GEOCHEMICAL STUDY OF ROCKS OF THE OKARARA AREA UZOCHUKWU CHIDINMA Department of Geology, University of Calabar Calabar, Nigeria ABSTRACT Granite gneiss, migamatitic gneiss, biotite gneiss and pegmatite constitute part of the major lithologic units occurring in the eastern part of Oban massif, southeastern Nigeria which is where the study area (Okarara) is situated. The rock suite is associated with charnockite and quartzite. They are medium to coarse grained in texture and consists mainly of quartz, microcline, plagioclase, orthoclase, oligoclase, biotite and accessory beryl in the pegmatite. This work seek to examine the petrochemical characteristics of the rocks of Okarara area (granite gneiss, migmatitic gneiss, biotite gneiss and pegmatite) so as to constrain its evolutionary history based on a comprehensive set of geochemical data of the rocks. This is based on the understanding that detailed geochemical data of crystalline rock units often yield valuable insight into their evolutionary history. The rocks are characterized by slight silica saturation, moderate to elevated mafic compositions, dominant metaluminous character and depleted HREE, which favour a deeper ultimate source (possibly the mantle domain) for the parental magmatic melts. The rocks, with exception of the biotite gneiss, are of granitic composition. Their geochemical evolution can be accounted for by fractional crystallization of magmatic melt that was generated by partial melting of basaltic materials, possibly in the mantle region. The upward migration of these mantle derived magma most likely induced partial melting in the lower continental crust to produce felsic melts that contaminated the mafic magma. A publication of The Geology World.com www.thegeologyworld.com
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GEOCHEMICAL STUDY OF ROCKS OF THE OKARARA AREA
UZOCHUKWU CHIDINMA
Department of Geology, University of Calabar
Calabar, Nigeria
ABSTRACT
Granite gneiss, migamatitic gneiss, biotite gneiss and pegmatite constitute
part of the major lithologic units occurring in the eastern part of Oban massif,
southeastern Nigeria which is where the study area (Okarara) is situated. The
rock suite is associated with charnockite and quartzite. They are medium to
coarse grained in texture and consists mainly of quartz, microcline,
plagioclase, orthoclase, oligoclase, biotite and accessory beryl in the
pegmatite. This work seek to examine the petrochemical characteristics of the
rocks of Okarara area (granite gneiss, migmatitic gneiss, biotite gneiss and
pegmatite) so as to constrain its evolutionary history based on a
comprehensive set of geochemical data of the rocks. This is based on the
understanding that detailed geochemical data of crystalline rock units often
yield valuable insight into their evolutionary history. The rocks are
characterized by slight silica saturation, moderate to elevated mafic
compositions, dominant metaluminous character and depleted HREE, which
favour a deeper ultimate source (possibly the mantle domain) for the parental
magmatic melts. The rocks, with exception of the biotite gneiss, are of granitic
composition. Their geochemical evolution can be accounted for by fractional
crystallization of magmatic melt that was generated by partial melting of
basaltic materials, possibly in the mantle region. The upward migration of
these mantle derived magma most likely induced partial melting in the lower
continental crust to produce felsic melts that contaminated the mafic magma.
A publication of The Geology World.com
www.thegeologyworld.com
LIST OF FIGURES FIGURE 1: Map of southeastern Nigeria basement complex showing the study
area
FIGURE 2: Plots of Okarara rocks (Oban massif, southeastern Nigeria) in
AFM discrimination diagram
FIGURE 3: Plots of Okarara rocks (Oban massif, southeastern Nigeria) in
Alkali- SiO2 discrimination diagram.
LIST OF TABLES
TABLE 1: Major element oxide composition of metamorphic rocks of Okarara
area, southeastern Nigeria.
TABLE 2: Summarized Niggli norm values for the analysed rocks
TABLE 3: Recalculated geochemical data
TABLE OF CONTENTS
ABSTRAST
LIST OF FIGURES
LIST OF TABLES
CHAPTER ONE
1.0 Introduction
CHAPTER TWO
2.0 Geological setting
CHAPTER THREE
3.0 Sampling, dressing and analytical procedures
CHAPTER FOUR
4.0 Results and discussion
4.1 Geochemical composition of analysed rocks
4.2 Calculation of the Niggli norm values of analysed rocks
4.3 Geochemical recalculations and plotting of discrimination diagrams for the
analysed rocks using their Niggli values
4.4 Discussion and interpretation of whole rock geochemical data
CHAPTER FIVE
4.0 Summary and conclusion
REFERENCES
CHAPTER ONE
1.0 INTRODUCTION
The Okarara area is located north-east of Akamkpa Local Government Area,
Cross River State. It forms part of the Oban Massif which is one out of the only
two basement complex in the whole of southeastern Nigeria (see figure 1).
The study area (Okarara) occupies about 67.07km2 in the Oban massif. It is in
the eastern part of Oban massif and as such is dominated by migmatite gneiss
and granite gneiss. Quartzites, biotite and pegmatite are also found though
they are not as much as the granite and migmatite gneisses. These rocks are
intruded by pegmatites and quartz veins. Intrusion of veins is dominant in the
gneisses.
The basement rocks in the southeastern part of Nigeria have only recently
started to receive some attention. The thick tropical rain forest and the rugged
topography of Oban massif especially in Okarara area have remained a barrier
to detailed geological studies. In this field work, however, attempts were
made to carry out a detailed geological study of the Okarara area with a view
to presenting the geochemical characteristics of the rocks in the area. The
more common rock constituents are nearly oxides. Chlorides, sulfides and
fluorides are the only important exceptions to this and their total amount in
any rock is usually much less than 1%. F. W. Clarke has calculated that a little
more than 47% of the Earth’s crust consists of oxygen. It occurs principally in
combinations of oxides of which the chief are silica, alumina, iron, and various
carbonates (calcium carbonate, magnesium carbonate, sodium carbonate, and
potassium carbonate). The silica functions principally as an acid forming
silicates, and all the commonest minerals of igneous rocks and metamorphic
rocks are of this nature. From a computation based on 1672 analyses of
numerous kinds of rocks, Clarke arrived at the following as the average
percentage composition:
SiO2 = 59.71
Al2O3 = 15.41
Fe2O3 = 2.63
FeO = 3.52
MgO = 4.36
CaO = 4.90
Na2O = 3.55
K2O = 2.80
H2O = 1.52
TiO2 = 0.60
P2O5 = 0.22
TOTAL = 99.22%
All the other constituents occur only in very small quantities usually much
less than 1%. These oxides combine in a haphazard way to form minerals. For
example, magnesium carbonates and iron oxides with silica crystallize as
olivine or enstatite, or with alumina and lime to form the complex
ferromagnesian silicates of which the pyroxenes, amphiboles, and biotite are
the chief. Any excess of silica above what is required to neutralize the bases
will separate out as quartz; excess of alumina crystallizes as corundum. These,
however, must be regarded only as general tendencies. It is possible by rock
analysis, to say approximately what minerals the rock contains. Hence we may
say that except in acid or siliceous rocks containing 66% of silica and above,
quartz will not be abundant. In basic rocks (containing 20% of silica or less)
quartz is rare and accidental. If magnesia and iron be above the average while
silica is low, olivine may be expected; where silica is present in greater
quantity over ferromagnesian minerals, augite, hornblende, enstatite or
biotite occurs rather than olivine. Unless potash is high and silica relatively
low, leucite will not be present for leucite does not occur with free quartz.
Nepheline, likewise, is usually found in rocks with much soda and
comparatively little silica. With high alkalis, soda-bearing pyroxenes and
amphiboles may be present. The lower the percentage of silica and alkalis, the
greater is the prevalence of calcium feldspar as contracted with soda or
potash feldspar. Clarke has calculated the relative abundance of the principal
rock forming minerals with the following results: apatite=0.6, titanium
minerals=1.5, quartz=12.0, feldspars=59.5, biotite=3.8, hornblende and
pyroxenes=16.8 and total=94.2%. This, however, can only be a rough
approximation.
FIGURE 1: MAP OF SOUTHEASTERN NIGERIA BASEMENT COMPLEX
SHOWING THE STUDY AREA
Okarara
CHAPTER TWO
2.0 GEOLOGICAL SETTING
The Oban massif in which the study area (Okarara) is located is
unconformably overlain to the south by the Calabar Flank which consists of
Cretaceous- tertiary sediments. It is separated to the north from the Obudu
plateau by the Ikom-Mamfe Embayment which consists of Cretaceous
sediments and basic volcanic/intrusives. It is thought that Oban massif and
Obudu plateau was continuous Pre-Cambrian basement feature before the
depression and deposition of sediments in the Ikom-Mamfe Embayment
during the Cretaceous (Petters et al 1987).
Within the context of the geology of the Nigerian basement complex, three
broad lithological units are distinguishable, namely gneiss-migmatite
basement, fine to medium grained metasedimentary metavolcanic units, and
syn- to late- tectonic Older Granite suites (Fitches et al., 1985; Ajibade and
Wright, 1989; Ekwueme, 1990; Annor, 1995). The Older Granite suites were
so named to distinguish them from tin-bearing anorogenic Younger Granite
suites, which are volcanic granitic ring complexes in the Jos Plateau area of
northcentral Nigeria. According to Ajibade (1982), the Older Granite suites
which are mostly Pan African in age, are commonly emplaced into migmatites,
gneisses and schists of Liberian (2700Ma), Eburnean (2000 – 2700Ma) and
probably Kibaran (1100Ma) ages.
Rocks of the gneiss-migmatite basement constitute more than 50% of the
study area. They display foliation that trends in the NE-SW direction, and this
reflects their possible remobilization during the Pan African (600Ma)
Orogeny.
Geological mapping of the Okarara area was carried out and detailed studies
covering the petrology, structural geology and geochemistry of rocks of the
Okarara area were undertaken. A cursory appraisal of these studies shows
that the Okarara area is essentially characterized by the occurrence of
regionally metamorphosed rock successions, pervasive migmatization and
granite plutonism. Accordingly, granitic gneisses and migmatitic gneisses
dominate the geology of the Okarara area. The migmatitic gneisses which are
quartzofeldspathic in composition constitute the dominant rock types in the
study area. They form the basement which has been deformed at most
localities as a result of extensive invasions by magmatic rocks of mostly
granitic and pegmatitic compositions.
CHAPTER THREE
3.0 SAMPLING AND ANALYTICAL PROCEDURES
A total of four representative rock samples were employed for the
geochemical analysis. Prior to the geochemical analysis proper, approximately
1 kg of each of the selected representative rock samples were broken into two
thumb-nail sized pieces with a hardened-steel hammer; one part was kept for
reference purposes, while the other piece was crushed and ground to reach a
particle size as fine as – 60 mesh, with the aid of a “jaw-crusher”. After coning
and quartering, the samples were powdered in an agate mortar, to – 200
mesh, and thoroughly homogenized. Every possible precaution, including
cleaning of all the crushing, grinding and homogenization equipment with
brush, compressed-air, distilled water and acetone to remove possible
remains from previously crushed sample, was adhered to in order to minimize
cross-contamination between samples. Admittedly, the variability in grain size
of the pulverized product from sample to sample may contribute to slight
errors in the analyses, this was however considered negligible. All sample
preparation and treatments were carried out at the Thin–Section Workshop of
Department of Geology, University of Calabar, Calabar- Nigeria
The geochemical analyses undertaken include: determination of whole – rock
geochemistry parameters and loss on ignition (LOI). Determination of whole –
rock geochemistry parameters was performed on pressed rock-powder
pellets using an XRF method at the United Cement Company (UNICEM),
Mfamosing, near Calabar in Cross River State of Nigeria. Ti, Mn and P were not
analyzed for. Essentially, 5 grams of the rock powder of each of the sample
was weighed out and mixed with a few drops of polyvinyl alcohol and the
sample placed in a die, and spread out to form a "puck". Subsequently, boric
acid (backing) was placed on top of the rock powder and a pellet formed by
applying pressure of 15 tons for about 15 seconds. After drying, the pellets
were placed in the sample holder of the XRF spectrometer, and the
fluorescence measured at eight element channels. The elements measured (as
oxides) include Si02, Al2O3, Fe2O3, MgO, CaO, Na2O, K2O, and SO3. Each channel
was calibrated using certified international reference rock materials.
For the loss on ignition (LOI), 1.0 gm of each powdered sample was weighed
into a porcelain crucible. Crucibles containing the samples were loaded on a
Silica tray and placed in a furnace that had been preheated to 350OC. The
temperature was then raised to 1100OC and the samples held at this
temperature for 2.5 – 3 hours. Afterward, the furnace was allowed to cool to
approximately 6500C and the samples removed and placed in a dessicator.
When cooled to room temperature, the crucibles were weighed and the
weight loss (LOI) recorded. When the %LOI is added to the total % element
oxides, the sum was found to be close to 100. The detection limit for all the
major element oxides is 0.01%. The only exceptions are Fe2O3 and K2O which
have a detection limit of 0.04%.
CHAPTER FOUR
4.0 RESULTS AND DISCUSSION
4.1 GEOCHEMICAL COMPOSITION OF ANALYSED ROCKS
The concentrations of major element oxides and other related chemical data
of the analyzed metamorphic rocks of the study area are presented in the
table below
UC/GLG/L2 UC/GLG/L51 UC/GLG/L45 UC/GLG/L68 SiO2 62.49 83.24 85.79 80.85 TiO2 Na na na na Al2O3 10.71 3.16 3.47 4.59 Fe2O3 11.18 0.72 0.48 1.46 MnO Na na na na MgO 4.03 1.81 2.22 2.22 CaO 7.85 3.93 2.8 1.12 K2O 0.38 6.12 4.11 6.94 Na2O 2.22 0 0 0.06 SO3 0.38 0.08 0.03 1.16 LOI 0.5 0.66 0.84 1.31 TOTAL 99.74 99.72 99.74 99.71 TABLE 1: MAJOR ELEMENT OXIDE COMPOSITION OF METAMORPHIC ROCKS
OF OKARARA AREA, SOUTHEASTERN NIGERIA.
UC/GLG/L2 = Biotite gneiss
UC/GLG/L51 = Granite gneiss
UC/GLG/L45 = Pegmatite
UC/GLG/L68 = Migmatic gneiss
4.2 CALCULATION OF THE NIGGLI NORM VALUES OF THE ANALYSED