Petrology and the Western Rift of Central Africa

1932 ◽  
Vol 69 (11) ◽  
pp. 497-510 ◽  
Author(s):  
A. W. Groves
Keyword(s):  
Rift Valley ◽  
Volcanic Rocks ◽  
Central Africa ◽  
Volcanic Field ◽  
Thin Sections ◽  
Basement Rocks ◽  
Geochemical Study ◽  
Northern Half ◽  
Tectonic Significance ◽  
Near Future

IN the course of several journeys between Masindi and Butiaba, and one from Butiaba to Hoima (via Waki Camp), specimens of the various gneisses were collected from roadside exposures. Thin sections of these were subsequently studied by the writer in England with the result that they were found to yield evidence of tectonic significance. In addition, the writer has, during the last three years, examined petrographically a large number of other rocks from those parts of Uganda bordering the Lake Albert Depression. In particular he has made a special study of the Charnockite Series, which are widely spread over the northern half of the Protectorate, with the result that it has been possible to observe the effects of the rift valley movements on these rocks in the regions adjacent to Lake Albert. It is hoped to publish a petrological and geochemical study of the Charnockite Series of Uganda in the near future. In the following pages it is proposed in Part I to present petrological evidence from the basement rocks of various districts in the hinterland of the Lake Albert scarps and then in Part II to point out the tectonic significance of this evidence in the light of the various theories of rift valley formation. In Part III the relation between the petrological characters of the volcanic rocks and the rift valley tectonics will be briefly reviewed. In the case of the basement rocks the remarks will be confined to the Lake Albert Depression, but in the case of the volcanic rocks the discussion will extend also to Lakes George and Edward and the Birunga volcanic field.

Contributions to the Petrology of British East Africa

1903 ◽  
Vol 13 (61) ◽  
pp. 228-263 ◽  
Author(s):  
G. T. Prior
Keyword(s):  
East Africa ◽  
Rift Valley ◽  
Volcanic Rocks ◽  
Central Africa ◽  
British Museum ◽  
Basaltic Rocks ◽  
Basement Rocks ◽  
Great Rift Valley ◽  
Rock Specimens

The following notes on the petrology of British East Africa are the result of an examination of rock-specimens collected by Professor J. W. Gregory on his well-known expedition from Mombasa to Mt. Kenya and Lake Baringo in 1892-3, and of rock-collections from the Uganda Protectorate which have been recently presented to the British Museum by Sir Harry Johnston.The collections include examples of the Archaean gneisses, schists, and granites which constitute the prevailing basement rocks of Central Africa; of ferruginotls schists, coarse sandstones, and quartzites belonging to the Palaeozoic Karagwe series ; and of an interesting series of Tertiary volcanic rocks comprising phonolites, phonolltic traehytes, riebeckite-rhyolites, kenytes, and basalts from the volcanoes of the Great Rift Valley, as well as of nephelinites and basaltic rocks containing melilite and perofskite from Mr. Elgon.

Geology of the Springdale Group: a newly recognized Silurian epicontinental-type caldera in Newfoundland

1987 ◽  
Vol 24 (6) ◽  
pp. 1135-1148 ◽  
Author(s):  
Marylou Coyle ◽  
D. F. Strong
Keyword(s):  
Large Scale ◽  
Basaltic Magma ◽  
Volcanic Rocks ◽  
Sedimentary Facies ◽  
Volcanic Field ◽  
Silica Content ◽  
Large Area ◽  
Basement Rocks ◽  
Iapetus Ocean ◽  
High Silica

Volcanic–sedimentary facies and structural relationships of the Silurian Springdale Group in west-central Newfoundland are indicative of a large collapse caldera with an area of more than 2000 km2. Basaltic flows, andesite flows and pyroclastic rocks, silicic ash-flow tuffs, high-silica rhyolite domes, and volcanically derived debris flows and breccias, fluviatile red sandstones, and conglomerates make up the group. It is bounded on the east and west by up-faulted basement rocks, which include gneisses, amphibolites, and pillow lavas, and in the northwest it unconformably overlies Lower Orodovician submarine volcanics. These margins are intruded by cogenetic and younger granitoid rocks. The volcanic rocks form a calc-alkaline series, although gaps in silica content at 52–56, 67–68, and 73–74% separate them into four groups: basalts, andesites–dacites, rhyolites, and high-silica rhyolites.The high-silica rhyolites are chemically comparable to melts thought to form the upper parts of large, layered silicic magma chambers of epicontinental regions. Such an environment is also suggested by the large area of the Springdale caldera and the fact that it is one of a number of calderas that make up a large Silurian volcanic field in western Newfoundland. An epicontinental tectonothermal environment for central Newfoundland in Silurian–Devonian times is readily explained by the fact that this magmatic activity followed a period of destruction and closure of the early Paleozoic Iapetus Ocean, with trapped heat and basaltic magma causing large-scale melting of thickened and subducted continental crust in an overall transpressional tectonic regime.

Petrochemistry and tectonic significance of Carboniferous volcanic rocks in New Brunswick

1986 ◽  
Vol 23 (9) ◽  
pp. 1243-1256 ◽  
Author(s):  
L. R. Fyffe ◽  
S. M. Barr
Keyword(s):  
New Brunswick ◽  
Tectonic Setting ◽  
Volcanic Rocks ◽  
Age Groups ◽  
Late Paleozoic ◽  
Basement Rocks ◽  
Basaltic Composition ◽  
Tectonic Significance ◽  
Maritimes Basin ◽  
Alkalic Basalts

Carboniferous volcanic rocks from the New Brunswick Platform in the Maritimes Basin are divided into three age groups. Late Tournaisian to early Visean volcanic rocks are tholeiitic basalts and andesites that, in southern New Brunswick, are inter-bedded with abundant calc-alkalic rhyolite. Late Visean to Namurian volcanic rocks consist of an interbedded sequence of alkalic basalts and trachyandesites. Late Westphalian volcanic rocks change in composition up section from trachyte to peralkalic rhyolite. All three age groups display petrochemical features indicative of an intraplate tectonic setting. The volcanic geochemistry is consistent with the development of the Maritimes Basin either as a failed rift formed along the margin of a late Paleozoic ocean or as a rhomb graben formed within a transcurrent zone; the former model is preferred. The change in basaltic composition from tholeiitic to alkalic apparently coincided with a decrease in rate of extension between the Tournaisian and Namurian. Local peralkalic volcanism occurred during regional sagging of the basin as extension ceased and basement rocks cooled in the Late Carboniferous.

scholarly journals Geochemical characteristics of Precambrian Basement rocks from southeastern Nigeria: an approach to their petrogenesis and tectonic setting.

2020 ◽  
Vol 18 ◽  
pp. 1-13
Author(s):  
CHINEDU UDUMA IBE
Keyword(s):  
Partial Melting ◽  
Tectonic Setting ◽  
Southeastern Nigeria ◽  
Eu Anomaly ◽  
Basement Rocks ◽  
Geochemical Study ◽  
Tectonic Significance ◽  
Granitic Intrusions ◽  
Ree Patterns

Trace and Rare-Earth element geochemical study of twenty samples of migmatitic banded gneisses, garnet biotite schists, dolerites, granites and rhyolites was carried out in a bid to determine their petrogenetic and tectonic significance in the evolution of the southeastern Basement complex of Nigeria. The data shows that partial melting (crustal anatexis) of migmatitic gneisses and schists played a significant role in the evolution of the granitic intrusions. This is supported by the high incompatible (Rb/Sr = 0.16 to 1.31 and Ba/Sr = 0.75 to 6.21) elements ratio in the granitic intrusions than that of the migmatitic gneisses and schists (Rb/Sr, 0.051 to 0.824; Ba/Sr, 0.7 to 5). High ratios of Ba/Sr and Rb/Sr and lesser values of Ba/Rb ratios in some granitic intrusions than in others suggests increasing fractionation during the anatexis. The role of partial melting is also evident in the smooth REE patterns shown by most of these rocks and the negative Eu anomaly as indicated by the values of Eu/Eu* (0.097 to 0.7). LREE enrichment is evident in the high values of Ce/YbN (12.08-174.5), La/YbN (15.2-228.4) and La/SmN (2.6-7.2) in the granitic intrusions. Tectonic discrimination diagrams of the rocks indicate that the basement rocks were most probably formed in a post-collision orogenic setting while the dolerite and the rhyolite were formed in within-plate anorogenic setting.

Early Cretaceous volcanic rocks and Early Cenozoic extrusions of Cape Mary, Schmidt Peninsula, north Sakhalin: Geochemical study

2007 ◽  
Vol 1 (3) ◽  
pp. 265-275 ◽  
Author(s):  
V. P. Simanenko ◽  
S. V. Rasskazov ◽  
T. A. Yasnygina ◽  
A. I. Malinovskii ◽  
A. A. Chashchin
Keyword(s):  
Early Cretaceous ◽  
Volcanic Rocks ◽  
Geochemical Study ◽  
Early Cenozoic

scholarly journals Mineralogical and Geochemical Trends in the Residual Soils above Basement Rocks in Ore Area, Southwestern Nigeria

2017 ◽  
Vol 9 (3) ◽  
pp. 42
Author(s):  
Adewole John Adeola ◽  
Emmanuel Tamunobelema Tubonemi
Keyword(s):  
Inductively Coupled Plasma ◽  
Soil Layer ◽  
Clay Mineralogy ◽  
Lateritic Soil ◽  
Residual Soils ◽  
Southwestern Nigeria ◽  
Thin Sections ◽  
Tropical Regions ◽  
Basement Rocks ◽  
Parent Rocks

Residual clays and laterite of economic values often occur within weathering profiles above basement rocks in tropical regions due to supergene enrichment and leaching of liable components. Previous studies in Ore area mainly on geochemistry of the basement rocks with scanty information on the weathered profiles. This study was carried out to determine the compositional characteristics of the basement rocks, the geochemical trends within the profiles above the parent rocks and the evaluation of their economic potentials.Petrographic study was carried out on thin sections of the rock samples. Elemental compositions of the rocks, clay, laterite, and top-soil were determined using Inductively Coupled Plasma-Mass Spectrometer (ICP-MS). Clay mineralogy was determined using X-ray Diffraction (XRD) analysis. Chemical index of Alterations (CIA) was calculated from geochemical data.Weathering of granite and banded gneiss in Ore resulted in the formation of soil layer, which ranged 0-0.5m, laterite 1.2-3m and clayey zone 2.9-3.0m. Quartz, plagioclase feldspars, microcline, biotite and hornblende were the essential minerals in the parent rocks. Granite and banded gneiss is high SiO2 (>65%) but low in MgO (<2.0%) and CaO (<4.0%). Kaolinite (60-80%), goethite (3-12%) and microcline (4-10%) were the dominant minerals in the XRD of the weathering profiles. Traces of illite were present only in granite. The CIA was generally > 85 indicating advanced state of weathering producing lateritic soil. The lateritic profiles over granite and banded gneiss of Ore area varied with the composition of the parent rocks. The clay layer has economic potential for ceramics, fertilizer and structural wares.

Mineralogical characterization of fibrous zeolites from the Kahrizak volcanic suite, south Tehran, Iran

Clay Minerals ◽  
2010 ◽  
Vol 45 (4) ◽  
pp. 507-517 ◽  
Author(s):  
M. Kusehlar ◽  
F. Tutti ◽  
H. Mirnejad ◽  
A. E. Lalonde
Keyword(s):  
Volcanic Rocks ◽  
Hydrothermal Activity ◽  
Scattered Electron ◽  
Northern Iran ◽  
Mineralogical Characterization ◽  
Thin Sections ◽  
Rich Solution ◽  
Cavity Filling ◽  
Fibrous Zeolites

AbstractSelected samples of large cavity filling and vein-type fibrous zeolites from Eocene volcanic rocks in the Kahrizak region, northern Iran, have been studied for their mineralogical and chemical characteristics. X-ray powder diffraction and electron microprobe analyses confirmed the presence of natrolite, mesolite and scolecite with compositions of [Na14.922Ca0.202K0.015Ba0.002] [Al15.697Si24.267O80]·nH2O, [Ca15.714Na14.224][Al46.431Si73.398O240]·nH2O and [Ca7.804Na0.142K0.024Ba0.012Mg0.006][Al15.320Si24.437O80]·nH2O, respectively. In addition, examination of textural relationships in thin sections and back-scattered electron images reveals a paragenetic order in which the Ca-rich zeolites crystallized first. It is most probable that the fibrous zeolites of Kahrizak were formed during two pulses of hydrothermal activity in the area. Scolecite and mesolite were precipitated from Ca-rich solution, whereas the second stage Na-rich, low-temperature fluid crystallized natrolite and reacted with Ca-species.

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