Neoproterozoic arc–back–arc system analog to modern arc–back–arc systems: evidence from tholeiite–boninite association, serpentinite mudflows and across-arc geochemical trends in Eritrea, southern Arabian-Nubian shield

Abstract

Neoproterozoic volcano–sedimentary–plutonic associations in Eritrea are part of the Arabian-Nubian Shield. In central Eritrea, the dominant low-grade supracrustal rocks are of volcanic origin, associated with subordinate sedimentary rocks and lenses of serpentinites. Pre- to syn-kinematic diorites and tonalites of ca. 810 Ma age and late- to post-kinematic granites of 585 Ma age intrude these supracrustal rocks. Low-K tholeiite–boninite metavolcanic rocks have trace element characteristics that are similar to modern oceanic primitive arc volcanic rocks. Boninites are found together with serpentinite bodies that are very similar to serpentinite mudflows occurring on modern forearcs, indicating a palaeo-forearc setting for central Eritrea. In contrast, the predominant rocks in western Eritrea are supracrustal sequences of sedimentary origin, with subordinate volcanic rocks. Tholeiitic metavolcanic rocks from western Eritrea have trace element characteristics that are similar to modern back–arc basin basalts. The timing of magmatism in both areas (ca. 850 and 800–850 Ma, western and central Eritrea plus northern Ethiopia, respectively) is broadly coincident. In the absence of major crustal discontinuities that might separate major terranes along the central-western Eritrean transect, this large Neoproterozoic tract may represent an 800–850 Ma NW-palaeo oceanic arc–back–arc system. Systematic spatial variations in Ti, Nb, Y and Zr concentrations from forearc, through arc, to back–arc provide further evidence for the north-west direction of subduction.