K-Ar dating of late Mesozoic volcanism and geochemistry of volcanic gravels in the North Huaiyang Belt, Dabie orogen: Constraints on the stratigraphic framework and exhumation of the northern Dabie orthogneiss complex

Two eruption episodes are identified through systematic field investigations and K-Ar dating of the late Mesozoic volcanic rocks in the North Huaiyang belt (NHB), Dabie orogenic belt, of which the earlier volcanic suite termed Maotanchang Fm. (Fm.) occurring at Jinzhai, Xianhualing and Maotanchang, etc., was erupted from 149 Ma to 138 Ma. The other named Xiaotian Fm. mainly distributed at Xiaotian, Shucheng, etc., was formed between 132 Ma and 116 Ma. During the eruption gap of the two volcanic suites deposited a volcano-sedimentary conglomerate layer, which are composed of the multi-compositional gravels, including the North Dabie orthogneiss complex (NDOC), volcanic gravels, etc. These volcanic gravels in the conglomerate layer show identical geochemical and isotopic compositions (87Sr/86Sr(t) =0.7084–0.7092, eNd (t) = −21.8–−24.4) to the Maotanchang Fm. volcanic rocks (87Sr/86Sr = 0.7086–0.7102, eNd= −19.2–−24.4), but significantly distinct from those of Xiaotian Fm. (87Sr/86Sr = 0.7076–0.7084, eNd= −17.2–−19.2). K-Ar dating results of its underlying and overlying volcanic sequences indicate that the conglomerate layers were deposited at −135 Ma. This suggests that the NDOC was rapidly exhumed to the surface during or shortly before −135 Ma and became the important provenance of the late Mesozoic volcano-sedimentary basins in the NHB. In combination with the regional volcano-sedimentary correlation, we divided the Mesozoic stratigraphic sequence in the NHB from base to top into Fanghushan Fm. (>160 Ma), Yuantongshan Fm. (/middlelower segment of Sanjianpu Fm.) (160–149 Ma), Maotanchang Fm. (/Zhougongshan Fm./upper segment of Sanjianpu Fm./Fenghuangtai Fm.) (149–135 Ma) and Xiaotian Fm. (/Baidafan Fm./Heshidu Fm.) (135–116 Ma).