排序方式: 共有17条查询结果,搜索用时 31 毫秒
1.
Yohannes Aberra 《The Geographical journal》2004,170(3):226-237
The Mekele Plateau, located in the semi-arid northern highland region of Ethiopia, is a drought-prone, highly degraded area. It has long been neglected by policymakers and thus recurring famine, widespread poverty, armed conflicts and mass outmigration have contributed to the near collapse of the farming systems and the livelihoods of the population. Traditional stream diversion irrigation has been practised in this area for several centuries. The systems of water management in particular may be considered inappropriate owing to the need for thriftiness in water use imposed by the local environment. The systems of headworks and canals are crude and wasteful on water. Recent interventions to improve the small-scale irrigation systems of the area have not been successful. Most interventions have preferred to introduce rainwater harvesting systems using micro-dams rather than trying to upgrade the traditional diversion schemes. Evaluation of irrigation structures in the field, interviews with key informants in government, analysis of documents and discussions with irrigators indicate that both the Ministry of Agriculture and SAERT-Commission's rainwater harvesting programmes have generally failed. The case of the most recent intervention by SAERT has failed because of myopic project planning, inadequate engineering studies, unsound designs, too short a time for implementation, and lack of full farmer involvement at different levels of project planning and implementation. 相似文献
2.
3.
4.
5.
Summary The formation of cummingtonite in two Ca and Al-poor and Mg-rich amphibolites from the Austroalpine Schneeberg complex occurred at a maximum temperature of about 550°C (5 kb). This is a result of the amphibolite facies Alpine overprint in this part of the Eastern Alps.Textural and chemical relations suggest (Mg–1Si–1Al2)-continuous reactions in the bivariant CMASH-assemblageCam-Cum-Chl
* followed by the discontinuous reactionCam+Chl+Qu=Cum+Plg+H2O to be responsible for the formation of cummingtonite in these samples.The Mg–Fe distribution coefficient
with values of 0.6–0.7 is similar to cummingtonite-Ca-amphibole pairs from amphibolites with oligoclase+quartz reported in the literature. The Mg/(Mg+Fe) ratio of the calcic amphiboles is lower (0.539–0.555) than the coexisting cummingtonites (0.648–0.662).
With 4 Figures 相似文献
Koexistierende Cummingtonite und Hornblenden in Amphiboliten des Schneeberger Zuges, Tirol, Österreich
Zusammenfassung In zwei Ca- und Al-armen Amphiboliten des nördlichen Schneebergerzuges (Rotmoostal) bildete sich Cummingtonit bei Maximaltemperaturen von 550°C (5 kb) bei der Altalpidschen Metamorphose.Texturelle und chemische Beziehungen lassen vermuten, daß sich Cummingtonite sowohl nach kontinuierlichen Reaktionen (in bezug auf den Tschermak-Vektor Mg–1Si–1Al2) gebildet hat, als auch aus Hornblende und Chlorit nach der diskontinuierlichen ACF-ReaktionCam+Chl+Qu=Cum+Plg+H2O hervorgegangen ist.Der Mg–Fe-Verteilungskoeffizient zwischen Hornblende und Cummingtonit entspricht den aus der Literatur bekannten Werten. Er beträgt zwischen 0.6–0.7 für die beginnende Amphibolitfazies. Die Mg/(Mg+Fe)-Verhältnisse sind höher in Cummingtonit (0.648–0.662) als in der koexistierenden Hornblende (0.539–0.555).
With 4 Figures 相似文献
6.
Ophiolite complexes, formed in a suprasubduction zone environment during Neoproterozoic time, are widely distributed in the
Eastern Desert of Egypt. Their mantle sections provide important information on the origin and tectonic history of ocean basins
these complexes represent. The geochemistry and mineralogy of the mantle section of the Wizer ophiolite complex, represented
by serpentinites after harzburgite containing minor dunite bodies, are presented. Presence of antigorite together with the
incipient alteration of chromite and absence of chlorite suggests that serpentinization occurred in the mantle wedge above
a Neoproterozoic subduction zone. Wizer peridotites have a wide range of spinel compositions. Spinel Cr# [100Cr/(Cr + Al)]
decrease gradually from dunite bodies (Cr# = 81–87) and their host highly depleted harzburgites (Cr# = 67–79) to the less
depleted harzburgites (Cr# = 57–63). Such decreases in mantle refractory character are accompanied by higher Al and Ti contents
in bulk compositions. Estimated parental melt compositions point to an equilibration with melts of boninitic composition for
the dunite bodies (TiO2 = ~<0.07–0.22 wt%; Al2O3 = 9.4–10.6 wt%), boninitic-arc tholeiite for the highly depleted harzburgites (TiO2 = <0.09–0.28 wt%; Al2O3 = 11.2–14.1 wt%) and more MORB-like affinities for the less depleted harzburgites (TiO2 = ~<0.38–0.51 wt%; Al2O3 = 14.5–15.3 wt%). Estimated equilibrium melts are found in the overlying volcanic sequence, which shows a transitional MORB–island
arc geochemical signature with a few boninitic samples. Enrichment of some chromites in TiO2 and identification of sulfides in highly depleted peridotites imply interaction with an impregnating melt. A two-stage partial
melting/melt–rock reaction model is advocated, whereby, melting of a depleted mantle source by reaction with MORB-like melts
is followed by a second stage melting by interaction with melts of IAT–boninitic affinities in a suprasubduction zone environment
to generate the highly depleted harzburgites and dunite bodies. The shift from MORB to island arc/boninitic affinities within
the mantle lithosphere of the Wizer ophiolite sequence suggests generation in a protoarc-forearc environment. This, together
with the systematic latitudinal change in composition of ophiolitic lavas in the Central Eastern Desert (CED) of Egypt from
IAT–boninitic affinities to more MORB-like signature, implies that the CED could represent a disrupted forearc-arc-backarc
system above a southeast-dipping subduction zone. 相似文献
7.
8.
Florian Gallien Aberra Mogessie Ernesto Bjerg Sergio Delpino Brigida Castro de Machuca Martin Thöni Urs Klötzli 《Lithos》2010,114(1-2):229-252
Migmatitic paragneisses of the Valle Fértil–La Huerta Ranges at the Western margin of the Sierras Pampeanas are composed of garnet–cordierite–plagioclase–biotite–quartz-bearing units that experienced peak metamorphic conditions of ca. 800 °C at 6–7 kbar. Based on petrological studies, pseudosection modeling and petrographic observations, an anticlockwise P–T path with a small pressure increment is proposed. Rare earth element LA-ICP-MS patterns acquired from rutile bearing garnets suggest a single stage of garnet growth at high-T at pressures above the ilmenite–rutile transition. U–Pb dating of zircon rims from the migmatites indicates two distinct metamorphic U–Pb ages of 525 ± 9 Ma and 478 ± 9 Ma. The older age is suggested to record an amphibolite facies event of the Pampean orogeny. The younger metamorphic age is contemporary with igneous zircons from metatonalites and pegmatites that yield 478 ± 4 Ma. We suggest that the prograde high-T metamorphic Famatinian event is associated with the emplacement of large magmatic bodies in which large-scale magmatic activity gave rise to an increased geothermal gradient of about 35 °C/km. Sm–Nd garnet ages of 447 ± 3 Ma indicate a time span of around 30 Ma for which temperatures above the garnet closure temperature prevailed. Using U–Pb, Sm–Nd and Rb–Sr isotope systems, a cooling rate of 3 to 6 °C/Myr is inferred. 相似文献
9.
The Gabbro Akarem (Late Precambrian) intrusion is concentrically zoned with a dunite core surrounded by lherzolite–clinopyroxenite
enveloped by olivine–plagioclase hornblendite and plagioclase hornblendite. Cu–Ni–PGE mineralization is closely associated
with peridotite, especially in the inner, olivine-rich core (dunite pipes) where net-textured and massive sulfides (pyrrhotite,
pentlandite, chalcopyrite) are found in association with Al–Mg-rich spinel and Cr-magnetite. Primary magmatic textures are
well preserved; however, deformation and mobilization due to shearing are locally observed. Platinum-group minerals (PGM)
documented from the deposit are: merenskyite (PdTe2) and michenerite (PdTeBi), as well as palladian bismuthian melonite (Ni,Pd) (Te,Bi)2. These minerals occur in intimate association with hessite (Ag2Te) and electrum (Au0.65Ag0.31Bi0.04) in two distinct textural positions: (1) as inclusions in pyrrhotite, pentlandite, and rarely chalcopyrite and (2) at sulfide–silicate
grain boundaries and on microfractures in base-metal sulfides (BMS) and olivine associated with serpentine and secondary magnetite.
Textural features suggest that PGM were exsolved from monosulfide solid solution over a wide range of temperatures. Late-stage,
low-temperature hydrothermal solutions led to redistribution of PGE. Mineralized samples show Ni/Cu ratios ranging from 0.2
to 2 with an average of 1.0. The (Pt + Pd + Rh)/(Os + Ir + Ru) ratio is generally >6 in most samples, and Os, Ru, and Ir are
below the detection limit (2 ppb). The PGE contents show positive correlation with S only at low sulfur contents. The PGE
patterns of Gabbro Akarem are similar to those of Alaskan-type deposits. Compared with stratiform deposits, Gabbro Akarem
is depleted in PGE. The consistently low PGE contents of the mineralization and their uniform distribution in the ultramafic
rocks despite the high sulfur content of the rock is attributed to rapid crystallization of sulfides in a highly dynamic environment.
Received: 3 November 1999 / Accepted: 29 July 2000 相似文献
10.
Kálmán Török Bianka Németh Friedrich Koller Júlia Dégi Eszter Badenszki Csaba Szabó Aberra Mogessie 《Mineralogy and Petrology》2014,108(1):33-47
Felsic to mafic granulite xenoliths from late Neogene basalt pyroclastics in four localities of the western Pannonian Basin (Beistein, Kapfenstein, Szigliget and Káptalantóti (Sabar-hegy) were studied to find out their metamorphic and fluid history. The characteristic mineral assemblage of the granulites consists of Pl + Opx + Qtz ± Cpx ± Bt ± Grt ± Kfs. Based on abundant magmatic relic microstructural domains occurring in these rocks, the potential precursors might have been predominantly felsic igneous or high to ultrahigh temperature rocks. Ternary feldspar thermometry provides a rough estimate of temperatures of about 920–1070 °C. The first fluid invasion event, which is linked with this early high to ultrahigh temperature stage is characterised by primary pure CO2 inclusions in apatite and zircon. The densest primary CO2 inclusions indicate 0.52–0.64 GPa pressure at the estimated temperature range of crystallization. According to mineral equilibria and geothermobarometry, the high to ultrahigh temperature rock cooled and crystallized to granulite of predominantly felsic composition at about 750–870 °C and 0.50–0.75 GPa in the middle crust, between 20 and 29 km depths. The second fluid invasion event is recorded by primary CO2-rich fluid inclusions hosted in the granulitic mineral assemblage (plagioclase, quartz and orthopyroxene). In addition to CO2, Raman spectroscopy revealed the presence of minor N2, H2S, CO and H2O in these inclusions. Partial melting of biotite-bearing assemblages could be connected to the next fluid invasion shown by secondary CO2-rich fluids recorded along with healed fractures in plagioclase, clinopyroxene and orthopyroxene. This event could have happened at depths similar to the previous ones. The final step in the granulite evolution was the sampling in the middle crust and transportation to the surface in form of xenoliths by mafic melt. This event generated temperature increase and pressure decrease and thus, limited melting of the xenoliths. The youngest fluid inclusion generation, observed mostly in healed fractures of felsic minerals, could be associated with this event. 相似文献