Ar-Ar dating results, petrography, major and trace element and Sr-Nd-Pb isotopic data of the late Mesozoic volcanic rocks in the southeastern Jilin Province are presented in this paper with aims to understand the origin of the extensive volcanism and the possible geological implications. Major conclusions are summarized below.1 The Mesozoic volcanic rocks in the southeastern Jilin Province mainly erupted in early Cretaceous (106-125Ma), temporarily consistent with the late Mesozoic extensive magmatism in eastern China. There did no exist so-called "late Triassic" and "early Jurassic" volcanic rocks in previous work, and the adakitic rocks from the Quanshuicun Formation (Fm.) erupted in Palaeocene (55-58 Ma).2 On the basis of new Ar-Ar ages, the Mesozoic-early Cenozoic volcano-sedimentary sequence in the Yanji area is rebuilt as: Sanxianling Fm./Tuntianying Fm. (118-115 Ma), Malugou Fm./Tianqiaoling Fm. (K1), Jingouling Fm. (108-106 Ma), Changcai Fm. (K2), Quanshuicun Fm. (55 Ma), and Dalazi Fm.3 Both groups (Guosong Fm. and Sankeyushu Fm.) of intermediate-felsicvolcanic rocks from the Tonghua area exhibit high-K calc-alkaline to shoshoniticaffinities, characterized by light rare earth element (LREE) and large ion lithophileelement (LILE) enrichment and variable high field strength element (HFSE, e.g., Nb,Ta and Ti) depletion, and moderately radiogenic Sr and unradiogenic Nd and Pbisotopic compositions. Compared with the younger group (Sankeyushu Fm.), theearlier group (Guosong Fm.) of rocks have relatively higher K2O, A/CNK, HFSEconcentrations (e.g., Nb = 36.2-44.4 ppm in Group 1 compare with Nb = 12.2- 22.1ppm in Group 2), and Sr-Nd-Pb isotope ratios (87Sr/86Sr(i) = 0.70584 0.70656, εNd (t)= -11.9 -8.2,206Pb/204Pb(i) = 16.41 16.97 in Guosong Fm.; 87Sr/86Sr(i) = 0.70539...
|