大兴安岭南段三叠纪基性火山岩时代与构造环境

Age and tectonic setting of Triassic basic volcanic rocksin southern Da Hinggan Range.

应用火山岩锆石U-Pb LA-ICPMAS法和基质Ar-Ar法定年,表明大兴安岭南段存在三叠纪玄武质岩石,其年龄为214～250Ma。主量和微量元素地球化学反映三叠纪火山岩为亚碱性和低K拉斑玄武岩系列,部分岩石具岛弧火山岩地球化学性质,但主要呈造山后伸展环境下的岩浆活动特征。Sr-Nd-Pb同位素组成说明三叠纪基性火山岩岩浆源区主要为岩石圈地幔,但局部地区有软流圈物质加入;岩浆演化过程受部分熔融和分离结晶作用控制,并有少量地壳物质的混染。综合研究表明三叠纪大兴安岭南段处于中亚造山带后造山伸展阶段,基性岩浆形成于软流圈上涌诱发岩石圈地幔的部分熔融,拉斑玄武岩和辉绿玢岩脉即为伸张条件下基性岩浆活动的产物。结合区域构造演化,本文推测西拉木伦河-长春-延吉缝合带可能具有西段造山和造山后伸张作用稍早于东段的特征。

Using zircon U-Pb LA-ICPMS and matrix Ar-Ar dating for volcanics, it is indicated that some Triassic （214 -250 Ma） basic volcanics were distributed in southern Da Hinggan Range, The analysis of major and trace elements show that the volcanics are characterized by low K2O contents （0. 16% - 1. 70% ） , low MgO contents （2. 89% - 10. 30% ） , enrichment of LREE, no pronounced Eu anomaly （ δEu = 0.73 - 1.09 ） and weak negative Nb and Ta anomalies. These geochemical characteristics are similar to those of continental tholeiite, Discrimination diagrams of various trace elements indicate that these volcanic rocks were formed in post-orogen stage. Isotopic geochemistries show that values of εNd （t） and （^87 Sr/^86Sr）i of the volcanic range from 0.4 to 6, 9 and from 0. 7032 to 0. 7049, respectively, The ratios of ^206Pb/^204Pb,^207Pb/^204Pb and ^208Pb/^204Pb vary from 18.22 to 18.59, 15, 55 to 15.61 and 38.18 to 38, 52, respectively, These geochemistries show that the basaltic magam was formed in partial melting of lithoshperic mantle. Based on above mentioned characteristics, combined with the regional researching results, we suggest that the southern Da Hinggan Range was very likely to an extensional environment of post-orogen during the Triassic.