Nigerian Younger Granite Province Research Articles

Termination of anorogenic alkaline magmatism in Nigerian Younger Granite province: insights from Afu A-type granite complex

Termination of anorogenic alkaline magmatism in Nigerian Younger Granite province: insights from Afu A-type granite complex

New geochronological and geochemical constraints on the Banke, Dutsen Wai, Guraka, and Zuku alkaline granite complexes from the northcentral Nigerian younger granite province

ABSTRACT An integrated study was carried out on four anorogenic syenite-granite complexes (i.e. Banke, Dutsen Wai, Guraka, and Zuku) from the Nigerian Younger Granite (NaYG) province, in a bid to constrain their magma sources, rare metal mineralization potentials, and tectonic implications. These four complexes primarily consist of quartz syenite, granite porphyry, arfvedsonite aegirine granite, and biotite granite. The studied rocks are chemically characterized by high alkalis, HFSEs, Ga/Al ratios, and zircon saturation temperature (776–1127°C, average = 932°C), typically of A1-type granites. The zircon εHf(t) values of the arfvedsonite aegirine granite (−6.0 to −0.79) are higher than those of the granite porphyry (−9.57 to −7.67) and biotite granite (−9.49 to −5.86) suggesting moderate crustal contamination of mantle-derived melts for the origin of the peralkaline granites. Generation of the biotite granite likely involved inputs of both crustal- and mantle-derived melts and is evidenced by the presence of inherited Pan-African (600–610 Ma) zircons and low whole rock Nb/U and Nb/Ta ratios. Furthermore, input of crustal material likely increased the metal budget of the residual melt and enhanced Nb, Sn, and sulphide mineralization in the Banke granite porphyry and biotite granite and Dutsen Wai biotite granite. Zircon U-Pb dating results constrain the emplacement ages to early Mesozoic era, from the Banke granite porphyry of 180 ± 1 Ma, Dutsen Wai biotite granite of 177 ± 3 Ma, Guraka arfvedsonite aegirine granite of 227 ± 2 Ma and Zuku arfvedsonite granite of 186 ± 2 Ma. These age data indicate a local E-W age migration pattern for the alkaline magma emplacement in northcentral NaYG province. Integrated with other geological evidence, we favour a formation model for the anorogenic NaYG province likely driven by the reactivation of ancient shear zones and structures.

Structural Analysis of Selected Ring Complexes in Some Parts of the Nigerian Younger Granite Provinces

The study area falls within the basement complex of northcentral Nigeria and covers four adjacent quarter degree sheets of 126 (Ririwai), 127 (Kalatu), 147 (Lere) and 148 (Toro) in some parts of Kaduna, Bauchi and Plateau States, Nigeria. This paper is aimed at interpreting the Aeromagnetic data to delineate structures in some parts of the Nigerian younger granite province. This was achieved by determining depth to basement, and developing a 2D model of the shape, location and depth of structures in some parts of the younger granite province. The anomalies on the aeromagnetic map were defined by fitting a first order polynomial to the total fields, by the method of least squares to obtain the residual field data. First vertical derivative and analytic signal computed, defined distinct pattern of the magnetic signatures. Depths to the source of the geologic structures where obtained from Werner and Euler deconvolution solutions which gives an average depths range of 231.2 m to 1040 m, with very few solutions having depths less than 200 m, the most prominent structure particularly the Ririwai ring complex have a depth range of 337.5 m to 465.5 m. The depth to basement for Werner solution ranged from 60 m to 420 m and the depth to basement of the contact model is shallow with depth of (60 to 420m) as compared to the dike model (200 to 420 m).

Onset of the anorogenic alkaline magmatism in the Nigerian Younger Granite province: Constraints from the Daura and Dutse complexes

The Younger Granite province of Niger and Nigeria exemplifies a classical anorogenic alkaline ring granite belt with southward age migration from ca. 470 Ma to 140 Ma. However, the precise age transition from the older Nigerien Younger Granite (NrYG) to the younger Nigerian Younger Granite (NaYG) is poorly constrained. The Daura and Dutse complexes, located at the northernmost tip of the NaYG province, represent a temporal and spatial link between the NrYG and NaYG provinces. The Daura complex is composed of amphibole syenite and biotite granite, while the Dutse complex consists of fayalite syenite and arfvedsonite aegirine granite. Similar to other alkaline rocks of the NrYG and NaYG provinces, the Daura and Dutse samples all chemically show typical A1-type granite features, such as high alkalis (K2O + N2O = 8.33–11.27 wt%.), Ga/Al ratios, Zr + Nb + Ce + Y contents (527–2345 ppm) and FeOt/(FeOt+MgO) ratios (0.03–0.70) as well as elevated HFSEs. The NdHf and Pb isotopic systematics show that all the syenite-granites yield εNd (t) = −2.9 to +1.5, ɛHf (t) = −3.8 to +16.7, and 206Pb/204Pb = 17.818 to 19.555, suggestive of magmas derived from a slightly depleted mantle source with OIB imprints. A reduced to slightly oxidized redox state is invoked for the parental melt based on estimations from mineral chemistry. The Daura amphibole syenite is composed of >60% alkali feldspar, show high Eu/Eu* (>1), Ba/Zr (>3.5) values and cumulate textures, resembling an alkali feldspar cumulate rock. The other alkaline granitic rocks were likely generated from variable fractional crystallization of the residual melt with distinct crustal contaminations. Zircon UPb dating reveal that the Daura amphibole syenite and biotite granite were emplaced at 324 ± 1 Ma and 321 ± 3 Ma, respectively, whereas the Dutse granitoids showed episodic intrusive periods of ca. 264 Ma and 232 Ma. All these results predate other anorogenic alkaline complexes of the NaYG and update the oldest age (ca. 214 Ma in previous references) recorded in the alkaline granite province. To the north, it connects well with ages from the Zinder complex (~330 Ma) in the southernmost NrYG province, reflecting a significant link between the NrYG and NaYG provinces. From a broad view, a linear age migration from north to south in the Niger-Nigeria Younger Granite province remains well preserved, although temporal and spatial intervals exist in local region. This emplacement pattern can be traced to the periodic and repeated reactivation of lithospheric shear zones and transcurrent faults.

Existence of a possible subsurface ring complex in the northwestern part of the Nigerian Younger Granite province: a contribution from aeromagnetic and remote sensing data

The study area lies within the north-western part of the Nigerian Younger Granite province. It is bounded by latitude 10°30′N and 11°00′N and longitude 8°00′E and 9°00′E. Previous geological and geophysical studies that were carried out in the study area focused mainly on the ring complexes with surface exposure while nothing is known about those without the surface exposure. This has resulted in gap in geo-scientific knowledge which this study aimed to fulfill. First vertical derivative (FVD) and Euler deconvolution of the aeromagnetic data along with processed DEM were used to achieve this purpose. Results of the interpretation revealed a magnetic anomaly, almost circular in shape, with characteristics that are similar to the anomalies that spatially coincide with the prominent ring complexes but without the outcrops that are associated with the complexes. This anomaly is interpreted to be an unexposed ring. Lineaments revealed by the results are mainly oriented in the E–W, NE–SW and NW–SE. The lineaments in the NE–SW and its conjugate are the most developed lineament system. Presence of ring complexes on and along the lineaments favours the opinion that the emplacement sites of the complexes are influenced by the reactivated Pan-African shear zones and transcurrent faults. In particular, the cross-over of major lineaments at the non-outcropping anomaly may suggest a very weak zone that probably allowed the volcanic eruption that initiated the emplacement of the unexposed ring complex. Results of the Euler deconvolution suggest a cylindrical-like body for the unexposed ring complex, which is consistent with pluton geometry, and a depth that range from outcrop to 1,200 m for it.

An Interpretation of Structures from the Aeromagnetic Field over a Region in the Nigerian Younger Granite Province

An interpretation of high resolution aeromagnetic data was carried out to provide new information about the study area. Located between latitude 10°30'N and 11°00'N and longitude 8°30'E and 9°00'E, the interpretation revealed previously unknown folds and magnetic anomaly with a signature that is similar to those associated with ring complexes in the study area but without the surface manifestation that is associated with the complexes. Qualitatively interpreted as unexposed ring complex, the magnetic anomaly is underlain by cross-over of major lineaments; implying presence of weak zone that probably allowed the volcanic eruption that initiated the emplacement of the source of the anomaly. A depth that ranges from outcrop to 1200 m and from outcrop to 1800 m is respectively suggested for the unexposed ring complex and folds by the Euler deconvolution results.

Hydrothermal fluids responsible for the formation of precious minerals in the Nigerian Younger Granite Province

Preliminary investigations in the Younger Granite Province of Nigeria have revealed that precious and semi-precious minerals like rubies, sapphires, emeralds, aquamarine, zircon and fluorite can be found in the region. The gem minerals are shown to have been produced either by direct deposition along fissures, veins and greisens by hydrothermal fluids or as a result of hydrothermal fluids reacting with wall-rocks. These wall rocks are either biotite granites from which the hydrothermal fluids originated or basement rocks or any other rocks which the biotite granites intrude and their residual hydrothermal fluids have invaded. The hydrothermal fluids appear to have been rich in alkalis (Na+, K+, etc.), rare elements (Be, Zr, F, REE, etc.) and siliceous. As these fluids rose through fractures and channel ways through the rocks, they either deposited the gem minerals in the fractures at the appropriate stability conditions or reacted with the wall-rocks producing the gem minerals at the expense of elements like Ca and A1 in the minerals of these rocks.

Magma distribution and crustal extension in the Nigerian younger granite province: evidence from the wase area

In the Wase area, situated E of the main belt of Jurassic (Younger Granite) ring complexes in Nigeria, the following sequence of events associated with Younger Granite magmatism took place: (1) Emplacement of minor syenite-granite central complexes and local felsite and microgranite dyke swarms; (2) dextral movement on the ENE-trending Dengi shear zone; (3) emplacement of an extensive swarm of mafic dykes. The S-shaped distribution pattern of ring complexes in the Nigerian Younger Granite province resembles a N-S spreading ridge truncated by ENE-trending transform faults. In the Wase area, the Dengi shear zone can be interpreted as an eastward continuation of the southern transform fault, developed in response to crustal extension in the N-S ‘ridge’ sector—the ring complex belt. The later mafic dyke swarm is correlated with a widespread Jurassic thermal event, known from basement rejuvenation ages W of the ring complex belt, and providing evidence for a much wider development of magma than is indicated by the closely grouped ring complexes.

Li, F and Rb contents and Ba/Rb and Rb/Sr ratios as indicators of postmagmatic alteration and mineralization in the granitic rocks of the Banke and Ririwai Younger Granite complexes, Northern Nigeria

The Banke and Ririwai complexes have plutonic phases of igneous activity composed mainly of granitic rocks. These granitic ring complexes are associated with Sn-Nb mineralization and are characterized by high Li, F and Rb contents and Rb/Sr ratios, and low Ba and Sr contents and Ba/Rb ratios. — The altered and mineralized granites have variable Rb/Sr and Ba/Rb ratios differing significantly from those of fresh rocks. These ratios as well as the Li, F and Rb concentrations are good indicators of granitic rocks associated with postmagmatic alteration and mineralization providing valuable tools for Sn-Nb exploration within the Nigerian Younger Granite province.

Postmagmatic arfvedsonite-aenigmatite paragenesis in the ring-dyke of the Burra Centre, Ningi-Burra complex, Nigeria

The SW segment of the peripheral ring-dyke of the Burra Centre contains a ferrohedenbergite quartz porphyry in which fayalite has destabilized and arfvedsonite has partially replaced the pyroxene. Arfvedsonite coexists with a later formed aenigmatite, this pair is thus considered as a new subsolidus assemblage developing at the expense of fayalite and clinopyroxene. The pair also crystallizes in textures considered to be typical vapour phase postmagmatic products. This is the first recorded occurrence of aenigmatite in the porphyry ring-dykes of the Nigerian Younger Granite province. Some representative chemical analyses of the rocks and minerals are presented. While other factors such as oxygen fugacity, temperature and volatile content must have contributed to the occurrence of aenigmatite, the single most important factor discriminating between host rocks is peralkalinity, developed at the magmatic stage but initiated mineralogically here only at the postmagmatic stage. It is suggested that the arfvedsonite-aenigmatite paragenesis marks a continuation of the postmagmatic development of peralkalinity in the residual fluids. Similar pathways were initiated by the sodian hedenbergite-ferrorichterite trend found in similar porphyritic facies in the adjoining Tibchi complex.

Three-dimensional gravity interpretation of the dutsen-wai complex, nigerian younger granite province

Three-dimensional modelling of gravity data shows that the Dutsen-Wai ring complex of the Nigerian Younger Granite Province can be a high-level, thin, tabular body with subsurface extension of felsic material, dominantly biotite granite, well beyond the outcrop limits of the complex in all directions. This geometry is consistent with emplacement of the body by near-surface lateral spreading of an intrusive diapir. Emplacement by this mechanism creates horizontally directed tensile stresses in the overburden which leads to the formation of ring fractures above the spreading intrusive.

Tuffisite with topaz from the Nigerian Younger Granite province — the Balfour Hill ‘sediments’

SummaryNear one of the Younger Granite intrusives of the Jos Plateau is a series of steeply dipping dark and predominantly fine grained rocks. Originally interpreted as argillaceous sediments thermally metamorphosed by the adjacent granite, with development of andalusite, later work established their volcanic origin but did not account for the andalusite. Field relationships and petrography of the rocks suggest they are tuffisites, emplaced by volcanic fluidization processes into part of the fracture system associated with the Younger Granite magmatism. The ‘andalusite’ is topaz, widespread in this province, mainly in the Younger Granite tin veins and greisens, sometimes as an accessory in the granites themselves, but not known from the volcanics — yet abundant in some of these tuffisites. It occurs in well formed crystals, displaying a ‘chiastolite-cross’ arrangement of inclusions, characteristic of andalusite but appearing not to have been previously recorded in topaz. The origin of the topaz must lie either in late-volcanic emanations or in hydrothermal fluids from the mineralizing stage of Younger Granite emplacement.

A gravity profile across the banke ring complex, Nigeria

The results of gravity measurements across the Banke ring complex indicate that a local negative gravity anomaly is associated with the complex. Supporting evidence for the association is provided by density determinations which indicate that the rocks of the ring complex are lighter than those of the surrounding basement complex. The anomaly can be explained if the main biotite-granite pluton of the ring complex extends in a southeasterly direction to a distance of some 5 km beyond the marginal ring-dyke, beneath a thin basement cover. It appears, therefore, that the surface structure of the Banke ring complex differs considerably from its structure at depth. In view of the association of such minerals as cassiterite and wolframite with the biotite-granites of the Nigerian Younger Granite Province, the area overlying the postulated extension of the Banke granite is suggested as a possible target for the exploration of these minerals.

Gravity Measurements over the Nigerian Younger Granite Province

THE “Younger Granites”, dated as Jurassic1, intrude the Nigerian basement complex and occur characteristically as ring-complexes, which are prominent topographic features in this part of Nigeria (Fig. 1). These complexes are acidic in composition and include a wide variety of biotite granites, alkaline granites and rhyolites2,3. The great variety of different rock types and the economic importance of the region have stimulated much geological interest. Most of the minerals mined in commercial quantities in Nigeria, including cassiterite, columbite, monazite, tantalite and zircon, are associated with the Younger Granites.

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.