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Basement Complex

Posted by on 2/20/2004 2:15:53 PM |

Basement complex rocks are subdivided into migmatite-gneiss complexes; the older metasedi- ments; the younger metasediments; the older gran- ites; and the younger granite alkaline ring complex- es and volcanic rocks. The migmatite gneiss complex is the common- est rock type in the Nigerian Basement complex. It comprises two main types of gneisses: the biotite gneiss and the banded gneiss. Very widespread, the biotitic gneisses are normally fine-grained with strong foliation caused by the parallel arrangement of alternating dark and light minerals.

The banded gneisses show alternating light-coloured and dark bands and exhibit intricate folding of their bands. The migmatite gneiss complex is the oldest basement rock, and is believed to be of sedimentary origin but was later profoundly altered into metamorphic and granite conditions. The older metasediments were also among the earliest rocks to form on the Nigerian Basement Complex. Initially of sedimentary origin, with a more extensive distribution, the older metasediments underwent prolonged, repeated metamor- phism; and now occur as quaitzites (ancient sand- stones), marble (ancient limestones), and other calcareous and relics of highly altered clayey sediments and igneous rocks.

Most parts of the Basement complex are under- lain by belts of roughly north-south trending, slight- ly metamorphosed ancient Pre-Cambrian sedimentary and volcanic rocks known as the younger metasediments. The major rock types are ancient shaly rocks which are now referred to as quartz- biotite-muscovite schist. These change laterally into coarse-grained feldspar-bearing micaceous schists. Schists with graphite, phyllites and chlorite are common. Ferruginous quartzites and tale schists also occur. The younger metasediments contain most of the gold deposits in Nigeria in the northwest around Maru and Anka, and at Zuru; near Kaduna, and also at llesha in southwestern Nigeria.

Older granites are widespread throughout the Basement Complex and occur as large circular masses within the schists and the older migmite- gneiss complexes. The older granites vary exten- sively in composition. The younger granite complexes in Nigeria are found mainly on the Jos Plateau, forming a distinc- tive group of intrusive and volcanic rocks that are bounded by ring dykes or ring faults. Other occur- rences approximate a north-south belt towards the middle Benue in the south where the ages are younger, and towards Niger Republic in the north where the younger granites are older. There is enormous variety in the granite composition of these rocks.

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