1.0 Introduction
The study area is situated in Akampka Local Government Area of cross river state (figure 1) and the area forms part of the Oban Massif which constitutes the basement complex of south eastern Nigeria the area lies between latitude 050 221N and 050 261N and longitude 0080 361E and 0080 381E greenwich and covers about 72km2
The study area consist of such Mappable units of the amphibolitesfacies rocks and they include gneisses, amphibolites intruded by quartz veins, Pegmatites veins and dolerite (Rahman et al, 1981, Ekwueme, 1990)
The Oban massif is surrounded in the North by the Mamfe Embayment, in the west by the Benue trough, in the south by the Calabar flank and extending into the Cameroon in the East. The Oban Massif is overlain by cretaceous tertiary sediments of the CalabarFlank (Ekwueme, 1990)
1.1 Location And Accessibility
The study area is restricted toAking-Westand its environs and covers an area of land consisting of three main settlements namely, Osomba, Mankorand Aking,all in Akamkpa L.G.A of cross river state.
The study area is accessible through the major road (the Calabar-EkangRoad) which extends to the Cameroonborder. Several other minor roads facilitate accessibility into various locations within the study area.
Review Of Literature
The Nigerian basement complex which includes Oban massif in the southern part of Nigeria where the study area is located shows the least investigated and therefore presented as an undifferentiated basement complex in the geological map of the region (geological survey Nigeria sheet 50). Oyawoye (1964) recognized three subdivisions in the Nigeria Basement.
- Ancient Metasediments
- Gneisses, Migmatites, Older Granite
- Younger Metasediments
Rahman, (1976) also recognized four major Petrologicalunits in the Basement Complex of Nigeria as follows:
- Migmatic — gneiss — quartzite complex (Esurnean,2000ma)
- Meta-igneous rocks
- Slightly magmatized to non-magmatizedparaschist
- Older granite and diorites
Rahman Et al (1981) also reveals from preliminary studies of the west and north western part of the Oban Massif the following Major lithologic Rock Units recognized:
- Unmetamorphoseddolorite to meso-diorite intrusive
- Magmatitic and sheared gneissic rocks paraschist,phyllites,quartzites and metaconglomerates, amphibolites and metadiorite, aplites and foliated pegmatites.
- Older granite intrusive series comprising granodioritemetadiorite, mellitus to granitic rocks, weakly foliated to unfoliatedpegmatites, aplites and quartz veins.
Nigeria lies in the Pan-African belt which has been assigned an age of 450-750Ma though Cohen et al (1984)suggested 450-1100matheoccurrence of Bauchite - Charnokiteintrusive at Akor along Calabar-Ekang Road.The rocks here exhibit striking similarity in terms of mineralogy and petrography with those that have been reported in the northern part of Nigeria. This striking similarities, the Lithologicand Lithotectonicsetting between the basement of Oban massif and those in the Northern part of Nigeria suggest that the metamorphic evens tectonism, magmatism and metasomatism which the basement rocks were subjected to were the same (Rahmanet al, 1980).
Ekwueme and Onyeagocha, (1986) showed two classes of metamorphic rocks in the age of the rocks in Uwetarea (Oban Massif) that extends to the study area, these are older and younger metasediments.
- The older metasedimentary series were deposited 2,500ma age, and are made up of gneiss and migmatites of low grade metamorphism that ranges from middle green schist to uppermost amphibolitesfacies grade(products of barrovian type of regional metamorphism) which some authors refer to as gneiss migmatites — quartzite complex.
- The younger metasediments are low grade metasediment deposited some 1000-800ma referred to as the newer sedimentary series that comprises of the pelites and semi-peleles.
This age relationship between metasedimentarysequence and the occurrence of garnet, staurolite, hornblende, biotite and fayatite conforms with the view of Rahman, (1986)
Whole rock rubidium - strontium has been used by Ekwueme Et al, (1988) on the basis of geochronology to obtain ages of 527±16ma and 676±26ma for gneisses and schist in parts of Oban massif respectively. The dataof 676±26ma represents the main phase of Pan — African orogeny in the area whereas 527±16Ma depicts retrogression, may be during the warring stages of the same orogeny. The age data above are used to correlate other similar rocks elsewhere in Nigeria.
In terms of crustal evolution, the region is underlain by rocks affected by the pan — African thermotectonic event (650 — 450ma),which involve mostly a reactivation of an older crust. Ekwueme, (1988) reported Pb — Sr whole rockisochron age of 128±153 and 546±24Ma from the charnochite of eastern Oban massif and consider them to be kibiran. Also Ekwueme, (1995) on the basis of recent isotopic studies reported that there exist early protozoic crustal components in Oban massif using the 207 Pb/206Pb evaporation techniques on single zircons. The banded gneiss in the oban village gave a zircon age of 584±20Ma.
Ekwueme and Onyeagucha, (1986) reported on the structural trend of foliation, lineation and fold axis in the Oban basement complex rocks and reveal that the regional strike of Nigerian basement complex is approximately constant in the N-S direction with variations between NW-SE and NE-SW directions.
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