Gravity interpretation of the Zaranda Younger Granite ring complex, Nigeria

Abstract

Gravity and geological data were collected over the Zaranda complex situated near Bauchi, Nigeria for a critical analysis of the deep structure in the area. The result of the study shows that, in addition to the host migmatitic gneiss, five major rock types form the complex. These are leixocratic and melanocratic syenites, granite porphyry, trachytes and basalts, and mafic-rich enclaves in the basement. A negative residual gravity anomaly of about − 10.6 mGal is centred on the complex. This negative residual is bordered on the eastern and southern margins by relatively positive residual anomalies of up to +5.2 mGal. The main negative anomaly displays a prominent NE-SW trend which is similar to those of the major structures in the basement, suggesting that the complex might have intruded the host migmatic gneiss through pre-existing structures. There is also good correlation between the residual anomalies and the geology, with the negative anomalies occurring over areas underlain by low density rocks corresponding to trachyte (2.56 × 10 3 kg m −3), syenite (2.62 × 10 3 kg m −3); while positive anomalies correspond to areas of basic rocks (2.76−2.92 × 10 3 kg m −3). 2-D and 3-D gravity modelling showed that the major pluton of the complex is the syenite body which extends to a depth of about 5.5 km. The mineralogy and chemistry of the rocks in the Zaranda complex suggest that the complex may be underlain by a zone of mafic rocks, as shown by field occurrence of hornblende-rich enclaves in the basement.