Prolonged arc accretion during growth of juvenile crust in the Arabian Nubian Shield: Insights from the granitoids of northern Ethiopia

Abstract

The Arabian Nubian Shield (ANS) is one of the largest juvenile continental crust formed after the Archean. To determine the timing and understand the mechanism of its early crustal growth, we conducted geochronological and geochemical studies on granitoids from northern Ethiopia. A plagiogranite, dated at 929.7 ± 2.2 Ma, represents one of the early granitoids from the ANS. It shows low K₂O, TiO₂, and REE contents, flat REE pattern and absence of Eu anomaly. Moreover, it exhibits (⁸⁷Sr/⁸⁶Sr)_i, Ɛ_Nd(t) and Ɛ_Hf(t) values of 0.70341, +5.0 and +8.4, respectively. Its zircon δ¹⁸O value is 4.8‰, lower than the mantle value. These characteristics suggest that it was derived from partial melting of altered oceanic crust. The oxygen fugacity obtained from zircon trace elements is ΔFMQ+1.3, consistent with a subduction setting.

Other plutons have much younger ages ranging from 860 to 840 Ma and are mainly medium K, I-type granitoids. These granitoids show low MgO, Ni and Cr contents, low Sr/Y, (La/Yb)_N, Nb/Ta and Zr/Sm ratios. They exhibit whole-rock (⁸⁷Sr/⁸⁶Sr)_i values of 0.70236–0.70304, Ɛ_Nd(t) values of 4.2–5.2, Ɛ_Hf(t) values of 5.8–7.6 and zircon Ɛ_Hf(t) values of 5.5–10.4, slightly lower than coeval depleted mantle. Furthermore, they exhibit fractionated REE patterns, enrichments in LILE and depletion in HFSE. These geochemical features suggest that these granitoids were generated by remelting of arc crust. Compiled data show that the associated basaltic rocks are of arc affinity, which confirms the coeval development of arc magmatism in the southern ANS. Our results suggest that the growth of juvenile crust in the southern ANS started from the early Neoproterozoic and peaked at ca. 880 to 800 Ma, much earlier than those in the northern ANS. The prolonged history accounts for the growth of the vast juvenile crust in the ANS.