Geochemistry and U-Pb zircon geochronology of Late-Mesozoic lavas from Xishan,Beijing  

作　　者：YUAN Honglin1, LIU Xiaoming1, LIU Yongsheng2, GAO Shan1,2 & LING Wenli2 1. Key Laboratory of Continental Dynamics, Northwest University, Xi’an 710069, China 2. Faculty of Earth Sciences, China University of Geosciences, Wuhan 430074, China 

出　　处：《Science China Earth Sciences》2006年第1期50-67,共18页中国科学（地球科学英文版）

基　　金：the National Natural Science Foundation of China(Grant Nos.40302015,40472099 and 40373013);the Program for Changjiang Scholars and Innovative Research Team in University.

摘　　要：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.Zircon U-Pb dating by both SHRIMP and LA-ICP-MS and geochemical study of the Tiaojishan Formation and the Donglintai Formation from Xishan, Beijing, reveal that ages of upper lavas 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 between these two formations is less than 5 Ma. These lines of evidence prove that the two volcanoes erupted within a short time. The age of the Tiaojishan Formation from Xishan, Beijing is distinctively different from that of the Chende Basin. This indicated that the ages of Tiaojishan lavas varied in different regions. The Tiaojishan Formation consists of typical adakite (SiO2=56%, Na2O = 3.99-6.17, Na2O/K2O = 2.2-3.1, Sr = 680-1074×10?6, Y = 13.2-16.3×10?6, Yb = 1.13-1.52×10?6, Sr/Y = 43-66), high-Mg adakite and high-Mg andesite (Mg# = 54-55). Features of continental crust of adakite from the Tiao-jishan Formation and its syngeneric middle silicic vocanic rocks, such as typical Nd-Ta negative ab-normality and Pb possive abnormality, indicate that these lavas are originated from partial melts of continental crust. These results suggest that the adakite from the Tiaojishan Formation of Xishan, Beijing derived from thickened eclogitic lower crust and lithosphere beneath the North China craton at mesozoic that was foundered into the aesthenosphere, and subsequenctly partially melted and in-teracted with mantle olivine during melts upward migration. The age of lavas from the Tiaojishan Formation restrained the foundation which should last at least until 137 Ma. Lavas of the Donglintai Formation are rhyolith and andesite with normal Mg# and thus they did not interact with the mantle. These lavas represent remobilized melts of lower crust material caused by mantle aesthenosphere upwelling migration induced by foundation.

关 键 词：adakite  eclogite  geochemistry  North China craton  lower crust foundation. 

分 类 号：P588.14[天文地球—岩石学] P597.3[天文地球—地质学]