-age magmatic event registered in the Western Sierras Pampeanas. The geodynamic history in both study areas reveals a complex orogenic evolution, dominated by convergent tectonics and accretion of juvenile oceanic arcs to the continent.  相似文献   

18.
Formation of the Urals Volcanic-Hosted Massive Sulphide (VHMS) deposits is considered to be related with the intra-oceanic stage of the island arc(s) development in Late Ordovician – Middle Devonian time (ca. 460–385 Ma) based on the biostratigraphic record of ore-hosting sedimentary rocks. However, the known radiometric ages of ore hosting volcanics are very limited. Here we present direct dating results of sulphide mineralisation from the Yaman-Kasy and Kul-Yurt-Tau VHMS deposits using Re-Os isotope systematics showing similar mineralisation ages of 362 ± 9 Ma and 363 ± 1 Ma. These ages coincide with the previous Re-Os dating of the Alexandrinskoe (355 ± 15 Ma) and Dergamysh (366 ± 2 Ma) VHMS deposits. This Late Devonian (Famennian) age corresponds to the late stage of the ‘Magnitogorsk arc – Laurussia continent’ collision event and coincides with a beginning of large scale subduction-related granitoid magmatism. The younger mineralisation age relative to the biostratigraphic ages of host rocks is interpreted as one of the latest episodes of the multi-stage history of VHMS deposits development. Ar-Ar ages of sericites from metasomatic rocks of Barsuchi Log and Babaryk deposits show even younger ages clustering around 345 Ma, and testify another late hydrothermal event in the history of the Urals VHMS deposits.  相似文献   

19.
Tropical glaciers are considered along three meridional profiles, for the Australasian sector, Africa and the Americas. Evaluated are the annual mean freezing level 0 °C, modern equilibrium line altitude MEL, and past equilibrium line altitude PEL. The calculation of 0 °C is based on a 1958–1997 global data set; MEL refers to estimates concerning the first half of the 20th century; and the timing of the PEL is not generally known. 0 °C stands around 4000–5000 m, with lower levels in the outer tropics. MEL is reached in the Australasian sector on four mountains, and in Africa on three mountains, near the Equator, near or above 0 °C. In the American cordilleras many peaks are still glaciated, above 0 °C, but in the arid southern tropical Andes even summits above 6000 m do not reach MEL. The PEL stands between 3000 m and 5000 m, high in the equatorial zone, but highest in the arid southern tropical Andes. The height difference between MEL and PEL is of order 1000 m, with regional differences. Deglaciation dates range between 15,000 and 8000 years BP, with later timing towards the higher elevations. This synopsis suggests priority targets for further research: morphological mapping and age determination in the High Atlas of Morocco; timing of deglaciation in the High Semyen of Ethiopia and the Altos de Cuchumatanes of Guatemala; exploration in the highlands of Lesotho and of the Dominican Republic; and glaciation in the arid southern tropical Andes.  相似文献   

20.
Rrapo Ormeni 《Tectonophysics》2011,497(1-4):114-121
This paper describes the one-dimensional (1D) velocity model computed by VELEST in the SEISAN seismic analysis system, inverting re-picked P-wave and S-wave arrival times recorded during 2002–2006 by the Albanian, Montenegro, Thessalonica and Macedonia seismic networks. The re-picked data yield P-wave and S-wave velocities proved to be more suitable compared to bulletin data for this detailed inversion study. Seismic phases recorded by the Albania seismic network and integrated with data from the Montenegro, Thessalonica and Macedonia networks are used to prepare the Albanian seismic bulletin. Earthquake hypocenters from the Albanian bulletins have also location errors that are negligible for civil protection purposes, large scale seismotectonic analyses and more accurate hypocentral determinations which are necessary for detailed seismotectonic and geodynamic studies.It was noted that the smoothness of the velocity variation increased with depth. A velocity of 5.5 km/s was calculated for the upper crust, 6.1 km/s was calculated for the middle crust and 6.9 km/s was computed for the lower crust. P wave velocity was 7.85 km/s at depth of 50 km and for the upper mantle it is 8.28 km/s. Using the improved velocity model, the earthquakes which occurred in Albania in the past 5 years were able to be relocated, achieving constrained hypocentral determinations for events in Albania. The interpretation of the 1 D velocity models infers interesting features of the deep structure of Albania. These results represent an important step towards more detailed seismotectonic analyses.  相似文献   

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