Abstract

In north-central Nigeria a group of rocks of alkalic affinities, known as the Younger Intrusive Series, invades a metamorphic terrain. The classic sequence of magmatic differentiation is observed, but the proportion of felsic to mafic rock is unusually high. The appearance of riebeckite, aegirine, hedenbergite, iron-rich biotite, and fayalite in the felsic rocks corresponds well with the theoretical products of differentiation. The Younger Intrusive Series of Nigeria is highly discordant to pre-existing structures. The clean sharp contacts, locally chilled, irregular but not intricate, confirm the magmatic origin suggested by the petrology. Some of the rhyolites of the Series are now believed to be extrusive. The rocks are post-tectonic, and have emplaced themselves chiefly by large-scale stoping along concentric and radial fractures relatively close to the surface. Many bodies of intrusive rhyolite occupy crescentic fractures, and granite porphyry develops ring-structures. The granite stocks afford illuminating evidence of intrusion by large-scale stoping, in which areas of country rock are almost severed by arms and apophyses from the main granite mass.

The alkalic rocks of New England are remarkably similar to the Younger Intrusive Series of Nigeria in history, structure, and petrology. Mineralogical parallels extend even to minor constituents. In both areas, the older rocks consist of metamorphosed sediments invaded by concordant syntectonic bodies, the so-called “Older Granites”. The chief group of alkalic rocks in New England is the White Mountain Magma Series of New Hampshire, consisting of ring-structures and other complexes comparable in size and extent to those of Nigeria. One difference from the Nigerian occurrences is the presence of a few feldspathoid rocks in New England. In both areas, the mineral evolution shows a progression from calcic to sodic plagioclase, with quartz appearing in the younger rocks, and a corresponding increase of the ratio of iron to magnesia in the dark minerals. These trends suggest derivation of the various rocks by magmatic differentiation, but such differentiation is quantitatively inadequate to explain the great preponderance of felsic rocks. It is suggested that, concomitant with the regional metamorphism and orogeny, there was partial melting at depth, contributing to the formation of a hybrid magma in the post-tectonic phase.

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