Geology, geochemistry and Sr–Nd constraints of selected metavolcanic rocks from the eastern boundary of the Saharan Metacraton, southern Sudan : a possible revision of the eastern boundary

Abstract

Neoproterozoic, Pan-African low-grade metavolcanic rocks and associated mafic and ultramafic rocks of ophiolitic origin have long been identified within the pre-Neoproterozoic Saharan Metacraton (SMC). These low-grade rocks within generally high-grade (upper amphibolite facies) gneiss and schist have not yet been fully investigated, and their geological and geotectonic significance have been recognised only in a very few localities: (1) the Delgo–Atmur ophiolite and low-grade volcano-sedimentary belt, (2) the Rahib ophiolite and low-grade sedimentary fold and thrust belt, both in northern Sudan along the eastern boundary of the Saharan Metacraton and (3) the low-grade volcano-sedimentary rocks in the Central African Republic. Dismembered and low-grade metamorphosed occurrences of mafic extrusive and intrusive and minor ultramafic rocks, grouped as the Arid unit, similar to those of the Arabian Nubian Shield (ANS), are reported here for the first time in the westernmost part of the Nuba Mountains, southeastern Sudan. These occurrences are interpreted to represent part of an ophiolite sequence with a lower cumulate layer composed of layered gabbro and minor cumulate hornblendite and a top layer of thick massive gabbro, pillowed basalt and basaltic andesite. The Arid unit is structurally underlain by basaltic-andesite and andesite and a metasedimentary sequence identified as turbidite and both grouped as the Abutulu unit. All of the rocks are slightly sheared, deformed and metamorphosed under low-grade greenschist facies to epidote amphibolite sub-facies. New geochemical and Sr–Nd isotope data reveal that the low-grade metavolcanic rocks of the westernmost Nuba Mountains represent a Neoproterozoic oceanic arc/backarc assemblage. The massive gabbro and pillowed basalt of the Arid unit show the geochemical characteristics of HFSE-depleted tholeiitic basalt while the co-genetic and more evolved meta-andesite of Abutulu unit show a calc-alkaline signature. Both units display a REE pattern characterized by LILE enrichment indicating formation in an arc/back-arc environment. This arc was active at around 778 ± 90 Ma (Sm– Nd 12 WR isochron) that is similar in age to the arc magmatism in the ANS. The close interval between the TDM Nd model age (average of 10 metavolcanic samples is 814 Ma) and the crystallization age (778 ± 90 Ma) is indicative of little or no involvement of older material. The western Nuba Mountains metavolcanic rocks have eNd values of +5.9 at 778 Ma (average of 12 samples) indicating a depleted mantle source similar to that of the ANS (published range from +6.5 to +8.4). The eNd values of the metavolcanic rocks are different from previously published ages of high-grade basement rocks that occupy the area west of the Kabus suture and east of Abutulu (+2.2 and +3.5 for the Rashad and Abbassyia). It is proposed that the metavolcanic and associated plutonic mafic rocks represent a unique Neoproterozoic entity named the Abutulu terrane that developed in a marginal back-arc basin west of the medium-grade gneiss of the Nuba Mountains. If the Abutulu terrane is included as a part of the ANS, then the eastern boundary of the SMC is adjacent to the western edge of the ANS along the Abutulu suture.