Journal of Mining and Geology Vol. 48(1) 2012, pp. 13–29© Nigerian Mining and Geosciences Society (NMGS) - Printed in Nigeria

1116-2775

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IntroductionGranitic rocks belong to one of the three major rock units that constitute the Precambrian Basement Complex in Nigeria. The other two units are the migmatites, gneisses and migmatitic gneisses, which are the basement sensu stricto and the schists (Oyawoye, 1964, Obiora, 2005). These granitic rocks, also referred to as Precambrian granites or the Older Granites, consist of porphyritic/porphyroblastic muscovite granites, biotite granites, hornblende-biotite granites, non-porphyritic/non-porphyroblastic granites, aplites, granodiorites, diorites, quartz diorites, syenites, quartz-hypersthene granites and hypersthene granites (charnockites) (Oyawoye, 1964, 1972; Rahaman, 1976; Onyeagocha, 1984; Makanjuola, 1982; Ekwueme, 1987, 1994; Olarewaju, 1987; Obiora, 2005, 2006). The Precambrian Basement Complex rocks in the part of the extension of the Bamenda massif into Southeastern Nigeria where the present study area is located are largely not well known. Hitherto, the publication on the radiometric age measurement of 539 ± 8 Ma obtained for biotite granite from Mkar and pyroxene-bearing hornblende-biotite granite from northeast of Katsina-Ala by Umeji and Caen-Vachette (1984) appears to be the only published work on the rocks around the area. Geochemical data required for proper classification and characterization of these granitic rocks are generally scarce. In this work, major, trace and rare-earth elements (REE) data on seven relatively unknown granitic plutons have been used to assess their petrogenetic evolution and tectonic setting.

Regional Tectonics and GeologyThe Nigerian Precambrian Basement Complex lies within the reactivated part of the Pan-African belt which resulted from the collision of the passive continental margin of the West-African craton and the active margin of the Pharusian belt (Tuareg shield), about 600Ma (Burke and Dewey, 1972; Leblanc, 1981; Black et al., 1979; Caby et al., 1981) (Fig.1). Evidence for the collision includes the presence of basic to ultrabasic rocks believed to be either remnants of mantle diapirs or a paleo-oceanic crust, which has characteristics of an ophiolitic complex and a high positive gravity anomaly. These occur in a narrow zone within the Dahomeyide orogen, which is located at the southeastern margin of the West African Craton in Togo and Benin Republic (Schluter, 2005). This collision orogenesis referred to as 'Trans-Saharan Pan-African Orogen' (Ferre et al., 1998) is characterised by early thrust-nappe development, high grade metamorphism, voluminous granite plutonism and late orogen-parallel tectonics (Boullier et al., 1986; Caby, 1989; Black and Lie´geois, 1993). The Precambrian ages recorded within the Nigerian Basement Complex are therefore mostly related to the Neoproterozoic to Early Paleozoic [Pan-African (900 450Ma)] thermotectonic events, with few imprints of older events such as the Meso-archean to Palaeo-proterozoic [Liberian (3000 2400 Ma)], Palaeo- to Meso-proterozoic [Eburnean (2400 1600 Ma)] and Meso- to Neo-proterozoic [Kibaran (1600 900 Ma)] (Obiora, 2005, 2008).

Chemical Characterization And Tectonic Evolution of Hornblende-Biotite Granitoids From The Precambrian Basement Complex Around Ityowanye and Katsina-Ala, Southeastern Nigeria

Smart Chika ObioraDepartment of Geology, University of Nigeria, Nsukka

Abstract

Geochemical studies on some porphyritic and non-porphyritic hornblende- biotite granitoids, which intruded the Precambrian Basement Complex around Ityowanye and Katsina- Ala in southeastern Nigeria, indicate that they are generally high-K calc-alkaline transalkaline to non-alkaline, alkalic-calcic to alkalic, ferroan and metaluminous. They are characterized by LILE enrichment, strong Nb and Ti anomalies, high fractionation factor (La/Yb) (8.06 to 84.43) and pronounced negative Eu-anomalies. N

They display similar trace-elements and REE patterns, suggesting that they are co-genetic. Their overall geochemical features indicate that they were most likely derived from partial melting of hornblende-rich crustal sources in an orogenic (syn- to late/post-collisional) tectonic setting.

The granites and granitoids in the Precambrian Basement Complex of Nigeria generally consist of quartz, K-feldspars, plagioclase of oligoclase composition (An ) and biotite. Hornblende, 22-28

hypersthene and muscovite are common in some varieties, namely: amphibole (hornblende)-biotite granites, hypersthene granites (charnockites) and muscov i te g ran i te s. Typ ica l minera l s o f metasedimentary rocks such as garnet and staurolite are also commonly encountered in some of the granitic rocks which are often weakly foliated. In NW Nigeria, the rocks include syenites and biotite granites. They have been described as calcalkaline, I-type granites

(Olarewaju and Rahaman, 1982; Egbuniwe et al., 1985; Fitches et al., 1985). The syenites have mildly alkaline affinities (Egbuniwe et al., 1985). In NE Nigeria (Northern Plateau), the rocks which consist of biotite ± muscovite ± garnet granites, porphyritic biotite ± amphibole- monzo- and syenogranites, porphyritic amphibole-biotite-clinopyroxene ± orthopyroxene ± fayalite quartz-monzonites and equigranular amphibole-biotite-clinopyroxene orthopyroxene quartz monzodiorites are metaluminous, transalkaline to non-alkaline, high K, ferriferous to highly ferriferous. They show an enrichment of incompatible trace-elements (Rb, Ba

1. Geological Map of parts of West Africa showing the position of Nigeria and its Pan-African basement, the Congo-Gabon craton, the West African craton and the Tuareg shield

(Adapted from Wright et al., 1985 in Obiora, 2006).

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and Th) with respect to the LREE, strong Nb and Ti anomalies for the most fractionated rocks, high fractionation factor, (La/Yb) and moderate negative N

Eu-anomalies (Ferre et al., 1998). In northcentral

Nigeria (NW Akwanga), the rocks are two-mica (muscovite-biotite) granites. They are Al-rich to slightly Al-excess and characterized by negative Eu anomaly, high Th contents, abundance of partially digested xenoliths of older schists and gneisses. They also have REE abundances and REE trends similar to those of rocks known to have been derived through partial melting of crustal rocks (Onyeagocha, 1986). Close to the southernmost basement-sediment contact in northcentral Nigeria around Nassarawan Eggon, the granitic rocks, namely: hypersthene-bearing biotite granites are magnesian, calc-alkalic and strongly peraluminous (Obiora and Ukaegbu, 2009). In the Oban Massif (southeastern Nigeria) the rocks are mainly granodioritic in composition (porphyroblastic hornblende-biotite granodiorites) with local variations of dioritic or tonalitic to granitic compositions. They are subalkaline (calc-alkaline), peraluminous and highly rich in K O (Rahman et al., 1988). In Ogoja 2

areas, also in southeastern Nigeria, the granitic rocks which include biotite granites, hornblende-biotite granites and granodiorite are subalkaline, highly potassic and metaluminous (Obiora, 2006). In SW Nigeria, the rocks are highly potassic, Fe-enriched and largely metaluminous porphyritic granites and granitic gneisses (Rahaman et al., 1983; Okonkwo and Winchester, 2004).

The granites and granitoids in the Precambrian Basement Complex, like most of the other Basement Complex rocks (migmatites, gneisses, migmatitic gneisses, schists) date Neoproterozoic to Paleozoic (See Table 1).

Field Occurrence and CharactersThe study area is underlain by banded gneiss, within which three bodies of porphyritic hornblende-biotite granites (P1, P2, P3), one porphyritic hornblende-biotite granodiorite (P4), two non-porphyritic hornblende-biotite-granodiorites (NP1 and NP2), and one non-porphyritic hornblende-biotite syenogranite (NP3) have been emplaced (Fig. 2). The banded gneiss is mesocratic and medium-grained, characterized by gneissose foliation or conspicuous alternations of light and dark coloured bands. Occasionally, it develops some features which are common in migmatite terrains such as augen structures, contorted foliations and quartzo-feldspathic veins which are often

pegmatitic. They can therefore be described as ''migmatitic'' based on these features. The light bands consist mainly of quartz and feldspars while the dark bands are enriched in mica (biotite). This constitutes the Basement sensu stricto in the study area and is very deeply weathered to clayey to loamy regoliths. Fresh outcrops are scarce and only encountered in the southeastern part of the area. It is also covered by alluvium along the banks of the River Katsina-Ala.

The porphyritic granites are generally leucocratic and coarse-grained, containing euhedral crystals of K-feldspars as phenocrysts, with quartz, plagioclase, biotite and hornblende in the coarse-grained groundmass. They are also weakly foliated. The granite body, P1 is generally circular in outline with average diameter of about 64 m and rises about 1.5 m above the surroundings; P2 is about 700 m long, 400 m wide and 1.5 m high with an essentially NW-SE orientation; P3 is about 700 m long, 300 m wide and over 3 m high, with NE-SW trend while P4 constitutes a circular body of diameter 150 m, rising about 5 m above the surroundings. The non-porphyritic granitic rocks (NP1, NP2, NP3) contain similar minerals as the porphyritic types and are much similar in dimensions to P4.

Petrographic CharacteristicsThe modal compositions of the banded gneiss and the granitic intrusions estimated from thin section studies are shown in Table 2. The feldspars present in the banded gneiss are plagioclase (An , andesine), and 43

microcline. The banded gneiss also contains two micas (biotite and muscovite), together with hornblende. The porphyritic granitic rocks consist of phenocrysts of subhedral to euhedral crystals of feldspars (microcline, orthoclase perthite and plagioclase) in a groundmass of anhedral quartz, plagioclase, dark brown biotite and hornblende. The plagioclase is oligoclase (An ). The feldspars 28

(plagioclase and microcline) in the rocks are colourless and cloudy in thin section. Plagioclase shows the characteristic albite polysynthetic twinning while microcline shows the characteristic cross-hatched (grid) twinning. Quartz is colourless and very clear in thin section and shows first-order interference colours of grey and white. Biotite is dark brown and strongly pleochroic from dark brown to light brown. It occurs in sections lath-shaped crystals showing perfect cleavages in one direction. The sections undergo parallel extinction. Muscovite differs from biotite in its lack of colour and the possession of third-

15

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order bright polarization colours of green. Hornblende mainly occurs in longitudinal sections which are brown, exhibiting strong pleochroism from light brown to yellowish brown. A few basal sections showing characteristic cleavages were also encountered. The longitudinal sections show inclined extinction, with angles between 20 and 22º. The hornblende-biotite-granodiorites (P4, NP1 and NP2) and the hornblende-biotite-syenogranite (NP3) contain large euhedral crystals of orthopyroxene(enstatite), which are colourless in thin section, showing perfect longitudinal

ndcleavages, 2 order interference colours and parallel extinction.

Geochemical CharacteristicsMethods of analysesGeochemical analyses on twelve representative samples of the granitoids and two samples of the banded gneiss were carried out at the Geochemistry Department of the Geological Division, National Geophysical Research Institute, Hyderabad, India. Major elements oxides were determined by a Philips MagiX PRO, Model PW 2440, wavelength dispersive X-ray Fluorescence Spectrometer, coupled with an automatic sample changer PW 2540 and using online SUPER Q 3.0 software (Philips, Eindhoven, The Netherlands). The analyses were carried out on

2. Geological Map of Ityowanye and Katsina- Ala areas.

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pressed pellets (40 mm diameter) prepared by using collapsible aluminium cups. The cups were filled with boric acid and covered by about 2 g of finely powdered rock samples, thereafter pressed under a hydraulic press at 55-ton pressure to obtain a pellet using the Hydraulic Press (Herzog, Germany). Loss on ignition (LOI) was determined by heating 1.0 g of powdered samples, placed in porcelain crucibles, in a furnace

0 heated to a temperature of 950 C for about 30 minutes and determining 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). 10ml acid mixture (HF:HNO :HCLO ) in the ratio of 7:3:1 was 3 4

added to 0.050g 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

0(~150 C) until they were nearly dry to form a completely crystalline paste. 20 ml of 1:1 (HNO : 3

distilled water) was thereafter added to each sample and warmed to dissolve the crystalline paste, followed

103by the addition of 5ml of 1 ppm Rhodium ( 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 labeling. Certified values of JG-2(Granite) (Govindaraju, 1994) were used as reference standard

for the analyses of all the rocks.

ResultsThe major element oxides, with CIPW norms and the trace-elements data are presented on Table 3a while the REE data are on Table 3b.

Major elements oxidesThe samples of the banded gneiss (BGM1 and BGM2) contain SiO (69.54 and 70.33%) while the 2

granitic rocks contain SiO in the range of 65.4 to 2

69.89 %; they are generally enriched in TiO (0.34 to 2

0.81 %), Fe O (t) (3.37 to 7.21 %), MgO (0.17 to 0.49 2 3

%) and CaO (1.24 to 2.46 %) compared to values of 0.03, 0.90, 0.03 and 0.79 %, respectively in the reference standard (JG-2). The enrichment in these oxides correlates positively with their high contents of mafic minerals (biot i te, hor nblende and or thopyroxene). The occurrence of only orthopyroxene in the mode of the rocks is also very apparent in the norm (Table 3a). They plot in the fields of granite, alkali granite and syenite on the TAS diagram of Cox et al. (1979) adapted by Wilson (1989) for chemical classification and nomenclature of plutonic rocks; most of them lie at the dividing line between alkaline and subalkaline rocks (Fig. 3a). On the TAS diagram of Middlemost (1997), they plot as transalkaline to subalkaline (calcalkaline + tholeiitic) (Fig. 3b). They are High-K calc-alkaline and metaluminous granitic rocks (Figs. 3c and 3d). On the plot of the Fe-number [Fe O tot/ (Fe O tot + MgO)] 2 3 2 3

40 50 60 70

SiO2(wt%)

0

2

4

6

8

10

12

14

16

Na

2O

+K

2O

(wt%

)

Gabbro

Gabbro

GabbroIjolite

Syeno - diorite

Syenite

Nepheline syenite

Syenite

Alkali granite

GraniteQuartzdiorite

Diorite

3a.Chemical classification and nomenclature of the granitoids as granites, alkali granites and syenites on the TAS diagram of Cox et al. (1979) adapted by Wilson (1989). Solid rhombus = granitoids; open triangles = migmatitic banded gneiss.

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versus SiO , the granitic rocks classify as ferroan (after 2

Frost et al., 2001) (Fig. 3e) and as alkalic- calcic to alkalic on the Modified Alkali-Lime Index (MALI) plot (also after Frost et al., 2001) (Fig. 3f).

N-type MORB normalized trace-elements patterns The granitic rocks (both porphyritic and non-porphyritic) all show similar patterns and contain 5 to 10 times higher amounts of the trace-elements than the gneisses, although both show somewhat similar trends

45 50 55 60 65 70 75

SiO2(wt%)

0

2

4

6

8

10

12

14

16

Na

2O

+K

2O

(wt%

)

calcalkaline + tholeiitic

transalkaline

alkaline

3b. Plots of the granitoids as subalkaline (calcalkaline + tholeiitic) to transalkaline on the TAS diagram of Middlemost (1997). Symbols as in Fig. 3a.

in which there is overall enrichment of the large ion lithophile elements (LILE: Rb, Ba, Th, U, K) and depletion of the high field strength elements (HFSE: Hf, Zr, Ti, Y). Particularly, the granitic rocks show strong positive anomalies in Th, K, La, Ce, Nd, Sm, Tb and negative anomalies in Ba, U, Nb, Ta, Sr, Hf, Zr, and Ti while the gneisses show strong positive anomalies in Rb, K, La, Sm and negative anomalies in Th, Ta, Zr and Y(Fig. 4).

45 50 55 60 65 70 75 80

SiO2(wt%)

0

1

2

3

4

5

6

7

8

K2O

(wt%

)

Low-K tholeiitic

Medium-K calcalkaline

High-K calcalkaline

3c. Plots of the granitoids as High- K calcalkaline rocks (after Rickwood, 1989). Symbols as in Fig. 3a.

0.5 1 1.5

Al2O3 / (CaO + Na2O + K2O)

0.5

1

1.5

2

2.5

3

Al 2

O3

/(N

a2O

+K

2O

)

Metaluminous

Peraluminous

Peralkaline

3d. Plots of the granitoids as metaluminous rocks (after Maniar and Piccoli, 1989). Symbols as in Fig. 3a.

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50 60 70 80 90

SiO2(wt%)

0.4

0.6

0.8

1

Fe

*=

FeO

tot

/(F

eO

tot

+M

gO

)(w

t%)

magnesian

ferroan

3e. Plots of the granitoids showing their ferroan character (after Frost et al., 2001). Symbols as in Fig. 3a.

50 60 70 80 90

SiO2(wt%)

-8

-4

0

4

8

12

Na

2O

+K

2O

-C

aO

(wt%

)

calcic

alkalic

alkalic

- calcic

calcic

- alkalic

3f. Plots of the granitoids in the field of alkalic- calcic to alkalic on the Modified Alkali-Lime Index plot (after Frost

et al., 2001). Symbols as in Fig. 3a.

0.1

1

10

100

1000

Tra

ce

-ele

men

tsc

on

ce

ntr

ati

on

/N

-ty

pe

MO

RB

RbBa Th U K NbTaLaCe SrNd P Hf Zr SmTi Tb Y

4. N-type MORB normalized trace-elements patterns for the granitoids showing strong enrichments in the LILE and depletions in HFSE. The normalizing values are those of

Saunders and Tarney (1984) with additions from Sun (1980). Symbols as in Fig. 3a.

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REE patterns The granitic rocks also show similar REE patterns and contents which are also higher than those of the gneisses by a factor of 4 to 10. Generally, the REE patterns for both the banded gneiss and the granitic rocks display LREE enrichment relative to the MREE and HREE. The granitic rocks display patterns with distinct negative Eu-anomalies with inclined MREE

and flat HREE whereas those of the gneisses lack the negative anomalies, although they show inclined MREE and flat HREE (Fig. 5). Specifically, the Eu-anomalies, expressed as (Eu/Eu*) ranges from 0.2 to 0.45 in the granitic rocks and from 0.89 to 1.29 in the gneisses. The La /Yb ratios in the granitic rocks N N

range from 8.06 to 84.43 whereas the range is from 39.22 to 67.97 in the gneisses.

DiscussionThe High-K calc-alkaline, alkali-calcic to alkalic and metaluminous character of the granitic rocks make them similar to the Caledonian granitoids, otherwise variously known as High-K alkali-calcic granitoids, Post-orogenic granitoids, Shoshonitic granitoids and K-rich calc-alkaline granitoids (Frost et al., 2001). The granitic rocks in this study, however, differ from the Caledonian granitoids in their ferroan (iron enrichment) character (Frost et al., 2001). A comparison of the geochemical characteristics of the granitic rocks in this study with those from other parts of Nigeria and the world (Table 4) reveal that they are very much similar to those of the Pan-African granitic

0.1

1

10

100

1000

RE

Eco

nce

ntr

ati

on

/C

ho

nd

rite

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

5. Chondrite- normalized REE patterns for the granitoids showing distinct negative Eu-anomalies with inclined MREE and flat HREE. The normalizing values are those of Nakamura (1974) with additions

from Haskin et al. (1968). Symbols as in Fig. 3a.

rocks which occur together with Jurassic (Younger) Granites in the Northern Plateau areas (NE Nigeria). The granitic rocks in this study and those in the Northern Plateau are both located in the Eastern terrane of Nigeria (Ferre et al., 1998). These characteristics include metaluminous, transalkaline, High-K character, with iron enrichment coupled with enrichment in LILE, high fractionation factor (La/Yb) (8.06 to 84.43) and strong Nb and Ti N

anomalies. The granitic rocks are also similar those in Igbeti and Jebba areas (SW Nigeria) in their metaluminous, highly potassic, Fe-enriched character (Rahaman et al., 1983; Okonkwo and Winchester, 2004). According to Ferre et al. (1998), metaluminous,

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P1 P2(1) P2(2) P2(3) P2(4) P3(1) P3(2) P4(1) P4(2) NP1 NP2 NP3 BGM1 BGM2

Major element oxides(wt%)

SiO2 68.97 65.4 66.05 69.89 68.31 67.87 69.86 68.25 67.76 69.67 66.26 66.31 69.54 70.33

TiO2 0.55 0.56 0.38 0.34 0.42 0.62 0.35 0.62 0.63 0.52 0.6 0.81 0.39 0.22

Al2O3 12.41 14.09 15.42 13.79 14.29 12.14 13.92 12.23 12.82 12.48 13.68 12.09 14.01 14.23

Fe2O3(t) 5.34 5.58 3.86 3.37 4.21 5.39 3.57 5.99 6.08 4.84 5.77 7.21 3.56 2.75

MnO 0.06 0.07 0.04 0.04 0.05 0.05 0.04 0.07 0.07 0.06 0.07 0.09 0.04 0.04MgO 0.32 0.29 0.21 0.17 0.21 0.49 0.33 0.37 0.35 0.27 0.34 0.47 1.53 1.16

CaO 1.92 2 1.43 1.27 1.42 2 1.24 2.34 2.47 1.93 2.46 2.79 3.89 3.97

Na2O 2.27 2.73 2.85 2.68 2.91 2.07 2.23 2.5 2.56 2.52 2.87 2.54 3.67 4.11

K2O 5.71 6.62 7.54 6.83 6.21 5.2 6.17 5.3 4.85 5.64 5.54 4.9 1.3 0.73

P2O5 0.21 0.19 0.11 0.11 0.12 0.2 0.11 0.24 0.22 0.17 0.2 0.34 0.09 0.04

LOI 0.8 0.5 0.61 0.21 0.66 1.98 0.64 0.51 0.57 0.46 0.41 0.46 0.55 0.46TOTAL 98.56 98.03 98.5 98.7 98.81 98.01 98.46 98.42 98.38 98.56 98.2 98.01 98.57 98.04

C.I.P.W.Norm

Q 24.53 12.48 9.09 16.7 16.98 33.6 22.97 22.66 23.04 22.73 15.8 22.02 25.37 25.38Pl 22.67 28.86 26.24 26.49 26.66 10.63 20.21 28.14 29.43 27 32.69 29.71 46.11 50.83

Or 35.32 40.78 45.17 40.84 37.53 32.84 37.44 33.05 30.28 34.57 34.27 31.17 7.98 4.42C 0.91 0.27 1.08 0.26 1.32 3.11 2.02 - 0.48 0.12 - - 0.65 0.41

Di - - - - - - - 0.24 - - 0.16 1.08 - -Hy 12.37 13.94 14.8 13.18 13.81 12.63 13.82 12.23 12.91 12.26 13.58 11.95 16.5 16.04

il 0.59 0.6 0.4 0.35 0.45 0.67 0.37 0.67 0.68 0.55 0.64 0.9 0.42 0.23mt 1.41 1.59 1.69 1.51 1.58 1.4 1.54 1.39 1.46 1.4 1.55 1.41 1.57 1.59

ap 0.4 0.36 0.2 0.2 0.22 0.39 0.21 0.46 0.42 0.32 0.38 0.68 0.17 0.07TOTAL 98.2 98.88 98.67 99.53 98.55 96.87 98.58 98.84 98.7 98.95 99.07 98.92 98.77 98.97

Trace elements (ppm)

Sc 9.49 8.453 6.003 5.099 6.244 9.008 7.383 8.866 9.673 6.851 8.583 12.599 4.6 3.956V 5.442 5.257 4.507 2.668 3.417 5.408 4.184 5.051 6.577 4.248 4.464 7.792 16.826 12.369

Cr 4.835 5.9 4.801 4.154 5.843 4.914 6.747 5.917 5.782 5.138 5.29 6.241 9.106 8.404Co 4.421 4.223 2.933 2.466 4.007 4.009 2.539 4.477 4.658 3.587 4.118 6.571 8.358 6.067

Ni 0.733 0.775 0.686 0.641 0.678 0.67 0.676 0.812 0.742 0.71 0.699 0.98 1.76 1.626Cu 0.449 0.516 0.325 0.354 0.319 0.466 0.307 0.463 0.433 0.415 0.419 0.614 0.486 0.22

Zn 29.926 28.64 24.381 19.508 26.208 28.074 19.88 29.673 30.012 26.446 27.313 37.383 17.966 14.143Ga 23.661 25.122 28.199 20.733 29.773 22.241 20.328 23.312 24.572 21.824 23.778 25.003 18.563 17.235

Rb 134.718 144.664 173.209 144.482 178.776 122.77 137.097 113.79 106.707 119.248 110.675 110.604 29.62 12.463

Sr 105.458 129.271 136.107 123.427 158.064 95.821 104.856 125.1 124.467 129.962 129.027 134.928 136.795 120.87Y 82.485 69.775 55.507 42.423 52.269 72.326 50.777 66.352 70.954 49.449 60.87 83.314 3.92 1.75

Zr 50.944 41.625 49.095 27.633 50.324 72.271 37.9 53.12 65.033 38.627 47.995 38.68 10.052 13.265Nb 41.609 36.686 23.561 21.656 33.602 37.261 23.5 35.445 39.049 30.252 35.372 48.515 2.768 2.308

Cs 1.216 1.144 1.813 1.149 1.626 1.112 1.22 0.9 0.845 0.687 0.746 0.771 0.315 0.124Ba 523.064 679.083 1013.244 717.016 1179.714 500.013 586.313 618.138 555.456 694.942 636.396 628.838 92.958 56.886

Hf 2.393 1.95 2.159 1.23 2.032 2.973 1.722 2.23 2.517 1.583 1.995 1.924 0.354 0.476Ta 1.466 1.638 1.132 0.98 1.498 1.156 1.331 0.139 1.793 1.286 1.543 1.765 0.107 0.107

Pb 6.498 6.925 7.865 7.497 7.649 5.74 6.299 6.724 6.485 6.193 6.395 6.811 3.372 3.487Th 22.956 13.292 60.823 6.673 18.045 19.881 5.063 12.194 24.375 10.621 14.729 11.165 0.188 0.238

U 2.108 2.031 2.416 1.306 1.765 2.961 1.388 1.728 2.184 1.183 1.577 1.517 0.255 0.905Rb/Sr 1.28 1.12 1.27 1.17 1.13 1.28 1.31 0.91 0.86 0.92 0.86 0.82 0.22 0.1

Ba/Sr 4.96 5.25 7.44 5.81 7.46 5.22 5.59 4.94 4.46 5.35 4.93 4.66 0.68 0.47Ba/Rb 3.88 4.69 5.85 4.96 6.6 4.07 4.28 5.43 5.21 5.83 5.75 5.69 3.14 4.56

Table 3a. Major element oxides, with C.I.P.W. norms and trace elements data on the granitic

intrusions and the host rock (banded gneiss) in Ityowanye and Katsina- Ala areas.

Explanation:

P1, P2, P3, P4 are porphyritic granitoids; NP1, NP2, NP3 are non-

porphyritic granitoids; BGM1 and BGM2 are banded gneiss.

23

24

Table 3b. REE data (ppm) on the granitic intrusions and the host rock (banded gneiss)in Ityowanye and Katsina-Ala areas.

25

transalkaline, ferro-potassic character is common to most alkaline granite suites, which usually have SiO -2

contents around 74%. However, the granitoids in this study have SiO -contents in the range of about 65 to 2

70%; this makes it difficult to compare them with alkaline granites. The common occurrence of only orthopyroxene, high K-feldspar content and the absence of clinopyroxene in the rocks is a characteristic feature of silicic charnockites, as opposed to that of intermediate charnockites (Rajesh, 2007). Contrary to the observation by Frost et al.(2001) regarding lack of iron enrichment in post-orogenic granitoids, Hyndman (1972) had reported that post-orogenic associations undergo pronounced iron-enrichment during most of the period of crystallization, presumably due to a low oxygen pressure, P , which is probably as a result of O2

low water-content in the magma at the time of its generation; the low concentration of oxygen inhibits the oxidation of iron to other elements, thereby enriching the magma in iron throughout the crystallization period. Also, in the later stages of crystallization, the trend turns rapidly towards higher alkalis.

The orogenic nature of the granitic rocks in this study is shown in the FeO (t) +MgO (wt %) versus CaO (wt %) diagram of Maniar and Piccoli (1989) (Fig. 6a)

while their post-orogenic character is supported by their plots on the Rb versus Y + Nb of Pearce et al. (1984) as modified by Pearce (1996) (Fig.6b). However, on the Rb/30 Hf (Ta x 3) diagram of Harris et al. (1986) (Fig. 6c), the granitic rocks spread in the fields of Syn-collisional to Late/post-collisional granitoids.

The similar trace-elements spidergrams and REE patterns for all the granitic rocks is evidence of their co-genetic origin. The enrichment in light REE relative to heavy REE could be caused by the presence of hornblende in the felsic melts (Rollinson, 1993). The negative Eu-anomalies reflect the crystallization of feldspars from the melt. Nb and Ti negative anomalies are characteristic of magmas connected with subduction areas (Bachlinski, 2007). Negative Nb anomalies are also characteristic of the continental crust and may be an indicator of crustal involvement in magma processes; negative Zr anomalies reflect fractionation of zircon (Rollinson, 1993; Baginski et al., 2007).

Metaluminous character of granitic rocks precludes an origin from the melting of strongly peraluminous sedimentary protoliths, but favours an origin from a metaluminous protolith such as the mantle or other igneous rocks which would not have

0 4 8 12 16 20 24 28

CaO (wt%)

0

10

20

30

40

50

60

Fe

O(t

)+

Mg

O(w

t%)

I

A

II

A = AMBIGUOUS ZONE

II = ANOROGENIC GRANTOID (rift-related and continental epeirogenic uplift; Maniar and Piccoli, 1989)

I = OROGENIC GRANITOID (island and continental arcs, collision; Maniar and Piccoli, 1989)

6a. Plots of the rocks from Ityowanye and Katsina- Ala areas as orogenic granitoids on the FeO (t) +MgO (wt %) versus CaO (wt %) diagram of

Maniar and Piccoli (1989). Symbols as in Fig. 3a.

10 100 1000

Y + Nb (ppm)

1

10

100

1000

Rb

(pp

m)

syn- COLG

WPG

post- COLG

VAG

ORG

6b. Post-orogenic character of the granitoids shown on the Rb versus Y + Nb of Pearce et al. (1984). syn-

COLG= syn-Collision Granites; post-COLG= post-Collision Granites; WPG= Within Plate Granites; VAG= Volcanic Arc Granites; ORG= Ocean Ridge

Granites. Symbols as in Fig. 3a.

26

undergone alterations, leading to increase in A/CNK ratios (Ferre et al., 1998). The overall geochemical features of the granitic rocks in this study therefore suggest their derivation from partial melting of hornblende-rich (igneous) crustal sources. From the tectonic setting analyses using the geochemical discrimination diagrams, the rocks are seen to be related to compressional(syn- to post-collissional) tectonism, rather than extensional. They are therefore related to the Pan-African Granites, otherwise known as the Older Granites which were emplaced during the Pan-African Orogenic/thermotectonic event. The granitic rocks in this study were most likely emplaced towards the end of the reactivation event, resulting from the collision of the West African Craton with the Tuareg shield, about 600 Ma. This collision was preceded by the subduction of the lithosphere beneath an ancient oceanic crust at the eastern margin of the West African craton underneath the Tuareg shield (Obiora, 2006). The existence of this paleo-oceanic crust is supported by the presence of basic to ultrabasic rocks with characteristics of an ophilitic complex and a high positive gravity anomaly in a narrow zone within the Dahomeyide orogen, located at the southeastern margin of the West African Craton in Togo and Benin Republic (Schluter, 2005).

Summary and ConclusionsGeochemical studies on the porphyritic and non-porphyritic amphibole- biotite granitoids in this study indicate that they are High-K calc-alkaline, alkali-calcic to alkalic and metaluminous suite of granitic rocks. This makes them similar to the Caledonian granitoids, otherwise variously known as High-K alkali-calcic granitoids, Post-orogenic granitoids, Shoshonitic granitoids and K-rich calc-alkaline granitoids. They however, differ from the Caledonian granitoids in their ferroan (iron enrichment) character. The common occurrence of only orthopyroxene, high K-feldspar content and the absence of clinopyroxene in the rocks is a characteristic feature of silicic charnockites, as opposed to that of intermediate charnockites. Although these granitic rocks are also similar to most alkaline granites in their metaluminous, transalkaline, ferro-potassic character, it is difficult to compare them to alkaline granites due to their generally low SiO -contents in the range of about 65 2

to 70%, as opposed to values which are up to 74% in alkaline granites. Other geochemical features of the granitoids include enrichment in LILE, strong Nb and Ti anomalies, high fractionation factor (La/Yb) N

(8.06 to 84.43) and pronounced negative Eu-anomalies. These suggest derivation from partial melting of hornblende-rich crustal sources. Tectonic setting analyses using discrimination diagrams indicate origin in an orogenic (syn-collisional to late/post-collisional environment, related to the collision of the West African Craton with the Tuareg shield, about 600 Ma.

Acknowledgements The author is grateful to Messrs Onah Fidelis, Edoka Ucheora and Miss Uwom Chineyem who assisted in the field work. He also thanks 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 at the Geological Division (Geochemical Section) of the Institute during the period, July 2008 to June 2009. Dr. Nirmal S. Charan (Scientist G), the contact Scientist to the author during his stay in NGRI facilitated the use of the laboratory facilities.The geochemical analyses were carried out by Drs. T. G. Rao and Keshav Krishna of NGRI.

Ta x 3

Rb/30

Hf

Syn- collisional

Late and post-collisional

Within plateVolcanic arc

6c. Plots of the rocks in the fields of Syn-collisional to Late/post-collisional granitoids on the Rb/30 Hf (Ta x 3)

diagram of Harris et al. (1986). Symbols as in Fig.3a.

27

Bachlinski, R., 2007. Kudowa Olesnice granitoid massif. Granitoids in Poland, AM Monograph No. 1, pp. 275 286.

Baginski, B., Duchesne, J., Martin, H. and Wiszniewska, J., 2007. Isotopic and geochemical constraints on the evolution of the Mazury granitoids, NE Poland. AM Monogragh No. 1, pp. 11 30.

Black, R., Caby, R., Moussine- Pouchkine, A., Bayer, R., Betrand, J. M., Boullier, M.M., Fabre, J. and Resquer, A., 1979. Evidence for late Precambrian plate tectonics in West Africa. Nature, 278, 5701, pp. 223-227.

Black, R. and Liegeois, J. P., 1993. Cratons, mobile belts, alkaline rocks and continental lithospheric mantle, the Pan- African testimony. Journal of the Geological Society, London, 150, pp. 89 -98.

Boullier, A.M., Lie´geois, J.P., Black, R., Fabre, J., Sauvage, M., Bertrand, J.M., 1986. Late Pan-African tectonics marking the transition from subduction-related calc-alkaline magmatism to within-plate alkaline granitoids (Adrar des Iforas, Mali). Tectonophysics, 132, pp. 233246.

Burke, K.C. and Dewey, J.F., 1972. Orogeny in Africa. In: Dessavagie, T.F.J. and Whiteman, A.J. (Eds) African Geology, Ibadan: Ibadan University Press, pp.583-608.

Caby, R., 1989. Precambrian terranes of Benin-Nigeria and Northeast Brazil and the late proterozoic south Atlantic fit. Geological Society of America Special Paper, 230, pp. 145 158.

Caby, R., Betrand, J. M. L. and Black, R., 1981. Pan-African ocean closure and

continental collision in the Hogger-Iforas segment, central sahara. In: Kroner, A. (ed) Precambrian plate tectonics, Elsevier Amsterdam, pp. 407- 437.

Caen-Vachette, M. and Umeji, A.C., 1983. Whole-rock Rb-Sr dating of two Monzogranites in Southern Nigeria and their implications on the age of the Pan-African orogenic cycle. Journal African Earth Sciences, 1, pp. 339-342.

Cox, K. G., Bell, J. D., Pankhurst, R. J., 1979. The interpretation of igneous rocks. George, Allen and Unwin, London.

Dada, S.S., Lancelot, J.R., Briqueu, L., 1989. Age and origin of the annular charnockitic complex at Toro, Northern Nigeria: UPb and RbSr evidence. Journal of African Earth Sciences, 9, pp. 227234.

Egbuniwe, I., Fitches, W., Bentley, M., and Snelling, N., 1985. Late Pan-African syenite- granite plutons from NW Nigeria. Journal of African Earth Sciences, 3(4), pp. 427435.

Ekwueme, B. N., 1987. Structural orientation and Precambrian deformational episode of Uwet area, Oban Massif, S.E. Nigeria. Precambrian Research, 34, pp. 269- 289.

Ekwueme, B. N., 1994. Structural features of Obudu Plateau, Bamenda Massif, Eastern Nigeria:

Preliminary Interpretation. Journal of Mining and Geology, 30(1), pp. 45 59.

Ekwueme, B.N. and Kroner, A., 1998. Single zircon evaporation ages from the Oban Massif, southeastern Nigeria. Journal of African Earth Sciences, 26, 195 205.

Ferre, E. C., Caby, R., Peucat, J. J., Capdevila, I. R., and Monie, P., 1998. Pan-African post-collisional, ferro-potassic granite and quartz-monzonite plutons of Eastern Nigeria. Lithos, 45, pp. 255 278.

Fitches, W. R., Ajibade, A. C., Egbuniwe, I. G., Holt, R. W. and Wright, J. B., 1985. Late Proterozoic schist belts and plutonism in NW Nigeria. Journal of Geological Society of London, 142, pp. 312 337.

Frost, B. R., Barnes, C. G., Collins, W. J., Arculus, R. J., Ellis, D. J. and Frost, C.D., 2001. A Geochemical Classification for Granitic Rocks. Journal of Petrology, 42(11), pp. 2033 -2048.

Govindaraju, K. 1994. Compilation of working values and sample description for 383 geostandards. Geostandards Newsletter XVIII, pp. 1 158.

Harris, N. B. W., Pearce, J. A. and Tindle, A. G., 1986. Geochemical characteristics of collision-zone magmatism. In : Coward, M. P. and Reis, A. C. (eds.)Collision tectonics. Special Publication of the Geological Society, 19, pp. 67 -81.

Haskin, L. A., Haskin, M. A., Frey, F. A., Wildman, T. R., 1968. Relative and absolute terrestrial abundances of the rare-earths. In: Ahrens, L. A. (ed.), Origin and distribution of the elements, volume 1. Pergamon, Oxford, pp. 889-911.

Hyndman, D. W., 1972. Petrology of Igneous and Metamorphic Rocks (International Series in the Earth and Planetary Sciences). McGrawHill Book Company, New York, 533p.

Karacik, Z., Yilmaz, Y., Pearce, J. A. and Ece, O. I., 2008. Petrochemistry of the south Marmara granitoids, northwest Anatolia, Turkey. International Journal of Earth Sciences( Geol Rundsch), 97, pp. 1191-1200.

Lar, U.A., 1988. Ge´ochronologie U-Pb sur zircons et monazites de l'encaissant (orthogneiss, migmatite) du Complexe charnockitique de Toro (Nige´ria). unpublished MSc Thesis, Universite´ des Sciences et Techniques du Languedoc, Montpellier, 93p.

Leblanc, M., 1981. The late Proterozoic ophiolites of Bou Azzer (Morocco): evidence for Pan-African plate tectonics. In: Kroner, A., (ed.) Precambrian plate tectonics, Elsevier Amsterdam, pp. 435 -451.

Makanjuola, A.A. 1982. A Review of the Petrology of the Nigerian syenites. Journal of Mining and Geology, 19(2), 1 14.

Middlemost, E.A.K., 1997. Magmas, Rocks and Planetary Development. Longman, Harlow, 299 p.

Maniar, P. D. and Piccoli, P. M., 1989. Tectonic discrimination of granitoids. Geological Society of American Bulletin, 101, pp. 635 -643.

REFERENCES

28

Nakamura, N., 1974. Determination of REE, Ba, Fe, Mg, Na and K in carbonaceous and ordinary chondrites. Geochimica et Cosmochimica Acta, 38, pp. 757- 775.

Obiora, S. C., 2005. Field Descriptions of Hard Rocks, with examples from the Nigerian Basement Complex. SNAAP Press (Nig.) Ltd., Enugu, 44p.

Obiora, S. C., 2006. Petrology and Geotectonic Setting of Basement Complex Rocks Around Ogoja, Southeastern Nigeria. Ghana Journal of Science, 46, pp. 13 - 25.

Obiora, S. C., 2008. Geology and Mineral Resources of the Precambrian Basement of Nigeria. A talk presented at the National Geophysical Research Institute (NGRI) Uppal Road, Hyderabad-500 007, India, (55 slides).

Obiora, S. C. and Ukaegbu, V. U. 2009. Petrology and Geochemical Characteristics of Granitic Precambrian Basement Complex Rocks in the southernmost part of nor th-central Nigeria. Chinese Journal of Geochemistry, Vol. 28, No. 4, pp. 377- 385.

Okonkwo, C. T. and Winchester, J. A., 2004. Geochemistry of granitic rocks in Jebba area, southwestern Nigeria. Journal of Mining and Geology, Vol. 40(2), pp. 95 -100.

Olarewaju, V., 1987. Charnockite granite association in SW Nigeria: rapakivi granite type and charnockite plutonism in Nigeria? Journal of African Earth Sciences, 6(1), pp. 67 -77.

Olarewaju, V. O. and Rahaman, M. A., 1982. Petrology and Geochemistry of Older Granites from some partsof Northern Nigeria. Journal of Mining and Geology, 18(2), pp. 16-28.

Onyeagocha, A. C., 1984. Petrology and geologic history of NW Akwanga in Northern Nigeria. Journal of African Earth Sciences, 2(1), pp. 41 -50.

Onyeagocha, A. C., 1986. Geochemistry of basement granitic rocks from northcentral Nigeria, Journal of African Earth Sciences, 5(6), pp. 651 -657.

Oyawoye, M.O., 1972. The Basement Complex of Nigeria in Dessauvagie, T.F.J. and Whiteman, A.J. (eds). African Geology. Ibadan University Press, Ibadan, pp. 67 -99.

Oyawoye, M.O., 1964. The Geology of the Nigerian Basement Complex A Survey of our Present Knowledge of them. Nigerian Mining, Geological and Metallurgical Society, 1, pp. 87 102.

Pearce, J., 1996. Sources and settings of granitic rocks. Episodes, 19(4), 120-125.

Pearce, J. A., Harris, N. B.W., Tindle, A.G., 1984. Trace element discrimination diagrams for the tectonic interpretation of granitic rocks. Journal of Petrology, 25, pp. 956-983.

Rahman, A. M. S., Ekwere, S. J., Azmatullah, M. and Ukpong, E. E., 1988. Petrology and geochemistry of granitic intrusive rocks from the western part of Oban Massif, Southeastern Nigeria. Journal of African Earth Sciences, 7(1), pp. 149 157.

Rahaman, M. A., 1976. Review of Basement Geology of

Southwestern Nigeria In: Kogbe, C. A., (ed.), Geology of Nigeria. Elizabethan Press, Lagos, pp. 41 -57.

Rahaman, M. A., Emofurieta, W. O. and Caen-Vachette, M., 1983. The potassic of Igbeti area: Further evidence of the polycyclic evolution of the Pan-African belt southwestern Nigeria. Precambrian Research, 22, pp. 75 -92.

Rajesh, H. M., 2007. The petrogenetic characterization of intermediate and silicic charnockites in high-grade terrains: a case study from southern India. Contributions to Mineralogy and Petrology, 154, pp. 591-606.

Rickwood, P.C., 1989. Boundary lines within petrologic diagrams which use oxides of major and minor elements. Lithos 22, pp. 247-263.

Rollinson, H. R., 1993. Using Geochemical Data: Evaluation, Presentation and Interpretation. Longman, UK, 352p.

Roy, P., Balaram, V., Kumar, A., Satyanarayanan, M. and Rao, T. G. (2007) New REE and trace data on two kimberlitic reference materials by ICP MS. Geostandards and Geoanalytical Research 31, pp. 261 273.

Saunders, A. D. and Tarney, J., 1984. Geochemical characteristics of basaltic volcanism within back-arc basins. In : Kokelaar, B. P. and Howells, M. F. (eds.) Marginal basin geology, Special Publication of the Geological Society of London, 16, pp. 59-76.

Schluter, T., 2005. Geological Atlas of Africa: With Notes on Stratigraphy, Tectonics, Economic Geology, Geohazards, Geosites and Geoscientific Education of Each Country. 2nd Edition. Springer, 272p.

Sun, S. S., 1980. Lead isotopic study of young volcanic rocks from mid- ocean ridges, ocean islands and island arcs. Philosophical transactions of the Royal Society, A297, pp. 409 -445.

Tubosun, I. A., Lancelot, J. R., Rahaman, M. A. and Ocan, 1984. U Pb Pan African ages of two Charnockite-Granite Associations from SW Nigeria. Contributions to Mineralogy and Petrology, 88, pp. 188 -195.

Umeji, A.C. and Caen-Vachette, M., 984. Geochronology of Pan-African Nassarawa-Eggon and Mkar-Gboko Granites, Southeast Nigeria. Precambrian Research, 23, pp. 317- 324.

van Breemen, O., Pidgeon, R.T., Bowden, P., 1977. Age and isotopic studies of some Pan-African granites from Northcentral Nigeria. Precambrian Research 4, 307-319.

Wilson, M., 1989. Igneous Petrogenesis. Unwin Hyman, London.

Wright, J. B., 1985. Geology and Mineral Resources of West Africa. George Allen AND UNWIN, LONDON., 187P.

Received 23rd August, 2010; Revision accepted 7th June 2011

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