Kasila Group Research Articles

Substitution of Kasila Group Basalt with the Archean Man Gneiss in Asphalt and Hydraulic Concrete Mix Design (Sierra Leone)

The study of the performances of the Archean of Man gneiss aggregates with the addition of filler to replace the basalt of Kasila group in the asphalt and concrete mix design of southern Sierra Leone is presented in this document. The goal is to compare the results of the asphalt and concrete mix design with gneiss and basalt aggregate. The applied methods and design used are 1) Volumetric design and Marshall method for the asphalt, 2) French Dreux-Gorisse Method for the concrete. We added 2% of gneissic filler and 2% portland cement type 42.5 R to the asphalt hot mix with the gneiss aggregates to follow the criteria variation. The Marshall, the diametric compression and the Duriez tests require us to perform four different types of mix design. The four mix designs meet the requirements but F2 and F4 give the best mechanical properties. F2 (gneiss + 2% filler) and F4 (basalt) have many similarities from which we can conclude their interchangeability. F2 gives 5255 of optimal bitumen content. In regards to hydraulic concrete, the results of the compressive strength test (cement content 350 kg CMI 42.5 R/m3) with the gneiss and basalt aggregates are respectively 40 MPa and 45 MPa at 28 days curing: these values are greater than 35 MPa required by the technical specifications. The use of the Super Fluid &#174 Thermoplast 120 admixture, to increase the concrete compressive strength, is justified by the requirement of a minimum of 80% Rc28 at 24 hours. For both types of concrete, we have at 24 hours, 34 and 35 MPa which are higher than the minimum of 32 MPa (in 24 h). These results meet the requirements of the technical specifications.

New insights on proterozoic tectonics and sedimentation along the peri-Gondwanan West African margin based on zircon U–Pb SHRIMP geochronology

New mapping and age data from rocks of the West African Craton and its western margin, as well as a review of published age data for the region indicate that the southwestern margin of the craton is composed of an Archaean basement of TTG gneisses and granitoid rocks that formed over a long period of time between 3.54 and 2.64Ga. The age data refute the previously held notion of two craton-forming events, the ∼3.0Ga Leonian and ∼2.7Ga Liberian cycles, but instead indicate a pulse at ∼3.4Ga, followed by near-continuous crustal growth between ∼3.05 and 2.64Ga. Age data for the Kenema Assemblage, a strip of Archaean rocks separated from the main Archaean exposure further east by Neoproterozoic cover, suggest that this represents a tectonic window.To the west of the Archaean craton, the Rokel-Kasila Belt occurs, which incorporates two tectonic terranes: the Palaeoproterozoic Kasila terrane, accreted to the Archaean craton and comprised of granulite-facies paragneiss units of the Kasila Group, and the Meso- to Neoproterozoic Marampa terrane, thrust on top of the Archaean basement, and comprised of greenschist-facies schists and metabasites of the Marampa Group. A metavolcanic unit in the Kasila Group provides an age of 1941±4Ma interpreted to date the emplacement of the Kasila succession. Xenocrystic zircon in the sample suggest the presence of cryptic older crust with components of 2.7–2.6Ga and 2.2Ga, and the Kasila Terrane is interpreted to represent a Ganderian-type peri-Gondwanan terrane left attached to the West African Craton. Detrital age data on two schist units of the Marampa Group suggest a maximum age of deposition of between 1076–1030Ma. The detrital age peaks indicate source terranes in part consistent with a West African affinity, but also comprising significant sources of between 2.0 and 1.0Ga for which no suitable source terranes are known in West Africa. We suggest that at the time the Marampa Group was deposited along the West African margin at ∼1.05Ga, a source terrane with significant matching components of Palaeoproterozoic and Mesoproterozoic source rocks, was present to provide the sedimentary input, possibly the Amazonian Craton.

The Geology, Mineral Resources of Sierra Leone and how the Resources can be Used to Develop the Nation

Sierra Leone forms part of the West African Craton whose counterpart is the Guyana shield. Two main structural divisions are recognized; (a) the Liberian granite-greenstone terrain and (b) the pene-contemporaneous Kasila group mobile belt. Radiometric ages from both divisions show a spread from 2100 Ma to over 3400Ma. The rocks in the country are predominantly Archaeanconsisting of a granitic basement containing elements of sedimentary, mafic formations and a group of supracrustal greenstone belts with banded ironstones and detrital sediments. In common with most Archaean terrains, the country has considerable mineral deposits and reviews of these deposits have been based on similarities between the Archaean of Sierra Leone and that of the Superior province in Canada or the Rhodesian craton in South Africa. In this study, the country's primary mineral resources which are diamonds, rutile, gold, bauxite, and iron oreare discussed. The production or mining of these resources contributed about 20% of GDP and up to 15% of fiscal revenues until the closure of some mines before the civil war and the others during the war. It is believed that if the economy of this post conflict nation is to grow stronger the mining industry will have to serve as an engine to its economic growth.Four recommendations have been proposed on how current and prospective Sierra Leonean governments will achieve their developmental objectives using revenue generated from the mining of these mineral resources.

Oxygen isotopic systematics of an open-system magma chamber:: An example from the Freetown Layered Complex of Sierra Leone

The Freetown Layered Complex of Sierra Leone is a 7-km thick, rift-related tholeiitic intrusion that formed ∼193 Ma ago during the opening of the middle Atlantic Ocean. The Complex was emplaced above Archean basement gneisses, granulites, and schists of the Kasila Group at pressures of 2.8 to 5.1 kbar. The δ 18O values of bulk cumulates (5.5–6.7), separated plagioclase (5.7–6.0), olivine (5.0–5.7), and clinopyroxene (5.1–5.8) spanning the entire stratigraphic section indicate that the Complex has preserved its primary magmatic oxygen isotopic composition. The δ 18O values of whole-rocks are uniform in Zone 1 (average = 5.74 ± 0.01) but vary widely in Zones 2 and 3 (average = 6.18 ± 0.52 and 5.71 ± 0.32, respectively). Variations in whole-rock δ 18O with stratigraphic height correlate well with plagioclase mode, with δ 18O values being highest in the strongly laminated anorthosites of Zones 2 and 3, and lowest in olivine gabbro from the cyclically layered subzone of Zone 3. The overall pattern of oxygen isotopic variation with stratigraphic height in the intrusion appears to the related to the accumulation of high-δ 18O, plagioclase-rich rocks that are overlain by low-δ 18O, olivine or pyroxene-rich rock types. Fractional crystallization in combination with mineral accumulation can explain the observed oxygen isotopic variations in the Complex. Oxygen isotopic thermometry yields equilibration temperatures of 1040 to 1290°C, which are similar to temperatures (1045 to 1381°C) estimated from the plagioclase-liquid thermometer applied to the Freetown bulk magma for each zone obtained by geochemical summation. The oxygen isotopic temperatures are highest where major influxes of new magmas have occurred, indicating negligible subsolidus resetting of mineral compositions. Contemporary diabase dikes intruding the Complex have whole-rock and plagioclase δ 18O values of 6.6 and 6.7, respectively, suggesting minimal interaction of the dikes with hydrothermal fluids. Archean gneisses from the Kasila Group have widely varying δ 18O values of 6.2 to 11.4 for whole rocks and ∼6.8 for quartz. The whole-rock δ 18O values for the gneisses suggest that the Kasila Group may have been selectively altered by hydrothermal fluids during retrograde (M3) metamorphism associated with the Pan African event. However, the Kasila appears not to have significantly exchanged oxygen with the Freetown magma. The calculated average δ 18O value for the original Freetown magma is 6.0 ± 0.2, which is similar to the values reported for magmas parental to the Kiglapait, Stillwater, and Duluth Complex intrusions.

Emplacement and reworking of the Marampa Group allothchon, northwestern Sierra Leone, West Africa

The structural evolution and relative age of the Precambrian Marampa Group, a 60 km wide north-northwest trending fold thrust belt is described in detail. The Marampa Group is shown to be unconformably overlain by the Rokel River Group which lies immediately to the east and is separated by a major crustal shear zone from gneisses and amphibolites of the Kasila Group to the west. Previous workers have interpreted the fold thrust belt as a klippe of the adjacent Kasila Group derived from the west or as an autochthonous volcano-sedimentary deposit engulfed by granitic. basement. Ages ranging from 500 to > 2700 Ma have been suggested. Evidence is presented to show that the important deformation of the Marampa Group clearly predates the deposition of the Rokel River Group and must represent a significant earlier orogenic event. Constraints on the relationship of this older deformation to the 2700–2750 Ma deformation of the Kasila Group are discussed. The earliest structures consist of flat lying thrusts which transported Marampa Group metasediments, with or without their basal metavolcanic formation, eastward from their source basin over the basin margin and onto a flanking heterogeneously deformed older granitic gneiss basement. Subsequent intrusion of megacrystic, now porphiyroclastic granites was followed by a major period of crustal extension during which sediments and volcanics of the Rokel River Group were deposited in rift basins. Renewed east-west crustal shortening ascribed to the Pan-African event inverted earlier extensional structures thrusting the Rokel River Group westward over -the Marampa Group and leading to local facing confrontations where east dipping faults were reactivated. The relationship of the Marampa Group to the greenstone belts of Guinea, Liberia and Sierra Leone remains unresolved.

A Tectonic Linkage between the Rodelide Orogen (Sierra Leone) and the St. Lucie Metamorphic Complex in the Florida Subsurface

The St. Lucie Metamorphic Complex comprises part of the non-Laurentian, Paleozoic Suwannee terrane beneath the Gulf and Atlantic Coastal Plains of the southeastern United States. Hornblende concentrates prepared from cuttings from two deep test wells penetrating the complex display internally concordant $$^{40}Ar/^{39}Ar$$ incremental-release spectra defining plateau ages of $$510.8 \pm 1.1 Ma$$ and $$513.1 \pm 1.8 Ma$$, which are interpreted to date post-metamorphic cooling through temperatures required for intracrystalline argon retention. The Kasila Group constitutes the western segment of the Rokelide orogen in Sierra Leone. Four hornblende concentrates prepared from amphibolite within the Kasila Group yield $$^{40}Ar/^{39}Ar$$Ar plateau ages of $$505.0 \pm 5.2$$, $$508.2 \pm 2.1$$, $$510.5 \pm 2.6$$, and $$546.1 \pm 6.8 Ma$$. They are interpreted to date post-metamorphic cooling through appropriate argon closure temperatures following a ca. 550-560 Ma, Pan-African II phase of tectonothermal activity....

Geology and mineral chemistry of the Bantoro Leucogabbro, Kasila group, western Sierra Leone

The Bantoro Leucogabbro, one of a number of coarse grained, inhomogeneously deformed and highly metamorphosed sheet-like Archaean intrusions, lies within the Kasila Group supracrustals, a linear zone of high grade metabasites bordering the West African Archaean Craton. Mineral and textural analyses indicate metamorphic recrystallisation of an original mineralogy of bytownite, orthopyroxene, clinopyroxene and pargasite that probably precipitated from a hydrous basaltic magma. Exchange reaction between melt, amphibole and plagioclase occurred during crystallisation of the magma. Tectonic burial to a depth of 30 km and associated deformation at high pressure and temperature induced partial metamorphic equilibration before thrusting brought the leucogabbro to its present position.

Late Proterozoic to Early Palaeozoic supracrustal succession of Sierra Leone: an aulacogen at the western margin of the West African Craton

Three parallel zones of Upper Proterozoic to Lower Palaeozoic sedimentary-volcanic cover rocks have been recognized in western Sierra Leone. The Kasila Group m the SW ist metamorphosed to the granulite facies and grades into the Marampa Schist Group, and this in turn grades into the unmetamorphosed Rokel River Group in the NE. Prior to this investigation, these rocks were thought to belong to different ages; the most highly metamorphosed was thought to be the oldest and the unmetamorphosed, the youngest. This view is unrealistic since field mapping has failed to reveal any sharp contacts between them and all available age data indicate their involvement in the Pan-African (ca. 550 Ma) Orogeny A model is proposed involving an intracontinental graben (aulacogen) extending from western Sierra Leone into the adjacent territories developed about 1000 Ma ago. During the Pan-African Orogeny a portion of the aulacogen fill overlying the remobilized basement was subjected to a regional metamorphism and deformation. The grade of metamorphism and the degree of deformation decreased steadily from the centre of the Pan-African domain to the contact with the West African craton. The unmetamorphosed and mildly deformed to undeformed equivalent rests on the craton.

The archaean kasila group of Western Sierra Leone: Geology and relations with adjacent granite-greenstone terrane

The Kasila Group, an Archaean highly metamorphosed supracrustal succession composed of basaltic igneous, clastic and chemical sedimentary rocks, outlines the western border of the granite-greenstone terrane of the West African Craton in Sierra Leone. The contact between the two tectonic units is characterised by intense overthrust ductile shear deformation operating during the late Archaean at high metamorphic grade, followed by increasingly brittle shear motions of the same sense indicating prolonged eastwards directed overthrusting subsequent to the formation of the adjacent granite-greenstone terrane to the east. The Kasila Group, poor in granitic intrusives, is unlike Archaean granite-greenstone and high-grade gneiss terranes and may represent the deep-seated remains of a telescoped supracrustal succession, perhaps resulting from the collision and suturing of Archaean terranes now represented by the West African Craton and the Guyana Shield.

Structural terranes and their relationships in Sierra Leone

Sierra Leone, composed mainly of Archaean granite-greenstone terrane, is bounded in the west by a westward dipping zone of intense, ductile, simple shear deformation which produced very fine-grained, high grade rocks. This zone has been interpreted as a possible Archaean suture developed following the collision of the Guyana Shield and the West African Craton. Granulite facies metamorphic supracrustals of the Kasila Group occur to the west of the sheared zone. Marampa Group lower grade metamorphics were thrust eastwards during the collision event. Late Precambrian rifting, well to the east of the mylonite zone and subsequent compression, preserved very low grade to unmetamorphosed Rokel River Group sediments and volcanics. Limited Pan-African tectonic transport of Archaean and late Precambrian material was again toward the east. All structural and stratigraphic units can be traced northward into Guinea where they disappear beneath the Paleozoic sediments of the Bové Basin. To the south, the Kasila Group, the granite-greenstone terrane and the mylonitized zone can be traced into Liberia. The Gibi Mountain Formation of Liberia is probably laterally equivalent to the lower portions of the Rokel River Group. This interpretation of the geology of Sierra Leone differs greatly from that of Guinea where the mylonitized zone, associated with a positive gravity anomaly, has been interpreted as a suture zone resulting from Pan-African continent-continent collision.

“Exposed cross-sections through the continental crust: implications for crustal structure, petrology and evolution”, by D.M. Fountain and M.H. Salisbury—a discussion

Fountain and Salisbury [1] examine geological and geophysical evidence for the nature of the deep continental crust, and emphasise the significance of deep crustal profiles uplifted and exposed along major overthrusts such as the Ivrea and Strona zones (North Italy), Pikwitonei belt (Manitoba), Kasila Group (West Africa) and Fraser and Musgrave Ranges (Australia). On the other hand these authors state that Archaean terrains not usually expose a cross-section through the crust and therefore do not provide data concerning changes in the crust with depth [1, p. 263]. To explain the overthrusts the authors refer to possible obduction, subduction and continental collision processes. The authors view Precambrian continental evolution in terms of repeated crustal recycling and reworking. The purpose of this comment is to point out evidence for (1) the exposure of vertical crustal sections in Archaean cratons; (2) the stability and little resetting of large sectors of the crust since the Archaean, and (3) for limits on the applicability of Mesozoic /Cainozoictype plate tectonics to Precambrian situations. Seismic refraction studies in the Yilgarn [2] and Pilbara [3] blocks in Western Australia indicate regional dips of the intracrustal (Conrad type) and Moho discontinuities. In the Yilgarn and eastward slope is accompanied by a general decrease in metamorphic grade in this direction [4]. In the southern Indian Shield an overall northward

A shear-mobile transform belt in the Precambrian Gondwanaland of Africa — South America

An Archaean-Lower Proterozoic mobile belt can be recognized in the West African — Brazilian — Southern African portions of Gondwanaland extending from the Kasila Group of Sierra Leone, through the Goias Belt of Central Brasil to the Namaqua Mobile Belt of Namibia (KGN). This belt is characterised by high shear strain deformation and high grade metamorphism and may represent a transform-shear mobile belt. Shear movement and displacement along the KGN during the Precambrian has affected the older cratons and controlled major secondary tensional and shear belts which are now recognized as younger Proterozoic mobile belts.

The Archaean geology of Sierra Leone

Abstract The Archaean of Sierra Leone is divisible into two pene-contemporaneous tectonothermal units, the Liberian granite—greenstone terrain, and the Kasila Group Mobile Belt with its associated klippen, the Marampa Group. The granite—greenstone terrain consists of the Sula Group supracrustals, deformed and metamorphosed over an underlying gneissose granite migmatite Basement, yielding ages of about 2700 Ma. Late kinematic granites are found at the margins of the greenstone belts, and as high-level developments of the reactivated Basement. The Kasila Group is a larger scale supracrustal belt which developed into a zone of extreme orogenic activity now forming the southwest margin of the West African Archaean Craton.

African Archaean mobile belts and granite–greenstone terrain

ARCHAEAN granite–greenstone terrains such as the west African and Rhodesian Cratons, are frequently bordered by pene-contemporaneous, long-lived, high-grade mobile belts containing granulite facies supracrustals, enderbites, granite gneisses and anorthosites. For example, in Sierra Leone1, the granite–greenstone terrain contains low-grade supracrustal belts along NNE–SSW trends, while the discordant Kasila Group granulite facies belt trends NW–SE, and forms the coastal border of the West African Craton. A similar relationship is found between the Rhodesian and Kaapvaal Cratons, which are separated by the east–west trending Limpopo Belt2,3. The metamorphic, structural and lithological contrasts between mobile belts and granite–greenstone terrain have been ascribed to contrasts in heat-flow regime. I attempt here to correlate field and experimental data to elucidate Archaean mobile belt–craton relationships.

The Kasila Group, Sierra Leone, an interpretation of new data

Recent geological mapping and U/Pb age determinations show that the Kasila Group has many of the characteristics shown by the Limpopo Belt and other high-grade linear metamorphic belts. The Kasila Group appears to form the southwestern periphery of the dominantly low-grade West African Archaean Craton.

Liberian Age Province (about 2,700 m.y.)and Adjacent Provinces in Liberia and Sierra Leone

Research Article| December 01, 1971 Liberian Age Province (about 2,700 m.y.)and Adjacent Provinces in Liberia and Sierra Leone P. M HURLEY; P. M HURLEY Department of Earth and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 Search for other works by this author on: GSW Google Scholar G. W LEO; G. W LEO U.S. Geological Survey, Washington, D.C. 20242 Search for other works by this author on: GSW Google Scholar R. W WHITE; R. W WHITE U.S. Geological Survey, Denver, Colorado 80225 Search for other works by this author on: GSW Google Scholar H. W FAIRBAIRN H. W FAIRBAIRN Department of Earth and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 Search for other works by this author on: GSW Google Scholar Author and Article Information P. M HURLEY Department of Earth and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 G. W LEO U.S. Geological Survey, Washington, D.C. 20242 R. W WHITE U.S. Geological Survey, Denver, Colorado 80225 H. W FAIRBAIRN Department of Earth and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 Publisher: Geological Society of America Received: 03 Feb 1971 Revision Received: 29 Jun 1971 First Online: 02 Mar 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Copyright © 1971, The Geological Society of America, Inc. Copyright is not claimed on any material prepared by U.S. government employees within the scope of their employment. GSA Bulletin (1971) 82 (12): 3483–3490. https://doi.org/10.1130/0016-7606(1971)82[3483:LAPAMA]2.0.CO;2 Article history Received: 03 Feb 1971 Revision Received: 29 Jun 1971 First Online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation P. M HURLEY, G. W LEO, R. W WHITE, H. W FAIRBAIRN; Liberian Age Province (about 2,700 m.y.)and Adjacent Provinces in Liberia and Sierra Leone. GSA Bulletin 1971;; 82 (12): 3483–3490. doi: https://doi.org/10.1130/0016-7606(1971)82[3483:LAPAMA]2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Whole-rock Rb-Sr dating of rocks from the crystalline basement has disclosed an age province in Liberia and Sierra Leone of about 2,700 m.y. The approximate eastern boundary of this province with the adjoining Eburnean age province of about 2,000 m.y. has been found in eastern Liberia. Much younger rocks of PanAfrican age (about 550 m.y.) bound the ancient province in Sierra Leone and western Liberia in a belt adjacent to the coast. Groups of infolded metasedimentary and metavolcanic rocks have been tentatively identified as associated with two of these ages. The Kambui Schists of Sierra Leone show whole-rock Rb-Sr ages of about 2,700 m.y. The pelitic and iron-bearing metamorphic rocks in the Marampa Formation of Sierra Leone, and similar sections in the Nimba Range, Liberia, appear to be about 2,200 m.y. old, and may fall within the typical Eburnean age range. The age values are scattered, however, and the Nimba rocks may be equivalent t o the Kambui. The Kasila Group of Sierra Leone, and a coastal belt in Liberia believed to be a continuation of the Kasila, yield typical PanAfrican ages of about 550 m.y. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.