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Pyroclastics from the lower Benue trough of Nigeria and their tectonic implications
Journal of African Earth Sciences, Vol. 2, No. 4, pp. 351 to 358, 1984 Printed in Great Britain
1)731-7247/84 $3.00 + 0.00 © 1984 Pergamon Press Ltd.
Pyroclastics from the lower Benue trough of Nigeria and their tectonic implications M . HOQUE Department of Geology, University of Nigeria, Nsukka, Nigeria (Received 1 June 1984) Abstract--Two most controversial aspects of Nigerian geology are the petrology and stratigraphic position of the Abakaliki pyroclastics and the origin and evolution of the Benue trough. The pyroclastics are considered by some workers as lower Benue's oldest volcanic rocks, formed during the rifting of the Afro-Brazilian plate in early Cretaceous time that led to the origin of the Benue trough. The rocks were described variously as andesitic tufts. degraded alkali basalts, or spilites. This study shows that the deposits are discrete, elongate or oval shaped bodies, emplaced as valley fills. They show several types of primary structures, such as parallel lamination, cross-lamination, and graded bedding and have abundant mudrock xenoliths derived from older rocks of the Asu River group. The rocks are mostly lithic or scoriaceous lapillistone whose composition has undergone extensive alterations. The geochemical study indicates that the pyroclastics were derived from a silica-undersaturated alkaline magma. The rocks overlie unconformably folded Santonian and pre-Santonian formations. These and other findings lead to the conclusion that the pyroclastics are much younger (late Santonian) than the Benue trough (early Cretaceous) and are therefore unrelated to the event that led to the origin of the trough.
INTRODUCTION Two ~osT controversial topics of Nigerian geology are the petrology and stratigraphic position of the Abakaliki pyroclastics of southeastern Nigeria and the origin of the Benue trough. The two problems became interwoven when several workers of Nigerian geology attempted to use the petrologic data and the time of emplacement of the pyroclastics in constructing a model for the origin and evolution of the Benue trough. The pyroclastics are prominently exposed in and around the town of Abakaliki in southeastern Nigeria (Fig. 1). Isolated outcrops of pyroclastics have also been reported from other parts of the lower Benue trough. Around Abakaliki, however, they form the few prominent physiographic features in an otherwise flat, swampy and rice-growing region composed of monotonous shale sequences of the Asu River group of perhaps Neocomian age (Ramanathan and Kumaran 1981). The Abakaliki pyroclastics have variously been reported to be preAlbian or Aptian (Uzuakpunwa 1974). Olade (1979) considered the unit to be older than Albian Asu River shales and to be overlying the Precambrian basement. IX
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McConnell (1949) and De Swardt (1950) reported it to be interstratified with Albian shales. A post-Albian age was advocated by Tattam (1930), Farrington (1952) and Pergeter (1957). Pickering (1947, cited by Okezie 1965) thought it to be a Recent valley-fill. Okezie (1965) in an admirable report on these and other igneous rocks in the area argued in favour of a post-Santonian age. The rocks have also been described variously as pyroclastic flows, intrusive breccias, submarine spilites, andesitic tufts, basaltic agglomerates or degraded alkali basalts (McConnell 1949, Tattam 1930, Okezie 1957, 1965, Uzuakpunwa 1974, Olade 1979). Poor exposures, deep lateritic weathering and inadequate access to most of the areas prevented earlier workers from a sustained investigation of the rock and, as a result, exact petrology and tectonic significance of the pyroclastics have not been properly evaluated. Extensive highway construction in the region has recently exposed several outcrops and provided excellent opportunity to study the unit in a greater detail. This report is intended to present results of the study and evaluate tectonic implications of several of the findings.
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351
352
M.
HOQUE
S T R U C T U R E S AND P E T R O L O G Y OF P Y R O C L A S T I C S
The Abakaliki pyroclastic deposits show several types of primary structures. These include parallel lamination, cross-lamination, graded bedding and alternations of coarse- and fine-grained layers. There are also intricate small-scale recumbent folds, micro-faults and deformed laminae. Locally, beds are found to be inclined to as much as 35° with an azimuth unrelated to dip-azimuths of surrounding shale beds. Nowhere does the rock show any tectonic deformation. The deposits are usually discrete, elongate or oval shaped bodies from several meters to hundreds of meters in length, often overlain by a thick lateritized or weathered profile. Onion-like spheroidal weathering is quite common. Abundant xenoliths of mudrocks, siltstone and shale have been observed in many fresh exposures (Fig. 2). The size of xenoliths varies from tens of centimeters to a millimeter or less in length. Many shale or mudrock xenoliths retain their high angularity; such xenoliths may represent small residence time in, and also minimal reaction with, the magma which was perhaps ascending at a very fast rate due to its explosive character. The pyroclastic grains show a wide range of sorting and the sorting varies greatly from bed to bed. Some finely laminated rocks appear to be slightly better sorted than others. Some beds contain chaotic assemblages of fine ash to large blocks. Some very poorly sorted units are essentially breccia, or more appropriately may be called pyrobreccia (a term used in this study to distinguish the volcanogenic breccia from the breccia of sedimentary or tectonic origin). However, similar generally thick and poorly sorted units have been described by others as unwelded ignimbrites (Sparks 1976); they are basically pyroclastic flow deposits. Their poor sorting is attributed to high particle concentration. Some beds show normal grading. Each graded unit may be a centimeter or less to a few centimeters in thickness; they may alternate between a fine-grained and a coarse-grained graded unit. These graded units may be pyroclastic fall deposits. Occasionally some units show cross-lamination and ripple structures. These units are usually thin (several centimeters in thickness), finegrained and poorly sorted, and abruptly overlie a coarsegrained unit. Their lateral extent is usually limited to a few meters only. The thinly cross-laminated or rippled beds are thought to have formed by ground surges (also called pyroclastic surges) rather than by tractive sedimentary processes (Schmincke etal. 1973, Bond and Sparks 1976, Sparks 1976, Wohletz and Sheridan 1979). These primary structures which are products of pure volcanic processes have earlier been considered to have formed by reworking of pyroclastic materials in fluvial or marine environment (Olade 1975, 1979). Subaerial pyroclastic deposits are usually classified into three genetic types: 11) fall, (2) surge and (3) flow (Sparks and Walker 1973, Sheridan 1979). It appears that the Abakaliki pyroclastics represented all of them. New road-cuts and quarries reveal variable thick-
nesses of the deposit, ranging from a few meters to about a hundred meters. At several places, unambiguous unconformable contacts between the underlying Asu River shales and the overlying pyroclastics have been observed for the first time (Fig. 3). Away from the central part of the Abakaliki anticlinorium (which is composed of Albian or pre-Albian Asu River shales), pyroclastic bodies are found to be surrounded by the Turonian Eze-Aku Formation at the flanks [Fig. I(B)]. Xenoliths of micrite fragments, presumably derived from the Nkalagu limestone member of the Eze-Aku Formation, have also been identified within the pyroclastics. Thin-section study of samples collected from various locations indicates that they are mostly fine lithic or scoriaceous lapillistone. The matrix consists mainly of microlaths of plagioclase with glassy or aphanitic materials, giving it a hyalocrystalline texture. The original composition of the rock however has undergone extensive alteration in the form of albitization; development of quartz and carbonate veinlets as well as filling up of vesicular and scoriaceous fragments with mosaics of feldspars, calcite and quartz are some of the evidences of post-depositional alterations. Scanning electron microscopy reveals extensive devitrification, sericitization, and growth of zeolites and other secondary minerals. Large-scale secondary alterations of volcanic debris into microlites of plagioclase are quite conspicuous. The mineralogy of the rock can therefore be divided into two groups: the primary and the secondary. The primary ones are plagioclase, vitric fragments, opaques of irontitanium minerals and a few poorly preserved hornblende or pyroxene. Measurements of extinction angles in a few primary plagioclase suggest the composition to be about An30_35. Large xenocrysts of hornblende and plagioclase have been observed in one location (Ajaba quarry) situated at the southwestern end of the Abakaliki anticlinorium. The secondary minerals are either alteration products or cavity-fillings. These are calcite, zeolites, plagioclase, quartz and chlorite (penninites). Microlaths of plagioclase are randomly oriented in a groundmass of vitric ash. In a few cases, however, they seem to show flow banding. Microprobe analyses of a few of these microlaths show them to be albitic plagioclases (Table 1). Calcite is by far the most abundant of the cavity-filling and vein-filling secondary minerals. T a b l e 1. M i c r o p r o b e a n a l y s e s of p l a g i o c l a s e laths Oxides SiO, A126~ FeO MgO CaO Na~O K~O TiO, MnO Total Or Ab An
624AA 711.57 19.30 0.23 0.04 {}.01 11.85 {I.02 0.{}2 -102.04
99.70
6381 7{1.[11 19.56 0.48 0.22 {}.{14 11.39 {}.{}7 {}.{}2 {}.ill 1(}1.8(} 0.52 99.22 0.26
638M (}8.52 1938 0.62 0.32 {}.03 11.{}4 0.07 {}.02 . . . 10{}.0(} 0.42 99.47 0.11
638V
638U
69.13 19.59 {1.03 0.04 (}.{13 11.66 {}.{}4 0.{12 . . . . 100.54
69.79 19.98 0.113 0.02 0.04 11.32 {/.05
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0.53 99.15 0.32
Pyroclastics of the lower Benue trough
Fig. 2. Layered mudrock xenolith in pyroclastics(Pecuno quarry, Abakaliki).
353
354
M . HOQUE
Fig. 3. Unconformable contact between pyroclastics and the underlying shales of the Asu River group (western slope of Ezza-Agu Hill, Abakaliki).
355
Pyroclastics of the lower Benue trough Table 2. Geochemical analyses of pyroclastic rock samples
Spl
SiO:
AI203
A01
42.69 47.10 42.12 4(}.43 50.17 49.38 42.84 41.25 43.42 48.35 46.48 39.14 46.5(I 47.56 40.54 39.76 52.9(I 46.06 53.39 46.95 45,93
15.90 14.66 14.8(} 17.31 15.20 14.88 15.76 12.78 13.99 16.15 14.05 14.64 14.97 15.79 15.87 15.97 15.9(I 15.27 15.76 18.48 17.81
A02 A03 A04 A05 A06 A07 A08 A09
AIO A11 AI2 AI3 AI4 A15 AI6 A17 A18 A19 A20 A21
Fe20~
FeO
MgO
CaO
Na20
K,O
TiO~
P/O 5
MnO
Loss
Total
D.I.
2.80 4.19 0.91 0.86 1.20 1.22 3.00 1.16 2.36 4.51 1.06 2.39 2.22 1.45 2.66 5.26 1.10 tr l.(10 4.92 5.48
7.02 4.89 7.47 11.06 7.69 7,6(/ 6.12 6.19 6.01 8.23 7.25 7.25 7.(11 7.69 8.61 6.47 5.21 9.34 7.54 4.55 5.(/3
10.65 9.94 7.21 13.72 8.13 8.81 7.33 6.76 6.34 4.77 9.6(i 8.28 6.03 5.0() 9.2(I 8.71 3.71 6.13 3.87 6.52 7.33
2.14 4.36 7.08 1.53 2,32 2.5(/ 5.61 8.89 9.8(/ 9.81 3.77 7.29 6.79 6.00 4.08 5.85 6.14 6.6(/ 4.56 6.15 5.75
4.39 2.93 5.55 2.16 3.93 3.72 5.54 5.13 3.75 3.74 3.72 4.89 4.45 5.78 6.15 3.94 6.(/4 4.28 4.62 4.66 4.61
(t.09 0.08 (/.15 0.10 0.03 0.10 0.20 0.25 0.12 0.88 0.11 0.16 0.01 0.04 0.16 -0.04 0.20 1.38 0.17 --
2.46 2.23 2.1(} 2.54 1.98 1.98 2.26 1.56 1.47 2.83 1.81 2.09 2.1 l 1.73 2.65 0.04 1.75 2.16 2.29 1.34 1.53
0.32 2.13 1.47 (}.19 0.37 0.35 0.36 (I.28 0.30 0.52 0.69 (/.49 0.44 (/.37 0.23 1.30 0.37 (/.52 0.35 0.21 0.24
0.1l 0.07 0.20 0.06 0.(/8 0.08 0.09 0.18 0.17 (/.21 0.11 0.17 0.13 0.14 (I.08 0.09 0.14 0.12 0.12 0.14 0.17
11.10 8.27 11.44 10.22 8.8(/ 9.38 11.49 14.59 12.22 -11.43 14.(19 9.12 8.46 10.60 10.15 7.41 10.32 5.12 6.1(I 6.12
99.7 100.9 l(X).5 100.2 99.(} 10().0 100.6 99.(/ 99.0 100.1 100.1 100.9 99.8 101.0 l(10.8 10(t.4 99.7 101.0 10().0 100.2 100.0
37.80 35.91 39.66 18.92 40.13 37.26 42.46 37.4(/ 30.33 36.48 32.01 33.79 34.77 45.89 40.53 33.56 50.99 37.30 50.26 4(/.44 38.81
Note: Fe,_O3/FeO based on wet chemical analysis; other oxides on XRF. D . I . - - differentiation index of Thornton and Tuttle ( 19601. Loss--loss on ignition; tr--trace amount.
There are many small and medium size hypabyssal sills and dykes intruding country rocks of the Abakaliki anticlinorium. A few of these intrusives are also emplaced within or very near to the pyroclastics. It is thought that the intrusives and the pyroclastics have formed during the same volcanic event, and would therefore be coeval (Burke et al. 1971; Nwachukwu, 1972, Olade 1979). Two sets of samples were selected for geochemical study, one from the pyroclastics and the other from the intrusives which are also relatively fresh (Tables 2 and 3). Variation diagrams based on total alkali versus silica (Kuno 1966) and silica versus differentiation index (Thornton and Tuttle 1960) show that both sets of rocks have a distinct alkaline and silica-undersaturated basal-
tic affinity (Figs 4 and 5). Although there are a few deviations in the plots, indicating perhaps considerable redistribution of elements due to alterations in the pyroclastics, the alkaline nature of the rock can still be established. Pearce et al. (1975) suggested a TiOz-K20-P205 ternary diagram as a method of discriminating between oceanic and continental alkali basalts. Figure 6 shows the plot of pyroclastic and intrusive samples. There is a considerable scatter of points of pyroclastic samples which is indicative of mobility as well as depletion of potassium. The intrusives which are more fresh than the pyroclastics show greater concentration of points in and very near to the continental basalt field. Following Pearce and Cann (1973), Olade (1979) used a Ti-Zr-Y
Table 3. Geochemical analyses of intrusive rock samples
Spl
SiO,
AleO~
101 I(/2 103 I04 105 106 I07 108 109 110 I1 I II2 113 I14 115 116 117 118 119 120 121
53.27 54.89 47.86 49.1(I 49.04 44.21 52.70 52.95 52.44 53.02 49,25 47,51 46,85 54,24 44,43 50,63 46,30 46,21 46.64 46,84 46.96
14.34 13.86 13.57 14.20 15.58 16.38 15.(/(} 14.85 15.0(( 15.34 13.49 16.54 19.46 14.91 15.15 14.43 14.48 18.00 15.78 17.22 16.82
Fe:O~
FeO
MgO
CaO
Na:O
K20
TiO~
P,O
MnO
Misc
Total
D.I.
3.22 2.98 3.41 3.52 3.61 (t.57 3.(}5 3.14 1.79 2.52 1.79 3.57 3.63 2.74 2.16 6.83 1.36 3.48 1.32 3.7(I 2.70
8.(t0 5.56 8.07 7.58 7,66 8,55 7,12 7,(t2 8,15 7.14 8,61 8.33 5.65 8.00 7.85 6.84 6.14 4.32 7.56 5.48 6.28
6.86 4.18 10.47 9.00 6.66 5.82 5.65 5.40 5.0(I 5.40 9.42 3.96 4.22 4.43 8.50 3.70 9.72 4.88 6.08 7.64 7.71
8.81 5.9(t 8.09 8.27 8.44 7.29 8.06 8.22 7.87 8.(t6 9.49 10.48 7.90 6.03 9.44 8.31 4.53 9.51 12.35 9.11 8.68
2.99 5.70 3.27 4.10 4.71 4.18 3.16 3.(/9 3.43 3.13 3.09 4.29 5.40 6.75 3.80 5.13 2.86 4.91 3.31 2.58 2.87
0.35 (/.69 1.30 0.08 0.84 0.04 0.50 (I.45 0.66 (}.42 1.10 (I.75 2.08 0.03 1.73 0.40 0.57 1.09 0.09 1.27 1.02
1.73 2.62 3.11 3.(}8 2.51 2.50 1.64 1.63 1.84 1.72 2.22 3.89 3.33 2.26 4.26 2.63 1.86 2.13 2.39 2,41 2.67
(I.25 (1.48 (}.69 0.88 0.8(/ 0.80 0.25 0.20 (}.3(I 0.23 (/.47 (I.52 1.33 0.47 0.66 0.83 0.76 0.40 0.27 (}.57 (}.6(t
0.18 0.14 0.16 0.19 0.15 0.13 0.15 0.17 0.18 (}.62 0.17 (I.15 0. t5 0.14 0.16 0.27 0.41 0.14 0.16 0.16 (I.15
1.0 --1.5 -9.52 2.81 2.95 3.40 2.49 1.00 ---1.94 -11.28 4.88 4.05 3.19 3.59
101.0 100.0 100.0 101.5 100.0 99.9 100.1 100.1 101.0 100.1 101.t 99.0 100.0 101.0 100.1 100.0 100.3 99.8 100.0 101.0 99.0
33.01 53.00 35.59 35.16 42.72 35.14 34.66 36.41 37.13 35.73 32.60 37.74 49.28 56.82 33.94 45.74 32.09 40.52 27.43 29.23 3(}.01
Note: D.l.---differentiation index of Thornton and Tuttle (196(/); Misc--includes H:O, CO2 and loss on ignition. F e : O J F e O data based on wet chemical analysis.
M. HOQUE
356 I
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ternary diagram to distinguish within-plate basalts (i.e. ocean island and continental basalts) from ocean-floor basalts (i.e. plate-margin basalts); he was able to demonstrate clearly that the Abakaliki pyroclastics belonged to "within-plate" continental basalt field.
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I PYROCLASTIC ROCK SAMPLES
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STRATIGRAPHIC POSITION OF PYROCLASTICS
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• INTRUSIVE ROCK SAMPLES i I l 50 55 60
{ 45
Si02 Wtl PERCENT Fig. 4. Variation diagram of total alkali against silica (after Kuno 1966). Dashed lines show boundaries of Japanese basalt types and lettered points refer to average [avas of Cascade province, North America (B, basalt; BA, basaltic andesite; A, andesite, after Carmichael et aL 1974, figs 11-2b and 11-6).
80
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• INTRUSIVE ROCK SAMPLES • PYROCLASTIC ROCK SAMPLES i I I I 80 60 40 20
0
DIFFERENTIATION INDEX Fig. 5. Plot of silica vs. differentiation index (Thornton and Tuttle 1960) showing the composition field of the pyroclastics and the intrusives.
Ti02
The following observations are relevant in establishing the stratigraphic position of the pyroclastics. (A) the pyroclastics rest unconformably on the Asu River shales (Albian or pre-Albian) in the central part of the Abakaliki anticlinorium, and are preserved mostly as valley-fills [Fig. I(B) crosssection], (B) as one moves away from the central part to the flanks of the anticlinorium, outcrops of pyroclastics are found to be surrounded by rocks of the Eze-Aku Formation (Turonian), (C) there are abundant xenoliths of mudrock and shale and occasionally micrites, derived from the underlying rocks of the Asu River group and the Eze-Aku Formation, (D) although the underlying formations were folded by the Santonian orogeny, the pyroclastics do not show evidence of any tectonic deformation, (E) pyroclastics as well as many intrusives and lead-zinc mineralized veins in the area are concentrated along the axes of the anticlinorium; these rock bodies are totally absent in the tectonically undeformed Anambra basin (of late Cretaceous to early Paleogene age), situated at the edge of the western flank of the folded Benue trough (Fig. 1), (F) the Pb-Zn mineralization in the Abakaliki area is fault controlled, and the Pb-Zn deposit is assigned a post-Santonian age (Okezie 1965, Nwachukwu 1972). These faults also cut across pyroclastic deposits at Juju Hill near Abakaliki. It appears therefore that volcanic activity, mineralization and faulting are episodes all related to a major tectonic event in the area and the event was of late Santonian in age. From the above observations, it can be concluded that the pyroclastic deposits were emplaced on an eroded surface of the folded rocks of the lower Benue trough, and have been preserved in most cases as valley-fills, perhaps as remnants of what was once a thick and extensive deposit. Their distribution, restricted within the folded trough along its NE-SW oriented fold axes and their absence in the Anambra basin suggest that the Santonian (and perhaps older) fracture systems might have controlled their emplacement. This also indicates a late Santonian age for the pyroclastics. TECTONIC IMPLICATIONS AND CONCLUDING REMARKS
K20
P205
Fig. 6. Triangular plot of TiO2-K20-P205 (after Pearce et al. 1975) showing distribution of pyroclastic and intrusive rock samples.
The age, stratigraphic relationship and lithologic character of the pyroclastics, as presented in this study,
Pyroclastics of the lower Benue trough have implications on earlier thoughts on the origin and evolution of the Benue trough (see: Burke et al. 1971, 1972, Kogbe 1976, Hoque 1981, Wright 1981). Most workers relate genesis of the trough to the splitting of Afro-Brazilian plate in early Cretaceous time. The trough is portrayed as a rifted depression (King 1950, McConnell 1969), or a failed arm of an RRR triple junction involving the Gulf of Guinea, the South Atlantic and the Benue trough (Burke etal. 1971, 1972, Burke and Dewey 1973). Within the concept of plate tectonics, Burke et al. postulated an active oceanic spreading along the Benue trough and formation of about 150-200 km wide oceanic crust beneath the lower Benue [Fig. I(A)], followed by a subduction motion along a Benioff zone which gave rise to more than 1300 m of andesitic, basaltic and pyroclastic rocks (Burke et al. 1971). Wright (1976, 1981) presented a modified RRR triple junction model in which the Benue trough formed an arm with a limited spreading amounting to no more than a few kilometers of crustal stretching. Following Burke and Dewey (1973) and Hoffman etal. (1974), Olade (1975, 1979) advocated an aulacogen model for the Benue trough; he cited Abakaliki pyroclastics and associated intrusives as an evidence of initial volcanic activity related to a plumegenerated rifting of the Afro-Brazilian plate, and regarded these volcanics forming a substratum of the rifted basin. The composition of the pyroclastics and the associated intrusives, as shown in the variation diagrams, suggests that the generating magma of these rocks was dominantly alkaline and silica-undersaturated and had no affinity with a magma characteristic of an andesitic volcanism at a convergent plate boundary, or with a tholeiitic and silica-oversaturated magma typical of a zone of oceanic spreading (Bailey 1974, 1977). There is also no evidence of thrust faults, or an ophiolitic sequence in the basin (Nwachukwu 1972). The late Santonian stratigraphic position of the pyroclastics, as established in this study, also invalidates the postulate of Olade (1979) that these rocks "typify" oldest volcanic activity associated with initial rifting of the African continental plate over a hot-spot. It appears, therefore, that neither the Burke model of spreading and subduction volcanism, nor the Olade model of aulacogen with a basaltic substratum satisfactorily explains the twin problem of the origin and pyroclastic volcanism of the Benue trough. It is suggested that these two events are neither coeval nor genetically related to each other, and therefore each should be independently evaluated within the context of a grahen basin in an intracratonic tectonic setting. Acknowledgements--This work was supported by the University of Nigeria Senate Research Grant No. 00248/76. I sincerely thank Dr M. T. Hussain of the University of Jos for collaboration during field work and for geochemical analyses of several rock samples, Dr A. C. Onyeagoeha for useful discussions, and Prof. E. G. Lidiak of the University of Pittsburgh for his aid in preparation of the manuscript. Prof. E. A. Vincent of the University of Oxford deserves special thanks for his generous assistance in microprobe and XRF analyses of many samples and for his warm hospitality during my stay at Oxford. I also wish to thank Mr P. J. Jackson and Mr K. A. Parish for SEM work, Mr S. E. Ani for Fe2OJFeO analyses and Mr F. Ozoani for drafting the illustrations.
357 REFERENCES
Bailey, D. I. 1974. Continental rifting and alkaline magmatism. In: The Alkaline Rocks (Edited by Sorensen, H.), pp. 148-159. Wiley, London. Bailey, D. I. 1977. Lithosphere control of continental rift magmatism. J. Geol. Soc. Lond. 133, 103-106. Bond, A. and Sparks, R. S. J. 1976. The Minoan eruption of Santorini, Greece. J. Geol. Soc. Lond. 132, 1-16. Burke, K. and Dewey, J. F. 1973. Plume-generated triple junctions. J. Geol. 81,406-433. Burke, K,, Dessauvagie, T. F. J. and Whiteman, A. J. 197l. Opening of the Gulf of Guinea and geophysical history of the Benue depression and Niger delta. Nature, Lond. 233, 51-55. Burke, K,, Dessauvagie, T. F. J. and Whiteman, A. J. 1972. Geological history of the Benue valley and adjacent areas. In: African Geology (Edited by Dessauvagie, T. F. J. and Whiteman, A. J.), pp. 187-205. Department of Geology, University of lbadan, Nigeria. Carmichael, I. S. E., Turner, F. J. and Verhoogen, J. 1974, Igneous Petrology, pp. 532-533. McGraw-Hill, New York. De Swardt, A. J. M. 1950. Geological plan of the Ameke lode. Unpublished Geol. Surv. Nigeria Report No. 18/2. Farrington, J. L. 1952. Preliminary description of the Nigerian leadzinc field. Econ. Geol. 47,583-608. Hoffman, P., Dewey, J. F. and Burke, K. C. 1974. Aulacogen and their genetic relation to geosynclines, with a Proterozoic example from Great Slave Lake, Canada. In: Modern and Ancient Geosynclihal Sedimentation (Edited by Dott Jr., R. H. and Shaver, R. H.), pp. 38-55. Soc. Econ. Paleont. Mineral. Special Publication No. 19. Hoque, M. 1981. Pyroclastics, volcanism and tectonics of southern Benue trough, Nigeria. IA VCEI Syrup. Arc Volcanism, Tokyo, 1981, Abstract vol., pp. 134-136. King, L. C. 1950. Outline and disruption of Gondwanaland. Geol. Mag. 87,353-359. Kogbe, C. A. (Ed.) 1976. Geology of Nigeria. Elizabethan Pub. Co., Lagos, Nigeria. Kuno, H. 1966. Lateral variation of basaltic magma type across continental margins and island arcs. Bull. volcan 29,195-222. McConnell, R. B. 1949. Notes on the lead-zinc deposits of Nigeria and the Cretaceous stratigraphy of the Benue and Cross River valleys. Unpublished Geol. Surv. Nigeria Report No. 752. McConnell, R. B. 1969. Fundamental fault zones in the Guinea and West African shields in relation to presumed axes of Atlantic spreading. Bull. geol. Soc. Am. 80, 1775-1782. Nwachukwu, S. O. 1972. The tectonic evolution of the southern portion of the Benue trough, Nigeria. Geol. Mag. 109,411-419. Okezie, C. N. 1957. The geology of the Izekwe area of Ogoja division: provisional interim report No. 1 on the mapping of sheet No. 289. Unpublished Geol. Surv. Nigeria Report. Okezie, C. N. 1965. A preliminary report on the igneous rocks of Abakaliki town and environs and their relation to lead-zinc mineralisation. Unpublished Geol. Surv. Report No. 1349. Olade, M. A. 1975. Evolution of Nigeria's Benue trough (aulacogen): a tectonic model. Geol. Mag. 112,575-583. Olade, M. A. 1979. The Abakaliki pyroclastics of southern Benue trough, Nigeria. J. Min. Geol. 16, 17-25. Pearce, J. A. and Cann, J. R. 1973. Tectonic setting of basic volcanic rocks determined using trace element analysis. Earth Planet, Sci. Lett. 19,290-300. Pearce, T. H., Gorman, B. E. and Birket, T. C. 1975. The TiO2-K20-P2Os diagram: a method of discriminating oceanic and non-oceanic basalts. Earth Planet. Sci. Lett. 24,419--426. Pergeter, R. C. 1957. Progress report (1956-1957). Geol. Surv. Nigeria Report. Ramanathan, Rm. and Kumaran, K. P. N. 1981. Age and paleoecology of MI well in Calabar flank, southeastern Nigeria. J. Min. Geol. 18, 163-171. Schmincke, H-Ulrich, Fisher, R. V. and Waters, A. C. 1973. Antidune and chute and pool structures in the base surge deposits of the Laacher See area, Germany. Sedimentology 20,553-574. Sheridan, M. F. 1979. Emplacement of pyroclastic flows: a review. In: Ash-flow tufts, (Edited by Chapin, C. E. and Elston, W. E.), pp. 125'-136. Geol. Soc. Am. Special Paper 180. Sparks, R. S. J. 1976. Grain size variations in ignimbrites and implications for the transport of pyroclastic flows. Sedimentology 23,147188. Sparks, R. S. J. and Walker, G. P. L. 1973. The ground surge deposit: a third type of pyroclastic rock. Nature Phy. Sci. 241, 62-64. Tattam, C. M. 1930. The lead-zinc deposits of Ogoja province. Geol. Surv. Nigeria Ann. Report.
358
M . HOQUE
Thornton, C. P. and Tuttle, O. F. 1960. Chemistry of igneous rocks.-I. Differentiation index. Am. J. Sci. 258,664--684. Uzuakpunwa, A. B. 1974. The Abakaliki pyroclastics---eastern Nigeria: new age and tectonic implications. GeoL Mag. 111, 65-70. Wohletz, K. H. and Sheridan, M. F. 1979. A model of pyroclastic surge. In: Ash-flow Tufts (Edited by Chapin, C. E. and Elston, W.
E.), pp. 177-194. Geol. Soc. Am. Special Paper 180. Wright, J. B. 1976. Origin of the Benue trough a critical review. In: Geology of Nigeria (Edited by Kogbe, C. A.), pp. 309-317. Elizabethan Pub. Co., Lagos, Nigeria. Wright, J. B. 1981. Review of the origin and evolution of the Benue trough in Nigeria. Earth Evolution Sci. 2, 98-103.
1)731-7247/84 $3.00 + 0.00 © 1984 Pergamon Press Ltd.
Pyroclastics from the lower Benue trough of Nigeria and their tectonic implications M . HOQUE Department of Geology, University of Nigeria, Nsukka, Nigeria (Received 1 June 1984) Abstract--Two most controversial aspects of Nigerian geology are the petrology and stratigraphic position of the Abakaliki pyroclastics and the origin and evolution of the Benue trough. The pyroclastics are considered by some workers as lower Benue's oldest volcanic rocks, formed during the rifting of the Afro-Brazilian plate in early Cretaceous time that led to the origin of the Benue trough. The rocks were described variously as andesitic tufts. degraded alkali basalts, or spilites. This study shows that the deposits are discrete, elongate or oval shaped bodies, emplaced as valley fills. They show several types of primary structures, such as parallel lamination, cross-lamination, and graded bedding and have abundant mudrock xenoliths derived from older rocks of the Asu River group. The rocks are mostly lithic or scoriaceous lapillistone whose composition has undergone extensive alterations. The geochemical study indicates that the pyroclastics were derived from a silica-undersaturated alkaline magma. The rocks overlie unconformably folded Santonian and pre-Santonian formations. These and other findings lead to the conclusion that the pyroclastics are much younger (late Santonian) than the Benue trough (early Cretaceous) and are therefore unrelated to the event that led to the origin of the trough.
INTRODUCTION Two ~osT controversial topics of Nigerian geology are the petrology and stratigraphic position of the Abakaliki pyroclastics of southeastern Nigeria and the origin of the Benue trough. The two problems became interwoven when several workers of Nigerian geology attempted to use the petrologic data and the time of emplacement of the pyroclastics in constructing a model for the origin and evolution of the Benue trough. The pyroclastics are prominently exposed in and around the town of Abakaliki in southeastern Nigeria (Fig. 1). Isolated outcrops of pyroclastics have also been reported from other parts of the lower Benue trough. Around Abakaliki, however, they form the few prominent physiographic features in an otherwise flat, swampy and rice-growing region composed of monotonous shale sequences of the Asu River group of perhaps Neocomian age (Ramanathan and Kumaran 1981). The Abakaliki pyroclastics have variously been reported to be preAlbian or Aptian (Uzuakpunwa 1974). Olade (1979) considered the unit to be older than Albian Asu River shales and to be overlying the Precambrian basement. IX
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McConnell (1949) and De Swardt (1950) reported it to be interstratified with Albian shales. A post-Albian age was advocated by Tattam (1930), Farrington (1952) and Pergeter (1957). Pickering (1947, cited by Okezie 1965) thought it to be a Recent valley-fill. Okezie (1965) in an admirable report on these and other igneous rocks in the area argued in favour of a post-Santonian age. The rocks have also been described variously as pyroclastic flows, intrusive breccias, submarine spilites, andesitic tufts, basaltic agglomerates or degraded alkali basalts (McConnell 1949, Tattam 1930, Okezie 1957, 1965, Uzuakpunwa 1974, Olade 1979). Poor exposures, deep lateritic weathering and inadequate access to most of the areas prevented earlier workers from a sustained investigation of the rock and, as a result, exact petrology and tectonic significance of the pyroclastics have not been properly evaluated. Extensive highway construction in the region has recently exposed several outcrops and provided excellent opportunity to study the unit in a greater detail. This report is intended to present results of the study and evaluate tectonic implications of several of the findings.
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351
352
M.
HOQUE
S T R U C T U R E S AND P E T R O L O G Y OF P Y R O C L A S T I C S
The Abakaliki pyroclastic deposits show several types of primary structures. These include parallel lamination, cross-lamination, graded bedding and alternations of coarse- and fine-grained layers. There are also intricate small-scale recumbent folds, micro-faults and deformed laminae. Locally, beds are found to be inclined to as much as 35° with an azimuth unrelated to dip-azimuths of surrounding shale beds. Nowhere does the rock show any tectonic deformation. The deposits are usually discrete, elongate or oval shaped bodies from several meters to hundreds of meters in length, often overlain by a thick lateritized or weathered profile. Onion-like spheroidal weathering is quite common. Abundant xenoliths of mudrocks, siltstone and shale have been observed in many fresh exposures (Fig. 2). The size of xenoliths varies from tens of centimeters to a millimeter or less in length. Many shale or mudrock xenoliths retain their high angularity; such xenoliths may represent small residence time in, and also minimal reaction with, the magma which was perhaps ascending at a very fast rate due to its explosive character. The pyroclastic grains show a wide range of sorting and the sorting varies greatly from bed to bed. Some finely laminated rocks appear to be slightly better sorted than others. Some beds contain chaotic assemblages of fine ash to large blocks. Some very poorly sorted units are essentially breccia, or more appropriately may be called pyrobreccia (a term used in this study to distinguish the volcanogenic breccia from the breccia of sedimentary or tectonic origin). However, similar generally thick and poorly sorted units have been described by others as unwelded ignimbrites (Sparks 1976); they are basically pyroclastic flow deposits. Their poor sorting is attributed to high particle concentration. Some beds show normal grading. Each graded unit may be a centimeter or less to a few centimeters in thickness; they may alternate between a fine-grained and a coarse-grained graded unit. These graded units may be pyroclastic fall deposits. Occasionally some units show cross-lamination and ripple structures. These units are usually thin (several centimeters in thickness), finegrained and poorly sorted, and abruptly overlie a coarsegrained unit. Their lateral extent is usually limited to a few meters only. The thinly cross-laminated or rippled beds are thought to have formed by ground surges (also called pyroclastic surges) rather than by tractive sedimentary processes (Schmincke etal. 1973, Bond and Sparks 1976, Sparks 1976, Wohletz and Sheridan 1979). These primary structures which are products of pure volcanic processes have earlier been considered to have formed by reworking of pyroclastic materials in fluvial or marine environment (Olade 1975, 1979). Subaerial pyroclastic deposits are usually classified into three genetic types: 11) fall, (2) surge and (3) flow (Sparks and Walker 1973, Sheridan 1979). It appears that the Abakaliki pyroclastics represented all of them. New road-cuts and quarries reveal variable thick-
nesses of the deposit, ranging from a few meters to about a hundred meters. At several places, unambiguous unconformable contacts between the underlying Asu River shales and the overlying pyroclastics have been observed for the first time (Fig. 3). Away from the central part of the Abakaliki anticlinorium (which is composed of Albian or pre-Albian Asu River shales), pyroclastic bodies are found to be surrounded by the Turonian Eze-Aku Formation at the flanks [Fig. I(B)]. Xenoliths of micrite fragments, presumably derived from the Nkalagu limestone member of the Eze-Aku Formation, have also been identified within the pyroclastics. Thin-section study of samples collected from various locations indicates that they are mostly fine lithic or scoriaceous lapillistone. The matrix consists mainly of microlaths of plagioclase with glassy or aphanitic materials, giving it a hyalocrystalline texture. The original composition of the rock however has undergone extensive alteration in the form of albitization; development of quartz and carbonate veinlets as well as filling up of vesicular and scoriaceous fragments with mosaics of feldspars, calcite and quartz are some of the evidences of post-depositional alterations. Scanning electron microscopy reveals extensive devitrification, sericitization, and growth of zeolites and other secondary minerals. Large-scale secondary alterations of volcanic debris into microlites of plagioclase are quite conspicuous. The mineralogy of the rock can therefore be divided into two groups: the primary and the secondary. The primary ones are plagioclase, vitric fragments, opaques of irontitanium minerals and a few poorly preserved hornblende or pyroxene. Measurements of extinction angles in a few primary plagioclase suggest the composition to be about An30_35. Large xenocrysts of hornblende and plagioclase have been observed in one location (Ajaba quarry) situated at the southwestern end of the Abakaliki anticlinorium. The secondary minerals are either alteration products or cavity-fillings. These are calcite, zeolites, plagioclase, quartz and chlorite (penninites). Microlaths of plagioclase are randomly oriented in a groundmass of vitric ash. In a few cases, however, they seem to show flow banding. Microprobe analyses of a few of these microlaths show them to be albitic plagioclases (Table 1). Calcite is by far the most abundant of the cavity-filling and vein-filling secondary minerals. T a b l e 1. M i c r o p r o b e a n a l y s e s of p l a g i o c l a s e laths Oxides SiO, A126~ FeO MgO CaO Na~O K~O TiO, MnO Total Or Ab An
624AA 711.57 19.30 0.23 0.04 {}.01 11.85 {I.02 0.{}2 -102.04
99.70
6381 7{1.[11 19.56 0.48 0.22 {}.{14 11.39 {}.{}7 {}.{}2 {}.ill 1(}1.8(} 0.52 99.22 0.26
638M (}8.52 1938 0.62 0.32 {}.03 11.{}4 0.07 {}.02 . . . 10{}.0(} 0.42 99.47 0.11
638V
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69.13 19.59 {1.03 0.04 (}.{13 11.66 {}.{}4 0.{12 . . . . 100.54
69.79 19.98 0.113 0.02 0.04 11.32 {/.05
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Pyroclastics of the lower Benue trough
Fig. 2. Layered mudrock xenolith in pyroclastics(Pecuno quarry, Abakaliki).
353
354
M . HOQUE
Fig. 3. Unconformable contact between pyroclastics and the underlying shales of the Asu River group (western slope of Ezza-Agu Hill, Abakaliki).
355
Pyroclastics of the lower Benue trough Table 2. Geochemical analyses of pyroclastic rock samples
Spl
SiO:
AI203
A01
42.69 47.10 42.12 4(}.43 50.17 49.38 42.84 41.25 43.42 48.35 46.48 39.14 46.5(I 47.56 40.54 39.76 52.9(I 46.06 53.39 46.95 45,93
15.90 14.66 14.8(} 17.31 15.20 14.88 15.76 12.78 13.99 16.15 14.05 14.64 14.97 15.79 15.87 15.97 15.9(I 15.27 15.76 18.48 17.81
A02 A03 A04 A05 A06 A07 A08 A09
AIO A11 AI2 AI3 AI4 A15 AI6 A17 A18 A19 A20 A21
Fe20~
FeO
MgO
CaO
Na20
K,O
TiO~
P/O 5
MnO
Loss
Total
D.I.
2.80 4.19 0.91 0.86 1.20 1.22 3.00 1.16 2.36 4.51 1.06 2.39 2.22 1.45 2.66 5.26 1.10 tr l.(10 4.92 5.48
7.02 4.89 7.47 11.06 7.69 7,6(/ 6.12 6.19 6.01 8.23 7.25 7.25 7.(11 7.69 8.61 6.47 5.21 9.34 7.54 4.55 5.(/3
10.65 9.94 7.21 13.72 8.13 8.81 7.33 6.76 6.34 4.77 9.6(i 8.28 6.03 5.0() 9.2(I 8.71 3.71 6.13 3.87 6.52 7.33
2.14 4.36 7.08 1.53 2,32 2.5(/ 5.61 8.89 9.8(/ 9.81 3.77 7.29 6.79 6.00 4.08 5.85 6.14 6.6(/ 4.56 6.15 5.75
4.39 2.93 5.55 2.16 3.93 3.72 5.54 5.13 3.75 3.74 3.72 4.89 4.45 5.78 6.15 3.94 6.(/4 4.28 4.62 4.66 4.61
(t.09 0.08 (/.15 0.10 0.03 0.10 0.20 0.25 0.12 0.88 0.11 0.16 0.01 0.04 0.16 -0.04 0.20 1.38 0.17 --
2.46 2.23 2.1(} 2.54 1.98 1.98 2.26 1.56 1.47 2.83 1.81 2.09 2.1 l 1.73 2.65 0.04 1.75 2.16 2.29 1.34 1.53
0.32 2.13 1.47 (}.19 0.37 0.35 0.36 (I.28 0.30 0.52 0.69 (/.49 0.44 (/.37 0.23 1.30 0.37 (/.52 0.35 0.21 0.24
0.1l 0.07 0.20 0.06 0.(/8 0.08 0.09 0.18 0.17 (/.21 0.11 0.17 0.13 0.14 (I.08 0.09 0.14 0.12 0.12 0.14 0.17
11.10 8.27 11.44 10.22 8.8(/ 9.38 11.49 14.59 12.22 -11.43 14.(19 9.12 8.46 10.60 10.15 7.41 10.32 5.12 6.1(I 6.12
99.7 100.9 l(X).5 100.2 99.(} 10().0 100.6 99.(/ 99.0 100.1 100.1 100.9 99.8 101.0 l(10.8 10(t.4 99.7 101.0 10().0 100.2 100.0
37.80 35.91 39.66 18.92 40.13 37.26 42.46 37.4(/ 30.33 36.48 32.01 33.79 34.77 45.89 40.53 33.56 50.99 37.30 50.26 4(/.44 38.81
Note: Fe,_O3/FeO based on wet chemical analysis; other oxides on XRF. D . I . - - differentiation index of Thornton and Tuttle ( 19601. Loss--loss on ignition; tr--trace amount.
There are many small and medium size hypabyssal sills and dykes intruding country rocks of the Abakaliki anticlinorium. A few of these intrusives are also emplaced within or very near to the pyroclastics. It is thought that the intrusives and the pyroclastics have formed during the same volcanic event, and would therefore be coeval (Burke et al. 1971; Nwachukwu, 1972, Olade 1979). Two sets of samples were selected for geochemical study, one from the pyroclastics and the other from the intrusives which are also relatively fresh (Tables 2 and 3). Variation diagrams based on total alkali versus silica (Kuno 1966) and silica versus differentiation index (Thornton and Tuttle 1960) show that both sets of rocks have a distinct alkaline and silica-undersaturated basal-
tic affinity (Figs 4 and 5). Although there are a few deviations in the plots, indicating perhaps considerable redistribution of elements due to alterations in the pyroclastics, the alkaline nature of the rock can still be established. Pearce et al. (1975) suggested a TiOz-K20-P205 ternary diagram as a method of discriminating between oceanic and continental alkali basalts. Figure 6 shows the plot of pyroclastic and intrusive samples. There is a considerable scatter of points of pyroclastic samples which is indicative of mobility as well as depletion of potassium. The intrusives which are more fresh than the pyroclastics show greater concentration of points in and very near to the continental basalt field. Following Pearce and Cann (1973), Olade (1979) used a Ti-Zr-Y
Table 3. Geochemical analyses of intrusive rock samples
Spl
SiO,
AleO~
101 I(/2 103 I04 105 106 I07 108 109 110 I1 I II2 113 I14 115 116 117 118 119 120 121
53.27 54.89 47.86 49.1(I 49.04 44.21 52.70 52.95 52.44 53.02 49,25 47,51 46,85 54,24 44,43 50,63 46,30 46,21 46.64 46,84 46.96
14.34 13.86 13.57 14.20 15.58 16.38 15.(/(} 14.85 15.0(( 15.34 13.49 16.54 19.46 14.91 15.15 14.43 14.48 18.00 15.78 17.22 16.82
Fe:O~
FeO
MgO
CaO
Na:O
K20
TiO~
P,O
MnO
Misc
Total
D.I.
3.22 2.98 3.41 3.52 3.61 (t.57 3.(}5 3.14 1.79 2.52 1.79 3.57 3.63 2.74 2.16 6.83 1.36 3.48 1.32 3.7(I 2.70
8.(t0 5.56 8.07 7.58 7,66 8,55 7,12 7,(t2 8,15 7.14 8,61 8.33 5.65 8.00 7.85 6.84 6.14 4.32 7.56 5.48 6.28
6.86 4.18 10.47 9.00 6.66 5.82 5.65 5.40 5.0(I 5.40 9.42 3.96 4.22 4.43 8.50 3.70 9.72 4.88 6.08 7.64 7.71
8.81 5.9(t 8.09 8.27 8.44 7.29 8.06 8.22 7.87 8.(t6 9.49 10.48 7.90 6.03 9.44 8.31 4.53 9.51 12.35 9.11 8.68
2.99 5.70 3.27 4.10 4.71 4.18 3.16 3.(/9 3.43 3.13 3.09 4.29 5.40 6.75 3.80 5.13 2.86 4.91 3.31 2.58 2.87
0.35 (/.69 1.30 0.08 0.84 0.04 0.50 (I.45 0.66 (}.42 1.10 (I.75 2.08 0.03 1.73 0.40 0.57 1.09 0.09 1.27 1.02
1.73 2.62 3.11 3.(}8 2.51 2.50 1.64 1.63 1.84 1.72 2.22 3.89 3.33 2.26 4.26 2.63 1.86 2.13 2.39 2,41 2.67
(I.25 (1.48 (}.69 0.88 0.8(/ 0.80 0.25 0.20 (}.3(I 0.23 (/.47 (I.52 1.33 0.47 0.66 0.83 0.76 0.40 0.27 (}.57 (}.6(t
0.18 0.14 0.16 0.19 0.15 0.13 0.15 0.17 0.18 (}.62 0.17 (I.15 0. t5 0.14 0.16 0.27 0.41 0.14 0.16 0.16 (I.15
1.0 --1.5 -9.52 2.81 2.95 3.40 2.49 1.00 ---1.94 -11.28 4.88 4.05 3.19 3.59
101.0 100.0 100.0 101.5 100.0 99.9 100.1 100.1 101.0 100.1 101.t 99.0 100.0 101.0 100.1 100.0 100.3 99.8 100.0 101.0 99.0
33.01 53.00 35.59 35.16 42.72 35.14 34.66 36.41 37.13 35.73 32.60 37.74 49.28 56.82 33.94 45.74 32.09 40.52 27.43 29.23 3(}.01
Note: D.l.---differentiation index of Thornton and Tuttle (196(/); Misc--includes H:O, CO2 and loss on ignition. F e : O J F e O data based on wet chemical analysis.
M. HOQUE
356 I
{
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ternary diagram to distinguish within-plate basalts (i.e. ocean island and continental basalts) from ocean-floor basalts (i.e. plate-margin basalts); he was able to demonstrate clearly that the Abakaliki pyroclastics belonged to "within-plate" continental basalt field.
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I PYROCLASTIC ROCK SAMPLES
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STRATIGRAPHIC POSITION OF PYROCLASTICS
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DIFFERENTIATION INDEX Fig. 5. Plot of silica vs. differentiation index (Thornton and Tuttle 1960) showing the composition field of the pyroclastics and the intrusives.
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The following observations are relevant in establishing the stratigraphic position of the pyroclastics. (A) the pyroclastics rest unconformably on the Asu River shales (Albian or pre-Albian) in the central part of the Abakaliki anticlinorium, and are preserved mostly as valley-fills [Fig. I(B) crosssection], (B) as one moves away from the central part to the flanks of the anticlinorium, outcrops of pyroclastics are found to be surrounded by rocks of the Eze-Aku Formation (Turonian), (C) there are abundant xenoliths of mudrock and shale and occasionally micrites, derived from the underlying rocks of the Asu River group and the Eze-Aku Formation, (D) although the underlying formations were folded by the Santonian orogeny, the pyroclastics do not show evidence of any tectonic deformation, (E) pyroclastics as well as many intrusives and lead-zinc mineralized veins in the area are concentrated along the axes of the anticlinorium; these rock bodies are totally absent in the tectonically undeformed Anambra basin (of late Cretaceous to early Paleogene age), situated at the edge of the western flank of the folded Benue trough (Fig. 1), (F) the Pb-Zn mineralization in the Abakaliki area is fault controlled, and the Pb-Zn deposit is assigned a post-Santonian age (Okezie 1965, Nwachukwu 1972). These faults also cut across pyroclastic deposits at Juju Hill near Abakaliki. It appears therefore that volcanic activity, mineralization and faulting are episodes all related to a major tectonic event in the area and the event was of late Santonian in age. From the above observations, it can be concluded that the pyroclastic deposits were emplaced on an eroded surface of the folded rocks of the lower Benue trough, and have been preserved in most cases as valley-fills, perhaps as remnants of what was once a thick and extensive deposit. Their distribution, restricted within the folded trough along its NE-SW oriented fold axes and their absence in the Anambra basin suggest that the Santonian (and perhaps older) fracture systems might have controlled their emplacement. This also indicates a late Santonian age for the pyroclastics. TECTONIC IMPLICATIONS AND CONCLUDING REMARKS
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P205
Fig. 6. Triangular plot of TiO2-K20-P205 (after Pearce et al. 1975) showing distribution of pyroclastic and intrusive rock samples.
The age, stratigraphic relationship and lithologic character of the pyroclastics, as presented in this study,
Pyroclastics of the lower Benue trough have implications on earlier thoughts on the origin and evolution of the Benue trough (see: Burke et al. 1971, 1972, Kogbe 1976, Hoque 1981, Wright 1981). Most workers relate genesis of the trough to the splitting of Afro-Brazilian plate in early Cretaceous time. The trough is portrayed as a rifted depression (King 1950, McConnell 1969), or a failed arm of an RRR triple junction involving the Gulf of Guinea, the South Atlantic and the Benue trough (Burke etal. 1971, 1972, Burke and Dewey 1973). Within the concept of plate tectonics, Burke et al. postulated an active oceanic spreading along the Benue trough and formation of about 150-200 km wide oceanic crust beneath the lower Benue [Fig. I(A)], followed by a subduction motion along a Benioff zone which gave rise to more than 1300 m of andesitic, basaltic and pyroclastic rocks (Burke et al. 1971). Wright (1976, 1981) presented a modified RRR triple junction model in which the Benue trough formed an arm with a limited spreading amounting to no more than a few kilometers of crustal stretching. Following Burke and Dewey (1973) and Hoffman etal. (1974), Olade (1975, 1979) advocated an aulacogen model for the Benue trough; he cited Abakaliki pyroclastics and associated intrusives as an evidence of initial volcanic activity related to a plumegenerated rifting of the Afro-Brazilian plate, and regarded these volcanics forming a substratum of the rifted basin. The composition of the pyroclastics and the associated intrusives, as shown in the variation diagrams, suggests that the generating magma of these rocks was dominantly alkaline and silica-undersaturated and had no affinity with a magma characteristic of an andesitic volcanism at a convergent plate boundary, or with a tholeiitic and silica-oversaturated magma typical of a zone of oceanic spreading (Bailey 1974, 1977). There is also no evidence of thrust faults, or an ophiolitic sequence in the basin (Nwachukwu 1972). The late Santonian stratigraphic position of the pyroclastics, as established in this study, also invalidates the postulate of Olade (1979) that these rocks "typify" oldest volcanic activity associated with initial rifting of the African continental plate over a hot-spot. It appears, therefore, that neither the Burke model of spreading and subduction volcanism, nor the Olade model of aulacogen with a basaltic substratum satisfactorily explains the twin problem of the origin and pyroclastic volcanism of the Benue trough. It is suggested that these two events are neither coeval nor genetically related to each other, and therefore each should be independently evaluated within the context of a grahen basin in an intracratonic tectonic setting. Acknowledgements--This work was supported by the University of Nigeria Senate Research Grant No. 00248/76. I sincerely thank Dr M. T. Hussain of the University of Jos for collaboration during field work and for geochemical analyses of several rock samples, Dr A. C. Onyeagoeha for useful discussions, and Prof. E. G. Lidiak of the University of Pittsburgh for his aid in preparation of the manuscript. Prof. E. A. Vincent of the University of Oxford deserves special thanks for his generous assistance in microprobe and XRF analyses of many samples and for his warm hospitality during my stay at Oxford. I also wish to thank Mr P. J. Jackson and Mr K. A. Parish for SEM work, Mr S. E. Ani for Fe2OJFeO analyses and Mr F. Ozoani for drafting the illustrations.
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