The early Cretaceous trachytes of Donglingtai Formation in Xishan, Beijing are characterized by slight Eu negative anomaly and significant enrichment in LREE, LILE (Ba, K and Sr) and depletion of Nb-Ta-Ti, Th-U. These trachytes have been highly enriched by Sr and Nd isotopic signatures (87Sr/86Sr(t)= 0.70638～ 0.70672, εNd(t) = -16.3～ -15.7), overlapping Sr-Nd isotopic range of late Mesozoic mafic igneous rocks in the region. Taking into account Nb-Ta fractionation and high Zr/Sm ratio for these trachytes, we consider that the trachytes of Donglingtai Formation are derived from the garnet-bearing amphibolite in the lower crust composed of garnet + plagioclase ± amphibole ± pyroxene residual phases. On the basis of the melting experimental results of crustal materials and regional extensional tectonics during late Mesozoic, it is concluded that the thermal input from underplating basaltic melt results in the partial melting of lower continental crust to generate the trachytes of Donglingtai Formation. And the characteristics of high Sr and Ba for these trachytes suggest that part of underplating basalts should take a share in partial melting at least.
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,εNd(t) =-21.8--24.4) to the Maotanchang Fm. volcanic rocks (87Sr/86Sr = 0.7086-0.7102,εNd = -19.2-24.4), but significantly distinct from those of Xiaotian Fm. (87Sr/86Sr = 0.7076-0.7084, εNd=
