International Journal of Engineering Science Invention ISSN (Online): 2319 – 6734, ISSN (Print): 2319 – 6726 www.ijesi.org ||Volume 5 Issue 12|| December 2016 || PP. 25-29 www.ijesi.org 25 | Page The Paleomagnetic Pole for Nairobi Phonolites W.C. Kirui 1 , J.P. Patel 2 1 (Department of Physics, Masinde muliro university of science and technology, P.O. Box 190- 50100, Kakamega , Kenya) 2 (Department of Physics, University of Nairobi, P.O. Box 30197-00100, Nairobi, Kenya) ________________________________________________________________________________________ Abstract: Specimens of the Nairobi phonolites, of lower tertiary age dated at 5.2 my, from four sites were sampled and treated in alternating field up to 100mT, stable primary components of the natural remanence isolated and various magnetic parameters analyzed. The cleaned mean directions have been classified as intermediate or reversed. The mean direction and corresponding pole position of the phonolites, for the intermediately magnetized sites is calculated at declination D=251.0 o , inclination I=0.1 o and longitude 126.49 o E, latitude 18.9 o S and for the reversedly magnetized sites at D=169.6 o , I=2.5 o ( α 95 =9.6 o ) and 127.3 o E, 79.6 o S with errors (δm=9.6 o , δp=4.8 o ) , respectively. These results may assist in stratigraphic correlation of Nairobi area rocks. Keywords: Amphiboles, Remanence, Stratigraphy, Phenocrysts. I. Introduction The geology of the east African rift system as a whole has been summarized by Baker et al. (1972) and the sequences and geochronology of the Kenya rift system discussed in Baker et al. (1971). The Kenya rift volcanic erupted nearly continuously from early Miocene to holocene times producing mainly nephelinites, alkali basalts and phonolites in the Miocene period. Pliocene activity was trachytic, nephelinitic to basaltic in most parts of the Kenyan rift system. The formations of which the country surrounding Nairobi area is built for the quaternary and tertiary include limuru trachytes, Nairobi phonolites, ngong basalts, tuffs and agglomerates, mbagathi phonolitic trachytes, Nairobi trachytes and kerichwa valley trachytes. The Nairobi phonolites are believed to have been caused by isolated, spasmodic local eruptions which gave rise to coarse and fine grained types of bedded tuffs. They are exposed over 260 km 2 of the athi plains including parts having a thin covering at a very low gradient. It overlies the Mbagathi phonolitic trachytes in the Mbagathi river valley and its tributaries, appears to be about 120 m thick under Nairobi and may consist of two main sheets as believed by Sikes (1939). The field appearance of the phonolites exhibit long conspicuous crystals of feldspar and small nephelines. A fissile trachytic texture is sometimes found with fluxion arrangement of minute feldspar laths around microphenocrysts of nepheline, which themselves often have a border of soda- amphiboles. Nepheline occurs in small phenocrysts, but is frequently replaced by sodalite. II. Sampling The Nairobi phonolites crop out to the east of Nairobi , Kenya. Four sites were sampled at a road cutting on the Nairobi- Mombasa road about 8 km from Nairobi, yielding 10 oriented block samples. It was observed that most surface units of the natural outcrops were badly weathered and were therefore not sampled. Average sampling location was at latitude 1 0 21'S , longitude 36°54'E. A site is described as a small area of exposure of a single lava flow or intrusion from which a number of oriented block samples have been taken and which is assumed to represent a single point in time and a spot reading of the paleomagnetic field. Freshly exposed rocks at road cuttings, stream beds, railway cuttings or quarries were sampled whenever possible but outcrops with obvious geological displacements (e.g. boulders) or those which are badly weathered were avoided, as they are unsuitable for paleomagnetic analysis. The block samples were first dislodged from the parent position with a sledge hammer, then carefully replaced in their original position before noting the orientation angles. The collection and orientation of the block samples has been done as described by Collinson et al. (1964). Each individual block sample was oriented in situ using a compass and an inclinometer, recording the dip and strike of the formation. The bearing, latitude and longitude were also noted for every sample. The maximum error in strike, dip and position angles is less than 2% of a degree while in bearing measurement it is up to 1° (degree).