Petrology and geochronology of Mesoproterozoic mafic-intermediate plutonic rocks from Mitwaba (D. R. Congo): implications for the evolution of the Kibaran belt in central Africa

摘要

Mesoproterozoic supracrustal rocks in the Kibaride belt (southeast Congo) were intruded by mafic-intermediate plutonic rocks. These igneous rocks were affected by greenschist- to amphibolite-facies metamorphism during the Mesoproterozoic Kibaran orogenesis. U-Pb single zircon dating of the Mitwaba mafic-intermediate plutonic rocks yields an emplacement age of 1.38 Ga. The compositions of the Mitwaba mafic-intermediate plutonic rocks range from gabbro to diorite and show the following elemental concentrations: SiO2: 49-58 wt %, TiO2: 0.53-0.92 wt %, Al2O3: 13.1-18.68 wt %, Zr: 45-142 ppm, Y: 13-43 ppm. Mg no. 40-66 indicates variable degrees of fractionation of the magmas. The rocks are marked by high and variable Th/Ta (3-14), La/Nb (2-5) and low Ce/Pb (0.3-12.8) and Ti/V (10-19). Chondrite-normalized REE patterns exhibit enrichment in LREE relative to HREE ((La/Yb)(N) = 2.9-5.8). Primordial mantle-normalized spider diagrams show negative slopes with gradual decrease from LIL to HFS elements and are marked by Nb and Ti negative anomalies. Immobile trace-element contents indicate a continental arc setting for these mafic-intermediate igneous rocks. They are inferred to have originated from a mantle wedge enriched by fluid from a subducting slab, with possibly an additional contribution from subducted sediments. Low, sub-chondritic Nb/Ta ratios in these mafic rocks support this interpretation. A model involving underplating of mafic-intermediate arc magma into the crust, triggering partial melting of Mitwaba group metasedimentary rocks during the accretionary stage of the Kibaran orogeny, is proposed to explain the coeval emplacement of mafic-intermediate arc magmas and peraluminous S-type granitoids in the Kibaride belt of central Africa.