GEOCHEMISTRY EVOLUTION OF SCHISTS OF NORTHWEST OBUDU AREA SOUTHEASTERN NIGERIA

Geochemistry of schists of Obudu area was carried out using ICP-MS and ICP-ES techniques in order to determine the geochemical evolution of the area. 40 samples were analyzed for their major, trace and REE composition. Field mapping revealed that gneisses, amphibolites and schists comprising migmatitic schists (MS), quartz-mica schists (QMS), garnet-mica schists (GMS), and hornblende biotite schists (HBS), intruded by granites, granodiorites, quartzofeldspathic rocks and dolerites occur in the area. Structural studies revealed that the schists trend approximately NE– SW (5 – 30 o ) indicating the Pan-African event. Modal analysis revealed that the schists have average concentration of quartz (15vol.%), plagioclase (An45-19 vol.%), biotite (15vol.%), garnet (9.0vol.%) and muscovite (6vol.%), the remaining consists of accessory minerals. Geochemistry showed that all the schists have molecular Al2O3 > CaO+K2O+Na2O, indicating they are peraluminous metasedimentary pelites. Trace and REE element results show that all the analyzed schist samples are depleted in Hg, Ag, Be, Bi, and Sb below < 1.0ppm, but relatively enriched in Ba, Sr and Zr with average concentration of 996, 675.73, 243.13 ppm respective. The HREE are depleted with ΣHREE < 10.2, but the LREE are relatively enriched with ΣLREE > 289.54. The ΣLREE/ΣHREE ratio ranges from 9.17 to 33.4, with a large positive delta V at Eu. These findings indicate that the schists of Northwest Obudu area are highly fractionated and had attained at least the uppermost amphibolite metamorphic grade. The schists had contributed to the development of the Pan-African continent.


INTRODUCTION
Northwest Obudu is a part of the Obudu, Oban, Bamenda massif which extends from western Cameroun into Southeastern Nigeria in the West African sub-region (Toteu et al., 1987). The area is particularly delineated within Latitudes 5 (Fig.1). The schists of Northwest Obudu area occur in association with other basement rocks, including migmatitic gneiss, granite gneiss, amphibolites, meta-quartzites and other rocks Obioha (2014); Obioha and Ekwueme (2012). The basement rocks had been intruded by various igneous rocks including granites, granodiorites, aplites, dolerites and quartzo-feldspathic rocks. Several scholars including Ekwueme (1985), Ejimofor (1988), Ejimofor et al., (1996), Shaw (1994), and many others have worked in Obudu area. Recent works in the area include those of Ekwueme (2010), Ekwueme (2011, 2012), Obioha et al., (2013). However, more work needs to be done for a better classification of the geochemistry of the schists of Obudu area. This study uses the major, trace and rare earth element of the schists to unravel the geochemical evolution of the area.

Location of Study Area
The area of study, Northwest Obudu-Bamenda Massif, is geographically situated between Latitudes 6 0 45'N -7 0 00'N and Longitudes 9 0 00'E -9 0 16'E ( Fig. 1). It covers a surface area of about 860.32 km 2 (Obioha, 2014) extending from Ushongo -Kwande and Vandeikya Local Government Areas in southern Benue State, to parts of Obudu and Bekwara Local Government Areas in northern Cross River State of Southeastern Nigeria (Obioha, 2014). The area is bounded in the west and northwest by Benue Trough, in the south by Mamfe Embayment, which separates it from the Oban Massif in Cross River State, and on the east by the Bamenda Massif of western Cameroon from which it extends into Southeastern Nigeria (Toteu et al., 1987 vegetation is the rain forest type which is gradually giving way to the Savanna grass land type in places due to intense agricultural activities in the area.

MATERIALS AND METHOD
The study involved geological field mapping and sampling and laboratory analysis. The field mapping was carried out between January and March when the environment was dry and the outcrops were well exposed. 40 representative rock samples were collected, processed and analyzed.

Sample Preparation and Analytical Method
The samples for whole rock geochemical analysis were prepared by the conventional dressing techniques. The analytical packages are the G4A-G4B techniques, carried out at the ACEME Analytical Laboratory, Vancouver Canada. Two separate analytical methods; inductively coupled plasma mass spectrometry (ICP-MS) and inductively coupled plasma emission spectrometry (ICP-ES) were employed. The cutting, grinding, pulverization and weighing were done by special electronic handling (SEH). The digestion was by Li-Borate and Li-tetra-borate and HCl. The resolution was 0.001ppm for the trace and rare earth elements and 100% for the major elements quoted as their oxides wt. %.

RESULT AND DISCUSSION
The sample location map is shown in Fig. 1, while Fig. 2 shows the geological map of the area (Obioha, 2014). The modal composition of the schists of Northwest Obudu area are presented in Table 1. The table shows average result for ten (10) runs for each of the samples of the schists mapped; the migmatitic (MS), biotite mica (BMS), quartz mica (QMS) and hornblende biotite schist (HBS) (Obioha et al., 2013). The modal compositions of the schists are summarized in Table 1. Fig. 3 shows the research flow sequence. Figs. 4 a-c, show the field relationship of the various schists mapped in the area.

Hornblende Biotite Schist
The hornblende biotite schist (HBS; Table 1 while opaques and accessory minerals constitute the rest of the modal volume (Table 1).

Major Element Geochemistry
The whole rock major element geochemistry of the schists of Northwest Obudu together with their CIPW and Niggli Norms are shown in (  (Table 2).

Trace Element Composition
Results of the trace elements abundance and distribution in the schists of Northwest Obudu-Bamenda area are presented in  (Taylor, 1965;Taylor and McLennan, 1985). Similarly, Rb varies from 53.6 ppm in HBS, through 117.8 ppm in the GMS to 121.7 ppm in the QMS. The Sr shows an enrichment trend from 504.5 ppm in the HBS through 751.6 in the QMS to 771.1 ppm in the GMS (Table 2) (Obioha and Ekwueme, 2013).  The Sr value ranges from 504.5 in HBS, through 751.6 ppm in the QMS to 771.1ppm in the GMS ( Table 2). The Tl content ranges from 0.3 to 0.5ppm indicating derivation from shale or crustal greywacke parent sources (Taylor, 1965;Ekwueme, 1993). Ga varies narrowly from 19.4 ppm to 19.6 ppm in all the analyzed schists sample, Sn varies from 1 -2 ppm. The Zn concentration ranges from 52 ppm in the HBS to maximum value of 81 ppm in the QMS ( Table 2). The Se, As, Cd, Sb, Bi, and Ag were depleted in all the analyzed samples < 1.0 ppm, while Hg was < 0.01ppm (Table 2). These geochemical trends indicate a derivation from crustal materials of pelitic origin, such as shale, clays and mudstones (Taylor, 1965;Ekwueme, 1993;Ukaegbu and Ekwueme, 2005). The chalcophile elements Cd, Sb, as well as Bi and the precious metals Ag and Hg are depleted in all the analyzed schist samples with values < 0.1 ppm, indicating derivation from a common primitive protolith.

REE Composition
Results of the rare earth element (REE) geochemistry of schists of Northwest Obudu area are presented in (Table 3). The table shows that the QMS and GMS samples are almost homogenous in composition. There is a general enrichment of the LREE: La (16.7-73.3ppm), Ce (39.9 -136.6ppm), Pr (5.39 -15.24ppm), Nb (24.4 -55.6ppm), Sm (4.37 -7.48ppm), and marked -ve anomaly at europium (Eu -1.16 -1.32ppm mean range), and average value of 1.14ppm. The HREE is highly depleted, with ΣHREE range of 8.2 -10.03 ppm (Table 3; Obioha, 2014). The table shows that the highest REE depletion occurs in the HBS (ΣHREE = 8.03), while the QMS shows the highest enrichment value of ΣLREE (289.54 ppm). Lu shows the least depletion of 0.1 in the (QMS and GMS), with average concentration of 0.13ppm (Table 3). Correlation shows that the REE results of the Obudu-Bamenda area are in conformity with the petrography, major elements and trace elements analytical results (Tables 1, 2 and 3).

Petrogenesis
The REE versus Chondrite normalized spidergrams plots of schists of Northwest Obudu area shows the individual plot of the schists (Fig. 8). All the schists show enrichment from La through Sm, a +ve delta V at Eu, a marked constriction at Ga and Tb, followed by a general depletion from Dy to Lu (Fig. 8). The areal plot of the REE versus Chondrite normalized spidergrams plot (Fig.  9) shows a homogenous REE composition. This strongly shows that all the schist samples were derived from similar and / or same parental protolith.
In the ACNK versus ANK binary diagram (Fig. 10) for discrimination of rocks of Northwest Obudu area > 80% of the schists fell in the peraluminous field, while ~20 fell in the metaluminous field, showing that the schists of NW. Obudu area are predominantly peraluminous. This corroborates both the petrography and the major element results. In the SiO 2 versus Na 2 O+K 2 O binary discriminant for rocks of Northwest Obudu area (Fig.  11), > 90% of the analyzed schists of NW. Obudu area fell in the sub-alkaline field. These show that the major, the trace and the REE results are in conformity with the petrogenetic analysis.

CONCLUSIONS AND RECOMMEMDATIONS
Petro-Geochemical investigation of rocks of Northwest Obudu area Southeastern Nigeria, has shown a high grade metamorphic terrain that has been subjected to polyphase deformations and metamorphism, with occurrences of gneisses, schists, amphibolites and intruded by granites, granodiorites, aplites and gabbros (Obioha, 2014). The field study shows that the schists comprised quartz mica schist QMS, biotite mica schist BMS, and hornblende biotite schists (HBS) of peraluminous to subalkaline composition (Obioha and Ekwueme, 2012). Trace element geochemistry shows that the Ba concentration ranged from 1803ppm in the QMS to 1814ppm in the BMS, with average of 995.67ppm. These Ba concentrations are about 2 to 12 times the crustal threshold of Ba (400ppm Taylor, 1965), 150ppm in basalt and 550pp in andesite.