The Newly Recognized Ca. 1.23–1.21 Ga Dolerite Sills and Flood Basalts from Fanhe Basin in the Northeastern North China Craton: Petrogenesis and Tectonic Implications

The Ectasion Period (1400-1200 Ma) represents a transition stage from Columbia (Nuna) to Rodinia supercontinents. LIPs and mafic magmatic events during this period can provide important constraints on deep processes and paleogeographic reconstructions during this transition. In this paper, we firstly reported some ca. 1.23-1.21 Ga flood basalts coeval with mafic sills from the Fanhe Basin in the northeastern North China Craton (NCC). New zircon/baddeleyite U-Pb/Pb-Pb dating on dolerite sills emplaced into the Fanhe Group yields emplacement ages from 1231 +/- 9 Ma to 1206 +/- 20 Ma. Whole-rock Ar-40/Ar-39 dating of two basalts from the Erdaogou Formation yields approximate total gas ages of ca. 1250 Ma and ca. 1210 Ma. These new geochronological results revealed that the Erdaogou basalts are coeval with some dolerite sills emplaced into the Kuangzhuangzi, Guanmenshan and Tongjiajie Formations and the dolerite sills emplaced into the Hutouling and Erdaogou Formations. These newly recognized ca. 1.23-1.21 Ga mafic rocks can be linked to the broader Licheng LIP of the previous studies. The ca. 1.23-1.21 Ga mafic rocks in the Fanhe basin can be divided into alkaline and sub-alkaline series and these two series exhibit distinct trace element and Nd isotopic compositions. The alkaline rocks show strongly fractionated chondrite-normalized REE patterns with high LaN/YbN values of 12.69-15.05. The sub-alkaline samples display less fractionated REE patterns with lower LaN/YbN values of 2.74-10.13. The alkaline rocks have OIB-like trace element patterns and slightly negative to positive eNd(t) values from 0.7 to 0.7. An OIB-like, asthenospheric mantle source is suggested as the source of the alkaline rocks. In contrast, the sub-alkaline rocks exhibit low negative eNd(t) values from 10.9 to 4.6 and are interpreted to be derived from a subduction-modified subcontinental lithospheric mantle. By comparisons with the coeval mafic magmatism in other parts of the NCC and other cratons, we suggest that the ca. 1.23-1.21 Ga mafic magmatism or LIP in the NCC was either related to the mantle plume that resulted in the Marnda Moorn LIP in Western Australia or related to a back-arc extension prior to the formation of a Grenvillian orogen located in another continent connected to the southeastern margin of the NCC during assembly of the Rodinia supercontinent.