J. B. Wright

Geochemistry of calc-alkaline volcanics in northwestern Nigeria, and a possible Pan-African suture zone

Abstract Major and trace element characteristics of dacites and rhyolites overlying and intruding basement rocks in northwestern Nigeria most closely resemble those of intracontinental orogenic volcanic associations. REE patterns point to a deep-seated source for the magmas, perhaps involving garnet fractionation at mantle depths and low-pressure plagioclase fractionation. The occurrence of calc-alkaline volcanics, small basic-ultrabasic complexes and major transcurrent faulting, is consistent with the presence of a Pan-African suture zone in northwestern Nigeria.

K-Ar Retention Ages from the Pan-African of Northern Nigeria

Twenty-four mineral and whole-rock K-Ar ages have been determined from twenty-two samples collected during recent mapping in northern Nigeria. With three exceptions, the samples are from narrow synclinorial belts of relatively low-grade metasedimentary rocks within the crystalline basement complex. The whole sequence is believed to have been extensively folded and metamorphosed during the Pan-African thermo-tectonic episode. No K-Ar ages older than 700 m.y. were found in the metasedimentary rocks, suggesting they may belong to the Katangan depositional cycle. The majority of metamorphic mica and whole-rock K-Ar ages range from 550 to 490 m.y. and are indicative of a period of post-metamorphic, epeirogenic uplift and cooling initiated in Middle Cambrian times. Late Cambrian deposition of molasse-type sediments in southern Ghana is believed to be the result of this uplift. A single K-Ar age of 656 m.y., obtained from a pegmatitic muscovite, suggests that metamorphic recrystallization was completed during late Precambrian times. K-Ar hornblende ages (690 to 650 m.y.) would appear to support this conclusion, but the presence of excess 40 Ar in the analyzed hornblende samples is indicated by an isochron plot.

On Place and Time in Orogenic Granite Plutonism

In a recently mapped region of northern Nigeria, typically syntectonic (“low-level”) granites, surrounded by gneisses, are exposed at the same crustal level as typically late-tectonic (“high-level”) granites, cutting migmatites and metasediments. Comparison with relationships in high-grade metamorphic terranes elsewhere suggests: (1) that low-level granites are root zones of granite masses which rose to comparatively high crustal levels during an orogenic climax; (2) that high-level granites are roof zones of bodies which were emplaced during waning stages of orogenesis and, therefore, did not rise so high. It is our contention that these terms have been inappropriately used to refer to level of granite emplacement, rather than to level of exposure within a granite mass.

The Cwm Dulyn rhyolite, Snowdonia; an extrusive dome? [letter]

SIR It appears that in the Ordovician volcanic province of North Wales, extrusive rhyolites are rare and recognized only as lava flows; any dome-like structures are evidently interpreted as being of intrusive origin (Rast, 1969, p. 307). During the Geological Society of Londons Volcanic Studies Group field meeting in Snowdonia, in May 1971, participants were taken to the Cwm Dulyn rhyolite, for the purpose of examining one of the allegedly most convincing examples of an intrusive rhyolite body in the whole volcanic province.

Tholeiite from the Cenozoic alkaline volcanic province of Nigeria

Bouldery outcrops of coarse-grained olivine-free lava, containing abundant orthopyroxene and clinopyroxene, mostly in glomerophyric aggregates, occur over a small area among the Tertiary to Recent volcanics of the Jos Plateau, Nigeria. This is the only rock of tholeiitic affinity in an otherwise completely alkaline volcanic province. Petrographically it does not greatly resemble tholeiitic occurrences reported from such provinces elsewhere, but that is believed to be due more to a relatively complex crystallization history than to any fundamentally different process of magma generation. It is not regarded as representing a tholeiitic parent for the alkaline lavas of the Jos Plateau region.