Abstract

New high spatial resolution secondary ion mass spectrometry (SIMS) zircon dating from the Dire Dawa Precambrian basement yields crystallization ages at c. 790 Ma and 600 – 560 Ma. Two of the youngest samples are pervasively deformed, indicating that orogenesis continued until c. 560 Ma. SIMS δ18Ozrn shows bimodality, with the oldest sample (c. 790 Ma) and inherited zircons of that age in the younger samples having values of 7.8 – 9.6‰, whereas the Ediacaran samples have δ18Ozrn values of 4.9 – 7.2‰. These δ18Ozrn ratios are higher than mantle values and indicate a supracrustal input to the source of the Dire Dawa granitoids. All samples have unradiogenic εNd(t) values of −10.3 to −5.8 and Nd model ages of 1.72 – 1.42 Ga. These attributes suggest that the Dire Dawa granitoids were mostly derived from reworking of long-lived crustal sources. The occurrence of c. 580 – 550 Ma orogenesis in both the Dire Dawa basement and the juvenile Western Ethiopian Shield and the confinement of c. 630 Ma metamorphism to only the latter indicate that these two lithospheric blocks of contrasting isotopic compositions amalgamated at c. 580 – 550 Ma. This suggests that the Mozambique Ocean, which separated these two lithospheric blocks, was completely consumed during the late Ediacaran to early Cambrian.

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