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
85 volcanic rocks of the Yixian Formation from the Sihetun type section werecollected and analyzed for geochemical and isotopic compositions. Major element compositionsindicate that the Sihetun volcanic rocks are high magnesium andesites with some basalts occurring atthe bottom of section. The Sihetun high magnesium andesites (SiO_2 = 52.82-59.31 wt%, Al_2O_3=14.15-16.35wt%) show many characteristics of adakites such as depletion in heavy rare-earth elements(HREE; Yb = 1.03-1.88 μg/g) and Y(12-20 μg/g) and high Sr (620-1323 μg/g) and Sr/Y(32-88), withhigh La_N/Yb_N ratio (10-25). They share similar major and trace element characteristics to volcanicrocks from the Xinglonggou Formation except their lower Nd isotope ratios (^(143)Nd/^(144)Nd(130Ma) = 0.5118-0.5119, ε_(Nd) (130Ma) = -11.6-13.8, ^(87)Sr/^(86)Sr (130 Ma) = 0.7058-0.7064.They were interpreted in a way that eclogite that formed at the base of thickened Archean lowercrust of the North China craton foundered into the convecting mantle and subsequently melted andinteracted with peridotite. However, compared to the Xinglonggou volcanic rocks, the source of theSihetun magma contained more ancient continental crustal material in order to explain its evolved Ndisotopes. The age of the Sihetun Formation was 120 to 130 Ma, and this indicates that delaminationlasted to the early Cretaceous period. The Sr contents and Sr/Y ratios of the Sihetun high-Mgandesites show significant negative correlations with SiO_2 for samples with SiO_2 > 56%. Thesesuggest that the Sr and Sr/Y values were reduced due to fractional crystallization of plagioclase.Accordingly, the effect of crystallization on volcanic Sr and Sr/Y ratio has to be taken intoaccount.
WANG Xiaorui1, GAO Shan1,2, LIU Xiaoming1, YUAN Honglin1, HU Zhaochu1, ZHANG Hong1,3 & WANG Xuance4 1. State Key Laboratory of Continental Dynamics, Department of Geology, Northwest University, Xi`an 710069, China
