Younger Granite Province Research Articles

Volcanic and intrusive structures in the Kila-Warji ring-complex, northern Nigeria

The Kila-Warji massif is one of a large number of ring-complexes of Jurassic age which cut the basement rocks of northern Nigeria. It is the only large member of this Nigerian Younger Granite province in which rocks of intermediate composition are dominant over acidic rocks. Structurally the complex presents several features of interest. The southern part——Warji—is a cauldron subsidence structure of volcanic rocks invaded by partly concordant intrusions. In the north the Kila ring-complex consists of a cluster of almost superimposed intrusive ring-structures. The complex lies in the extreme north of the Nigerian Younger Granite province (Fig. 1) and forms a group of low hills rising from an almost level plain developed on basement rocks. Farther north a few small complexes are exposed near the southern fringes of the Plio-Pleistocene sediments of the Chad basin, on the northern flanks of which Younger Granites reappear as large massifs in the Zinder-Gouré area of Niger ( Black 1963 ) and at Matsena in Bornu Province (Turner, in press). The Kila-Warji complex, composed mainly of acid and intermediate volcanics, syenites and peralkaline granites, has closer affinities to these northern complexes than to the main part of the Nigerian province, where biotite-granites are the most abundant rock type. Previous work on the area includes a preliminary survey by Raeburn (Falconer & Raeburn 1923) and an account of the petrography of some of the rocks by Beer (1952) . Field work for the present study was carried out from October 1965 to January 1966. This paper

DISCUSSION OF PAPERS PUBLISHED IN PREVIOUS PARTS OF THE QUARTERLY JOURNAL

From Mr C. N. Okezie: The writer recently mapped a small portion (about 200 square miles) of the basement complex of the Kafanchan area, in the Nigerian Younger Granite province. He also mapped a very small Younger Granite stock, the Nok granite, within this area. As a result of this work it became apparent to the writer that the trends of some intrusive Older Granites, their consanguineous granitic pegmatite dykes and sills, and the few basic dykes found in the area appear to have been controlled by four main structural planes of weakness in the basement rocks. These planes are the roughly north-south schistosity and foliation and three inferred sets of master-joints striking roughly nw–se, ne–sw , and e -w. Careful examination of the Younger Granite complexes of this province as seen on the 1 : 2 000 000 geological map of Nigeria or on pl. vii of the Society’s Memoir .No . 1 shows also that these four planes of weakness appear to have exercised a significant control in the emplacement of the Younger Granites. This is shown by the fact that groups of these complexes are often aligned while individual complexes are often elongated parallel to any one of these four directions. The position is illustrated as follows: Aligned complexes (1) Banke-Kudaru-Rishua-Kerku-Nok-Afu (N-S) (2) Banke-Liruei-Suku-Tongolo-Dagga Allah-Kwandon-Kaya-Kofayi ( nw-se ) (3) Kwandon-Kaya-Jos-Bukuru-Vom-Kagoro ( ne -sw) (4) Burra-Ningi-Kila-Wurji-Fagam ( e -w) (5) Zuku-Saiya-Shokobo-Jere-Sanga-Buji-Jos-Bukuru-Rop ( n-s ) (6) Sara-Fier-Sha-Kaleri-South-Mada-Afu ( ne-sw ) Elongated complexes (1) Liruei, Kwandon-Kaya, Kofayi ( nw-se ) (2) Afu, Fagam ( ne -sw) (3) Sara–Fier, Nok, Rishua, Kudaru ( n––s ) (4)

Ring-structures in the Sara-Fier Younger Granite complex, Northern Nigeria

The Sara-Fier complex is composed mainly of granites, but also includes minor acid volcanic rocks and a few small intrusions of basic and intermediate composition. The acid intrusive rocks of the complex form five distinct ring-centres which overlap in a north-south line. An ideal order of events in these ring-centres, applicable to the Nigerian Younger Granite province as a whole is: (i) early volcanic rocks, mainly rhyolitic; (2) ring-fracture, cauldron-subsidence, and the intrusion of a ring-dyke, often of hornblende-granite-porphyry or horn-blende-pyroxene-fayalite-granite; (3) the emplacement of a succession of granites within this ring-fault, generally in a concentric fashion, and including hornblende-biotite-, biotite-, and riebeckite-granites. Intrusive activity seems normally to have been confined within the outer ring-dyke and to have been completed in each centre before the beginning of a new centre. The early ring-dykes are steep structures and since they may be up to a mile wide their emplacement cannot be explained by simple cauldron-subsidence. Later ring-dyke-like bodies and stocks do, however, show outward-dipping contacts, and their emplacement seems to have been controlled by underground cauldron-subsidence beneath dome-shaped fractures.

Ring Complexes in the Younger Granite Province of Northern Nigeria

Summary This memoir presents a summary of recent studies on the Nigerian Younger Granite complexes. Sufficient information is now available to indicate that the province includes some of the finest examples of granitic ring-complexes in the world. The cycle of igneous activity was initiated by a volcanic phase during which large volumes of rhyolitic lavas were extruded. These lavas were succeeded by series of granitic intrusions whose emplacement and form have been mainly controlled by ring-fracturing and major block subsidence. As a result of the extensive down-faulting accompanying the granite emplacement, some of the rhyolites have been preserved at the present erosion level. Basic and intermediate rocks occur in some of the complexes. The memoir is divided into two main sections. The first includes a general discussion of the essential structural, petrographic and geochemical characteristics of the province. The second section is devoted to the description of individual complexes which have been selected to illustrate the varieties of structure and rock type which are encountered.