LITHO
DESCRIPTION AND FIELD RELATION
The Nigeria basement complex have been
conformed to comprise of three major rock units of which are; migmatite,
gneiss, migmatite gneiss, the schist belt and older granite and younger granite complexes and all of these are seen in different location
of the studied area. From this description of rocks units and also the younger
granite intrusion.
GNEISS AND MIGMATITE/MIGMATITE
GNEISS.
The rocks exhibit great variation in
the percentage of light and dark mineral components. These resulted from the
protolith from which they were derived from and pressure temperature condition
which they formed.
GNEISS: These are metamorphic material of
felsic and mafic composition which shows alternating colour of light and dark
bands that yield a structure called gneissose foliation. They are medium to
coarse grained. In respect to the already established geochemistry most of the
gneisses are meta-sedimentary in origin as shown by their mineral constituent
of aluminiosilicate, ioderite, and garnet. There are quartzo feldsparthic
component which at time of deformation described as Augen gneisses due to their
eye shaped structure. These lithologic or rock units are seen in location 1, 5,
6 and 11 within the studied area.
MIGMATITE: These rocks are mixture of gneissic –
schistose rock with more or less foliated granites or granitites. They are
frequently cut by granitic components. The rock units consist of
quartzo-feldspathic portion and the dark coloured maffic portion which are
identical with igneous and metamorphic rock respectively. From the conclusion
drawn above, the migmatite are primarily of metamorphic material which
cross-out a veined granitic material. In some cases, the migmatite gneiss are
inter layered with amphibolite and banded iron formation.
These
rocks units are also encountered at location 1, 5, 6 all in Akwanga –Lafia road
and Agbonju Hill etc.
The migmatite have undergone series of fracture and folding as shown in most of
the location within the studied area.
THE SCHIST BELT
This
is a belt of meta-sedimentary material which are seen with subordinate mafic
and ultramafic rock units. It is mainly of schist with major schist features of
schistocity. It may be mica schist as seen as in location 10 and very well
foliated and consist of k-feldspar, calcium feldspar, biotite and quartz. At
the location are elongate enhedral shape minerals emplaced boldly all over the
schist belt.
OLDER GRANITE
The older granites of Nigeria are
calc-alkaline in chemical composition.
These
are also granite rock seen to have intrude the basement complex.
Analysis
have shown that rock units were porphyritic/porphyroblastic, muscovite,
granite, biotite granite aplite, quartz diorite etc. they show weak alignment
of constituent platy minerals which a times result to their weak foliation.
Older granite is seen within much location in the studied area.
YOUNGER GRANITE COMPLEX
The younger granite is generally
alkaline or basic granite composition and poor in an aluminum and mafic
component and is called Jurassic in age.
They
mostly occur as tubular intrusion mostly within the sedimentary basement
contact.
The rocks that are found in younger
granite province are basalt, trachyte and phenolite.
PEGMATITES
Simple pegmatites composed of microcline and quartz are wide spread
throughout the basement complex. They form dykes in the migmatite – gneiss
complex. Generally, there are only few centimeters or one meter wide and seldom
more than a few meters long.
Some
of the minerals identified in the studied rocks include quartz, microcline,
muscovite, biotite and plagioclase.
Quartz
is normally found at the center of microcline.
This
pegmatite was seen on outcrop at Agbonju hill.