Abstract:
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. New geochemical and Sr-Nd
isotope data reveal that the low-grade metavolcanic rocks of the westernmost Nuba
Mountains represent a Neoproterozoic oceanic arc/back-arc 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 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 crustal material. The western Nuba
Mountains metavolcanic rocks have εNd 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 metavolcanic rocks are structurally bounded from the eastern side by mediumgrade
orthogneisses and supracrustal metasediments intruded by syn to late-orogenic
granite. Based on their geochemistry and isotopic characteristics these gneisses are
interpreted to be evolved from an arc magma source of early Neoproterozoic age
976±58 Ma corresponds to εNd value of +6.3±0.9 (six point Nd isochron). The tight
clustering of εNd values and the TDM Nd model ages preclude significant involvement of
older crustal material and hence these gneisses are assigned to intra-oceanic arc
environments.
