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

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 +/- 14Ma and 851 +/- 2Ma, respectively. The Emba-Derho granitoids show high and a narrow range of SiO2 (71.3-73.6wt.%) and Al2O3 (14.9-15.9wt.%) contents, but relatively lower K2O+Na2O (4.8-5.5wt.%). They are characterized by high Sr (306-367ppm) and very low Y (3.57-6.03ppm) and Yb (0.38-0.61ppm), and high Sr/Y (42-70) ratios, implying they have adakitic signatures. Their chondrite-normalized REE patterns are highly fractionated with high La/Yb-N (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 Sr-87/Sr-86 (0.70084 to 0.70263), higher positive ?Nd (+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 (67.9-78.4wt.%), Al2O3 (11.05-16.51wt.%), and K2O+Na2O (5.86-8.76%) contents. The rocks have extremely low initial Sr-87/Sr-86 (0.68685 to 0.70099) and high ?Nd (+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.