Norman Kennedy Grant

South Atlantic, Benue Trough, and Gulf of Guinea Cretaceous Triple Junction

During the early separation of Africa from South America, the South Atlantic and the transform faults of the Gulf of Guinea were joined by a short-lived line of lithosphere spreading running northeast from the Niger delta, under the Benue trough. The separation of the continents was established by Aptian times, and the spreading under the Benue trough lasted from the Albian to the Santonian. The early separation of West Africa from northern Brazil was characterized by lateral movements of the two continents against each other along the St. Paul9s and the Romanche oceanic transform faults. Some 20 m.y. of transform fault motion may have elapsed before West Africa was completely separated by oceanic crust from northern Brazil. The South Atlantic, Benue trough, and the Gulf of Guinea formed an unstable RRF triple junction, which may have caused internal strain in the African plate. It may also have resulted in possible dilation of the Gulf of Guinea transform faults which, together with the short intervening ridge segments, served to localize the Cretaceous volcanicity thought to be responsible for the recently discovered North Brazilian Ridge.

Potassium-argon ages and strontium isotope ratio measurements from volcanic rocks in northeastern Nigeria

The Cenozoic volcanic activity in northeastern Nigeria began with the intrusion into the Benue trough of a trachyte-phonolite suite of plugs 22-11 m.y. ago. Later activity, which was more widespread and dominantly basaltic in character, began some 7 m.y. ago and has continued until very recent times. It resulted in basaltic plugs and lava plateaux within the Benue trough, and cinder cones and lavas on the Jos Plateau.The initial 87Sr/86Sr ratios of nine of the fifteen analyzed basic and alkalic rocks lie in the range 0.7025–0.7032, and the highest ratio measured is 0.7129.The main group of trachytes and phonolites are considered to be fractional melts derived from the upper mantle, modified in small part by potassium feldspar crystal fractionation. Two Sr-rich phonolite plugs may have a separate origin from the main group of trachytes and phonolites.The Cenozoic volcanic activity in northeastern Nigeria is probably related to the nearby Cameroun volcanic line. The concentration of plugs within the Benue trough may reflect internal adjustments along old lines of weakness within the African lithosphere plate, in response to synchronous changes affecting the plates external dimensions and internal structure, such as the growth of the Red Sea and the Gulf of Aden, and the volcanism of the east African rift valley.

Geochronology of Precambrian basement rocks from Ibadan, Southwestern Nigeria

Abstract A Sb-Sr isochron age of 2205 ± 70 my given by a granite gneiss demonstrates that Eburnean granitic and older metamorphic rocks are widespread in southwestern Nigeria. In the Ibadan area the reactivation due to the Pan-African thermo-tectonic event (600 ± 150my) appears to have been largely thermal in character. Its termination some 480 my ago is dated by K-Ar ages on separated minerals and on one member of an unmetamorphosed basalt dyke suite. The host rocks to the Eburnean granite gneiss may have crystallized during the 2800 ± 200 my Liberian cycle.

The structure and sequence of geological events in the Basement Complex of the Ibadan area, Western Nigeria

Abstract The Basement Complex in Western Nigeria in general, and in the Ibadan area in particular, is composed primarily of a banded gneiss in which hornblende-biotite rich bands alternate with quartz-oligoclase rich bands. The banded gneiss, which originated as part of a sedimentary sequence, contains large lenses of granite gneiss and thin intercolated layers of quartzite and amphibolite. Two distinct major structural events can be clearly identified in the early geological history of the Ibadan area. Detailed field studies suggest that the formation of the Ibadan Granite Gneiss, which had yielded an Eburnean Rb-Sr isochron age, was associated with the later of these events. Five phases of dyke or vein formation, two of which pre-date the formation of the Granite Gneiss, have also been identified giving an overall sequence of geological events the first of which may correspond to the beginning of the Liberian orogeny, around 3000 m.y. ago, and the last of which reflects the waning of the Pan-African thermo-tectonic event about 500 m.y. ago.

Dissociation of somatostatin effects. Peptides inhibiting the release of growth hormone but not glucagon or insulin in rats

Abstract Two analogs of somatostatin were tested for their effects on release of growth hormone, glucagon, and insulin after subcutaneous injection into rats. These peptides significantly suppressed pentobarbital-stimulated growth hormone release but showed no effect on arginine-stimulated glucagon or insulin release at dosages greater than 2 mg/kg. Somotostatin acts on all three secretions at dosages below 200 μg/kg.

Orogeny and Reactivation to the West and Southeast of the West African Craton

The term “Pan-African thermotectonic event,” or “orogeny,” has been applied by Kennedy (1964, 1965) to a shield-wide series of metamorphic belts with ages in the range 450–700 m. y., which wrap around and enclose older undisturbed cratonic Precambrian complexes (Fig. 1). This reticulate group of belts has been recognized principally by mineral radiometric ages, which for most part give little information on the earlier history of the belts. In some cases they include folded and metamorphosed equivalents to cratonic sediments and crystalline basement rocks, but in others they consist of metasedimentary, gneissic, and granitic rocks of largely unknown age.

The isotopic composition of strontium and oxygen in lavas from St. Helena, South Atlantic

Strontium and oxygen isotope measurements on the alkali basalt-trachyte-phonolite suite of St. Helena show that some of the late-fractionated rocks are enriched in 87Sr and depleted in 18O relative to the older basalts. The data rule out both the formation of the late-fractionated rocks by the partial melting of hydrothermally altered oceanic crust and the contamination of the volcanic rocks by oceanic sediment. It also appears to be incompatible with models based either on the melting of previously fractionated and crystallized liquids in the volcanic pile, or the long-term fractionation of lavas over several millions of years in a sub-volcanic magma chamber. It is concluded that hydrothermal interaction with meteoric water is the most important cause of the 18O depletion. If the interaction occurred at widely differing temperatures, and involved meteoric and seawaters, it might conceivably have caused both the oxygen and strontium isotope heterogeneities.