Subduction to accretion in the southern Irumide Belt: Perspective from geochronology and petrology of granulites and charnockites in NW Mozambique

Abstract

In the Mesoproterozoic Southern Irumide Belt, Tete Province, NW Mozambique, granulite-facies metavolcaniclastic rocks and orthogneisses from the Chidzolomondo Complex provide new constraints on the subduction and accretionary phases of the belt evolution. The complex comprises a metavolcano-sedimentary succession intruded by syn- to post-orogenic granitoids, charnockites and gabbros. The paragneisses exhibit a calc-alkaline chemical composition akin to intermediate magmatic rocks found in volcanic arcs. They are interpreted as products of a three-component mixture, consisting of pelite-aluminous, quartz, and reworked mafic volcanic rocks. A paragneiss sample contains detrital zircon with ages mainly between 1344 and 1098 Ma and εHf(t) values between −10 to +7, defining a maximum depositional age of 1121 ± 2 Ma. A charnockitic gneiss, dated at 1097 ± 3 Ma, and a metagabbro, dated at 1061 ± 3 Ma, respectively, represent the syn- and late-orogenic magmatic events. Both rocks exhibit juvenile εHf(t) signatures ranging from +1.5 to +4.5. The mineral assemblage of cordierite + garnet + orthopyroxene in metavolcaniclastic rocks, and plagioclase + hypersthene + diopside in the metagabbro, provide evidence of the HT-LP metamorphic conditions. Phase equilibria modeling of a paragneiss indicates that the peak metamorphic conditions reached approximately 785–795 °C and 3.7–4.2 kbar. By dating of zircon overgrowths in granulite-facies gneisses, we characterized the orogenic (M1) and thermal (M2) metamorphisms that affected the complex. The M1 event records the subduction-type and accretion metamorphism dated between 1092 and 1060 Ma. The M2 event, identified in the metagabbro sample close to the contact with a charnockite intrusion, yielded an age of 1023 ± 12 Ma for the thermal metamorphism. Intrusive charnockite and a gabbro record the post-orogenic magmatism with ages of 1026 ± 4 Ma and 1026 ± 12 Ma, respectively. The results provide a timeline for the last orogenic events of the Southern Irumide Belt, including the maximum depositional age of the Chidzolomondo Complex at ca. 1120 Ma, syn-orogenic magmatism and metamorphism until ca. 1060 Ma, and late- to post-orogenic magmatism and metamorphism between 1050 and 1020 Ma.