Petrogenesis and tectonic implications of the Neoproterozoic adakitic and A-type granitoids in the southern Arabian-Nubian shield
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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.
KeywordsGranitoid Neoproterozoic Petrogenesis Arc setting Arabian-Nubian shield
We are grateful to Yosief Tadesse senior geologist in Koka Gold Mine, Eritrea, and Woldegabriel Genzebu, assistant professor, Department of Earth Sciences at EIT, for their support and assistance during the field work. We also thank Dr. Zhang Hao, Beijing Yanduzhongshi Geological Analyses Laboratory, Institute of Mineral Resources, Chinese Academy of Geosciences, for his assistance with LA-ICP-MS zircon U–Pb dating. We also highly acknowledge the two anonymous reviewers for their constructive comments and suggestions.
This study was funded by the China Geological Survey (Wuhan Center) under the project number DD20160109.
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