GRAVITY AND GEOLOGICAL INVESTIGATIONS OVER THE ZARANDA YOUNGER GRANITE RING COMPLEX OF NORTHERN NIGERIA
GRAVITY AND GEOLOGICAL INVESTIGATIONS OVER THE ZARANDA YOUNGER GRANITE RING COMPLEX OF NORTHERN NIGERIA
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Date
1982-10
Authors
Ibe, Kalu Kalu
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Abstract
Gravity and geological i n v e s t i g a t i o n s were c a r r i e d out over
t h e Zaranda h i l l s i t u a t e d on the Bauchi Sheet (sheet 149 NW) and
l o c a t e d between l a t i t u d e s 10° 9-20' N and 10° 25.00' N and between
longitudes 9° 26.00' E and 9° 45.00' E. The i n v e s t i g a t i o n s were
aimed at a d e t a i l e d analysis of a magnetic low centred on the area
on the aeromagnetic t o t a l magnetic i n t e n s i t y map of the Bauchi area.
Petrographic, density, s t r u c t u r a l and chemical analysis
based on X-ray fluorescopy were c a r r i e d out on the samples in
a d d i t i o n to f i e l d measurements. These analysis show t h a t:
(a) The major structural trend of the units in the complex
is NNE - SSW and these dip steeply with values ranging
from 75° to 90°.
(b) There are at l e a s t five major rock types in the survey
area and the Zaranda complex has four recognizable
units which intruded the Precambrian crystalline
basement made up of gneisses and migmatites. These are:
( i ) the precaldera trachyte
( i i ) the faylite-bearing hornblende syenite
( i i i ) the porphyritic granite; and
(iv) basic rocks of intermediate group with some
extrusives
In addition, widespread occurrences of muscovite-biotite
older granite were observed in the vicinity of the complex. The
units are emplaced in a r i n g - l i k e structure with the older precaldera
trachyte at the core and the syenite and granite porphyry emplaced
l a t e r . The basic rocks form the outermost ring and are emplaced
at some distance from the other u n i t s.
Analysis of the gravity data shows that Bouguer anomalies
ranging from -433 g.u. to - 6l3g.u. with predominant NNE - SSW
trending axes lie in the area and that over the complex a residual
anomaly of the range from -20 to -106 g.u, exist. The main
residual anomaly of -106 g.u. trending NNE - SSW is flanked by
relatively positive residual anomalies ranging from -10 to 52 g.u.
on the eastern, southern and southwestern margins.
Interpretation of the gravity anomalies using two and threedimensional
models indicate that the Zaranda complex is a tabular
6yenitic body which extends to a depth of about 5.5 km with a
central mass of trachytic rock extending to a depth of about
2.5 km. The positive anomalies forming a ring structure were
modelled as shallow tabular basic intrusions.
The gravity and geological investigations correlate well.
The low gravity anomalies correspond to areas underlain by acid
rocks while those areas underlain by the basic rocks have positive
anomalies.
The ring-like structure of the Zaranda complex and the
emplacement of the basic dykes gave rise to the existence of
natural aquifers which could further be investigated. Occurrences
of kaolinite pits were found in small amounts, but the existence
of large occurrence could also be investigated.
Description
Thesis submitted in partial fulfillment of the requirements for
the award of the degree of Master of Science, Applied Geophysics.
Department of Physics
Ahmadu Bello University
Zaria
Keywords
GRAVITY,, GEOLOGICAL,, INVESTIGATIONS,, ZARANDA,, YOUNGER,, GRANITE,, RING,, COMPLEX,, NORTHERN NIGERIA