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, 8Nd (t) = -21.8--24.4) to the Maotanchang Fm. volcanic rocks (87Srl 6 Sr = 0.7086---0.7102, &Nd = -19.2-24.4), but significantly distinct from those of Xiaotian Fm. (87Sr/86Sr = 0.7076---0.7084, 8Nd= -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. (/middle-lower segment of Sanjianpu Fm.) (160-149 Ma), Maotanchang Fm. (/Zhougongshan Fm.lupper segment of Sanjianpu Fm.lFenghuangtai Fm.) (149-135 Ma) and Xiaotian Fm. (/Baidafan Fm./Heshidu Fm.) (135-116 Ma).
1688There are different viewpoints on the metamorphic age of the high-to ultra-high pressure (HP-UHP) rocks in the Dabie orogenic belt [I-7], but numerous geochronological data marked the rapid exhumation/uplifting of HP-UHP rocks since the Indo-Sinian [3][4][5][6][7][8][9][10][11][12]. Recent studies [8,9] showed that HP-UHP metamorphic rocks in the Dabie orogenic belt encountered a peak metamorphism during the early Triassic, and then rapidly exhumed to middle/lower crustal levels (~700°C) at 230-210 Ma, accomplished by the eclogite-facies recrystallization and/or granulite-facies superimposed metamorphism. This was followed by a later exhumation under the amphibolite-facies retro-metamorphism at ca. 200 Ma [8,9] or by a rapid exhumation of the HP-UHP rocks and their hosted terrain from middle-lower crust to middle-upper crust levels at 13]. However, the precise age when the HP-UHP rocks and their hosting rocks were exhumed to the surface is still poorly constrained [14,15].In recent years evidence from detritus composition of Jurassic sediment in Hefei-Liu'an basins, C-O isotopic composition of marble gravels, and from composition of detrital garnet indicates that there was other unidentified material source except for the NDOC by the beginning of late Jurassic [16][17][18]. Alt...
