Petrogenesis and tectonic implications of the Neoproterozoic adakitic and A-type granitoids in the southern Arabian-Nubian shield

Abstract

Both adakitic and A-type granitoid have been recently identified in the southern ANS. The Emba-Derho and Koka granitoids yielded the same zircon U–Pb ages of 851 ± 14 Ma and 851 ± 2 Ma, respectively. The Emba-Derho granitoids show high and a narrow range of SiO2 (71.3–73.6 wt.%) and Al2O3 (14.9–15.9 wt.%) contents, but relatively lower K2O + Na2O (4.8–5.5 wt.%). They are characterized by high Sr (306–367 ppm) and very low Y (3.57–6.03 ppm) and Yb (0.38–0.61 ppm), and high Sr/Y (42–70) ratios, implying they have adakitic signatures. Their chondrite-normalized REE patterns are highly fractionated with high La/YbN (14.03 to 29.82) ratio without significant Eu anomalies (Eu/Eu* = 0.89–1.07). Their primitive mantle-normalized trace element diagram is characterized by enrichment of LILE and depletion of HREE (Ta, Nb, and Ti). The samples have low initial 87Sr/86Sr (0.70084 to 0.70263), higher positive ƐNd (+ 5.0 to + 8.2), and more radiogenic Pb isotopes. These lines of geochemical evidence indicate that the Emba-Derho granitoids were generated by partial melting of a subducted oceanic slab at the stability field of garnet. In contrast, the Koka granitoids display wide range of SiO2 (67.9–78.4 wt.%), Al2O3 (11.05–16.51 wt.%), and K2O + Na2O (5.86–8.76%) contents. The rocks have extremely low initial 87Sr/86Sr (0.68685 to 0.70099) and high ƐNd (+ 5.43 to + 5.78) and relatively less radiogenic Pb isotopic compositions compared with the Emba-Derho granitoids. They also show A2-type geochemical characteristics, suggesting that the Koka granitoids were originated from the juvenile continental crust. Both the adakitic and A-type granitoids in this study may have been formed in an arc-back-arc setting resulted from NW dipping subduction of the oceanic slab.