Petrogenesis of the Fe-Ti intrusive complexes in the Sierra Leone region, Central Atlantic

The study of melt inclusions in Cr-spinels from melanocratic troctolites provided the first direct information on the physicochemical parameters of enriched magmatic systems that produced high-Fe and high-Ti intrusive complexes in the Sierra-Leone region (Central Atlantic, 6°N). These complexes are made up of predominating hornblende Fe-Ti oxide gabbronorites and gabbrodiorites with subordinate amount of ultramafics, diorites, quartz diorites, and trondhjemites. The study of melt inclusions and rocks showed that the majority of gabbroids of the Central Atlantic (Sierra Leone area and 15°20′ Fracture Zone) were derived from N-MORB-type melts, whereas differentiated Fe-Ti-oxide rocks were crystallized from other melts, which were preserved as inclusions in the Cr-spinels from the melanocratic troctolites of the Sierra Leone region. The ion-microprobe study of these inclusions yield direct evidence on the elevated water content (up to 1.24–1.77 wt %) in the parental melts of Fe-Ti oxide rocks. Data on trace and rare-earth element distribution together with high (La/Sm)_N and (Ce/Yb)_N ratios in the inclusions indicate the possible influence of deep plume source on the generation of these magmas. Simulation based on melt inclusion data testifies that high-Fe intrusions of the Sierra Leone area were crystallized from the water-saturated magmas at relatively low temperatures (1020–1240°C). It was shown that the geochemically enriched Fe-Ti melts were presumably formed regardless of N-MORB-type magmatism predominant in Central Atlantic, under the influence of new mantle plume that caused melting of hydrated oceanic lithosphere.     

Magmatic rocks in the Charlie-Gibbs Fracture Zone, North Atlantic Ocean

Two basalt groups corresponding to different geodynamic environments were distinguished based on results of additional geochemical studies of materials obtained during Cruise 15 of the R/V Akademik Kurchatov and literature data on the Charlie-Gibbs Fracture Zone: (1) predominantly low-Ti tholeiites that are most closely similar to spreading basalts generated during the initial opening of the North Atlantic (southeastern margin of Greenland and the Vöring and Rockall plateaus) and (2) high-Ti differentiated basalts of variable alkalinity, which are deep, low-volume partial melts likely derived with the participation of a mantle plume. Compositional variations in the basalts were studied in the active and passive parts of the fracture zone. Our results bridge the gap in detailed geochemical data on oceanic crustal rocks and can be used in reconstructions of ancient geodynamic environments.     

Peculiarities of accessory zircon from the carter seamount tuffs (Sierra Leone Rise in the East Atlantic)

In volcanic tuffs, dredged during Cruise 23 of the R/V Akademik Nikolaj Strakhov, accessory zircon was found; except for the mineral-forming components, there were ∼2% of ThO₂ and 0.75% of Ce₂O₃ in zircon. During rapid uplift of magmatic masses to the ocean bottom surface, admixture elements isolated into specific minerals. As a result, destruction structures were formed in the rim parts of primary zircon crystals, and the new-formed association of zircon + thorite + cerite + thoriante + baddeleyite appeared.     

Age of Plutonic Rocks from the Vema Fracture Zone (Central Atlantic) and Nature of Their Mantle Sources

This study presents the results of petrographic, geochemical, and isotope geochronological analyses of rock samples from the Southern flank of the Vema transform fault (Atlantic), which were dredged on cruises 19-th and 22-nd of the R/V Akademik Nikolai Strakhov. The sample suite includes both fresh and metamorphosed gabbros, dolerites, serpentinites, metapyroxenites. Zircons separated from three gabbro samples recovered at three different stations were used for in situ U–Pb dating by LA-ICP-MS. The ages reveal a strong linear relationship with a distance from the axis of the Mid-Atlantic ridge, which allowed us to estimate the rate of spreading in this segment of the Mid-Atlantic Ridge. It can be concluded that the estimated spreading rate of 16.2 ± 0.8 mm/yr was constant over the past 15 Myr. The mutual consistency of all U-Pb zircon and ³⁹Ar–⁴⁰Ar amphibole ages (Cipriani et al., 2009) obtained from the sampled transect suggests the temporal continuity of magmatic events that led to the formation of the original gabbroic rocks and their transformation during subsequent metamorphism. Rb—Sr isotope data show that hydrothermal activity took place in the presence of seawater between 14.7 and 9 Ma in the spreading axis region. Variations in the Nd isotopic composition in the time sequence of magmatic events indicate a high degree of chemical and isotopic heterogeneity of the ascending mantle material which became later entrained in the melting region beneath a spreading zone. Melting of the sources with primitive mantle composition (ε_Nd ~ + 8 to +9) as well as enriched sources took place in the time interval between ~ 17 and 14.7 Ma and at about 8 Ma. The enriched source material is most likely represented by ancient mafic substratum.     

Melt migration in the upper mantle along the Romanche Fracture Zone (Equatorial Atlantic)

Textural and petrological data of mantle peridotites sampled in the central and western parts of the Romanche Fracture Zone (Equatorial Atlantic) during the oceanographic expedition PRIMAR-96 (Russian R/V Gelendzhik) are presented. The studied rocks are mantle peridotites carrying patches, pockets and veins/dikes of magmatic origin, interpreted to be the product of various extents of magma impregnation on mantle partial melting residues. Estimated partial melting degrees based on clinopyroxene Ti/Zr ratios are in the ranges 5–13% and 18–20%. In highly impregnated samples, refertilization of residual peridotite minerals precludes a correct evaluation of the degree of melting. Magmatic products occur as pl±cpx±opx±ol±sp aggregates with various textural features. Interstitial pl-rich patches and gabbroic pockets are interpreted to derive from magma migration through the upper mantle by diffusive porous flow in the ductile part of the lithosphere and melt–rock reactions. Metasomatism of the host peridotites is testified by Ti and Cr increase in spinel and Ti, Sr, Zr, Y and LREE increase in clinopyroxene. Veins and dikes reflect channeled magma migration focused by brittle failures at shallower lithospheric levels. Minor or no chemical changes occurred in peridotites impregnated along fractures. The compositions of magmatic minerals in impregnated peridotites are consistent with derivation from variably fractionated melts of probably MORB type. Barometric estimates suggest that the Romanche peridotites were impregnated at minimum depths of ca. 9–12 km. Thermometric estimates for the peridotite hosts are in the range 750–1050 °C. The spread in temperature values is partly ascribed to localized heating by migrating melts of relatively cold peridotites. Our data and the occurrence of both fertile and depleted peridotites in a neighbouring area along the western Romanche FZ are in accord with the hypothesis of small-scale (<100 km) mantle heterogeneity along this fracture zone.     

Geological and Geophysical Studies of the Charlie Gibbs Fracture Zone (North Atlantic)

Doklady Earth Sciences - The geological and geophysical data obtained during the 50th cruise of R/V&nbsp;Akademik Nikolaj Strakhov&nbsp;on the Charlie Gibbs megatransform system structure...     

Towards an understanding of man-firewood relations in Freetown (Sierra Leone)

This paper examines ‘folk’ evaluation and classification of firewood as a means towards understanding man-firewood relations in Freetown. It demonstrates how access to ‘wood views’ renders comprehensible identifiable spatial and temporal patterns in firewood exploitation and utilisation. It is argued that the extension of the ethno-scientific approach beyond mere identification of wood categories to the examination of the relationship between cognition and action generates information capable of informing efforts towards short-term gains and/or long-term development policy aimed at a more efficient management and utilisation of the firewood resource. The article concludes by emphasising the need for an understanding of the view both from ‘above’ (technological forestry) and ‘below’ (folk ecology) of man-firewood relationships in the Freetown area in order that meaningful co-operative modifications based on a blend of sodo-cultural and ecological considerations, on the one hand, and economic and technological factors on the other, can be effected.     

Ferromanganese Crusts of the Doldrums Fracture Zone,Central Atlantic: New Data on the Chemical Composition

Doklady Earth Sciences - New data on the morphology, chemical composition, and age of the ferromanganese crusts of the Doldrums Fracture Zone, Central Atlantic, collected during the 45th cruise of...     

Background to the Liberia and Sierra Leone implosions

Liberia and Sierra Leone are tragic examples of what happens when central authority collapses and warlords emerge as de-facto rulers over large portions of the national territory. Horrors inflicted on non-combatants and the well publicized trading in conflict diamonds served to focus world attention on these two small countries sharing a common border. Both countries have experienced mixed success with outside military intervention for peace keeping and nation building purposes. What has happened is all the more distressing when one considers each countrys prospects at birth under the political and economic aegis of arguably two of the most powerful and enlightened countries of the time, Great Britain in the case of Sierra Leone and the United States in Liberia.     

Background to the Liberia and Sierra Leone implosions

Liberia and Sierra Leone are tragic examples of what happens when central authority collapses and warlords emerge as de-facto rulers over large portions of the national territory. Horrors inflicted on non-combatants and the well publicized trading in conflict diamonds served to focus world attention on these two small countries sharing a common border. Both countries have experienced mixed success with outside military intervention for peace keeping and nation building purposes. What has happened is all the more distressing when one considers each countrys prospects at birth under the political and economic aegis of arguably two of the most powerful and enlightened countries of the time, Great Britain in the case of Sierra Leone and the United States in Liberia.     

New Data on the Geological Structure of the Eastern Flank of the Charlie Gibbs Fracture Zone (North Atlantic)

Doklady Earth Sciences - The geological structure of a large volcanic edifice in the eastern flank of the large Charlie Gibbs Fracture Zone in the region of the Eastern Thule submarine rise (North...     

现代海底热液成矿作用

从现代块状硫化物矿床成矿特征对比角度,总结分析了世界现代海底喷流的块状硫化物成矿堆积,综述了现代海底块状硫化物成矿主要形成于洋壳和岛弧环境的实际观察结果,突出强调了洋壳环境和岛弧或陆壳环境两种成矿环境对成矿类型分类的意义。对上地幔部分熔融岩浆来源与地壳物质可能带人、火山喷发岩浆系列的演化和对热液成矿作用的控制进行了讨论,对比分析了岩浆流体对成矿的重要贡献和控制作用,以及成矿热液循环体系形成的条件和模式。     

Temporary labour migration and sustainable post-conflict return in Sierra Leone

Economic and social development indicators suggest that the small West African state of Sierra Leone is among the poorest countries in the world. Sierra Leone’s economy and quality of life deteriorated rapidly during a decade of political instability and civil war in the 1990s, when many people fled their homes and abandoned their livelihoods due to the rebel insurgency. This paper examines the post-war reconstruction scenario in Sierra Leone and presents recent evidence from two rural communities in the Eastern Province that were badly affected by the conflict. The paper considers the links between the farming and diamond mining sectors, which, despite severe dislocation during the conflict period, have proved to be remarkably resilient. It is argued that seasonal labour mobility associated with this dual economy has not only continued to be a key ingredient in sustaining livelihood portfolios, but is actually an essential pre-condition for the creation of an enabling environment for sustainable post-conflict return.

黔中断裂带特征及其与铀矿成矿作用探讨

黔中断裂带形成于早古生代之前,是黔中重要的控岩-控矿构造复合带.本文通过对该断裂带特征及铀成矿关系分析研究,运用现代热液成矿理论,打破以往沉积-表生成矿思维束缚,认为断裂带内铀成矿的核心是构造和热液作用的叠合,成矿机制是幔源含U流体活动的参与和深部热液流体多次叠加改造,铀源主要为富铀的黑色岩系和晚中生代玄武岩;铀矿体主...     

Petrochemistry of eclogites from the Koidu Kimberlite Complex,Sierra Leone

Petrography, mineral and bulk chemistry of upper mantle-derived eclogites (garnet and clinopyroxene) from the Koidu Kimberlite Complex, Sierra Leone, are presented in the first comprehensive study of these xenoliths from West Africa. Although peridotite-suite xenoliths are generally more common in kimberlites, the upper mantle sample preserved in Pipe Number 1 at Koidu is exclusively eclogitic, making this the fifth locality in which eclogite is the sole polymineralic xenolith in kimberlite. Over 2000 xenoliths were collected, of which 47 are described in detail that include diamond, graphite, kyanite, corundum, quartz after coesite, and amphibole eclogites. Grossular-pyrope-almandine garnets are chromium-poor (<0.72 wt% Cr₂O₃) and fall into two distinct groups based on magnesium content. High-MgO garnets have an average composition of Pyr₆₇Alm₂₂Gross₁₁, low-MgO garnets are grossular- and almandine-rich with an average composition of Gross₃₄Pyr₃₃Alm₃₃. Clinopyroxenes are omphacitic with a range in jadeite contents from 7.7 to 70.1 mol%. Three eclogites contain zoned and mantled garnets with almandine-rich cores and pyrope-rich rims, and zoned clinopyroxenes with diopside-rich cores and jadeite-rich rims, and are among a very rare group of eclogites reported on a world-wide basis. The bulk compositions of eclogites have ranges comparable to that of basalts. High-MgO eclogites (16–20 wt% MgO) have close chemical affinities to picrites, whereas low-MgO eclogites (6–13 wt% MgO) are similar to alkali basalts. High-MgO eclogites contain high-MgO garnets and jadeiterich clinopyroxenes. Low-MgO eclogites contain low-MgO garnets, diopside and omphacite, and the group of primary accessory phases (diamond, graphite, quartz after coesite, kyanite, and corundum); grospydites are peraluminous. Estimated temperatures and pressures of equilibration of diamond-bearing eclogites, using the diamond-graphite stability curve and the Ellis and Green (1979) geothermometer, are 1031°–1363° C at 45–50 kb.K _D values of Fe-Mg in garnet and clinopyroxene range from 2.3 to 12.2. Diamonds in eclogites are green, yellow, and clear, and range from cube to octahedral morphologies; the entire spectrum in color and morphology is present in a single metasomatized eclogite with zoned garnet and clinopyroxene. Ages estimated from Sm-Nd mineral isochrons range from 92–247 Ma. _Nd values range from +4.05 to 5.23. Values of specific gravity range from 3.06–3.60 g/cc, with calculated seismic Vp of 7.4–8.7 km/s. Petrographie, mineral, and bulk chemical data demonstrate an overall close similarity between the Koidu xenolith suite and upper mantle eclogites from other districts in Africa, Siberia and the United States. At least two origins are implied byP-T, bulk chemistry and mineral compositions: low-MgO eclogites, with diamond and other accessory minerals, are considered to have formed from melts trapped and metamorphically equilibrated in the lithosphere; high-MgO eclogites are picritic and are the products of large degrees of partial melting, with equilibration in the asthenosphere. Fluid or diluted melt metasomatism is pervasive and contributed here and elsewhere to the LIL and refractory silicate incompatible element signature in kimberlites and lamproites, and to secondary diamond growth.     

A primary platinum occurrence in the Freetown Layered Intrusion, Sierra Leone

The primary source of the alluvial platinum-group minerals (PGM) of the Freetown Layered Intrusion, Sierra Leone, has been unknown since their discovery 70 years ago. This is a report on work in progress to establish a primary occurrence related to the igneous layering. New whole-rock major and minor element analyses and precious metal assays show that at key horizons there is a significant change in the silicate mineral compositions and in Cu, Cr and Ni, as well as in Pt and Pd, comparable to those encountered in other PGM-bearing layered intrusions. Anomalous (0.1 to 0.4 g/t) Pt levels occur in pyroxene troctolites, which maintain these levels along strike. Received: 30 July 1999 / Accepted: 2 December 1999     

从太平洋中部热液矿物的表现探讨热液活动的广泛性

位于太平洋中部的克利帕顿和克拉里昂断裂带之间的研究区处于远离大陆的远洋深水环境，区内断裂构造和火山活动非常发育，热水活动强烈，由此形成一系列与热液活动及蚀变沉积有关的矿物，如磷灰石、碳磷灰石、稀土磷灰石、磷铝锶石、重晶石、碳酸钡矿、泡饿矿及非晶质ＳｉＯ２球。这都说明大洋中热水溶液活动并不仅限于洋脊轴部和裂谷地区，只要有较大规模的断裂存在，就会有热水溶液活动或海底热泉出现，随之后形成一系列与热液活动     

Mineralization and deformation of the Malanjkhand terrane (2,490–2,440 Ma) along the southern margin of the Central Indian Tectonic Zone

The Malanjkhand Cu–Mo–Au deposit, located near the northwest margin of the Malanjkhand batholith (terrane), is a strategic and significant porphyry-style deposit that experienced a protracted 50 m.y. deformational history shortly after its formation at 2,490±8 Ma (Stein et al. 2004). In a recent study, Panigrahi et al. (2004) averaged U–Pb SHRIMP zircon data from a pooled set of samples from the Malanjkhand batholith to advocate a meaningless intermediate age of ~2,476 Ma for the Malanjkhand granitoid and its Cu–Mo–Au deposit. In the northwest part of the Malanjkhand batholith, Re–Os dating of occurrence-specific molybdenite captures not only the age of porphyry-style mineralization and associated magmatism, but also elucidates a complex deformational history that extends to ~2,450 Ma. In the central part of the Malanjkhand batholith, Re–Os dating of delicate spindles of accessory molybdenite occurring with pristine muscovite in miarolitic cavities within the undeformed microgranitoid at the Devgaon Mo prospect unequivocally shows that deformation ceased at this location no later than 2,470–2,465 Ma. The deformational history recorded at the Malanjkhand deposit in the northwest most likely reflects prolonged transpressive convergence and docking of the Malanjkhand terrane with units in the poorly understood (proto) Central Indian Tectonic Zone (CITZ) along its southern margin, the Central Indian shear zone. The timing for this convergence is Late Archean–Early Paleoproterozoic.Comment on “Age of granitic activity associated with copper–molybdenum mineralization at Malanjkhand, Central India” by Panigrahi MK et al. (Mineralium Deposita 39:670–677)     

Early Pliocene Paleoceanography of the Vityaz Fracture Zone, Central Indian Ridge

Planktic foraminifera from the calcareous substrate of a ferromanganese crust in the Vityaz Fracture Zone （VFZ）, Central Indian Ridge were studied to reconstruct the early Pliocene paleoceanography of this region. Eleven species of planktic foraminifera were encountered, among them Globorotalia menardii, Neogloboquadrina dutertrei, Globigerina bulloides and Globigerinoides tuber are prominent. Predominance of N. dutertrei in the top 3 cm of the carbonate substrate is attributed to an influx of fresh water which eventually triggered their productivity by increasing the nutrient level. The presence of G. bulloides and G. menardii in significant proportions in deeper layers suggests the prevalence of open ocean upwelling. The bulk chemical compositions of the substrate at different depth intervals indicates higher enrichment of trace metals in the upper sections which could have been supplied through oceanic water by the chemical weathering of terrestrial matter during the peak of Pliocene Asian monsoon. Thus, it is concluded that during the early Pliocene the biogenic components of the substrate were distinctly contributed by both upwelling and productivity triggered by an influx of fresh water originating from the intensification of the Asian monsoon during the early Pliocene Period.