Petrography and geochemistry of Neoproterozoic charnockite–granite association and metasedimentary rocks around Okpella, southwestern Nigeria

Abstract

The Neoproterozoic charnockite-granite association of Okpella intrudes metasedimentary and migmatite-gneiss complex rocks in the eastern Igarra Schist Belt, southwestern Nigeria. In order to unravel the complex processes involved in the formation and tectonic evolution of the Igarra Schist Belt during the Neoproterozoic, detailed field, petrographic and whole-rock geochemical study of the charnockite-granite association and metasedimentary rocks around Okpella was conducted. Published data on the metasedimentary rocks and Pan-African granitoids in different sectors of the belt were also compiled for detailed interpretations. Results of the study revealed that the charnockites and granites of Okpella show primary magmatic mineralogy and geochemical characteristics. They are silicic ( 63 wt. % SiO2), metaluminous to peraluminous, high-K calcalkaline, ferroan, post-collisional granitoids. The garnet-biotite schist, calc-silicate gneiss and quartzite in the area have paragenesis characteristic of low-medium grade metasedimentary rocks. The mineralogy and geochemistry of the charnockites suggest that the charnockitic melt was derived by partial melting of underplated basaltic (mantle-derived) magma and assimilation-fractionation processes at lower crustal levels.The granites probably originated from mixed melts derived from lower-to mid-crustal tonalites-trondhjemitesgranodiorites and/or subducted metagreywackes and mantle-derived magmas (probably the charnockitic melt progenitor). The granites and charnockites are coeval and were presumably emplaced during the post-collisional stages of the Pan-African Orogeny. The garnet-biotite schist, calc-silicate gneiss and quartzite represent 1 metamorphosed immature to slightly mature sedimentary rocks, probably greywacke, marl and subarkose, respectively, that were sourced from intermediate-acidic rocks which underwent low-moderate chemical weathering with minor contribution from recycled sediment sources and deposited in active continental environments. The Neoproterozoic evolution of the Igarra Schist Belt, therefore, involved deposition and infolding of sediments in active continental margin during the early Pan-African followed by upwelling of basaltic magma from the mantle which underplated the lower crust to generate the charnockitic and granitic melts through mantle-crust interaction during the late Pan-African.

Geohydraulic and vulnerability assessment of tropically weathered and fractured gneissic aquifers using combined electrical resistivity and geostatistical methods

“…The Younger Granites, i.e., the Mesozoic anorogenic calc-alkaline ring complexes, as shown in Fig. 1b, intruded the Nigerian Eastern Basement Complex terrain before the formation of any of the sedimentary basins in the eastern and western terranes of Nigeria [26][27][28]. showing the study area in the Nigerian Southwestern Basement Complex (modified after [28]).…”

“…1a. The Southwestern Basement Complex of Nigeria is made up of three major rock suites, namely the Migmatite-Gneiss Complex, ranging in age from Neoproterozoic to Paleoproterozoic and Archean, e.g., [22][23][24]; the Neoproterozoic Schist Belts, consisting of low-grade, younger metasedimentary, and metavolcanic rocks with ages ranging between 690 and 489 Ma, e.g., [23,24], and the Pan-African Older Granites, which intruded the two earlier lithologies, have ages ranging between 650 and 580 Ma, e.g., [26,27].…”

Geohydraulic characteristics and groundwater vulnerability assessment of tropically weathered and fractured gneissic aquifers using combined georesistivity and geostatistical methods

“…The Southwestern Basement Complex of Nigeria comprises three major rock suites, namely the Migmatite-Gneiss Complex, the Schist belt, and the Older Granitoid. The Migmatite-Gneiss Complex rocks range in age from Neoproterozoic to Paleoproterozoic and Archean [20,21,22]; the Neoproterozoic Schist Belts, consisting of low-grade, younger metasedimentary, and metavolcanic rocks with ages ranging between 690 and 489 Ma [21,24,25], and the Pan-African Older Granites, which intruded the two earlier lithologies, have ages ranging between 650 and 580 Ma [25,26,27,28].…”

Geohydraulic and vulnerability assessment of tropically weathered and fractured gneissic aquifers using combined electrical resistivity and geostatistical methods

“…1a. The Southwestern Basement Complex of Nigeria is made up of three major rock suites, namely the Migmatite-Gneiss Complex, ranging in age from Neoproterozoic to Paleoproterozoic and Archean, e.g., [22][23][24]; the Neoproterozoic Schist Belts, consisting of low-grade, younger metasedimentary, and metavolcanic rocks with ages ranging between 690 and 489 Ma, e.g., [23,24], and the Pan-African Older Granites, which intruded the two earlier lithologies, have ages ranging between 650 and 580 Ma, e.g., [26,27]. Early magmatic phases dating from 790 to 709 Ma have also been reported in some of the Older Granites rocks.…”

“…The Younger Granites, i.e., the Mesozoic anorogenic calcalkaline ring complexes, as shown in Fig. 1b, intruded the Nigerian Eastern Basement Complex terrain before the formation of any of the sedimentary basins in the eastern and western terranes of Nigeria [26][27][28].…”