Geochemical data of pyroclastics rocks from Ishiagu, Lokpaukwu and Uturu are presented. The occurrence of compositional variation in the samples indicates that high-level fractionation or alteration occurred. The data indicate that samples are Picrite to Basanite (GroupA), Hawaiite (Group B) and Nephelinite (Group C). Such variation would suggest that rocks of these three localities do not form part of continuous series and could have resulted from partial melting process generated from the mantle and later passed through the process of magma contamination/ assimilation. The rocks can be separated into three magmatic groups: mildly tholeiitic, transitional tholeitic and calcalkaline. Petrographic characterization shows that the rocks are composed of plagioclase (Labradiorite and andesine), clinopyroxene (cpx) (Augite), orthopyroxene (opx) (hypersthene), iron ores, olivine, nepheline and k-spar (in three samples) and quartz (in five samples). Major element geochemistry indicates that all the pyroclastics in the study areas are deficient in CaO and MgO and high in Na2O, TiO2 and LOi. K2O content is low in the pyroclastic rocks of Ishiagu and Uturu, while the pyroclastics in Lopkaukwu are exceptionally high in K2O, implying that the magma was eventually modified by high K-alkaline chemistry, probably from the widening of the proto-oceanic South Atlantic before final emplacement as volcanic in a sediment dominated continental rift setting.

The Lalmai hilly area, Comilla, lying between the Ishiagu, Lokpaukwu and uturu are located within the lower Benue trough of Nigeria (Figure 1), and it accommodates discontinuous exposures of eroded volcanic and hyperbyssal features. The Benue trough was formed as a result of a series of tectonism, accompanied by magmatism and repetitive sedimentation in the Cretaceous during the separation of South America from Africa. The determination of the source of the intruding magma and the history of the emplacement processes of the pyroclastics can be useful in the interpretation of the petrogenesis and geotectonics of the Benue trough. To achieve this, a sample analysis has been done in order to determine the character of the melting process of the intruding magma and how the magma was emplaced in southern Benue trough since magmas can be characterized by their constituent immobile elements in rocks (O’Hara, 1965; and Wood et al., 1979). This study is essentially a support project to the ongoing research on magmatism and its relationship to the development of the Benue trough.

Earth Sciences Journal, Palaeoenvironmental Studies, Rajapuram and Somanapally Sandstones, Palaeocurrent Analysis, Crossbedding, Proterozoic Rocks, Godavari Valley, Pakhal Sandstones, Polycrystalline Grains, Precambrian Sediments, Bimodal System.