Pan-African, post-collisional, ferro-potassic granite and quartz-monzoniteplutons of Eastern Nigeria

Three Pan-African hypersthene-bearing monzogranitic and quartz-monzonitic p lutons from the Eastern terrane of Nigeria have been investigated in detail . New major, trace and REE data, used to constrain their origin and nature, indicate that they display chemical features of ferro-potassic trans-alkal ine affinity. Further trace element discrimination suggests (i) production of calc-alkaline medium-K diorite magmas by partial melting of fluid-metaso matised mantle wedge possibly combined with melts from the dehydration part ial melting of altered oceanic crust; (ii) simultaneously production of the granite-quartz-monzonite ferro-potassic magmas from partial melting of hor nblende-bearing granodioritic crustal sources; (iii) mixing of the two magm as, Sr initial ratios of 0.707 to 0.711 witness that the source of the gran ite magmas is the lower crust. Ages of the lower crustal granulitic protoli ths is bracketed by Nd model ages between 1.9 and 2.2 Ga. Pb evaporation ag es on single zircons constrain the emplacement of the three plutons around 580 Ma. Ar-40/Ar-39 ages of amphiboles at about 560 Ma suggest cooling rate s around 15 degrees C/Ma. Extensive field work has established that pluton emplacement occurred during a regional north-south dextral strike-slip tect onics following the 630-610 Ma stage of oblique continent-continent collisi on in this part of west Africa. (C) 1998 Elsevier Science B.V. All rights r eserved.