Zircon U-Pb dating by both SHRIMP and LA-ICP-MS and geochemical study of theTiaojishan Formation and the Donglintai Formation from Xishan, Beijing, reveal that ages of upperlavas of Tiaojishan Formation and Middle of Donglintai Formation are 137.1 ± 4.5 Ma(2σ) and 130—134 Ma, respectively. The fomer is slightly older than the latter and the age difference betweenthese two formations is less than 5 Ma. These lines of evidence prove that the two volcanoes eruptedwithin a short time. The age of the Tiaojishan Formation from Xishan, Beijing is distinctivelydifferent from that of the Chende Basin. This indicated that the ages of Tiaojishan lavas varied indifferent regions. The Tiaojishan Formation consists of typical adakite (SiO_2 = 56%, Na_2O =3.99—6.17, Na_2O/K_2O = 2.2 -3.1, Sr = 680-1074 x 10^(-6), Y = 13.2-16.3 x 10^(-6), Yb = 1.13-1.52x 10^(-6), Sr/Y = 43-66), high-Mg adakite and high-Mg andesite (Mg# = 54—55). Features ofcontinental crust of adakite from the Tiaojishan Formation and its syngeneric middle silicic vocanicrocks, such as typical Nd-Ta negative abnormality and Pb possive abnormality, indicate that theselavas are originated from partial melts of continental crust. These results suggest that the adakitefrom the Tiaojishan Formation of Xishan, Beijing derived from thickened eclogitic lower crust andlithosphere beneath the North China craton at mesozoic that was foundered into the aesthenosphere,and subsequenctly partially melted and interacted with mantle olivine during melts upward migration.The age of lavas from the Tiaojishan Formation restrained the foundation which should last at leastuntil 137 Ma. Lavas of the Donglintai Formation are rhyolith and andesite with normal Mg# and thusthey did not interact with the mantle. These lavas represent remobilized melts of lower crustmaterial caused by mantle aesthenosphere upwelling migration induced by foundation.
YUAN Honglin1, LIU Xiaoming1, LIU Yongsheng2, GAO Shan1,2 & LING Wenli2 1. Key Laboratory of Continental Dynamics, Northwest University, Xi’an 710069, China
The Wudangshan, Yaolinghe volcanic-sedimentary sequences and doleritic-gabbroic sills comprise the largest exposed Precambrian basement in South Qinling. Zircons separated from 5 volcanic-pyroclas- tic samples of the Wudangshan Group, 2 volcanic samples of the Yaolinghe Group and one sample for the mafic sills were used for U-Pb dating by laser ablation-inductively coupled plasma mass spec- trometry (LA-ICPMS). The results reveal that the Wudangshan volcanic sequence was formed at (755±3) Ma (a weighted mean from the 5 samples, MSWD=0.47), whereas the Yaolinghe volcanic suite and the mafic sill were crystallized at (685±5) (2 samples, MSWD=0.36) and (679±3) Ma (MSWD=1.6), respectively, which are equal to each other within analysis errors. These ages are markedly younger than those previously documented for the rocks. The newly obtained ages for the Wudangshan and Yaolinghe Groups are identical to those of the bottom Liantuo and slightly older than those of the Nantuo Forma- tions, respectively, lower strata of the Nanhua (middle to late Neoproterozoic) stratotype section in eastern Three Gorges, Yangtze craton. A range of inherited magmatic zircons was recognized with ages of 830 to 780 Ma, which are typical of Neoprotzrozoic magmatisms recorded along the margins and interior of the Yangtze craton. Thus, there is Neoproterozoic basement comprising 830―780 Ma igne- ous suites in South Qinling; the inherited zircons were detrital sediments derived from the northern margin of the Yangtze craton. Accordingly, it is suggested that the South Qinling is a segment of the Yangtze craton before the Qinling Orogeny.
