Petrology and Rare Earth Elements (REE) Distribution Patterns of Magmatic Rocks In Gboko Area, Lower Benue Trough Nigeria: Implication For Tectonic Evolution
Volcanic rocks can be subdivided according to tectonic setting into oceanic, sub-oceanic, sub-continental and continental. The relationship between the magma chemistry and the tectonic settings determines the end products (rocks) by magmatic fractionation and/or progressive crustal contamination. Attempts to explain the magmatic and tectonic evolution of the Gboko area is considered in terms of the magmatic end products and the REE distribution patterns. Volcanic and intermediate rocks such as basalts, trachyandesites, trachytes, dacites, rhyolites, nepheline syenites and quartz diorites occur within the area intermittently. The distribution of the chondrite normalized values is used to determine the effect due to magmatic fractionation and the crustal contamination.
The REE distribution shows relative enrichment of the LREE compared to the chondrites and the HREE except for one of the rhyolites. The distribution patterns show similar trend for basalts, trachyandesites, dacites and quartz diorites. The trends show slight Ce, Pr, Sm, Dy and Tm negative anomalies. In the quartz diorites there is comparatively more depletion of the LREE and a significant negative anomaly of Tm. Closely similar to the trends of basalts are those of trachyte and nepheline syenite. The trachtyes and nepheline syenites have similar negative Sm, Eu, Tb, Dy, and Tm anomalies and slight positive Gd and Er anomalies as well as higher relative depletion of HREE. The rhyolites display similar trends with significant negative Eu anomaly, except in one of the rhyolite where there is relative enrichment of the HREE and a significant negative Ce anomaly. The trends of the basalts are comparable to theoleiitic basalts and their similarity with the other rocks indicates the same source magma. In theoleiitic provinces, basalts coexist with trachyandesites and rhyolites as residual products of differentiation of basic magmas as confirmed by the similar trends. The significant Ce and Eu negative anomalies in the rhyolites and quartz diorites indicate that mineral fractionation and crustal contamination has played a part.