Geochemical constraints on the origin of some intrusive igneous rocks from the Lower Benue rift, Southeastern Nigeria

Journal of African Earth Sciences 58 (2010) 197–210

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Geochemical constraints on the origin of some intrusive igneous rocks from the Lower Benue rift, Southeastern Nigeria S.C. Obiora a,*, S.N. Charan b a b

Department of Geology, University of Nigeria, Nsukka, Nigeria Geological Studies Division, National Geophysical Research Institute, Hyderabad 500 007, India

a r t i c l e

i n f o

Article history: Received 29 April 2009 Received in revised form 23 February 2010 Accepted 8 March 2010 Available online 11 March 2010 Keywords: Dolerites and dioritic rocks Alkali basalt magma series High fractionation indices Garnet-bearing HIMU/enriched mantle sources Crustal contamination CO2-rich hydrous phase

a b s t r a c t Major elements oxides, trace elements and REE data on some dolerites and dioritic rocks from the Lower Benue rift indicate that they are mainly basic in composition and belong to the alkali basalt magma series. They are characterized by enrichments in incompatible elements and high fractionation indices, (La/Yb)N, 5.7–14.72 which reflect LREE enrichment and indicate the presence of garnet in the source of the rocks. The ratios (Tb/Yb)N (1.63–2.6)and Dy/Yb (2.37–3.34) also indicate garnet-bearing lherzolite sources. The HFSE ratios Zr/Nb (0.88–2.6) and Y/Nb (0.57–1.57) suggest that the rocks were derived from enriched source(s) with dominant HIMU signature. The rocks were most likely formed in a within-plate setting of the intracontinental rift-type, similar to the Kenyan rift. Crustal contamination and interactions of the rocks with aqueous fluids are suggested by depletions in the more mobile LILE (Ba, Rb, K and Sr). The major element (K), some of the trace elements (Th, Co) and the REE (La, Ce) were heavily mobilized in some of the rocks (highly altered dolerites and dioritic rocks) during a low-grade metamorphic alteration. This is thought to have been caused by an intense hydrothermal activity involving a CO2-rich hydrous phase contained in the crustal materials. This hydrous phase is possibly related to the saline groundwater common in the area with high concentrations of Ca2+, Na+, Cl , Sr+, Ba+ and K+, thought to have been pressed out from the calcareous marine sediments during their burial. Ó 2010 Elsevier Ltd. All rights reserved.

1. Introduction The Lower Benue rift is the southeastern part of the Benue rift (otherwise, Benue Trough), an intracontinental rift which extends from the Gulf of Guinea northeastwards through the eastern part of Nigeria to the northern part of Cameroun. It stretches for a distance of about 1000 km in an essentially NE–SW trend and has a width of about 80 km. The origin has, therefore, been traced to the opening of the Gulf of Guinea and South Atlantic during the separation of South American plate from the African plate in the Mesozoic era (Burke et al., 1971; Grant, 1971). The rift is filled by Late Aptian to Coniacian sedimentary rocks some of which have been slightly regionally metamorphosed and are host to numerous bodies of alkaline igneous rocks (Obiora, 2002; Obiora and Umeji, 2004). The occurrence of igneous rocks in the Benue rift was first reported by Wilson and Bain (1928) who described rocks exposed at Lokpanta during the construction of a railway from Port Harcourt to Enugu, all in the southern part of Nigeria. A good number of igneous bodies has been referred to

* Corresponding author. E-mail addresses: [email protected] (S.C. Obiora), [email protected] (S.N. Charan). 1464-343X/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.jafrearsci.2010.03.002

and described in the area by Farrington (1952), Okezie (1957, 1961), Cratchley and Jones (1965), Olade (1979), Hossain (1981), Umeji (1985), Benkhelil (1986), Obiora (1994, 2002), Obiora and Umeji (1995, 2005), Maluski et al. (1995) and Coulon et al. (1996). The igneous rock bodies are over one hundred and twenty (120) in number and occur mostly as intrusive rocks of basic to intermediate compositions. There are also a few pyroclastic rocks and rare lava flows. Most of the accounts of the igneous rocks largely cover the petrography and major element oxide compositions of these rocks which commonly exhibit different degrees of alterations in their mineral constituents (Gunthert and Richards, 1960; Obiora and Umeji, 1995, 2005). Data on trace elements, including the rare-earth elements (REE) which are considered very useful in tracing the origins of igneous rocks are scarcely available. In this paper, geochemical data, including trace elements and REE on some of the intrusive igneous rocks in the study area have been used to constrain their origin, source characteristics and tectonic setting. 2. Regional distribution and ages of the igneous rocks in the Lower Benue rift The igneous rocks in the Lower Benue rift occur in four major magmatic districts, including southwest of Gboko,

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Ejekwe–Wanikande, Abakaliki and Okigwe–Ishiagu districts, from the north to the south (Fig. 1). The igneous rocks range in ages from Albian to Tertiary. A summary of the forms/types, number and ages of the igneous rocks, and the regionally metamorphosed host rocks is presented in Table 1. The igneous rocks are closely associated with galena (PbS)–sphalerite (ZnS)–baryte (BaSO4) and saline groundwater mineralization in an essentially NE–SW trending belt.

3. Field occurrence and characteristics of the intrusive igneous rocks The igneous rocks in this study occur mainly in the Ejekwe– Wanikande and Okigwe–Ishiagu districts where they intruded very

low-grade regionally metamorphosed calcareous shales and silty? Shales belonging to Late Aptian to Albian and Late Cenomanian to Early Turonian formations, locally known as the Asu River and EzeAku Shale Groups, respectively (Fig. 2). The outcrops of the igneous rocks are shown at locations 1–11 (Fig. 2) with the details of the samples on Table 2. The rocks crop out as relatively small to large bodies, generally ranging from 40 cm to 1.2 m long and 10 m to 210 m wide. The large rock bodies occur at locations 4 and 8. The one at location 8 is exceptionally large, being over 2.0 km long and 0.8 km wide and covering two communities known as Oferekpe-1 and Egefi-Ndiezioke, all in Izzi Local Government Area (Abakaliki Division) of Ebonyi State of Nigeria. The rocks can be subdivided into two varieties, namely: dolerites (locations 2, 3, 5, 6, 9) and dioritic rocks (locations 1, 4, 7, 8, 10, 11). The dolerites are generally medium-grained and melanocratic whereas the

Fig. 1. Geological Map of Nigeria showing the subdivisions of the Benue rift into Lower, Middle and Upper parts with the four major magmatic districts in the Lower part.

Table 1 Distribution and ages of the igneous rocks in the Lower Benue rift (after Obiora (2002)). Districts

Form/type of igneous rocks

Number of outcrops of igneous rocks

Age of igneous rocks

Host regionally metamorphosed sedimentary rocks

(1) South West of Gboko

Subvolcanic intrusions, namely: phonotephrites, phonolites and trachytes

Up to 40

86 Ma (Foyum monzo-syenite)

Slates from shales of the Albian Asu River and Turonian Eze-Aku Groups

61 Ma (Abata syenite) 59 Ma (Agyra syenite) (Umeji, 2000) (2) Ejekwe–Wanikande

(a) Non-fragmental igneous rocks, namely: alkali dioritic rocks, microgabbros/dolerites, nepheline syenites, basalts, basaltic sills, trachytes and phonolites (b) Pyroclastics, namely: tuffs and lapilli tuffs of basaltic to trachybasaltic compositions

More than 70

Up to 30

105 & 104 Ma (Wanikande syenite) (Snelling, 1965 & Benkhelil, 1986), 94 Ma (basaltic sill, valley of Anyim River), 88.9 Ma (olivine microgabbro, Ameka) 80.7 Ma (basic rock, Workum Hills) (Benkhelil, 1986)

Mostly in the slates from the Asu River Group and less commonly in those from the Eze-Aku Group

(3) Abakaliki

Pyroclastics (tuffs and lapilli tuffs of basaltic to trachybasaltic compositions and rare basalts

Up to 10

Not available

Slates from shales of the Asu River Group

4) Okigwe–Ishiagu

Dolerite sills, alkali dioritic plutons, agglomeratic pyroclastics

Less than 10

87 Ma (dolerite, Leru), 74 & 76 Ma (diorites, Eziator), (Umeji, 2000)

Slates from the Asu River Group; less commonly in the Eze-Aku Group

S.C. Obiora, S.N. Charan / Journal of African Earth Sciences 58 (2010) 197–210

199

Fig. 2. Geological Map of the Lower Benue rift showing locations of the intrusive igneous rocks in this study.

4. Petrographic characteristics of the intrusive igneous rocks Table 2 Details of samples of the intrusive igneous rocks and their locations. Sample No.

Location No. (Fig. 2)

Location name

Name of rock

1ASCO 1BSCO

1

Eziator (coarse part) Eziator (fine part)

Dioritic rock Dioritic rock

2ASCO 2BSCO

2

Ikoti-ibenta (Oyoba village road)

Dioritic rock

3ASCO 3BSCO

3

Ipuole Ebo farm

Dolerite

4ASCO 4BSCO 4CSCO

4

Woleche Ebo

Dioritic rock

5SCO

5

Izekwe

Dolerite

6ASCO 6BSCO

6

Okpodom No. 3/Wanakom

Dolerite

7ASCO 7BSCO

7

Oferekpe-2

Dioritic rock

8ASCO 8BSCO

8

Oferekpe-1

Dioritic rock Dolerite

9SCO

9

Anyim Ogbara Asaa

10SCO

10

Mkpumeakwaokuko

Dioritic rock

11SCO

11

Opankwa

Dioritic rock

dioritic rocks are medium to coarse-grained and mesocratic to leucocratic. The larger intrusions (locations 4 and 8) are finer-grained and melanocratic towards the peripheries, and coarse-grained, mesocratic to leucocratic towards the centre. One feature common to most of the rocks is the presence of welldeveloped crystals of calcite, with or without pyrite which appear like cavity-fillings. Details of the characteristics of the individual intrusions are presented in Obiora (2002) and Obiora and Umeji (2005).

The rocks are subdivided into two groups, namely: dolerites and dioritic rocks, on the basis of their texture and mineral constituents, identified from both thin section and X-ray Diffraction studies. The dolerites possess doleritic to sub-ophitic textures in which clinopyroxene is in intersertal positions or are partially enclosed by plagioclase (Fig. 3a–c). The plagioclase is identified to be mostly oligoclase and less commonly, labradorite. Accessory apatite is present. The dolerites show varying degrees of alteration in their primary mineral constituents giving rise to the presence of calcite, analcime, thuringite (chlorite), serpentinized olivine, tremolite, epidote and the opaques (pyrite and hematite), with calcite being the most common (Fig. 3d and e). The dioritic rocks have fabrics and mineral constituents which are similar to those of the dolerites. They consist of randomly oriented plagioclase crystals which are in sub-ophitic to ophitic and rarely intersertal relationship with the clinopyroxene. The plagioclase is sodic andesine. Hornblende is rare to absent. Biotite is also a common essential mineral, but occurs as pseudomorphs after hornblende in the rock from location 10. The name, ‘dioritic rocks’ has been adopted for these medium- to coarse-grained rocks because of their mesocratic to leucocratic colours and presence of andesine with general absence of hornblende as an essential mafic mineral. All the secondary minerals in the dolerites, with the exception of serpentinized olivine and analcime are also common in these dioritic rocks. Details of the mineral constituents of the rocks are presented in Obiora (2002) and Obiora and Umeji (2005). 5. Geochemistry 5.1. Analytical procedure Major elements oxides were determined from pressed pellets (40 mm diameter) prepared by using collapsible aluminium cups. The preparation of the pellets involved filling of the aluminium cups with boric acid powder and covering the powder

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pl

a

b cp

pl cp pl

cp

cp 0.5 mm

0.5 mm

c

cp

pl

pl

0.5 mm

e

d

pl

c c

c

pl

c

pl

pl pl

pl 0.5 mm

c 0.5 mm

Fig. 3. (a) Photomicrograph of dioritic rock from location 4 showing large crystals of clinopyroxene (cp) in a sub-ophitic relationship with small lath-shaped crystals of plagioclase (pl). (b) Photomicrograph of dioritic rock from location 7 showing large crystals of clinopyroxene (cp) in a sub-ophitic relationship with small lath-shaped crystals of plagioclase. (c) Photomicrograph of dioritic rock from location 8 showing a large crystal (longitudinal) of clinopyroxene (cp) in a sub-ophitic relationship with small lathshaped crystals of plagioclase (pl). (d) Photomicrograph of dolerite from location 4 showing a well-developed crystal of calcite (c) developed from intersertal clinopyroxene between randomly oriented plagioclase crystals (pl). (e) Photomicrograph of dolerite from location 2 showing numerous well-developed crystals of calcite (c) developed from intersertal clinopyroxene between randomly oriented plagioclase crystals (pl).

with 2 g of finely powdered rock samples, thereafter pressing under a hydraulic press at 55-ton pressure using the Hydraulic Press (Herzog, Germany). A Philips MagiX PRO, Model PW 2440, wavelength dispersive X-ray Fluorescence Spectrometer, coupled with an automatic sample changer PW 2540 and with online SUPER Q 3.0 software (Philips, Eindhoven, The Netherlands), was used for the analyses. Loss on ignition (LOI) was determined by heating 1.0 g of powdered samples, placed in a

porcelain crucible, in a furnace heated to a temperature of 950 °C for about 30 min and measuring the percentage weight loss after cooling the samples in a dessicator. The trace elements and the REEs were determined by use of an Inductively Coupled Plasma Mass Spectrometer (ICP-MS) model ELAN DRC II (Perkin–Elmer Sciex Instrument, USA). The samples analyzed were prepared following the procedure for open-acid digestion outlined by Roy et al. (2007). Ten millilitres (10 ml) of acid

Table 3 Major-element oxides and trace-elements data on the intrusive igneous rocks. Dolerites (fresh) 3BSCO

SiO2 TiO2 Al2O3 Fe2O3(t) MnO MgO CaO Na2O K2O P2O5 LOI

44.17 3.62 12.47 16 0.18 8.91 7.18 3 0.02 0.5 3.85

45.96 2.79 12.1 13.26 0.14 9.05 8.66 2.54 0.64 0.43 4.03

TOTAL

99.9

99.6

Sc V Cr Co Ni Cu Zn Ga Rb Sr Y Zr Nb Sn Sb Cs Ba Hf Ta W Tl Pb Th U Ni/Co Zr/Nb Y/Nb La/Nb Th/Nb Th/La Ba/La

26.053 277.35 379.919 53.919 153.021 38.765 123.45 18.905 0.612 356.993 28.252 37.001 37.714 8.739 0.87 0.039 41.379 1.107 0.382 3.55 0.073 6.579 1.801 0.476 2.84 0.98 0.75 0.61 0.05 0.08 1.81

25.326 217.662 431.095 44.508 117.647 34.34 115.904 15.858 13.111 859.826 22.209 33.492 30.568 4.164 0.138 0.501 311.939 0.888 2.033 5.128 0.09 6.083 1.524 0.414 2.64 1.1 0.73 0.57 0.05 0.09 17.81

Dioritic rocks (fresh) 5SCO

Dolerites (altered)

1ASCO

4ASCO

4BSCO

4CSCO

7ASCO

7BSCO

8ASCO

8BSCO

2ASCO

2BSCO

6ASCO

6BSCO

45.23 2.94 11.69 14.87 0.14 9.2 8.13 2.65 0.09 0.53 3.63

46.96 2.96 13.98 14.41 0.18 6.57 4.75 3.59 1.4 0.92 3.51

49.97 2.25 13.97 14.25 0.15 4.2 6.47 2.7 0.87 0.41 3.83

46.97 2.52 13.95 12.48 0.15 7.11 8.79 2.61 0.74 0.38 3.37

47.71 3.33 13.25 13.62 0.22 6.22 7.04 4.37 0.12 1.16 2.84

46.31 2.01 13.02 12.16 0.16 8.17 11.22 2.25 0.33 0.39 3.94

46.98 2.73 13.75 12.78 0.13 6.99 8.84 3.04 0.58 0.42 3.24

46.97 1.33 14.95 10.75 0.14 7.02 11.96 2.39 0.63 0.22 2.89

46.82 1.97 13.06 12.47 0.17 8.08 9.87 2.44 0.39 0.42 3.74

41.86 2.46 10.14 14.93 0.17 6.23 11.13 2.87 0.21 0.36 8.66

39.71 2.54 9.98 14.76 0.17 6.23 13.08 2.68 0.16 0.28 9.73

41.92 1.76 11.43 14.63 0.17 11.21 7.37 2.45 0.06 0.35 9.22

42.03 2.37 13.81 11.24 0.16 5.33 10.36 2.89 1.44 0.75 9.12

41.49 2.11 11.65 11.01 0.16 10.08 8.01 2.65 0.86 0.64 10.95

99.1

99.23

99.07

99.07

99.88

99.96

99.48

99.25

99.43

99.02

99.32

100.57

99.5

99.61

19.449 165.691 20.255 28.123 8.167 14.846 149.451 25.241 19.115 554.312 50.621 106.062 64.367 6.075 0.11 0.271 972.913 2.513 2.926 0.595 0.091 6.165 3.107 0.892 0.29 1.65 0.79 0.6 0.05 0.08 25.17

18.96 201.795 35.109 29.799 19.091 26.3 119.965 20.474 16.941 174.03 34.027 56.471 21.716 3.556 0.101 0.231 128.425 1.411 0.805 0.658 0.142 6.031 2.105 0.552 0.64 2.6 1.57 0.92 0.1 0.11 6.46

27.343 229.847 290.799 40.448 82.86 43.142 103.432 18.745 14.655 751.107 26.569 37.5 29.727 1.827 0.13 0.141 175.191 1.014 1.113 0.342 0.079 6.407 1.474 0.431 2.05 1.26 0.89 0.67 0.05 0.07 8.85

14.463 243.677 89.035 33.905 22.454 32.387 128.629 18.642 4.844 695.367 37.658 65.784 72.174 5.549 0.148 0.097 197.603 1.535 1.361 0.488 0.06 6.355 3.262 0.865 0.66 0.91 0.52 0.69 0.05 0.07 3.97

32.397 242.616 258.259 39.108 71.304 40.38 80.501 14.931 5.978 552.981 24.254 34.318 34.992 4.95 0.122 0.064 205.178 0.869 0.817 0.542 0.075 5.516 1.977 0.508 1.82 0.98 0.69 0.58 0.06 0.1 10.05

26.557 277.163 155.655 44.047 69.426 51.142 122.098 20.992 13.196 708.411 25.891 32.602 37.061 6.15 0.136 0.169 190.818 0.889 0.666 0.249 0.089 6.653 1.478 0.366 1.58 0.88 0.7 0.63 0.04 0.06 8.11

44.827 223.247 124.35 35.173 53.654 60.695 78.88 13.816 11.859 412.84 16.2 18.148 14.529 5.317 0.104 0.235 166.081 0.529 0.607 0.297 0.082 6.009 0.992 0.266 1.53 1.25 1.12 0.67 0.07 0.1 17.04

30.812 235.654 275.233 39.027 73.01 38.087 86.874 14.429 8.591 585.224 23.765 33.667 34.655 4.75 0.159 0.091 202.101 0.848 1.146 0.255 0.069 7.127 1.959 0.527 1.87 0.97 0.69 0.57 0.06 0.1 10.16

32.164 316.947 44.914 36.138 31.673 38.019 130.107 19.867 4.399 186.996 41.562 185.939 38.059 Nil Nil 0.369 29.603 4.546 17.878 Nil Nil 1.227 2.249 0.357 0.88 4.89 1.09 0.46 0.06 0.13 1.68

32.121 339.057 35.228 36.222 27.884 42.895 126.777 19.851 3.396 185.814 37.248 158.475 34.506 Nil Nil 0.373 24.099 3.891 15.628 Nil Nil 1.357 2.035 0.316 0.77 4.59 1.08 0.47 0.06 0.13 1.5

25.541 247.697 551.76 53.645 242.462 86.508 134.485 18.483 2.826 362.009 25.076 99.561 33.514 Nil Nil 0.69 34.289 2.627 7.714 Nil Nil 1.657 1.759 0.316 4.52 2.97 0.75 0.51 0.05 0.1 2

25.345 235.698 431.568 50.094 168.288 34.435 113.838 17.151 3.104 679.152 27.096 27.883 30.905 4.903 0.14 0.16 119.749 0.813 1.441 0.462 0.054 6.524 1.354 0.342 3.36 0.9 0.88 0.65 0.05 0.07 5.97

14.829 226.274 62.704 26.93 31.403 40.655 106.928 19.264 47.769 572.618 29.372 233.196 91.505 Nil Nil 1.248 50.412 4.658 34.997 Nil Nil 1.536 5.125 0.83 1.17 2.55 0.32 0.47 0.06 0.12 1.17

9SCO

16.682 185.81 318.705 38.345 228.36 36.425 74.497 12.743 18.832 954.538 19.314 29.311 44.182 5.187 0.154 1.499 681.898 0.621 0.687 0.363 0.119 7.119 2.323 0.696 5.96 0.66 0.44 0.52 0.05 0.1 29.82

S.C. Obiora, S.N. Charan / Journal of African Earth Sciences 58 (2010) 197–210

3ASCO

201

202

Table 4 REE data on the intrusive igneous rocks.

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu P LREE P HREE P REE P P LREE/ HREE LaN/YbN CeN/YbN LaN/SmN GdN/YbN TbN/YbN Dy/Yb Eu/Eu

Dioritic rocks (fresh)

Dolerites (altered)

Dioritic rocks (altered)

3ASCO

3BSCO

5SCO

1ASCO

4ASCO

4BSCO

4CSCO

7ASCO

7BSCO

8ASCO

8BSCO

2ASCO

2BSCO

6ASCO

6BSCO

9SCO

1BSCO

10SCO

11SCO

22.852 50.582 8.138 29.415 7.837 2.486 7.249 0.924 5.894 0.783 1.955 0.363 1.841 0.438 118.824 19.447 138.271 6.11 8.3 6.99 1.8 3.14 2.35 3.2 1.01

17.517 38.785 6.137 22.402 5.761 2.015 5.552 0.7 4.582 0.6 1.558 0.282 1.445 0.354 90.602 15.073 105.675 6.01 8.11 6.83 1.88 3.06 2.27 3.17 1.1

20.071 39.578 6.698 24.543 6.632 2.19 6.474 0.793 5.129 0.694 1.741 0.302 1.551 0.379 97.522 17.063 114.585 5.72 8.65 6.49 1.87 3.33 2.39 3.31 1.03

38.654 89.604 15.214 57.011 14.703 4.694 13.231 1.603 10.411 1.394 3.612 0.667 3.439 0.857 215.186 35.214 250.4 6.11 7.52 6.63 1.62 3.07 2.18 3.03 1.03

19.886 42.936 7.015 25.615 7.476 2.378 7.416 1.001 6.472 0.9 2.37 0.471 2.331 0.595 102.928 21.556 124.484 4.77 5.7 4.68 1.64 2.54 2.01 2.78 0.98

19.791 41.235 6.803 24.714 6.604 2.225 6.286 0.798 5.335 0.704 1.838 0.337 1.723 0.421 99.147 17.442 116.589 5.68 7.68 6.09 1.85 2.91 2.17 3.1 1.06

49.775 100.045 15.465 52.41 11.781 3.663 10.784 1.254 7.562 1.012 2.635 0.47 2.261 0.566 229.476 26.544 256.02 8.65 14.72 11.25 2.61 3.8 2.6 3.34 1

20.407 41.581 6.382 21.78 5.158 1.73 5.254 0.667 4.47 0.647 1.795 0.363 1.889 0.481 95.308 15.566 110.874 6.12 7.22 5.6 2.44 2.22 1.65 2.37 1.02

23.519 49.35 7.694 27.578 7.017 2.44 6.848 0.867 5.324 0.708 1.827 0.328 1.626 0.381 115.158 17.909 133.067 6.43 9.67 7.72 2.07 3.36 2.5 3.27 1.08

9.745 20.331 3.194 11.441 3.05 1.145 3.135 0.436 3.125 0.44 1.194 0.226 1.245 0.325 47.761 10.126 57.887 4.72 5.23 4.15 1.97 2.01 1.64 2.51 1.14

19.889 40.719 6.211 21.126 5.099 1.684 5.019 0.639 4.362 0.641 1.737 0.332 1.833 0.461 93.044 15.024 108.068 6.19 7.26 5.65 2.41 2.18 1.63 2.38 1.02

17.651 38.589 4.657 24.933 6.519 1.941 6.755 1.065 7.143 1.455 3.51 0.406 2.327 0.448 92.349 23.109 184.698 4 5.07 4.22 1.67 2.31 2.14 3.07 0.9

16.053 34.948 4.241 22.204 6.004 1.762 6.084 0.939 6.349 1.296 3.159 0.358 2.007 0.387 83.45 20.579 104.029 4.06 5.35 4.43 1.65 2.42 2.19 3.16 0.9

17.13 34.046 3.902 19.403 4.874 1.583 4.722 0.682 4.562 0.847 2.102 0.239 1.304 0.26 79.355 14.718 94.073 5.39 8.78 6.64 2.17 2.89 2.45 3.5 1.01

43.211 77.206 7.93 35.353 6.855 2.029 6.454 0.84 5.158 0.994 2.457 0.284 1.557 0.318 170.555 18.062 188.617 9.44 18.56 12.61 3.89 3.3 2.53 3.31 0.94

22.869 44.641 6.462 21.319 4.81 1.639 4.585 0.55 3.616 0.525 1.392 0.27 1.47 0.355 100.101 12.763 112.864 7.84 10.4 7.72 2.93 2.49 1.75 2.46 1.07

11.55 25.544 4.043 15.219 4.435 1.599 4.404 0.584 3.958 0.541 1.402 0.279 1.41 0.36 60.791 12.938 73.729 4.7 5.48 4.61 1.61 2.49 1.94 2.81 1.11

74.346 131.048 16.87 48.051 8.642 2.523 8.049 0.822 4.753 0.625 1.731 0.312 1.601 0.396 278.957 18.289 297.246 15.25 31.05 20.82 5.31 4.01 2.4 2.97 0.93

13.127 27.505 4.243 14.717 3.937 1.282 3.926 0.545 3.745 0.548 1.435 0.289 1.526 0.373 63.529 12.387 75.916 5.13 5.75 4.58 2.06 2.05 1.67 2.45 1

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Dolerites (fresh)

Table 5 Correlation matrix of HFSE, selected trace elements and major elements oxides versus Zr in relatively fresh intrusive igneous rocks. Zr

y

1.00 0.97 0.99 0.39 0.75 0.70 0.76 0.83 0.90 0.94 0.95 0.97 0.98 0.63 0.61 0.62 0.69 0.62 0.87 0.33 0.39 0.69 0.59 0.48

Hf 1.00 0.98 0.52 0.77 0.63 0.80 0.86 0.93 0.97 0.98 0.99 0.96 0.52 0.54 0.50 0.77 0.49 0.86 0.27 0.52 0.73 0.46 0.44

Ti

1.00 0.45 0.73 0.68 0.74 0.82 0.90 0.95 0.96 0.98 0.97 0.61 0.57 0.55 0.69 0.56 0.84 0.28 0.45 0.68 0.58 0.45

1.00 0.63 0.26 0.62 0.63 0.64 0.65 0.62 0.57 0.32 0.23 0.22 0.31 0.74 0.40 0.44 0.38 1.00 0.66 0.26 0.20

Nb

1.00 0.53 0.97 0.97 0.94 0.87 0.82 0.79 0.69 0.12 0.33 0.36 0.68 0.26 0.89 0.01 0.63 0.89 0.04 0.11

Ta

1.00 0.45 0.52 0.61 0.64 0.61 0.64 0.61 0.74 0.12 0.24 0.43 0.37 0.51 0.02 0.26 0.35 0.57 0.03

La

1.00 0.99 0.95 0.88 0.84 0.80 0.69 0.13 0.43 0.44 0.76 0.34 0.92 0.16 0.62 0.95 0.03 0.29

Ce

1.00 0.98 0.93 0.90 0.87 0.76 0.21 0.46 0.46 0.77 0.37 0.93 0.15 0.63 0.94 0.14 0.30

Nd

1.00 0.98 0.96 0.94 0.84 0.32 0.46 0.45 0.77 0.38 0.91 0.13 0.64 0.90 0.26 0.30

Sm

1.00 0.99 0.98 0.88 0.39 0.47 0.43 0.79 0.38 0.87 0.15 0.65 0.85 0.36 0.33

Tb

1.00 0.99 0.91 0.42 0.51 0.46 0.81 0.42 0.85 0.22 0.62 0.81 0.41 0.41

Dy

1.00 0.94 0.49 0.52 0.47 0.77 0.45 0.84 0.22 0.57 0.76 0.47 0.41

Yb

V

1.00 0.61 0.61 0.60 0.63 0.61 0.85 0.33 0.32 0.58 0.57 0.47

Cr

1.00 0.43 0.52 0.18 0.73 0.36 0.45 0.23 0.06 0.79 0.40

1.00 0.94 0.21 0.87 0.54 0.77 0.22 0.46 0.57 0.89

Ni

1.00 0.22 0.95 0.60 0.82 0.31 0.40 0.62 0.84

Sc

1.00 0.18 0.73 0.27 0.74 0.72 0.09 0.33

Co

1.00 0.55 0.82 0.40 0.28 0.69 0.80

Th

1.00 0.31 0.44 0.80 0.21 0.41

SiO2

1.00 0.38 0.16 0.50 0.93

TiO2

1.00 0.66 0.26 0.20

Na2O

1.00 0.02 0.34

K2O

1.00 0.51

MgO

1.00

Table 6 Correlation matrix of HFSE, selected trace elements and major elements oxides versus Zr in highly altered intrusive igneous rocks. Zr Zr Y Hf Ti Nb Ta La Ce Nd Sm Tb Dy Yb V Cr Ni Sc Co Th SiO2 TiO2 Na2O K2O MgO

1.00 0.81 0.99 0.72 0.29 0.96 0.01 0.02 0.22 0.38 0.77 0.77 0.54 0.62 0.52 0.32 0.26 0.02 0.04 0.48 0.72 0.31 0.07 0.38

Y

Hf 1.00 0.87 0.75 0.06 0.65 0.10 0.07 0.14 0.40 0.94 0.99 0.90 0.79 0.61 0.42 0.52 0.05 0.05 0.43 0.75 0.17 0.20 0.39

1.00 0.71 0.17 0.91 0.09 0.08 0.13 0.32 0.81 0.83 0.61 0.72 0.48 0.28 0.39 0.13 0.06 0.54 0.71 0.36 0.07 0.31

Ti

1.00 0.29 0.62 0.11 0.14 0.32 0.51 0.78 0.76 0.70 0.42 0.78 0.41 0.12 0.28 0.16 0.55 1.00 0.19 0.23 0.55

Nb

1.00 0.37 0.94 0.94 0.96 0.87 0.34 0.12 0.02 0.44 0.45 0.33 0.80 0.70 0.94 0.45 0.29 0.62 0.93 0.54

Ta

1.00 0.07 0.07 0.25 0.34 0.62 0.60 0.38 0.49 0.50 0.33 0.12 0.07 0.09 0.40 0.62 0.26 0.20 0.40

La

1.00 1.00 0.97 0.86 0.23 0.00 0.09 0.61 0.38 0.35 0.86 0.78 1.00 0.65 0.11 0.80 0.90 0.54

Ce

1.00 0.98 0.88 0.26 0.03 0.05 0.59 0.40 0.37 0.85 0.78 1.00 0.64 0.14 0.81 0.89 0.55

Nd

1.00 0.95 0.45 0.23 0.11 0.43 0.52 0.43 0.75 0.76 0.98 0.52 0.32 0.72 0.87 0.62

Sm

1.00 0.68 0.49 0.37 0.20 0.66 0.54 0.53 0.71 0.89 0.36 0.51 0.64 0.72 0.71

Tb

1.00 0.97 0.85 0.54 0.73 0.55 0.23 0.23 0.29 0.20 0.78 0.10 0.09 0.59

Dy

1.00 0.90 0.71 0.66 0.48 0.45 0.05 0.05 0.35 0.76 0.07 0.14 0.48

Yb

1.00 0.67 0.72 0.54 0.49 0.15 0.03 0.26 0.70 0.03 0.19 0.49

V

Cr

1.00 0.21 0.10 0.88 0.50 0.58 0.66 0.42 0.53 0.67 0.04

1.00 0.85 0.06 0.71 0.43 0.03 0.78 0.35 0.40 0.91

Ni

1.00 0.02 0.74 0.41 0.26 0.41 0.53 0.26 0.95

Sc

1.00 0.65 0.83 0.60 0.12 0.61 0.92 0.24

Co

1.00 0.80 0.55 0.28 0.78 0.78 0.85

Th

1.00 0.64 0.16 0.81 0.88 0.59

SiO2

1.00 0.55 0.90 0.45 0.25

TiO2

1.00 0.19 0.23 0.55

Na2O

1.00 0.58 0.55

K2O

1.00 0.50

MgO

S.C. Obiora, S.N. Charan / Journal of African Earth Sciences 58 (2010) 197–210

Zr Y Hf Ti Nb Ta La Ce Nd Sm Tb Dy Yb V Cr Ni Sc Co Th SiO2 TiO2 Na2O K2O MgO

1.00 203

S.C. Obiora, S.N. Charan / Journal of African Earth Sciences 58 (2010) 197–210

mixture (HF:HNO3:HClO4) in the ratio of 7:3:1 was added to 0.050 g of pulverized rock samples placed in clean and dry PTFE Teflon beakers and kept overnight for digestion. The samples were placed on a hot plate (1500 °C) until they were nearly dry to form a completely crystalline paste. Twenty millilitres (20 ml) of 1:1 (HNO3:distilled water) was thereafter added to each sample and warmed to dissolve the crystalline paste, followed by the addition of 5 ml of 1 ppm Rhodium (103 Rh) used as internal standard. The volume was then made up to 250 ml with double distilled water and the sample solution stored in 60 ml HDPE sample bottles after proper labelling.

1 Commendite

Rhyolite Trachyte

0.1

Dacite Trachyandesite Andesite

5.2. Results 0.01

The major elements oxides and trace-elements data are presented in Table 3 while the REE data is shown in Table 4. The rocks are all basic in composition as can be seen from their SiO2 (39.71–49.97%), MgO (4.2–11.21%). The rocks can be subdivided into two using high contents of LOI as an indication of some degree of alteration and the presence of secondary volatile and carbonate phases (Sayit and Goncouglu, 2009). The first group contains low LOI (2.84–3.94%) while the second group contains high LOI (7.39–10.95%). This alteration revealed by the high contents of LOI in the samples belonging to the second group (1BSCO, 2ASCO, 2BSCO, 6ASCO, 6BSCO, 9SCO, 10SCO, 11SCO) is consistent with their highly altered nature. Alteration of the rocks is also shown by their contents of well-developed crystals of calcite, along with pyrite and hematite which are formed from the breakdown of the interstitial clinopyroxenes in the doleritic to sub-ophitic textures in the rocks. 5.2.1. Element mobility in altered rocks It is widely known that some major elements such as Si, Na, K, and Ca, trace elements (e.g. Cs, Rb, Ba and Sr) and the transition metals Mn, Zn and Cu are easily mobilized by late and/or post-magmatic processes involving fluids and also during metamorphism (Pearce and Cann, 1973; Sayit and Goncouglu, 2009). Conversely, the HFSE (Y, Zr, Hf, Ti, Nb, Ta), the REE, the transition elements (V, Cr, Ni, Sc, Co), the LILE (Th) and the Nd isotopic composition are generally unchanged under a wide range of metamorphic conditions (up to the medium grade/lower amphibolite facies), including seafloor alteration (Edwards, 1978; Rollinson, 1993; Bienvenu et al., 1990 as cited in Viruete et al., 2009). These, otherwise, relatively immobile elements are considered more reliable for petrogenetic studies. There are however, special cases where some of these relatively immobile elements (e.g. Ti, Y, Zr and the REE) have been known to be mobilized such as in the alteration of glassy basalts (e.g. pillow lavas) and in the dehydration of the subducted oceanfloor (Edwards, 1978; Rollinson, 1993). Also, the REE may be mobilized by halogen-rich or carbonate-rich mineralizing fluids (Rollinson, 1993). On account of this possible mobility of some of these elements, their reliability for use in petrogenetic studies is commonly assessed based on their co-variance with Zr, which is also a relatively immobile element, but characterized by abundances which vary systematically and significantly with igneous processes (Edwards, 1978; Viruete et al., 2009). Correlation matrices of the relatively immobile elements coupled with some important major element oxides have been calculated for the two populations of rocks in this study, namely: relatively fresh samples (Group 1) and the highly altered ones (Group 2) for this purpose (Tables 5 and 6). All the elements in the matrix, namely: HFSE (Y, Zr, Hf, Ti, Nb, Ta), the REE (La, Ce, Nd, Tb, Dy, and Yb), the transition elements (V, Cr, Ni, Sc, Co), the LILE (Th) and the major element oxides (SiO2, TiO2, MgO, Na2O, K2O), generally show very good correlation with Zr in Group 1. In Group 2, Y, Zr, Hf, Ti, Ta, Ti, Tb, Dy, V, Cr, SiO2 and TiO2 also

Phonolite

Pantellerite

Zr / TiO2

204

Andesite basalt Subalkaline basalt

Alkali basalt

Basanite

0.001 0.01

0.1

Nb / Y

1

10

Fig. 4. Plots of the rocks on the Zr/TiO2 versus Nb/Y diagram of Winchester and Floyd (1977) for the discrimination of different magma series and their differentiation products. Solid circles = relatively fresh dolerites; solid triangles = relatively fresh dioritic rocks; open circles = highly altered dolerites; open triangles = highly altered dioritic rocks.

generally show very good correlation, Nb, Sm, MgO and Na2O show fairly good correlation while La and Ce show virtually no correlation. These results of the correlation indicate that most of the elements, with the exception of La, Ce and K in the highly altered samples (Group 2) are reliable for use in making petrogenetic inferences for the rocks.

5.2.2. Geochemical classification using the immobile trace elements The rocks plot mainly in the alkali basalt field on the Zr/TiO2 versus Nb/Y diagram of Winchester and Floyd (1977) for the discrimination of different magma series and their differentiation products (Fig. 4). The concentrations of the HFSE are expected to be higher in the more fractionated (intermediate and acid) members of a rock suite than in the less fractionated (basic) members (Govindaraju, 1994). Considering the concentrations of the HFSE (Zr, Y, Hf, and Ta) which showed negligible mobility in the rocks in this study, it is observed that samples 1ASCO, 2ASCO, 2BSCO, 4ASCO, 4BSCO, 4CSCO, 6ASCO and 6BSCO have higher concentrations of these elements than the rest of the samples. The ranges are: Zr (37.5–233.2 ppm), Y (25.08–50.62 ppm), Hf (1.01–4.66 ppm), Ta (1.11–35.0 ppm) while the rest contain Zr (18.15–37.0 ppm), Y (16.2–28.25 ppm), Hf (0.53–0.90 ppm) and Ta (0.28–2.03 ppm). Unlike the concentrations of the HFSE which increases with fractionation, Ni/Co ratio generally decreases with fractionation (Taylor, 1965). The ratios could be 26 in ultrabasic rocks, 1.6/4.52 in diabase/dolerite, 0.43 in diorites, 1.0 in syenite and 0.5 in granite (Computed from the Geostandards published by Govindaraju, 1994 and data in Taylor, 1965). Considering only the relatively fresh samples in which Ni and Co are not reasonably mobilized, samples 1ASCO, 4ASCO and 4SCO have low Ni/Co ratios (0.29–0.66) suggesting more fractionation while the rest show less fractionation with high ratios (1.52–3.36). Based on the petrographic characteristics (sub-ophitic to ophitic fabrics, presence of essential plagioclase and clinopyrox-

S.C. Obiora, S.N. Charan / Journal of African Earth Sciences 58 (2010) 197–210

10000

10000

a

b

1000

Rock / Chondrite

1000

Rock / Chondrite

205

100

10

100

10

1

1

0.1

0.1

Ba Th Nb La Sr P Zr Ti Y Yb Rb K Ta Ce Nd Sm Hf Tb Tm 10000

Ba Th Nb La Sr P Zr Ti Y Yb Rb K Ta Ce Nd Sm Hf Tb Tm

c

Rock / Chondrite

1000

100

10

1

0.1

Ba Th Nb La Sr P Zr Ti Y Yb Rb K Ta Ce Nd Sm Hf Tb Tm Fig. 5. (a) Spidergrams of the rocks showing strong depletions in Zr and Hf. (b) Spidergrams of the rocks showing strong depletions in Zr, Hf, Ta, Rb and K. (c) Spidergrams of the rocks showing strong depletions in Ba and K, with strong enrichment in Ta (symbols as for Fig. 4).

ene, without hornblende) and the geochemical classification in the field of alkali basalts using the Zr/TiO2 versus Nb/Y diagram (Fig. 4), HFSE contents and Ni/Co ratios, most of the rocks are basic (dolerites) in composition. The samples from locations 1, 2, 4 and 6 which show higher concentrations of HFSE and low Ni/Co ratios are more fractionated. The presence of plagioclase of andesine composition makes the rocks from locations 1 and 4 geochemically much closer in composition to diorites, although mineralogically, they do not contain essential hornblende. With reference to the classification and glossary of terms for igneous based on the recommendations of IUGS (Le Maitre, 2002), the rock, ‘diorite’ differs from dolerites and gabbros in its contents of essential hornblende and plagioclase of andesine composition.

5.2.3. Multi-elements diagrams (Spidergrams) Spidergrams consisting of the incompatible trace elements and selected REE are presented in Fig. 5a–c. The concentrations of these elements in the rocks were normalized to the values of chondrites after Thompson (1982). The patterns generally show enrichments in the incompatible trace elements, similar to the Oceanic Island Basalts (OIB). The rocks can be subdivided into three different groups based on the similarity of the patterns. The first group (Fig. 5a) is characterized by strong depletion in Zr and Hf; the second group (Fig. 5b) also shows strong depletion in these two elements as well as in Ta, Rb and K while the third group (Fig. 5c) shows strong depletion in Ba and K (with mild depletion in Sr) and strong enrichment in Ta. The first group consists mainly of relatively fresh dolerite (sample 3BSCO) and dioritic rocks (1ASCO,

206

S.C. Obiora, S.N. Charan / Journal of African Earth Sciences 58 (2010) 197–210

a

b 100

Rock / Chondrite

Rock / Chondrite

100

10

1

10

1

0.1

0.1

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

c

Rock / Chondrite

100

10

1

0.1

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Fig. 6. (a) Parallel and inclined REE patterns of the intrusive igneous rocks. (b) REE patterns of the highly altered rocks in which there is a drop in La and Ce. (c) REE patterns of the highly altered rocks in which there is an increase in La and Ce (symbols as for Fig. 4).

4ASCO, 4BSCO, 7BCSO, 8ASCO, 8BSCO), as well as highly altered dolerite (9SCO) and dioritic rock (1BSCO). The second group also consists of the relatively fresh dolerites (3ASCO, 5SCO) and dioritic rocks (4CSCO, 7ASCO) as well as highly altered dioritic rocks (10SCO and 11SCO). The third group comprises only altered dolerites (2ASCO, 2BSCO, 6ASCO, 6BSCO). 5.2.4. Rare-earth-elements patterns Chondrite-normalized REE patterns for the rocks are presented in Fig. 6a–c. The normalizing values were obtained from Nakamura (1974) with addition from Haskin et al. (1968). The rocks generally show similar patterns which are parallel and inclined due to enrichment in the light rare-earth elements (LREE) relative to the heavy REE (HREE). Negative Eu-anomlay is totally absent in the patterns. Minor differences in the patterns are observed

in the highly altered dolerites and alkali diorites, which have been subdivided into two groups. The first group (Fig. 6b) which consists of highly altered dolerites (2ASCO, 2BSCO, 6ASCO) shows a drop in La and Ce while in the second group (Fig. 6c) comprising one highly altered dolerite (6BSCO) and one highly altered dioritic rock (10SCO) there is a sharp increase in these two elements, compared to their values in the relatively fresh equivalents. The ratios of ‘‘total” HREE to ‘‘total” LREE in all the samples, excluding these five (2ABSCO, 2BSCO, 6ASCO, 6BSCO and 10SCO) that exhibit abnormal contents of La and Ce, range from 5.72 to 7.84 in the dolerites and 4.72 to 8.65 in the dioritic rocks. The quantitative value of Eu-anomaly, Eu/Eu, ranges from 0.98 to 1.11; this corroborates the absence of negative Eu-anomaly in the REE patterns. The fractionation indices, given by (La/Yb)N, in these rocks in which La and Ce are not mobilized

S.C. Obiora, S.N. Charan / Journal of African Earth Sciences 58 (2010) 197–210

10

Plume sources

Nb / Y

1

0.1

Non - plume sources

0.01 1

10

Zr / Y Fig. 7. Plots of the rocks on the Nb/Y versus Zr/Y log–log diagram of Fitton et al. (1997) for discrimination of mantle plume and non-plume sources (symbols as for Fig. 4). The parallel lines define the upper and lower bounds of the Icelandic basalts.

generally range from 8.11 to 8.65 in the dolerites and 5.7 to 14.72 in the dioritic rocks; the indices in the highly altered rocks range from 5.07 to 10.4 for the dolerites and 5.48 to 31.05 for the diorites. 6. Discussion The rocks examined in this study are alkali dolerites based on petrographical characteristics and their high Nb/Y coupled with low Zr/TiO2. This is apparent in their plots within the field of alkali basalts on the Zr/TiO2 versus Nb/Y diagram of Winchester and Floyd (1977). Only the rocks from locations 1 and 4 have HFSE concentrations and Ni/Co ratios which are comparable to those of intermediate rocks (diorites) in geostandards of Govindaraju (1994) and geochemical data in Taylor (1965). Although they contain plagioclase of andesine composition, they lack hornblende which is an essential mafic constituent in diorites (Le Maitre, 2002). The alkaline affinity of the rocks is supported by their enrichments in incompatible elements and high fractionation indices, (La/Yb)N, which range from 5.7 to 14.72. These high values of the fractionation index correspond to considerable depletion in HREE relative to LREE (otherwise, LREE enrichment) and indicate the presence of garnet as a residual phase in the sources of the rocks. A garnet-bearing source is also indicated by the ratios of (Tb/Yb)N (1.63–2.6) while the ratios of Dy/Yb (2.37–3.34) suggest garnet-bearing lherzolite source (cf Sayit and Goncouglu, 2009). The ratios of Zr/Nb in the range of 0.88–2.6 (for the relatively fresh samples) are similar to that of the High l mantle region (HIMU) (Tatar et al., 2007; Wilson, 1993). Ratios, which cover the range of both HIMU (2.7–5.5) and Enriched mantle sources (EM1: 3.5–13.1; EM2: 4.4–7.8) have been obtained for other igneous rocks in the Benue rift (Obiora and Ugwuonah, 2008; Obiora and Charan, in submission). Low ratios of Y/Nb (0.57–1.57) also suggest that the rocks may have been associated with enriched mantle sources as suggested by Sayit and Goncouglu (2009).

207

The ratios of La/Nb, Th/Nb and Th/La (Table 3) in all the rocks are within the range of HIMU and enriched mantle sources given by Tatar et al. (2007). Plots of the rocks on the Nb/Y versus Zr/Y log–log diagram of Fitton et al. (1997) as in Fig. 7 show that they are related to mantle plume sources. The overall similarities in both the spidergrams and the REE patterns are strong indications that they were derived from the same magma source and most probably co-genetic. The rocks were emplaced in a within-plate tectonic setting as shown by the discrimination diagrams using mostly the less mobile HFSE (Fig. 8a–c). On the Th/Yb versus Nb/Yb diagram of Pearce and Peate (1995) and Sayit and Goncouglu (2009) (Fig. 8d), the rocks plot mainly at the lower margin of the MORB field, close to the within-plate enrichment trend of Sayit and Goncouglu (2009). The within-plate setting was an intracontinental rift, similar to the Kenyan rift within the East African rift system as revealed by the La/10–Y/15–Nb/8 discrimination diagram of Cabanis and Lecolle (1989) shown in Rollinson (1993) (Fig. 8e). The depletion in the less mobile HFSE (Zr, Hf and Ta) in the spidergrams indicates the crystallization of zircon, ilmenite and sphene whereas the enrichment in Ta in some of the rocks (e.g. Fig. 5c) is evidence that ilmenite and zircon were not removed from the melt. On the other hand, the depletion in the more mobile LILE (Ba, Rb, K and Sr) is evidence of interaction with aqueous fluids and also indication of crustal contamination (Rollinson, 1993). The mobilization of the major elements and some of the HFSE, including La and Ce in the highly altered rocks most likely resulted from the low-grade metamorphic alteration in the presence of CO2-rich hydrous phase contained in the crustal materials. The involvement of a CO2-rich hydrous phase in the alteration of the intrusive igneous rocks in the study area has been suggested by Obiora and Umeji (2005). This alteration was responsible for the formation of calcite, tremolite, epidote, thuringite (chlorite) and analcime in the rocks. The plagioclase of oligoclase composition in the dolerites might have resulted from the breakdown of clinopyroxene and calcic plagioclase in the presence of the carbonate-rich hydrous fluid as demonstrated by Obiora and Umeji (2005) using chemical equations. The evidence of mobilization of the REE in the rocks suggests a kind of alteration in which the water/rock ratio was high. According to Rollinson (1993, p. 137–138), hydrothermal activity is expected to have a major effect on the REE contents only when there is high water/rock ratio because hydrothermal solutions contain between 5  102 and 106 times less REE than the rock through which they have passed. Wright (1976) had thought that the alteration in the diorites from Eziator (location 1) was caused by soda and carbonate-rich hydrothermal fluids upon emplacement of the rock in partly consolidated, but still wet sediments. Obiora and Umeji (2005), however were of the opinion that the wet state of the sediments during the emplacement of the highly altered rocks was unlikely since most of the rocks gave radiometric ages (Santonian to Campanian) which are much younger than the Albian age of the sediments, coupled with the presence of contact relations including chilling effects and baking of the sedimentary rocks. We suggest in this paper that the carbonated-rich hydrous phase responsible for the alteration in some of the rocks could be related to the saline groundwater with high concentrations of Ca2+, Na+, Cl , Sr+, Ba+ and K+. These fluids are thought to have been driven out of the predominantly calcareous marine sediments due to compaction and diagenesis accompanying their burial (Obiora and Umeji, 2004). This reasoning is supported by the Santonian age ascribed to the regional metamorphic alteration of the sedimentary rocks in the northernmost part of the study area by Benkhelil (1986, 1987), which is in line with the radiometric ages of the rocks.

208

S.C. Obiora, S.N. Charan / Journal of African Earth Sciences 58 (2010) 197–210

a 7.5

5

Zr / Y

Within - plate basalt Plate margin basalt

2.5

0 0

500

1000

Ti / Y

b

c

Hf / 3

12

Subduction zone enrichment 8

Rb / Y

A

B

Within - plate enrichment

4

D

Rb / Nb = 1

C 0 0

Th

Ta

1

2

3

4

5

6

7

8

Nb / Y

Fig. 8. (a) Zr/Y versus Ti/Y discrimination diagram for the rocks (symbols as for Fig. 4). (b) Plots of the rocks on the Th–Hf–Ta diagram of Wood (1980). Field A = N-type MORB; B = E-type MORB and within-plate tholeiites; C = alkaline within-plate basalts; D = volcanic arc basalts (symbols as for Fig. 4). (c) Rb/Y versus Nb/Y diagram (after Termel et al., 1998) showing within-plate enrichment trend for the rocks (symbols as for Fig 4). (d) Th/Yb versus Nb/Yb diagram of Pearce and Peate (1995) and Sayit and Goncouglu (2009) (symbols as for Fig 4). (e) Tectonic setting discrimination of the rocks on the La/10–Y/15–Nb/8 diagram of Cabanis and Lecolle (1989). Field 1A = calc-alkali basalts; 1C = volcanic arc tholeiites; 1B = an area of overlap between 1A and 1C; 2A = continental basalts; 2B = back-arc basalts; 3A = alkali basalts from intracontinental rifts (e.g. Kenya rift); 3B and 3C = E-type MORB (3B enriched, 3C weakly enriched); 3D = N-type MORB (symbols as for Fig. 4).

7. Conclusions Geochemical data from the intrusive igneous rocks in this study which are classified megascopically and microscopically as dolerites and dioritic rocks suggest that they are mainly dolerites. A few however, have trace elements concentrations comparable to those in diorites. All the rocks plot in the field of alkali basalts on the Zr/TiO2 versus Nb/Y diagram of Winchester and Floyd (1977). The alkaline affinity of the rocks is supported by their enrichments in incompatible elements and high fractionation indices, (La/Yb)N, which range from 5.7 to 14.72. These high fractionation indices, which reflect LREE enrichment, indicate the presence of garnet in

the source of the rocks. Garnet-bearing sources are also indicated by the ratios of (Tb/Yb)N (1.63–2.6) and Dy/Yb (2.37–3.34). Ratios of Zr/Nb (0.88–2.6) and Y/Nb (0.57–1.57) suggest that the rocks may have been associated with HIMU and more enriched mantle sources. The rocks were formed in a within-plate setting of the intracontinental rift-type, similar to the Kenyan rift. Crustal contamination and interactions of the rocks with aqueous fluids are suggested by depletion in the more mobile LILE. The major element (K), trace elements (Th, Co) and the REE (La, Ce) were heavily mobilized in some of the rocks during a low-grade metamorphic alteration, thought to have been caused by an intense hydrothermal activity involving a CO2-rich hydrous phase contained in the

S.C. Obiora, S.N. Charan / Journal of African Earth Sciences 58 (2010) 197–210

d

10

subduction zone enrichment 1

Th / Yb

ric en 0.1

MORB

0.01

de

et pl

e at pl in ith W

ed

d he

t en m h ric n e

0.001 0.1

1

e

Nb / Yb

10

100

Y / 15

3D 1C

1B

2B

3C

3B

1A

2A 3A

La / 10

Nb / 8 Fig. 8 (continued)

crustal materials. This hydrous phase is possibly related to the Carich saline groundwater that was pressed out from the calcareous marine sediments during their burial. Acknowledgements The first author is grateful to Dr. V.P. Dimri, Director, National Geophysical Research Institute (NGRI), Hyderabad, India for accepting him as a CSIR-TWAS Postdoctoral Fellow and permission to use the laboratory facilities during the period, July 2008 to June 2009. The ICP-MS analyses were carried out by Dr. M. Satyanarayanan and Mr. Sawant while the XRF was done by Keshav Krishna, all of NGRI. The maps were drawn by Mr. S. Jamaluddin, also of NGRI. References Benkhelil, J., 1986. Structure and Geodynamic Evolution of the Intracontinental Benue Trough (Nigeria). Ph.D. Thesis, University of Nice. Published by Elf Nigeria Ltd., SNEA(P), 236p.

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