Petrochemistry and Geotectonic Setting of Granitic Rocks in Aderan Area , S . W . Nigeria

Aderan area, southwestern Nigeria is underlain by metaigneous and metasedimentary rocks which have been intruded by granitic rocks of probable Pan-African (ca. 600 Ma) age. Four types of granitic rocks have been identified in the area, including granitic gneisses, medium-grained granite, porphyritic granite and granodiorite. Geochemical analysis show that the rocks are largely calc-alkaline. The granodiorite is less siliceous and more calcic, and also contains less Ba, Nb and Rb, and more Sr, Ce and La than the granites. Tectonically, the rocks classify as volcanic arc and syn-collisional and possibly late-topost-collisional granitic rocks with respect to the Pan-African orogeny. Chemical characteristics indicate that these rocks were derived from partial melting of mafic to semi-pelitic (metasedimentary) crustal rocks under conditions of intermediate oxygen fugacity and activities of H2O.

Granite magmatism is commonly associated with several tectonic settings and various stages during orogenic evolution (Pitcher, 1983;Pearce, Harris, & Tindle, 1984;Whalen, Currie, & Chappell, 1987;Maniar & Piccoli, 1989;C. Frost, B. Frost, Chamberlain, & Edwards, 1999).This paper considers the petrochemistry of the variably deformed and undeformed granitic rocks of Aderan area, southwestern Nigeria in terms of their origin and geotectonic setting.It is a contribution to the understanding of the geodynamic environment of the granitoid magmatism in this sector of the Nigerian basement complex.

Geology of Aderan Area
Aderan area is located at the northwestern margin of the southwestern sector of the Nigerian Basement Complex .The area is underlain by metaigneous and metasedimentary rocks which have subjected to polyphase deformation and metamorphism in the range of upper greenschist to amphibolite facies.These metamorphic rocks have intruded by granitic rocks of probable Neoproterozoic age.
In the area four lithostratigraphic units have identified (Figure 2).In the south, a belt of migmatitic gneisses characterized by concordant quartzofeldspathic bands associated with dark bands rich in biotitie, plagioclase and occasionally, hornblende.Locally, this rock type is intercalated with augen gneiss.The intercalated augen gneiss is similar to rocks dated at ca 1900 Ma in the Igbeti area ca 50km to the west.Two narrow N-S trending discontinuous belts of quartzites occurs in the central portion of the area.These quartzites have been locally cut by N-S trending, transcurrent shear zones.In the east are discontinuous belts of quartz-mica schists which locally contain thin psammitic bands probably reflecting original sedimentary, lithological variation.Also intercalated with the metamorphic rocks are thin, concordant bands of amphibolite.Intruding all these metamorrphic rocks are granitic rocks varying in composition from granodiorites to granites and pegmatites (Figure 2).
Field evidence, including cross-cutting relations and presence of xenoliths of older rock types, indicate that the foliated (gneissic) granites are relatively, the oldest and the medium-grained grantes are older than the porphyritic granites while the relative age of the granodiorite is not yet known.Rahaman et al. (1983) obtained a whole-rock Rb/Sr isochron age of 617±37 Ma for porphyritic granites of Igbeti area, about 50 km to the west which are similar to those of this study area.
Two episodes of ductile deformation and one of brittle -ductile deformation has affected the rocks of the area.
The first two gave rise to the development of penetrative foliations in the metamorphic rocks and to the formation of asymmetric folds in the rocks.The third episode involved the formation of dextral, transcurrent shear zones which cut all the rocks including some of the Neoproterozoic granitoids.

Granodiorite
This rock type occurs in the southwestern part of the area (Figure 2).It is dark grey and medium-grained.It consists of plagioclase (40-45%), quartz (25-35%) and biotite (12-15%).Some of the plagioclase feldspar crystals are zoned have been partially sericitized while some biotite grains have been chloritized.Epidote, magnetite, apatite and zircon occur as accessory minerals.

Granitic Gneiss (Gneissose Granite)
This rock type occurs mainly in the northeastern and central parts of the area close to the major shear zones (faults), (Figure 2).These rocks are foliated with strong planar preferred orientation of the constituent minerals especially, biotite (5%), microcline (40%), plagioclase (10%), and quartz (40%).Some varieties locally contain minor amounts of muscovite.Accessory minerals include titanite, zircon, and magnetite.Some sericite and chlorite also occur as partial replacements of plagioclase and biotite, respectively.

Medium-grained Granite
This rock type is the most widely distributed in the area.It is made of microcline, quartz, biotite and plagioclase feldspar with accessory amounts of titanite, zircon, apatite and magnetite.Sericite, epidote and chlorite occur as minor replacements of plagioclase and biotite, respectively.

Geochemistry
Six samples of the granitic gneiss, four of the granodiorite, four samples of the medium-grained granite as well as two samples of the porphyritic granite were analysed for major and trace elements at Keele University, England and at the Technical University, Berlin, Germany by X-Ray Fluorescence Spectrometry.Analytical methods and precision have been described in Okonkwo and Winchester (1995).The major and trace elements compositions of the rocks are presented in Table 1.

Major Elements
The granodiorite generally has the lowest concentrations of SiO 2 and K 2 O and higher concentration of CaO compared with the other rock types.The granitic gneiss generally has lower concentrations of Al 2 O 3 , Fe 2 O 3 , MgO, CaO, and higher SiO 2 compared with the other rock types (Table 1).
On the Na 2 O + K 2 O versus SiO 2 plot (Figure 3a) after Cox, Bell and Pankhurst (1979) the granitic gneisses and the medium-grained granites plot in the field of granite, while the granodiorites and the porphyritic granites plot in the field of quartz diorite (or granodiorite).Peccerillo and Taylor (1976) showing the classification of the granitic rocks.Symbols as in Figure 3a On the K 2 O versus SiO 2 diagram (Figure 3b) after Peccerillo and Taylor (1976), the granitic gneisses, medium-granites and the porphyritic granites plot largely in the high-K calc-alkaline to shoshonitic fields while the granodiorite plots in the calc-alkaline, medium-K field.On the Na 2 O+K 2 O-CaO versus SiO 2 diagram (Figure 3c) after B. Frost, Barnes, Collins, Arculus, Ellis and C. Frost (2001), the granitic gneisses, medium-grained granites and the porphyritic granites plot in the alkali-calcic to calc-alkali fields while the granodiorite plots in the calcic field .Also, on the plot of Fetotal/(Fetotal+MgO) against SiO 2 (Figure 3d) after Frost et al. (2001), all the granitic rocks plot largely in a field overlapping the ferroan and magnesian rocks.4a) after Maniar and Piccoli (1989), the granitic gneisses plot in the peraluminous field , the granodiorites are slightly peraluminous, the porphyritic granites plot in the metaluminous field while the medium-grained granites are largely metaluminous.Also on the A/C+N+K (ASI) versus SiO 2 plot (Figure 4b) after White and Chappell (1977) the granitic rocks plot largely in the field of I-type granitoids except for most of the samples of the granitic gneiss which plot in the field of S-type granitoids.On the AFM diagram (Figure 4c), the granitic rocks follow a calc-alkaline trend with two clusters -a more alkaline cluster of granitic gneisses and medium-grained granites, and a more iron-rich cluster of the granodiorites and porphyritic granites.In Harker variation diagrams SiO 2 shows very strong negative correlations with Al 2 O 3 , (Figure 5a), CaO (Figure 5b) and a positive correlation with Na 2 O (Figure 5c) indicating plagioclase fractionation; and a very strong negative correlation with Fe 2 O 3 (Figure 5d) and MgO (Figure 5e) also suggesting pyroxene and hornblende fractionation.All these indicate the importance of fractional crystallization in the evolution of this magmatic suite.SiO 2 also shows a good positive correlation with K 2 O (Figure 5f) and with Rb (Figure 5g) also supporting the role of fractional crystallization.The granitic gneiss and the medium-grained granites are the more evolved of the magmatic suite.

Trace Elements
The granodiorite generally has the highest concentrations of Sr, La and Ce but lower Rb than the other rock types.On the other hand the granitic gneisses generally have the highest concentrations of Rb and lower values of the heavy rare-earth elements-La, Ce and Nd, compared with the other rock types (Table 1).
In the normalized multi-element spiderdiagram (primordial mantle of Wood et al., 1985, Figure 6a ), the granitic rocks show an enrichment in large ion lithophile (LIL) elements relative to the high field strength (HFS) elements as well as negative anomalies of Nb, Ce, P and Ti.Some of the granitic gneisses and some of the medium-grained granites show some negative Ba anomaly.The element concentrations generally fall within the range for mature continental arcs (Brown, Thorpe, & Webb, 1984).(1984) the rocks also plot largely in the field of Volcanic Arc Granites.On the Rb/Zr versus SiO 2 plot (Figure 6d) after Harris, Pearce, and Tindle (1986) these granitic rocks plot within the field of the Group III (Late to Post-Collisional) Granites and Volcanic-Arc Granites.It has been suggested Harris et al. (1986) that Group III is post-collisional and forms calc-alkaline suites with trace element characteristics similar to Volcanic-arc magmatism.They are believed to form like volcanic-arc magmas, from the LIL-enriched mantle wedge above subducted oceanic lithosphere which have probably been contaminated with melts from the lower crust.The crustal melts may have resulted from thermal relaxation in the lower crust and the mantle-derived magmas by adiabatic decompression in the upper mantle (Harris et al., 1986).

Discussion
The granites of the Nigerian basement complex occur in the Neoproterozoic mobile belt formed during the convergence of lithospheric blocks during the Pan-African orogeny.This is borne out by their calc-alkaline characteristics as well as their affinity with volcanic arc and syn-collisional or post-collisonal granites.The early and deformed variety-the granitic gneiss-is probably syntectonic while the later medium-grained, granodiorite and porphyritic granite possibly mark the later stages-i.e. the late to post-collisional period.
The peraluminous nature of the granitic gneiss suggest derivation from partial melting of semipelitic metasedimentary rocks ( Holtz & Johannes, 1991) and the slightly peraluminous of the granodiorites and the metaluminous nature of the porphyritic granite suggest derivation of their magmas from either melting of mafic rocks or ammphibolites (Ellis & Thompson, 1986;Patino Douce, 1999).The slightly metaluminous nature of the granodiorite may also be due to some assimilation of of metasedimentary material by their magma (Frost et al., 1999).
Ferroan (Fe-enriched) granites are closely associated with conditions of limited availability of H 2 O and low oxygen fugacity during partial melting of their source rocks (Frost et al., 2001) as well as the crystallization of anhydrous silicates.On the other hand, magnesian granites are associated with relatively hydrous magmas and oxidizing differentiation trends (Frost & Lindsley, 1991).It would therefore appear that these granitic rocks evolved under largely intermediate conditions with respect to these parameters.
These granitic rocks in Aderan area are similar petrochemically to the granitic suite documented in the Jebba area to the east by Okonkwo and Winchester (2004).
The porphyritic granites are also similar petrologically and chemically to the coarse, porphyritic granites of Igbeti area, southwestern Nigeria dated 617±37 Ma (Rahaman et al., 1983).In northeastern Nigeria, Ferre et al. (2002) recognized two granite suites which intruded the area during the Pan-African orogeny-early, peraluminous, biotite-muscovite granites dated ca 605 Ma and younger, porphyritic, metaluminous, hornblende-biotite granites emplaced at ca 580 Ma.These suites thus correspond petrographically and geochemically to the two main types of granites observed in the Aderan area.
In southeast Nigeria, Ukwang and Ekwueme (2009) have documented granitic rocks in Obudu Plateau with volcanic arc and syncollisional affinity emplaced with regard to the Pan-African orogeny In the northeast of Nigeria, Ferre et al. (1998) have described hypersthene-bearing monzogranitic and quartz-monzonitic rocks of Neoprterozoic age with ferro-potassic transalkaline and metaluminous characteristics which have affinity with within-plate or post-collisional granites.They assigned these rocks to the post-collisional stage of the Pan-African orogeny.

Conclusions
The foliated granitic gneisses of Aderan area are calc-alkaline, probably syn-collisional peraluminous granites emplaced during the Pan-African orogeny.The more calcic granodiorites and the largely shoshonitic, alkali-calcic porphyritic granites are probably late-to post-collisional with respect to the Pan-African orogeny.
The predominance of these granitoid rocks in the Aderan area indicate their emplacement in mature continental arcs associated with thick continental crust probably during the Pan-African orogeny.

Figure 1 .
Figure 1.Index map showing the location of the study area in Nigeria between the West African craton and the Congo craton

Figure 3d .
Figure 3d.FeOt/(FeOt+MgO) versus SiO 2 diagram after Frost et al. (2001) showing the distribution of the granitic rocks with respect to the ferroan and magnesian fields.Symbols as in Figure 3a

Figure 4a .
Figure 4a.Molecular Al 2 O 3 /Na 2 O+K 2 O versus molecular Al 2 O 3 /CaO+Na 2 O+K 2 O diagram afterManiar and Piccoli (1989) showing the classification of the rocks into metalumnous and peraluminous fields.Symbols as in Figure3a

Figure 5a .
Figure 5a.Plot of Al 2 O 3 versus SiO 2 for the granitic rocks.Symbols as in Figure 3a

Figure 5b .
Figure 5b.Plot of CaO versus SiO 2 .Symbols as in Figure 3a

Figure 5e .
Figure 5e.Plot of MgO versus SiO 2 .Symbols as in Figure 3a

Figure 6b .
Figure 6b.Nb versus Y tectonic discrimination diagram afterPearce et al. (1984) showing the tectonic affinities of the rocks.Symbols as in Figure3a

Figure 6c .
Figure 6c.Rb versus Y+Nb tectonic discrimination diagram after Pearce et al. (1984) showing the volcanic are affinity of the rocks.Symbols as in Figure 3a

Figure 6d .
Figure 6d.Zr versus SiO 2 diagram after Harris et al. (1986) indicating the Group III (post-collisional) affinity of the rocks.Volcanic arc granites also plot in this field.Symbols as in Figure 3a

Table 1 .
Chemical compositions of the granitic rocks of Aderan area