geochemical study of rocks of the okarara area...

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.

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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

ROCKS

Biotite gneiss (UC/GLG/L2)

Weight (%) Mol weight Mol pro˟1000 Groupings Niggli values

SiO2=62.49 60.07 1040

TiO2=na 79.89 -

Al2O3=10.71 101.82 105 105 al = 16

Fe2O3=11.18 159.68 70(2) =140

FeO=10.06 72.00 140 380 fm = 57

MnO=na 70.93 -

MgO=4.03 40.31 100

CaO=7.85 56.07 140 140 c = 21

K2O=0.38 94.20 4

Na2O=2.22 61.31 36 40 alk = 6

SO3=0.38 80.07 5

LOI=0.5

670 total=100

Mol weight= molecular weight

Mol pro= molecular proportion

FeO= Fe2O3 (0.8998) =11.18(0.8998) =10.06

Si= mol pro of SiO2 (1000) ˟100 =1040 ˟ 100 = 155

Total groupings 1 670 1

Ti= mol pro of TiO2 (1000) ˟100 =0

Total groupings 1

K= K2O = 4 = 0.1

K2O+Na2O 40

Mg= MgO = 100= 0.42

FeO+MnO+MgO 240

i-

ut =100+4alk= 124

Therefore, Q= 155-124=31

Granite gneiss (UC/GLG/L51)


SiO2=83.24 60.07 1040

TiO2=na 79.89 -

Al2O3=3.16 101.82 31 193 al = 49

Fe2O3=0.72 159.68 5(2) =10

FeO=0.65 72.00 9 64 fm = 16

MnO=na 70.93 -

MgO=1.81 40.31 45

CaO=3.93 56.07 70 70 c = 18

K2O=6.12 94.20 65

Na2O=0 61.31 - 65 alk = 17

SO3=0.08 80.07 1

LOI=0.66

392 total=100



FeO= Fe2O3 (0.8998) =0.72(0.8998)=0.65

Si= mol pro of SiO2 (1000) ˟100 =1386 ˟ 100 = 354


Ti= mol pro of TiO2 (1000) ˟100 =0

Total groupings 1

K= K2O = 65 = 1

K2O+Na2O 65

Mg= MgO = 45= 0.83

FeO+MnO+MgO 54

i-

ut =100+4alk= 168


Pegmatite (UC/GLG/L45)


SiO2=85.79 60.07 1428

TiO2=na 79.89 -

Al2O3=3.47 101.82 34 34 al = 17

Fe2O3=0.48 159.68 3(2) =6

FeO=0.43 72.00 6 67 fm = 34

MnO=na 70.93 -

MgO=2.22 40.31 55

CaO=2.8 56.07 50 50 c = 26

K2O=4.11 94.20 44

Na2O=0 61.31 - 44 alk = 23

SO3=0.03 80.07 0

LOI=0.84

195 total=100



FeO= Fe2O3 (0.8998) =0.48(0.8998)=0.43

Si= mol pro of SiO2(1000) ˟100 =1428 ˟ 100 = 732


Ti= mol pro of TiO2(1000) ˟100 =0

Total groupings 1

K= K2O = 44 = 1

K2O+Na2O 44

Mg= MgO = 55= 0.90

FeO+MnO+MgO 61

i-

ut =100+4alk= 192


Magmatic gneiss (UC/GLG/L68)


SiO2=80.85 60.07 1346

TiO2=na 79.89 -

Al2O3=4.59 101.82 45 45 al = 17

Fe2O3=1.46 159.68 9(2) =18

FeO=1.31 72.00 18 91 fm = 34

MnO=na 70.93 -

MgO=2.22 40.31 55

CaO=1.12 56.07 20 20 c = 26

K2O=6.94 94.20 74

Na2O=0.06 61.31 1 75 alk = 23

SO3=1.16 80.07 15

LOI=1.31

231 total=100



FeO= Fe2O3 (0.8998) =1.46(0.8998) =1.31

Si= mol pro of SiO2 (1000) ˟100 =1346 ˟ 100 = 583


Ti= mol pro of TiO2 (1000) ˟100 =0

Total groupings 1

K= K2O = 74 = 1

K2O+Na2O 75

Mg= MgO = 55= 0.75

FeO+MnO+MgO 73

i-

ut =100+4alk= 236


The niggli norm values for the analysed rocks are summarized in the table

below

Biotite gneiss Granite gneiss pegmatite Magmatic gneiss al 16 49 17 20 fm 57 16 34 39 c 21 18 26 9 alk 6 17 23 33 si 155 354 732 583 ti 0 0 0 0 k 0.1 1 1 1 mg 0.42 0.90 0.90 0.75 q 31 186 540 347 TABLE 2: SUMMARIZED NIGGLI NORM VALUES FOR THE ANALYSED ROCKS

4.3 GEOCHEMICAL RECALCULATIONS AND PLOTTING OF

DISCRIMINATION DIAGRAMS FOR THE ANALYSED ROCKS USING THEIR

NIGGLI VALUES

These diagrams which are also called variation diagrams are one of the most

useful tools in interpreting the petrogenesis and tectonic setting of both

metamorphic and igneous rocks. The geochemical data of the analysed rocks

are plotted in these diagrams after which interpretation of the plots are made.

The diagrams are either rectangular or triangular and values plotted include

weight percentages, molecular proportions, niggli values etc. Each diagram

has its objectives. (Ekwueme, 1993)

The recalculated values used in the plotting is summarized in the table below

Biotite gneiss Granite gneiss pegmatite Magmatic gneiss Na2O/Al2O3(wt%) 0.21 0 0 0 K2O/Al2O3 (wt%) 0.04 2.0 1.2 1.5 A (%) 9 66 57 58 F (%) 76 15 13 23 M (%) 15 19 30 19 SiO2 (wt%) 62.49 83.24 85.79 80.85 FeO/FeO+MgO (%) 0.7 0.3 0.2 0.4 TABLE 3: RECALCULATED GEOCHEMICAL DATA

A= Na2O + K2O

F= FeO + Fe2O3

M= MgO

4.4 DISCUSSION AND INTERPRETATION OF THE WHOLE ROCK

GEOCHEMICAL DATA

The Niggli norm tells us from the geochemical data the actual mineral in a

rock. For instance, the positive value of q (quartz index) shows that the rocks

contains quartz in its mode and therefore are siliceous. Also, the high values of

CaO (7.85) and Na2O (2.22) and lower value of K2O (0.38) in the biotite gneiss

shows that it contains more of plagioclase feldspar and little of potassium

feldspar varieties; while the high values of K2O (4.11 – 6.98) and lower values

of NaO2 (0 – 0.06) and CaO (1.12 – 2.93) in granite gneiss, pegmatite and

magmatic gneiss shows that it contains more of the potassium feldspar and

little or no plagioclase feldspar.

The Niggli norm values also tell us the nature of the magma from which rocks

evolved. For instance, biotite gneiss is more enriched in the heavier elements

and this tells us that the magma it evolved from is a mafic magma, possibly it

evolved from tholeiitic magmatism (as confirmed in the AFM diagram, see Fig

2). Further supporting this is the high FeO/FeO+MgO value (0.7) recorded for

biotite gneiss. This value precludes considerations of peridotitic materials as a

likely source for the parental magmatic melts generation. This is because

FeO/FeO+MgO value from 0.7 suggests that the magma under consideration

could not have been in equilibrium with typical olivine of peridotite unless the

oxygen fugacity (⨏O2 ) was exceptionally high approaching that of the

magnetite-hematite buffer (Mysen, 1973-1974; Mysen and Boettcher, 1975).

It therefore became reasonable at this stage to propose partial melting of

basaltic materials other than peridotite, leaving behind residual assemblages

(Arth and Hanson, 1972; Gromet and Silver, 1987), as the most likely

processes that were involved in the generation of the parental magmatic melt

of the biotite gneiss. On the other hand, the depletion of these heavy elements

in granite gneiss, pegmatite and magmatic gneiss suggests that they evolved

from felsic magma. The upward migrations of the mantle-derived magma

probably induced partial melting in the lower continental crust to produce

felsic melts (Hildreth and Moarbath, 1988; Huppert and Sparks, 1988), which

contaminate the mafic melts with partial ‘transfer’ of geochemical traits.

From the geochemical data of the rocks of Okarara, K2O is in excess of Na2O.

The rocks are undersaturated in Na2O compared with silica; hence they are

sub- alkaline rocks. This fact is further confirmed in the cross-plots of

(Na2O+K2O) versus SiO2 (Fig. 3) as it clearly plots the rocks in the sub-

alkaline field. The AFM plots (Fig. 2) puts biotite gneiss in the tholeiitic magma

series field while granite gneiss, pegmatite and magmatic gneiss plots in the

calc-alkaline magma series field. This further supports that biotite gneiss

originated from partial melting of basaltic materials while granite gneiss,

pegmatite and magmatic gneiss originate from felsic magma. A magma series

is a series of compositions that describes the evolution of a mafic magma,

which is high in magnesium and iron and produces basalt or gabbro, as it

fractionally crystallizes to become a felsic magma which is low in magnesium

and iron and produces rhyolite or granite. Accordingly, biotite gneiss is high in

magnesium and iron as observed from the geochemical data (see table 1)

while granite gneiss, pegmatite and magmatic gneiss are low in both

magnesium and iron (see table 1). Little wonder the petrography of granite

gneiss, pegmatite, and magmatic gneiss showed that they have granitic

composition. The rocks of the study area have no TiO2 value. The higher

values of FeO, Fe2O3, and MgO; and the lower value of SiO2 of biotite gneiss

further prove its igneous origin. The lower values of FeO, Fe2O3, and MgO;

and higher values of SiO2 in granite gneiss, pegmatite and magmatic gneiss

prove their metasedimentary origin. Also, the lower value of K2O (0.38) of

biotite gneiss and its higher values (4.11-6.94) in granite gneiss, pegmatite,

and magmatic gneiss aided in proving that biotite gneiss has an igneous origin

while granite gneiss, pegmatite and magmatic gneiss has a sedimentary origin.

CHAPTER FIVE

5.0 SUMMARY AND CONCLUSION

The Okarara area is a basement terrain forming part of the Oban massif in the

southeastern Nigeria. Dominating the study area are biotite gneiss, granite

gneiss, pegmatite and migmatic gneiss. Their trend in the NE –SW direction

indicates the relevance of Pan African thermotectonic events in the

evolutionary history of the rocks. The rocks are all characterized by silica

saturation and low (for granite gneiss, pegmatite and magmatic gneiss) to

high (for biotite gneiss) concentration of mafic components. The observed

chemical trend on the AFM (Fig 2) diagram shows that biotite gneiss is

tholeiitic while granite gneiss, pegmatite and magmatic gneiss are calc-

alkaline. Finally, the enrichment of the heavier elements in biotite gneiss

suggests its evolution from mafic magma, while the depletion of these

elements in granite gneiss, pegmatite and magmatic gneiss suggests felsic

magma origin.

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geochemical study of rocks of the okarara area...

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