Cretaceous – Cenozoic Magmatism and Volcanism

Chapter 3 Cretaceous – Cenozoic Magmatism and Volcanism Basaltic lava plateaus, trachyte plugs and domes, large central volcanoes and small basalt cinder cones with thin flows are all found among the more southerly mani- festations of Cenozoic volcanism in West Africa (Wright, 1985). This province also includes the remarkable offshore continuation of the Cameroon volcanic line, the four islands situated in the Gulf of Guinea itself. Areas of basement doming include the Jos Plateau, southeast of the Benue Trough, with probably still greater uplifts; and the Adamawa Highlands further east, where lavas of the Ngaoundere Plateau overlie the Ngaoundere fault zone, which was reactivated in the Cretaceous. The flood basalts of the Biu Plateau are situated on the Zambuk Ridge of the Upper Benue Trough, and the smaller Longuda Plateau lies near the bifurcation of the Upper Benue Trough, which has been identified as a possible secondary triple junction (Fig. 3.1). On the Jos Plateau, the earliest eruptions were basaltic lavas of the somewhat enigmatic Fluvio-Volcanic Series. These deposits form laterite-capped sheet and residual flat-topped hills at elevations varying from about 1,100 to 1,400 m, and consist of fluvio-lacustrine gravels, sands and clays, interbedded with or overlain by yellow and purple clays representing kaolinised and bauxitised basalt lavas. The laterite capping to these beds has been correlated with the laterite developed on the Paleocene Kerri–Kerri Formation of the Upper Benue Trough, such that the Fluvio- Volcanic Series may also be of Lower Tertiary age. The sediments and volcanics reach a maximum thickness of 300 m and were deposited in depressions on an ancient landscape of moderate relief, burying all but the highest hills. Basalts post-dating the fluvio-volcanics on the Jos Plateatu have been divided into Older and Newer Basalts on the somewhat subjective criteria of relative degrees of weathering and lateritization. Radiometric dating suggests that they are not older than 3 Ma and there was probably more or less continuous activity from late Pliocene to very recent time. Some of the cinder cones and their thin valley-filling flows look very young and there are all gradations to heavily eroded cones and to lava sheets and remnants without a discernible source. Several cones are aligned along presumed basement fracture trends. The youngest cones contain abundant ultramafic nodules and megacryst phase of upper mantle origin. Rounded fist-sized fragments of diopsidic pyroxene and magnesium (pyropic) garnet are especially striking. Other minerals include magnesium ilmenite, alkali amphibole and sodic N.G. Obaje, Geology and Mineral Resources of Nigeria, Lecture Notes 49 in Earth Sciences 120, DOI 10.1007/978-3-540-92685-6 4, C Springer-Verlag Berlin Heidelberg 2009 50 3 Cretaceous – Cenozoic Magmatism and Volcanism Fig. 3.1 Cretaceous-Tertiary igneous activities generally located within the Benue Trough in Nigeria (Captured from Wright, 1985) plagioclase. The most unusual megacryst phases occur in extensive basalt flows immediately south of the Jos Plateau, which contain large number of zircon and corundum crystals, many of gem quality (Wright, 1985). A single plug of trachytic phonolite on the Jos Plateau probably pre-dates the basalts. It is of particular interest for its rich content of upper mantle and crustal inclusions (peridotite, gabbro, anorthosite, syenite, granite) and large megacrysts of anorthoclase feldspar. In the Middle and Upper Benue Trough and in the Yola arm, plugs and domes (tholoids of trachyte and phonolite) form characteristically steep– sided hills. Samples of these rocks have given ages of between 12 and 22 Ma, plac- ing them in the Miocene. In the Benue Trough, basaltic volcanism probably did not begin until the end of the Miocene, when the Biu and Longuda Plateaus were built. The bulk of the Biu Plateau consists of Pliocene basalts, erupted from small vent or fissures and spreading thinly over wide areas. There are numerous small pyroclastic cones within the sequence, interpreted as the result of explosive eruption, caused by ground water percolating into lava pile and coming in contact with fresh lava (Wright, 1985). There was a break in activity at the end of the Pliocene, with weathering, erosion and laterite formation. According to Wright (1985) activities resumed in the Quaternary, in the form of thin valley-filling basalt flows from small cinder cones, some of which have very youthful aspects. These cones are also notable for their abundant inclusions of upper mantle peridotite and plentiful megacrysts of minerals similar to those found in Quaternary basalts on the Jos Plateau. The chronology of the Biu Plateau is well controlled by radiometric dating with sample of the plateau basalts falling in the 7–2 Ma range, the Quaternary basalts giving 1 Ma or less. The smaller Longuda Plateau to the southwest has not been dated, but its geomorphology suggests that it is also built of Pliocene basalts, though there are no later Quaternary cones there. In contrast, the area to the southeast of The Cameroon Line 51 the Biu Plateau consists entirely of Quaternary flows and cones aligned WNW – ESE, parallel to the boundary between basement and Yola arm of the Benue Trough. There are great numbers of basaltic necks and plugs and flow remnants among the Cretaceous sediments in the Middle and Upper Benue Trough, extending in to the Yola arm (Fig. 3.1). Although their physiographic aspect might suggest that they are similar in age to the phonolite and trachyte bodies, radiometric dating of some of them has given Pliocene ages, such that they are contemporaneous with the growth of the Biu and Longuda Plateaus. The Cenozoic volcanics of the Benue Trough can almost invariably be distinguished from the Cretaceous igneous rocks by their fresh and unaltered aspect, both in outcrop and in thin section. The Cameroon Line The main Cameroon Line extends from the islands in the Gulf of Guinea through Mount Cameroon and the Bamenda Highlands, and across the Yola arm of the Benue Trough towards Lake Chad. It is defined both by Lower Tertiary Younger Granite complexes and by the Cenozoic volcanics. An eastern branch diverges north of the Bamenda Highlands and extends to the Ngaoundere Plateau of the Adamawa High- lands, which lies on the Ngaoundere fault zone. Volcanism was spare along the northern third of the line, where domes, plugs and necks of phonolite, trachyte and basalt may belong to the Miocene and Pliocene episodes of the Benue Trough. The oldest lavas on the Bamenda Highlands are basalts that have been altered to multi-coloured clays resembling those of the Fluvio-Volcanic Series. Most of the volcanic activities in Cameroon was probably Pliocene, producing mainly basalts, but also flows and plugs of trachyte, rhyolite and subordinate phonolite in the Bamenda Highlands and the Ngaoundere Plateau. There are large basaltic and trachyte – rhyolite shield volcanoes with summit calderas in the Bamenda Highlands and basalt trachyte volcanism characterizes the Manenguba Mountains to the south (Wright, 1985). Signs of recent basaltic activity in several places show that volcanism persisted through the Quaternary, and Mount Cameroun is still active. It is the largest mountain in West Africa, rising some 4,000 m above sea level, built mainly of basaltic lavas and ashes, with cinder cones scattered about its flanks. The last significant eruption was in 1922, when lavas emanated from small cones on the upper slopes and reached the sea on the west side of the mountain. The most recent eruptions were in 1954, 1959 and 1982. The adjacent offshore island of Bioko (formerly Fernando Po) is also built mainly of basalts, probably mostly of Pliocene age. The more distant offshore islands are also formed mainly of basalt flows, overlain and intruded by phonolites and trachytes, sometimes followed by more basalts. Radiometric dates and field relationships suggest that volcanism commenced as long ago as the late Oligocene (c. 30 Ma), but at least some of the phonolites and trachytes are Pliocene (3–3 Ma). There is a striking resemblance between the distribution of the main 52 3 Cretaceous – Cenozoic Magmatism and Volcanism Pliocene-Quaternary volcanics in the Gulf of Guinea province and Y-shape of the Benue Trough, and the two patterns can be superimposed. When the South Atlantic opened around 100 Ma ago, the Benue Trough originated as the failed arm of the Gulf of Guinea triple junction. A zone of crustal attenuation and elevated heat flow asthenosphere, bifurcating in the northeast to form the Yola branch of the trough. Over this zone, the sedimentary basin developed, accompanied by some igneous activity. Analysis of magnetic anomaly patterns, transform faults and bathymetry in the south Atlantic has been used to suggest that, about 80 Ma ago, the African plate was rotated clockwise slightly. It is proposed that as a result the linear plume in the asthenosphere came to lie southeast of the Benue Trough and from its new position it subsequently generated the Cenozioc volcanism of the Cameroon Line. Controls of Cenozoic Magmatism The difficulties of relating magmatic activity of continental regions to the influence of long-lived stationary mantle plumes or hot spots were reviewed by Wright (1985). The basin and swell structure of the African continent began to take shape soon after the end of the Pan African event, as the continental crust and lithosphere adjusted to lateral inhomogeneities of structure and composition. Some parts became warmer and thicker and more buoyant than others, possibly in part because of regional vari- ations in the concentration of radioactive heat-producing elements. The swells have been the site of uplift and potential or actual igneous activities at intervals through- out the succeeding several hundred million years. Their positions are fixed with respect to the lithospheric plate itself and must move with it over the underlying mantle.

Load more