Discovery of Archaean crust within the Akitkan orogenic belt of the Siberian craton: New insight into its architecture and history

The Akitkan orogenic belt separates the Aldan and Anabar superterranes of the Siberian craton and was previously suggested to consist exclusively of Palaeoproterozoic juvenile crust. Foliated granitoids in the Baikal basement salient within the Akitkan belt were studied geochronologically and geochemically. Thermal ionisation mass spectrometry (TIMS) analysis of six zircons of a tonalite sample yielded a discordia line with an upper concordia intercept age of 2884 ± 12 Ma which is interpreted as the time of tonalite emplacement. Geochemical data of the foliated granitoids confirm their affinity to the tonalite–trondhjemite–granodiorite (TTG) series. They are characterized by high Al2O3 contents (14.4–17.2 wt.%), high Sr/Y ratios (18–99) and show significant rare-earth element (REE) fractionation with Lan/Ybn = 11–89. The ɛNd(T) values vary from −1.6 to +0.8 and Nd mean crustal residence ages range from 3.23 to 3.07 Ga. Geochemical and isotopic data confirm their genesis through interaction of mantle and crustal material under high pressure near the base of thickened crust either after accretion of ancient continental crust and an oceanic plateau, or after underplating of oceanic plateau-like basaltic magmas. itkan orogenic belt is a composite crustal unit of Archaean and Palaeoproterozoic age. We propose two possible scenarios of its evolution. In the first scenario Palaeoproterozoic complexes developed along the active margin of the Archaean Anabar superterrane at 2.1–2.0 Ga. Alternatively Archaean and Palaeoproterozoic components of the Akitkan belt could have a distal origin and were accreted to the Anabar superterrane. These accretional processes were followed by a major collision of the Aldan and Anabar superterranes at ∼1.9 Ga finalised by post-collisional granitoid magmatism at 1.87–1.84 Ga.