柴北缘滩间山群玄武岩锆石定年及其地质意义

Zircon U-Pb Dating for Basaltic-andesite of Tanjianshan Group in the Lvliangshan Region of Qaidam Basin North Margin and Its Geological Significance

为较确切的确定柴北缘滩间山群成岩年龄,利用锆石LA-ICP-MSU-Pb定年及岩石化学分析方法对绿梁山地区玄武安山岩进行了研究。岩石化学分析结果显示,绿梁山玄武安山岩SiO2含量为51.92%~52.55%,ALK较低,为1.51%~2.23%,Na2O/K2O=14.10~26.30;稀土元素含量为30.04×10^-6~47.57×10^-6,LREE/HREE=1.03~1.51,δEu=0.89~0.99,具平缓的稀土元素配分曲线;微量元素蛛网图中,玄武安山岩相对亏损P、Ti等高场强元素,富集Sr、Rb、Ba等大离子亲石元素,具大陆边缘环境成因特点。通过锆石LA-ICP-MSU-Pb定年,获得绿梁山玄武安山岩206Pb/238U年龄加权平均值为(431.5±5.7)Ma,MSWD=0.35,代表了岩浆结晶年龄。研究表明,绿梁山玄武安山岩形成于闭合期的弧后盆地环境。据此,推测柴北缘滩间山群经历了自大洋俯冲至陆陆碰撞整个造山过程,其形成时间大致持续了80Ma。

In order to precisely determine the diagenetic age of Tanjianshan group,LA-ICP-MS technique for zircon U-Pb dating and rock Chemical analysis was applied on Lvliangshan basalt.The chemical analysis results of basaltic-andesite in Liangshan showed that of content SiO 2 ranged from 51.92%to 52.55%,ALK was from 1.51%to 2.23%,and Na 2O/K 2O ranged from 14.10 to 26.30;REE content changes at 30.04×10^-6 to 47.57×10^-6,LREE/HREE ranged from 1.03 to 1.51,δEu ranged from 0.89 to 0.99,with flat REE patterns.Trace element distribution pattern displayed that basaltic-andesite was relative loss of HFSE like P,Ti,etc.,rich of LILE like Sr,Rb,Ba,etc.,and have the genetic characteristics of the continental margin.By Zircon LA-ICP-MS U-Pb dating,LvLiangshan basaltic-andesite have 206 Pb/238 U weighted mean age of(431.5±5.7)Ma,and MSWD=0.35,which respresented the magmatic crystallization age.Studies have shown that,basaltic-andesite in LvLiangshan formed at the closure time of back-arc basin environment.It speculated that from the beginning of ocean diving to continental-continental collision,the formation of Tanjianshan group of North Qaidam margin approximately continued 80 Ma accordingly.